Polymer backbone curvature has been shown to significantly alter mixing and interchain stacking in polymer:fullerene blends, which effects charge mobility and photovoltaic device efficiency.[1] Using the polymer curvature as a ‘control-handle’ for a series of closely related alternating thiophene-benzothiadiazole copolymers, this work probes the link between the nature of polymer-fullerene intermixing and charge generation pathways using ultrafast broadband transient absorption spectroscopy. We demonstrate that free charge generation is most efficient in a 3-phase morphology that features intimately mixed polymer:PCBM regions amongst neat polymer and PCBM phases.
Through analysis of the polymer ground state bleaching spectral dynamics, as well as near IR photoinduced absorption signatures, we are able to determine whether holes occupy disordered or crystalline polymer chains and thereby track their migration. For the polymers whose rigid backbones lead to 2-phase blends lacking an intermixed region, the observation of minimal spectral dynamics, along with polarization anisotropy retention, shows that holes are immobile. Rapid geminate charge recombination and poor photovoltaic performance are observed in such blends. In contrast, the 3-phase morphology supported by the polymer with a curved backbone exhibits highly mobile holes that are observed to migrate from the intermixed domains to phase-pure polymer domains. Such blends feature longer-lived populations of extractable charges and concomitantly better device performance. The energy gradient between the intermixed and phase-pure regions may be sufficient to drive efficient separation of charge pairs initially generated in intermixed regions, with free charges subsequently percolating through these phase-pure domains.
These results provide valuable insight into the pathway of efficient charge separation and its link to blend morphology and polymer structure.
[1] Lee, W.; Kim, G.-H.; Ko, S.; Yum, S.; Hwang, S.; Cho, S.; Shin, Y.; Kim,
J. Y.; Woo, H. Y. Macromolecules 2014, 47, 1604–1612.