Effect of polarity of small molecule interlayer materials on the open circuit voltage and power conversion efficiency of polymer solar cells

A series of small molecule based interlayer materials with different functional groups were synthesized and evaluated as cathode modifiers at the interface between the poly(3-hexylthiophene) (P3HT):[6,6]-phenyl C61-butyric acid methyl ester (PCBM) active layer and Al cathode. The device performances of the organic solar cells (OSCs) were correlated with the interface properties between the small molecule interface modifiers and the Al cathode. High open circuit voltage (V_oc) was obtained in the OSC with highly polar phosphine oxide compound, while low V_oc was observed in the OSC with less polar phosphine chalconide interlayer. The interface between the interlayer and Al was analyzed with ultraviolet photoelectron spectroscopy, and the energy level shift at the interface was correlated with the V_oc and polarity of the interlayer. Highly polar phosphine oxide compound induced large energy level shift, while less polar phosphine chalconide compound induced small energy level shift at the interface. Therefore, the use of the highly polar interlayer was effective to improve the V_oc of OSCs.