Numerical simulations of the current-voltage characteristics of PECVD-microcrystalline silicon based p-i-n diodes were performed to study the affect of defect density and mobility on solar cell performance. Depending on the combination of both parameters the ideality factor increases or decreases with applied forward bias. The reason is the variable contribution of volume recombination to the total diode current and space charge stored in defect states. The decrease in dark current with reduced hydrogen dilution can partly be attributed to a decrease in recombination centers by the same factor as predicted for midgap defect states by the analytic diode theory. Microcrystalline silicon solar cells deposited in the highly crystalline regime (high H-dilution) are limited by recombination of photogenerated carriers and high dark current. Both can be attributed to a large number of recombination centers. The fill factor of our state-of-theart solar cell is limited by the dark current for small illumination intensities, by series resistance for high illumination levels and by both at its maximum under AM1.5 illumination. Short-circuit current and open-circuit voltage pairs measured under intensities from 10-6 to 30 suns reveal a diode characteristic corresponding to an ideality factor of one at large forward bias.