Boosting the Performance and Stability of All-Polymer Photomultiplication-Type Organic Photodiode: A Synthetic Approach

We demonstrate how achieving near-ideal spatial isolation of the polymer acceptor through a synthetic method can greatly improve not only the EQE but also the operational stability of an all-polymer photomultiplication-type organic photodiode (PM-OPD). Due to their extraordinarily high external quantum efficiency (EQE, often higher than 10,000%), photomultiplication-type organic photodiodes (PM-OPDs) have recently attracted a lot of attention and can be used as efficient self-amplifying photodiodes for the detection of weak light intensity. The trap-assisted photomultiplication mechanism in the PM-OPD is artificially activated, which sets it apart from traditional photovoltaic devices. The photoactive layer in a typical PM-OPD structure consists of a donor:acceptor (100:1, w:w) ratio to produce spatially segregated acceptor domains. Next, holes are gathered along the percolation pathway following the separation of photogenerated excitons at the donor/acceptor interface, and electrons are captured by the localized acceptor. Here, the Schottky barrier thins at the Al contact due to the trapped electrons, enabling hole injection through tunneling under reverse bias. Additional hole injection is necessary for current continuity; this is triggered by the narrowing of the Schottky barrier once a photogenerated hole is gathered by the counter electrode while the electron is still trapped. In order for PM-OPD to obtain a high EQE, "efficient electron trapping" is therefore a crucial requirement. In terms of effective electron trapping in PM-OPD, poly(3-hexylthiophene) (P3HT) as a donor and [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) as an acceptor have so far proven to be the most effective active layer combination. We hypothesize that this is mostly because the donor polymer semiconductors have better spatial distribution and localization of tiny molecule acceptors. We propose to address the simple phase separation of active layers with small molecular acceptors mixed with polymer donors during PM-OPD operation despite their high molecular diffusivity and the high operating voltage of a typical PM-OPD. To obtain not only a high EQE but also a high operational stability of PM-OPD, all-polymer bulk heterojunction (BHJ) should be researched in this regard, much like the organic photovoltaic field. Here, a series of naphthalenediimide (NDI)-based copolymers with variously alkylated benzenes are used in a synthetic method to examine the impacts of a polymer acceptor structure on PM-OPD performance. PNDI-Ph, PNDI-Tol, and PNDI-Xy were used as the polymer acceptors to create high-performance all-polymer PM-OPDs. PNDIs were subjected to systematic analyses using UV-vis absorption spectroscopy, density functional theory (DFT) calculations, and two-dimensional grazing-incidence X-ray diffraction (2D-GIXD). These analyses showed that structural features influence not only their optical, electrochemical, and microstructural properties but also their miscibility with donor polymers, and ultimately, the performance of the constructed all-polymer PM-OPD devices. This was demonstrated by effective Schottky barrier height measurement and drift-diffusion simulation. We showed that the crucial factor in the performance of an all-polymer PM-OPD device is spatial isolation of the acceptor domain inside the donor matrix, not high crystallinity or favorable electron transport characteristics. This led to the realization of an optimized PNDI-Xy based PM-OPD, which is greater than any previous all-polymer PM-OPD disclosed, with a high EQE of 770,000% and a high specific detectivity of 3.06 1013 Jones. We also discuss the improved operational stability of all-polymer PM-OPD devices against traditional small-molecule acceptor-based PM-OPD devices.