A STUDY OF CZTS THIN FILMS FOR SOLAR CELL APPLICATIONS
ADISSERTATION SUBMITTED TO THE DEPARTMENT OF MATERIALS SCIENCE AND ENGINEERING AND THE COMMITTEE ON GRADUATE STUDIES OF STANFORD UNIVERSITY IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY
Vardaan Chawla December 2011
© 2011 by Vardaan Chawla. All Rights Reserved. Re-distributed by Stanford University under license with the author.
This work is licensed under a Creative Commons Attribution- Noncommercial 3.0 United States License. http://creativecommons.org/licenses/by-nc/3.0/us/
This dissertation is online at: http://purl.stanford.edu/pd193nf2703
ii I certify that I have read this dissertation and that, in my opinion, it is fully adequate in scope and quality as a dissertation for the degree of Doctor of Philosophy.
Bruce Clemens, Primary Adviser
I certify that I have read this dissertation and that, in my opinion, it is fully adequate in scope and quality as a dissertation for the degree of Doctor of Philosophy.
Stacey Bent
I certify that I have read this dissertation and that, in my opinion, it is fully adequate in scope and quality as a dissertation for the degree of Doctor of Philosophy.
Michael McGehee
Approved for the Stanford University Committee on Graduate Studies. Patricia J. Gumport, Vice Provost Graduate Education
This signature page was generated electronically upon submission of this dissertation in electronic format. An original signed hard copy of the signature page is on file in University Archives.
iii Abstract
Recently, a lot of attention has been given to the link between the rise in fossil fuel consumption, CO2 emissions, and the average temperature of the planet. Multiple models have shown that if this trend is allowed to continue, the consequences for the environment could be quite significant. Furthermore, even if we are able to reduce our consumption of fossil fuels and reduce the CO2 levels, it will not change the fact that fossil fuels are a limited resource and are likely to be exhausted within the next 100- 150 years. Addressing this problem will require the development of a renewable source of energy that is carbon neutral. Solar radiation is one of the most abundant forms of energy available on the planet, but harnessing this energy has been hampered by the high costs of current solar technologies. These costs stem from the use of non-optimal (silicon), toxic (cadmium) and expensive (indium) materials. To further reduce the costs of solar energy, a novel, earth abundant, non-toxic and inexpensive material is required that can make this energy source competitive with fossil fuels.
Previous work on Cu2ZnSn(S,Se)4 (CZTSSe) has shown that it is an excellent can- didate for use in thin film solar cells due to its earth abundant, inexpensive, non-toxic constituents and optimal material properties. The focus of this work was to use sput- ter deposition to synthesize this material as a thin film, incorporate it into a device and develop techniques to improve e ciency. Reactive sputtering was used to grow thin films of the pure sulfide Cu2ZnSnS4 (CZTS) material while compound sputtering was used to incorporate selenium into the film and grow the sulfo-selenide hybrid ma- terial Cu2ZnSn(S,Se)4 (CZTSSe). The thin films were incorporated into devices based on the the widely accepted Cu(InGa)Se2 (CIGS) device stack. The composition of the films was analyzed using inductively coupled plasma optical emission spectroscopy
iv and Auger electron spectroscopy. Morphology and interfaces between device layers were imaged using scanning electron microscopy. Phase analysis was conducted us- ing x-ray di↵raction and Raman spectroscopy. The devices were characterized using current-voltage and external quantum e ciency measurements. Reactive sputtering of CZTS films results in a unique morphology that can be controlled by varying the deposition parameters. Short circuit current (Isc)andef-
ficiency were highly dependent on film composition while open circuit voltage (Voc) was not. The films phase separated when grown o↵stoichiometry but the secondary phases detected did not always agree with the established phase diagram. This phase separation was also controlled using deposition parameters and used to form nano- structured CZTS-ZnS films. Compound sputtering of CZTSSe films resulted in signif- icantly larger grains as compared with CZTS films. The device performance was also significantly higher with all device parameters (Isc,Voc etc.) showing improvement. The secondary phases detected in this material did agree with the established phase diagram and it was shown that copper sulfide phase (Cu2S) is very detrimental to device performance while SnS2 and ZnS are not. Thus, the ideal composition regime for growth of CZTSSe would be copper poor and zinc/tin rich. The best e ciencies achieved during this work were 3.4% for CZTS and 9.3% for CZTSSe.
v Acknowledgements
This work could not have been completed without the guidance of my advisor, Dr. Bruce Clemens. Over the years he has challenged me, encouraged me and when needed, provided a swift kick in the rear to keep me on the right track.
This project has been funded by the Global Climate and Energy Project (GCEP), Center on Nanostructuring for E cient Energy Conversion (CNEEC) and AQTSolar. I thank GCEP and CNEEC for their support and the team at AQTSolar for providing not just the funds to continue this work, but also equipment and guidance to make it a success.
Thank you to the students in the Clemens, McGehee and Bent groups (Gloria, Randy, Steve, Melody, Chinmay, Yesheng, Joel, CJ, Mike, Zach, Je↵, Jonathan, Art and more.) for support with equipment and help with experiments and to Dr. Mike McGehee and Dr. Stacey Bent for use of their facilities and for providing comments on my dissertation.
I would also like to thank the people that kept me sane through 5+ years of Ph.D. work. The taco tuesday group for weekly entertainment, my roommate Danielle for the many wine breaks while we were both writing our dissertations, friends from OEP for the many weekend adventures, and my family for their words of encouragement.
And most important, my sister and brother-in-law who made a large sacrifice to raise me and without whom, I would not have been able to complete this work.
vi Contents
Abstract iv
Acknowledgements vi
1Motivation 1 1.1 The Energy Problem ...... 1 1.2 The Solar Solution ...... 3 1.2.1 Crystalline Silicon ...... 3 1.2.2 AmorphousSilicon ...... 5 1.2.3 ThinFilm...... 5
1.3 Cu2ZnSnS4 (CZTS) ...... 6
2 Background 9 2.1 Sputtering ...... 9 2.1.1 ReactiveSputtering...... 9 2.1.2 CompoundSputtering ...... 11 2.2 Materials Characterization Techniques ...... 11 2.2.1 Scanning Electron Microscopy (SEM) ...... 11 2.2.2 AugerElectronSpectroscopy(AES)...... 12 2.2.3 Inductively Coupled Plasma Optical Emission Spectroscopy (ICP) 14 2.2.4 X-ray Di↵raction(XRD)...... 14 2.2.5 Raman Spectroscopy (RS) ...... 17 2.3 Solar Cells ...... 19 2.3.1 Solar Spectrum ...... 19
vii 2.3.2 p-nJunctions ...... 19 2.3.3 Heterojunctions...... 24 2.3.4 Equivalent Circuit ...... 25 2.4 Device Characterization Techniques ...... 26 2.4.1 CurrentVoltageMeasurement(IV) ...... 26 2.4.2 External Quantum E ciency Measurement (EQE) ...... 28
2.5 Cu2ZnSn(S,Se)4 (CZTSSe) ...... 30 2.5.1 CZTS vs. CZTSe vs. CZTSSe ...... 30 2.5.2 Material Properties ...... 31 2.5.3 Phase Diagram ...... 31 2.5.4 PhaseAnalysis ...... 33 2.5.5 Device Stack ...... 33
3 Materials and Methods 36 3.1 AbsorberDepositionandAnnealing ...... 36 3.1.1 Reactive co-sputtering at high temperature ...... 36 3.1.2 Reactive co-sputtering at low temperature followed by a post deposition anneal ...... 37 3.1.3 Compound sputtering at room temperature followed by a post deposition anneal ...... 37 3.1.4 Deposition Chambers ...... 38 3.2 Device Fabrication ...... 39 3.3 Characterization Techniques ...... 40 3.3.1 Scanning Electron Microscopy (SEM) ...... 40 3.3.2 AugerElectronSpectroscopy(AES)...... 40 3.3.3 Inductively Coupled Plasma Optical Emission Spectroscopy (ICP) 40 3.3.4 X-ray Di↵raction(XRD)...... 41 3.3.5 Raman Spectroscopy (RS) ...... 42 3.3.6 Current Voltage Measurements ...... 42 3.3.7 External Quantum E ciency Measurements ...... 42
viii 4 Results and Discussion 43
4.1 Cu2ZnSnS4 (CZTS) ...... 43 4.1.1 Growth Temperature Study ...... 43 4.1.2 PhaseAnalysis ...... 47 4.1.3 Morphology ...... 58 4.1.4 Devices ...... 60 4.1.5 NanostructuredCZTS-ZnSFilms ...... 63
4.2 Cu2ZnSn(S,Se)4 (CZTSSe) ...... 70 4.2.1 AdditionofSelenium ...... 70 4.2.2 NoteonPhaseAnalysis ...... 71 4.2.3 Morphology ...... 71 4.2.4 High E ciencyDevices...... 73
5 Conclusion 83
Bibliography 86
ix List of Tables
4.1 FilmstoichiometryfromICP-OESMeasurement ...... 46 4.2 CZTSSe device parameters for devices with varying Se/S ratio. . . . . 78
x List of Figures
1.1 Global CO2 emissions and di↵erent emission stabilization scenarios (left) and the calculated increase in temperatures of the planet (right) for di↵erent stabilization targets as determined by the Intergovernmen- tal Panel on Climate Change (IPCC). From [1]...... 2 1.2 Global exergy flux and reservoirs of di↵erent forms of exergy available on the planet. From [2]...... 4 1.3 Relative abundance (top) and bulk cost (bottom) of elements com- monly used in the production of solar cells. From [3]...... 7 1.4 Relative global production of elements commonly used for production of solar cells. From [3]...... 8
2.1 Schematic of the basic sputter process. A voltage is applied between the target and the substrate to accelerate argon ions into the target and eject target atoms. The target atoms deposit on the substrate, creating a thin film...... 10 2.2 A schematic of the Auger e↵ect. An electron is knocked out of one of the core orbitals and the hole is filled with an electron from one of the higher energy orbitals. Energy is released in this process and results in ejection of an electron from one of the outer orbitals...... 13 2.3 Graphical representation of Bragg’s law: n =2dsin(✓)...... 15 2.4 The di↵erence between the x-ray path in symmetric and grazing angle XRD configurations. The x-ray travels through much more film in the grazing angle configuration...... 18
xi 2.5 The AM0 and AM1.5 solar spectrum (green curves, left axis). Inte- grating these curves gives the total power density of incident light (blue curves, right axis)...... 20 2.6 The formation of a p-n junction. A p-type and an n-type semiconductor as placed in contact, which results in di↵usional flux of electrons and holes and the formation of a built in potential...... 21 2.7 A p-n junction when illuminated by one photon. An electron-hole pair is generated and split using the built in electric field...... 23 2.8 The accepted band diagram for a CIGS heterojunction solar cell. There are three materials (CIGS, CdS and ZnO) that form the junction. From [4]...... 24 2.9 The equivalent circuit used to represent solar cells. It consists of a current source, a diode, a shunt resistor and a series resistor...... 26
2.10 Example IV Curve. The short circuit current density (Jsc), open circuit
voltage (Voc)andmaximumpowerpoint(Pmax)aremarked...... 27 2.11 Example EQE curve. The di↵erent loss mechanisms are marked and discussed in the text. From [4]...... 29 2.12 The chalcopyrite crystal structure shared by CIS, CIGS, CZTS, CZTSSe and CZTSe. From [5]...... 30 2.13 A) Maximum theoretical e ciency as a function of bandgap of absorber material. B) Absorption coe cient of CZTS and CZTSe as a function of photon energy. From [6] and [5] respectively...... 32
2.14 Isothermal section of Cu2S-ZnS-SnS2 ternary phase diagram at 670K. Region 1 shows the composition range under which CZTS can form as
a pure phase (left). Binary phase diagram of the Cu2SnS3-ZnS system. CZTS is the line compound at 0.5 mol% ZnS (right). From [7]. . . . . 32
2.15 Simulated XRD patterns of CZTS, Cu2SnS3 and ZnS [8] and locations of Raman peaks [9–13] of phases that have been identified in CZTS films by other groups...... 34 2.16 The CIGS device stack that has been adapted for CZTS...... 35
xii 3.1 Schematic of the sputter chamber designed for reactive sputtering with
H2S. It is equipped with three sources for the metallic components of CZTS and a secondary chamber with a molybdenum source for depo- sition of a back contact...... 39
4.1 Symmetric XRD patterns of CZTS thin films grown at A) 100 C, B)
200 C, C) 300 CandD)400 C...... 44
4.2 AES depth profiles of CZTS films grown at A) 100 C, B) 200 C, C)
300 CandD)400 C. The surface of the film is at the origin on the x-axis...... 45
4.3 Isothermal section of Cu2S-ZnS-SnS2 ternary phase diagram at 670K. From [7]. The red, green and blue lines show the areas of the phase diagramthatwereexploredinthisstudy...... 48 4.4 Symmetric XRD patterns of CZTS films with varying A) Cu/(Zn+Sn) ratio and B) Zn/(Cu+Sn) ratio...... 50 4.5 Grazing angle XRD pattern of a stoichiometric CZTS film...... 51 4.6 GIX patterns of CZTS films with varying Cu/(Zn+Sn) ratio and con- stant Zn/Sn ratio...... 51 4.7 GIX patterns of CZTS films with varying Sn/(Zn+Cu) ratio and con- stant Cu/Zn ratio...... 52 4.8 GIX patterns of CZTS films with varying Zn/(Cu+Sn) ratio and con- stant Cu/Sn ratio...... 53 4.9 Raman spectra of CZTS films with varying Cu/(Zn+Sn) ratio and constant Zn/Sn ratio. Raman peaks of some relevant phases are marked. 54 4.10 Raman spectra of CZTS films with varying Zn/(Cu+Sn) ratio and constant Cu/Sn ratio. Raman peaks of some relevant phases are marked. 55 4.11 AES image maps of zinc rich CZTS thin film: A) SEM image, B) copper image map, C) zinc image map, D) tin image map, E) overlay of copper, zinc and tin image maps...... 57
4.12 SEM images of CZTS films grown at A) 400 Cand40mtorrandB)
550 Cand12mtorr...... 58
xiii 4.13 SEM images of CZTS films grown at room temperature and then an-
nealed at 550 Cin10%H2Sat40mtorr...... 59 4.14 A) IV and B) EQE measurements of the best CZTS device grown using reactive sputtering...... 61
4.15 A) Open circuit voltage (Voc), B) short circuit current density (Jsc) and C) e ciencyasafunctionofCu/(Zn+Sn)ratio...... 62 4.16 Comparison of a conventional CZTS device (left) with a CZTS nanos- tructured device (right) ...... 64 4.17 A) Raman spectrum of a CZTS-ZnS film, B) Symmetric XRD pattern of a CZTS-ZnS film...... 66 4.18 AES image maps of a nanostructured CZTS-ZnS film: A) copper, B) zinc, C) tin, D) cadmium, E) overlay of copper, zinc and tin, F) overlay ofcadmiumandtin...... 67 4.19 IV and EQE measurements of a nanostructured CZTS-ZnS device. . . 69 4.20 AES depth profile for a nanostructured CZTS film. The cadmium signal can be seen throughout the depth of the film...... 70 4.21 XRD patterns of films with varying selenium to sulfur ratio and a ZnS phase that can be identified after the CZTSSe pattern shifts . . . . . 72 4.22 Cross-sectionSEMimagesoftwoCZTSSefilms...... 73 4.23 E ciency vs. Zn/(Cu+Sn) ratio (top) and the XRD identification of the ZnS (110) peak (bottom)...... 75 4.24 E ciency vs. Sn/(Cu+Zn) ratio (top) and the XRD identification of
the SnS2Speak(bottom)...... 76 4.25 E ciency vs. Cu/(Zn+Sn) ratio (top) and the XRD identification of
the Cu2Speak(bottom)...... 78 4.26 IV curves of higher e ciency films with varying Cu/(Zn+Sn) ratio. . 79 4.27 A) IV curves and B) EQE spectra for devices with varying Se-S ratio. 80 4.28 A) IV curve and B) EQE spectrum for the best CZTSSe device, E↵=9.3% 82
xiv Chapter 1
Motivation
1.1 The Energy Problem
If we take a step back and look at the “big picture,” the motivation for this work comes from the drastic rise in CO2 emissions and the current climate crisis threatening the world. Multiple independent agencies have shown that CO2 emissions have been increasing significantly since the dawn of the 20th century. This is only natural since the majority of the world’s energy comes from carbon based fuels and any modern civilization requires inexpensive, abundant energy to continue to grow. However, asidee↵ectoftheseemissionsisthegreenhousee↵ectandoverallwarmingofthe planet. If this trend continues it is likely that the consequences will be drastic changes to the planet’s climate.
