Towards Cost-Effective Crystalline Silicon Based Flexible Solar Cells

Towards Cost-Effective Crystalline Silicon Based Flexible Solar Cells:

Integration Strategy by Rational Design of Materials, Process, and

Devices

Dissertation by

Rabab R. Bahabry

In Partial Fulfillment of the Requirements

For the Degree of

Doctor of Philosophy of Science

King Abdullah University of Science and Technology

Thuwal, Kingdom of Saudi Arabia

November, 2017 2

EXAMINATION COMMITTEE PAGE

The dissertation/thesis of Rabab R. Bahabry is approved by the examination committee.

Committee Chairperson: Prof. Muhammad Mustafa Hussain Committee Co-Chair: Prof. Udo Schwingenschlögl Committee Members: Prof. Osman Bakr, Prof. Santosh (External)

Rabab R. Bahabry

ABSTRACT

Towards Cost-Effective Crystalline Silicon Based Flexible Solar Cells:

Integration Strategy by Rational Design of Materials, Process, and Devices

Rabab R. Bahabry

The solar cells market has an annual growth of more than 30 percent over the past

15 years. At the same time, the cost of the solar modules diminished to meet both of the rapid global demand and the technological improvements. In particular for the crystalline silicon solar cells, the workhorse of this technology. The objective of this doctoral thesis is enhancing the efficiency of c-Si solar cells while exploring the cost reduction via innovative techniques. Contact metallization and ultra-flexible wafer based c-Si solar cells are the main areas under investigation.

First, Silicon-based solar cells typically utilize screen printed Silver (Ag) metal contacts which affect the optimal electrical performance. To date, metal silicide-based ohmic contacts are occasionally used for the front contact grid lines. In this work, investigation of the microstructure and the electrical characteristics of nickel monosilicide

(NiSi) ohmic contacts on the rear side of c-Si solar cells has been carried out. Significant enhancement in the fill factor leading to increasing the total power conversion efficiency is observed.

Second, advanced classes of modern application require a new generation of 5 versatile solar cells showcasing extreme mechanical resilience. However, silicon is a brittle material with a fracture strains <1%. Highly flexible Si-based solar cells are available in the form thin films which seem to be disadvantageous over thick Si solar cells due to the reduction of the optical absorption with less active Si material. Here, a complementary metal oxide semiconductor (CMOS) technology based integration strategy is designed where corrugation architecture to enable an ultra-flexible solar cell module from bulk mono-crystalline silicon solar wafer with 17% efficiency. This periodic corrugated array benefits from an interchangeable solar cell segmentation scheme which preserves the active silicon thickness and achieves flexibility via interdigitated back contacts. These cells can reversibly withstand high mechanical stress as the screen-printed metals have fracture strain >15%. Furthermore, the integration of the cells is demonstrated on curved surfaces for a fully functional system.

Finally, the developed flexing approach is used to fabricate three-dimensional dome-shaped cells to reduce the optical coupling losses without the use of the expensive solar tracking/tilting systems.

ACKNOWLEDGEMENTS

All praise is to Allah the almighty, the most merciful and the most beneficent.

My sincerest thanks are given to my research advisor and mentor, Prof. Muhammed

Mustafa Hussain for giving me the opportunity of pursuing an amazing research area. His support, guidance and valuable suggestions have contributed significantly to the completion of this endeavor. Next, I would like to give my deepest appreciation and gratitude to Prof. Udo Schwingenschlögl, Prof. Osman Baker and Prof. Santosh Kurinec for taking the time to serve in my defense committee.

I want to extend my special thanks to the college of the integrated nanotechnology laboratory (INL), especially to Mr. Melvin Cruz and friends: Amani Almuslem, Maha Nour,

Joanna Nassar, Sohail Shaikh and Sherjeel Khan. Furthermore, I wish to express my deep thanks to my friend and sister Arwa Kutbee for her kindness, support and for sharing many nice and difficult moments of our Ph.D. Journey.

I highly appreciate the efforts of the staff of the Advanced Nanofabrication, Imaging and

Characterization Facilities of KAUST, especially Mr. Ibrahim Mohd, Mr. Ahad Syed, Mr.

Nimer Wehbi, Mr. Elhadj M. Diallo, Mrs. Nini Wei and Mr. Ulrich Buttner for their incredible support. Moreover, I would like to thank Dr. Abdulrahman ElLabban from

KAUST solar cell center.

Furthermore, I owe my deepest gratitude to my dear husband Khaled Alamoudi, my caring mom Khadija Qouta, adorable kids Raghad, Shahad, Jouri, Ahmed and

Mouhammad and my thoughtful siblings: Rayid, Rudhab, Rihab and Reem for their 7 continuous encouragement, patience, and support. Their high confidence in me was always a vast impulsion in my academic life. Thank you all for your support and never- ending love; this achievement in every bit is yours as much as it is mine.

TABLE OF CONTENTS

EXAMINATION COMMITTEE PAGE ...... 2

ABSTRACT ...... 4

ACKNOWLEDGEMENTS ...... 6

Table of Contents ...... 8

LIST OF ABBREVIATIONS ...... 12

LIST OF SYMBOLS ...... 14

LIST OF ILLUSTRATION ...... 15

LIST OF TABLES ...... 22

1- Introduction ...... 24

1.1 Global energy outlook ...... 24

1.2 Energy harvesters for IOT ...... 26

1.3 Classification of Solar Cells ...... 28

1.4 Solar cells Physics ...... 30

1.5 Structure and Principle ...... 30

1.6 Current Technology Challenges ...... 34

1.7 Dissertation Outline and chapters overview ...... 35

2- Low-Cost Nickel Silicide Rear Ohmic Contact for c-Si solar cells ...... 38 9

2.1 Introduction ...... 41

2.2 Experimental ...... 45

2.2.1 Nickel-mono silicide (NiSi) optimization: ...... 45

2.2.2 Screen printed Ag removal ...... 48

2.2.3 SIMS depth profiling of NiSi/Cu rear contact ...... 49

2.3 Current-Voltage curves of NiSi/Cu and Al rear contacts ...... 51

2.4 Series resistance determination (Rs) methods ...... 58

2.4.1 Comparison of dark and one sun JV-curves ...... 58

2.4.2 Comparison of two JV-curves measured at one and 0.7 sun illumination

intensities ...... 59

2.5 Fabrication process of in-house c-Si solar cell ...... 62

2.5.1 J-V curves of rigid c-Si solar cells using NiSi/Cu contacts ...... 64

2.5.2 Flexible c-Si solar cell (in-house fabrication) ...... 66

2.6 NiSi/Cu rear contact for thin c-Si ...... 68

2.6.1 Experimental ...... 68

2.6.2 I/V measurements For the NiSi/Cu contacts on the soft polished samples ...... 70

2.7 Conclusion ...... 72

3- Ultra-Flexible High-Efficiency Mono-crystalline Silicon Solar Cell ...... 73

3.1 Introduction ...... 74 10

3.2 Results and Discussion ...... 76

3.2.1 Ultra-Flexible c-Si solar cell ...... 76

3.2.2 Solar cell efficiency...... 83

3.2.3 Flexible LEDs matrix and system on chip integrated with the flexible c-Si solar cells

using indoor illumination ...... 87

3.2.4 Mechanical flexibility of interdigitated back contact...... 89

3.3 Benchmarking of flexible Si-based solar cells efficiency...... 96

3.3.1 Academic research ...... 96

3.3.2 Our process vs the industrial approach ...... 98

3.4 Experimental ...... 102

3.4.1 Corrugation architecture technique ...... 102

3.4.2 The laser-based dicing method ...... 103

3.4.3 Device characterization...... 104

3.4.4 System on chip (SOC) ...... 105

3.5 Conclusion ...... 106

4 -3D models of c-Si solar cell ...... 107

4.1 Introduction ...... 107

4.2 Experimental ...... 109

4.2.1 Laser engraving for low-cost design printing ...... 109

4.2.2 Microlithography ...... 110 11

4.3 Results and Discussion ...... 111

4.3.1 Design printed via the Laser engraving ...... 111

4.3.2 Design transferred via lithography ...... 114

4.3.3 Electrical characteristics of the dome-shaped c-Si cells ...... 114

4.3.4 Kirigami design c-Si solar cells ...... 119

4.4 Conclusion ...... 121

5 CONCLUSIONS AND FUTURE DIRECTION ...... 122

References ...... 123

PUBLICATIONS ...... 136

LIST OF ABBREVIATIONS

AFM Atomic Force Microscopy

ARC Anti-reflective coating

BSF Back surface field

CIGS Copper indium gallium selenide

CMOS Complementary Metal Oxide Semiconductor

C-Si Crystalline Silicon

EDS Energy-dispersive X-ray spectroscopy

FF Fill factor

HIT Heterojunction

HRTEM High-resolution transmission electron microscopy

IBC Interdigitated back contact

I/I Ion Implantation

IPV integrated photovoltaics

Jsc Short circuit current

PV Photovoltaic 13

Ra Roughness average

Rs Sheet resistance and series resistance

Rsh shunt resistance

Rms, Roughness root mean squared

RPM Revolutions Per Minute

SEM Scanning Electron Microscopy

SIMS Secondary Ion Mass Spectrometry

Sq Roughness Root Mean Square Gradient

STC Standard Test Condition

TEM Transmission Electron Microscopy

UHV Ultra-high vacuum

Voc Open circuit voltage

 mean-free recombination time

LIST OF SYMBOLS

d diameter

Eg bandgap

ɛ dielectric constant

ħ Dirac’s constant e elementary charge

τ carrier lifetime

λ wavelength

Rc critical radius

Voc open-circuit voltage

Isc short-circuit current t time

VMPP maximum power point voltage

IMPP maximum power point current

ɳ power conversion efficiency

LIST OF ILLUSTRATION

Figure 1.1 The gradual transition of the primary energy sources over the next 20 years [2].

...... 25

Figure 1.2 summary of the correlation between the effect of process temperature and substrates on the efficiency of several common thin film solar cells (TFSCs) [22] ...... 27

Figure 1.3 Electrical parameters of C-Si PV for different Si thicknesses [25] ...... 28

Figure 1.4 Timeline of solar cells generations [27] ...... 29

Figure 1.7: Si-based solar cell structure ...... 31

Figure 1.8 The current-voltage (black) and power-voltage (grey) characteristic of an ideal solar cell ...... 34

Figure 2.1 Schematic illustration of crystalline silicon solar cells with (a) traditional screen- printed Ag and (b) NiSi/Cu rear contacts. Scanning electron microscope (SEM) cross- sectional images of (c) the screen printed Ag and (d) NiSi/Cu contacts on the textured silicon rear surface. (e) Sheet resistance of (NixSiy) crystalline silicon (100) as a function of

Rapid Thermal Annealing (RTA) temperatures. (f) Glancing incidence x-ray diffraction

(GIXRD) of NiSi and Ni (as-deposited). (g) Transmission electron microscopy (TEM) cross- sectional image of NiSi on silicon with thickness of ~ 44 nm ...... 44

Figure 2.2 HADDF-STEM image of NiSi layer (b) Energy-dispersive X-ray (EDX) of NiSi in terms of weight percentages...... 46

Figure 2.3 Cross-sectional TEM image Si/NiSi/Cu stack ...... 47

Figure 2.4 Mass spectrum of the Ag etched region ...... 49 16

Figure 2.5 SIMS depth profiling of NiSi/Cu on the rear side of the silicon ...... 50

Figure 2.6 SEM image of NiSi/Cu rear contacts on the textured silicon...... 51

Figure 2.7 Output current density vs. voltage of NiSi/Cu (red circles) and screen printed

Ag (black squares) under (a) one-sun illumination showing both of current and power density, ...... 52

Figure 2.8 Output current density vs. voltage of NiSi/Cu (red circles) and screen printed

Ag (black squares) under one-sun and shifted dark measurements ...... 53

Figure 2.9 Output current density vs. voltage of NiSi/Cu (red circles) and screen printed

Ag (black squares) under different illumination levels: 1 and 0.7 illumination intensities

...... 54

Figure 2.10 Comparison of JV curves of the dark current: (a) experimental measurements

(inset is equivalent circuit including series (Rs) and shunt (Rsh) resistance). (b) simulated data as a function contact resistance ...... 56

Figure 2.11 JV-curves of one sun and Jsc shifted dark measurements. (a) NiSi/Cu rear contact (b) screen printed Ag...... 58

Figure 2.12 JV-curves of one sun and Jsc shifted dark measurements. (a) NiSi/Cu rear contact (b) screen printed Ag...... 60

Figure 2.13 Process flow (a) creating PN diode on Si(100) (b) lithography steps for NiSi formation.(c) Passivation layer using Al2O3 (d) Interdigitated metallization on top of the silicidation ...... 63

Figure 2.14 Electrical performance of fabricated interdigital PN junction with ohmic contact using NiSi/Cu stack vs Schottky using Cr/Au measured on 2 cm2 samples area . 64 17

Figure 2.15 Releasing process using lift-off method (a) transferring micro holes using lithography to etch the Si using DRIE (b) the top portion of the Si sheet is released with the solar cell on top by XeF2 etching. (c) released thin flexible PV(d) cross-sectional SEM for the released PV. (e) Top view of the micro-holes’ pattern. (F) optical image of the flexible PV ...... 67

Figure 2.16 surface roughness measurements on mechanically polished samples (a,b)

SEM images of course grinding and soft polishing. (c,d) AFM roughness images of course grinding and soft polishing.(e,f) Optical profiler of course grinding and soft polishing ... 69

Figure 2.17 Electrical performance of NiSi/Cu contacts on the soft polished samples ... 70

Figure 2.18 Electrical characteristics of the NiSi/Cu contacts on the soft polished samples

...... 71

Figure 3.1 (a) Scheme of: the ultra-flexible corrugated c-Si solar cell (b) The interdigitated back contact (IBC) c-Si solar cell structure ...... 76

Figure 3.2 optical Image of the corrugated architecture c-Si solar cell (5” wafer): consists of 16 parallel grooves of linear channels (~0.86 mm wide) and 18 remaining active segments of the c-Si solar cells (~ 6.24 mm wide) except the solar cells covering the common electrode on both edges which has a width of 8.81 mm...... 77

Figure 3.3 Ultra-flexible c-Si solar cells on a five-inch wafer scale. Optical images of 5” wafer of c-Si IBC solar cell illustrates: a) high bending flexibility, b) Zigzag module, c) folded wafer in half to obtain bifacial demonstration and d) The zig-zag side view and the internal angle (). Scanning electron microscopy images: e) Cross-sectional shows zig-zag deformation with = 71.4° and f) top view reveals the deformation of the interdigitated 18 contacts. f) Optical image illustrates the view of the corrugated architecture consists of

~6.2 mm active solar cell segments and 0.8 mm grooves width. SEM images of: g) The minimum bending radii of curvature (R=<140 um) during folding h,i) Top view of the folded Aluminum contact and the Silicon segments...... 79

Figure 3.4 Scheme of the progression flow of the deep reactive ion etching (DRIE) and corrugation technique process ...... 80

Figure 3.5 a) Optical image of the 5” wafer post the DRIE process and during peeling off the Kapton tape. b) Optical microscope image of the interdigitated back contact (IBC) after the DRIE. C) Optical profiler measured on the IBC for surface roughness analysis . 81

Figure 3.6 a) DRIE etching of the exposed silicon area vs the thickness reduction (each groove area=1270.86 mm2). b) and c) Cross-sectional scanning electron microscopy

(SEM) images taken after 250 and 450 etching cycles, respectively ...... 82

Figure 3.7 Secondary Ion Mass Spectrometry (SIMS) spectra of the IBC post the DRIE in both positive and negative modes...... 83

Figure 3.8 Electrical performance of the flexible c-Si solar cells compared to the rigid ones. a) J-V performance of the flexible cells enabled via the corrugation technique compared to the rigid devices under solar simulator illumination (1 sun) at the room temperature. b) Power density (mW/cm2) compression ...... 84

Figure 3.9 Electrical characteristics comparisons of: open circuit voltage (Voc), Efficiency

(), short circuit current (Jsc) and FF...... 85

Figure 3.10 Weight and total power (mW) measured of devices of an area= 127×15 mm2.

