High Speed Response Organic Photodetectors with Cascade Buffer Layers

Organic photodiodes (OPDs) with improved photo-response characteristics are reported, which are an important performance indicator. The photo response characteristics of OPDs depend on the decay rate of current due to charge carriers remaining inside the device even after interruption of the irradiated light. In this study, the effect of improving the response speed of OPDs is experimentally investigated by applying a cascade HTL to the hole-extraction interface and investigated that the introduction of multi-layered hole transport layers (HTLs) with a stepwise energy profile, called cascade shape profile, is effective in reducing the decay time of the remaining charges. For a two-layered cascade HTL OPD, the amount of remaining photo-current is minimized, equivalent to less than 70% that of single-layered HTL OPDs, when the highest-occupied-molecular-orbital (HOMO) level gaps among anode (indium tin oxide), HTL1, HTL2, and the p-type semiconductor are close to equal (phi 1 approximate to phi 2 approximate to phi 3). A cascade HTL does not negatively affect the sensitivity of OPDs, suggesting that it provides a promising means of achieving practical organic image sensors.