Investigation of Molecular Dynamics of a PTB7:PCBM Polymer Blend with Quasi-Elastic Neutron Scattering

In organic photovoltaics, bulk heterojunctions (BHJs) of organic semiconductor substances such as conjugated polymers and fullerenes are frequently used as active layers in which light is transformed into free charge carriers. The performance of the respective solar cells is critically influenced by the inner morphology of the active layer. Besides structural, also dynamical properties are important but by far less frequently investigated. In this study, we probe the polymer dynamics of an active layer composed of a low band gap polymer and a fullerene derivate. The acceptor [6,6]-phenyl-C61-butyric-acid-methyl-ester (PCBM) and the donor 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), as well as their 1:1 blend, are probed with time-of-flight quasi-elastic neutron scattering. Our observable timescale window of motions covers a range from 1 ps to approximately 50 ps. We probe a temperature range from 150 to 400 K, which covers the reported maximum power conversion efficiency of the respective BHJ solar cells. Within this temperature range, PCBM does not show any dynamics on the observable timescale. Blending with PCBM (wt. ratio 1:1) causes frustration of the PTB7 side-chain dynamics, which manifests in increased relaxation times and decreased diffusion coefficients. Important for solar cell applications, temperature variations do have a major impact on the polymer dynamics, which in some circumstances are far more pronounced than the influence of blending with PCBM. Furthermore, we show that this temperature dependence follows an Arrhenius-type behavior.