Facile chemical spray deposition of Ag-nanowire films: tailoring their structural, optical, and electrical properties for application as TCEs

Abstract

High-quality silver nanowires (Ag-NWs) with diameters below 200 nm were successfully deposited on glass substrates using a facile spray coating technique, forming transparent conductive electrodes (TCEs) for use in perovskite solar cells (PSCs). The impact of film thickness on the structural purity, surface morphology, optical behavior, and electrical transport properties of the Ag-NW films was thoroughly examined using advanced characterization techniques, including XRD, XPS, FE-SEM, FIB, AFM, UV-visible-NIR spectroscopy, Hall effect analysis, and four-probe resistance studies. The FE-SEM and FIB analyses revealed that the Ag-NWs possessed diameters ranging from 42 to 180 nm and lengths from 2.01 µm to 2.5 µm. Notably, the Ag-3 NW film demonstrated enhanced optical and electrical transport characteristics, achieving an exceptional figure of merit (45.02 × 10⁻⁴ Ω⁻¹) and low sheet resistance (18.1 Ω □⁻¹). The PSC devices incorporating the Ag-NW electrodes exhibited a remarkable efficiency of 11.6%, highlighting their potential for next-generation solar energy applications. Hence, the results obtained confirm the viability of Ag-NW thin films in advancing PSC technology.

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1. Introduction

Transparent conducting electrodes (TCEs) are beneficial for the fabrication of optoelectronic gadgets because of their distinctive qualities, such as good optical transparency and high electrical conductivity.¹ TCEs serve as an essential functional layer in solar cells, light-emitting diodes (LEDs), batteries, touchscreen displays, etc.^1,2 Smart modern electronic gadgets such as smartphones, smart TVs, digital navigators, smart computers, smart sensors, and smart wearable devices are some of the important advancements achieved with the application of TCEs.^1,3,4 The most widely utilized and commercially accessible TCE materials in optoelectronic platforms are fluorine (F)-doped tin oxide (SnO₂) (FTO) and indium-tin oxide (ITO) owing to their low sheet resistance of 10–20 Ω □⁻¹ and good optical transmittance values above 90%.^5,6 Nevertheless, the use of FTO/ITO in flexible devices is subject to numerous restrictions, including their brittleness and lack of mechanical flexibility.^3,7 Furthermore, the processing costs associated with manufacturing FTO/ITO are high due to the high vacuum requirements. Also, indium element sources are scarce due to the widespread use of ITO. Compared to ITO systems, the optimization procedure of the FTO electrodes is comparatively more challenging, and their low transparency is mainly due to anion doping and the requirement for a thicker glass substrate.^3,7 Our motivation to explore low-cost, alternative TCE materials without FTO/ITO was sparked by recent developments in other oxides, metal nanowires, carbon nanotubes (CNTs), graphene oxide (GO), and organic materials.^1,8,9 However, alternative oxide-based TCOs such as Al-doped ZnO and Sb-doped SnO₂ also exhibit poor mechanical properties, which limit their usage in flexible devices.^10,11 In the case of CNTs and GO system-based organic TCE materials, their usage is limited due to their low figure of merit values.¹² The nanowires reported thus far are metallic (Cu, Pd, Au, and Ag), semiconducting (InP and GaN), and superconducting (YBCO), which have attracted considerable research interest from researchers in recent years.^2,13 Presently, newly explored metal nanowires have shown promising results from a TCE point of view. In particular, metal nanowires (Cu and Ag) exhibit good electrical and optical properties, along with excellent mechanical behavior, facilitating their application in flexible devices.^14,15 Given that Ag-NWs have distinct physiochemical, optical, and mechanical properties surpassing that of their bulk counterparts, they have been the subject of greatest research.³ In recent years, several alternative methods have been explored for synthesizing Ag NWs.^2,13 These methods include electrochemical approaches, hydrothermal method, wet chemical synthesis, solvothermal method, polyol method, and thin film techniques.^16,17 Among them, thin film technology (physical vapor deposition [PVD] and chemical vapor deposition [CVD]) offers an effective way to fine-tune the optical and electrical properties of metal nanowires.¹⁸ Ag-based thin-film nanomaterials have been prepared using PVD techniques such as magnetron sputtering, PLD, and E-beam evaporation.^19–21 The primary benefit of chemical synthesis techniques over vacuum-based physical approaches is the uniform distribution of formed nanostructure. Margulis et al. reported the synthesis of Ag NWs with atomic dimensions (0.4 nm diameter) and lengths in the micrometer range using the soft template approach.³ The most popular technique for producing silver nanowires is CVD, and in the beginning, the usage of templates, such as hard and soft templates, were well studied.¹⁶ Several studies have been conducted to improve the optoelectronic properties of Ag-NWs. Among them, the spin coating-based CVD technique is one of the simplest ways to coat metallic nanomaterials.²² Ag-NW thin films exhibit similar TCE properties to FTO/ITO electrodes, along with good mechanical properties such as flexibility and bendability.²³ For example, a nanowire structure with a diameter in the nanometer (nm) scale, length in the micrometer (µm) scale and a good figure of merit value leads to better TCE properties.^21,24 Ag nanowires are frequently used in the research and development of novel materials used as catalysts and functional materials.^25,26 Spray pyrolysis is one of the ways to deposit metallic nanowires, even on large areas.²⁷ Moreover, it is feasible to effectively apply broad-scale nanowire harvesting using this technique. The advantages of the spray technique for the creation of nanostructures were briefly presented by Das et al.²⁸ The adoption of this method is motivated by its affordability, ease of use, vast production capacity for industrial application, no need for a template, and high output with improved control over dimensions.

In this study, high-quality silver nanowire thin films were fabricated using the spray pyrolysis method and their thickness optimized on glass substrates for use as TCEs in PSC devices. Optimal tuning of the concentration of silver, PVP, and cupric chloride ensured the formation of Ag-NWs with diameters below 200 nm. Advanced characterization techniques including XRD, XPS, FE-SEM, FIB, AFM, UV-visible-NIR spectroscopy, Hall effect measurements, and resistance analysis were employed to assess the properties of the deposited Ag-NWs. Their charge transport behavior was studied based on film thickness, demonstrating the viability of the optimized low-thickness Ag-NW films as bottom electrodes in PSC devices. The photovoltaic performance of the Ag-NW-based PSCs was systematically compared with commercially available FTO/ITO reference electrodes through efficiency measurements and electrochemical impedance spectroscopy, confirming the potential of Ag-NW films for application in next-generation solar technology.

2. Experimental details and characterization

2.1. Materials

Silver nitrate (AgNO₃ 99%, MW: 169.30 g mol⁻¹), ethylene glycol (EG, 99%, MW: 62.07 g mol⁻¹), cupric chloride (CuCl₂·2H₂O 98.5%, MW: 170.48 g mol⁻¹) and polyvinylpyrrolidone (PVP K90, MW: 360 000 g mol⁻¹) were acquired from Sigma Aldrich. Ethanol (99.9%, MW: 46.07 g mol⁻¹) and acetone (99%, MW: 58.08 g mol⁻¹) were purchased from Finar Chemical.

To deposit the optimized Ag nanowire thin films, the results of the corresponding electrode-based PSC devices were compared to commercial FTO with a sheet resistance of 13 (Ω □⁻¹) and ITO with a sheet resistance of 12 (Ω □⁻¹), which were used as the bottom electrode in the reference devices. The electron transport layer (ETL), perovskite active layer, hole transport layer (HTL), and Au top electrode in the (n–i–p) device structure were deposited using the following chemicals: TiCl₄ (99.999), methylammonium iodide, lead(II) iodide (PbI₂), lithium bis(trifluoromethane) sulfonimide salt (Li-TSFI, 99.95%), tetrahydrofuran (THF), chlorobenzene methylamine ethanol solution (30 wt%, ME) and 4-test-butyl pyridine (TBP) were purchased from Merck-Sigma-Aldrich. Dimethyl sulfoxide (DMSO) and dimethylformamide (DMF) solvents were purchased from Sigma-Aldrich and were used exactly as supplied, requiring no further purification. All chemicals were analytical grade and used as received without further purification.

2.2. Synthesis and spray deposition of silver nanowires

The silver-based spray precursor solution was prepared using the polyol technique (Fig. 1a), where ethylene glycol (EG) served as both a reducing agent and solvent, silver nitrate (AgNO₃ 99%) acted as a precursor, PVP functioned as a capping agent, and chloride salts regulated the reaction. In the preparation process, 10 mL of ethylene glycol (99%) was mixed with 0.2 M silver nitrate and stirred for 30 min. Simultaneously, 10 mL of ethylene glycol was combined with 0.6 mL of cupric chloride and 0.2 mL of polyvinylpyrrolidone, followed by magnetic stirring for 1 h. The resulting solution was then carefully added dropwise (0.2 µL min⁻¹) into the silver nitrate solution with continuous stirring for 2 h. The final mixture was transferred to a Teflon-lined autoclave and heated at 180 °C for 2 h, allowing the reaction to proceed. Upon spontaneous cooling to room temperature, the Ag-NWs were purified using DI water and acetone solvents. To eliminate residual moisture, the purified solution was subjected to vacuum drying at 150 °C for 2 h, ensuring the production of high-quality Ag-NWs for further applications.

2AgNO₃ + CuCl₂ + 2H₂O = 2Ag + Cu (NO₃)₂·2H₂O + Cl₂↑ (1)

Fig. 1 (a) Schematic of the synthesis of Ag-NWs using the polyol method, (b) spray deposition of Ag-NW thin film, and (c) fabrication of Ag-NW electrode-based PSC device.

A 0.05 M spray precursor solution was prepared by dispersing 1.348 g of silver nanowires in a binary solvent system comprising 200 mL of ethanol and 50 mL of ethylene glycol. The choice of solvent critically influences the physicochemical environment during deposition. Ethanol, characterized by its high volatility, facilitates rapid solvent evaporation, thereby intensifying the local concentration gradient at the substrate interface and promoting anisotropic film growth.²⁹ In contrast, ethylene glycol is a polar solvent, it has high-boiling-point and acts as a stabilizing medium for the intermediate silver nanostructure and modulates the reduction kinetics through its coordination behavior. During the nucleation process, ethylene glycol effectively controls the size of the silver nanowires at a suitable thermodynamical processing temperature. This dual-solvent strategy effectively governs both the nucleation rate and the diffusion-limited growth regime, enabling controlled elongation and alignment of the formed nanowires.²⁹ Equally, the size of the Ag NWs is determined by the balance between nucleation density and silver concentration.³⁰ An excessive nucleation density, often driven by a high precursor concentration, leads to competition for growth resources and results in polydisperse, entangled morphologies.²⁹ Conversely, insufficient nucleation yields sparse growth and incomplete wire formation. A finely tuned silver concentration ensures that each nucleation site receives adequate feedstock for directional growth, enabling the formation of a uniform nanowire morphology with minimal branching or agglomeration.³⁰ This balance is essential for achieving reproducible, high-aspect-ratio Ag nanowires suitable for high-performing devices.

