Ternary Bulk Heterojunction Photovoltaic Cells Composed
of Small Molecule Donor Additive as Cascade Material
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Abstract
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 (PC<sub>61</sub>BM), 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/PC<sub>61</sub>BM 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 PC<sub>61</sub>BM. 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/PC<sub>61</sub>BM interface