The present work reports the fabrication and detailed electrical properties of heterojunction diodes based on n-type Si and poly(3,4- ethylenedioxythiophene) doped with poly(4-styrenesulfonate) (PEDOT:PSS) having the reduced graphene oxide (RGO). This heterojunction diode showed a good rectifying behavior with an ideality factor of 1.2. A photocurrent decay model is presented that addresses the charge trapping effect and doping mechanisms for composite PEDOT:PSS films having RGO sheets. The enhanced dark conductivity was observed by incorporating RGO into PEDOT:PSS. For heterojunction diodes, the high photocurrent density originates from efficient hole transport combined with electron trapping with long-second lifetime.