China, United States and India were the three largest CO2 emitting countries in 2009. [14] While U.S. emissions are relatively static from year to year, Chinese and Indian emissions are growing exponentially. Thus, overall CO2 emissions are expected to continue to grow as the economies of developing countries require more energy. Figure 1.1 shows the results of modeling by the Intergovernmental Panel on Climate Change (IPCC) to determine the expected rise in the planet’s average temperature if we are able to stabilize CO2 at di↵erent levels. Even if we are able to stop the increase in emission and stabilize the rate at the current value of 29 Gt/yr, there will still be a 4 Cincreaseintheaveragetemperatureoftheplanet.
1 CHAPTER 1. MOTIVATION 2
Figure 1.1: Global CO2 emissions and di↵erent emission stabilization scenarios (left) and the calculated increase in temperatures of the planet (right) for di↵erent stabi- lization targets as determined by the Intergovernmental Panel on Climate Change (IPCC). From [1].
Even if it were possible to reduce our consumption of fossil fuels and stop the increase in the warming of the planet, the fact still remains that fossil fuels are a limited resource. A simple calculation of reserves-to-production ratio (current known reserves divided by current production rate) for each fossil fuel (coal, oil, natural gas) shows that, at our current consumption rate, we are likely to run out of known oil reserves in approximately 40-50 years, coal in 120-150 years and natural gas in 50-60 years. [15] While this simple calculation does not take into account prospecting for new reserves or increases in consumption rate, it does provide an idea of how limited fossil fuels are and how urgent the energy crisis is. CHAPTER 1. MOTIVATION 3
1.2 The Solar Solution
Solar power, on the other hand, is an entirely renewable, abundant resource. Figure 1.2 shows the global exergy1 flux and exergy reservoirs on the planet. The green arrows and bubbles are the fossil fuels, while the large orange arrows that start at the top left of the figure are the solar radiation. As can be seen, fossil fuels are a very small portion of the total exergy available, while solar radiation is one of the abundant forms of exergy on the planet. Furthermore, the majority of fossil fuels are in limited reservoirs, while solar radiation is a renewable, constant flux. So why aren’t solar panels the primary source of energy for our planet? The answer to this question lies with the way the solar flux is harnessed. A number of technologies exist for this task, but each one has enough drawbacks to keep it from becoming competitive with fossil fuels. For a solar technology to make a large impact on the energy consumption of the world, it needs to be cheap, non-toxic and utilize only abundant materials. The currently commercialized technologies and their drawbacks will be discussed here.
1.2.1 Crystalline Silicon
Silicon solar cells are currently the dominant solar technology. Eighty percent of all solar cells are silicon based and it is the oldest technology (first generation) with the largest research base. However, silicon is an indirect bandgap semiconductor, which means that thick layers (200µm) of this material are required to absorb all the sunlight available. Furthermore, the silicon must be of a very high purity level, which, combined with the large quantity required, increases costs significantly. As a result, even though silicon technology has been around for over 30 years, with record e ciencies as high as 25%, it has yet to reach grid parity or make a large impact on the total energy usage of the world. [16]
1Exergy is the useful portion of an energy source. While energy must be conserved, exergy can be destroyed when it is converted from one form of energy to another. CHAPTER 1. MOTIVATION 4
Global Exergy Flux, Reservoirs, and Destruction
34000 62500 Atmospheric Reflection Extra-Solar Radiation Wind Energy 162000 0.06 Ocean Thermal Gradient Atmospheric 60 Waves Solar Absorption 0 Wave 0 Radiation Energy Absorption 100 31000 870 Wind OTEC 5000 41000 Evaporation 0.36 86000 300 7.2 Hydro- Surface Surface Incident 5.4 electricity Reflection Surface Heating 90 Scattering 43000 Clouds Rivers
90 Photosynthesis 1.2 Traditional Biomass 3.7 Tides 0.016 Solar Energy 0.15 Commercial Biofuels 30 ZJ 3100 ZJ 3.5 Ocean Tides 0.04 Carbon Burial Plants 1000 ZJ Lithium
Terrestrial Environment Tidal 1 0.0005 Energy 3.6 Coal Uranium Nuclear Fuel 270 ZJ Coal Solid Earth 0.2 Tides 5.0 Oil 300 ZJ 360000 ZJ Methane Hydrate 110 ZJ Oil Thorium Seawater Geothermal Energy Uranium 200 ZJ 3.2 Gas 1E10 ZJ Deuterium 0.03 50 ZJ Gas
32 Crustal Thermal Thermal Kinetic Natural Exergy Destruction Exergy Accumulation [ZJ] (=10 21 J) Energy 1.5E7 ZJ KEY Nuclear Chemical Human Use for Energy Services Exergy Flux [TW] (=10 12 W) Radiation Gravitational
Exergy is the useful portion of energy that allows us to do work and perform energy services. We gather exergy from energy-carrying substances in the natural world we call energy resources. While energy is conserved, the exergetic portion can be destroyed when it undergoes an energy conversion. This diagram summarizes the exergy reservoirs and flows in our sphere of influence including their interconnections, conversions, and eventual natural or anthropogenic destruction. Because the choice of energy resource and the method of resource utilization have environmental consequences, knowing the full range of energy options available to our growing world population and economy may assist in efforts to decouple energy use from environmental damage. Prepared by Wes Hermann and A.J. Simon Global Climate and Energy Project at Stanford University (http://gcep.stanford.edu) Ver. 1.1 © GCEP 2005, 2007
Figure 1.2: Global exergy flux and reservoirs of di↵erent forms of exergy available on the planet. From [2]. CHAPTER 1. MOTIVATION 5
1.2.2 Amorphous Silicon
Amorphous silicon is a noncrystalline semiconductor in which the lattice structure of crystalline silicon is eliminated while keeping the chemical bonding intact. This changes the absorbance characteristics of the material and can allow thinner layers to be used for devices. An amorphous silicon solar cell uses a p-i-n (or n-i-p) structure which consists of very thin p-type and n-type layers and a thicker intrinsic layer where the majority of the light is absorbed. These structures can even be stacked on top of each other and tuned to improve the e ciency of these cells while using only 1% of the silicon required for crystalline silicon cells. However, creating these multilayer stacks increases the complexity of the production process and as a result, significantly increases the costs. Furthermore, amorphous silicon cells su↵er from low e ciencies that are further degraded (by about 20% of the initial value) when the cell is placed in sunlight for a long period of time.
1.2.3 Thin Film
Thin film solar technology (second generation) was developed as a response to the high material usage and costs of silicon solar cells. It utilizes materials that have high absorption coe cients to minimize the quantity of material required. It is also focussed on the use of polycrystalline materials and “dirty” (as compared with sil- icon) processing to further reduce costs. However, this reduction in costs has also led to a reduction in overall e ciency, with the best thin film cell e ciencies limited to 20.3%. [17] This technology is currently being commercialized using two mate- rial systems: cadmium telluride (CdTe) and copper indium gallium selenide (CIGS). While CdTe and CIGS solar cells are the cheapest available on the market, there are some significant disadvantages to both of these technologies. Figure 1.3 shows acomparisonoftherelativeabundanceintheearth’scrustandcostin$/ton of some common elements used in thin film solar cells. Tellurium (used in CdTe) is by 3 far the least abundant material in the earth’s crust (10 ppm) and has one of the highest costs ($400000/tonne). Indium and Gallium, used for CIGS, are reasonably abundant (0.9ppm and 30ppm respectively) but are the two most expensive elements CHAPTER 1. MOTIVATION 6
($700000/tonne and $800000/tonne respectively). Wadia et al used these costs and a simple cost model to determine the raw material cost (Cr)forCdTeandCIGScells to be 0.097¢/W and and 0.023¢/W respectively and the for CZTS to be 0.0049¢/W, which is an order of magnitude lower. These numbers appear to be a very small fraction of the overall cost of a panel (100¢/W) because their model does not take into account the costs of processing (Cp)thematerialforphotovoltaicapplications. These costs are very complex and hard to predict for novel and evolving industries like photovoltaics. However, it has been shown that the Cp/Cr ratio is generally uniform and Cr is good predictor of the cost of a complete solar panel. [18]
1.3 Cu2ZnSnS4 (CZTS)
For solar energy to become the primary source of energy used in the world, a new approach is required that addresses the drawbacks of the current technologies: high material usage, high material costs and the use of toxic materials. Recently, the semi- conductor Cu2ZnSn(S,Se)4 (CZTSSe) has attracted attention as a promising material for use in solar cells. It has a high theoretical e ciency (based on bandgap) and can be used as a thin film to limit material usage. It is formed entirely from earth abun- dant, inexpensive, non-toxic elements. Figure 1.3 shows the abundance and cost of copper, zinc, tin and sulfur as compared with some elements that are currently used to make thin film solar cells. The lower abundance and highest cost element in this set is tin (4ppm and $3000/tonne respectively) which is still significantly better than the elements used in the current technologies (Cd, Te, In, Ga). Furthermore, the CZTS elements are mined in large quantities in many countries of the world. Figure 1.4 shows the annual global production of these elements. A large global production is necessary for the solar industry to continue to grow rather than be limited by mate- rial shortages. The CdTe industry is a prime example of an industry that is likely to be limited by material production. Tellurium, one of the primary components of this material, has a very low annual production (250-300 tones/yr). At this production rate, if all the tellurium is converted directly into solar panels which no losses, the industry would only be able to produce 4-5 GWp of solar panels per year. CHAPTER 1. MOTIVATION 7
Downloaded from rsta.royalsocietypublishing.org on June 22, 2011
Review. Sustainable photovoltaics 1841
103 S
(ppm) 102 Cu Zn Ga 10 Sn 1 In Cd Bi 10–1 Se
10–2 Te 10–3
102
103
104
) 5
–1 10
6 ($ tonne 10
Figure 1. Occurrence in the Earth’s crust and current costs of some of the elements relevant to Figurethin-film 1.3: photovoltaics. Relative abundance Note that logarithmic (top) and scales bulk have cost been (bottom) used for of bothelementsy axes. commonly Current usedtechnologies in the includeproduction copper of indium solar cells. gallium From diselenide [3]. (CIGS) and cadmium telluride (CdTe). Promising alternatives include copper zinc tin sulphide (CZTS). (Online version in colour.)
2 calculated from the solar constant (1.366 kW m− ) and the cross-sectional area of the Earth. The result is 1.4 1017 W, i.e. around 5000 times the estimated primary power requirement for 2050.× If we assume that we could cover 1 per cent of the Earth’s land surface with solar arrays operating at a power efficiency of 10 per cent, a rough calculation based on the land area illuminated by the Sun and losses owing to weather and seasons indicates that photovoltaics (PVs) could generate around 25 TW. The constraints imposed by mineral scarcity become apparent from a calculation which shows that the amount of cadmium that would be required to fabricate enough CdTe solar cells to generate this power exceeds the identified world reserves [3] by more than a factor of 100. In other words, resource limitations appear to restrict the possible contribution of CdTe solar cells to less than 1 per cent of the total additional power required by 2050. Clearly, planning for terawatt deployment of PVs requires careful consideration of sustainability issues including material scarcity and cost. The availability and price of a range of elements used in PV cells are contrasted in figure 1.
Phil. Trans. R. Soc. A (2011) CHAPTER 1. MOTIVATION 8
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1842 L. M. Peter
108 S Cu Zn 107
6 ) 10 –1 Sn yr 105 Cd 4 (tonnes 10 Bi
103 Se Te 102 In Ga 10
Figure 2. Annual production of some of the elements relevant for photovoltaics. Note the Figure 1.4: Relative globallogarithmic productiony scale. of (Online elements version commonly in colour.) used for production of solar cells. From [3].
