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Light trapping solar TiO_2 film preparation and characterization

Author: RongLinYan
Tutor: ZhouJiCheng
School: Central South University
Course: Electronic Science and Technology
Keywords: Direct current reactive magnetron sputtering TiO2 thin film Light trapping thin film Solar cell
CLC: TM914.4
Type: Master's thesis
Year: 2010
Downloads: 218
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Abstract

At present, improving conversion efficiency of solar cells is an imperative problem to be solved, depositing a light trapping thin film matching with optical property on solar cells to reduce the solar cells’ light reflection and increase light transmission is an ideal scheme. TiO2 thin films have optimal optical characteristics with optimal refractive index and low absorption index in the visible region, and its transparent center wavelength (λ=550nm) well coincides with the visible band of sunlight. Therefore, TiO2 is a very competive material as a light trapping thin film.In this paper, TiO2 thin films were deposited on glass and Si wafers substrates by direct current (DC) reactive magnetron sputtering, and its process conditions were optimized. The morphology, structure, optical properties of TiO2 thin films were analyzed by atomic force microscopy, X-ray diffraction, ultraviolet and visible spectrophotometer, respectively. Effects of deposition conditions on thin films structure and properties were investigated. The method of one-step preparing double lays light trapping thin film by changing process conditions was proposed, which can be applied to reduce cost and improve conversion efficiency of solar cells.The experimental results show that the deposition rate first increases and then decreases as increasing of oxygen flow ratio, and increases as increasing of sputtering power, and decreases as increasing of sputtering pressure. Surface of the TiO2 thin films is flat and no defect. After RTA at 500℃, the grain size tends to be uniform, and surface of thin films becomes smooth and compact; after RTA at 900℃, atom displacement takes place, TiO2 thin films transform from columnar grain to prismatic grain, and its surface becomes rough. As-deposited TiO2 thin films are amorphous, and after RTA at 300℃were identified to become anatase, another transition from anatase to rutile occurred at 900℃.TiO2 thin films with a high transmission in visible and near infrared spectrum, as well as refractive indices ranging from 1.51 to 2.72 at a wavelength of 600 nm, were obtained. The low refractive index thin films were obtained by increasing oxygen flow ratio, while the high refractive index thin films were achieved by annealing the as-deposited samples at high temperature. The extinction coefficient increases as increasing of anneal temperature. The optical property of anatase is better than rutile’s, therefore, anatase is more suitable for light trapping thin film. When the thickness of thin film is 60nm, TiO2 thin films as light trapping thin films show good antireflection performance, and the weighted average reflectance is only 3.37%.

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