HUST develops the low-temperature conformal SnO2 coating to enable efficient printable mesoscopic perovskite solar cells with industrial scalability

Printable mesoscopic perovskite solar cells exhibit immense potential for photovoltaic industrialization, thanks to their inherent advantages: hole-transport-layer-free architecture, low-cost carbon electrodes, and all-solution processing. However, state-of-the-art mesoporous TiO₂ electron transport layers suffer from inefficient charge transport and severe interfacial recombination, which bottleneck device performance enhancement. Passivating the inner surface defects of mesoporous TiO₂ with low-temperature SnO₂, a high-performance material with superior electrical properties, is hindered by SnO₂ performance degradation induced by high-temperature post-treatment processes.

To tackle this issue, Kai Chen, a PhD candidate at the Wuhan National Laboratory for Optoelectronics (WNLO), Huazhong University of Science and Technology (HUST), developed a low-temperature fabrication strategy. By engineering a TiO₂@SnO₂ bilayer composite mesoporous electron transport layer, device performance was significantly optimized and enhanced.

Chen explained, "We use a pre-sintered three-layer porous scaffold (mesoporous TiO₂/mesoporous ZrO₂/porous carbon) as the substrate and employ chemical bath deposition (CBD) for the precise modification of SnO₂. Through the selection of specific tin precursors and solvents, the reaction rate is modulated via the slow diffusion of atmospheric moisture, enabling uniform and conformal SnO₂ deposition on the inner surface of mesoporous TiO₂, followed by mild annealing for crystallization and densification. This low-temperature process not only effectively mitigates the degradation of SnO₂ electrical properties caused by high temperatures but also achieves selective SnO₂ deposition, thereby suppressing charge leakage at the spacer layer."

Co-author Dr. Jiale Liu, a postdoctoral fellow at WNLO, HUST, noted, "The TiO₂@SnO₂ bilayer structure boosts device performance through multi-faceted synergistic effects: it optimizes interfacial energy level alignment, reduces defect state density, suppresses non-radiative recombination, and enhances transport layer conductivity, while simultaneously improving perovskite crystallization quality and pore filling efficiency. Furthermore, this fabrication strategy holds great promise for large-scale industrialization, and the lab-scale photovoltaic modules prepared via this approach exhibit excellent process compatibility."

A key breakthrough of this work is the realization of site-selective and conformal SnO₂ deposition on mesoporous TiO₂ surfaces. The research team innovatively designed a slow CBD method for SnO₂ coating: tin tert-butoxide molecules first adsorb onto the TiO₂ surface to form a self-assembled monolayer; as atmospheric moisture diffuses into the CBD solution, a gradual hydrolysis reaction occurs to form a conformal SnO₂ layer; finally, low-temperature heat treatment is performed to remove residual organics, yielding the desired TiO₂@SnO₂ core-shell structure.

For printable mesoscopic perovskite solar cells, this TiO₂@SnO₂ bilayer composite mesoporous electron transport layer strategy delivers a power conversion efficiency (PCE) of 22.5% for small-area cells with an active area of ~0.1 cm², and a PCE of 18.8% for large-area modules with an aperture area of 57.33 cm².

Accelerated stability tests under continuous operation were also conducted: devices were subjected to simulated 1-sun illumination at 55 ± 5 °C in ambient air without UV filtration. TiO₂@SnO₂-based devices demonstrated exceptional durability—encapsulated devices retained 90% of their initial PCE after 2000 hours of maximum power point tracking (MPPT), in stark contrast to only 1000 hours for pristine TiO₂-based control devices.

This work has been published in the journal PhotoniX in a paper entitled Low-Temperature Conformal SnO₂ Coating Enables Efficient Printable Mesoscopic Perovskite Solar Cells with Industrial Scalability.