Thin Film Cdte Based Pv Cells English Language Essay

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Thin-film CdTe based PV cells are one of the leading contenders for providing for high efficiency, low-cost and stable PV cells. The formation of a stable, low resistance, non-rectifying contact to p-CdTe thin-film is possibly the most critical challenge associated with this technology in the fabrication of efficient and stable solar cells. However, the use of back surface field (BSF) material is found to be better, effective and easier one among several methods still used to overcome the ohmic contacting problem. This paper numerically explores the possibility of high efficiency, ultra-thin and stable CdTe cells with Cu2Te BSF. The cell performances (efficiency, FF, Voc, Jsc, temperature stability) are investigated by recognized simulator 'Analysis of Micro-electronics and Photonic Structures' (AMPS-1D). A modified structure of CdTe based PV cell SnO2/Zn2SnO4/CdS/CdTe/Cu2Te/Ni has been proposed over the reference structure SnO2/ Zn2Sn04/CdS/CdTe/Cu. The results have shown that the use of Cu2Te as BSF layer enhances the cell performances and have no adverse effect on cell stability. In this analysis the highest conversion efficiency of CdTe based PV cell without BSF has been found around 17% using CdTe absorber thickness of 5µm, on the other hand with the BSF the conversion efficiency has been found to be as high as 19.4% using only 0.6 µm thick CdTe absorber, which opens a new window for ultra-thin CdTe PV cells. Furthermore it was also found that the cell normalized efficiency linearly decreased with the increasing operating temperature at the gradient of -0.35%/C, which indicate better stability of the proposed CdTe PV cell.

PV cell; Thin film; CdTe; BSF layers; Ultra-thin.

Introduction (Heading 1)

Already, the CdTe PV cell has gained a lot of attraction to the researchers and commercials for its low cost, higher efficiency, highly stable and large scale fabrication opportunity. It is a worldwide belief that thin film CdTe solar cell will dominate the future PV market as it has high efficiency under AM1.5 illumination and shown long-term stable performance [1] for commercial usage. Polycrystalline CdTe has a direct optical bandgap of 1.45 eV, which is very close to the optimum bandgap for solar cells. It has a high absorption coefficient of 5Ã-105/cm, which means that all the photons with energy greater than the bandgap (Eg) can be absorbed within a very thin CdTe absorber layer. Hence, the lesser thickness requirement for an absorber layer can lead to economized cell material usage and reduced the cost of fabrication. Apparently, the main aspiration of today's PV research is to use less semiconductor material by making the cells thinner. Commercially available leading CdTe cell has an efficiency of 16.5% [2]. This cell uses a modified structure of (CTO/ZTO/CdS/CdTe/Cu:HgTe:CuXTe) having 10μm of CdTe and 0.1μm CdS layer. Recently First Solar announced a new world record for CdTe photovoltaic (PV) cell having an efficiency of 17.3% [3]. The most severe issue related with the CdTe solar cell technology is the formation of an efficient and stable ohmic contact on the p-CdTe layer [4]. Since CdTe is a p-type semiconductor with a high band-gap (1.45 eV) and high electron affinity (χ= 4.5 eV). Typically a stable ohmic contact to CdTe requires either a metal with a work function more than 5.7 eV or a sufficiently narrow Schottky barrier to enable tunneling. There is no metal having such high work functions to make good ohmic contacts to p-CdTe, instead they tend to form Schottky or blocking barriers [5]. Generelly Cu is used as back contact, but back-contacts containing Cu are not stable, it leads to efficiency degradation with time due to Cu diffusion to the front contact which causes shunting effect [6]. There are many allowed states within the forbidden gap at the back surface in CdTe solar cell, and recombination can efficiently take place through these states. When the minority-carrier diffusion length is longer than the cell thickness, back-surface recombination may become the primary limitation on cell performance.The use of an electron reflector is a strategy to overcome these obstacles. An electron reflector is a conduction-band energy barrier at the back surface of the solar cell, which can reduce the recombination due to the electron flow to the back surface & help formation of stable back contact. Typical techniques to create an electron re

To moderate the width of barrier height by heavily doping extra layer of back surface field (BSF) with appropriate material.

To reduce the barrier height between the CdTe absorber layer and final metal back contact.

Among these the use of BSF layers at back surface has been found quiet easy & efficient. The importance of BSF layers attracted some attention in the 1980s [7, 8, 9]. The BSF layer accommodates the photo-generated minority carriers and keeps them within the reach of the p-n junction for efficient collection. Throughout this study using 'AMPS-1D' simulator, we aimed to achieve higher efficiency using BSF, and study the overall effect on cell performance. The effect of temperature variation is also described to understand the environmental effect on the cell performance.

ZnTe is the most common BSF material.Romeo et al. at the University of Parma (Italy) has proposed two more potential BSF material for CdTe solar cells, these are Sb2Te3[10] & As2Te3[11]. In this paper, we have introduced Copper Telluride (Cu2Te) as an efficient BSF material & investigated its effect on cell parameter. This analysis has shown that Sn02/Zn2Sn04 front contact with Cu2Te BSF layer with Nickel (Ni) as back contact, an ultra-thin (<1µm) CdTe cell is possible maintaining improved performance & stability.

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