Numerical Simulation and Performance Optimization of a Solar Cell Based on WO₃/CdTe Heterostructure Using NiO as HTL Layer by SCAPS 1D

Abstract

In this paper, a solar cell based on ${\mathrm{W}\mathrm{O}}_3$/CdTe heterojunction was analyzed and optimized, for which the following structure of the Al/AZO/${\mathrm{W}\mathrm{O}}_3$/CdTe/NiO/Ni device was proposed, which was numerically simulated by the SCAPS 1-D software. Using the software, the effect of the thickness and carrier concentration of the absorber layer (CdTe) and the window layer (${\mathrm{W}\mathrm{O}}_3$) was analyzed, and the optimal value of these parameters was found to be 2 µm and ${10}^{15}{\mathrm{c}\mathrm{m}}^{-3}$ for the CdTe layer and 10 nm and ${10}^{19}{\mathrm{c}\mathrm{m}}^{-3}$ for the ${\mathrm{W}\mathrm{O}}_3$ layer, respectively. The influence of the defect density of the ${\mathrm{W}\mathrm{O}}_3$/CdTe interface on the performance of the proposed cell was also analyzed, simulating from ${10}^{10}$ to ${10}^{16}$ ${\mathrm{c}\mathrm{m}}^{-2}$, obtaining better device performance at lower interface defect density. Another parameter analyzed was the operating temperature on the photovoltaic performance of the device, observing that the solar cell has a better performance at lower temperatures. Finally, a maximum optimized PCE of 19.87% is obtained with a V_oc = 0.85 V, J_sc = 28.45 $\mathrm{m}\mathrm{A}/{\mathrm{c}\mathrm{m}}^2$, and FF = 82.03%, which makes the ${\mathrm{W}\mathrm{O}}_3$/CdTe heterojunction an interesting alternative for the development of CdTe-based solar cells.

1. Introduction

CdTe is a p-type semiconductor material with a band gap of $~$1.49 eV [1] and an absorption coefficient of ∼${10}^5{\mathrm{c}\mathrm{m}}^{-1}$, which allows it to absorb more than 99% of the incident photons with an energy greater than its band gap in films as thin as $~$2 µm [2]. In addition, CdTe can be produced on a large scale with good cost efficiency due to its adaptability to manufacturing processes [3]. These characteristics allow considering CdTe as a suitable material for the development of thin-film solar cells.

Generally, a CdTe thin-film solar cell is manufactured in a superstrate configuration, with a glass/TCO/CdS/CdTe/Back contact structure [4]. CdTe is the absorbing part of the cell where the most carrier generation and accumulation take place. In order to improve the efficiency of solar cells based on this heterojunction, alternatives are being used in cell architecture that incorporates nanostructured materials, such as the case of Di Carlo V. et al. [5], who report the growth of CdTe nanowires with a gap energy of 1.539 eV. Meanwhile, Hongmei Dang et al. [6] report the fabrication and characterization of a solar cell based on the CdS nanowires/CdTe heterojunction and found substantial improvements.

The CdS, with a bandgap of ~2.4 eV, is a good choice as a window layer, since it allows most of the incident photons of sunlight to pass through it. Until now, it has been the most widely used semiconductor to form the pn junction together with CdTe to carry out the photovoltaic effect [7]. However, due to its energy bandgap, photons whose energy is above this energy bandgap value are absorbed by it, and the photogenerated carriers in this layer are not collected, generating a parasitic absorption that harms the short-circuit current density (J_sc) [2,8].

For its part, ZnO:Al is an ideal candidate to be used as a transparent conductive oxide (TCO), since it has a wide band gap (3.3 eV) and excellent optical and electrical properties [9], in addition to being abundant materials in nature.

Along with the experimental work, in recent years, simulations of solar cells have been carried out, which has made it possible to optimize the time and cost of the experimental processes, as in the case reported by Montoya de los Santos et al. [10], who report a theoretical increase in the efficiency of a solar cell based on the CdS/CdTe heterojunction using ZnO:Al and CuSCN nanolayers.

