Fabrication of NiO/Fe2O3/PSi/p-Si nanocomposite Heterojunction for Application in Solar Cells

INTRODUCTION

Nowadays, considerable attention has been attracted toward nanoparticles due to their special physical properties, which are different from those of bulk materials [1]. In previous decades, researchers have used metal oxide NP to implement a considerable number of developments in scientific applications. The applications of NP’s could be in sensors, catalysts, medical sciences [2], batteries [3], capacitors and semiconductors. In the past years, iron oxide Fe₂O₃ is an n-type semiconductor in the hematite phase and has been attractive to many researchers’ interests. It has many advantages, such as availability, low cost, non-toxicity, thermal stability and a narrow band gap energy near 2.2 eV [4, 5]. Fe₂O₃ has significant importance in solar cell applications, optoelectronics and energy conversion [6-11]. Fe₂O₃ nanoparticles can be formed by implementing many techniques, such as the chemical method [12], co-precipitation method [13], microwave-assisted synthesis [14], hydrothermal method [15], thermal decomposition [16], laser pyrolysis [17], and pulsed laser ablation [18, 19].

The heterojunction technique will largely increase efficiency due to the lowering of energy loss by increasing the capture of sunlight [20-25], high performing, stability, high electrical production in both hot environmental and low sunlight conditions, and advantages over conventional solar cells. The (HJ) solar cells are used to limit the charge carrier flow or to collect them [26].

MATERIALS AND METHODS

Preparation of Fe₂O₃ and NiO materials

The material Nickel oxide (NiO) was prepared by using a chemical method. This molarity is recommended value for many researchers [27-29]. The nickel nitrate and sodium hydroxide serving as precursor materials to get 0.1 M. Initially, 2.9 g of nickel nitrate was dissolved in 100 mL deionized water, the previous solution was placed on a magnetic stirrer at 70 °C for one hour to ensure complete dissolution. Subsequently, a sodium hydroxide solution was prepared by dissolving 4 g of NaOH in 100 mL deionized water. The NaOH solution was then added dropwise to the nickel nitrate Ni(NO₃)₂ solution under continuous stirring with heating at 300 °C. A noticeable colour change was observed, indicating that the reaction and the formation of nickel oxide occurred. Approximately 20 mL of the sodium hydroxide solution was required to complete the reaction and until obtain a required NiO, colour then left the solution for 30 minutes.

NiO thin films were deposited on glass substrates using the same deposition conditions, utilizing a single concentration, while preserving identical spraying and drying parameters. The prepared samples were then used for structural, morphological, and optical tests before moving on to the cell-making stage.

All molar concentrations (0.05, 0.10, 0.15 and 0.20 M) have been used to deposit thin films of Fe₂O₃ on the glass substrates separately. The iron (III) oxide (Fe₂O₃) was dissolved with different molar concentrations by weighing appropriate amounts of Fe₂O₃ powder. Each concentration was dissolved separately in 100 mL of deionized water; the weight according to each concentration was (0.798, 1.59, 2.4 and 3.2 g), respectively, for each concentration. Before spraying, the Fe₂O₃ solution was heated to 70 °C to avoid inhomogeneity and to reduce the impact of thermal shock on the glass substrates. After that, the solution was put into a hand-held spray device (Airbrush Kit) as shown in Fig. 1. The deposition was done utilizing a manual spray deposition method, with a fixed spraying position and normal incidence angle at 90° toward the substrates. The solution was sprayed in successive bursts at a rate of four to five sprays per cycle, allowing the substrates to dry for a few minutes between each spraying cycle. The positions of the glass substrates were moved from time to time during the deposition process with tweezers to make sure that all samples were sprayed was in same conditions. Each glass substrate was exposed to approximately twenty spray cycles in total; this method gives an indication that all thin films prepared are approximately the same. The duration of deposition for each concentration was half an hour. Same way was followed for all Fe₂O₃ concentrations. After the deposition, the samples were left to dry at room temperature.

The density of the Fe2O3 and NiO are equal to (5.25, 6.67 g/cm3) respectively, the thin films thickness was approximately (0.9mm).

Preparation of Porous Silicon by Electrochemical Etching

The etching procedure starts after the washing of the silicon slides, where the silicon is immersed in a mixture of hydrofluoric acid (46%) and ethanol (99.99) in a 1:1 volume ratio (20 mL HF : 20 mL ethanol) at ambient temperature. The inclusion of ethanol serves to eliminate the Hydrogen bubbles formed on the surface, obstructing the process. The cell utilized in this method is constructed from Teflon, which is resistant to hydrofluoric acid. Teflon remains impervious to acid owing to its saturated condition and the presence of robust fluorine bonds.

where the silicon served as the positive electrode (anode), and gold was used as the negative electrode (cathode). The etching process was performed at a current density of 15 mA/cm² for a duration of 15 minutes. where the result after the etching process appeared to produce an etched area of the sample of almost (0.785) cm² as shown in Fig. 2.

