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Development of wide-band gap indium gallium nitride solar cells for high-efficiency photovoltaics

Dissertation
Author: Omkar K. Jani
Abstract:
Main objective of the present work is to develop wide-band gap InGaN solar cells in the 2.4-2.9 eV range that can be an integral component of photovoltaic devices to achieve efficiencies greater than 50%. The III-nitride semiconductor material system, which consists of InN, GaN, AlN and their alloys, offers a substantial potential in developing ultra-high efficiency photovoltaics mainly due to its wide range of direct-band gap, and other electronic, optical and mechanical properties. However, this novel InGaN material system poses challenges from theoretical, as well as technological standpoints, which are further extended into the performance of InGaN devices. In the present work, these challenges are identified and overcome individually to build basic design blocks, and later, optimized comprehensively to develop high-performance InGaN solar cells. One of the major challenges from the theoretical aspect arises due to unavailability of a suitable modeling program for InGaN solar cells. As spontaneous and piezoelectric polarization can substantially influence transport of carriers in the III-nitrides, these phenomena are studied and incorporated at a source-code level in the PC1D simulation program to accurately model InGaN solar cells. On the technological front, InGaN with indium compositions up to 30% (2.5 eV band gap) are developed for photovoltaic applications by controlling defects and phase separation using metal-organic chemical vapor deposition. InGaN with band gap of 2.5 eV is also successfully doped to achieve acceptor carrier concentration of 10 18 cm-3 . A robust fabrication scheme for III-nitride solar cells is established to increase reliability and yield; various schemes including interdigitated grid contact and current spreading contacts are developed to yield low-resistance Ohmic contacts for InGaN solar cells. Preliminary solar cells are developed using a standard design to optimize the InGaN material, where the band gap of InGaN is progressively lowered. Subsequent generations of solar cell designs involve an evolutionary approach to enhance the open-circuit voltage and internal quantum efficiency of the solar cell. The suitability of p-type InGaN with band gaps as low as 2.5 eV is established by incorporating in a solar cell and measuring an open-circuit voltage of 2.1 V. Second generation InGaN solar cell design involving a 2.9 eV InGaN p-n junction sandwiched between p- and n-GaN layers yields internal quantum efficiencies as high as 50%; while sixth generation devices utilizing the novel n-GaN strained window-layer enhance the open circuit voltage of a 2.9 eV InGaN solar cell to 2 V. Finally, key aspects to further InGaN solar cell research, including integration of various designs, are recommended to improve the efficiency of InGaN solar cells. These results establish the potential of III-nitrides in ultra-high efficiency photovoltaics.

TABLE OF CONTENTS

ACKNOWLEDGEMENTS .......................................................................................... iv LIST OF TABLES .......................................................................................................vii LIST OF FIGURES ................................................................................................... viii LIST OF SYMBOLS and ABBREVIATIONS ............................................................ xi SUMMARY ................................................................................................................ xiii 1. INTRODUCTION AND RESEARCH OBJECTIVES............................................ 1 1.1

T HE STRUGGLE FOR ‘ POWER ’...................................................................... 1

1.1.1 Consequences of burning oil ....................................................................................... 1 1.1.2 Harnessing the power of the Sun ................................................................................ 4 1.1.3 High efficiency for economic viability........................................................................ 5 1.1.4 VHESC – An ultra-high efficiency approach ............................................................ 8 1.2

S PECIFIC RESEARCH OBJECTIVES ................................................................ 9

1.2.1 Task 1: Develop an accurate modeling tool for III-nitride solar cells ................. 10 1.2.2 Task 2: Optimize MOCVD growth of InGaN for band gaps as low as 2.4 eV 10 1.2.3 Task 3: Develop efficient fabrication scheme for InGaN solar cells ................... 11 1.2.4 Task 4: Understand loss mechanisms in InGaN solar cells due to material quality and fabrication issues .................................................................................... 12 1.2.5 Task 5: Design efficient InGaN solar cell in the 2.4 – 2.9 eV range ................... 12 1.3

