Solar cells are photodiodes with very large surface areas. The large surface area makes the device more sensitive to incoming light, as well as more powerful (high currents and voltages) than photodiodes. For example, silicon alone can create a potential of 0.5 V, which can supply up to 0.1 A when exposed to bright light. They can be used to power small devices, such as solar calculators, or can be added in series to charge nickel-cadmium batteries. Often solar cells are used as light-sensitive elements in detectors of visible and near-infrared light (for example, light indicators, photosensitive trigger mechanism for a relay).
Like photodiodes, they have a positive and negative lead, which should be connected to more positive and more negative voltage areas inside the circuit. The typical response time for a solar cell is about 20 ms. Like batteries, cells can be combined in series or in parallel. Each solar cell produces an open circuit voltage from about 0.45 to 0.5 V and can generate up to 0.1 A in bright light. By adding cells in series, the output voltage becomes the sum of the individual cell voltages. When cells are placed in parallel, the output current increases.
Each cell provides 0.5 V, so the total voltage is 4.5 V minus 0.6-V drop due to the diode). A diode is added to the circuit to prevent NiCd cells from discharging through the solar cell during darkness. It is important not to exceed the safe rate of charging NiCd-elements. To slow down the charge rate, you can add a resistor installed in series with the batteries. Photothyristors are light-thyristors. Two common photothyristors include light activated SCR (LASCR) and a light-active triac.
LASCR acts as a switch that changes states whenever it is exposed to a light pulse. Even when the light is removed, the LASCR remains on until the polarity of the anode and cathode is canceled or the power is removed. The light-active triac is similar to LASCR, but is designed to handle AC currents. The equivalent circuit shown here helps explain how LASCR works. Again, like another pn junction optoelectronic device, the photon will collide with an electron on the p-semiconductor side, and the electron will be ejected through the pn-junction to the n-th side.
When a series of photons release several electrons through a junction, a sufficiently large current is generated at the base to turn on the transistors. Even when photons are eliminated, LASCR will remain on until the polarities of the anode and cathode are canceled or the power is turned off. (This is due to the fact that the fundamentals of transistors are continuously modeled by the main current flowing through the anode and cathode leads).