As the preceding discussion of CdTe makes clear, the material requirements for large-scale deployment of PVs are demanding. Figure 2 compares current annual production of the same set of elements. The impact on production requirements of increasing production of thin-film PVs would clearly be largest for CIGS, as the annual production of indium and gallium is tiny when compared with metals such as copper, zinc and tin, which could be used as the alternative material, CZTS. Solar cells need sun. The amount of solar radiation available for PV energy conversion varies considerably. In Europe, Spain and Italy have particularly high levels of sunshine that is not too different from those in North Africa. In the UK, irradiation levels are more modest but still usable. Figure 3 shows the European Commission’s map of annual irradiation levels for Europe [4], which shows that the south of the UK receives more than 50 per cent of the annual amount arriving in North Africa. It is worth noting that the role played by PVs in national energy strategies is not related simply to the local levels of solar radiation. Germany, which receives a level of irradiation similar to that of the UK, had achieved an installed PV capacity of 9.8 GWp1 by 2009, whereas the UK was far down the European list with only around 33 MWp installed [5]. The current contribution of PVs to the total world energy requirement is still very small, with total worldwide installed capacity just over 20 GWp [6]. However, as figure 4 illustrates, the total installed PV capacity is rising steeply, and it is clear that consideration of the sustainability implications of further rapid expansion should already be influencing research directions. The search for new materials forms an important part of this emerging scenario.
1Wp (watts peak) refers to the nominal power output of a solar panel when illuminated with 2 AM (air mass) 1.5 radiation at a power density of 1 kW m− .
Phil. Trans. R. Soc. A (2011) Chapter 2
Background
2.1 Sputtering
Sputtering is a process in which a target material is bombarded with energetic par- ticles with the purpose of ejecting target atoms onto a substrate that is to be coated with a thin film. The process is dependent upon collisions between particles and electrons and the ensuing momentum exchange. Figure 2.1 shows a schematic of this process. A voltage is applied between the target material (cathode) and the sub- strate (anode). Due to the voltage, electrons are accelerated away from the target and collide with atoms of the gas (usually an inert gas like argon) occupying the space between the cathode and anode. The gas atoms are ionized and accelerated into the target material by the applied voltage. Collision of the gas ions with the target cause energy to be transferred from the gas ion to the target material. If this energy is greater than the binding energy of the target atoms, an atom will be ejected onto the substrate. In steady state, a constant flux of atoms leaves the target and deposits on the substrate. [19]
2.1.1 Reactive Sputtering
Reactive sputtering is a process for deposition of compound thin films where one or more components of the compound are incorporated from the ambient sputtering gas.
9 CHAPTER 2. BACKGROUND 10
Target Target Substrate Ar+$ atom Anode
Cathode
Ar+$ Thin Film
Electric Field
V$
Figure 2.1: Schematic of the basic sputter process. A voltage is applied between the target and the substrate to accelerate argon ions into the target and eject target atoms. The target atoms deposit on the substrate, creating a thin film. CHAPTER 2. BACKGROUND 11
The inert gas (usually argon) used for regular sputtering is replaced by a mixture of areactivegasandtheinertgas.Forexample,TiNcanbegrownusingreactivesput- tering with a titanium target and a mixture of argon and nitrogen gas. The titanium reacts with nitrogen at the target and at the substrate to form the compound. The advantage of using reactive sputtering over compound sputtering (discussed in the next section) is that metallic targets are cheaper and significantly easier to manufac- ture in high purities. The disadvantage is that the targets are a↵ected by the reactive gas, and the sputter rate of the target can change significantly as the compound forms on the target surface. Furthermore, it is di cult to control the quantity of the reactive element incorporated into the film.
2.1.2 Compound Sputtering
Compound sputtering is another process used for deposition of compound thin films. Unlike reactive sputtering, compound sputtering is performed in an inert gas (usu- ally argon) and all species of the thin film are present in the target at the desired stoichiometry. The primary advantage of compound sputtering is that the flux of each component exiting the target can be accurately controlled and is equivalent to the target stoichiometry. Once the targets reach steady state there are no changes in sputter rate as is sometimes seen with reactive sputtering. The disadvantage of this process arises from manufacturing of complex compound targets. It is complicated and expensive to create targets of multi-component compounds that form a singular phase.
2.2 Materials Characterization Techniques
2.2.1 Scanning Electron Microscopy (SEM)
SEM is the one of the best tools for imaging the morphology of thin films and the interfaces between thin film layers. Unlike transmission electron microscopy (TEM), SEM sample preparation is almost as simple (and sometimes simpler) as optical mi- croscopy and the technique is capable of reaching much higher magnifications. All CHAPTER 2. BACKGROUND 12
microscopy techniques have a finite limit beyond which they are unable to resolve any features. For optical microscopy this limit is given by: [20]
0.61 d = (2.1) NA where d is the minimum resolvable distance, is the wavelength of the light, and NA is the numerical aperture of the lens used for imaging. Using some real numbers for NA and a wavelength on the lower end of the visible spectrums yields a minimum resolvable distance of about 100-200nm. An SEM, on the other hand, uses electrons rather than light at energies of about 10-30 keV. At these energies, the wavelength 3 of an electron is about 10 nm and the microscope is capable of significantly lower resolvable distances (0.05nm). An SEM usually consists of a filament for generation of electrons, a number of magnetic lenses to focus the beam, and an electron detector to detect the electrons that interact with the sample.
2.2.2 Auger Electron Spectroscopy (AES)
AES is a common technique used for the compositional analysis of surfaces. It is similar to SEM in that an electron beam is focused onto the sample and electrons ejected from the sample are analyzed. The di↵erence is in the type of electrons being analyzed and what they are analyzed for. AES detectors are tuned for electrons that have undergone the Auger e↵ect, which is a series of electronic transitions in the orbitals of an atom. Figure 2.2 shows an example of an Auger transition. An electron from the incoming electron beam interacts with a core electron in the atom, ejecting it from its orbital. Another electron from one of the outer orbitals (high energy state) will then drop into the space of the originally ejected electron while releasing its excess energy. If this energy is greater than the binding energy of the outer shell electrons, one of them will be emitted from the atom with an energy given by:
E = E E E (2.2) Auger K L1 L2
where EAuger is the kinetic energy of the Auger electron that will be detected,
EK is the binding energy for core shell electrons, EL1 is the binding energy of the CHAPTER 2. BACKGROUND 13
Figure 2.2: A schematic of the Auger e↵ect. An electron is knocked out of one of the core orbitals and the hole is filled with an electron from one of the higher energy orbitals. Energy is released in this process and results in ejection of an electron from one of the outer orbitals.
L1 orbital and EL2 is the binding energy of the outer orbital from which the Auger electron is ejected. Each of these energies is characteristic of the electronic structure of an element, and thus the measured Auger electron energies allow the user to detect the chemical composition of the surface being scanned. AES analysis is conducted in a number of di↵erent forms to gain similar but complementary information about the surface being scanned:
• Depth profiles are used to determine the composition as a function of depth into a sample. One of the advantages of AES is that it is a very surface sensitive technique. This means that Auger electrons are only ejected from 1-5nm into the surface and the composition analysis is limited to those depths. [21] A depth profile is generated by using an ion gun to sputter away material from the surface and take a composition measurement at many di↵erent depths into the sample. The data is usually plotted as composition vs. sputter time, but if the sputter rate is known it can be converted into composition as a function of depth into CHAPTER 2. BACKGROUND 14
the sample.
• Linescans are used to determine the composition of a film along a line on the surface of the sample. The data is plotted as composition vs. distance along the line. This is a quick way to determine if the surface of the sample is homogenous.
• Image maps are used to determine the composition of the surface of a film as a function of x and y coordinates. A secondary electron detector (used in regular SEM) is used to capture an image of the surface of a sample. The electron beam is then scanned over the surface of the sample while collecting composition data at each point. The data is plotted for separate elements as a function of position on the surface, with the intensity of the color in the image map related to the quantity of the element present at any point.
2.2.3 Inductively Coupled Plasma Optical Emission Spec- troscopy (ICP)
ICP is a technique that is used for compositional analysis of samples. It is very accu- rate with high sensitivities for the majority of elements. The sample to be analyzed is dissolved into an acidic solution that is likely to keep it from precipitating out. The solution is introduced into a nebulizer where it is converted into a mist. An intense electromagnetic field is used to create a high energy plasma using argon gas. The sample mist is introduced into this plasma and is broken down into charged ions of its constituent elements. The ions continue to go through changes in the energy state of their electrons, emitting radiation with characteristic wavelengths. A monochrometer is used to separate wavelengths that correspond to specific elements and direct them toward a detector. The intensity at the detector is proportional to the quantity of the element present in the sample. [22]
2.2.4 X-ray Di↵raction (XRD)
XRD is a technique used to characterize the crystal structure of materials. X-rays are electromagnetic radiation with energies in the range of 0.1-100 keV. When directed CHAPTER 2. BACKGROUND 15
Figure 2.3: Graphical representation of Bragg’s law: n =2dsin(✓).
at a material, x-rays interact with the electrons in the material and are elastically scattered. If the material is crystalline (i.e. it consists of repeating planes of atoms), the interaction will produce a di↵raction pattern consisting of maxima where the x- rays constructively interfere. Figure 2.3 shows the interaction of two x-rays with a repeating planes of atoms. The angle of incidence of the x-rays is defined as ✓ and the spacing between the planes is d. The di↵erence in traveled path length between the x-ray that is interacting with the first plane of atoms and the x-ray that is interacting with the second plane of atoms, is equal to 2dsin(✓). If the di↵erence is equal to an integral number of wavelengths of the x-rays they will constructively interfere. This is called Bragg’s law and can be written as:
n =2dsin(✓)(2.3)
where n is some arbitrary integer and is the wavelength of x-rays used in the experiment. Therefore, if the angle of incidence (✓)isscannedoveralargerange,itis possible to create a di↵raction pattern than shows the constructive interference peaks from almost all the di↵erent crystal planes. This pattern is di↵erent for di↵erent crystal structures and acts almost as a fingerprint for any material. CHAPTER 2. BACKGROUND 16
Symmetric vs. Grazing Angle
XRD was used in this work in two configurations: symmetric and grazing angle (also known as glancing angle). In general, symmetric scans are used for bulk samples since it is the technique that provides the most information. However, for thin films, grazing angle scans can be more useful due to their unique geometry. Figure 2.4 shows the di↵erence between a symmetric configuration and a grazing angle configuration. In a symmetric scan, the angle of incidence of the x-ray beam and the angle the detector makes with the substrate are the same and both are varied together during the measurement. This is a problem for thin films because there is a very small amount of material present in the path of the x-ray and the majority of the di↵racted x-ray intensity comes from the substrate rather than the thin film. The grazing angle configuration is a way to increase the path length of the x-ray in the thin film and improve the surface sensitivity of XRD. When an x-ray is incident on an interface between two materials, in this case air and the sample being analyzed, part of the x-ray energy is reflected and part of it is carried by a refracted wave traveling into the sample. The refracted wave continues into the sample and is scattered by the atoms in the lattice. The angle of incidence and the angle of refraction are related by the indices of refraction of the two materials:
n cos(↵0)= cos(↵)(2.4) n0
where ↵ and ↵0 are the angles of incidence and refraction respectively and n and n’ are the indices of refraction of air and the sample material respectively. Then, for any incident x-ray, there is a critical angle ↵c:
1 n0 ↵ = cos (2.5) c n below which there is total external reflection and the refracted wave travels along the surface of the sample. If the refracted wave is confined to the surface of the sample, all di↵racted beam intensity must now also come only from the surface of the sample. For ↵<↵c,thepenetrationdepthoftherefractedwavecanbecalculated using: CHAPTER 2. BACKGROUND 17
1 D = (2.6) Imk | 0| where Imk’ is the imaginary part of the refracted wave vector in the direction perpendicular to the sample surface and is given by:
2 n0 2 k0 = k cos ↵ (2.7) s n2 where k is the incident wave vector. However, in this work, grazing angle XRD is used in configurations where ↵>↵c.Inthiscase,thepenetrationdepthisdetermined primarily by the photo absorption of the sample and can be expressed as:
sin↵ D = (2.8) µ ! where ↵ is the angle of the incident x-ray and the µ is the linear mass absorption coe cient of the material being analyzed. Thus, by varying ↵ the penetration depth can be controlled to constrain the x-rays to the sample and scan almost any range of depths from a few nanometers to the entire thickness of the sample. While this configuration still allows the full range of d-spacings to be scanned it does have some drawbacks. Sputtered thin films can often be textured, which means that the grains in the film are not randomly oriented. For example, in a film that has (100) texture, the majority of grains are oriented such that the (100) direction is perpendicular to the substrate. This texture can be detected in a symmetric scan because all d-spacings are scanned in the same orientation (usually perpendicular to the substrate). Thus, if a majority of the grains are oriented in the (100) direction, a symmetric scan would show a more intense (100) peak. In a grazing angle scan, the scan orientation varies with the angle of the detector and the di↵raction pattern will only be influenced by the grains that are oriented randomly in the film.
2.2.5 Raman Spectroscopy (RS)
Raman spectroscopy is a characterization technique that involves inelastic scattering of light with matter. It is used heavily in chemistry and for organic molecules since CHAPTER 2. BACKGROUND 18
Symmetric)Scan)X-ray)Path)) Grazing)Angle)X-ray)Path)
Substrate) Substrate)
Figure 2.4: The di↵erence between the x-ray path in symmetric and grazing angle XRD configurations. The x-ray travels through much more film in the grazing angle configuration. it is ideal for studying vibrational modes in a system. When light is incident on any matter, it interacts with vibrational modes in the system, which results in a change in the wavelength of the reflected light. This change can be measured and then used to calculate the Raman shift using the following equation:
1 1 w = (2.9) ✓ 0 1 ◆
where w is the Raman shift in units of inverse length, 0 is the wavelength of incident light, and 1 is the wavelength of reflected light. Typically, laser light is used to illuminate a sample. Reflected light from the illuminated spot is then sent through filters to remove the light that does not go through a frequency change, and then through a monochrometer to split it into singular wavelengths. The generated Raman spectrum is characteristic of a specific molecule. In crystalline thin films (like those synthesized in this work), the vibrational modes in the system are bond vibrations (phonons). Since crystalline materials have well defined bond energies, they have sharp Raman peaks that can be used to identify specific phases. Predicting the Raman spectrum of a multicomponent system is relatively complex, and phases as usually identified by compared experimental spectra with data from reference materials. CHAPTER 2. BACKGROUND 19
2.3 Solar Cells
2.3.1 Solar Spectrum
Solar energy is incident on the earth as a spectrum of photons with a range of energies which are determined by the properties of the sun. Outside the earth’s atmosphere the intensity of this spectrum is 1.35 kW/m2.Atthesurfaceoftheearth,dueto absorption in the atmosphere, the intensity drops to 1.01 kW/m2.Thesespectraare designated AM0 and AM1.5 respectively. AM stands for Air Mass and is a measure of how much light is absorbed in the atmosphere. It is defined by the following equation:
1 AM = (2.10) cos(✓) where ✓ is the angle of incident sunlight. These AM spectra are used as the standard testing condition for solar panels depending on where the solar panel will be used. For example, AM0 (which implies that there is no atmosphere) would be used for solar panels in space and AM1.5 is generally used for terrestrial solar panels. Figure 2.5 shows both these spectra in irradiance as a function of wavelength (in green, corresponding to the left y-axis). The integrated form of each of these spectra are shown in blue and correspond the right y-axis.
