Experimental and numerical simulation studies of CuO thin films based solar cells

Spray pyrolysis was used to prepare CuO thin films, (CuCl₂, H₂O) as a precursor, it was dissolved in 15 ml, 20 ml and 25 ml of distilled water. Just before deposition, ethanol and distilled water were applied to the glass substrates to clean them of any impurities or contaminants. The concentration solution is 0.2 M and it was stirring for 15 min, that one was manually applied to the 300 °C preheated glass substrate using a scented atomizer. Besides that, Thermal treatment was applied to the resulting sprayed films for 1 h at 300 °C Then, after this reaction, the creation of the CuO thin films begins [10]:

$$\begin{eqnarray}&&{{\rm{CuCl}}}_{2({\rm{s}})}+{{\rm{H}}}_{2}{\rm{O}}\to {{\rm{CuO}}}_{({\rm{s}})}+{{\rm{HCl}}}_{({\rm{g}})}\end{eqnarray} \tag{ 1 }$$

All other parameters were maintained, and we merely changed the volume of the sprayed solution in order to change the thickness of CuO thin films.

Over the past few decades, the simulation tool or software has been increasingly important and necessary, especially in the field of photovoltaics. Numerical simulation software for solar cells has proved extremely helpful understanding and predicting in the behavior of devices by exposing behind fundamental principles. To minimize the amount of experiments while creating the optimum solar cell layout, several researchers use simulation. Additionally, real instruments like solar cell technology can be designed, developed, and improved using these simulations. The impact of changing each layer's physical characteristics, such as thickness, bandgap, electron affinity, and dielectric permittivity, on solar cells are being studied. This paper present a numerical study of photovoltaic solar cell based on CuO thin film using SCAPS-1D software (Solar cell capacitance simulator) [21]. SCAPS-1D is a program created by Marc Burgelman at the University of Gent's department of electronics and information systems [22] to simulate the proposed solar cell. SCAPS-1D resolves the basic semiconductors equations, for example the poisson equation (2) and hole and electron continuity equations [23].

$$\begin{eqnarray}&&\frac{{d}^{2}}{d{x}^{2}}{\rm{\Psi }}=\frac{{\rm{e}}}{{\varepsilon }_{0}{\varepsilon }_{{\rm{r}}}}\left({\rm{p}}\left({\rm{x}}\right)-{\rm{n}}\left({\rm{x}}\right)+{{\rm{N}}}_{{\rm{D}}}-{{\rm{N}}}_{{\rm{A}}}+{\rho }_{{\rm{P}}}+{\rho }_{{\rm{n}}}\right)\end{eqnarray} \tag{ 2 }$$

$$\begin{eqnarray}&&\frac{d{j}_{p}}{{dx}}=G-R\end{eqnarray} \tag{ 3 }$$

$$\begin{eqnarray}&&\frac{d{j}_{n}}{{dx}}=G-R\end{eqnarray} \tag{ 4 }$$

Where:

N_A and N_D are the charge impurities of acceptor and donor type, Ψ is electrostatic potential, e is the electrical charge, p and n are the hole and electron concentrations, ${{\rm{\unicode{x02107}}}}_{0}$ and ${\unicode{x02107}}_{r}$ are the vacuum and relative permittivity ${\rho }_{p}$ is holes distribution and ${\rho }_{n}$ is electrons distribution.

The fill factor and the efficiency (PCE) are showing in the following equations.

$$\begin{eqnarray}&&{FF}=\frac{{I}_{m}{V}_{m}}{{J}_{{sc}}{V}_{{oc}}}\end{eqnarray} \tag{ 5 }$$

$$\begin{eqnarray}&&{PCE}=\frac{{I}_{m}{V}_{m}}{{P}_{{in}}}\end{eqnarray} \tag{ 6 }$$

With:

I_m: Photocurrent

V_m : the highest possible photovoltaic power output.

J_sc : Short circuit photocurrent,

V_oc : Open circuit voltage.

Figure 1 shows the x-ray diffractions of CuO thin films deposited by spray pyrolysis methods and deposited with different thickness. As a result of having multiple peaks, the films were polycrystalline with monoclinic crystal structure (ICDD card no 00-005-0254). The peaks intensity clearly rise as the film thickness increases. Moreover, CuO films preferred diffractions were seen at 2θ = 35.4° and 2θ = 38.7°, corresponding to the (0 0 2) and (1 1 1) plans, respectively. In addition, because of the high annealing temperature or the present of impurities, secondary phases are believed to be responsible for the appearance of additional peaks at 2θ = 31.75°.

