A one-dimensional opto-electro-thermal simulator for an a-Si:H based thin-film solar cell is developed. The simulator includes optical, electrical, and thermal modelling for a complete modelling of energy conversion in a-Si:H based solar cells. Particularly, the thermal impact on the performance of the cell has been studied. Cell performance is worse when thermal simulation is included compared to simulations where temperature is kept constant. This implication suggests that cell performance is typically overestimated in simulations where the thermal effect is ignored. A simplified optical model was used, reducing the computational resources required in the study. The electrical model proposed in this study extends the classical drift-diffusion model to include the effects of temperature. The proposed thermal model considers the energy conservation in a non-thermal equilibrium condition between electron, optical phonons, and acoustic phonons. While simplifications have been used to reduce the complexity of the simulations, the program captures the essential behaviour of the cell with reasonable accuracy. Hence, the developed program is useful for a-Si:H based thin-film solar cell design and optimization.