Researchers from Sungkyunkwan University, the Korea Research Institute of Chemical Technology (KRICT), the Massachusetts Institute of Technology (MIT), the Korea Advanced Institute of Science and Technology (KAIST), Ajou University, and the Ulsan National Institute of Science and Technology (UNIST) have developed an excess ligand strategy based on chemical bath deposition (CBD) of tin dioxide (SnO₂). This strategy addresses common limitations of CBD, such as prolonged deposition times, uneven film formation on large-area substrates, and susceptibility to oxidation.
Conventional CBD synthesis of SnO₂ typically proceeds through two competing nucleation pathways: cluster-by-cluster aggregation and ion-by-ion growth. Unfortunately, the cluster-by-cluster pathway often dominates, leading to heterogeneous deposition characterized by incomplete surface coverage and the formation of defects detrimental to charge transport and recombination dynamics. The new method enables the rapid synthesis of high-quality SnO₂ electron transport layers (ETLs) by suppressing the cluster-by-cluster pathway while promoting the ion-by-ion pathway to produce uniform films.
The resulting SnO₂ films exhibit excellent optoelectronic properties, including a low surface recombination velocity (5.5 cm/s) and a high electroluminescence efficiency of 24.8%. These improvements lead to power conversion efficiencies of up to 26.4% for perovskite solar cells, 23% for perovskite modules, and 23.1% for carbon-based perovskite cells.
In this new approach, suppressing the cluster aggregation pathway involves deliberately introducing an excess of ligand molecules compared to conventional protocols. These ligands coordinate with tin ions, stabilizing them and regulating nucleation kinetics. This molecular-level control preferentially directs the growth direction towards direct ion-by-ion deposition onto the substrate, avoiding the formation of colloidal SnO₂ clusters that compromise film integrity.
The ligand-rich environment also provides biochemical passivation of surface defects, which act as recombination centers in the final film. By saturating these sites during growth, the excess ligand method mitigates mid-gap states and traps that traditionally plague ETLs derived from wet-chemical methods. As a result, charge carrier transport through the ETL is smoother, enhancing the device’s open-circuit voltage and fill factor.
The rapidity of this deposition method marks an additional industrial advantage. Traditional CBD techniques require longer durations to achieve uniform coverage, a major bottleneck towards manufacturing scale. By altering the reaction pathway kinetics, the excess ligand strategy compresses processing time without sacrificing film quality or uniformity. The reduction in synthesis time directly translates into cost savings and higher throughput in manufacturing environments.
Importantly, this study also addresses oxidation-related degradation issues. In conventionally solution-processed SnO₂ films, uncontrolled oxidative growth induces variable stoichiometry and local defects. The controlled ligand environment buffers the chemical environment, resulting in stoichiometrically consistent, phase-pure SnO₂ layers with improved chemical stability, a crucial factor influencing device lifespan.
This research not only bridges the gap between laboratory-scale device fabrication and industrially viable production but also enhances the fundamental understanding of nucleation kinetics in chemical bath deposition. Regulating ligand content as a lever to control nucleation pathways opens the door to similar approaches in other oxide semiconductors, potentially revolutionizing the fabrication of ETLs beyond SnO₂.
Its implications extend to the broader context of perovskite photovoltaics, where enhancing interface quality is crucial for overcoming stability and efficiency bottlenecks. Defect suppression at the ETL/perovskite interface reduces hysteresis and photodegradation pathways, two persistent challenges hindering the wider adoption of perovskite solar technology. By addressing these issues through material synthesis innovation, the study brings the industry closer to realizing commercially viable perovskite solar cell modules.
Looking ahead, combining the excess ligand CBD method with roll-to-roll processing and other scalable deposition techniques opens a promising route towards achieving flexible, lightweight, and low-cost solar components. The combination of superior performance metrics and scalable manufacturing processes could accelerate the deployment of perovskite-based photovoltaics in large-scale energy projects.
In conclusion, this excess ligand strategy in SnO₂ chemical bath deposition represents a new avenue for fabricating electron transport layers for perovskite solar cells. By tailoring nucleation pathways to prioritize ion-by-ion growth over cluster aggregation, researchers have obtained uniform, defect-minimized films with superior optoelectronic properties. This advancement translates into higher device efficiencies and scalable production capabilities, fostering new possibilities for the sustainable energy sector. As the photovoltaic industry intensifies its pursuit of high-quality materials and processes, this work may inspire future innovations that bridge the gap between research breakthroughs and practical applications.
