Sunlight to Electricity
A solar cell is a large-area PN junction optimized to absorb sunlight and convert it into electrical current. When a photon with energy exceeding the band gap is absorbed, it creates an electron-hole pair. The built-in electric field of the junction sweeps electrons to the N side and holes to the P side, establishing a photovoltage. Connect an external load and current flows — a clean, silent conversion of light into electricity with no moving parts.
The IV Curve and Maximum Power Point
The current-voltage characteristic of an illuminated solar cell is described by I = I_ph − I_0(exp(V/V_T) − 1). At zero voltage (short circuit), all photogenerated current flows through the circuit as I_sc. At zero current (open circuit), the voltage reaches V_oc. The maximum power point sits at the knee of the IV curve, and the fill factor FF = P_max/(V_oc × I_sc) measures how rectangular the curve is — higher fill factor means more power extraction.
Temperature: The Silent Efficiency Killer
Solar cells lose efficiency as they heat up, which is ironic given they sit in direct sunlight. The dominant mechanism is the exponential increase in dark saturation current I_0 with temperature, which reduces V_oc by about 2.2 mV per degree Celsius for silicon. A cell at 60°C produces roughly 15% less power than at standard test conditions (25°C). This drives the design of cooling systems, elevated mounting, and the preference for light-colored back sheets.
Beyond the Single Junction
The Shockley-Queisser limit caps single-junction efficiency at 33.7% because sub-bandgap photons pass through unabsorbed and above-bandgap photon energy is wasted as heat. Multi-junction cells stack semiconductors with decreasing band gaps, each absorbing a different part of the spectrum. Triple-junction cells on satellites exceed 38% efficiency, and laboratory tandems using perovskite on silicon have surpassed 33%, pointing toward a future of cheap, ultra-efficient solar power.