S UMMARY .................................................................................................... 12

2. InGaN FOR HIGH-EFFICIENCY PHOTOVOLTAICS ..................................... 14 2.1

E FFICIENCY LIMITS IN A SINGLE JUNCTION SOLAR CELL ....................... 14

2.2

H IGH - EFFICIENCY APPROACHES .............................................................. 16

2.2.1 Tandem solar cells ....................................................................................................... 16 2.2.2 Quantum-well solar cells ............................................................................................ 17 2.2.3 Other high-efficiency concepts ................................................................................. 19 2.3

T HE I N G A N MATERIAL SYSTEM ............................................................... 20

2.4

C HALLENGES IN THE I N G A N TECHNOLOGY ......................................... 22

2.4.1 Substrates and crystalline quality ............................................................................... 22 2.4.2 Phase separation .......................................................................................................... 24 2.4.3 P-type doping ............................................................................................................... 25 2.4.4 Ohmic contact to p-InGaN ....................................................................................... 26 2.5

S UMMARY .................................................................................................... 28

v 3. EXPERIMENTAL APPROACH AND DESIGN OF InGaN SOLAR CELLS .... 30 3.1

G ENERAL SOLAR CELL DESIGN PRINCIPLES ............................................ 30

3.1.1 Light absorption ............................................................................................................... 30 3.1.2 Collection efficiency ........................................................................................................ 32 3.1.3 Open-circuit voltage .................................................................................................... 35 3.1.4 Resistance...................................................................................................................... 35 3.2

P RELIMINARY I N G A N SOLAR CELL DESIGN ........................................... 36

3.3

S IMULATION OF I N G A N SOLAR CELLS .................................................... 39

3.3.1 PC1D files for III-nitrides .......................................................................................... 40 3.2.2 Simulation of test GaN/InGaN solar cell ............................................................... 41 3.4

S UMMARY .................................................................................................... 43

4. POLARIZATION MODEL FOR InGaN SOLAR CELLS .................................... 44 4.1

T HEORY OF POLARIZATION M ODEL IN III- NITRIDES ........................... 44

4.1.1 Origin of polarization in the III-nitrides .................................................................. 45 4.1.2 Spontaneous polarization ........................................................................................... 46 4.1.3 Strain model and piezoelectric polarization ............................................................. 48 4.1.4 Effect of net polarization ........................................................................................... 52 4.2

I MPLEMENTATION OF POLARIZATION IN PC1D .................................... 53

4.2.1 PC1D solver method .................................................................................................. 53 4.2.2 Incorporation of polarization parameters ................................................................ 55 4.2.3 Modification of PC1D solver routine ....................................................................... 57 4.3

S OME IMPORTANT POLARIZATION RESULTS ........................................... 58

4.3.1 Spontaneous polarization in InGaN/GaN p-i-n solar cell ................................... 58 4.3.2 Piezoelectric polarization in GaN window layers ................................................... 63 4.4

S UMMARY .................................................................................................... 66

5. EPITAXIAL GROWTH AND FABRICATION OF InGaN SOLAR CELLS ....... 67 5.1

MOCVD GROWTH OF G A N TEMPLATES ................................................ 68

5.1.1 MOCVD growth apparatus ....................................................................................... 68 5.1.2 Epitaxy of GaN templates.......................................................................................... 71 5.2

E PITAXY OF I N G A N FOR SOLAR CELLS ................................................... 73

5.2.1 Preliminary growth of InGaN ................................................................................... 73 5.2.2 Suppression of phase separation in InGaN ............................................................. 75 5.2.3 Absorption coefficient of InGaN ............................................................................. 78 5.3

F ABRICATION OF I N G A N SOLAR CELLS .................................................. 82

5.3.1 Mg-activation in InGaN ................................................................................................. 82 5.3.2 Metal contact schemes ................................................................................................ 82 5.3.3 Device processing ........................................................................................................ 84 5.4

S UMMARY .................................................................................................... 87

6. InGaN SOLAR CELL RESULTS ........................................................................... 88 6.1

P RELIMINARY I N G A N SOLAR CELL RESULTS .......................................... 88

6.1.1 Suitability of InGaN for photovoltaics .................................................................... 88 6.1.2 Identification of major loss mechanisms in InGaN solar cells ............................. 96

vi 6.1.3 Demonstration of 2.5 eV p-type InGaN for solar cells ......................................... 98 6.2

A DVANCED I N G A N SOLAR CELLS ......................................................... 101

6.2.1 Evolution of InGaN solar cell design ................................................................... 101 6.2.2 Optimization of n-GaN strained window layer ................................................... 107 6.3

