Investigation of thin-film CdTe/Getandem solar cells

Won-Jae LeeB.Eng, M.Eng.

This thesis is presented for the degree of Doctor of Philosophy of TheUniversity of Western Australia

School of Electrical, Electronic and Computer Engineering

The University of Western Australia

2017

Abstract

The rapid growth of the world’s economy and population needs a tremendous amount

of energy. Moreover, global warming, fossil fuel depletion and international conflict related

with territories involving natural resources are a great concern all over the world. To solve

these energy problems that the world is facing, the development of alternative energy sources

is required. A thin-film based solar cell is one of the leading contenders for providing cost-

effective and pollution-free energy. Furthermore, the efficiency of thin-film solar cells can

be improved with a tandem structure. This thesis describes the development of solar cells

employing a tandem structure with CdTe and Ge thin films, from crystallization of thin film

materials to simulation of tandem solar cell performance.

The cost-effective use of materials is essential if the solar cell industry is to provide en-

ergy cheaper than fossil fuel sources. In order to render the fabrication process low-cost and

simple, poly-Ge thin films have been investigated using electron beam (E-beam) evapora-

tion and solid phase crystallization (SPC). For all crystallized Ge thin films as a photovoltaic

material, the electrical properties were found to improve with increasing SPC temperature.

After crystallization of the Ge layer through annealing at 600◦C, p-Ge/n-Si heterojunction

devices obtained an on/off current ratio of 106, an ideality factor of 1.25, and a built-in po-

tential of 0.58eV. The improvement in device performance is correlated with the degree of

crystallization of the Ge thin film, which indicated that it can be considered as a suitable

candidate for optoelectronic applications.

For CdS/CdTe solar cells, n-type CdS thin films for use as a window layer were prepared

and investigated using thermal evaporation and various characterization techniques. It is

found that post-deposition annealing temperature is more effective than deposition substrate

temperature in improving the electrical and optical properties of the CdS thin films. For

the case of increasing substrate temperature during deposition, the formation of defects is

determined by the effective Cd/S ratio, whereas the post-deposition annealing temperature

determines defect migration and annihilation, which have a strong influence on the electrical

vi

and optical properties of CdS thin films. The electrical and optical properties of CdS thin

films obtained herein are shown to be suitable for thin-film solar cell applications, which is

demonstrated by fabricating n-CdS/p-Si and CdS/CdTe heterojunction devices.

To form the absorber in CdS/CdTe solar cells, CdTe films were deposited by thermal

evaporation in a high vacuum onto CdS/ITO/glass substrates in superstrate configuration.

The CdTe layers were then recrystallized with CdCl2 and annealed in air. The properties of

CdTe films were found to improve at higher deposition substrate temperature and with CdCl2

treatment. In the n-CdS/p-CdTe heterojunction solar cells, the photovoltaic cell parameters

indicated that deposition of CdS films at room temperature resulted in higher performing

cells; with the substrate temperature required during thermal deposition of CdTe acting to

effectively anneal the underlying CdS thin film. Under 1 sun illumination (AM 1.5G), the

fabricated solar cells achieved a highest efficiency of > 11% with MgF2 anti-reflection coat-

ing, a short-circuit current of 24.16mA/cm2, and an open-circuit voltage of 0.765V.

Since the experimental effort required to optimize the overall fabrication process and

tunnel junctions are beyond the scope of this project, the CdTe/Ge tandem solar cells were

modeled by incorporating possible improvements as well as experimental data to improve

the modeling conditions and achieve realistic outcomes. One approach applied to model

the tunnel junction is the use of a-Si:H as the p+ recombination layer and a Ge n+ layer.

The calculated highest performance of the tandem cells was around 25.3% in efficiency

with Voc = 1.2V, Jsc = 26.3mA/cm2, and FF = 81.9%. For stand-alone cells, the efficiency

of the CdTe top cell is 21.12% (Voc = 0.98V, Jsc = 26.3mA/cm2, and FF of 81.9%), and

the efficiency of the Ge bottom cell is 4.58% (Voc = 0.25V, Jsc = 27.7mA/cm2, and FF of

66.13%). This thesis has proposed and laid the groundwork for further development of a

novel thin-film tandem solar cell structure, consisting of the combination of a thin-film CdTe

solar cell, a tunnel junction and a crystallized thin-film Ge solar cell. Via experimental results

and comprehensive device modeling, it has been shown that tandem cell device efficiencies

approaching or exceeding 25% are achievable.

Acknowledgements

First of all, I would like to thank Prof. Lorenzo Faraone, Prof. John Dell and Prof.

Gilberto Umana-Membreno for their support, advice and supervision of my doctoral thesis,

and with allowing me to study with excellent facilities on the beautiful campus at the Univer-

sity of Western Australia. I would like to thank all current and former MRG members listed

in random order below for support and fruitful discussions (Ms. Sabina Betts, Dr. Dilusha

Silva, Dr. Jarek Antoszewski, Dr, Mariusz Martyniuk, Dr. Fei Jiang, Dr. Gino Putrino, Dr.

Wen Lei, Dr. Yongling Ren, Dr. Adrian Keating, Dr. Renjie Gu, Dr. Nima Dehdashti, Ms.

Karen Kader, Jing Zhang, Nir Zvison, Hemendra Kala, Haifeng Mao, Farah Muhammad

Khir, James Sharp, Dhirendra Tripathi, Radha Krishnan Nachimuthu, Rohit Sharda, Anna

Podolska, Amit Choudhary, Balaji Sankarshanan, Ben Cheah, Imtiaz Madni, Michal Zaw-

ierta, Xiao Sun). I would like to thank many people who I met and had a good time with in

Perth listed in random order (Dr. Kim Young-ho, Lee Joonmo, Kanchan Chaudhury, Patrick

Ho, Dr. Jang Ugeun, Dr. Oh Se-Heon, Dr. Rhee Jonghwan, Prof. Eun-Jung Holden, Prof.

Lee Mi-kyung, Dr. Lee Jong-Ku, Dr. Moon Seongkon, Dr. Choi Yusuk, Dr. Kim Duyong,

Jeon Minjung, and their families). I would also like to thank my wife’s previous bosses and

friends (Chloe Lee, Hally Kim and their families). I am sorry if I did not mention all of you,

but I really appreciate all of you. Last but not least I would like to thank my wife, Mijin

Kwon, for her support during my PhD years, and my beloved family including my little son,

parents and brother.

