High Efficiency Solar Cells Base on Organic-inorganic Perovskites Materials

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I. INTRODUCTION
In recent years, organic-inorganic perovskite solar cells have attracted great interest in the photovoltaic research community due to their ease of manufacturing, low production costs, superb light harvesting characteristics and high performance, making them more preferable than other current solar cell materials. Solar cells have achieved extremely high-power conversion efficiencies (PCEs) of more than 20 percent in solution-processed perovskite (PSC), but practical application of this photovoltaic technology requires further advances in both long-term stability and large-area demonstration materials. The perovskite-based type of solar cells have gained everincreasing prominence in the type solar cells, because of their outstanding physical properties: (1) strong optical absorption (2) high charge carrier mobility (3) high photovoltaic conversion efficiency. Methylammonium (MA) lead halide (e.g. CH3NH3PbI3), the most commonly employed material in (PSCs) solar cells, was reported to have high absorption coefficients (direct bandgap ∼1.55 eV) as well as high mobilities for electrons and holes. [1] SCAPS analyzes the physics of the model and defines the recombination profiles, the electric field distribution, the transport mechanism of the carrier. and the individual current densities. The impact of various layer characteristics on cell performance, such as thickness and defect density, have been thoroughly investigated. The thickness of the absorber and buffer (TiO₂) layers influences the short-circuit current density (Jsc) and efficiency (PCE ) of solar cells. The PSCs use organic and inorganic materials of different forms, such as TiO2, ZnO, SnO2, and PCBM, such as ETM [2] [3]. By filling pinholes and vacancies between perovskite grains, PCBM plays a critical role in improving the efficiency of the lightabsorbing layer, resulting in a film with wide grains and fewer grain borders [4].

II. PEROVSKITE SOLAR CELLS
Nature published an article on a perovskite solar cell with an efficiency greater than 10% in 2012. Since then, due to their high performance, low manufacturing costs, and strong marketing ability, Perovskite solar cells have attracted a lot of attention [5]. The composition of perovskite solar cells consists generally of 5 layers which are a cathode layer of silicon, the anode-based layer of the hole material (HTM), perovskite layer and, anode layer (FTO/ITO). A typical perovskite solar cell with a planar structure is shown in Figure.1. The perovskite layer is used in the unit, where photon excitation occurs, as a light absorber. The produced electrons and holes are separated and transferred to the ETM layer and HTM layer respectively. The function of the ETM layer is that electrons and block holes are extracted and transferred. The HTM layer is used to remove and pass holes and block electrons. This implies high electron mobility in the ETM layer, high mobility in the HTM layer for holes, and appropriate band offsets between the ETM layer/perovskite layer/HTM layer necessary for high efficiency. [6] [7]. The major processes of charge transport in perovskite solar cells are shown in Figure.2. The green arrows are desirable energy transfer processes, including photon excitation, the transport of electrons from the perovskite layer to the ETM layer, and the transport of holes from the perovskite layer to the HTM layer. The red arrows show the unwanted power loss processes for perovskite solar cells involving the combination of charging carriers, ETM layer back electron flow, and HTM layer hole flow with the perovskite layer A challenge has been the determination of charge carriers in organic-inorganic hybrid perovskite solar cells. The existence of both organic and inorganic components is largely due to this Edri et al [10] experimentally examined this ambiguity about the existence of carriers in perovskite material . It was found that the lead halide perovskite solar cells (thin film and/or inert mesoporous configuration) operate generally as p-i-n junction. An intrinsic semiconductor material has a p-i-n junction sandwiched between an extrinsic semiconductor type p and n. Their study was carried out on a solar cell with mixed halide lead based perovskite, CH₃NH₃PbI₃ -xClx absorber., Minemoto etal. and Tanaka et al [11]. They found that a typical Wannier-type exciton is the dominant charge carrier in lead halide perovskite. This is similar to the type of charge carriers observed in inorganic materials. Wannier type excitons occur when the electric field screening tends to reduce the Coulomb interaction between electrons and holes. The screening reduces the binding energy of the electron-hole pair and makes them move independently, unlike Frenkel excitons which occur in organic materials [12].

