Platform Leader: Timothy Schmidt
Deputy Platform Leader: Ken Ghiggino
The solar spectrum arrives at the Earth’s surface as broadband, white light characterised by the sun’s temperature of 6000 kelvin. Naturally, it contains all the colours of the rainbow, as well as colours we can’t see, like ultraviolet and infrared. However, solar cells and other excitonic applications only use one colour of light efficiently, that which is near their absorption threshold. For perovskite solar cells, like those in Platform 1.2, this is in the near infrared.
The goal of this platform is to tame the solar spectrum, by controlling the energy and spatial dimension of light. By doing this we aim to exceed the 30% Shockley-Queisser efficiency limit for light-to-electricity energy conversion.
Photon Upconversion is the process of converting two low-energy photons into one of higher energy. Designing materials which can exploit this process would allow us to utilise energy from the low energy (infrared) part of the Sun’s spectrum (which is currently wasted using existing technology) and transform it into higher energy so it can be converted into electric current.
Luminescent solar concentration is a process whereby the flux (energy density) of light hitting a surface can be increased by concentrating the light absorbed over a large area into a much smaller area by means of waveguiding. A Luminescent Solar Concentrator (LSC) can improve the efficiency of upconversion and also allow solar energy collection to be integrated into building architecture.
This platform aims to design high efficiency, solid-state, thin-film upconversion materials for solar energy applications. Our aim is to develop a hybrid material which will demonstrate 45% internal quantum efficiency under 10-sun equivalent illumination. This platform also aims to deliver robust and efficient materials and devices for LSCs. Within the first three years the researchers in this platform have designed, synthesised and characterised a range of new highly luminescent materials and incorporated these materials into LSC devices. The team has created several large-scale LSC devices, with one of those having the highest reported efficiency for a large area (400 cm2) LSC.1
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Exciton Science Research Groups
The Schmidt group fabricates and characterises upconvertors. Postdoctoral Fellow Thilini Ishwara has been working towards demonstrating efficient upconversion using conjugated polymer emitters supplied by PhD student Riley O’Shea of the Wong group at UniMelb. PhD student Rosina Pelosi has been working towards linking quantum dots for LSC applications.
The Ghiggino group is working towards organic nanoparticle upconvertors under the guidance of Postdoctoral Fellow Siobhan Bradley and in collaboration with PhD students Bolong Zhang and Rehana Pervin of the Wong group on efficient LSCs using perylene diimide dyes.
CI Funston and her group built a SNOM apparatus for the characterisation of triplet mobilities.
As detailed above, the Wong group is working with both Schmidt and Ghiggino groups both on upconvertors and LSCs.
The McCamey group provides magnetic fields for the detailed characterisation of triplet-triplet annihilation in upconversion. McCamey and Schmidt worked together on assessing LSCs as a technology for device powering under low light with industrial partner Hunter Valley Signs.
The Widmer-Cooper group has been working with the Mulvaney group at UniMelb on the alignment of semiconductor nanorods for LSCs.
The Mulvaney group through the work of Masters student Hanbo Yang is working together with AI Gary Rosengarten, PhD student Timothy Warner, towards large scale, quantum dot LSCs.
CI Cole works closely with CI Schmidt on the interpretation of magnetic field data, and also with CI Russo on theoretical support for the design of conjugated polymer upconvertors with Postdoctoral Fellow Igor Lyskov.
Schmidt and McCamey have engaged actively with HiVis (trading as Hunter Valley Signs) and were granted $116k through a NSW Innovation Connection grant with HiVis to solve the problem of low-light device powering, so that smart roadsigns and advanced safety warning signs can be solar powered in dark locations such as forests. The project will finish in early 2020.
John E. Anthony (University of Kentucky, USA) - synthesising of ligands for quantum dot sensitized upconversion.
Felix Castellano (North Carolina State University, USA)
We made strident progress on LSCs in 2019, commencing the year with a summer school on photovoltaics and design featuring Professor Angele Reinders from T. U. Eindhoven. The design school resulted in a publication featuring students who designed a product as part of the practical sessions at the school. The publication, titled “Designing with Luminescence Solar Concentrator Photovoltaics”, which focussed on an interdisciplinary design project using advanced luminescent solar concentrator photovoltaic (LSC PV) technologies in new contexts of use such as the built environment, mobility and consumer products. In the past years LSC PV technologies have been rapidly maturing showing increasing efficiencies up to 10% with high expectations regarding further improvements at low costs. In this context in 2019 a summer school of one week took place aimed at optimally combining design features of LSC PV devices and further enhancements of their performance by scientific research.2 This was only one paper that originated out of the school, discussions on the headland at Coogee even led to a joint publication between Exciton Science AI Ned Ekins-Daukes, and Reinders on novel configuration for LSC PV devices, with vertically placed bifacial PV solar cells made of mono-crystalline silicon (mono c-Si). This LSC PV device comprises multiple rectangular cuboid lightguides, made of poly (methyl methacrylate) (PMMA), containing Lumogen dyes, in particular, either Lumogen red 305 or orange 240. The bifacial solar cells are located in between these lightguide cubes and can, therefore, receive irradiance at both of their surfaces.3
In the laboratory, the UniMelb node reported a high efficiency, large-format LSC, coupled to perovskite solar cells.1 This work has highlighted the importance of establishing an appropriate LSC ‘figures of merit’ that will allow any useful comparisons with literature. The UniMelb node made further progress on energy transfer mixed organic dye systems that increase Stokes shift and fluorescence.
