High-Sensitivity Solution-Processed Organic Phototransistor Based on a Bulk Heterojunction with a Persistent Radical as the Electron Acceptor

In bilayer organic phototransistors (OPTs), charge transport and light-sensing functionalities are separately performed and optimized in two different layers. For optimizing the sensitivity of solution-processed bilayer OPTs, the approach of using a donor-acceptor bulk heterojunction (BHJ) as the light-sensing layer is well established in the literature, but the choice of the electron-accepting materials is often limited to fullerene-soluble derivatives or to standard nonfullerene acceptors. Herein, we report the unprecedented use of an organic persistent radical as an electron acceptor in the BHJ light-sensing layer of solution-processed bilayer OPTs. The radical acceptor is coupled at different donor:acceptor ratios to a low-band-gap polymer that absorbs in the near-infrared (NIR) region. At a donor:acceptor ratio of 1:3, the organic radical forms isolated domains within the BHJ. Such a morphology, coupled with the strong electron-accepting characteristics of the radical, leads to efficient trapping of electrons and efficient hole transport within the BHJ, as measured in charge-selective devices operated in the space-charge limited current (SCLC) range. This, together with the chemical and photostability of the persistent radical, allows us to obtain an OPT with photosensitivity (P) of 1 × 105 in response to NIR irradiation at 2 mW/cm2 and excellent photostability over time.