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How do photons excite electrons inside a solar cell?

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Diagram of a solar cell showing labeled layers, including a front electrical contact grid, antireflection layer, absorber layer, back junction layer, and back electrical contact layer, with arrows indicating light entering and terminals connecting to a circuit. — solar cell A commonly used solar cell structure. In many such cells, the absorber layer and the back junction layer are both made of the same material.

In a solar cell, photons excite electrons through the photovoltaic effect, a process facilitated by the semiconductor materials that form the core of the device.12

Within these materials, electrons are typically bound to atoms in a lower-energy state known as the valence band. When incident light strikes the solar cell, photons—the energy packets of light—transfer their energy to these bound electrons. If a photon possesses energy greater than the material's band gap, it provides sufficient energy to excite the electron, causing it to jump from the valence band to a higher-energy state called the conduction band.431

Once in the conduction band, the electron becomes relatively free to move through the solid. This excitation also creates a "hole" in the valence band, representing a missing negative charge. To generate a usable electric current, a p-n junction creates a built-in electric field that separates these free electrons and holes, forcing them to flow in a specific direction toward electrical contacts and into an external circuit.4312