Cu₂ZnSnS₄ nanoparticulate thin films and their solar cell characteristics

Abstract

Thin film technology for solar cells has emerged as the most promising alternative since it can beat the absorption limit of crystalline silicon solar cells and has also an ideal band gap as compared to silicon. However, current thin films are based on rare and toxic elements such as cadmium and indium. Therefore, during the past 5-7 years, there has been a sudden increase in the interest to develop Cu₂ZnSnS₄ (CZTS) thin films (based on all non-toxic abundant elements) to facilitate the increasing demand for the electricity and its production. CZTS demonstrate a high optical absorption coefficient (10⁴ cmˉ¹), a suitable band gap (1.4-1.6 eV) and a high electrical conductivity, making it an ideal material for solar energy conversion applications. The aim of this study is to fabricate and characterize CZTS thin films and evaluate their solar cell performance. In this work, CZTS thin films were grown using two different processes such as electrodeposition and chemical synthesis approach. The first objective is to fabricate high quality quaternary CZTS thin films by single step electro deposition method followed by sulfurization at 550°C. For this, we systematically studied the role of deposition potential over compositional, structural and optical properties of the CZTS films. We discuss the nucleation and growth mechanism of the CZTS thin film and propose an optimum deposition potential of -1.4 V vs. Ag/AgCl to achieve an ideal composition, uniform growth (void free) with tunable optical properties. Using the chrono-amperometric data and the Scharifker and Hill model we found that the nucleation mechanism for CZTS thin film is instantaneous. Optical properties demonstrated the optimum band gap of 1.5 eV for kesterite CZTS films prepared from a precursor electrodeposited at -1.4 V vs. Ag/AgCl. CZTS thin films deposited at optimum potential and assembled into the solar cell structure demonstrated an efficiency of 5.0%. The work also includes the synthesis of size controlled kesterite phase CZTS nanoparticles. The nanoparticle size has been controlled from 2-8 (± 0.5) nm by a simple control of amine-to-precursor molar ratio. We demonstrate the synthesis of as much as 20 gm of quaternary chalcogenide nanoparticles powder in a single reaction, without a size-sorting process. The fabricated devices exhibit efficiencies ranging from 3.6% to 4.5% depending on the CZTS particles average diameter. Comparison of EQE data suggests that our current is mainly restricted by the collection losses. Further, we have incorporated the extrinsic impurities such as Fe and Mn (to replace Zn) or Se (to replace S) into CZTS in order to control secondary phase formation, micro structural and band gap variation of CZTS. It is demonstrated that the crystal structure, band gap, and photo response of CZTS thin films are affected by substitution of anion/cations. The absorption characteristics show the earth abundant compound Cu₂MSnS₄/Se₄ (M=Zn, Mn and Fe) band gaps are in the range of 1.0 to 1.5 eV with high optical absorption coefficients (~10⁴ cmˉ¹) in the visible region. The efficiency of CZTS solar cell is enhanced significantly from 4.5% to 7.6% with selenium doping. The variation of device performance may be ascribed to the changes in the micro-structure and band gap. A further improvement in the power conversion efficiency was achieved by embedding the silica nanoparticles (150 nm) into the CZTS absorber layer; we studied the effect of silica particles depth in the CZTS absorber layer on optical absorption and solar cell performance theoretically and experimentally as well. We fabricated the CZTS solar cells with 150 nm sized silica nanoparticles at the top, middle and bottom of the absorber layer for light-trapping. The best optical absorption and electrical efficiency (η=5.3%) is observed for particles placed at the middle-level of the absorber layer; this is 18% more than that of the reference cell (η=4.5%). Particles placed on the top lead to higher optical absorption but lower conversion efficiency compared to particles at the bottom. Thesis submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy of the Indian Institute of Technology Bombay, India and Monash University, Australia.