The Influence of the Blend Ratio, Solvent Additive, and Post-production Treatment on the Polymer Dynamics in PTB7:PCBM Blend Films

The polymer dynamics inside a bulk heterojunction (BHJ), as used in organic solar cells, are investigated with quasielastic neutron scattering to study hydrogen motion in the polymer side chains. Different blend ratios of the polymer 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) and the small molecule acceptor [6,6]­phenyl-C₇₁-butyric acid methyl ester (PCBM) are investigated. In addition, the influence of performance-enhancing measures, such as the use of the solvent additive 1,8-diiodooctane (DIO) and the post-production treatment of the BHJ films with methanol, on the polymer dynamics is studied. The analysis of mean square displacements as well as relaxation times of diffusional motions of the hydrogen atoms, located mainly in the polymer side chains, shows a gradual stiffening of the PTB7 side chains for higher PCBM loading in the BHJ films. The presence of DIO significantly increases diffusive mobility inside the films, while the methanol treatment does not affect hydrogen motions.