Ultrafast Spectroscopic Identification of Hole Transfer in All-Polymer Blend Films of Poly(1-{4,8-bis[5-(2-ethylhexyl)thiophen-2-yl]-benzo[1,2‑<i>b</i>:4,5‑<i>b</i>′]dithiophen-2-yl}-3-methyl-5-(4-octylphenyl)‑4<i>H</i>‑thieno[3,4‑<i>c</i>]pyrrole-4,6(5<i>H</i>)‑dione) and Poly[1,8-bis(dicarboximide)-2,6-diyl]-<i>alt</i>-5,5′-(2,2′-bithiophene)]

All-polymer solar cells composed of wide-band-gap polymer poly­(1-{4,8-bis­[5-(2-ethylhexyl)­thiophen-2-yl]-benzo­[1,2-<i>b</i>:4,5-<i>b</i>′]­dithiophen-2-yl}-3-methyl-5-(4-octylphenyl)-4<i>H</i>-thieno­[3,4-<i>c</i>]­pyrrole-4,6­(5<i>H</i>)-dione) (PTP8) as the donor and poly­[1,8-bis­(dicarboximide)-2,6-diyl]-<i>alt</i>-5,5′-(2,2′-bithiophene)] [P­(NDI2OD-T2), also known as Activink N2200] as the acceptor exhibit a broad absorbance in the range 300–900 nm, thanks to complementary absorption of near-infrared light by N2200. Although N2200 shows reasonably high electron mobility, the contribution of the photogenerated excitons in N2200 to the power conversion of the PTP8/N2200 solar cell is insignificant. Here, the hole transfer from N2200 to PTP8 in PTP8/N2200 blend films was investigated by utilizing ultrafast transient absorption spectroscopy. The spectral fingerprints of ground-state bleaching and hole polaron-induced absorption of PTP8 are identified under selective excitation of the N2200 component and unambiguously indicate hole transfer from N2200 to PTP8. The hole transfer is slow (∼100 ps), comparable to the geminate exciton recombination rate, consequently limiting the transfer efficiency and carrier generation. The hole-transfer efficiency depends on the PTP8/N2200 weight ratio, showing a highest value of ∼14.1% in the 3:2 film

Thermal Annealing Effect on Ultrafast Charge Transfer in All-Polymer Solar Cells with a Non-Fullerene Acceptor N2200

Ultrafast transient absorption (TA) spectroscopy was employed to investigate the thermal annealing effect on the charge transfer (CT) in bulk heterojunction (BHJ) all-polymer solar cells (all-PSCs) utilizing an n-type polymer P­(NDI2OD-T2) (Polyera, N2200) as acceptor and a low bandgap polymer PBPT as donor. The CT generates hole polarons residing in the PBPT and electron polarons belonging to N2200, manifested in the TA spectra of the BHJ films as the long-lived absorption peak centered at ∼850 nm. The CT is most efficient in the film annealed at 160 °C and its efficiency declines monotonically when enhancing or reducing the annealing temperature, displaying a positive correlation with the power conversion efficiency (PCE) of the corresponding solar cell devices. This correlation is analyzed in terms of the crystallinity and phase separation, which are the key factors determining the performance of all-PSCs. Our results can provide valuable guidance for the fabrication of BHJ all-PSCs to improve their PCE