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Development and Characterization of High-Performance Solution-Processed Bulk Heterojunction Organic Photodetectors


The motto of University of California is “let there be light.” But how do we know whether there is light, especially the light that we cannot see with our eyes? The answer is photodetectors. Photodetection has long play an indispensable role in modern technologies. The research endeavors on organic photodetectors (OPDs) have been continuously increasing in the recent years since they promise great potential as a platform for photosensing, imaging, and optical communication. Compared to the established crystalline inorganic counterparts, OPDs show their unique attractiveness: tunable spectral activity, low-temperature and low-cost fabrication, light weight and mechanical flexibility, large-area solution/printing processibility, and versatile integration into complicated optoelectronics. In this thesis, three studies are carried out to enhance the sensing performance, as well as to better understand the device physics of bulk heterojunction (BHJ) OPDs. Notwithstanding the exciting progress made thus far for OPDs, efficient broadband detection, especially in the near infrared (NIR) region, still needs to be improved due to the challenges in simultaneously securing high photoresponse and low dark/noise current for narrow bandgap systems. For the first part, the focus is placed on achieving high NIR sensing performance for BHJ organic photodiodes. A novel ultranarrow bandgap (~1.2 eV) small molecule is used, which present the highest responsivity of 0.45-0.52 A/W in the spectral region of 920 – 940 nm. With well suppressed charge injection under reverse bias using thick junction approach, high NIR specific detectivity of ~1012 Jones can be obtained even at a large bias of 2 V. Subsequently, we touch an instability problem found for inverted structure OPDs, where a large inconsistency of dark current can be universally found between before- and after-illumination conditions, negatively impacting the crucial figures-of-merit such as specific detectivity. Systematic control tests reveal that the widely used zinc oxide electron transport layer has caused the issue, which is triggered by exposure to high energy photons. We identify an alternative electron transport layer, optimized as “multilayer” tin oxide film, which is not only immune to the illumination-induced dark current inconsistency, but also simultaneously retain well suppressed low dark/noise current, uncompromised photoresponse, and rapid response speed. With the success of regular photodiode based OPDs studied in the first two parts, the focus is then shifted to introducing photomultiplication (PM) gain to BHJ OPDs, aiming at external quantum efficiency over 100%. This goal is accompanied by the targets of simultaneously achieving broad spectral response, low operating voltage, and applicable photoresponse speed, the aspects where the current approaches of making PM OPDs show shortcomings. Introduction of electron trapping agent (tetracyanoquinodimethane, TCNQ) into the BHJ blend has resulted in PM OPDs that can deliver not only large EQE over 104 % at low operation voltages, but also a wide span of spectral response from ultraviolet to NIR. The origin of the observed large photocurrent gain is revealed to originate from the interfacial metal-organic interaction and the enhanced electron trapping in the bulk of the BHJ layer, both induced by the intentionally added TCNQ. These effects essentially convert the device behavior from that of a regular photodiode to a photoconductor with excessive trapping sites of electrons.

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