The relentless pursuit of sustainable and cost-effective energy solutions has positioned thin film solar panels as a critical technology in the global photovoltaic landscape. Among the various absorber materials, Cu2ZnSn(S,Se)4 (CZTSSe), a member of the kesterite family, has emerged as a highly promising candidate for next-generation thin film solar panels. Its appeal lies in the abundance and low toxicity of its constituent elements, a tunable direct bandgap (approximately 1.0–1.5 eV), and a high absorption coefficient (>104 cm−1). These properties make it an ideal absorber layer for efficient thin film solar panels. While the theoretical efficiency limit for single-junction CZTSSe devices approaches the Shockley–Queisser limit, the current certified record efficiency stands at 12.62%, achieved via vacuum-based sputtering. To realize the true potential of CZTSSe for low-cost, large-scale photovoltaic deployment, non-vacuum, solution-based fabrication methods are indispensable. These approaches offer inherent advantages such as high material utilization, simple processing, low capital investment, and compatibility with roll-to-roll manufacturing, which are essential for producing affordable thin film solar panels.
This review provides a comprehensive analysis of the progress in fabricating kesterite CZTSSe absorbers using solution-based techniques, which are pivotal for the future of thin film solar panels. The discussion is systematically organized into four principal categories: Spray Pyrolysis, Bath-Based Aqueous Methods, Nanoparticle-Based Inks, and Direct Solution Coating. For each method, we delve into the fundamental processing principles, key optimization strategies—such as cationic composition tuning, annealing/selection conditions, and extrinsic doping—and the resulting photovoltaic performance. The ultimate goal is to elucidate the pathways and challenges toward achieving high-efficiency, solution-processed CZTSSe thin film solar panels.
1. Spray Pyrolysis: Aerosol-Assisted Deposition
Spray pyrolysis involves atomizing a precursor solution containing metal salts and chalcogen sources (e.g., thiourea) into a fine mist, which is then directed onto a heated substrate. The droplets undergo pyrolysis, leading to the direct formation of a thin film. This method is attractive for its simplicity and scalability in manufacturing thin film solar panels.
The technique’s efficacy heavily depends on precursor chemistry and processing parameters. The choice of solvent—such as dimethylformamide (DMF), dimethyl sulfoxide (DMSO), or water–alcohol mixtures—significantly influences film morphology and purity. For instance, DMF-based solutions have yielded films with superior phase purity and denser grains compared to other solvents. A critical parameter is the chalcogen ratio during annealing. Research indicates that an optimal S/(S+Se) ratio of approximately 0.2 maximizes device performance by favorably influencing grain growth and electronic properties. Post-deposition annealing in chalcogen atmosphere (sulfurization/selenization) is crucial for crystallinity and stoichiometry control. Performance has been further enhanced by employing strategies like depositing a CuGe seed layer on Mo back contact to suppress the formation of a thick, resistive Mo(S,Se)2 interfacial layer. Doping with elements like Ag and Mn has also been explored to passivate defects and improve carrier concentration. Through such optimizations, spray pyrolysis has achieved a notable power conversion efficiency (PCE) of 10.04% for CZTSSe thin film solar panels, demonstrating its viability as a fabrication route.
2. Bath-Based Aqueous Methods: Electrochemical and Chemical Growth
This category encompasses techniques where the absorber film is grown from an aqueous bath, offering excellent control over film thickness and composition at potentially low temperatures, beneficial for flexible thin film solar panels.
2.1 Electrodeposition (ED): ED involves the electrochemical reduction of metal ions onto a conductive substrate (typically Mo-coated glass). It can be performed as sequential stacking of elemental (Cu/Zn/Sn) or compound layers, or as a single-step co-deposition of all elements. While stacking can lead to inhomogeneities, co-deposition improves interfacial adhesion. Key optimizations include precise control of the cationic ratio [Cu/(Zn+Sn) and Zn/Sn] and post-deposition selection under controlled temperature profiles. Efficiency enhancements have been achieved by smoothing the precursor surface to eliminate pinholes and reduce charge recombination. The current record PCE for electrodeposited CZTSSe thin film solar panels is 9.9%.
2.2 Chemical Bath Deposition (CBD) & Successive Ionic Layer Adsorption and Reaction (SILAR): CBD relies on controlled precipitation reactions from a solution, while SILAR involves alternating immersion of the substrate in cationic and anionic precursor solutions. These methods can produce uniform and adherent films. However, for CZTSSe, challenges such as small grain size, pin-holes, and poor interfacial morphology with the Mo back contact have limited device performance. Optimizing parameters like pH, temperature, and annealing conditions is essential. To date, CBD and SILAR have reported maximum PCEs of 3.0% and 3.74% for CZTSSe thin film solar panels, respectively, indicating significant room for improvement in absorber quality.
