New Homologous Series of the 2D Hybrid Lead Iodide System and Its Implications for Photovoltaic Applications

Usually, an aliphatic chain or an aromatic ring is used as the organic spacer (A) to form two-dimensional (2D) lead halide compounds with n ≥ 2 in the general formula A₂(MA)_n−1Pb_nI_3n+1 or A(MA)_n−1Pb_nI_3n+1. Departing from this practice to address their limitations, we use a cyclic amine, cyclohexanemethylamine (CMA), to synthesize a new homologous series of 2D hybrid lead iodides, (CMA)₂(MA)_n−1Pb_nI_3n+1, with n = 1–4. While electronic and dielectric confinements enhance both bandgap and exciton binding energies in this family of compounds compared to the 3D compounds, as also in other low-dimensional hybrid lead halide systems, the present n = 2 compound has the lowest exciton binding energy of 58 meV among all n = 2 hybrid lead halide 2D systems reported so far. Interestingly, time-resolved photoluminescence measurements reveal a longer lifetime (0.4–186 ns depending on n) in these compounds compared to those (generally in the range of 0.1–0.3 ns) for all other such 2D lead halide systems; the longer lifetime becomes increasingly more prominent with increasing n, indicating slower recombination and improved carrier transport than any other 2D system reported so far. Prompted by this observation, we use spin-coating of CMAI ligands on the active material to grow an integrated 2D surface/3D bulk structure, improving all solar photovoltaic parameters, including stability, and leading to an average PCE of 23.8% and a champion PCE of 24.3%, compared to photovoltaic solar cells made in the absence of the CMAI ligands but keeping all other fabrication parameters the same, achieving an average PCE of 22.6% and a champion cell PCE of 23.03%.