Theoretical Optical Performances of Semiconductor Nanocrystals for Image Sensors and Photovoltaics Applications

Colloidal quantum dot (CQD) thin films are emerging materials that are expected to be used more and more in the microelectronics industry. One of their greatest strengths lies in the capability to tune their optical properties across a wide spectral range by changing their size, shape, and composition. Due to these excellent optical properties, research on QDs has experienced extremely rapid development during the last few decades, accompanied by the emergence of various semiconductor materials (II-VI, III-V, IV-VI, and group IV) and various technologies relying on QD thin films (LEDs, displays, photovoltaics, image sensors), driven also by their low-cost synthesis and easy integration into conventional microelectronic fabrication flows.<br/><br/>QD thin films are made of chemically-synthesized semiconductor nanocrystals, embedded in an insulating matrix made of organic and/or inorganic ligands. The role of these ligands is to passivate dangling bonds on the surface of the QDs, stabilize both mechanically and chemically the QDs, and allow charge carrier transfer between QDs without affecting their optical absorptions. As a consequence, it is important to model the dielectric behavior of the QD thin films and see how the properties of the ligands matrix and the choice of the nanocrystals' shape, composition, and size impact the final performances of the CQD thin films.<br/><br/>In this presentation, we will present a methodology to predict the optical properties of CQD thin films based on nanocrystal quantum simulations coupled with effective medium theory. By using Tight-Binding (TB) simulations, we computed the electronic structure of several nanocrystals of different sizes and made of various bulk semiconductors to find empirical laws describing the evolution of the QDs’ optical bandgap as a function of their diameter. For all these materials, we then determined the influence of the QD size on the oscillator strength [1], enabling us to establish empirical laws to forecast the absorption coefficient and optical dielectric permittivity of single QDs. We then used the Bruggeman formula [2] to extrapolate the complex optical indices of thin films made of QDs embedded in a ligand matrix [3]. This allowed us to assess the influence of the ligand and volume fraction on the optical performances. Adjusting the model parameters (QD size, ligand length, and volume fraction), we provided an abacus giving the maximum theoretical optical performances of CQD thin films for various semiconductor materials.<br/>Finally, using the optical indices calculated previously, we performed stack diode simulations using the transfer matrix method (TMM), to see how intrinsic CQD film absorption impacts the design and the quantum efficiency of photodiodes embedding QDs.<br/><br/>The presented work thus provides a detailed understanding of the various physical and chemical parameters at the origin of the optical properties of QDs as well as elements allowing to compare the performances of QDs with other photosensitive materials used in photonics, image sensors, and photovoltaics. It provides a useful resource and a guide for chemists and physicists synthesizing and characterizing new types of QD.<br/><br/><br/>[1] Moreels, I.; Lambert, K.; Smeets, D.; De Muynck, D.; Nollet, T.; Martins, J. C.; Vanhaecke, F.; Vantomme, A.; Delerue, C.; Allan, G.; et al. Size-Dependent Optical Properties of Colloidal PbS Quantum Dots. <i>ACS Nano</i> <b>2009</b>, <i>3</i>, 3023–3030<br/>[2] Choy, T. C. <i>Effective Medium Theory</i>; Oxford University Press, 2015<br/>[3] Chehaibou, B., Izquierdo, E., Abadie, C., Cavallo, M., Khalili, A., Dang, T. H., ... & Delerue, C. Complex Optical Index of PbS Nanocrystal Thin Film and their Use for Short Wave Infrared Sensor Design. <i>Nanoscale </i><b>2022, </b><i>7</i>