Solar cells represent a cornerstone in the global transition toward renewable energy, with ongoing efforts to improve their efficiency, sustainability, and scalability. Among the plethora of materials investigated, kesterite compounds based on Cu₂ZnSn(S,Se)₄ (CZTSSe) have emerged as particularly promising candidates. Their appeal lies in their composition of abundant, non-toxic elements, which contrasts sharply with other thin-film photovoltaic technologies reliant on scarce or hazardous materials. Despite this promise, kesterite solar cells have historically lagged behind in power conversion efficiency, posing a persistent challenge to scientists and engineers alike.
At the heart of this challenge is the complex chemistry and physics of multinary semiconductor materials like CZTSSe. Unlike simpler binary or ternary compounds, these materials consist of four or more elements whose interactions determine critical properties such as bandgap, carrier mobility, and defect formation. Consequently, the synthesis routes and formation pathways exert profound influence on the ultimate device performance. Recent advances have spotlighted the synthesis stage, particularly the design and use of molecular inks, as a pivotal aspect of kesterite fabrication that can unlock higher efficiencies.
Molecular inks are precursor solutions containing metal complexes and chalcogen sources that, upon deposition and thermal processing, form the kesterite thin film. This approach enables finer control over elemental distribution and uniformity at the nanoscale, which is indispensable for producing defect-minimized absorber layers. By tailoring the chemical state of these inks—through the choice of ligands, solvent environment, and precursor ratios—researchers can influence nucleation dynamics and crystallization pathways. This precise control mitigates the formation of detrimental point and extended defects, which historically limited photovoltaic performance by acting as recombination centers.
One of the notable breakthroughs reported in recent research is the crossing of the 15% efficiency threshold using molecular ink-based synthesis. This milestone signifies a critical step towards making kesterite solar cells viable competitors to established thin-film technologies like CdTe and CIGS. Achieving this level of performance required not only optimization of the ink chemistry but also a deep understanding of the post-deposition annealing and crystallization kinetics. Controlling these parameters allowed for the deliberate engineering of grain boundaries and the reduction of secondary phases, which often impair charge transport and extraction.
A central focus of the latest studies centers on defect chemistry in CZTSSe films. Unlike single-element semiconductors, multinary compounds are prone to complex defect configurations due to their multiple constituent atoms. The interplay between copper, zinc, tin, sulfur, and selenium can generate intrinsic defects that act as electron or hole traps. The molecular ink strategy aids in managing this complexity by ensuring homogeneous precursor mixing and facilitating optimal stoichiometry control. Such advancements directly translate to improved open-circuit voltage (Voc) and fill factor (FF) metrics in finished solar cells.
The synthesis temperature and atmosphere also play decisive roles in the quality of the kesterite absorber layers. High-temperature annealing under controlled environments promotes grain growth and defect passivation but can also risk the evaporation or segregation of volatile components. Fine-tuning these conditions in combination with molecular ink chemistry has allowed researchers to circumvent these drawbacks, preserving the desirable phase purity and enhancing device stability. Understanding these thermodynamic and kinetic processes at a granular level is vital for replicating laboratory successes at industry-relevant scales.
Furthermore, the use of molecular inks paves the way for low-cost, scalable fabrication techniques compatible with large-area substrates and roll-to-roll manufacturing. This aspect is critical for the commercial viability of kesterite photovoltaics, as it promises the reduction of material wastage and energy input during synthesis. Compared to vacuum-based deposition techniques common in other thin-film photovoltaics, ink-based methods present an attractive alternative that aligns with sustainable manufacturing goals.
The evolution of CZTSSe solar cells is also marked by the integration of sophisticated characterization tools that elucidate the material’s microstructure and electronic properties. Techniques such as time-resolved photoluminescence, scanning transmission electron microscopy, and X-ray diffraction mapping provide insights into defect distribution, phase segregation, and carrier dynamics. These analyses have been instrumental in refining molecular ink formulations and processing protocols, leading to solar cells with enhanced electron lifetimes and mobility.
Future directions in kesterite research, inspired by the molecular ink paradigm, include the exploration of novel ligands and solvent systems that further improve precursor solubility and stability. Some efforts are focused on incorporating additives that passivate defects or promote preferential crystallographic orientations to improve charge transport. Additionally, the development of multi-step annealing and selenization processes tailored to the ink chemistry offers pathways to engineer absorber layers with superior optoelectronic quality.
Moreover, understanding the fundamental thermodynamic principles governing the formation of secondary phases remains a critical research area. Unwanted phases such as ZnSe, Cu₂SnSe₃, or SnS can both consume active materials and create electronic barriers at interfaces. Molecular ink strategies enable dynamic compositional adjustments during synthesis, potentially minimizing these phases and optimizing absorber homogeneity. This fine balance between precursor chemistry and final film properties is key to pushing efficiencies beyond the current limits.
Beyond photovoltaic applications, the insights gained from the study of molecular ink chemistry and formation pathways in multinary semiconductors have broader implications. Similar methodologies could be applied to other emerging materials systems for optoelectronics, thermoelectrics, or photocatalysis. The foundational understanding of how precursor chemistry influences crystallization and defect landscapes could accelerate the discovery and optimization of materials with complex elemental compositions.
In conclusion, the breakthrough achievements in kesterite solar cells owe much to the meticulous control over precursor chemistry afforded by molecular inks. This synthesis pathway offers a robust platform for addressing the longstanding challenges in CZTSSe photovoltaic technology, including defect mitigation, phase purity, and large-scale manufacturability. As research continues to harness these advantages, the prospect of affordable, efficient, and environmentally benign solar energy conversion via kesterite cells appears increasingly within reach.
The path forward involves not only continued refinement of molecular ink formulations but also innovative device architectures and interface engineering to maximize power conversion efficiencies. Coupling these advances with computational modeling and machine learning could further accelerate the optimization process, tailoring synthesis parameters for custom applications. The confluence of chemistry, materials science, and engineering in this interdisciplinary effort is emblematic of the future of sustainable energy research.
Ultimately, the story of kesterite solar cells exemplifies how fundamental chemistry and careful materials design converge to solve complex technological challenges. As these solar cells edge closer to commercial viability, their success will represent a triumph of both scientific ingenuity and practical innovation, enabling a cleaner energy future powered by Earth-abundant materials.
Subject of Research: Synthesis and formation pathways of high-efficiency kesterite solar cells through molecular ink chemistry.
Article Title: Formation pathway of high-efficiency kesterite solar cells fabricated through molecular ink chemistry.
Article References:
Jimenez-Arguijo, A., Gong, Y., Caño, I. et al. Formation pathway of high-efficiency kesterite solar cells fabricated through molecular ink chemistry. Nat Energy (2026). https://doi.org/10.1038/s41560-025-01900-y
Image Credits: AI Generated