Ag-NW thin films were deposited onto glass substrates (7.5 × 2.5 cm²) using the spray pyrolysis technique. The glass substrates were subjected to a meticulous cleaning process, including rinsing with distilled water, washing with soap solution, and ultrasonic treatment for 10 min. To further eliminate organic contaminants, ultrasonic cleaning in IPA and distilled water was performed for another 10 min, followed by a final acetone ultrasonic treatment. The cleaned substrates were stored under vacuum and dried at 150 °C to ensure their purity. The spray coating process was carried out in dynamic mode (Fig. 1b), with constant deposition parameters, as follows: substrate temperature (250 °C), precursor solution volume (50 mL), aerosol pressure (2 bar), substrate-to-nozzle distance (18 cm), spray coverage area (10 × 5 cm²), and spray duration (60 s). During thin film formation, the nucleation thermodynamics is critically influenced by the substrate temperature as well as the physicochemical properties of the solvent, particularly polarity and viscosity. Solvents with high viscosity, such as ethylene glycol, limit the precursor mobility and reduce the likelihood of particle agglomeration, thereby promoting uniform nucleation. In contrast, low-viscosity solvents such as ethanol facilitate the enhanced lateral growth of adatoms, favoring two-dimensional nanowire growth.³¹ The subsequent pyrolytic decomposition pathways are outlined as follows:²⁹

The film thickness was controlled by adjusting the number of spray cycles, with Ag-NW thin films sprayed 5, 10, 15, 20, and 25 times to achieve different thickness levels. A knife-edge method was used to mask the substrate corners for thickness measurements. A stylus profiler confirmed that the films deposited at 5, 10, 15, 20, and 25 spray cycles had thicknesses of approximately 86 nm, 112 nm, 47 nm, 135 nm, and 186 nm, respectively. Film thickness plays a critical role in determining the bonding mechanism between the thin film layer and the glass substrate, directly influencing the mechanical properties, thermal stability, and long-term performance of the device.³⁰ Thin films with a lower thickness may exhibit poor adhesion due to their limited contact area and elevated residual stress, causing them to crack during the nucleation process. In contrast, Ag NW thin films deposited with a higher film thickness offer better stress relaxation and stronger van der Waals and chemical bonding, enhancing their interfacial adhesion and the device durability.³⁰ Ag nanowire thin films also help accommodate the thermal expansion mismatch between the film and glass, reducing interfacial strain during the formation of the films. Therefore, selecting the optimal thickness range is essential to ensure robust bonding, minimize interfacial defects, and support an effective charge transfer process, especially in metal oxide-based semiconducting devices.³⁰ These films are designated as Ag-1, Ag-2, Ag-3, Ag-4, and Ag-5, with varying properties tailored for optimal TCE applications in PSC devices.

2.3. Characterization

The analysis of the structure and crystallinity of Ag-NWs was conducted using a range of advanced characterization techniques. X-ray diffraction (XRD) patterns were recorded using a Bruker D8 Advance X-ray diffractometer with Cu Kα radiation (λ = 0.15418 nm), while the chemical composition was investigated via SPECS X-ray photoelectron spectroscopy (XPS) using non-monochromatic Al Kα radiation (1486.6 eV). The cross-sectional surface morphology and compositional distribution of the Ag-NW films were examined through focused ion beam (FIB; Thermo Fisher® Helios G4-UX) and field-emission scanning electron microscopy (FE-SEM). Surface characteristics were further analyzed using atomic force microscopy (AFM; ANTON PARR) with a probe radius of >10 nm and a scan area of 15 × 15 µm². To explore the structural and nanocrystalline properties, high-resolution transmission electron microscopy (HR-TEM; Thermo Fisher® Titan™ Themis) was employed at an operating voltage of 300 kV. The optical properties of the Ag-NW films were assessed through UV-Vis transmittance measurements using a Shimadzu UV-2401PC spectrometer in near-normal incident mode. Estimation of the film thickness was carried out using an Ambios Technology Inc. XP-1 surface profiler, while optical bandgaps were determined via the Tauc relation using the spectral transmittance data. Electrical charge transport properties were measured at room temperature using the Hall effect (Ecopia, HMS 3000) in the conventional van der Pauw geometry. To evaluate the resistance stability of the Ag-NW films, temperature-dependent four-probe analysis was performed. All measurements were conducted at room temperature to ensure consistency and accuracy in the characterization process.

2.4. Fabrication of Ag-nanowire electrode-based PSC devices

The reference PSC was initially fabricated using commercially available FTO and ITO electrodes, exhibiting a sheet resistance of 13 Ω □⁻¹ and 12 Ω □⁻¹, respectively. As a promising alternative, we introduced a spray-deposited Ag-NW electrode, demonstrating superior conductivity (18 Ω □⁻¹) and excellent transparency (above 90%) for the fabrication of PSCs. Both devices were fabricated using identical processes and conditions to ensure a consistent comparison. The PSC devices incorporating the FTO/ITO electrodes were constructed based on previously reported methodologies, validating the effectiveness of the proposed Ag-NW electrode in advancing solar cell technology.³² The process for the fabrication of the spray-deposited Ag-NW thin film electrode-based PSC device closely followed that of the FTO/ITO electrodes, with minor variations during the preparation of the photoanode. Commercially available TiO₂ was employed as the electron transport layer (ETL) across all devices. The TiO₂ ETL solution was spin-coated onto the Ag-NW/FTO/ITO electrodes at 4000 rpm for 30 s, followed by in situ annealing at 450 °C for 2 h, allowing them to cool naturally to room temperature.

The electrodes were systematically transferred into a controlled glove box environment (VTI-vacuum technology) to facilitate the precise deposition of the perovskite (methylammonium lead iodide, MAPI) layer and hole transport layer (HTL) (Spiro-MeOTAD) using an advanced spin-coating technique. A two-step spin coating process was meticulously employed to ensure the formation of highly uniform and defect-free MAPI₃ perovskite and Spiro-MeOTAD layers, optimizing the morphology and interfacial properties of the film. The precursor solutions for MAPbI₃ and Spiro-MeOTAD were carefully engineered based on established research protocols, achieving a finely tuned composition tailored for enhanced charge transport and superior photovoltaic performances.³² Several trials were performed to optimize the thickness of the perovskite and HTL layer coatings. Eventually, an appropriate mask was used for the gold (Au) electrode with a 120 nm film deposited by a sputtering process. Three series of devices were fabricated and tested following similar fabrication procedures.

2.5. PSC testing

Utilizing a Sol3A Class AAA Solar Simulator, the power conversion efficiency (PCE, η%) of the Ag nanowire thin film and conventional FTO/ITO electrode-based PSC devices was meticulously analyzed through photocurrent–voltage (J–V) characterization under standard AM1.5 illumination conditions. A 150 W Xe light lamp with a precisely controlled light intensity of 100 mW cm⁻² was employed for measurement accuracy, with an Oriel SRC1000TC reference cell utilized for calibration to ensure reliability. Furthermore, the electrochemical impedance spectroscopy (EIS) analysis was conducted using a PARSTAT4000A AMETEK Scientific Instruments electrochemical workstation, operating at 10 mV across a broad frequency range of 0.1 Hz to 1 MHz. This comprehensive approach enabled the in-depth assessment of the charge transport dynamics and interfacial resistance, facilitating deeper insights into the electrical performance of the fabricated PSC devices.

3. Results and discussion

3.1. Structural analysis

The XRD analysis of the Ag thin films revealed a well-defined nanostructured film with a distinct polycrystalline nature (Fig. 2a). The diffraction peaks were systematically indexed to correspond with a face-centered cubic (FCC) crystal structure, exhibiting a high degree of crystallinity and phase purity. The identified space group, Fm-3m, aligns precisely with ICSD card No. 44387, confirming the structural integrity and lattice arrangement of the Ag thin films. This correlation underscores the successful fabrication of Ag nanostructures, optimized for advanced optoelectronic applications.²² The phase purity of the Ag-NW films was confirmed by the absence of secondary phase peaks associated with Ag₂O₃ and other Ag-based oxides in the XRD patterns, highlighting the successful synthesis of silver nanowires. However, a notable variation in crystallographic characteristics was observed as a function of film thickness, particularly with an increasing number of spray coating cycles. Specifically, the intensity of the (111) peak was progressively enhanced with an increase in the film thickness, indicating improved crystallinity and structural coherence, which plays a crucial role in optimizing the electrical and optical properties of the film for advanced applications.²⁵ Also, the XRD peak intensity exhibited a progressive enhancement with an increase in the width (radius) of the Ag-NW thin film as the number of spray coating cycles increased, correlating with an increase in the film thickness. Notably, the intensified peaks observed for the higher thickness films indicate an improvement in crystallinity, suggesting enhanced structural order and phase purity. In contrast, the film with a lower thickness displayed lower peak intensities, which is attributed to its relatively narrow nanowire width, signifying its nanocrystalline nature with smaller grain structures. Additionally, a critical trend was observed as the film thickness increased, where the Ag-NW length was reduced, particularly in Ag-4 and Ag-5 films, suggesting an interplay between the deposition conditions and nanowire growth dynamics. This variation in nanowire length and width significantly influenced the XRD peak position, demonstrating a direct correlation between the morphological characteristics and diffraction pattern shifts. A subtle shift in peak position was further noted in the XRD spectra as a function of Ag-NW film thickness (Fig. 2b), reinforcing the impact of structural modulation on the crystallographic orientation. For precise lattice parameter evaluation, Rietveld refinement was conducted using the GSAS-II software, with the results summarized in Table 1. The Rietveld analysis of the Ag-NW thin films (Fig. 2c) revealed minor variations in their lattice parameters compared to the standard Ag thin films, reflecting subtle structural adjustments induced by variations in the film thickness. The lattice parameters of the Ag NWs thin films significantly varied compared to the standard Ag lattice parameter value of a = b = c = 4.112 Å.³³ Table 1 shows the variation in lattice parameters on with an increase in film thickness. Table 1 and Fig. 2d illustrate the average crystallite size (L) values, derived from the full width at half maximum (FWHM) of the key diffraction planes of (111), (200), (220), (311), and (222). These values were precisely calculated using the Scherrer equation, ensuring the accurate estimation of the nanoscale crystalline dimensions.

Fig. 2 (a) XRD pattern of the spray-deposited Ag-NW thin films with different film thicknesses. (b) Variation in XRD peak shift upon increasing the film thickness. (c) Rietveld analysis of the Ag-3 thin film. (d) Crystallite size-texture behavior of the Ag thin films as a function of film thickness.

Table 1 The structural, surface, and optical characteristics of the Ag NW thin films compared to the standard values

Samples	Lattice parameter (Å) a = b = c	D (nm)	TChkl	Transmittance (%)	Thickness (nm)	Roughness (nm)	Eg (eV)	Ref.
Ag bulk	4.071	22.3	—	>90	24	8.9	2.5	2 and 39
Ag film	4.18	30.6	—	>90	6.7	5.4		14 and 40
Ag-1	4.15	26.0	1.015	66.5	86	28	2.55	Present work
Ag-2	4.13	32.0	1.077	74.5	112	32	2.50
Ag-3	4.12	22.8	1.113	97.8	47	22	2.49
Ag-4	4.14	34.8	1.102	81.8	135	38	2.54
Ag-5	4.17	38.4	1.013	66.7	186	52	2.57

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The systematic evaluation of these diffraction peaks provides deep insights into the structural refinement, crystallinity, and grain size distribution of the Ag-NW thin films, further reinforcing their suitability for advanced optoelectronic applications.²² The crystallite size of the Ag-NW films exhibited a systematic variation with an increase in the film thickness, which was primarily influenced by the peak broadening effect observed in the XRD patterns. Notably, the Ag-3 NW film displayed the smallest crystallite size of 22.8 nm, whereas the Ag-5 NW film demonstrated a significantly larger crystallite size of 38.4 nm, highlighting the direct correlation between film thickness and crystalline growth dynamics. Furthermore, the significant variation in film thickness controls the texture behavior (orientated growth) Ag-NWs, indicating their structural reorganization during the nucleation process at the nanoscale level. To quantify these changes, the texture coefficient (TC_hkl) of the films was meticulously computed using the Harris equation, providing insights into the preferred crystallographic orientations and their impact on film performance,³⁴ as follows:

where N (5) is the number of reflections seen in the XRD pattern of the deposited films, I_hkl is the measured intensity of the hkl plane, and I_0hkl is the standard intensity of the hkl plane from the ICSD No: 44387 file.²² The Ag-NW thin films demonstrated a direct correlation between texture coefficient (TC_hkl) and crystallographic orientation, where the films with higher TC_hkl values exhibited a stronger preferred orientation, while those with lower TC_hkl values displayed a more random structural arrangement. The estimated TC_hkl values for the Ag-NW thin films are systematically presented in Fig. 2d, with specific numerical data detailed in Table 1. Furthermore, the XRD analysis confirmed that the (111) plane serves as the primary textured (TC_hkl) plane in the Ag-NW films, reinforcing its role in defining the overall crystallographic alignment and material properties.³⁵ However, the Ag NW films exhibit an increasing trend in TC_hkl values up to the Ag-3 film, beyond which the TC_hkl values decreased due to several structural defects. At lower film thicknesses, the interfacial region strongly affects the film orientation due to the dominant nanoscale interactions and strain from the substrate. As the film thickness increases, these effects weaken, resulting in a more relaxed structure and less directional texture. Also, the preferred orientation of spray-deposited Ag-NW thin films depends on several factors, including the selected precursor, spray rate, and substrate temperature.³⁶ Hence, the texture of the film has a strong influence on its charge transport properties.

