Thermal residual stresses in silicon thin film solar cells under operational cyclic thermal loading: A finite element analysis

Abstract

In manufacturing amorphous silicon solar cells, thin films are deposited at high temperatures (200–400°C) on a thick substrate using sputtering and plasma enhanced chemical vapor deposition (PECVD) methods. Since the thin films and substrate have different thermal expansion coefficients, cooling the system from deposition temperature to room temperature induces thermal residual stresses in both the films and substrate. In addition, these stresses, especially those having been induced in the amorphous silicon layer can change the carrier mobility and band gap energy of the silicon and consequently affect the solar cell efficiency. In this paper, a 2D finite element model is proposed to calculate these thermal residual stresses. The model is verified by the available analytical results in the literature. Then using the model, the thermal residual stresses are studied in a commercial amorphous silicon thin film solar cell for different deposition temperatures, and subsequently, the simulation results are validated with experimental results. It is shown that for the deposition temperatures of 200 and 300°C, the biaxial thermal stress reaches the values of −367 and −578MPa, respectively, in the amorphous silicon layer. Finally, the model is utilized to study the cyclic thermal stresses arising in the solar cell installed in Tehran as a sample city during its operational time due to the ambient temperature changes and photovoltaic process. The results are reported for different months based on the minimum and maximum temperatures published during a year. It is demonstrated that the temperature changes can induce mean stress with stress amplitude of −526 and 60MPa, respectively, in the amorphous silicon layer during June, for example. The calculated loading history can be used by the manufactures in thermal fatigue life assessment of the solar cell.