Organic photovoltaics (OPVs) with nonfullerene acceptors (NFAs) feature excellent device performance and device stability. However, they are facing problems when the amine-rich polyelectrolytes are used as cathode interfacial layers. In this work, a small molecule, ethanedithiol (EDT) at the polyethyleneimine ethoxylated (PEIE)/active layer interface is inserted for mitigating the undesirable reaction between amine-rich groups and electron-acceptor moieties in NFA. The main role of EDT is to passivate the PEIE surface and prevent electron flow to NFA and the unwanted reaction can be mitigated. It improves the performance of OPV devices by reducing the work function, decreasing trap-assisted recombination, and improving electron-mobility. As a result, the flexible device with the PEIE interfacial layer with a power conversion efficiency (PCE) of 7.20% can be improved to 10.11% after the inclusion of EDT. Moreover, EDT-modified device can retain 98.18% after it is bent for 200 cycles and can maintain 80.83% of its initial PCE under continuous light illuminated in ambient conditions without any encapsulation. Based on these findings, the proposed strategy constitutes a crucial step toward highly efficient flexible OPVs.
Solution-processed flexible organic optoelectronic devices have great potential as low-cost organic photovoltaics for energy harvesting, and in organic light-emitting diodes as a lighting source. However, a major challenge for improving device performance and stability is the different interfacial characteristics of the hydrophobic organic layers and hydrophilic transparent electrodes, particularly for flexible devices. Surface wetting controlled interfacial engineering can provide a useful method to develop highly efficient flexible organic devices. Here, an unsaturated fatty acid-modified ethoxylated polyethyleneimine organic interfacial layer is designed, which is hydrophobic or hydrophilic on different interfaces. This interlayer results in a power conversion efficiency of 10.57% for rigid and 9.04% for flexible photovoltaic devices. Furthermore, the long-term air storage stability for 250 h is substantially improved, retaining 87.75% efficiency without encapsulation, due to the wettability driven improvement of the optical and electronic properties of the cathode interfacial layer. The performance of organic light emitting diodes also benefitted from the interlayer. This study provides a strategy to simultaneously improve efficiency and stability by controlling the wettability of the interfacial layer.
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