Lead-Free Chalcogenides for Eco-Friendly Photovoltaics

P. K. Patil^a, Dr. D. B. Salunkhe^b*, Dr. H. S. Gavale^c*

^aDept. Of Physics, S.S.V.P.S’s L. K. Dr. P. R. Ghogrey Science College, Dhule

^bDepartment of Physics, KVPS Kisan ACS College Parola Dist Jalgaon 425111

^c Dept Of Physics, Z. B. Patil College, Dhule

Abstract:
The production of semiconductor materials which are lead free and eco-friendly has become a more focus of research into sustainable and renewable energy. The need for safe, eco-friendly and sustainable solar energy materials has increased interest in lead-free photovoltaic technologies. Silver bismuth chalcogenides, namely AgBiS₂ and AgBiSe₂, are promising absorber materials because they are environmentally friendly, chemically stable, and made from relatively abundant elements. These materials have suitable band gaps, absorb light strongly, and can tolerate crystal defects, which are important for efficient solar cell performance. AgBiS₂ and AgBiSe₂ thin films can be produced using low-cost and scalable methods such as spin coating, successive ionic layer adsorption and reaction (SILAR), hydrothermal synthesis, and other solution-based techniques. These methods are suitable for large-area and flexible solar devices. In addition, AgBiS₂ and AgBiSe₂ show better thermal and environmental stability compared to lead-based perovskite materials. This review summarizes recent progress in their synthesis, structural and optical properties, and photovoltaic performance. The main challenges, including charge transport and interface losses, are discussed, along with future research directions to improve efficiency and long-term stability.

Keywords:AgBiX₂, AgBiS₂, AgBiSe₂, eco-friendly, lead-free chalcogenides, photovoltaics, sustainable energy, solar devices.

1.Introduction:

The rising global demand for energy and increasing environmental concerns have intensified research into renewable energy sources. Solar energy is considered one of the most promising options because it is clean, abundant, and sustainable [1,2]. However, conventional silicon-based solar cells involve high-temperature processing and costly manufacturing steps, which limit their economic feasibility for large-scale deployment [3]. This has motivated the search for alternative photovoltaic materials that are efficient, low-cost, and environmentally benign [4,5].

Lead-based perovskite solar cells have demonstrated rapid improvements in power conversion efficiency in recent years [6,7]. Despite this progress, their commercial application is hindered by the presence of toxic lead and poor long-term stability under moisture, heat, and continuous light exposure [8–10]. These issues have driven significant efforts toward the development of lead-free photovoltaic materials with improved environmental safety and operational stability [11,12].

Chalcogenide semiconductors have emerged as attractive candidates for lead-free solar cells due to their high optical absorption, tunable band gaps, and good chemical stability [13–15]. Among them, silver bismuth chalcogenides, particularly AgBiS₂ and AgBiSe₂, have received growing attention as sustainable absorber materials [9-13]. These compounds consist of relatively non-toxic and earth-abundant elements and possess band gap energies well suited for solar energy conversion [8-12]. Their strong light absorption enables efficient photon harvesting in thin films, while their defect-tolerant nature helps suppress non-radiative recombination losses [2].

AgBiS₂ and AgBiSe₂ are also compatible with low-cost and scalable fabrication techniques, including spin coating, successive ionic layer adsorption and reaction (SILAR), and hydrothermal synthesis [14]. These solution-based methods allow large-area deposition and integration with flexible substrates [9]. In addition, silver bismuth chalcogenides exhibit improved thermal and environmental stability compared to lead-based perovskite materials, making them promising for long-term photovoltaic applications [8].

This review presents an overview of recent progress in AgBiS₂ and AgBiSe₂-based photovoltaic materials, covering synthesis routes, structural, optical, and electrical properties, and device performance [10,11]. Key challenges related to charge transport, interface engineering, and efficiency optimization are discussed, and future research directions are proposed to advance stable, efficient, and environmentally friendly solar cell technologies [11].

2. Properties and Crystal Structure:
1. Crystal Structure

AgBiS₂ and AgBiSe₂ are ternary chalcogenide semiconductors composed of silver, bismuth, and sulfur or selenium. These materials generally crystallize in a cubic or near-cubic crystal structure, which is favorable for uniform thin-film formation [1,2]. In the crystal lattice, Ag⁺ and Bi³⁺ ions occupy metal sites and are coordinated by S²⁻ or Se²⁻ anions [3].

