Wavelength‐Selective Organic Photodetectors

Abstract

Spectroscopic sensing combined with optical imaging is crucial with respect to today's ever-growing demand for instant analytical techniques to be incorporated in various handheld and wearable devices. Further miniaturization and integration of such types of sensors is critical and wavelength-selective organic photodetectors (OPDs) may provide the required technology. In this progress report, some early OPD applications and their potential are presented. Crucial device parameters such as the specific detectivity, external quantum efficiency, and dark current density of visible and near-infrared wavelength-selective pho-todetectors are compared and assayed to theoretical and semi-empirical limits. The different organic detector approaches include the use of inherently narrow-band absorbers as well as internally filtered and microcavity devices. Each of these strategies comes with its own specific material and device design criteria, around which material development and selection should be centered to move beyond the current state of the art. As OPD technology matures, device stability becomes important and is hence also briefly discussed. Via this perspective, it is aimed to provide the reader with critical insights into the device physics and chemistry of wavelength-selective OPDs, hereby providing leverage for new ideas to bring this technology to the market. The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/adfm.202104060. manipulate the detection wavelength and transparency range. Additionally, organic photo-active materials can be deposited via thermal evaporation or from solution via inkjet printing, blade, or slot-die coating at ambient temperatures. [2] These mild depo-sition conditions allow increased flexibility in detector design on various substrates. This includes fabrication directly on complementary metal-oxide semiconductor (CMOS) readout circuits, [3,4] as well as the use of polymer substrates for lightweight and flexible detectors. [5,6] To date, photodetectors employing organic semiconductors with optical gaps spanning the visible (VIS) and near-infrared (NIR) range from 400 up to 2500 nm have been reported. [7-9] High-performance devices have been demonstrated in the VIS and significant research efforts now focus on OPDs with detection wavelengths beyond the silicon regime (> 1000 nm), which at this time requires costly inorganic materials which are not compatible with direct integration on CMOS readout circuits. [10-12] However, to be interesting for NIR applications, OPDs will need significant improvements in dark and noise currents as well as photon-to-electron conversion efficiencies. Furthermore, only a few studies exist on their long-term operational stability and degradation mechanisms. In this progress report, we discuss the status and recent progress in approaches to achieve wavelength-specific and tun-able OPDs. We focus on devices with an intrinsically narrow detection range and a rather simple device architecture and omit filtered broad-band detectors. For a full overview of the chemistry, physics, and technology of broad-band OPDs, we refer to several recent reviews. [5,8,13,14] The benefits of filter-less narrow-band OPDs are illustrated through a selection of recent proof-of-concept demonstrations. Progress in the physical and chemical methods to achieve narrow-band OPDs is reported and we discuss performance limits and how to improve the different performance parameters, including device stability. As such, we hope to provide the reader with critical insights and inspiration for the tailored design of organic semiconductors for innovative optical sensing applications. 2. Selected Proof-Of-Concept Demonstrations of Wavelength-Selective OPDs As opposed to inorganic semiconductors, organic materials can be designed to have a strongly peaked absorption centered