Low power electronics are an ideal application for organic photovoltaics (OPV) where a low-cost OPV device can be integrated directly with a battery to provide a constant power source. We demonstrate ultra-high voltage small molecule multijunction devices with open circuit voltage (VOC) values of up to 7V. Optical modelling is employed to aid the optimisation of the complex multi-layer stacks and ensure current balancing is achieved between sub-cells, and optimised multijunction devices show power conversion efficiencies of up to 3.4% which is a modest increase over the single junction devices. Sub-cell donor/acceptor pairs of boron subphthalocyanine chloride (SubPc)/fullerene (C60) and SubPc/Cl6-SubPc were selected both for their high VOC in order to minimise the required number of junctions, but also for their absorption overlap to reduce the spectral dependence of the device performance. As a result, the devices are shown to directly charge a micro-energy cell type battery under both low illumination intensity white light and monochromatic illumination.
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