Note that all the J-V characteristics measured when the devices are flat ...... 86 19

Figure 3.11 Integration of the corrugated c-Si solar cells. Five cells are connected in series and installed on convex glass curvature. Optical images of: a) Output voltage using two bulb lights. b) KAUST logo display using LEDs matrix powered by the flexible solar cells via a desk light. c) Flexible cells are installed on a glass mug used to supply power to a fully functional system placed on a plant leaf and subjected to a humid condition. d) Circuit diagram. e) Humidity sensor response. f) Red LED on a flexible board with notification.

...... 87

Figure 3.12 Extreme Mechanical Flexibility: 3D strain modeling of the IBC (width=1 mm and 25 m thickness) located between silicon solar cells with thickness of 250 m during bending with different angles (). a) The maximum and minimum strain of IBC as a function of the bending angle. b) and c) Strain compression of =0 and 180, respectively

...... 90

Figure 3.13 a) J-V characteristics of the devices at flat position and using convex bending

(R= 9 cm) under 1 sun illumination compared to the measurements under the sun (noon) measured at (Google Map coordinates: 22.316521, 39.095746). b) Electrical characteristics of the devices without any consideration of the projection area. c) J-V characteristics of the devices using convex bending (R= 6, 5 and 3 cm)...... 91

Figure 3.14 a) Scheme of projected area key components. b) Optical photo of the flexible cells installed on concave and convex glass curvatures (Rbending= 6 cm). c) Projected area as a function of the bending radii. The scatter points are calculated with Equations (3) and

(4)...... 93

Figure 3.15. Electrical characteristics of the devices using concave and convex bending 20 compared to the flat devices...... 94

Figure 3.16 a) Electrical characteristics of the devices using concave and convex bending compared to the flat devices without considering the variation of the projection area during the bending process. b) Scheme of zigzag module c) optical image of the devices deformed in zigzag shape. d) Electrical characteristics of the zigzag-shaped devices as a function of the interior angle of the shape without considering the variation of the projection area...... 95

Figure 3.17 (a) Optical image of the 3D printed cyclic bending apparatus in both cases: pre-bent (top) and after bending (bottom). b) Electrical characteristics of the devices as a function of the bending cycles...... 96

Figure 3.18 Comparison with literature data. Both of the efficiency and the area are taken from the published electrical characteristics of varies Si-based solar cells...... 97

Figure 3.19 comparison between our flexing approach and the commercialized method for Si based solar cells (a) schematics illustration (b) optical photos...... 100

Figure 3.20 The individual cells (15×127 mm2) for rigid (top) and its flexible version

(bottom) ...... 105

Figure 4.1 The coupling efficiency c Vs the source angle...... 108

Figure 4.2 four-way grid dome design ...... 109

Figure 4.3 four-way grid dome mask ...... 111

Figure 4.4 laser engraving profiler for different laser power and speed values...... 112

Figure 4.5 Optical photos of the laser engraving method to transfer the four-way grid dome on IBC solar cells...... 113 21

Figure 4.6 Optical photo of the dome-shaped c-Si solar cell...... 114

Figure 4.7 Electrical characteristics of the solar cells distributed on a 3D printed dome- shaped substrate...... 115

Figure 4.8 Current and voltage values of the cells distributed on a dome-shaped sphere compared to the same cells as a flat plate...... 116

Figure 4.9 The effect of dome-shaped geometry on the power generation compared with the fixed planar cells ...... 117

Figure 4.10 polyamide sheet 0.25 m after cutting using Co2(a) flat (b) stretched ...... 120

Figure 4.11 flexible c-Si solar cells mounted on polyamide sheet...... 120

LIST OF TABLES

Table 2-1: Elemental quantification of P2 and P3 as shown in (Error! Reference source not found.) ...... 47

Table 2-2 Output parameters of (c-Si) solar cells with screen-printed Ag vs. NiSi/Cu rear contact of 1 cm2 as measured under AM1.5 spectrum (100 mW/cm2) at 25 °C, means ±

SD...... 57

Table 2-3 Variation of electrical performance parameters at the maximum power mpp value taken for series resistance determination (RS;light_dark) for dark and 1 sun intensity illumination ...... 59

Table 2-4 Variation of electrical performance parameters measured with respect to short circuit current density JSC (mA/cm2) taken for determination of the average series resistance (Rs;int:var) (Ω.cm2) value for two different light intensities (1 and 0.7 sun) . 61

Table 2-5 The simulation parameters used in investigating the contact resistance impact on the JV-behavior...... 62

Table 2-6 Electrical characteristics of fabricated bulk interdigital PN junction ...... 65

Table 2-7 Grinding and polishing steps ...... 68

Table 2-8 Surface roughness measurements taken for different etching steps ...... 69

Table 2-9 Electrical characteristics of the NiSi/Cu contacts on the soft polished samples

...... 71

• Table 3-1 Comparison between our flexing approach (left) and the commercialized method (right)...... 99 23

Table 3-2 Flexible industrial solar panel characteristics vs our ultra-flexible solar cell. 101

Table 4-1 Electrical characteristics of the dome-shaped c-Si cells vs the fixed planar cells

...... 118

1 Introduction

1.1 Global energy outlook

The global energy landscape is changing due to the world productivity growth, environmental concerns, and the technological improvements. The world’s population is projected to increase by around 1.5 billion people to reach nearly 8.8 billion people by

2035. Therefore, global demand for energy is expected to double by 2050 [1, 2].

Currently, the world economy is overwhelmingly dependent on fossil fuels such as oil, natural gas, and coal. Fossil fuels are the leading greenhouse gas emitters in the world, contributing more than 75% of all carbon, methane and other greenhouse gas emissions

[3]. Burning coal at high temperature to generate the electrical energy leads to large concentrations of pollutants in both air and water. For example, air pollution is the reason behind the death of 18,000 people each day. Moreover, the number of deaths attributed to air pollution each year is 6.5 million deaths according to the World Health Organization

(WHO) which is much greater than the number of HIV/AIDS, tuberculosis and road injuries combined [4]. Moreover, The human-enhanced greenhouse effect trapped more than

25% of the sun’s radiation inside the atmosphere. The additional heat warms the atmosphere and the plant’s surface above the normal temperature causing the global warming [5].

Renewable energy sources such as photovoltaics (PV), wind, geothermal, hydropower are the fastest growing fuel source and expected to quadruple over the next

20 years as shown in Figure 1.1. On the other hand, fossil fuels remain the dominant 25 source of global energy supplies (77%) in 2035. Therefore, our knowledge of the longer- term energy transition that is taking place to ensure we can continue to meet the energy needs of a changing world [4, 6, 7].

Figure 1.1 The gradual transition of the primary energy sources over the next 20 years

[2].

• The following content is prepared as a submitted manuscript:

A. T. Kutbee, R. R. Bahabry, S. Shaikh, S. Khan, G. A. Torres Sevilla, M. M. Hussain,

"Energy and power management of IOT", Energy Technology (Wiley VCH), (Invited),

Manuscript prepared

1.2 Energy harvesters for IOT

In this new era of the Internet of Everything (IoE), physical and virtual things are interconnected into data network to smarties our everyday products and enhance the quality of our daily lives [8, 9]. One of the most essential energy-related technologies in the IoE is the integration of sustainable energy harvesters and storage devices with the major power grids [10]. Among the energy harvesters, flexible solar cell technology has paved the way for wide application spectrum ranging from wearable and implantable electronics, robotics and infrastructure/vehicle- integrated solar panels on curved surfaces to space applications [10, 11]. This is mainly triggered by the need of self- powered technologies which abandons cords and benefit from wireless technologies for the sheer feasibility and convenience of the wearer. [12-17] Recent research efforts have investigated the development of inorganic and organic flexible thin-film solar cells (TFSCs) through materials innovation and device engineering. Silicon-based solar cells are one of the most investigated materials in photovoltaics technology due to its natural abundance, non-toxicity, excellent reliability, mature manufacturing, and high performance.[18]

Flexible thin film Si-based solar cells have shown their advantage in applications in which weight is a critical factor. However, thin film Si-based solar cells seem to be disadvantageous over thick Si solar cells due to the reduction of the optical absorption with less active Si material. Nevertheless, these technologies can provide higher power conversion efficiencies as compared to polycrystalline compounds or organic solar cells, due to high material quality.[19, 20] 27

In this regard, two main approaches are explored extensively for producing TFSCs.

The first approach includes direct deposition of the TFSCs on a flexible substrates, such as polymers, fabrics, and paper.[11, 21] Such method is restricted by the maximum temperature of the substrate materials, impacting the solar cells performance. [22] The second approach is the transfer printing technique which utilizes the conventional rigid substrates for the fabrication and then transfers the TFSCs onto the resilient substrate.

[23, 24] Thus, the second method overcomes the performance limitation associated with the substrate thermal incompatibility with the fabrication of high-efficiency TFSCs.

However, it suffers from both of low throughput and high cost. Thus, a tradeoff between mechanical compliance, performance, and robustness of flexible photovoltaic cells remains a major challenge as shown in Figure 1.2, especially in high volume production for large surface coverage applications.

Figure 1.2 summary of the correlation between the effect of process temperature and 28

substrates on the efficiency of several common thin film solar cells (TFSCs) [22]

On the other hand, the electrical properties of thin C-Si P are poor compared to those achieved with bulk ones as shown in Figure 1.3.

Figure 1.3 Electrical parameters of C-Si PV for different Si thicknesses [25]

1.3 Classification of Solar Cells

Solar cells can be classified into four generations, the first-generation cells (G1) also called conventional or wafer-based cells which made of Silicon (Si). Si-based Photovoltaics

(PV) is the commercially powerful technology that includes materials such as polysilicon and monocrystalline silicon (C-Si) and known as high cost/high efficiency.

Second-generation cells (2G) are thin film solar cells that include amorphous 29 silicon, Cadmium Telluride (CdTe) and Copper indium gallium selenide (CIGS) which classified to be the low cost/ low efficiency. The third-generation (3G) of solar cells includes some thin-film technologies often described as emerging photovoltaics or called low cost/high efficiency [26, 27]. Finally, the fourth generation of the solar cells technology (4G) combines the low price with the flexibility of organic/inorganic thin films

[28, 29]. Among the various kinds of solar cell technologies, C-Si solar cells hold the prominent market share, due to the advantages of material abundance, wide spectral absorption range [30], high carrier motilities, and its mature and reliable manufacturing base CMOS technology. However, there is still room for further improvement in the context of affordability of crystalline Si PV while increasing the efficiency in order to sustain its competitive edge in the future [3, 31, 32].

Figure 1.4 Timeline of solar cells generations [27] 30

1.4 Solar cells Physics

Solar cells, or photovoltaics, are solid state devices that convert the energy of light directly into electrical energy. Light in the form of photon energy can excite electrons into higher energy states within the material, allowing them to act as charge carriers for an electric current. Photon energy depends on the color or wavelength of the light. This is the photoelectric effect explained by Einstein in 1905. Under normal conditions, the excited electrons will lose their energy quickly and relax back to their ground state. In a solar cell device, there is some built-in asymmetry which pulls the excited electrons away before they can relax, and feeds them to the external load [33, 34].

1.5 Structure and Principle

Solar cells are mostly made up of two opposite doped semiconductors – a planar p-n junction. The basic structure of a solar cell device includes a p-type semiconductor absorber layer, an n-type semiconductor emitter layer, front contact antireflective coating layer, and front and back metal contacts as shown in [35]. 31

Figure 1.5: Si-based solar cell structure

When placing a thick layer of p-type absorber layer (positive charges as majority carriers) together with a thin layer of n-type emitter layer (negative charges as majority carriers), the negative charges – electrons from the n-doped emitter - will diffuse to the p-doped absorber layer, and positive charges – holes from p-doped absorber layer - will diffuse to the n-doped emitter layer. As these majority charges diffuse and recombine with the others, they leave behind fixed charge sites. While these fixed charge sites build up, an internal electric field is created – with the direction from n emitter layer to p absorber layer. This internal built-in electric field has the opposite direction to the flow of these above majority charge carriers. When this built-in electric field is equal to the energy band gap of the semiconductor, thermal equilibrium is established. At thermal equilibrium, there is no net current within the device. The space charge region is also called the 32 depletion region. This space charge region has almost no free charge carriers, and it expands more into the less doped side. The width of this depletion layer depends on the doping level.

When a non-resistant wire is connected between the n emitter layer and the p absorber layer, the generated current that flows through the wire is called the short circuit current, Isc. For an ideal solar cell, the short-circuit current, and the light-generated current, Io are identical. Therefore, the short-circuit current is the largest current which may be drawn from the solar cell. For an ideal solar cell which has perfectly passivated surface and uniform generation, the equation for the short-circuit current can be approximated as:

Isc = qG(Ln + Lp)

where G is the generation rate, and Ln and Lp are the electron and hole diffusion lengths respectively. The short-circuit current depends strongly on the generation rate and collection of light-generated carriers or their diffusion lengths. [26]

When there is no wire connected, the electrons which got swept from the p side to the n side will have nowhere to go. They build up an electric field that forward biases the built-in electric field. This forward bias will be the force to drive the photogenerated current through the resistive load when one is connected. When the forward bias current is equal to the photogenerated current, the forward bias voltage is called the open circuit voltage, Voc. The maximum Voc is equal to the built-in voltage. Open circuit voltage is a measure of the amount of recombination in the device. 33

푛푘푇 퐼퐿 푉표푐 = ln ( + 1) 푞 퐼0

Solar cells’ current is the difference between the photogenerated current and the forward bias current.

