Research On Silicon Nitride Thin Film For Solar Cells And Hydrogen Passivation

Generating electricity by solar cells becomes one of the significant methods to solve the shortage of energy and environment pollution. Currently, more than 80% solar cells are made from crystalline silicon, so it is very important to develop high efficiency and low cost crystalline silicon solar cells. Preparing anti-reflective coating and hydrogen passivation are two key procedures in the process of high efficiency crystalline silicon solar cells.The research progress, future and unsolved problems of silicon nitride thin film for solar cells were systematically reviewed in this paper. By the PECVD (Plasma Enhanced Chemical Vapor Deposition) system and the reactants of silane and ammonia, silicon nitride thin film with excellent anti-reflective and passivation effects was prepared. The relatively optimum parameters for depositing SiNx thin film and the basic physical and chemical properties of SiNx were investigated. The effects of substrate temperature, the flow ratio of silane over ammonia and the RF power on the refractivity and deposition rate were researched. The passivation results of hydrogen plasma and SiNx thin film on the solar cell materials and devices were studied, respectively. The effects of post deposition annealing on solar cell materials and devices also were discussed primarily. These experimental results offered helpful reference and guide for developing the domestic solar cell process.The silicon nitride thin film deposited in this paper has excellent anti-reflective results and transmissivity. Its refractive index is about 2.1. The surface of thin film is very smooth, with a roughness of about 3 nm. The film is amorphous and hard to be crystallized. The atomic ratio of Si and N ranges from 1.1:1 to 1.4:1, indicating that the film is silicon-rich. The thermal stability of SiNx thin film was also investigated. Theexperiments show that the film contains a large mount of hydrogen, but it will diffuse from the film after annealing at high temperature. Furthermore, the film is found possible to be cracked after annealing at high temperature(1000 ).The experiments indicate that the deposition rate will increase with the increase of the flow ratio of SiHU/NHs, slightly decrease with the increase of substrate temperate, and increase obviously with the increase of RF power. The deposition rate is about 8.6 nm/min under some certain parameters. However, the refractive index will increase with the increase of the flow ratio of SiFU/NHs, slightly increase with the increase of substrate temperate, and decrease with the increase of RF power.By measuring the passivation results of hydrogen plasma and SiNx thin film, we found an evident improvement of minor carrier lifetime in polycrystalline silicon after hydrogen plasma treatment, although it has little to do with the annealing temperature and time. The hydrogen contained in SiNx thin film can enhance the carrier mobility of monocrystalline silicon, but after annealing at high temperature the mobility turns down. SiNx thin film can improve the minor carrier lifetime of both mono and poly silicon by the simultaneous surface and bulk passivation. Both hydrogen plasma and SiNx thin film can effectively enhance the short circuit current density of mono and poly silicon solar cells, which cause the improvement of the absolute transfer efficiency about 0.5%~2.9%. Depositing SiNx thin film followed by hydrogen plasma treatment will result in better passivation effect.The thickness of SiNx thin film will decrease and the refractive index will increase after annealing. But the refractive index will decrease dynamically at 1000 . The post deposition of SiNx at 400 will enhance the hydrogen diffusion and passivation. Hydrogen will loss heavily and the minor carrier lifetime will decrease quickly at the temperature higher than 400 . Hydrogen losses more obviously in the RTP (Rapid Thermal Process) than in the conventional furnace.