Photogenerated Charge-Aided Low-Voltage Operation of n-Channel Organic Transistors with n-Type Conjugated Polymers─Toward Photosensor Applications

Organic semiconducting materials with a low charge carrier mobility cannot properly act as a channel layer at low voltages for organic field-effect transistors (OFETs). Here, we demonstrate that an n-type conjugated polymer, poly[{5,5,11,11-tetrakis(5-(2-ethylhexyl)thiophene-2-yl)-dithieno[2,3-d:2′,3′-d′]-s-indaceno[1,2-b:5,6-b′]dithiophene}-co-{2,7-bis(2-octyldodecyl)benzo[lmn][3,8]phenanthroline-1,3,6,8(2H,7H)-tetraone}] (PIDTT-NDI), works as an n-channel layer at ≤5 V under light illumination, even though the devices are unable to operate as a transistor in the dark at low voltages. PIDTT-NDI layers with three different thicknesses (t = 50, 70, and 100 nm) are placed on poly(methyl methacrylate) (PMMA) gate-insulating layers in the OFET geometry of bottom-gate and top-source/drain electrodes. Two individual monochromatic light with wavelengths (λ) of 434 and 754 nm, which are chosen from the main peaks in the optical absorption spectra of the PIDTT-NDI films, are employed as a photoexcitation source. The results show that the light illumination delivers proper output and transfer curves of typical n-channel OFETs at ≤5 V, and the electrical conductance of devices gradually increased with the light intensity. In particular, a remarkable enhancement in the photosensitivity (ca. 14,000% at λ = 434 nm) is achieved for the OFETs with the 100 nm-thick PIDTT-NDI layers due to the extremely low drain current level in the dark (5 V).