ИСТИНА

Organic phototransistors (Fig. 1a) combine the properties of field-effect transistors and photodiodes. Efficient phototransistors should be ambipolar to allow all photogenerated electrons and holes to contribute to the photocurrent. In ambipolar transistors, the position of the electron-hole recombination zone in the channel can be controlled by the gate voltage, VG. The typical width of this zone is in the range of 15-200 nm.1 The ability to control the spatial position of the recombination zone can be used for studying the fundamental processes of charge injection, recombination, and transport in organic semiconductors,2 and also it is promising for development of new devices, such as optical image scanners with high spatial resolution.3 In ambipolar organic field-effect phototransistors, the recombination zone can act as a photosensitive region, because it is depleted with charge carriers and the maximum electric field is achieved there, which facilitates efficient separation of the photogenerated electron-hole pairs. Using numerical modeling, it was shown that the normalized photocurrent Jph/Jdark dependences on VG can reproduce the spatial profiles of incident light intensity across the phototransistor channel, after transformation of the VG-scale to x-scale in accordance with the position dependence of the electric field peak, xpeak, on VG (Fig. 1b).3 In this work, we present the organic phototransistor numerical model enhanced by accounting for time-dependence of model parameters in order to study the response times of the phototransistor. The modelling shows that the response times to a stepwise illumination are several nanoseconds (Fig. 1c), and upon switching VG from one value to another the response times are a few microseconds. We show that a comprehensive improvement of the phototransistor performance, namely, simultaneous improvement of the spatial resolution, external quantum efficiency (EQE), sensitivity, and response times, can be achieved by changing of only two parameters — the channel length and the intensity of incident radiation. Variation of the remaining phototransistor parameters affects the its performance ambiguously. For example, lowering the active layer dielectric constant and increasing the gate electric capacitance results in high spatial resolution and short response times; in particular, Jph/Jdark vs VG dependences reproduce the spatial position of an incident light beam with an accuracy of tens of nanometers and response times shorter than a few microseconds. However, such phototransistor will have relatively low values of photocurrent, EQE and sensitivity. The numerical modelling presented in this work is expected to facilitate the development of organic phototransistors with high spatial and temporal resolution. This work was supported by Russian Science Foundation (project 18-79-00341).