Preparation Ag / NiO/ Fe₂O₃/ PS / Si / Ag SHJ solar cell

Thin films of Fe₂O₃ at different concentrations (0.05, 0.10 and 0.15 M) were utilized. A single-channel adjustable micropipette (10–100 µL) was used to drop Fe2O3 solution on PSi substrate. 50 µL amount of Fe₂O₃ solution was poured directly on PSi surface and left to deposit. Then the samples were put on a heated plate (70 °C) for about a minute to start drying and make sure the solvent evaporated equally over the whole PSi surface. The samples were gently tapped on the heated plate to let trapped air bubbles out, make the thin film more homogeneous, and help the substance enter deeper into the porous silicon. This procedure helped prevent the formation of air bubbles and made it easy for Fe₂O₃ solution to enter the porous silicon structure.

Finally, the samples were put in an oven and heated to 100 °C for 1 hour to make sure the deposited layer stayed stable and stuck well to the substrate surface.

Nickel oxide (NiO) layer was added to each previous sample (using the same previous methodology). As shown in Fig. 3.

RESULTS AND DISCUSSION

Fig. 4 shows the crystalline structures of NiO and Fe₂O₃ with different concentration thin films, which were examined using X-ray diffraction (XRD) which has target (Cu-Kα) with wavelength (1.54060 Å). The XRD patterns of Fe₂O₃ thin film was taken by scanning 2θ between 9° and 80°. Fig. 4a shows the Fe2O3' s XRD pattern. The peaks were shown at angles of 26.6◦, 33.1◦, 35.7°, 40.8°, 49.7◦, 54.2◦, 57.6°, 62. 5◦, and 64.1◦, which matched to (012), (104), (110), (113), (024), (116), (018), (214) and (300) planes, respectively. The diffraction peaks of Fe₂O₃ are shown orthorhombic structures at dominant direction (104) for the highest intensity at 2θ = 33.099°. This result is in full agreement with [30-32], This pattern is aligned with the standard data from JCPDS card No. 01-1030 [30].

The XRD of NiO thin film shows the peaks at 2θ angles of 37.2◦, 43.3◦, 62.9°and 75.5°which points to (111), (200), (220) and (311) planes, respectively. NiO thin film refers to Face-Centered Cubic structures in the (200) direction for the highest intensity at 2θ = 47.4787°, This result is in full agreement with [33-34]. This pattern is aligned with the standard data from JCPDS card No. 04-0835 [35]. As shown in Fig. 4b.

The surface morphology was characterized by utilizing atomic force microscopy (Model TT-2 AFM workshop). as shown in Fig. 5. The AFM analysis in three dimensions for PSi, NiO and Fe₂O₃ with different concentration thin films, it shows an atomic force microscope with a scanning area of 2 μm. For PSi, and Fe₂O₃ there are irregular and randomly distributed hills and voids of nanocrystalline silicon throughout the whole surface.  On other hand, for NiO there is more regularly hills distribution and fewer voids. This morphology of (Fe₂O₃ , Psi) increased the adhesion among layers. Tab. 1 explains some information related to the AFM test as shown in Table 1.

The emission spectra of PS with an etching current density of 15 mA/cm² at a constant period of 15 min and exposed to an excitation wavelength range from 320-900 nm is appeared in Fig. 6.

Because of porous silicon nanowires, the photoluminescence spectrum peak location and intensity are dependent on the quantum size effect, which appears in nanostructures. The highest amount of photoluminescence is due to the increasing porosity; otherwise, the blue shifting of the peak position is caused by a decreases in the silicon nanocrystalline size [36, 37].

In the FTIR spectrum, there are Resonances related to various wavenumbers that correspond to the absorption sites of the porous silicon, NiO and Fe₂O₃ thin films. An FTIR test of the porous silicon sample is shown in Fig. 7a. The peaks inside the FTIR spectra from 610 to 640 cm^-1 are due to the bending vibration related to surface oxidation (Si-Si). The peaks from 800 to 880 cm^-1 are due to the symmetric stretching which may indicate silicon oxide (Si-CH₃ or Si-O-S)). The peaks from 1050 to 1100 cm^-1 are due to the asymmetric stretching vibration, which may indicate silicon oxide (Si-O-Si). The peaks from 1250 to 1270 cm^-1 are due to the bending vibration, which confirms the presence of organic coverage (Si-CH₃). The peak from 1630 to 1640 cm^-1 is due to the bending vibration, which confirms the water associated with the porous materials (H-O-H). The peaks from 2900 to 3000 cm^-1 are due to the stretching vibration, which indicates the presence of hydrocarbon groups (C-H). The peak from 3400 to 3600 cm^-1 is due to the stretching vibration, which suggests moisture absorption (O-H). This result is in full agreement with [38-40].

