Physically Adsorbed Fullerene Layer on Positively Charged Sites on Zinc Oxide Cathode Affords Efficiency Enhancement in Inverted Polymer Solar Cell

We present a novel idea for overcoming the drawback of poor contact between the ZnO cathode and active layer interface in an inverted polymer solar cell (i-PSC), simply by incorporating an electron-acceptor self-assembled monolayer (SAM)-tetrafluoroterephthalic acid (TFTPA)-on the ZnO cathode surface to create an electron-poor surface of TFTPA on ZnO. The TFTPA molecules on ZnO are anchored on the ZnO surface by reacting its carboxyl groups with hydroxyl groups on the ZnO surface, such that the tetrafluoroterephthalate moieties lay on the surface with plane-on electron-poor benzene rings acting as positive charge centers. Upon coating a layer of fullerenes on top of it, the fullerene molecules can be physically adsorbed by Coulombic interaction and facilitate a promoted electron collection from the bulk. The active layer is composed of the mid bandgap polymer poly(3-hexylthiophene) (P3HT) or low bandgap polymer, poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b']dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl) carbonyl]thieno[3,4-b]thiophenediyl]] (PTB7), as the donor and [6,6]-phenyl-C-61-butyric acid methyl ester (PC61BM) or [6,6]-phenyl-C-71-butyric acid methyl ester (PC71BM) as the acceptor. Significant enhancement in power conversion efficiency (PCE) was observed for the devices with the active layer P3HT:PC61BM (or PC71BM) by promoting from 3.20 to 4.03% (or from 3.27 to 4.04%) and with the active layer PTB7:PC71BM from 6.03 to 6.90%. This method should be also applicable to other types of active layer.