On the other hand, ${\mathrm{W}\mathrm{O}}_3$ is a chemically stable n-type semiconductor material with a wide band gap of ~3.15 eV [11], it can present different crystallographic phases depending on the synthesis temperature: tetragonal, orthorhombic, and monoclinic. ${\mathrm{W}\mathrm{O}}_3$ is a non-toxic, low-cost, and easily evaporable material and is generally used as an ETL layer in solar cell design [12].

With the aim of seeking alternatives, in this paper, we propose to introduce ${\mathrm{W}\mathrm{O}}_3$ to replace CdS as a window layer due to its low toxicity and wide band gap, seeking to reduce the absorption of incident photons in the window layer and thus study the feasibility of a substrate-type solar cell at CdTe base performing a heterojunction with ${\mathrm{W}\mathrm{O}}_3$ as layer n. Figure 1 shows the quantum efficiency (QE) of simulated CdS/CdTe and ${\mathrm{W}\mathrm{O}}_3$/CdTe heterojunctions, where it can be seen that when using ${\mathrm{W}\mathrm{O}}_3$ as a window layer, a QE of practically 100% is obtained at wavelengths of 300–520 nm. When using CdS as a window layer, there is a reduction in the simulated QE due to the band gap of the CdS, decreasing to 90% at wavelengths corresponding to incident photons with energy greater than 2.4 eV. The results obtained in this study reflect that using ${\mathrm{W}\mathrm{O}}_3$ as a window layer reduces parasitic absorption in this layer, obtaining a higher J_sc than when using CdS as a window layer, as reported in the following investigations: 23 $\mathrm{m}\mathrm{A}/{\mathrm{c}\mathrm{m}}^2$ [3], 24.18 $\mathrm{m}\mathrm{A}/{\mathrm{c}\mathrm{m}}^2$ [13], 23.75 $\mathrm{m}\mathrm{A}/{\mathrm{c}\mathrm{m}}^2$ [14], and 25.5 $\mathrm{m}\mathrm{A}/{\mathrm{c}\mathrm{m}}^2$ [15].

In this study, the effect of varying the thickness and carrier concentration of the CdTe layer and ${\mathrm{W}\mathrm{O}}_3$, as well as the effect of defect density at the ${\mathrm{W}\mathrm{O}}_3$/CdTe interface was simulated and analyzed on the device’s photovoltaic performance, and the operating temperature using SCAPS 1-D software. Said analysis was carried out through the main photovoltaic parameters: open circuit voltage (V_oc), short-circuit current density (J_sc), fill factor (FF), and efficiency (PCE) under standard lighting (AM 1.5 G 1000 W/${\mathrm{m}}^2$, 300 K).

2. Device Modeling and Simulation

2.1. Numerical Method

A numerical simulation is a tool that allows us to understand and analyze the influence of different physical parameters on the performance of a solar cell, which allows us to analyze and optimize different structures of solar cells based on crystalline, polycrystalline, and amorphous materials [16], minimizing the cost and time of manufacturing prototypes. SCAPS 1-D, being a software intended for the simulation of semiconductor material properties, allows us to analyze the influence of each layer that makes up the structure of the solar cell to be simulated.

Simulator of the capacitance of solar cells-1 dimension (SCAPS-1D) is a numerical modeling software designed to simulate the DC and AC electrical characteristics of thin-film heterojunction solar cells and has been specially developed for Cu(In,Ga)Se2 and CdTe solar cells [16].

The working principle of SCAPS-1D is based on Poisson’s equations, steady-state electron–hole continuity and the electron and hole current densities [17].

$$\frac{{\partial }^2\psi \left(x\right)}{{\partial x}^2}+\frac{q}{{\epsilon }_r{\epsilon }_0}\left[p\left(x\right)-n\left(x\right)+N_D^{+}\left(x\right)-N_A^{-}\left(x\right)+p_t\left(x\right)-n_t\left(x\right)\right]=0$$

(1)

where $\psi$ is the electrostatic potential; $N_D^{+}$ is the ionized donor concentration; $N_A^{-}$ is the ionized acceptor density; $n$ and $p$ are, respectively, hole and electron density; ${\epsilon }_r$ and ${\epsilon }_0$ are, respectively, relative and vacuum permittivity; $p_t$ and $n_t$ represent the holes and electrons trapped, respectively; $q$ is the electron charge; and $x$ is the position in the x-coordinate.

