Chemistry of K in Cu(In,Ga)Se2 photovoltaic absorbers: Effects of temperature on Cu-K-In-Se films

Abstract

Incorporation of K has led to world record Cu(In,Ga)Se2 photovoltaic power conversion efficiencies, but there is poor consensus about the role of phase impurities in these advances. This work lays a foundation for identifying and controlling these phase impurities. Films of Cu-K-In-Se were co-evaporated at varied K/(K + Cu) compositions and substrate temperatures (with constant (K + Cu)/In ∼ 0.85). Increased Na composition on the substrate's surface and decreased growth temperature were both found to favor Cu1-xKxInSe2 alloy formation, relative to two-phase CuInSe2+KInSe2 formation. Structures from X-ray diffraction (XRD), band gaps, resistivities, minority carrier lifetimes and carrier concentrations from time-resolved photoluminescence were in agreement with previous reports, where low K/(K + Cu) composition films exhibited properties promising for photovoltaic absorbers. Films grown at 400–500 °C were then annealed to 600 °C in a controlled Se ambient, which caused K loss by evaporation in proportion to the initial K/(K + Cu) composition. Similar to growth temperature, annealing drove Cu1-xKxInSe2 alloy consumption and CuInSe2+KInSe2 production, as evidenced by high temperature XRD. Annealing also decomposed KInSe2 and formed K2In12Se19. At high temperature, the KInSe2 crystal lattice gradually contracted as temperature and time increased, as well as just time. Evaporative loss of K during annealing could accompany the generation of vacancies on K lattice sites, and may explain the KInSe2 lattice contraction. This knowledge of Cu-K-In-Se material chemistry may be used to predict and control minor phase impurities in Cu(In,Ga)(Se,S)2 photovoltaic absorbers—where impurities below typical detection limits may have played a role in recent world record photovoltaic efficiencies that utilized KF post-deposition treatments.