How Solar Cells Work
The photovoltaic effect: how a silicon junction turns photons into current, why panel efficiency sits near 20%, and what actually limits it.
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A solar cell converts light directly into electricity with no moving parts and no fuel. The working idea is one junction and one bandgap: a semiconductor absorbs a photon, promotes an electron across the bandgap from the valence band to the conduction band, and a built-in electric field at the junction sweeps the freed electron one way and the hole the other. Collect that flow at metal contacts and you have current — no moving parts, no fuel, 25-year lifetimes.
The photovoltaic effect
Silicon absorbs a photon if its energy exceeds the bandgap (1.12 eV). The p-n junction's depletion region provides the field that separates charge; metal contacts collect it. Photons below the bandgap pass through (wasted as heat they cannot make); photons far above the bandgap waste their excess as heat. That two-front loss — plus reflection and resistive losses — is why the theoretical ceiling for a single silicon junction (Shockley–Queisser limit) is about 33%, and commercial modules sit at 18–22% efficient. Multi-junction cells stack materials with different bandgaps and exceed 45% but cost too much for rooftops; they power satellites instead.
From cell to system
A cell produces ~0.6 V; series-connected cells form a module; modules in series and parallel form an array whose DC output an inverter converts to grid AC. Real-world output is the nameplate derated for orientation, shading, temperature (hot panels produce less), and inverter efficiency — Ohm's law and Kirchhoff's circuit laws describe the electrical behavior once the photons have been converted. Levelized cost of solar electricity has fallen over 90% since 2010, which is why it now undercuts new fossil generation in most markets.
The tradeoffs
Intermittency and energy storage are the real engineering problem, not the panels — how batteries work covers the storage side. Manufacturing energy pays back in 1–3 years of operation, and panel lifetimes exceed 25 years, making the lifetime energy balance strongly positive.
Tags
energy photovoltaics physics solar