Finds documents with both search terms in any word order, permitting "n" words as a maximum distance between them. Best choose between 15 and 30 (e.g. NEAR(recruit, professionals, 20)).
Finds documents with the search term in word versions or composites. The asterisk * marks whether you wish them BEFORE, BEHIND, or BEFORE and BEHIND the search term (e.g. lightweight*, *lightweight, *lightweight*).
Activate our intelligent search to find suitable subject content or patents.
Select sections of text to find matching patents with Artificial Intelligence.
powered by
Select sections of text to find additional relevant content using AI-assisted search.
powered by
(Link opens in a new window)
Abstract
This article delves into the preparation and characterization of PbS thin films using the chemical bath deposition method, focusing on the effects of varying film thickness and microstructure. The study examines the structural and morphological properties of the films through X-ray diffraction and scanning electron microscopy, revealing the impact of thickness on crystallite size and surface morphology. Optical properties, including transmission, reflectance, and energy band gap, are thoroughly analyzed, showing a decrease in band gap with increasing film thickness due to quantum confinement effects. The article also explores the photoconductive performance of the films, highlighting the potential of thinner films for photodetection applications. The research concludes that while thicker films exhibit higher absorption, their photodetection capabilities are limited by interfacial disorder and trap-limited transport. This comprehensive study provides valuable insights into the optimization of PbS thin films for various optoelectronic applications.
AI Generated
This summary of the content was generated with the help of AI.
Abstract
This work presents the deposition of lead sulfide (PbS) thin films on glass substrates prepared via chemical bath deposition (CBD). The influence of film thickness on surface morphology and structure is investigated. Film thicknesses (300-1200 nm) were prepared layer-by-layer. The films exhibit shiny, continuous, and homogeneous appearances. X-ray diffraction (XRD) reveals that the prepared films were polycrystalline with (200) preferred crystal orientation. Scanning Electron Microscopy (SEM) shows that the agglomeration of grains becomes bigger (increased from 90 ~ 225 nm) with film thickness. Optical transmission and reflectance were employed to study the optical properties of the films in the spectral range 200 to 2500 nm. The optical band gap of the films decreases from 1.55 eV down to 0.93 eV, corresponding to wavelengths 800 nm to 1.3 µm, as the thickness of the PbS film increases from 300 to 1200 nm. The absorption coefficient of the films varies from 104 to 105 cm-1 in the spectral range of 300 to 2500 nm. The films’ refractive indices in the 800-850 nm range are similar for film thicknesses up to 600 nm, but they show a discontinuity at 850 nm for thicker films. Photoconductive response occurs for film thicknesses of 600 nm or less. In contrast, thicker films do not respond to light.
Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
1 Introduction
Today, energy issues are one of the main challenges facing humanity[1]. Clean energy systems are essential for meeting the growing global energy demand while reducing dependence on fossil fuels and the associated environmental damage[2]. Identifying renewable and environmentally sustainable energy sources is essential to meet the increasing demands of humanity. A hydrogen fuel cell offers an efficient way to fulfill future global energy needs. In this system, hydrogen reacts with oxygen from the air to produce energy, with water as a by-product. The photoelectrochemical (PEC) method for hydrogen production from water plays a vital role in addressing future energy crises. [3] The effectiveness of PEC for hydrogen production depends on the performance of photoelectrodes, including their ability for light absorption, charge separation, and surface area for electrochemical reactions [4]. Notably, lead sulfide (PbS) thin films are a promising IR photo detector material that can be integrated with PEC electrodes to convert IR photons into measurable photocurrent. It is increasingly studied as a potential solution for energy crises, especially in thermoelectric energy harvesting and infrared photodetection [5], since it has distinctive features that make it suitable for such a research area. Lead Sulfide is a vital IV–VI semiconductor, with a narrow direct band gap of 0.41 eV at room temperature [6, 7]. It has a large exciton Bohr radius of 18 nm [8], which results in strong quantum confinement for electrons and holes, even for large particles[9]10. When the PbS particle size decreases to the nanoscale, its optical band gap increases from 0.41 eV to as high as 5.2 eV, making it suitable for optical sensors with adjustable properties and UV photodetectors. [11] and optical switches as well [12].
PbS is a promising material for optical and photonic device applications like solar cells. [13, 14], gas sensors [15], Photocatalysts[16‐18], and also infrared optoelectronic devices [19, 20]. PbS thin films can be prepared by different physical and chemical methods, such as pulsed laser deposition (PLD) [21], sputtering [22], chemical vapor deposition[23], vacuum evaporation [24], electro deposition[25], and chemical bath deposition (CBD) [26]. The CBD technique has many advantages over other preparation methods, such as being simple, low-cost, easy to handle, suitable for room temperature, extensive area deposition, and depositing thin films on different substrates.[10, 21]. The CBD method can use the ion-by-ion or cluster method for film growth. Different conditions of preparation can change the properties and the application of thin films.[27]. Recently, researchers have extensively investigated the fabrication of PbS thin films using CBD. Mingyang Liu et al, [28]prepared PbS thin films by chemical bath deposition in a completely organic environment. Narayani et al,[3] Synthesized PbS thin films by an economical chemical method on glass substrates at room temperature to investigate the influence of deposition time on physicochemical and optoelectronic properties, to evaluate its utility as an ammonia gas sensor.. Guodong Zhang et al,[29] prepared PbS thin films on a glass substrate using an innovative wet chemical deposition technique. They noted that there is a strong infrared sensitivity in the as-grown monolayer film without high-temperature sensitization. The one-step production method for NIR PbS detectors was straightforward and user-friendly. Kiran E et al, [30] prepared Pb-doped CdS thin films (PbxCd1-xS) by affordable chemical bath techniques. XRD spectra indicate that Pb is substituting for Cd in the lattice sites of the CdS phase.
