Schematic of PTB7-Th:PCBM OSCs in an inverted geometry with ZnO, ZnO/Ba(OH)2 and ZnO:Ba(OH)2 nanocomposites as ETLs.

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Inefficient light absorption and poor charge separation are considered as two major bottlenecks for achieving highly efficient bulk heterojunction organic solar cells (BHJ OSCs). In the present study, we have introduced an additional phenyl-C71-butyric acid methyl ester (PC 70 BM) layer to modify the interface between ZnO based electron transport layer (ETL) and photoactive layer comprised of Poly[4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2b;4,5-b']dithiophene-2,6-diyl-alt-(4-(2-ethylhexyl)-3-fluorothieno[3,4-b]thio-phene-)-2-carboxylate-2-6-diyl)] (PTB7-Th):PC 70 BM. This interface engineering has quenched the electron-hole recombination at the interface and has improved power conversion efficiency (PCE) from 6.65 to 7.74%. Devices were fabricated in an inverted geometry having a structure ITO/ZnO (40nm)/PC 70 BM (5nm)/PTB7-Th:PC 70 BM (70nm)/MoO 3 (10nm)/Ag (100nm). Additionally, V-grooved textured PDMS films were attached to the backside of OSC substrates which has further improved PCE to 9.12%. Our study suggests that the performance enhancement as observed in OSCs with V-grooved textured PDMS films could be due to increased total optical path length of the incident light within the device.