푞푉 퐼 = 퐼 [푒푥푝 ( − 1)] − 퐼 0 푛푘푇 퐿

where IL= light generated current.

Current-Voltage Curve

The efficiency of a solar cell, η, is calculated as the maximum generated power,

Pmax divided by the incident power, Pin.

푃푚푎푥 휂 = 푃𝑖푛

The maximum generated power, Pmax, is defined as the product of open circuit voltage, short circuit current and fill factor, FF.

푃푚푎푥 = 푉표푐퐼푠푐퐹퐹

The fill factor, FF, is the area of the most significant rectangle which is fit in the IV curve of a solar cell. FF is also defined to be the ratio between maximum operation points between short circuit current and open circuit voltage.

푉푀푃퐼푀푃 퐹퐹 = 푉푂퐶퐼푆퐶

푉푂퐶퐼푆퐶퐹퐹 휂 = 푃𝑖푛

These four quantities: Jsc, Voc, FF, and η are the key performance characteristics of a solar cell. The Standard Test Condition (STC) for solar cells is the Air Mass 1.5 spectrum, 34 an incident power density of 1000 Wm-2, and a temperature of 25oC [36].

Figure 1.6 The current-voltage (black) and power-voltage (grey) characteristic of an ideal

solar cell

1.6 Current Technology Challenges

The research on increasing the efficiency has been covering three main areas for C-

Si PV. The first area is exploring the surface recombination loss in order to enhance both short circuit current and open circuit voltage of the working cell [37]. Therefore, a better front surface passivation layer or more effective back surface field has been considered in order to reduce the surface recombination losses on both sides of the working cell.

The second area is to minimize the optical loss to enhance the light absorption of the working cell. Also, optimizing the contact pattern has been studied using better surface 35 texturing or minimizing the cost of the antireflective coating layer (ARC) to decrease the shading loss [36]. The third area is to reduce the resistive loss of the working cell to increase output current [36, 38]. This area has been attracting high interest from the industry lately [39-46]. The photovoltaic industry has been satisfied with the current low- cost, and simple screen printed front metallization process. On the other hand, in order to increase the overall cell efficiency and reduce the contact resistance with low-cost, investigating more effective contact is needed to compensate the resistive loss from using larger wafer side [47]. Overall, contact metallization contributes to all three areas.

As highlighted before, a low-cost metallization with lower resistance is desired in order to replace the current screen-printed Ag paste contact for the future crystalline Si-based solar cells.

1.7 Dissertation Outline and chapters overview

This doctoral thesis investigates novel approaches to enhance the efficiency for c-

Si-based solar cells in different key areas for the next generation of thinner and larger Si wafers. First, chapter one gives a brief introduction to the global energy outlook, classification of the solar cells, the physics of c-Si solar cells and the current technology challenges.

In the second chapter 2, the contact engineering in c-Si solar cells is the investigated area. The industrial Si-based cells utilize screen printed silver (Ag) as the primary metallization technology due to its remarkable current collecting properties with the relative simplicity of the procedure. On the other hand, screen printed Ag exhibits high 36 contact resistance. The Metal silicides based on ohmic contact formation, have proven to be great contact materials in complementary metal oxide semiconductor (CMOS) technology because of their low specific resistivity, minimal junction penetration, and high thermal stability. In this chapter, we investigated Nickel mono-silicide NiSi/Cu ohmic contact on the rear side for the first time, especially that 49% of the recombination losses are taking place there at the maximum power point. We have observed experimental improvement of both of the Fill factor and efficiency by 6.5% and 1.2% respectively for the NiSi/Cu compared to Ag paste. This is attributed to the average reduction of the Rs ~

0.737 Ω.cm2 which supported by performing simulations to understand the effect of the contact resistance on the dark electrical performance of the cells.

Moreover, the impact of Interdigitated back contacts NiSi/Cu is explored compared to Cr/Ag. Finally, is interdigitated back contact for flexible mono-crystalline silicon solar cell. We’re reporting the efficiency for different released thickness using a novel flexible-

Si process, which control the thickness to reduce the required diffusion path for the generated minority carriers to reach the P-N junction. Additionally, nickel silicidation has been formed before the metal contact deposition

In the third chapter, we developed an innovative approach to transforming the rigid industrial c-Si solar cells into an ultra-flexible one. We show a complementary metal oxide semiconductor (CMOS) based integration strategy where corrugation architecture enables an ultra-flexible and low-cost solar cell module from bulk mono-crystalline silicon solar wafer with 17% power conversion efficiency. This periodic corrugated array benefits from an interchangeable solar cell segmentation scheme which preserves the active 37 silicon thickness of 240 m and achieves flexibility via interdigitated back contacts. These cells can reversibly withstand high mechanical stress as the screen-printed metals have fracture strain >15%. These cells can reversibly withstand high mechanical stress and can be deformed to zigzag and bifacial modules. Theses corrugation silicon-based solar cells offer ultra-flexibility with high stability over 1000 bending cycles including convex and concave bending to broaden the application spectrum with a bending radius of curvature lower than 140 m. Finally, the integration of the flexible solar cells is demonstrated on a curved surface for supplying power both flexible 88 LED Matrix and paper-based fully functional system using indoor illumination.

Chapter four continues where chapter three leaves off; we employ our novel flexing approach to fabricate three-dimensional dome-shaped c-Si cells to reduce the optical coupling losses. Conventional planar solar modules have a limited ability to capture the maximum potential of the solar energy due to the misalignment angle between the modules and the sun. Therefore, this chapter is exploring some promising 3D modules without the use of the expensive solar tracking/tilting systems. Furthermore, the laser engraving technique is

At the last chapter, the main findings and achievements of this dissertation are summarized with directions to future work in evolving this workhouse of the solar cell technology.

2 Low-Cost Nickel Silicide Rear Ohmic Contact for c-Si solar cells

The silicon-based solar cell is one of the most important enablers toward high efficiency and low-cost clean energy resource. Metallization of silicon-based solar cells typically utilizes fairly expensive screen printed silver (Ag) which degrades the optimal electrical performance. To date, metal silicide-based ohmic contacts are occasionally used as an alternative candidate only to the front contact grid lines in crystalline silicon

(c-Si) based solar cells. In this chapter, we investigate the microstructure and the electrical characteristics of nickel mono-silicide (NiSi) ohmic contact on the rear side of c-Si solar cells. We observe a significant enhancement in the fill factor of around 6.5% for NiSi/Cu rear contacts leading to increasing the efficiency by 1.2% compared to silver.

This is attributed to the improvement of the parasitic resistance in which the series resistance decreased by 0.737 Ω.cm2. Further, we complement experimental observation with a simulation of different contact resistance values, which manifests

NiSi/Cu rear contact as a promising low-cost metallization for c-Si solar cells with enhanced efficiency.

This chapter was published as:

1 R. R. Bahabry, A. Gumus, A. T. Kutbee, N. Wehbe, S. M. Ahmed, M. T.

Ghoneim, K.–T. Lee, J. A. Rogers, M. M. Hussain, “Current Enhancement in

Crystalline Silicon Photovoltaic by Low-Cost Nickel Silicide Back Contact”, 43RD

IEEE PHOTOVOLTAIC SPECIALISTS CONFERENCE (IEEE PVSC 2016), Portland,

OR, USA 5 – 10 June, 2016 39

2 R. R. Bahabry, A. Hanna . T. Kutbee, N. Wehbe, M. M. Hussain, “Enhanced

efficiecny of c-Si solar cells using NiSi/Cu rear contacts” submitted to Energy

Technology (Wiley VCH).

3 R. R. Bahabry, A. Gumus, A. T. Kutbee, N. Wehbe, S. M. Ahmed, M. T.

Ghoneim, K.–T. Lee, J. A. Rogers, M. M. Hussain, “Novel Contact Engineering

for Low-Cost and Competitive Efficiency Crystalline Silicon Photovoltaics using

Nickel Mono-Silicide Metallization” KAUST-NSF 2016, Thwall, SA

4 R. R. Bahabry, A. Gumus, A. T. Kutbee, N. Wehb, S. M. Ahmed, M. T.

Ghoneim, K.–T. Lee, J. A. Rogers, M. M. Hussain, “Toward Efficient and Low-

Cost Metallization for Crystalline Silicon Photovoltaics”, 2016 C3E Women in

Clean Energy Symposium, Stanford University in Palo Alto, California, on May

31, 2016

5 R. R, Bahabry, M. M. Hussain, “Mono-crystalline Bulk Silicon Based High-

Efficiency Flexible Solar Cell”, TMS Middle East - Mediterranean Materials

Congress on Energy and Infrastructure Systems (MEMA 2015), January 11-14,

2015 • Doha, Qatar

6 Rabab Bahabry, Jhonathan Rojas, Aftab Hussain, Muhammad Mustafa

Hussain, “Novel interdigitated back contact for flexible mono-crystalline

silicon solar cells”, MRS Fall Meeting 2014, Boston, MA, USA

7 Rabab R. Bahabry, Jhonathan P. Rojas, Aftab Hussain, Muhammad M.

Hussain, “Silicidation In Flexible Si-Based Photovoltaics For Enhanced PV

Efficiency, KAUST-NSF 2015, Thwall, SA 40

8 M. M. Hussain, R. R. Bahabry, J. P. Rojas, D. P. Singh, A. C. Sepulveda, I.

Wicaksono, A. T. Kutbee, “Freeform Thin Film Energy Harvesters”, Thin Films

2016, Singapore, Singapore, 12 – 15 July 2016. [Session on Functional Thin

Films for Energy Photonics.

2.1 Introduction

Crystalline silicon (c-Si) solar cell has proven reliability and efficiency, holding the dominant market share compared to the varied Photovoltaics (PV) technology market due to its advantages like non-toxicity, abundance, and stability. Silicon has an energy band gap of 1.12 eV, corresponding to a broad spectral absorption range with a cut-off wavelength of about 1160 nm. Thus, silicon has a very close optimum solar-to-electric energy conversion using single semiconductor optical absorber[32, 48]. Yet further performance per cost enhancement signifies a great effort to fulfill the global demand for renewable energy. Improving the efficiency of c-Si solar cells, while reducing the total cost is thus critically important field [39, 40, 43-46, 49, 50]. In that regard, one key area is contact engineering in c-Si solar cells, which utilize screen printed silver (Ag) as the primary metallization technology due to its remarkable current collecting properties with the relative simplicity of the procedure. On the other hand, screen printed Ag exhibits high contact resistance since the current must flow through Ag crystallites formed during firing, then tunnel through an insulating glass layer,[51] organic binders and solvents to reach the metal finger bulk, resulting in metallization- induced recombination losses.

Furthermore, Ag paste costs nearly 40% of the total cell price because of the material cost and thickness. [52] In 2010 alone, 7% of the of the total annual world Ag supply was dedicated to the silicon-based solar cells industry, indicating a necessity to substitute the

Ag with low-cost and better metallization properties. [53]

Recent advances in silicon solar cells research are heightening an evolutionary 42 approach to increase the cell efficiency of the industry via exploring silicidation on the front contacts. [54, 55] Metal silicides based on ohmic contact formation principle, have proven to be great contact materials in complementary metal oxide semiconductor

(CMOS) technology because of their low specific resistivity, minimal junction penetration, and high thermal stability. [56, 57] Additionally, metal silicides have demonstrated great promise in energy harvesting devices.[58] In particular, Nickel silicide has been used for high-efficiency c-Si PV on the front side current emitter through contact line metallization.

[59, 60] Plated Ni/Cu contacts have also been used to achieve 18.1% on large area multi- crystalline substrates using a single-sided buried contact design.[42, 45, 52, 61]

In this work, we investigated Nickel mono-silicide NiSi/Cu ohmic contact on the rear side, especially that 49% of the recombination losses are taking place there at the maximum power point. [62] Our approach is unique in nature on the back side to discover its impact on enhancing the solar cell’s efficiency by increasing the Fill Factor (FF) which is defined by:

푃푚 퐹퐹 = (1) 푉표푐퐼푠푐

Where 푃푚 is the maximum output power, 푉표푐 is the open circuit voltage and 퐼푠푐 is the short circuit current. The power of the solar cells is dissipated through the resistance of the contacts and through a leakage current around the sides of the device as can be shown below:

푅푠ℎ 푃 = 퐼2 (푅푠 + ) (2) 푚 푚푝 푋 43

퐼0푅푠ℎ 푉푚푝 + 퐼푚푝푅푠 Where 푋 = 푒푥푝 ( ) + 1 (3) 푛푉푡ℎ 푛푉푡ℎ

푅푠 is the series resistance, 푅푠ℎ is the shunt resistance, 퐼0 is reverse saturation current of the diode, n is the diode quality factor, 푉푚푝 and 퐼푚푝 are the voltage and current at the maximum power point, respectively.[63] We assume that using the nickel silicide ohmic contact on the rear side will reduce the FF due to its influence on the 푅푠 as can be seen from equations (1, 2 and 3), especially that nickel silicide forms ohmic contact with p-type silicon24 with a barrier height of 0.5 eV. Also, NiSi works as an appropriate barrier to prevent copper diffusion in silicon.[64]

This investigation includes studying the electrical and microstructure characterization of NiSi on the rear side of silicon solar cells compared to the conventional screen printed Ag as the schematics illustrate in. We have used textured C-Si solar cells.