Their atomic arrangement resembles a rock-salt-type framework, where metal–chalcogen bonds form a compact and symmetric network [4]. This structural symmetry allows the materials to accommodate a certain level of lattice disorder without severe degradation of electronic properties [5]. Such defect tolerance is particularly beneficial for solution-processed films, where perfect crystallinity is difficult to achieve [6].

2. Optical and Electrical Properties

AgBiS₂ and AgBiSe₂ exhibit strong absorption in the visible region, with absorption coefficients high enough to enable efficient light harvesting in thin absorber layers [7,8]. AgBiS₂ mainly absorbs visible light, while AgBiSe₂ has a slightly narrower band gap, allowing absorption to extend into the near-infrared region [9,10].

1. Optical Absorption Behaviour

AgBiX₂ (X = S, Se) materials show strong light absorption in the visible and near-infrared (NIR) regions, which is essential for efficient solar energy harvesting. UV–Vis absorption studies typically reveal a clear and sharp absorption edge, indicating good crystallinity and a well-defined electronic band structure. These materials possess high absorption coefficients in the range of about 10²–10⁵ cm⁻¹, allowing effective photon absorption even with very thin films. This property is particularly beneficial for low-cost and flexible photovoltaic devices.

Replacing sulfur (S) with selenium (Se) causes the absorption edge to shift toward longer wavelengths. This red shift occurs due to the larger atomic size and higher polarizability of selenium. As a result, the material can absorb a broader portion of the solar spectrum, improving light utilization in photovoltaic applications.

• Band Gap Energy

The optical band gap (Eg) of AgBiX₂ compounds is commonly determined using Tauc plots derived from UV–Vis absorption measurements. These materials typically exhibit direct or quasi-direct band gap characteristics, which are advantageous for optoelectronic and photovoltaic applications.

AgBiS₂ generally shows a band gap in the range of 1.2 to 1.6 eV. In comparison, AgBiSe₂ has a smaller band gap, usually between 0.9 and 1.2 eV. This reduction in band gap allows AgBiSe₂ to absorb light over a wider wavelength range.

Both band gap values fall close to the optimal range required for efficient solar energy conversion, enabling effective utilization of the solar spectrum [2–3]. Furthermore, the band gap of these materials can be tuned through anion substitution (S to Se), nanostructure formation, and defect engineering, enhancing their suitability for photovoltaic devices.

• Photoluminescence (PL) Characteristics

Photoluminescence analysis provides valuable insight into charge carrier recombination processes and the presence of defects in AgBiX₂ materials. These compounds generally show weak to moderate PL emission, which indicates reduced radiative recombination and efficient separation of photo-generated charge carriers. Such behavior is highly desirable for applications in solar cells and photocatalysis.

The observed PL emission peaks are commonly attributed to recombination occurring near the band edge as well as defect-related states, including sulfur or selenium vacancies and antisite defects. Lower PL intensity suggests suppressed electron–hole recombination, which contributes to improved photovoltaic and photocatalytic performance [6].

Electrical Properties and Charge Transport:

Electrical characterization reveals that AgBiX₂ materials generally show p-type conductivity, primarily arising from intrinsic defects such as silver vacancies [1,2]. Synthesis conditions and post-deposition treatments have a strong influence on charge carrier concentration, mobility, and electrical resistivity [3,4]. Enhanced crystallinity and reduced defect density improve charge transport and suppress recombination losses, resulting in improved electrical performance [5–7]Both compounds behave as semiconductors and show effective generation of charge carriers under illumination [11]. Their electronic structure supports the transport of electrons and holes with relatively low recombination losses. Importantly, AgBiS₂ and AgBiSe₂ are known for their defect-tolerant nature, where common point defects do not form deep trap states that severely limit carrier lifetime.

3. Thermal and Environmental Stability

A major advantage of AgBiS₂ and AgBiSe₂ is their high thermal and environmental stability compared to lead-based perovskite absorbers [12,13]. These materials retain their structural and optical properties when exposed to air, moisture, and moderate heating conditions [7].

The strong metal–chalcogen bonds in silver bismuth chalcogenides provide chemical robustness, reducing the risk of phase degradation or decomposition during long-term operation [18,19]. Several studies have reported stable performance of AgBiS₂ and AgBiSe₂ thin films under continuous light exposure and extended storage periods [20–22]. This stability makes them suitable candidates for durable and reliable photovoltaic devices.

