We report on modeling the evolution of the spatial defect profile in the i-layer of a-Si:H solar cells combined with tests of the model against the efficiency decay of amorphous silicon (a-Si:H) p-i-n solar cells under various light-soaking conditions. We use a finite-element device simulator. The defect density during light-soaking is calculated as a function of time and of position. The defect density evolves due to the combined effects of light-induced generation, assumed proportional to the product of free carrier densities, and of light-induced annealing, assumed proportional to the sum of carrier densities. We find that in thick cells defects close to the p-i interface affect cell efficiency less than bulk defects, and that in thin cells the defect density increases significantly in the central portion of the i-layer. However, in thin cells the high electric field masks the effect of the defect density increase.