The Effect of Temperature on Cu-K-In-Se Thin Films

TL;DR

This study investigates how temperature and potassium content influence the phase formation, structural properties, and K loss in Cu-K-In-Se thin films, providing insights for optimizing photovoltaic absorber materials.

Contribution

It offers new understanding of temperature effects on phase stability and K evaporation in Cu-K-In-Se films, aiding in controlling impurities for better PV performance.

Findings

Increased Na and lower temperature favor CKIS alloy formation.

Annealing at 600°C causes K loss and phase transformation.

K evaporation may create vacancies affecting lattice structure.

Abstract

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.