Narrow Band Gap Hybrid Copper(I)Iodides: Designer Materials for Optoelectronic Applications

In recent years, there has been a concerted effort in developing narrow band gap semiconductors that exhibit excellent physical properties and optoelectronic performance, as well as enhanced solution processability and structural stability. Herein, we report a new series of copper(I)iodide-based ionic hybrid semiconductors with narrow band gaps (similar to 1.5-1.8 eV). These compounds are systematically designed by using pyrazine derivatives as cationic ligands and various 1D-Cu m I n chains as anionic inorganic motifs to form one-dimensional (1D) structures. They demonstrate high optical absorption coefficients, decent electrical conductivity, excellent air/moisture/thermal stability, and superb solution processability, enabling the fabrication of high-quality thin films via simple solution processes. Additionally, we have carried out a comprehensive photoelectron spectroscopic study on highly orientated thin film samples of selected hybrid compounds to experimentally verify, for the first time, that the photoexcitation process in such materials involves an anion-to-cation through-space charge transfer (TSCT), consistent with the calculated electronic structures. Overall, these narrow band gap CuI-based hybrid semiconductors define a new subclass of low-cost, highly stable, and efficient light-absorbing materials promising for applications in optoelectronics.