Photo-gain optimization in multilayer organic phototransistors by study of space-charge limited current

Solution-processed organic phototransistors show the best of their potential in multilayer device architectures where each layer is devoted to a specific function, i.e. field-effect charge transport and light sensing. However, the optimization of the multilayer stack is often under-performed, due to limitations posed by the solution processing to avoid mutual dissolution of the underneath layers, and due to the lack of simple screening methods capable to assess the performance of the devices. In this work, figures of merit such as sensitivity and photo-gain of fully solution-processed multilayer organic phototransistors are optimized by studying in hole-only and electron-only diodes the space-charge limited current behaviour of different bulk heterojunctions, which constitute the photoactive layers of the phototransistors. The different bulk heterojunctions, based on two different fullerene derivatives (acceptors) and two different processing solvents/co-solvents, show different electrical behaviours depending on the bulk-heterojunction morphological characteristics. The proposed approach, which simplifies the study and allows the optimization of the functional and structural characteristics of the photoactive layer, allowed to increase the overall sensitivity of the multilayer phototransistor by 6 times.