IV. NUMERICAL EMULATION
Simulation is a crucial technique for realizing a deep insight into the physical activity, the viability of the suggested physical interpretation, and the effect of physical changes on the solar cell devices' performance. Different models of simulation exist for solar cell simulation (SCAPS, AMPS, SCAP, etc). SCAPS is a one-dimensional program for simulation with seven input layers of semiconductors manufactured by a group of researchers from Ghent University, Belgium (Solar cell power simulator) [13]. It is not possible to build a solar cell without stimulatory operation, as time and resources are wasted. It does not only minimize the risk, time, and money but also analyzes the properties of the layers and their function to maximize the efficiency of the solar cell. To simulate a computer all the basic input parameters should be well specified to function as a real counterpart. The different structure has been used by perovskite-based solar cells with inorganic semiconductor solar cells, such as CIGS, and exciting form Wannier is contained in perovskite. Thus, SCAPS like a 1D simulator can be used to model solar cells based on perovskite SCAPS is a numerical solution of the two poison equations and the continuity equations on the limits of one dimension or two [15]. Express the following equations.  (1) Calculates the amount of change in voltage as a result of the difference in the density of the charge in three directions. The higher it is, the more volatile, but stable with time, and this causes an increase in the movement of the charge and the speed of its transfer in the center or the area of depletion. Equation 2 is the net gain of the current as a result of the processes of generation and recombination, the final net represents the current and when taking a grad is to reveal the extent of change or activity of this process in the three dimensions Equations 5 and 6 represent the current density of the hole and the electron. Where Vb and Vn are the potential effectives expressed in Equations 6 and 7. ∆G is band structure variation, μn, μp represents electron and hole mobility, respectively.

V. SOLAR CELL DEVICE CHARACTERIZATION PARAMETERS
The main parameters that are used to characterize the performance of solar cells are the (Power Conversion Efficiency,( Ƞ) ,the short-circuit current density, (Jsc) .

A. .Short-Circuit Current Density (Jsc)
The short-circuit current density (is the maximum photogenerated current delivered by a solar cell when the terminals of the solar cell are in contact with each other (i.e. [8]. [14]. 25 short-circuited)). The solution for (Jsc) can be gotten from the net current density(J(V)) in the solar cell given as. dar (8) where dar (V) is the dark current density. This is given by: dar ( ) (9) Set a solar cell under a standard light source, when the output is in short circuit state, which means the voltage value is 0, the current is the maximum output current called short-circuit current. It is given as: Or equivalently, the short circuit current given by: ( ) (10) Where V is the voltage across the junction, T is the absolute temperature, is the dark saturation current density and m is the ideality factor with values between 1 and 2.

B. Power Conversion Efficiency (Ƞ)
Power conversion efficiency (η) is the percentage of incident power transformed into useable electricity by solar cells.
The ratio between the generated maximum power and the incident power is used to calculate the power conversion efficiency. This is given by:

VI. SCAPS SIMULATION OF FTO/PCBM/PEROVSKITE/P3HT
Notice that all simulation parameters are carefully chosen from the recorded experimental data and other literature for each layer of the architecture. Table .1. Summarizes all primary simulation parameters.  Tables shows the values of the most useful cell parameters required for the simulation. These values have been selected on the basis of or in certain cases realistic projections of theoretical factors, experimental evidence and current literature. Most of the parameters used for the absorber and the thicknesses of the other layers were extracted from the literature while the parameters for other layers in the Table were gotten from the works reported by [16] and [17].

VII. RESULT AND DISCUSSION
The results derived from numerical simulations are discussed in this part. The cell output variations that occur in the cell parameter variance are discussed. The parameters investigated are Effect of the (thickness of ETL Layer, effective density of states, annealing Temperatures, Different Back Contacts) for FTO/P3HT/CH3NH3PbI3/PCBM solar cells device.

A. Effect of the thickness of PCBM on the solar cell.
The absorber layer should be set for optimum thickness to absorb the maximum number of photons and to produce electron-hole pairs. Absorber layer thickness has been ranged from 100nm to 2600nm. When the thickness of the absorber layer increases the longer wavelength of the illumination creates a fair amount of electron-hole pair generation. Figure.4. shows a difference in the thickness of the PV parameter absorber layer. By looking at the JSC / thickness graph we can say that by increasing its thickness JSC , increased and the efficiency is obtained, The Convert the light to excitons depends on the energy gap, and since the thickness increases, this means that the number of excitons generated increases so the current increases to a certain thickness and then decreases due to the decrease in the transmitted radiation due to the darkening with increasing thickness. And that all the light spectrum may be scattered and lost, except for the red color. It reaches deeper depths by penetration, and by converting it into an exciton, the energy gap must be within the spectrum IR. In addition to increase amount of light which absorb inside the absorber layer due to increase the thickness in specific range. The highest value of efficiency is obtained at the thickness of 1200nm.