3. Nanoparticle-Based Ink Methods: Building from Nanocrystals
This approach involves synthesizing CZTS or CZTSSe nanocrystals (NCs) in solution, which are then dispersed in a solvent to form an ink for coating (e.g., spin-coating, blade-coating). The nanocrystals are typically synthesized via hot-injection, solvothermal, or microwave-assisted methods. Hot-injection is particularly favored for producing monodisperse, phase-pure NCs with controlled size and composition.
The nanocrystal ink is deposited to form a particulate precursor film, which is then subjected to a high-temperature annealing step under S/Se atmosphere to sinter the nanoparticles, remove organic ligands, and foster large grain growth. The initial NC composition and the annealing profile are critical. This method benefits from the pre-formed kesterite phase in the NCs, which can reduce the formation of secondary phases during annealing. Significant progress has been made through compositional engineering, such as partial substitution of Sn with Ge to widen the bandgap and reduce band tailing. Nanoparticle-based methods have consistently yielded high-quality absorbers, with reported efficiencies reaching 9.4% for Ge-alloyed devices. The primary challenge lies in the complexity of the synthesis and the potential for carbonaceous residue from ligands, which can hinder grain growth and increase series resistance in the final thin film solar panel.
4. Direct Solution Coating: Molecular Precursor Inks
This is the most promising and widely researched solution route for high-efficiency thin film solar panels. It involves dissolving or complexing metal and chalcogen sources in a solvent to create a homogeneous molecular precursor ink, which is coated directly and then thermally processed to form the polycrystalline absorber.
4.1 Hydrazine-Based Processing: Hydrazine (N2H4) is a uniquely powerful solvent that can dissolve elemental metals and chalcogens, enabling the formulation of pure precursor solutions. This method produces exceptionally high-quality films with large, dense grains and clean grain boundaries. It holds the current solution-processed record PCE of 12.6% for CZTSSe thin film solar panels, a testament to the superb film quality achievable. However, hydrazine’s extreme toxicity and explosiveness severely limit its practical application and scalability for commercial thin film solar panel production.
4.2 Non-Hydrazine, Green-Solvent Processing: The search for safer, environmentally benign alternatives has driven the development of precursor inks based on solvents like DMSO, DMF, and amine-thiol mixtures. These solvents dissolve metal salts (e.g., chlorides, acetates) and chalcogen sources (thiourea, selenourea) through complexation.
- DMSO/DMF Routes: These polar aprotic solvents are effective at dissolving precursors. The key innovation has been establishing a redox equilibrium in the precursor solution, which improves solution stability and leads to better-grained films after selenization. Careful control of the chalcogen content and the use of additives (e.g., water, other solvents) to modify ink rheology and substrate wetting have been crucial. Efficiencies above 11% have been achieved with DMSO-based inks.
- Thiol-Amine Solvent Systems: Mixtures like thiols (e.g., 1,2-ethanedithiol) and amines (e.g., ethylenediamine) are highly effective at dissolving metal oxides, salts, and even elemental powders. This chemistry allows for flexible tuning of the precursor. A landmark achievement was the development of a water-based ammonia-thioglycolic acid system, which offers a truly green and low-cost pathway. Through meticulous optimization of metal–thiol coordination and post-deposition treatments like alkali metal doping (Na, Li), this approach has yielded a certified efficiency of 12.0% (12.3% lab-measured), rivaling the hydrazine-processed record and marking a major breakthrough for sustainable thin film solar panel fabrication.
The performance progression of various solution methods for CZTSSe thin film solar panels is summarized in the table below, highlighting the superior performance of direct solution coating techniques, particularly those employing molecular precursor inks.
