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Thin Film Photovoltaics by Spectroscopic Ellipsometry

Thin Film Photovoltaics by Spectroscopic Ellipsometry

SE01

Thin Film by Spectroscopic Ellipsometry

What do we mean by photovoltaics? First used in about 1890, the word has two parts: photo, derived from the Greek word for , and volt, relating to pioneer Alessandro Volta. So, photovoltaics could literally be translated as light-electricity. And that's what photovoltaic (PV) materials and devices do - they con- vert light into electrical energy (), as French physicist discovered as early as 1839. Commonly known as solar cells, individual PV cells are electricity-producing devices made of materials. PV cells come in many sizes and shapes - from smaller than a postage stamp to several inches across. They are often connected together to form PV modules that may be up to several feet long and a few feet wide. Modules, in turn, can be combined and connected to form PV arrays. Did you know that PV systems are already an important part of our lives? Simple PV systems provide power for many small consumer items, such as calculators and wristwatches. More complicated systems provide power for communications satellites, water pumps, and the , appliances, and machines in some people's homes and workplaces. Many road and traffic signs along highways are now powered by PV. In many cases, PV power is the least expensive form of electricity for performing these tasks.

The Photoelectric Effect Light and the PV Cell The photoelectric effect is the basic physical process by which a PV cell converts sunlight into electricity. When light shines on a PV cell, it may be reflected, absorbed, or pass right through. But only the absorbed light generates electricity. The energy of the absorbed light is transferred to in the of the PV cell. With their newfound energy, these electrons escape from their normal positions in the atoms of the semicon- ductor PV material and become part of the electrical flow, or cur- rent, in an electrical circuit.

The sun emits almost all of its energy in a range of from about 2x10-7 to 4x10-6 meters. Most of this energy is in the visible light region. Solar cells respond differently to the different wavelengths of light. In summary, light that is too high or low in en- ergy is not usable by a cell to produce electricity. Rather, it is trans- formed into heat. Bandgap of and Light Only with a certain level of energy can free electrons in To induce the built-in within a PV cell, two layers of the semiconductor material from their atomic bonds to produce an somewhat differing semiconductor materials are placed in contact electric current. This level of energy is known as the «bandgap en- with one another. One layer is an «n-type» semiconductor with an ergy,». To free an , the energy of a must be at least abundance of electrons, which have a negative electrical charge. as great as the bandgap energy. However, photons with more en- The other layer is a «p-type» semiconductor with an abundance of ergy than the bandgap energy will expend that extra amount as «holes», which have a positive electrical charge. Sandwiching these heat when freeing electrons. So, it's important for a PV cell to be together creates a p/n junction at their interface, thereby creating «tuned»-through slight modifications to the silicon's molecular an electric field. SE01 structure-to optimize the . A key to obtaining an effi- cient PV cell is to convert as much sunlight as possible into electric- ity. has a bandgap energy of 1.1 electron-volts (eV). (An electron-volt is equal to the energy gained by an electron when it passes through a potential of 1 volt in a vacuum.) The bandgap energies of other effective PV semiconductors range from 1.0 to 1.6 eV. In this range, electrons can be freed without creating extra heat. The photon energy of light varies according to the different wave- lengths of the light. The entire spectrum of sunlight, from to , covers a range of about 0.5 eV to about 2.9 eV. For example, red light has an energy of about 1.7 eV, and blue light has an energy of about 2.7 eV. Most PV cells cannot use about 55% of the energy of sunlight, because this energy is either below the bandgap of the material or carries excess energy.

Polycrystalline thin-film cells have a heterojunction structure, in which the top layer is made of a different semiconductor material than the bottom semiconductor layer. The top layer, usually n-type, is a window that allows almost all the light through to the absorbing layer, usually p- type. An «ohmic contact» is often used to provide a good electrical connection to the substrate.