Advertisement
In the 1.3 µm wavelength range, several well-established photodetectors are commonly used, especially in applications like telecommunications, medical imaging, and environmental sensing. These detectors are known for their efficiency, reliability, and widespread acceptance in the industry. Here are some of the most prominent ones: InGaAs photodiodes. [31], silicon photodiodes[32], germanium photodetectors [33], and PbS and PbSe photodetectors. Although PbS thin films as a photodetector don’t match the performance of widely accepted industrial detectors, they offer several advantages over well-established photodetectors at wavelengths ≤ 1.3 µm. PbS thin films have potential benefits; they can be more economical to produce than InGaAs detectors. This cost advantage can be significant in applications requiring large-area detectors or high-volume production [34]. PbS films exhibit broadband absorption; they have an inherently broad absorption range in the near-infrared (NIR) spectrum. This makes them suitable for applications needing broad spectral responsiveness[33]. Besides, PbS films hold huge scalability and integration[32]. Although PbS films grown by CBD have long been explored for low‑cost infrared sensing, most reports either focus on growth recipes and basic optical gaps or on device demonstrations using additional processing and electrical characterization. However, using the CBD to grow a simple PbS thin-film broadband photodetector for the spectral range 750 nm to 1300 nm hasn’t been investigated to the best of our knowledge (Figs. 1,2,3).
Fig. 1
Schematic illustration of the CBD reaction with substrates fixed vertically in the chemical bath.
In this study, we demonstrate the fabrication of four PbS thin-film samples with varying thicknesses using CBD. The experimental procedure for growing the PbS films was designed to incrementally increase the film layer-by-layer, aiming to create a clear difference in thickness among the samples. Our goal is to achieve a higher energy gap shift reaching 0.93 eV, corresponding to 1.3 µm. The structural, morphological, and optical properties of the deposited films were examined, and their potential application in photodetection was also investigated.
2 Experimental work
2.1 Materials for thin film preparation
Lead nitrate Pb(NO3)2 with a molecular weight of 331.2098 g/mol, Thiourea NH2CSNH2 as a source of sulfur with a molecular weight of 76.117 g/mol, and sodium hydroxide as a base medium with a molecular weight of 39.997 g/mol were purchased from El-Merey company.
2.2 Sample preparation
The PbS thin films were grown on standard glass substrates of various sizes to suit different measurements. Before depositing the PbS films, the substrates were cleaned by washing with aqua regia and rinsed with distilled water. Then, they were placed in acetone for 10 minutes, washed again with distilled water, immersed in pure ethanol for another 10 minutes, and rinsed with distilled water one last time. For drying, nitrogen gas was used, and afterward, the substrates were prepared for further deposition.
Advertisement
The preparation method can be summarized in the following steps: First, 60 mL of NaOH solution with a molarity of 0.57 M was prepared in a 500 mL beaker and kept under stirring. Second, 20 mL of Pb (NO3)2 with a molarity of 0.175 M was added to the NaOH solution. Then, 250 mL of distilled water was added to dilute the reaction mixture, and the pre-cleaned glass substrates were immersed vertically in the solution. Finally, 20 mL of NH2CSNH2 at a concentration of 1.0 M was added, and stirring was stopped after 20 seconds. After 30 minutes, the substrates were removed from the solution. This method follows the procedure performed by John L. Davis et al [20]. The grown films were rinsed with distilled water and dried with nitrogen gas. They were very mirror-like in uniformity and homogeneous.
In the CBD bath, Pb(NO₃)₂ supplies Pb2⁺ while thiourea releases sulfide under alkaline hydrolysis; the resulting reaction deposits PbS on the substrate, with NaOH controlling pH and lead speciation to moderate nucleation and film growth”. The reaction is expressed in Eqs. 1–5
In this work, successive reactions were performed to grow layers of PbS one after another at room temperature. Starting with four substrates placed in the initial reaction, after 30 minutes, PbS films were grown on each substrate. One of the prepared films was set aside as S1 (stored in a desiccator), while the other three PbS films underwent a second repeated reaction to double the film thickness. Then, after 30 minutes, one more sample was set aside as S2 (doubled PbS layer). This process was repeated two more times, producing S3 and S4. The series includes four samples, each with a higher layer than the previous one. Each time, the films were rinsed with distilled water and dried before re-growing the film on them again. This is schematically illustrated in
3 Characterization techniques
3.1 X-ray diffraction (XRD)
The crystal structure of the thin films was analyzed by X-ray diffraction (XRD) using an Empyrean powder diffractometer. Measurements were performed in the θ-2θ geometry and recorded in the range of 20-80º using Cu Kα radiation (λ=1.5406Å) at 45 kV and 30 mA.
3.2 Scan electron microscopy (SEM) and EDX
The surface morphology of the prepared films was examined by using a Quanta 250 FEG scanning electron microscopy equipped with an Energy Dispersive X-ray (EDX) Analysis Unit
3.3 A UV-VIS-NIR spectrophotometer
The transmission and reflectance spectra of the films in the range 200-2500 nm were measured using a JASCO V-570 spectrophotometer (JASCO Corp.).
3.4 Thin film measurement
The film thickness was measured by a Dektak 150 stylus profilometer. This contact -mode instrument, measures height differences (steps profiles) across the sample surface.
