Abstract

In this research work, a copper bismuth oxide- (CuBi2O4-) based thin-film solar cell has been proposed for the lead and toxic-free (Al/ITO/TiO2/CuBi2O4/Mo) structure simulated in SCAPS-1D software. The main aim of this work to make an ecofriendly and highly efficient thin-film solar cell. The absorber layer CuBi2O4, buffer layer TiO2, and the electron transport layer (ETL) ITO have been used in this simulation. The performance of the suggested photovoltaic devices was quantitatively evaluated using variations in thickness such as absorber, buffer, defect density, operating temperature, back contact work function, series, shunt resistances, acceptor density, and donor density. The absorber layer thickness is fixed at 2.0 μm, the buffer layer at 0.05 μm, and the electron transport layer at 0.23 μm, respectively. The CuBi2O4 absorber layer produces a solar cell efficiency of 31.21%, an open-circuit voltage () of 1.36 V, short-circuit current density () of 25.81 mA/cm2, and a fill factor (FF) of 88.77%, respectively. It is recommended that the proposed CuBi2O4-based structure can be used as a potential for thin-film solar cells that are both inexpensive and highly efficient.

1. Introduction

The rising depletion of fossil fuels and their detrimental influence on the environment create concerns about the improvement of clean and renewable energy supplies [13]. Solar energy is a practical alternative energy source given the declining use of fossil fuels. Due to their comparatively cheap production costs, high absorption coefficient, low surface recombination rates, and comparatively high efficiency, thin-film solar cells witnessed a major boost in their power conversion efficiency (PCE) a little over ten years ago [4, 5]. A thin-film solar cell is a second-generation solar cell that is made by depositing one or more thin layers, or thin film (TF) of photovoltaic material on a substrate, such as glass, plastic, or metal. Thin-film solar cells are commercially used in several technologies, including cadmium telluride (CdTe), copper indium gallium diselenide (CIGS), and amorphous thin-film silicon (a-Si, TF-Si) [68]. On the other hand, cadmium (Cd) has frequently been used as an absorber layer material [912]. But it is also unfit for the environment. To overcome these drawbacks, a CuBi2O4-based thin-film solar cell has been proposed in this study. CuBi2O4 is used as an absorber layer material in a few studies with traditional efficiencies. Another reason for selecting this structure is that it is environmentally friendly. Bismuth (CuBi2O4) metal has low toxicity to humans and poses minimum threat to the environment. Bismuth compounds generally have very low solubility, but they should be handled with care, as there is only limited information on their effects and fate in the environment [13]. Copper-based ternary oxide semiconductors with tiny energy bandgaps have lately drawn a lot of attention as a photocathode material for solar energy conversion [14]. In Cu, a chemical element with the atomic number 29, group 11, period 4, and a very high thermal and electrical conductivity, is present in CuBi2O4 materials [15]. The absorber layer CuBi2O4 is one of the most promising photocathode materials for solar cell technology because of its low cost and ease of manufacturing [14, 16, 17]. The CuBi2O4 possesses properties that are close to the optimal standard for solar cell application nontoxicity and abundant resource [1820]. One benefit of CuBi2O4 is its tiny energy bandgap, which varies from 1.4 to 1.8 eV. Previous CuBi2O4-based work structures Al/FTO/CdS/CuBi2O4/Ni, ITO/SnS/CBO/Au, and ITO/WS2/CBO/Au are fabricated by SCAPS-1D software. Their efficiencies are 22.84%, 26%, and 27.73%, respectively [21]. In this study, the structure Al/ITO/TiO2/CuBi2O4/Mo is designed for the first time, and we obtain the efficiency 31.2%. TiO2 is used as the buffer layer. Titanium dioxide (TiO2), a wide-bandgap semiconductor with a high refractive index, has a tremendous potential for use in electronics and optoelectronics. This is because of its transparency in the visible spectral range. TiO2 is renowned for its wide range of applications and bandgap of more than 3 eV. However, the pure bandgap of TiO2 is 3.2 eV. Titanium dioxide (TiO2) is a natural oxide of the element titanium with low toxicity and negligible biological effects. This is why TiO2 is used here instead of CdS [22]. Several hole-transport layer (HTL) materials with a thin-film absorber layer were used in earlier investigations. In addition to improve the performance of the solar cell, HTL is free in this structure. Eliminating the hole-transport layer (HTL) in this solar cell structure avoids oxidation, reduces costs, and provides better stability and more consistent results [23]. ETL plays an important role in thin-film solar cell. Hence, an appropriate selection of good ETL material for getting an efficient thin film is the need of the hour. Several ETL materials like ZnO, ITO, and CdS were tried to improve the efficiency [2426]. In this case, ITO acts as an electron transport layer, because we got the maximum efficiency in ITO. The materials for the front and back contacts are aluminum and molybdenum, respectively. By using SCAPS-1D, we investigate the performance parameters such as open-circuit voltage (), short-circuit current (), fill factor (FF), and the efficiency (η). The effects of temperature, series and shunt resistances, energy bandgap, defect density, and the ETL (absorber layer, buffer layer) thickness on device performance are discussed in this paper. The creation of nontoxic, ecofriendly thin-film solar cells in suitable places may benefit from these investigations.

