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-b:4,5-b′]dithiophen-2-yl}-3-methyl-5-(4-octylphenyl)-4H-thieno[3,4-c]pyrrole-4,6(5H)-dione) and Poly[1,8-bis(dicarboximide)-2,6-diyl]-alt-5,5′-(2,2′-bithiophene)

Abstract

The all-polymer solar cells composed of wide band gap polymer PTP8 as the donor and N2200 as the acceptor exhibit a broad absorbance in the range of 300-900 nm, thanks to the complementary absorption of the near-infrared light by N2200. Although N2200 shows reasonably high electron mobility, the contribution of the photo-generated excitons in N2200 to the power conversion of the PTP8/N2200 solar cell is insignificant. Here, the hole transfer from the N2200 to PTP8 in PTP8/N2200 blend films was investigated by utilizing the ultrafast transient absorption spectroscopy. The spectral fingerprints of the ground state bleaching and the hole polaron-induced absorption of the PTP8 are identified under selective excitation of the N2200 component, which unambiguously indicate the hole transfer from the 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...