Highly efficient organic p-i-n photovoltaic cells based on tetraphenyldibenzoperiflanthene and fullerene C70

In this work, tetraphenyldibenzoperiflanthene (DBP) and fullerene C-70 were used to construct organic photovoltaic cells. A power conversion efficiency of 5.19% was obtained by inserting a blend layer of DBP and C-70 between the two neat layers, i.e. the so-called p-i-n structure, while the bilayer cell has an efficiency of 3.56%. The significant enhancement in photocurrent was mainly ascribed to absorption of the blend film. Interestingly optical characterization revealed that DBP molecules in the blend film showed a preferable orientation parallel to the substrate surface, suggesting that anisotropic properties can be realized in vacuum deposited donor-acceptor blend films for efficient light harvesting. Photoresponse of the p-i-n cells extended into the spectral range longer than band-edge absorption of the two materials, which is clearly associated with formation of charge transfer states between the DBP donor and C-70 acceptor. On the other hand, the charge transfer states cause increase of saturation current of the diodes, and thus decrease of open circuit voltage. Our results indicate that the controversial effects of such charge transfer states should be considered in organic solar cells with respect to material selection and device design.