2. Device Architecture and Simulation

Figure 1 shows the Al/ITO/TiO2/CuBi2O4/Mo structural characterization of the device which is created and simulated using the SCAPS-1D software program. In the proposed structure, TiO2 was used as buffer layer, and ITO was used as ETL in CuBi2O4-based solar cell. Table 1 shows the required parameters for the simulation.

At the Department of Electronics and Information Systems (EIS) of the University of Gent in Belgium, a program named SCAPS-1D for “Solar Cell Capacitance Simulator One-Dimensional” was developed to simulate solar cells [27, 28].

Poisson’s equation and the continuity equation for the free electrons and holes in the conduction and valence bands are given. The continuity equations for electrons and holes are where and are electron and hole current densities, is the recombination rate, and is the generation rate.

The Poisson equation is where is the electrostatic potential, is the electrical charge, is the relative and is the vacuum permittivity, and are hole and electron concentrations, is charged impurities of donor, is the acceptor type, and and are hole and electron distributions, respectively [29, 30].

The SCAPS-1D program was used to extract the energy band diagram for the suggested Al/ITO/TiO2/CuBi2O4/Mo thin-film solar cell. The optical characteristics of solar cells are discussed using the energy band diagram (Figure 2).

3. Result and Discussions

3.1. J-V Characteristics

By varying the CuBi2O4 thickness of the absorber layer, Figure 3 demonstrates the J-V characteristics. The buffer layer and ETL settings are fixed, whereas the absorber layer’s thickness varies here from 0.1 to 2 μm, and current and voltage rise with an increasing efficiency of 6.4% to 31.21%. From the J-V curve, it can be found that efficiency increases continuously as absorber thickness increases. Increased current and voltage are caused by the addition of the electron-hole pair during the thickness of the absorber layer.

3.2. Quantum Efficiency

The ratio of carriers collected by the solar cell to photon incident on the solar cell at a specific energy is here described as the quantum efficiency. It might also be stated as a function of wavelength or as an energy value [32]. In Figure 4, the QE rises at longer wavelengths as the absorber layer thickness increases. This is because there are less electron-hole pairs produced by photons inside the absorber layer. Additionally, for longer wavelengths of low energy, light is not absorbed below bandgaps, which causes quantum efficiency to drop to zero for wavelengths greater than 810 nm. The CuBi2O4 absorber layer’s thickness changed from 0.1 to 2.0 μm, and the ITO and TiO2 layers’ thicknesses were fixed at 0.23 μm and 0.05 μm, respectively.

3.3. Effect of Variation in Absorber Layer Thickness

The thickness of the active layer is one of the most important parameters in increasing the performance of solar cells [33, 34]. The models were simulated at various absorber layer thicknesses, and then, a regression analysis model was made to determine the efficiency and fill factor at each specified value. According to Figure 5, the fill factor, current density, and open-circuit voltage of this arrangement rise as the thickness of the absorber layer increases. When the absorber layer thickness is 0.1 μm, the values of the efficiency, , , and FF are 6.4%, 1.24 V, 6.32 mA/cm2, and 81.06%, respectively. When absorber layer thickness is 2.0 μm, the values of the efficiency, , , and FF are 31.2%, 1.36 V, 25.81 mA/cm2, and 88.76%, respectively. This might be accounted for by the fact that as the CuBi2O4 layer becomes thicker, more short-wavelength photons are absorbed, which promotes the photogeneration of additional free carriers [35].

3.4. Effect of the Buffer Layer Thickness

The impact of TiO2 thickness on CuBi2O4-based solar cells is displayed in Figure 6. Here, the absorber layer thickness was fixed at 2 μm while the TiO2 thickness was increased from 0.01 μm to 0.2 μm. Here, when the thickness is 0.01 μm, in that time, the efficiency is 31.18%, 1.36 V, 25.8 mA/cm2, and the fill factor 88.76%, and when the thickness is 0.2 μm, then the efficiency is 31.21%, 1.36 V,25.82 mA/cm2, and the fill factor 88.76%. It is seen that all the parameters are stable for all the points. It is because fewer photons will reach the absorber through the thicker buffer layer [3638].

