Lithium-doping inverts the nanoscale electric field at the grain boundaries in Cu2ZnSn(S,Se)4 and increases photovoltaic efficiency

Abstract

Passive grain boundaries (GBs) are essential for polycrystalline solar cells to reach high efficiency. However, the GBs in Cu₂ZnSn(S,Se)₄ have less favorable defect chemistry compared to CuInGaSe₂. Here, using scanning probe microscopy we show that lithium doping of Cu₂ZnSn(S,Se)₄ changes the polarity of the electric field at the GB such that minority carrier electrons are repelled from the GB. Solar cells with lithium-doping show improved performance and yield a new efficiency record of 11.8% for hydrazine-free solution-processed Cu₂ZnSn(S,Se)₄. We propose that lithium competes for copper vacancies (forming benign isoelectronic Li_Cu defects) decreasing the concentration of Zn_Cu donors and competes for zinc vacancies (forming a Li_Zn acceptor that is likely shallower than Cu_Zn). Both phenomena may explain the order of magnitude increase in conductivity. Further, the effects of lithium doping reported here establish that extrinsic species are able to alter the nanoscale electric fields near the GBs in Cu₂ZnSn(S,Se)₄. This will be essential for this low-cost Earth abundant element semiconductor to achieve efficiencies that compete with CuInGaSe₂ and CdTe.