Table of contents

Table of contents xi

List of figures xv

List of tables xxi

1 Introduction 1

1.1 Motivation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

1.1.1 World energy demand . . . . . . . . . . . . . . . . . . . . . . . . 1

1.1.2 Solar energy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

1.2 Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

1.2.1 Solar cell physics and operation . . . . . . . . . . . . . . . . . . . 6

1.2.2 Thin-film solar cells (TFSCs) . . . . . . . . . . . . . . . . . . . . 9

1.2.3 Tandem (multi-junction) solar cells . . . . . . . . . . . . . . . . . 13

1.3 Toward CdTe/Ge tandem solar cells . . . . . . . . . . . . . . . . . . . . . 16

1.4 Required specifications (see Fig. 1.19) . . . . . . . . . . . . . . . . . . . . 17

1.5 Thesis outline and scope . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

2 Review of CdTe and Ge single and tandem solar cells 23

2.1 CdTe solar cells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

2.1.1 Highest efficiency . . . . . . . . . . . . . . . . . . . . . . . . . . 25

2.1.2 CdTe solar cells on flexible substrates . . . . . . . . . . . . . . . . 27

2.2 Ge solar cells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

2.2.1 Highest efficiency . . . . . . . . . . . . . . . . . . . . . . . . . . 28

2.2.2 Ge-on-glass solar cells . . . . . . . . . . . . . . . . . . . . . . . . 30

2.3 CdTe/Ge tandem solar cells . . . . . . . . . . . . . . . . . . . . . . . . . . 31

xii Table of contents

2.4 Summary and discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

3 Experimental and characterization techniques 35

3.1 Review of deposition methods . . . . . . . . . . . . . . . . . . . . . . . . 36

3.1.1 Ge deposition techniques . . . . . . . . . . . . . . . . . . . . . . . 36

3.1.2 CdS deposition techniques . . . . . . . . . . . . . . . . . . . . . . 40

3.1.3 CdTe deposition techniques . . . . . . . . . . . . . . . . . . . . . 44

3.2 Substrate preparation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47

3.3 Structural characterization . . . . . . . . . . . . . . . . . . . . . . . . . . 48

3.3.1 X-Ray Diffraction (XRD) . . . . . . . . . . . . . . . . . . . . . . 48

3.3.2 Scanning Electron Microscopy (SEM) . . . . . . . . . . . . . . . . 50

3.4 Electrical characterization . . . . . . . . . . . . . . . . . . . . . . . . . . 51

3.4.1 Hall effect . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51

3.4.2 Current-voltage (I-V) . . . . . . . . . . . . . . . . . . . . . . . . . 53

3.4.3 Capacitance-voltage (C-V) . . . . . . . . . . . . . . . . . . . . . . 57

3.5 Optical characterization . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58

3.5.1 Spectroscopy for optical properties . . . . . . . . . . . . . . . . . 58

3.5.2 Spatial photocurrent mapping . . . . . . . . . . . . . . . . . . . . 59

3.6 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60

4 Thin film Ge and devices 63

4.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63

4.2 Ge thin film deposition and device preparation . . . . . . . . . . . . . . . . 64

4.3 Characterization of crystallized Ge thin films . . . . . . . . . . . . . . . . 66

4.3.1 Hall effect measurement . . . . . . . . . . . . . . . . . . . . . . . 66

4.3.2 SEM analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69

4.3.3 XRD analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70

4.3.4 Optical transmission . . . . . . . . . . . . . . . . . . . . . . . . . 73

4.4 Characterization of crystallized Ge/Si hetero-junction diodes . . . . . . . . 73

4.4.1 I-V measurements . . . . . . . . . . . . . . . . . . . . . . . . . . 73

4.4.2 C-V measurements . . . . . . . . . . . . . . . . . . . . . . . . . . 75

4.4.3 Spatial photocurrent map . . . . . . . . . . . . . . . . . . . . . . . 76

4.5 Summary and discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . 78

Table of contents xiii

5 Thin film CdS and devices 79

5.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79

5.2 CdS thin film deposition and device preparation . . . . . . . . . . . . . . . 80

5.3 Characterization of evaporated CdS . . . . . . . . . . . . . . . . . . . . . 81

5.3.1 Hall effect measurement . . . . . . . . . . . . . . . . . . . . . . . 81

5.3.2 SEM analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84

5.3.3 X-Ray Diffraction . . . . . . . . . . . . . . . . . . . . . . . . . . 87

5.3.4 Optical characterization . . . . . . . . . . . . . . . . . . . . . . . 89

5.4 Characteristics of CdS/Si hetero-junction devices . . . . . . . . . . . . . . 93

5.5 Summary and discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . 95

6 Thin film CdTe and devices 97

6.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97

6.1.1 Technological challenges . . . . . . . . . . . . . . . . . . . . . . . 98

6.2 CdTe thin film deposition and device preparation . . . . . . . . . . . . . . 100

6.3 Characterization of evaporated CdTe . . . . . . . . . . . . . . . . . . . . . 102

6.3.1 X-Ray Diffraction . . . . . . . . . . . . . . . . . . . . . . . . . . 102

6.3.2 Optical transmittance . . . . . . . . . . . . . . . . . . . . . . . . . 103

6.3.3 SEM image analysis . . . . . . . . . . . . . . . . . . . . . . . . . 105

6.4 The role of CdCl2 treatment . . . . . . . . . . . . . . . . . . . . . . . . . 105

6.4.1 Effect of CdCl2 treatment on device properties . . . . . . . . . . . 108

6.5 Summary and discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . 109

7 Performance of CdS/CdTe solar cells 111

7.1 Back contact . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111

7.1.1 Back contact on CdTe films . . . . . . . . . . . . . . . . . . . . . 111

7.1.2 Contact improvement with Cu . . . . . . . . . . . . . . . . . . . . 113

7.1.3 Spice modeling of back-contact effect in solar cells . . . . . . . . . 113

7.2 Effect of CdS window layer . . . . . . . . . . . . . . . . . . . . . . . . . . 117

7.2.1 Role of CdS film as a window layer . . . . . . . . . . . . . . . . . 117

7.2.2 Effect of CdS film preparation on photovoltaic properties . . . . . . 119

7.3 Optical losses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121

7.3.1 Reflection losses . . . . . . . . . . . . . . . . . . . . . . . . . . . 121

xiv Table of contents

7.3.2 Anti-reflection coating using MgF2 . . . . . . . . . . . . . . . . . 125

7.4 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 126

8 Potential and limitations of CdTe/Ge tandem solar cells 129

8.1 Obstacles limiting high-efficiency CdTe/Ge tandem solar cell technologies . 129

8.1.1 Requirements for the CdTe top cell . . . . . . . . . . . . . . . . . 129

8.1.2 Tunnel junction or recombination layer . . . . . . . . . . . . . . . 137

8.1.3 Ge bottom cell . . . . . . . . . . . . . . . . . . . . . . . . . . . . 138

8.2 Simulation of CdTe/Ge tandem cells . . . . . . . . . . . . . . . . . . . . . 139

8.3 Summary and discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . 143

9 Summary, conclusions and future work 145

9.1 Summary and conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . 145

9.2 Future work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 147

Appendix A Updating scanning laser microscopy (SLM) system 149

A.1 Updating SLM machine . . . . . . . . . . . . . . . . . . . . . . . . . . . . 149

A.2 Measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 150

A.2.1 Laser beam induced current (LBIC) . . . . . . . . . . . . . . . . . 152

A.2.2 Spatial photocurrent mapping . . . . . . . . . . . . . . . . . . . . 153

A.2.3 Carrier lifetime . . . . . . . . . . . . . . . . . . . . . . . . . . . . 154

Appendix B Publications arising from this thesis 155

References 157

List of figures

1.1 World population growth . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

1.2 Global GDP growth estimated by Oxford Economics, IMF World Economic

Outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

1.3 Comparison of various energy sources . . . . . . . . . . . . . . . . . . . . 3

1.4 Global (AM0 and AM1.5G) standard spectrum . . . . . . . . . . . . . . . 4

1.5 Spectrum of the radiation interrupted by the earth’s atmosphere . . . . . . . 4

1.6 The path length in units of Air Mass, changes with the zenith angle . . . . . 5

1.7 Bandgap structure and a solar cell with a resistive load . . . . . . . . . . . 7

1.8 Solar cell voltage-current characteristic under illumination . . . . . . . . . 8

1.9 Equivalent circuit of a solar cell . . . . . . . . . . . . . . . . . . . . . . . 9

1.10 Best efficiency research solar cells . . . . . . . . . . . . . . . . . . . . . . 10

1.11 Flexible thin film solar cell . . . . . . . . . . . . . . . . . . . . . . . . . . 11

1.12 Solar PV crystalline silicon and thin-film module cost learning curve . . . . 11

1.13 Absorption coefficient of various materials . . . . . . . . . . . . . . . . . . 12

1.14 Maximum theoretical efficiency (Shockley–Queisser limit) for thin film solar

cells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

1.15 Losses by thermalization and non-absorption of low-energy-photons . . . . 13

1.16 Absorption of solar spectrum by different energy bandgap materials in a tan-

dem solar cell . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

1.17 Tandem solar cell operation with different connection (parallel and serial

connection) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

1.18 The maximum efficiency for a double-junction tandem cell under the AM1.5G

spectrum . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

1.19 Band diagram for a tunnel junction and details of a tandem structure . . . . 18

1.20 Chapter description illustrated through a CdTe/Ge tandem cell structure . . 19

xvi List of figures

2.1 The best efficiency of CdTe solar cells and modules . . . . . . . . . . . . . 24

2.2 Fabrication methods for thin film solar cells . . . . . . . . . . . . . . . . . 24

2.3 J-V comparison between the best efficiency record and SQ limits . . . . . . 26

2.4 Quantum efficiencies of record cells . . . . . . . . . . . . . . . . . . . . . 26

2.5 Quantum efficiencies of milestone cells on flexible substrates . . . . . . . . 28

2.6 I-V measurement of the highest efficiency Ge solar cell . . . . . . . . . . . 29

2.7 Phosphorous diffusion into Ge at different temperature and time . . . . . . 29

2.8 Fabrication and characteristics of Ge-on-glass solar cells . . . . . . . . . . 30

2.9 A suggested structure of CdTe/Ge tandem solar cells . . . . . . . . . . . . 31

3.1 Schematic illustration of solid phase crystallization processes in a-Si . . . . 37

3.2 Phase diagram describing crystal status of 200 nm thick undoped Ge films

processed by MIC technique . . . . . . . . . . . . . . . . . . . . . . . . . 39

3.3 Operation of electron beam evaporation . . . . . . . . . . . . . . . . . . . 40

3.4 Operation of thermal evaporation . . . . . . . . . . . . . . . . . . . . . . . 43

3.5 Schematic diagram of various techniques for CdTe thin film deposition . . . 45

3.6 Measured optical properties of various substrates . . . . . . . . . . . . . . 48

3.7 X-ray diffraction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49

3.8 Ge surface image by SEM with short working distance and low acceleration

voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51

3.9 Geometry for measuring the Hall effect . . . . . . . . . . . . . . . . . . . 52

3.10 Schematic diagrams of resistivity and Hall effect measurements by the van

der Pauw method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52

3.11 The 2T Hall effect measurement setup . . . . . . . . . . . . . . . . . . . . 54

3.12 Extraction of ideality factor with modeling . . . . . . . . . . . . . . . . . . 55

3.13 Extraction of parasitic resistances (Rs and Rsh) under illumination) . . . . . 56

3.14 Photovoltaic I-V curve under illumination . . . . . . . . . . . . . . . . . . 57

3.15 A typical graph of 1/C′2 versus voltage . . . . . . . . . . . . . . . . . . . . 58

3.16 Schematic diagram of SLM system for 2D current mapping . . . . . . . . . 60

4.1 Pinholes on Ge thin films prepared at high deposition rate . . . . . . . . . . 64

4.2 Schematics of a poly-Ge heterojunction device on silicon substrate . . . . . 65

4.3 Mask design for fabrication of p-Ge/n-Si devices . . . . . . . . . . . . . . 66

List of figures xvii

4.4 Results of Hall-effect and resistivity measurements on Ge thin films de-

posited on glass substrates . . . . . . . . . . . . . . . . . . . . . . . . . . 66

4.5 SEM images of 600 nm thick Ge films deposited and annealed on silicon . . 68

4.6 SEM images of 1 μm and 200 nm thick Ge films deposited and annealed on

glass . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69

4.7 Cross-sectional SEM images of 1 μm thick Ge films deposited on glass slides 70

4.8 XRD spectra of crystallized Ge thin films . . . . . . . . . . . . . . . . . . 71

4.9 Optical transmittance spectra of crystallized Ge annealed for 30min . . . . 72

4.10 Bandgap structure of a single crystalline Ge/Si junction . . . . . . . . . . . 73

4.11 Current-voltage characteristics of p-Ge/n-Si heterojunction diodes as a func-

tion of annealing temperature . . . . . . . . . . . . . . . . . . . . . . . . . 74

