Colloidal quantum dots (CQDs) are nanoscale single crystals of semiconductors dispersed in an organic solvent. Low-dimensionality endows them with size tunability of optical bandgap and other optoelectronic properties. CQDs have been extensively explored for electronic applications, including solar cells, light-emitting diodes, lasers, and field-effect transistors. Research has focused at chemically synthesizing high-quality CQDs, and finding robust processing schemes of packing these CQDs into dense thin-films to form electronically-conductive assemblies. Figure 1 illustrates the common schemes used to convert as-synthesized CQDs into electronic devices. Read my review article for details that also highlights automated synthesis as a key step in the journey of CQDs to the market.
Figure 1. CQD inks can be converted into thin-films using scalable coating techniques. The initial organic/insulating ligands (blue) capping the CQD surfaces are clipped and replaced with shorter ligands (green) to increase electrical conductivity of the CQD film.
PhD Highlight: My PhD research highlighted the impact CQD surfaces have on optoelectronic device performance. I used Photoemission Spectroscopy (PES) to understand how processing solvents modify these surfaces and impact electronic band structure. These studies also involved using synchrotron PES capabilities at the Canadian Light Source (CLS).
A major highlight of my PhD was one of the first demonstrations of a blade-coated CQD solar cell. This cell was fully fabricated in a high moisture ambient (>50% RH) without any humidity control using high-speed blade-coating of ~18 meter/minute. Device performances at par with the lab-based spin-coating were achieved whilst using 25x less CQD ink. Although moisture is damaging to CQD surfaces, we devised a post-processing scheme to undo the moisture-damage. This was an important demonstration as it conclusively showed that it is possible to scalably fabricate CQD optoelectronics in an ambient with no moisture control without compromising device performance.
Figure 2. In-situ X-ray scattering experiments performed by me while at NIST to explore self-assembly of CQDs during blade-coating.
CQDs present a unique opportunity to enable emergent properties in devices made out of them. These exotic phenomenon occur where CQDs are allowed to form a self-assembled ‘supracrystal’ – crystals of CQDs (Figure 2). This includes phenomenon such as superflourescence, and band-like charge transport. When CQDs arrange in an ordered fashion, disorder and band tails get suppressed revealing phenomenon beyond the reach of disorder. Electronic devices assembled using CQD superlattice are a holy grail of this promising field.
Ahmad R. Kirmani, PhD 