Abstract A new phenomenological model for fitting the illuminated I-V curves of a-Si:H PIN solar cells is proposed, in which the relation between photocurrent and applied bias voltage is used. The I-V curve of the a-Si:H PIN solar cells were measured and analyzed by IBM personal computer and the Marquardt mathematic method. The experiments show that the fitting results were excellent. The fitting error was less than one of other models and the model parameters could be modified automatically, so that reproduction and reliability of experiment data were good.