4.12 Experimental 1/C′2 versus reverse bias characteristics for the p-Ge/n-Si diodes 75

4.13 2D spatial photocurrent maps of p-Ge/n-Si heterojunction devices . . . . . 76

4.14 Measured photocurrent for 100 nm p-Ge/n-Si devices with transmission of

100 nm thick crystallized Ge thin film and absorption of 100 nm p-Ge/n-Si

modeled using optical properties of single crystal materials . . . . . . . . . 77

5.1 The mask design and schematic diagram for fabrication of n-CdS/p-Si het-

erojunction device . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81

5.2 Hall effect measurements on evaporated CdS films . . . . . . . . . . . . . 82

5.3 SEM surface images of CdS thin films . . . . . . . . . . . . . . . . . . . . 85

5.4 Cross-sectional SEM images of CdS thin films . . . . . . . . . . . . . . . . 86

5.5 SEM images of CdS thin films deposited and annealed on silicon substrates 88

5.6 XRD spectra of CdS thin films . . . . . . . . . . . . . . . . . . . . . . . . 89

5.7 Transmission spectra of 1μm thick CdS thin films on glass slides . . . . . . 90

5.8 Extraction of optical bandgap for CdS films . . . . . . . . . . . . . . . . . 91

5.9 Refractive index as a function of temperature . . . . . . . . . . . . . . . . 92

5.10 I-V curves of Ohmic contacts formed using aluminum on both n-CdS films

and p-Si substrates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93

5.11 I-V characteristics of CdS/Si heterojunction devices . . . . . . . . . . . . . 94

6.1 A 3.5μm thick CdTe film with absorption coefficient . . . . . . . . . . . . 98

xviii List of figures

6.2 Mask design and schematic diagram for fabrication of CdS/CdTe heterojunc-

tion devices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101

6.3 SEM images for n-CdS/p-CdTe heterojunction solar cell on ITO-coated glass 101

6.4 XRD spectra of CdTe thin films . . . . . . . . . . . . . . . . . . . . . . . 102

6.5 Optical transmission of 1 μm thick CdTe thin films on glass slides . . . . . 103

6.6 SEM images of CdTe thin films on glass slides . . . . . . . . . . . . . . . 104

6.7 XRD spectra of CdTe films with and without CdCl2 treatment . . . . . . . 106

6.8 SEM surface images of CdTe thin films on glass slides . . . . . . . . . . . 107

6.9 I-V characteristics of CdS/CdTe devices with and without CdCl2 treatment . 109

7.1 Energy band diagram of semiconductor-metal junction . . . . . . . . . . . 112

7.2 Experimental back-contact barrier effects on solar cell parameters . . . . . 112

7.3 A two-diode equivalent circuit model for the CdS/CdTe solar cell . . . . . . 114

7.4 Spice modeling to extract back-contact barrier of two different solar cells

with different back contacts . . . . . . . . . . . . . . . . . . . . . . . . . . 116

7.5 Calculated barrier height extracted from the contact saturation current . . . 116

7.6 Role of CdS thin films . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117

7.7 Representative photovoltaic I-V curves under 1 sun illumination . . . . . . 118

7.8 Experimental photovoltaic properties (Voc,Jsc, FF and efficiency) of solar cells119

7.9 Structure with multiple thin film layers . . . . . . . . . . . . . . . . . . . . 122

7.10 Optical properties of a fabricated CdS/CdTe solar cell with modeling data . 124

7.11 Experimental optical transmission and absorption of CdTe solar cells as a

function of thickness of MgF2 AR coating . . . . . . . . . . . . . . . . . . 124

7.12 Observed improvement in CdTe solar cell performance with an optimized

thickness (70nm) of MgF2 AR coating . . . . . . . . . . . . . . . . . . . . 125

7.13 Observed improvement in solar cell parameters due to the application of an

anti-reflection coating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 126

7.14 Typical I-V characteristics of fabricated solar cells . . . . . . . . . . . . . . 127

8.1 Optical constants for modeling, and comparison of modeled optical proper-

ties with experimental data . . . . . . . . . . . . . . . . . . . . . . . . . . 131

8.2 Modeled absorption in CdS and CdTe layers as a function of CdS film thick-

ness. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 132

List of figures xix

8.3 Modeled solar spectra absorbed in CdTe layer and generated photocurrent as

a function of thickness of CdS thin films . . . . . . . . . . . . . . . . . . . 133

8.4 Calculated open-circuit voltage as a function of carrier concentration of CdTe

film . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 135

8.5 Simulation of solar cell performance using the high Jsc and high Voc . . . . 135

8.6 Measured optical properties of CdTe solar cells . . . . . . . . . . . . . . . 136

8.7 Modeled optical properties of CdTe solar cells with ITO/TiO2 double TCO

layer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 137

8.8 Tandem cell structure and energy band diagram for CdTe/Ge solar cell . . . 140

8.9 Modeled photocurrent and absorption of the solar spectrum as a function of

Ge thickness . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 141

8.10 Modeled quantum efficiency of CdTe top cell and Ge bottom cell . . . . . . 142

8.11 Modeled photovoltaic properties of CdTe/Ge tandem solar cells and individ-

ual top and bottom cells . . . . . . . . . . . . . . . . . . . . . . . . . . . . 143

A.1 Schematic diagram of SLM system . . . . . . . . . . . . . . . . . . . . . . 150

A.2 Noise improvement after replacement of old parts . . . . . . . . . . . . . . 151

A.3 Temperature calibration for precise measurement . . . . . . . . . . . . . . 151

A.4 Specific device geometry to measure LBIC . . . . . . . . . . . . . . . . . 152

A.5 Specific device geometry to measure spatial photocurrent . . . . . . . . . . 153

A.6 Specific device geometry to measure transient carrier lifetime . . . . . . . . 153

List of tables

1.1 Confirmed terrestrial solar module efficiencies measured under the AM1.5G

spectrum . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

1.2 Required specifications for a CdTe/Ge tandem cell . . . . . . . . . . . . . 17

2.1 Photovoltaic properties of the best cell and module . . . . . . . . . . . . . 25

2.2 A comparison of flexible cells with record device parameters . . . . . . . . 27

2.3 Predicted photovoltaic performance of the CdTe/Ge tandem cell in the pre-

vious work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

3.1 Summary of Ge crystallization using a variety of techniques . . . . . . . . 39

3.2 Optical bandgap of sputtered CdS films . . . . . . . . . . . . . . . . . . . 42

3.3 Specifics of various substrates . . . . . . . . . . . . . . . . . . . . . . . . 47

4.1 Summary of properties of Ge thin films crystallized using various techniques 67

4.2 Extracted diode ideality factor, built-in potential and n-Si doping concentra-

tion of p-Ge/n-Si devices . . . . . . . . . . . . . . . . . . . . . . . . . . . 76

5.1 Extracted energy bandgap of CdS thin films at different deposition and an-

nealing temperatures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92

6.1 Extracted solar cell parameters dependent on CdCl2 treatment . . . . . . . 108

8.1 Layer properties for simulation . . . . . . . . . . . . . . . . . . . . . . . . 134

8.2 Ge properties for simulation . . . . . . . . . . . . . . . . . . . . . . . . . 140

8.3 Modeled results of tandem solar cell and individual top and bottom cells . . 143

Chapter 1

Introduction

1.1 Motivation

1.1.1 World energy demand

If the 20th century was the era of the Cold War, energy has become the next battlefield

since the late 20th century as countries seek to protect their natural resources [1]. Use of en-

ergy is necessary to sustain human life and economic development. However, global warm-

ing, fossil fuel depletion, and international conflict are associated with this energy use [1, 2].

Worldwide energy consumption was around 12,730.4 Mtoe (toe : Tonnage of Oil Equiv-

alent) in 2013, with approximately 87% of this sourced from the combustion of fossil fu-

els [3]. The world’s growing population, as well as an increase in the average gross domestic

product (GDP), will lead to an ongoing increase in energy consumption [4]. Moreover, the

world’s population (see Fig. 1.1) is expected to reach 9 billion between 2040 and 2050 [5],

and the average world GDP (see Fig. 1.2) is increasing at 3-4% annually, as predicted by the

International Monetary Fund (IMF) [6]. Thus, world energy demand is increasing, driven

by population and world GDP growth, and is estimated that it will reach 18,000 Mtoe by

2035 [3, 7].

This continuous increase in the world’s energy demand and consumption seems unavoid-

able in the near future. Similarly, global warming will eventually lead to substantial changes

in the world’s climate, if most of the energy is generated by the combustion of fossil fu-

2 Introduction

Fig. 1.1 World population growth [8].

Fig. 1.2 Global GDP growth estimated by Oxford Economics, IMF WorldEconomic Outlook, EIU [9].

els [10]. In spite of this, the current world’s use of renewable energy sources, such as wind,

solar, biofuels, wave, tidal and geothermal power, account for just 2.4% of total energy gen-

eration capacity, excluding hydro [3]. Therefore, renewable energies such as solar and wind

need to be advanced if we are to reduce the use of fossil fuels, and save our environment.

1.1.2 Solar energy

“In 14 and a half seconds, the sun provides

as much energy to Earth as Humanity uses in a day.”

