Method to extract diffusion length from solar cell parameters - Application to polycrystalline silicon

A closed form, analytical expression for the interdependence of the effective diffusion length L-eff and the open-circuit voltage of solar cells is derived for the parallel connection of recombination in the space-charge region and in the neutral base region. This expression allows for the calculation of L-eff from the open-circuit voltage, the short-circuit current, and the base doping of the solar cell as the only quantities that need to be determined experimentally. Values of L-eff calculated with our method match with an accuracy of 35% values that are determined experimentally by quantum-efficiency measurements of silicon solar cells. The agreement holds in a range 0.3 mum <L-eff<300 mum. Alternatively, L-eff is calculated from the short-circuit current and from the overall light absorption in the solar cell via an analytical expression for the L-eff -dependence of carrier collection. Again, we find a good match between the measured L-eff and the values calculated by this method. We further analyze photovoltaic output parameters from literature data of polycrystalline silicon solar cells covering grain sizes from 10(-2) to 10(4) mum. We calculate L-eff for these solar cells with our method and interpret the results in terms of grain-boundary recombination velocity S-GB. We find that the data points split into two distinct groups, one with 10(5) cm/s<S-GB<10(7) cm/s, and one showing 10(1) cm/s<S-GB<10(3) cm/s. Since all cells with low S-GB have a {220} surface texture, we ascribe the low-recombination velocity of grain boundaries to the low-electronic activity of [110]-tilt grain boundaries in these films. (C) 2003 American Institute of Physics.