Silicon-based solar panels have become an integral part of our urban and rural landscapes, providing a substantial and growing supply of renewable energy, but their relatively low efficiency means that the search for new photovoltaic materials is still ongoing. One promising alternative to silicon-based photovoltaics is the utilisation of quantum dot composites, i.e. ordered assemblies of semiconducting nanoparticles. By organising quantum dots into ordered structures, one may exploit the local interactions between them to manipulate the behaviour of light. However, the interactions between quantum dots are highly sensitive to their separation distance and surrounding chemical environment, meaning that design and control over the assembly are of paramount importance. In practice control over the structure is particularly difficult because nano-particles tend to choose simple, high symmetry 3D arrangements or 2D monolayers. One of the primary challenges for creating photovoltaic quantum dot devices lies in overcoming these structural limitations to achieve light absorption, without subsequent re-emission.
In a recent publication in Chem, researchers from the Universities of Tohoku, Kyushu and Sheffield Universities report a revolutionary 3D array of CdS (cadmium sulfide) quantum dots, identified on the Materials and Magnetism beamline (I16) at Diamond Light Source. In their study, the quantum dots were coated with a specially designed double shell of molecules and were shown to self organise into a unique low-symmetry structure when heated. The study demonstrates the progressive changes in the material characteristics as the ordered phase begins to take shape. Once fully ordered the quantum dot array exhibits ‘high light absorption, low re-emission’ properties, representing a promising advance towards more efficient solar cells.
Figure 1: (a-c) GI-SAXS patterns of the thin film sample during the annealing process. (a) Initial state, (b) intermediate and (c) fully ordered state. The black spot in (c) shows indicates the systematically absent (100) peak. (d) A 2D slice of the ordered state showing the quantum dots (yellow), hydrocarbon chains (pink, red) and the dendrons (blue/green). Blue indicates the aromatic body of the dendrons. The cubic unit cell parameter is 11.86 nm
Introducing anisotropy to quantum dot arrays
Two molecular species were grafted to the surface of each CdS quantum dot, creating a unique two-layered shell around each of them. The inner shell consisted of hydrocarbon chains, while the outer shell consisted of dendrons (larger, tree-like molecules). When initially deposited from solvent, the thin film sample was highly fluorescent when optically excited. As the film was baked (annealed at 150 °C) it became less and less fluorescent with time until almost all absorbed light was retained.
To explore this transformation further, researchers from the University of Sheffield performed Grazing Incidence Small Angle X-ray Scattering (GI-SAXS) measurements on the sample during various stages of the annealing process. Warren Stevenson said: “station I16 was well suited for this experiment; the GI-SAXS setup allowed us to achieve high X-ray contrast from thin film samples, which was needed to resolve the all-important weak peaks”. Remarkably, with annealing time they observed the progressive structural development of a previously unseen, low symmetry 3D particle arrangement. It was shown that the fluorescence decreased as the degree order among particles increased.
The low symmetry of the structure is thought to arise from the composition of the two layer molecular shell. In order to fill space and support such an unusual structure the two molecular species have to separate and reposition on the CdS surface. This creates a non-spherical radial profile enabling new and more versatile modes of packing of spherical nanoparticles.
Fluorescence quenching mechanism in quantum dot composites
The structure dependent fluorescence is thought to arise from the behaviour of the dendrons in the outer shell of the quantum dots. In the disordered initial state the dendrons are likely to randomly distribute with the particles; however in the ordered state, the dendrons are forced to cluster into the gaps between quantum dots, leading to aromatic π-stacking. The altered electronic state of the dendrons offers an alternative de-excitation method, preventing re-emission of absorbed light.
Light retention and energy transfer inside the film are very promising attributes for quantum dot solar cells. Further work is now taking place to see if the absorbed energy can be extracted as electricity and if so, whether it can be done efficiently. Stevenson added: “We’re optimistic that self-assembled quantum dot arrays such as this one could be a big advance on current solar cell materials, but the research is still in its early stages”.
To find out more about the I16 beamline, or to discuss potential applications, please contact Principal Beamline Scientist Prof Steve Collins, firstname.lastname@example.org.
Matusbara, M.; Stevenson, W.; Yabuki, J.; Zeng, X.B.; Dong, H.; Kojima, K.; Chichibu, S.F.; Tamada, K.; Muramatsu, A.; Ungar, G.; Kanie, K., A Low-Symmetry Cubic Mesophase of Dendronized CdS Nanoparticles and Their Structure-Dependent Photoluminescence, Chem, 2, 860–876 (2017). DOI: 10.1016/j.chempr.2017.05.001
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