Solar Cell

Structure. A solar cell (or photovoltaic cell) is, physically, a p-n junction very similar to a photodiode -- a large-area junction exposed to light -- but built with NO external bias battery anywhere in its circuit. A fine metal grid of thin conducting fingers is deposited across the illuminated top face (to collect current with minimal loss of the light-collecting area to shading), and a solid metal contact covers the entire back face.

The photovoltaic effect. When sunlight (or any sufficiently energetic light) falls on the exposed junction, it generates electron-hole pairs inside and near the depletion region, exactly as in the photodiode of Section 9.14. But because the depletion region already contains a built-in electric field (Section 9.9), these newly generated carriers are swept apart by that field EVEN WITH NO EXTERNAL BATTERY CONNECTED AT ALL: the field drives the generated electrons toward the n-side and the generated holes toward the p-side, and this ongoing, light-driven separation of charge is exactly what builds up a measurable, USABLE potential difference across the two external terminals of the cell -- the cell itself becomes a small source of EMF, capable of driving a current through an external load resistor with no other power source needed. This direct conversion of light energy into electrical energy, with no external bias required, is called the PHOTOVOLTAIC EFFECT, and is what distinguishes a solar cell's mode of operation from a photodiode's (which needs an external reverse-bias supply and merely modulates a small current drawn from it). …