Spectroscopic Study of Charge Transport at Organic Solid-Water Interface

Charge transport in an organic solid and its coupling with the neighboring aqueous biological environment dictates the performance of many organic bioelectronic devices. Understanding how the transport property at the solid-water interface is influenced by the surface structure characteristics of the organic solid is essential for rational design of organic bioelectronics and chemical sensors. However, in situ probing such structure-property relationships has been difficult due to lack of experimental techniques with sufficient sensitivity to the water-buried interface. Here, we demonstrate a charge accumulation spectroscopy (CAS)-based protocol, exploiting water-gated organic field-effect transistor as the testing platform, to investigate the structure-dependent localization of polaronic charge carriers at the organic semiconductor-liquid interface. Our results reveal that the degree of charge delocalization is reduced drastically when the charge carriers are moved from the bulk semiconductor to the semiconductor-water interface, suggesting the existence of a highly disordered surface layer in contact with water. It is also found that the charge delocalization could be further reduced by intercalation of chloride ions (from salt water) in the semiconductor surface layer. This study suggests that the spectroscopic signatures of polaronic charge carriers could be a sensitive probe to detect the structure-dependent charge localization at organic solid-liquid interfaces.