Figure 2.1 (a,b) depicts all the components of a standard dopant-diffused silicon homo- junction solar cell where electrons and holes are generated in the p-type silicon, then extracted via phosphorus-doped (front, yellow) and boron-doped (back, gray) regions with an n-type emitter layer and screen printed Ag grids/bus bar as the front and rear contacts besides the aluminum back surface field (BSF) (Ag/green). Cross-sectional scanning electron microscope (SEM) image was obtained to measure the total thickness of the Ag paste using FEI QuantaTM 3D as in Fig. 2.1 (c), indicating an average thickness of Ag paste of ~10 m. 44

Figure 2.1 Schematic illustration of crystalline silicon solar cells with (a) traditional

screen-printed Ag and (b) NiSi/Cu rear contacts. Scanning electron microscope (SEM)

cross-sectional images of (c) the screen printed Ag and (d) NiSi/Cu contacts on the textured silicon rear surface. (e) Sheet resistance of (NixSiy) crystalline silicon (100) as a

function of Rapid Thermal Annealing (RTA) temperatures. (f) Glancing incidence x-ray

diffraction (GIXRD) of NiSi and Ni (as-deposited). (g) Transmission electron microscopy

(TEM) cross-sectional image of NiSi on silicon with thickness of ~ 44 nm

Fabrication of NiSi/Cu rear contact was accomplished in three stages: (1) formation of optimized NiSi, (2) removal of screen-printed Ag, and (3) formation of NiSi/Cu contacts on the rear side. In the first stage, two different thicknesses (20 nm and 50 nm) of nickel were deposited to achieve the minimum resistance using electron-beam deposition (0.5

Å/s deposition rate) on a lightly doped, p-type c-Si (100) substrates with a sheet resistance

(ρsh) of 330 Ω/sq. Next, the samples were annealed in an inert Argon (Ar) atmosphere at 45 different temperatures ranging from 300 °C to 750 °C using rapid thermal annealing (RTP) for 60 seconds. Finally, removal of unreacted Ni was done using piranha (H2SO4:H2O2) at

120 °C for 120 seconds. Fig. 1(e) plot shows the sheet resistance as a function of the annealing temperature. For temperatures below 350 °C, Nickel disilicide (Ni2Si) was formed first until all Ni was reacted. This formation was followed by Nickel mono-silicide

(NiSi) phase transformation at temperatures between 400–600 °C.

2.2 Experimental

2.2.1 Nickel-mono silicide (NiSi) optimization:

Characterization is carried out on the samples which achieves the minimum Rsh value of 2.8 Ω/sq by depositing 50 nm of Ni and annealed at 450 °C. We performed near glancing incidence x-ray diffraction (GIXRD) by fixing the angle of incidence of the X-ray generator with reference to the plane of sample at 3° while moving the detector with respect to the sample at a 2-theta angle. Figure 2.1(f) confirms the nickel mono-silicide formation of the optimized formation recipe. Also, Transmission electron microscopy

(TEM) samples are prepared via dual-beam using Pt/C deposition for sample protection.

Then TEM images were obtained using an FEI Titan ST electron microscope operated at

300 kV used to study the microstructure of NiSi phase. Figure 1.1(g) shows the cross- sectional TEM image of the resulting structure of NiSi with a thickness of ~ 44 nm. Energy- dispersive X-ray (EDX) spectroscopy in Figure 2.2 shows the elemental composition of silicon and nickel which have an approximately 49.24% and 50.76% atomic percentages, 46 respectively, indicating that around 22 nm of silicon is consumed for NiSi structure as shown in Figure 2.2 (b) for details for the EDX study of NiSi layer.

Figure 2.2 HADDF-STEM image of NiSi layer (b) Energy-dispersive X-ray (EDX) of NiSi in

terms of weight percentages.

In addition, TEM images show the desired stack of (Si/NiSi/Cu). However, it can be seen that thin SiO2 layer was formed between NiSi/Cu layer. This phenomenon has been reported where Si diffuses through the silicide layer and reacts with O2 forming SiO2 [57].

Therefore, wet oxide etching step was devolved after the silicide formation step. Figure

2.3 and Table 2-1 show the cross-sectional TEM and HRTEM of the stack. 47

Figure 2.3 Cross-sectional TEM image Si/NiSi/Cu stack

Table 2-1: Elemental quantification of P2 and P3 as shown in (Error! Reference source

not found.)

point Element Line Weight Atom%

Si Ka 0.32 49.24 P2 Ni Ka 0.68 50.76

Si Ka 0.56 69.39 P3 O Ka 0.44 30.61

2.2.2 Screen printed Ag removal

The second stage was etching the screen-printed Ag from the back side. This step started with protecting the emitter side as well as the Al (BSF) back side using 10 m of

Photoresist (PR-AZ 9260) opening only the rear Ag bus bar windows using photolithography (Exposure: 1800 mJ/cm2, 4 min). A subsequent wet-chemical etching of the exposed Ag paste was performed by immersing 30 × 50 mm cleaved pieces in an etching solution. The solution contains mixtures of 1-5 % HNO3 (for Ag/Al oxidation), 65-

75 % H3PO4 (for oxide dissolution) and 5-10 % CH3COOH (for wetting and buffering) in H2O diluent. The etching was performed at 40 °C for three hours to ensure the removal of

Ag/Al out of the exposed areas (see Fig S2. In the supplementary material for SIMS mass spectrum of Ag etched region).

Secondary ion mass spectroscopy (SIMS) mass spectrum was performed after the

Ag past etched chemically. In Figure 2.4 revealed the absence of Ag peak from the etched reign using the optimized etching recipe.

Figure 2.4 Mass spectrum of the Ag etched region

2.2.3 SIMS depth profiling of NiSi/Cu rear contact

The fabrication of the (NiSi/Cu) contact on the rear textured silicon surface was verified by performing SIMS. The depth profiling experiment was carried out using a

Dynamic-SIMS instrument (Hiden Analytical Ltd., Warrington, UK) under UHV conditions

(10-9 Torr). The depth profiles of Cu, NiSi, Ni ,B and silicon as shown in Figure 2.5. However, the depth profiling has low accuracy in terms of the depth and sharp interfaces due to the high roughness of the rear side because of the random pyramidal Si surface. 50

Figure 2.5 SIMS depth profiling of NiSi/Cu on the rear side of the silicon

The third stage after removing the Ag was cleaning the exposed heavily boron doped silicon with buffer oxide etchant (BOE) for 60 seconds to etch the native oxide before forming the silicide. After that, the samples were placed upside down inside the e-beam chamber to form the nickel mono-silicide as described in the first step. Subsequently, an oxide etching step was required to remove the silicon oxide layer which grows during the annealing step using (BOE) for 120 seconds. Finally, we deposited 0.5 m of Cu on the

NiSi using magnetron sputtering process (400 W, 25 sccm, 5 mTorr). Cross-sectional SEM image was obtained for NiSi/Cu rear contact on the textured silicon as shown in Figure

2.6. 51

Figure 2.6 SEM image of NiSi/Cu rear contacts on the textured silicon.

2.3 Current-Voltage curves of NiSi/Cu and Al rear contacts

Electrical characterization of the samples was done under simulated AM 1.5 sunlight

(Spectra-Physics 91160–1000), calibrated to give 100 mW/cm2 using an NREL–KG5- filtered silicon reference cell. The current density and voltage (JV) curves were recorded with a Keithley 2400. Figure 2.8 compares the output current for the screen printed Ag and NiSi/Cu rear contacts of the optimized solar cells with an area of 1 cm2. We observed significant increment in FF of approximately 6.5 % for NiSi/Cu rear contacts led to increased efficiency by 1.2 % compared to screen printed Ag solar cells. However, a

2 modest improvement of ~ 0.01 V and 0.32 mA/cm in Voc and Jsc were found. This is typically expected for a lowered series resistance Rs as it mainly affects FF and not Voc or

Jsc. [65] Since the silicide formation significantly affect the metal-specific contact resistance. Thus, it is important to correctly extract the series resistance value Rs to validate this hypothesis. 52

)

2 2 NiSi/Cu Current density 30 Screen printed Ag current 20 NiSi/Cu Power Screen printed Ag Power 20

Power density Power (mW/cm

Current density Current (mA/cm

0 0 0.0 0.1 0.2 0.3 0.4 0.5 0.6 Voltage (V)

Figure 2.7 Output current density vs. voltage of NiSi/Cu (red circles) and screen printed Ag

(black squares) under (a) one-sun illumination showing both of current and power density,

In this work, we used two different methods for Rs determination with high accuracy

depending on the JV-curves under different illumination intensities (dark, 0.7 and 1 sun)

comparing both of NiSi/Cu and screen-printed Ag rear contacts. [66-68] The first method

depends on comparing one-sun with the dark JV-curves as shown in Figure 2.8. The

principle of this method is based on a voltage difference at maximum power point (mmp)

of one sun and the dark JV-curves.

푉푑푎푟푘,푚푚푝 − 푉푙𝑖𝑔ℎ푡,푚푚푝 푅푠:푙𝑖𝑔ℎ푡_푑푎푟푘 = (4) |퐽푚푚푝|

Applying this method shows that RS:light_dark is reduced for NiSi/Cu rear contact by

0.68 Ω.cm2 compared to screen printed Ag. 53

0 1.0 sun current (Ag) shifted dark current (Ag)

) 1.0 sun current (NiSi/Cu) 2 -10 shifted dark current (NiSi/Cu)

-20

2 J (Ag)= 32.99 mA/cm mpp V (Ag)= 0.066 V -30 light_dark 2 J (NiSi/Cu) = 36.87 mA/cm mpp V (NiSi/Cu)= 0.051 V Current density Current (mA/cm light_dark -40

0.0 0.1 0.2 0.3 0.4 0.5 0.6 Voltage (V)

Figure 2.8 Output current density vs. voltage of NiSi/Cu (red circles) and screen printed

Ag (black squares) under one-sun and shifted dark measurements

The second method of Rs determination depends on the comparison of two JV- curves measured at different illumination intensities as shown in Figure 2.11. Measuring the JV-curves at different illumination intensities ends in two shifts between them. The first shift is in the current density due to the difference in the photo-generated current which is proportional to the incident illumination intensity as explained in Figure 2.10 and of electrical parameters variation in supplementary material). The second shift is in voltage which is caused by the smaller series resistance loss, at a lower light intensity:

∆푉 = 푅푠,푙𝑖𝑔ℎ푡∆퐽푠푐. Where ∆퐽푠푐is the variance in the two short-circuit current densities.

The series resistance value using this method (푅푠,𝑖푛푡:푣푎푟) can be calculated using the following equation:

∆푉 푅푠,𝑖푛푡:푣푎푟 = | | (5) ∆퐽푠푐 54

2 The second method confirms that RS,int:var is reduced for NiSi/Cu with 0.794 Ω.cm compared to the screen printed Ag. The previous calculations using both methods reveal the RS reduction and approve the impact using the NiSi/Cu ohmic contact on improving the FF which results in enhancing the efficiency of the solar cells as proposed in equations

(1, 2 and 3).

0.7 sun illuminati (Ag) )

2 1.0 sun illumination (Ag) 0.7 sun illumination (NiSi/Cu) -10 1.0 sun illumination (NiSi/Cu)

-20

-30 Current density (mA/cm density Current

-40

0.0 0.1 0.2 0.3 0.4 0.5 0.6 Voltage (V)

Figure 2.9 Output current density vs. voltage of NiSi/Cu (red circles) and screen printed

Ag (black squares) under different illumination levels: 1 and 0.7 illumination intensities

Further investigation of the solar cells dark measurements was studied since the series resistance near the open circuit is strongly affects the JV curves. We explored the dark current for the screen-printed Ag and NiSi/Cu cells in Figure 2.10(a). For voltages above VOC ~0.6 V, we see an increase in the dark current for the NiSi/Cu cells as compared to the screen-printed Ag. Additionally, Rs values extracted according to PV simulator software (nanohub) from dark JV characteristics.[69] The results confirmed a series 55 resistance reduction from 0.966 Ω.cm2 for the screen printed Ag to 0.55 Ω.cm2 for NiSi/Cu rear contact.

To verify the contact resistance effect, we have simulated a solar cell of similar structure which is in contact with the screen-printed Ag and the NiSi/Cu contacts using

TCAD Synopsys Sentaurus software (Table 2-5). ohmic contact was assumed for the emitter contact with zero contact resistance. However, the p++ collector contact was simulated with different contact resistivity values to simulate the effect of series resistance on dark JV curves. Previous studies showed that the screen-printed Ag–Al

-4 -5 2 pastes have specific contact resistance (휌푐) is in the range of 10 –10 Ω·cm on the heavily doped silicon. [70-72] On the other hand, the 휌푐 of NiSi ohmic contact is in the range of 10-7–10-8 Ω·cm2 on the same highly doping concentration. Figure 2.10 (b) shows that the current value is affected by the contact resistivity, showing similar trend to the experimental behavior inFigure 2.10(a). According to the results in Fig. 3(b), as the contact resistance increases from 10-4 to 10-7 Ω/cm2, the current increases by 17% from 152 A/cm2 to 179 A/cm2. Although the simulation overestimates the current density, due to not taking into account an experimentally validated mobility model and patristic recombination effects, it still qualitatively shows the effect of the series resistance reduction on the dark JV which we believe to play a significant role in the case of NiSi/Cu cell.

300 NiSi/Cu rear contact

screen printed Ag ) 2 250

200

Rs 150 + decreasing s R R 100 sh

J Jdark sc -

Current density (mA/cm density Current 50

0.0 0.2 0.4 0.6 0.8 1.0 Voltage (V) Ohmic Contact 200 Contact Resistivity = 10-7 . cm-2

Contact Resistivity = 10-4 . cm-2 )

2 -2 -2

150 Contact Resistivity = 10 . cm A/cm

( 200

) 2

100 150

A/cm (

100 Current 50 Current Current 50

0.8 0.9 1.0 Voltage (V)

0 0.0 0.2 0.4 0.6 0.8 1.0 Voltage (V)

Figure 2.10 Comparison of JV curves of the dark current: (a) experimental measurements (inset is equivalent circuit including series (Rs) and shunt (Rsh)

resistance). (b) simulated data as a function contact resistance

Table 2-2 Output parameters of (c-Si) solar cells with screen-printed Ag vs. NiSi/Cu rear contact of 1 cm2 as measured under AM1.5 spectrum (100 mW/cm2) at 25 °C, means ±

SD.

Solar Cell NiSi/Cu Printed Ag

Parameters

Jsc(mA/cm) 39.883 ± 0.213 39.562 ± 0.135

Voc (V) 0.598 ± 0.0014 0.588 ± 0.0014

FF (%) 67.886 ±1.945 61.383 ± 1.917

Efficiency (%) 16.254± 0.68 14.746 ± 0.443

2 RS;int:var (Ω.cm ) 1.143 ± 0.676 1.937 ± 0.860

RS;light_dark 1.38 2.06

(Ω.cm2)

2.4 Series resistance determination (Rs) methods

2.4.1 Comparison of dark and one sun JV-curves

The principle of this method is based on a voltage difference at maximum power point (mmp) of one sun and the dark JV-curves. Figure 2.11 and Table 2-3 show the variation of the electrical performance parameters at the maximum power

|mpp|. Applying this method shows that RS:light_dark is reduced for NiSi/Cu rear contact by

0.68 Ω.cm2 compared to screen printed Ag.

