High-performance Photomultiplication-type Organic Photodiodes: External Quantum Efficiency, Bandwidth, Stability, and Color Selectivity

Abstract

The photomultiplication-type organic photodiodes (PM-OPDs) can overcome the photocurrent limit of the conventional organic photodiodes by selfamplification effect with an external quantum efficiency (EQE) over 100%. PMOPDs generate photoconductive gain by inducing injection and circulation of holes (electrons) by artificially trapping electrons (holes), so the longer the electron (hole) recombination lifetime, the higher EQE. Therefore, it has a fatal disadvantage of a slow response speed. In addition, a lot of reported results based on various materials also show a limit of convergence to about 100,000% of EQE. Also, in general, in the case of a photoactive layer composed of donor:acceptor (100:1, w/w), since electrons are trapped by the isolated acceptor domains, the isolated morphology plays an important role in generating PM mechanism, and the stability of the device without phase separation is more important than that of a general OPDs. Therefore, studies were conducted to overcome the limitations of PM-OPDs. Chapter 1 describes the research background of this thesis, and Chapter 2 describes the experimental methods used in this research. In Chapter 3, a method to overcome the slow response speed of PM-OPD is introduced. Methods of obtaining a low hole trap density of the photoactive layer from 1) the regioregularity of the donor polymer and 2) a pre-deposited interlayer are introduced. It is shown that a low hole trap density can induce a fast response speed even with a long carrier recombination lifetime, suggesting that there is no direct trade-off relation between the response speed and the photoconductive gain. To demonstrate the feasibility of the fabricated PM-OPD with a high operation speed, a plethysmography measurement was performed. In Chapter 4, a newly designed structure of PM-OPD which can achieve extremely high EQE is proposed. Methods of inducing electrostatic attraction to stabilize trapped electrons 1) by adding fluorinated azide to the photoactive layer and 2) by inducing ionic electrolytes as an interlayer are introduced. An ionic polyelectrolyte is introduced as an interlayer of PM-OPD to induce interfacial electrostatic interactions between the exposed cations and the trapped electrons within isolated acceptor domains. As a result, they showed exceptionally high EQEs. The feasibility of the fabricated high-EQE PM-OPDs especially for weak light detection is demonstrated via a pixelated prototype image sensor. In Chapter 5, a method for realizing a stable PM-OPD is introduced. It is shown that fluorinated azide can enhance an operational stability by freezing the morphology of the photoactive layer. The fabricated device shows improved stability in operational stability and damp heat test. In conclusion, in this study, PM-OPDs with fast response speed, high EQE, and high stability are realized through material and structural approaches.