B. Effect of the effective density of states on the solar cell
Recently, the low effective mass and consequently the low density of electronic states in the valence band and conduction band found experimentally and theoretically [18][19][20][21][22][23] for lead halide perovskites has been named as a possible ingredient to the success of metal-halide perovskites.
[24] In addition, the band structure may involve direct and indirect transitions in a very near energy range which has also been stated to be beneficial for photovoltaic efficiency [25] [26] .Given that the effective mass or effective density of states are features of the band structure that are always calculated in the context of computational material screening efforts, it is of huge relevance for the community investigating novel materials for photovoltaics, whether these basic features of semiconductor band structures would be beneficial for photovoltaics at least on the level of theory where parabolic approximations to the energy vs. momentum relations of the bands are sensible. A detailed look at the key properties of solar cell absorber material, i.e. absorption, charge transport and recombination reveals that the effective mass or effective density of states enters a large number of equations that potentially affect device performance. According to Eq (12) it is clear that the effective density of state depends on effective mass, which is determined by the material, and the value of Nc can affect the density of electrons and holes directly.
where, c is effective density of state, m is effective mass, is the Boltzmann constant, T is temperature, ℎ is the Planck constant.
Through this research, we note that an increase in the density of states leads to a decrease in the efficiency and the output current, as shown in the following figures.

C. Effect of annealing Temperatures for CH₃NH₃PbI₃
The study of solar cell with temperature (T) behavior is significant since they are commonly subjected to temperatures from 15°C (243.15 k) to 50°C in terrestrial applications (333.15 K). Most modeling of semiconductor is carried out at 300 K since it is close to room temperature and a convenient number. Usually, however, solar cells are about 2 degrees lower at 25°C (298.15 K). The discrepancy is mostly negligible and both are known to be room temperature. Really, the operating temperature varies depending on the latitude, altitude, year day and time of the location in question. In addition, the operating temperature also affects the efficiency of a solar cell. Solar cells are sensitive to temperature, like any other semiconductor unit. The results of simulation I-V characteristic such as PCE and Jsc of the perovskite solar cells with varying environment temperature as shown in Table 6 where the highest efficiency is 9.59% with JSC = 12.012678 mA/cm² is achieved when the temperature at 213.15 K, therefore the best result at very low temperature to be very appropriate to work in space. If the temperature rises from 333.15 K to 213.15 K, the PCE and JSC are responsible for the decreased generation of the electron-hole pairs of perovskite materials, as seen in Figure 7.
From Figure 7 the open-circuit current increases gradually with increasing temperature, it can adjust the efficiency with temperature due to control of the recombination, generation, and collection of the charge carriers so the best temperature of perovskites solar cells with CH3NH3Pbl3 as PVSC is 213.15 .   28

D. Effect of Different Back Contacts
Simulations have been carried out with the aid of silver (Ag), iron, copper (Cu) graphite alloy, gold (Au), nickel (Ni), and platinum (Pt). Figure 8 illustrates the impact of different back contact with efficiency. The simulated output of solar cells improves but saturates after a certain value with the rising function of the work. The majority carrier barrier height (relative to Ef) decreases due to band bending at the metal-semiconductor interface, thus making contact more ohmic [27]. As the metal 's work function increases, perovskite solar cell efficiency is also increasing. Holes traveling towards the electrode are energetically unfavorable because the electric field around HTM / back contact becomes negative. For this reason, it can be expected that the lower function of the work is responsible for the Stability of efficiency. The simulation results show that Pt is one of the potential rear contact materials that can develop PSC performance

VIII. CONCLUSIONS
In this study, the thickness of the PCBM(from 100 to 2600 nm) , the CB and VB of the perovskite(from 2.2E12 to 2.2E20), and the temperature were changed, and we obtained the best efficiency of 9.54% and JSC=12.011455mA/cm2 at thickness (1200 nm for PCBM,50 nm for P3hT,100 nm for MAPbl₃), and we found that the optimum value of efficiency corresponding to value of VB and CB (CB=2.