0 0 NiSi/Cu rear contact screen printed Ag rear contact

1.0 sun current (NiSi/Cu) 1.0 sun current (Ag)

) ) 2 2 shifted dark current (NiSi/Cu) shifted dark current (Ag) -10 -10 V Vlight_ dark 2 light_ dark 2 Rs  1.37 / cm Rs;light_ dark   2.0 / cm ;light_ dark J J mpp mpp -20 -20

Vlight,mpp=0.433 V Vlightmpp=0.470 V -30 -30 2 2 Jmpp= 32.99 mA/cm

Jmpp= -36.87 mA/cm Current density Current (mA/cm Current density Current (mA/cm V = 0.499 V Vdark,mpp= 0.521 V dark,mpp V = 0.066 V -40 Vlight_dark= 0.051 V -40 light_dark

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 Voltage (V) Voltage (V)

Figure 2.11 JV-curves of one sun and Jsc shifted dark measurements. (a) NiSi/Cu rear

contact (b) screen printed Ag.

Table 2-3 Variation of electrical performance parameters at the maximum power

|mpp| value taken for series resistance determination (RS;light_dark) for dark and 1 sun

intensity illumination

shifted dark 1.0 sun illumination

measurements

Rear |mpp| Vdark,mpp |mpp| Vlight,mpp Jmpp ∆Vlight_dark RS;light_da

contact 2 2 2 (mΩ.cm ) (V) (mΩ.cm ) (V) (A/cm ) (V) rk

(Ω.cm2)

NiSi/Cu 19.148 0.521 17.33 0.470 0.0368 0.051 1.38

Screen

printed 17.52 0.499 14.73 0.433 0.0329 0.066 2.06

2.4.2 Comparison of two JV-curves measured at one and 0.7 sun illumination

intensities

This method is based on the comparison of two JV-curves measured at different illumination intensities as shown in Figure 2.12 for NiSi/Cu rear contact and screen- printed Ag, respectively. Measuring the JV-curves at different illumination intensities (1 and 0.7 sun) ends in two shifts between them. The first shift is in the current density due to the difference in the photo-generated current which is proportional to the incident 60 illumination intensity. The second shift is in voltage which is caused by the smaller series resistance loss, at a lower light intensity: ∆푉 = 푅푠,푙𝑖𝑔ℎ푡∆퐽푠푐. Where ∆퐽푠푐is the variance in the two short-circuit current densities. Each JV point [Jx,Vx] lies at a fixed distance ∆J (10,

15 and 20 mA) from the short-circuit current (e.g. Jx = JSC–∆J) where ∆J is marked on each curve. Table S1 presents the [Jx,Vx] points and the corresponding Rs;int:var calculated from:

∆푉 푅푠,𝑖푛푡:푣푎푟 = | | ∆퐽푠푐

0 0

0.7 sun illumination (NiSi/Cu) 0.7 sun illuminati (Ag) ) 2 1.0 sun illumination (NiSi/Cu) 1.0 sun illumination (Ag)

V =0.012 V ) I = 20 mA 3 2 I = 20 mA -10 3 -10 3 V3=0.025 V 2 Rs3= 0.60cm 2 Rs3= 1.25cm I2= 15 mA I2= 15 mA

I = 10 mA I1= 10 mA -20 1 V2=0.014 V -20 V2=0.024 V Rs = 0.93 cm2 2  2 Rs2= 1.66 cm

-30 -30

Current density (mA/cm density Current V =0.019 V V =0.029 V 1 (mA/cm desity Current 1 Rs = 1.9cm2 2 1 Rs1= 2.9 cm -40 -40

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 Voltage (V) Voltage (V)

Figure 2.12 JV-curves of one sun and Jsc shifted dark measurements. (a) NiSi/Cu rear

contact (b) screen printed Ag.

Table 2-4 Variation of electrical performance parameters measured with respect to short circuit current density JSC (mA/cm2) taken for determination of the average series

resistance (Rs;int:var) (Ω.cm2) value for two different light intensities (1 and 0.7 sun)

Table 2-5 The simulation parameters used in investigating the contact resistance impact

on the JV-behavior.

Properties Doping (푐푚−3)

22 n-layer (emitter) Nd=5×10 (1um thick)

c-Si (p-type) 2×1015

22 p-layer (collector) NA=5×10

2.5 Fabrication process of in-house c-Si solar cell

The fabrication starts with a P-type Si < 100 > wafer (1–30 Ω-cm, 500 μ m thickness).

The first step was depositing SiO2 (950 nm) on the whole wafer using Plasma Enhanced

Chemical Vapor Deposition (PECVD). Respectively, n+ areas were defined through lithography patterning after etching the SiO2 in Reactive Ion Etching (RIE). Source diffusion

(Phosphorous TP-250) was used to create N+/P lateral junction. After the N+/P lateral junction formed, lithography patterning was used to lift the silicidation off metallurgical junction position and creates periodic holes (12-14 µm) through the whole contact areas.

The silicidation formed by depositing Ni/TiN (40/25) nm using Magnetron Sputtering

Deposition, followed by lifting off the deposited film from the defined previous positions, respectively, 450 °C annealing for 300 secs was performed in order to form nickel silicide.

The unreacted metal was stripped in Piranha solution for 5 min. Structural 63 characterization of NiSi was done using Scanning Electron Microscopy (SEM). Sheet

Electrical Resistance has reduced after NiSi formation from 2.5 kΩ/sq to 2.5 Ω/sq.

Additionally, the sa1me lithography patterns which used for the silicidation was reused for the depositing contact metallization by electron beam evaporation where 350 nm of

Cupper was deposited. Fabrication process is explained in Figure 2.13 .

( ( a) b)

( ( c) d)

Figure 2.13 Process flow (a) creating PN diode on Si(100) (b) lithography steps for NiSi formation.(c) Passivation layer using Al2O3 (d) Interdigitated metallization on top of the

silicidation

P- Si wafer Mono NiSi

N+ well metallization on N+ area

Passivation layer metallization on P- area

2.5.1 J-V curves of rigid c-Si solar cells using NiSi/Cu contacts

The electrical characterization on the bulk rigid interdigitated PN junction is shown in Figure 2.14 and Table 2-6 Electrical characteristics of fabricated bulk interdigital PN junction .It demonstrations that the ohmic contact using NiSi/Cu stack has an efficiency of 5.3% due to the desired increment in both of the fill factor and in the Voc in comparison to the Schottky junction using Cr/Au contact which has an efficiency of 1.5%.

5.00E-02 Cr/Au with no 4.50E-02 silicidation 4.00E-02 3.50E-02 3.00E-02 NiSi/Cu 2.50E-02 2.00E-02

Current(A) 1.50E-02 1.00E-02 NiSi/Cu/ 5.00E-03 Al2O3 0.00E+00 passivation -0.1 0 0.1 0.2 0.3 0.4 0.5 0.6 Voltage (V)

Figure 2.14 Electrical performance of fabricated interdigital PN junction with ohmic

contact using NiSi/Cu stack vs Schottky using Cr/Au measured on 2 cm2 samples area

Table 2-6 Electrical characteristics of fabricated bulk interdigital PN junction

contact Voc (V) Jsc Fill Factor Efficiency (%)

(mA/cm2) (%)

NiSi/Cu with 0.52 23 45 5.3

Al2O3

passivation

NiSi/Cu without 0.5 23.01 43 5

passivation

Cr/Au without 0.44 9.374 36.167 1.5

silicidation

2.5.2 Flexible c-Si solar cell (in-house fabrication)

Here, a novel process of flexing c-Si PV has been demonstrated which results in thin photoactive layers with high flexibility, periodic arrays of micro-structured junctions that has the potential of amplifying the absorption while decreasing system parasitic resistance with the formation of the low-cost metallization using NiSi/Cu. This texture can be engineered further to reduce reflections and enhance light trapping. By contrast to conventional techniques for texturing, this process happens as a natural consequence of the release process, thereby avoiding any additional cost. The lift-off technique shown in

Figure 2.15 leads to the production of textured (micro-holes shaped) surfaces at both sides of the released ultra-thin layers.

Unfortunately, after the releasing step, the cells didn’t work due to the holes distribution among the samples which destroy the p-n junction and shorten the cells.

a) b)

c) d)

30 um

( ( e) f)

20 um

for the released PV. (e) Top view of the micro-holes’ pattern. (F) optical image of the

flexible PV 2.6 NiSi/Cu rear contact for thin c-Si

Current standard c-Si PV is using highly doped front and back areas in order to achieve

decent ohmic contact with the current screen-printed metallization. Here, we

investigate fabricating the NiSi/Cu contacts on the rear side of the cells which can help

to minimize the surface recombination loss after removing the Ag/Al paste via

mechanical etching.

2.6.1 Experimental

Further investigation on the impact of forming NiSi/Cu Ohmic contact will be exploded on the lightly doped and thinned (C-Si) PV. In this part of the work, the removal of Ag/Al back contact will be applied using the grinding and thinning process which performed by using (Allied, MultiPrep. Polishing System-8”). The polishing steps are illustrated in [Table 4]. The surface morphology of the samples was explored and analyzed by atomic force microscopy (AFM, Agilent 5400 SPM) and the optical profile (Zygo) as shown in Figure 2.16.

Table 2-7 Grinding and polishing steps

Grinding Diamond RPM time steps lapping film Coarse 30 um grinding 15 um 9 um Fine grinding 300 3min 6 um Soft 3 um polishing 1 um3

2 69

Table 2-8 Surface roughness measurements taken for different etching steps

Roughness Optical profiler AFM measurement tool (Zygo) Height parameters Sq Sa rms Ra

Coarse grinding 37 nm 27.6 nm 230 nm 187 nm Soft polishing 9.76 nm 5.76 nm 4 nm 3 nm

Figure 2.16 surface roughness measurements on mechanically polished samples (a,b)

SEM images of course grinding and soft polishing. (c,d) AFM roughness images of

course grinding and soft polishing.(e,f) Optical profiler of course grinding and soft

polishing

2.6.2 I/V measurements For the NiSi/Cu contacts on the soft polished samples

Figure 2.17 Electrical performance of NiSi/Cu contacts on the soft polished samples

Table 2-9 Electrical characteristics of the NiSi/Cu contacts on the soft polished samples

Si Voc Jsc FF η Thickness (V) (mA/cm2) m

As received 200 0.6 37.1 70 15.8 (Ag/Al) Back metallization 1 150 0.525 43 57.5 13.00

2 130 0.538 44.2 58.36 13.87

3 120 0.521 38.52 63 12.64 Ag/Ni/NiSi

4 80 0.541 35.5 60.28 11.57

5 66 0.521 35 59.00 10.75

Figure 2.18 Electrical characteristics of the NiSi/Cu contacts on the soft polished samples 72

2.7 Conclusion

In summary, we found that the low-cost NiSi/Cu ohmic contact is an efficient metallization candidate on the rear side of c-Si solar cells instead of the conventional Ag paste. We have reported experimental improvement of both of the FF and efficiency for

NiSi/Cu compared to Ag paste by 6.5% and 1.2%, respectively. This is attributed to the

2 average reduction of the Rs ~ 0.737 Ω.cm which supported by performing simulations to understand the effect of the contact resistance on the dark electrical performance of the cells. Future work will be directed to replace the Ag paste by NiSi/Cu on both of front and rear contacts using advanced electrodeposition of a nickel seed layer to meet the automatic production line for manufacturing solar cells.

3 Ultra-Flexible High-Efficiency Mono-crystalline Silicon Solar Cell

Advanced classes of modern application require new generation of versatile solar cells showcasing extreme mechanical resilience, large scale, low cost and excellent power conversion efficiency. Conventional crystalline silicon-based solar cell offer one of the most highly-efficient power sources but a key challenge remains to attain its mechanical resilience whilst preserving electrical performance. Here we show, a complementary metal oxide semiconductor (CMOS) based integration strategy where corrugation architecture enables ultra-flexible and low-cost solar cell modules from bulk monocrystalline large scale (127127 cm2) silicon solar wafer with 17% power conversion efficiency. This periodic corrugated array benefit from an interchangeable solar cell segmentation scheme which preserves the active silicon thickness of 240 m and achieves flexibility via interdigitated back contacts. These cells can reversibly withstand high mechanical stress and can be deformed to zigzag and bifacial modules. Theses corrugation silicon-based solar cells offer ultra-flexibility with high stability over 1000 bending cycles including convex and concave bending to broaden the application spectrum with a bending radius of curvature lower than 140 m.

This chapter has submitted to advance energy material journal and U.S patent office:

• Rabab R. Bahabry, Arwa T. Kutbee, Sherjeel Khan, Adrian C. Sepulveda, Irmandy

Wicaksono, Maha Nour, Nimer Wehbe, Amani S. Almislem, Mohamed T. Ghoneim,

Galo A. Torres Sevilla, Ahad syed, Sohail F. Shaikh and Muhammad M. Hussain. 74

“Corrugation Architecture Enabled Ultra-Flexible Wafer-Scale High-Efficiency

Mono-crystalline Silicon Solar Cell”. (DOI: 10.1002/aenm.201702221)

• Rabab R. Bahabry and Muhammad M. Hussain. U.S. Patent Provisional Application

filed KAUST Ref: 2017-133-01 (JULY 14, 2018 )

3.1 Introduction

Flexible solar cell technology have paved the way for wide application spectrum ranging from wearable and implantable electronics, robotics and infrastructure/vehicle- integrated solar panels on curved surfaces to space applications.[17, 73-75] This is mainly triggered by the need of self-powered technologies which abandons cords and benefit from wireless technologies for the sheer feasibility and convenience of the wearer. [12-

17] Recent research efforts have investigated the development of inorganic and organic flexible thin-film solar cells (TFSCs) through materials innovation and device engineering.

Silicon-based solar cells is one of the most investigated materials in photovoltaics technology due to its natural abundance, non-toxicity, excellent reliability, mature manufacturing and high performance.[18] Flexible thin film Si-based solar cells have shown their advantage in applications in which weight is a critical factor. However, thin film Si-based solar cells seem to be disadvantageous over thick Si solar cells due to the reduction of the optical absorption with less active Si material. Nevertheless, these technologies can provide higher power conversion efficiencies as compared to polycrystalline compounds or organic solar cells, due to high material quality.[19, 20] 75

In this regard, two main approaches are explored extensively for producing TFSCs.

