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