Utilizing reactive sputtering and MO-sputtering prepare the absorber of Cu-III-VI2-based thin film solar cells

• Main Authors: Tang-Xuan Zhung, 鍾堂軒
• Other Authors: Prof. Huey-Liang Hwang
• Format: Others
• Language: en_US
• Published: 2001
• Online Access: http://ndltd.ncl.edu.tw/handle/16715584959038046063

碩士 === 國立清華大學 === 電子工程研究所 === 89 === Cu-III-VI2 ternary compound semiconductor is one of the most promising materials for thin film photovoltalic devices, especially solar cells, due to the suitability of its band gap and its high absorption coefficient. Recently, the National Renewable Energy Laboratory (NREL) has reported that the total-area efficiency of the solar cell based on Cu(In,Ga)(Se,S)2 has reached 18.8% ,growing by 3-stage evaporation method. Martin A. Green indicated in his book that the efficiency of a tandem solar cell with three-layer structures, in which energy gaps are 1.0eV, 1.6eV, and 2,2 eV, respectively, could reach 35%. This could be reached by Cu-III-VI2 material due to its band gap can be varied from 1 to 3.5eV by composition control. Therefore, how to develop a suitable industry technique to produce a large area, uniform, high throughput, and inexpensive Cu-III-VI2- based solar cell is very important in 21th’s century. In order to develop a suitable industry technology to prepare the absorber of Cu-III-IV2-based solar cells, we choose the sputtering method as our experimental method. In our experiments, reactive sputtering method with different CuIn alloy targets was utilized to prepare the CuInS2 films. We prepared CuInS2 thin films under different process conditions such as substrate temperatures, RF power, Ar and H2S flow rates. Generally, Cu-rich films we prepared have good crystalline quality and low resitivity. But, it also limed by it rough surface and the problem resulting from surface secondary phases. In-rich films we prepared have smooth surface morphology, but it also suffer from low carrier mobility and smaller grain size. Near-stoichiometric films could be prepared by utilizing a target with Cu/In=0.95 at atomic percentage. The best process condition occurred at a small H2S flow rate ( <=7.5 sccm) and a temperature of 400 °C. In this condition, the films have good crystal quality without surface secordary phases. To control the composition of the films, futher, we also try to add a negative bias to the Mo-coated glass. Although the details of the bias sputtering are not always clearly understood, there is a little doubt that the weak-bounded species grown on the surface of the films may be resputtered due to a low-energy ion bombardment. Increasing the magnitude of the negative bias results that the Cu/In become smaller than 1 and the surface become more smooth. Besides preparing CuInS2 by the reactive sputtering method, we also developed a new technology, named MO-sputtering process, which added a small amount of metal-organic gas during sputtering process to solve the disadvantages that composition control is difficult in the sputtering process using an alloy target. TMGa has been utilized as our metal organic source to prepare Cu(In,Ga)S2 thin films. We have prove that the mo-sputtering can be developed as a new technique to prepare Cu-III-VI2 solar cells.