2.3.2 p-n Junctions
At the heart of any (or at least most) solar cells is a p-n junction. A p-type semi- conductor has a high concentration of holes and a n-type semiconductor has a high concentration of electrons. A p-n junction is formed when a p-type semiconductor and an n-type semiconductor are brought into contact with each other in such a way that allows for the flow of charge carriers (shown in figure 2.6). Electrons and holes di↵use to opposite sides due to the concentration gradients and leave behind positive and negative ions respectively. The positive and negative ions create an electric field which leads to drift of electrons and holes until the di↵usional fluxes are canceled out. Eventually, the p-n junction reaches a steady state condition that is charge neutral but has a built in electric field. This built in electric field is the key to capturing the CHAPTER 2. BACKGROUND 20
1400 2.0
1.8 1200 Power Density (W m Density Power ) 1.6 -1 1000
nm 1.4 AM0 -2 AM1.5 1.2 800 Integrated AM0 1.0 Integrated AM1.5 600
0.8 -2
0.6 400 ) Irradiancem (W 0.4 200 0.2 0.0 0 400 800 1200 1600 2000 2400 Wavelength (nm)
Figure 2.5: The AM0 and AM1.5 solar spectrum (green curves, left axis). Integrating these curves gives the total power density of incident light (blue curves, right axis). CHAPTER 2. BACKGROUND 21
n"type' p"type'
Electron'diffusion'
Hole'diffusion' +'+'+'+'+' "' "' "' "' "' +'+'+'+'+' "' "' "' "' "' +'+'+'+'+' "' "' "' "' "' +'+'+'+'+' "' "' "' "' "' Space'charge'region' E'field"
Diffusion'flux'of'electrons' E"field'flux'of'electrons"
E"field'flux'of'holes" Diffusion'flux'of'holes'
Figure 2.6: The formation of a p-n junction. A p-type and an n-type semiconductor as placed in contact, which results in di↵usional flux of electrons and holes and the formation of a built in potential. solar radiation incident on the junction and converting it to usable electricity. Figure 2.7 shows the band diagram for a p-n junction and what happens in the junction when it is exposed to one photon of light. The light excites an electron from the valence band to the conduction band, creating an electron-hole pair. Due to the built in electric field, the electron (negatively charged) will flow down the potential gradient while the hole flows up it (positively charged), creating a current under steady state illumination. By placing a load between these two charge carriers, it is then possible to extract usable electrical power from the junction. However, it is important to take note that only photons with energy above the band gap of the CHAPTER 2. BACKGROUND 22
absorbing material will excite electron-hole pairs. Unfortunately, solar radiation is incident on the earth as a full spectrum of energies (above and below any given band gap) and as such, any photons with energy below the band gap will not be utilized by the solar cell. This ”mismatch” between the solar spectrum and a p-n junction is possibly the largest source of ine ciency in a solar cell. To calculate the magnitude of power lost due to this ine ciency, we need to calculated the maximum power density a solar cell is capable of producing, assuming there are no losses in the device. Recall that P=VJ and we will assume that the maximum voltage a solar cell can produce is equal to its band gap (Eg). Thus, the power produced by a solar cell is then:
P = Eg(J)(2.11)
where J is the current density produced by a solar cell with band gap Eg.To calculate the current density we use the irradiance function shown in figure 2.5. This function can be converted to photon flux density (or the # of photons incident at some energy) using the following method:
I( )d = I(E)dE (2.12) where I(E)istheirradianceasafunctionofenergy.Wavelengthandenergyare related by E=hc/ ,wherehisPlanck’sconstantandcisthespeedoflight.Taking the derivative and plugging back into the original equation we get:
hc d hc = = E ! dE E2
hc I(E)=I( ) (2.13) E2 Dividing I(E) by the energy of one photon E, gives:
I(E) hc n(E)= = I( ) (2.14) E E3 where n(E) is the photon flux density. If we assume that every photon above the CHAPTER 2. BACKGROUND 23
Incoming' Ec# photon'
Efi#
Ef#
Ev#
n"type' p"type'
Load'
Figure 2.7: A p-n junction when illuminated by one photon. An electron-hole pair is generated and split using the built in electric field. band gap produces one electron we can integrate this curve to get the current density as a function of band gap.
1 J(Eg)=e n(E)dE (2.15) ZEg where e is the charge of one electron. Plugging back into equation 2.11 gives power density as a function of band gap:
1 P (Eg)=e Eg n(E)dE (2.16) ⇤ ZEg The maximum of this function occurs at a band gap of 1.11ev and power density of 0.441kW/m2. If the power density of light incident on the earth is 1.01kW/m2 we have lost almost 57% of the power due to the ”mismatch” between our junction and the spectrum of solar radiation. A more accurate calculation using a thermodynamic method is performed by Shockley and Queisser and shows that the loss due to the p-n junction is actually 66%. [6] The di↵erence between these two calculations comes from radiative recombination which occurs in every material. CHAPTER 2. BACKGROUND 24
Figure 2.8: The accepted band diagram for a CIGS heterojunction solar cell. There are three materials (CIGS, CdS and ZnO) that form the junction. From [4].
2.3.3 Heterojunctions
In a p-n junction, the semiconductor used for the p-type and n-type parts is usually the same material that is doped with p and n dopants. This means that the band gap throughout the material is homogenous and the only change is the type of car- riers present. The solar cells discussed in this work are heterojunctions rather than regular p-n junctions. A heterojunction is created by combining p-ype and n-type semiconductors that are not the same material. This means that the band gap is no longer homogenous and it is possible to have o↵sets/barriers in the bands. Figure 2.8 shows the band diagram for a Cu(InGa)Se2 (CIGS) solar cell (the device stack for this technology is discussed in section 2.5.5 and is very similar to CZTSSe). Notice that this junction is made from three separate materials (CIGS, CdS, ZnO) but still operates in the same way as a regular p-n junction when exposed to light. However, unlike a homogenous p-n junction, there will be electronic losses associated with the o↵set in the conduction band between CIGS and CdS. CHAPTER 2. BACKGROUND 25
2.3.4 Equivalent Circuit
Real solar cells are more complicated and cannot be represented simply by a p-n junction. They are measured under illumination, have other factors that contribute to power losses and are represented by the more complicated circuit seen in figure 2.9. This circuit consists of:
1. A current source which represents the current generated due to incident light.
2. A diode which represents the p-n junction (or heterojunction).
3. A shunt resistor, Rsh,whichrepresentsshuntingpathsinthesolarcellfor current to leak through. These can be grain boundaries, defects or even large physical shunts.
4. A series resistor, Rs,whichrepresentsresistanceintheelectrodesandcontacts to the cell.
5. A load resistor Rload which is used to dissipate the power from the solar cell.
Using this circuit we can then develop an equation for I, the current flowing through the load resistor, by subtracting out the current flowing through the diode and the shunt resistor from the current generated by the light. This derivation is shown below:
I = I I I (2.17) L F sh
e(V + IRs) V + IRs I = IL I0 exp 1 (2.18) " mkBT ! # Rsh
where I is the current flowing through the load resistor. IL is the current generated due to incident light. IF is the current flowing through the diode and is replaced with the equation for an ideal diode. Ish is the current flowing through the shunt resistor. I0 is the saturation current for the diode, which comes from the material characteristics (carrier concentration, di↵usivity, di↵usion length) of the components CHAPTER 2. BACKGROUND 26
Rs!
I! IL!
I! Rsh! Rload!
IF! Ish!
Figure 2.9: The equivalent circuit used to represent solar cells. It consists of a current source, a diode, a shunt resistor and a series resistor. of the p-n junction. ”m” is the non-ideality factor associated with the diode which comes from recombination of carriers in the depleted region and ranges from 1 to 2. This equation can be used to fit the IV response of real solar cells and determine each one of these solar cell parameters (I0,Rsh,Rs, m). This makes it possible to determine where/how power is being lost in a solar cell and what improvements can be made to a device.
2.4 Device Characterization Techniques
2.4.1 Current Voltage Measurement (IV)
Current voltage measurements are the primary way to characterize solar cells and compare them against each other. A voltage is applied to the solar cell under illumi- nation (usually under AM1.5 solar radiation) and the current response is measured. Plotting this response for a range of voltages gives the curve shown in figure 2.10. From this curve we can determine some important solar cell parameters like short cir- cuit current (Isc), open circuit voltage (Voc), fill factor (FF), maximum power point
(Pmax)andmostimportantofall,e ciency(⌘). Isc is the maximum current the solar cell is capable of producing (at zero voltage) while Voc is the maximum voltage the solar cell can produce (at zero current). The maximum power point is the voltage CHAPTER 2. BACKGROUND 27
Pmax!
Jsc!
Voc!
Voltage (V)
Figure 2.10: Example IV Curve. The short circuit current density (Jsc), open circuit voltage (Voc)andmaximumpowerpoint(Pmax)aremarked. and current along this curve where the solar cell produces the most power. The fill factor is a measure of the “squareness” of the curve and is defined by:
P FF = max (2.19) VocIsc E ciency is a measure of how well a solar cell converts the power of sunlight into usable electrical power. It is the primary way that the e cacy of a solar cell is measured and is defined by:
P FFV I ⌘ = max = oc sc (2.20) Pin Pin
where Pin is the power of the sunlight incident on the panel. E ciency is also used to compare one solar cell to another since it is independent of any convoluting factors (e.g. size etc.). CHAPTER 2. BACKGROUND 28
2.4.2 External Quantum E ciency Measurement (EQE)
EQE is a measure of how well a solar cell converts an incident photon into an electron and is defined as electrons generated divided by photons incident on the device. The measurement is performed by recording the current response of a solar cell to monochromatic light over a large range of wavelengths. It is defined by:
I ( ) EQE ( )= L (2.21) qN ( )
where IL is the current generated by the light as a function of wavelength, N is the number of photons incident on the cell and q is the charge of an electron. It is useful because di↵erent parts of the EQE curve provide information about di↵erent loss mechanisms in the device. Figure 2.11 shows an example of an EQE curve for a CIGS device (the device stack for this technology is discussed in section 2.5.5 and is very similar to a CZTSSe stack). The numbers (1-6) correspond to the following loss mechanisms: [4]
1. Shading from the top grid.
2. Reflection from the front surface. Most high quality cells have an anti-reflection coating to minimize this.
3. Absorption in the top conducting electrode (ZnO).
4. Absorption in the bu↵er layer (CdS).
5. Incomplete absorption in the absorber (CIGS or CZTS).
6. Recombination in the device. CHAPTER 2. BACKGROUND 29
Figure 2.11: Example EQE curve. The di↵erent loss mechanisms are marked and discussed in the text. From [4]. CHAPTER 2. BACKGROUND 30
Grey% Black% Blue% Red%
CZTSSe% Cu% Zn% Sn% S,Se%
CIGS% Cu% In,Ga% In,Ga% Se%
Figure 2.12: The chalcopyrite crystal structure shared by CIS, CIGS, CZTS, CZTSSe and CZTSe. From [5].
2.5 Cu2ZnSn(S,Se)4 (CZTSSe)
2.5.1 CZTS vs. CZTSe vs. CZTSSe
CZTSSe is a compound formed by mixing pure sulfide CZTS and pure selenide CZTSe materials. While these two materials have some di↵erences, they have very similar optical and electronic properties and the same crystal structure. [5] Furthermore, when the two compounds are mixed to create a hybrid sulfo-selenide material, it has been reported that the crystal structure is retained through the entire range of sulfur to selenium ratios. [23] Figure 2.12 shows the crystal structure for these materials. The grey atoms are copper, black atoms zinc, blue atoms tin and red atoms can be either sulfur or selenium. CHAPTER 2. BACKGROUND 31
2.5.2 Material Properties
For a material to be considered promising for solar work, it must have a high absorp- tion coe cient and an ideal bandgap. A high absorption coe cient allows a material to be used as a thin film which greatly reduces costs, and the bandgap of a material controls the highest theoretical e ciency of solar cells that are based on it. Figure 2.13A shows the theoretical e ciency of a solar cell (as calculated by Shockley and Queisser [6]) as a function of bandgap of the absorber material. The dark red lines and red area show the bandgap of CZTS, CZTSe and CZTSSe respectively. CZTS and CZTSe straddle the peak of this curve while the bandgap of CZTSSe can be ma- nipulated to be anywhere in between. Figure 2.13B shows the absorption coe cient for CZTS and CZTSSe as a function of photon energy. Both of these materials have absorption coe cients high enough to allow them to be used as thin films. The dotted line shows the absorption coe cient for CuInSe2 (CIS is a thin film material that is currently being commercialized). Notice that the absorption coe cient for CZTSe (which has a similar bandgap to CIS) increases much faster than that for CIS as the photon energy in increased above the bandgap.
2.5.3 Phase Diagram
CZTS is a compound that forms in the Cu2S-ZnS-SnS2 system. Figure 2.14 (left) shows an isothermal section of the accepted phase diagram for this system with the CZTS phase denoted by region 1. The composition range over which a pure CZTS phase is stable is limited to between 2 and 5 mol%. Outside of region 1, this system phase separates into CZTS and another secondary phase (i.e. ZnS, Cu2S, SnS2). A binary slice of this ternary phase diagram that shows the equilibria between Cu2SnS3 and ZnS is in figure 2.14 on the right. According to this binary section, CZTS is a line compound and any deviation from stoichiometry at low temperatures would result in the formation of a secondary phase. CHAPTER 2. BACKGROUND 32
A CZTSe& CZTS& B )& CZT(S,Se
Figure 2.13: A) Maximum theoretical e ciency as a function of bandgap of absorber material. B) Absorption coe cient of CZTS and CZTSe as a function of photon energy. From [6] and [5] respectively.