$$-\frac{\partial J_n}{\partial x}+G-R=0$$

(2)

$$-\frac{\partial J_p}{\partial x}+G-R=0$$

(3)

G is the carrier generation rate, R is the net recombination from direct and indirect band, and $J_p$ and $J_n$ are, respectively, hole and electron current densities

$$J_n=qn{\mu }_{n}E+q{D}_n\frac{\partial n}{dx}$$

(4)

$$J_p=qp{\mu }_{p}E-q{D}_p\frac{\partial p}{dx}$$

(5)

$D_p$ and $D_n$ are, respectively, hole and electron diffusion coefficients; $E$ is the electric field; and ${\mu }_p$ and ${\mu }_n$ are, respectively, hole and electron mobilities. In this research work, software version 3.3.10 was used.

2.2. Device Structure and Simulation Parameters

Figure 2 shows the structure of the simulated device, at the top of the aluminum front contact, with AZO as transparent conductive oxide (TCO), the n-type layer or window layer of ${\mathrm{W}\mathrm{O}}_3$, the CdTe absorber layer, a hole transporter layer (HTL) of NiO, and Ni as back contact at the bottom of the device. Due to the high work function of CdTe ($~$5.5 eV) [2], a metal with a higher work function than CdTe is required to achieve an ohmic contact, so NiO was incorporated as an HTL layer between the CdTe absorber layer and the Ni back contact. The high concentration of acceptor carriers and work function ($~$5.2 eV) of NiO help to extract the carrier from the back contact, improving the quality of the ohmic contact.

Figure 3a shows the energy band diagram of the simulated structure using ${\mathrm{W}\mathrm{O}}_3$ as a window layer the difference between the conduction and valence bands between the CdTe and ${\mathrm{W}\mathrm{O}}_3$ layers are $∆E_c=$ 0.27 and $∆E_v=$ 1.93 eV, respectively. The thickness of the depletion zone (W) is 0.731 µm. The power barrier V_bi is 1.03 V. The Fermi level for CdTe and ${\mathrm{W}\mathrm{O}}_3$ with respect to the valence band is 0.195 and 3.14 eV, respectively. The work function of CdTe is 5.5 eV, while the work function of ${\mathrm{W}\mathrm{O}}_3$ is 4.5 eV. Figure 3b shows the band diagram using CdS as a window layer, where we have an $∆E_c=$ 0.38 eV, $∆E_v=$ 0.55 eV, V_bi = 1.32 V, and W = 0.817 µm. It can also be seen that using ${\mathrm{W}\mathrm{O}}_3$ as a window layer does not generate a peak in the conduction band because ${\mathrm{W}\mathrm{O}}_3$ has a higher electron affinity than CdTe, favoring transport by diffusion. On the other hand, when using CdS as a window layer, a peak is generated in the conduction band, which limits the transport of electrons by diffusion from the n region to the p region, having as the dominant transport mechanism, potential barrier overcoming, tunneling and recombination at the interface. The work function of CdS is ~4 eV, so to have an ohmic contact with this material, a metal with a work function of less than 4 eV is needed. However, given the work function of ${\mathrm{W}\mathrm{O}}_3$, it allows us to obtain an ohmic contact when using metals like Al and Ag as front contacts.

To carry out the simulation process, the SCAPS-1D software requires the input of physical parameters of the layers that make up the device to be simulated (Figure 2), which are described in

Table 1. Parameters used in the simulation of the structure Al/AZO/${\mathrm{W}\mathrm{O}}_3/$CdTe/NiO/Ni.

