Revealing the Beneficial Role of K in Grain Interiors, Grain Boundaries, and at the Buffer Interface for Highly Efficient CuInSe2 Solar Cells

K incorporation within grain boundaries, grain interiors, and interfaces has been studied within CuInSe₂ solar cells to better understand the beneficial or detrimental role of K distribution among these regions in chalcopyrite-based solar cells. Solar cells have been fabricated with intentional K introduction into specific regions of the device including the CuInSe₂/CdS interface (CuInSe₂/KInSe₂/CdS) and the grain interiors (Cu_0.93K_0.07InSe₂/CdS). A control CuInSe₂/CdS device was also studied to separate effects of K originating from the soda-lime glass substrate from those of intentionally introduced K. The experiment was designed to understand K effects in Cu(In,Ga)Se₂ solar cells while mitigating complications from multiple elements in the 3⁺ site. The distribution of all elements within these samples has been directly observed with sub-nm resolution via atom probe tomography. In addition, electron beam–induced current measurements have been performed to correlate the atom probe tomography compositional profiles to the nanoscale carrier collection properties. The experiments show that a large decrease in the Cu/In ratio at the CdS interface can be achieved by forming KInSe₂ at the absorber surface, which drastically improves the device efficiency. The results presented here show a direct link between K concentration, Cu depletion, and In accumulation, such that the Cu/In ratio significantly reduces with K incorporation. The findings help clarify the mechanism behind K-induced efficiency enhancement.