| Solution Method Category | Key Characteristics | Typical PCE Range (%) | Record PCE (%) | Primary Advantages | Primary Challenges |
|---|---|---|---|---|---|
| Spray Pyrolysis | Aerosol deposition, in-situ pyrolysis | 2–8 | 10.04 | Scalable, simple setup | Control over stoichiometry, film porosity |
| Electrodeposition | Electrochemical growth from aqueous bath | 6–9 | 9.9 | High material use, low temp deposition | Film homogeneity, post-annealing required |
| Nanoparticle Inks | Coating of pre-synthesized nanocrystals | 7–9.5 | 9.4 (Ge-doped) | Good phase purity from start | Complex synthesis, ligand removal |
| Direct Solution (Hydrazine) | Molecular precursor ink, high-quality films | 10–12.6 | 12.6 | Excellent film morphology, record efficiency | Extremely toxic and hazardous solvent |
| Direct Solution (Non-Hydrazine) | Molecular precursor ink, green solvents | 8–12.3 | 12.3 | Safe, scalable, high efficiency potential | Ink stability, defect control |
5. Critical Challenges and Path Forward for Solution-Processed Thin Film Solar Panels
Despite remarkable progress, the efficiency of solution-processed CZTSSe thin film solar panels still lags behind their vacuum-deposited counterparts and other established thin film technologies like CIGS. This “efficiency gap” is primarily attributed to the high open-circuit voltage (VOC) deficit, defined as $$ V_{OC,def} = \frac{E_g}{q} – V_{OC} $$ where \(E_g\) is the bandgap and \(q\) is the elementary charge. This deficit stems from several intrinsic and process-related factors:
1. Point Defects and Band Tailing: The kesterite structure is prone to a high density of intrinsic point defects (e.g., CuZn antisites, VCu vacancies) and related defect complexes. These create band tailing and deep-level recombination centers, which severely limit VOC. The defect formation energy is inherently low in this quaternary system.
2. Secondary Phases and Interface Recombination: The narrow phase stability window of CZTSSe makes it challenging to avoid secondary phases like Zn(S,Se) or Cu2Sn(S,Se)3 during solution processing and annealing. These phases, often at grain boundaries or interfaces, act as recombination sites. Furthermore, the interface with the CdS buffer layer and the formation of a Mo(S,Se)2 layer at the back contact must be carefully managed to minimize recombination losses.
3. Grain Boundary Recombination: While grain sizes have increased significantly, non-radiative recombination at grain boundaries remains a critical issue, especially in solution-processed films where impurity segregation may occur.
The future research direction for solution-processed CZTSSe thin film solar panels must focus on a multi-pronged strategy to address these challenges:
- Advanced Defect Engineering: Moving beyond simple cationic ratio adjustments (Cu-poor, Zn-rich). Targeted extrinsic doping with elements like Li, Na, K, Ag, or Cd-substitution for Zn has shown promise in passivating defects, modifying band structure, and improving carrier concentration. Computational modeling combined with high-throughput experimental screening will be vital to identify optimal doping schemes.
- Precursor Chemistry and Reaction Pathway Control: The quality of the final absorber is fundamentally determined by the molecular structure of the precursor ink and the transformation pathway during annealing. Future work must delve deeper into understanding and controlling these pathways—such as intermediate phase evolution and chalcogen incorporation kinetics—to promote the growth of a more ordered, defect-suppressed kesterite lattice. Developing novel chelating agents and solvent systems that promote ideal molecular precursors is key.
- Interface and Band Alignment Engineering: Exploring alternative buffer layers (e.g., Zn(O,S), (Zn,Mg)O) and interface passivation strategies to reduce interface recombination and improve band alignment. Optimizing the back contact interface to control Mo(S,Se)2 formation is equally important.
- Process Innovation: Refining annealing profiles (temperature, time, atmosphere pressure) and exploring advanced techniques like rapid thermal processing, photonic curing, or multi-step annealing sequences to enhance grain growth while suppressing volatile element loss and secondary phase formation.
6. Conclusion
Solution-based fabrication methods have undeniably established themselves as a cornerstone for the future development of low-cost, high-performance kesterite thin film solar panels. From spray pyrolysis and electrochemical methods to advanced direct solution coating with green solvents, significant strides have been made in understanding and controlling the complex processes that govern film formation and device performance. The recent achievement of over 12% efficiency using a benign water-ammonia-thiol based precursor ink marks a pivotal moment, proving that solution processing can rival vacuum techniques without the associated toxicity and cost hurdles. The central challenge remains the mitigation of the high VOC deficit, which is rooted in the material’s intrinsic defect physics. The path forward lies in a concerted effort combining sophisticated defect engineering through doping and substitution, precise control over precursor chemistry and transformation pathways, and meticulous interface management. By addressing these fundamental materials science challenges, solution-processed Cu2ZnSn(S,Se)4 thin film solar panels have a clear pathway to breaking the 15% efficiency barrier and beyond, ultimately fulfilling their promise as a sustainable, earth-abundant, and commercially viable photovoltaic technology.