Surface Engineering of Tin Oxide Nanoparticles by pH Modulation Facilitates Homogeneous Film Formation for Efficient Perovskite Solar Modules

Producing clean energy in a sustainable manner is a critical global imperative. Solar energy provides notable potential due to its unlimited supply, high power density, and environmental benefits, distinguishing it from other energy sources. Although Si-based solar cells dominate the market, the development of alternative solar cell materials continues due to high processing costs and limited application areas. Perovskite solar cells (PSCs) exhibit outstanding photovoltaic properties, including high light absorption coefficients in the visible spectrum, low exciton binding energy, and long carrier diffusion lengths. When integrated with solution-based technologies that offer low production costs, PSCs have achieved an efficiency exceeding 26.1% within just 15 years, establishing them as one of the most promising candidates for replacing Si-based solar cells. To commercialize PSCs, various research efforts are being directed towards bifacial and flexible devices, tandem cells, and other areas. In all these fields of research, large-area fabrication emerges as a crucial factor. However, there are significant efficiency variations between small-scale cells and large-scale modules. Therefore, research on perovskite solar modules (PSMs) is necessary to improve efficiency and stability. Achieving high-efficiency PSMs necessitates uniform film deposition to minimize performance losses typically seen with PSCs. Defects such as microscopic pinholes may not pose significant issues at the cell level but can critically affect performance and durability at the module scale. In addition to improving the quality of perovskite films, the formation of homogeneous and defect-controlled charge transport layers is an essential component for achieving high-performance PSMs. Tin oxide (SnO₂) films generally used as the electron transport layer (ETL) in solution-processed PSCs introduce numerous microscopic defects at the ETL/perovskite interface.<br/>Herein, we describe a simple approach to forming a homogeneous film and reducing hydroxyl groups on SnO₂ film surface by adding nitric acid (HNO₃) to SnO₂ dispersion. The surface of SnO₂ nanoparticles (NPs) initially exists as oxide ions (O^-) under basic conditions at high pH. Modulating the pH of the SnO₂ NP colloidal dispersion with a small amount of HNO₃induces hydroxyl groups on the surface of the NPs. The induced hydroxyl groups on the surface of the SnO₂ NPs lead to the formation of oxo groups (Sn-O-Sn) between NPs by condensation reaction. The formation of oxo groups on SnO₂ NP surfaces reduces surface oxide ions. These oxide ions generate hydroxyl groups on the SnO₂ film surface after the annealing process at ambient conditions. By modulating the pH, the reduced oxide ions consequently minimize the hydroxyl groups on the SnO₂ film, which act as electronic defects.<br/>By surface engineering of SnO₂ NPs, high performance has been achieved: 23.7% efficiency for a unit cell, 20.3% efficiency for a 24.5 cm² minimodule, and 19.0% efficiency for a 214.7 cm² submodule. These efficiencies are averages from results obtained by reverse/forward scans. In outdoor tests, the target PSM generated 16.5% higher cumulative electricity over a month compared to a control PSM. Furthermore, under damp heat conditions (85°C and 85% RH), the target PSM maintained 80% of its initial efficiency over 1080 hours. These results demonstrate the significance of the proposed surface engineering strategy in the chemical modification and uniformity of the ETL in large-scale PSMs.^[1] In future studies, we will aim to apply this strategy for homogeneous ETL at low temperatures into roll-to-roll process. This approach is expected to accelerate the commercialization of all-printing PSMs.<br/><br/><b>Reference.</b><br/>Yun, H.-S., Seo, Y.-H., Seo, C.-E., Kim, H. S., Yoo, S. B., Kang, B. J., Jeon, N. J., & Jung, E. H. <i>Adv. Energy Mater. </i>2400791 (2024). https://doi.org/10.1002/aenm.202400791