Figure 2.14: Isothermal section of Cu2S-ZnS-SnS2 ternary phase diagram at 670K. Region 1 shows the composition range under which CZTS can form as a pure phase (left). Binary phase diagram of the Cu2SnS3-ZnS system. CZTS is the line compound at 0.5 mol% ZnS (right). From [7]. CHAPTER 2. BACKGROUND 33
2.5.4 Phase Analysis
Even though a number of deposition processes have been attempted for CZTS, there are only a handful of groups capable of producing devices with e ciencies greater than 6%. [23–27] One explanation that has been put forth for this is that CZTS is unstable at high temperatures without an appropriate gaseous atmosphere. [27,28] This can lead to the breakdown of the primary phase and the formation of secondary phases that can be detrimental to device e ciency. Unfortunately, conclusive detec- tion of these phases and identification of the phases that are detrimental to device performance is still lacking. Two primary characterization techniques have been applied to this problem: x-ray di↵raction (XRD) [29,30] and Raman spectroscopy (RS) [9–13,31], but the results have been limited by the similarities between CZTS and the binary and ternary sulfide phases in this material system (ie. ZnS, Cu2SnS3,CuxS). Figure 2.15 shows the simulated XRD patterns (top) [8] and the locations of the RS peaks (bottom) of some of the secondary phases that can form in this system. [9] The XRD pattern for CZTS shares primary peaks with ZnS and Cu2SnS3,whichmakesidentification of these two phases exceptionally challenging. If there is any CZTS present in the
film, it becomes impossible to identify ZnS and Cu2SnS3 because they have no unique peaks. The Raman spectrum for CZTS once again shares a number of peaks with other phases in this system. Generally, due to lack of resolution in experimental setups, identifying phases using RS is also very challenging.
2.5.5 Device Stack
The standard device stack used for CZTSSe comes directly from the CIGS stack developed at the National Renewable Energy Laboratory (NREL). [32] Figure 2.16 shows the di↵erent layers in this stack, which starts with a soda lime glass substrate. A molybdenum layer is then sputtered onto the glass as the back electrode. This layer is followed by the absorber (CZTSSe/CIGS) and the CdS bu↵er layer. The purpose of the bu↵er layer is to complete the p-n junction and protect the absorber from damage during deposition of subsequent layers. The next layer is a thin i:ZnO CHAPTER 2. BACKGROUND 34
Figure 2.15: Simulated XRD patterns of CZTS, Cu2SnS3 and ZnS [8] and locations of Raman peaks [9–13] of phases that have been identified in CZTS films by other groups. CHAPTER 2. BACKGROUND 35
Figure 2.16: The CIGS device stack that has been adapted for CZTS. layer deposited to increase the shunt resistance of the devices and is followed by a transparent conductive top electrode. While the best CZTSSe devices are made using this device stack, it is unlikely that it is ideal for CZTSSe since it was originally developed for a di↵erent material. Chapter 3
Materials and Methods
3.1 Absorber Deposition and Annealing
Two di↵erent sputter based processes were used for preparation of thin films in this work (the basics of sputtering are covered in section 2.1). The majority of the CZTS work was performed at Stanford University on chamber #1 using reactive sputtering while the majority of the CZTSSe work was performed at AQT Solar (www.aqtsolar.com) on chamber #2 using compound sputtering. The specifics are described below.
3.1.1 Reactive co-sputtering at high temperature
Three con-focal one inch sputter guns in chamber #1 (described below) were outfitted with 99.99% copper, zinc and tin targets. Advanced Energy MDX 1KW DC power supplies were used for the copper and tin gun, while an Advanced Energy RF5S power supply was used for the zinc gun. Sputter fluxes were determined using a rate monitor and set to yield films with the required composition. Growth pressures and temperatures ranging from 2.5 - 40 mtorr and 100 - 550 Crespectivelywere attempted. The ratio of H2Stoargonwasvariedfrom0.14to0.5.
36 CHAPTER 3. MATERIALS AND METHODS 37
3.1.2 Reactive co-sputtering at low temperature followed by a post deposition anneal
Six sputter guns in chamber #2 (described below) were outfitted with 99.99% copper, zinc and tin targets. Advanced Energy MDX 500W DC power supplies were used for the copper and tin gun while AJA 100/300 power supplies were used for the zinc guns. Sputter fluxes were determined by deposition of thin films and thickness measurements using x-ray reflectivity and set to yield films with close to the ideal CZTS composition. Growth pressures and temperatures ranging from 2.5 to 5 mtorr and 25 to 250 Crespectivelywereattempted.TheratioofH2Stoargonwasheld constant at 0.5 during the deposition. The films were annealed at 550 Cina10%
H2S/argon mixture at a total pressure of 40 mtorr.
3.1.3 Compound sputtering at room temperature followed by a post deposition anneal
Three sputter guns in chamber #2 (described below) were outfitted with 99.99% cop- per selenide (CuSe2), zinc sulfide (ZnS) and tin sulfide (SnS2) targets. An advanced
Energy MDX 500W DC power supply was used for the CuSe2 gun while AJA 100/300 power supplies were used for the ZnS and SnS2 guns. Sputter fluxes were determined by deposition of thin films and thickness measurements using x-ray reflectivity and set to yield films with close to the ideal CZTS composition. The growth pressure and temperature were 5 mtorr and 25 C respectively. The targets were sputtered in argon only and the sulfur/selenium were incorporated into the film from the targets. The
films were annealed at 580 Cwithdi↵erentquantitiesofsolidSnS.ThesolidSnSis volatile and creates a SnS gas atmosphere during the anneal. The initial Se/S ratio and starting quantity of solid SnS can be used to control the Se/S ratio in the final thin film. CHAPTER 3. MATERIALS AND METHODS 38
3.1.4 Deposition Chambers
Sputter Chamber #1 (Stanford Unversity)
Figure 3.1 shows a schematic of the sputter chamber used at Stanford University that was specifically designed and built for deposition of CZTS. It is equipped with:
• Athree-sourceflangefordepositionofcopper,zincandtin.Thesourcesare con-focal to minimize composition gradients.
• H2Sandargongaslineswithmassflowcontrollersthatallowforthegeneration
of di↵erent H2Stoargonratiosandacapacitancemanometertodetermine pressure.
• A load lock for quick sample loading/unloading.
• A secondary chamber that contains a molybdenum source so that the back electrode may be deposited without breaking vacuum.
• A heating stage capable of reaching temperatures up to 700 C.
Sputter Chamber #2 (AQT Solar)
The sputter chamber used for deposition of CZTSSe was provided as part of a col- laboration with AQT Solar (www.aqtsolar.com), a bay area startup. It has similar features to the chamber at Stanford:
• Six sources for deposition of metal/compound targets.
• Substrate rotation to increase uniformity.
• H2Sandargongaslineswithmassflowcontrollersthatallowforthegeneration
of di↵erent H2Stoargonratiosandacapacitancemanometertodetermine pressure.
• A load lock for quick sample loading/unloading.
• A lamp heater capable of reaching temperatures up to 800 C. CHAPTER 3. MATERIALS AND METHODS 39
Figure 3.1: Schematic of the sputter chamber designed for reactive sputtering with H2S. It is equipped with three sources for the metallic components of CZTS and a secondary chamber with a molybdenum source for deposition of a back contact.
3.2 Device Fabrication
A testable device was prepared by incorporating CZTSSe films into the well estab- lished CIGS device stack described in section 2.5.5. The molybdenum layer was either deposited in chamber #1 or bought from UHV Sputtering Inc. (the stress state of this layer was not controlled). After CZTSSe deposition, a 65nm CdS film was de- posited using the Chemical Bath Deposition (CBD) method developed at NREL. [33] To increase shunt resistance, a 85nm i:ZnO layer was deposited using RF sputtering and a conductive 250nm Al:ZnO layer was deposited using DC sputtering to act as a top electrode. A metal grid was screen printed onto the Al:ZnO to minimize series resistance further. CHAPTER 3. MATERIALS AND METHODS 40
3.3 Characterization Techniques
3.3.1 Scanning Electron Microscopy (SEM)
SEM was used in this work as the primary way to study the morphology of the thin films being deposited. Roughness and grain sizes were estimated and inter- faces between layers were viewed to better understand the device. Samples for SEM were prepared by marking the back of the substrate with a diamond scribe and then breaking it over a sharp knife edge. The purpose of this procedure was to ensure that the cleaved edge of the thin film was preserved so that it might be viewed in the microscope. The instrument used was a FEI XL30 Sirion with FEG source.
3.3.2 Auger Electron Spectroscopy (AES)
The similarities and di↵erences between AES and SEM are discussed in section 2.2. AES was used in this work for depth profiling and to generate image maps of sam- ple surfaces. Image maps were generated as di↵erent element components and then overlaid to determine if there was any compositional separation in the film. Samples were prepared by sputter etching the surface to remove any carbon, oxygen or other device layers to ensure that the bulk CZTS film was being scanned. The instrument used was a PHI 700 Scanning Auger Nanoprobe.
3.3.3 Inductively Coupled Plasma Optical Emission Spec- troscopy (ICP)
ICP was used to determine the average composition of the thin films. Samples were prepared by dissolving the film in a fully concentrated mixture of aqua regia (1:3 2 volume ratio of HNO3 and HCl). The films were broken into small pieces (4mm ) and placed in a test tube with a small quantity of aqua regia. The test tube was placed in a sonic bath to help dissolve the films. After the films were dissolved, the mixture was then diluted with deionized water to a concentration appropriate for the ICP system. The instrument used was a Thermo Scientific ICAP 6300 Duo View CHAPTER 3. MATERIALS AND METHODS 41
Spectrometer.
3.3.4 X-ray Di↵raction (XRD)
XRD was used for phase identification in this work in symmetric as well as grazing angle mode. The di↵ractometer used was a PANalytical X’Pert equipped with Cu K-↵ x-ray tube and an x-ray mirror as the optics used to focus the incident beam.
A1/2 divergence slit and 10mm beam mask were used to obtain a beam height and width of 1.2mm and 10mm respectively. This beam width and height were chosen to ensure that the beam is incident only on the sample during the measurement.
Aparallelplatecollimatorwitha0.27 acceptance angle was use for the di↵racted beam optics. The x-ray detector used was a sealed proportional detector filled with axenon/methanegasmixture.Samplesweremountedtothesamplestageusing3M magic tape. The samples were aligned with the x-ray beam by moving the sample into the beam and rocking !,theanglethesamplemakeswiththeincidentbeam.1 Symmetric scans were conducted by setting the angle of the incident and di↵racted beam equal to each other and measuring the intensity of the di↵racted beam for angles ranging from 15 to 80 . The incident and di↵racted angle are held equal to each other by rotating the detector at twice the angular rate as the sample. The scattering vector was kept perpendicular to the sample throughout the scan and only the size of the vector was changed to scan through the d-spacings of di↵erent planes.
Grazing angle scans were conducted by setting the incident beam angle to 2 and scanning the detector from 15 to 80 .The2 incident angle yielded a penetration depth of 2-2.2um and ensured that the entire thickness of the film was scanned. The result of both these techniques was a plot of intensity as a function of angle with peaks in intensity at angles where there is constructive interference. Bragg’s law was used to convert the angles to interplanar d-spacings (discussed in section 2.2.4). The list of d-spacings vs. intensity were compared with observed and calculated di↵raction patterns obtained from the International Centre for Di↵raction Data to identify the di↵erent phases present.
1The alignment procedure and more information about the equipment used can be found at http://www.stanford.edu/group/glam/xlab/Main.htm CHAPTER 3. MATERIALS AND METHODS 42
3.3.5 Raman Spectroscopy (RS)
RS was used for phase identification in some of the thin films. Samples were scanned 1 1 from 150cm to 600cm since all primary peaks for CZTS and other phases are present in this range. The wavelength of the laser used was 532nm.
3.3.6 Current Voltage Measurements
The devices were connected to a PV Measurements Inc. IVQE measurement system using probe tips for the top and bottom electrode. The measurements were done under an AM1.5 solar light simulator. The simulator was flashed on and the sample measured by scanning the voltage from -0.5 to 0.6 V and measuring current. The area used to determine the e ciency of the device from this measurement was the total area of the cell minus the area shaded by the top grid.
3.3.7 External Quantum E ciency Measurements
The devices were once again connected using probe tips for the top and bottom electrode and AM1.5 radiation was used to simulate illuminated conditions. The current response of the cell was then measured for monochromatic light from 300nm to 1200nm. Chapter 4
Results and Discussion
4.1 Cu2ZnSnS4 (CZTS)
4.1.1 Growth Temperature Study
At the beginning of this work (2007-2008), it was still unknown what temperature would be required to form the CZTS phase. The few papers that had been published stated annealing temperatures in the 500-600 C range. [34–37] However, the goal with this work was to try and evolve the CZTS phase in one step using reactive sputtering at high temperature. This had not been attempted before and it was unclear what temperature would be required to form the CZTS phase since sputtering is already a highly energetic process.
Four samples were grown at temperatures ranging from 100-400 C. The sputter conditions were held constant (to yield films close to the stoichiometric value) for all four samples and only the temperature was varied. The phase was analyzed using symmetric XRD and the composition was profiled using AES. Figure 4.1 shows the XRD patterns for each of these films. The primary CZTS peak (112) is present in each of these films with the intensity of the peak increasing with temperature. More of the other CZTS peaks (200, 220, 312, 224) are also evolved in the higher temperature samples. However, the ratio between these peaks is not consistent with the literature [8] and it is likely that the films are textured in the (112) direction.
43 CHAPTER 4. RESULTS AND DISCUSSION 44
Figure 4.1: Symmetric XRD patterns of CZTS thin films grown at A) 100 C, B) 200 C, C) 300 CandD)400 C.
Conclusive phase identification is further complicated by the problems discussed in section 2.5.4, and it is possible that the films grown at the lower temperatures are a multi-phase mixture rather than CZTS. There is a decrease in the FWHM of the primary peak with increasing temperature that is likely due to increasing grain size. There is also a shift in the molybdenum peak, which implies that the stress state of the molybdenum film is changing with temperature.
