Interface engineering through electron transport layer modification for high efficiency organic solar cells.

In the present study, we have compared the device performance of poly[4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2- b ;4,5- b ']dithiophene-2,6-diyl- alt -(4-(2-ethylhexyl)-3-fluorothieno[3,4- b ]thio-phene-)-2-carb-oxylate-2-6-diyl)] (PTB7-Th):phenyl-C71-butyric acid methyl ester (PCBM) organic solar cells (OSCs) in an inverted geometry with ZnO, a bilayer of ZnO and Ba(OH) 2 [ZnO/Ba(OH) 2 ] and a nanocomposite of ZnO and Ba(OH) 2 [ZnO:Ba(OH) 2 ] as electron transport layers (ETLs). Our study reveals that the performance of the devices with the ZnO/Ba(OH) 2 and ZnO:Ba(OH) 2 nanocomposite as ETL supersedes that of devices with only ZnO as ETL. The plausible reasons for the improved performance of these devices are identified using morphological studies, contact angle measurements, X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS) and photo-electrochemical impedance spectroscopy (EIS) measurements. It is observed that films of ZnO/Ba(OH) 2 and ZnO:Ba(OH) 2 nanocomposites have a low work function and are slightly more smooth and hydrophobic than ZnO films. This might have suppressed the charge recombination and thereby improved the charge collection as has been confirmed by EIS measurements.