Effect of UV irradiation on solution processed low voltage flexible organic field-effect transistors

Effect of ultra-violet (UV) irradiation (λpeak = 365 nm) on the electrical characteristics of solution processed flexible TIPS-pentacene organic field-effect transistors (OFETs) has been investigated. Pristine TIPS-pentacene OFETs demonstrated average field-effect mobility of 0.1 cm2 V−1 s−1, with near zero threshold voltage and current on-off ratio of ∼104. On UV irradiation, OFETs displayed a joint photoconductive and photovoltaic effect due to photo-generated excitons. The maximum current modulation and photo-responsivity obtained from these OFETs were ∼500 and ∼43 mA/W respectively at an intensity of 1.8 mW/cm2 while operating at a low voltage of −5 V. On increasing the irradiation time, a positive shift in the threshold voltage was observed. At larger values of irradiation time, a roll-off in maximum drain current and mobility values were observed, which was attributed to slight deterioration in crystallinity due to prolonged UV exposure, as confirmed from X-ray diffraction studies. Similar trend was observed for mobility and threshold voltage values, when gate bias during UV irradiation was increased. In addition, transistors exhibited a repeatable dynamic response to periodic pulses of UV irradiation.