Surface and Bulk Effects of K in Highly Efficient Cu1-xKxInSe2 Solar Cells

To advance knowledge of the beneficial effects of K 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 Cu_1-xK_xInSe₂ absorbers at low K/(K+Cu) compositions (0 ≤ x ≤ 0.30) was studied. Efficiency, open-circuit voltage (V_OC), and fill factor (FF) were greatly enhanced for x ~ 0.07, resulting in an officially-measured 15.0%-efficient solar cell, matching 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 (J_SC), 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%), V_OC, 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 Cu_1-xK_xInSe₂ absorber. These characteristics demonstrate that K may benefit PV performance by different mechanisms at the surface and in the absorber bulk.