Temperature-independent carrier formation dynamics in bulk heterojunction

We investigated the effects of temperature on the carrier formation dynamics in a small-molecular blend film, 2,5-di-(2-ethylhexyl)-3,6-bis-(5\u27\u27-n-hexy-[2,2\u27,5\u27,2\u27\u27]terthiophen-5-yl)-pyrrolo[3,4-c]pyrrolo-1,4-dione (SMDPPEH)/[6,6]-phenyl C71-butyric acid methyl ester (PC71BM). We spectroscopically determined the absolute numbers of donor ($n_{\text{D}^{*}}$) and acceptor ($n_{\text{A}^{*}}$) excitons per absorbed photon as functions of the delay time (t), in addition to the relative number of donor carries ($n_{\text{D}^{+}}$). We found that the carrier formation dynamics is independent of temperature at 300 and 80 K: the carrier formation time (τrise = 0.4 ps) is much faster than the decay time (τdecay ≈ 2.5 ps) of donor excitons. The temperature independence strongly suggests that only excitons created near the donor–acceptor interface contribute to the carrier formation