S UMMARY .................................................................................................. 108

7. CONCLUSION ..................................................................................................... 109 7.1

O VERVIEW OF CONTRIBUTION IN I N G A N PHOTOVOLTAICS ............. 109

7.2

R ECOMMENDATION FOR FURTHER WORK ............................................ 113

7.2.1 Integration of present work .................................................................................... 113 7.2.2 Optimize the top contacting scheme ..................................................................... 114 7.2.3 Lower band gap of InGaN to increase absorption ............................................. 114 7.2.4 Improve convergence of the modified PC1D software ..................................... 114 7.3

S UMMARY .................................................................................................. 115

APPENDIX A: PC1D files for III-nitrides ................................................................. 116 APPENDIX B: Fabrication procedure for InGaN/GaN solar cells ......................... 122 BIBLIOGRAPHY ...................................................................................................... 126 PUBLICATION LIST ............................................................................................... 136

vii

LIST OF TABLES

Table 2.1: Detailed balance calculations of band gaps and achievable efficiencies of 3 to 8 stack tandem solar cells under black body radiation at 6000K, 500x ......... 20 Table 2.2: Lattice mismatch and thermal expansion coefficient mismatch of GaN with common substrates. .............................................................................................. 23

Table 4.1: Spontaneous polarization coefficients in III-nitrides. .............................................. 48 Table 4.2: Coefficients pertaining to piezoelectric polarization III-nitrides. ........................... 52

Table 5.1: Fitting parameters used for absorption coefficient of InGaN samples of band gap 2.95 eV (Sample 1) and 2.45 eV (Sample 2). ............................................. 81 Table 5.2: Optimized anneal conditions for InGaN. .................................................................. 82

Table 6.1: Summary of InGaN test solar cell performance for consecutive generations. ................................................................................................................... 106

Table 7.1: Summary of contribution to InGaN photovoltaics in present work. .................. 111

viii

LIST OF FIGURES

Figure 1.1: Historic perspective of global atmospheric concentrations of (a) carbon dioxide, and (b) methane. ............................................................................................ 2 Figure 1.2: Multi-model averages and assessed ranges predicted for global warming. ......................................................................................................................................... 2 Figure 1.3: Annual production scenarios of world crude oil production with 2% growth rates and different resource levels. ............................................................... 3 Figure 1.4: World PV cell/module ................................................................................................ 5 Figure 1.5: Efficiency and cost projections for first, second and third generation PV. .................................................................................................................................. 6 Figure 1.6: Best research-cell efficiencies. .................................................................................... 7 Figure 1.7: Schematic of the architecture of VHESC. ................................................................ 8 Figure 1.8: Predicted contributions of each solar cell in the proposed VHESC design.............................................................................................................................. 9

Figure 2.1: Major loss processes in a single-junction solar cell under forward bias: ............ 14 Figure 2.2: Concept of tandem cell. ............................................................................................. 17 Figure 2.3: Quantum-well solar cell structure and band diagram. .......................................... 18 Figure 2.4: Band gap Vs. lattice constant of common semiconductor materials. ................. 21 Figure 2.5: Schematic comparison of band structures of (a) an ideal material, and (b) a phase separated material. .................................................................................. 24 Figure 2.6: Work function of common metals used to obtain Ohmic contacts with respect to conduction and valence band energies of InxGa1-xN for 0

Figure 3.1: Geometrical comparison of (a) smooth surface, and (b) textured surface in solar cells indicating higher absorption for textured surface due to lower reflection and increase in optical path length. ................................. 32 Figure 3.2: Role of (a) p+-region, and (b) window layer for generating front surface fields and passivation.................................................................................... 33 Figure 3.3: (a) A p-n solar cell compared to (b) a p-i-n solar cell illustrating extension of depletion region electric field. ............................................................ 34 Figure 3.4: Simulated band diagram of InGaN p-i-n solar cell with i-region thickness of .................................................................................................................. 37 Figure 3.5: Optimized structure of an InGaN (a) p-i-n solar cell, and (b) quantum- well ................................................................................................................................ 38 Figure 3.6: GaN/InGaN test device for PC1D. ........................................................................ 41

ix Figure 3.7: PC1D simulation results for test GaN/InGaN device indicating (a) I-V characteristics, (b) band diagram at maximum power, and (c) quantum efficiency. ..................................................................................................................... 42

