High Mobility and Low Trap Density in 3D PbS Quantum Dots Superlattices by PbI₂ Passivation

Lead chalcogenide colloidal quantum dots (CQDs) are one of the most promising materials to revolutionize the field of optoelectronics. This is due to their unique optical properties, namely the bandgap tunability in a wide wavelength range (800 – 3000 nm). Even if the past two decades of research resulted in significant development of this technology, these materials still show moderate mobility and high trap densities, which are a limitation for their applications. Most of the problems stem from the disordered nature of the deposited films and poor control over the ligand exchange (LE). It is predicted that ordered assemblies of CQDs, the so-called superlattices (SLs), should display mobilities up to four orders of magnitude higher than the disordered counterparts. Recently, we demonstrated mobilities above 270 cm²/Vs in 3D SL of PbSe CQDs^[1], when surface traps are filled using an ionic-gel gate. Nevertheless, using PbSe CQDs is undesirable for applications due to its tendency to oxidise very easily. On the other hand, PbS shows much higher stability even in air when proper passivation strategies are adopted. While there are several reports in the literature on PbS-based superlattices, the focus has been on structural characterisation and often transport properties are not measured. A recent report on 2D square SL with PbS CQDs demonstrated electron mobilities of 15 cm²/Vs with ionic liquid gating.<br/><br/>Here we demonstrate for the first time highly ordered 3D SLs of electronically coupled PbS CQDs and an effective trap passivation with iodide-based ligands. To be suitable for photodetection, we grow films up to 220 nm of thickness with outstanding coherent ordering, both in-plane and along the thickness, as proved by electron microscopy and advanced x-ray scattering techniques. To achieve electronic coupling, we perform LE with ethylenediamine which, despite the noticeable thickness, removes the oleic acid ligand. We finally test the transport properties with an ionic-gel gated field-effect transistor and observe n-type transport, high conductance, and modulation. The measured electron mobilities achieve a maximum of 220 cm²/Vs, which is one order of magnitude above the state-of-the-art for PbS SLs with comparable gating, proving how the 3D structures outperform the 2D ones thanks to the better ordering. Finally, a passivation treatment with PbI₂ is shown to reduce the surface trap density as demonstrated by a significant reduction of the subthreshold swing. The combination of strong absorption in the short-wavelength infrared range and the record charge mobilities make these metamaterials an excellent candidate for fast-response photodetector in a wavelength range where traditional semiconductors perform poorly despite the high cost. We also demonstrate how surface trap passivation will be a key element to control in order to implement these 3D SLs in optoelectronic devices.<br/><br/>[1] J. Pinna, R. M. Koushki, D. S. Gavhane, M. Ahmadi, S. Mutalik, M. Zohaib, L. Protesescu, B. J. Kooi, G. Portale, M. A. Loi, <i>Advanced Materials</i> <b>2023</b>, <i>35</i>, 2207364.