Different PV materials have different energy band gaps. Photons with PV structures characterized by energy equal to the energy are absorbed to create free spectroscopic ellipsometry electrons. Photons with less energy than the band gap energy pass through the material. Proper understanding of thin film materials and thin film stack, tai- loring numerous properties of thin films required for an efficient so- lar cell demands a range of characterization techniques. Thin Film Solar Cells Spectroscopic ellipsometry is an optical technique used for the measurement of thin film thickness and optical constants. The Thin film solar cells can be made from: technique provides the advantages to be non-destructive, fast, pre- • Polycrystalline thin films-including indium diselenide cise to the Angstrom level, and the capability to measure multi-lay- (CIS), telluride (CdTe), and thin-film silicon er stacks. • Single-crystalline thin films-including high-efficiency material 1 Characterization of anti-reflective coatings such as gallium arsenide (GaAs) Silicon is a shiny gray material and can act as a mirror, reflecting Polycrystalline Thin Film more than 30% of the light that shines on it. To improve the con- version efficiency of a , antireflection (AR) coating is ap- Thin film cells use much less material: the cell's active area is usu- plied to the top layer of the cell helping to optimize light ally only 1 to 10 micrometers thick, whereas thick films typically are absorption. Single AR layer or multiple AR layers basically use SiOx 100 to 300 micrometers thick. Also, thin-film cells can usually be and TiOx materials. Another way to reduce is to texture manufactured in a large-area process, which can be an automat- the top surface of the cell, which causes reflected light to strike a ed, continuous production process. Finally, they can be deposited second surface before it can escape, thus increasing the probabili- on flexible substrate materials. ty of absorption. Unlike most single- cells, a typical thin-film device doesn't have a grid for the top electrical contact. Instead, it uses a thin layer of a transparent conducting oxide, such as tin oxide. These oxides are highly transparent and conduct electricity very well. A separate antireflection coating might top off the device, un- less the transparent conducting oxide serves that function. The typical polycrystalline thin film cell has a very thin (less than 0.1 micron) layer on top called the «window» layer. The window layer's role is to absorb light energy from only the high-energy end of the spectrum. It must be thin enough and have a wide enough bandgap (2.8 eV or more) to let all available light through the in- terface (heterojunction) to the absorbing layer. The absorbing layer under the window, usually doped p-type, must have a high absorp- tivity (ability to absorb photons) for high current and a suitable band gap to provide a good voltage. Still, it is typically just 1 to 2 To make an efficient solar cell, we try to maximize absorption, minimize microns thick. reflection and recombination, and thus maximize conduction. SE01

Experimental Conditions - Spectroscopic Ellipsometer: UVISEL SiNx Mapping - Spectral range: 0.75 - 5.0 eV ⇔ 248 - 1653 nm (n value at 633 nm)

SiO2 158 Å Graded TiO 1242 Å 2 Full structure characterization SiO 500 Å 2 9 Thicknesses Glass 9 TiO Graded optical constants Cr 2

TiO2 Graded Optical Constants 4 1.4 3.8 3 Characterization of TCO film 1.2 3.6 The properties of the contacts and windows layers are critical to 1 device performance. 3.4 n 0.8 k At least one contact must be electrically conducting and transpar- 3.2 ent to photons in the spectral range where the absorber creates 0.6 carriers. Transparent conducting oxides are the material of choice 3 0.4 for this purpose (ZnO, ITO, TiO2, SnO2). 2.8 The highest efficiency devices rely on diffusion to transport carriers 0.2 to the junction of collection. 2.6 0 1 2 3 4 5 Photon Energy (eV) Experimental Conditions

tio2_film top.dsp (n) tio2_film bottom.dsp (n) - Spectroscopic Ellipsometer: UVISEL tio2_film top.dsp (k) tio2_film bottom.dsp (k) - Spectral range: 0.6 - 4.0 eV ⇔ 306 - 2066 nm

One can notice that the bottom of the TiO2 film exhibits a higher refractive index than the top of the film. Full structure characterization 9 Thicknesses Roughness 532 Å 9 SiCO optical constants 2 Thickness mapping of SiN thin film x x SnOGraded2 SnO2 6582 Å 9 SnO2 Graded optical constants SiCO 256 Å Experimental Conditions x 9 Surface roughness Glass - Spectroscopic Ellipsometer: UVISEL - Spectral range: 0.6 - 6.5 eV ⇔ 190 - 2100 nm