3.5. Effect of Defect Density on the Absorber Layer

The defect density in the absorber layer was changing from 106 to 1013 cm-3 while holding other parameters fixed (Figure 7). All cell parameters are significantly impacted by the absorber defect; when the defect density is 106 cm-3, the values of the efficiency, , , and fill factor are 24.07%, 1.36 V, 25.81 mA/cm2, and 88.76%, respectively. When the defect density is 1013, the values of efficiency, , , and FF are 14.73%, 1.29 V, 13.84 mA/cm2, and 82.36%, respectively. The open-circuit voltage decreases as the defect concentration increases. The short-circuit current decreases as a result of the carrier recombination [39]. The fill factor (FF), as shown in Figure 7, decreases in a manner similar to that of the external quantum efficiency (%) [40].

3.6. Effect of Temperature

Operating temperature has a big impact on how well solar cells work [41]. Temperature variations have an effect on the solar cell’s overall performance. Efficiency is affected by temperature, as seen in Figure 8. As shown in the figure for both configurations, the working temperature was then varied between 300 K and 450 K to assess its effects on PCE, , , and FF for the ideal absorber thickness. When the temperature is at 300 K, the values of the efficiency, , and FF are 31.2%, 1.36 V, and 88.76%, respectively. When the temperature is at 450 K, the values of the efficiency, , and FF are 27.19%, 1.23 V, and 85.69%, respectively. is stable here. It is observed that by increasing working temperature, the efficiency was decreasing. The velocity of charged particles increases as temperature rises [42]. The rate of electron and hole recombination rises as temperature rises because there are fewer free electrons and holes available [43].

3.7. Effects of Series and Shunt Resistances

The performance of a device is greatly influenced by the series and shunt resistances. Efficiency changed in this example due to the effect of changing series resistance. Keeping set at 105Ω-cm2, the series resistance performance is explored by changing 0 to 5 Ω-cm2. As series resistance rises, the efficiency decreased. The initial efficiency was 31.2%. At the final resistance, the efficiency was 28.1%, FF was 80%, and and were flat by varying the resistance. The series and shunt resistances of a solar cell should be zero and infinite, but in practice, things operate quite differently.

Solar cells’ parasitic components and series and shunt resistances exhibit losses. Since recombination in defect states is the primary cause of shunt resistance, the defect state reduces as the device’s shunt resistance increases [44]. Series resistance lowers the PV properties of the photovoltaic cell. Figure 9 illustrates the effect of series resistance when the shunt resistance is set at 105Ω-cm2 and the series resistance is changed. Figure 10 illustrates the outcome of shunt resistance. The shunt resistance performance is explored by changing from 101 to 106Ω cm2, and remains constant at 0.5 Ω cm2. When is 101Ω-cm2, its efficiency increases 1.58% to 30.89% when increases to a higher value. FF, , and are observed at 24.96%, 24.58 mA/cm2, and 0.25 V, and when is 106Ω-cm2, these output parameters varied to 87.88%, 25.81 mA/cm2, and 1.36 V.

3.8. Effects of Back Contact Work Function

The changing of back contact material work function (eV) and its influence of the parameters are present here. Figure 11 shows the influence of back contact work function on solar cell performance parameters. The electric field of the depletion area produces and separates carriers, which must be collected at metal contacts to power the load. Imperfections in the metal-semiconductor interface will affect the efficiency of solar cells if they are recombined [4547]. Solar cell efficiency will be aided by the surface field produced by this strongly doped back end. Materials used as back contact include copper, carbon, molybdenum, beryllium, nickel, and gold, with work functions ranging from 4.7 to 5.1. When the work function is 4.7, then the efficiency is 25.85%, 1.14 V, 25.81 mA/cm2, and FF 87.44%, and when the work function is 5.1, then the efficiency is 31.48%, 1.37 V, 25.82 mA/cm2, and FF 88.93%. The proposed solar cell structure with metal back contact gold (Au) got the PCE of 31.51% and with molybdenum (Mo) got PCE of 31.21%. The efficiency was not so high with gold (Au) as compared to molybdenum (Mo) as metal back contact, and gold is expensive than molybdenum; that is why we did not use gold (Au) as metal back contact layer with our structure. The efficiency of solar cells will be impacted over the long run if they are recombined as a result of flaws in the metal-semiconductor interface [45].

