Surface and Bulk Effects of K in Cu1-xKxIn1-yGaySe2 Solar Cells

Two strategies for enhancing photovoltaic (PV) performance in chalcopyrite solar cells were investigated: Cu_1−xK_xIn_1−yGa_ySe₂ absorbers with low K content (K/(K+Cu), or x ~ 0.07) distributed throughout the bulk, and CuIn_1−yGa_ySe₂ absorbers with KIn_1−yGa_ySe₂ grown on their surfaces. Distributing K throughout the bulk absorbers improved power conversion efficiency, open-circuit voltage (V_OC) and fill factor (FF) for Ga/(Ga+In) of 0, 0.3 and 0.5. Surface KIn_1−yGa_ySe₂ and bulk x ~ 0.07 Cu_1−xK_xIn_1−yGa_ySe₂ films with Ga/(Ga+In), or y of 0.3 and 0.5 also had improved efficiency, V_OC, and FF, relative to CuIn_1−yGa_ySe₂ baselines. On the other hand, y ~ 1 absorbers did not benefit from K introduction. Similar to Cu_1−xK_xInSe₂, the formation of Cu_1−xK_xGaSe₂ alloys was favored at low temperatures and high Na supply by the substrate, relative to the formation of mixed-phase CuGaSe₂ + KGaSe₂. KIn_1−yGa_ySe₂ alloys were grown for the first time, as evidenced by X-ray diffraction and ultraviolet/visible spectroscopy. For all Ga/(Ga+In) compositions, the surface KIn_1−yGa_ySe₂ absorbers had superior PV performance in buffered and buffer-free devices. However, the bulk x ~ 0.07 absorbers only outperformed the baselines in buffered devices. The data demonstrate that KIn_1−yGa_ySe₂ passivates the surface of CuIn_1−yGa_ySe₂ to increase efficiency, V_OC, and FF, while bulk Cu_1−xK_xIn_1−yGa_ySe₂ absorbers with x ~ 0.07 enhance efficiency, V_OC, and FF by some other mechanism.