Pyridinium salt-based molecules as cathode interlayers for enhanced performance in polymer solar cells

A series of water/alcohol-soluble small molecules based on electron-deficient pyridinium salts namely BTPS, BnPS and F8PS were successfully synthesized. Their photophysical and electrochemical properties were thoroughly studied. Due to its good film-forming ability, F8PS was employed as a cathode interlayer in an organic photovoltaic cell. Simultaneous enhancements in open-circuit voltage (V-oc), short circuit current density (J(sc)) and fill factor (FF) were achieved, and the power conversion efficiency (PCE) was increased from 4.32% to 6.56% compared to the device based on the bare Al cathode. V-oc was significantly improved from 0.76 V to 0.94 V, and it is one of the best results reported in literature to date for polymer solar cells (PSCs) based on the active layer of poly [N-9'-heptadecanyl-2,7-carbazole-alt-5,5-(4',7'-di-2-thienyl-2',1',3'-benzothiadiazole)] (PCDTBT):[6,6]-phenyl-C-71-butyric acid methylester (PC71BM). The greatly increased V-oc may be due to the interface dipoles generated by F8PS. It was also demonstrated that post treatment of the active layer with ethanol gave an improvement of the overall device efficiency from the initial 4.32% to 5.55%, compared to the device with the bare Al cathode. Therefore, the improvement in performance after pyridinium salt deposition may be due to a combination of the effects of ethanol treatment and the presence of the thin pyridinium salt layer. The good water/alcohol solubility, ideal HOMO/LUMO energy levels and the excellent electron transfer/collection ability of the hydrophilic pyridinium salt derivatives makes them a promising family of electron transport materials for highly efficient PSCs.