Investigating the Influence of Bath Temperature on the Chemical Bath Deposition of Nanosynthesized Lead Selenide Thin Films for Photovoltaic Application

Abstract

Thin films of CBD are formed on metal surfaces from an aqueous solution containing Pb(NO₃)₂ and Na₂O₄Se. The impact of the bath temperature upon lead selenide NPs is analyzed. The evaluation of X-ray diffraction demonstrates that the produced NPs were polycrystalline with (111) orientation. The morphological analysis of the surface shows that the grains are spherical gemstones. As the bath temperature was raised from 20 to 85 degrees Celsius, the energy bandgap decreased from 2.4 to 1.2 eV, indicating a reduction in the band gap. Micron-sized nanoparticles produced at 85 degrees Celsius exhibited the best crystallinity and were uniformly spread across the surface of the substrate with excellent particle sizes. If the solution bath temperature increases from 20°C to 85°C, the average strength of PL decreases. The maximum photoluminescence strength is predominantly because of self-trapped exciton recombination, formed from O₂ vacancy and particle size called defect centers, for the deposited thin films at 45°C and 85°C. The photoluminescence intensity rises sequentially with all temperatures. Therefore, the finest solution temperature is 85 degrees Celsius.

This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

1. Introduction

Currently, the world is in trouble of air pollution released from nonrenewable sources of energy such as coal, natural gas, fossil fuels, and fabrics [1]. A wide-ranging investigation has been dedicated to producing numerous kinds of semiconductor thin films which are applicable in renewable sources of energy like solar cells [2]. This is because of their potential uses in the production of photovoltaic materials, optical-electronic devices, sensors, and infrared indicator instruments [3]. The lead selenide thin films appeal consideration of many scholars because they are low cost, exist in abundance, and retain semiconducting material goods [4, 5]. There have been numerous ways for the fabrication of thin films of lead selenide that have been discovered, which comprise electrodeposition, CBD, electrochemical atomic layer, photochemical, molecular beam epitaxial, and pulsed laser deposition method [6], amongst other techniques. Thin films created by chemical methods are frequently considerably costly than thin films produced by capital-intensive physical techniques. CBD methods were specially picked for their several advantages, including their minimal price, broad area manufacturing, and ease of use in equipment setup [7]. Several other semiconductor thin films, notably [8–10] ZnS, have been successfully deposited using a chemical approach in the past few decades and also SnS, PbSe, CdSe, ZnSe, CdS, PbS, Cu₂S, CuInS, and CuBiS₂ on glass substrates [11–15], and they have no longer quality. There are very few chemically prepared lead selenide thin films reported by using glass substrates in an alkaline base. But this can make deletions of films and affect the quality of prepared films. They release hydroxide chemicals during their depositions [16–20]. For the present work, we have used metallic substrates to grow PbSe thin films by chemical bath deposition procedure in an acidic medium.

2. Materials and Methods

2.1. Materials and Fabrication of the Sample

Chemical bath deposition was used to deposit PbSe thin films on a metallic substrate ($30\times 70\times 1$ mm) using a chemical bath deposition process. Before depositing, the metal substrate was thoroughly washed in ethanol for 15 minutes after that, ultrasonically cleaned with a deionized solution for the next 20 minutes, and then finally desiccated in warm air. An aqueous solution of lead nitrate was used as a metallic precursor source that is lead, sodium selenite as the sulfide ion source, and [(HOC₂H₄)3N] as a complexing agent for the synthesis of lead sulfide thin films [10, 11]. All of the chemicals that were used in the deposition were of analytical quality. Many of the treatments were made with deionized water as a base. In a step-by-step deposition procedure, 25 ml of 0.2 M lead nitrate was mixed with 10 ml of triethanolamine agent to form a complex. This has been followed by the addition of 15 milliliters of 0.2 M sodium selenite; the pH of the resulting mixture was adjusted to 4 by adding a droplet of sulfuric acid [12], and swirling it was gradually introduced to the reaction mixture constantly for several minutes. The clean metal substrates were produced in order to grow nanoparticles of lead selenide on the surface of the metal. The temperatures were 20°C, 45 degrees Celsius, and 85 degrees Celsius, respectively. After the deposition time of 95 min, we fetch the water from the chemical solution by using the syringe. The bottom of the glass of the bath solution is left with molten lead selenide nanoparticles. Next, the metal substrates were coated with molten lead selenide, and it was dried in the air, splashed with deionized water, and dehydrated in desiccators for even more analysis.

