Effect of sodium on recrystallization and photovoltaic properties of CdTe solar cells

During processing of CdTe solar cells, sodium can diffuse from the glass substrate into the active layers and can considerably affect CdTe solar cell properties. In this paper, the effects of sodium on structural and photovoltaic properties of low temperature grown CdTe solar cells are investigated in detail. Presence of sodium during the CdCl2 treatment enhances CdTe grain growth and causes widening of grain boundaries, creating pinholes and possible shunt paths in the solar cell. Furthermore, even small concentrations of sodium strongly enhance CdS–CdTe intermixing and a recrystallization leading to CdS island formation, which results in direct contacts between absorber and transparent front contact. Due to sodium present during the CdCl2 treatment the space charge region width strongly decreased, indicating the possibility of increasing acceptor density in CdTe. In spite of the doping effect, cells, which were CdCl2-treated in the presence of sodium, showed deterioration of photovoltaic performance due to excessive CdS–CdTe intermixing and electrical shunting. A sufficiently thick and large grained TCO layer of ZnO:Al can effectively suppress sodium diffusion in low temperature grown CdTe solar cells on soda-lime glass preventing the deterioration of solar cell properties due to sodium. This enabled the growth of 13.5% efficiency (VOC=853 mV, FF=74.4%, JSC=21.3 mA cm−2, without anti-reflection coating) CdTe solar cells on low cost soda-lime glass without additional diffusion barrier layer in spite of large sodium content of the substrate.