Correlating dilute solvent interactions to morphology and OPV device performance

• Solvent additives affect crystalline content of P3HT and PCBM in BHJ film.
• BHJ film cross-links when annealed at 300 °C in the presence of 1,8-diiodooctane.
• Pristine P3HT and PCBM films also cross-link when annealed under similar conditions.
• NMR spectroscopy for quantitative analysis of trace solvents in BHJ films.

Solvent additives have been explored as a reliable way to control the morphology in bulk-heterojunction (BHJ) layers for improved device performance. We show that the choice of solvent additives has direct implications on morphological evolution, i.e. poly(3-hexylthiophene) (P3HT): [6,6]-phenyl C61-butyric acid methyl ester (PCBM) BHJ films processed with a small amount of 1,8-diiodooctane or 1-chloronaphthalene have more crystalline PCBM domains compared to crystalline P3HT domains, while the opposite is true for films cast with nitrobenzene additive and films cast purely from chlorobenzene. The BHJ film cross-links when annealed at 300 °C in the presence of 1,8-diiodooctane. Cross-linking is found to occur even in pristine P3HT and PCBM films annealed under similar conditions. NMR spectroscopy is presented as a viable technique for quantitative analysis of the amount of solvents left in the BHJ films before and after heat treatment. Despite differences in the ways the additives affect the morphology of the BHJ layer, device performance remained stable over 300 h for all additives tested.

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