Surface passivation of perovskite thin films through sequential physical vapour deposition for improved stability, structural, optical and morphological properties

In this work, direct passivation of methylammonium tin iodide thin films (MASnI3) with 4-fluorophenethylammonium iodide (4-FPEAI) eliminating use of toxic solvents and without annealing through sequential physical vapour deposition (SPVD) is reported. The stoichiometry of MASnI3 was achieved by depositing 100 nm of SnI2 and 300 nm of MAI, then annealing for 40 min. Subsequently, 4-FPEAI was deposited on top of MASnI3 with varying thicknesses of 5, 7 and 9 nm. The film's crystallinity was enhanced as FPEAI thickness increased from 5 nm to 7 nm, then reduced to 9 nm, as depicted by XRD. Consequently, the film's crystallite size increased from 35.51, 49.70, and 52.35 nm at 0, 5, and 7 nm FPEAI treatment and decreased to 42.46 nm at 9 nm FPEAI passivation, respectively. Pristine MASnI3 films showed many pinholes and grain boundaries, while FPEAI modified films exhibited a smooth surface with larger grains and fused grain boundaries (GBs), as revealed by SEM and AFM images. FPEAI filled the voids in the film and acted as a centre for nucleation, enhancing grain growth and lowering surface roughness. The grain sizes obtained were 392.9, 408.9, 444.6 nm, and 308.7 nm for 0, 5, 7 and 9 nm FPEAI treated films. Surface defects and GBs were significantly suppressed in the 4-FPEAI modified film, as revealed by a strong PL emission and reduced defect density. XRD measured after 24 h of nitrogen storage showed that FPEAI modified films displayed better stability where the perovskite peaks were more intense than the bare film, which degraded quickly. FPEAI, being hydrophobic, prevented moisture infiltration into the film through defect passivation, enhancing its stability. Hence, FPEAI passivation improved the optical, morphological and structural properties as well as the stability of MASnI3 films, which is favourable for application in optoelectronic devices.