Poly(methyl methacrylate)-assisted construction for enhanced optical absorption nonlinearities in two-dimensional Dion-Jacobson perovskite films

Two-dimensional (2D) Ruddlesden-Popper (RP) and Dion-Jacobson (DJ) perovskites are attractive candidates for nonlinear photonic applications, owing to their unique “multiple quantum wells” structures. However, the nonlinear optical (NLO) absorption properties of DJ perovskites with higher structural stability and charge transport capability are still not well known. Additionally, the defects at grain boundaries are a key issue limiting the optoelectronic performance of perovskite films. In this work, three n = 1 phase 2D DJ perovskite films with representative organic cations were prepared, which are (BDA)PbI₄, (AMP)PbI₄, and (PDMA)PbI₄, respectively (BDA: 1,4-butadiammonium, AMP: 4-(aminomethyl)piperidine, PDMA: 1,4-phenylenedimethanammonium). The impact of the interlayer organic cation steric effect and conjugation effect on their NLO absorption properties was systematically explored. Subsequently, a novel poly(methyl methacrylate) (PMMA) passivation strategy was proposed that improved crystal quality and reduced perovskite ion defects. NLO absorption measurements demonstrate all pristine perovskite films manifest saturable absorption (SA) responses under femtosecond (fs) laser pulses at 515 nm and turn to reverse saturable absorption (RSA) behaviors at 800 nm. These can be attributed to the quantum and dielectric confinement effects of 2D perovskites, while the better interlayer charge transport of 2D DJ perovskites also contributes to the prominent nonlinear absorption performance. After PMMA passivation treatment, 2D DJ perovskite films exhibited significantly enhanced nonlinear absorption properties under wide-band ultrafast lasers excitation, which benefit from better crystal quality and reduced trap states, implying good universality of this strategy. Owing to the hydrophobicity of PMMA, its addition also induces better ambient stability in passivated films, improving the feasibility of this material in the practical development of photonic devices and thus having broad application prospects. This work offers new insights and a more systematic mechanism explanation for the NLO absorption properties of 2D DJ perovskite films, and presents a feasible passivation strategy for optimizing their NLO absorption performance.