Advances in Interface-induced photomultiplication-type organic photodetectors: device physics, design strategies and multifunctional applications

Photomultiplication-type organic photodetector (PM-OPD), which converts the faint photon signals into amplified photocurrents, combines the benefits of intrinsic solution-processability and tailorable spectral response inherited from organic active materials, as well as compact footprint without external pre-amplifiers. Density and distribution of engineered traps within and nearby photoactive layers are keys for efficient PM behaviors in PM-OPDs, which induces trap-induced band bending and facilitates unobstructed carrier injection via the tunneling effect. Hitherto, incorporation of trap states inside photosensitive layer by doping materials or applying a disproportionate electron donor-acceptor ratio within active layer is the widely used method to fabricate PM-OPDs. Despite the great performance enhancements in these PM-OPDs, high bias voltage and slow response speed severely limit their commercial applications. Furthermore, the absence of carrier blocking layers leads to an increase in dark current, which eventually elevates noise level and results in a lower detectivity for weak light signals. To enhance PM device performance, various interface and morphology engineering strategies have been developed, alongside novel multifunctional devices for diverse applications. In this review, we outline recent progress in constructing high-performance PM-OPDs via interface engineering and vertical morphology control of the active layer, focusing on smart structural design and advances in stacking architectures for multifunctional applications. This review not only introduces the latest advances in fabrication strategies and device mechanisms for stacked PM-OPDs, but also provides significant insights into the fabrication of multifunctional applications based on these engineered PM-OPDs, including dual-mode, dual-band devices with distinct spectral range and operation modes, micro-spectrometers, up conversion optical imagers that convert invisible light into visible emission and polarization detectors.