SCAPS 1D Simulation of a Lead-Free Perovskite Photovoltaic Solar Cell Using Hematite as Electron Transport Layer ()
1. Introduction
The demand for energy at the global level continues to increase, and one of the direct consequences of this is the depletion of fossil fuels [1]. In addition, their use contributes to the greenhouse effect. Thus causing global warming [2]-[5]. It is, therefore, urgent for States to diversify their “energy mix” by increasing the share of renewable energies, in particular, photovoltaic solar energy, which offers a very promising means of producing electricity because the sun is abundant, free and almost unlimited on the earth [6] [7]. In addition, to make the most of solar energy, photovoltaic technology must address three important factors: efficiency, stability, and low efficiency. Thus, the semiconductors used in the manufacture of these solar cells are the subject of multiple research. So far, silicon is the most commonly used material for the manufacture of photovoltaic cells, accounting for more than 80% of the world’s production [4] [8]. Due to its very energy-intensive and expensive production method, other materials now appear to be competitive, in particular, perovskite, which represents a real alternative because of its low cost and thin thickness to create the photovoltaic effect, which makes it possible to produce a large number of solar cells with little material [5] [9]-[12]. The recent implementation of CH3NH3PbX3 perovskite absorbers (X = I, Cl, Br) with the organic hole conductor 2,2’,7,7’-tetrakis (N, N-di-p-methoxyphenylamine) 9,9’-spirobifluorene (OMETAD spiro) has achieved energy conversion efficiencies (PCE) higher than 23% and has been recognized as an emerging revolution in photovoltaic technologies [13] [14]. However, spiro-OMETAD is costly because its synthesis is more complex and requires rigorous purification steps, which increases production costs [15]. Moreover, lead in perovskite is a limitation due to its potential toxicity and environmental pollution [16]. There are environmental and human health-related complications when lead-based materials are used in perovskite solar cells [13]. So, to solve the problem of lead, we replaced it with tin in perovskite, giving the molecule CH3NH3SnI3. Also, in order to use cheaper materials, we opted for PEDOT: PSS as the hole transport layer, replacing the Spiro-Ometad, and hematite (α-Fe2O3) instead of TiO2 [17] [18], which is widely used as an electron transport layer. Hematite is used here for the first time as an electron transport layer. Hematite is the α polymorph of Fe2O3. Its gap energy is around 2.1 eV [19] [20]. Moreover, between 600 and 1000 nm, the transparency of α-Fe2O3 thin films varies between 70% and 85%, giving them transparency in the visible range [21]. Moreover, hematite is widely available and inexpensive to produce. Hematite could ensure the efficient transfer of electrons generated by light absorption to the electrode, thus contributing to the production of electric current in solar cells.
Before implementing physical devices experimentally, it is often necessary to use simulation to establish orders of magnitude for the device parameters. Numerical simulation is a powerful tool for understanding and describing a physical device. Photovoltaic solar cells are governed by the properties of semiconductors. So, it is possible to understand the parameters that limit or improve the performance of photovoltaic solar cells through simulation, as mentioned by [8]. In this study, we used SCAPS to simulate photovoltaic solar cells with a view to optimizing them [22]-[24].
The main objective of this study is to highlight the effect of several physical parameters on the photovoltaic performance of the solar cell based on the perovskite CH3NH3SnI3 as an absorbent layer with the SCAPS 1D simulation tool. Particular attention will be paid to the study of the effect of the thickness, defect density and doping concentration of each layer of the cell on the characteristics of the solar cell in order to predict the optimal parameters for obtaining high electrical conversion efficiency.
2. Device Modelling Parameters
The α-Fe2O3/CH3NH3SnI3/PEDOT: PSS structure studied is a planar structure given in Figure 1(a). Therefore, it is composed of a hematite (α-Fe2O3) layer of type n, used as an electron transport material (ETL) and at the same time as a front contact (anode), hybrid halogenated perovskite (CH3NH3SnI3) of type p which was used as an absorbent material and PEDOT: PSS as a hole transport material (HTL). The energy band alignment is shown in Figure 1(b). The conduction band offset at α-Fe2O3/ CH3NH3SnI3 interface is +0.27 eV, which is crucial for the migration of generated photoelectrons to the front contact. A similar study is carried out by [25], where the PCBM is the electron transport layer. Table 1 summarises the simulation parameters for the configuration. The given parameters Eg is the energy bandgap, εr is the relative permittivity, χ being the electron affinity, μn and μp are the electron and hole mobilities, Nt is the defect density, respectively. Na and Nd are the densities of acceptor and donor materials, whereas Nc and Nv are the effective densities of the conduction band and valence band. The parameter values not included in the table are set identically for all layers. Neutral Gaussian distribution defect is adopted with characteristic energy being set to 0.1 eV [26]. The hole capture cross section is set to 1 × 10−15 cm2, with the thermal velocity of all carriers is fixed at 107 cm/s [27]. No optical reflectance is considered in the simulation and we have optimized the numerical parameters used in the study using the control variable method.
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Figure 1. Simulated solar cell: (a) configuration of the cell; (b) energy band alignment.
After performing simulations in SCAPS using the parameters in Table 1, we obtained the solar cell output parameters: Short circuit current density (Jsc), Open circuit Voltage (Voc), Fill factor (FF), and Power conversion efficiency (PCE).
Table 1. Simulation parameters for solar cell material layers.
