Dielectric Surface-Dependent Photogating Phenomenon and Interface Trap Engineering in C8-BTBT Leading to Photo-Induced Synaptic Applications

Organic phototransistors (OPTs) are being adopted in considerable research field due to their excellent photo-sensing capabilities, high compatibility with flexible substrates, and simple manufacturing processes. To improve the photoresponsive characteristics, previous OPTs studies have focused on the development of organic semiconductor layers. Exploring a suitable dielectric surface, however, is also an important factor in controlling these photoresponsive properties in OPTs. Nonetheless, research on dielectrics is still lacking. In this talk, we present the dielectric-surface-dependent photogating behavior of 2,7-Dioctyl[1]benzothieno[3,2-b][1]benzothiophene (C8-BTBT) phototransistors. Photogating effect was observed depending on the trap states between C8-BTBT and various dielectric interfaces: cyclic transparent optical polymer (CYTOP), poly(methyl methacrylate) (PMMA), aluminum oxide (Al₂O₃), and silicon dioxide (SiO₂). The photogating effect was not observed in hydrophobic dielectric-based OPTs (CYTOP-based OPTs and PMMA-based OPTs), while the photogating effect was observed in hydrophilic dielectric-based OPTs (Al₂O₃-based OPTs and SiO₂-based OPTs), achieving high sensitivity of >7.34×10⁵ A A^-1, detectivity of >6.32×10¹¹ Jones, and 20 V shift of turn-on voltage under 400 nm light irradiation. The operation principle of photogating effect was elucidated through a comprehensive study, including an analysis of roughness, morphological investigation, hydroxyl group, and surface energy, according to the dielectric surface. Al₂O₃- and SiO₂-based OPTs have more hydroxyl group than CYTOP- and PMMA-based OPTs, resulting in relatively high surface energies and interfacial trap density of states (N_it) values. Through the nearly identical surface roughness (Ra) values for each dielectric by AFM analysis, we demonstrated that the photogating effect is dependent on the interfacial charge trap state by the surface energy, not because of the specific morphological variation between each dielectric surface and semiconductor interface. In addition, we implemented an artificial synapses application using the photogating effect. Optoelectronic artificial synaptic operation using SiO₂-based OPTs accomplishes a recognition rate of more than 95%, whereas, despite the same semiconductor, CYTOP-based OPTs accomplish a recognition rate of 0%. As a result, this research suggests that optimizing the dielectric layer surface characteristics is an important component in modulating photoresponsive properties, and through the implementation of artificial synapses, the possibility of applying optoelectronic devices.