Nanoscale J-aggregates of poly(3-hexylthiophene): Key to electronic interface interactions with graphene oxide as revealed by KPFM
The performance of organic thin film optoelectronic devices strongly relies on the nanoscale aggregate structure of the employed conjugated polymer. Their impact on electronic interface interactions with adjacent layers of graphene, widely reported to improve the device characteristics, yet remains an open issue, which needs to be addressed by an appropriate benchmark system. Here, we prepared discrete ensembles of poly(3-hexylthiophene) nanoparticles and graphene oxide sheets (P3HTNPs–GO) with well defined aggregate structures of either J- or H- type and imaged their photogenerated charge transfer dynamics across their interface by Kelvin probe force microscopy (KPFM). A distinctive inversion of the sign of the surface potential and surface photovoltage (SPV) demonstrates that J-aggregates are decisive for establishing charge transfer interactions with GO. These enable efficient injection of photogenerated holes from P3HTNPs into GO sheets over a range of tens of nanometers, causing a slow SPV relaxation dynamics, and define their operation as efficient hole-transport layer (HTL). Conversely, H-type aggregates do not facilitate specific interactions and entrust GO sheets the role of charge-blocking layers (CBL). The direct effect of P3HT’s aggregate structure on GO’s functional operation as HTL or CBL thus establishes clear criteria towards the rational design of improved organic optoelectronic devices.