A Comparative Study of the Thin-Film CdTe Solar Cells with ZnSe/TCO and the CdS/TCO Buffer Layers
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A Comparative Study of the Thin-Film CdTe Solar Cells with ZnSe/TCO and the CdS/TCO Buffer Layers Tamara Potlog1, Nicolae Spalatu1 Arvo Mere2, Jaan Hiie2, Valdek Mikli2 1 Physics Department, Moldova State University, A. Mateevici str. 60, Chisinau, MD 2009 Republic of Moldova 2 Department of Materials Science, Tallinn University of Technology, Ehitajate tee 5, Tallinn, 19086 Estonia ABSTRACT The growth of ZnSe and CdTe thin films by close spaced sublimation is examined. The investigations show that ZnSe films deposited on glass substrates are polycrystalline and exhibit wurtzite-zinc-blende polytypism. The CdTe films grown on glass/SnO2/ZnSe are polycrystalline and have an f.c.c. zinc-blende structure as in the case of a glass/SnO2/CdS buffer layer. The electric and photovoltaic parameters of ZnSe/CdTe solar cells depend on the ZnSe film thickness. Furthermore, it is shown for the first time that the best photovoltaic parameters are achieved using a Zn buffer layer at the interface between ZnSe and CdTe. INTRODUCTION The investigation of different types of semiconductor heterojunctions for easy photovoltaic solar energy conversion with simple fabrication and low cost has assumed special significance in recent years. As a result, many heterojunction solar cells are currently being explored. The solar cells based on CdTe material received wide attention. CdTe has a large optical absorption coefficient (>104 cm-1). A small amount of CdTe (2-8 μm thick) is needed for the absorber layer (100 times thinner than typical crystalline-Si solar cells). For example, until now, the high efficiency (16.5%) of CdTe based solar cells has been achieved using a CdS/CdTe heterojunction [1]. However, the band gap energy of CdS (2.4 eV) is relatively small for the CdS/CdTe heterojunction solar cells, since 0.1 m of a CdS film absorbs 36% of the incident radiation with an energy higher than 2.42 eV. Therefore, it is widely thought that it is possible to improve the efficiency of solar cells based on CdTe using alternative window materials. One of them is ZnSe. It has a larger bandgap (Eg =2.7 eV) than CdS and should allow more light to pass without being absorbed. Researchers [2] calculated the lattice mismatch using the electron affinity of the semiconductors and found CdTe/ZnSe to be 12.5%. The incorporation of S into ZnSe will form ZnSexSe1-x with a wider energy band gap. The previous paper [3] indicates that the contact potential of the formed junction between ZnSexSe1-x and CdTe is almost the highest among the II-VI alloys/compounds available to make window layers; hence, this combination has an excellent potential. The aim of this work is to grow a ZnSe thin film layer to be used with CdTe absorber layers. This layer will be deposited utilizing the same close sublimation (CSS) method as in the case of CdS/CdTe thin film cells [4]. The optimal technological conditions will be determined and correlated to the formation of ZnSe/CdTe devices with good photovoltaic
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parameters. The optimized high-efficiency ZnSe/CdTe photovoltaic d
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