Figure 4.1: (a) Ga-face, and (b) N-face GaN ............................................................................. 45 Figure 4.2: Direction of spontaneous polarization jn Ga-face GaN. ..................................... 47 Figure 4.3: Schematic representation of (a) biaxial strain in the basal plane on a crystal, and (b) resultant deformation. ..................................................................... 50 Figure 4.4: Strain relaxation profile of GaN on AlN template for (a) perfectly pseudomorphic epilayer, and (b) epilayer relaxed by 95% ................................... 51 Figure 4.5: Polarization parameters dialogue box...................................................................... 55 Figure 4.6: PC1D user interface identifying the three methods to access polarization model : (1) by clicking on the ‘Polarization Model Disabled’ line in the ‘Parameter View’, (2) by selecting ‘polarization…’ under the ‘Device’ tab of the command bar, or (3) directly through the toolbar. ......................................................................................................................... 56 Figure 4.7: GaN/InGaN p-i-n test structure (a) without and (b) with spontaneous polarization effects. .................................................................................................... 59 Figure 4.8: Comparison of p-GaN/u-InGaN interface for two cases, (a) without, and (b) with spontaneous polarization through their (c) and (d) band structures, (e) and (f) carrier densities, and (g) and (h) electric fields at zero bias calculated using the modified PC1D software. ..................................... 60 Figure 4.9: Comparison of u-InGaN/n-GaN interface for two cases, (a) without, and (b) with spontaneous polarization through their (c) and (d) band structures, (e) and (f) carrier densities, and (g) and (h) electric fields at zero bias calculated using the modified PC1D software. ..................................... 62 Figure 4.10: Schematic of a p-InGaN layer with a strained p-GaN cap layer indicating the direction of spontaneous and piezoelectric polarization. ............ 63 Figure 4.11: Energy band and electric field diagram for p-GaN/p-InGaN heterostructure for p-GaN layer under variable strain relaxation. ...................... 64

Figure 5.1: (a) Photograph, and (b) schematic of the Emcore MOCVD growth reactor. ......................................................................................................................... 69 Figure 5.2: (a) Chronological temperature profile, and (b) in-situ reflectometry data for a typical two-step GaN template growth. ......................................................... 72 Figure 5.3: Summary of X-ray diffraction data for InGaN grown by MOCVD with indium composition ranging from 0 to 35%. ......................................................... 73 Figure 5.4: Photoluminescence vs. indium composition for InGaN grown by MOCVD. ..................................................................................................................... 74 Figure 5.5: Summary of FWHM for (0002) ω-scan and PL for InGaN grown by MOCVD as a function of indium composition. .................................................... 75 Figure 5.6: (a) XRD scans and (b) indium compositions for InGaN grown at variable TEGa flow rates. ......................................................................................... 76 Figure 5.7: PL of In0.07Ga0.93N grown at variable TMIn flow rates................................... 77 Figure 5.8: PL obtained for InGaN with variable thickness .................................................... 77 Figure 5.9: Measured transmission and absorption data for InGaN of band gap 2.95 eV (Sample 1) and 2.45 eV (Sample 2). .......................................................... 79

x Figure 5.10: Measured absorption coefficient and theoretical fit for InGaN samples of band gap 2.95 eV (Sample 1) and 2.45 eV (Sample 2). .................................... 80 Figure 5.11: Contacting schemes used for InGaN solar cells that involve (a) top current spreading layer, (b) interdigitated grid contacts, and (c) solid opaque contacts. ......................................................................................................... 83 Figure 5.12: Typical fabrication process sequence for an (a) InGaN p-i-n solar cell involving (b) mesa etch, 9c) n-contact deposition, (d) n-contact anneal, (e) current spreading layer deposition, (f) p-contact deposition, and (g) p-contact anneal. ......................................................................................................... 85 Figure 5.13: (a) Macroscopic view of fabricated 2” InGaN solar cell wafer, and (b) microscopic view of grid contacts in a device with interdigitated grids. ............ 87

