Solar cells with porous silicon: modification of surface-recombination velocity

Laser-induced transient-grating measurements were performed to monitor the influence of porous silicon on the surface recombination of a highly doped n(+)-silicon emitter of solar cells. With this technique, photocarrier diff usion and recombination with a time resolution of some tens of picoseconds can be studied. Using pulses of the second- and third-harmonic radiation fr om an Nd3+:YAG laser (quantum energy 2.34 and 3.51 eV, respectively), two d ifferent-depth regions of the emitter were excited. Using a kinetic model, which includes carrier diffusion and recombination at the surface and in th e bulk of the emitter, surface-recombination velocities in a series of samp les typical for each successive operation of solar-cell technology with dif ferent surface-doping level and surface preparation were evaluated. From th e analysis, we conclude that porous silicon formed on the emitter passivate s the surface of the silicon layer, i.e. reduces the rate of surface recomb ination at the porous silicon-crystalline silicon interface. Ytterbium as a co-dopant of the emitter increases the surface recombination velocity.