Lithium-doping inverts the nanoscale electric field at the grain boundaries in Cu sub(2)ZnSn(S,Se) sub(4) and increases photovoltaic efficiency

Passive grain boundaries (GBs) are essential for polycrystalline solar cells to reach high efficiency. However, the GBs in Cu sub(2)ZnSn(S,Se) sub(4) have less favorable defect chemistry compared to CuInGaSe sub(2). Here, using scanning probe microscopy we show that lithium doping of Cu sub(2)ZnSn(S,Se) sub(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 Cu sub(2)ZnSn(S,Se) sub(4). We propose that lithium competes for copper vacancies (forming benign isoelectronic Li sub(Cu) defects) decreasing the concentration of Zn sub(Cu) donors and competes for zinc vacancies (forming a Li sub(Zn) acceptor that is likely shallower than Cu sub(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 Cu sub(2)ZnSn(S,Se) sub(4). This will be essential for this low-cost Earth abundant element semiconductor to achieve efficiencies that compete with CuInGaSe sub(2) and CdTe.