Phase control for quasi-2D blue emitters by spacer cation engineering
Effect of the alkylammonium tail length in phenyl-alkyl spacer cations in quasi-2D Ruddlesden-Popper perovskites on the phase distribution in low n films (n=2 stoichiometry) is investigated. The increase in the alkyl chain length suppresses the formation of n=1 phase (and consequently higher n phases), which is attributed to the change in the packing arrangement of spacer cations from parallel (one and two carbon atoms alkyl chain) to non-parallel (3 carbon atoms alkyl chain). Single blue emission peak corresponding to n=3 phase (466 nm) and n=2 phase (436 nm) is obtained in the PL spectra of propylphenylammonium quasi-2D perovskites with methylammonium (MA) and formamidinium (FA) cations, respectively. The same trends in phase distribution, namely the reduction in the proportion of n=1 phase with increasing alkyl chain length are observed for both MA- and FA- based perovskites. However, FA-based samples exhibit higher crystallinity but worsened morphology (more pinholes) and less efficient funneling compared to MA-based samples. Consequently, efficient sky-blue LEDs with the highest EQE of 3.35% are obtained for PPA2MAPb2Br7 perovskite.