Abstract
A photomultiplication-type organic photodiode (PM-OPD), where an electric double layer (EDL) is strategically embedded, is demonstrated, with an exceptionally high external quantum efficiency (EQE) of 2210000%, responsivity of 11200 A W-1 , specific detectivity of 2.11 × 1014 Jones, and gain-bandwidth product of 1.92 × 107 Hz, as well as high reproducibility. A polymer electrolyte, poly(9,9-bis(3'-(N,N-dimethyl)-N-ethylammoinium-propyl-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene))dibromide is employed as a work-function-modifying layer of indium tin oxide (ITO) to construct an EDL-embedded Schottky junction with p-type polymer semiconductor, poly(3-hexylthiophene-diyl), resulting in not only advantageous tuning of the work function of ITO but also an enhancement of the electron-trapping efficiency due to electrostatic interaction between exposed cations and trapped electrons within isolated acceptor domains. The effects of the EDL on the energetics of the trapped electron states and thus on the gain generation mechanism are confirmed by numerical simulations based on the drift-diffusion approximation of charge carriers. The feasibility of the fabricated high-EQE PM-OPD especially for weak light detection is demonstrated via a pixelated prototype image sensor. It is believed that this new OPD platform opens up the possibility for the ultrahigh-sensitivity organic image sensors, while maintaining the advantageous properties of organics.
Highlights
Research regarding photomultiplication-type organic photodiodes (PM-OPDs) with external quantum efficiencies (EQEs) exceeding 100% have been actively conducted[1,2,3,4,5,6,7,8,9,10]
The high EQE of PM-OPDs is a result of photoconductive gain generation, where the gain is an index describing the number of conducting charges that can be induced by one generated photon
The photoactive layer consists of a P3HT:phenyl-C71-butyric acid methyl ester (PC71BM) (100:1, w/w) bulk heterojunction, which is typical for PM-OPDs where PC71BM is localized within the P3HT matrix without forming electron percolation pathways
Summary
Research regarding photomultiplication-type organic photodiodes (PM-OPDs) with external quantum efficiencies (EQEs) exceeding 100% have been actively conducted[1,2,3,4,5,6,7,8,9,10]. The high EQE of PM-OPDs is a result of photoconductive gain generation, where the gain is an index describing the number of conducting charges that can be induced by one generated photon. More recently reported alternative theory predicts that the high gain observed in the experiments can originate from other external effects such as defects, surface conditions, and surface depletion regions that may localize excess minority carriers, resulting in accumulated excess majority carriers creating gain, or extrinsic gain generation mechanism[13]. In this case, the gain (for a p-type photoconductor) is determined by:
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