Ramez Naam – Scientific American

The available energy from the sun is orders of magnitude larger than all other energy

1.1 Motivation 3

Fig. 1.3 Comparison of various energy sources [2].

sources, as evident from Fig. 1.3. Even a small fraction of the available solar energy reaching

the earth’s surface would be enough to satisfy the expected global energy demand. Solar

energy is the only renewable source that can meet world energy consumption with a small

fraction of the available total [2]. Furthermore, solar energy is virtually infinite and clean,

which can solve our environmental problems as well.

Radiation from the sun sustains life on earth and controls climate. The sun has a surface

temperature of around 5800K, so the spectrum of the radiation from the sun is similar to that

of a 5800K blackbody [11]. The values of irradiance from the sun on the outer atmosphere

are about 1360 W/m2 [12]. Most of the energy from the sun is concentrated in the visible

and near-visible range of the spectrum, as shown in Fig. 1.4. The visible light, between 380

and 780 nm, represents 48% of the total energy from the sun. Shorter wavelength ultraviolet

(UV) accounts for 6.4% of the total. The remaining 45.6% of the radiant energy is spread

over the infrared region [13].

Various components of the earth’s atmosphere prevent solar radiation from penetrating

it and reaching the earth’s surface, since some of the radiation is absorbed and scattered by

the atmosphere. Oxygen and nitrogen absorb very short wave radiation. Ultraviolet (UV)

radiation is blocked by ozone in the atmosphere and some of it reaches the earth’s surface.

4 Introduction

Fig. 1.4 Global (AM0 and AM1.5G) standard spectrum [14].

Fig. 1.5 Spectrum of the radiation interrupted by the earth’s atmo-sphere [18].

Water vapor, carbon dioxide and, to a lesser extent, oxygen, selectively absorb in the near

infrared [15] as shown in Fig. 1.5. As a result of reflection, scattering, and absorption of

radiation, the solar energy that reaches the earth’s surface is much reduced in intensity. The

energy associated with direct sunlight thus depends on the altitude of the sun, and also varies

with such factors as latitude, season, and cloudiness [15–17]. Particularly these seasonal- and

daytime-dependent variations of the sun spectrum will affect current matching in a tandem

structure, thus reducing efficiency more than single cells.

1.1 Motivation 5

Fig. 1.6 The path length in units of Air Mass, changes with the zenithangle.

The atmospheric path for any zenith angle is simply described relative to the overhead

air mass as shown in Fig. 1.6. This pathlength can be approximated by 1/cosθz, where θz is

the angle between the sun and the point directly overhead [16, 19]. Therefore:

AM = 1/cosθz (1.1)

The spectrum outside the atmosphere is designated by Air Mass zero (AM0) because it

passes through no air mass. Air Mass 1 Direct (AM1D, θz = 0) is direct radiation that

reaches the ground after passing through the entire atmosphere overhead. The direct portion

of the solar radiation is collimated with an angle of approximately 0.53° (full angle), while

the "diffuse" portion is incident from the hemispheric sky and from ground reflections and

scatter [20]. The standard spectrum at the Earth’s surface is called AM1.5G ("Global" which

includes both direct and diffuse radiation, θz = 48.19) or AM1.5D (direct radiation only).

AM1.5G has been calculated to be approximately 970 W/m2, which has been standardized as

1 kW/m2 (equal to 100 mW/cm2) when used for rating photovoltaic products. Therefore, the

sun provides 1 kW/m2 of free, non-polluting power for several hours every day [16, 17, 19].

However, as shown in Table 1.1, the highest efficiency of commercial solar modules

produced in industry is still less than 20% especially for multi-crystalline materials, and

there is much scope for converting more solar energy into electricity [21, 22]. Therefore,

technologies for converting solar energy into electricity need to be improved. Some new and

6 Introduction

Table 1.1 Confirmed terrestrial solar module efficiencies measured underthe global AM1.5 spectrum (1000 W/m2) [21].

Material Eff.[%] Area(cm2) Voc[V] Isc[A] FFSi (crystalline) 22.9 778 5.60 3.97 80.3

Si (large crystalline) 22.8 15,738 69.36 6.459 80.0Si (multi-crystalline) 19.2 15,126 77.93 4.726 78.9

GaAs (thin film) 24.1 858.5 10.89 2.255 84.2CdTe (thin film) 18.6 7038.8 110.6 1.533 74.2CIGS (Cd free) 17.5 808 47.6 0.408 72.8CIGS (thin film) 15.7 9,703 28.24 7.254 72.5

a-Si/nc-Si (tandem) 12.3 14,322 280.1 0.902 69.9Organic 8.7 802 17.47 0.1065 70.4

emerging developments, such as organic and multi-junction solar cells, have the potential to

change the situation [21–25].

1.2 Background

1.2.1 Solar cell physics and operation

An inorganic photovoltaic cell is basically a semiconductor diode that converts sunlight

directly into electricity [17, 26–28]. When a photon with energy greater than the bandgap is

absorbed, it can induce electron-hole pairs by excitation of electrons out of the valence band

into the conduction band as depicted in Fig. 1.7a. In the second step of the energy conversion

process, the photogenerated electron-hole pairs are separated by the internal electric field in

the space-charge layer of the diode structure of the solar cell with electrons drifting to one

of the electrodes and holes drifting to the other electrode with diffusion becoming crucial in

carrier transport at the neutral region. Fig. 1.7b shows a solar cell with a resistive load. When

light is incident through the window layer, the electron-hole pairs generated in the absorber

layer will be swept out of the layer to produce the photocurrent Jph due to the electric field.

As a consequence, Jph produces a voltage drop across the resistive load. The net current of

this pn-junction solar cell, is

J = JF − Jph = J0[

exp(

eVnkT

)−1

]− Jph (1.2)

when the ideal-diode equation is used. The forward-bias voltage produces a forward-bias

current JF with a direction opposite to Jph.

1.2 Background 7

(a)

(b)

Fig. 1.7 (a) Bandgap structure, (b) A solar cell with a resistive load.

The primary parameters employed to describe the performance of a photovoltaic device

are the short-circuit current density (Jsc), open-circuit voltage (Voc), fill–factor (FF) and

conversion efficiency (η). Jsc is the current density that flows through the junction under

illumination under short circuit condition (bias is never applied to a solar cell). In the ideal

case it equals the photogenerated current density (Jph) and is proportional to the incident

number of photons or, alternatively, the intensity of illumination. Voc is the voltage across

the junction when the current through the junction is zero (i.e. open, no circuit load), and

can be expressed as

Voc =nkT

eln(

JphJ0

+1)

(1.3)

by rearranging equation 1.2 with J = 0. The point on the J–V curve that yields the maximum

power is referred to as the maximum power point (mp), the corresponding current density

and voltage are Jmp and Vmp as shown in Fig. 1.8. The fill–factor (FF) is a measure of the

8 Introduction

Fig. 1.8 Solar cell voltage-current characteristic under illumination.

"squareness" of the J–V curve, and is given by

FF =VmpJmpVocJsc

(1.4)

The standard test conditions are when Psun is 100 mW/cm2 of a normally incident air-mass

(AM) 1.5 spectrum at 25◦C. The efficiency of a solar cell is defined as the ratio of the max-

imum output power Pmax to the input power (incident power) Pincident , and can be expressed

as

η =Pmax

Pincident=

VocJscFFpincident

(1.5)

A real solar cell has a parasitic series resistance (Rs) and shunt resistance (Rsh). There

are several physical mechanisms responsible for these resistances. Series resistance Rs is

composed of the bulk resistance of the semiconductor materials and the resistance of the

front and back contacts. Shunt resistance Rsh is caused by leakage across the p–n junction

and around the edge of the cell. An ideal cell will have infinite shunt resistance (Rsh = ∞) and

zero series resistance (Rs = 0). A modest value of Rs mostly affects the far–forward voltage

region above Vmp, whereas the open–circuit voltage is not affected by Rs because no current

flows at Voc. The influence of Rsh is visible in the low voltage range (near zero and reverse

voltage). Both, Rs and Rsh can reduce the FF by a predictable amount. High values of Rs

and low values of Rsh can also reduce Jsc and Voc, respectively. Under illumination, the J-V

1.2 Background 9

characteristics of a solar cell with parasitic resistance can be described by

J = J0

[exp

(e(V −RsJ)

nkT

)−1

]− Jph + (V −RsJ)Rsh (1.6)

The electronic behavior of such a solar cell can be represented by an equivalent circuit

model, shown in Fig. 1.9, based on discrete electrical components. The photo-generation

mechanism is represented by the current generator, and the dark current is represented by the

diode which is oriented opposite to the current generator. The resistor parallel to the diode

represents the shunt resistance Rsh and the resistor that is in series with the rest of the circuit

represents the series resistance Rs.

Fig. 1.9 Equivalent circuit of a solar cell.

1.2.2 Thin-film solar cells (TFSCs)

Thin-film solar cells (TFSCs) have improved significantly over the years, with efficien-

cies approaching 23% in 2017, including cadmium telluride (CdTe) : 22.1% , copper indium

gallium diselenide (CIGS) : 22.6%, and amorphous thin-film silicon (a-Si) : 14%, as shown

in Fig. 1.10. The general advantages of thin-film technologies over crystalline silicon re-

late to the simple deposition processes on large area and cheap substrates such as glass,

lower material consumption, and the possibility of using flexible substrates in roll-to-roll

processes as shown in Fig. 1.11 [29]. Thin-films can be deposited by various techniques,

both physically and chemically. Film thickness varies from a few nanometers (nm) to tens

of micrometers (μm), much thinner than conventional poly-silicon solar cells which are hun-

dreds of micrometers (μm) thick. Therefore, thin film technologies allow solar cells to be

flexible, portable and stackable. All photovoltaic (PV) technologies are focused on low-cost

and high-conversion efficiency. In this respect, thin film PV technology is the best candidate,

10 Introduction

Fig.