Parameters	CdTe [1]	${\mathbf{W}\mathbf{O}}_3$ [12,18,19]	NiO [1,20]	AZO [20,21]
Thickness (µm)	0.5–3	0.01–0.15	0.020	0.050
Bandgap, ${\mathit{E}}_{\mathit{g}}$ (eV)	1.49	3.15	3.8	3.3
Electron Affinity, $\mathit{x}$ (eV)	4.28	4.55	1.46	4.55 8.12
Relative permittivity, ${\mathit{\epsilon }}_{\mathit{r}}$	9.4	10	10
Effective CB density of states, ${\mathit{N}}_{\mathit{c}}$ ${(\mathbf{c}\mathbf{m}}^{\mathbf{-}\mathbf{3}})$	${8\times 10}^{17}$	${4.2\times 10}^{18}$	${2.8\times 10}^{19}$	${4.1\times 10}^{18}$
Effective VB density of states, ${\mathit{N}}_{\mathit{v}}$ $\left({\mathbf{c}\mathbf{m}}^{\mathbf{-}\mathbf{3}}\right)$	${1.8\times 10}^{19}$	${9\times 10}^{18}$	${1\times 10}^{19}$	$8.2{\times 10}^{18}$
Electron mobility, ${\mathit{\mu }}_{\mathit{n}}$ ${(\mathbf{c}\mathbf{m}}^{\mathbf{2}}/\mathbf{V}\mathbf{s})$	500	20	12	100
Hole mobility, ${\mathit{\mu }}_{\mathit{p}}$ ${(\mathbf{c}\mathbf{m}}^{\mathbf{2}}/\mathbf{V}\mathbf{s})$	60	10	2.8	20
Electron Thermal Velocity ($\mathbf{c}\mathbf{m}/\mathbf{s}$)	${10}^7$	${10}^7$	${10}^7$	$2.2\times {10}^7$
Hole Thermal Velocity ($\mathbf{c}\mathbf{m}/\mathbf{s}$)	${10}^7$	${10}^7$	${10}^7$	${1.5\times 10}^7$
Donor concentration, ${\mathit{N}}_{\mathit{D}}$ ${(\mathbf{c}\mathbf{m}}^{\mathbf{-}\mathbf{3}})$	0	${10}^{13}–{10}^{19}$	0	${10}^{21}$
Acceptor concentration, ${\mathit{N}}_{\mathit{A}}$ $\left({\mathbf{c}\mathbf{m}}^{\mathbf{-}\mathbf{3}}\right)$	${10}^{13}–{10}^{17}$	0	${10}^{21}$	0
Defect density, ${\mathit{N}}_{\mathit{t}}$ ($\mathbf{1}/{\mathbf{c}\mathbf{m}}^{\mathbf{3}})$ Defect type	${10}^{15}$ acceptor	${10}^{12}$ donor	${10}^{14}$ acceptor	$5\times {10}^{14}$ acceptor

. SCAPS-1D also allows the electrical properties of the back and front contacts to be incorporated into the simulation process, as well as the defect density ($N_t$) of the ${\mathrm{W}\mathrm{O}}_3$/CdTe interface (

Table 2. Simulation Parameters for Interface Defects and Contacts.

Interface Defect Density [12,19]	-
${\mathrm{W}\mathrm{O}}_3$/CdTe	${10}^{10}–{10}^{16}{\mathrm{c}\mathrm{m}}^{-2}$
Defect type	Acceptor
Capture cross-section electrons/holes	${10}^{-15}{\mathrm{c}\mathrm{m}}^{-2}$
Back Contact Electrical Properties [22,23]	-
Work function of Ni	5.15 eV
Surface recombination velocity of electrons	${10}^5$ cm/s
Surface recombination velocity of holes	${10}^7$ cm/s
Front Contact Electrical Properties [22,23]	-
Work function of Al	4.2 eV
Surface recombination velocity of electrons	${10}^7$ cm/s
Surface recombination velocity of holes	${10}^5$ cm/s

), which are generated by the network mismatch between both layers, promoting free links that favor the appearance of recombination centers [18].

Regarding the working conditions of the simulation, it was performed with AM 1.5 G standard illumination and a temperature of 300 K, except in the section where these parameters are discussed. A default mesh of 51 points with a step size of 0.02 V was used for the I-V analysis since the SCAPS-1D algorithm is designed in such a way that it provides a larger number of points in the mesh in regions where the materials properties experience a greater variation (near interfaces/contacts), but in areas where the properties are expected to remain practically constant (in the bulk), the algorithm provides fewer points for the discretization of the mesh [17]. SCAPS-1D allows an increase or decrease in the number of points to optimize the mesh; however, in this investigation, we work with the number of points in the mesh predetermined by the software because when increasing or decreasing the number of points in the mesh, there were practically no changes in the results obtained.

