Sub-micron Cu(In,Ga)Se2 solar cells with an efficiency of 18.2% enabled by a hole transport layer

Abstract

Reducing the thickness of Cu(In,Ga)Se₂ solar cells is a key objective to reduce production cost and to improve sustainability. The major challenge for sub-micron Cu(In,Ga)Se₂ cells is the recombination at the backside. In standard Cu(In,Ga)Se₂ backside recombination is suppressed by a bandgap gradient, acting as a back surface field. This gradient is difficult to maintain in sub-micron absorbers. In this study, a hole transport layer passivates the back contact and enables efficient sub-micron Cu(In,Ga)Se₂ solar cells without the need for a Ga gradient. The backside passivation by the hole transport layer is as effective as an optimized Ga gradient, resulting in a significant increase in open-circuit voltage by 80 mV in comparison to the reference sample. Moreover, the hole transport layer exhibits good transport properties, leading to a fill factor as high as 77%. A photoluminescence quantum yield of 0.15% and efficiency above 18% are demonstrated in sub-micron Cu(In,Ga)Se₂ absorbers.