SnO2 Graded Optical Constants 2.2 1.1 9 Film thickness uniformity 2 1 SiNx ~ 700 Å 9 Optical constants (n,k)uniformity 1.8 0.9 mc-Si 0.8 1.6 0.7 1.4 n 0.6 k 1.2 0.5

SiNx Thickness Mapping 1 0.4 0.3 0.8 0.2 0.6 0.1 0.4 500 1 000 1 500 2 000 (nm)

SnO2_film top.dsp (n) SnO2_film bottom.dsp (n) SnO2_film top.dsp (k) SnO2_film bottom.dsp (k) SE01

Experimental Conditions SiCxOy Optical Constants 4.4 - Spectroscopic Ellipsometer: UVISEL 4.2 - Spectral range: 0.75 - 5.0 eV ⇔ 248 - 1653 nm

4

3.8 9 Thicknesses 3.6 Roughness 83 Å 9 Micro crystalline Si optical constants 3.4 µm c-Sic-Si n 278 Å 9 Surface roughness 3.2 c-Si 3

2.8

2.6 Optical C onstants of M icro C rystalline Silicon 2.4 5 4 2.2 500 1 000 1 500 2 000 3.5 Wavelength (nm) 4.5

3 4 2.5 Experimental Conditions 3.5 n k 2 - Spectroscopic Ellipsometer: UVISEL 3 - Spectral range: 0.6 - 6.5 eV ⇔ 190 - 2100 nm Optical Bandgap 1.5 2.5 1

Full structure characterization 2 0.5 9 Thicknesses Roughness 27 Å 0 9 Graded ZnO optical constants 1 2 3 4 5 Graded ZnO Photon Energy (eV) 900 Å 9 Surface roughness c-Si Experimental Conditions

ZnO Graded Optical Constants - Spectroscopic Ellipsometer: UVISEL 2.4 0.9 - Spectral range: 0.6 - 6.5 eV ⇔ 190 - 2100 nm 2.2 0.8

2 0.7 0.6 1.8 9 Thickness 0.5 n 1.6 k Graded µm Si 1226 Å 9 Graded Micro crystalline Si 0.4 1.4 c-Si optical constants 0.3 1.2 0.2 1 0.1 M icro C rystalline Silicon Graded Optical C onstants 0.8 0 1 2 3 4 5 6 Photon Energy (eV) 4 3

ZnO film top.clc (n) ZnO film bottom.clc (n) 3.5 ZnO film top.clc (k) ZnO film bottom.clc (k) 2.5 3 2 n 2.5 k 1.5 2 4 Characterization of Micro crystalline & Amorphous 1 Silicon thin films 1.5 0.5 Microstructure of silicon films can be varied from one extreme of 1 amorphous/nanocrystalline to highly oriented and/or epitaxial 1 2 3 4 5 6 growth. Photon Energy (eV)

The characterization of silicon thin films by spectroscopic ellipsom- microSi film top.clc (n) microSi film bottom.clc (n) etry provides a wealth of information such as: microSi film top.clc (k) microSi film bottom.clc (k) - Optical bandgap - Inhomogeneities of silicon films (gradient) - The shape of optical constants is directly linked to the microstruc- ture of silicon materials SE01

Experimental Conditions Experimental Conditions - Spectroscopic Ellipsometer: UVISEL - Spectroscopic Ellipsometer: UVISEL - Spectral range: 1.14 - 3.54 eV ⇔ 350 - 1088 nm - Spectral range: 0.6 - 4.2 eV ⇔ 295 - 2066 nm

9 Thicknesses Full CIGS PV Structure Characterization Roughness 26 Å 9 Amorphous Si optical constants ITO 109 nm 9 Film Thicknesses am Si 2518 Å 9 Surface roughness ZnO 110 nm Glass 9 CIGS, ITO, ZnO Optical constants Graded CIGS 2218 nm 9 CIGS Film gradient Mo-SS Optical Constants of

