Study of Efficiencies CdTe/CdS Photovoltaic Solar Cell According to Electrical Properties by Scaps Simulation

The photovoltaic performance (efficiency η) of an ITO/CdS/CdTe structure cell is studied in this article according to its electrical properties. The study is carried out by simulation with SCAPS (Solar Cell Capacitance Simulator) whose mathematical model is based on solving the equations of Poisson and continuity of electrons and holes. An electrical conversion efficiency of 23.58% is obtained by optimizing the mobility of the electrons (100 cm 2 /Vs), that of the holes (25 cm 2 /Vs), the density of electrons (10 15 cm −3 ), the density of the effective states in the conduction band (7.9 × 10 17 cm −3 ) and the electronic affinity (3.85 eV) of the CdTe absorbent layer.


Introduction
The very high solar potentials in Sahelian countries (5.7 kWh/m 2 /day on the ground of solar energy) [1], do not relate the equivalent photovoltaic solar performances of the panels installed in these places. They are far lower than the powers obtained by the standard test conditions (irradiation: 1000 W·m −2 , module temperature: 25˚C, spectral distribution of the radiation: AM 1: 5). There are many reasons, in particular the fact that very high local temperatures are not taken into account, which are environmental conditions dissimilar to those of How to cite this paper: Seck, S.M., Ndiaye, E.N., Fall, M. and Charvet, S. (2020) Study of Efficiencies CdTe/CdS Photovoltaic Solar Cell According to Electrical Properties by Scaps Simulation. Natural Resources, the tests. A photovoltaic panel whose surface temperature exceeds 25 degrees, loses up to half a percent of efficiency per additional degree [2] The production of photovoltaic electricity is obtained by the conversion of solar energy (solar radiation) with photovoltaic solar panels or modules made up of series/parallel assemblies of photovoltaic solar cells obtained from organic inorganic or hybrid photovoltaic materials [3] [4]. Moreover, thin-film solar cells have many advantages, in particular their ease of synthesis, their low cost in a wide range and their adaptability in industrial production. Indeed, mono-junctions of monocrystalline silicon and of polycrystalline silicon form the first solar technologies offering respectively yields going up to 21% and 14% [5].
With the development of technology and the creation of new basic materials, high yields are obtained [6]. As a result, the performance of photovoltaic modules is strongly linked to the intrinsic parameters of the basic materials constituting each element that a photovoltaic solar cell makes up (chemical nature, gap, doping rate, carrier mobility). Controlling these parameters will therefore effectively improve the performance of solar panels. This paper presents a study by simulation of heterojunction CdTe solar cells by SCAPS simulation according to the parameters (charge carrier densities, mobilities and electronic affinities) of the components of solar cells. It links the influence of parameters on electrical performance under standard test conditions. The use of techniques for simulating the performance of a solar cell has several advantages. It minimizes the cost of manufacturing prototypes by offering the possibility of varying all parameters. They make it possible to better understand the impact of different physical parameters on cell performance, and to design and optimize different cells without the need to systematically use experimental procedures which can prove to be expensive. Finally, they make it possible to assess the performance of a structure and its maximum theoretical yield. It also offers the opportunity to study the influence of each layer of the cell in the case of interdependent parameters.

Presentation of SCAPS (Solar Cell Capacitance Simulator)
SCAPS allows simulation of one-dimensional solar cells [7]. SCAPS software was originally developed to model and simulate the electrical characteristics of hete- And: In Equation (1) and Equation (2), φ describes electrical potential, q is the unit charge, ε 0 and ε r are the dielectric constants of vacuum and semiconductor, respectively; n and p are electron and hole density, respectively; A N − and D N + are the density of ionized donors and acceptors, ρ t the charge density of defects, J n and J p the electron and hole current density; G is the generation rate and R n(p) is the recombination loss.
The transportation of carriers is shown by: where μ n(p) is the electron/hole mobility and D n(p) is the diffusion coefficient.
The generation rate is given by: where φ 0 is the incident photon flux, x is a depth and α the absorption coefficient of the active material and λ, the absorbed photon wave length. Cell performance was achieved with scaps under standard conditions (a cell temperature of 25˚C and radiation of 1000 W/m 2 with an air mass spectrum of 1.5 (AM1.5)).