3.2. X-ray photoelectron spectroscopy study

To gain deeper insight into the fundamental charge transfer mechanisms, the oxidation states of the elements in the Ag-NW thin films were meticulously analyzed using XPS study. The survey scan spectra of the selected Ag-3 and Ag-5 films (Fig. 3a) reveal distinct variations in the Ag peak intensity, directly correlating with the film thickness. This observed intensity shift underscores the influence of the structural parameters on electronic properties, further validating the role of Ag-NW film thickness in modulating the charge transport behavior.¹⁴ Also, the C 1s peak observed at 285 eV is identified when the film was exposed to the surrounding air.³⁷ To determine the appropriate charge state of the Ag element, Gaussian peak fitting was applied to the narrow scan spectrum. The narrow scan XPS spectra for the Ag 3d_3/2 and Ag 3d_5/2 peaks of the Ag-3 and Ag-5 nanowire thin films are presented in Fig. 3b, revealing distinct electronic states and binding energy variations. The presence of Ag in the 2+ oxidation state contributes to the higher binding energy observed at 374.3 eV for the Ag 3d_3/2 peak, while the Ag 3d_5/2 peak appears at 368.4 eV, indicating strong electronic interactions within the nanowire structure. These findings highlight the influence of film thickness on the electronic properties of Ag-NWs, reinforcing their role in the optimizing charge transport dynamics for advanced optoelectronic applications.³⁸ Variations in the oxidation state of silver significantly influence the electrical properties of Ag-based films, particularly in Ag nanowire-based electrodes.¹⁴ Metallic silver (Ag⁰) provides excellent conductivity and forms continuous charge transport pathways, whereas oxidized forms such as Ag⁺ introduce semiconducting behavior, which disrupts these pathways and increase the contact resistance, especially at nanowire junctions.³⁸ Even partial oxidation can reduce the conductivity by over 50%, as observed in Ag-NW transparent electrodes exposed to air or elevated temperatures. Maintaining a uniform and predominantly metallic oxidation state across the film enhances the charge transport by preserving low-resistance junctions and stable percolation networks.³⁸ It was identified that the film thickness increased from Ag-3 to Ag-5 film, and a slight shift in their peaks to the lower binding energy direction was observed. Also, it is noted that this variation in peak shift is not much affected by the charge state of the Ag element.¹⁴ At a higher film thickness, the binding energy decreased, which may lead to improved electrical and optical properties in Ag-NW films.¹⁴ It has been reported that the most suitable film thickness may increase the free carrier concentration and lead to higher mobility values.³⁸ However, the variation in peak shift may also be due to the variation in the deposition parameters/conditions.

Fig. 3 (a) X-ray photoelectron spectra of the selected Ag-3 and Ag-5 NW films obtained by survey scan. (b) Core level X-ray photoelectron spectra of the Ag-3 and Ag-5 NW thin films.

3.3. FE-SEM and EDS analysis

Fig. 4a–e present the high-resolution FE-SEM images, illustrating the morphological evolution of the Ag-NW thin films deposited via spray-doping at varying thickness levels. A transformation from silver nanoparticles to well-defined nanowire structures is observed as a direct function of film thickness, indicating a progressive shift in surface topology and nanowire formation. This structural transition highlights the influence of the deposition parameters on the growth of the nanowires, demonstrating the critical role of thickness modulation in optimizing the uniformity and conductivity of films for advanced applications.⁴¹ Fig. 4c reveals the presence of highly oriented Ag-NWs with a uniform size distribution across the entire measurement area, demonstrating exceptional structural consistency. During the formation of Ag nanowire thin films, the nucleation process is effectively controlled by the deposition temperature, where Ag atoms migrate along the nanowire surface due to the deposition temperature, causing the wires to fragment into spherical particles, a process known as spheroidization.⁴² This disrupts the percolation pathways essential for conductivity. Additionally, thermal stress can weaken the junctions between nanowires, further compromising the film homogeneity. Ag-NW films typically fail beyond 200–250 °C, but protective coatings such as SnO₂ shells have been shown to significantly enhance their thermal stability. For instance, Ag@SnO₂ core–shell structures remained stable up to 500 °C for short durations and maintained their performance for over 40 h at 250 °C.⁴² Compared to the higher film thickness Ag-NW coating (Ag-5 film), the Ag-3 NW film exhibits significantly enhanced homogeneity, indicating its optimal deposition conditions. Notably, in the films with a thickness exceeding 47 nm (Ag-3), Ag nanoparticles are almost entirely absent, signifying improved nanowire formation. Furthermore, an increase in film thickness leads to a substantial enhancement in the Ag-NW dimensions, with the diameter expanding from 46 nm to 198 nm and length increasing from 2.2 nm to an impressive 14.2 µm (Fig. 4f). In contrast, the films with lower thickness levels (Ag-1 and Ag-2) show sparse Ag nanowire formation, which is attributed to the inadequate concentration of silver ions necessary for effective nucleation. These findings underscore the critical role of film thickness in determining the nanowire uniformity, growth dynamics, and overall optoelectronic performance. Films deposited at higher thicknesses demonstrated enhanced yield, achieving an exceptional aspect ratio optimized for superior performance. The morphological characteristics of Ag-NWs were found to be highly sensitive to the spray solution concentration and deposition parameters, directly influencing the nanowire formation dynamics. In the spray solution, the growth-regulating agent played a pivotal role in the precise synthesis of high-quality Ag-NWs, ensuring controlled nanowire elongation, suppression of aggregation, enhancement of structural uniformity, and long-term stability. This strategic optimization of the synthesis conditions is instrumental in tailoring Ag-NWs for next-generation optoelectronic and energy applications.³ In this process, polyvinyl pyrrolidone (PVP) played a crucial role as a capping agent, effectively inhibiting nanoparticle aggregation, while promoting anisotropic growth by selectively binding to specific crystallographic facets. This strategic interaction facilitated the controlled elongation of the silver nanowires, ensuring uniformity in their structural morphology and optimizing their functional properties for advanced optoelectronic applications.⁴³ A CuCl₂·2H₂O-based growth-regulating agent is essential for directing the morphological transformation of silver nuclei into well-defined nanowire structures. During the nucleation phase, silver nanoparticles predominantly evolve into two distinct forms, i.e., twinned particles and single-crystalline particles, which serve as critical intermediates in the early stages of Ag-NW formation. The presence of CuCl₂·2H₂O plays a pivotal role in modulating the growth kinetics, ensuring controlled anisotropic development and enhancing the overall uniformity and stability of the resulting nanowires.⁴³ The Ostwald ripening process causes twinning particles to become nanowires or nanorods.⁴⁴ One-dimensional nanostructures, such as nanowires and nanorods, are typically formed through the textured growth of multi-twinned particles, a process that drives controlled anisotropic development. In the case of Ag-NW thin films, their pronounced texture behavior is attributed to the strong nucleation mechanism facilitated by Ostwald ripening of multi-twinned particles, enabling enhanced crystallographic alignment and structural refinement. This effect plays a crucial role in optimizing the nanowire morphology, improving the conductivity, and ensuring superior film uniformity, making it highly suitable for advanced optoelectronic applications.⁴⁴ According to nucleation theory, the formation of nanoparticles during the synthesis process is inevitable, as it is a fundamental aspect of the particle growth dynamics. To precisely quantify the dimensional characteristics of Ag nanowires, the ImageJ software was employed for the estimation of their diameter (radius) and length.

Fig. 4 (a–e) FE-SEM images showing the surface morphology of the Ag1–5 Ag-NW thin films as a function of film thickness. (f) Variation in the diameter and length of the Ag-NW thin films as a function of film thickness.

A clear trend emerged, where the diameter of the nanowires progressively decreased with an increase in film thickness up to 46 nm. However, beyond this threshold, the opposite behavior was observed, with the nanowire diameter increasing (Fig. 4). Conversely, the nanowire length exhibited a steady increase with an increase in film thickness, reinforcing the direct influence of the deposition parameters on the nanowire morphology and aspect ratio optimization. This interplay between film thickness and nanowire structural evolution highlights the significance of controlled synthesis conditions in achieving tailored material properties for high-performance applications. The Ag-3 film demonstrated a highly uniform and densely packed nanowire network, outperforming the other films in structural consistency. In contrast, the Ag-4 film exhibited a lower nanowire density, accompanied by silver nanoparticles of varying sizes, indicating the heterogeneity in the deposition process. As the film thickness increased further (Ag-5 film), the presence of multi-twinned nanoparticles led to the formation of larger-sized nanowires, signifying a transition in the growth dynamics. Critically, the films with a high-density and uniform nanowire structure exhibited superior charge transport efficiency, surpassing those with merely larger diameters. This correlation underscores the pivotal role of nanowire density and structural uniformity in optimizing the electrical conductivity for high-performance device applications.

Focused ion beam (FIB) cross-sectional measurements were conducted to precisely determine the nanowire width and length, as depicted in Fig. 5a–e. The comparative analysis revealed that the Ag-3 thin film exhibited a relatively lower nanowire width (radius) compared to the other films, indicating its finer structural formation. The nanowire lengths for the Ag-1, Ag-2, Ag-4, and Ag-5 films were measured at 2.01 µm, 2.31 µm, 2.33 µm, and 2.35 µm, respectively, whereas the Ag-3 film demonstrated an extended length of approximately 2.5 µm. These values align closely with that estimated from the FE-SEM measurements, reinforcing the accuracy of the structural assessment.

Fig. 5 (a–e) The FIB cross-sectional images of the spray-deposited Ag NW thin films as a function of film thickness for Ag1–5.

Furthermore, the films deposited at higher thicknesses exhibited better interfacial adhesion with the glass substrate, which is attributed to the formation of a compact and well-integrated nanowire layer, enhancing the mechanical stability and charge transport efficiency in the fabricated structure.⁴⁵ The films deposited at lower thicknesses exhibited weaker interfacial formation with the glass substrate, primarily due to the initial emergence of island growth caused by a lower number of spray cycles. In its early phase, island growth is characterized by poor adhesion forces, which are significantly lower than that observed in layer growth, leading to reduced structural integrity. However, as the number of spray cycles increased, a gradual transition from island growth to layer growth was observed, driven by the Ostwald ripening process, which facilitates the densification and uniform distribution of Ag nanowires. Crucially, the interface between the Ag-NW film and the glass substrate plays a pivotal role in the charge transport properties, directly influencing the electrical conductivity, carrier mobility, and overall device performance in advanced optoelectronic applications.⁴⁵

Energy dispersive X-ray analysis (EDXA) measurement was conducted to precisely verify the stoichiometric composition of the chemical constituents in the selected Ag-3 and Ag-5 thin films. The analysis demonstrated that the Ag-NW films exhibited an exact stoichiometric ratio, closely aligning with the standard reference sample (Fig. 6a and b). Notably, achieving precise elemental composition in thin films deposited via conventional techniques such as sputtering and thermal evaporation remains challenging due to their inherent deposition variations. However, the spray-coating technique offers remarkably accurate compositional control, ensuring a uniform elemental distribution that closely matches the standard, making it an optimal method for the fabrication of high-performance nanostructured films.⁴⁵ Ag-NWs exhibited an exact compositional ratio as a result of many factors, such as the deposition condition/parameters.⁴⁵ Variations in the deposition process such as precursor concentration, substrate temperature, deposition rate, and ambient atmosphere can significantly impact the elemental distribution within thin films, influencing both their uniformity and stoichiometry.⁴⁵ A non-uniform elemental distribution may lead to phase segregation, compositional gradient, or localized defects, which in turn affect the electrical, optical, and mechanical properties. For example, in spray pyrolysis-coated oxide films, inconsistent precursor flow or temperature gradients can result in uneven metal-to-oxygen ratios across the substrate, compromising the device performance.⁴⁵ Therefore, optimizing the deposition protocols is essential not only for functional performance but also for ensuring batch-to-batch consistency and cost-effective scalability in commercial production environments.⁴⁵ The EDX elemental distribution mapping images of the selected Ag-3 and Ag-5 thin films, as presented in Fig. 6c and d, respectively, reveal a uniform elemental distribution across the entire scanned region. However, a notable distinction in film morphology and composition between the Ag-3 and Ag-5 films is observed. The Ag-3 film demonstrates a highly uniform and densely packed nanowire formation, signifying a well-controlled deposition process, which optimizes its structural homogeneity. In contrast, the Ag-5 film exhibits a broader particle distribution, confirming the formation of larger nanowires, likely resulting from its increased nucleation density and film thickness. Additionally, the elemental concentration of the Ag-NW thin films aligns closely with the surface characteristics observed in the FE-SEM images, reinforcing the correlation between deposition parameters, nanowire growth dynamics, and compositional uniformity in the fabricated films.