Figure 6.1: Band diagram of p-i-n GaN solar cell with In0.4Ga0.6N quantum wells ......... 89 Figure 6.2: Fabricated preliminary InGaN solar cell. ................................................................ 90 Figure 6.3: (a) XRD and (b) PL of In0.07Ga0.93N p-i-n solar cell, ....................................... 91 Figure 6.4: UV lamp source illumination compared to common light sources. ................... 92 Figure 6.5: I-V characteristics comparison of (a) In0.07Ga0.93N p-i-n solar cell, (b) In0.4Ga0.6N p-i-n solar cell, and (c) In0.4Ga0.6N quantum-well solar cell. ...................................................................................................................... 93 Figure 6.6: Photoemission spectrum from a biased In0.07Ga0.93N p-i-n solar cell. .......... 94 Figure 6.7: Reflection corrected quantum efficiencies of In0.07Ga0.93N p-i-n, In0.4Ga0.6N p-i-n, and In0.4Ga0.6N quantum well solar cells. ......................... 95 Figure 6.8: I-V characteristic of a GaN p-i-n solar cell with In0.05Ga0.95N as the i-region with (a) top current spreading layer, and (b) grid contacts. ................... 96 Figure 6.9: IQE of an In0.05Ga0.95N /GaN p-i-n solar cell with as the i-region. ............. 97 Figure 6.10: Fabricated 2.5 eV In0.28Ga0.72N solar cell........................................................... 99 Figure 6.11: (a) I-V, and (b) QE measurement data of the 2.5 eV In0.28Ga0.72N solar cell. .................................................................................................................... 100 Figure 6.12: Generations of InGaN solar cell evolution. ......................................................... 102 Figure 6.13: Sample I-V characteristics of (a) first-generation, and (b) second generation test solar cells. ........................................................................................ 104 Figure 6.14: Internal quantum efficiency comparison of first and second-generation test solar cells. ........................................................................................................... 104 Figure 6.15: Schematic comparison of band diagrams of (a) fifth (p-GaN window), and (b) sixth-generation (n-GaN window) solar cells. ........................................ 105 Figure 6.16: Comparison of I-V curves for sixth-generation test solar cells employing (a) Ni/Au, and (b) Ti/Al/Ti/Au as top contacts. ........................... 107

Figure 7.1: InGaN solar cell with VOC of 1.858 V measured using a multimeter under a........................................................................................................................ 112 Figure 7.2: Integrated solar cell design to yield higher performance. ................................... 113

xi

LIST OF SYMBOLS and ABBREVIATIONS

(Symbol) (Unit)

(Description)

α cm -1

absorption coefficient α o (eV 1/2 )/cm

absorption coefficient fitting parameter ε -

strain ε x -

strain in direction of basal a-plane of wurtzite crystal ε z -

strain perpendicular to basal a-plane of wurtzite crystal η %

efficiency λ μm, nm

wavelength μ cm 2 /V•s

mobility of electron or hole σ P C/m 2

polarization-induced charge density τ s, ms, μs, ns

minority carrier lifetime Φ eV

work-function φ n V

electron quasi-fermi potential φ p V

hole quasi-fermi potential χ eV

electron affinity ψ V

electrostatic potential

2DEG -

two-dimensional electron gas 2DHG -

two-dimensional hole gas a Å

lattice constant of unit cell a e

Å

lattice constant ‘a’ of epilayer a e ’ Å

strained lattice constant ‘a’ of epilayer a s Å

lattice constant ‘a’ of substrate b (variable)

bowing factor (used during interpolation of band gap, polarization, etc.) c Å

lattice constant of unit cell C 11 GPa

elastic constant C 13 GPa

elastic constant D C/m 2

electric flux density E V/cm

electric field e 31 C/m 2

piezoelectric constant e 33

C/m 2

piezoelectric constant E C eV

conduction band energy level E F

eV

semiconductor fermi energy level EG eV

band gap E i

eV

semiconductor intrinsic energy level E V

eV

valence band energy level FF %

fill factor

xii In x Ga 1-x N

-

indium gallium nitride with indium composition of ‘x’ and gallium composition of ‘1-x’ (0< x< 1) IQE %

