Directly probing the molecular order of conjugated polymer in OPV blends induced by different film thicknesses, substrates and additives

Abstract

In organic bulk heterojunction photovoltaic (OPV) devices, formation of a phase-separated morphology of the blend thin film with a high degree of molecular order is required for efficient device performance. Using resonant Raman spectroscopy we monitor in situ the P3HT molecular order in P3HT:PCBM blend films influenced by the substrate, film thickness and additives. We report that molecular order depends on substrate for as-cast films, consistent with vertical phase separation driven by a surface energy gradient, but is standardised to a highly ordered state by thermal annealing. In situ Raman spectroscopy reveals this phase transition to a more ordered state begins at 40–60 °C for ∼120 nm thick blend films, which corresponds to the glass transition temperature (T_g). Ultra-thin (<10 nm thick) blend films had greater P3HT order than the bulk and reorganised at lower temperatures, which we propose is due to a P3HT-rich interfacial layer at the film/air interface, and that extra disordered component retained despite annealing is due to P3HT trapped in a disordered state within the corresponding PCBM-rich substrate interface. Finally we probe how the 1,8-octanedithiol (ODT) additive improves P3HT molecular order in blends by increasing phase separation during deposition, finding that 3% ODT by volume presents a saturation point for improving molecular order, and the improvement is comparable to that by thermal annealing. Through in situ experiments and varied fabrication conditions, we have built an understanding of how processing conditions determine conjugated polymer molecular order in blends, with the aim of controlling morphology for higher OPV efficiencies.