The first approach includes direct deposition of the TFSCs on flexible substrate, such as polymers, fabrics and paper.[11, 21] Such method is restricted by the maximum temperature of the substrate materials, impacting the solar cells performance. [22] The second approach is the transfer printing technique which utilizes the conventional rigid substrates for the fabrication and then transfer the TFSCs onto resilient substrate. [23,

24, 76] Thus, the second method overcomes the performance limitation associated with the substrate thermal incompatibility with fabrication of high-efficiency TFSCs. However, it suffers from both of low throughput and high cost. Thus, a tradeoff between mechanical compliance, performance and robustness of flexible photovoltaic cells remains as a major challenge, especially in high volume production for large surface coverage applications.

Here, we show a novel approach for flexing crystalline (c-Si) solar cells which overcome the previous major limitations associated with the material thickness reduction and the high cost of flexing process. Our approach is inspired by the corrugated architecture where thin areas are grooved between thicker areas which eventually makes the platform in-hand flexible. We have grooved series of linear channels (~1 mm wide) without photolithography to reduce the thickness of silicon in those areas while keeping the remaining areas intact in silicon solar cell (~ 6.2 mm wide). This corrugated architecture silicon solar cells show extreme bending radius  140 m with one of the highest power conversion efficiency among c-Si flexible solar cells that reaches 17.2 % on a wafer scale (127127 mm). We also demonstrate high mechanical stability of the cells during 1000 cycles of bending. Finally, the integration of the flexible solar cells is 76 demonstrated on a curved surface for supplying power both flexible 88 LED Matrix and paper-based fully functional system using indoor illumination.

3.2 Results and Discussion

3.2.1 Ultra-Flexible c-Si solar cell

The mechanical adaptability of the corrugated c-Si solar cells concept is illustrated in Figure 3.1a. We have used interdigitated back contact (IBC) solar cells with an area of

127127 mm2. Each wafer comprises an interdigitated p and n regions on a bulk silicon wafer with ~ 260 m thickness as depicted in Figure 3.1.[31]

Figure 3.1 (a) Scheme of: the ultra-flexible corrugated c-Si solar cell (b) The

interdigitated back contact (IBC) c-Si solar cell structure

The IBC solar cell modules are highly attractive due to their high efficiency and 77 advanced photons absorption attributed to less contact-shading compared to the standard front side contacts modules. Additionally, the IBC configuration simplifies our flexing process without the utilization of expensive lithography techniques where the silicon thinning process is executed on the whole front side without any precautions needed for handling the contacts. In order to achieve extreme flexibility, corrugation architecture or segmentation was utilized on the IBC wafers with an alternating regions of thinned and rigid Si discontinuities as shown in Figure 3.2.

Figure 3.2 optical Image of the corrugated architecture c-Si solar cell (5” wafer): consists

of 16 parallel grooves of linear channels (~0.86 mm wide) and 18 remaining active

segments of the c-Si solar cells (~ 6.24 mm wide) except the solar cells covering the

common electrode on both edges which has a width of 8.81 mm.

The optical image of the corrugated c-Si solar cell is shown in Figure 3.3a demonstrating extreme mechanical bending flexibility. It can not only be-bent but also 78 can be deformed into different configurations including a zigzag and bifacial modules as shown in Figure 3.3b and 3.3c, respectively. An optical front view image of the zigzag configuration depicted in Figure 3.3.d in which an extraordinary flexibility and plastic deformation of the interdigitated back contact with the acute bending angle of 71.4° as observed using scanning electron microscopy imaging (SEM) (Figure 3.3 e). Further SEM imaging of the top view of the sample is revealed in Figure 3.3f showing both of the negative and positive IBC electrodes with a width of 0.42 mm and 1.6 mm respectively.

Figure 3.3g demonstrates the extreme bending flexibility of the solar cell, folded around a radius  140 m. Moreover, the microstructure of the active solar cell and the bent contact is shown in Figure 3.3h with no presence of microcracks in the contact region. 79

Figure 3.3 Ultra-flexible c-Si solar cells on a five-inch wafer scale. Optical images of 5”

wafer of c-Si IBC solar cell illustrates: a) high bending flexibility, b) Zigzag module, c)

folded wafer in half to obtain bifacial demonstration and d) The zig-zag side view and

the internal angle (). Scanning electron microscopy images: e) Cross-sectional shows

zig-zag deformation with = 71.4° and f) top view reveals the deformation of the

interdigitated contacts. f) Optical image illustrates the view of the corrugated architecture consists of ~6.2 mm active solar cell segments and 0.8 mm grooves width. 80

SEM images of: g) The minimum bending radii of curvature (R=<140 um) during folding

h,i) Top view of the folded Aluminum contact and the Silicon segments.

In order to facilitate the industrial c-Si solar cells with the flexible version of the modules and a comparable efficiency, our corrugated flexing technique is designed to be applied on 5” wafer size with a lithography less approach. The main flexing steps are depicted in Figure 3.4 (See Methods for details).

The combination of the photoresist (PR) and the Kapton tape work as a hard mask against the plasma etching. Moreover, the PR layer enables smooth peel-off of the Kapton with no residues on the active solar area after the flexing process is completed.

Figure 3.4 Scheme of the progression flow of the deep reactive ion etching (DRIE) and

corrugation technique process

To assess the plasma etching effect of the silicon etching progression and the underlying IBC surface, cross section SEM imaging, Zygo profiler measurements, for IBC 81 surface roughness, and Secondary Ion Mass Spectrometry spectra (SIMS) analysis, were carried out. Figure 3.5a shows the optical photo of the wafer after the plasma etching is completed. Figure 3.5b and 3.5c present the optical microscopy image of the IBC contacts with the root mean square average (rms) of 1.269 m.

Figure 3.5 a) Optical image of the 5” wafer post the DRIE process and during peeling off

the Kapton tape. b) Optical microscope image of the interdigitated back contact (IBC)

after the DRIE. C) Optical profiler measured on the IBC for surface roughness analysis

The silicon thickness reduces during the DRIE process from 240 m at the start, to

170 m at 250th cycle (54 min), to 71 m at 450th cycle (99 min), and finally the silicon is etched totally at 650th cycle. Thickness reduction againstt DRIE etching cycles along with consumed etching time is plotted in Figure 3e. In addition, SEM images are carried out in

Figure 3f and 3g after 54 and 99 min of DRIE etching, respectively. 82

Figure 3.6 a) DRIE etching of the exposed silicon area vs the thickness reduction (each

groove area=1270.86 mm2). b) and c) Cross-sectional scanning electron microscopy

(SEM) images taken after 250 and 450 etching cycles, respectively

In order to verify the silicon etching process, the etched grooves were analyzed using SIMS in both Positive and Negative modes. In Figure 3.7 the analyzed film is composed mainly of aluminum, in addition to alkali and alkaline earth (Na, K, Ca) and halogen metals (F) commonly detected as contaminant in SIMS. The positive and negative spectra are complementary since the most intense peak in the positive mode is Al+ (mass

27) whereas AlO– (mass 43) is the most intense peak in the negative mode. Although the aluminum film is expected to oxidize after contact with air, the intense aluminum oxide signals detected in both modes are also due to the usage of oxygen as a sputtering beam. 83

7 Positive SIMS Spectrum Negative SIMS Spectrum 1x10 + - Al AlO+ AlO

8x106 1.5x104

AlO+ - 2 AlO2

6x106 4 + 1.0x10 Al2 4x106

+ Ion Signal Ion (c/s) Al2O 5.0x103 2x106

50 60 70 Ca+

Na+ F+ + 0 K 20 30 40 50 60 70 0.040 50 60 70 Mass (amu) Mass (amu)

Figure 3.7 Secondary Ion Mass Spectrometry (SIMS) spectra of the IBC post the DRIE in

both positive and negative modes.

3.2.2 Solar cell efficiency

The J–V characteristics are measured under a solar simulator (approximately the

A.M. 1.5 Global Spectrum with 100 mW cm− 2 intensity and spectral mismatch correction at the room temperature), and in the dark using a four-point probe set up to reduce the resistance associated with the high current collected from 15×127 mm2 c-Si solar cell. The measured cells were diced using a laser with a power of 100%, speed of 60% and 500 kHz.

For each cell, the reported figures of merit for calculating each of the: current density (J), power density (P), and power conversion efficiency (η) are based on active solar cells area only excluding the area of grooves where no photocurrent is generated which was found to be around 16.65 cm2. The J–V characteristics of averaged optimized flexible cells are 84 shown in Figure 3.8 compared to rigid ones on flat surface where the error is the standard deviation from ten devices.

Figure 3.8 Electrical performance of the flexible c-Si solar cells compared to the rigid

ones. a) J-V performance of the flexible cells enabled via the corrugation technique

compared to the rigid devices under solar simulator illumination (1 sun) at the room

temperature. b) Power density (mW/cm2) compression 85

Figure 3.9 Electrical characteristics comparisons of: open circuit voltage (Voc), Efficiency

(), short circuit current (Jsc) and FF.

The open circuit voltage (Voc)= 0.620±0.02 V, short circuit current (Jsc)= 38.055±1.2 mA/cm2, fill factor (FF)= 72.0139±1.5%, = 17.017±0.8% and P= 17.00 mW/cm2 under one sun illumination compared to the rigid IBC solar cells which gives the following

2 performance in average: Voc= 0.622±0.02 V, Jsc= 38.607±1.2 mA/cm , FF= 71.9413±1.5 %,

= 17.294±0.8 % and P= 17.30 mW/cm2 as illustrated in Figure 3.9. 86

Figure 3.10 Weight and total power (mW) measured of devices of an area= 127×15

mm2. Note that all the J-V characteristics measured when the devices are flat

2 The total power (mW) which is generated from each flexible cell (15×127 mm ) is reduced with 13.7% compared to the rigid ones as the total power generated from the rigid cell is 328 mW and reduced to 283 mW for the flexible corrugated architecture ones as plotted in Figure 3d. This is attributed to the active solar cell loss at the grooves area where total weight loss is 13.3 % of each cell as the rigid cells weight is reduced from 1.5 gram into 1.3 gram for the flexible cells. Comparison of the weight of the rigid and flexible

5” wafer is shown inFigure 3.10.

3.2.3 Flexible LEDs matrix and system on chip integrated with the flexible c-Si solar

cells using indoor illumination

The integration of a system consists of five flexible solar cells (each cell area ~ 127

15 mm2) connected in series is demonstrated to explore a seamless integration of the developed corrugation cells using indoor illumination. The integration is explored with both of LEDs matrix and a paper-based humidity sensor while the flexible cells are installed on convex curvatures.

Figure 3.11 Integration of the corrugated c-Si solar cells. Five cells are connected in series and installed on convex glass curvature. Optical images of: a) Output voltage using

two bulb lights. b) KAUST logo display using LEDs matrix powered by the flexible solar cells via a desk light. c) Flexible cells are installed on a glass mug used to supply power to 88

a fully functional system placed on a plant leaf and subjected to a humid condition. d)

Circuit diagram. e) Humidity sensor response. f) Red LED on a flexible board with

notification.

Figure 3.11 (a) displays the output voltage (3–2.9 V) of the five cells connected in series using two (light bulb) office lamps installed on convex bending glass substrate (R=

6 cm). Figure 3.11 (b) presents the system connected to 88 LEDs matrix displaying KAUST logo using (blue, yellow, green and red) LEDs (Additional display of the LEDs matrix using the flexible solar cells can be seen in the Supplementary Video 1). Furthermore, the cells connected in series are installed on a glass mug used to supply power to a fully functional system as shown in the optical image in Figure 5c. We interfaced a paper-based humidity sensor[77, 78] with Cypress's Bluetooth Low Energy (BLE) PSoC® 4 BLE System-on-Chip

(SoC) (See methods for details). The system was then placed on a leaf of a plant and subjected to humid conditions by blowing air from the mouth on the sensors via a pipette.

The humidity sensor changes capacitance based upon the relative humidity of the environment. PSOC’s Current Digital to Converter (IDAC) was used to find the capacitance changes of the humidity sensor. For the purpose of demonstration, we only show the detection of a humid condition both with a red LED on the flexible board and notification on the smartphone (as can be seen in the Supplementary Video2). Figure 3.11 (d) illustrates the full circuit diagram. When the sensor sensed humidity, its capacitance changes which is recorded by IDAC in PSOC as demonstrated in Figure 3.11 (e). PSOC then turns on the LED and sends notification to a smartphone as shown in Figure 3.11(f). 89

3.2.4 Mechanical flexibility of interdigitated back contact

Our corrugation design consists of several active solar cell segments connected through the IBC (screen printed Aluminum) which serves the purpose of the carrier substrate. The segmentation approach has been used recently to obtain flexibility and stretchability even with brittle materials as Si (fracture strains <1%) as the stretchable electronics is attracting tremendous attentions.[79, 80] In our lithography less corrugated approach, the interconnects (grooves) width are ~ 0.86 mm. To obtain the minimum bending radius for the interconnect, we assume that its length (L) is the perimeter of a circle with radius (Rbending):

L R = (1) bending 2π

Hence, R bending is 137 m, which is consistent with the experimental value obtained via folding the solar cell as SEM image reveals in (Figure 1l). As the device is bent with the previous bending radii, its fundamental strain () can be estimated from:

푡 max = (2) 2Rbending

Where t is the interconnects thickness (~ 25 um) and max is the maximum stain.

Hence, The obtained strain is below 10% which remains below the rupture strain of the screen printed metals that is >20%.[81, 82] Comparative simulation study is carried out with COMSOL™ for the maximum strain in the interconnects as a function of bending angles () which is defined by the angle between the active solar cells segment during bending and the neutral axis as shown in Figure 3.12. The maximum strain is below 8% 90 in the interconnects even with the maximum bending angle is equal to 180 (both of the active solar cells are folded).

Figure 3.12 Extreme Mechanical Flexibility: 3D strain modeling of the IBC (width=1 mm

and 25 m thickness) located between silicon solar cells with thickness of 250 m during bending with different angles (). a) The maximum and minimum strain of IBC as

a function of the bending angle. b) and c) Strain compression of =0 and 180,

respectively

For further enhancement and future work, the lowest estimated bending radius which can be achieved without exceeding 20% of the interconnects stain would be 62.5

µm with the length of 393 µm as designed using the Equation 1 and 2. 91

In Figure 3.13, the flexible solar cells show little hysteresis in Jsc as the bending radius decreases with light illumination for both of the concave and convex bending.

However, both of the FF and Voc remains in the average value of 72% and 0.62 V, respectively.

Figure 3.13 a) J-V characteristics of the devices at flat position and using convex bending (R= 9 cm) under 1 sun illumination compared to the measurements under the sun (noon) measured at (Google Map coordinates: 22.316521, 39.095746). b) Electrical characteristics of the devices without any consideration of the projection area. c) J-V characteristics of the devices using convex bending (R= 6, 5 and 3 cm).

