Platform Leader: Girish Lakhwani
Deputy Platform Leader: Anthony Chesman (CSIRO)
This Platform aims to deliver solutions for future lighting and display technologies by developing materials and devices beyond current efficiency, brightness and stability limits with spectral coverage from the ultraviolet to visible and infrared range. These next-generation light-emitting devices (LEDs) will open up new architectures and applications, such as tunable lasers.
In this platform, we employ a combined theoretical and experimental approach towards the realisation of (a) a stable blue LED that has been challenging traditionally due to triplet losses and defect emission, and (b) an electrically pumped polariton laser that remains the ‘Holy Grail’ in light emitting devices. Polariton lasing does not require population inversion, instead needing only strong coupling between the exciton state and a cavity photon mode.
Key aims are to:
This platform has two major research thrusts whose activities are highly collaborative involving two research universities, USyd and UoM, and partners at CSIRO - one on the development of blue-emitting quantum dot (QD) LEDs and the second toward the demonstration of the first electrically-injected polariton laser.
We completed fabrication of an LED based on blue-emitting quantum dots, with full characterisation of its lifetime and spectral properties. We have identified the best QD processing conditions and developed a ligand exchange process, which leads to enhanced performance of the QD-LEDs. In-depth analysis of the electrical properties of LEDs with different ligand systems has revealed the key role of ligands and provided a pathway for future device improvements.
At the start of 2020, CSIRO staff commenced discussions with members of the Bach Group (Monash) regarding the development of a back-contact perovskite LED. This work would use the quasi-interdigitated back-contact electrode architecture developed by the Bach Group for perovskite solar cells, and use high band gap perovskites as the light emitting layer. The fabrication of this device architecture would be a world first and would eliminate the parasitic absorption from charge transport layers in a planar perovskite LEDs.
Continuing on our efforts from last year on screening new dye molecules that can support a high density of polaritons in a conducting matrix, we identified a DPP:F8BT system to demonstrate that efficient energy transfer is possible while still maintaining strong coupling and polariton emission. This host:guest blend approach will help pave the way toward reduced polariton lasing thresholds and the eventual demonstration of electrically injected organic lasers.
We demonstrated optically pumped polariton lasing from molecularly insulated PDI dispersed in a PS matrix sandwiched between partly home-made DBR mirrors (Q factor ~ 600). The emission exhibits threshold behaviour, strong directionality, and slight blueshifting with respect to the fluorescence spectrum. We expect that continued exploration of these dyes will allow improved performance in organic polariton lasing with electrical injection.
We did not set out to study the following phenomena in this platform, but it came to fruition while addressing other aspects of laser function and in other cases, due to Exciton Science’s increasing global presence and personnel expertise that attracted new collaborations.
The project is divided into five work packages - Towards Flexible LEDs, Materials Discovery for Polaritons, Device Engineering, Theory of Exciton Polaritons and Device Simulations and lastly, Industry Research. The activities have not changed scope and have been on track towards the four-year objectives.
We have been working on new CdSe- and ZnSe-based quantum wells or nanoplatelets. These materials offer high QY and their flat geometry may solve some of the interfacing problems in QD LEDs. This work is ongoing and will need some fabrication at CSIRO FE Lab. Dr. Nick Kirkwood can oversee that work in collaboration with CSIRO staff. We are also attempting to make GaN nanocrystals. These blue emitting materials could solve some of the key stability issues associated with chalcogenides.
On lasing, we made in-roads into the identification of champion materials for polariton lasing, identifying key material parameters that underpin strong exciton-photon coupling and demonstration of optically pumped lasing. Going forward, greater efforts will be dedicated to demonstrating low thresholds for optical pumping lasing and proof-of-concept low thresholds through electrical injection – this will be the focus of the next two years. For the latter, we have identified suitable device geometry, supported by Lumerical simulation work.
Personnel commitment in this platform is thinner compared to others and given the recent departure of key postdocs - Randy Sabatini (USyd), PhD students Christian Blauth and Bolong Zhang (UMelb) and forthcoming departure of Patrick Conaghan (USyd) in July 2021, it has not been easy to retain momentum. Furthermore, limitations around travel restrictions due to Covid-19 are proving challenging to hire competitive international candidates with expertise in polariton science, LEFETs and material synthesis. We are the only people in Australia who work on polariton devices using organic materials, so there is no domestic talent that can fill this gap easily while international competition in the field of organic polaritonics has become fierce, with numerous renowned groups and new groups stepping into this field.
