Solution-Processed Ternary Organic Photodetectors with Ambipolar Small-Bandgap Polymer for Near-Infrared Sensing

Abstract

Organic photodetectors (OPDs) detecting light in the near-infrared (NIR) range from 900 to 1200 nm offer numerous applications in biomedical imaging and health monitoring. However, an ultra-low bandgap of the electron donor compound required to achieve NIR detection poses a unique challenge in selecting a complementary acceptor material with a suitable energy-level offset. To tackle this, a solution-processed, fullerene-dominated, ternary device is engineered by adding an ultra-low bandgap (0.6–0.8 eV) ambipolar polymer, polybenzobisthiadiazole-dithienocyclopentane (PBBTCD), into the active layer of visible-light-responsive OPDs (bandgap of 1.8 eV) to form a ternary blend. The resulting OPD benefits from the extended absorption beyond 1000 nm. The cascaded energy level alignment within the ternary blend and the applied reverse bias both improve the overall NIR photocurrent responsivity by 2 orders of magnitude, reaching 0.4 mA W−1 at 1050 nm and −2 V for ternary devices. Furthermore, a photovoltage responsivity of 0.3 mV m2 W−1 along with significant open-circuit voltage (Voc) of 0.12 V allow NIR detection in the Voc mode. Prominently, this ability is accomplished with a minimal presence of PBBTCD. Taken together, this work indicates potential strategies for extending the spectral activity of conventional OPDs through introduction of an ambipolar ultra-low bandgap polymer as a minor element.