We consider that the reduction of the Jsc is due to the variation of the projected area of the bent cells. Given the length of the solar cells L0 in its flat state (11.3 cm), the 92 projected area S of the rectangular device under light illumination is:

Sin(L0/2Rcave) S = S0 (3) L0/2Rcave

2 Where S0 is the area of the solar cell when it’s flat (16.65 cm ), Rcave is the concave curvature bending radius as illustrated in Figure 3.13 (a). On the other hand, the convex curvature bending radius (Rvex) of the same glass curvature is smaller than Rcave due to the glass thickness (~3.6 mm). Therefore, an approximation of Rcave is expressed as:

Rcave= Rvex–2t (4)

Where t is the thickness of the glass. Figure 3.13 (b) shows the optical image of both of the bent cells using convex and concave glass curvatures. Figure 3.13 (c) displays the projected area associated with the bending radius of: 6, 5.5, 5 and 4.4 cm for the convex and concave bending. 93

Figure 3.14 a) Scheme of projected area key components. b) Optical photo of the

flexible cells installed on concave and convex glass curvatures (Rbending= 6 cm). c)

Projected area as a function of the bending radii. The scatter points are calculated with

Equations (3) and (4).

Figure 3.14 demonstrates the electrical characterization of a flexible cells with an excellent bending durability after considering the projected area in the Jsc calculations which eliminates any hysteresis and confirms that the projected area is the one key factor of changing of the flexible device performance.[83] 94

70 70 FF (%) Concave bending 60 Convex bending 60 50 50

40 40 2 J (mA/cm ) sc 30 30

20 20  10 10 V (V) oc 0 0

4.5 5.0 5.5 6.0 Flat6.5 Bending Radii (cm)

Figure 3.15. Electrical characteristics of the devices using concave and convex bending

compared to the flat devices.

In addition, the performance of the zigzag model is measured under illumination as a function of the interior angles of the zigzag without taking in consideration the projection area variation in Figure 3.16. 95

FF (%) a 70 70

60 Concave bending 60 Convex bending 50 50

40 40 2 J (mA/cm ) sc 30 30

20 20  10 10 V (V) oc 0 0

3.5 4.0 4.5 5.0 5.5 6.0 Flat6.5 Bending Radii (cm) d b c 80 80 FF(%)70 70

60 60

50 50

40 40 2 J (mA/cm ) 1 sc 30 30 20 20 .2 cm  10 10 V (V) oc 0 0

100 120 140 Flat Interior angles(

The flexible cells are bent for: 100, 250, 500, 750 and 1000 cycles to investigate the durability under repeated cyclic bending. Figure 4h displays the optical images of the 3D printed test apparatus in both cases: pre-bent (top) and after bending (bottom). The electrical characterization of the cells under illumination versus the bending cycles are shown in Figure 3.17. It showed a robust performance of the cells without any gradual decrease in the output characteristics. 96

Figure 3.17 (a) Optical image of the 3D printed cyclic bending apparatus in both cases: pre-bent (top) and after bending (bottom). b) Electrical characteristics of the devices as

a function of the bending cycles.

3.3 Benchmarking of flexible Si-based solar cells efficiency

3.3.1 Academic research

our achieved efficiency using the measured area of the flexible c-Si solar cells is compared with the data literature in Figure 3.18.[81, 84-88] 97

Figure 3.18 Comparison with literature data. Both of the efficiency and the area are

taken from the published electrical characteristics of varies Si-based solar cells.

3.3.2 Our process vs the industrial approach

The commercialized grade semi-flexible solar cells intended to form a semi-flexible solar panel. Their method relays on assembling multiple cells on a semi-flexible handling carrier and reconnecting them to form one panel. Additionally, the total thickness of whole product is in the milliliters range[89].

On the other hand, we are transforming a monocrystalline silicon solar cell into its ultra-flexible version without any additional components while preserving the original efficiency of the cell using the corrugation architecture method. In our approach, the screen printed interdigitated back contact (IBC) which is a main layer of the solar cell structure serves as both the carrier of the c-Si segments and metal interconnection. Our process offers new possibilities in the flexible electronics as the screen-printed metals has fracture strain >20 which allows high flexibility and complete foldability as shown in Figure

1F. Moreover, the resulting ultra-flexible cells have a total thickness in the micrometers range. We believe that our approach is suitable for the Internet of things (IoT) era and can meet the needs of its wide application spectrum. Table 3-1 and Figure 3.19 show a comparison between our flexing approach and the commercialized method for Si-based solar cells. Detailed description of previous methods is listed in Table 3-2.

• Table 3-1 Comparison between our flexing approach (left) and the

commercialized method (right).

Our approach Industrial approach

One wafer to Multiple cells to one panel multiple segments

Only segmentation (No assumbling, Step 2: Assumble Step 3: Reconnect no addtional Step 1: Cut on a handler via metal handler substarate substarte interconnection and no reconnecting)

100

c-Si solar wafer Step 3:

IBC Step 1 Front ribbon which is a main connections of the cells layer of the solar Dicing cell structure the solar cells

Only segmentation step is needed

Step 2:

Assembling the diced cells on top of semi flexible handling substrate

IBC which is a main External layer of the solar carrier substrate cell structure with limited flexibility.

Figure 3.19 comparison between our flexing approach and the commercialized method for Si based

solar cells (a) schematics illustration (b) optical photos.

101

Table 3-2 Flexible industrial solar panel characteristics vs our ultra-flexible solar cell.

Semi-flexible industrial solar

This work panel basic concept The ultra-flexibility is The semi flexibility is

obtained via the back contact obtained via an external

which is one of the structure sheet used to assemble the

layers of solar cells. rigid cells on it.

Flexibility High Semi

Assembling step None Yes

Handling substrate None Oxidation resistance PCB or

aluminum sheets.

Thickness 0.26 mm ~ 2.5 mm size 5 inch- wafer (127127) mm. 1100570 mm

Any wafer size can be used

Solar cell IBC solar cells Conventional vertical configuration junction solr cells.

102

3.4 Experimental

3.4.1 Corrugation architecture technique

This technique is applied on 5” industrial supplied ion implanted monocrystalline n- type silicon solar cells with IBC configuration. First, positive PR 9260 is spun on the front side at 2,400 r.m.p for 60s. Pyrolysis bake is then carried out at 110C for 180. second.

The same previous step used on the backside yielding 10 m-thick PR on both faces. After that double layer of Kapotin (1/4” Polyimide film with silicon adhesive) tape is applied in perpendicular lines (15 lines) across the wafer with ~1 mm gap between the tapes and with wider tapes located at the wafer edges (~0.9 mm) to prevent thinning the silicon located on top of the IBC common electrodes. The wafer was exposed to broadband stepper at 1.800 mJ/cm2 followed by development using AZ 726 MIF for 6 min. DRIE step is executed on the top side including SiN (ARC) layer and where the whole silicon is etched using Bosch process[90] of successive deposition and etching cycles (~650 cycles) using cryogenic cooling at −20 °C for smooth sidewalls. The initial Ar plasma strike was performed using10 sccm C4F8, 20 sccm SF6, and 50 sccm Ar at 15 mTorr, 2000 W ICP, and

20 W RF for 10 s. Principal etching of Si surface takes the following reaction:

− − 푆퐹6 + 푒 → 푆퐹3 + 3퐹 + 푒 (1)

푆푖(푠표푙𝑖푑) + 4퐹 → (푆푖퐹3)(𝑔푎푠) (2)

The deposition step was composed of 100 sccm C4F8 and 5 sccm SF6 at 30 mTorr,

1300 W ICP, and 5 W RF for 3 s and the etching step was composed of 5 sccm C4F8 and 103

100 sccm SF6 at 30 mTorr, 1300 W ICP, and 35 W RF for 8 s. Plasma ploymerization reaction is:

− 퐶4퐹8 + 푒 → 2퐶2퐹4 → 퐶퐹2 → 퐶퐹3 → (3)

퐶2퐹4, 퐶퐹2 → (퐶퐹)푥 (4)

Finally, Kapton tape was peeled out and the wafer was cleaned using Acetone and

IPA for PR removal.

3.4.2 Material investigation

The mass spectra were recorded on interconnects post the DRIE process using

Secondary Ion Mass Spectrometry (Dynamic SIMS) instrument from Hiden Analytical company (Warrington-UK) operated under ultra-high vacuum conditions, typically 10-9

+ Torr. A continuous O2 beam of 2 keV energy was employed to sputter the surface while the selected ions were sequentially collected using a MAXIM spectrometer equipped with a quadrupole analyzer.

3.4.2 The laser-based dicing method

To dice the solar cells into area of 12715 mm2, we used 1.06 m ytterbium-doped fiber laser (PLS6MW Multi-Wavelength Laser Platform, Universal Laser Systems, USA).

The laser interacting with the material strongly depends upon the wavelength, speed, power level of the laser and the absorption characteristic the material. The calibration of the laser dicing parameters is carried out to obtain sufficient beam penetration through the entire thickness (260 m) of the device as shown in (Figure S3). Based on the sharp 104 profile of the laser beam with power with 100%, speed of 60% and 500kHz is used for the cells dicing. Finally, we gently cleaved cells by hand.

3.4.3 Device characterization

The devices were first characterized in the dark and under illumination of a solar simulator (ORIEL A.M. 1.5 Global Spectrum with 100 mW cm− 2 intensity. Prior each measurement, the intensity of the solar simulator was calibrated with a reference Si solar cell and a readout meter for solar simulator irradiance (Newport). J-V characteristics were recorded using a Keithley 2420-C Source Meter.

All the measured samples are individual sub-cells diced using the laser including the rigid cells and the ones which we have applied our process on them. The available Keithley can’t measure the high current generated from a 5-inch wafer. Thus, we had to dice the wafer into individual cells (15×127 mm2).

Moreover, we used a four-point probe setup to reduce the resistance associated with the high current collected from each of the individual cells. 105

Figure 3.20 The individual cells (15×127 mm2) for rigid (top) and its flexible version

(bottom)

Glass bending curvatures with radius of 9, 6, 3 and 2.5 cm used for the electrical bending study. Scanning Electron Microscope imaging is used at acceleration voltage of 5 kV.

3.4.4 System on chip (SOC)

The BLE module comes with integrated antenna, crystal & Flash/EEPROM. The circuit was printed on a flexible Polyimide substrate, followed by bonding of paper sensor and BLE chip on the substrate.

106

3.5 Conclusion

In conclusion, corrugation architecture enabled ultra-flexible, high performance c-

Si solar cells were demonstrated on a five-inch wafer via a lithography less CMOS compatible technique. The flexible cells have the power conversion efficiency of 17.2% which is comparable to the rigid IBC module efficiency. The corrugation approach consists of active solar cells array (each cell area ~ 1276.2 mm2) and array of grooves (each groove area= 127~0.86 mm2) while the active solar cells are connected via IBC screen printed Aluminium which plays the key role as the carrier for the solar cells segments. The screen-printed metals can withstand high strain exceeds 20%. Moreover, a bending radius of 140 m can be achieved with the groove width of 0.86 mm. Hence, the flexible cells showed a consistent electrical performance for the convex and concave bending after the projection area calculations are taken in consideration. Furthermore, the whole five-inch wafer can be deformed in zigzag and a bifacial module. Moreover, the flexible cells show a high mechanical stability over 1000 cyclic bending. We have demonstrated the integration of the several flexible cells in series with a flexible LEDs matrix and full functional system using indoor illumination. Our corrugation flexible solar cells can broaden the application spectrum and may lead towards unconventional areas of applications.

107

4 3D models of c-Si solar cell

Nonplanner solar cells are capable of capturing sunlight from all the directions compared to the traditional flat wafers based technology. The efficiency of wafer-based technology relies on their relative position to the sun. Therefore, some advances in this field addressed this issue with motorized frames that follow the sun’s path.

Here, we create a new approach that depends on newly designed 3D shaped c-Si solar cells which can take a dome and Kirigami shaped designs. Thus, we overcome the need of using expensive motors.

Our approach depends highly on exploring the laser cutting and engraving techniques in addition to the DRIE in order to transfer the new designs on the wafers to transform them into 3D shapes.

4.1 Introduction

Conventional planar solar cell modules have a limited ability to capture the maximum protentional of the solar energy due to the misalignment angle between the modules and the sun called optical coupling losses as illustrated in Figure 4.1. The projected area (W) of the solar cell is decreased and scaled down due to the relative position of the sun to the cell [91, 92]. Advanced research to maximize the solar power output and the mitigate the optical coupling losses depends on tilting the position of the panel using tracking system over the course of the day and/or the year.[93-95] 108

The geographic location of the system defines if the solar panel tilting motors require one/two tracking axes as the conventional trackers can boost the yearly solar power between 20-40% compared to the planar solar panels as shown in table 4-1. Also, the integration of the concentrators with solar trackers supports further absorption factor of a wide range of sun angles. [96-101] On the other hand, solar tracking systems enhance the total cost by ~12%, weight and requires additional space to align the panels.[102] Thus, the implementation of the solar tracking systems is limited worldwide. On contrast, the solar panels costs keep decreasing to meet the global demand.

Figure 4.1 The coupling efficiency c Vs the source angle.

• The following content is on-going to be published as:

R. R. Bahabry, A. T. Kutbee, N. Qaiser, S. Shaikh, S. Khan, M. M. Hussain, "Dome shaped c-Si solar cells for enhanced coupling efficiency”.

109

4.2 Experimental

The required 3D dome-shaped solar cells are going to be fabricated using the IBC solar cells transformed into 3D shape via applying the four-way grid dome design [103] illustrated in figure 4.2. This design is going to be printed using two main experimental methods: laser engraving and microlithography.

Figure 4.2 four-way grid dome design

4.2.1 Laser engraving for low-cost design printing

A laser is a device that produces a beam of coherent light through an optical amplification process based on stimulated emission of electromagnetic radiation. There are many types of lasers share the basic components set including: gas lasers, fiber lasers, solid state lasers, dye lasers, diode lasers and excimer lasers.

The laser interaction with materials depends strongly upon the laser energy, and the absorption prperties of the material.

Common wavelengths for laser material processing are 10.6-9.3 m emitted by 110

CO2 lasers and 1.06 m produced by fiber lasers. However, the material absorption characteristics and its chemical composition influence highly the results of the

laser/material interaction. CO2 lasers with 10.6-micron wavelength are primarily used for

material removal to engrave non-metal materials.CO2 lasers are not typically used for metal engraving because most of the laser energy is reflected. However, fiber lasers with

1.06-micron wavelength can be used for shallow engraving into metal.

