Photoelectrical behavior of perylene derivative/π-conjugated polymer system

Organic photovoltaic (PV) cells composed of a perylene derivative, N,N′-di(pentan-3-yl)-perylene-3,4:9,10-bis(dicarboximide), PTCDI, acting as an acceptor and a π-conjugated polymer poly{1-phenyl-2-[4-(trimethylsilyl)phenyl]acetylene}, PDPA, acting as a donor of photogenerated free electrons are presented. Two types of starting double-layer samples were prepared: samples I composed of a spin-cast PDPA thin film bottom layer and a PTCDI upper layer deposited by the vacuum sublimation, and samples II composed of a spin-cast mixed PDPA/PTCDI layer and a vacuum-sublimation deposited PTCDI upper layer. The samples were treated with toluene vapors to induce changes in the morphology of heterojunctions due to recrystallization of PTCDI, which have been monitored by the optical absorption spectra and AFM images, and the influence of the morphology changes on photovoltaic characteristics of the samples has been studied. A short-time (2-5 min) toluene-vapor treatment positively influences the PV efficiency of system I due to a formation of larger PTCDI crystals, which improves the collection efficiency of the upper layer. However, a longer-time treatment of a system I results in a decrease in its PV efficiency owing to a significant decrease in the PDPA-PTCDI interfacial area accompanying further growth of PTCDI crystals, which lowers the photogeneration efficiency of the system. A combination of molecularly dissolved (in PDPA) and crystalline PTCDI in the system II has been proved to provide better PV efficiency due to a combination of improved free charge carrier photogeneration in the composite bottom layer and the collection efficiency of charge carriers of the upper layer.