Harvesting of Infrared Part of Sunlight to Enhance Polaron Transport and Solar Water Splitting

There is increasing evidence that defects such as oxygen vacancies are a double-edged sword for photoelectrochemical (PEC) water splitting. Although surface oxygen vacancies can largely improve the catalytic activity, their bulk counterparts may bind polarons, drag down the carrier transport, and thus degrade the PEC performance. However, it is very challenging to precisely control the spatial and energy distributions of defects. Instead, the infrared part of sunlight, normally discarded in PEC water splitting, is harvested to thermally activate the polarons. With the prototypical BiVO4 photoanode (absorbs blue-ultraviolet light), even when is undoped, a high solar-to-hydrogen efficiency of 5.3% is achieved in conjunction with a photothermal substrate (absorbs infrared light) and in tandem with a perovskite solar cell (absorbs red-green). Detailed characterizations reveal that the temperature increase of the system can not only accelerate the polaron hopping, but also activate the polarons bound by defects. This study, by demonstrating the use of the PEC-inactive infrared part of sunlight to enhance transport kinetics at the device level, allows for panchromatic sunlight harvesting to improve the overall PEC performance.