The first solar cells were developed by D. Chapin, G.L. Pearson and C. Fuller in the USA. In the Horological Journal of April 1958, the following can be read: "One of their electric wrist watches has been modified by Hamilton Watch Co. to operate on power provided by the sun. A small silicon cell converts light into enough power to store sufficient electricity to run the watch. No commercial development is planned". A number of firms, for instance Patek Philippe (1952) and Lip, used solar cells in small clocks.
| 1920 | The first research on selenium starts. |
| 1930 | The first photocells are invented by the physicists B. Lange and W. Schottky. |
| 1953 | Patek Philippe presents the first light powered clock at the Basle Fair. The clock was equipped with photocells made of selenium. |
| 1976 | The first Swiss-made solar powered cells for watches are manufactured by Esotron S.A. of La Chaux-de-Fonds, Switzerland. |
The first solar cells were made of silicium and selenium. When light falls on a special kind of semiconductor diode, the photons create pairs of holes and electrons. The negative electrons go in the depletion-layer to the N-silicium and the positive holes to the P-silicium, and a voltage difference of about 0.3 to 0.5 Volt will be created. Four or five solar cells connected together in series, generate 1.5 Volt; the voltage required to make a watch run or recharge its battery.