1.10

Bes

teffi

cien

cyre

sear

chso

larc

ells

[30]

.

1.2 Background 11

Fig. 1.11 Flexible thin film solar cell [31].

because thin-film solar cells can be produced using cheap large-area deposition techniques

and low-cost substrates combined with schemes of monolithically interconnecting solar cells

in series for high efficiency [32].

Fig. 1.12 shows exactly cost effectiveness of thin-film solar cells in module price com-

pared to c-Si solar cells. Conventional c-Si PV modules are the most expensive PV tech-

nology, although they have the highest commercial efficiency. However, CIGS modules are

approaching the efficiency levels of c-Si modules [33]. Therefore, advantage of thin-film

solar cells on manufacturing cost is unquestionable.

In thin film solar cells, the absorber is the most important layer where the absorbed light

generates pairs of free electrons and holes, which can then be extracted to the contacts and

Fig. 1.12 Solar PV crystalline silicon and thin-film module cost learningcurve [33].

12 Introduction

Fig. 1.13 Absorption coefficient of various materials [34].

can contribute to the electrical current (photocurrent of the device). Thus, the layers must

absorb as much light as possible and be fabricated as thin as possible. The absorber layer

thickness is dependent on the absorption coefficient of the materials, as shown in Fig. 1.13.

The absorption coefficient in the semiconductor is a very strong function of photon energy

and bandgap energy. The intensity of the photon flux decreases exponentially with distance

through the semiconductor material. The relation between an incident photon intensity, I0,

and the photon flux at a position x, I(x), can be given by

I(x) = I0exp−αx (1.7)

where α is the absorption coefficient and x is the material thickness. Thus, for an absorption

coefficient of 1×104 cm−1, a film 2.3 μm thick absorbs about 90% of incident light. In orderto achieve the high absorption of light and reduction of layer thickness at the same time, the

materials must have a high absorption coefficient.

Since thin film solar cells consist of more than two thin layers, optical losses such as a

reflection loss in devices and optical absorption loss in window layers need to be reduced.

Commonly, an anti-reflection coating is deposited on thin-film solar cells using low refractive

index materials such as MgF2 to reduce the reflectance loss. TCO (transparent conductive

oxide) is generally used as a front contact in thin film solar cells, which need to have high

conductivity and transparency to improve efficiency of thin film solar cells by preventing any

1.2 Background 13

optical losses [35].

1.2.3 Tandem (multi-junction) solar cells

The theoretical maximum efficiency of single-junction cells has been calculated as a

function of bandgap energy, as depicted in Fig. 1.14 [36, 37]. The two main mechanisms

limiting solar cell efficiency are; losses by thermalization, and non-absorption of low-energy-

photons. In the case of thermalization, the excess energy of absorbed photons is transferred

to the active material via phonons. This energy is effectively lost to the photovoltaic conver-

sion process. In contrast, photons with energy lower than the bandgap cannot be absorbed

(see Fig. 1.15) [38]. Therefore, in single cell devices, a trade-off has to be found between

Fig. 1.14 Maximum theoretical efficiency (Shockley–Queisser limit) forthin film solar cells under AM1.5 illumination. Note that the bandgap ofthe CIGS system can be tuned by controlling the In/Ga ratio [37].

Fig. 1.15 Losses by thermalization and non-absorption of low-energy-photons.

14 Introduction

thermalization loss and non-absorption loss minimized by using wide and narrow bandgap

materials, respectively.

One promising approach to improve the efficiency of thin film photovoltaic devices is

multi-junction solar cells, which are comprised of a stack of devices made of different ma-

terials. The idea of a tandem cell is to achieve higher absorption efficiency using materials

with different bandgaps. One material is optimized to collect the higher energetic photons

and the other, with a narrower bandgap than the first one, is optimized to absorb photons

with lower energy [39]. Fig. 1.16 shows the ideal picture of the relation between the ab-

sorption spectra of the three solar cells used in a triple-junction tandem cell. As shown in

Fig. 1.16b, each solar cell with different bandgap material in a tandem solar cell absorbs and

produces electrical current in response to different wavelength of light. The efficiency of

this triple-junction tandem solar cell shows higher efficiency via absorption of wide range of

solar spectrum. Therefore, it is possible to enhance the efficiency by balancing the optical

(a)

(b)

Fig. 1.16 (a) Absorption of solar spectrum by different energy bandgapmaterials in a tandem solar cell, (b) Spectra absorption on each componentcell in a tandem solar cell.

1.2 Background 15

and electrical properties of each cell in a tandem cell.

There are two different methods to connect component cells: parallel or serial connec-

tions. For parallel connections [see Fig. 1.17, left-hand side], intermediate electrodes en-

sure charge collection for each individual cell. These back and front electrodes have to be

transparent to minimize photon losses and highly conducting to maximize charge carrier

collection. An obvious material for such electrodes would be a transparent conductive oxide

(TCO). Serial connection is more realizable (see Fig. 1.17, right-hand side), since it only re-

quires thin, non-continuous, non-absorbing metallic layers (tunnel or recombination layers)

to match currents of the different cells and collect the carriers at two electrodes. Tandem

cells in this thesis represent series connection only (monolithic tandem cell) fabricated by

stacking one solar cell on top of the other.

Although power conversion efficiency of solar cells has been increasing steadily (as

shown in Fig. 1.10) and their production costs have been decreasing, there is still further

room for improvement (for example, there is 10% absolute efficiency gap in CdTe solar

cells between practical and theoretical efficiency as shown in Fig. 1.10 and Fig. 1.14 respec-

tively). With increasing efficiency of individual solar cells, tandem solar cells will provide

more opportunities to improve the overall efficiency by combining these cells.

Fig. 1.17 Tandem solar cell operation with different connection (paralleland serial connection).

16 Introduction

1.3 Toward CdTe/Ge tandem solar cells

The efficiency of CdTe solar cells has reached beyond 20% in 2014 through over 40

years research, as shown in Fig. 1.10. Since it is difficult to improve solar cell efficiency

within a short time period, attempts to achieve higher efficiency solar cells have generally

taken the paths of: using novel materials (such as : Perovskites) and development of tandem

cells combining semiconductor materials with different energy bandgaps. In this work, a

tandem cell structure that utilizes Ge as a bottom cell and CdTe as a top cell is proposed. All

materials (CdS, CdTe and Ge) can be deposited by simple thermal evaporation and recrys-

tallized by thermal annealing to make the thin films required by solar cells. This process has

been demonstrated in previous work over a wide range of temperatures, lending credence

to the viability of a CdTe/Ge/low-cost-substrate structure. Although the bandgap of CdTe

(∼1.45 eV) and Ge (∼0.67 eV) are not optimal for a double-junction tandem cell due tophotocurrent mismatching, as shown in Fig. 1.18, more than 30% can be achieved theoreti-

cally, and they can serve as the basis for the development of triple-junction solar cells.

Since the data in Fig. 1.18 were calculated using bandgaps of materials without any opti-

cal and electrical losses [41], optical and electrical properties should be considered together

in practice and modeling. In the modeling results, which will be presented in Chapter 8, it

will be shown that current matching between a CdTe top cell and a Ge bottom cell can be

optimized by controlling the thickness of the individual absorbers. It should be noted that

the tandem-cell structure proposed in this thesis is formed employing thin-film technologies,

Fig. 1.18 The maximum efficiency for a double-junction tandem cell underAM1.5G spectrum [40]

1.4 Required specifications (see Fig. 1.19) 17

and should be realizable using deposition techniques that are, in principle, much simpler and

lower cost than those used for the fabrication of tandem cells based on ternary and quaternary

single-crystal semiconductors.

The success of a tandem cell depends not only on photocurrent matching, but also on

ensuring that the interface between the two cells allows continuity of the photocurrent. This

is usually achieved by creating a tunnel junction at the interface. In such a junction, the

holes constituting the photogenerated current in the top cell recombine, via band-to-band

tunneling, with photogenerated electrons from the bottom cell. Such a tunnel junction can

often be formed using degenerately doped regions.

1.4 Required specifications (see Fig. 1.19)

In order to optimize the tandem cell, the basic parameters (thickness and doping concen-

tration) listed in Table 1.2 and Fig. 1.19b need to be realized. The equilibrium band-diagram

for the tunnel junction is shown in Fig. 1.19a. The n+ emitter of the Ge cell forms one side

of the tunnel junction, and a p+ CdTe region was introduced to form the other side. These

parameters are for the basic tandem structure to be fabricated with an expected improvement

in photovoltaic conversion efficiency up to a value in excess of 20% modeled in previous

work [42]. However, as the record efficiency of solar cells continues to be improved and sev-

eral requirements are not practical, such as p+ CdTe layers, the expected efficiency needs to

be confirmed through experimental data. The achievable properties of thin films for tandem

cells will be demonstrated and modeled more thoroughly throughout this thesis.

Table 1.2 Required conditions for a CdTe/Ge tandem cell.

Individual cell Layers Doping Thickness

Top cell n-CdS ∼1×1018cm−3 100 - 150nmp-CdTe ∼1×1015cm−3 2.5 - 3μm

Tunnel junction p+ CdTe >2×1019cm−3 ∼50nmn+ Ge >2×1019cm−3 ∼50nm

Bottom cell n-Ge ∼2×1018cm−3 ∼100nmp-Ge ∼5×1017cm−3 5 - 20μm

18 Introduction

(a)

(b)

Fig. 1.19 (a) Band diagram for the tunnel junction and (b) details of aproposed tandem structure [42].

