Light-transformable and -healable triboelectric nanogenerators

Triboelectric nanogenerators (TENGs) have attracted significant interest due to their energy harvesting performances using relatively versatile and robust device architectures (i.e., a pair of stacked distinct layers connected to electrodes). The materials and device architectures of TENGs are highly compatible with emerging wearable technologies. Recently, rational surface morphology designs were considered as a key to enhancing the output efficiency. Surface relief structures can increase an effective surface area, enhance the mechanical contact area between two stacked materials, and therefore enhance the output voltage of a TENG. Unfortunately, surface reliefs in soft polymeric materials generally degrade under repetitive mechanical contact stress. This work introduces a TENG that is transformable and healable via light irradiation-induced material migration (also called by photofluidization). This technique deterministically maximizes the energy harvesting performance and remotely repairs morphological damage. As a representative example, the photo-transformable azobenzene polymer (azopolymer) was implemented in a TENG. The azopolymer surfaces were sculpted to form surface reliefs with a controlled dimensionality and periodicity. The output voltage and current obtained from the TENGs were linearly proportional to the surface area, as expected. During TENG operation, however, the surface relief collapsed and flattened. As a result, the output signals of the TENGs degraded continuously. The light-powered reversible transformation perfectly healed the surface morphology and the resultant TENG performance. The light-powered transformation and healing of the TENGs provides a facile, large-area, and reliable method for designing future flexible energy-harvesting devices.