4 Results and discussion
The film thicknesses of S1, S2, S3, and S4 were measured using a Dektak 150 stylus profilometer and found to be 300 nm, 600 nm, 900 nm, and 1200 nm, respectively
4.1 Morphology and structure analysis
The XRD patterns of PbS thin films with different thicknesses are shown in Fig 4. All the films are polycrystalline with (200) preferred crystal orientation. The characteristic reflection peaks of PbS are observed at 25.99°, 30,04°, 43.07°, 51.01°, 62.45° and 71.1° for (111), (200), (220), (311), (400) and (420) Miller planes of the PbS phase with cubic structure (galena) planes, respectively. All samples have nearly the same X-ray peak positions. The inter-planar distance (d) calculated from 2ϴ of the XRD lines are 0.3411, 0.2966, 0.2099 and 0.1788 nm. They are in good agreement with the standard (JCPDS:05-592) interplanar distance of 0.3429, 0.2969, 0.2099, and 0.1790, respectively, corresponding to the planes of cubic PbS (111), (200), (220), and (311).
Fig. 4
X-ray diffraction (XRD) patterns of the PbS thin films for samples S1–S4.
The peaks have nearly the same intensities with increasing film thickness. As PbS films are grown on amorphous glass substrates, the broadening of XRD lines is due to the grain size of the crystallites [35]. The average crystalline size (D) was calculated using Debye-Scherrer's formula [36, 37]
$$D = \frac{k\lambda }{{\beta \cos \theta }}$$
(6)
Where k is Scherer's constant and is equal to ~ 0.9, λ is the X-ray wavelength of CuKα radiation and equals 0.154 nm, θ is the Bragg diffraction angle, and β is the FWHM of the XRD peak appearing at the diffraction angle θ. Peaks (111) and (200) were used in calculating the crystallite size because they are the most intense x-ray peaks, and the averaged values are shown in Table 1. PbS films exhibited a slight increase in the average crystallite size with increasing film thickness. The averaged values calculated from equation 6 are 51 nm, 54 nm, 56 nm, and 66 nm for S1, S2, S3, and S4, respectively. PbS thin films exhibit an increase in crystallite size as thickness increases, which can be attributed to the coalescence between neighboring islands during deposition.
Table 1
summarizes all the calculated values for the four samples which are comparable to the bulk lattice parameter (a = 5.936)
Sample
(hkl)
FWHM β (degree)
d (Å)
a (Å)
Crystallite size D (nm)
Dislocation density ( δ) × 10-4 (nm-2)
Strain ( \(\varepsilon\)) × 10-4
S1
111
0.16
3.428
5.9374
51
3.8
6.8
200
0.16
2.975
5.95
51
3.8
6.7
220
0.21
2.098
5.9340
41
6.0
8.5
311
0.22
1.79
5.9367
40
6.2
8.7
400
0.07
1.4817
5.9268
133
5.6
2.6
420
0.40
1.328
5.9389
24
1.7
14.2
S2
111
0.1535
3.427
5.9357
53.2
3.5
6.5
200
0.1535
2.9707
5.9414
54.9
3.3
6.3
220
0.1535
2.098
5.9340
55.7
3.2
6.2
311
0.255
1.79
5.9367
34.6
8.4
10.0
400
0.307
1.484
5.936
30.3
10.9
11.4
420
0.409
1.327
5.9345
23.9
17.5
14.5
S3
111
0.1279
3.427
5.9357
63.8
2.5
5.4
200
0.1535
2.97
5.94
49.6
4.1
7.0
220
0.0768
2.099
5.9368
111.3
0.8
3.1
311
0.2047
1.79
5.9367
43.0
5.4
8.1
400
0.2047
1.4857
7.16
45.4
4.8
7.6
420
0.307
1.327
5.9345
31.8
9.9
10.9
S4
111
0.1023
3.42
5.9236
79.8
1.6
4.3
200
0.1535
2.971
5.942
53.6
3.5
6.5
220
0.1535
2.1
5.9396
55.7
3.2
6.2
311
0.1535
1.79
5.9367
57.3
3.0
6.0
400
0.2047
1.43
5.72
45.4
4.8
7.6
420
0.1535
1.328
5.9389
63.5
2.5
5.5
Mular indices (hkl), interplanar distance (d), lattice parameter (a), crystallite size (D), dislocation (δ), and strain( ε) for the PbS with different thicknesses
The dislocation density can be derived from the crystallite size using the following formula. [38]:
$$\delta = \frac{1}{{ D^{2} }}$$
(7)
Using the most dominant X-ray peak 200, it is found that the averaged dislocation density for the samples is nearly the same order as the thickness increases. It ranges from 3-4 \(\times\) 10-4 nm-2
The lattice strain is calculated using the following relation. [39] :
$$\varepsilon = \frac{\beta \cos \theta }{4}$$
(8)
The calculated lattice strain is nearly the same with increasing thickness. The average strain is 6 \(\times\) 10-4, however, it increases slightly for S3
The lattice parameter for the cubic structure is given by Eq. ( 9)[39]
Table1 summarizes the calculated interplanar distance and lattice parameter for PbS thin films with different thicknesses for all measured diffraction peaks and the crystallite size, dislocation density, and lattice strain for all the diffraction peaks. However, when comparing parameter values among samples, peaks (111) and (200) are regarded as the most prominent in the XRD for accuracy purposes. As film thickness increases, the (111) and (200) XRD peaks narrow, and the calculated crystallite size grows; the corresponding reductions in estimated dislocation density and micro-strain indicate improved crystalline order.
The surface morphology of the films was examined using scanning electron microscopy (SEM). Fig 5 shows the SEM images of the PbS samples with different thicknesses. Sample S1, with the least film thickness, has regularly well-organised small grains. As the thickness increases, the agglomeration of grains becomes bigger. It is noticeable that the surface roughness increases with increasing thickness due to the growth of isolated islands and together.
Fig. 5
SEM images for PbS films with different thicknesses of investigated samples.
