A solution-processed barium hydroxide modified aluminum doped zinc oxide layer for highly efficient inverted organic solar cells
Inverted organic solar cells (iOSCs) with air stable interface materials and top electrodes and an efficiency of 6.01% are achieved by inserting a barium hydroxide (Ba(OH)2) layer between the aluminum doped zinc oxide (AZO) electron extraction layer and the active layer. A low bandgap diketopyrrolopyrrole–quinquethiophene alternating copolymer (pDPP5T-2) and phenyl-C61-butyric acid methyl ester (PC61BM) were chosen as the active layer compounds. Compared to the control device without Ba(OH)2, insertion of a few nm thick Ba(OH)2 layer results in an enhanced VOC of 10%, JSC of 28%, FF of 28% and PCE of 80%. Modification of AZO with a solution processed low-cost Ba(OH)2 layer increased the efficiency of the inverted device by dominantly reducing the energy barrier for electron extraction from PC61BM, and consequently, reduced charge recombination is observed. The drastic improvement in device efficiency and the simplicity of fabrication by solution processing suggest Ba(OH)2 as a promising and practical route to reduce interface induced recombination losses at the cathode of organic solar cells.