Temperature effects on carrier formation dynamics in organic heterojunction solar cell

Abstract

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[1,2b:4,5-b 0 ] dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b] thiophenediyl]] (PTB7), and the C 70 acceptor (A). We spectroscopically determined the absolute number of donor exciton (n DÃ), acceptor exciton (n AÃ), and carrier (n Dþ) per an absorbed photon against the delay time (t). At 300 K, we found that the decay time (s decay ¼ 3.5 ps) of A* is much longer than the carrier formation time (s form ¼ 1.1 ps), indicating that the late A* component does not contribute to the carrier formation process. The elongated s form (¼1.5 ps) at 80 K is ascribed to the exciton migration process, not to the exciton dissociation process. V

Carrier Formation Dynamics in Prototypical Organic Solar Cells as Investigated by Transient Absorption Spectroscopy

“…The PIA spectra in the late stage (>1 ps) show broad PIA around 1150 nm. The PIA is originated from the photoinduced carriers (D + ) of PTB7 [57][58][59][60]. Actually, the spectral profile is similar to that of the electrochemical differential absorption spectra of the PTB7 neat film (vide infra).…”

“…The transient absorption spectroscopy is one of the powerful tools used to clarify the dynamics of the exciton, CT state, and carrier. The spectroscopy has been applied to the OSCs with BHJ active layers [42,43,[46][47][48][49][50][51][52][53][54][55][56][57][58][59][60][61]. Figure 2 shows schematic diagram of the transient absorption spectroscopy.…”

“…The length scale of the phase separation was 24 nm [72]. The recent development of the low-band gap donor polymers further increased PCE beyond 5%, which motivated intensive spectroscopic investigation of the low-band gap donor polymers [56][57][58][59][60][61][62][63][64]. In particular, Yu and coworkers [73,74] have developed a series of donor polymers based on alternating ester-substituted [75]).…”