1.5 Thesis outline and scope

This thesis was part of a larger project that aims to determine the viability of practical

CdTe/Ge thin film tandem solar cells. While the thin film solar cells should be comprised

of either single-crystalline or poly-crystalline thin films, work presented in this thesis is

restricted to poly-crystalline to demonstrate cost-effective techniques using a simple evapo-

ration system. The objective of the present work was to overcome the limitations of solar

cell efficiency through a tandem structure. Device structures and processing methods were

developed to reduce the optical and electrical losses and enable the production of low-cost

photovoltaics with high conversion efficiency. This thesis consists of 9 chapters addressing

different aspects of the CdTe/Ge thin film tandem solar cell as shown in Fig. 1.20.

1.5 Thesis outline and scope 19

Chapter 2 : A literature review is presented in this chapter to understand previous process

development of CdTe and Ge stand-alone solar cells, and CdTe/Ge tandem solar cells. For

CdTe solar cells, the process development from the early stage of devices through to the

present technology is reviewed. Since there is only one paper regarding CdTe/Ge tandem

solar cells and few papers on Ge solar cells, the proposed design of tandem cells and process

methods of Ge cells in the papers are reviewed.

Chapter 3 : This chapter describes the experimental and characterization techniques used

to prepare thin films (Ge, CdS and CdTe), and to characterize their properties for fabrica-

tion of tandem solar cells. Since the preparation of different materials and characterization

of their properties are essential to fabricate devices, various deposition techniques were re-

viewed to find an adequate technique for thin films and devices. Characterization of crystal-

lized thin films was conducted from the structural, optical and electrical points of view. In

this work, all process methods are simple and cost-effective for photovoltaic devices.

Chapter 4 : Germanium (Ge) as a thin film material for the bottom cell in a tandem struc-

ture as shown in Fig. 1.20 was characterized and utilized to fabricate optoelectronic devices.

The Ge thin films used in this study were prepared by electron beam (ebeam) evaporation

and crystallized through solid phase crystallization (SPC) employing conventional thermal

annealing at different temperatures. The p-Ge/n-Si heterojunction devices using crystallized

Ge thin films demonstrated good optoelectronic performance. The results herein presented

demonstrate that crystallized germanium thin films can be employed to realize low-cost p-Ge

for electronic and optoelectronic applications. Furthermore, the potential for Ge thin films

Fig. 1.20 Chapter description illustrated through a CdTe/Ge tandem cellstructure.

20 Introduction

to be used in photovoltaic devices is feasible, if some conditions can be satisfied.

Chapter 5 : CdS thin films as a window layer in thin film solar cells as shown in Fig. 1.20

were studied in this chapter. To investigate the effect of the process conditions, the CdS films

were prepared by thermal evaporation at different substrate temperatures, and at different

post-deposition annealing temperatures. The high substrate deposition temperature improved

the thin film optical properties but degraded the electrical properties, while post-deposition

annealing was found to improve both the electrical and optical properties of CdS thin films

simultaneously. n-CdS/p-silicon heterojunction devices showed an improvement of diode

characteristics at the higher post-deposition annealing temperature. Evaporated CdS thin

films demonstrated in this work showed optical and electrical improvements under certain

process conditions. However, the transmittance for wavelengths in blue range of the solar

spectrum needs to be improved for CdTe solar cells.

Chapter 6 : CdTe thin films as an absorber layer in thin film solar cells were investigated

by preparing the films using a thermal evaporation system. The deposition temperature is

critical for the films to have large grain size with post-deposition annealing, as demonstrated

through XRD and SEM analysis. It is found that a CdCl2 treatment is essential to improve

electrical properties of CdTe thin films, although further study and understanding are still

needed. Using an evaporation system, CdS and CdTe films can be deposited in the same

chamber without breaking vacuum.

Chapter 7 : n-CdS/p-CdTe solar cells were fabricated and optimized as a top cell in a

tandem structure. Improvement of open-circuit voltage (Voc) and short-circuit current (Isc)

was obtained by applying post-deposition annealing and anti-reflection coating (ARC) using

MgF2, respectively. Since the presence of a back-contact barrier can significantly affect the

current-voltage characteristics of CdTe-based solar cells, the effect of metal back contact was

studied by Spice modeling. Additionally, optical losses were studied from the viewpoint of

CdTe solar cells.

Chapter 8 : In this chapter, several challenges and solutions suggested for the fabrication

of CdTe/Ge tandem solar cells are reviewed. With the proposed solutions, a CdTe/Ge tandem

solar cell was modeled using experimental optical properties of the thin films. It is found that

optical and electrical properties of both top and bottom cells need to be taken into account to

optimize tandem solar cells. Moreover, to match photocurrent between the top and bottom

cells, the thickness of each cell needs to be optimized, considering the individual optical

1.5 Thesis outline and scope 21

properties (absorption coefficient) of materials. Finally, several requirements were derived

from the simulation to optimize the efficiency of tandem solar cells approaching or exceeding

25% conversion efficiency.

Chapter 9 : In this chapter, summary and conclusions of this thesis, as well as suggestions

for future work, are presented.

Chapter 2

Review of CdTe and Ge single and

tandem solar cells

2.1 CdTe solar cells

Cadmium telluride (CdTe) solar cells are regarded as the leading thin-film photovoltaic

(PV) technology because it was the first PV technology to achieve a price per watt peak (Wp)

below $1 ($0.85) in 2009, commercialized by First Solar, Inc. [43]. CdTe thin film solar cells

have been studied for more than half a century, but the efficiency achieved remained at less

than 20% for a long time as shown by the progress of the technology in Fig. 2.1 [43, 44].

In 2015, the solar cell efficiency of 21.5% was achieved for a research cell, and 17.5% for a

module [21], thus making CdTe the leading thin film photovoltaic technology.

The first single-crystal and poly-crystal CdTe solar cells were made without CdS window

layers and transparent conductive oxide (TCO) contacts. In 1963, Cusano reported a CdTe

solar cell that employed a p-Cu2Te/n-CdTe heterojunction, and succeeded in obtaining 5.4%

efficiency [44]. Since Cu diffusion led to instabilities in the devices, CdS was combined with

CdTe to form a p-n heterojunction with efficiencies around 6% [45]. These cells were fabri-

cated in substrate configuration, having a Mo back contact and a CdTe thickness of more than

10 μm [45]. The substrate configuration is a traditional method to fabricate semiconductor

devices by stacking up materials on the bottom substrate as shown in Fig. 2.2a. The domi-

24 Review of CdTe and Ge single and tandem solar cells

Fig. 2.1 The best efficiency of CdTe solar cells and modules.

nant issues of CdTe solar cell development, such as the difficulty of doping p-type CdTe, the

difficulty in obtaining low-resistance contacts to p-type CdTe, and the recombination losses

associated with the junction interface became obstacles to achieve more than 6% efficiency.

Ten years later in 1982, Tyan et al. [46] presented a thin film cell fabricated in superstrate

configuration with more than 10% efficiency. This cell was grown by closed-space sublima-

tion. Soon after the realization of the first CdS/CdTe device, it was recognized that a cell

fabricated in superstrate configuration is more efficient. In the superstrate configuration the

materials are stacked, in reverse order to that of the conventional substrate configuration, on

top of a transparent support material such as glass or plastic, as shown in Fig. 2.2b. Another

important milestone was the discovery that chlorine treatment (CdCl2) of CdTe thin-films

substantially increased device efficiency. Such optimizations resulted in laboratory scale

solar cells with efficiencies exceeding 15% demonstrated by Ferekides et al. [47]. It took

(a) (b)

Fig. 2.2 Fabrication methods: (a) substrate, (b) superstrate configuration.

2.1 CdTe solar cells 25

Table 2.1 Photovoltaic properties of the best CdTe cell and module [21].

Material Eff.[%] Voc[V] Isc[A] FFCdTe cell 21.0 0.876 30.25m 79.4

CdTe module 17.5 103.1 1.553 76.6

almost 10 years before the efficiency reached 16.5%, as shown in Fig. 2.1. The research

groups at the National Renewable Energy Laboratory (NREL) and at the University of South

Florida (USF) are those that pushed the efficiency to the range of 16%.

It is remarkable that the highest efficiency CdTe PV devices are fabricated using poly-

crystalline rather than single crystalline CdTe. This is because grain boundaries enhance the

collection of photogenerated minority carriers, as demonstrated by many researchers [48–

50]. It is thus with great effort that the highest efficiencies at the cell and module level

available to date, as shown in Table 2.1, have been made possible. Interestingly, a non-

technical challenge that CdTe technology faces is that Cd and, to a lesser extent, Te are

considered as toxic materials. However, CdTe is very stable compound, which is not soluble

in water and with a high melting temperature (1092◦C), and once encapsulated it is definitely

harmless [51].

2.1.1 Highest efficiency

First Solar, Inc. announced their highest efficiency CdTe solar cell in 2015. Although the

process conditions have not been disclosed, the record solar cell performance was compared

with that of other materials and with the Shockley–Queisser (SQ) limit by Russell et al. [52].

The J-V curve of the record CdTe solar cell was also compared with other poly-crystalline

cells and the SQ limits at standard test conditions [21]. From the results shown in Fig. 2.3,

the current density of the CdTe record cell was found to be about 92% of the fundamental

limit, while Voc is approaching 80% of the fundamental limit. It is noticeable that there is

more room for Voc improvement. Therefore, the carrier concentration needs to be increased

in the CdTe layer by overcoming dopant compensation without additional current losses to

achieve higher Voc.

