Kutkan Seda, Dhanabalan Balaji, Lin Miao-Ling, Tan Ping-Heng, Schleusener Alexander, Arciniegas Milena P, Krahne Roman
Italian Institute of Technology (IIT), Via Morego 30, 16163 Genoa, Italy.
State Key Laboratory of Superlattices and Microstructures, Institute of Semiconductors, Chinese Academy of Sciences, 100083 Beijing, China.
Nanoscale. 2023 Aug 10;15(31):12880-12888. doi: 10.1039/d3nr02172f.
Organic-inorganic low-dimensional layered metal-halide perovskites are semiconductors in which the optoelectronic properties can be tuned by the material composition and the design of the layered architecture. While the electronic band structure is mainly determined by the inorganic octahedra lattice, the binding and conformation of the organic cations induces related lattice distortions that can break the symmetry and lead to the splitting of the exciton energy levels, and influence the dielectric confinement. Furthermore, organic-induced lattice deformations lead to offsets in -space (where is the wavevector) that go along with the exciton energy level splitting. Hence, the electronic transitions between these levels require the momentum contribution of phonons, and contributions of phonons in the exciton recombination dynamics result in thermal broadening of the emission linewidth. In this work, we investigate the band-edge emission of two-dimensional Ruddlesden-Popper lead-bromide perovskites synthesized with different organic cations that vary in their binding head group and their alkyl chain length. We find several peaks in the low-temperature photoluminescence spectra, and the number of peaks in the band-edge emission and their decay dynamics depend strongly on the type of organic cation in the material, which we relate to the difference in the inorganic lattice distortions that the cations induce. For two-dimensional layered perovskites with mainly in-plane distortions, induced by short primary ammonium molecules, we find a two-fold splitting of the band edge emission at low temperatures. If also out-of-plane distortions are present, as for the long-chain primary ammoniums, a three-fold splitting is observed. Interestingly, the low-energy peaks of the split series merge into the highest energy peak with increasing temperature. Thermal broadening analysis of the temperature-dependent photoluminescence linewidth in the structures with out-of-plane distortions yields energies that are larger than those reported for the inorganic lattice phonons. This indicates the involvement of either high-frequency oscillations involving the organic cations, or the broadening might be related to higher order phonon scattering processes in the excitonic recombination process. The strong directionality of the phonon modes in the octahedral lattice could promote the involvement of multiple electron-phonon scattering processes in the exciton relaxation dynamics, for example involving modes with orthogonal directionality.
有机-无机低维层状金属卤化物钙钛矿是一类半导体，其光电性质可通过材料组成和层状结构设计进行调控。虽然电子能带结构主要由无机八面体晶格决定，但有机阳离子的键合和构象会引发相关的晶格畸变，从而打破对称性，导致激子能级分裂，并影响介电限域。此外，有机诱导的晶格形变会导致在波矢空间中的偏移，这与激子能级分裂相伴。因此，这些能级之间的电子跃迁需要声子的动量贡献，并且声子在激子复合动力学中的贡献导致发射线宽的热展宽。在这项工作中，我们研究了用不同有机阳离子合成的二维Ruddlesden-Popper溴化铅钙钛矿的带边发射，这些有机阳离子在其键合头基和烷基链长度上有所不同。我们在低温光致发光光谱中发现了几个峰，并且带边发射中的峰数及其衰减动力学强烈依赖于材料中有机阳离子的类型，我们将其与阳离子诱导的无机晶格畸变差异相关联。对于主要由短伯铵分子诱导产生面内畸变的二维层状钙钛矿，我们在低温下发现带边发射有双重分裂。如果还存在面外畸变，如对于长链伯铵，则观察到三重分裂。有趣的是，随着温度升高，分裂系列的低能峰合并为最高能峰。对具有面外畸变结构的温度相关光致发光线宽进行热展宽分析，得到的能量大于无机晶格声子报道的能量。这表明要么涉及有机阳离子的高频振荡，要么展宽可能与激子复合过程中的高阶声子散射过程有关。八面体晶格中声子模式的强方向性可能会促进激子弛豫动力学中多个电子-声子散射过程的参与，例如涉及具有正交方向性的模式。