Earlier this year, we started a new roving postdoc scheme to support collaborative work across different nodes. Together with CSIRO, CI Lakhwani has been successful in securing a three-year project funding jointly supported by Exciton Science and CSIRO. This three-year postdoc will provide the necessary continuity to this project. We are in the process of canvassing domestic candidates with transistor device experience who can help us achieve the electrical injection side of the project. Covid-19 disruptions have also had a huge impact on the synthetic efforts of CI Wong. The Melbourne Covid lockdown resulted in more than six months of lab time lost in 2020. A postdoc from the UK, William Kendrick, was assigned to work on materials synthesis for this platform. However, he did not arrive in Melbourne in March because of travel restrictions and it is unlikely he will arrive until well into 2021. CI Wong will be hiring a local research assistant in 2021 to boost synthetic efforts.
In the future work section of last year’s report, we discussed possibilities to explore (a) singlet fission in pentacene dimers and (b) device physics of OPVs in an optical cavity. While we were able to identify appropriate pentacene dimers and donor/acceptor blends, respectively, to test our hypothesis, unfortunately, due to limited access to labs, we couldn’t continue the experiments. We should be able to bring these goals to fruition in 2021.
Dingchen Wen is working on GaN nanocrystals in an effort to solve the stability issues identified by Blauth during his PhD. We will also examine graphene as a replacement for ITO in devices.
Efforts towards the development of QD-LEDs have slowed following Blauth completing his studies. Unfortunately, with the effects of COVID-19 limiting our ability to recruit new students in this area the work has been severely disrupted. To continue our innovation in this area, we have explored a new opportunity which leverages the expertise of other researchers in the Centre, namely the Bach Group’s world-leading back-contact electrode design, to develop a novel LED architecture. This new avenue of investigation presents a distinct point of differentiation from the competition while allowing us to still work towards the aims of the LED area.
We successfully established a new research partnership with BluGlass Pty Ltd. BluGlass contracted the Lakhwani group for a research consulting project (worth $48K) involving;
Since the tail end of 2019 and carrying on to 2020, we have been successful in developing new national and international collaborations.
Dr Akshay Rao (University of Cambridge) – ultrafast spectroscopy on singlet fission dimers in optical cavities. Idea is to demonstrate change in fission rate (S1 -> TT pair) due to strong coupling induces change in potential energy surface
Prof Ted Sargent (University of Toronto) and Prof Weibo Gao (National University of Singapore) on chiral perovskites. Though not directly related to this project, we were approached by them due to our increasing global presence, our successful work on Faraday rotation in perovskites and Lakhwani’s expertise in chiroptical spectroscopy.
Dr Jenny Clark (Sheffield) and Prof Ulrich Scherf (Wuppertal) on polariton scattering in MeLPPP. The aim is to study the mechanism of polariton scattering via optically pumped polariton lasers.
Prof Satish Patil (IISc Bangalore) and A/Prof Jyotishman Dasgupta (TIFR Mumbai) on polaritons in twisted perylene dimers and ultrafast Raman on coupled vibrations.
Prof Anita Ho-Baillie (UNSW, now University of Sydney) on Faraday rotation work discussed in Task E. Anita provided us with mm long lead bromide perovskite crystals.
A/Prof Shih-Chun Lo and A/Prof Ebinazar Namdas (University of Queensland) on polariton work discussed in Task C. They provided us with DPP dyes.
Prof. Hiroshi Uji-I, (Hokkaido) on vibro-polaritonic chemistry
The Australia-China Centre for Graphene Optoelectronics will assist through the development of graphene-based devices. These will be flexible as well as offering improved device stability.
In the next two years, our key focus is on the following:
To work towards demonstrating electrically injected polariton lasing. We plan to achieve this by integrating LEFETs with DFB structures. To this end, we have already witnessed exciton confinement in DFB structures. We have been making progress in fabricating transistors and expect the hiring of roving postdoc, as discussed above, will accelerate our efforts.
To exploit strong exciton-photon coupling to change the rate of singlet fission, impact charge dissociation in solar cells and enhance the rate of chemical reactions by breaking chemical bonds. In 2020, we strengthened our team with the addition of AI Hutchison (UMelb), who is an expert in the polariton assisted chemistry. We aim to work closely with him towards these goals. A/Prof Ivan Kassal (USyd) is also a recent AI addition who brings expertise on Monte Carlo simulations and quantum chemistry which will help us simulate polariton scattering and exciton diffusion, aiding device physics work.
Since starting from scratch in 2018, we have come a long way on polariton research. We have not only identified a new material class for lasing but also demonstrated optically pumped lasing. With the addition of new AIs and the increasing involvement of other CIs, we are poised to take polariton research to new heights in the Exciton Science rebid. We believe polaritons will play a critical role in championing new technological advancements. We have already started brainstorming on research directions and will be investing significant time in the first half of next year in crystalising ideas into a proposal due in June 2021.