Conjugated Polymer Heteroatom Engineering Enables High Detectivity Symmetric Ambipolar Phototransistors

Solution-processed high-performing ambipolar organic phototransistors (OPTs) can enable low-cost integrated circuits. Here, a heteroatom engineering approach to modify the electron affinity of a low band gap diketopyrrolopyrole (DPP) co-polymer, resulting in well-balanced charge transport, a more preferential edge-on orientation and higher crystallinity, is demonstrated. Changing the comonomer heteroatom from sulfur (benzothiadiazole (BT)) to oxygen (benzooxadiazole (BO)) leads to an increased electron affinity and introduces higher ambipolarity. Organic thin film transistors fabricated from the novel PDPP-BO exhibit charge carrier mobility of 0.6 and 0.3 cm² Vs⁻¹ for electrons and holes, respectively. Due to the high sensitivity of the PDPP-based material and the balanced transport in PDPP-BO, its application as an NIR detector in an OPT architecture is presented. By maintaining a high on/off ratio (9 × 10⁴), ambipolar OPTs are shown with photoresponsivity of 69 and 99 A W⁻¹ and specific detectivity of 8 × 10⁷ for the p-type operation and 4 × 10⁹ Jones for the n-type regime. The high symmetric NIR-ambipolar OPTs are also evaluated as ambipolar photo-inverters, and show a 46% gain enhancement under illumination.