Ligand Engineering of SnO2 Nanocrystal Ink for Low-Cost Inverted Flexible Perovskite Solar Cells

The integration of low-temperature, solution-processed inorganic electron transport layers (ETLs) into inverted flexible perovskite solar cells (f-PSCs) remains a critical challenge due to the polar solvent-induced damage of the underlying perovskite film. Conventional SnO2 nanocrystal ink based on polar solvents inevitably causes irreversible damage to the perovskite layer in inverted PSCs. Herein, we propose a nonpolar ink based on oleate modified SnO2 nanocrystals. Exchanging the hydrophilic surface groups with long-chain oleate ligands allows the SnO2 nanocrystals to be stably dispersed in a nonpolar solvent of n-hexane. This avoids polar solvent-induced damage to the perovskite film and enables the deposition of a dense, pinhole-free SnO2 film directly onto the perovskite layer. The oleate ligands ensure solvent orthogonality, and their long alkyl chains enable more complete displacement of surface hydroxyls and more thorough passivation of surface defects than short-chain ligands through interchain van der Waals interactions. Consequently, the resulting inverted f-PSCs exhibit suppressed nonradiative recombination and improved carrier extraction. This yields a record efficiency of 22.19%, the highest value reported to date for inverted f-PSCs using solution-processed SnO2 nanocrystal ETLs. Furthermore, the dense film morphology combined with the hydrophobic alkyl chains significantly enhances the environmental and mechanical stability of the inverted f-PSCs. This study demonstrates that ligand engineering of nanocrystals enables the solution-processed fabrication of inorganic ETLs with nonpolar solvents without compromising interfacial optoelectronic quality, offering a viable route toward all-solution-processed inverted f-PSCs.