Alkali segregation and matrix concentrations in thin film Cu(In,Ga)Se2 at targeted interfaces characterized in 3-D at the nanoscale

Recently, the highest laboratory efficiencies of polycrystalline Cu(In,Ga)Se₂ thin film solar cells have been achieved by introducing alkali impurity atoms, namely K and Na. This contribution will discuss a novel technique used to target grain boundaries with known structure to relate quantified K and Na segregation to matrix elements concentrations. We used this technique to select from roughly 20 grain boundaries with known misorientations, extracted from electron backscattered diffraction (EBSD) micrographs, and then chemically analyzed them in 3-D using atom probe tomography (APT). The grain boundaries analyzed exhibited a correlation of K and Na segregation to matrix elements concentrations and grain boundary misorientations. Alkali metal segregation at the interface of CuS/Cu(In,Ga)Se₂ is also discussed.