internal quantum efficiency I SC A

short-circuit current J SC

A/cm 2

short-circuit current density MBE -

molecular beam epitaxy MOCVD -

metal-organic chemical vapor deposition n s m -2

surface carrier concentration P C/m 2

polarization P PZ C/m 2

piezoelectric polarization P SP C/m 2

spontaneous polarization QE %

quantum efficiency R -

interface relaxation factor (0

strain relaxation coefficient V bi V

built-in potential V OC

V

open-circuit voltage x m

thickness

xiii

SUMMARY

Main objective of the present work is to develop wide-band gap InGaN solar cells in the 2.4 – 2.9 eV range that can be an integral component of photovoltaic devices to achieve efficiencies greater than 50%. The III-nitride semiconductor material system, which consists of InN, GaN, AlN and their alloys, offers a substantial potential in developing ultra-high efficiency photovoltaics mainly due to its wide range of direct-band gap, and other electronic, optical and mechanical properties. However, this novel InGaN material system poses challenges from theoretical, as well as technological standpoints, which are further extended into the performance of InGaN devices. In the present work, these challenges are identified and overcome individually to build basic design blocks, and later, optimized comprehensively to develop high-performance InGaN solar cells. One of the major challenges from the theoretical aspect arises due to unavailability of a suitable modeling program for InGaN solar cells. As spontaneous and piezoelectric polarization can substantially influence transport of carriers in the III-nitrides, these phenomena are studied and incorporated at a source-code level in the PC1D simulation program to accurately model InGaN solar cells. On the technological front, InGaN with indium compositions up to 30% (2.5 eV band gap) are developed for photovoltaic applications by controlling defects and phase separation using metal-organic chemical vapor deposition. InGaN with band gap of 2.5 eV is also successfully doped to achieve acceptor carrier concentration of 10 18 cm -3 . A robust

xiv fabrication scheme for III-nitride solar cells is established to increase reliability and yield; various schemes including interdigitated grid contact and current spreading contacts are developed to yield low-resistance Ohmic contacts for InGaN solar cells. Preliminary solar cells are developed using a standard design to optimize the InGaN material, where the band gap of InGaN is progressively lowered. Subsequent generations of solar cell designs involve an evolutionary approach to enhance the open-circuit voltage and internal quantum efficiency of the solar cell. The suitability of p-type InGaN with band gaps as low as 2.5 eV is established by incorporating in a solar cell and measuring an open-circuit voltage of 2.1 V. Second generation InGaN solar cell design involving a 2.9 eV InGaN p-n junction sandwiched between p- and n-GaN layers yields internal quantum efficiencies as high as 50%; while sixth generation devices utilizing the novel n-GaN strained window-layer enhance the open circuit voltage of a 2.9 eV InGaN solar cell to 2 V. Finally, key aspects to further InGaN solar cell research, including integration of various designs, are recommended to improve the efficiency of InGaN solar cells. These results establish the potential of III-nitrides in ultra-high efficiency photovoltaics.

1

1. INTRODUCTION AND RESEARCH OBJECTIVES

1.1 T HE STRUGGLE FOR ‘ POWER ’ Our history is directed by countless struggles for power, and the victorious, always glorified. Till the last millennium, power, which leads to the notion of supremacy, was appropriately glorified analogous to pseudo-significance of gold. Gold, although signifies prosperity, is ultimately just a shiny chunk of metal – beautiful to gaze at, but has no practical functionality! With the dawn of the new millennium rose the realization of a more specific and practical form of power. This time, prosperity was realized in the form of black-gold, or crude oil. This is now the new form of power, which not only dominates world politics, but also directly supports the grass-root activity of the society and has the potential to uplift human standards of living. However, this era of new power does not come without a new set of challenges – degradation of the environment, long-term sustainability and national security issues. Hence, our struggle for power persists as we explore alternate ways to secure our future.

1.1.1 Consequences of burning oil Power form natural resources such as coal and crude oil produce greenhouse gases as a byproduct. As a result and owing to human activities, global atmospheric concentrations of carbon dioxide, methane and nitrous oxides have increased markedly since 1750, and now

2

Figure 1.1: Historic perspective of global atmospheric concentrations of (a) carbon dioxide, and (b) methane.

Figure 1.2: Multi-model averages and assessed ranges predicted for global warming.