On upconversion, the platform has fabricated and characterised thin film upconvertors based on conjugated polymers. These demonstrate the importance of film plasticity for triplet mobility. A publication is in preparation. Conjugated polymers were provided by UniMelb to UNSW for incorporation into upconversion devices.
Quantum dots have been prepared at Monash and UNSW and organic nanocrystals were prepared at UniMelb. Polymer nanoparticles (DPA polymer/Ru(mbp)3 sensitizer) have been prepared at UniMelb, but preliminary results indicate triplet quenching and back energy transfer reduce upconversion efficiencies.
RMIT have studied the triplet mobility in MEH-PPV and found it to be very fast. Postdoctoral Fellow Laszlo Frazer at Monash has published a theoretical analysis and code for upconvertor simulation. With Murad Tayebjee, Schmidt has published a theoretical appraisal of an up-and-downconverting solar cell.
A notable advance in 2019 was the first demonstration of photochemical upconversion from below the bandgap of silicon and the energy of singlet oxygen. We showed that, contrary to the usual expectation, the admission of oxygen enhances the intensity of upconverted light and significantly speeds up the photochemical processes involved. These results establish a new strategy for circumventing the problem of oxygen in photochemical upconversion and lay the foundation for an expansion of this process into new applications. A publication has been submitted and is in advanced stages of review.
1. Bolong Zhang, ‘High-Performance Large-Area Luminescence Solar Concentrator Incorporating a Donor–Emitter Fluorophore System’, ACS Energy Letters 2019, 1839 – 1844.
2. A. Reinders et al., "Designing with Luminescent Solar Concentrator Photovoltaics," 2019 IEEE 46th Photovoltaic Specialists Conference (PVSC), Chicago, IL, USA, 2019, pp. 0221-0226.
3. M. Aghaei et al, “Simulation of a Novel Configuration for Luminescent Solar Concentrator Photovoltaic Devices Using Bifacial Silicon Solar Cells”, Appl. Sci. 2020, 10(3), 871
Overall the platform has continued to work towards achieving its initial aims and objectives. Several objectives have been achieved (simulation codes, new materials, high efficiency LSCs, LSC device development and polymer film upconversion). The original platform 1.1 has met most of the milestones laid out in the Work Package document. It has not yet met some of the milestones set out for the two year mark. The scope has not changed, but spin-off projects include upconversion below the silicon bandgap, and working towards a proof-of-principle bifacial, upconverting silicon device.
Initial Platforms 1.1 and 1.2 were integrated into a single platform in 2019 to recognise the overlap in light harvesting objectives of both platforms. Less progress has been made in expanding the spectral range of LSCs but new work with quantum dots in this area has been initiated. Research to date has suggested directions for collaborations across platforms – e.g. integration of perovskite solar cells with LSC devices, stability/lifetime evaluation, quantum dot development and assessment, integration of upconversion with LSCs, and integration of upconversion with perovskite solar cells.
Organic dyes, in particular perylene diimides, has been optimised to maximise Photoluminescent Quantum Yield (PLQY) and minimise reabsorption with emission maximum at 600 nm. We believe we have the best performing system in this wavelength range and we are now looking at device stability tests. Quantum dots are being investigated to optimise their dispersibility in polymer matrices. The next stage is to synthesise quantum dots that have emission in the Near Infrared (800-900 nm) with high PLQY and integrate them into LSCs. There are also efforts to combine organic dyes with quantum dots more for scientific interest than for LSC improvement.
UniMelb and CSIRO are currently combining flexible LSC films with flexible solar cells working towards the idea of an LSC sticker. This new project has been initiated and provided with seed funding through the Exciton Science, Science Committee Collaborative Funding scheme.
The HiVis project provides Exciton Science with the opportunity to design a fit-for-purpose LSC for charging smart roadsigns in a forest environment.
Upconversion using polymers has not provided the improved efficiencies expected. Individual typology angle (ITA) spectroscopy will be applied to resolve the bottlenecks and, based on the outcomes, new polymer structures will be developed. Quantum dot-sensitized polymer upconversion experiments are also planned.
A series of conjugated polymers, derivatives of MEH-PPV, has been synthesised. Higher PLQY was achieved for the derivatives but there is a trade-off between the wavelength of emission, PLQY and upconversion efficiency. Nevertheless, upconversion performance of the new PPV materials are substantially higher than MEH-PPV. Next generation of PPV emitters are being synthesised. Conjugated polymers as triplet sensitisers are also under investigation.
Upconversion from below the silicon bandgap has been achieved. We are working towards demonstrating this with a bifacial silicon device.
An efficient relationship for quantum dot materials was not established. Schmidt and his group has now started making these materials in-house.
Some preliminary experiments have been performed at UniMelb to test this idea with currently available materials and expertise.
Platforms 1.1 and 1.2 were combined during 2019 to provide further focus for the overlapping programs on light harvesting and light concentration. Loss of staff and completing PhD students means the platform will need new recruitment of students and Postdoctoral Fellows to move its program into 2020.