In summary, AgBiS₂ and AgBiSe₂ possess favorable crystal structures, strong optical absorption, suitable band gaps, and defect-tolerant electronic properties [21,22]. Their excellent resistance to thermal and environmental degradation further enhances their potential as lead-free absorber materials for sustainable photovoltaic applications [15-18].

3.1Chemical Synthesis Approaches for AgBiS₂ and AgBiSe₂ Thin Films:

1. Successive Ionic Layer Adsorption and Reaction (SILAR)

SILAR is a solution-based deposition technique that is widely applied for the preparation of AgBiS₂ and AgBiSe₂ thin films because of its simplicity and low processing cost [1,2]. The method involves repeated dipping of the substrate into cationic and anionic solutions, separated by rinsing steps. Silver and bismuth ions are adsorbed from metal salt solutions, followed by reaction with sulfur or selenium ions to form the chalcogenide layer on the substrate surface [3].

The thickness and composition of the films can be adjusted by controlling the number of deposition cycles, solution concentration, and immersion time [4]. Doping can be conveniently introduced by adding suitable dopant ions into the metal precursor solution, allowing easy modification of the film properties without complex processing steps [5,6]. Due to its low-temperature operation and suitability for large substrates, SILAR is well suited for cost-effective photovoltaic fabrication.

2. Chemical Bath Deposition (CBD)

Chemical bath deposition is a commonly used technique for producing chalcogenide semiconductor films with uniform coverage [7]. In this method, the substrate is placed in a reaction bath containing metal precursors, a sulfur or selenium source, and complexing agents that regulate the release of ions into the solution [8]. Controlled chemical reactions in the bath lead to gradual film growth on the substrate.

Film quality, including thickness, grain size, and stoichiometry, can be tailored by varying parameters such as bath temperature, pH, and deposition duration [9]. Doping is achieved by introducing small amounts of dopant salts into the bath, enabling uniform incorporation during film growth [10,11]. Post-deposition heat treatment is often applied to improve crystallinity and electrical performance [12].

3. Hydrothermal Method

The hydrothermal method involves chemical reactions carried out in sealed vessels at elevated temperature and pressure [13]. This approach allows the synthesis of AgBiS₂ and AgBiSe₂ materials with high crystallinity and controlled morphology [14]. Metal salts and chalcogen sources are dissolved in aqueous or mixed solvents and heated under carefully controlled conditions.

Dopant elements can be added directly to the precursor solution, leading to uniform dopant distribution throughout the material [15,16]. Although hydrothermal synthesis produces high-quality materials, its use in large-area thin-film deposition is limited. Therefore, it is mainly employed for nanostructured absorbers and fundamental material studies.

4. Spin Coating of TiO₂ Base Layer

Spin coating is a widely adopted technique for depositing compact and uniform TiO₂ layers that act as electron transport layers in photovoltaic devices [17]. A TiO₂ precursor solution or diluted paste is dropped onto the substrate and spread evenly by rapid rotation [18].

The final film thickness is influenced by the spin speed, spinning time, and solution viscosity [19]. After coating, thermal treatment is usually applied to enhance film densification and charge transport properties [20]. A well-prepared TiO₂ base layer improves interfacial contact and facilitates efficient electron extraction from AgBiS₂ or AgBiSe₂ absorber layers [21,22].

In summary, SILAR and CBD are particularly effective for depositing doped AgBiS₂ and AgBiSe₂ thin films using low-cost and scalable solution-based techniques. The hydrothermal method provides high-quality crystalline materials but is less suitable for large-area films. Spin coating remains an efficient and reliable approach for preparing TiO₂ base layers, contributing to improved photovoltaic device performance

3.3 Post-Deposition Treatments and Performance Enhancement

After film deposition, additional processing steps are often required to improve the quality and performance of AgBiS₂ and AgBiSe₂ thin films. These post-deposition treatments help enhance crystal structure, reduce defects, and improve charge transport within the photovoltaic device [1,2].

A) Heat Treatment (Annealing)

Heat treatment is widely applied to improve the structural properties of AgBiS₂ and AgBiSe₂ films [3]. Annealing is typically performed in air, inert atmospheres, or sulfur- or selenium-rich environments. This process allows atoms within the film to rearrange into a more ordered structure, leading to larger grain sizes and improved crystallinity [4].

