Correlation between local morphology and electronic states in organic field-effect transistors investigated by charge modulation microscopy

The morphology of the thin active layer is one of the main parameters that influence charge transport properties and thus the functioning of organic field-effect transistors (OFETs). To understand this relationship, it is desirable to have techniques able to locally probe the electronic properties along the channel of a working device. In this contribution we show that with Charge Modulation Microscopy (CMM) it is possible to optically map the charge density in a high-mobility n-type OFET with a lateral resolution of about 500 nm; this is achieved by measuring the local polaronic absorption and bleaching features in a laser scanning confocal microscope. Interestingly, the signal is sensible only to the few-nanometers thick accumulation layer, allowing the investigation of the properties of the electronic states at the semiconductor-dielectric interface involved in charge transport processes. By using a polarized light probe, we highlight the presence of a micrometric texture in the map of the charge induced features, which can be related to regions of preferential alignment of the polymeric backbones. These regions of preferential alignment could play a crucial role in the transport properties of polymeric semiconductors, especially in novel n-type high-mobility materials in which the influence of morphology at different scales is currently highly debated.