INVESTIGATION OF DIFFERENT BUFFER LAYERS, FRONT AND BACK CONTACTS FOR CdS/CdTe PV FROM NUMERICAL ANALYSIS M. A. Matin 1 , Nowshad Amin1,2 and Kamaruzzaman Sopian 2 1Department of Electrical, Electronics & System Engineering, of Engineering and Built Environment Solar Energy Research Institute National University of Malaysia, UKM, Bangi 43600, Selangor, Malaysia. ABSTRACT Polycrystalline thin film CdTe shows great promise for efficient, low-cost photovoltaics (PV) cell. A numerical analysis was conducted utilizing AMPS simulator to explore the possibility of higher efficiency and stable CdS/CdTe cell among seven different cell structures with tin oxide (Sn02) and cadmium stannate (Cd2Sn04) as front contact layer, zinc oxide (ZnO) and zinc stannate (Zn2Sn04) as buffer layer and Ag or antimony telluride (Sb2Te3)with Mo as back contact material. It was found that the structure of CTO/ZTO/CdS/CdTe/Ag produced best efficiency over 17°t'o. This analysis has also shown that Cd2Sn04 front contact, Zn2Sn04 buffer layer and Sb2Te3 back contact materials are suitable for high efficiency (>15.5%) and stable CdTe based cells. Moreover, it was found that the cell normalized efficiency linearly decreased at the temperature gradient of -0.3%tC. 1. INTRODUCTION The two key properties of CdTe are its near ideal PV bandgap of 1.45 eV and its high optical absorption coefficient over 5x1OS/cm that are potential for realization of higher efficiency and low cost PV cell. From optoelectronic and chemical properties, CdS is the best suited n-type hetero- junction partner to CdTe for thin film polycrystalline solar cells. The polycrystalline layers of a CdS/CdTe cell can be deposited using a variety of different low cost techniques, such as close- space sublimation (CSS), chemical vapor deposition (CVD), chemical bath deposition (CBD), and sputtering. Clearly one of the main goals of today's PV cell research is to use less semiconductor material by making the cells thinner. Thinning will not only save material, but will also lower production time, and the energy needed to produce the PV cells. All of these factors will decrease the production cost. Moreover, the CdTe thin film solar cells have shown long-term stable performance and high efficiency under AM1.5 illumination for terrestrial uses. In 1972 Bonnet et al. published an interesting paper reporting an efficiency of 6°t'o on CdTe/CdS thin film solar cells [1]. The 10°t'o efficiency value was overcome by Tyan et al. [2] and finally an efficiency of 15.8% was reached by Ferekides et al. [3]. In 2001 a group from NREL reported a record efficiency of 16.5% [4], which is the highest until today. In the last decade the record efficiency has changed from 15.8°t'o to 16.5% only. This 16.5°t'o efficient CdS/CdTe cell used modified structure with 0.1 JJm CdS and 10 JJm of CdTe layer fabricated using three different technology CSS for CdTe film, CBD for CdS film and magnetron sputtering for all other layers. This champion cell used too thick layers of CdTe and CdS materials. However, there are scopes to reduce the thickness to save materials and to increase the efficiency by improving open circuit voltage (Voc), short circuit current density (Jsc) and fill factor (FF) with different buffer layers, front and back contacts 978-1-4244-2950-9/09/$25.00 ©2009 IEEE materials. The conventional superstrate structure of a thin film CdTe/CdS solar cell is composed of 4 layers: a transparent and conducting oxide (TCO) which acts as a front contact, a n- CdS film which is the so-called window layer, a p-CdTe film which is the absorber layer made on top of CdS and finally the back contact on top of the CdTe layer. By incorporating a very thin resistive buffer layer, the CdS layer thickness can be reduced to 50 nm, which significantly improves the blue response, CdS film morphology and conversion efficiency of the CdTe devices [5]. There are scopes to improve the stability of the cell at higher operating temperature by applying stable back contact materials like Sb2Te3. All of the above ideas were modeled in this work with seven different possible structures and numerical analysis was done by utilizing AMPS 1D simulator to achieve the best possible structure of CdS/CdTe cell for higher efficiency and stability. Efficiency as high as 17.0°t'o (Structure D) was found with 1 JJm of CdTe layer and 100 nm Zn2Sn04 buffer layer without Sb2Te3 back contact. However, i-ZnO buffer layer showed low conversion efficiency of 6.84% and 11.26°t'o, respectively with and without Sb2Te3back contact. It was found that 1 JJ m of CdTe absorber layer, 50 nm of CdS window layer, 100 nm of Zn2Sn04 buffer layer and 100 nm Sb2Te3back contact layer are enough for high efficiency (>15.5°t'o) and stable CdTe based cells. It was also found that the cell normalized efficiency linearly decreased with operating temperature. 2. MODELING AND SIMULATION Modeling is widely used in analysis of single-crystal solar cells. Due to the complex nature of CdS/CdTe thin-film polycrystalline solar cells, the need for numerical modeling methods is higher. Numerical modeling techniques can help the understanding of solar cells, and should give the additional ideas to vary structures and cell parameters to improve the cell performance. The number of parameters that can be varied in the particular solar-cell model is larger than 50 [6]. Obviously, a problem with 50 variables is too ambiguous to solve reliably. It is therefore necessary to minimize the number of variable parameters by fixing many of them at "reasonable" values. It was a tough challenge to choose the appropriate parameters. Many of them depend on fabrication techniques and deposition methods and can thus vary among devices. A three-layer device model of a Sn02/CdS/CdTe solar cell is the starting point for the simulation in this work. A baseline case was utilized to approximate the highest-efficiency CdS/CdTe solar cells at that time, and it was slightly modified in this work to explore the seven different structures. Fig. 1 illustrates the typical superstrate structure of CdS/CdTe baseline case solar cell and the modified structure with different buffer layers, front and back contact materials. 001628