Influence of the metal phthalocyanine molecular orientation on charge separation at the organic donor/acceptor interface

Abstract

To achieve highly efficient organic photovoltaics (OPVs), control of the molecular orientation is one of the prime important factors, for the interfacial dipole orientation and energy offset at the donor/acceptor (D/A) interface influence the device performance. We examine the effect of the planar-shape molecular orientation on the D/A interface, by comparing two different metal phthalocyanine molecules under identical process conditions. From the noticeable changes in the near edge X-ray absorption fine structure and ionization potential, copper phthalocyanine (CuPc) on a thin CuI layer reveals a strong change in molecular orientation compared to zinc phthalocyanine (ZnPc). To account for the different templating effect between CuPc and ZnPc, Cu d-d orbital coupling is evidenced as for the interaction between CuI and CuPc. The apparent change of the CuPc orientation reveals a relatively large energy offset at the CuPc/C-60 D/A interface and increased open-circuit voltage in the corresponding OPV device. The lying-down orientation of CuPc on CuI or face-on geometry at the C-60/CuPc interface induces strong electron-electron coupling and long-lived charge transfer exciton states, which is directly related to better charge separation and correspondingly good OPV performance.