Crystalline Si Surface Passivated Of Solar Cells By Al₂O₃Thin Films Deposited By Atomic Layer Deposition

Energy is the driving force of social development.However, the environmentalissues caused by over-exploitation and over-utilization of fossil energy forced thehunmanbeings to seek renewable energy sources as a andidate. Solar energy is a mainclean energy for our future and its efficiency is the ultimate goal of the researchers. Inrecent years, there is a growing demand of high efficiency solar cells for the PVmarket so that people develop new materials and new technology to improveconversion efficiency of solar cells.The solar cells require excellent surface passivation because they become thinnerand thinner. Good passivation can reduce the surface recombination velocity （SRV） ofminority carriers, and improve Iscand Vocof solar cells, and thus the photoelectricconversion efficiency can be improved as well. Atomic Layer Deposition （ALD） is anew technology of thin film deposition process.The Al₂O₃thin films deposited byALD have been applied in the solar cells, and obtained high conversion efficiency inthe laboratory. However, the passivation mechanism and treatment process need to befurther studied and optimized. In this thesis, researches of passivation andantireflection behavior of the Al₂O₃thin films deposited by atomic layer deposition toimprove the efficiency of solar cells.Specific studies are as follows:（1） Crystalline silicon surface passivated by Al₂O₃thin films deposited byALD. Al₂O₃thin films of different thickness prepared by ALD were deposited onn-type and p-type Czochralski （CZ） silicon surface respectively. And then weoptimized the process conditions of an annealing step.We analysisd passivationmechanism of Al₂O₃to draw the passivation performance of influence of the filmthickness, annealing temperature and annealing time. In addition, samples before andafter annealling was investigated by X-ray photoelectron spectroscopy （XPS）.Theresults proved that there is a certain amount of Al-OH bond in Al₂O₃thin filmsprepared by ALD. Al-OH turn into the Al-O after annealing and released a Hatom.The H atoms can passivate the dangling bonds on Si surface. The O/Al were1.68and1.53based on the XPS peak intensity before and after annealing respectively,i.e. the O/Al was more close to the stoichiometric ratio of Al₂O₃. It also proved thepresence of Al-OH in before annealing.（2） Dual functions of anti-reflectance and surface passivation of the atomic layer deposited Al₂O₃films on crystalline silicon wafers. The Al₂O₃films weredeposited by ALD on the textured monocrystalline silicon substrates of differentthickness and obtain their reflectance curve. The average reflectance of originalmonocrystalline silicon wafers and a standard SiNx were obtained as comparison.There were obvious results show that the30nm and70nm Al₂O₃films can makeaverage reflectance of the textured surface reduced from14.2%to10.6%and4.2%,i.e a drop of25%and70%. Meaningfully, the reflectance for100nm Al₂O₃coated ontextured Si are quite similar to that of the SiNxsample, i.e2.8%,a drop of80%.Therefore, Al₂O₃shows it’s potential of passivation and antireflection.Ourresults show the dual fuctions of surface passivation and anti-reflection of Al₂O₃inC-Si solar cells applications.