Performance characterization: The current density (J-V) characteristics of the devices were measured using a fully automated combinatorial high-throughput solar cell measurement system that was reported by us previously. To simulate solar light, an ABET 3000 solar simulator with a xenon arc lamp, fed with 1000 W input power was used. The light intensity was calibrated using a professional reference silicon cell with an IR-cut off filter (KG5, Schott).