太阳能光伏讲义Lecturenote5.doc

PHOTOVOLTAIC TECHNOLOGY Absorption of light If a photon has Eph Eg ( absorption. “Blue” light (short ( 500nm) is absorbed more strongly than red (( ~ 650nm) or IR (( 750 nm) radiation in Si. That is in a Si solar cell blue light is absorbed closer to the front surface than red light. Generation rate of electron-hole (e-h) pairs per unit volume, G [‘number of e-h’ / cm3/s]. N = # of photons/cm2/s at surface G = αNe –αx α = absorption coefficient [cm-1] x = distance [cm] Recombination Electron- hole pairs generated by light in a semiconductor layer will eventually recombine (R)- “reverse” process of the generation process (G). The carrier lifetime =average time for recombination to occur after electron- hole generation.~ 10 μs for Si. The carrier diffusion length = average distance a carrier will move (due to thermal collisions – diffusion) prior to recombining ~ 100 – 300 μm for Si P-N Junctions formation A p-n junction is formed by doping adjacent regions in a semiconductor with p and n type impurities. P and N type material join majority carrier cross over carriers occupy near junction saturation of carriers at junction depletion layer diffusion ceased L 5 4