Abstract
Nanocrystal quantum dots (QD) functionalised with active organic ligands hold significant promise as solar energy conversion materials, capable of multiexcitonic processes that could improve the efficiencies of single-junction photovoltaic devices. Small-angle X-ray and neutron scattering (SAXS and SANS) were used to characterize the structure of lead sulphide QDs post ligand-exchange with model acene-carboxylic acid ligands (benzoic acid, hydrocinnamic acid and naphthoic acid). Results demonstrate that hydrocinnamic acid and naphthoic acid ligated QDs form monolayer ligand shells, whilst benzoic acid ligated QDs possess ligand shells thicker than a monolayer. Further, the formation of a range of nanocomposite materials through the self-assembly of such acene-ligated QDs with an organic small-molecule semiconductor [5,12-bis((triisopropylsilyl)ethynyl)tetracene (TIPS-Tc)] is investigated. These materials are representative of a wider set of functional solar energy materials; here the focus is on structural studies, and their optoelectronic function is not investigated. As TIPS-Tc concentrations are increased, approaching the solubility limit, SANS data show that QD fractal-like features form, with structures possibly consistent with a diffusion limited aggregation mechanism. These, it is likely, act as heterogeneous nucleation agents for TIPS-Tc crystallization, generating agglomerates containing both QDs and TIPS-Tc. Within the TIPS-Tc crystals there seem to be three distinct QD morphologies: (i) at the crystallite centre (fractal-like QD aggregates acting as nucleating agents), (ii) trapped within the growing crystallite (giving rise to QD features ordered as sticky hard spheres), and (iii) a population of aggregate QDs at the periphery of the crystalline interface that were expelled from the growing TIPS-Tc crystal. Exposure of the QD:TIPS-Tc crystals to DMF vapour, a solvent known to be able to strip ligands from QDs, alters the spacing between PbS-hydrocinnamic acid and PbS-naphthoic acid ligated QD aggregate features. In contrast, for PbS-benzoic acid ligated QDs, DMF vapour exposure promotes the formation of ordered QD colloidal crystal type phases. This work thus demonstrates how different QD ligand chemistries control the interactions between QDs and an organic small molecule, leading to widely differing self-assembly processes. It highlights the unique capabilities of multiscale X-ray and neutron scattering in characterising such composite materials.
Highlights
Solar energy conversion systems based upon small-molecule organic semiconductors (OSCs) with inorganic semiconducting quantum dots (QDs) have attracted significant interest due to their potential to work in tandem with existing photovoltaic technologies and boost device efficiencies.[1,2,3,4,5,6,7]
The work presented employs a combination of SANS and SAXS to provide complementary insights into: (i) structural studies of ligand exchange from as as-synthesized PbS–oleic acid (OA) QDs to PbS–benzoic acid (BA), PbS–naphthoic acid (NA) and PbS–hydrocinnamic acid (HCA) QDs, (ii) the early stages of self-assembly of small molecules and QDs from solution where the concentration of the small molecule is increased, approaching the solubility limit, generating aggregated structures; and (iii) characterizing and understanding the complex morphologies of QD:TIPS-Tc nanocomposite materials formed via selfassembly
In either SAXS or SANS, the contrast between two components is proportional to the square of the scattering length density (SLD) difference
Summary
Solar energy conversion systems based upon small-molecule organic semiconductors (OSCs) with inorganic semiconducting quantum dots (QDs) have attracted significant interest due to their potential to work in tandem with existing photovoltaic technologies and boost device efficiencies.[1,2,3,4,5,6,7] One of the most promising solar energy conversion processes, ‘‘photon multiplication’’, harnesses the ability of 5,12-bis((triisopropylsilyl)ethynyl)tetracene (TIPS-Tc) and a family of similar molecules to undergo singlet exciton fission (SF), whereby one singlet exciton is converted into two triplet excitons that are 7970 | Soft Matter, 2020, 16, 7970--7981. Small angle scattering (both X-ray [SAXS] and neutron [SANS]) have provided quantitative structural insight into a range of QDs systems, from the size and distribution of the inorganic core radii (via SAXS) and the coverage and conformation of the native oleic acid (OA) ligand layer of as-synthesized QDs (via SANS).[19,20,21,22,23] Here, the ability of such scattering techniques to probe multiple lengthscales is employed to quantify QD ligand exchange processes and consequent OSC:QD self-assembly. Sample solutions of a desired QD concentration were prepared through drying aliquots of stock solutions of PbS–OA under nitrogen, followed by the addition of the appropriate solvent (d-toluene or h/d-toluene blend) This is a fixed-geometry time-of-flight instrument which utilizes neutrons with wavelengths between 2.2 and 10 Å. PbS QD:TIPS-Tc crystals suspended via Kapton tape where placed in a 10 mL glass beaker containing 200 mL of DMF and exposed to vapours for 30 minutes
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