3 far exceed pre-industrial values determined from ice cores spanning many thousands of years as indicated in Figure 1.1 [1]. Moreover, eleven of the last twelve years (1995 -2006) rank among the twelve warmest years in the instrumental record of global surface temperature (since 1850). This warming of the climate system, indicated in Figure 1.2, is unequivocal, as is now evident from observations of increases in global average air and ocean temperatures, widespread melting of snow and ice, and rising global average sea level. Burning conventional energy sources also has a direct impact on humans: each 1% loss of total ozone due to environmental pollution leads to a 3 – 5% increase in skin cancer cases [2]. While humans might manage to bite the bullet momentarily, the disruption in the ecological food chain will ultimately catch up to affect a healthy human life. Our dependence on conventional energy resources has grown so much that it poses a question how well we will be able to cope in their absence. We have already become sensitive to the supply of oil as experienced through volatile petroleum prices. According to

Figure 1.3: Annual production scenarios of world crude oil production with 2% growth rates

4 the United States Geological Survey (USGS) mean resource estimate, conventional crude oil production would be expected to peak in 2037 as shown in Figure 1.3 [3]. While bio-fuels may seem to be a potential solution to the oil shortage, they also contribute to greenhouse gases. Hence, this is the right time to start transitioning our dependence on the correct choice of non-conventional energy. Finally, domestic and international conflicts for oil, like the civil wars in Africa and the ‘Iraq situation’, have become a routine segment in our news today. As nations realize that the cause of these tensions may also be motivated to amass energy resources, the necessity for self reliance for a prosperous and sustainable future becomes obvious. While we wander around gaping left and right, searching the corners of science for alternative energy, the solution lies directly on top of us – the Sun!

1.1.2 Harnessing the power of the Sun The Sun is one of the most essential elements in nature required to sustain life. It has been recognized and worshipped for centuries, starting with the Harappan, Greek and Aztec civilizations, for its light, energy and cleansing powers. Today we recognize additional functionality of the Sun for processes such as photosynthesis and formation of fossil fuels. Among the various options for alternative sources of energy, the Sun poses to be appealing due to its essentially infinite and omnipresent nature. In fact, the solar energy resource is much greater than all other renewable and fossil-fuel based energy resources combined [4]. Sunlight reaching the earth’s surface is almost 6,000 times the average power consumed by humans. These figures encourage us to look for ways to harness solar energy and convert it into other convenient forms.

5

Photovoltaics (PV) is the direct process of converting sunlight into electricity. This process is highly reliable; easy to install; thrives on low operation cost; very safe; generally has no moving parts; while PV systems can be stand-alone, grid-connected, as well as modular. Moreover, this process does not involve any combustion or greenhouse-gas emission, thus making it safe for the environment.

1.1.3 High efficiency for economic viability The global solar electricity market is currently more than $10 billion/year, and the industry is growing at more than 30% per annum [5][6] as indicated in Figure 1.4. In spite of this steady growth of the PV market, the current total global PV installed capacity is about 9 GWP [7], which accounts for only 0.04% of the world energy usage. While the current

Figure 1.4: World PV cell/module

6 generation cost of solar electricity in the United States is around $0.20/kWh [8] (prior to receiving subsidies), it still is about four times greater than its fossil-based competitor. This high price of solar-generated electricity compared to other conventional energy sources is primarily due to high manufacturing and installing costs; on the other hand, challenges such as system reliability, system integration and storage are continually being improved. To economically reach its most competitive long-term position, it is argued that PV must push towards ever-increasing energy conversion efficiency while retaining low areal processing costs [9]. This leads to the concept of “third-generation” PV, which has a performance potential beyond that of single junction cells, while retaining the areal cost advantage of “second-generation” thin-film solar cells. This third-generation PV has the potential to achieve beyond the $1/W target as shown in Figure 1.5, which is equivalent to the price of fossil-based electricity.

Figure 1.5: Efficiency and cost projections for first, second and third generation PV [9].

7

The progressive development of record-efficiency solar cells is summarized in Figure 1.6 [10], where the emerging multi-junction concentrator solar cell efficiencies have clearly dominated over other technologies. The current efficiency record is held by a triple-junction tandem solar cell at 40.7% [11][12]; however, modeling results indicate that such structures are approaching their theoretical efficiency limits. Overcoming the 50% efficiency barrier demands exploration of new material systems and probably novel solar cell architectures. The Indium Gallium Nitride (InGaN) material system, although in its rudimentary stages of development, demonstrates the versatility and promise as a successful high-efficiency photovoltaic material.