Fig. 6 (a and b) EDXA images of the spray-deposited selected Ag-3 and Ag-5 NW thin films. (c and d) Elemental distribution mapping images of the selected Ag-3 and Ag-5 NW thin films.

3.4. Effect of roughness analysis

Atomic force microscopy (AFM) was employed to systematically evaluate the surface roughness and homogeneity of the Ag-NW thin films, providing high-resolution insights into their structural characteristics. The 3D surface topography images, as displayed in Fig. 7a–e, illustrate the distinct morphological evolution of Ag-NWs as a function of film thickness, highlighting significant variations in their nanostructural features. Notably, the Ag-1 and Ag-2 films exhibit needle-like structures, indicative of anisotropic growth mechanisms, while the Ag-3 film demonstrates a more uniform granular surface morphology, promoting enhanced surface smoothness and homogeneity. This transition in nanostructure formation suggests improved interfacial uniformity, which can significantly influence the electrical, optical, and charge transport properties of the Ag-NW thin films, making them suitable for advanced device applications.⁴⁶ In general, needle-like surface features contribute to higher surface roughness compared to a uniform granular structure, leading to increased topographical irregularities, which influence the material properties. Fig. 7a–e provide a precise visualization of the variation in surface roughness (R_a) value as a function of film thickness, highlighting key differences in morphological behavior. Among the analyzed films, the Ag-3 thin film exhibited the lowest roughness and film thickness, which is attributed to its homogeneous and densely packed nanowire arrangement. This optimized structural uniformity enhances the stability, conductivity, and overall performance of the film, making it highly suitable for advanced optoelectronic applications.⁴⁶ In the case of the films deposited with a higher thickness (Ag-4 and Ag-5), they showed an increment in irregular and bigger granular structures.⁴⁷ Fig. 7a–e present the root-mean-square (RMS) surface roughness of the Ag-NW films as a function of film thickness, providing critical insights into their structural uniformity and interfacial properties. The Ag-3 film exhibited an exceptionally low RMS value of 22 nm (Fig. 5c), highlighting its superior surface homogeneity. A significant variation in RMS value was observed across the different film thicknesses, demonstrating the direct correlation between deposition conditions and surface characteristics. Specifically, the Ag-1 and Ag-2 films recorded RMS values of 28 nm and 32 nm, while the Ag-4 and Ag-5 films exhibited higher RMS values of 38 nm and 52 nm, respectively. This trend suggests that the films with increased thickness exhibit a lower interfacial defect influence, reinforcing their structural stability. Furthermore, surface roughness plays a pivotal role in modulating the transport properties of Ag-NW films, directly impacting their electrical conductivity, charge carrier dynamics, and overall functional performance in advanced applications.⁴⁸ The charge carrier mobility may be affected by the increased surface roughness of the Ag-NW thin films; these findings are in line with the FE-SEM particle analysis investigation.

Fig. 7 (a–e) AFM images of the Ag-NW thin films spray deposited on a glass substrate at different film thicknesses.

3.5. HR-TEM analysis

The crystalline properties of the Ag-NW films were rigorously analyzed using high-resolution transmission electron microscopy (HR-TEM), providing deep structural insights into their nanowire architecture. Fig. 8a and b show the HR-TEM surface images of the Ag-3 and Ag-5 nanowire thin films, respectively, captured at 200 nm magnification, revealing well-defined crystalline nanostructures. The spray-deposited films exhibit a distinct nanowire morphology, with crystallite diameters consistently measured below 200 nm, indicating their controlled nucleation and growth dynamics. Notably, the extended nanowire length plays a crucial role in facilitating efficient charge transport, enhancing the electronic conductivity and optimizing the interfacial charge transfer mechanisms in advanced optoelectronic applications.⁴⁹ The Ag-5 film exhibited a relatively larger nanowire diameter compared to Ag-3, but the Ag-3 film displayed a more intricate and interconnected nanowire network, highlighting their distinct growth mechanisms and structural variations. Fig. 8c and d present the HR-TEM images of the Ag-3 and Ag-5 NW films, respectively, captured via selected area electron diffraction (SAED) at a magnification of 5 nm, offering high-resolution insights into the polycrystalline nature of the films. The diffraction pattern analysis reveals bright spots and subtle circular patterns, confirming their nanocrystalline structure with polycrystalline nature, indicative of well-defined grain boundaries and multi-domain lattice orientation. Fig. 8c shows the SEAD pattern of the Ag NW thin film, which is consistent with the XRD results. This structural distinction plays a crucial role in influencing the electronic transport properties, optical behavior, and overall functionality of the nanowires in advanced applications.⁵⁰ The crystalline nature of the Ag-NW films was meticulously examined using high-resolution transmission electron microscopy (HRTEM) Bragg diffraction imaging, providing comprehensive verification of their structural integrity. Notably, the Ag-3 thin film exhibited more pronounced bright spots, indicating its higher crystallinity, whereas the Ag-5 thin film demonstrated relatively reduced intensity, suggesting slight variations in its atomic alignment and structural coherence. Furthermore, precise determination of the interlayer plane spacings in the Ag-NW films confirmed their well-defined lattice arrangements. The measured spacings for the (111), (200), and (220) crystallographic planes were identified as 0.24 nm, 0.27 nm, and 0.324 (Fig. 8e and f), respectively.

Fig. 8 (a and b) HR-TEM surface morphology of the selected Ag-3 and Ag-5 films. (c and d) Selected area electron diffraction (SAED) patterns of the Ag NW thin films. (e and f) HR-TEM Bragg diffraction planes of the Ag NWs films.

These results validate the enhanced structural order, phase purity, and lattice configuration of Ag-NWs, reinforcing their optimized growth dynamics for advanced electronic and optical applications.²² The HR-TEM Bragg diffraction image of the selected Ag-3 NW thin film (Fig. 8e) reveals a high concentration of crystallographic planes aligned in a single particle plane direction, signifying pronounced texture behavior and enhanced structural coherence. In contrast, the Ag-5 NW film (Fig. 8f) exhibits a divergent arrangement of crystallographic planes, confirming its polycrystalline nature, where multiple orientations contribute to a more complex lattice structure. Furthermore, the Ag-NW films display multidirectional orientations across various crystallographic planes; however, in the case of the Ag-3 film, the majority of aligned planes are oriented along the (111) direction, emphasizing its dominant texture and preferential lattice arrangement, which play a significant role in optimizing its electronic transport and structural stability.⁵⁰ The HR-TEM results are consistent with the XRD data analysis. Hence, the HR-TEM results for the Ag NW thin films indicate the formation of a nanostructure.

3.6. Optical properties

Fig. 9a illustrates the correlation between film thickness and optical transmittance of the Ag-NW thin films, revealing a pronounced variation in transmittance as a function of structural parameters and film thickness. The thickness of the Ag nanowire films significantly influences their optical properties, particularly their transparency across the UV-visible-NIR spectral range. As their thickness increases, the light absorption and scattering generally increase, leading to a reduced transmittance. The Ag-3 NW film typically exhibits higher transparency due to its minimal absorption and interference from surface defects, making it ideal for applications such as transparent electrodes. In contrast, the Ag-1 and Ag-5 thin films exhibited low transmittance due to their lower and higher film thickness, respectively, which affect the surface homogeneity and improve light scattering. The Ag-1, Ag-2, Ag-4, and Ag-5 films, characterized by nanowires with larger diameters, exhibited higher light scattering effects, leading to a decrease in their optical transmittance due to increased photon dispersion. Furthermore, the films with greater surface roughness inherently demonstrate enhanced scattering, further reducing their light transmission efficiency. In contrast, the Ag-3 NW film, which features a highly uniform, densely packed, and low-diameter nanowire architecture, facilitated superior optical transmittance by minimizing scattering and optimizing light propagation through the film matrix. This structural refinement plays a critical role in enhancing the optical clarity, making the Ag-3 film particularly suitable for applications requiring high transparency and controlled light modulation. The Ag-3 NW thin film demonstrated an exceptional optical performance, achieving an average transmittance (Tr) exceeding 90% at 550 nm. This remarkable transparency is attributed to its highly uniform and densely packed surface morphology, which effectively minimizes light scattering and enhances photon transmission. The optimized nanowire arrangement facilitates superior optical clarity, making the Ag-3 film ideal for applications demanding high transparency and precise light modulation in advanced optoelectronic devices.³⁶ The Ag-NW thin films demonstrated exceptional near-infrared (NIR) transmittance, consistently exceeding 90%, confirming their optimized optical properties. Furthermore, across all films, a strong absorption profile was observed at lower wavelengths in their UV-visible spectra, aligning with prior experimental findings. This behavior underscores the intrinsic electronic transitions and plasmonic interactions governing light absorption, making these films highly suitable for optoelectronic and photonic applications requiring precise spectral modulation.⁵¹ The Tauc plot was used to estimate the optical bandgap energy (E_g) of the Ag-NW thin films, using the following relationship:³⁹

(αhv)ⁿ = A(h − E_g) (4)

where A is a constant, α represents the absorption coefficient, h represents Planck's constant, and n represents a variable (n = 2 denotes direct conversion and n = 1/2 denotes indirect conversion).⁵¹ Fig. 9b illustrates the fitting of the direct bandgap values of the Ag-NW films as a function of film thickness.

Fig. 9 (a) UV-visible transmittance spectra of Ag-NW thin films as a function of thickness (insert is the average transmittance as a function of film thickness). (b) Band gap of the Ag-NW thin films as a function of film thickness.

The direct bandgap (E_g) value exhibited a progressive decrease with an increase in film thickness up to the Ag-3 NW film, indicating the enhanced electronic interactions within the nanowire network. However, beyond this threshold, the opposite trend was observed, where the bandgap increased with a further increase in the film thickness. This phenomenon is attributed to the increase in the diameter and surface roughness of the nanowires, which influence their quantum confinement effect and carrier mobility (Table 1). Additionally, the variations in the nucleation dynamics, deposition parameters, and growth conditions played a critical role in modulating the bandgap values of the films, further demonstrating the interdependent relationship between structural evolution and optoelectronic properties in silver nanowire systems.⁵² The shift in the Fermi level toward the conduction band is also confirmed by the variation in the band gap. This Fermi level shifting in nanomaterials is called the Burstein–Moss (B–M) effect.⁵³ The shift in the Fermi level of a material induces a significant modification in its optical properties, directly influencing its electronic transitions and charge carrier dynamics. One of the primary contributors to the reduction in optical bandgap is the presence of surface defects, which disrupt the structural coherence in a material. According to the E_g variation plot, it is evident that the Ag-1, Ag-2, Ag-4, and Ag-5 NW thin films exhibit higher bandgap values, suggesting an increase in the prevalence of surface defects within these films. These structural imperfections play a critical role in governing both the optical transparency and charge transport mechanisms in the films. Moreover, the variation in surface defects facilitates the formation of free carrier levels near the lower edge of the conduction band, significantly influencing electronic behavior and optoelectronic efficiency. Understanding and controlling these defect-induced phenomena is fundamental in tailoring nanowire films for high-performance optoelectronic and photonic applications.^50,51 An increased concentration of free carriers can induce the emergence of defect-induced energy states near the conduction band, altering the charge transport dynamics and impacting the electronic and optical properties. These localized states facilitate trap-assisted recombination, influencing the conductivity and device performance in optoelectronic applications.⁵¹ Hence, the variation in the band gap values of Ag-NWs result in a strong variation in device performance.