2.2. Characterization Methods

The crystalline geometries of the nanosynthesized PbSe thin films were investigated employing an X-ray diffraction (XRD) apparatus from PANalytical, model US [13], to determine their composition. The diffractometer was connected with a CuK source that operates at 35 kV and 23 mA; all scans were obtained in a 2$\theta$ range from 20 to 85°. An optical absorption measurement was executed using a Jeneway 6850 UV/visible spectrophotometer in the range from 226 to 2250 nm. The morphology of nanoparticles size was characterized by scanning electron microscopy (SEM) in a Hitachi SU5000 with an operating voltage of 20 kV. Photoluminescence of the prepared material was analyzed by using a photoluminescence spectrophotometer.

3. Results and Discussion

Images of X-ray diffraction (XRD) structures of films deposited at different bath temperatures, including 20°C, 45°C, and 85°C, are shown in Figure 1. Exactly four peaks were observed on the film made at 45°C, which happened at $2\theta =26.5$°C and 29.6°C. Raising bath temperature (to 85°C) enhanced the strength of the PbSe-related peaks, but only at the expense of the overall peak intensity. Diffraction across the (111) plane exhibits the maximum intensity with a well-defined sharp peak, suggesting that the material processed has a high degree of crystallinity. This suggests that when the temperature of the bath rises, the particle sizes of the thin film that is being deposited grow in proportion. When the bath temperature was raised, the number of PbSe peaks expanded as well. PbSe structure with (111) planes has been discovered, as indicated by the peaks that have been produced. Every one of these peaks, which corresponded to the cubic phase of lead diselenide, was a good match with the conventional data [12]. The values of the three lattice constants are all the same (6.128). Contrarily, it is the metal substrate that was utilized to conduct the analysis that is responsible for the occurrence of the Iron dioxide peak in the XRD pattern. At the 2$\theta$ values of 40.3, 52.8, and 67.3, four peaks were seen. Iron oxide has an orthorhombic structure, and the peaks indicated with solid triangles were connected with mirrors of the cubic structure of lead selenide, while the peaks indicated with undefended diamonds are related with reflections of the orthorhombic structure of lead selenide, and this result witnessed with scanning electron microscope analysis which is homogeneous with a cubic structure agrees with previous reports.

[figure omitted; refer to PDF]

The size of the particle was premeditated by using Scherer’s formula; it is given by $$\begin{array}{}\text{(1)}& D=\frac{0.9\lambda }{\beta Cos\theta },\end{array}$$where $\lambda$ shows wavelength, $\beta$ stands for (FWHM) in rad, and $\theta$ represents the angle diffraction (Bragg angle). Comparison of evaluated and standard “$d$” and $2\theta$ values for nanosynthesized PbSe thin films is listed in Table 1.

Table 1

Comparison of evaluated and standard “$d$” and $2\theta$ values for nanosynthesized PbSe thin films with varied bath temperatures (20°C, 45°C, and 85°C) and deposition time at 95 minutes.

SI. No.	Bath temperature (°C)	Miller index (hkl) plane	$2\theta$ values	Lattice constants ($\mathring{A}$)	Particle size (nm)
1	20	(111)	25.43	6.128	52.9
2	45	(222)	26.21	6.128	62.7
3	85	(444)	28.54	6.128	70.65

An investigation into the surface morphology of a film was conducted utilizing scanning electron microscopy (SEM). It can be used to determine the degree of grain and the architecture of produced films. The surface morphologies of the films produced at different bath temperatures ranging between 20 to 85 degrees Celsius are depicted in Figure 2. Every one of the samples was obtained at a voltage of 10.00 kV and a magnification of 20.71x. These were observed that the morphology of the samples changed in response to the bath temperature [14]. While all of the lead selenide thin films generated at all temperatures completely covered the metal substrates, the thin film deposited at 20°C is not relatively homogeneous and does not have excellent smoothness of the surface, as revealed by scanning electron microscopy (Figure 2(a)). In these films, it was shown that the grains were extremely minute in size, with clearly defined grain borders between them. For the films deposited at 45°C and 85°C, well-defined grains can be seen in the images in Figure 2(b) and 2(c), respectively. The surfaces of the metal substrates were coated with a cubic-shaped PbSe crystal. PbSe surfaces grow more homogenous in appearance when the temperature of the bath is raised. Furthermore, it was shown that the grain amounts increased gradually. Once the tiny polycrystalline components have agglomerated together, they produce PbSe grains when heated to 85 degrees Celsius. The grains created a compact morphological characteristic across the substrate and had been tightly packed together in some way. In the present study, it was discovered that the temperature of the mixture played an important impact on the efficiency of the nanofabricated PbSe films.

