An efficient organic near-infrared (NIR) to visible upconversion imaging device is obtained by integrating a photoactive buffer layer between two organic light-emitting diodes in a tandem configuration. As both types of photo-carriers (electrons and holes) are harvested to generate visible photons, this tandem device exhibits a significantly higher On/Off switching and has higher photon-photon conversion efficiency than conventional upconversion devices. We show that a high 5% photon-photon conversion rate is possible by optimizing the various functional layers in the tandem structure. We also demonstrate that a pixel-less NIR imaging chip can be made simply by using a large area single tandem organic upconversion device.
We have successfully developed a C60:LiF/BCP (bathocuproine) bilayer-buffer structure to optimize both the light absorption enhancement and the exciton-blocking at the cathode interface of a small molecular photovoltaic cell based on the archetypical CuPc/C60 structure. The function of the C60:LiF layer is to serve as an optical spacer and is found to yield a peak power conversion efficiency (PCE) with a 50 wt. % LiF at a thickness of 30 nm. A BCP layer is added between the optimized C60:LiF layer and the Al metal electrode to function as a barrier for excitons. This combined bilayer-buffer structure yields an optimal performance in short circuit current (Jsc) and PCE. A detailed study using a single carrier electron-only structure and a numerical simulation of optical electric-field distribution suggests that C60:LiF layer enhances light absorption at long wavelengths.
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