Simulation and Investigation of the Optical Properties of CdS/CdTe Solar Cells
Parinaz Khaledi1*, Yousefali Abedini2
1 Ph.D. student of Physics, Urmia University of Technology, Urmia, Iran, [email protected]
2 Associate ProfessorPhysics Department, University of Zanjan and Center for Research in Climate Change and Global Warming
(CRCC), Institute for Advanced Studies in Basic Sciences (IASBS), Zanjan, Iran, [email protected] Abstract- Being one of the most important techniques, significant research has been conducted in solar cell efficiency improvement. Simulation of various structures and materials of solar cells provides a deeper understanding of device operation and ways to improve their efficiency. Over the last two decades, polycrystalline thin-film Cadmium- Sulfide and Cadmium-Telluride (CdS/CdTe) solar cells fabricated on glass substrates have been considered as one of the most promising candidates in the photovoltaic technologies, for their similar efficiency and low costs when compared to traditional silicon-based solar cells. In this workg CdS/CdTe solar cells simulated using MATLAB software and the optical properties of this structure have been investigated.
Keywords: solar cell, CdS/CdTe, simulated, photonics, optical properties.
1. Introduction
The considerable energy level produced by energy can heat oceans, create atmospheric events and evaporation cycle, flow rivers, and destroy natural lands due to storms and hurricanes. The 1906 San Francisco earthquake with magnitude of 7.8 at Richter scale led to release of 1017 Joule that is equal to the generated solar energy within one second.
Oil reservoirs of the earth is about 3 trillion barrels consisting of 1.7×1022 joule energy while such amount energy can be generated by sun during half and one day.
Human annual consumption is about 4.6×1025 joule that is equal to the energy generated by sun within one hour. The sun is constantly generating 1.2×1025 trillion watt (tw) energy that is highly more than any other generation source, either renewable or non- renewable sources. This energy is significantly more than the energy that is required for human that is about 13 trillion watt. Covering 0.16% of lands by 10% solar cells it is possible to obtain 20tw energy that is two times greater that world fossil fuel consumption besides energy of various nuclear reactor power plants [1]. Solar energy is robust while a minor amount of it is directly used in human activities. About 80-85% of the whole consumed energy is obtained from fossil resources. These resources are not renewable, are reducing, and generating greenhouse gases and other environmental pollutions [2]. Care for the environment is one of the most underlying responsibilities when using and choosing any source from preferred energy resources. Fossil fuels emit a large volume of greenhouses such as CO2 leading to disturbance of the ecological balance.
These pollutions are increasing due to overuse of fossil fuels and expansion of human community needs. The solution to this problem is use of fossil fuels with carbon separation that is a difficult method, because large spaces are required in this method to store the obtained greenhouse gases and maintenance of these gases leads to considerable problems. Nuclear power plants may seem to be a suitable choice but construction and maintenance of several thousand 1GW nuclear power plants al around the world is a controversial issue within providing 10tw energy demanded. Uranium deposits of these power plants will be finished after 10 years then seawaters should be used that is also an exhausted resource. On the other hand, use of renewable energy sources is an ideal method and solar energy is the most suitable energy resource compared with other renewable energy sources as it is not an exhausted energy resource and is environment- friendly [1]. Quantitative constraint and finish-ability of fossil fuels make use choosing renewable energy resource in future. in Figure 1 The concentrated heat is then used as a source of a conventional power plant, where steam drives generators for electricity.
Figure 1 An illustration of Concentrated Solar Power Systems [3]
A photovoltaic cell, or solar cell, is a device that converts light into electric current directly by utilizing photoelectric effect. Although the history of solar cells can be dated back to 1880s, Pearson, Fuller and Chapin started the whole new chapter of photovoltaics by creating the first silicon solar cell in 1954[4].
2. Equilibrium Simulation Results
Figure 2 Equilibrium energy band diagram By solving the Poisson equation solely
equilibrium results can be achieved. Shown in Figures 2-4 are the energy band diagram, the electric field profile and the carrier densities. The back contact was assumed to be Ohmic, so that flat band is observed. All energy levels above are referenced with respect to the Fermi level, which equals to zero along the entire device.
Figure 3 Electric field profile for uniform and non-uniform mesh at equilibrium.
Figure4 Carrier distributions at Equilibrium
The number of grid points has been reduced from 688 to 74 with the non-uniform strategy we developed. We also modeled the equilibrium with Schottky contact and the results of these simulations are as shown in Figure 5-7 The number of grid points increased to 79 dues to the increasing electric field near Schottky contact. Both band bending and depleted majority carrier concentration were observed.
Figure 5 Equilibrium band diagram with Schottky contact applied.
Figure 6 Electric field profiles at equilibrium with Schottky contact applied.
Figure 7 Carrier distributions at equilibrium with Schottky contact applied We could also achieve the accumulation type Ohmic contact by adjusting the barrier height to near zero value, as depicted in Figure 8 below.
Figure 8 Equilibrium band diagram with accumulation type Ohmic contact 3. Photocurrent Transient
Similarly, to the step bias response, a variety step functions of illumination have been applied to the standard solar cell under short circuit conditions, so that the current decay and carriers’ transients can be analysed. Shown below are the charging and discharging processes in solar cells due to on and off illumination. The natural decay of current has been reproduced in Figure 10
Figure 9 Photocurrent transients.
Figure 10 Exponential decay of photocurrents
Figure 11 Current transient under 10 Sun illumination with 30 ns pulse width.
4.Conclusions
To conclude, a drift – diffusion model has been developed from scratch to simulate the steady state and transient operation of CdS/CdTe solar cells. The self-consistent solutions of potential and carrier distributions are obtained by solving the coupled Poissons’ equation and the continuity equations. The configuration of the solar cell is a SnO/CdS/CdTe heterostructure, with an n+-n+-p doping profile. The effect of Schottky contact was observed in both dark current and light current simulations.
REFERENCES
[1] G. W. Crabtree and N. S. Lewis, "Basic research needs for solar energy utilization,"
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[2] G. W. Crabtree and N. S. Lewis, "Solar Energy Conversion," Physics Today, vol. 60, pp. 37-42, 2007.
[3] https://fa.wikipedia.org