The average agglomeration of grain size from the SEM is about 91nm, 163 nm, 185 nm, and 224 nm for S1, S2, S3, and S4, respectively. SEM micrographs reveal that the microstructure of the PbS films evolves significantly with thickness. Thicker films (S3 and S4) exhibit a more irregular morphology, possibly due to grain coalescence or amorphous intergranular regions
It is worth noting that there is no contradiction between those size values and the crystallite size values calculated above from the X-ray peaks. The grain is larger than the crystallite, as it can be polycrystalline. Moreover, the crystallite size is the main factor affecting the manifestation of the quantization behavior observed in the optical properties of materials when its size is less than 100 nm [40], XRD crystallite size is the size of a coherently diffracting domain; SEM grain size is the projected particle or agglomerate seen on the surface. A single SEM grain can contain many smaller crystallites (subgrains) separated by low‑angle boundaries or defects.
Quantitative and compositional analysis of the samples was conducted using energy-dispersive X-ray (EDX) measurements. EDX analysis was performed for Pb and S at various points on the samples, and Fig 6 Displays the EDX patterns for the investigated samples.
Fig. 6
Energy Dispersive X-ray (EDX) measurements for the investigated samples S1,S2, S3 and S4.
The EDX data indicate that samples S1 and S2 are slightly rich in sulfur, while S3 and S4 are rich in Pb. Strong signals for Si and O were observed due to the glass substrates. The Pb2+ and S2- signals for the film structures are listed in patterns showing multiple main peaks at different energy levels corresponding to the Pb and S elements. The atomic concentrations of each element remain roughly constant as the film thickness increases, likely due to the rate of chemical reaction. Overall, the average atomic percentage of Pb/S is not exactly 1, indicating slight off-stoichiometry with either S or Pb being slightly excessive, see Table 2 This may cause trapped mid-energy within the energy gap of those samples.
Table 2
Atomic concentrations of Pb and S in PbS films at different thicknesses as determined by EDX analysis.
Thickness (nm)
Pb%
S%
S/Pb
comment
300
47
53
1.13
Slight S rich
600
48
52
1.08
Near stoichiometric
900
56
44
0.78
Pb-rich non-stoichiometric
1200
51
49
0.96
Slight Pb-rich
5 Optical properties
The optical properties of the thin films were examined by measuring the transmission (T) and reflectance (R) of the samples. T and R curves are shown in Fig 7 and Fig 8 . According to Lambert's law, the optical transmission spectra decrease noticeably with increasing film thickness. With increasing film thickness, the transmission edge shifts toward longer wavelengths and becomes more gradual with thickness, which may be related to tail states. The transmission spectra of S3 and S4 reach a low value, lower than 20%. Besides, their cutting edges nearly coincide, indicating a close energy gap between them.
Fig. 7
Transmission spectra for PbS films with different thicknesses.
The observed interference behavior in T and R spectra is related to the film thickness and surface morphology. Interference fringes in the transmission and reflection spectra originate from the incident light wave between the air-film, film, and substrate–air interfaces. The appearance of the interference fringes also confirms that PbS films are uniform and smooth. The reflectance of S1 is the highest among all the samples, while it decreases with film thickness in the spectral range (300 to 2500 nm). This can be attributed to the morphological quality of the films. An explanation for that can be related to the grain size and the surface roughness, which increase with thickness, leading to a decrease in reflectance as reported in [41].
5.1 The Energy bands and the absorption coefficient
The energy band gap and absorption coefficient are significant parameters for semiconductor thin films since they are helpful in optoelectronic applications. The energy band is the minimum energy required to free electrons from the semiconductor film's valence band to the conduction band. If a photon is to be absorbed by the film, its energy needs to be at least equal to the value of the energy gap.
The absorption coefficients (α) of films at different wavelengths are determined from the expression [41].
Where d is the film thickness, R and T are the specular reflectance and transmission of the films, respectively Tauc's relations for the direct band gap are used to determine the value of the band gap for PbS thin films with different thicknesses [42], as given in (11)
Where: A is a constant, hν is the incident photon energy, and Eg is the band gap.
The variation of (αhυ)2 versus the photon energy (hυ) of the prepared films with different thicknesses is shown in Fig 9. The band gap Fig 9 Eg of each film was determined from the respective Tauc plot by extrapolating the linear part to the x-axis ((αhν)2 = 0).
As the film thickness increases, the band gap decreases. The values of the energy gap of the prepared films are found to be 1.55 eV, 1.14 eV, 0.97 eV, and 0.93 eV for S1, S2, S3, and S4, as shown in Fig 9. The decrease in the band gap with increasing film thickness is due to the increment in the crystallite size with thickness increase, as indicated from the X-ray peaks calculated above. The relation between film thickness and average crystallite size, as well as the film's energy gap, is shown in Fig 10 . The reported band gap energy of bulk PbS ranges from 0.4 to 0.6 eV[41, 43] .The obtained energy gap values are much higher due to the quantum confinement effect. This manifests in a crystallite size lower than 100 nm [44]. Others reported similar behavior[10, 44, 45].
Fig. 10
Variation in energy gap and crystallite size with film thickness.
Table 3 summarizes a comparison of our results for S1 (the thinnest sample in our study) in thickness, crystallite size, and band gap with previous studies. Our results are consistent with the fact that as film thickness increases, its crystallite size increases and its energy gap decreases.