Zoom In Zoom Out Reset image size

Zoom In Zoom Out Reset image size

The following equations were utilized to calculate the microstructural parameters [24] using the Debye–Scherrer's formula to determine crystallite size (D) (7), dislocation density (δ), The crystalline number (N), and microstrain (ε) which appears in table 1.

$$\begin{eqnarray}&&D=\frac{{\rm{k}}\lambda }{\beta \cos \theta }\end{eqnarray} \tag{ 7 }$$

$$\begin{eqnarray}&&\delta =\frac{1}{{{\rm{D}}}^{2}}\end{eqnarray} \tag{ 8 }$$

$$\begin{eqnarray}&&N=\frac{{\rm{t}}}{{{\rm{D}}}^{3}}\end{eqnarray} \tag{ 9 }$$

$$\begin{eqnarray}&&\varepsilon =\frac{\beta \cos \theta }{4}\end{eqnarray} \tag{ 10 }$$

With

K: is a constant that typically equals 0.9.

t: the thin film's thickness

λ : X-ray wavelength (λ_Cu-Kα1 = 0.154056 nm)

β : the diffraction peaks' full-width at half-maximum (FWHM),

θ : the diffraction angle.

The crystallite size was found to be higher for t = 234.1 nm (figure 2) which caused a reduction in dislocation density, microstrain and the crystalline number. We deduce that the optimal thickness for obtaining CuO thin films with good crystallographic quality is t = 234.1 nm. These findings provide strong support for earlier research[25, 26].

The surface morphology of CuO thin films produced by spray pyrolysis technique is shown in figure 3. As demonstrates that the sprayed layer has a dense and consistent morphology, and the nanoparticles are evenly spaced across the glass substrate. The cross-section photographs (table 2) were used to measure the thickness of the CuO thin films.

Zoom In Zoom Out Reset image size

3.3.1. Transmittance / absorbance and reflectance spectra

The optical characteristics of spray pyrolysis-deposited CuO thin films were characterized by UV-visible spectrophotometer. The transmittance spectra in the 300–1000 nm wavelength range are depicted in figure 4. It is clear that the transmittance spectra are influenced by the film thickness. Indeed, the films present low transmittance in visible region never exceeds 10%. The film deposited with t = 234.1 nm present the highest transmittance about 35% near infrared region.

Zoom In Zoom Out Reset image size

Figure 5 shows absorbance spectra as a function of Wavelength and photon energy. As illustrated in figure 5(a) with high absorption at wavelength range 300–600 nm and these high values of absorbance are corresponding to very low transmittance values. This result could be due to the electronic transition between the valence band and the conduction band [26]. Contrarily, though, at 400–600 nm wavelength range, the optical absorbance grows as the thickness of the film, which can be linked to the changes in morphological properties and thickness growth [3]. These results are similar to those of CuO deposited using spray pyrolysis [2] and other methods [9]. Figure 6 shows the optical reflectivity calculated by the equation (11) of CuO thin films [27]:

$$\begin{eqnarray}&&R=1-{T}^{\frac{1}{2}}{e}^{-\frac{\alpha d}{2}}\end{eqnarray} \tag{ 11 }$$

All of the copper oxide thin films' reflectance spectra exhibit high reflectance in the 300–700 nm wavelength region. CuO thin films, on the other hand, exhibit significant changes in reflectance values around the infrared spectrum with t = 234.1 nm. This is due to the numerous absorption and reflectance induced by the various CuO thin films, which is morphology- and roughness-dependent.

Zoom In Zoom Out Reset image size

Zoom In Zoom Out Reset image size

3.3.2. Optical band gap energy

The band gap energy (Eg) of copper oxide thin films was determined by Tauc's relation [28].

$$\begin{eqnarray}&&{(\alpha h\nu )}^{2}=A\left(h\nu -{E}_{g}\right)\,\end{eqnarray} \tag{ 12 }$$

Eg is the optical band gap and A is a characteristic parameter independent of photon energy. Figure 7 shows (αhυ)² = f(hυ) for the determination of the direct band gap of the CuO semiconductor by extrapolating the linear part of the graph. As illustrated, the band gap energy of film deposited by different thickness to be found 1.14 eV, 1.61 eV and 1.53 eV for t = 175.5 nm, t = 234.1 nm and t = 292 nm respectively. This results, may be explained by the morphological properties and the interband transition because of the film thickness. The suitable value of band gap energy is 1.53 eV this is not far from the absorber layer's 1.5 eV optimum for solar conversion [29] and seen at t = 292 nm.

Zoom In Zoom Out Reset image size

The electrical characteristics of thin CuO films created via spray pyrolysis techniques were evaluated by the four-point probe. A current has been passed through the two outer probes and the voltage will have reported by the inner probes. The formula below can be used to calculate electrical square resistance Rs [23]:

$$\begin{eqnarray}&&{R}_{\square }=\frac{\pi }{\mathrm{ln}\left(2\right)}\left(\frac{{\rm{V}}}{{\rm{I}}}\right)\end{eqnarray} \tag{ 13 }$$

The instrument measures the Rs square resistance. The calculation of electrical resistivity was performed via the formula shown below [30]:

$$\begin{eqnarray}&&\rho =d* R{\rm{\square }}\end{eqnarray} \tag{ 14 }$$

d is the film's thickness and the following equation has been used to calculate conductivity:

$$\begin{eqnarray}&&\sigma =\frac{1}{\rho }\end{eqnarray} \tag{ 15 }$$

The results are presented in table 3. It is clear that, film deposited by film thickness t = 292 nm has the high conductivity with low resistivity.

