High-efficient Cd-free CZTS solar cells achieved by nanoscale atomic layer deposited aluminium oxide  
Xin Cui, Kaiwen Sun, Jialiang Huang, Chang-Yeh Lee, Chang Yan, Heng Sun, Yuanfang Zhang, Martin Green, Bram Hoex, Xiaojing Hao
UNSW, Sydney, Australia

Earth-abundant and non-toxic kesterite Cu2ZnSnS4 (CZTS) solar cells are considered as a promising alternative candidate for thin film solar cells, but still being hampered by a relatively low VOC.  Here, we propose a heterojunction modification strategy by applying atomic layer deposited aluminium oxide (Al2O3) for heterojunction interface passivation. Through post-treatment of CZTS absorber by nanoscale ALD Al2O3, the effective lifetime of the minority carrier at the heterojunction region is improved, indicating effectively passivating by nanoscale alumina. With a significant gain of Voc by 50 mV, we demonstrated a device efficiency of 9.9 %, the highest efficiency ever reported for Cd-free CZTS solar cells.