2 4.8 1.8

1.6 4.6 1.4

4.4 1.2 n k 1 4.2 0.8

0.6 4 0.4

3.8 0.2

0 400 500 600 700 800 900 1 000 Wavelength (nm)

5CIGS PV Cell

Copper Indium Selenide (CuInSe2) have extremely high optical ab- sorption coefficients that allow nearly all of the available light with- ZnO Optical Constants in the terrestrial spectrum to be absorbed in the first micrometer of 1.95 0.5 the material. Therefore, the total thickness of the active layers is in the order of 2 micrometers, resulting in efficient use of materials 1.9 without negatively impacting the conversion efficiency. 1.85 0.4 The addition of controlled amounts of gallium and/or sulfur into 1.8 1.75 the CuInSe2 absorber layer allows to adjust its energy gap to pro- 0.3 vide device with higher voltage, better carrier collection and higher 1.7 n k conversion efficiency. 1.65

1.6 0.2 Typical GIGS PV Cell 1.55

1.5 0.1 Transparent Layer 1.45

1.4 0 CdS 2 3 4 Photon Energy (eV)

CuInGaSe2 Contact

Substrate SE01

ITO Optical Constants P3HT Optical Constants 2.4 1.64 0.8 0.32 2.3 0.7 1.6 0.3 2.2 0.6 1.56 0.28 2.1 0.5 1.52 0.26 2 n k n k 0.4 0.24 1.9 1.48 1.8 0.22 0.3 1.44 1.7 0.2 0.2 1.4 1.6 0.18 0.1 1.5 1.36 0.16 0 2 3 4 1 2 3 4 5 Photon Energy (eV) Photon Energy (eV)

6 Polymer solar cells ZnO Optical Constants Organic solar cells and polymer solar cells are built from thin films 1.760 0.220 (typically 100 nm) of organic semiconductors such as polymers 0.200 1.720 and small-molecule compounds like polyphenylene vinylene, cop- 0.180 per phthalocyanine and fullerenes. 1.680 0.160 Energy conversion efficiencies achieved to date using conductive 0.140 polymers are low at 6% efficiency. However, these cells could be 1.640 0.120 beneficial for some applications where mechanical flexibility and n k disposability are important. 1.600 0.100 0.080 Experimental Conditions 1.560 0.060 366 - 03/2008 366 - 0.040 - Spectroscopic Ellipsometer: UVISEL 1.520 0.020 - Spectral range: 0.6 - 5.0 eV ⇔ 248 - 2066 nm 1.480 0.000 1 2 3 4 5 Photon Energy (eV) Organic PV Structure Characterization ZnO 167 Å P3HT 143 Å 9 Film thicknesses Conclusion PEDOT:PSS 543 Å 9 PEDOT: PSS, P3HT, ZnO Optical Glass constants (n,k) Spectroscopic ellipsometry is an ideal technique to characterize film thicknesses and optical constants and optical bandgap for photovoltaic applications. Spectroscopic ellipsometers are also sensitive to the presence of rough overlayer and graded optical PEDOT:PSS Uniaxial Anisotropic Optical Constants constants. The technique provides the advantage to be fast, simple to operate

2.1 0.3 and non-destructive for the characterization of the samples. - Printed in France - ©HORIBA Jobin Yvon - RCS EVRY B 837 150 - Printed in France EVRY B 837 - ©HORIBA Jobin Yvon RCS 2 0.25 Sources and Acknowledgements - http://www1.eere.energy.gov/solar/tf_polycrystalline.html 1.9 0.2

n k - Céline Eypert, Li Yan, Michel Stchakovsky, Marzouk Kloul, Assia 1.8 0.15 Shagaleeva, Roland Seitz, application engineers - HORIBA Jobin Yvon ellipsometry. 1.7 0.1

1.6 0.05

1.5 0 2 3 4 5 6 Photon Energy (eV)

pedot o.udf (n) pedot e.udf (n) pedot o.udf (k) pedot e.udf (k) This document is not conractually binding under anyThis document is not circumstances

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