Lithium-doping inverts the nanoscale electric field at the grain boundaries in Cu2ZnSn(S,Se)4 and increases photovoltaic efficiencyElectronic supplementary information (ESI) available: X-ray diffraction (XRD), scanning electron microscopy (SEM), inductively coupled plasma mass spectroscopy (ICPMS), drive-level capacitance profiling (DLCP), X-ray photoelectron spectroscopy (XPS), recrystallizaiton of thiourea, and Li doping concentration study. See DOI: 10.1039/c5cp04707b

Passive grain boundaries (GBs) are essential for polycrystalline solar cells to reach high efficiency. However, the GBs in Cu 2 ZnSn(S,Se) 4 have less favorable defect chemistry compared to CuInGaSe 2 . Here, using scanning probe microscopy we show that lithium doping of Cu 2 ZnSn(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 Cu 2 ZnSn(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 Cu 2 ZnSn(S,Se) 4 . This will be essential for this low-cost Earth abundant element semiconductor to achieve efficiencies that compete with CuInGaSe 2 and CdTe. Lithium doping changes the electric field at the GBs and improves DMSO solution processed Cu 2 ZnSn(S,Se) 4 solar cell efficiency to 11.8%.