Annealing also removes residual solvents and improves film compactness, which enhances electrical conductivity and reduces carrier recombination [5]. However, excessive heating may cause chalcogen loss or phase instability, making careful optimization of annealing conditions essential [6].

B) Sulfurization and Selenization Treatments

Exposure of deposited films to sulfur or selenium vapor is commonly used to correct compositional deficiencies and improve phase quality [7]. Such treatments help compensate for sulfur or selenium vacancies that can form during film growth [8].

By reducing these vacancies, carrier transport properties are improved, resulting in enhanced photovoltaic performance [9]. Chalcogen-rich treatments are particularly beneficial for films prepared by solution-based methods, where slight non-stoichiometry is often observed [10].

C)Surface and Interface Modification

Surface treatments are important for minimizing charge losses caused by surface defects [11]. Chemical passivation techniques can reduce dangling bonds and surface trap states, leading to improved carrier lifetime [12].Engineering the interface between AgBiS₂/AgBiSe₂ absorber layers and the TiO₂ electron transport layer is also crucial. Improved interface quality enhances charge transfer and suppresses interfacial recombination, contributing to higher device efficiency [13,14].

D) Post-Treatment of TiO₂ Base Layer

The performance of the TiO₂ base layer can be significantly improved through post-deposition treatment [19]. Thermal annealing enhances TiO₂ crystallinity and electron mobility, while surface treatments reduce trap states at the TiO₂ surface [20].

An optimized TiO₂ layer provides better electronic contact with the absorber material, enabling efficient electron extraction and reducing recombination losses at the interface [21,22].

4.Morphological and Structural Characteristics

AgBiS₂ and AgBiSe₂ thin films generally exhibit smooth and well-covered surfaces with uniform grain distribution when prepared using solution-based techniques. Optimized deposition conditions and post-deposition heat treatment promote grain growth, resulting in fewer grain boundaries that support improved charge transport. The film thickness typically lies in the sub-micron to micron range, providing effective and uniform light absorption across the absorber layer. Structural studies, such as X-ray diffraction, confirm that these materials commonly crystallize in cubic or near-cubic phases. Thermal treatment further enhances crystallinity and phase stability. A reduced density of structural defects contributes to better electrical transport properties. Together, these morphological and structural features play a crucial role in achieving efficient and stable photovoltaic device performance.

7Applications for Sustainable Energy

Silver bismuth chalcogenides such as AgBiS₂ and AgBiSe₂ are increasingly explored for sustainable energy applications because they are free from toxic lead, absorb light efficiently, and show good operational stability [1,2]. Their suitable band gap energies make them effective light-absorbing layers for thin-film solar cells [3]. In addition, these materials can be deposited using inexpensive and scalable solution-based techniques, allowing the fabrication of large-area and flexible photovoltaic devices [4].

Beyond solar cells, AgBiX₂ materials have demonstrated potential in photocatalytic processes, including solar-driven water splitting and the breakdown of environmental pollutants, due to their strong visible-light response and effective charge carrier separation [5,6]. Their chemical robustness supports stable performance under prolonged illumination [7]. Overall, silver bismuth chalcogenides offer a promising and environmentally friendly pathway for advancing next-generation sustainable energy technologies [8–10].

8Problems and Future Prospects

Despite the significant progress achieved with AgBiX₂ materials, several challenges still need to be addressed. These include achieving precise control over material stoichiometry, suppressing the formation of secondary phases, and ensuring long-term device stability. Improving performance further will require effective strategies such as controlled doping, interface engineering, and defect passivation. Future research should focus on developing scalable fabrication methods and gaining a deeper understanding of defect-related physics, which are essential steps toward the commercial realization of AgBiX₂-based energy devices.

9.Conclusion

Silver bismuth chalcogenides, namely AgBiS₂ and AgBiSe₂, have gained considerable attention as lead-free materials for sustainable energy applications. Their suitable band gap energies, strong optical absorption, defect-tolerant behavior, and good thermal and environmental stability make them highly promising for use in photovoltaic and photocatalytic systems. Moreover, these materials can be synthesized using low-cost and scalable solution-based methods, enabling their application in large-area devices. With further advancements in controlled synthesis, doping techniques, and interface engineering, the performance of AgBiS₂ and AgBiSe₂ is expected to improve further, strengthening their potential as environmentally friendly alternatives for next-generation solar energy technologies.

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