Fig. 3

Photodetection performance of dual-gate organic phototransistor. a Transient response of drain current with and without top gate bias (V_TG = 0, −20 V), operating at fixed source-drain bias of 5 V and V_BG of 20 V. A monochromatic light source was employed for excitation (543 nm, 10 mW cm⁻²), modulated at 10 Hz using a mechanical chopper. At such high intensity, the twofold increase in photocurrent is likely due to the improved separation of photogenerated electron-hole pairs by the vertical electric field introduced by the oppositely biased gates. b Normalized photocurrent showing the rise and decay kinetics at high (7.5 mW cm⁻²) and low (0.5 mW cm⁻²) intensities. At both biases, slower response time was observed at low intensity, which is consistent with the filling of energetically distributed trap states. With negative V_TG, slower rise time is likely due to the increase in electron-hole separation by the vertical electric field which extends time taken for the charge trapping/detrapping processes to reach steady state. In dark, the faster decay time can be explained by the sweep out of trapped holes by the top p channel which is not allowed at V_TG = 0 V. c Photo-responsivity, noise current spectral density I_N and specific detectivity D^* as a function of frequency at various V_TG. Responsivity, at 1 mW cm⁻², was reduced with increasing frequency since it is limited by the response times, and little difference was observed with/without negative V_TG. I_N also decreased with increasing frequency, showing 1/f characteristics. A significant reduction in I_N was observed at increasingly negative V_TG (3 orders of magnitude from 0 to −20 V). Such improvement in noise is also reflected in the resulted specific detectivity, which is nearly constant in the displayed frequency range since the reduction in responsivity is compensated by the improved noise. The solid lines serve as guides for the eye