Active SnO₂ Crystal Planes Enable Efficient and Ultra-Bendable n-i-p Perovskite Solar Cells with Record Certificated Power Conversion Efficiency

The tin (IV) oxide (SnO₂) electron transport layer (ETL) has been widely employed to fabricate high-performance perovskite solar cells (PSCs). It has been reported that carbon quantum dots (CQDs) can be used to enhance electron mobility of SnO₂. However, an in-depth understanding of the driving force in this process is still lacking. Here, a high-angle annular dark-field scanning transmission electron microscope (HAADF-STEM) is employed, for the first time, to reveal the SnO₂ crystal face changes with one new type of CQD doping. Synchrotron-based grazing incidence wide-angle X-ray scattering (GIWAXS) can penetrate the flexible substrate to detect the buried region of the perovskite layer, showing the crystallinity and phase purity of the perovskite are significantly improved with CQD-modified SnO₂. The flexible n-i-p PSCs delivers a power conversion efficiency (PCE) up to 23.57% (22.75%, certificated), which is one of the highest values for single-junction n-i-p flexible PSCs. The corresponding n-i-p flexible modules achieve a PCE of 17.79% with aperture area ~ 24 cm². Furthermore, the flexible PSCs show excellent stability, preserving ≈95% of their initial efficiency after 1200 h under 40% relative humidity and 1-sun light irradiation at 25 °C, and maintained > 90% of initial efficiency after 2500 bending cycles at a bending radius of 6 mm.