3.7. Analysis of electrical charge transport properties

To investigate the electrical transport characteristics of the Ag-NW thin films, Hall effect measurements were performed at room temperature. Table 2 represents the estimated resistivity, Hall mobility, and carrier concentration of the Ag-NW thin films deposited at various film thicknesses. It was identified that the Ag-NW film carrier concentration varied as a function of film thickness (Table 2). Among the films, the Ag-3 NW film exhibited the highest carrier concentration of 1.58 × 10²² cm⁻³. In the case of the Ag-1 and Ag-5 thin films, they showed relatively lower carrier concentrations of 3.48 × 10²¹ and 2.58 × 10²¹ cm⁻³, respectively. The mobility of the films increased with film thickness up to the Ag-3 NW film (36.8 cm² V⁻¹ s⁻¹), which decreased with a further increase in film thickness due to the influence of surface defects (Table 2). The impact of thin-film thickness on carrier mobility arises from the complex interplay among crystallinity, electrical transport properties, and surface morphology.⁵⁴ As the film thickness increases, materials often transition from an inhomogeneous nanostructure to homogeneous surface structure, enhancing the lattice ordering and reducing the defect density, thereby improving the carrier mobility.⁵⁴ For instance, ZnO thin films deposited at higher temperatures showed a sharp increase in mobility from 1.76 cm² V⁻¹ s⁻¹ in 194 nm films to 13.89 cm² V⁻¹ s⁻¹ in 662 nm films due to their improved homogeneous surface structure.⁵⁴ Additionally, nanostructure thin films may fall below the electrical percolation threshold, where disconnected grains hinder charge transport; beyond a critical thickness, continuous conductive pathways emerge, as observed in ITO films where the mobility only became saturated after achieving percolation.⁵⁵ Although the surface area increases with roughness, its role in enhancing the mobility is secondary unless surface interactions dominate. Thus, optimizing the film thickness is essential to balance crystallinity and percolation, especially in oxide semiconductors and flexible electronics where mobility governs the device performance.⁵⁵ The Ag-3 thin film has a uniform, dense, low-diameter and high-length nanowires, which can promote effective charge transport, whereas the other thin films showed a higher diameter and relatively low uniform surface behaviors, which can affect their charge transport properties.¹⁴ Also, this low surface behavior affects the mobility of charge carriers due to free carrier collision and scattering by other defects.⁵⁶ The resistivity of the Ag-NW films decreased up to the Ag-3 film (8.54 × 10⁻⁵ Ω cm) and then improved for the film deposited at a higher film thickness (Table 2). The decrease in carrier concentration and mobility caused by free carrier collision and the scattering effect are the main reasons for an increase in resistance. Sheet resistance is one of the unique properties of thin film materials, and thus the applicability of the TCE materials is characterized by their sheet resistance. The sheet resistance of a film is different from the bulk resistance; sheet resistance is closely related to the geometry of materials. Generally, the sheet resistance (R_S) of Ag-NW thin films is calculated using the following relation:⁵⁷where V is the measured voltage, I is the applied current, and 4.532 is the geometric structure constant (F) of the cubic sample. Given that the film thickness is significantly less than the spacing between the probes, a correction factor of 4.532 is applied for probes that are evenly spaced (≈1 mm) and a current of 1 mA applied. Table 2 shows that the Ag-3 NW film has the lowest sheet resistance value of 18.1 Ω □⁻¹ among the films. The results obtained for the Ag NW thin films are consistent with earlier reports, and also demonstrate that they are suitable for optoelectronic devices.

Table 2 Electrical transport properties of the Ag NW thin films as a function of film thickness

Samples	Carrier concentration (cm^−3)	Mobility (cm^2 V^−1 s^−1)	Resistivity (Ω cm)	Sheet resistance (Ω □^−1)	FOM × 10^−4 Ω^−1	Ref.
Ag NWs	—	—	—	10	390.08	58
Ag NWs	—	—	—	56.5	117.42	59
Ag NWs	—	—	—	8.1	9.77	60
Ag-1	3.48 × 10^21	14.5	8.14 × 10^−4	94.6	5.858	Present work
Ag-2	8.75 × 10^21	27.1	2.51 × 10^−4	22.4	27.71
Ag-3	1.58 × 10^22	36.8	8.54 × 10^−5	18.1	45.02
Ag-4	5.15 × 10^21	21.7	4.61 × 10^−4	34.2	19.93
Ag-5	2.58 × 10^21	8.22	9.14 × 10^−4	49.5	11.22

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3.8. Figure of merit analysis

The figure of merit (FoM) is an important parameter used to determine the applicability of TCE materials in various optoelectronic devices. In general, the FoM is defined from the relation between sheet resistance and transmittance of thin film.⁶¹ The dimensionless FoM of a TCE material is expressed using the newly proposed Haacke high-resolution figure of merit (FOM_(H-HR)) relation, as follows:⁶¹where T_r is the transmittance at λ = 550 nm (mean wavelength of the visible region), the value of n is 12, and R_S is the sheet resistance. An ideal material must have an FOM_(H-HR) as high as feasible, but one of the two characteristics determining FOM_(H-HR) must be compromised based on the requirements and type of application.⁵⁷ A device should ideally have minimal sheet resistance and maximum transparency; however, achieving these qualities simultaneously is challenging. Table 2 displays the calculated FOM_(H-HR) values for the Ag-NW thin-films as a function of film thickness. Among them, the Ag-3 NW film exhibited the highest FOM_(H-HR) of 45.02 × 10⁻⁴ Ω⁻¹, which indicates its applicability as a TCE material. The other Ag-NW films showed lower FOM_(H-HR) values, and hence they cannot be used as TCE electrodes. For example, Wang et al. reported better transmittance and sheet resistance values for silver nanowires deposited using the UV irradiation technique, exhibiting an FOM_(H-HR) value of 117.42 × 10⁻⁴ Ω⁻¹.⁵⁹ Moreover, highly conductive and uniform silver films with a critical thickness lower than 15 nm and a sheet resistance of 1.6 Ω □⁻¹ for the 40 nm-thin film, corresponding to a resistivity of 6.4 µΩ cm, were reported by Zhang et al.⁶² These aspects might be the reason for the slightly lower values of transmittance and sheet resistance in the present case. To further deposit a large area of Ag-3 NW films, similar deposition conditions/parameters were chosen.

3.9. Resistance stability analysis of Ag-NW thin films

Temperature-dependent resistance is an essential property for employing materials in various optoelectronic, energy, and battery devices. Generally, the performance of an optoelectronic device is strongly affected by the resistance stability of its TCE materials.⁵² The Ag-NW thin films were annealed at different temperatures (from 200 °C to 450 °C at intervals of 50 °C) and for different durations (from 30 min to 120 min at intervals of 30 min) to understand their resistance stability.⁶³ The Ag-NW thin films were analyzed for their resistance stability based on the sheet resistance measurement carried out by a four-probe technique after post-annealing at varying temperatures for different durations (Fig. 10a–e), respectively. The Ag-NW thin films exhibit a slight increment in sheet resistance as the annealing temperature increased from 200 °C to 450 °C due to their metallic behavior. The Ag-3 NW thin film exhibited a relatively lower sheet resistance of 19.7 Ω □⁻¹ when annealed at 200 °C and 32.3 Ω □⁻¹ at 450 °C for 30 min.

Fig. 10 (a–f) Resistance stability of the Ag-NW thin films as a function of film thickness with different annealing temperatures and duration vs. sheet resistance.

In the case of the Ag-5 film, it exhibited a relatively higher variation in sheet resistance from 47.4 Ω □⁻¹ when annealed at 200 °C to 56.2 Ω □⁻¹ at 450 °C for 30 min. However, a similar trend is also witnessed for the other Ag-NW films. In metallic systems, their electrical transport properties decline due to charge carrier scattering effects and free carrier collision can affect the mobility of the charge carriers.⁵³ Scattering processes such as phonon scattering, grain boundary scattering, and ionized impurity scattering have a significant impact on the mobility of free carriers.⁶⁴ The sheet resistance of Ag-NW thin films increased linearly with the annealing duration (Fig. 10f). This might be because a higher carrier concentration of electrons is affected by a higher annealing temperature.⁶⁵ Matthiessen's rule is used to identify the factors that influence the electrical resistivity of films.⁶⁵ Therefore, using Matthiessen's rule, the electrical resistivity of the Ag-NW films is expressed in terms of various scattering processes, as follows:⁶⁵

where µ_g, µ_i, and µ_l represent the grain boundary, ionized, and phonon scattering mobility values, respectively. Grain boundary scattering and free carrier trapping at the interface have a greater impact on the mobility of free carriers.⁵³ In contrast, photon scattering does not have much impact on the carrier mobility. Hence, the Ag-NW thin films with higher diameters possibly possess more grain boundaries, which can affect their carrier mobility. In particular, the Ag-2, Ag-4, and Ag-5 NW films exhibited larger diameters, which is the main reason for the decrease in their electrical transport properties. It was reported that grain boundary scattering also impacts the free carrier mobility, according to Muraoka et al.⁶⁶ In the case of the Ag-3 film, the presence of more uniform and longer-length nanowires is the main reason for its improved electrical transport properties. Hence, the Ag-3 NW thin film showed the best resistance stability, which may prove to be a promising electrode for various optoelectronic devices.

3.10. Surface resistance variation analysis of Ag-NW thin films

The Ag-3 NW thin film prepared using the optimized spray deposition conditions/parameters was chosen as a large-area (5 × 5 cm²) coating on electrodes for the fabrication of perovskite solar cells. To select the uniform surface area of Ag-3 NW film, the variation in its sheet resistance was measured across the 5 × 5 cm² area film at 1 × 1 cm² area sections of the entire region. A working demonstration of the large-area spray-deposited Ag-3 NW film is shown in Fig. 11a in a light glowing experiment. The gradual increase in sheet resistance variation observed throughout the measured region is depicted in the 2D and 3D contour maps in Fig. 11b and c, respectively.

Fig. 11 (a) Photograph of the spray-deposited large-area (5 × 5 cm²) Ag-NW thin film working demonstration with the help of a glowing LED light. Variation in sheet resistance shown as 2D and 3D contour maps (b and c).

The center region has a more uniform sheet resistance compared to the corner regions, and also the center regions were found to have lower sheet resistance than the corners. This variation in sheet resistance is mostly caused by the spray dynamics.¹¹ During the spraying process, there is a difference in pressure depending on how far the substrate is from the center of the spray nozzle.⁶⁷ The substrate area just beneath the spray nozzle has better homogeneity and uniformity than other film sections. The 3D image in Fig. 11b clearly shows that the core of the film has a low sheet resistance of about ∼18 Ω □⁻¹. Conversely, the film at the corners of the substrate shows a large variation in sheet resistance from ∼36 to 47 Ω □⁻¹. Consequently, it is established that the sheet resistance of the film is influenced by its surface properties and spray dynamics. Additionally, significant surface variation in the film morphology and surface roughness is evident from the findings of the FE-SEM and AFM experiments. The mobility of the free carriers would be reduced as a result of the increased variations in surface roughness. Importantly, to minimize carrier scattering, a homogeneous coating with low surface roughness is preferable.⁶⁶ The obtained results provided several indications of the Ag-NW thin-film stabilization with desired properties. Further, a series of trials were conducted for the deposition of the large area (5 × 5 cm²), uniform and dense Ag-NW thin film for electrode applications. Hence, the central regions of the film (2.5 × 2.5 cm² area) were selected for the fabrication of perovskite solar cells.

