Organic Multibit Phototransistor Memories with High External Quantum Efficiency

Abstract This work reports phototransistor memory devices fabricated using organic planar heterojunctions of two organic semiconductor materials, namely poly(2,5‐bis(2‐octyldodecyl)−3,6‐di(pyridin‐2‐yl)‐pyrrolo[3,4‐c]pyrrole‐1,4(2H,5H)‐dione‐alt‐2,2′‐bithiophene) (PDBPyBT) and 2,9‐didecyldinaphtho[2,3‐b:2′,3′‐f]thieno[3,2‐b]thiophene (C10‐DNTT). The results of the study show a broad spectral sensitivity, ranging from UV light to red light. It is found that both organic layers are photoactive but also both play a role in the device's charge‐trapping properties: the PDBPyBT layer is effective in trapping holes while the C10‐DNTT layer traps electrons. The thickness of the PDBPyBT layer is identified as a crucial factor affecting the device performance, with the devices featuring a ≈20 nm thick PDBPyBT layer exhibiting a light‐induced hysteresis of ≈40 V. While the devices with a “ribbon” structured PDBPyBT layer demonstrate a linear response to the increasing light exposures duration. Furthermore, a new method is introduced to evaluate the external quantum efficiency (EQE) of the phototransistor memories in order to provide a robust metric that allows a benchmarking comparison of such devices' performance. When assessed using this EQE metric, these devices exhibit exceptionally high efficiency of trapping photogenerated charges compared to other reports. This study also demonstrates that the devices possess multibit programming capabilities, suggesting their potential use in dosimetry applications.