Platform Leader: Girish Lakhwani
Deputy Platform Leader: Anthony Chesman (CSIRO)
This Platform will 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 will open up new architectures and applications such as tunable lasers. We will employ a combined theoretical and experimental approach towards the realisation of an electrically pumped polariton laser, in which ultra-low energy thresholds are possible, increasing efficiency and limiting thermal decomposition.
To this end, we will be investigating a range of organic semiconductors to determine parameters that are critical to demonstrate lasing, and to develop a theory to identify different polariton modes and characteristic stop-band. We will also be investigating hybrid materials (e.g. Perovskites) to identify strategies towards electrical injection. Finding key industrial partner(s) will lead to technology development, particularly through the fabrication of high-precision optical cavities.
The key partner in this platform is CSIRO, which is providing facilities and expertise support through its Flexible Electronics Laboratory. CSIRO’s printing and device fabrication capability is essential for positive outcomes.
Thin film light-emitting devices (LEDs) have come a long way since the first observations of electroluminescence in organic diodes. Various key developments have led to the commercialisation of the organic LED technology in the form of ultra-thin, light-weight, high-resolution displays. However, many fundamental questions regarding materials and device design, particularly for applications that require high efficiency, brightness and stability, remain.
At the same time, LEDs, such as lasers, have led to a multitude of technological developments including telecommunications, data storage, medicine and many areas of analytical science. Electrically pumped organic lasers remain one of the key goals in optoelectronics, and their development would launch a revolution in low-cost, flexible display devices. To date, however, even optically pumped, continuous wave, solid state lasers using organic molecules are yet to be realised. As the excitation pulse width or frequency is increased, triplet states are formed, which prevent stimulated emission and thereby increase the lasing threshold, often leading to material degradation. The work being undertaken within this Platform has the potential to further our progress towards realising the ambitious objective of designing a functional electrically pumped organic laser.
Exciton Science Research Groups
We studied a series of Perylene Dimide (PDI) analogues synthesised by the Wong Group and identified an analogue that showed high Rabi Splitting (~ 140 meV), where the molecular isolation reduced aggregation and allowed for efficient emission making it suitable candidate for polariton lasing. 1 We also started a collaborative project with the Namdas and Lo Group in UQ to investigate DPP-based dyes for polariton lasing. We found these dyes to show high Rabi splitting (~ 100 meV) within a conducting matrix of F8BT polymer, offering an opportunity for a host-guest system where electrical injection of charges and formation of excitons is supported by F8BT, while emission emerges from the lower polariton branch of DPP as a consequence of decay of excitons transferred from F8BT polymer.
We have been working together with the Lakhwani Group investigating device geometries including DFBs and LEFET devices for optically pumped and electrically injected lasing, respectively.
We have been synthesising a range of PDI materials as potential candidates for polariton lasing and have been working with the Lakhwani Group in their characterisation. These PDI molecules have also proven to be useful as LSC materials in Platform 1.1. Lately, we have developed strategies towards scale-up of these materials.
Tadahiko Hirai, a device physicist with extensive experience in LED fabrication, co-supervised a PhD student, Christian Blauth, from the Mulvaney Group. Their work focused on the fabrication and characterisation of blue QD-LEDs. 2, 3, 4
The Lakhwani Group have established a new research partnership with BluGlass Pty Ltd. Bluglass contracted the group for a research consulting project that involves cavity design of GaN edge- emitting lasers using numerical simulations.
The Lakhwani Group collaborated with the Gao Group in Singapore to explore the use of perovskites for lasing. While the work is still ongoing, we meanwhile contributed to a theory paper to formulate design rules for 3D chiral perovskite that could be useful for lasing, among other optoelectronic applications5
Significant research has been carried out in 2019 within this platform towards the development of next-generation light-emitting devices. The platform has two 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 towards the demonstration of the first electrically injected polariton laser.
Within the QD LEDs research thrust, we successfully completed their fabrication together with a full characterisation of device lifetime and spectral properties. We identified the best QD processing conditions and developed a ligand exchange process, which led to enhanced performance of the QD-LEDs. In-depth analysis of the electrical properties of LEDs with different ligand systems revealed the key role of ligands and provided a pathway for future device improvements. Three papers were published based on this work.
Within the polariton lasing research thrust, we directed our efforts in 2019 primarily towards materials discovery of organic molecular systems that can support an appreciable density of exciton-polaritons, i.e. systems that can demonstrate a strong exciton-photon coupling, whose strength - often characterised by Rabi Splitting - should exceed 100 meV, while still maintaining an appreciable emission quantum yield. To this end, we identified two dye systems, perylene dimides and DPPs, as candidates. One published paper and two new collaborations - one national and one international - emerged from this work.
We also devoted considerable efforts to learn and optimise the fabrication of high quality DFBs and DBRs optical cavities and LEFET device architectures for optical pumped and electrically injected lasing, respectively. Furthermore, we also employed optical simulations using Lumerical to survey different laser architectures.
1. Randy Sabatini et al. J Mat Chem C, 7, 2954 (2019)
2. Christian Blauth et al. J. Appl. Phys. 125, 195501 (2019)
3. Christian Blauth et al. J. Appl. Phys. 126, 075501 (2019)
4. Christian Blauth et al. Appl. Phys. Lett. 115, 173505 (2019)
5. Guankui Long et al. Adv. Mater. 31, 1807628 (2019)
This platform started slower than others because it required all personnel to learn and adapt to a new field of polaritonics. Over the last couple of years it has gradually evolved and 2019 has proved to be an important year for this platform because of two key aspects: Firstly, it established a close collaborative network between the Wong and Lakhwani Groups involving the synthesis and characterisation of new polariton materials, and between the Mulvaney, Lakhwani and Chesman Groups on QD-LED and LEFET device fabrication, which also involved the Australian National Fabrication Facility (ANFF) and Melbourne Centre for Nanofabrication (MCN). Secondly, the work produced here and presented at conferences got us a good introduction in the polariton research community.
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 aims of the four-year objectives. It has made progress in all its research objectives, including demonstration of blue QD-LEDs with stable emission and improved device efficiencies, identification of champion materials for polariton lasing, and identifying key material parameters that underpin strong exciton-photon coupling. Going forward, greater efforts will be dedicated to demonstrating low thresholds for polariton lasing both by optical pumping and through electrical injection.
In 2020, we will focus on fabricating devices using perylene and DPP molecules to demonstrate optically pumped polariton lasing at room temperature, and identify clear thresholds for polariton lasing and conventional photon lasing. State of the art light-emitting devices, including lasers, have limited device efficiencies due to a high density of triplet build-up, which results in non-radiative decay. Recently a new material class has emerged that demonstrates thermally activated delayed fluorescence (TADF) which results from harvesting of triplets back into radiative singlet states by reverse intersystem crossing. We will dedicate considerable efforts to synthesising a range of TADF molecules that are also able to support strong exciton-photon coupling.
We will also explore new areas where polaritons could be exploited, such as in organic solar cells and spin-based devices. We also aim to strengthen our partnership with industrial partners, such as BluGlass, by building on our success in an ongoing seed project on cavity design for lasers.
The first recruits (postdocs and students) hired in 2017 within different platforms including this one are close to finishing their PhD degrees and/or in process of moving into new jobs elsewhere. To mitigate the risk of losing research momentum and technology and material transfer especially in this platform where personnel commitment is relatively thin, it’s important to have a few personnel with overlapping skill set and training at any given time to ensure continuity of research projects.