The device configuration is one of the most crucial factors for evaluating the performance of perovskite solar cells. The photovoltaic structure studied consist of transparent conductive oxide (TCO)/buffer (TiO₂/absorber (perovskite)/hole transporter material (HTM). The cell is illuminated schematically as shown in Figure 1. In this structure the perovskite solar cell has an architecture where the perovskite material is sandwiched between an electron extractor (negative charge carriers) who is TiO₂ and a layer extractor of hole (positive charge carriers) the HTM.

We enter parameters to definite materials of each layer that compose the solar cell. The input parameters for the simulation study are presented in Table 1. These values are based on experimental results and their references are cited. We have introduced in both interfaces the different parameters shown in Table 2 and for each layer the defect parameters in Table 3. For the optical absorption α (hν), we use combinaison of the six sub-models (back ground, Eg-step, Eg-sqrt, power law 1, power law 2, sub-bandgap) for each layer in order to obtain a better

result. When we set all these parameters of each layer we launch scaps.

At first we varied the thickness from 100 nm to 1000 nm of perovskite keeping the other parameters invariable. Then we varied the doping concentration of perovskite from 10¹⁰ cm⁻³ to 10¹⁸ cm⁻³. For each simulation four photovoltaic parameters are displayed: the open-circuit voltage (V_oc), the current density (J_sc), the fill factor (FF) and the power conversion efficiency (PCE). The curves are plotted with these results to evaluate the performance of the cell (Figure 3, Figure 4).

· Current density (J_sc)

The short-circuit current (J_sc) is the maximum current flowing through the solar cell, i.e. when the cell is short-circuited without load. The highest J_sc corresponds to the zero voltage of the circuit as shown in the graphical representation of short circuit current given in Figure 2 [5] .

· Open circuit voltage (V_oc)

The open circuit voltage (V_oc) is the maximum voltage measured from a solar cell. When the cell is in open circuit, no current flows then the voltage at its terminals is V_oc as shown in Figure 2 [5] .

· Fill factor (FF)

FF is the quality measure for solar cell performance by taking the ratio of maximum power delivered by a solar cell to the theoretical power of a solar cell. Fill factor can also be interpreted as the ratio of rectangular areas depicted in Figure 2 for maximum power point and (MPP) and theoretical maximum power P max = J m p ⋅ V m p . Fill factor is defined in Equation (3) [5] .

FF = J m p ⋅ V m p J s c ⋅ V o c = P max J s c V o c (3)

where V_mp and J_mp are the maximum point voltage and current density generated by a solar cell.

· Power Conversion Efficiency (PCE)

Power Conversion efficiency (PCE) expressed as a percentage is the ratio of the maximum power delivered by the PV cell and the incident light power (P_in) [5] .

PCE is defined in Equation (4).

PCE = P max P i n = F F × V o c × J s c P i n (4)

As absorber layer, perovskite plays a critical part to cell performance. The thickness of absorber is one of the most crucial parameters and has remarkable influence on the device. In the simulation, we focus on device performance parameters (i.e. J_sc, V_oc, FF and PCE) as a function of absorber thickness (varying from 100 to 1000 nm).

To absorb the maximum number of photons and generate electronn-hole pair, absorber layer should set for optimum thickness. So performance solar cell depends mainly on thickness of absorber layer [14] . Illumination spectrum is global AM 1.5 and operation temperature is 300 K. We keep the dopant concentration to Na = 10¹⁵ cm⁻³ and thicknesses of all HTM to 50 nm, their dopant concentration to Na = 10²⁰ cm⁻³. We do the simulation with six holes transporting materials (HTM): three inorganics and three organics. The HTM used are 2,2',7,7'-tetrakis-(N,N-di-p-methoxyphenylamine), 9,9'-spirobifluorene (spiro-OMETAD), poly(3-hexylthiophène-2,5diyl(P3HT), copper (I) oxide (Cu₂O), poly(triarylamine) (PTAA), nickel oxide (NiO), copper iodide (CuI).

The graphs presented in Figures 3(a)-(d) show respectively the variations of open-circuit voltage (V_oc), short circuit current (J_sc), fill factor (FF) and efficiency (PCE) of different holes transporting layers (HTM) with the thicknesses of the absorber layer.

The V_oc plots of all HTM used are similar. The simulated result show that with increasing the absorber thickness, V_oc values continously increase by varying the thickness from 100 nm to 1000 nm (Figure 3(a)). The best V_oc values of each HTM are summarized in Table 4. These values are greater than that of silicon (0.6 v) for all HTMs except for the case of NiO (0.55 v).

Let us analyze the plots of J_sc and PCE (Figure 3(c) & Figure 3(d)). The plots J_sc and PCE are almost similar. It is found that with spiro-OMETAD, CuI and

PTAA the increase of thickness leads to an increase of the values of J_sc and PCE of the cell. The highest values of J_sc and PCE obtained with these HTM correspond to the largest thickness (1000 nm); spiro-OMETAD (J_sc: 40.05 mA/cm², PCE: 23.76%); PTAA (J_sc: 40.03 mA/cm², PCE: 23.58%); CuI (J_sc: 37.15 mA/cm², PCE: 18.6%).

