How it works
A solar cell is a large-area PN junction — typically 156 mm × 156 mm — illuminated from the P side through an anti-reflection coating. Photons above the bandgap create electron-hole pairs; the junction field separates them before recombination, driving conventional current from N to P internally (or from P to N in the external circuit). The I-V characteristic is I = I_ph − I_0(e^(qV/kT) − 1), where I_ph is photocurrent proportional to irradiance and I_0 is the diode's dark saturation current. Short-circuit current I_SC is the maximum current (at V = 0) and open-circuit voltage V_OC is the maximum voltage (at I = 0).
Key points to remember
Fill factor FF = P_max / (V_OC × I_SC) quantifies I-V curve squareness; commercial silicon cells have FF between 0.70 and 0.82. Theoretical maximum efficiency for a single-junction silicon cell (Shockley-Queisser limit) is about 29.4%; best lab cells reach 26.7% while commercial panels sit at 20–23%. Series resistance in the cell (contact resistance, bulk resistance) reduces FF and is the main efficiency loss mechanism at high irradiance. AM1.5 spectrum (1000 W/m²) is the standard test condition. Multi-junction concentrator cells using GaInP/GaInAs/Ge can exceed 40% efficiency.
Exam tip
Anna University exam questions on solar cells almost always ask you to define and calculate fill factor, so memorise FF = P_MPP / (V_OC · I_SC) and know that a higher fill factor means a more ideal cell with lower series resistance.