Long-term thermally stable nanoconfining networks for efficient fully conjugated semicrystalline/amorphous diblock copolymer photovoltaics

Abstract

Abstract In this study, a new fully π-conjugated diblock copolymer comprising of major semicrystalline poly(3-butyl thiophene) (P3BT) and minor amorphous poly(2,5-dihexyloxy-p-phenylene) (PPP) block chains was rationally designed and further used as synergistic charge-transporting networks for PCBM based solar cells. The light-harvesting P3BT block was carefully chosen due to its high melting temperature and greater hole mobility to afford better thermal stability. Despite lower molecular weight P3BT block in the copolymer compared with that of a P3BT homopolymer counterpart, the copolymer exhibited long-range ordered fibrils with larger P3BT crystals and higher total crystallinity, indicating that the minor PPP chains promote the ordering and crystallization of P3BT. In addition, the strong immiscibility and difference in the crystallographic kinetics among the two main-chain moieties allow the copolymer/PCBM hybrid to spontaneously self-assemble into an interpenetrating nanostructure, while permitting PCBM nanoparticles to preferentially locate in the amorphous PPP domains to form bicontinuous conduits for efficient dual-charge transport. Notably, the copolymer/PCBM hybrids exhibited superior long term thermal stability against degradation and PCBM aggregation upon accelerated aging of the devices. At stringently high aging temperature of 180 °C for 4 h for a thermal stress test, the copolymer devices retain nearly 90% of their maximum PCE, whereas those of the reference P3BT/PCBM solar cells decrease severely to be less than 50% of their maximum. These results demonstrate the advantage of using thermally stable all π-conjugated block copolymers to provide substantial enhancement in charge dissociation, transport as well as extended thermal stability due to the nanoconfining effect of its self-assembled copolymer structure.