Facet-dependent exfoliation feasibility and optoelectronic properties of two-dimensional all-inorganic halide perovskites

Two-dimensional (2D) all-inorganic halide perovskites exhibit promise for optoelectronic applications, yet selective exfoliation along specific crystallographic planes remains a critical challenge for performance optimization. Using first-principles calculations combined with device simulations, we systematically investigated the structural stability, exfoliation feasibility, and optoelectronic properties of 24 all-inorganic 2D perovskites derived from the (100) and (111) planes of cubic perovskites, specifically the A2BX4 and A3B′2X9 series (A = Cs, Rb; B = Pb, Sn; B′ = Bi, Sb; X = Cl, Br, I). Our results demonstrate that (111)-derived A3B′2X9 perovskites exhibit significantly lower exfoliation energies (23.1−62.1 meV/Å2) than (100)-derived A2BX4 counterparts (59.7−174.0 meV/Å2), attributed to weaker van der Waals interlayer coupling in the former. Rb3Bi2I9 possesses an ultralow exfoliation energy of 23.1 meV/Å2, rivaling that of graphene and demonstrating exceptional potential for mechanical exfoliation of high-quality monolayers. A2BX4 monolayers exhibit direct band gaps, which are favorable for optoelectronic applications; whereas A3B′2X9 monolayers display indirect band gaps. Among all investigated materials, monolayer Rb2SnBr4 emerges as an outstanding candidate, featuring an ideal direct band gap of 1.34 eV (HSE06) that perfectly matches the Shockley−Queisser limit for single-junction solar cells. SCAPS-1D device simulations further predict that optimized Rb2SnBr4-based solar cells can achieve a remarkable theoretical power conversion efficiency of 27.10% under defect densities below 1014 cm−3. This work establishes that (111) plane cleavage is optimal for synthesizing exfoliable 2D perovskites, while (100) plane orientation enables superior direct band gap characteristics for photovoltaic applications, providing critical design principles for crystallographic plane engineering in halide perovskite devices.