Ternary Bulk Heterojunction Photovoltaic Cells Composed of Small Molecule Donor Additive as Cascade Material

To explore the potential of ternary blend bulk heterojunction (BHJ) solar cells as a general platform for improving the performance of organic photovoltaics, we studied a ternary BHJ system based on poly­(3-hexylthiophene) (P3HT), [6,6]-phenyl C61 butyric acid methyl ester (PC61BM), and DTDCTB. The optimized ternary structure containing a weight ratio of 20% DTDCTB as the cascade material demonstrates a ∼25% improvement of the power conversion efficiency (PCE) as compared to the binary P3HT/PC61BM solar cells. A systematic spectroscopic study is carried out to elucidate the underlying mechanism of charge transfer in the ternary system. Wavelength-dependent external quantum efficiency measurement confirms the contribution of DTDCTB to the enhanced photocurrent. Photoinduced absorption spectroscopy and transient photovoltage measurement reveal unambiguously that charges generated in DTDCTB are efficiently transferred to and subsequently transported in P3HT and PC61BM. The results also suggest that despite the realization of cascade charge transfer, the bimolecular charge recombination process in the ternary system is still dominated by the P3HT/PC61BM interface.