Novel Organic Shortwave Infrared Photosensors
A low-cost and scalable short-wavelength infrared (SWIR: λ = 1–3 μm) photosensor will be widely deployable and transformative for a wide range of spectroscopic systems and optoelectronics that form the foundation for scientific, industrial, medical, and defense applications. Conventional SWIR photosensors are cost-prohibitive due to complex fabrication involving epitaxial growth of inorganic crystals. To make SWIR photodetectors affordable for ubiquitous sensing, this thesis aims to realize low-cost organic SWIR bulk heterojunction (BHJ) photodiodes by using a novel class of modular narrow bandgap conjugated polymers, and to demonstrate the direct solution deposition of organic thin films can replace complex manufacturing processes and produce the comparable performance.
While polymeric semiconducting materials have been widely used and extensively studied in organic photovoltaics, different device behaviors accompany the progressive shrinkage of polymer bandgap to extend spectral absorption out to SWIR region. A better understanding of the fundamental properties of SWIR organic photodiode (OPD) is therefore needed to predict and advance performance. This thesis focuses on understanding different aspects of OPD performance that centers around specific detectivity, which ultimately defines the signal-to-noise ratio.
Firstly, in Chapter 3, the challenges in the dark noise increase associated with the low-bandgap organic system are discussed, followed by an introduction to different noise suppression methods. An emphasis is made on the importance of direct noise measurement to stay out of the pitfall of overestimating detectivity. Two approaches including interface engineering and electrode work function tuning are demonstrated to suppress the noise current in SWIR OPDs. Secondly, in Chapter 4, an improved model to decouple dissociation and collection efficiency is proposed to pinpoint the limiting factor of SWIR OPDs, and exemplary device engineering methods are shown to improve the dissociation bottleneck. Then, in Chapter 5, two recombination mechanisms limiting the device efficiency are shed light upon by connecting the optoelectronic properties to the materials composition. Lastly, in Chapter 6, three applications are demonstrated to show the practicality of solution-processed SWIR OPDs and the potential in realizing the low-cost and large-scale SWIR sensing.