Heavy metal free emitters for new-generation light sources

Project Information

MEGA

Grant agreement ID: 823720

DOI

10.3030/823720

Project closed

EC signature date 4 October 2018

Start date 1 January 2019

End date 31 December 2023

Funded under

• EXCELLENT SCIENCE - Marie Skłodowska-Curie Actions

Total cost

€ 1 633 000,00

EU contribution

€ 1 614 600,00

Coordinated by

KAUNO TECHNOLOGIJOS UNIVERSITETAS
Lithuania

Periodic Reporting for period 2 - MEGA (Heavy metal free emitters for new-generation light sources)

Reporting period: 2021-01-01 to 2023-12-31

Summary of the context and overall objectives of the project

Organic heavy metal free fluorescent materials show exceptional potential for use in new-generation light sources, such as organic light-emitting devices (OLEDs) and organic lasers. It is anticipated that these new materials will enable organic electronic devices to be constructed with higher efficiency, simpler device structures, lower fabrication costs, and reduced environmental impact. Amongst the different types of materials currently being investigated, two show particular promise:

• Fluorescence materials exhibiting thermally activated delayed fluorescence (TADF) for use in OLEDs in displays and lighting devices.
• Fluorescent materials with low thresholds for amplified spontaneous emission (ASE) for use in organic lasers in spectroscopy and telecommunication.

However, in order to develop these materials for commercial industrial use, several challenges still remain to be overcome, including:

• Theories explaining TADF and ASE are still in their infancy.
• Organic material samples need to be extremely pure (>99.5%). Consequently, new synthesis routes need to be developed.
• TADF emitters for OLEDs have lifetimes that fall well short of industry requirements.
• Fluorescence emitters for lasers need high available optical gain, solution processability and narrow emission spectra with high efficiency.
• Properties of TADF and lasing materials are very sensitive to structural changes.

Thus, the overall goal of the MEGA project was to help develop organic heavy metal free fluorescent materials for commercial use by tackling these challenges. In order to develop the new materials, the following scientific and technical objectives will be targeted:

• Objective 1: Screen compounds with TADF or lasing properties by means of molecular modelling.
• Objective 2: Synthesise most promising compounds with TADF or lasing properties.
• Objective 3: Characterise most promising compounds with TADF or lasing properties.
• Objective 4: Test materials in device structures to meet industry requirement.

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

The main achievements during the project have been the following:

WP1 Theory and Screening

• 61 new low-molar-mass semiconductors for thermally activated delayed fluorescence (TADF) and lasing were designed.
• Using theoretical methods, a selection of compounds for TADF and lasing was accomplished.
• New donor-acceptor TADF molecules were designed.
• Insights were gained into the physical phenomena impacting singlet-triplet energy difference, leading to the development of new design rules for tuning energy gaps.

WP2 Synthesis

• Development and expansion of various classes of donor-acceptor emitters, including boron-containing and tricarbazolylamine-based compounds.
• Introduction of new moieties such as BODIPY in the synthesis of organic compounds for lasers, to improve energy transfer efficiency and photoluminescence.
• Enhanced understanding of the relationship between emitter properties and structure, leading to the design and synthesis of new materials with improved photoluminescence characteristics.
• Modifications to improve solubility and reaction yields of compounds like triphenylboron compound DG7, and investigation of different molecular structures for potential TADF emitters.

WP3 Characterisation

• Synthesis and characterization of various organic compounds, including fluoro- and trifluoromethyl-substituted derivatives, for applications in OLEDs and solar cells.
• Development and analysis of new donor materials for organic electronics, focusing on carboline-based and other novel molecular structures.
• Optimization and thorough characterization of materials for lasing applications, including incorporation of BODIPY and other efficient moieties.
• Extensive study of the photophysical properties of synthesized compounds, leading to improved understanding of their emission characteristics and quantum yields.
• Exploration and characterization of novel TADF emitters and hosts, advancing the knowledge in the field of organic light-emitting diodes.

WP4 Material Testing in Device Structures

• A protocol for characterizing OLEDs was established.
• Various OLED test stacks were developed, considering the optoelectronic requirements of different TADF emitter families, leading to the creation of specific device structures optimized for these emitters.
• Detailed analysis of the optical properties of all functional layers used in OLEDs, including a focus on TADF emitters in various device structures.
• Initial tests and development of new compounds within the MEGA project, assessed for their potential in laser applications.
• Extensive study and characterization of lasing properties of specific compounds, including analysis of their suitability for organic laser applications.

WP5 Project Co-ordination, Management and Dissemination

• 328.1 secondment months implemented.
• 32 international journal papers published.
• 10 conference papers presented.

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

During the project, substantial progress beyond the state of the art was made in the fields of “theory” and “synthesis” of thermally activated delayed fluorescence (TADF) and amplified spontaneous emission (ASE) materials.

Theory:

(i) Replacement of "static" energy diagrams with "dynamical" energy diagrams
This is due to joint contributions from different studies and is based on including the effect of the molecular vibrations in the energy diagrams. The difference with respect to the static diagrams (consisting of three well-distinguished energy levels) resides on two new aspects: (a) a continuum of triplet excited state energy levels is considered in the energy diagram of a given compound, and (b) due to the intra-molecular vibrations, the sense of energy-evolution through the continuum (up or down) is crucial for a better understanding when comparing several compounds.

(ii) TADF in boron nitrogen (BN) based compounds
This aspect concerns a recently discovered class of TADF compounds, in which donor- and acceptor atoms are intercalated in the same block instead of compounds consisting of separated donor- and acceptor blocks. This difference results in the crucial advantage of the BN compounds: both parameters mentioned above can be jointly improved in the BN compounds, as compared to the compounds designed as donor-acceptor intercalated blocks, for which improving one of the parameters can be only achieved at the expense of deteriorating the other parameter.

The new insights achieved by our studies bring to light the reasons why the two parameters can be improved independently: while the energy parameter (EP) can be improved through space extension of the molecular pi-system, the intensity parameter (IP) is by design-concept destined to be large, given that the space-localizations of the electron and hole in the excited state are imbricated in the same block due to the resonance effect, in turn enhanced by the intercalated donor- and acceptor atoms.

Synthesis:

(i) An optimized synthetic route has been developed for the preparation of donor-acceptor-donor type TADF emitters using readily available acceptor and donor moieties by efficient Buchwald-Hartwig cross-coupling reactions.
(ii) It was discovered that, for the design and synthesis of stable boron containing emitters, the boron-oxygen bond should be exploited as it gives a higher thermal stability for the target emitters. Such boron-chelate acceptor moieties can be used in various TADF molecular systems as efficient acceptors.

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