The theory that the films grown at lower temperatures (100-200 C) are a multi- phase mixture is supported by the AES depth profiles shown in figure 4.2. The films grown at low temperature are not compositionally uniform through the depth of the
film. Uniformity does improve as temperature increases, and the film grown at 400 C is relatively uniform through its depth. All films show an increase in copper con- centration at the film-molybdenum interface, implying that the copper is segregating there or penetrating further into the molybdenum than is zinc or tin. This could simply be due to preferential sputtering of zinc and tin from the surface of the sam- ple, but that is unlikely. If that were the case, the copper content in the film would CHAPTER 4. RESULTS AND DISCUSSION 45
Figure 4.2: AES depth profiles of CZTS films grown at A) 100 C, B) 200 C, C) 300 C and D) 400 C. The surface of the film is at the origin on the x-axis. appear high throughout the film rather than just at the film-molybdenum interface. It was stated earlier that these films were grown to be stoichiometric by calibrating the sputter fluxes during deposition. Furthermore, ICP measurements were used to establish that the overall stoichiometry of the film is very close to the ideal value. These measurements are shown in table 4.1. However, the AES data shows that the copper content in these films is significantly higher than the ideal value. There are three possible explanations for this:
1. Preferential sputtering of zinc and tin. Zinc and tin have higher sputter yields than copper and this could be increasing the copper content at the surface of the film. Since Auger is a surface sensitive technique, the film would also appear CHAPTER 4. RESULTS AND DISCUSSION 46
Table 4.1: Film stoichiometry from ICP-OES Measurement
Sample Growth Temperature ( C) Cu/Zn Sn/Zn Zn A 100 2.00 0.96 1.00 B 200 2.09 1.05 1.00 C 300 2.08 1.01 1.00 D 400 2.01 1.09 1.00
to be copper rich.
2. Film non-uniformity. In a depth profile, only the composition of a small spot (less than 100nm) is scanned as a function of depth in the film. It is possible that the film composition is non-uniform laterally as well and the 100 nm spot being scanned is in a copper rich section of the film.
3. The AES system is not calibrated. Accurate stoichiometry measurements with this technique require that the AES system be calibrated using a CZTS standard sample. This is especially important for samples containing zinc and copper because the primary copper peak (922ev) overlaps with the primary zinc peak (908 ev) and any zinc signal will enhance the copper measurement. This system was not calibrated and as such the accuracy (but not the precision) of the stoichiometry measurements is unreliable. This is the most likely explanation for the discrepancy between the AES and ICP data.
The XRD patterns in figure 4.1 show that the five primary CZTS peaks are present in these films. However, these peaks are shared between CZTS, ZnS and Cu2SnS3 (CTS) and no unique CZTS peak is observed (002 or 110). Thus, it is possible that the films are actually a two phase mixture of ZnS and CTS rather than CZTS. Since this study was completed, other groups have shown (using in-situ XRD) that, in vacuum, the CZTS phase does not form until the sample reaches 500 C. [28,38] As a result, all future experiments are conducted at temperatures higher than 500 C. CHAPTER 4. RESULTS AND DISCUSSION 47
4.1.2 Phase Analysis
Symmetric X-ray Di↵raction (XRD)
Some of the challenges associated with use of XRD for phase analysis in this material system were discussed in section 2.5.4. Identification of ZnS and Cu2SnS3 in a CZTS thin film is almost impossible due to the similarities in crystal structures. However, XRD is the primary materials characterization technique for phase analysis and it was initially used in this work in the symmetric configuration. CZTS films were purpose- fully grown o↵-stoichiometry to explore the extraneous phases that might appear in these films. Two metal components of the film were held constant while varying the third to limit the number of secondary phases that were likely to appear. The sulfur content in the films was held constant at close to the stoichiometric value. Figure 4.3 shows an isothermal section of the Cu2S-ZnS-SnS2 ternary phase diagram at 670K. The colored lines indicate the areas of the phase diagram that were investigated in an attempt to form specific binary phases. For example, in the case of the green line, the Cu/(Sn+Zn) ratio is varied while holding the Sn/Zn ratio constant at the stoichiometric value. The phases likely to form in this case would be directly related to the copper content in the film. The same analysis applies to the red and blue lines. The films used in this study were reactively co-sputtered in a one step process at high temperature. Figure 4.4 shows the XRD patterns of CZTS thin films growth with varying Cu/(Zn+Sn) and Zn/(Cu+Sn) ratios, respectively. According to the
Cu2S-ZnS-SnS2 ternary phase diagram, if the Sn-Zn ratio is held constant at the stoichiometric value, a copper rich film should contain excess Cu2SalongwithCZTS and a copper poor film should contain CZTS, ZnS and SnS2,sincethereisnosingular Zn-Sn-S phase. If the Cu-Sn ratio is kept constant, a zinc rich film should contain
CZTS and ZnS, while a zinc poor film should contain CZTS and Cu2SnS3.[7] Figure 4.4A shows the symmetric XRD patterns of films in which the Cu/(Zn+Sn) ratio is varied from 0.38 to 1.35 (stoichiometric value = 1.0). The Sn-Zn ratio is held constant across all the films to ensure that the changes in the XRD pattern are only related to the copper content in the film. There is no evidence of the ZnS or SnS2 phase in the copper poor films. However, it is likely that these phases exist but cannot CHAPTER 4. RESULTS AND DISCUSSION 48
Figure 4.3: Isothermal section of Cu2S-ZnS-SnS2 ternary phase diagram at 670K. From [7]. The red, green and blue lines show the areas of the phase diagram that were explored in this study. CHAPTER 4. RESULTS AND DISCUSSION 49
be seen in the XRD data because the ZnS pattern is completely obscured by the CZTS pattern as shown in figure 2.15 and because there is not enough of any SnS2 phase present to be seen as this is a thin film. This is due to the scattering geometry of symmetric scans and is discussed in section 2.2.4. However, this technique is capable of identifying a Cu2S phase in the copper rich film (sample A) which is in agreement with theory. In the experiment shown in 4.4B, the Zn/(Cu+Sn) ratio is varied from 0.14 (zinc poor) to 1.06 (zinc rich). Unfortunately there is no evidence of the ZnS
(zinc rich) phase or the Cu2SnS3 (zinc poor) phase that should exist in these films. Once again, this is likely because these phases do not have any unique peaks that are not obscured by the CZTS pattern.
Grazing Angle X-ray Di↵raction (GIX)
While symmetric XRD scans did not prove to be very e↵ective for phase identification, it was possible to identify a number of extraneous phases using GIX. Figure 4.5 shows the GIX pattern for a stoichiometric CZTS film. All the peaks are indexed using the accepted di↵raction pattern for CZTS from the International Center for Di↵raction Data (ICDD). [8] There are no secondary peaks in this film that could be attributed to impurities or other phases from this system. This sample is used as the control/stoichiometric sample for the experiments in this section. Figure 4.6 shows the results of an experiment where the Cu/(Zn+Sn) ratio is varied. In this figure, sample B is the stoichiometric CZTS sample while sample A is afilmwithaCu/(Zn+Sn)ratiohigherthanthestoichiometricvalue(i.e.afilmthat is copper rich). All unmarked peaks in this figure correspond to CZTS peaks and only the extraneous peaks have been marked as identified using references from the ICDD. [8] The extraneous peaks at 27.83, 32.23, 46.28 and 54.78 can be attributed to the Cu(2 x)Sphasewhilethepeaksat31.88,52.58and48.23canbeattributedtothe CuS phase. [8] This result disagrees with the phase diagram presented by Olekseyuk et al. which predicts the formation of only Cu2Sinthecopperrichgrowthregime.[7] In figure 4.7, sample E is the stoichiometric CZTS sample. Samples C and D are grown with excess tin content while the copper to zinc ratio remains constant (thus varying the Sn/(Cu+Zn) ratio). The excess peaks in these samples (22.18, 26.18, CHAPTER 4. RESULTS AND DISCUSSION 50
Figure 4.4: Symmetric XRD patterns of CZTS films with varying A) Cu/(Zn+Sn) ratio and B) Zn/(Cu+Sn) ratio. CHAPTER 4. RESULTS AND DISCUSSION 51
Figure 4.5: Grazing angle XRD pattern of a stoichiometric CZTS film.
Figure 4.6: GIX patterns of CZTS films with varying Cu/(Zn+Sn) ratio and constant Zn/Sn ratio. CHAPTER 4. RESULTS AND DISCUSSION 52
Figure 4.7: GIX patterns of CZTS films with varying Sn/(Zn+Cu) ratio and constant Cu/Zn ratio.
27.28, 30.53, 31.53, 39.03, 42.93, 45.03, 48.88, 50.98, 52.88, 54.18) can all be attributed to the same ↵-SnS phase. [8] A comparison of samples C and D shows that there is also an increase in peak intensity of the extraneous peaks with increasing Sn/(Cu+Zn), which further supports the conclusion that there is a SnS phase present. Once again, this result disagrees with the phase diagram, which predicts the formation of only the SnS2 phase in the tin rich regime. The likely cause for this discrepancy is that the films are grown/annealed in vacuum while the phase diagram is for systems at atmospheric pressure. Furthermore, the phase diagram is only applicable if the sulfur content in the film is at the stoichiometric fraction 50%. ICP-OES measurements showed that the sulfur fraction in these films ranged from 45-48% rather than 50%. It has also been shown by multiple groups that films annealed in vacuum can lead to sulfur deficient films due to evaporation of sulfur. Furthermore, an atmosphere that is sulfur rich (ie. the chemical potential of sulfur gas is equal to or greater than the chemical potential of sulfur in the CZTS film), is required to ensure that no sulfur escapes from the CZTS film. [27,28] CHAPTER 4. RESULTS AND DISCUSSION 53
Figure 4.8: GIX patterns of CZTS films with varying Zn/(Cu+Sn) ratio and constant Cu/Sn ratio.
Figure 4.8 shows the results of an attempt to evolve and identify the ZnS phase. As mentioned in section 2.5.4, ZnS shares all of its peaks with the CZTS phase. Thus, to identify this phase, it is necessary to focus on one of the three primary CZTS/ZnS peaks (in this case the 112 peak). The zinc content in the film is increased while holding the copper to tin ratio constant. The only di↵erence seen is the “shoulders” that develop around this peak. However, using this technique, it is possible to identify the ZnS phase using XRD.
Raman Spectroscopy (RS)
Challenges associated with use of RS for phase analysis in this material system were discussed in section 2.5.4. As with XRD, RS using a 532nm laser has the problem of many overlapping peaks as shown in figure 2.15. Experiments similar to those described in the previous section were performed for this RS study. Once again, CZTS films were purposefully grown o↵-stoichiometry to try and identify extraneous phases using RS. CHAPTER 4. RESULTS AND DISCUSSION 54
Figure 4.9: Raman spectra of CZTS films with varying Cu/(Zn+Sn) ratio and con- stant Zn/Sn ratio. Raman peaks of some relevant phases are marked.
Figure 4.9 shows the Raman spectra of films with varying Cu/(Zn+Sn) ratio and 1 constant Zn/Sn ratio. The primary CZTS peak can be seen at 336-337 cm in all of the films. However, even though the Cu/(Zn+Sn) ratio is varied from 0.38 (copper poor) to 1.35 (copper rich), no other singular peak can be conclusively identified. This is unusual because a Cu(2 x)S phase has been identified in other copper rich samples by Fernandes et al. [11] and was also identified in these films using XRD. However, there are multiple Cu(2 x)Sphases(andthemajorityofwhicharenotvisibleina Raman spectrum) and it is likely that the phase evolved in this work di↵ers from that identified in the literature. There is a shoulder present to the left of the primary peak in the most copper poor film (D), which is likely due to a convolution of peaks from the ZnS and Sn2S3 phases. According to the phase diagram, the Sn phase should be
SnS2 rather than Sn2S3 as seen in these films. This observation can be explained by a lack of sulfur in the films. ICP-OES measurements showed that the sulfur content in the films ranged from 45%-48% rather than 50%. Even though measurement of gaseous species using ICP-OES can be unreliable, the presence of the sulfur poor CHAPTER 4. RESULTS AND DISCUSSION 55
Figure 4.10: Raman spectra of CZTS films with varying Zn/(Cu+Sn) ratio and con- stant Cu/Sn ratio. Raman peaks of some relevant phases are marked.
Sn2S3 phase could mean that the films are in fact sulfur poor. For the films with varying Zn/(Cu+Sn) ratios and constant Cu-Sn ratios (figure 1 4.10), once again, the primary CZTS peak can be seen at 336-337 cm in all the
films. According to the phase diagram, there should be evidence of Cu2SnS3 in the zinc poor films and ZnS in the zinc rich films. However, these films show very small di↵erences in their respective Raman spectra with no other identifiable phases even though the Zn/(Cu+Sn) ratio is varied from 0.10 (zinc poor) to 0.51 (zinc rich). It is likely that the peaks of these phases are too close to the primary CZTS peak and are only contributing to the broadening of this peak.
Auger Electron Spectroscopy (AES)
It has been established in the previous two sections that XRD and RS are insu cient for phase analysis in this system. AES is generally not considered a tool capable of identifying extraneous phases. However, it is capable of identifying very small compositional variations on a surface. The capability of AES to detect compositional CHAPTER 4. RESULTS AND DISCUSSION 56
variation, coupled with a complimentary technique like XRD, can then be used to identify previously hidden phases. Figure 4.11A shows the SEM image of the surface of a CZTS film that has been grown with zinc content above the stoichiometric value (established using ICP-OES) and sputtered lightly to remove any oxide and carbon contaminants. Figures 4.11B, 4.11C and 4.11D show the concentration of copper, zinc and tin as a function of loca- tion on the sample. The intensity of the color in these images is directly proportional to the quantity of the element present. In Figure 4.11C, the bright red areas corre- spond to a zinc concentration of almost 50% while the darker red areas correspond to a zinc concentration of close to 12%. In the case of tin (figure 4.11D), the bright blue area corresponds to a concentration of 12%, while the dark blue (almost black) area corresponds to negligible tin signal (< 2%). A comparison of the zinc and tin image maps shows that areas that contain 50% zinc do not contain any tin, while the areas that contain more tin do still contain some zinc. Figure 4.11B shows the image map for copper, where the bright green areas cor- respond to 25% copper, while the green-black areas correspond to almost no copper (< 2%). From the map it appears that there is copper present almost everywhere in the film (i.e. it is not segregating from the zinc as the tin does). However, zinc and copper share overlapping Auger peaks (922 ev and 908 ev) and anywhere there is zinc present will show some copper signal. It is more likely that the film is phase separating into ZnS and CZTS, and the excess copper seen in the ZnS area is an artifact of the Auger analysis. When these images are overlaid in figure 4.11E, it can be seen that the composition is split into areas (blue green) that contain copper, zinc and tin (CZTS) and areas (orange) that contain zinc only. This is in agreement with the phase diagram mentioned earlier which states that a zinc rich CZTS film should segregate into CZTS and ZnS as long as there is enough sulfur present. [7] Unfortunately, due to the similarities between the ZnS phase and CZTS, it seems that this is one of the few non-exotic techniques that can be used to identify ZnS in a CZTS thin film. The sulfur content throughout the film was constant at 47%-48% and as a result is not shown here. CHAPTER 4. RESULTS AND DISCUSSION 57
Figure 4.11: AES image maps of zinc rich CZTS thin film: A) SEM image, B) copper image map, C) zinc image map, D) tin image map, E) overlay of copper, zinc and tin image maps. CHAPTER 4. RESULTS AND DISCUSSION 58
Figure 4.12: SEM images of CZTS films grown at A) 400 Cand40mtorrandB) 550 Cand12mtorr.
