Interface Engineering in PBTTT-C14 OFETs: Unified Control of Voltage- and Photogating Performance

In contrast to the conventional voltage-gating mechanism observed in organic field-effect transistors (OFETs), the photomediated gating effect presents a promising strategy for controlling charge transport within the semiconductor channels. This effect holds significant potential for enhancing the functionality of OFETs, particularly in image and optical pattern recognition. In this work, we provide a detailed analysis of photogated OFETs based on poly[2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2-b]thiophene] (PBTTT-C14) and compare its performance with that of voltage-gated counterparts. Notably, we observe that the photoelectric conversion efficiency of the OFETs improves when the dielectric surface is passivated using the self-assembled monolayers (SAMs) including [2-(3,6-dimethoxy-9H-carbazol-9-yl)ethyl] phosphonic acid (MeO-2PACz), hexamethyldisilazane (HMDS), and octyl trichlorosilane (OTS). We attribute this enhanced photogating performance to a built-in negative pseudo-gate potential developed in the channel of the OFETs. Generally, the photogenerated charge carriers become trapped by interfacial/bulk trap sites, and due to their specific spatial distribution, they possess the ability to induce an additional electric field, similar to a gate voltage, which leads to the modulation of channel conductivity. The OFETs exhibit superior photogating efficiency and smaller electrical equivalent subthreshold swing when compared to their performance under voltage gating. The characteristics of the photogated OFET measured at different wavelengths of light exhibit improved performance under low-wavelength illumination. The potential and challenges of photogated OFETs invite great fundamental as well as applied research interest in the domain of dual-responsive transistors.