Here is how such a lantern looks like:
The lid not only bears the solar cells, it contains also a rechargeable battery, a bit of electronics, and a white light-emitting diode (LED) that produces the light at night.
The LED is actually quite tiny, but it gives off a very bright and concentrated spot of light. This light is refracted and dispersed by the corrugated casing of transparent plastic.
Photons of sunlight hitting the semiconducting material of the photovoltaic solar cell can promote electric charges from the valence to the conduction band, where they can move and create a small direct electric voltage, if the material is suitably engineered. This voltage is used to charge the battery. When no light falls on the solar cells, the electronics allows the battery to discharge via the LED; where just the inverse process takes place: Electric charges recombine across the band gap, thereby emitting photons. LEDs that can give off white light are a relative recent development, and rely on quite sophisticated recipes to combine different semiconducting materials.
The solar constant, the flux of energy from the sun hitting the Earth, is 1.37 kW/m² at the top of the atmosphere, but there is of course absorption, clouds blocking the light, the constant change of day and night, the height of the Sun varying with the seasons... - as a result, the annual average of solar power arriving at ground level is about 120 W/m² for Germany (or 1 MWh/m² per year). The solar cell of the lamp has an area of roughly 50 cm² (7 × 7 cm²), so it can harvest roughly 0.6 W. Assuming an efficiency of 15% for a standard multicrystalline Si solar cell, the solar panel produces a power of about 10 mW on average over the year. During the summer months, this power may be higher by a factor of two or so, but it is still quite small - yet big enough to light steps in the garden path all night long.
- For more on LEDs, see our post The blue LED
- Germany may not be the sunniest place on Earth, but it is leading in the production and application of photovoltaics.
Physics, Solar Cells, LED