  To advance knowledge of K bonding in Cu(In,Ga)(Se,S)2 (CIGS) photovoltaic
(PV) absorbers, recent Cu-K-In-Se phase growth studies have been extended to PV
performance. First, the effect of distributing K throughout bulk Cu1-xKxInSe2
absorbers at low K/(K+Cu) compositions (0 <= x <= 0.30) was studied.
Efficiency, open-circuit voltage (VOC), and fill factor (FF) were greatly
enhanced for x ~ 0.07, resulting in an officially-measured 15.0%-efficient
solar cell, matching to the world record CuInSe2 efficiency. The improvements
were a result of reduced interface and bulk recombination, relative to CuInSe2
(x ~ 0). However, higher x compositions had reduced efficiency, short-circuit
current density (JSC), and FF due to greatly increased interface recombination,
relative to the x ~ 0 baseline. Next, the effect of confining K at the
absorber/buffer interface at high K/(K+Cu) compositions (0.30 <= x <= 0.92) was
researched. Previous work showed that these surface layer growth conditions
produced CuInSe2 with a large phase fraction of KInSe2. After optimization (75
nm surface layer with x ~ 0.41), these KInSe2 surface samples exhibited
increased efficiency (officially 14.9%), VOC, and FF as a result of decreased
interface recombination. The KInSe2 surfaces had features similar to previous
reports for KF post-deposition treatments (PDTs) used in world record CIGS
solar cells-taken as indirect evidence that KInSe2 can form during these PDTs.
Both the bulk and surface growth processes greatly reduced interface
recombination. However, the KInSe2 surface had higher K levels near the
surface, greater lifetimes, and increased inversion near the buffer interface,
relative to the champion bulk CKIS absorber. These characteristics demonstrate
that K may benefit PV performance by different mechanisms at the surface and in
the absorber bulk.
