Properties of Cu1-x K x InSe2 alloys

Adding potassium to Cu(In,Ga)Se-2 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 a parts per thousand yen 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.