Properties of Cu1−x K x InSe2 alloys

Abstract

Adding potassium to Cu(In,Ga)Se2 absorbers has been shown to enhance photovoltaic power conversion efficiency. To illuminate possible mechanisms for this enhancement and limits to beneficial K incorporation, the properties of Cu1−x K x InSe2 (CKIS) thin-film alloys have been studied. Films with K/(K + Cu), or x, from 0 to 1 were grown by co-evaporation, and probed by XRF, EPMA, SEM, XRD, UV–Visible spectroscopy, current–voltage, and TRPL measurements. Composition from in situ quartz crystal and EIES monitoring was well correlated with final film composition. Crystal lattice parameters showed linear dependence on x, indicating complete K incorporation and coherent structural character at all compositions in the <100> and <010> lattice directions, despite the different symmetries of CuInSe2 and KInSe2. The band gap energy showed pronounced bowing with x composition, in excellent agreement with experimental reports and semiconductor theory. Films of Mo/CKIS/Ni were non-ohmic, and increasing x from 0 to 0.58 decreased the apparent CKIS resistivity. Further evidence of decreased CKIS resistivity was observed with photoluminescence response, which increased by about half a decade for x > 0, and indicates increased majority carrier concentration. Minority carrier lifetimes increased by about an order of magnitude for films grown at x = 0.07 and 0.14, relative to CuInSe2 and x ≥ 0.30. This is the first report of a Cu-K-In-Se film with >1 at.% K, and the observed property changes at increased x (wider band gap; lower resistivity; increased lifetime) comprise valuable photovoltaic performance-enhancement strategies, suggesting that CKIS alloys have a role to play in future engineering advances.