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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 PVperformance. First, the effect of distributing K throughout bulk Cu1-xKxInSe2absorbers at low K/(K+Cu) compositions (0 <= x <= 0.30) was studied.Efficiency, open-circuit voltage (VOC), and fill factor (FF) were greatlyenhanced for x ~ 0.07, resulting in an officially-measured 15.0%-efficientsolar cell, matching to the world record CuInSe2 efficiency. The improvementswere a result of reduced interface and bulk recombination, relative to CuInSe2(x ~ 0). However, higher x compositions had reduced efficiency, short-circuitcurrent density (JSC), and FF due to greatly increased interface recombination,relative to the x ~ 0 baseline. Next, the effect of confining K at theabsorber/buffer interface at high K/(K+Cu) compositions (0.30 <= x <= 0.92) wasresearched. Previous work showed that these surface layer growth conditionsproduced CuInSe2 with a large phase fraction of KInSe2. After optimization (75nm surface layer with x ~ 0.41), these KInSe2 surface samples exhibitedincreased efficiency (officially 14.9%), VOC, and FF as a result of decreasedinterface recombination. The KInSe2 surfaces had features similar to previousreports for KF post-deposition treatments (PDTs) used in world record CIGSsolar cells-taken as indirect evidence that KInSe2 can form during these PDTs.Both the bulk and surface growth processes greatly reduced interfacerecombination. However, the KInSe2 surface had higher K levels near thesurface, greater lifetimes, and increased inversion near the buffer interface,relative to the champion bulk CKIS absorber. These characteristics demonstratethat K may benefit PV performance by different mechanisms at the surface and inthe absorber bulk.

Surface and bulk effects of K in highly efficient Cu$_{1-x}$K$_x$InSe$_2$ solar cells