Organic photodiodes: device engineering and applications

Frontiers of Optoelectronics (2022) 15:49 https://doi.org/10.1007/s12200-022-00049-w REVIEW ARTICLE Organic photodiodes: device engineering and applications Shan Tong1 · Xiao Hou1 · Xiaokuan Yin1 · Xiaojun Guo1 Received: 19 May 2022 / Accepted: 9 August 2022 © The Author(s) 2022 Abstract Organic photodiodes (OPDs) have shown great promise for potential applications in optical imaging, sensing, and communication due to their wide-range tunable photoelectrical properties, low-temperature facile processes, and excellent mechanical flexibility. Extensive research work has been carried out on exploring materials, device structures, physical mechanisms, and processing approaches to improve the performance of OPDs to the level of their inorganic counterparts. In addition, various system prototypes have been built based on the exhibited and attractive features of OPDs. It is vital to link the device optimal design and engineering to the system requirements and examine the existing deficiencies of OPDs towards practical applications, so this review starts from discussions on the required key performance metrics for different envisioned applications. Then the fundamentals of the OPD device structures and operation mechanisms are briefly introduced, and the latest development of OPDs for improving the key performance merits is reviewed. Finally, the trials of OPDs for various applications including wearable medical diagnostics, optical imagers, spectrometers, and light communications are reviewed, and both the promises and challenges are revealed. Keywords Organic photodiodes · Wearable electronics · Photoplethysmography · Optical imagers · Spectrometers · Optical communications 1 Introduction Photodetectors based on various inorganic semiconductors, including silicon, III-V semiconductors, metal oxides, and semiconducting alloys, have been extensively explored in optical sensing or imaging systems for medical, security, and industrial applications [1–5]. With advantages of widerange tunable photoelectrical properties, low-temperature facile processes, and excellent mechanical flexibility, organic semiconductor (OSC) photodetectors have received wide attention as promising technology choices for developing optical sensing or imaging interfaces in emerging applications where the existing inorganic devices may not meet requirements [1, 6–9]. Among various organic photodetector device configurations, the organic photodiode (OPD) is most widely investigated, due to its fast response, high sensitivity, and making full use of the existing research basis of organic photovoltaics (OPVs) [6, 8, 9]. Extensive research * Xiaojun Guo 1 School of Electronic Information and Electrical Engineering, Shanghai Jiao Tong University, Shanghai 200240, China work has been carried out on exploring materials, device structures, physical mechanisms, and processing approaches to improve the performance of OPDs to the level of their inorganic counterparts. To date, OPDs with spectra spanning from ultraviolet (UV) to near-infrared (NIR) have been reported [10–13]. The performance of state-of-art OPDs even rivals that of commercialized low-noise silicon photodiodes (PDs) within the visible spectral range [14]. Large area, ultra-thin and flexible OPDs have also been fabricated, showing benefits for creating optical sensing interfaces in new form factors [15]. Additionally, various system prototypes of OPDs have been built to seek potential applications in wearable health sensing devices, optical imagers, spectrometers, light communication systems, etc. [16–29]. Due to inherent mechanical flexibility and process compatibility with low Young’s modulus plastic substrates, one promising application of OPDs is in skin-conformal optical sensors for wearable health monitoring and medical diagnostics (e.g., photoplethysmography (PPG) and cardiovascular sensing) with minimal invasiveness [15]. With low-temperature facile processes, the OPD is also more suitable than the hydrogenated amorphous silicon (α-Si:H) PD for direct integration on top 13 Vol.:(0123456789) 49 Page 2 of 33 of thin-film transistor (TFT) backplanes for large-area/flexible high-resolution optical imagers [30]. There has been intensive research on OPD-based active-matrix imagers for medical imaging and biometric authentication applications, which used inorganic TFT backplanes, including α-Si:H, amorphous indium-gallium-zinc-oxide (a-IGZO), and lowtemperature polycrystalline silicon (LTPS) to leverage the industry-standard processes [31, 32]. All-organic integration of OPDs on top of the organic TFT (OTFT) backplane was also developed to achieve thermal and mechanical matching of the whole material stack with common plastic films for ubiquitous optical imagers of highly customizable form factors [33, 34]. With tailorable photoelectrical properties, miniaturized spectrometer prototypes were made by integrating customized wavelength-selective OPD pixels into compact modules for handheld or wearable spectrum measurements [35, 36]. Fast response with bandwidth up to MHz and some specific photo-response properties of OPDs make them potential in the development of various light communication systems, including indoor navigation and data communication (high indoor photo-generation efficiency) [37], encryption communication (spectral selectivity) [38, 39], multichannel visible light communication (multi-wavelength response) [40], and remote control (NIR response) [41]. The research progress as shown above has established a solid material and device basis for developing high-performance OPDs for integrated systems, and has also presented great promise of OPDs for many emerging applications. There have been several reviews in the literature on the advances of OPD-related research, covering topics of materials, device structures, physics, processing methods, and applications with OPDs, respectively [1, 6, 7, 9, 42–45]. It is vital to link the device optimal design and engineering to the system requirements, and examine the existing deficiencies of OPDs for practical applications. Therefore, this review will start from discussions on the required performance metrics for different applications. Then the fundamentals of the OPD device structures and physics are briefly introduced, and the latest development of OPDs for improving the key performance metrics is reviewed. Finally, the trials of OPDs for various applications are reviewed, and both the promises and challenges are revealed. 2 Performance metrics Defining proper performance metrics is key to link device-level optimal design to specific application requirements. The key metrics for typical optical sensing or imaging applications should cover performance in four aspects: photon-to-electron conversion efficiency, transient response, detection range, and spectral responsivity. High photon-to-electron conversion efficiency is 13 Frontiers of Optoelectronics (2022) 15:49 generally required f (...truncated)