Impact of crystallinity on the circular and linear dichroism signals in chiral perovskite

Abstract

Chiral perovskites owing to their broken mirror symmetry exhibit selective absorption of circularly polarized light manifesting a strong circular dichroism (CD). CD spectroscopy has been a key technique to understand chiral perovskites and how these semiconductors achieve chirality at the molecular level. However, there is a debate on whether the observed CD is intrinsic to the chiral crystal structures or is modulated by extrinsic phenomena particularly linear dichroism (LD) and linear birefringence (LB) effects. This work investigates the chiroptical properties of $(R-/S-\text{MBA})_2\mathrm{CuCl}_4$ (MBA = Methylbenzyl ammonium) series by thoroughly studying the contribution from LD and LB to the observed CD signals. The comparison of highly oriented and randomly oriented films exhibits notable LD and LB contributions to the observed CD, which are caused by orientation-dependent electric-field interactions and local anisotropy. Both randomly and highly oriented films exhibit distinct CD responses, with LD--LB effects largely dominating the CD in highly oriented films. This has been revealed by the obvious shift in the observed CD signals to the below-absorption-edge ($\sim 430$ nm) regime and broadening of features. Our findings demonstrate that careful consideration of crystal orientation and structural effects is necessary for appropriate interpretation of CD spectra in chiral perovskite thin films.