Photoluminescence characterization of Cu2Sn1-xGexS3 bulk single crystals

Naoya Aihara,Kunihiko Tanaka

doi:10.1063/1.5050033

Naoya Aihara, Kunihiko Tanaka

Open Access

https://doi.org/10.1063/1.5050033

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Journal: AIP Advances	Publication Date: Sep 1, 2018
Citations: 6	License type: cc-by

Affiliation: Nagaoka University of Technology

Abstract

Cu2Sn1-xGexS3 (CTGS) which is composed of earth-abundant and non-toxic elements is a promising material for the absorber layer of thin-film solar cells. In this study, the optical properties of CTGS bulk single crystals with varying germanium content were investigated by temperature and excitation power dependent photoluminescence (PL) measurements. At low-temperature, excitons and broad defect-related bands were observed in the PL spectra from all samples. These PL bands indicate a blue-shift with an increase in the germanium content, which suggested the formation of solid-solutions in CTGS. The broad band was dominated by donor-acceptor pair (DAP) recombination luminescence. The DAP bands were due to the transition of carriers between shallow acceptors and relatively deep donors for any alloy composition. Band-to-band (BB) recombination luminescence was also observed from all samples at room temperature. The band gap energies were varied from 0.933 to 1.544 eV with an increase in the germanium content, which was determined by spectral fitting of the BB bands. In addition, a small optical bowing parameter b, of ca. 0.1 eV was determined, which indicates that the band gap energy of CTGS can be controlled almost linearly by varying the alloy composition. Therefore, the optimum band gap energy for single-junction solar cells can be achieved by control of the CTGS alloy composition.

Highlights

Copper chalcogenide semiconductors such as Cu(In,Ga)Se2 and Cu2ZnSn(S,Se)[4] have recently been considered as good candidates for the absorber layer in thin-film solar cells
Cu2Sn1-xGexS3 (CTGS) which is composed of earth-abundant and non-toxic elements is a promising material for the absorber layer of thin-film solar cells
CTGS is a p-type semiconductor with tunable direct band gap energy (Eg) from 0.923 to 1.6 eV4 due to the formation of a solid solution between Cu2SnS3 (CTS) and Cu2GeS3 (CGS) and a large absorption coefficient higher than 104 cm-1.5–7 Some research groups have fabricated CTGS-based solar cells,[8,9,10,11] and CTGS-based thin-film solar cells with power conversion efficiencies of 6.7% have recently been demonstrated with a graded band gap structure in the absorber layer.[12]

Summary

INTRODUCTION

Copper chalcogenide semiconductors such as Cu(In,Ga)Se2 and Cu2ZnSn(S,Se)[4] have recently been considered as good candidates for the absorber layer in thin-film solar cells. Cu(In,Ga)Se2 and Cu2ZnSn(S,Se)4-based thin-film solar cells have been demonstrated with respective power conversion efficiencies of 22.6%1 and 12.6%.2. These materials consist of rare (indium, gallium, and selenium) and toxic (selenium) elements. CTGS is a p-type semiconductor with tunable direct band gap energy (Eg) from 0.923 to 1.6 eV4 due to the formation of a solid solution between Cu2SnS3 (CTS) and Cu2GeS3 (CGS) and a large absorption coefficient higher than 104 cm-1.5–7 Some research groups have fabricated CTGS-based solar cells,[8,9,10,11] and CTGS-based thin-film solar cells with power conversion efficiencies of 6.7% have recently been demonstrated with a graded band gap structure in the absorber layer.[12] the highest reported conversion efficiency is only approximately one-fifth of the theoretical limit.[13] a deep understanding of the fundamental properties of CTGS is necessary for further improvement of the power conversion efficiency. Eg is one of the most significant parameters for photovoltaic absorber materials; PL observation was performed to determine Eg for all of the CTGS samples at 298 K, which is the typical operation temperature of photovoltaic cells

EXPERIMENTAL

RESULTS AND DISCUSSION

CONCLUSIONS