Temperature effects on carrier formation dynamics in organic heterojunction solar cell

The femto-second time-resolved spectroscopy was performed on the heterojunction (HJ) solar cell which consists of prototypical low-band gap donor (D), poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-$b′$] dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b] thiophenediyl]] (PTB7), and the C₇₀ acceptor (A). We spectroscopically determined the absolute number of donor exciton (⁠$nD*$⁠), acceptor exciton (⁠$nA*$⁠), and carrier (⁠$nD+$⁠) per an absorbed photon against the delay time (t). At 300 K, we found that the decay time (⁠$τdecay$ = 3.5 ps) of A* is much longer than the carrier formation time (⁠$τform$ = 1.1 ps), indicating that the late A* component does not contribute to the carrier formation process. The elongated $τform$ (=1.5 ps) at 80 K is ascribed to the exciton migration process, not to the exciton dissociation process.