The aim of this paper deal with the mathematical modelling approach and behaviour of thin-film a-Si: H solar cells. The current-voltage (I-V) curve presents double linearities on the cell parameters and the exponential. So, thin-film P-I-N solar cell (a-Si-H) based on the classic combination of diode with an exponential I-V characteristics and adding a new current named recombination loss. The fundamental understanding of parasitic parameters will be discussed, and an equivalent circuit used in the research to study thin-film P-I-N structures is described. This work will show the mathematical modelling and prediction of dynamic resistance and short circuit current and open-circuit voltage resistance. The impact of parameters on the performance of an a-Si: H solar cell in different solar irradiations is more difficult to comprehend than that of crystalline silicon. As a result, adding an additional diode to the model can help for a more thorough investigation of the degradation process, particularly the Staebler-Worski effect. The parasitic resistance and dynamic resistance are determining in open-circuit voltage and short circuit current. Finally, the experimental I-V and P-V curves are displayed to investigate the various irradiation intensities.