The templating effect of diammonium cations on the structural and optical properties of lead bromide perovskites: a guide to design broad light emitters

The templating role in the formation of 2D metal halide perovskites of diammonium cations, such as 1,3-phenylenediammonium (1,3-PDA), 1,3-xylylenediammonium (1,3-XDA), 1,4-phenylenediammonium (1,4-PDA), and 1,4-xylylenediammonium (1,4-XDA) cations, has been investigated. Single-crystal X-ray diffraction confirms the formation of 2D Dion-Jacobon (DJ) perovskites for all the cations except for the 1,3-XDA cation leading to a so-called 0D perovskitoid. The analysis of the structural data showed a higher distortion degree for the systems containing a shorter cation, namely 1,4-PDA. A detailed spectroscopic investigation, with both static and time-resolved photoluminescence spectroscopy, revealed a broadband emissive component at room temperature with hundreds of nanometers of bandwidth when 1,3-PDA and 1,4-PDA cations are present, while a narrow peak is found for the longer cation, i.e. 1,4-XDA. A broadband emission arises in this last sample as well by decreasing temperature. Such a broad emission, as indicated by the analysis of the power and temperature dependence of the PL, can be attributed to trap-mediated excitonic recombination or STEs, and the trend in emissive properties can be correlated with the different levels of octahedral distortions. The novel systems reported in this work enlarge the family of broadband emitters and add further insights to develop efficient perovskite-based broadband and white light emitters.