The surface morphology image of the metal oxides Fe₂O₃ nanoparticles is shown in Fig. 7b; it shows the chemical bonds as well as the compound's functional groups. For the infrared spectrum (FTIR) for Fe₂O₃ nanoparticles at different molarities, the large and wide band at 3438cm^-1 is related to the O-H stretching vibration for O-H groups.  The absorption peaks near 1632cm^-1 and 1384cm^-1 are classified as the vibration of C=O, which are asymmetric and symmetric bending, and an absorption peak around 1270cm^-1. The band lower than 700 cm^-1 is referred to Fe-O stretching mode.  The band related to Fe-O stretching mode for Fe₂O₃ is shown at 576 cm^-1. This result is in full agreement with [33-35].

While, for NiO, which appeared many significant absorption peaks. It is shown in Fig. 7c, The peaks inside the FTIR spectra near 443.54 and 663cm^-1 is due to the vibrations related to nickel oxygen bond stretching. The wide wideness of a peak gives an indicate that the NiO is crystalline in nature. 2346 cm^-1 peak indicates the absorption band, which is due to the symmetric and asymmetric stretching modes related to the vibrations of the CO₂ molecule that is absorbed from the air [41]. Also, the broad absorption band centered at 3437, 1384 cm⁻¹ is attributable to the band O–H stretching vibrations and the weak band near 1632 cm⁻¹ is assigned to H–O–H bending vibration mode. The bands at 1115 cm^-1 are related to (C=O) group [42-45]. This result is in full agreement with [30-31].

The optical characteristics are crucial when studying solar cell efficiency. The direct and indirect band gap (Eg) can be obtained by linear fitting and extrapolating the line to the x-axis ((αhv)² as a function of (hν)), as shown in Fig. 8a-b. The direct energy gap of Fe₂O₃ at different concentration is about 2.65, 2.70, 2.75 and 2.80 eV, respectively, with concentration. This result is fully agreement with references [36, 46-48].  While for NiO is about 3.62 eV as shown in Fig. 8 c. This result is fully agreement with references [33, 35]. As shown in Fig. 8 c.

The electrical measurement results related to the (I-V curve) for the Ag/NiO/Fe₂O₃/Psi/p-Si/Ag at different Fe₂O₃ concentration devices that were done under forward and reverse biasing in both dark and light conditions are shown in Fig. 9. The cell is tested under standard test conditions (STC), which specify a light intensity of 1000 W/m² (AM1. 5G spectrum) and a cell temperature of 25°C. These measurements are principally to describe the device's performance. The decreasing width of depletion region's as forward bias voltage increases. In contrast, the increasing width of depletion region's as reverse bias voltage, so there is no current flows.

When applied forward bias voltage, the majority charge carriers in both side of potential barrier are injected inside the depletion region, as a result, the width of depletion region was reduced and number of majority and minority carriers per unit volume in increased. In the case of illumination, a heterojunction was exposed to visible light which passed through NiO layer, but it absorbed by the Fe₂O₃ and PSi and Si substrate. This will lead to generate the e-h pairs near the NiO/Fe₂O₃/Psi/p-Si interface and increasing photocurrent with increasing bias voltage [49–51].

In Fig. 10, it is clear that the values of current in illuminated case is higher than dark case at reverse bias at different Fe₂O₃ concentration, so the current is independent on the applied voltage in this case, but it is due to the enhance in the absorption of incident photons because the increasing of reflections from the nanowire on the surface of porous silicon then give the Fe₂O₃ film more ability to absorb the photons.

The (I-V) characteristics of the solar cell Ag/ NiO/Fe₂O₃/Psi/p-Si /Ag at different Fe₂O₃ concentrations are shown in Fig. 11, which explains a schematic diagram for Ag/ NiO/Fe₂O₃/Psi/p-Si /Ag solar cells. It displays that the open-circuit voltage (V_OC) was measured at I = 0, and the short-circuit current value (I_SC) was obtained at V = 0, as indicated in Table 2, where the fill factor (F.F.) as well as photovoltaic conversion solar cell efficiency (η) are calculated using the following formulas (Eqs. 1 and 2) [52]. The intensity of the Philips Halogen lamp (P_in) used for illumination was 100 mW/cm²:

This result means that this synthesis of a heterojunction gets good performance related to Ag/NiO/Fe₂O₃/PSi/p-Si/Ag solar cells.

CONCLUSION

The characterization of precipitated Iron Oxide and Nickel oxide films was done by the drop casting method, which is a simple method; these films were deposited on glass substrates. The optical properties were examined using the UV-spectrum, and with using Taus equation, the electronic transmissions (direct and indirect transmissions) were defined with the presence of two different optical energy gaps; both locations were in the visible region.

Ag/NiO/Fe₂O₃/PSi/p-Si/Ag solar cells were manufactured using electrochemical etching of p-type silicon. NiO had good transparency in the spectral range (400-1100) nm. The electrical properties were dependent on the energy gap of nanomaterials. The result has been got in this work (F.F, h) are higher and better than reported papers.

CONFLICT OF INTEREST
The authors declare that there is no conflict of interests regarding the publication of this manuscript.