  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 (CKIS) alloy formation, relative to
mixed-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 (PV) absorbers. Films grown at 400-500 C were then
annealed to 600 C under Se, which caused K loss by evaporation in proportion to
initial K/(K+Cu) composition. Similar to growth temperature, annealing drove
CKIS 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 PV absorbers-where impurities below typical detection limits
may have played a role in recent world record PV efficiencies that utilized KF
post-deposition treatments.