With HTM P3HT, Cu₂O and NiO when we increase the thickness the values of J_sc and PCE increase to an optimal value before dropping. Thus the optimum thickness is 200 nm for P3HT (J_sc: 32.23 mA/cm², PCE: 17.98%) and NiO (J_sc: 31.45 mA/cm², PCE: 12.41%). For Cu₂O, the optimal thickness is reached at 300nm with J_sc: 32.06 mA/cm², PCE: 17.91%.

By increasing thickness, the short circuit current (J_sc) increases because a thicker absorber layer will absorb more photons, which will create more electron-hole pair. But with a thicker absorbing layer, the chances of recombination also increase because the charges have to travel a greater distance for diffusion. Therefore, after a certain point, the efficiency decreases with increasing thickness. With increasing thickness, recombination multiplies [10] .

A thinner absorbent layer results in less electron-hole recombination. A thinner absorber layer can produce a higher power conversion efficiency (PCE) than a thicker layer e.g. P3HT, NiO, Cu₂O layers (Figure 3(d)).

Fill Factor is considered as an ability to deliver available power a load generated by a cell, in another word, the internal power depletion. In thicker absorbers the internal power depletion enhances and reduces the fill factor. Fill factor is observed maximum value for 100 nm thickness (P3HT: 75.05%, PTAA: 74.14%, Cu₂O: 73.87%, NiO: 71.84%, CuI: 77.02%) and for 200 nm (spiro-OMETAD: 70.82%).

The simulation results show us that the highest efficiencies are reached with spiro-OMETAD: 23.76%, PTAA: 23.58% (Table 4). The results suggest that an optimal thickness of 200 nm for absorber layer could result in high efficiency tin perovskite solar cells with P3HT, 300 nm with NiO or Cu₂O. If the absorber thickness surpasses the optimal value, it results in more excess carriers and more traps which give more opportunity for occurrence of recombination. For spiro-OMETAD and PTAA the simulation indicates that the optimal thickness is more than 1000 nm.

Doping is a very important process used to improve the properties of semiconductors devices, such as photovoltaic [15] . 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 [16] .

The dopant concentrations of perovskite were varied from 10¹⁰ to 10¹⁸ cm⁻³ in order to identify their effect toward solar cell performances with various HTM. Others parameters are kept constant such as perovskite thickness to 300 nm, all HTM thickness to 50 nm, their doping concentration to Na = 10²⁰ cm⁻³, temperature to 300 K.

Figure 4 below shows the dependences of J_sc, FF, V_oc, and efficiency on dopant concentration. At low dopant concentration (10¹⁰ cm⁻³ < Na < 10¹⁴ cm⁻³), the solar cell parameters almost do not depend on doping. From 10¹⁵ cm⁻³ to 10¹⁸ cm⁻³ a drop of less than 10% of V_oc is noted. From 10¹⁴ cm⁻³ to 10¹⁶ cm⁻³, observe an increase of less than 10% of the J_sc, values for all HTMs, then a fall of more than 50% of all these values.

The concentration of 10¹⁵ cm⁻³ represents the optimum value to reach the hightest performance with spiro-OMETAD, and PTAA while Cu₂O, P3HT, CuI, NiO requires a concentration of 10¹⁶ cm⁻³. These values are summarized in Table 5 below.

From 10¹⁴ cm⁻³ to 10¹⁶ cm⁻³, the increase of 10% of the J_sc revealed is offset by the drop of 10% in V_oc. PCE increases around 10% to reach a maximum value at 10¹⁵ cm⁻³ for spiro-OMETAD and PTAA then at 10¹⁶ cm⁻³ for the other HTMs. After these maximum values occur a fall of more than 30% of PCE is recorded.

It can then be said that increasing the doping concentration to 10¹⁴ cm⁻³ has no effect on cell performance. This behavior of the cell can be explained by a defect growth such as the recombinations that cancel the new charges created. After this concentration value, the number of photogenerated charges accumulates

and dominates the defects that appear in the cell, which improves the performance of the cell. This improvement evolved to a concentration of 10¹⁵ cm⁻³ or 10¹⁶ cm⁻³ where the recombination defects take precedence over the created charges. From this moment, a deterioration of the performance of the cell is noted. The best values obtained by our simulation for each HTM used are shown in Table 5.

To better understand the performance of the cells thus formed, we fixed doping concentration constant 10¹⁰, 10¹¹, 10¹²... 10¹⁸ cm⁻³ and we vary the thickness from 100 nm to 1000 nm. The best results are co-signed in Table 6. With HTM PTAA (PCE: 23.74%), CuI (PCE: 23.25%) performance results are obtained similar to those of spiro-OMETAD (PCE: 23.76%).