Developing high efficiency Cu2ZnSnS4 (CZTS) thin film solar cells by sputtering

Kesterite Cu2ZnSnS4 (CZTS) has demonstrated high potential and value as anew promising absorber material for thin film photovoltaics recently. It possesses themerits of earth abundant element constituent, facilitating its low-cost and large-scaleproduction. Moreover, as a high bandgap material (Eg~1.5eV), CZTS is able to becombined with other photovoltaic cells with lower bandgap (like Si) for tandem solarcells, enabling higher power conversion efficiency beyond the single junction solar cellShockley-Queisser efficiency limit. To be widely deployed in either case, thedevelopment of high efficiency single junction CZTS solar cells is the first priority.This thesis aims to investigate, understand and address the present efficiency-limitingissues for the sake of boosting the efficiency of CZTS solar cells.Firstly, the effect of CZTS composition has been carefully studied. The Cucontent can greatly affect the microstructure of CZTS grains and, the electrical propertyof CZTS film such as carrier concentration and minority carrier lifetime, therebyinfluencing the CZTS solar cell efficiency.Secondly, the effects of sulfurization annealing atmosphere and post-heattreatment have been studied. The “epitaxial” CdS/CZTS interface and moderate Cddiffusion effects were discovered, found to play critical roles in improving the cellefficiency.Thirdly, the conduction band alignments of different buffer materials with CZTShave been carefully investigated. Conduction band offset (CBO) at the CdS/CZTShetero-interface was found to be unfavourably “cliff-like” whereas CBO of In2S3/CZTSwas confirmed to be “spike-like”. Based on these results, the CdS/In2S3 hybrid bufferswith different stacking sequences were studied, among which CdS/In2S3/CZTS showsthe best cell performance.Last but not least, Cd alloying with CZTS has been studied. Cd alloyed CZTS(i.e. CZCTS (Cu2ZnxCd1-xSnS4)) can effectively boost energy conversion efficiency toover 10% (active area). The improved efficiency is believed to arise from better qualityof the CZCTS absorber, i.e. larger grains, longer minority carrier lifetime and reducedband tailing issues.Based on above combined processing strategies, a full-sized 7.6% efficientCZTS solar cell (with total area over 1.0 cm2) has been achieved (certified by NREL),setting a new world record for such a standard CZTS solar cell