3. Results and Discussion

3.1. Effect of the Thickness and Concentration of Carriers ($N_D$) of the Window Layer ${WO}_3$

To analyze the effect of the thickness and concentration of carriers $(N_D$) of the window layer ${\mathrm{W}\mathrm{O}}_3$ on the performance of the devices, these were varied from 10 to 150 nm and from ${10}^{13}$ to ${10}^{19}$ ${\mathrm{c}\mathrm{m}}^{-3}$, respectively. The other parameters were kept constant during the simulation.

In Figure 4a, it can be seen that J_sc decreases from 28.45 to 28.33 $\mathrm{m}\mathrm{A}/{\mathrm{c}\mathrm{m}}^2$ when the thickness of the window layer increases from 10 to 150 nm, since having a greater thickness in this layer favors a greater absorption of the incident photons, transmitting fewer photons to the absorbing CdTe layer and thus decreasing J_sc. V_oc remains practically constant (Figure 4a) as the thickness of the window layer increases since the carrier diffusion length allows the carriers separated by the electric field generated by the pn junction to reach frontal contact, thus maintaining the separation of charges. FF has a small decrease when the window layer’s thickness increases, going from 81.95 to 81.13%. Since J_sc and FF decrease, PCE also decreases (Equation (6)) with increasing window layer thickness, decreasing from 19.87 to 18.97% (Figure 4a).

$$PCE=\frac{J_{sc}{V}_{oc}FF}{P_{in}}$$

(6)

Figure 4b shows the behavior of V_oc, which increases from 0.73 to 0.85 V by increasing $N_D$ from ${10}^{13}$ to ${10}^{19}{\mathrm{c}\mathrm{m}}^{-3}$ because the depletion zone increases on the side of the absorber layer by increasing $N_D$, which favors a greater generation and separation of photogenerated carriers. J_sc increases from 28.03 to 28.46 $\mathrm{m}\mathrm{A}/{\mathrm{c}\mathrm{m}}^2$ when $N_D$ increases from ${10}^{13}$ to ${10}^{19}$ ${\mathrm{c}\mathrm{m}}^{-3}$ (Figure 4b). This is also due to the increase in the depletion zone in the absorber layer as $N_D$ increases, favoring an increase in the number of photogenerated carriers collected by the contacts. FF increases from 78.36 to 81.95% (Figure 4b) when $N_D$ increases from ${10}^{13}$ to ${10}^{19}$ ${\mathrm{c}\mathrm{m}}^{-3}$. This is due (Equation (7)) to the fact that the maximum cell voltage (V_m) increases from 0.64 to 0.75 V generating that the maximum power ($P_m$) of the simulated device increases with the increase in $N_D$. In the same figure, the PCE increases as $N_D$ increases, going from 16.20 to 19.87%, since the photogenerated electrons in the absorber layer will have a higher probability of passing through the junction and being collected by the frontal contact.

$$FF=\frac{P_m}{V_{oc}{J}_{sc}}$$

(7)

3.2. Effect of the Thickness and Acceptor Carrier Concentration ($N_A$) in CdTe Absorber Layer

In this section, the effect of the thickness and carrier concentration ($N_A$) in the CdTe absorber layer was studied, for which the thickness of the CdTe layer was varied from 0.5 to 3 µm, while the carrier´s concentration was varied from ${10}^{13}$ to ${10}^{17}$ ${\mathrm{c}\mathrm{m}}^{-3}$. The other parameters were kept constant during the simulation.

In Figure 5a, it can be seen that J_sc has a value of 25.80 $\mathrm{m}\mathrm{A}/{\mathrm{c}\mathrm{m}}^2$ at a thickness of 0.5 µm, reaching a maximum value of 2$8.45\mathrm{m}\mathrm{A}/{\mathrm{c}\mathrm{m}}^2$ from 2 µm, since having a thicker film has a higher absorption [2] because the probability that the photons are absorbed is higher due to a longer optical path. The behavior of V_oc remains practically constant when varying the thickness of the absorbent layer, having a negligible decrease in V_oc of 0.01 V as the thickness increases from 0.5 to 1 µm. This behavior can be explained by the fact that (Equation (8)) J_sc remains constant from 1.5 µm, and the saturation current density ($J_0$) is not affected by the increase in the thickness of the absorber layer. Since FF is inversely proportional (Equation (7)) to J_sc and V_oc, and since there is a greater increase in J_sc compared to the decrease in V_oc when increasing the thickness of the absorber layer, FF decreases from 85.78 to 81.97% when increasing the thickness of the absorbent layer from 0.5 to 3 µm (Figure 5a). PCE increases from 19.02 to 19.87% when the thickness of the absorber layer increases since there is a greater generation of electron–hole pairs given by a greater absorption capacity by having a thicker CdTe layer.

