Sunlight can be harvested through photovoltaic cells, which as the name indicates, transform sunlight directly into energy (volts), and photoelectrochemical cells, which transform sunlight into hydrogen fuel. The photovoltaic cells are problematic in that they can only generate energy during daylight hours and use rechargeable batteries to store energy at night. This method is cost-inefficient and only a short-term solution.
Photoelectrochemical cells however can store energy without exterior batteries; by using special semiconductors, light energy can split water molecules into oxygen and hydrogen, which can then be stored as fuel to later produce energy.
While this method should be efficient, the semiconductor often used, hematite, does not work quite as well as scientists predicted and the reason was unknown until recently.
The research team studied how hematite, an iron oxide like rust, works in the water-splitting process. They assumed that some of the electrons that were excited by the sunlight's incoming photons reached energy states that could not move in the material. Those photons were wasted and lost in the process, leading to a decrease in productivity.
The scientists developed a new method for testing this limitation on hematite, which could be applied to other materials as well. The discovery allows for a better understanding of semiconductors and the most efficient way to build photoelectrochemical cells, so that solar energy and storage can be better produced.