Researchers from Sungkyunkwan University, Korea Research Institute of Chemical Technology (KRICT), Massachusetts Institute of Technology (MIT), Korea Advanced Institute of Science and Technology (KAIST), Ajou University and Ulsan National Institute of Science and Technology (UNIST) have developed an excess ligand strategy based on the chemical bath deposition (CBD) of tin oxide (SnO2) that solves some of the common constraints of CBD like prolonged deposition times, non-uniform film formation over large-area substrates, and susceptibility to oxidation.
The conventional CBD synthesis of SnO₂ typically takes place via 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 approach enables rapid synthesis of high-quality SnO2 ETLs by suppressing the cluster-by-cluster pathway while facilitating the ion-by-ion pathway to create uniform films.
The resulting SnO2 thin films exhibit superior optoelectronic properties, including a low surface-recombination velocity (5.5 cm s−1) and a high electroluminescence efficiency of 24.8%. These improvements result in a high power-conversion efficiency of 26.4% for perovskite solar cells, an efficiency of 23% for perovskite modules and an efficiency of 23.1% for carbon-based perovskite cells.
In this novel method, the suppression of the cluster aggregation path involves the deliberate introduction of ligand molecules in excess relative to conventional protocols. These ligands coordinate with tin ions, stabilizing them and moderating nucleation kinetics. This molecular-level control steers the growth preferentially towards direct ion-by-ion deposition onto the substrate, circumventing the formation of colloidal SnO₂ clusters that compromise film integrity.
Such ligand-rich environments also provide 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. Consequently, charge carriers experience smoother transit through the ETL, enhancing open-circuit voltage and fill factor metrics in the integrated devices.
The rapidity of this deposition approach marks an additional industrial advantage. Conventional CBD techniques require extended durations to achieve coverage uniformity, a major bottleneck when moving toward manufacturing scale. By shifting the reaction pathway kinetics, the excess ligand strategy compresses processing times without sacrificing film quality or uniformity. This reduction in synthesis time translates directly to cost savings and higher throughput in manufacturing environments.
Importantly, the framework of this study also tackles oxidation-related degradation issues. In traditional solution-processed SnO₂ films, uncontrolled oxidative growth can induce variable stoichiometries and localized defects. The controlled ligand environment buffers the chemical milieu, leading to stoichiometrically consistent, phase-pure SnO₂ layers with improved chemical robustness, an essential factor for device longevity.
This research not only bridges the gap between laboratory-scale device fabrication and industrially feasible production but also enhances fundamental understanding of nucleation dynamics in chemical bath depositions. The manipulation of ligand content as a lever to control nucleation pathways opens avenues for similarly structured approaches in other oxide semiconductors, potentially revolutionizing the fabrication of electron-transport layers beyond SnO₂.
The implications extend into 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 phenomena and photodegradation pathways, two persistent challenges inhibiting broader adoption of perovskite solar technology. By addressing these through material synthesis innovation, the study brings the community closer to realizing commercially viable perovskite modules.
Looking forward, integrating the excess ligand CBD method with roll-to-roll processing and other scalable deposition techniques posits a promising route toward flexible, lightweight solar modules with low manufacturing costs. The combination of superior performance metrics and scalable fabrication processes could accelerate the deployment of perovskite-based photovoltaics in large-scale energy projects.
In conclusion, this excess ligand strategy in the chemical bath deposition of SnO₂ represents a new path in fabricating electron-transport layers for perovskite solar cells. By tailoring nucleation pathways to prioritize ion-by-ion growth over cluster aggregation, researchers have achieved uniform, defect-minimized films with exceptional optoelectronic attributes. This advancement translates into improved device efficiencies and scalable production capabilities, fostering new possibilities in the sustainable energy industry. As the photovoltaic sector intensifies its quest for superior materials and processes, this work could inspire future innovations that bridge the divide between research breakthroughs and real-world applications.
