Structural, Electronic and Optical Properties Of Cu2SnS3 Solar Absorber: A First-Principle Density Functional Theory Investigation

Structural, Electronic and Optical Properties Of CU2SNS3 Solar Absorber: A First-principle Density Functional Theory Investigation

ABSTRACT

The development of inexpensive, non-toxic, high efficiency and earth-abundant solar absorbers is critical for terawatt scale implementation of photovoltaics. Cu2SnS3 is a promising earth abundant absorber material that is attracting attention recently for optoelectronic application including photovoltaic solar cells. However, very little is known about the relationship between structural and optical properties such as the absorbance, reflectivity, refractive index, extinction coefficient and energy loss function. These information are however, very essential for the design and fabrication of Cu2SnS3 photovoltaic devices to achieve higher power conversion efficiencies.

In this thesis, first-principles calculation based on state-of-the-art methodology of screened hybrid density functional theory (DFT) have been employed to comprehensively characterize the structural, electronic, and optical properties of Cu2SnS3 material. From band structure analysis, Cu2SnS3 is demonstrated to be a direct band gap materials with a predicted band gap of 0.9 eV, which is in good agreement with available experimental values of 0.9 – 1.3 eV.

It is evident from the calculated partial density of states (PDOS) that the anti-bonding Cu-d, Sn-p and S-p states are involved in the transition from valence to conduction band. On the basis of the calculated optical absorbance (in the order 105 cm?1), reflectivity (approx. 23%), refractive index (approx. 2.90) and extinction coefficient and energy loss function, Cu2SnS3 is demonstrated to be an attractive non-toxic, earth-abundant, and cost effective material for scalable thin-film PV applications.

TABLE OF CONTENTS

Abstract i
Acknowledgments ii
Dedication iii
Table of Contents v
List of Abbreviation vi
List of Figures vii
List of Tables viii
1 INTRODUCTION 1
1.1 Issues facing traditional photovoltaics . . . . . . . . . . . . . . . . . . 1
1.2 Earth abundant thin film photovoltaics . . . . . . . . . . . . . . . . . 2
1.2.1 Cu-Sn-S system ternary materials . . . . . . . . . . . . . . . . 3
1.3 Problem statement . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.4 Aims and objectives . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
2 THEORETICAL BACKGROUND 5
2.1 First-Principles Calculations . . . . . . . . . . . . . . . . . . . . . . . 6
2.2 Schrödinger Equation . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
2.2.1 Born-Oppenheimer approximation . . . . . . . . . . . . . . . . 8
2.2.2 Density Functional Theory . . . . . . . . . . . . . . . . . . . . 9
2.2.3 The Thomas-Fermi (TF) Model . . . . . . . . . . . . . . . . . 11
2.2.4 The Hohenberg-Kohn (HK) Theorem . . . . . . . . . . . . . . 11
2.3 Wave-Function Based Methods . . . . . . . . . . . . . . . . . . . . . 13
2.3.1 The Hartree-Fock Formalism . . . . . . . . . . . . . . . . . . . 14
2.4 The Kohn-Sham (KS) Scheme . . . . . . . . . . . . . . . . . . . . . . 17
2.5 Exchange-correlation functional . . . . . . . . . . . . . . . . . . . . . 19
2.5.1 The Local-Density Approximation (LDA) . . . . . . . . . . . . 20
2.5.2 The Generalized-Gradient Approximation (GGA) . . . . . . . 21
iv
2.5.3 Meta-GGA (mGGA) . . . . . . . . . . . . . . . . . . . . . . . 22
2.5.4 Hybrid Schemes . . . . . . . . . . . . . . . . . . . . . . . . . . 23
3 METHODOLOGY AND COMPUTATIONAL DETAILS 25
3.1 Basis Sets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
3.1.1 Plane Waves . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
3.1.2 Pseudo-potentials . . . . . . . . . . . . . . . . . . . . . . . . . 27
3.2 Computational Details . . . . . . . . . . . . . . . . . . . . . . . . . . 30
4 RESULTS AND DISCUSSION 32
4.1 Structural Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
4.2 Electronic Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
4.2.1 Band Structure . . . . . . . . . . . . . . . . . . . . . . . . . . 33
4.2.2 The Density of States(DOS) . . . . . . . . . . . . . . . . . . . 35
4.3 Optical Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
4.3.1 Dielectric Function . . . . . . . . . . . . . . . . . . . . . . . . 36
4.3.2 The Absorption Coefficient . . . . . . . . . . . . . . . . . . . . 37
4.3.3 Refractive index . . . . . . . . . . . . . . . . . . . . . . . . . . 38
4.3.4 The Reflectivity . . . . . . . . . . . . . . . . . . . . . . . . . . 39
4.3.5 The Extinction Coefficient . . . . . . . . . . . . . . . . . . . . 40
4.3.6 Energy Loss Function . . . . . . . . . . . . . . . . . . . . . . . 41
5 SUMMARY AND CONCLUSION 43
Bibliography 44