Solution-processed zinc oxide nanocrystals (ZnO NCs) hybridized with insulating poly(ethylene glycol) (PEG) are introduced as a cathode interlayer in bulk heterojunction organic photovoltaic cells based on poly(3-hexylthiophene) (P3HT):(6,6)-phenyl-C61 butyric acid methyl ester (PC61BM) blends. The performance of devices with ZnO-PEG interlayers exhibit an excellent maximum power conversion efficiency (PCE) of 4.4% with a fill factor (FF) of 0.69 under optimized conditions. This enhanced device performance is attributed to decreased series resistance from the hole blocking properties of ZnO, as well as the facilitated electron transport due to the reduced area of ZnO domain boundaries upon addition of PEG. The addition of PEG also lowers the electron affinity of ZnO, which leads to a nearly Ohmic contact at the polymer/metal interface. Moreover, the ZnO-PEG interlayer serves as an optical spacer that enhances light absorption and thereby increases the photocurrent. The addition of PEG permits control over layer thickness and refractive indices. Improved photon energy absorption is supported by optical simulations. Devices with highly stable metals such as Ag and Au also show dramatically enhanced performance comparable to conventional devices with Al cathode. Due to its simplicity and excellent characteristics, this multifunctional interlayer is suitable for high performance printed photovoltaic cells.
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