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 (Cu0.93K0.07InSe2/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)Se-2 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.