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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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For solution-based small-molecule organic photovoltaic cells (SM BHJ), thermal annealing is often regarded as a very vital processing to yield high-efficiency devices. Here, the device with the photoactive p-DTS(FBTTh 2 ) 2 :PC 60 BM blends has been studied by varying the annealing temperature. The highest power conversion efficiency (PCE), fill factor (FF), and short-circuit current (J sc ) has been achieved after the device was annealed at the temperature of 80 8C. X-ray diffraction (XRD) displayed that the p-DTS(FBTTh 2 ) 2 tends to form big islands with well-defined crystalline structure (long coherence length) surrounded by small islands of the PC 60 BM after annealing, and reached the largest grain size of 27 nm after annealing at 80 8C. AFM images demonstrated the most sufficient phase separation within the photoactive blend layer after annealing at 80 8C, which provides good photogenerated excitons dissociation. Such improvements have proved beneficial to decrease the ohmic loss and recombination in carrier-transport process via measuring series resistance and shunt resistance as well as hole mobility. This work opens up a new way to understand the relationship between thermal annealing and molecular interaction from the viewpoint of lattice relaxation.

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