https://phys.org/news/2018-06-energy-storing-solar-cell-reality.html

In two joint projects, the scientists are exploring new ideas for using molecules to store solar energy and are investigating molecules and processes that allow energy to be stored efficiently and released in a controlled manner when required. It is even conceivable that stored chemical energy could be converted directly into electrical energy.

The research is based on the so-called norbornadiene-quadricyclane storage system. Norbornadiene (NBD) and quadricyclane (QC) are hydrocarbons and have been under discussion among experts as potential candidates for storing solar energy for some time now. Under the influence of light, a reaction within the norbornadiene molecule is triggered, causing the molecule to transform into quadricyclane. The reaction produces an energy density similar to that of a high-performance battery. Thanks to this property, quadricyclane is also known as "solar fuel."