F4TCNQ Doping of P3HT:PCBM Photovoltaic Devices

Photovoltaic properties of a conventional bulk heterojunction organic solar cell composed of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) were investigated as a function of the addition amount of a p-type doping reagent, 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ). Addition of a small amount of F4TCNQ to P3HT induced spectral changes with the appearance of low energy polaron bands, suggesting the efficient doping of the p-type semiconductor. The doping also resulted in the improved hole conduction, as revealed by current density-voltage (J-V) curves in the dark. A systematic study of the devices indicated that the soft doping with F4TCNQ and the utilization of a hole-transporting PEDOT:PSS layer on an ITO electrode sometimes improve the short-circuit current density (JSC) and fill factor (FF). In contrast, the hard doping always generates hole-trapping sites. Based on this knowledge, the device performances were successfully optimized.