Benzodithiophene‐Based Spacers for Layered Perovskites

Incorporating extended pi-conjugated organic cations in layered lead halide perovskites is a recent trend promising to merge the fields of organic semiconductors and lead halide perovskites and can offer new materials with supplementary semiconducting properties such as controlled electronic conductivity, increased light absorption, enhanced photo-induced charge transfer, and tunable quantum well electronic structures. However, the ability to successfully incorporate conjugated organics in layered lead halide perovskites is severely limited by their size and their intramolecular pi-pi stacking. In a new report with from first author Barbara Primera Darwich, benzodithiophene (BDT) was incorporated into Ruddlesden-Popper (RP) layered and quasi-layered lead iodide thin films (with methylammonium, MA) of the form (BDT)₂MA_n–1PbnI_3n+1. The importance of tuning the ligand chemical structure is shown as an alkyl chain length of at least 6 carbons is required to form a photoactive RP (n = 1) phase. In quasi-layered form, the BDT cation gave a power conversion efficiency of up to 14.7% while transient photocurrent, UV photoelectron spectroscopy and Fourier transform infrared spectroscopy point to improved charge transport in the device active layer and additional electronic states close to the valence band suggesting the formation of a Lewis adduct between the BDT and surface iodide vacancies. These results underline both the importance of molecular engineering of pi-conjugated organic cations to tune their ability of incorporation in layered lead halide perovskite thin films. The findings further highlight the possibility to harness functionality in pi-conjugated organic cations to improve performance in layered and quasi-layered lead halide perovskite solar cells.