The quantum efficiency (QE) of record cells are compared in Fig. 2.4, showing that op-

tical losses have been largely minimized. The quantum efficiency (QE) is the ratio of the

number of carriers collected by the solar cell to the number of photons given from sunlight

on the solar cell. QE normally represents EQE (external quantum efficiency) including the

26 Review of CdTe and Ge single and tandem solar cells

Fig. 2.3 J-V comparison between the best efficiency record and SQ lim-its [52].

effect of optical losses such as transmission and reflection. Internal quantum efficiency (IQE)

refers to the efficiency with which photons absorbed in the cell can generate collectable car-

riers. Notably, losses in the CdS window layer have been essentially eliminated, resulting

in relatively square QE curves that are limited by the optical absorption of the window lay-

ers (glass/TCO/CdS) at short wavelengths. Optical properties of CdTe have been improved

through what appears as a reduction in the bandgap of the CdTe absorber for long wave-

lengths [53]. It indicates that optical properties of the films were influenced by process

conditions such as deposition temperature, CdCl2 treatment and CdS-CdTe interface. Thus,

the process optimization is essential to realize optimal bandgap. The effective CdTe bandgap

of the cells was calculated from the 35% point of the published QE graphs, as tabulated in

the legend of Fig. 2.4.

Fig. 2.4 Quantum efficiencies of record cells [52].

2.2 Ge solar cells 27

2.1.2 CdTe solar cells on flexible substrates

One of the advantages of thin film solar cells is their potential application in flexible

devices. A 16.4% high-efficiency CdTe solar cell on flexible substrates was achieved by

Mahabaduge et al. in 2015 [54]. Ultra-thin glass was used as a substrate for fabrication of the

flexible CdTe solar cell. The improvement was achieved by using sputtered CdS:O and co-

evaporated ZnTe:Cu with rapid thermal annealing. Optical properties of window materials

are very important to reduce absorption losses in the window layers. Using a sputtering

system, the CdS film was deposited in oxygen ambient to have high transmission in the

blue region of the solar spectrum, which enhanced the short-circuit current of solar cells.

As shown in Fig. 2.5 and Table 2.2, devices with sputtered CdS:O have a noticeably higher

QE for shorter wavelengths than devices with the CBD CdS, resulting in the 1.2 mA/cm2

improvement in short-circuit current density [54]. In this work, presented in Chapter 8, the

thickness of the CdS film was reduced in order to achieve high transmission in the blue region

of the spectrum. ZnTe:Cu was co-evaporated as a back contact in this paper, which enhanced

the Voc and FF . Since Cu diffusion affects solar cell performance during heat treatment, the

heat treatment after forming the back contacts with Cu should be controlled and optimized

carefully. If thermal treatments during or after Cu contact formation are conducted at high

temperature and/or for long process time, Cu diffusion will be induced and enhanced.

Table 2.2 A comparison of flexible cells with record device parame-ters [54].

Material Eff.[%] Voc[mV] Jsc[mA/cm2] FFOld best 14.1 822 24.3 70.3New best 16.4 831 25.5 77.4

2.2 Ge solar cells

Ge was used for semiconductor devices and photodetectors earlier than silicon because

of its high charge-carrier mobility and relatively high-absorption coefficient [55]. Further-

more, Ge is attractive for several reasons: it is a well-characterized material due to long-term

research; it is an elemental semiconductor; it can be deposited and recrystallized over a wide

range of temperatures, as demonstrated in the past as well as in this work. However, Ge

stand-alone solar cells suffer from poor performance due to thermalization losses, a con-

28 Review of CdTe and Ge single and tandem solar cells

Fig. 2.5 Quantum efficiencies of milestone cells on flexible substrates [54].

sequence of its relatively narrow bandgap (0.67 eV). Thus, as explained in Chapter 1, Ge

is mostly used as a substrate and a bottom cell material in monolithically stacked high-

efficiency multi-junction solar cells developed for space applications [56].

In a tandem cell structure, if the short-circuit current of two (or more) solar cells is not

exactly identical, one of the solar cells has to sacrifice its short circuit current, as shown in

Fig. 1.17. This sacrifice limits the contribution of one of the solar cells in the tandem cell.

As an alternative, a mechanically stacked solar cell in combination with a stand-alone Ge

bottom cell is under development for applications in space modules and terrestrial concen-

trator systems (even though the top cell, in a mechanically stacked solar cell, needs to have

transparent electrodes for both front and back contacts) [57]. Above all, since most research

on Ge solar cells has used single crystal, which is not cost-effective, it may prove to be

beneficial to study poly-crystalline Ge as a thin-film material for photovoltaic applications.

Therefore, crystallization of Ge films has been studied in this thesis, and will be presented in

detail in Chapter 4.

2.2.1 Highest efficiency

For fabrication of a high-efficiency stand-alone Ge solar cells, the most important steps

are forming a shallow emitter and a good passivation, because Ge has high absorption co-

efficient for photon energies above 0.8 eV and high surface recombination velocity at the

front and rear surfaces of the cell (when unpassivated). Both thick emitter and high sur-

face recombination will reduce the photocurrent of devices. The highest efficiency Ge solar

2.2 Ge solar cells 29

Fig. 2.6 I-V measurement of the highest efficiency Ge solar cell [55].

cell (7.8%) was obtained using an emitter formed by phosphorous diffusion from a spin-on

dopant (SOD) source as shown in Fig. 2.6 [55]. Firstly, the n+ emitter was formed in a p-

type Ge substrate using a spin-on dopant (SOD). Various conditions were studied to make

shallow emitter as shown in Fig. 2.7. Subsequently, using evaporated aluminum, the back

contact and the back surface field (BSF) were realized in a single process by subsequent

annealing at temperatures above the Al-Ge eutectic temperature (426◦C) to form a highly

doped p+ zone. After isolating devices by mesa etching, a thin layer of amorphous silicon

(a-Si) was deposited for surface passivation. The front contact and an antireflective coating

were formed by diffusion of Pd/Ag layer through this passivation layer and by evaporation

of ZnS and MgF2 respectively. From the literature, several useful methods could be adopted

Fig. 2.7 Phosphorous diffusion into Ge at different temperature andtime [55].

30 Review of CdTe and Ge single and tandem solar cells

in our work. Firstly, SOD is a low-cost method to dope Ge films simply by spinning dopants

and annealing. This can be applied in doping phosphorus to convert p-type poly-Ge into

n-type. Aluminum is a reliable candidate to form back contacts with a back surface field. As

a bottom cell in a tandem cell, a Ge cell does not require passivation separately since other

layers (tunnel junction) will be deposited on top of the Ge cell and metal contacts will be

formed on the bottom.

2.2.2 Ge-on-glass solar cells

For fabrication of Ge solar cells on a glass substrate using single-crystal Ge, epitaxial

growth and wafer bonding were conducted [58]. In order to fabricate the solar cells, six steps

were conducted, as schematically summarized in Fig. 2.8a : (a) growth of a Ge epilayer on a

Ge substrate, (b) H-implantation, (c) wafer bonding to glass, (d) layer-splitting and chemical

etch-back, (e) growth of the solar element, (f) mesa definition and contact lithography. As

shown in Fig. 2.8b, the highest efficiency of 2.6% of Ge-on-glass solar cells is not as good

as traditional Ge solar cells of 7.8% efficiency reported in Ref. [55]. However, it may be

not directly comparable because of the different process, substrate and thickness of layers.

Although epitaxial growth, wafer bonding and layer splitting are not likely to be adopted

in cost-effective fabrication, they are unavoidable if glass substrates and single-crystal Ge

need to be used at the same time. However, Ge deposited by simple thermal evaporation

(a)(b)

Fig. 2.8 (a) Fabrication of Ge on glass solar cells, (b) I-V characteristicsof the highest efficiency of Ge-on-glass solar cells under AM1.5G illumi-nation [58].

2.3 CdTe/Ge tandem solar cells 31

and crystallized by thermal annealing can be utilized on glass substrates without any high-

cost methods. Although completed Ge solar cells have not been fabricated in our work,

crystallized Ge on glass substrates were investigated and Ge/Si heterojunction devices were

fabricated, using solid-phase Ge crystallization, which will be presented in Chapter 4. There-

fore, crystallized poly-Ge provides a much simpler fabrication process at a lower cost than

single crystal Ge.

2.3 CdTe/Ge tandem solar cells

A CdTe/Ge tandem solar cell was proposed by Pulfrey et al. [59] to enhance CdTe solar

cell performance by adding a thin-film Ge solar cell. The proposed structure is depicted in

Fig. 2.9. As mentioned in Chapter 1 (Section 1.3), while the bandgaps of CdTe (∼1.45 eV)and of Ge (∼0.67 eV) are not optimal for a two-component tandem cell, the Ge bottom cellhas the required capability of generating a photocurrent to match that of the best CdTe cells.

As modeled in Chapter 8 (Section 8.2), current matching can be optimized by controlling

the Ge thickness, since optical absorption is a function of thickness. The optical and elec-

trical modeling was conducted simultaneously, and the calculated performance results are

summarized in Table 2.3.

CdTe/Ge tandem solar cells were modeled in detail and described by Sharp et al. [42]

Fig. 2.9 A suggested structure of CdTe/Ge tandem solar cells [59].

Table 2.3 Predicted photovoltaic performance of the CdTe/Ge tandem cellin the previous work [59].

