Shortwave Infrared (SWIR) light imaging is critical in a variety of applications ranging from medical diagnosis, industrial inspection and safety monitoring, etc. due to their information-rich merits. This report presents an organic up-conversion imager for SWIR light imaging. The imager integrates an organic SWIR photodiode (OPD) and an organic light emitting diode (OLED) to convert invisible SWIR image directly into visible image without sophisticated data acquisition and processing electronics. Our imager is capable of imaging light signal with wavelength up to 1400 nm, due to the sensitivity of the novel OPD. The design guidelines for attaining highly sensitive and low voltage up-conversion imagers are revealed by studying the photo responsivity and current-voltage characteristics of the SWIR PD and OLED. The results show that the elimination of deep trap states in the SWIR PD favors the photo sensitivity and reduces the operating voltage of the up-conversion imager.
Short wavelength infrared (SWIR) sensors are important to applications in environmental monitoring, medical diagnosis and optical communications, but there are only a few organic semiconductors that show optoelectronic response in the SWIR region. Recently we demonstrated a family of novel donor-acceptor polymers with narrow bandgap responsive in the SWIR region, and the bulk heterojunction photodiodes based on these polymers show detectivity up to 1E11 Jones at a wavelength of 1.37 micron, with absorption edge extending out to 1.7 micron. A SWIR photodiode was incorporated into the etalon-array reconstructive spectroscopy system to demonstrate its imaging capabilities.
As the initial performance is very promising, we proceed to investigate the stability of the encapsulated devices and to infer the degradation mechanisms. The performance of photodiodes were monitored by IV measurement, external quantum efficiency (EQE) and electrochemical impedance spectroscopy. The IV measurement and electrochemical impedance spectroscopy were conducted both in the dark and under illumination, to track over several weeks the change in charge generation and recombination processes under the short circuit and open circuit conditions. The characteristics from band-to-band absorption and from absorption in charge-transfer states were compared to quantify the lifetime and recombination losses of photogenerated carriers in these devices.
KEYWORDS: Short wave infrared radiation, Infrared radiation, Infrared photography, Chemical analysis, Polymers, Transistors, Electronics, Infrared detectors, Signal to noise ratio, Analytical research
Low-cost infrared photo-transistors with improved detectivity (i.e. higher signal-to-noise ratio) could find further use in spectral analysis, which is important for chemical identifications, as well as other applications from environmental monitoring to optical communications. Accordingly, the main goal of this research is to advance printed, flexible photo-transistors by using a family of novel donor-acceptor polymers with narrow bandgap that are responsive in the short wavelength infrared (SWIR) region. In particular, the transistors show optical response extending out to a wavelength of 1.8 micrometer. The external quantum efficiency and the rectification ratio are used to characterize the performance of devices with different polymer layer thickness, in order to optimize detectivity. The individual transistors could further be exploited for the fabrication of integrated arrays for bio-medical and/ or robotic applications. It paves the way to large-area, conformal designs that are currently not achievable with conventional inorganic SWIR materials.
Photosensors responsive to the short wavelength infrared (SWIR) spectra are used in a variety of applications including environmental monitoring, medical diagnosis and optical communications. However, most organic semiconductors do not absorb in the SWIR region. Here we show novel donor-acceptor polymers with narrow bandgap responsive in the SWIR region, and the polymers are processed into photodiodes with structure of ITO/PEDOT:PSS/Bulk Heterojunction (BHJ)/Al. The performance of devices with different polymer structures are compared through metrics including detectivity, quantum efficiency, response time and rectification ratio, to determine the mechanisms of charge recombination loss in charge transfer states and charge transport process. We also use different solution-processed interfacial functional layers (e.g. ZnO, MoO3, TiO2) as electrode interface structures. The results provide guideline for selecting suitable polymers and design of device structures, to enable high performance SWIR photosensor via scalable solution-processed fabrication.
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