Two-step charge photogeneration dynamics in polymer/fullerene blends for photovoltaic applications

Abstract

We measured the picoseconds (ps) transient dynamics of photoexcitations in blends of poly(3-hexyl-thiophene) (P3HT; donors-D) and fullerene [6,6]-phenyl-C61-butyric acid methyl ester (PCBM; acceptor-A), using the transient pump/probe photomodulation technique in an unprecedented broad spectral range from 0.25 to 2.5 eV, and compared the results with organic solar cell performance based on the same blends. In D-A blends with maximum domain separation such as regio-regular P3HT/PCBM with (1.2:1) weight ratio having solar cell power conversion efficiency of \ensuremath{\sim}4$%$, we found that, although the photogenerated intrachain excitons in the polymer nano-domains decay within \ensuremath{\sim}10 ps, no charge polarons are generated on their expense up to \ensuremath{\sim}2 ns. Instead, there is a buildup of charge transfer (CT) excitons at the D-A interfaces having the same kinetics as the exciton decay, which dissociate into separate polarons in the D and A domains at a much later time ($\ensuremath{\gg}\phantom{\rule{-0.16em}{0ex}}\phantom{\rule{-0.16em}{0ex}}1$ ns). This two-step charge photogeneration process may be typical in organic bulk heterojunction cells. Although the CT excitons are photogenerated on the exciton expense much faster in D-A blends having smaller domain size such as in regio-random P3HT/PCBM, their dissociation is less efficient because of larger binding energy. This explains the poor solar cell power conversion efficiency (0.1$%$) based on this blend. Our results support the two-step charge photogeneration mechanism in polymer/fullerene blends and emphasize the important role of the CT binding energy in generating free charge polarons in organic solar cells.