The Impact of Stoichiometry on the Photophysical Properties of Ruddlesden-Popper Perovskites

2D Ruddlesden-Popper perovskites are interesting for a variety of applications owing to their tunable optical properties and their excellent ambient stability. As these materials are processable from solution, they hold the promise of procuring flexible and cost-effective films through large-scale fabrication techniques. However, such solution-based deposition techniques often induce large degrees of heterogeneity due to poorly controlled crystallization. The microscopic properties of films of (PEA)(2)PbI4 cast from precursor solutions of different stoichiometry are therefore investigated. The stoichiometry of the precursor solution is found to have a large impact on the crystallinity, morphology, and optical properties of the resulting thin films. Even for films cast from stoichiometric precursors, differences in photoluminescence intensities occur on a subgranular level. The heterogeneity in these films is found to be thermally activated with an activation energy of 0.4 eV for the emergence of local variations in nonradiative recombination rates. The spatial variation in the distribution of trap states is attributed to local fluctuations in the stoichiometry. In line with this, the surface can successfully be passivated by providing an excess of phenylethylammonium iodide (PEAI) to an as-cast film, enhancing the photoluminescence by as much as 85% without significantly altering the film's morphology.