Table 3
comparison crystallite size and band gap of our work with other literature
Extinction coefficient and refractive index are optical constants that play a key role in a material's properties for optoelectronic applications. The extinction coefficient indicates the attenuation of electromagnetic waves while propagating through the material. The refractive index measures the speed of an electromagnetic wave through a material. For a semiconductor film, the refractive index (nf) measures its transparency to incident spectral radiation and how it is reflected. [41], 49. The complex refractive index is expressed as :
$$n_{f} = n + ik$$
(12)
Where n is the real refractive index and k is the imaginary refractive index (extinction coefficient). The following relations give k and n [41]
The prepared films' extinction coefficient and refractive index as a function of the incident wavelength can be calculated from equations (13) and (14). Fig 11 shows both the absorption and extinction coefficients of the films α (λ), k (λ) as a function of wavelength. Both directly depend on the thickness of the film (i., a thicker film has higher α and k values), except for S4, which has an absorption coefficient lower than S3.
Fig. 11
(a) Absorption coefficient as a function of wavelength derived from the transmission and reflectance spectra. (b) Extinction coefficient of PbS for the films of varying thicknesses.
The observed decrease in the S4 absorption coefficient and the extinction coefficients shown above may be caused by the apparent reduction of extracted absorption and extinction coefficients for thicker films, which is mainly due to increased surface roughness and diffuse scattering that lowers the measured specular reflection reflectivity.
The refractive indices n(λ) of the films as a function of wavelength are plotted in
Figure 12 Equation (14) calculates the refractive index based on wavelength using the measured values of T and R. The refractive index is a critical parameter in optoelectronics, where light waves travel through the material. As a function of wavelength, it greatly influences the propagation of wave behavior.
Fig 12
a Refractive index dispersion curves for PbS films S1–S4. b refractive indices dispersion of the films in the vicinity of 825 nm, showing an optical anomaly for S3 and S4
Figure 12a shows the refractive index dispersion curves exhibiting oscillatory behavior due to interference effects. Refractive index curves for S1 and S2 exhibit near-smooth n(λ) behavior, while S3 and S4 show higher discontinuities Δn at ~ 850 nm, indicating optical inhomogeneity of both samples near this wavelength see
Figure 12b. The value of Δn for S3 is ~ 0.16 and for S4 is ~ 0.12. while for S1and S2 there is slight refractive index change( Δn is only ~ 0.02-~ 0.03). The observation of a small discontinuity in the refractive index at 825 nm that increases with film thickness can be explained as follows. This feature lies close to the optical bandgap (~800 nm) of S1 and may arise from a shift of the absorption edge, reduced Urbach tail, or from thickness‑dependent composition/density changes, as confirmed by EDX Table 2 . Measurement artifacts such as interference, increased surface roughness and diffuse or scattering, can also amplify the apparent discontinuity. To clarify the origin, further investigation the absorption coefficient and Urbach energy need to be done in future
5.3 Electro-Optic Application of As-Prepared Films
One application of PbS films is photo-detection. We tried using the as-prepared films (without surface treatment) as a photo detector. When a semiconductor absorbs a light photon, electron-hole pairs are created inside the semiconductor material (an electron from the valence band will be excited to the conduction band). When an external electric field is applied to that semiconductor (two electrodes with a space between them), charge carriers will move in opposite directions, producing the photocurrent or signal current in the external circuit. This current induces a voltage drop, constituting the detector signal [50].
In an ideal semiconductor, photons with energies below the bandgap, hνmin < Eg, cannot be absorbed, and the relation therefore defines the maximum detectable wavelength is
However, in real thin films, the absorption edge is not perfectly sharp. Various physical processes, including Urbach tail states, defect or impurity levels, phonon-assisted transitions[51, 52], and carrier trapped levels[53] , can extend the absorption into sub-bandgap energies. In addition, measurement artifacts such as scattering or substrate contributions may further broaden the apparent edge. Collectively, these mechanisms smear the bandgap region and introduce an effective broadening. \(\Delta E\), so that the optical absorption onset occurs over a finite energy range rather than at a sharp threshold.
We experimentally studied the photoconductivity of the prepared films using the setup in Fig 13. The as-deposited films were wired for electrodes with drop casting of silver paste on the surface of the films. The distance between electrodes was 30 mm. A 12 V (100-watt) halogen lamp served as the light source. Spax 750 monochromator was used with an IR grating in the 800 nm to 1800 nm spectral range. An amplifying circuit was employed to bias the samples. The signal obtained from the four samples with different thicknesses was then amplified using a Stan/SR510 lock-in amplifier.
Fig. 13
Experimental setup for detecting the signals of the as-deposited films (author’s master’s thesis)
The lock-in amplifier eliminates the presence of a background current inside our sample photocell, which is produced from released charge carriers by heating or background light, not by light absorption. [50]
Figure 14 displays the detected signal from the prepared films when used as a photodetector. As shown in Fig 14, no signals are detected for S1, S2, or S3. Only very noisy signals are observed from S4. This setup should detect a signal generated by the photocurrent produced in the film when light is absorbed and read it as a voltage on the lock-in amplifier. According to equation (15), the energy gap of S4 is about 0.93 eV, so the maximum wavelength can be detected is about 1330 nm. Wavelengths longer than this will not be absorbed or detected. (see Fig. 10). This confirms that S4 cannot detect the photo signal from ~1330 to 1800 nm, indicating that the observed signal in that range Fig. 14 is not a true photo-detected signal. Similarly, there is an upper wavelength limit for photodetection in S1, S2, and S3. This limit shifts to about 800 nm, 1094 nm, and 1280 nm, respectively.No signals were observed at wavelengths lower than those. S4 detected a very weak signal, which can’t be photocurrent as it is detected at wavelengths beyond the cutoff wavelength (1330nm for S4).
Fig. 14
Signal detected from photocurrent for the four different film thicknesses. The shaded area shows the expected absorption range for each sample.
