In the relentless pursuit of renewable energy technologies, the enhancement of perovskite solar cell performance remains a critical milestone. Recent advancements have underscored chemical bath deposition (CBD) as a promising route for fabricating electron-transport layers (ETLs), pivotal in defining the efficiency and stability of these photovoltaic systems. However, traditional CBD methods have been constrained by several inherent challenges, such as prolonged deposition times, non-uniform film formation over large-area substrates, and susceptibility to oxidation, all of which undermine their viability for scalable, high-efficiency applications. A groundbreaking study has now unveiled an innovative “excess ligand strategy” in the chemical bath deposition of tin oxide (SnO₂), marking a transformative step toward overcoming these limitations.
Electron-transport layers crafted with SnO₂ via CBD serve as critical mediators in efficiently extracting and transporting electrons generated in the perovskite absorber layer. Central to achieving high device performance is the formation of ETLs exhibiting uniform morphologies, minimal defect densities, and robust interfacial properties. The conventional CBD synthesis of SnO₂ typically proceeds 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 novel approach employs an excess ligand environment that fundamentally alters the deposition kinetics, effectively suppressing the cluster-by-cluster mechanism while preferentially facilitating the ion-by-ion growth pathway. This strategic modulation of growth dynamics enables the formation of highly uniform, conformal SnO₂ thin films, even on large substrates, within markedly reduced synthesis times. The resulting films exhibit substantially suppressed defect densities, manifesting superior optoelectronic properties necessary for high-performance solar devices.
A pivotal metric reflecting the quality of these SnO₂ films is the surface recombination velocity, which is critically lowered to 5.5 cm s⁻¹ in this work. The surface recombination velocity quantifies the propensity for photogenerated carriers to recombine at interfaces before contributing to photocurrent. Lowering this parameter illustrates the efficacy of the excess ligand strategy in passivating surface states and minimizing trap-assisted recombination, an achievement rarely realized with conventional CBD films.
Moreover, the luminescent properties of these SnO₂ films set new benchmarks in the realm of perovskite photovoltaics. Exhibiting an electroluminescence efficiency of 24.8%, these ETLs contribute not only to improved charge extraction but also function effectively as recombination suppression layers, which is vital for device stability and long-term operational performance. This high electroluminescence yield reflects a pronounced reduction of non-radiative recombination pathways, underscoring the quality of the interface engineered via this method.
The impact of these advanced SnO₂ ETLs permeates through the entire solar cell architecture, culminating in impressive photovoltaic efficiencies. The study reports a power-conversion efficiency (PCE) of 26.4% for champion perovskite solar cells, which aligns with or surpasses state-of-the-art figures. Such efficiencies herald the maturity of the excess ligand CBD method as a scalable technique capable of supporting commercial-level device fabrication.
Extending beyond individual cells, the practical scalability of this technique is demonstrated by achieving a 23% efficiency in perovskite solar modules, devices that integrate multiple cells to deliver higher voltage and practical power outputs suitable for real-world applications. Furthermore, the method’s compatibility with carbon-based perovskite cells yielding an efficiency of 23.1% showcases its versatility and adaptability to diverse device architectures, broadening the potential market impact.
Technically, 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 our 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, the discovery of an excess ligand strategy in the chemical bath deposition of SnO₂ represents a paradigm shift 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 directly into record-setting 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 stands poised to inspire future innovations that bridge the divide between research breakthroughs and real-world applications.
Subject of Research: Electron-transport layers in perovskite solar cells; chemical bath deposition of tin oxide.
Article Title: Efficient and luminescent perovskite solar cells using defect-suppressed SnO2 via excess ligand strategy.
Article References:
Seo, G., Yoo, J.J., Nam, S. et al. Efficient and luminescent perovskite solar cells using defect-suppressed SnO2 via excess ligand strategy. Nat Energy (2025). https://doi.org/10.1038/s41560-025-01781-1
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