4. Testing of Ag-NW electrode-based PSC devices

4.1. Analysis of photocurrent–voltage (J–V) characterization

The power conversion efficiency of the spray-deposited Ag-NW thin film electrode-based PSC devices was studied utilizing several photovoltaic parameters, including fill factor (FF), open-circuit voltage (V_oc), and short circuit photocurrent density (J_sc). Furthermore, the photovoltaic performances of the spray-deposited NW thin film electrode-based PSC device were compared with the performance of the standard FTO/ITO reference electrode-based PSC device.⁶⁸ Fig. 12a–c show the estimated results from the J–V characteristic measurement, schematic of the fabricated PSC device structure, and energy band diagram of the Ag-NW electrode-based PSC device, respectively. The Ag-NW thin film-based PSC device exhibited a power conversion efficiency of 11.6%, which is calculated from the values of V_oc of 916.3 mV, J_sc of 19.3 mA cm⁻², and FF of 64.7%. A standard reference FTO electrode-based PSC device has a V_oc of 1065.6 mV, J_sc of 22.1 mA cm⁻², FF of 65.2%, and a power conversion efficiency of 15.3%. Similarly, the standard reference ITO electrode-based PSC device exhibited a V_oc of 892.4 mV, J_sc of 18.7 mA cm⁻², FF of 58.1%, and power conversion efficiency of 9.7%. It should be noted that the commercial FTO electrode-based reference PSC device exhibited a relatively better performance than the Ag-NW electrode and ITO electrode-based PSC devices. The obtained photovoltaic results are similar to that of previously reported FTO electrode- and Ag electrode-based PSC devices (Table 3). Recently, Xie et al. found that structured Ag-NW provides a superior electron transport channel, which inhibits charge recombination by establishing an energy barrier between the transport layer and perovskite.⁶⁹ It was also reported that an Ag-NW thin film improved the photovoltaic performance of an ITO electrode with the maximum efficiency of 8.44% achieved, according to Kim et al., which was attributed to its large specific surface area and numerous active adsorption/interaction sites, increasing light harvesting.⁷⁰ The Ag-NW-based PSC device exhibited a good power conversion efficiency due to the higher current density and fill factor achieved by the better charge transfer process, and low charge transport resistance in the interface of the electrode and charge transport layer.⁷¹ Ag-NW thin film and FTO/ITO electrode-based PSC devices were fabricated with an active light exposure area of about 0.16 cm². Additionally, the device performance is greatly influenced by the charge carrier injection, recombination mechanism, light absorption, and charge carrier generation.⁶⁹

Fig. 12 (a) The J–V characteristic curves of the Ag-NW thin film electrode- and commercial FTO/ITO reference electrode-based fabricated PSC devices. (b) Schematic of the fabricated Ag-NW thin film electrode-based PSC device structure. (c) Energy band alignment of the Ag-NW-based PSC device.

Table 3 Photovoltaic performances of the Ag NW thin-film electrode- and reference FTO/ITO electrode-based PSC devices in comparison with the reported Ag, FTO/ITO electrode-based PSC devices

Device	Voc (mV)	Jsc (mA cm^−2)	FF (%)	PCE (%)	Ref.
FTO	1065.6	22.1	65.2	15.3	Present work
ITO	892.4	18.7	58.1	9.7
Ag-NW	916.3	19.6	64.7	11.6
e-Ag	0.77	17.97	0.48	6.64	69
Ag/PH1000	0.47	12.09	55	3.13	72
ITO/Ag-NW	1.04	13.17	61.83	8.44	70
ITO	0.76	19.1	66	9.77	73
ITO	1.05	16.1	66	11.2	74
FTO	0.98	24.3	57.3	13.6	75

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4.2. Charge transport mechanism

Fig. 13a–c schematically depict the device architecture of the Ag-NW thin film electrode-based PSCs and charge transfer process and energy band structure alignment in the absence of light illumination and presence of light illumination, respectively. The suitable conduction and valence band energies of each layer are highly beneficial for a better charge transfer process.⁷⁰ The Ag nanowire architectures (1D) cause a major alteration in the band structure, resulting in the formation of suitable band matching.⁷¹ The Ag NW thin film cubic structure has a narrow band gap of 2.5 eV, which causes effective charge transport behavior.⁶⁹ The conduction band (−4.5 eV) and valence band (−5.6 eV) energies of the Ag-NW thin film with a narrow bandgap of ∼2.5 eV match well with the bandgap (3.6 eV) and conduction band (−4.12 eV) energy values of TiO₂ (Fig. 10c).

Fig. 13 (a) Charge transfer process in Ag-NW electrode-based PSC device structure. (b) Energy band alignment in the absence of light illumination and (c) energy band alignment in the presence of light illumination.

However, in comparison to the Ag NW thin film, the FTO electrode system exhibits a greater conduction band of −4.32 eV and a comparatively wide bandgap of ∼3.5 eV, which is more suitable with the TiO₂ ETL layer.⁷¹ Excitons are created and separated, and then move toward the appropriate electrodes when the perovskite active layer is exposed to sunlight. In this procedure, the charge carriers produced by the photogenerated process move to the electron/hole transport layers, and then move to the appropriate electrodes. The electrode layer should separate these produced electrons before they recombine.⁷² The Ag-thin film nanowire structure effectively improves the charge transfer process. In the nanowire structure, the charge carriers only move in one direction, and hence the free carrier collision can be effectively controlled.⁷² Also, the nanowire structure offers effective electron drift, which can facilitate a higher photocurrent carrier density and high V_OC value.⁷⁰ This nanowire-based charge transfer process is also advantageous for the suppression of the charge carriers to avoid the recombination process at the perovskite and photoanode interface.⁷⁶ The Ag-NW thin film demonstrates the dense formation of longer nanowires, which facilitate significant charge carrier transfer, and also can regulate the recombination process.⁶⁸ Consequently, the Ag nanowire-based PSC device achieves lower charge transport resistance and offers a greater photocurrent density. However, compared to the reference FTO electrode-based device, the power conversion efficiency of the Ag-nanowire-based device is comparatively lower. This could be because high metallic behavior can indirectly increase the diffusion resistance, which can affect the hole mobility.¹⁷ Hence, the nanowire surface structure plays an important role in the charge transfer process of the PSC device.⁷¹

4.3. Electrochemical impedance spectroscopy analysis

Electrochemical impedance spectroscopy (EIS) confirmed the variation in the charge transportation process in the Ag-NW thin film electrodes in comparison with the standard FTO/ITO electrode-based PSC devices during light illumination. Fig. 14a–c illustrate the Nyquist plots used to determine the charge transfer process of the different layers in the PSC devices. To determine the impedance value, an equivalent circuit was fitted with non-linear least squares (NLLS) fit, as shown in the insert in Fig. 14a. The sheet resistance (R_S) values of the standard reference FTO, ITO, and Ag-NW thin film electrode-based devices were 70 Ω, 202 Ω, and 163 Ω (Fig. 14a–c), respectively. Compared to the ITO electrode, the Ag-NW thin film exhibits a similar sheet resistance, indicating its good charge transfer process as a photoanode.⁷⁶

Fig. 14 Electrochemical impedance analysis of the Ag-NW thin film electrode-based PSC devices. Nyquist plots along with the equivalent circuits of (a) Ag-NW, (b) reference FTO (STD), and (c) reference ITO (STD). (d) The Bode plot of the fabricated PSC devices.

The impedance spectrum exhibited a single semicircle for all the different (FTO, ITO, and Ag-NW thin film) electrode-based devices, which indicates the charge-transfer resistance (R₁) at the perovskite–photoanode interface at the measured frequency in the range of 0.1 Hz to 1 MHz. At higher frequencies, they also exhibited higher diffusion resistance at the perovskite-top electrode interface (R₂). The charge transfer resistance (R₁) for the FTO, ITO, and Ag-NW thin-film electrodes is significantly different with values of 22.5 Ω, 103.6 Ω, and 37.1 Ω, respectively. The diffusion resistance (R₂) for the FTO, ITO, and Ag-NW thin-film electrodes is significantly different with values of 1970 Ω, 3458 Ω, and 2386 Ω, respectively. Compared to the FTO electrode, the ITO and Ag-NW thin-film electrodes show a large diffusion resistance, indicating their lower hole transfer process. The nanowire surface structure has a major impact on the charge transfer resistance in the interface layers. Furthermore, the Bode plot helps to understand the charge transport process. The lifetime of the charge carriers, which was determined using the formula τ = 1/2pf_max, where f_max is the low cutoff frequency, provides the frequency for the Bode plot. The cutoff frequencies of the FTO, ITO, and Ag-NW thin-film electrodes can be seen in Fig. 14d, which are 533 Hz, 885 Hz, and 210 Hz, respectively. In the case of the FTO, ITO, and Ag-NW thin-film samples, the predicted lifetime (τ) of the charge carrier is 3.72 ms, 2.26 ms, and 9.54 ms, respectively. Because of the nanowire structure, the longer lifetime of the Ag film helps to effectively suppress the process of charge carrier recombination.⁷¹ Additionally, it can be seen that the Ag-NW thin-film electrode-based PSC device has a higher lifetime value, which suggests that there is less recombination resistance at the interface between the photoanode and the perovskite, which lowers the carrier density.⁶⁹ The J–V and EIS characterization results of the PSC devices utilizing the ITO and Ag-NW thin-film electrodes are contrasted with that of earlier studies.⁷² Therefore, the improved efficiency of the Ag-NW-based PSC devices is similar to the standard reference electrode-based PSC devices due to the nanowire surface structure. Compared to conventional FTO electrodes, Ag-NW films are more susceptible to surface diffusion and spheroidization at elevated temperatures, which can disrupt their conductive pathways and reduce the device efficiency over time. However, recent advances in encapsulation such as SnO₂ coatings have significantly improved the durability of Ag-NW electrodes, resulting in comparable lifetimes and enhanced mechanical flexibility.⁴² These findings suggest that with appropriate surface passivation and interface engineering, Ag-NW electrodes can evolve into viable alternatives to brittle oxide-based contacts for next-generation flexible and transparent PSC applications.

5. Conclusion

This study demonstrated that Ag-NW thin films on glass substrates have been successfully optimized via facile chemical spray pyrolysis. The Ag-NW film was identified to possess a face-centered cubic crystal structure with a space group of Fm-3m according to its XRD spectra. The XPS results indicate that Ag has a charge state of 2+, which improves the concentration of free carriers. The surface structure features of the nanowires varied as a function of film thickness. A small diameter of 42 nm and long length of 2.5 µm were achieved in the Ag-3NW thin film. It was found that the thin Ag-3 NW film showed a more uniform and dense film formation, leading to good optical and electrical properties. The Ag-3 NW thin film was found to have a good optical transmittance of 90% (550 nm) in the UV-visible to NIR range. The film thickness greatly modifies the bandgap, and the calculated bandgap value of the Ag-3 NW film is 2.49 eV. The Ag-3 film exhibits n-type conductivity, as indicated by Hall effect measurements, with a lower sheet resistance (18.1 Ω □⁻¹), resistivity (8.54 × 10⁻⁵ Ω cm), greater carrier concentration (1.58 × 10²² cm⁻³), and enhanced mobility (36.8 cm² V⁻¹ s⁻¹). The Ag-thin films showed the maximum resistance stability at 400 °C, and the resistance stability of the Ag-NW films has to be improved with an increase in the film thickness. A PSC device was fabricated successfully using a spray-deposited Ag-3 NW thin layer as the bottom electrode. The experimental findings were compared with commercially available FTO/ITO electrodes. The Ag/AgCl electrode-based PSC device showed a PCE of 11.6%, which was calculated from the open-circuit voltage of 916.3 mV and short circuit photocurrent density of 19.6 mA cm⁻². The Ag-NW electrode-based device has a lower efficiency compared to the reference FTO electrode-based device (15.3%) and a better efficiency than the reference ITO electrode (9.7%). Thus, to achieve a greater power conversion efficiency, it is still necessary to enhance the electrical and surface qualities of the Ag-NW films. Overall, owing to the characteristics of the Ag-NW thin-film electrode, it shows promise, and therefore more research into it as a substitute TCE to support commercially available ITO/FTO electrodes should be done.

Conflicts of interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Data availability

The data supporting the findings of this study are available upon request. Due to privacy, ethical, or proprietary considerations, the data supporting this study are not publicly available. However, summary statistics and relevant findings are provided within the article.

Acknowledgements

The authors gratefully acknowledge financial support from MANIT-Bhopal through the SEED grant. They also extend their sincere thanks to the Department of Physics at the Maulana Azad National Institute of Technology, Bhopal and the Department of Metallurgical Engineering and Materials Science at the Indian Institute of Technology, Indore, for providing the research facilities. Special appreciation is due to the HORIBA-IISc Technical Center at the Department of Inorganic and Physical Chemistry, IISc Bengaluru, for the instrumental support.

