Influence of surface passivation aiming towards enhancing the performance of organic phototransistor fabricated using oriented thin films of PFO-DBT

Oriented and uniform thin films of solution-processable organic semiconducting copolymer PFO-DBT were fabricated using a recently developed unidirectional floating film transfer (FTM) method and used as an active semiconductor on surface passivated Si/SiO₂ as gate dielectric to fabricate organic phototransistor (OPT) in the bottom gate top contact device architecture. The influence of different passivation of SiO₂ such as an ultrathin spin-coated cyclic transparent optical polymer (CYTOP) and the self-assembled monolayer of hexamethyldisilazane (HMDS) on anisotropic charge transport and device performance of OPTs have been systematically investigated. FTM-processed thin films provided good optical anisotropy in thin films and electrical anisotropy in the OPTs. With improved surface properties and enhanced anisotropies, the device showed a charge carrier mobility of 7.8 × 10^-3 cm²/V-sec, on/off ratio in the order of 10⁵, and a threshold voltage of -4V under dark conditions. OPTs fabricated in this work demonstrated photosensitivity nearly of order 10⁴ and a corresponding responsivity of 17 A/W under the illumination of 525 nm monochromatic green light with a low power density of 0.15 mW/cm². The results are promising for making OPTs with enhanced mobility, photosensitivity and photo-responsivity in the visible region.