$$V_{oc}=\frac{kT}{q}ln\left(\frac{J_{sc}}{J_0}+1\right)$$

(8)

Figure 5b shows the behavior of J_sc as a function of the concentration of $N_A$ carriers in the absorber layer, which increases from 21.38 to 28.45 $\mathrm{m}\mathrm{A}/{\mathrm{c}\mathrm{m}}^2$ as $N_A$ increases from ${10}^{13}$ to ${10}^{15}$ ${\mathrm{c}\mathrm{m}}^{-3}$ since having a higher concentration of carriers improves the conductivity of the material, thus favoring a better extraction of the carrier from the back contact; however, J_sc decreases when exceeding a $N_A$ of ${10}^{15}$ ${\mathrm{c}\mathrm{m}}^{-3}$, decreasing its value to 24.57 $\mathrm{m}\mathrm{A}/{\mathrm{c}\mathrm{m}}^2$ with a $N_A$ of ${10}^{17}$ ${\mathrm{c}\mathrm{m}}^{-3}$, since the fermi level of the holes on the p side decreases, thus increasing the power barrier V_bi making it difficult for carriers to flow through the junction. V_oc decreases from 1.12 to 0.93 V when $N_A$ increases from ${10}^{13}$ to ${10}^{17}$ ${\mathrm{c}\mathrm{m}}^{-3}$. This is because the mobility of the carriers decreases when $N_A$ increases [24], which reduces the diffusion length of the carrier ($L_n$) and the equilibrium electron density on the p side ($n_{p0}$) thus increasing an increase in the saturation current $J_0$ (Equation (9)).

$$J_0=\frac{q{D}_p{p}_{n0}}{L_p}+\frac{q{D}_n{n}_{p0}}{L_n}$$

(9)

FF increases from 33.97 to 82.03% (Figure 5b) by increasing $N_A$ from ${10}^{13}$ to ${10}^{15}$ ${\mathrm{c}\mathrm{m}}^{-3}$ mainly due to the behavior of V_oc (Equation (7)); however, by increasing $N_A$ to ${10}^{17}$ ${\mathrm{c}\mathrm{m}}^{-3}$, FF decreases to 76.74% mainly due to the maximum current density of the device (J_m) decreasing from 26.44 to 21.68 $\mathrm{m}\mathrm{A}/{\mathrm{c}\mathrm{m}}^2$, causing P_m to decrease, for which FF decreases (Equation (7)). PCE has its maximum value of 19.87% at a $N_A$ of ${10}^{15}$ ${\mathrm{c}\mathrm{m}}^{-3}$ due to the increase in J_sc and FF; however, PCE decreases when $N_A$ exceeds ${10}^{15}$ ${\mathrm{c}\mathrm{m}}^{-3}$. This is due to the decrease in J_sc and FF.

3.3. Effect of Defect Density ($N_t$) at the ${WO}_3/CdTe$ Interface

To understand the effect of the density of states of the ${\mathrm{W}\mathrm{O}}_3/CdTe$ interface, it was varied from ${10}^{10}$ to ${10}^{16}{\mathrm{c}\mathrm{m}}^{-2}$, the other parameters were kept constant during the simulation. Figure 6 shows that the performance of the solar cell is negatively affected by a higher defect density in the interface. V_oc is the one that has a greater decrease when increasing the density of defects in the interface, decreasing from 0.85 to 0.60 V, since having a greater density of defects has a greater number of recombination centers [18], which generates that the probability that the photogenerated carriers are separated is lower. J_sc has a decrease of 0.26 $\mathrm{m}\mathrm{A}/{\mathrm{c}\mathrm{m}}^2$ with increasing defect density from ${10}^{10}$ to ${10}^{16}{\mathrm{c}\mathrm{m}}^{-2}$. By increasing $N_t$, there is a greater probability that the photogenerated carriers recombine at the junction interface, causing V_m to decrease from 0.75 to 0.51 V, therefore the maximum power (P_m) of the cell decreases and, consequently FF decreases (Equation (7)) from 82.03 to 77.93% even if J_sc and V_oc decrease. PCE decreases from 19.87 to 13.27% as $N_t$ increases, mainly due to the decrease in V_oc.

