Origin of performance loss in post-deposition Na-treated Cu(In,Ga)Se2 solar cell

The post-deposition Na-treated Cu(In,Ga)Se2 photovoltaic absorber layer after film growth was experimentally compared with the conventional Cu(In,Ga)Se2 grown under Na environment. The post-deposited Na was found to function in a similar way to the conventional Na, which greatly increased hole concentration of the absorber. However, the post-deposition treatment deteriorated the double-graded bandgap profile and induced a lattice contraction of the Cu(In,Ga)Se2 grown under Na-free condition, which may be responsible for the inferior open-circuit voltage and fill factor compared to the conventional Cu(In,Ga)Se2 absorber grown on sodalime glass substrate.