^{1}

^{*}

^{2}

^{3}

^{1}

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.

The very high solar potentials in Sahelian countries (5.7 kWh/m^{2}/day on the ground of solar energy) [^{−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 the tests. A photovoltaic panel whose surface temperature exceeds 25 degrees, loses up to half a percent of efficiency per additional degree [

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 [

With the development of technology and the creation of new basic materials, high yields are obtained [

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.

SCAPS allows simulation of one-dimensional solar cells [

The equations that govern the studied model are the equations of Poisson and continuity for electrons and holes.

d 2 φ d x 2 = − q ε 0 ε r ( p − n − N A − + N D + + ρ t q ) (1)

And:

{ 1 q d J x d x = G − R n − 1 q d J p d x = G − R p (2)

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; N A − and N D + 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:

{ J n = − q D n d n d x + q μ n d φ d x J p = − q D p d p d x − q μ p d φ d x (3)

where μ_{n}_{(}_{p}_{)} is the electron/hole mobility and D_{n}_{(p)} is the diffusion coefficient.

The generation rate is given by:

G ( λ , x ) = φ 0 ( λ ) e − α ( λ ) ⋅ x (4)

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)).

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 [

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

With the data, we can save the variations of Jsc, Voc, FF and analyze the efficiency η. the J-V characteristic modeled in

The efficiency of the CdTe/CdS solar cell reached 21.34% similar to that of a practical CdTe based solar cell [

The efficiency (η) is analyzed as a function of the electrons and holes mobility (

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 (

Parameters | Layers | ||
---|---|---|---|

ITO | CdS | CdTe | |

Permittivity | 9.0 | 10.0 | 10.2 |

Effective state density in the BC (cm^{−3}) | 2.20 × 10^{+18} | 2.20 × 10^{+18} | 8.0 × 10^{+17} |

Effective density of states in BV (cm^{−3}) | 1.80 × 10^{+19} | 1.80 × 10^{+19} | 1.80 × 10^{+19} |

Electrons mobility (cm^{2}/Vs) | 1.0 × 10^{+2} | 1.0 × 10^{+2} | 3.2 × 10^{+2} |

Holes mobility (cm^{2}/Vs) | 2.5 × 10^{+1} | 2.5 × 10^{+1} | 4.0 × 10^{+1} |

Donor density Nd (cm^{−3}) | 1.0 × 10^{+17} | 1.0 × 10^{+17} | 0.0 |

Acceptor density Na (cm^{−3}) | 0.0 | 0.0 | 1.0 × 10^{+14} |

Absorption coefficient (cm^{−1}) | 1.0 × 10^{+6} | SCAPS | 1.0 × 10^{+5} |

Electron Mobility (cm^{2}/Vs) | 100 | 100 | 320 | 700 |
---|---|---|---|---|

Holes Mobility (cm^{2}/Vs) | 20 | 25 | 40 | 65 |

η (%) | 22.03 | 22.10 | 21.34 | 20.82 |

We found that the optimum torque value of carrier mobility is required to obtain maximum efficiency of the cell.

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 (^{16} cm^{−3} is the optimum value which would increase 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 [^{17} cm^{−3} is the best value.

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 (

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.

However, for CdS/CdTe cell, the values of ΔEc (χ_{CdS} − χ_{CdTe}) lie between 0.20 and 0.25 eV and are found to be independent of the process of cell fabrication [

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:

• A couple of mobility of charge carriers of the CdTe layer (mobility of the electrons of 100 cm^{2}/Vs and that of the holes of 25 cm^{2}/Vs).

• A density of the electrons in the CdTe absorbent layer which allows us to obtain coherent results and a good yield of 10^{15} cm^{−3}.

• A density of the effective states in the conduction band in the absorbent layer CdTe of 7.9 10^{17} cm^{−3}.

• A reasonable and acceptable dielectric permittivity of the absorber layer CdTe of 8.

• An electronic affinity of the absorber layer CdTe = 3.85 eV.

However, the increase in the optical gap of the CdTe absorber layer reduces the intensity of the photocurrent while the open circuit voltage increases. The compromise between these two phenomena gives an optimum value of the optical gap EgCdTeopt = 1.47 eV which corresponds to the maximum efficiency of the cell.

Thus, we were able to show the impact of the studied characteristics (charge carriers mobility, the density of carriers, the density of effective states, the dielectric permittivity and the electronic affinity) on the efficiency of the solar cell with SCAPS. The results of simulation of the thin film solar cell based on CdTe, are in good agreement with those of the literature, which show that the parameters of the photovoltaic cell have significant effects on the photovoltaic performances (Voc, Jsc, FF and η).

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.

We would like to thank all the members of the physical department of Alioune Diop University in Bambey Senegal for the collaboration with the interuniversity master of renewable energies (MIER) and Professor M. Burgelman and colleagues for develop SCAPS available to us (ulis.be).

The authors declare no conflicts of interest regarding the publication of 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, 11, 147-155. https://doi.org/10.4236/nr.2020.114009