3.4. Effect of Operating Temperature

To study the behavior of the solar cell depending on the temperature to which it will be subjected. This was varied from 270 to 370 K since the solar cells in their applications can be exposed to this temperature range outdoors. The other parameters analyzed were kept constant at their optimized values.

Being more specific, V_oc decreases by 0.15 V when the operating temperature increases by 100 K, as can be seen in Figure 7d. This behavior may be due to the fact that, as the temperature increases the density of carriers increases, which reduces their mobility as observed in Equation (8), thus increasing $J_0$ and consequently decreasing V_oc (Equation (8)). J_sc has a practically constant behavior when increasing the temperature from 270 to 370 K (Figure 7c). Figure 7b shows that FF has a small decrease of 0.73% when the operating temperature increases by 100 K, due to the behavior of V_oc and to the fact that Vm decreases from 0.77 to 0.62 V with increasing temperature. In Figure 7a, it can be seen that PCE decreases from 20.27 to 16.82% when the temperature increases by 100 K, given that when the temperature of a material is increased, its conductivity is affected and that phonons are generated [23], which cause scattering in charge carriers thus affecting PCE.

4. Conclusions

In this paper, we simulate and analyze the photovoltaic performance of a CdTe-based solar cell using ${\mathrm{W}\mathrm{O}}_3$ as a window layer, with a device structure of Al/AZO/${\mathrm{W}\mathrm{O}}_3$/CdTe/NiO/Ni, using the SCAPS 1-D software. It was found that the concentration of $N_A$ carriers in the absorber layer plays an important role in the solar cell´s performance, obtaining the maximum PCE at a $N_A$ of ${10}^{15}{\mathrm{c}\mathrm{m}}^{-3}$. It was also observed that the photovoltaic performance of the simulated solar cell has an almost constant behavior from 1.5 µm thick on the absorber layer. As the thickness of the window layer increases, PCE decreases; however, it is not such a determining factor for the photovoltaic performance of the cell. On the other hand, the concentration of $N_D$ carriers of the window layer is a very determining factor, obtaining a maximum PCE at a $N_D$ of ${10}^{19}{\mathrm{c}\mathrm{m}}^{-3}$. The increase in the density of $N_t$ defects at the ${\mathrm{W}\mathrm{O}}_3$/CdTe interface negatively affects PCE, decreasing by 6.60% when increasing $N_t$ from ${10}^{10}$ to ${10}^{16}{\mathrm{c}\mathrm{m}}^{-2}$. The solar cell´s performance is impaired with the increase in operating temperature, since at lower temperatures, a higher efficiency is obtained, such that at 270 K, a PCE of 20.27% is obtained. PCE decreases by 3.45% as the operating temperature increases until 370 k, having an acceptable performance at extreme temperatures. The other sections were simulated at standard temperatures of 300 K.

A maximum PCE of 19.87% was obtained with a V_oc = 0.85 V, J_sc = $28.45\mathrm{m}\mathrm{A}/{\mathrm{c}\mathrm{m}}^2$, and FF = 82.03%, achieved with a thickness and carrier concentration of 2 µm and ${10}^{15}$ ${\mathrm{c}\mathrm{m}}^{-3}$, respectively, for the absorber layer and the window layer a thickness of 10 nm and a carrier concentration of ${10}^{19}$ ${\mathrm{c}\mathrm{m}}^{-3}$. CdS remains theoretically and experimentally one of the best options to be used as a window layer in heterojunction with CdTe; however, based on the results obtained in this research, ${\mathrm{W}\mathrm{O}}_3$ has the potential to become a viable alternative as a window layer for the development of CdTe-based solar cells, since it can favor the increase in J_sc due to its wide band gap, in addition to being a non-toxic and low-cost material.