The structural tunability of perovskites has paved the way for applications such as photovoltaics, light-emitting devices, and lasing. Some 2D perovskites are also ferroelectric, making such materials attractive for resistive memory devices. 2D perovskites are of interest for light emission because the emission color of 2D perovskites can be tuned across the visible spectrum. Emission profiles of 2D perovskites typically fall into two categories: “narrow” emission, in which the linewidth of the emitter is < 100 nm, and “broad” emission, for which a single peak can span hundreds of nm. While there is consensus that narrow emission comes from free excitons, the origins of broad emission are under debate. Proposed origins include self-trapped excitons (STEs), phonon replicas, and coupling of STEs to specific point defects. Regardless, many broad-emitting features appear to have strong exciton-phonon coupling. Because perovskite devices typically employ polycrystalline thin films, it is crucial to understand how such light-matter interactions are affected by thin film growth.

Here, we investigate thin film growth of the broad-emitting and ferroelectric perovskite (EA)₄Pb₃Br₁₀. [1] This phase exhibits three emission features; we find that two of them are phonon-coupled. Notably, film strain turns off the broad, phonon-coupled emission. This broad emission can be recovered by slowing the growth kinetics of the thin film and eliminating strains. Photothermal deflection spectroscopy shows that strain increases electronic disorder near the free exciton absorbance onset. In addition, the challenge of making phase-pure films in low-dimensional Ruddlesden-Popper structure ((A’)2(A)n−1BnX3n+1) is overcome by using a single A/A’-site cation, ethylammonium (EA), whose optimal size also prohibits formation of off-target phases. These results reveal 1) a way to tune phonon-coupled emission in films that is not available to bulk crystals, 2) that care should be taken to account for the effects of strain when studying exciton-phonon coupling in thin films and 3) that such film strain should be accounted for when making resistive memory devices, as ferroelectric domains can be affected by strain.

[1] Kennard, R.M., Dahlman, C.J., Chung, J., Cotts, B.L., Mikhailovsky, A.A, Mao, L., DeCrescent, R.A., Stone, K.H., Venkatesan, N.R., Mohtashami, Y., Assadi, S., Salleo, A., Schuller, J.A., Seshadri, R. and Chabinyc, M.L. Growth-Controlled Broad Emission in Phase-Pure Two-Dimensional Hybrid Perovskite Films. Accepted to Chemistry of Materials, 2021.