Material Eff.[%] Voc[mV] Isc[mA/cm2] FFStand-alone Ge cell 4.8 216 24.3 70.0

CdTe/Ge tandem cell 21.8 1113 25.5 76.2

32 Review of CdTe and Ge single and tandem solar cells

with different thin-film CdTe/Ge tandem structures. CdTe/Ge tandem solar cell efficiencies

exceeding 20% were calculated. However, a thin tunnel junction based on degenerated CdTe

films, as suggested in the literature, is not realizable because dopant compensation limits the

highest effective doping possible [60]. Moreover, since the efficiency of a stand-alone CdTe

single cell now exceeds 20%, previous calculations may need to be revisited. Two signifi-

cant factors have led to improvement in CdTe-based solar cell performance (as indicated in

Subsection 2.1.1): improved optical properties of CdS films (high transmission in the blue

region of the solar spectrum), and improved optical properties of CdTe films (from bandgap

reduction). Optical transmission of CdS thin films was improved to allow the absorber layer

to collect more photons from the blue region of the solar spectrum. Deposition conditions

influence the optical properties of CdTe films, as well as their energy bandgap. Additionally,

higher doping in CdTe films at the absorber or back surface field (BSF) can be employed to

improve Voc. This is possibly how a high Voc was achieved in the highest record cell, which

approach 80% of the fundamental limit, as shown in Fig. 2.3. Hence, prior performance

predictions will need to be revisited, taking into account recent progress in material quality.

2.4 Summary and discussion

In this chapter, research on CdTe and Ge stand-alone solar cells, and tandem cells has

been reviewed. CdTe solar cells have been studied for more than half a century and have

now reached an efficiency of 21.5%. The theoretical efficiency limit of around 30% for

CdTe solar cells is still far from experimental results in state-of-the-art cells, but provides

an optimistic motivation. While the efficiency achieved of stand-alone Ge solar cells can

be further increased (e.g., employing shallow emitter region, improved surface passivation,

etc.), a stand-alone Ge solar cell will always underperform CdTe cells because of thermal-

ization losses arising from the narrow energy bandgap of Ge. However, Ge solar cells can

be employed as a bottom cell in a tandem cell arrangement, either in the monolithic or the

mechanically stacked configuration.

CdTe/Ge tandem solar cells have been proposed a relatively simple way of improving the

performance of CdTe solar cell technology. In the recent developments of CdTe solar cells

(∼21.0%), improvements in QE were achieved by reducing absorption losses in the windowlayers and by improving the crystal quality of the CdTe layer. These two optimization strate-

2.4 Summary and discussion 33

gies were employed in this work. Although previously modeled CdTe/Ge tandem cells are

not practical using current technology, there are opportunities for fabrication of a CdTe/Ge

tandem solar cells, as predicted by realistic simulations presented in Chapter 8.

Chapter 3

Experimental and characterization

techniques

There are various methods to deposit films for the fabrication of thin film solar cells. The

properties of the deposited thin films have been found to depend both on deposition method

and deposition conditions [61, 62]. Chemical-bath deposition (CBD) and close-spaced subli-

mation (CSS) are arguably the most popular techniques for the preparation of CdS and CdTe

thin films, respectively, due to the high quality of the films produced and their high through-

put [63]. For Ge films, different deposition techniques and recrystallization methods have

been utilized by many researchers [64]. In this work, evaporation techniques were employed

to fabricate solar cells, since these are relatively low-cost and low-temperature processes

that can be employed for all relevant materials (CdS, CdTe and Ge). The thermal evapo-

ration method was employed to deposit, and fabricate CdS and CdTe solar cells, on bare

glass or ITO-coated glass substrates; whereas for poly-Ge films, e-beam evaporation and

furnace annealing were employed for the deposition and solid phase crystallization (SPC),

respectively.

Furthermore, the material properties and fabrication process that impact the performance

parameters of thin-film solar cell need to be investigated. This demands the deployment of

various characterization techniques to gain insights that will enable optimization of material

characteristics and device performance. In this work, the required investigation of structural,

36 Experimental and characterization techniques

optical and electronic properties of evaporated Ge, CdS and CdTe was conducted. For the

characterization of electronic properties, Hall effect, current-voltage (I-V), and capacitance-

voltage (C-V) measurements were employed. I-V measurement in the dark and/or under

illumination were employed for the characterization of parasitic effects, and to identify loss

mechanisms and to quantify fundamental device properties. Structural characterization re-

lied on X-ray diffraction (XRD) and scanning electron microscopy (SEM) to investigate the

crystallinity, and the surface and cross-sectional morphology of deposited thin films. Optical

characterization required the use of transmission, absorption and reflection spectroscopy to

evaluate optical properties of thin films. Scanning laser microscopy (SLM) was employed to

evaluate spatial photocurrent distribution in heterojunction devices. Optical property param-

eters, such as refractive index and extinction coefficient, were extracted from the measured

characteristics to enable the modeling of thin film solar cells on the basis of experimentally

determined parameters.

All deposition, annealing and device fabrication processes were conducted at the Uni-

versity of Western Australian (UWA), employing facilities supported by Australian National

Fabrication Facility (ANFF).

3.1 Review of deposition methods

3.1.1 Ge deposition techniques

Single-crystal Ge ingots and wafers are produced via the Czochralski process, and ho-

moepitaxial and heteroepitaxial Ge layers can be deposited using various epitaxy meth-

ods. Homoepitaxial Ge layers have been grown by means of metal organic vapor phase

epitaxy (MOVPE) using iso-butyl germane (iBuGe) as metal-organic precursor [65]. Het-

eroepitaxial Ge layers have been grown selectively on silicon substrates patterned using a

SiO2 mask for the realization of pMOSFETs [66]. However, despite the demand for Ge

films for applications such as tandem solar cells, these growth methods are not likely to be

utilized for the realization of a commercially viable Ge-based photovoltaic technology, due

to their high cost and low throughput. In this subsection, methods for the preparation of poly-

crystalline Ge (poly-Ge) thin films on low-cost substrates, such as glass, were investigated

to provide scope for a low-cost fabrication process for optoelectronic device applications,

especially monolithic tandem solar cells.

3.1 Review of deposition methods 37

In the realisation of high-quality poly-Ge thin films, post-deposition recrystallization ap-

pears to have greater impact on crystal quality than the deposition method. The recrystalliza-

tion process step is performed by thermal annealing after deposition of amorphous germa-

nium (a-Ge). Many studies have reported investigations of solid-phase crystallization (SPC)

in germanium thin films deposited and crystallized using various methods and techniques,

with most studies seeking to optimize optical and electrical properties of the material for

specific applications [67–74]. Generally, solid-phase crystallization (SPC) is a simple and

cost-effective method to crystallize amorphous materials deposited on substrate materials

such as glass [75]. The thermodynamics and kinetics of solid-phase crystallization from the

amorphous phase to the poly-crystalline phase can be explained employing classical nucle-

ation and growth theory, as depicted schematically in Fig. 3.1. When amorphous films are

annealed to a certain temperature, the film is transformed into a thermodynamically stable

crystalline phase through four steps: incubation, nucleation, growth, and steady state. In

this process, small crystallites are formed at nucleation sites, which then grow in size with

increasing time at the expense of the contiguous amorphous matrix [76]. Solid-phase epi-

Fig. 3.1 Schematic illustration of solid phase crystallization processes ina-Si: (a) random nucleation and growth, (b) solid phase epitaxy [76].

38 Experimental and characterization techniques

taxy (SPE) growth, in contrast, can be regarded as an interface mediated process, since the

rearrangement of atoms is influenced by the interface between the amorphous film and the

single-crystal template [77], as shown in Fig. 3.1.

In order to achieve lower costs and utilization in a wider range of applications, inex-

pensive materials such as glass are more attractive than quartz substrates. For the case of

glass substrates, material and device processing steps need to be limited to temperatures be-

low 550◦C. In this case, simple SPC of Ge films is a feasible process for the realization of

poly-Ge because a-Ge films start to crystallize at around 400◦C, as described in Chapter 4.

An alternative technique is rapid thermal annealing (RTA), which has the advantage of high

heating rates (up to 60◦C/s) that result in significant reduction of the total crystallization time.

During RTA, thermal radiation is applied in pulses to heat the sample surface, minimizing

heating of the glass substrate (which is transparent to the infrared radiation).

In an effort to reduce the crystallization temperature and crystallization time, and to

increase the grain size, metal-induced crystallization (MIC) has also been investigated as

an alternative crystallization process for thin-film device fabrication. MIC is in general

an interface-controlled phenomenon occurring in metal/amorphous semiconductor systems.

Initially the interfacial covalent bond–weakening effect, in the interface between the semi-

conductor and metals, generates a limited amount of high mobility semiconductor atoms.

These “free” semiconductor atoms may migrate along short-circuit fast diffusion paths such

as metal/semiconductor interfaces and metal GBs at low temperatures. The occurrence of in-

termediate wetting, the nucleation of crystallization, and the continued semiconductor crystal

growth are all governed by the interface thermodynamics under the constraint of a limited

amount of available “free” semiconductor atoms [70, 78–81].

The Ge MIC process involves the deposition of a-Ge films on top of which a layer of

suitable metal is deposited. This bilayer of metal and Ge is then annealed in a furnace at

temperatures ranging from 150 to 500◦C for durations between one minute to several hours,

leading to crystallization of the a-Ge. The growth rate depends on the annealing conditions

of the bilayer. In the MIC process, metals such as aluminum, nickel, gold, etc. are used to

decrease the crystallization temperature below 500◦C, as shown in Fig 3.2. In case of Pd

and Cu, it has been reported that crystal