Surface and Bulk Effects of K in Cu$_{1-x}$K$_x$In$_{1-y}$Ga$_y$Se$_2$ Solar Cells
Two strategies for enhancing photovoltaic (PV) performance in chalcopyrite solar cells were investigated: Cu1-xKxIn1-yGaySe2 absorbers with low K content (K/(K+Cu), or x ~ 0.07) distributed throughout the bulk, and CuIn1-yGaySe2 absorbers with KIn1-yGaySe2 grown on their surfaces. For the Ga-free case, increased temperature improved PV performance in the KInSe2 surface absorbers, but not in the bulk x ~ 0.07 absorbers. Growth temperature also increased KInSe2 phase fraction, relative to Cu1-xKxInSe2 alloys-evidence that surface KInSe2 improved performance more than bulk KInSe2. Surface KIn1-yGaySe2 and bulk x ~ 0.07 Cu1-xKxIn1-yGaySe2 films with Ga/(Ga+In), or y of 0.3 and 0.5 also had improved efficiency, open-circuit voltage (VOC), and fill factor (FF), relative to CuIn1-yGaySe2 baselines. On the other hand, y ~ 1 absorbers did not benefit from K introduction. Similar to Cu1-xKxInSe2, the formation of Cu1-xKxGaSe2 alloys was favored at low temperatures and high substrate Na content, relative to the formation of mixed-phase CuGaSe2 + KGaSe2. KIn1-yGaySe2 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 KIn1-yGaySe2 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 KIn1-yGaySe2 passivates the surface of CuIn1-yGaySe2 to increase efficiency, VOC, and FF, while bulk Cu1-xKxIn1-yGaySe2 absorbers with x ~ 0.07 enhance efficiency, VOC, and FF by some other mechanism.