Research On Anti-reflection Coating Based On Effective Medium Approximation Model

Solar cell is a kind of electrical device which directely converts the photons energyin the salar radiation into electricity. Nowadays, most of the solar cells in the market arecrystalline or polycrystalline silicon solar cells. However, silicon solar cells areconfronted with high cost and low conversion efficiency problems. One of the factorsdeterming solar cells conversion efficiency is the incident light reflection loss. While thelight gets to silicon (the refractive index of silicon is about4) solar cell from air (therefractive index of air is about1), the discontinuity of refractive index leads to over30%of incident light reflecting off the silicon solar cell without any anti-reflection process,which will cause the low conversion efficiency.In order to reduce the reflection loss, anti-reflection coating (ARC) is widely usedto deposit on silicon solar cell surface. With the consideration of cost and performanceenhancement, single layer ARC is widely used in commercial silicon solar cell.However, single layer ARC only can have a good anti-reflection performance in anarrow band wavelength range. To achieve a low reflectance in a wide wavelengthrange, a low cost three-layer ARC is studied and fabricated in this thesis. From the toplayer contacting the air to bottom layer contacting the silicon solar cell surface, all thethree layers are made of silicide, including silicon dioxide layer, silicon nitride layer andsilicon nitride layer embedded with silicon nanocrystals.In this thesis, firstly, the principle of ARC is studied and the most important factorsaffecting the anti-reflection performance are the optical constants and thickness of ARC.Then, with the effective medium approximation (EMA) theory, the relationship betweenthe optical constants of thin film and the volume fraction of nanocrystals embedded inthin film is studied, and the three-layer ARC mode is developed. Afterwards, based onplasma enhanced chemical vapor deposition (PECVD) process, a low cost fabricationgprocess to continuously deposit three layer thin films is achieved and this three-layerARC is deposited on silicon substrate. By utilizing the spectrometer, in the wavelengthrange of300nm to1100nm, the reflectance is only5.6%, which is much less than that(38.8%) of silicon substrate without any ARC. At last, the three-layer ARC proposed in this thesis is applied to commercialpolycrystalline silicon solar cell. The commercial polycrystalline silicon solar cell andsolar cell with the three-layer ARC conduct the measurements of anti-reflectionperformance, external quantum efficiency (EQE) and current density–voltagecharacteristic, respectively. The results show that the solar cell with the three-layer ARChas a better performance in all the three kinds of tests. After calculation, as comparedwith commercial polycrystalline silicon solar cell, the solar cell with the three-layerARC improves the conversion efficiency by9.7%.