[figures omitted; refer to PDF]

The optical immersion of the nanosynthesized PbSe thin films deposited from different bath temperatures was measured in the wavelength with the range between 226 and 2250 nm, as shown in Figure 3. The absorbance of the films expressively raised when the bath temperature increased within the deliberated range of wavelength. The highest absorption of thin films was witnessed in the visible wavelength range. The optical absorption coefficient of the PbSe thin films was obtained from Lambert’s equation: $$\begin{array}{}\text{(2)}& \alpha =2.30\frac{A}{t},\end{array}$$where $A$ is the optical absorbance, $\alpha$ is the optical absorption coefficient, and $t$ is the film thickness. The energy band gaps (Eg) of the PbSe thin films were calculated by utilizing Tauc’s relation [18–25]: $$\begin{array}{}\text{(3)}& {ahv}^n=khv-\text{Eg},\end{array}$$where $h$ is Planck’s number, $v$ stands for frequency, $k$ expresses the optical transition constant number, Eg is the energy of bandgap, and $n$ is the transition type. The value of $n$ is varying either 2 or 2/3 for direct allowed and forbidden transitions or 1/2 and 1/3 for indirect allowed and forbidden transitions, correspondingly [16]. The best linear fit for Equation (3) is given for $n=2$ in the main absorption edge, representing that the thin films have direct optical band gaps. The ($\alpha hv$) axis intercept attained by extrapolating the linear portion of the $\alpha hv2$ vs. $h\upsilon$ curve gives the Eg of the films as shown in Figure 3. The Eg of the nanosynthesized PbSe thin films declined from 2.4 eV to 1.2 eV as the bath temperature of the solution increased from 20°C to 85°C. The decrease in bandgap could be due to the rise in the crystallite dimension with increasing the temperature in the deposition bath, and this result is the same as reported in [17]. The maximum absorbance in the visible light section and the band gaps of the thin films within the range 2.6 eV to 1.2 eV in all PbSe thin films provide the film’s applicant materials as an absorber layer in photovoltaic thin-film solar cells as well as well-organized visible light photocatalyst [26–32].

[figure omitted; refer to PDF]

Inappropriate to discover the optical study of deposited PbSe nanoparticles, photoluminescence was similarly used. In the $\lambda$ range from 350 nm to 600 nm at different temperatures, the PL spectra of the nanosynthesized thin films were reported. If the solution bath temperature increases from 20°C to 85°C, the average strength of PL decreases [19]. The maximum photoluminescence strength is predominantly because of self-trapped exciton recombination, formed from O₂ vacancy and particle size, which we call defect centers, for the deposited thin films at 45°C and 85°C. The photoluminescence intensity rises sequentially with all temperatures.

Figure 4 expresses that the absorption of nanosynthesized lead selenide thin films is formed at different bath temperatures. All the samples reveal a gradual rising absorbance in the visible area, which provides the potential for these tools to be applicable in a photovoltaic solar cell. From the plotted figure, it shows that the samples synthesized at a greater temperature of the solution have greater absorption values related to other solution bath temperatures. Because of these, nanosynthesized thin films have the maximum uniform surface and well crystallinity thin films by comparing with other reported samples [19].

[figure omitted; refer to PDF]

4. Conclusions

The creation of nanoproduced lead selenide (PbSe) thin films in acidic media can be accomplished with a simple and inexpensive chemical bath deposition process. It is expected that the chemical solution formed from lead nitrate and sodium selenate molecules will serve as a source of lead and selenide ions, respectively. The triethanolamine was served as a complexing mediator through deposition procedure. The X-ray diffraction pattern tells the creation of a cubic crystal structure with the toughest peaks ascribed to (111) miller index plane of lead selenide. PL confirmed that photoluminescence emission of the deposited thin films increased with increasing bath temperature. The deposited film at 85°C demonstrated excellent crystallinity and was evenly spread throughout the surface of the material with larger grain sizes. In response to an increment in bath temperature between 20°C to 85°C, the energy band gap narrowed, decreasing from 2.4 to 1.2 eV, making it acceptable for photovoltaic solar cells.

Disclosure

This study was performed as a part of the employment of the authors from Dambi Dollo University, Ethiopia.

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