The laser speed impact is investigated using Si (100) at 40, 60, 80, 100%.

Additionally, the laser power engraving of Si (100) is studied at 30,40, 50,60,70,80,90,

100%. Then, the etching depth profiler is measured using the mechanical profiler.

4.2.2 Microlithography

The same previous step used on the backside yielding 10 m-thick PR on both faces. Then a 5” chrome mask is used to transfer the four-way grid dome on the IBC wafer solar cell as illustrated in Figure 4.3. 111

Figure 4.3 four-way grid dome mask

The wafer was exposed to broadband stepper at 1.800 mJ/cm2 followed by development using AZ 726 MIF for 6 min. DRIE step is executed on the top side including

SiN (ARC) layer and where the whole silicon is etched using Bosch process [90].

4.3 Results and Discussion

4.3.1 Design printed via the Laser engraving 112

Figure 4.4 laser engraving profiler for different laser power and speed values.

In order to optimize The laser engraving parameters on Si (100). Both of the laser power and speed etching depth profile is shown in Figure 4.4. The maximum engraving profiler is achieved at 60 and 100 % speed and power respectively.

The optimized laser parameters are applied on the IBC solar wafer which has a silicon thickness of 240 um . 113

Figure 4.5 Optical photos of the laser engraving method to transfer the four-way grid

dome on IBC solar cells.

Figure 4.5 presents the optical photos of the laser engraving method to transfer the

four-way grid dome on IBC solar cells. The etched wafers are fully flexible along x-

direction and semi-flexible along y-direction. Moreover, the IBC contacts become

extra weak to carry the solar cell array due to heat dissipation process during the laser

etching process.

114

4.3.2 Design transferred via lithography

The resulting wafer shows high flexibility and can be mounted on a 3D printed dome as shown in Figure 4.6.

Figure 4.6 Optical photo of the dome-shaped c-Si solar cell.

4.3.3 Electrical characteristics of the dome-shaped c-Si cells

The J–V characteristics are measured under a solar simulator (approximately the

A.M. 1.5 Global Spectrum with 100 mW cm− 2 intensity and spectral mismatch correction at the room temperature). Figure 4.7 plots the electrical characteristics of the solar cells distributed on a 3D printed dome-shaped substrate. As expected, the Jsc is the most affected parameter compared to the flat sample as the solar simulator is perpendicular 115 to position of the the sample. Therefore, studying the generated power of the cells is done un the sunlight through the time of the day. Figure 4.8 shows both of the current and voltage values of the cells distributed on a dome-shaped sphere compared to the same cells as a flat plate. The results illustrate that the sphere shape cells have a higher generated current compared to the flat samples. The effect of dome-shaped geometry on the power generation compared with the fixed planar cells is in Figure 4.9.

Figure 4.7 Electrical characteristics of the solar cells distributed on a 3D printed dome-

shaped substrate.

116

Dome shaped 0.6 Flat plate

0.5

0.4

0.3 Voltage (V) Voltage 0.2

0.1

0.0 6AM 8 AM 10 AM 12 PM 2 PM 4 PM 6 PM Time of the day

800 Dome shaped 700 Flat plate

600

500

400

300 Current (mA) Current

200

100

0 6AM 8 AM 10 AM 12 PM 2 PM 4 PM 6 PM Time of the day

Figure 4.8 Current and voltage values of the cells distributed on a dome-shaped sphere compared to the same cells as a flat plate. 117

500

400

300

Dome shaped

200 Flat plate Power (mW) Power

100

6AM 8 AM 10 AM 12 PM 2 PM 4 PM 6 PM Time of the day

Figure 4.9 The effect of dome-shaped geometry on the power generation compared with the fixed planar cells

118

Table 4-1 Electrical characteristics of the dome-shaped c-Si cells vs the fixed planar cells

Current Voltage Power Current Voltage Power Time of mA V mW mA V mW the day dome-shaped Flat plate

6AM 10.24 0.357 3.65568 8.22 0.399 3.27978

8 AM 668 0.559 373.412 645 0.556 358.62

10 AM 704 0.566 398.464 681 0.566 385.446

12 PM 738 0.575 424.35 702 0.576 404.352

2 PM 714 0.571 407.694 690 0.56 386.4

4 PM 668 0.559 373.412 645 0.556 358.62

6 PM 75.13 0.451 33.88363 66.5 0.446 29.659

119

4.3.4 Kirigami design c-Si solar cells

Kirigami is an altered art of origami which is the art of paper folding. Both words are derived from the Japanese words for cutting (kiru), folding (oru), and paper (kami).

Folding and cutting of paper to create ceremonial or decorative objects date back centuries in Asia and Europe. The modern notion of origami as an art and recreational craft (and eschewing all cuts in its purest form) took shape last century.

The origami usually atart by using a squared pristine sheet of paper which evolves through multiple folding steps where cutting and gluing are not allowed. On the other hand, kirigami depends strongly on the cutting and folding steps. Designs range from flat, symmetrical cut-out decorations like schoolroom snowflakes to elaborate patterns that form 3D models similar to book pop-ups. 120

Figure 4.10 polyamide sheet 0.25 m after cutting using Co2(a) flat (b) stretched

Figure 4.11 flexible c-Si solar cells mounted on polyamide sheet.

121

4.4 Conclusion

We have shown that 3D dome-shaped c-Si cells for the first time which can be used as a low-cost, lightweight approach to enhance the solar generated power. This geometry is advantages over the conventional mechanisms that requires heavy motors/supports which allow the relative position of the sun. Both of the current and voltage are enhanced benefits from this geometry which led to higher power generation.

Furthermore, we show an alternative approach to enable deformability through kirigami c-Si solar cells.

122

5 CONCLUSIONS AND FUTURE DIRECTION

For future work, integrating the developed NiSi/Cu formation into high efficiency commercial solar cell will be promising. For the thin flexible silicon, more study on the effect of different texturing techniques on rear side cells would help to further increase the solar cell efficiency.

In this work, we are transforming a monocrystalline silicon solar cell into its ultra- flexible version without any additional components while preserving the original efficiency of the cell using the corrugation architecture method. In our approach, the screen-printed IBC which is a main layer of the solar cell structure serves as both the carrier of the c-Si segments and metal interconnection. Our process offers new possibilities in the flexible electronics as the screen-printed metals has fracture strain >20 which allows high flexibility and complete foldability as shown in Figure 1F. Moreover, the resulting ultra-flexible cells have a total thickness in the micrometers range. We believe that our approach is suitable for IoT era and can meet the needs of its wide application spectrum.

We have demonstrated an example of 3D c-Si solar cells including dome shape and kirigami geometry. These nonplanar structures can enhance the solar-generated power while maintaining reasonably good performance. This approach opens an alternative direction of producing flexible, stretchable, and deformable electronics.

123

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136

PUBLICATIONS

Papers and conferences discussed in this dissertation:

1 Rabab R. Bahabry, Arwa T. Kutbee, Sherjeel Khan, Adrian C. Sepulveda,

Irmandy Wicaksono, Maha Nour, Nimer Wehbe, Amani S. Almislem, Mohamed

T. Ghoneim, Galo A. Torres Sevilla, Ahad syed, Sohail F. Shaikh and Muhammad

M. Hussain . “Corrugation Architecture Enabled Ultra-Flexible Wafer-Scale

High-Efficiency Mono-crystalline Silicon Solar Cell”. (DOI:

10.1002/aenm.201702221)

2 Rabab R. Bahabry and Muhammad M. Hussain. U.S. Patent Provisional

Application filed KAUST Ref: 2017-133-01 (JULY 14, 2017)

3 R. R. Bahabry, A. Hanna . T. Kutbee, N. Wehbe, M. M. Hussain, “Enhanced

efficiecny of c-Si solar cells using NiSi/Cu rear contacts” submitted to Energy

Technology (Wiley VCH).

4 R. R. Bahabry, A. T. Kutbee, N. Qaiser, S. Shaikh, S. Khan, M. M. Hussain,

"Dome shaped c-Si solar cells for enhanced coupling efficiency” (on-going).

5 R. R. Bahabry, A. Gumus, A. T. Kutbee, N. Wehbe, S. M. Ahmed, M. T.

Ghoneim, K.–T. Lee, J. A. Rogers, M. M. Hussain, “Current Enhancement in

Crystalline Silicon Photovoltaic by Low-Cost Nickel Silicide Back Contact”, 43RD

IEEE PHOTOVOLTAIC SPECIALISTS CONFERENCE (IEEE PVSC 2016), Portland,

OR, USA 5 – 10 June, 2016 137

6 R. R. Bahabry, A. Gumus, A. T. Kutbee, N. Wehbe, S. M. Ahmed, M. T.

Ghoneim, K.–T. Lee, J. A. Rogers, M. M. Hussain, “Novel Contact Engineering

for Low-Cost and Competitive Efficiency Crystalline Silicon Photovoltaics using

Nickel Mono-Silicide Metallization” KAUST-NSF 2016, Thwall, SA

7 R. R. Bahabry, A. Gumus, A. T. Kutbee, N. Wehb, S. M. Ahmed, M. T.

Ghoneim, K.–T. Lee, J. A. Rogers, M. M. Hussain, “Toward Efficient and Low-

Cost Metallization for Crystalline Silicon Photovoltaics”, 2016 C3E Women in

Clean Energy Symposium, Stanford University in Palo Alto, California, on May

31, 2016

8 M. M. Hussain, R. R. Bahabry, J. P. Rojas, D. P. Singh, A. C. Sepulveda, I.

Wicaksono, A. T. Kutbee, “Freeform Thin Film Energy Harvesters”, Thin Films

2016, Singapore, Singapore, 12 – 15 July 2016. [Session on Functional Thin

Films for Energy Photonics.

9 R. R, Bahabry, M. M. Hussain, “Mono-crystalline Bulk Silicon Based High-

Efficiency Flexible Solar Cell”, TMS Middle East - Mediterranean Materials

Congress on Energy and Infrastructure Systems (MEMA 2015), January 11-14,

2015 • Doha, Qatar

10 Rabab Bahabry, Jhonathan Rojas, Aftab Hussain, Muhammad Mustafa

Hussain, “Novel interdigitated back contact for flexible mono-crystalline

silicon solar cells”, MRS Fall Meeting 2014, Boston, MA, USA 138

11 Rabab R. Bahabry, Jhonathan P. Rojas, Aftab Hussain, Muhammad M.

Hussain, “Silicidation In Flexible Si Based Photovoltaics For Enhanced PV

Efficiency, KAUST-NSF 2015, Thwall, SA

12 A. T. Kutbee, R. R. Bahabry, S. Shaikh, S. Khan, G. A. Torres Sevilla, M. M.

Hussain, "Energy and power management of IOT", Energy Technology (Wiely

VCH), (Invited), Manuscript prepared

Other papers and conferences not discussed in this dissertation:

1 A.T. Kutbee, R. R. Bahabry, K. O. Alamoudi, M. T. Ghoneim, M. D. Cordero, A.

S. Almuslem, A. Gumus, E. M. Diallo, J. M. Nassar, A. M. Hussain, N. M.

Khachab, M.M. Hussain, “Flexible And Biocompatible High Performance Solid-

State Micro-Battery For Implantable Orthodontic System”, Nature Flexible

Electronics (npj), (2017), Accepted and In press

2 S. F. Shaikh, M. T. Ghoneim, R. R. Bahabry, S. M. Khan, M. M. Hussain*,

“Modular Lego-Electronics”, Adv. Mater. Tech. Accepted and In press

3 A. S. Almuslem, A. N. Hanna, T. Yapici, N. Wehbe, E. M. Diallo, A. T. Kutbee, R.

R. Bahabry, M. M. Hussain, “Water Soluble Nano-scale Transient Material

Germanium Oxide For Zero Toxic Waste Based Environmentally Benign Nano-

Manufacturing”, Appl. Phys. Lett. 110, 074103 (2017).

4 K.-T. Lee, Y. Yao, J. He, B. Fisher, X. Sheng, M. Lumb, L. Xu, M. A. Anderson, S.

Han, Y. Kang, A. Gumus, R. R. Bahabry, J. W. Lee, U. Paik, N. D. Bronstein, A. P.

Alivisatos, M. Meitl, S. Burroughs, M. M. Hussain, J. C. Lee, R. G. Nuzzo, J. A. 139

Rogers, “Concentrator Photovoltaic Module Architectures With Capabilities

for Capture and Conversion of Full Global Solar Radiation”, Proc. Natl. Aca.

Sci. doi: 10.1073/pnas.1617391113

5 A. N. Hanna, M. T. Ghoneim, R. R. Bahabry, A. M. Hussain, H. M. Fahad, M. M.

Hussain, “Area and Energy Efficient High Performance ZnO Wavy Channel Thin

Film Transistor”, IEEE Trans. Elect. Dev. 61(9), 3223–3228 (2014).

6 A. N. Hanna, M. T. Ghoneim, R. R. Bahabry, A. M. Hussain, H. M. Fahad, M. M.

Hussain, “Zinc Oxide Integrated Area Efficient High Performance Wavy

Channel Thin Film Transistor”, Appl. Phys. Lett. 103, 224101 (2013).

7 A. N. Hanna, M. T. Ghoneim, R. R. Bahabry, A. M. Hussain, H. M. Fahad, M. M.

Hussain, “Wavy Channel Thin Film Transistor Architecture for Area Efficient,

High Performance and Low Power Displays”, Phys. Status Solidi (RRL) 8(3),

248–251 (2013) [Cover Page].

8 M. M. Hussain, J. P. Rojas, G. A. Torres Sevilla, M. T. Ghoneim, A. M. Hussain,

S. M. Ahmed, J. M. Nassar, R. R. Bahabry, M. Nour, A. T. Kutbee, E. Byas, B. Al-

Saif, A. M. Alamri, “Transformational electronics: a powerful way to

revolutionize our information world”, Proc. SPIE 9083, Micro- and

Nanotechnology Sensors, Systems, and Applications VI, 90831K (June 4, 2014);

doi:10.1117/12.2050103, Baltimore, Maryland, USA

9 J. P. Rojas, G. A. Torres Sevilla, M. T. Ghoneim, A. M. Hussain, S. M. Ahmed, J.

M. Nassar, R. R. Bahabry, M. Nour, A. T. Kutbee, E. Byas, B. Al-Saif, A. M.

Alamri, M. M. Hussain, “Transformational electronics: a powerful way to 140 revolutionize our information world”, Proc. SPIE 9083, Micro- and

Nanotechnology Sensors, Systems, and Applications VI, 90831K (June 4, 2014); doi:10.1117/12.2050103, Baltimore, MD, USA [Invited]