Visible light effect on the performance of photocouplers/phototransistors based on pentacene

We report on the processing and characterization of organic photocouplers and phototransistors. These optoelectronic components were realized on the same transparent substrate by coupling an organic light emitting diode (OLED) with a photoconductive material such as pentacene. Various OLEDs with emission peaks in the green (550 nm) and the blue (450 nm) have been tested in order to optimize the photocoupler performance in terms of photocurrent gain (Iphotocurrent/Idark) and response times. In the first hand, we showed that the photocurrent gain increases as the wavelength decreases, and in the second hand, that the rise and decay times, corresponding, respectively to the illumination time of the component and to the time required for the device to come back to its initial state, are faster for photons of high energy: from a wavelength of 450 nm, the recombination processes are fast enough so that no photocurrent persists during relaxation. Moreover, we examined the performance of a phototransistor based on the coupling of an OLED (with an emission peak at 550 nm) as an input unit with a transistor (based on pentacene and polymethylmethacrylate as the dielectric gate) as an output unit. The characteristics of the transistor under illumination and at maximal luminance are consistent with a charge carrier mobility and a threshold voltage of about 0.03 cm2 V−1 s−1 and 12.5 V, respectively.