References

1. S. C. Dixon, D. O. Scanlon, C. J. Carmalt and I. P. Parkin, N-Type doped transparent conducting binary oxides: An overview, J. Mater. Chem. C, 2016, 4, 6946–6961,  10.1039/c6tc01881e.
2. S. Wack, P. Lunca Popa, N. Adjeroud, C. Vergne and R. Leturcq, Two-Step Approach for Conformal Chemical Vapor-Phase Deposition of Ultra-Thin Conductive Silver Films, ACS Appl. Mater. Interfaces, 2020, 12, 36329–36338,  DOI:10.1021/acsami.0c08606.
3. G. Y. Margulis, M. G. Christoforo, D. Lam, Z. M. Beiley, A. R. Bowring, C. D. Bailie, A. Salleo and M. D. McGehee, Spray deposition of silver nanowire electrodes for semitransparent solid-state dye-sensitized solar cells, Adv. Energy Mater., 2013, 3, 1657–1663,  DOI:10.1002/aenm.201300660.
4. K. K. Sears, M. Fievez, M. Gao, H. C. Weerasinghe, C. D. Easton and D. Vak, ITO-Free Flexible Perovskite Solar Cells Based on Roll-to-Roll, Slot-Die Coated Silver Nanowire Electrodes, Sol. RRL, 2017, 1, 1–9,  DOI:10.1002/solr.201700059.
5. S. T. Zhang, M. Foldyna, H. Roussel, V. Consonni, E. Pernot, L. Schmidt-Mende, L. Rapenne, C. Jiménez, J. L. Deschanvres, D. Muñoz-Rojas and D. Bellet, Tuning the properties of F:SnO₂ (FTO) nanocomposites with S:TiO₂ nanoparticles-promising hazy transparent electrodes for photovoltaics applications, J. Mater. Chem. C, 2017, 5, 91–102,  10.1039/c6tc04153a.
6. A. Chen, K. Zhu, H. Zhong, Q. Shao and G. Ge, A new investigation of oxygen flow influence on ITO thin films by magnetron sputtering, Sol. Energy Mater. Sol. Cells, 2014, 120, 157–162,  DOI:10.1016/j.solmat.2013.08.036.
7. M. V. Hohmann, A. Wachau and A. Klein, In situ Hall effect and conductivity measurements of ITO thin films, Solid State Ionics, 2014, 262, 636–639,  DOI:10.1016/j.ssi.2013.10.004.
8. P. Kartikay, D. Sadhukhan, A. Yella and S. Mallick, Enhanced charge transport in low temperature carbon-based n-i-p perovskite solar cells with NiOx-CNT hole transport material, Sol. Energy Mater. Sol. Cells, 2021, 230, 111241,  DOI:10.1016/j.solmat.2021.111241.
9. N. Selvakumar, U. Pradhan, S. B. Krupanidhi and H. C. Barshilia, Structural and optical properties of graphene oxide prepared by modified hummers' method, Comput., Mater. & Continua, 2016, 52, 173–184.
10. R. N. Chauhan, N. Tiwari, R. S. Anand and J. Kumar, Development of Al-doped ZnO thin film as a transparent cathode and anode for application in transparent organic light-emitting diodes, RSC Adv., 2016, 6, 86770–86781,  10.1039/c6ra14124b.
11. R. Ramarajan, M. Kovendhan, K. Thangaraju, D. P. Joseph and R. R. Babu, Facile deposition and characterization of large area highly conducting and transparent Sb-doped SnO2 thin film, Appl. Surf. Sci., 2019, 487, 1385–1393,  DOI:10.1016/j.apsusc.2019.05.079.
12. D. A. R. Barkhouse, O. Gunawan, T. Gokmen, T. K. Todorov and D. B. Mitzi, Yield predictions for photovoltaic power plants:empirical validation,recent advances and remaining uncertainties, Prog. Photovoltaics Res. Appl., 2015, 20, 6–11,  DOI:10.1002/pip.
13. H. Sung, N. Ahn, M. S. Jang, J. K. Lee, H. Yoon, N. G. Park and M. Choi, Transparent Conductive Oxide-Free Graphene-Based Perovskite Solar Cells with over 17% Efficiency, Adv. Energy Mater., 2016, 6, 2–7,  DOI:10.1002/aenm.201501873.
14. G. Zhao, W. Shen, E. Jeong, S. G. Lee, H. S. Chung, T. S. Bae, J. S. Bae, G. H. Lee, J. Tang and J. Yun, Nitrogen-Mediated Growth of Silver Nanocrystals to Form UltraThin, High-Purity Silver-Film Electrodes with Broad band Transparency for Solar Cells, ACS Appl. Mater. Interfaces, 2018, 10, 40901–40910,  DOI:10.1021/acsami.8b13377.
15. P. Ferrada, D. Rudolph, C. Portillo, A. Adrian, J. Correa-Puerta, R. Sierpe, V. del Campo, M. Flores, T. P. Corrales, R. Henríquez, M. J. Kogan and J. Lossen, Interface analysis of Ag/n-type Si contacts in n-type PERT solar cells, Prog. Photovoltaics Res. Appl., 2020, 28, 358–371,  DOI:10.1002/pip.3242.
16. K. Naito, N. Yoshinaga and Y. Akasaka, Material- and process-effects on homogeneity and electric properties of transparent conducting films composed of hydrazine-reduced graphene oxide and/or silver nanowire, Synth. Met., 2016, 215, 243–250,  DOI:10.1016/j.synthmet.2016.03.004.
17. E. Lee, J. Ahn, H. C. Kwon, S. Ma, K. Kim, S. Yun and J. Moon, All-Solution-Processed Silver Nanowire Window Electrode-Based Flexible Perovskite Solar Cells Enabled with Amorphous Metal Oxide Protection, Adv. Energy Mater., 2018, 8, 1–11,  DOI:10.1002/aenm.201702182.
18. J. Bonsak, J. Mayandi, A. Thøgersen, E. Stensrud Marstein and U. Mahalingam, Chemical synthesis of silver nanoparticles for solar cell applications, Phys. Status Solidi C, 2011, 8, 924–927,  DOI:10.1002/pssc.201000275.
19. S. Coskun, B. Aksoy and H. E. Unalan, Polyol synthesis of silver nanowires: An extensive parametric study, Cryst. Growth Des., 2011, 11, 4963–4969,  DOI:10.1021/cg200874g.
20. A. Kim, Y. Won, K. Woo, S. Jeong and J. Moon, All-solution-processed indium-free transparent composite electrodes based on Ag nanowire and metal oxide for thin-film solar cells, Adv. Funct. Mater., 2014, 24, 2462–2471,  DOI:10.1002/adfm.201303518.
21. H. Lee, Y. Li, P. Cui, L. Wang, H. Lee and K. Lee, Highly bendable, conductive, and transparent film by an enhanced adhesion of silver nanowires, ACS Appl. Mater. Interfaces, 2013, 5, 9155–9160,  DOI:10.1021/am402578d.
22. S. C. Dixon, J. A. Manzi, M. J. Powell, C. J. Carmalt and I. P. Parkin, Reflective silver thin film electrodes from commercial silver(i) triflate via aerosol-assisted chemical vapor deposition, ACS Appl. Nano Mater., 2018, 1, 3724–3732,  DOI:10.1021/acsanm.8b00949.
23. T. Akter and W. S. Kim, Reversibly stretchable transparent conductive coatings of spray-deposited silver nanowires, ACS Appl. Mater. Interfaces, 2012, 4, 1855–1859,  DOI:10.1021/am300058j.
24. S. Sun, W. Zha, C. Tian, Z. Wei, Q. Luo, C. Q. Ma, W. Liu and X. Zhu, Solution Processed Semi-Transparent Organic Solar Cells Over 50% Visible Transmittance Enabled by Silver Nanowire Electrode with Sandwich Structure, Adv. Mater., 2023, 35, 1–9,  DOI:10.1002/adma.202305092.
25. C. X. Kan, J. J. Zhu and X. G. Zhu, Silver nanostructures with well-controlled shapes: Synthesis, characterization and growth mechanisms, J. Phys. D Appl. Phys., 2008, 41, 155304,  DOI:10.1088/0022-3727/41/15/155304.
26. P. Kalakonda, Synthesis of silver nanowires for highly conductive and transparent films, Nanomater. Nanotechnol., 2016, 6, 1–6,  DOI:10.1177/1847980416663672.
27. R. Ramarajan, N. Purushothamreddy, R. K. Dileep, M. Kovendhan, G. Veerappan, K. Thangaraju and D. Paul Joseph, Large-area spray deposited Ta-doped SnO₂ thin film electrode for DSSC application, Sol. Energy, 2020, 211, 547–559,  DOI:10.1016/j.solener.2020.09.042.
28. S. Das and V. Jayaraman, SnO₂: A comprehensive review on structures and gas sensors, Prog. Mater. Sci., 2014, 66, 112–255,  DOI:10.1016/j.pmatsci.2014.06.003.
29. C. M. Mahajan, M. Pendharkar, Y. A. Chaudhari, S. S. Sawant, B. Ankamwar and M. G. Takwale, Spray deposited nanocrystalline ZnO transparent electrodes: Role of precursor solvent, J. Nano- Electron. Phys., 2016, 8(2), 02026,  DOI:10.21272/jnep.8(2).02026.
30. F. Zhang, B. Cheng, J. Liu, Y. Yue, Z. Huang, Y. Zhang and D. Zang, Facile synthesis of Ag nanowires enhanced PVB for transparent conductive film, J. Mater. Res. Technol., 2020, 9, 14509–14516,  DOI:10.1016/j.jmrt.2020.10.039.
31. C. M. Mahajan and M. G. Takwale, Precursor molarity dependent growth rate, microstructural, optical and electrical properties of spray pyrolytically deposited transparent conducting ZnO thin films, Micro Nanostruct., 2022, 163, 107131,  DOI:10.1016/j.spmi.2021.107131.
32. H. S. Ko, J. W. Lee and N. G. Park, 15.76% efficiency perovskite solar cells prepared under high relative humidity: Importance of PbI2 morphology in two-step deposition of CH3NH3PbI3, J. Mater. Chem. A, 2015, 3, 8808–8815,  10.1039/c5ta00658a.
33. C. Chen, L. Wang, G. Jiang, J. Zhou, X. Chen, H. Yu and Q. Yang, Study on the synthesis of silver nanowires with adjustable diameters through the polyol process, Nanotechnology, 2006, 17, 3933–3938,  DOI:10.1088/0957-4484/17/15/054.
34. P. Taylor and G. B. Harris, Philosophical Magazine Series 7 X Quantitative measurement of preferred orientation in rolled uranium bars, Communication, 2009, 37–41.
35. V. Biju, N. Sugathan, V. Vrinda and S. L. Salini, Estimation of lattice strain in nanocrystalline silver from X-ray diffraction line broadening, J. Mater. Sci., 2008, 43, 1175–1179,  DOI:10.1007/s10853-007-2300-8.
36. Q. H. Tran, D. T. Chu, V. H. Hoang, Q. T. Do, S. H. Pham, P. Leclère, T. D. Nguyen and D. C. Nguyen, Enhancement of electrical and thermal properties of silver nanowire transparent conductive electrode by Ag coating, Mater. Sci. Eng. B, 2022, 278, 1–9,  DOI:10.1016/j.mseb.2022.115640.
37. J. Xu, J. Hu, C. Peng, H. Liu and Y. Hu, A simple approach to the synthesis of silver nanowires by hydrothermal process in the presence of gemini surfactant, J. Colloid Interface Sci., 2006, 298, 689–693,  DOI:10.1016/j.jcis.2005.12.047.
38. E. Dimngaihvungi, M. Kumar, A. K. Singh, B. Pani, A. K. Singh and M. Singh, Multiple times synthesis of silver nanowires by recycling the waste left after standard polyol synthesis for flexible transparent heater, J. Taiwan Inst. Chem. Eng., 2024, 161, 105529,  DOI:10.1016/j.jtice.2024.105529.
