Complex Optical Index of PbS Nanocrystal Thin Film and their Use for Short Wave Infrared Sensor Design

• Published in: Nanoscale 2022-02, Vol.14 (7), p.2711-2721
• Main Authors: Chehaibou, Bilal, Izquierdo, Eva, Chu, Audrey, Abadie, Claire, Cavallo, Mariarosa, Khalili, Adrien, Dang, Tung Huu, Greboval, Charlie, Xiangzhen, Xu, Ithurria, Sandrine, Vincent, Grégory, Gallas, Bruno, Mugny, Gabriel, Arnaud, Arthur, Lhuillier, Emmanuel, Delerue, Christophe
• Format: Article
• Language: English
• Subjects: Condensed Matter; Physics
• ISSN: 2040-3364; 2040-3372
• DOI: 10.1039/D1NR07770H

As nanocrystals (NCs) gain maturity, they become central building blocks for optoelectronics in devices such as solar cells and, more recently, infrared focal plane arrays. Now that proof of concept of these devices has been established, their optimization requires a deeper understanding of their electronic and optical features to engineering their optoelectronic properties accurately. Though PbS NCs have been extensively investigated, the complex optical index of PbS NC thin films remains mostly unknown. Some previous works have unveiled the optical index for this type of material optimized for solar cells (excitonic peak at 940 nm), but longer wavelengths remain scarce and surface chemistry effects, which are known to be of central importance for the layer doping, are simply unexplored. Here, we conduct a systematic investigation of the complex optical index of PbS NC thin films using broadband spectrally resolved ellipsometry. The obtained results are then compared with simulations combining Tight-Binding (TB) modeling at the NC level and Bruggeman model to expand the results to the film scale. While TB calculation gives the NC optical indices, we extract keys NC film parameters as the NC volume fraction and ligand indices by fitting Bruggeman formula to ellipsometry measurement. We also bring evidence that this joint modeling method can be conducted without the need for ellipsometry data while preserving the main feature of the experimental result. Finally, the unveiled optical indices are used to model the absorption of short-wave infrared diode stack based on PbS NCs and are relevant for state-of-the-art devices. Our electromagnetic modeling shows that the absorption within the contact is now a major limitation of the current device operated at telecom wavelength