Colloidal Arenethiolate-Capped PbS Quantum Dots: Optoelectronic Properties, Self-Assembly, and Application in Solution-Cast Photovoltaics

Abstract

Suitable postsynthesis surface modification of lead-chalcogenide quantum dots (QDs) is crucial to enable their integration in photovoltaic devices. Here we exploit arenethiolate anions to completely replace pristine oleate ligands on PbS QDs in the solution phase, thus preserving the colloidal stability of QDs and allowing their solution-based processability into photoconductive thin films. Complete QD surface modification relies on the stronger acidic character of arenethiols compared to that of alkanethiols and is demonstrated by FTIR and UV−vis−NIR absorption spectroscopy analyses, which provide quantitative evaluation of stoichiometry and thermodynamic stability of the resulting system. Arenethiolate ligands induce a noticeable reduction of the optical band gap of PbS QDs, which is described and explained by charge transfer interactions occurring at the organic/inorganic interface that relax exciton confinement, and a large increase of QD molar absorption coefficient, achieved through the conjugated moiety of the replacing ligands. In addition, surface modification in the solution phase promotes switching of the symmetry of PbS QD self-assembled superlattices from hexagonal to cubic close packing, which is accompanied by further reduction of the optical band gap, ascribed to inter-QD exciton delocalization and dielectric effects, together with a drastic improvement of the charge transport properties in PbS QD solids. As a result, smooth dense-packed thin films of arenethiolatecapped PbS QDs can be integrated in heterojunction solar cells via a single solution-processing step. Such single PbS QD layers exhibit abated cracking upon thermal or chemical postdeposition treatment, and the corresponding devices generate remarkable photocurrent densities and overall efficiencies, thus representing an effective strategy toward low-cost processing for QD-based photovoltaics.

Colloidal Quantum Dot Photovoltaics Enhanced by Perovskite Shelling

“…These can be directly deposited, simplifying the manufacturing, and using materials and solvents more efficiently. [33][34][35] Unfortunately, the highest-performing devices reporetd to date based on this approach have been limited to PCEs of 6%, considerably below the best layer-by-layer CQD devices.Liganding CQDs using methylammonium iodide (MAI) was recently shown to offer the possibility of short-ligand-exchanged CQDs that could be directly deposited from a polar solvent. 20 Very recently, it was reported that PbS quantum dots and methylammonium lead triiodide perovskites (MAPbI 3 ) can exhibit coherence in their lattice fringes owing to their minimal lattice mismatch.…”

“…These can be directly deposited, simplifying the manufacturing, and using materials and solvents more efficiently. [33][34][35] Unfortunately, the highest-performing devices reporetd to date based on this approach have been limited to PCEs of 6%, considerably below the best layer-by-layer CQD devices.…”

Advancing colloidal quantum dot photovoltaic technology

“…Popular ligands for CQD electronic film applications include alkane and aromatic thiols, amines and carboxylic acids [16,85,[93][94][95][96], metal chalcogenide complexes [90] and halogen atoms [97]. Most organic ligands consist of three parts: an inner anchor to the quantum dot, a middle hydrophilic segment and an outer functional group.…”

“…Most high-performing devices utilize short thiol ligands [184] in combination with inorganic halide ligands [16], which are delivered through solid-state or solution-phase ligand exchanges. Improved ligand strategies typically lower the density of mid-gap electronic trap states while increasing diffusion lengths and carrier mobilities [16,95,134,135,185]. The highest certified PCE to date of 9.9% has been achieved in a modified DH architecture [102].…”

Ethylenediamine and ethylene glycol stabilized colloidal PbX (X=S, Se, Te) nanocrystals via amide- and ester-derivatives of 1-(propanoic acid)-3-methylimidazolium bromide ionic liquid

“…Sustained interests in colloidal lead chalcogenide, PbX (X¼S, Se, Te), nanocrystals (NCs) arise from the materials' large exciton Bohr radius, high molar absorption coefficients, small effective masses of charge carriers, and tunable interparticle spacing and morphology, which translate to suitability in photovoltaic and optoelectronic applications [1,2]. Synthesis of tailored NCs via solution chemistry routes with high accuracy and precision have been achieved by systematic control of thermodynamic parameters and growth kinetics, with the aid of careful selection of precursors, solvents, capping agents and catalysts [3][4][5].…”