3.9. Effects of Variations in ETL Thickness

The behavior of the proposed solar cell is also investigated on the basis of altering the ETL thickness. The effect of variations in ETL thickness on performance indicators including , , FF, and conversion efficiency is shown in Figure 12. ETL might range in thickness between 0.1 and 0.5 μm. While the ETL thickness was altered, the thickness of the absorber and buffer remained unchanged. The parameters of the solar cell are influenced by the electron transport layer because of ITO’s significant defect of intrinsic low mobility [48]. Charge accumulation and ultimately recombination will develop as the layer’s thickness does [49].

3.10. Effects of Variations in Bandgap

Numerous factors affect the efficiency and performance of PV cell layouts. A complex internal physical process that occurred is what caused the issue. The performance of PV cell designs must be taken into consideration in a solar cell model. The cause is a complicated internal physical mechanism that has taken place. To trust a solar cell model, we must consider a number of factors as well as many situations that can be tested and compared [50]. We have considered the impact of this on the CuBi2O4 absorber layer’s bandgap. Figure 13 illustrates the effects of bandgap changes. We altered the bandgap from 1 to 2 eV. When the bandgap is 1 eV, the efficiency is 26.8%, 1.34 V, 22.83 mA/cm2, and FF 87.75%. When the bandgap is 1.6 eV, then the efficiency is 30%, and when the bandgap is 2 eV, the desired efficiency is 31.2%, 1.36 V, 25.81 mA/cm2, and FF 88.76%. All the parameters increased with the increase of bandgap.

3.11. Effects of Acceptor Density

Figure 14 demonstrates how open-circuit voltage increases and short-circuit current decreases when CuBi2O4’s acceptor density varies. The fill factor (FF) increases once the acceptor density surpasses 1016 cm-3 and then slightly rises. The power conversion efficiency (PCE) increased as a result. We also investigated how acceptor density and photovoltaic efficiency are related. Here, when the acceptor density is , then the efficiency is 36.36%, 1.72 V, 28.52 mA/cm2, and FF 78.84%, and when the density is , then the efficiency is 33.07%, 1.42 V, 25.54 mA/cm2, and FF 90.70%. When the photogenerated minority carriers reach the depletion zone of the combination, an existing electrical field separates them [51]. Lower acceptor density would increase the device’s series resistance, while higher acceptor density would decrease the device’s shunt resistance, thus reducing the performance of the solar cell.

3.12. Effects of Donor Density on the Buffer Layer

Figure 15 shows that the solar cell output parameters , , FF, and are nearly constant as a function of the thickness of the TiO2 buffer layer due to the insufficient number of electron-hole pairs produced in the CuBi2O4 absorber layer. Since it is assumed that less light passes through the absorber, because of a thicker buffer layer resulting in a negligible current for insufficient production of photons which creates electrons and holes a thin buffer layer is predicted to give high solar cell activities. In this case, the thickness varied from 1014 to 1018.When the donor density is 1014 cm-3, then the efficiency is 31.20%, 1.36 V, 25.80 mA/cm2, and FF 88.82%, and when the density is 1018 cm-3, the efficiency is 31.19%, 1.36 V, 25.86 mA/cm2, and FF 88.54%. Here, graph for the donor density parameters was flat.

4. Conclusion

A numerical investigation of the CuBi2O4-based thin-film solar cells is carried out using the one-dimensional simulator SCAPS. In this work, SCAPS-1D was used to analyze the optimum nature of CuBi2O4-based thin-film solar cells with absorber, buffer, defect density, series resistance, shunt resistance, donor, acceptor density, bandgap variation, and ETL topologies. These findings show the creation of HTL free thin-film solar cell made of copper bismuth oxide that is ecologically effective. The defect density in the absorber layer was changing from 106 to 1013 cm-3 and the efficiency was reduced from 24.07% to 14.73%. At a working temperature of 300 K, the best result (31.21%) and an open-circuit voltage of 1.36 V, short-circuit current density of 25.81 mA/cm2, and a fill factor (FF) of 88.77% were attained. Additionally, it is discovered through research that using suitable back contact materials with a high work function can improve the output performance of the proposed device construction. Mo was used here because of its low cost. Therefore, this simulation study opens a new direction for a simpler, low-cost, highly efficient, and more stable cadmium free CuBi2O4-based thin-film solar cell.

Data Availability

The data that support the findings of this study are available from the corresponding author upon reasonable request.

Ethical Approval

This research is based on simulation. As a result, during research, this work did not cause any harm to humans, animals, or the environment. The used materials are nontoxic, and we hope this structure will not be harmful for further work.

Conflicts of Interest

There is no conflict of interest.

Acknowledgments

We thankfully acknowledge Dr. Marc Burgelman of the University of Gent in Belgium who has generously provided the SCAPS simulation program.