Defects passivation and H-diffusion controlled by emitter region in polysilicon solar cells submitted to hydrogen plasma

A significant cost reduction in photovoltaic cells could be achieved if they could be made from thin polycrystalline silicon (poly-Si) films. Despite hydrogenation treatments of poly-Si films are necessary to obtain high energy conversion, the role of the n+ emitter on defects passivation via hydrogen diffusion in n+pp+ polysilicon solar cells is not yet understood thoroughly. In this connection, influence of hydrogenation temperature and doping level of the n+ emitter on open-circuit voltage (VOC) were analyzed. It was found that VOC greatly improved by a factor of 2.9 and reached up to 430 mV at a microwave plasma power and hydrogenation temperature of 650 W and 400°C, respectively for a duration of 60 min. Moreover, slow cooling is more advantageous for high VOC compared to the rapid cooling. However, etching of the emitter region was observed, and this degradation is similar for both cooling methods. Furthermore, annealing of the hydrogenated cells in inert gas for 30 min revealed a slight increase in VOC, which reached 40-80 mV, depending on the annealing temperature. These results were explained by hydrogen atoms diffusing into the bulk of the material from subsurface defects that are generated during plasma hydrogenation process. Also, our findings show clearly that VOC values are much higher for a less doped phosphorus emitter compared to that of heavily doped. The origin of these behaviors was clarified in detail.