Light-powered watches work when light strikes a solar cell made from a semiconductor such as specially treated silicon, and frees up electrons that flow in the same direction to form an electric current which is then stored in the secondary battery and used to power the watch.
   Citizen produced the world's first analogue quartz watch using sunlight as a power source in 1976. This was followed by a digital type a year later.
   The original solar cells were made from adjoining layers of single crystal silicon and were therefore very thick. Furthermore, both primary and secondary (storage) batteries were not very reliable.
   The advent of the amorphous silicon solar cell and of large capacity condensers led to the development of solar watches that didn't use batteries, and in 1986 Citizen launched its Analog with Solar Cell watch. However, even when fully charged, the watch had sufficient power for no more than 10 days. Nevertheless, the era of "batteryless" watches had begun.

Citizen soldiered on to pioneer the use of a new secondary power source—a lithium-ion secondary battery. More popularly known as the MT cell, it is clean, pollutant-free, highly reliable and long-lasting. It increased performance and allowed for the creation of smaller and thinner watches. In 1996 Citizen introduced Eco-drive, which is powered by the MT cell.