The figure of merit is often used as a performance indicator for conducting films. Two critical metrics used to assess the quality of transparent conducting oxides are optical transmittance and electrical conductivity [31]. The figure of merit (F) correlate these two parameters by the following formula [32]:

$$\begin{eqnarray}&&F=\frac{-1}{\rho \mathrm{ln}{\rm{}}(T)}\end{eqnarray} \tag{ 16 }$$

ρ: the electrical resistivity

T : the typical transmittance in the 600–1000 nm wavelength region.

The figures of merit were determined to be 1.9 × 10⁻³ Ω⁻¹cm⁻¹, 3.5419 × 10⁻³ Ω⁻¹cm⁻¹, and 2.4 × 10⁻³ Ω⁻¹cm⁻¹, respectively, for the CuO thin films of 175.5 nm, 234.1 nm, and 292 nm. These findings concur with those made by Cho et al [11]. According to the experimental results (figure 8), depositing high-quality CuO films requires a growth thickness of 234.1 nm and that maybe related to the high transmittance.

Zoom In Zoom Out Reset image size

The present work uses SCAPS- 1D to simulate CuO/TiO₂ /FTO solar cell as shown in figure 9, which n-type window layer is of TiO₂, p-type absorber layer is of CuO, FTO is used as a front contact and Au is a back contact. The aim of this work is twofold: first, to identify the optimal bandgap energy values based on experimental results, and second, to expand the thickness range of the CuO absorber layer. This will be achieved through the use of SCAPS-1D. Layer thickness of TiO₂ was acquired from the literature [18]. Table 4 lists the material parameters used in the simulation were selected from earlier studies [17, 33, 34]. The back and front parameters of solar cells are listed in table 5. All the simulation experiments were obtained under 300 K of the surrounding air temperature, AM1,5 G of the solar spectrum with a light power of 1000 W m⁻² . Figure 10 compares the performance of CuO/TiO₂/FTO solar cell to depict the J-V characteristic. Table 6 displays the four solar cell characteristics Voc, Jsc, FF, and PCE using the values of the gap energy and the thickness of the absorbing layer that were determined from the results of the experiment. The CuO absorber layer with an energy gap value of 1.53 eV and thickness of 292 nm produces the highest efficiency of 13.38%, Voc = 0.94 V, Jsc = 17.31 cmA cm⁻² and FF = 81.47%. These findings could potentially be associated with the band gap energy of the CuO absorber layer, which is 1.53 eV, a value in close proximity to the optimal 1.5 eV for solar conversion [29], this energy value would provide a good balance between optical absorption and electrical conductivity for CuO-based solar cells.

Zoom In Zoom Out Reset image size

Zoom In Zoom Out Reset image size

3.5.1. Impact of absorber layer thickness

Layer absorber thickness has a direct impact on photovoltaic solar cell performance. Figure 11 provides a visual representation of the investigation into the cell's performance. This examination involved the variation of the CuO absorber layer's thickness, ranging from 0.1 μm to 1 μm, with the band gap energy set at 1.53 eV, while holding the remaining parameters constant.

Zoom In Zoom Out Reset image size

Figure 11 demonstrates that Voc, Jsc, FF and PCE rise with the thickness of the absorbent layer, which can be explained by the significant absorption of long-wavelength photons that boost photogeneration [35, 36]. However, the highest value is observed at 1 μm.

Figure 12 displays the quantum efficiency (QE) as a function of wavelength, where it is defined as the ratio of carriers collected by the solar cell to incident photons with a particular energy.

Zoom In Zoom Out Reset image size

How well a material transforms photons into an electrical current or charge is known as its quantum efficiency. A CuO absorber layer's quantum efficiency may peak at a wavelength around 370 nm, which could mean the material excels in converting photons of that particular wavelength into an electrical current or charge. As it can have seen the cell's reaction to incident sunlight becomes more favorable when the absorber layer thickness increases. Furthermore, the most favorable response is evident within the wavelength range of 400–800 nm, with absorption tapering off beyond 870 nm. The optimal value for absorber layer thickness is 1 μm.

3.5.2. Influence of temperature on solar cell performance

Figure 13 shows the temperature effect on n-TiO₂/p-CuO solar cell, Voc and Jsc are decreased when the temperature value increases. Moreover, the fill factor and efficiency indicate high values at low temperature. The high values of FF and PCE found to be 85.05% and 19.24% corresponding at 295 K. This drop of efficiency as temperature rises is due to an increase in the rate of electron and hole recombination [15]. According to this analysis, the proposed cell is more suitable for high efficiency in low-temperature parts of the world.

Zoom In Zoom Out Reset image size