4.1.3 Morphology
The morphology of a thin film can have a significant influence on its properties. This is specially true for solar cells, since grain boundaries and surface roughness can greatly a↵ect recombination in the film. This section will discuss some of the factors that a↵ect morphology in these films. CHAPTER 4. RESULTS AND DISCUSSION 59
Figure 4.13: SEM images of CZTS films grown at room temperature and then an- nealed at 550 Cin10%H2Sat40mtorr.
Growth Temperature and Pressure
Figure 4.12 shows the SEM cross sections of CZTS films with constant composition
(Zn-rich) reactively sputtered at A) 400 C, 40mtorr and B) 550 C, 12 mtorr. The films both have columnar grains that extend (in the case of the larger grains) from the substrate to the surface of the film. This is a unique morphology that has never been shown for this material system, as most other deposition processes involve post deposition annealing and yield spherical grains rather than columnar growth. As expected, the grain diameter increases with increasing temperature and decreasing pressure and can be controlled to produce columns ranging from 50-500nm. These results are in agreement with established models for grain growth during sputter deposition as described by Ohring. [19]
Post Deposition Annealing
Some of the CZTS films grown were reactively sputtered at room temperature and then annealed at 550 C to evolve the correct phase. Figure 4.13 shows cross section SEM images of these films. The morphology is significantly di↵erent from that pre- sented in the previous section and is closer to the morphology presented by other groups [10, 24, 39, 40]. This is likely because at room temperature the film that is CHAPTER 4. RESULTS AND DISCUSSION 60
deposited is amorphous and the CZTS phase is evolved during the anneal. This leads to di↵usion of the atoms in the solid film and nucleation and growth of spherical grains that increase in size by Ostwald ripening.
4.1.4 Devices
AnumberofCZTSfilmswereincorporatedintothewellestablishedCIGSdevice stack shown in figure 2.16. The IV (A) and EQE (B) measurements for the best device are shown in figure 4.14. The IV curve shows good diode behavior with a fill factor of 64% and e ciency of 3.4%. Fitting of this curve to the one diode equation 2 (discussed in section 2.3.4) showed that the series resistance (Rs) was 0.59 ohm*cm shunt resistance1,was953ohm*cm2 and the diode quality factor was 2.14. These values are similar to CIGS devices of much higher e ciency and it is not clear why the CZTS device e ciency is so severely limited. The world record CZTS devices are currently made by Wang et al. using co-evaporation. [26] They report that the device e ciency is limited due to high series resistance in the device. One explanation for the high Rs reported is the formation of a MoS2 layer at the molybdenum CZTS inter- face. However, no evidence of this layer is seen in this work and reactively sputtered CZTS devices do not su↵er from high series resistance. For CIGS, degraded device e ciency can sometimes be explained by defects at the CdS-absorber interface or a large conduction band o↵set between CdS and the absorber. [41,42] Unfortunately, such work has not yet been replicated for CZTS.
E↵ect of Composition
The composition of the devices was varied using the phase diagram shown in figure 4.3. The data resulting from the variation of, Zn/(Cu+Sn) and Sn/(Cu+Zn) ra- tio did not show any relevant trends in device performance. However, varying the Cu/(Zn+Sn) ratio did result in some significant trends. Figure 4.15 shows the open circuit voltage (Voc), short circuit current density (Jsc)ande ciencyofCZTSdevices
1The resistances are given in units of ohm*cm2 because they have been normalized for the area. This allows them to be compared with other devices with di↵erent active areas. CHAPTER 4. RESULTS AND DISCUSSION 61
Figure 4.14: A) IV and B) EQE measurements of the best CZTS device grown using reactive sputtering. CHAPTER 4. RESULTS AND DISCUSSION 62
Figure 4.15: A) Open circuit voltage (Voc), B) short circuit current density (Jsc)and C) e ciency as a function of Cu/(Zn+Sn) ratio. CHAPTER 4. RESULTS AND DISCUSSION 63
as a function of the Cu/(Zn+Sn) ratio. Voc data did not show any trend though the highest Voc was achieved in a copper poor film. Jsc and e ciency both increase as a function of copper content in the film with the best Jsc observed just below the stoi- chiometric Cu/(Zn+Sn) ratio. There is a steep drop-o↵in e ciency in the films that are copper rich. This behavior is in agreement with other groups that have observed the highest e ciencies of CZTS in the copper poor regime. [26,43] An explanation for this behavior comes from the extraneous phases that were observed in these films in section 4.1.2. In the copper poor regime the extraneous phases that form are ZnS and
SnS, while in the copper rich regime Cu2Swasobserved.Cu2Sisahighlyconductive phase and is known to be very detrimental in CIGS thin film solar cells. It is likely the e ciency of CZTS devices that are copper rich is also limited by this phase.
4.1.5 Nanostructured CZTS-ZnS Films
The best CZTS devices are produced by Wang et al. using thermal co-evaporation.
[26] This process is similar to that used by Repins et al. [32] to produce CuInGaSe2 cells at e ciencies as high as 19.9%. However, the best CZTS devices are still limited to 6.8%. This is likely due to the added level of complexity in the CZTS system. Unlike the psuedo-ternary CIGS, CZTS is a quaternary compound which adds another set of possible detrimental defects. [44,45] Recombination at these defects is the likely cause of the limited e ciencies that have been observed. [23,24,26,27,46] If this is the case, a radical change in the device structure will be required to achieve e ciencies close to the theoretical maximum from this material. One possible way to accomplish this is by nanostructuring the device to reduce the path length required for an electron to di↵use to the junction as shown in figure 4.16. There are multiple theoretical studies suggesting that this type of interdigitated structure can be used to improve e ciencies in materials with poor lifetimes. [47–50] The ideal periodic spacing is dependent on material properties but in most cases ranges from 100-300nm. Initial experimental results of devices fabricated based on these studies have also been very promising. [51,52] However, creating this type of nanostructure is generally a complicated process involving multiple steps and most CHAPTER 4. RESULTS AND DISCUSSION 64
Figure 4.16: Comparison of a conventional CZTS device (left) with a CZTS nanos- tructured device (right)
of the techniques that have been attempted are not scalable to large areas. An ideal process would involve only one step and be capable of deposition over large areas in ashorttime.
CZTS is a line compound that occurs in the Cu2SnS3-ZnS binary phase diagram shown in figure 2.14. [7] Theoretically, any thin film grown under zinc rich conditions, given a lack of kinetic barriers, should naturally phase separate into CZTS (p-type) and ZnS (n-type). ZnS has been shown to be a very e↵ective heterojunction partner for CIGS and should work for CZTS as well as long as it is doped appropriately. [53] Section 4.1.2 showed the results of AES image mapping of a CZTS thin film that is zinc rich. As can be seen from figure 4.11, CZTS films do indeed phase separate into ZnS and CZTS. Thus, the focus of this work is to use deposition parameters to control the phase separation to achieve the desired nanostructure in a one step process.
Nanostructure Control and Analysis
Section 4.1.3 discussed the e↵ect of growth temperature and pressure on the mor- phology of CZTS films. The images in figure 4.12 show that it is possible to get a CHAPTER 4. RESULTS AND DISCUSSION 65
columnar morphology with column widths ranging from 50-500nm. Thus, films that are grown zinc rich, should phase separate into distinct columnar grains of CZTS and ZnS. Temperature and pressure could then be used to grow the correct column width based on the material properties of CZTS. Figure 4.17A shows the Raman spectrum for a columnar CZTS film grown at
550 Cand12mtorrunderzincrichconditionswithagrainwidthof200-250nm.The 1 spectrum shows the primary CZTS peak at 338 cm ,whichisinagreementwith previously presented results. [9–11] Figure 4.17B shows the symmetric XRD pattern of the same film. The primary CZTS peaks are all present with no extraneous peaks belonging to other phases. [8] As mentioned in section 2.5.4, it is not possible to conclusively determine the di↵erence between the CZTS and ZnS phases using XRD due to the similarities between the two crystal structures. Only a combination of XRD and AES can be used to identify the ZnS phase, and thus to establish that a nanostructure is being evolved in the film. Since the XRD pattern in figure 4.17B shows no extraneous peaks, it is likely that the only phases present in this film are CZTS and ZnS. [8] Figure 4.18 shows the AES image maps of the surface of a CZTS device that has been grown with zinc content above the stoichiometric value (established using ICP-OES) and sputter etched to remove layers other than the absorber (CdS, ZnO, Al:ZnO). Figures 4.18A, 4.18B and 4.18C show the concentration of copper, zinc and tin as a function of location on the surface of the sputter etched sample. The intensity of the color in these images is directly proportional to the quantity of the element present. In Figure 4.18A, the bright red areas correspond to a copper concentrations of almost 25% while the dark red areas (almost black) correspond to negligible copper content (< 2%). In the case of zinc (figure 4.18B), there is minimal color variation throughout the image which implies that there is Zn present everywhere in the film in almost the same quantity. A more careful analysis reveals that the zinc content ranges from 12% to 16%. Figure 4.18C shows the tin content in the film with bright green areas correspond- ing to 12% tin and dark areas corresponding to negligible tin (¡ 2%). When these images are overlaid in figure 4.18E, we can see that the composition is split into areas CHAPTER 4. RESULTS AND DISCUSSION 66
Figure 4.17: A) Raman spectrum of a CZTS-ZnS film, B) Symmetric XRD pattern of a CZTS-ZnS film. CHAPTER 4. RESULTS AND DISCUSSION 67
Figure 4.18: AES image maps of a nanostructured CZTS-ZnS film: A) copper, B) zinc, C) tin, D) cadmium, E) overlay of copper, zinc and tin, F) overlay of cadmium and tin. CHAPTER 4. RESULTS AND DISCUSSION 68
(blue-green) that contain copper, zinc and tin (CZTS) and areas (dark blue) that contain only zinc. This is in agreement with the phase diagram mentioned earlier which states that a zinc rich CZTS film should segregate into CZTS and ZnS as long as their is enough sulfur present. [7] The scale of the segregation is on the order of 200- 300nm which is the size of the grains in the film as seen in the SEM images in figure 4.12B. Furthermore, this scale is close to the ideal calculated size for a nanostructured device. [47–50]
Device Performance
Phase separated films with the ideal nanostructure spacing (200-300nm) were incor- porated into the well established CIGS device stack (SLG / Mo / CZTS / CZTS-ZnS / CdS / i:ZnO / Al:ZnO). [32] Figure 4.19 shows the current-voltage (IV) measure- ment and external quantum e ciency (EQE) measurement of this cell. The Voc and 2 Isc were 343mV and 9.52mA/cm respectively. The devices had a fill factor and e ciency of 41.3% and 1.35% respectively. This is significantly worse than the con- ventional CZTS devices discussed earlier. Further analysis using AES showed one possible explanation. Figure 4.18D shows the image map for cadmium in this device. The Al:ZnO, ZnO, CdS and half the CZTS film have been sputter etched using an ion gun and the surface scanned for cadmium. The bright areas correspond to 32% Cd while the the dark areas correspond to negligible amounts of cadmium (¡ 2%). Figure 4.18F is an overlay of the cadmium signal and the tin signal and shows that there is no intermixing between the cadmium and tin in the film. Since it was established earlier that the tin is only present in the pure CZTS phase it is safe to conclude that the cadmium has only penetrated into the ZnS phase. This is thermodynamically feasible according to the phase diagram presented by Chen et al. [54] and has been observed in bath deposition before by Oladeji et al. [55] However, the image map is only a scan of one planar cut within the thickness of the film. An AES depth profile (shown in figure 4.20) was performed to determine how far the cadmium was penetrating into the thickness of the film. It shows that cadmium is present in the film all the way to molybdenum interface. As mentioned earlier in section 4.1.1, the composition in the AES profile is unlikely to be accurate but can be used to establish CHAPTER 4. RESULTS AND DISCUSSION 69
Figure 4.19: IV and EQE measurements of a nanostructured CZTS-ZnS device. CHAPTER 4. RESULTS AND DISCUSSION 70
Figure 4.20: AES depth profile for a nanostructured CZTS film. The cadmium signal can be seen throughout the depth of the film. the presence of cadmium and the overall compositional uniformity.
4.2 Cu2ZnSn(S,Se)4 (CZTSSe)
4.2.1 Addition of Selenium
Up until this point, this work has discussed pure sulfide CZTS thin films and the di culties associated with formation of this phase and characterization of this system. The majority of the di culties stem from the fact that it is a four component system with many secondary phases and electronic defect states. Thus, it does not seem very logical to add a fifth component (selenium) in an e↵ort to improve photovoltaic performance. However, recent results by Todorov et al. have shown that the device performance of these thin films can be significantly improved by introducing selenium into the system. [23] While the best pure sulfide films have been limited to 6-7% e ciency for the last 5 years [24–27], sulfo-selenide hybrid films (CZTSSe) have seen large improvements in e ciency with the best films achieving e ciencies as high as CHAPTER 4. RESULTS AND DISCUSSION 71
10.1% in a very short period of time. [56] This section of the work is focused on adding selenium to the CZTS system, improving the device e ciency and trying to identify a composition window for growth of high e ciency CZTS devices by determining which secondary phases are or are not detrimental to device performance.
4.2.2 Note on Phase Analysis
Section 2.5.4 discussed some of the complications associated with phase analysis in
CZTS films. The primary problem is that CZTS, ZnS and Cu2SnS3 share similar crystal structures and it is almost impossible to di↵erentiate these phases using XRD because of the overlapping peaks as shown in figure 2.15. Section 2.5.1 described the di↵erences (or lack thereof) between CZTS and CZTSSe. While substitution of selenium for sulfur in the lattice does not change the crystal structure, it does change the size of the lattice since selenium is a much larger atom. This increase in size, and as a result in lattice parameter causes the peaks in the XRD pattern to shift to the left. Figure 4.21 shows the primary peak shift for a number of samples with varying selenium to sulfur ratios. Once the CZTSSe pattern shifts, it becomes possible to identify some of the phases that were originally obscured. In this case, a ZnS phase that was previously covered by the CZTS pattern can be resolved in films with enough selenium.
