Electronic Properties of Cu-Phthalocyanine/Fullerene Planar and Bulk Hetereojunctions on PEDOT:PSS

In this paper, we report UV and X-ray photoelectron spectroscopy studies on layered planar and mixed bulk heterojunctions of Cu-phtalocyanine (CuPc) and C60, a prototypical material pair for organic photovoltaic cells (OPVCs). The respective heterojunctions were formed on poly(ethylene-dioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) substrates in order to achieve morphologies comparable to those in actual OPVCs. As a result of a CuPc-to-substrate electron transfer, the work function of pristine PEDOT:PSS is reduced from 5.4 to 4.4 eV. The deposition of C60 onto CuPc, however, leads to a work function increase of 0.2 eV. The codeposition of C60 and CuPc to form mixed bulk heterojunctions resulted in an effective anode work function of 4.6 eV. The energy offset between the highest occupied levels of CuPc and C60 was determined as 1.3 eV for both the layered planar and mixed bulk heterojunction. With reported values of the charge transport gap of C60, we estimate the upper limit of the open circuit voltage to be 1.0 eV for both types of heterojunctions. Our results demonstrate that the energy level offsets are independent of particular interface morphology in C60/CuPc heterojunctions grown on PEDOT:PSS, and that differences in device efficiency are due to other effects.