Chemical Origin of Exciton Self-trapping in Cs$_3$Cu$_2$X$_5$ Cesium Copper Halides

Abstract

Copper halides Cs3Cu2X5 (X=Cl, Br, I) are promising materials for optoelectronic applications due to their high photoluminescence efficiency, stability, and large Stokes shifts. In this work, we uncover the chemical bonding origin of the Stokes shift in these materials using density functional theory calculations. Upon excitation, one [Cu2X5]3- anion undergoes sizeable local distortions, driven by Cu-X and Cu-Cu bond formation. These structural changes coincide with the formation of a self-trapped exciton, where particularly the hole is strongly localized on one anion. Analysis of the electronic structure and bonding reveals reduced antibonding interactions and enhanced bonding character in the excited state, stabilizing the distorted geometry. Our results establish a direct link between orbital-specific hole localization and bond formation. It provides a fundamental understanding of the excitation mechanism in Cs3Cu2X5 and offers design principles to tune optical properties in 0D copper halides.

Figures (13)

• Figure 1: a. Cs$_3$Cu$_2$I$_5$ ground state structure. b. Proposed geometry change of [Cu2I5]3- ion upon excitation. The Cs ions are hidden to highlight the structural change. c. Most prominent changes in bond lengths in [Cu2I5]3- upon excitation.
• Figure 2: Top: density of states (DOS) and bottom: crystal orbital Hamilton population (COHP) of Cs$_3$Cu$_2$I$_5$ in the ground state. The zero level is put at the top of the valence band and is marked by a dashed line.
• Figure 3: COHP of the affected [Cu2I5]3- unit in Cs$_3$Cu$_2$I$_5$ in (a) the ground state, (b) one hole added, and the hole channels of the (c) singlet and (d)triplet states. The Fermi level is marked by the horizonal dash line.
• Figure 4: Wave functions of the lowest Cu($d$)--I($p$) bonding state (a,b) and the highest antibonding state (c,d) of the [Cu$_2$I$_5$]$^{2-}$ ion. Yellow and blue colors denote regions of opposite phase; (a,c) undistorted [Cu$_2$I$_5$]$^{3-}$ rhombus structure; (b,d) optimized [Cu$_2$I$_5$]$^{2-}$ bitetrahedral structure.
• Figure 5: COHP of hole spin channel of the triplet (top) and $\Delta n(\mathbf r) = n_\mathrm{triplet}(\mathbf r) - n_0(\mathbf r)$ (bottom) of Cs$_3$Cu$_2$X$_5$, X = (a) I, (b) Br, (c) Cl. The Cu$_2$X$_5$ ion with bitetrahedral geometry is marked by the white frame.