4.2.3 Morphology
The addition of selenium has a significant impact on the morphology of these thin films. As a comparison, figures 4.12 and 4.13 show two SEM cross sections of pure sulfide CZTS films. There is a large variation in grain size in these films but the largest grains are limited to 500nm. Figure 4.22 shows SEM cross sections for two CZTSSe films. The smallest grains in these films are 500nm and the largest grains are 1500nm and extend the full thickness of the film. It is not known why the addition of selenium causes the grain size in the films to increase so significantly. There are a number of possible explanations:
1. The larger selenium atom increases the size of the interstitial sites in the lattice, CHAPTER 4. RESULTS AND DISCUSSION 72
Figure 4.21: XRD patterns of films with varying selenium to sulfur ratio and a ZnS phase that can be identified after the CZTSSe pattern shifts
and as a result, decreases the barrier for di↵usion of the other atoms in the structure. Copper and zinc are significantly smaller atoms than selenium and the increase in lattice parameter may allow them to di↵use freely through the lattice.
2. The selenium reacts with the CZTS from the liquid phase. Selenium is a liquid at high temperature and it is possible that the reaction to form CZTSSe involves the liquid phase of selenium. Such a mixed phase reaction scheme has been shown to lead to large grains in CIGS. [57,58]
3. The melting points of CZTS and CZTSe are 990 Cand805 Crespectively. Addition of selenium reduces the melting point of the material, raising the homologous temperature and increasing di↵usion in the film.
4. The bond enthalpies for Cu-S, Zn-S and Sn-S (276, 205 and 464) are all higher than those for Cu-Se, Zn-Se and Sn-Se (251, 170 and 401) which could reduce the mobility of atoms in CZTS vs. CZTSe. CHAPTER 4. RESULTS AND DISCUSSION 73
Figure 4.22: Cross-section SEM images of two CZTSSe films.
4.2.4 High E ciency Devices
As stated earlier, the focus of this part of the work was to try and improve the e ciency of CZTS devices by adding selenium to these films and determining a com- position window for growth of high e ciency2 CZTSSe hybrid devices. To accomplish this in a controlled manner, the phase diagram was used as a guide to devise an ex- perimental plan. Figure 4.3 shows the phase diagram and how two components of the composition were held constant while the third was varied purposefully to determine the e↵ect of this variation on device e ciency. Furthermore, XRD was used to try and identify the secondary phases that form in these films and determine which ones are detrimental to device performance.
Zn/(Cu+Sn) Ratio
Figure 4.23 (top) shows the e ciency of four CZTSSe films grown with varying Zn/(Cu+Sn) ratios ranging from 0.29-0.47. As a point of comparison, a stoichio- metric CZTSSe film has a Zn/(Cu+Sn) ratio of 0.33. The best e ciency achieved was 7.9% with a Zn/(Cu+Sn) ratio of 0.36 (zinc rich). This result is in agreement
2The term “high e ciency” is used here because the devices presented have some of the highest e ciencies reported for this material. As of the writing of this dissertation, the best device produced as part of this work had an e ciency of 9.3%, while the best e ciency reported in literature was been 10.1% CHAPTER 4. RESULTS AND DISCUSSION 74
with a number of other research groups that have reported that the highest e ciencies are achieved in zinc rich and copper poor films. [23,24,46] The most likely explanation for this has to do with the secondary phase that forms in zinc rich films. Figure 4.23 (bottom) shows a closeup of the XRD pattern near the (220) CZTSSe peak of these films. The shoulder to the right of the peak can be identified as ZnS, [8] the inten- sity of which increases with increasing zinc content in the film. This is in agreement with the phase diagram presented earlier in this work, and the high resistivity of ZnS (104 ohm*m) explains why excess zinc is not very detrimental to device performance. Furthermore, the best e ciency is achieved for a film with detectable quantities of ZnS.
Sn/(Cu+Zn) Ratio
Asimilaranalysisisperformedfortincontentandshowninfigure4.24.Onceagain, the stoichiometric ratio is 0.33 and the range of Sn/(Cu+Zn) ratios explored is 0.39 to 0.53. There is a clear trend in the data with higher e ciencies being achieved for lower Sn/(Cu+Zn) ratios. However, tin content in these films is exceptionally hard to control due to the volatility of tin phases, and as a result this dataset is limited only to tin rich films. The highest e ciency achieved in this experiment was 7.1% at a Sn/(Cu+Zn) ratio of 0.39, and the decrease in e ciency as the tin content is increased is not precipitous. A look at a closeup of the XRD pattern (figure 4.24 bottom) reveals that the secondary phase being formed in these films is SnS2.SnS2 (105 ohm*m) has an even higher resistivity than ZnS and is likely to be electrically inactive in the device.
Cu/(Zn+Sn) Ratio
Figure 4.25 shows the e↵ect of copper content on the e ciency of CZTSSe thin films. The Cu/(Zn+Sn) ratio is varied from 0.61 to 1.24 and the stoichiometric ratio is 1.0. As mentioned before, it has been reported that films that are zinc rich and copper poor are ideal for device performance and a similar result is observed here. There is a very sharp drop in e ciency in films that are copper rich, while the best CHAPTER 4. RESULTS AND DISCUSSION 75
8.5 Stoichiometric Ratio 8.0
7.5
7.0
6.5 Efficiency (%) 6.0
5.5
5.0 0.28 0.32 0.36 0.40 0.44 0.48 Zn/(Cu+Sn) Ratio
Figure 4.23: E ciency vs. Zn/(Cu+Sn) ratio (top) and the XRD identification of the ZnS (110) peak (bottom). CHAPTER 4. RESULTS AND DISCUSSION 76
7.5
7.0
6.5
6.0
5.5 Efficiency (%) 5.0
4.5
4.0 0.40 0.44 0.48 0.52 Sn/(Cu+Zn)
Figure 4.24: E ciency vs. Sn/(Cu+Zn) ratio (top) and the XRD identification of the SnS2Speak(bottom). CHAPTER 4. RESULTS AND DISCUSSION 77
e ciency (4.6%) is achieved in a film that is noticeably copper poor. XRD was used to determine that the secondary phase that forms in copper rich films is Cu2S. The 4 resistivity of this phase is very low (10 ohm*m) and it has been reported to be very detrimental in CIGS films. The phase is so detrimental that even the film that has the correct stoichiometric ratio shows a large drop in e ciency. To further explore this phenomenon, the same experiment was performed with some higher e ciency films and over a smaller Cu/(Zn+Sn) range. Figure 4.26 shows the IV curves and e ciencies of these films. The Cu/(Zn+Sn) ratio is varied from 0.84 to 0.93 with the best e ciency (6.7%) achieved at 0.91. Once again, there is sharp drop in e ciency once the film reaches a Cu/(Zn+Sn) ratio close to the stoichiometric ratio. The most copper rich film behaves as a shunted resistor rather than a diode. Fitting of these curves to the one diode equation showed that the shunt resistance (normalized by area) of the film with the highest e ciency is 465 ohm*cm2 and the film with the lowest e ciency is 2.6 ohm*cm2.
Se-S Ratio
The selenium to sulfur ratio does not seem to have a significant impact on the e - ciency of CZTSSe devices. Once a small amount of selenium is introduced into the film, high e ciencies can be achieved. However, this ratio does have an impact on electronic characteristics of the device such as open circuit voltage and short circuit current. The device parameters for these films are shown in table 4.2. Figure 4.27A shows the IV curves for a number of devices with the Se-S ratio varying from 0.55 to 1.32. The e ciency and fill factor of these devices is almost the same and the only changes observed in the IV curve are a decrease in Voc and increase in Isc with increasing selenium content. This result can be explained by a change in the band gap of the material with changing Se-S ratio. It has been reported in literature that the band gap of CZTSSe changes from 1.0-1.5ev depending on the Se-S ratio, with higher selenium content leading to a lower band gap. [59,60] Figure 4.27B shows the EQE curves for these devices which further support this hypothesis. As the selenium content is increased, the absorption edge shifts towards higher wavelengths (or lower energies). CHAPTER 4. RESULTS AND DISCUSSION 78
5.0 Stoichiometric Ratio 4.5
4.0
3.5
3.0
2.5
Efficiency (%) 2.0
1.5
1.0
0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 Cu/(Zn+Sn) Ratio
Figure 4.25: E ciency vs. Cu/(Zn+Sn) ratio (top) and the XRD identification of the Cu2Speak(bottom).
Table 4.2: CZTSSe device parameters for devices with varying Se/S ratio. Sample Se/S E↵.(%) Voc(V) Jsc(mA) FF(%) A 1.32 7.9 0.46 31.30 56 B 0.80 8.0 0.51 28.90 53 C 0.55 7.9 0.54 29.11 54 CHAPTER 4. RESULTS AND DISCUSSION 79
B Cu/(Zn+Sn) 30 ideal = 1.0 0.85 ) 2 25 0.87 0.91 20 0.93
15 Efficiency (%) 10 3.5 4.5 5 6.7
Current Density (mA/cm 0.1 0
-5 -0.2 0.0 0.2 0.4 0.6
Voltage (V)
Figure 4.26: IV curves of higher e ciency films with varying Cu/(Zn+Sn) ratio.
Best Device
The best device produced in this section of the work had an e ciency of 9.3%. The Cu/(Zn+Sn) ratio was 0.91, the Zn/Sn ratio was 1.08 (zinc rich and copper poor) and the thickness was 1.9µm. Figure 4.28A shows the IV curve and pertinent device parameters for this device. The Voc,Isc, fill factor and e ciency were determined from the curve while J0,RS,RSh and n were calculated by fitting the data to the one diode equation for solar cells. [61] The shunt and series resistances have been normalized to area so that they may be compared with other cells. The device parameters are on par with the best CZTSSe devices currently produced by Barkhouse et al. [56] and are similar to CIGS and CdTe devices that have comparable e ciencies.
However, the poor diode quality factor (n) and high reverse saturation current (J0) imply that there is still a significant amount of recombination in the device. Further evidence of this can be seen in the EQE spectrum in figure 4.28B. The loss in EQE at higher wavelength light (750-950nm) implies that electronics generated by light that is absorbed further into the absorber layer recombine before they reach the junction. While it is possible that these wavelengths of light are just being reflected or absorbed CHAPTER 4. RESULTS AND DISCUSSION 80
Figure 4.27: A) IV curves and B) EQE spectra for devices with varying Se-S ratio. CHAPTER 4. RESULTS AND DISCUSSION 81
in one of the other layers of the device, this is unlikely since there is no evidence in the literature of this e↵ect. CHAPTER 4. RESULTS AND DISCUSSION 82
30 A 25 ) 2 20 Eff = 9.3%
15 Voc = 0.52 V 2 10 Jsc = 28.3 mA/cm
FF = 63% 5 2 J0 = 1.73E-3 mA/cm 0 2 CurrentDensity (mA/cm RS = 0.925 ohm*cm 2 R = 549 ohm*cm -5 Sh
n = 2.08 -10 -0.2 0.0 0.2 0.4 0.6 Votage (V) 100 B 90
80 70 60
50 40
30 20
External Quantum Efficiency (%) ExternalQuantum Efficiency 10
0 400 500 600 700 800 900 1000 1100 1200 Wavelength (nm)
Figure 4.28: A) IV curve and B) EQE spectrum for the best CZTSSe device, E↵=9.3% Chapter 5
Conclusion
It has been shown by multiple sources in the scientific community that there is a link between CO2 emissions from fossil fuels and the rising temperature of the planet. It has also been shown that the fossil fuel stores are expected to be entirely consumed within the next 100-200 year. Therefore, a renewable resource of energy is required to address the energy needs of the world. While solar radiation is one of the most abundant forms of energy, the current cost of solar energy is too high for it to make a large impact on the total energy use of the world. Continued deployment of solar en- ergy will required further reduction in costs and the use of earth abundant, non-toxic, inexpensive materials. CZTSSe is one such material and the goal of this work was to develop a sputter based process for deposition of CZTSSe thin films, characterize these thin films, incorporate them into devices and improve device e ciency. AreactivesputteringprocesswasdevelopedfordepositionofCZTSthinfilms.
This process utilizes H2Sasthereactivegasandyieldsfilmswithauniquecolumnar morphology that can be controlled using deposition parameters. It was shown that grazing angle XRD is significantly better than symmetric XRD for identification of secondary phases in this material system. This technique was used to conclusively identify Cu(2 x)S, CuS and SnS and showed some potential for identifying ZnS. RS was significantly less successful but was used to identify ZnS and Sn2S3.Sincethat work was completed other groups have shown that changing the wavelength of the laser used can significantly increase the e cacy of this technique. AES image maps
83 CHAPTER 5. CONCLUSION 84
were used to identify ZnS which is usually the most elusive phase in this system. It was shown that the phase diagram is not always accurate for thin films grown using this technique and non-equilbrium phases can exist in these films, either due to deviations from atmospheric pressure in growth conditions (the phase diagram is at standard temperature and pressure) or due to kinetic e↵ects. CZTS films were incorporated into the established CIGS device stack and the best e ciency achieved was 3.4%. Isc and e ciency were shown to be dependent on the stoichiometry of the
film while Voc was not. In an e↵ort to improve the e ciency, nano-structured CZTS thin films were also created using this reactive sputtering process. A combination of XRD, RS, SEM and AES were used to establish that the correct nanostructure had been evolved. The films were incorporated into devices and e ciencies as high as 1.35% were achieved. CdS penetration into the ZnS phase present in the films was observed and identified as a cause for the degraded performance observed in the nano-structured devices. AcompoundsputteringprocesswasdevelopedfordepositionofCZTSSethin films. Selenium was incorporated into the films from the compound targets and no reactive gas was needed. XRD was used to identify ZnS, Cu2SandSnS2 in these films which is in agreement with the phase diagram. The addition of selenium greatly increased the grain size and e ciency of these films (as compared with CZTS) with all device parameters (Isc,Voc etc.) showing improvement. A number of possible expla- nations were provided for this observation but more work is required to conclusively determine the cause. Devices showed an increase in Isc and a decrease in Voc with increasing Se-S ratio and vice versa. This is likely due to a decrease in the band gap of the film which allows the material to capture more of the solar spectrum. There was no correlation between selenium content and e ciency in the films once a minimum amount of selenium was added. Device e ciency was correlated with composition and the amount of the secondary phase present in the film. It was shown that ZnS and SnS2 are relatively benign in the film while Cu2Sisexceptionallydetrimentalto device performance. Recent work on CZTSSe has shown that this material is very promising for solar cell applications. The record e ciency has seen a steady increase over the last two CHAPTER 5. CONCLUSION 85
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