Effective diffusion length of multicrystalline solar cells
Previous definitions of effective diffusion length of a multicrystalline semiconductor are reviewed. A new definition is proposed, which is based on the comparison of the current collected from the base of a multicrystalline solar cell and a single-crystal cell for uniform carrier generation, and holds for any sample thickness. An analytical expression for is obtained from the solution of the three-dimensional equation for the charge collection probability inside a grain with square cross section. This expression is used to numerically study the dependence of on the bulk diffusion length, grain size and grain boundary recombination velocity, and to make a comparison with previous specifications of . The results indicate, in particular, that is the value that would be determined by the surface photovoltage method. The case of a cell with grains of different properties is briefly discussed and an average is defined. It is also shown that can be used to calculate the contribution of the base to the reverse saturation current of the cell.