Developing Efficient Cu2ZnSnS4 (CZTS) Thin Film Solar Cells by Heterojunction Engineering

Abstract

Kesterite Cu2ZnSnS4 (CZTS), having the Earth-abundant and environment-benign constituents, and stable structure, is regarded as a promising thin-film photovoltaic material. However, the current power conversion efficiency (PCE) of CZTS solar cells is far below the commercialization-viable level. One of the main issues restricting the efficiency is the severe Shockley-Read-Hall (SRH) recombination at the highly defective CdS-buffer/CZTS-absorber heterointerface and within the CZTS absorber layer, giving rise to a large open circuit voltage (Voc) deficit. This thesis aims to mitigate the SRH recombination within the CdS/CZTS heterojunction by novel post-deposition treatment technologies to facilitate the passivation of the local defects.

Firstly, the ultrathin intermediate stannic oxide (SnO2) layer was introduced at the CZTS/CdS heterointerface via a solution method. The employment of this layer enabled the effective passivation of the heterointerface, resulting in higher Voc, fill factor (FF) and thus PCE. Secondly, we applied our in-house developed moisture-assisted post-deposition annealing (MAPDA) treatment to modify the heterojunction by manipulating the trace element distributions. This technology enabled Na and K depletion in the CZTS film, which, in turn, facilitated the spontaneous Cd diffusion during the chemical bath deposition process for CdS buffer layer, driving a significant improvement in device performance. The heterojunction modification is attributed to the remarkable mitigation of local deep-level defect and the creation of the preferrable shallow acceptor copper vacancies. Peak efficiency at 9.40 % was obtained using the combined MAPDA and heterojunction air annealing (HJA) treatments, which further optimized the elemental distributions within the heterojunction region. Finally, the nanoscale optoelectronic characterization techniques, including Kelvin probe force microscopy (KPFM) and conductive-atomic force microscopy (C-AFM) were applied to investigate the impact of excess Na and K at the CZTS surface. The significant enhancement of quasi-Fermi level splitting and effective alleviation of SRH recombination through the combined MAPDA and HJA treatments were also revealed by surface photovoltage (SPV) analysis through KPFM.

These technologies with first-hand novelty explore new defect passivation routes in kesterite solar cells, which can also be widely applied to other thin-film solar cells.