Figure 1.6: Best research- cell efficiencies [10].

8 1.1.4 VHESC – An ultra-high efficiency approach One potential application of an InGaN photovoltaic device is in the Very High Efficiency Solar Cell, which targets an efficiency of greater than 50% [13]. High efficiency modules are being developed based on the co-design of the optics, interconnects and solar cells as shown in Figure 1.7. This architecture significantly increases the design space for high performance photovoltaic modules in terms of materials, device structures and manufacturing technology. It allows multiple benefits, including increased theoretical efficiency, new architectures which circumvent existing material/cost trade-offs, improved performance from non-ideal materials, device designs that can more closely approach ideal performance limits, reduced spectral mismatch losses and increased flexibility in material choices. An integrated optical/solar cell allows efficiency improvements while retaining low area costs, and hence expands the applications for photovoltaics. It allows a design approach which focuses first on performance, enabling the use of existing state-of-the-art photovoltaic

Mid Energy Solar Cell Front Lens Secondary Concentrator Dichroic Prism Hollow Pyramid Concentrator Silicon/ Low Energy Solar Cell

Figure 1.7: Schematic of the architecture of VHESC.

9 technology to design high performance, low cost multiple junction III-Vs for the high and low energy photons and a new silicon solar cell for the mid-energy photons, all while circumventing existing cost drivers through novel solar cell architectures and optical elements. Figure 1.8 breaks down the predicted efficiency contribution at each energy level of the VHESC to reach a practical efficiency of greater than 50%. InGaN is one of the few material systems that can provide band gaps of 2.4 eV or greater, which is critical to reach the 50% target as seen from the figure. Thus, it is vital to explore InGaN for photovoltaic applications.

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Abstract: Main objective of the present work is to develop wide-band gap InGaN solar cells in the 2.4-2.9 eV range that can be an integral component of photovoltaic devices to achieve efficiencies greater than 50%. The III-nitride semiconductor material system, which consists of InN, GaN, AlN and their alloys, offers a substantial potential in developing ultra-high efficiency photovoltaics mainly due to its wide range of direct-band gap, and other electronic, optical and mechanical properties. However, this novel InGaN material system poses challenges from theoretical, as well as technological standpoints, which are further extended into the performance of InGaN devices. In the present work, these challenges are identified and overcome individually to build basic design blocks, and later, optimized comprehensively to develop high-performance InGaN solar cells. One of the major challenges from the theoretical aspect arises due to unavailability of a suitable modeling program for InGaN solar cells. As spontaneous and piezoelectric polarization can substantially influence transport of carriers in the III-nitrides, these phenomena are studied and incorporated at a source-code level in the PC1D simulation program to accurately model InGaN solar cells. On the technological front, InGaN with indium compositions up to 30% (2.5 eV band gap) are developed for photovoltaic applications by controlling defects and phase separation using metal-organic chemical vapor deposition. InGaN with band gap of 2.5 eV is also successfully doped to achieve acceptor carrier concentration of 10 18 cm-3 . A robust fabrication scheme for III-nitride solar cells is established to increase reliability and yield; various schemes including interdigitated grid contact and current spreading contacts are developed to yield low-resistance Ohmic contacts for InGaN solar cells. Preliminary solar cells are developed using a standard design to optimize the InGaN material, where the band gap of InGaN is progressively lowered. Subsequent generations of solar cell designs involve an evolutionary approach to enhance the open-circuit voltage and internal quantum efficiency of the solar cell. The suitability of p-type InGaN with band gaps as low as 2.5 eV is established by incorporating in a solar cell and measuring an open-circuit voltage of 2.1 V. Second generation InGaN solar cell design involving a 2.9 eV InGaN p-n junction sandwiched between p- and n-GaN layers yields internal quantum efficiencies as high as 50%; while sixth generation devices utilizing the novel n-GaN strained window-layer enhance the open circuit voltage of a 2.9 eV InGaN solar cell to 2 V. Finally, key aspects to further InGaN solar cell research, including integration of various designs, are recommended to improve the efficiency of InGaN solar cells. These results establish the potential of III-nitrides in ultra-high efficiency photovoltaics.