The Energy Band Structures of Cd 1-x Zn x Te Polycrystalline Thin films and their Applications for Photovoltaic Devices
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M5.24.1
THE ENERGY BAND STRUCTURES OF Cd1-xZnx Te POLYCRYSTALLINE THIN FILMS AND THEIR APPLICATIONS FOR PHOTOVOLTAIC DEVICES *
ZHENG Jiagui, FENG Lianghuan , SHAO Ye, CAI Yaping. ZHANG jingquan, CAI Wei. WU LiIi.CAI DaoIin, LEN Wenjian, Zhu Jumu Department of Materials Science, Sichuan University, Chengdu 610064.P.R.China
ABSTRACT In the paper, Cd1-xZnxTe polycrystalline films have been prepared by co-evaporation, which can control the zinc content, using CdTe and ZnTe as the evaporation sources. The cubic phase in these films as-deposited and annealed has been demonstrated by XRD. The XRD peaks of Cd1-xZnxTe films are always between the corresponding peaks of CdTe and ZnTe. According to Vegard Formula, x values of Cd1-xZnxTe films have been confirmed from XRD data, which shows that both the calculated and experimental x values are almost identical. The optical transmission of Cd1-xZnxTe films has been measured, and the optical energy gaps have been calculated from the transmission. The band gaps vary in a quadratic way with x values, and increase with annealing temperatures. When application of Cd1-xZnxTe film as a buffer of around 80nm thick in CdS/CdTe solar cells, the structure of the cells becomes CdS/CdTe / Cd1-xZnxTe / ZnTe:Cu and their average conversion efficiency is improved by 40% where x is in the range of 0.55 to 0.65. The mentioned above results are discussed.
1. Introduction
The single crystals of Cd1-xZnxTe are important materials for x-ray and -ray detectors worked at room temperature. However, Cd1-xZnxTe can grow to be a crystal only for some special zinc content, which limits their applications. In general, Cd1-xZnxTe can be deposited as the polycrystalline films with any x values. Because the energy gaps of Cd1-xZnxTe polycrystalline films vary with x values, the films could be much useful for photo-electronic and photovoltaic devices. The purpose of using Cd1-xZnxTe films in CdTe solar cells is to modify the energy band at the interface between CdTe absorbed layer and ZnTe: Cu back-contact layer. In fact, there is an abrupt barrier of l.l eV at the interface. When introducing a Cd1-xZnxTe buffer layer, the abrupt barrier becomes two smaller barriers, which could be helpful to collect the photo-generated carriers. Cd1-xZnxTe films with different x values have been deposited by co-evaporation. The dependence of energy gap on zinc content in the films has been studied. The solar cells of two kinds CdS/CdTe/ZnTe: Cu and CdS/CdTe/ Cd1-xZnxTe/ ZnTe: Cu have been fabricated in the same conditions. Their performance of the solar cells has been measured and analyzed.
2. Experimental procedure A series of Cd1-xZnxTe films were prepared with a simultaneous evaporation system DM-400, which was designed by ourselves. The films were deposited on the glass and CdTe film substrates at 300K by use of simultaneous evaporation of CdTe *
Corresponding author
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and ZnTe from two horizontal sours. The system pressure is of –10-4 Pa The glass microscope slides were
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