3D/1D Architecture Using a 1-Hexyl-3-methylimidazolium Lead Triiodide Interlayer for Robust and Highly Performing Perovskite Solar Cells

The simultaneous improvement in the performance and stability of the perovskite solar cells (PSCs) remains a key challenge toward their commercialization. Herein, we explore the ionic liquid 1-hexyl-3-methylimidazolium iodide (HMImI) for the synthesis of a lead halide derivative, namely, (HMIm)PbI₃. According to single-crystal X-ray analysis, (HMIm)PbI₃ forms 1D chains of face-sharing [PbI₆]^4- octahedra and behaves as a semiconductor with a band gap of 2.85 eV. This compound, when deposited on top of the main 3D perovskite (Cs/FA/MA)PbI_3-xBr_x, passivates the surface of the absorber by lowering the density of the trap states, thus enhancing the radiative recombination and the open circuit voltage. In addition, the hydrophobic character of the alkyl chain of the imidazolium cation prohibits the penetration of the humidity and at the same time prevents ion migration from and toward the main perovskite absorber. Furthermore, the PSCs based on this 3D/1D solar cell architecture achieved a power conversion efficiency (PCE) of almost 20% and retained practically 80% of their initial efficiency after 1700 h of storage under dark and ambient conditions, outperforming the corresponding 3D reference device. This is attributed to the high quality of the perovskite layer, as confirmed by grazing-incidence wide-angle X-ray scattering, scanning electron microscopy, atomic force microscopy, and contact angle measurements. The obtained results clearly indicate that the dimensionality engineering approach involving ionic liquids with the appropriate choice of the organic cation is a very promising strategy for improving the efficiency and stability of the perovskite solar cells.