Solar Cell or Photovoltaic Cell

Solar energy can be harnessed in two broad ways: PHOTOTHERMAL devices, which convert solar energy into heat (mostly used for heating water), and PHOTOVOLTAIC devices, which convert solar energy directly into electrical energy using SOLAR CELLS (also called photovoltaic cells). Light incident on a solar cell produces both a current and a voltage, so it acts directly as a source of DC power -- useful anywhere from remote terrestrial locations to satellites and space stations.\n\nStructure (Fig. 16.13a): a solar cell is essentially a large-area p-n junction. Its n-side is THIN and faces the incident solar radiation directly, coated with an antireflection layer that also serves as the electrical FRONT CONTACT -- this coating lets visible light through while reflecting away IR (heat) radiation, protecting the cell's electronic properties from temperature-related degradation. The p-side is relatively THICK and forms the back of the cell, coated with a conducting BACK (or rear) contact. Making the n-side thin ensures incident light penetrates through to the depletion region, where electron-hole pair generation is most useful. A good solar-cell material must be photosensitive (able to absorb light and raise electrons to a higher energy state) and must allow those higher-energy electrons to be extracted into an external circuit, where they dissipate their energy usefully before returning to the cell.\n\nWorking: when light strikes the cell, (1) electron-hole pairs are photo-generated within the depletion region; (2) the junction's built-in electric field SEPARATES these carriers before they can recombine -- Fig. 16.13(b) -- sweeping electrons into the n-region (where they become majority carriers) and holes into the p-region (where they become majority carriers); (3) these separated charges accumulate on the p- and n-sides, building up a voltage across the cell; and (4) when an external circuit/load is connected, this accumulated charge drives a current -- called the light-generated or photo-generated current -- through it, dissipating usable power in the load. Crucially, this separation only works because the carriers are kept spatially apart by the depletion region's field; without that separation, the photo-generated electrons and holes would simply recombine and no net current or power would result.\n\nThe V-I characteristic (Fig. 16.14) is conventionally drawn in the fourth quadrant, since the cell SUPPLIES current to a load. Its intersection with the current axis gives the SHORT-CIRCUIT current $I_{sc}$ (maximum current, delivered when the load is shorted, so power delivered is zero); its intersection with the voltage axis gives the OPEN-CIRCUIT voltage $V_{oc}$ (maximum voltage, when no current flows, so power delivered is again zero); and somewhere between these two extremes lies the point of MAXIMUM POWER delivered to the load. Criteria for choosing a solar-cell material include: a band gap between about 1.0 and 1.8 eV, high optical absorption, good electrical conductivity, and ready availability. Most practical solar-cell materials cluster around a 1.5 eV band gap -- silicon (Eg = 1.1 eV), GaAs (Eg = 1.43 …