Materials |
CH3NH3SnI3 [10]-[12] |
PEDOT: PSS [12] |
α-Fe2O [19] [20] |
Eg (ev) |
1.3 |
2.2 |
2.3 |
X (ev) |
4.170 |
2.900 |
3.900 |
ε/ε0 |
8.200 |
3.000 |
9.800 |
Nc (cm−3) |
1 × 1018 |
1 × 1015 |
3 × 1018 |
Nv (cm−3) |
1 × 1018 |
1 × 1018 |
1 × 1019 |
µe (cm−2/Vs) |
2000 |
1 × 10−1 |
2 × 10−3 |
µh (cm−2/Vs) |
300 |
2 × 10−3 |
5 × 10−5 |
Nd (cm−3) |
- |
- |
4.003 × 1020 |
Na (cm−3) |
5 × 1014 |
1 × 1018 |
- |
σp (cm−2) |
8.200 |
3.000 |
9.800 |
Nt (cm−3) |
1.019E+13 |
1.042E+13 |
1.042E+13 |
3. Results and Discussions
3.1. Effect of Thickness of Absorber Layer CH3NH3SnI3
Figure 2. Variation of solar cell parameters with thickness of CH3NH3SnI3: (a) Jsc; (b) Voc; (c) FF; (d) PCE.
The thickness of the absorbent layer plays a very important role in the performance of the cell. Thus, to highlight this, we varied the thickness from 100 nm to 1000 nm. The simulation results are shown in Figure 2.
It can be seen that when the thickness of the absorbent layer (perovskite) is varied, the current density Jsc increases. This is because more photons are captured in the volume of the material. Indeed, when the number of absorbed photons increases in the volume of the material, there is an increase in the electron-hole pairs generated and consequently, the current also increases, hence, the increase in conversion efficiency. Thus, the Power Conversion Efficiency (PCE) can reach 34.54% at 700 nm. A slight decrease in Voc with thickness is due to the increased rate of recombination. The decrease in form factor with increasing absorber thickness can be attributed to increasing the series resistance of the absorber, as proved by [28] [29].
3.2. Effect of Doping Concentration of Absorber Layer
Doping is a very important process used to improve the properties of semiconductor devices, such as solar cells [30]. Doping of a photoactive material in the solar cell architecture decides the electrical behavior of the layers, which will affect the performance of the device [13]. For this study, the thickness of the perovskite layer is fixed at 700 nm, and the thickness of the hematite (the ETL layer) and PEDOT: PSS (the HTL layer) are fixed at 5 nm. We varied the perovskite doping from 1.1010 cm−3 to 1.1022 cm−3.
Figure 3. Variation of solar cell parameters with doping of CH3NH3SnI3: (a) Jsc; (b) Voc; (c) FF; (d) PCE.
Figure 3 above shows that when 1E+10 cm−3 ≤ Na < 1E+16 cm−3, the doping concentration of the absorber has no influence on the photovoltaic parameters of the cell. But, we can observe that from 1E+16 cm−3 to 1E+20 cm−3, the increase in absorber doping concentration leads to an improvement in solar cell parameters. This is due to a reduction in the potential barrier and, therefore, a reduction in the space charge region. Above 1E+20 cm−3, the solar cell parameters drop in value, especially on short-circuit current and efficiency, because of the decrease in the collection of generated photo carriers. This is possible due to the increased recombination rate of charge carriers inside the perovskite absorbing layer. So, the concentration of 1020 cm−3 represents the optimal value to achieve the maximum performance of our solar cell.
3.3. Effect of Defect Density Absorber Layer
Defects are inevitable in the absorbent layer; they can exist in the volume and on the surface. Point defects such as gaps, interstitial defects, Schottky and Frenkel [31]. We considered a neutral Gaussian distribution defect with 0.1 eV characteristic energy in the volume, as did Nithya K.S. and Sudheer K.S. in their study [26]. The defect density varies from 1013 to 1016 cm−3 in the absorbent layer (CH3NH3SnI3).
Figure 4. Variation of solar cell parameters with defect density Nt (cm−3): (a) Jsc; (b) Voc; (c) FF; (d) PCE.
As expected, the high defect densities in the absorbing layer material limit the performance characteristics of the solar cell, as related by [11] [26] [32]-[34]. According to the results of our simulation, all solar cell parameters are significantly reduced when the defect concentration of the absorbing layer increases. This is because these defects introduce deep or shallow levels into the energy bandgap. Because of these defects, charge carriers can become trapped and facilitate non-radiative electron-hole recombination [35]. (Figure 4)
Thus, in this study, to optimize the solar cell, it is advisable to retain a defect density of the order of 1013 cm−3 for the absorbent layer (CH3NH3SnI3).
4. Conclusions
The SCAPS 1D software allowed us to simulate the photoelectric behavior of the perovskite-based solar cell with structure α-Fe2O3/ CH3NH3SnI3/PEDOT: PSS. The photovoltaic solar cell is characterized by its J(V) curve. The parameters of the simulation relate to the thickness, the density of the defects and the doping concentration of the absorbent layer. The best performance of the cell is achieved for: Jsc = 33.948884 mA/cm2, Voc = 1.3724 V, FF = 90.80% and PCE = 42.3%, when the Perovskite absorbent layer has 700 nm thickness, doped with a Na acceptor concentration of 1020 cm−3; beyond this concentration, the cell yield drops.
In perspective, in order to verify these encouraging results, an experimental study should be carried out to understand the limits of the real solar cell.
Acknowledgements
The authors sincerely thank Dr. Marc Burgelmann and his team at the University of Gent, Belgium, for providing the scientific community with this simulation program.