Rubino Andrea, Lozano Gabriel, Calvo Mauricio E, Míguez Hernán
Instituto de Ciencia de Materiales de Sevilla (CSIC-US), C/Américo Vespucio 49, 41092 Sevilla, Spain.
Nanoscale. 2023 Feb 9;15(6):2553-2560. doi: 10.1039/d2nr05109e.
Precise knowledge of the optical constants of perovskite lead halide quantum dots (QDs) is required to both understand their interaction with light and to rationally design and optimize the devices based on them. However, their determination from colloidal nanocrystal suspensions, or films made out of them, remains elusive, as a result of the difficulty in disentangling the optical constants of the organic capping ligands and those of the semiconductor itself. In this work, we extract the refractive index and extinction coefficient of ligand-free methylammonium lead iodide (MAPbI) and bromide (MAPbBr) nanocrystals. In order to prevent the use of organic ligands in the preparation, we follow a scaffold assisted synthetic procedure, which yields a composite film of high optical quality that can be independently and precisely characterized and modelled. In this way, the contribution of the guest nanocrystals can be successfully discriminated from that of the host matrix. Using a Kramers-Kronig consistent dispersion model along with an effective medium approximation, it is possible to derive the optical constants of the QDs by fitting the spectral dependence of light transmitted and reflected at different angles and polarizations. Our results indicate a strong dependence of the optical constants on the QD size. Small nanocrystals show remarkably large values of the extinction coefficient compared to their bulk counterparts. This analysis opens the door to the rigorous modelling of solar cells and light-emitting diodes with active layers based on perovskite QDs.
为了理解钙钛矿型卤化铅量子点（QDs）与光的相互作用，并合理设计和优化基于它们的器件，需要精确了解其光学常数。然而，由于难以区分有机封端配体和半导体本身的光学常数，从胶体纳米晶体悬浮液或由它们制成的薄膜中确定这些常数仍然很困难。在这项工作中，我们提取了无配体的甲基碘化铅（MAPbI）和甲基溴化铅（MAPbBr）纳米晶体的折射率和消光系数。为了避免在制备过程中使用有机配体，我们采用了一种支架辅助合成方法，该方法产生了具有高光学质量的复合薄膜，该薄膜可以独立且精确地表征和建模。通过这种方式，可以成功地区分客体纳米晶体和主体基质的贡献。使用克拉默斯 - 克朗尼格一致色散模型以及有效介质近似，通过拟合不同角度和偏振下透射和反射光的光谱依赖性，可以推导出量子点的光学常数。我们的结果表明光学常数强烈依赖于量子点的尺寸。与体相材料相比，小纳米晶体的消光系数值显著较大。该分析为基于钙钛矿量子点的有源层的太阳能电池和发光二极管的严格建模打开了大门。