An ORIEL 71180 PbS commercial detector is used to detect the halogen light, as shown in Fig 15. The detector consists of bulk material (energy gap 0.4 eV), so its detectivity range opens to 3100 nm. As shown in Fig 15 The ORIEL PbS signal is three orders of magnitude higher than the S4 signal. An additional proof confirms that the detected signal from S4 is likely due to thermal drift or electrical noise rather than a real signal.
Fig. 15
Comparison between the signal obtained from the commercial PbS detector and the smooth signal from S4
When the halogen lamp was replaced with a diode laser emitting at 825 nm, S1 and S2 detected the laser spectrum. Although S1 has an energy gap at 1.55eV, equivalent to 800 nm, the detection of the 825 nm laser can be attributed to energy gap broadening by some mechanisms as mentioned earlier. The signals obtained from S1 and S2 match the wavelength used and are shown in Fig 16 .The peak's high intensity confirms the possibility of using our as-prepared films as photodetectors. We expect an improvement in the samples' detectivity if they are annealed. As noted in previous work, [54]
Fig. 16
The spectrum of the 825nm diode laser as detected from S1 and S2 samples
No signal has been detected from the higher thickness films S3 or S4. The absorption coefficients at 825 nm remain nearly constant throughout the entire samples. (see Fig 11). The reason for the signal detection failure of higher film thickness may be attributed to defect states or roughness affecting carrier recombination or wave propagation inside the thicker samples. This difference emphasizes the important role of post-absorption processes such as carrier lifetime, mobility, defects, and trap-assisted recombination in determining device performance. [55]. The poor photo-response of S3 and S4 can be linked to Pb-rich composition, which can induce inside defect carriers that increase scattering, preventing optical transition from taking part. This behavior can be responsible for the anomaly observed as refractive index discontinuity in S3 and S4, as shown in Fig. 12, the refractive index dispersion curves.
S3 and S4 are thicker films grown through a successive multilayer process; they can have more interfacial areas and surface roughness, which cause carriers to scatter rather than contribute to radiative transitions. The occurrence of a reduced photoconductive response despite increased absorption may be due to this. Our optical data reveal discontinuities in reflectivity and refractive index, indicating graded interfaces and scattering caused by roughness. This discontinuity aligns with trap accumulation at grain boundaries and interfacial regions, promoting nonradiative recombination and lowering carrier mobility. Our current work focuses on optical diagnostics without thermal or laser annealing. The lack of post-deposition treatment probably keeps defect states and compositional gradients, which contribute to the observed effects behavior. The discontinuities in reflectivity and refractive index support the presence of interface- and defect-mediated mechanisms; however, direct measurement of trap densities and depth-resolved composition is beyond this study’s scope. Future work may include heat or laser annealing to enhance the structural and optical properties of the films. Also, Further investigation of the absorption coefficient and Urbach energy, as well as measuring total R and T with an integrating sphere, can be considered for future work. One hypothesis for the detection of laser light but not broadband illumination is that the laser exposure may induce local annealing in the low-thickness sample, thereby improving the internal structure. This structural modification could explain the enhanced response to laser excitation compared to broadband light.[56]
6 Conclusion
PbS thin films with varying thicknesses (300–1200 nm) were deposited using chemical bath deposition and found to be polycrystalline with a preferred (200) orientation. The optical bandgap decreased as thickness increased, from 1.55 eV to 0.93 eV. Photoconductive response was observed only in the thinner films (300 and 600 nm), while thicker films showed suppressed photodetection due to interfacial disorder and trap-limited transport, despite higher absorption. Discontinuities in reflectivity and refractive index in the thicker samples suggest reduced microstructural integrity or stoichiometric deviations. Refractive index uniformity serves as a useful indicator of film quality and carrier transport efficiency.
Declarations
Conflict of interest
The authors declare no conflicts of interest regarding the research work reported in this manuscript.
Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/.
Publisher's Note
Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
C. Acar, I. Dincer, G.F. Naterer, Review of photocatalytic water-splitting methods for sustainable hydrogen production: review: photocatalysis for sustainable hydrogen. Int. J. Energy Res. 40(11), 1449–1473 (2016). https://doi.org/10.1002/er.3549CrossRef
3.
R. Joshi, N. Gosavi, K. Sali, and S. Gosavi, 2025 “Study of Synthesis Dependent Physicochemical and Optoelectronic Properties of Nanocrystalline Lead Sulfide PbS) Thin Films Deposited using Chemical Bath Deposition Method,” Progress in Physics of Applied Materials, https://doi.org/10.22075/ppam.2025.37005.1134.
4.
A.M. Ahmed, F. Mohamed, A.M. Ashraf, M. Shaban, A.A.P. Khan, A.M. Asiri, Enhanced photoelectrochemical water splitting activity of carbon nanotubes@TiO2 nanoribbons in different electrolytes. Chemosphere 238, 124554 (2020). https://doi.org/10.1016/j.chemosphere.2019.124554CrossRefPubMed
5.
Z.-H. Hou, X. Qian, Q.-J. Cui, S.-F. Wang, L.-D. Zhao, Strategies to advance thermoelectric performance of PbSe and PbS materials. Rare Met. 43(9), 4099–4114 (2024). https://doi.org/10.1007/s12598-024-02774-xCrossRef
S. Seghaier, N. Kamoun, R. Brini, A.B. Amara, Structural and optical properties of PbS thin films deposited by chemical bath deposition. Mater. Chem. Phys. 97(1), 71–80 (2006). https://doi.org/10.1016/j.matchemphys.2005.07.061CrossRef
8.
A.G. Midgett et al., Size and composition dependent multiple exciton generation efficiency in PbS, PbSe, and PbSxSe1–x alloyed quantum dots. Nano Lett. 13(7), 3078–3085 (2013). https://doi.org/10.1021/nl4009748CrossRefPubMed
9.
