Title:  High-Performance Organic Photodiodes and Their Applications

Committee:

Dr. Bernard Kippelen, ECE, Chair , Advisor

Dr. Gee-Kung Chang, ECE

Dr. Omer Inan, ECE

Dr. Benjamin Klein, ECE

Dr. Gregory Abowd, CoC

Abstract:

Low-cost high-performance photodetectors are in need to enable ubiquitous intelligence and computing systems. Organic photodiodes (OPDs) can be fabricated into devices with a highly conformal form factor using coating and printing techniques that are scalable. Hence, they enable large-area devices and arrays with simple fabrication processes in the use of versatile applications. In this work, we develop OPDs with an unprecedented level of performance that can rival that of low-noise silicon photodiodes (Si PDs). The magnitude and fluctuations in the dark current play a central role in low-light-level sensing devices. We unravel the origins of dark current and electronic noise in organic bulk heterojunction and demonstrate the proper selection of materials with weak electronic interactions between donors and acceptors that produces a low-noise photodiode with a measured specific detectivity value of 8 x 1013 Jones. Next, we demonstrate large-area OPDs fabricated on flexible substrates, yielding low dark current density values in the pA/cm2 range at low-voltage operation, which outperforms that of rigid their silicon counterparts of similar size. With this capability, we design these OPDs into an innovative geometry to record photoplethysmograms (PPG) for applications in physiological sensing. Their highly conformal form factors combined with the new geometry provide improved optical power collection compared to using conventional small-area Si PDs. Finally, we introduce a universal method using atomic layer deposition for reducing dark current density values for OPDs with low shunt resistance. This superficial treatment allows decreased dark current density in reverse bias by five orders of magnitude in thin p-doped OPDs, leading to a reduced noise equivalent power and a high detectivity value. This strategy provides a solution to the problem of sensing a continuous pulse of light with weak optical power.