Dispersion and alignment of semiconductor nanocrystals for luminescent solar concentrators
Creators: name: Warner, Timothy
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 400803
subject: Electrical energy generation (incl. renewables, excl. photovoltaics)
classificationCode: 401703
subject: Energy generation, conversion and storage (excl. chemical and electrical)

description: There is a growing need to increase the production of renewable energy, of which solar energy is a key component. To complement large scale deployment of solar panels, there should be a focus on compensating for the energy consumption of buildings through the use of building integrated photovoltaics (BIPV). Luminescent solar concentrators are a good candidate for BIPV applications as the aesthetics and transparency are highly controllable. Semiconductor nanocrystals, including quantum dots (QDs), nanorods (NRs) and nanoplatelets (NPLs), are an important emerging luminescent material for LSCs. Careful design of the nanocrystals can minimise loss mechanisms of the LSCs, however compared with organic dyes, extra attention should be paid to the dispersion of nanocrystals, as nanocrystal aggregation causes additional efficiency losses due to scattering. This thesis examines the topic of nanocrystal dispersion using three different fabrication methods, polymerisation, sheet extrusion and solvent casting. It also examines the effect of alignment of elongated nanocrystals, due to solvent casting and stretching, on LSC efficiency and provides the first reported validation of simulations predicting the effects. Finally, the thesis contains an analysis of the relationship between power conversion efficiency (PCE) and average visible transmittance (AVT) using the combination of fabrication of devices, characterisation, and Monte Carlo simulations. Further work was conducted on a published Monte Carlo ray tracing simulation software to allow predictions of PCE for different luminophore, lightguide and PV cell systems.

Good nanocrystal dispersion was achieved for polymersation and a method was developed using dynamic light scattering (DLS) to predict appropriate concentrations of QDs dispersed in monomers for ideal LSC fabrication using polymerisation. Effective nanocrystal dispersion was also achieved using solvent casting methods while improvements were made to the dispersion for sheet extrusion. Further improvements to the sheet extrusion method are required before it is a viable option for LSC fabrication, both in terms of the heat stability of the QDs, and the quality of the QD dispersion. While solvent casting was an effective, simple and reliable method for dispersing QDs there is a limit to the QD concentration before aggregation and scattering is observed. Additionally, it was found that solvent casting NRs and elongated NPLs resulted in alignment of nanoparticles which was detrimental to LSC efficiency. When utilising NRs or elongated NPLs there should either be a process induced alignment of the particles in a beneficial orientation, or at least a random orientation such as that which can be achieved using polymerisation. Furthermore, stretching of the solvent cast LSC films did not significantly change the LSC efficiency of the films but did result in polarised fluorescence, which may have applications in sensors or display devices. As expected, it was shown that LSC efficiency increased as the AVT decreased and that light scattering also increases LSC efficiency. However, it was also shown that scattering is a less effective method of improving LSC efficiency compared with LSCs with well dispersed nanocrystals, which highlights the importance of improving methods to disperse nanocrystals.

Further simulations investigated the importance of different fundamental properties of LSCs and the consequences on the PCE. This included conclusions about reabsorption, geometry, transparency and colour of LSC devices. The choice of device structure is more important when reabsorption by the luminophore occurs whereas luminophores with negligible reabsorption are equally effective in thin-film and bulk-doped LSC devices. QDs with broad absorption, such as CuInSe2/ZnS, have both higher PCE potential, even for equivalent AVT values, and more neutral colour which is important which considering LSCs for BIPV applications. Additionally it has been determined that even state-of-the-art CuInSe2/ZnS QDs require further improvements in terms of reabsorption losses and photoluminescence quantum yield (PLQY) in order to be competitive with the world record LSC efficiency of 7.1%.