T. Zeng et al., Scalable hybrid films of polyimide-animated quantum dots for high-temperature capacitive energy storage utilizing quantum confinement effect. Adv. Funct. Mater. 35(16), 2419278 (2025). https://doi.org/10.1002/adfm.202419278CrossRef
Y. Wang et al., Ultraviolet photodetectors based on nanometer-thick films of the narrow band gap semiconductor PbS. ACS Appl. Nano Mater. 5(7), 8894–8901 (2022). https://doi.org/10.1021/acsanm.2c01059CrossRef
12.
K.C. Preetha, T.L. Remadevi, Optimization of PbS thin films using different metal ion sources for photovoltaic applications. J. Mater. Sci. Mater. Electron. (2013). https://doi.org/10.1007/s10854-012-0887-2CrossRef
13.
S. Günes, K. Fritz, H. Neugebauer, N. Sariciftci, S. Kumar, G. Scholes, Hybrid solar cells using PbS nanoparticles. Sol. Energy Mater. Sol. Cells 91(5), 420–423 (2007). https://doi.org/10.1016/j.solmat.2006.10.016CrossRef
S. Zinatloo-Ajabshir, M. Salavati-Niasari, Preparation of magnetically retrievable CoFe2O4@SiO2@Dy2Ce2O7 nanocomposites as novel photocatalyst for highly efficient degradation of organic contaminants. Compos. Part B Eng. 174, 106930 (2019). https://doi.org/10.1016/j.compositesb.2019.106930CrossRef
18.
S. Zinatloo-Ajabshir et al., Novel rod-like [Cu(phen)(2)(OAc)]·PF(6) complex for high-performance visible-light-driven photocatalytic degradation of hazardous organic dyes: DFT approach, Hirshfeld and fingerprint plot analysis. J. Environ. Manage. 350, 119545 (2024). https://doi.org/10.1016/j.jenvman.2023.119545CrossRefPubMed
19.
A. Sharma et al., Synergistic improvement of narrow bandgap PbS quantum dot solar cells through surface ligand engineering, near-infrared spectral matching, and enhanced electrode transparency. ACS Appl. Mater. Interfaces 17(4), 6614–6625 (2025). https://doi.org/10.1021/acsami.4c22334CrossRefPubMed
20.
Y. Hao et al., Characteristics of carrier localization and their effects on minority carrier lifetime in InAs/In0.5Ga0.5As0.5Sb0.5 type II superlattices. Appl. Phys. Lett. 127(8), 082105 (2025). https://doi.org/10.1063/5.0273175CrossRef
21.
T. Beatriceveena, E. Prabhu, A. Murthy, V. Jayaraman, K. Gnanasekar, Highly selective PbS thin film based ammonia sensor for inert ambient: in-situ Hall and photoelectron studies. Appl. Surf. Sci. 456, 430–436 (2018). https://doi.org/10.1016/j.apsusc.2018.06.145CrossRef
22.
Y. Xiao et al., Study of the quasi-single crystalline lead sulfide film deposited by magnetron sputtering and its infrared detecting characteristics. J. Mater. Sci. Mater. Electron. 33(20), 16029–16044 (2022). https://doi.org/10.1007/s10854-022-08494-1CrossRef
23.
W. Zhang et al., Design and optimization of a high-efficiency MPCVD reactor for 4-inch diamond film deposition based on steady-state multiphysics modeling. Diamond Relat. Mater. 157, 112446 (2025). https://doi.org/10.1016/j.diamond.2025.112446CrossRef
24.
I.R.C. Urbiola, J.A.B. Martínez, J.H. Borja, C.E.P. García, R.R. Bon, Y.V. Vorobiev, Combined CBD-CVD technique for preparation of II-VI semiconductor films for solar cells. Energy Procedia 57, 24–31 (2014). https://doi.org/10.1016/j.egypro.2014.10.004CrossRef
25.
M. Alanyalıoğlu, F. Bayrakçeken, Ü. Demir, Preparation of PbS thin films: a new electrochemical route for underpotential deposition. Electrochim. Acta 54(26), 6554–6559 (2009). https://doi.org/10.1016/j.electacta.2009.06.056CrossRef
26.
M.A. Najim, M.A. Hassan, K.I. Hassoon, Benefits of seed layer-assisted deposition of cupric oxide prepared by chemical bath technique. J. Mater. Sci. Mater. Electron. 35(36), 2284 (2024). https://doi.org/10.1007/s10854-024-14026-wCrossRef
27.
S. Moshtaghi, S. Zinatloo-Ajabshir, M. Salavati-Niasari, Nanocrystalline barium stannate: facile morphology-controlled preparation, characterization and investigation of optical and photocatalytic properties. J. Mater. Sci. Mater. Electron. 27(1), 834–842 (2016). https://doi.org/10.1007/s10854-015-3824-3CrossRef
28.
M. Liu et al., Study of the growth mechanism of organic chemical bath-synthesized PbS thin films at atomic resolution. J. Phys. Chem. C 129(9), 4757–4764 (2025). https://doi.org/10.1021/acs.jpcc.4c08384CrossRef
29.
G. Zhang, D. Guan, J. Qiu, D. Huang, H. Shi, C. Leng, One-step high-performance CBD-PbS film deposition technology for room temperature near infrared detectors. Appl. Surf. Sci. 710, 163752 (2025). https://doi.org/10.1016/j.apsusc.2025.163752CrossRef
30.
