Investigation of Optimum Transparent Conductive Oxides (TCOs) for CdS:O/CdTe Thin Film Solar Cells (TFSCs) from Numerical Analysis.

Transparent Conductive Oxides (TCOs) are an increasingly important component of solar cells, where they act as front electrode elements.The structural templates, diffusion barriers and their work function controls the open circuit voltage(V oc ). In this paper, various transparent conductive oxide materials have been studied which are used as the front surface contacts of CdS:O/CdTe based thin film solar cells. Various electrical and optical parameters like work function, thickness, temperature etc., of some common transparent conductive oxides materials such as ZnO, FTO, SnO 2 etc., are studied. The main idea was to find an optimum conductive oxide layer for CdTe solar cell which shows the great potential in thin film area of solar cell. All the analysis was done by using the widely used simulator Analysis of Microelectronic and Photonic Structures (AMPS 1D). It was observed that both SnO 2 and ZnOshow similar performance under various conditions. Indium Tin Oxide (ITO) has shown the worst performance among them in all conditions. Fluorinated Tin Oxide (FTO) looked promising match for CdTe solar cells in some conditions.


M S Sadek.
Transparent Conductive Oxides (TCOs) are an increasingly important component of solar cells, where they act as front electrode elements.The structural templates, diffusion barriers and their work function controls the open circuit voltage(V oc ). In this paper, various transparent conductive oxide materials have been studied which are used as the front surface contacts of CdS:O/CdTe based thin film solar cells. Various electrical and optical parameters like work function, thickness, temperature etc., of some common transparent conductive oxides materials such as ZnO, FTO, SnO 2 etc., are studied. The main idea was to find an optimum conductive oxide layer for CdTe solar cell which shows the great potential in thin film area of solar cell. All the analysis was done by using the widely used simulator Analysis of Microelectronic and Photonic Structures (AMPS 1D). It was observed that both SnO 2 and ZnOshow similar performance under various conditions. Indium Tin Oxide (ITO) has shown the worst performance among them in all conditions. Fluorinated Tin Oxide (FTO) looked promising match for CdTe solar cells in some conditions. Copy Right, IJAR, 2016, All rights reserved.

Introduction: -
Cadmium Telluride (CdTe) is a promising material for solar cells as it has an ideal energy band gap (Eg) of 1.45 eV and larger absorption coefficient (>5×10 5 /cm) [M. Hadrich et al., J. Britt et al.]. The polycrystalline CdS was found to be the best-suited hetero-junction n-type partner with p-type CdTe absorber for CdS/CdTe solar cell. Small-area CdTe cells already achieved efficiency of 16.5% in laboratory and commercial modules have achieved efficiency of 10% [X. wu et al.]. CdTe solar cells use Transparent Conductive Oxides (TCO) as the front contact which is optically transparent and electrically conductive materials. [Hecht, D. S et al.] Major considerations in the choice of the TCO for the solar cell, besides the conductivity and transparency, are electronic compatibility with the adjacent layers in the cell, processing requirements, and stability under environmental conditions. A carrier concentration on the order of 10 20 cm -3 or higher and a band gap energy above 3eV are usually for high conductivity. As with all transparent conducting films, a tradeoff must be done between the conductivity and transparency, since increasing thickness and the carrier concentrations will increase the conductivity, but decreases the transparency.

Features of TCO: -
TCO for solar cell applications have been fabricated from both inorganic and organic materials. In case of organic solar cells, the TCO is made up of a layer of mostly Indium Tin Oxide (ITO), Fluorine doped Tin Oxide (FTO), doped Zinc Oxide (ZnO), SnO 2 etc. In order to be employed for large scale PV design, the basic characteristic features of a TCO material should be - High transparency in visible light and more than that to enhance efficiency.  High conductance.
 High carrier mobility.  Excellent long term stability.  Low cost.  High throughput deposition.  Capable of some sorts of light trapping.  Some may require device specific properties.

Different TCOMaterials: -
There are many different materials which are extensively used in solar cells as the front contact. Among them Indium Tin Oxide (ITO) is most widely used for differentdevices because of its two main properties, its electrical conductivity and optical transparency as well as the ease of deposition methods like Physical Vapor Deposition, Electron beam evaporation and sometimes sputter diffusion. ITO is a heavily doped n-type semiconductor with a bandgap of around 4eV. The distinguishable property of ITO is its chemical resistance of moisture. Because of its bandgap, it is mostly transparent in the visible part of the spectrum and its extinction coefficient in this wavelength range is zero. In the UV region, it is opaque, so it has non-zero k due to band to band absorption, similar as NIR and IR region of the spectrum.
ITO has some inherent drawbacks due to which novel alternative materials are searched for. High costs, limited resource of Indium in the earth's crusts, the fragility and also the lack of flexibility puts a limit on the uses of ITO.

Overview of CdS/CdTe Solar Cell: -
The polycrystalline CdS was found to be the best-suited hetero-junction n-type partner with p type CdTe absorber for CdS/CdTe solar cell but it has three main issues that limit device performance by limits the values of V oc and FF [T.L. Chu et al.] There is a crucial need for a higher band gap window material to further improve and overcome that limitations associate with the CdTe heterojunction solar cells. Oxygenated cadmium sulfide (CdS:O) seems to be a better candidate in this regards. However, the advantage of CdS:O window layer over the CdS has been reported elsewhere[Aminul Islam et al.]. Nano crystalline CdS:O is a promising window material as it has higher optical band gap (2.5-3.1 eV), a better lattice match with CdTe absorber and reduce inter-diffusion tendency at the PN junction [X. wu, R. G et al.].Moreover, the formation of stable, low resistance back contact to p-CdTe is another well-known major challenge associated with the fabrication of efficient CdTe based solar cells.  Then the TCO layers are changed by using different materials like ITO, FTO etc., and tries to find out the optimum TCO layers that best suit for the poly-crystalline CdTe based solar cell. Table 1 shows the description of all the parameters used in this analysis, these parameters were selected based on literature, theory, experimental data, or in some cases, reasonable estimation.  In this work, the band gap of CdS:O window layer has been taken 2.75 eV to be in the middle of the range.

Results and Discussion: -
The numerical analysis has been performed with the envision to detect the best suited transparent conductive oxides layers for ploy-crystalline CdTe based solar cell. The dependency of different TCO materials on the performance of the solar cell are investigated first by using AMPS-1D. Fig -3 shows the simulated efficiency of the solar cell design using different TCO layers, whose specification was given to the

Conclusion: -
In this numerical analysis, Cadmium Telluride (CdTe) based design of thin film solar cell which is the most prominent and most widely used thin film solar cell are taken with CdS being the window layer having lattice matched with the CdTe absorber. Different materials are used as the transparent conductive oxides (TCO) layers in order to find the maximum performance from the cell. It is obvious from the simulated results that, SnO 2 is the most suitable oxide layer for the cell providing the maximum efficiency about 11.233%. FTO and ZnO oxide layers also shows good performance resulting efficiency about 11.226% and 11.181% respectively. Temperature dependency of the open circuit voltage value is also a great concern. Because the device's fill factor (FF) is a function of both open circuit voltage(V oc ) and short circuit current density (J sc ) All of the oxides layers shows great resistance againstoperating temperature changes, which is the basic criteria for a good conductive oxide. The efficiency did not alter too much by thickness variation within a limited range.