Defect Control for 12.5% Efficiency Cu₂ ZnSnSe₄ Kesterite Thin-Film Solar Cells by Engineering of Local Chemical Environment.

Abstract

Kesterite-based Cu₂ ZnSn(S,Se)₄ semiconductors are emerging as promising materials for low-cost, environment-benign, and high-efficiency thin-film photovoltaics. However, the current state-of-the-art Cu₂ ZnSn(S,Se)₄ devices suffer from cation-disordering defects and defect clusters, which generally result in severe potential fluctuation, low minority carrier lifetime, and ultimately unsatisfactory performance. Herein, critical growth conditions are reported for obtaining high-quality Cu₂ ZnSnSe₄ absorber layers with the formation of detrimental intrinsic defects largely suppressed. By controlling the oxidation states of cations and modifying the local chemical composition, the local chemical environment is essentially modified during the synthesis of kesterite phase, thereby effectively suppressing detrimental intrinsic defects and activating desirable shallow acceptor Cu vacancies. Consequently, a confirmed 12.5% efficiency is demonstrated with a high V_OC of 491 mV, which is the new record efficiency of pure-selenide Cu₂ ZnSnSe₄ cells with lowest V_OC deficit in the kesterite family by E_g /q-Voc. These encouraging results demonstrate an essential route to overcome the long-standing challenge of defect control in kesterite semiconductors, which may also be generally applicable to other multinary compound semiconductors.