K.E. Suryawanshi, M.R. Sonawane, Effect of Pb concentration on D.C. conductivity, Seebeck coefficient and photoconductivity of PbxCd1-xS thin films using chemical bath deposition method. Next Res. 2, 100902 (2025). https://doi.org/10.1016/j.nexres.2025.100902CrossRef
31.
G. Cao et al., Multicolor broadband and fast photodetector based on InGaAs–insulator–graphene hybrid heterostructure. Adv. Electron. Mater. 6(3), 1901007 (2020). https://doi.org/10.1002/aelm.201901007CrossRef
D.K. Hwang et al., Ultrasensitive PbS quantum-dot-sensitized InGaZnO hybrid photoinverter for near-infrared detection and imaging with high photogain. NPG Asia Mater. 8(1), e233–e233 (2016). https://doi.org/10.1038/am.2015.137CrossRef
34.
Q. Xu, L. Meng, T. Zeng, K. Sinha, C. Dick, X. Wang, On-chip colloidal quantum dot devices with a CMOS compatible architecture for near-infrared light sensing. Opt. Lett. 44(2), 463–466 (2019). https://doi.org/10.1364/OL.44.000463CrossRefPubMed
S. Moshtaghi, S. Zinatloo-Ajabshir, M. Salavati-Niasari, Preparation and characterization of BaSnO3 nanostructures via a new simple surfactant-free route. J. Mater. Sci. Mater. Electron. 27(1), 425–435 (2016). https://doi.org/10.1007/s10854-015-3770-0CrossRef
A. Hussain, A. Begum, A. Rahman, Characterization of nanocrystalline lead sulphide thin films prepared by chemical bath deposition technique. Arab. J. Sci. Eng. 38(1), 169–174 (2012). https://doi.org/10.1007/s13369-012-0390-3CrossRef
39.
T. Tohidi, K. Jamshidi-Ghaleh, A. Namdar, R. Abdi-Ghaleh, Comparative studies on the structural, morphological, optical, and electrical properties of nanocrystalline PbS thin films grown by chemical bath deposition using two different bath compositions. Mater. Sci. Semicond. Process. 25, 197–206 (2014). https://doi.org/10.1016/j.mssp.2013.11.028CrossRef
D. Vankhade, T.K. Chaudhuri, Effect of thickness on structural and optical properties of spin-coated nanocrystalline PbS thin films. Opt. Mater. 98, 109491 (2019). https://doi.org/10.1016/j.optmat.2019.109491CrossRef
A.K. Mishra, C. Rana, S. Saha, Fabrication and comparison of heterojunction solar cells from CdS/PbS nanoparticles and CdS/PbS bulk. Nano Express 1(2), 020024 (2020). https://doi.org/10.1088/2632-959X/abab16CrossRef
44.
S. Gorer, A. Albuyaron, G. Hodes, Quantum-size effects in chemically deposited, nanocrystalline lead selenide films. J. Phys. Chem. 99(44), 16442–16448 (1995). https://doi.org/10.1021/j100044a036CrossRef
L.R. Singh, M.A. Hussain, Effect of doping concentration on the optical properties of nanocrystalline Zn doped PbS thin films deposited by CBD method. CL 17(11), 583–591 (2020). https://doi.org/10.15251/CL.2020.1711.583CrossRef
47.
B. Abdallah, A. Ismail, H. Kashoua, W. Zetoun, Effects of deposition time on the morphology, structure, and optical properties of PbS thin films prepared by chemical bath deposition. J. Nanomater. 2018(1), 1826959 (2018). https://doi.org/10.1155/2018/1826959CrossRef
48.
A.N. Fouda, M. Marzook, H.M. Abd El-Khalek, S. Ahmed, E.A. Eid, A.B. El Basaty, Structural and optical characterization of chemically deposited PbS thin films. SILICON 9(6), 809–816 (2017). https://doi.org/10.1007/s12633-015-9399-zCrossRef
Richard S. Quimby, “chapter 13: Optical Detectors,” in Photonics and Lasers: An Introduction, Published by John Wiley & Sons, Inc., Hoboken, New Jersey.
51.
I. Bouderbala, A. Guessoum, S. Rabhi, O. Bouhlassa, I. Bouras, Optical band-diagram, Urbach energy tails associated with photoluminescence emission in defected ZnO thin films deposited by sol-gel process dip-coating: effect of precursor concentration. Appl. Phys. A (2024). https://doi.org/10.1007/s00339-024-07366-1CrossRef
52.
M. Nagaraja, P. Raghu, H.M. Mahesh, J. Pattar, Structural, optical and Urbach energy properties of ITO/CdS and ITO/ZnO/CdS bi-layer thin films. J. Mater. Sci. Mater. Electron. 32(7), 8976–8982 (2021). https://doi.org/10.1007/s10854-021-05568-4CrossRef
53.
B. Choudhury, A. Choudhury, Oxygen defect dependent variation of band gap, Urbach energy and luminescence property of anatase, anatase–rutile mixed phase and of rutile phases of TiO2 nanoparticles. Phys. E Low-Dimens. Syst. Nanostruct. 56, 364–371 (2014). https://doi.org/10.1016/j.physe.2013.10.014CrossRef
S. Kouissa, A. Djemel, M. S. Aida, and Djouadi M. A., “Characterization and Simulation of PbS Photoconductors Prepared by Chemical Bath Deposition.,” in SENSORDEVICES 2015, Venice, Italy: International Academy, Research, and Industry Association (IARIA) POD Publisher Curran Associates, Inc. 2015, pp. 1–6.
56.
L. Fan, Z. Huang, Q. Lü, G. Liu, G. Qiao, J. Liu, Effect of two-step annealing on optoelectronic properties of lead sulfide thin films. Acta Opt. Sinica (2023). https://doi.org/10.3788/AOS222038CrossRef