39. A. Aziz, M. Khalid, M. S. Akhtar, M. Nadeem, Z. A. Gilani, H. M. N. Ul Huda Khan Asghar, J. Rehman, Z. Ullah and M. Saleem, Structural, morphological and optical investigations of silver nanoparticles synthesized by sol-gel auto-combustion method, Dig. J. Nanomater. Biostruct., 2018, 13, 679–683.
40. B. jia Li, H. min Zhang, J. jun Ruan, L. Wang, Z. yan Wang and L. jing Huang, Nanosecond pulsed laser etching anti-reflective gratings and synchronous laser annealing for performance optimization of Ag/AZO thin films, Ceram. Int., 2024, 50, 27176–27187,  DOI:10.1016/j.ceramint.2024.05.015.
41. U. T. Khatoon, G. V. S. N. Rao, K. M. Mantravadi and Y. Oztekin, Strategies to synthesize various nanostructures of silver and their applications - A review, RSC Adv., 2018, 8, 19739–19753,  10.1039/c8ra00440d.
42. A. Baranowska-Korczyc, E. Mackiewicz, K. Ranoszek-Soliwoda, A. Nejman, S. Trasobares, J. Grobelny, M. Cieślak and G. Celichowski, A SnO₂ shell for high environmental stability of Ag nanowires applied for thermal management, RSC Adv., 2021, 11, 4174–4185,  10.1039/d0ra10040d.
43. S. Hemmati, M. T. Harris and D. P. Barkey, Polyol Silver Nanowire Synthesis and the Outlook for a Green Process, J. Nanomater., 2020, 2020, 6–10,  DOI:10.1155/2020/9341983.
44. X. Lian, J. Chen, Y. Zhang, M. Qin, T. R. Andersen, J. Ling, G. Wu, X. Lu, D. Yang and H. Chen, Solvation effect in precursor solution enables over 16% efficiency in thick 2D perovskite solar cells, J. Mater. Chem. A, 2019, 7, 19423–19429,  10.1039/c9ta06009j.
45. D. C. Tsai, Z. C. Chang, B. H. Kuo, Y. H. Wang, E. C. Chen and F. S. Shieu, Thickness dependence of the structural, electrical, and optical properties of amorphous indium zinc oxide thin films, J. Alloys Compd., 2018, 743, 603–609,  DOI:10.1016/j.jallcom.2017.12.062.
46. J. Zheng, L. Hu, J. S. Yun, M. Zhang, C. F. J. Lau, J. Bing, X. Deng, Q. Ma, Y. Cho, W. Fu, C. Chen, M. A. Green, S. Huang and A. W. Y. Ho-Baillie, Solution-Processed, Silver-Doped NiOx as Hole Transporting Layer for High-Efficiency Inverted Perovskite Solar Cells, ACS Appl. Energy Mater., 2018, 1, 561–570,  DOI:10.1021/acsaem.7b00129.
47. Y. Yu, S. Zhang, A. M. Mio, B. Gault, A. Sheskin, C. Scheu, D. Raabe, F. Zu, M. Wuttig, Y. Amouyal and O. Cojocaru-Mirédin, Ag-Segregation to Dislocations in PbTe-Based Thermoelectric Materials, ACS Appl. Mater. Interfaces, 2018, 10, 3609–3615,  DOI:10.1021/acsami.7b17142.
48. M. C. Eze, G. Ugwuanyi, M. Li, H. U. Eze, G. M. Rodriguez, A. Evans, V. G. Rocha, Z. Li and G. Min, Optimum silver contact sputtering parameters for efficient perovskite solar cell fabrication, Sol. Energy Mater. Sol. Cells, 2021, 230, 111185,  DOI:10.1016/j.solmat.2021.111185.
49. J. Q. Hu, Q. Chen, Z. X. Xie, G. Bin Han, R. H. Wang, B. Ren, Y. Zhang, Z. L. Yang and Z. Q. Tian, A Simple and Effective Route for the Synthesis of Crystalline Silver Nanorods and Nanowires, Adv. Funct. Mater., 2004, 14, 183–189,  DOI:10.1002/adfm.200304421.
50. J. P. Deng, S. H. Li, S. Te Chang and Y. C. Shih, Hyaluronan-assisted synthesis of silver nanoparticles and nanowires, Res. Chem. Intermed., 2019, 45, 119–125,  DOI:10.1007/s11164-018-3631-6.
51. G. Turgut, Effect of Ta doping on the characteristic features of spray-coated SnO₂, Thin Solid Films, 2015, 594, 56–66,  DOI:10.1016/j.tsf.2015.10.011.
52. K. Keerthi, P. Tarachand, G. S. Okram, T. Shripathi, V. Ganesan, B. G. Nair, A. Abraham and R. R. Philip, Electrical and transport properties of nearly stoichiometric transparent n-type silver indium oxide thin films, Phys. Status Solidi B Basic Res., 2016, 253, 2230–2235,  DOI:10.1002/pssb.201600190.
53. R. Ramarajan, M. Kovendhan, K. Thangaraju and D. Paul Joseph, Substrate Temperature Dependent Physical Properties of Spray Deposited Antimony-Doped SnO₂ Thin Films, Thin Solid Films, 2020, 704, 137988,  DOI:10.1016/j.tsf.2020.137988.
54. J. Zhang, P. Yang, W. Peng, H. Dong and L. Li, The effect of thickness on the optical and electrical properties of Hf doped indium oxide thin films, Vacuum, 2023, 216, 112426,  DOI:10.1016/j.vacuum.2023.112426.
55. C. A. V. Huayhua, B. D. A. Huacarpuma, L. A. R. Junior, J. A. Guerra, F. F. H. Aragón and J. A. H. Coaquira, Tuning the structural, electrical, and optical properties of ITO thin films via thickness control and vacuum annealing, Mater. Today Commun., 2025, 48, 113605,  DOI:10.1016/j.mtcomm.2025.113605.
56. M. Behtash, P. H. Joo, S. Nazir and K. Yang, Electronic structures and formation energies of pentavalent-ion-doped SnO₂ : First-principles hybrid functional calculations, J. Appl. Phys., 2015, 117, 175101,  DOI:10.1063/1.4919422.
57. M. Huang, Z. Hameiri, H. Gong, W. C. Wong, A. G. Aberle and T. Mueller, Hybrid silver nanoparticle and transparent conductive oxide structure for silicon solar cell applications, Phys. Status Solidi RRL, 2014, 8, 399–403,  DOI:10.1002/pssr.201400005.
58. S. Zhang, X. Liu, T. Lin and P. He, A method to fabricate uniform silver nanowires transparent electrode using Meyer rod coating and dynamic heating, J. Mater. Sci.: Mater. Electron., 2019, 30, 18702–18709,  DOI:10.1007/s10854-019-02223-x.
59. S. Wang, H. Liu, Y. Pan, F. Xie, Y. Zhang, J. Zhao, S. Wen and F. Gao, Performance Enhancement of Silver Nanowire-Based Transparent Electrodes by Ultraviolet Irradiation, Nanomaterials, 2022, 12, 2956,  DOI:10.3390/nano12172956.
60. S. Yu, X. Ma, X. Li, J. Li, B. Gong and X. Wang, Enhanced adhesion of Ag nanowire based transparent conducting electrodes for application in flexible electrochromic devices, Opt. Mater., 2021, 120, 111414,  DOI:10.1016/j.optmat.2021.111414.
61. C. M. Mahajan and S. P. Gumfekar, Unveiling the evolution of microstructural and optoelectronic properties of spray-pyrolyzed ZnO thin films via tailoring the aerosol deposition time, Can. J. Chem. Eng., 2025, 1–13,  DOI:10.1002/cjce.70128.
62. M. Esro, S. Georgakopoulos, H. Lu, G. Vourlias, A. Krier, W. I. Milne, W. P. Gillin and G. Adamopoulos, Solution processed SnO₂:Sb transparent conductive oxide as an alternative to indium tin oxide for applications in organic light emitting diodes, J. Mater. Chem. C, 2016, 4, 3563–3570,  10.1039/c5tc04117a.
63. H. Ha, C. Amicucci, P. Matteini and B. Hwang, Mini review of synthesis strategies of silver nanowires and their applications, Colloid Interface Sci. Commun., 2022, 50, 100663,  DOI:10.1016/j.colcom.2022.100663.
64. R. Ramarajan, Thermal stability study of niobium doped SnO₂ thin film for transparent conducting oxide application, Superlattices Microstruct., 2019, 135, 106274,  DOI:10.1016/j.spmi.2019.106274.
65. E. Shanthi, A. Banerjee, V. Dutta and K. L. Chopra, Electrical and optical properties of tin oxide films doped with F and (Sb+F), J. Appl. Phys., 1982, 53, 1615–1621,  DOI:10.1063/1.330619.
66. Y. Muraoka, N. Takubo and Z. Hiroi, Photoinduced conductivity in tin dioxide thin films, J. Appl. Phys., 2009, 105, 103702,  DOI:10.1063/1.3126713.
67. P. Marchand, I. A. Hassan, I. P. Parkin and C. J. Carmalt, Aerosol-assisted delivery of precursors for chemical vapour deposition: Expanding the scope of CVD for materials fabrication, Dalton Trans., 2013, 42, 9406–9422,  10.1039/c3dt50607j.
68. Y. Jin, Y. Sun, K. Wang, Y. Chen, Z. Liang, Y. Xu and F. Xiao, Long-term stable silver nanowire transparent composite as bottom electrode for perovskite solar cells, Nano Res., 2018, 11, 1998–2011,  DOI:10.1007/s12274-017-1816-8.
69. M. Xie, H. Lu, L. Zhang, J. Wang, Q. Luo, J. Lin, L. Ba, H. Liu, W. Shen, L. Shi and C. Q. Ma, Fully Solution-Processed Semi-Transparent Perovskite Solar Cells With Ink-Jet Printed Silver Nanowires Top Electrode, Sol. RRL, 2018, 2, 1–10,  DOI:10.1002/solr.201700184.
70. A. Kim, H. Lee, H. C. Kwon, H. S. Jung, N. G. Park, S. Jeong and J. Moon, Fully solution-processed transparent electrodes based on silver nanowire composites for perovskite solar cells, Nanoscale, 2016, 8, 6308–6316,  10.1039/c5nr04585a.
71. M. Lee, Y. Ko, B. K. Min and Y. Jun, Silver Nanowire Top Electrodes in Flexible Perovskite Solar Cells using Titanium Metal as Substrate, ChemSusChem, 2016, 9, 31–35,  DOI:10.1002/cssc.201501332.
72. J. Wang, X. Chen, F. Jiang, Q. Luo, L. Zhang, M. Tan, M. Xie, Y. Q. Li, Y. Zhou, W. Su, Y. Li and C. Q. Ma, Electrochemical Corrosion of Ag Electrode in the Silver Grid Electrode-Based Flexible Perovskite Solar Cells and the Suppression Method, Sol. RRL, 2018, 2, 1–10,  DOI:10.1002/solr.201800118.
73. F. Zuo, S. T. Williams, P. W. Liang, C. C. Chueh, C. Y. Liao and A. K. Y. Jen, Binary-Metal Perovskites Toward High-Performance Planar-Heterojunction Hybrid Solar Cells, Adv. Mater., 2014, 26, 6454–6460,  DOI:10.1002/adma.201401641.
74. J. Liu, C. Gao, L. Luo, Q. Ye, X. He, L. Ouyang, X. Guo, D. Zhuang, C. Liao, J. Mei and W. Lau, Low-temperature, solution processed metal sulfide as an electron transport layer for efficient planar perovskite solar cells, J. Mater. Chem. A, 2015, 3, 11750–11755,  10.1039/c5ta01200g.
75. H. Lu, Y. Ma, B. Gu, W. Tian and L. Li, Identifying the optimum thickness of electron transport layers for highly efficient perovskite planar solar cells, J. Mater. Chem. A, 2015, 3, 16445–16452,  10.1039/c5ta03686k.
76. J. Wang, X. Chen, F. Jiang, Q. Luo, L. Zhang, M. Tan, M. Xie, Y. Q. Li, Y. Zhou, W. Su, Y. Li and C. Q. Ma, Electrochemical Corrosion of Ag Electrode in the Silver Grid Electrode-Based Flexible Perovskite Solar Cells and the Suppression Method, Sol. RRL, 2018, 2, 1–10,  DOI:10.1002/solr.201800118.

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