Boosting charge and thermal transport - role of insulators in stable and efficient n-type polymer transistors

Conjugated polymers are promising materials for flexible electronics. However, some prominent challenges remain and limit further commercialization. Among these issues, n-type polymers are known to be prone to electron trappings, which may lead to heat localization, unsustainable transport and ultimately device failure. In this contribution, three n-type polymers with representative electron-transporting moieties, double B←N bridged bipyridine (BNBP), naphthalene-diimide (NDI), and perylene-diimide (PDI), are selected and intentionally blended with a small amount of insulating polymer polystyrene (PS). In an organic field-effect transistor (OFET) structure, the blended semiconductors are shown to possess enhanced electron mobilities and device durability. The origin of the improved performance is investigated. Despite the thermally and electrically insulating properties of bulk PS, the blend films show improved heat transfer and electronic properties as revealed by scanning photothermal deflection and time-resolved photoluminescence. The counter-intuitive outcome is rationalized by a microstructure model in which PS blends inhomogeneously with the semiconductors. The added PS tends to mix with the amorphous phase, passivates phonons and charge trappings, and offers more efficient phonon and electron transport pathways. This work provides mechanistic insights into clinical device performance enhancement for semiconductor/insulator blends.