Bulk heterojunction (BHJ) based on a donor (D) polymer and an acceptor (A) fullerene derivative is a promising organic photovoltaic. Here, we investigated the femtosecond charge dynamics after acceptor excitation in 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)/[6,6]-phenyl C71-butyric acid methyl ester (PC70BM), which is a low-gap (Egap≈1.6 eV) organic photovoltaic. We observed fast conversion from the acceptor exciton (A*) to the donor polaron (D+) within τrise = 0.45 ps. The fast carrier formation is ascribed to the prompt charge transfer of an electron from the D highest occupied molecular orbital (HOMO) to the A HOMO.

A bulk heterojunction (BHJ) based on a donor (D) polymer and an acceptor (A) fullerene derivative is a promising organic photovoltaic (OPV). We investigated femtosecond charge dynamics after D (at 633 nm) and A (at 400 nm) excitations in a prototypical low-band-gap and highly efficient OPV, i.e., 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)/[6,6]-phenyl C71-butyric acid methyl ester (PC70BM). The photoinduced absorption (PIA) spectra of the blend film were decomposed into three PIAs, i.e., donor exciton (D*), acceptor exciton (A*), and carrier (D+) components. The analysis revealed that D*→D+ conversion predominates in 633 nm excitation, while A*→D+ conversion predominates in 400 nm excitation. The carrier formation times of both processes are nearly the same (τrise∼0.2–0.3 ps).

We studied organic photovoltaics (OPVs) in bulk-heterojunction cells (BHJ) based on a liquid-crystalline fluorine copolymer (F8T2) with the capability of forming periodic nanostructures as the donor and a soluble fullerene (PC 70 BM) as the acceptor. The surface morphology of the blend film shows that the F8T2 and PC 70 BM phases are separated into large islands, which grow in size with increasing fraction of PC 70 BM. The power conversion efficiencies (PCEs) of the cells exhibit a distinct maximum as a function of the PC 70 BM concentration in the BHJ OPVs; the PCE has a maximum at F8T2:PC 70 BM (1:2) and decreases at both lower and higher concentrations of PC 70 BM. The OPV with the highest performance has a PCE of 2.28%. In order to investigate the charge separation that occurs at the interface between F8T2 and PC 70 BM, we also performed ultrafast transient absorption spectroscopy on F8T2:PC 70 BM (1:2) blend films.

Bulk heterojunction (BHJ) based on a donor (D) polymer and acceptor (A) fullerene derivative is a promising organic photovoltaics. Here, we performed femtosecond time-resolved spectroscopy of poly(9,9'-dioctylfluorene-co-bithiophene) (F8T2)/[6,6]-phenyl C70-butyric acid methyl ester (PC70BM) blend film with distinct D/A interfaces. We decomposed differential absorption spectra into fast, slow, and constant components via two-exponential fitting at respective probe photon energies. The decomposition distinguished photoinduced absorptions (PIAs) due to singlet exciton, donor polaron, and interfacial charge-transfer (CT) state. Based on these assignments, we discussed the charge dynamics in the F8T2/PC70BM blend film.

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