  Co-evaporation of Cu-KF-In-Se was performed on substrates with varied surface
Na compositions. Compositions of interest for photovoltaic absorbers were
studied, with ratios of (K+Cu)/In ~ 0.85 and K/(K+Cu) ~ 0 - 0.57. Soda-lime
glass (SLG) substrates led to the most Na by secondary ion mass spectrometry,
while SLG/Mo and SLG/SiO2/Mo substrates led to 3x and 3,000x less Na in the
growing film, respectively. Increased Na content favored Cu1-xKxInSe2 (CKIS)
alloy formation by X-ray diffraction (XRD), while decreased Na favored the
formation of CuInSe2 + KInSe2 mixed-phase films. Scanning electron microscopy
and energy dispersive X-ray spectroscopy revealed the KInSe2 precipitates to be
readily recognizable planar crystals. Extrinsic KF addition also drove
diffusion of Na out from the various substrates and into the growing film, in
agreement with previous reports. Time-resolved photoluminescence showed
enhanced minority carrier lifetimes for films with moderate K compositions
(0.04 < K/(K+Cu) < 0.14) grown on Mo. Due to the relatively high detection
limit of KInSe2 by XRD and the low magnitude of chalcopyrite lattice shift for
CKIS alloys with these compositions, it is unclear if the lifetime gains were
associated with CKIS alloying, minor KInSe2 content, or both. The identified
Na-K interdependency can be used to engineer alkali metal bonding in
Cu(In,Ga)(Se,S)2 absorbers to optimize both initial and long-term photovoltaic
power generation.