Methodological Approach
We aim to optimize the efficiency of our solar cell by modifying the characteristics (mobility, doping) of CdTe with SCAPS. It makes it possible to study and analyze the evolution of characteristics such as Jsc, Voc, FF and therefore the efficiency η as a function of the intrinsic properties of doped CdTe.
The recommended method for introducing a solar cell is to use the graphical interface of SCAPS. In this way, we can interactively introduce all the parameters that will be controlled by SCAPS so as not to have impossible or inconsistent values [7].

Properties and Structure of Studied CdTe Solar Cell
We used a thin film solar cell in ITO/CdS/CdTe structure, where CdTe the base layer, CdS the emitter and ITO the antireflecting layer. The front contact is made of aluminum. All the thicknesses of ITO, CdS and CdTe layer were respectively 0.30, 0.08 and 4.00 µm. The parameters of the materials used in this modeling were selected from experimental data in the literature and from the characteristic values (see Table 1) [7] [10] [11].

J-V Characteristic of the CdTe Solar Cell
With the data, we can save the variations of Jsc, Voc, FF and analyze the efficiency η. the J-V characteristic modeled in Figure 1 uses the values in Table 1.
The efficiency of the CdTe/CdS solar cell reached 21.34% similar to that of a practical CdTe based solar cell [11]. This proves that our solar cell model is valid and can be used to perform a simulation analysis of its performance with different parameters of the material. Then, we look for the effect of the evolution of the electrical characteristics (Jsc, Voc, FF and η) as a function of the intrinsic properties of the material (doping, mobility) in order to optimize them and predict their behavior of the photocell.

Effect of the Mobility of CdTe Charge Carriers on Yield
The efficiency (η) is analyzed as a function of the electrons and holes mobility ( Figure 2). The efficiency of the cell decreases with the increase in mobility of the electrons (Figure 2(A)) and increases with the speed of the holes (Figure 2(B)). These results are in agreement with those of the literature. Because a couple of mobility of the charge carriers which is not optimized adds constraints on the thickness of the active layer and increases the losses by recombinations [12].
The conversion efficiency is evaluated based on carrier mobility couples. The yield is maximal and equal to 22.10% for a couple of mobility equal to 100 cm 2 /Vs for electrons and 25 cm 2 /Vs to that of the holes ( Table 2).   We found that the optimum torque value of carrier mobility is required to obtain maximum efficiency of the cell.

Effect of Carrier Density (for Acceptor) of CdTe
The acceptor density was varied from 10 12 cm −3 to 10 19 cm −3 in steps of 10 cm −3 .
The decrease in the acceptor density of CdTe greatly reduces the yield ( Figure  3). Recombination becomes important with an increase in the density of carriers. Therefore, an increase in density can drop the collection of free electrons generated by the incident radiation on front contact and contribute to lowering the value of Jsc [13]. In our conditions, a density of 10 16 cm −3 is the optimum value which would increase the yield.

Effect of the Density of Effective States in the BC of the CdTe Layer on the Yield
The density of the effective states was varied from 7.9 × 10 17 cm −3 to 22 × 10 17 cm −3 .
An effective state density in the smaller conduction band (CB) gives the best performance [14]. The more the density of the effective states in the CB increases the more the efficiency of the cell decreases ( Figure 4). A density of 7.9 × 10 17 cm −3 is the best value.

Effect of the Electronic Affinity of the CdTe Layer on the PV Conversion Efficiency
A low electronic affinity of the CdTe absorber indicates a low efficiency from 3.8 eV. For an electronic affinity between 3.8 eV and 3.85 eV, the efficiency equal to 21.37% is maximum and almost constant throughout the range before decreasing after 3.85 eV ( Figure 5). The results obtained are consistent because the differences in electronic affinities and band gap energies cause a discontinuity at the level of the connection of the conduction and valence bands. Therefore, the influence of the electronic affinity of the CdTe layer (CdTe gap = 1.45 eV vs CdS gap = 2.5 eV) in the solar cell, acts on the energy difference in the conduction band.

Conclusions
This work established an analysis of the relationship between the properties of the layers of the ITO/CdS/CdTe structure cell and their performance with an optimal electrical conversion efficiency of 23.58%.
In fact, the optimized values are: Therefore, we can say that the main factors limiting photovoltaic performance in African ecosystems, linked to the physical properties of materials, can be circumvented by also taking temperatures into account.