Solution-Processed PbS Quantum Dots and CsPbBr₃ Nanocrystals Based Bulk-Nano Heterojunction Enabled Efficient Optoelectronic Devices

Lead sulphide (PbS) colloidal quantum dots (CQDs) have shown promising applicability in optoelectronic devices owing to their advantageous properties, including large exciton Bohr radius, high absorption coefficient, infrared light absorption, low-temperature solution processability, scalability, and size-dependent bandgap tunability due to quantum confinement effect, etc.[1] Nonetheless, the performance of PbS CQD-based optoelectronic devices is constrained by trap states, which arise from the existence of dangling bonds at the non-passivated surfaces, improper washing, foreign atoms, etc. These traps generally increase with the size of PbS CQDs. As reported earlier, small-sized (< 2.7 nm) or large bandgap PbS CQDs consist of octahedron structures with all lead-rich facets (i.e., (111)).[2] The (111) facets exhibit a significant tendency to form bonds with ligands possessing one negative charge, such as oleate (OA^-). However, as the size of PbS CQDs increases more than 2.7 nm, the charge-neutral (200) facet begins to grow at the edge of the (111) facet.[2] The (200) facets of PbS CQDs consist of 1:1 (Pb: S) stoichiometry. Thus, these (200) facets can only be passivated by the charge-neutral oleic acid (OAH). This OAH can easily be removed from the (200) facets during the cleaning process or under the exposure of a vacuum.[3] Therefore, the non-passivated (200) facets create trap states by reacting the surface terminated atoms (i.e., Pb and S) with oxygen and forming PbO, PbSO₃, PbSO_4, and SO₂. In order to passivate the (200) facets, a few approaches have been used, such as ligand engineering, interface engineering, etc. In the case of ligand engineering, the researchers used several organic, inorganic, and hybrid organic-inorganic ligands. However, aggressive solvents used during the ligand exchange process limit the complete passivation of (200) facets. In the case of interface engineering, researchers used different types of perovskite precursors to passivate the (200) facets of PbS CQDs by providing suitable ions. These cations/anions make complete interfacial connections with PbS CQDs or create passivating electrical double layers. However, the thickness of passivating material is a critical parameter for charge carrier transport. In the case of complete passivation by perovskites hinders the charge transport properties of PbS CQDs as the thickness of passivating perovskite materials are significantly high, as compared to short chain organic and inorganic ligands.
Here, we have used both approaches to passivate the trap states and improve the performance of PbS CQDs, which have a size of more than 2.7 nm. In the first step (interface engineering), the PbS CQDs and cesium lead bromide (CsPbBr₃) nanocrystals (NCs) were synthesized using the hot injection method and room temperature solution process, respectively. Then, as synthesized, PbS CQDs and CsPbBr₃ NCs were mixed so that CsPbBr₃ should only contribute to surface passivation and form bulk nano heterojunction. In this process, CsPbBr₃ could passivate only one of the (200) facets due to its low lattice mismatching with PbS CQDs and larger size. In the second step (ligand engineering), hybrid organic-inorganic ligands were used to passivate the facets of PbS CQDs that had not been passivated by CsPbBr₃. The addition of CsPbBr₃ reduced trap density from 10²² to 2.5 x 10²¹ m^-3 and increased charge mobility by ~5 times. Furthermore, the photocurrent response of the NIR photodetector was enhanced by ~5 times, and the efficiency of solar cell devices was also improved by nearly 12%. Various studies showed that the combined ligand and interface engineering enhances surface properties of PbS CQDs by effectively passivating the (200) facets and significantly improving the optoelectronic device performance.

References:
[1] S. Pradhan, Nat. Nanotechnol., 14, 72–79 (2019).
[2] H. Beygi, Appl. Surf. Sci., 457, 1–10 (2018).
[3] D. Zherebetskyy, Science, 344, 1380–1384 (2014).