Ultrafast transient spectroscopy of nano-domains of polymer/fullerene blend for organic photovoltaic applications

We measured the picoseconds (ps) transient photomodulation (PM) dynamics of photoexcitations in blends of regio-regular poly(3-hexyl-thiophene) [RR-P3HT] (donors-D) and indene-C-60 bisadduct (fullerene derivative) [ICBA] (acceptor-A) that phase-separate into D- and A-nano-domains, in a broad spectral range from 0.25 to 2.5 eV; in comparison with steady state PM spectra. We correlate our measurements with organic photovoltaic solar cell performance made from the same D and A materials. In D-A blends of RR-P3HT/ICBA with (1.2:1) weight ratio having solar cell power conversion efficiency of similar to 5.1%, we found that although the intrachain excitons in the polymer nano-domains decay within similar to 10 ps, no charge polarons are generated on their expense up to similar to 1 ns. Instead, there is a built-up of charge-transfer (CT) excitons at the D-A domain interfaces that occurs with the same kinetics as the exciton decay. The CT excitons dissociate into separate polarons in the D-and A-nano-domains at a much later time (>> 1 ns). This two-step charge photogeneration process is typical in organic bulk heterojunction cells. Our results emphasize the important role of the CT state in generating free charge polarons in organic solar cells. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4769211]