Solar Cell Doubles as Battery

A practical solar energy system usually includes solar cells that convert light to electricity and batteries that store the energy for later use.

Scientists from Toin University of Yokohama in Japan have designed a single, compact device that can both convert solar energy to electricity and store the electricity. “We succeeded in incorporating both photovoltaic and storage functions in a single cell with a thin, sandwich-type structure,” said Tsutomu Miyasaka, a researcher at the University.

The researchers’ photocapacitor is also efficient at capturing energy from weak light sources like sunlight on cloudy or rainy days and indoor lighting.

The light-driven, self-charging capacitor could eventually be used to power portable electronic devices like phones, cameras, and PDAs, said Miyasaka. “Users can just bring the device anywhere and expose it to indoor and outdoor ambient light whether they need power or not [then] release the stored electricity anytime they want,” he said.

Solar cells convert light to electricity by absorbing photons and using their energy to move electrons. There are two basic types of solar cells. Conventional cells are solid-state devices usually made from silicon. It is also possible to capture the energy from photons using dye molecules.

The researchers’ device is an electro-chemical cell made up of a pair of electrodes sandwiching a liquid electrolyte. The electrolyte contains a high concentration of ions, or atoms that carry a charge because they have gained or lost an electron. The electrodes are glass plates with metal coatings on the inside surfaces. The top electrode sports a film of titanium dioxide semiconductor nanoparticles that has pores 15 to 30 nanometers in diameter and contains ruthenium dye molecules. Both electrodes have porous inner layers of carbon particles that are about 5,000 nanometers in diameter, which is about the size of a red blood cell. The carbon layers encase the electrolyte.

Dye-based solar cells use dye molecules to absorb photons, which causes negatively-charge electrons and positively-charged holes to separate in the semiconductor layer. The researchers’ photocapacitor transfers these charges to the carbon layers.

The electrons travel toward the bottom electrode, where they accumulate on the carbon surface near the electrolyte. A chemical reaction that restores the electrical balance of the dye also makes holes accumulate on the carbon surface of the top electrode. “Electrons and holes generated by light-excited organic dye can be directly accumulated on the large surface area of the carbon layer,” said Miyasaka.