Abstract
We measured the picoseconds (ps) transient dynamics of photoexcitations in blends of regio-regular poly(3-hexyl-thiophene) (RR-P3HT) (donors-D) and fullerene (PCBM) (acceptor-A) in an unprecedented broad spectral range of 0.25 to 2.5 eV. In D-A blends with maximum domain separation, such as RR-P3HT/PCBM, with (1.2:1) weight ratio having solar cell power conversion efficiency of ~4%, we found that although the intrachain excitons in the polymer domains decay within ~10 ps, no charge polarons are generated at their expense up to ~1 ns. Instead, there is a build-up of charge-transfer (CT) excitons at the D-A interfaces having the same kinetics as the exciton decay. The CT excitons dissociate into separate polarons in the D and A domains at a later time (>1 ns). This “two-step” charge photogeneration process may be typical in organic bulk heterojunction cells. We also report the effect of adding spin 1/2 radicals, Galvinoxyl on the ultrafast photoexcitation dynamics in annealed films of RR-P3HT/PCBM blend. The addition of Galvinoxyl radicals to the blend reduces the geminate recombination rate of photogenerated CT excitons. In addition, the photoexcitation dynamics in a new D-A blend of RR-P3HT/Indene C60 trisadduct (ICTA) has been studied and compared with the dynamics in RR-P3HT/PCBM.
Highlights
In recent years, organic solar cells have attracted widespread interest in research institutes and commercial companies because of their versatility, flexibility, ease of processing and low cost
We have found that the addition of few weight percentage of a spin-1/2 radical “Galvinoxyl” to the regular poly(3-hexyl-thiophene) (RR-polymer regio-regular (RR-)(3 hexyl thiophene) (P3HT))/phenyl-C61-butyric acid methyl ester (PCBM) blend improves the photovoltaic device performance of the solar cell based on this blend
In order to explore the origin of this organic photovoltaic (OPV) improvement, we studied the photoexcitation dynamics of RR-P3HT/PCBM blend
Summary
Organic solar cells have attracted widespread interest in research institutes and commercial companies because of their versatility, flexibility, ease of processing and low cost. In addition it has been shown that there exist some amorphous regions in the blend where polymer and fullerene are finely mixed [4,5] Despite these significant advances in device performance, the intricacies of charge carrier photogeneration and evolution in D-A blends for OPV applications are still the focus of fundamental research, with the goal of developing novel photoactive materials and improving the power conversion efficiency of solar cell devices [6,7]. We show that after the photoexcited excitons in the polymer domains reach the D-A interfaces, the charge generation process proceeds via the formation of CT excitons at the interfaces, which occurs within ~10 ps These findings are consistent with our recent report about ultrafast photophysics of RR-P3HT/Indene C60 bisadduct (ICBA) blend [9]. Our findings support the two-step process of the charge photogeneration mechanism in organic D-A blends proposed before [9], and emphasize the important role of the CT exciton binding energy in generating free charges in organic solar cells
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