Improvement of voltage deficit of Ge-incorporated kesterite solar cell with 12.3% conversion efficiency

We demonstrate the improved efficiency of a Cu2Zn(Sn1− x Ge x )Se4 (CZTGSe) thin-film solar cell with a conversion efficiency of 12.3%; this cell exhibits a greatly improved open-circuit voltage (V OC) deficit of 0.583 V and a fill factor (FF) of 0.73 compared with previously reported CZTGSe cells. The V OC deficit was found to be improved through a reduced band tailing via the control of the Ge/(Sn + Se) ratio. In addition, the high FF was mainly induced by a reduced carrier recombination at the absorber/buffer interface and/or in the space charge region, whereas parasitic resistive effects on FF were very small.