Researchers at Nanjing Tech University have developed a potassium chloride (KCl)-modified tin oxide (SnO2) electron transport layer (ETL) that suppresses interfacial defects and improves crystallization in carbon-based perovskite solar cells (PSCs). The modification, introduced during a low-temperature chemical bath deposition (CBD) process, raised the champion device's power conversion efficiency (PCE) to 15.13%, a 7.22% improvement over an unmodified reference device, while also improving long-term stability.
SnO2 has become a popular alternative to TiO2 as an ETL in both rigid and flexible PSCs, thanks to its high ionic conductivity and favorable energy alignment with perovskite films. Chemical bath deposition is a common way to fabricate SnO2 layers because it operates at under 100°C, uses little energy, and scales well. However, aqueous CBD-grown SnO2 films tend to suffer from non-uniform grain sizes and a high density of defect states, including oxygen vacancies and undercoordinated Sn4+ sites that trap charge carriers and reduce open-circuit voltage. To address this, the team introduced KCl directly into the aqueous CBD growth process.
The chloride ions passivate oxygen vacancies and undercoordinated Sn4+ defects in the SnO2 film, promote lateral grain growth, and lower the ETL's resistivity, which improves fill factor. At the ETL/perovskite interface, the reduced defect density also supports more vertically oriented perovskite grain growth, cutting interfacial recombination and improving charge extraction. Energy-dispersive X-ray spectroscopy confirmed the successful incorporation of potassium and chlorine into the SnO2 films, while X-ray diffraction showed improved crystallinity after the KCl treatment.
Beyond the efficiency gain, the modified devices showed reduced hysteresis and better moisture resistance. Unencapsulated devices retained 74.2% of their initial efficiency after 1,200 hours of storage under ambient conditions (20-25°C, 40-50% relative humidity), pointing to meaningfully improved environmental tolerance.
The researchers note that the CBD-based approach also offers simpler fabrication and lower production costs than conventional spin-coating methods, which they argue makes it a promising route toward scalable, low-cost carbon-based PSCs, a device architecture already valued for replacing noble-metal back contacts with cheaper, more stable carbon electrodes.
