Light-emitting Dendrimers
Submitting Institutions
University of St Andrews,
University of EdinburghUnit of Assessment
PhysicsSummary Impact Type
TechnologicalResearch Subject Area(s)
Chemical Sciences: Inorganic Chemistry, Macromolecular and Materials Chemistry
Engineering: Materials Engineering
Summary of the impact
Impact: Economic
The light-emittingdendrimers are a new class of materials for organic
light-emitting diodes, a major display technology. They have been
commercialised by Cambridge Display Technology (CDT), the leading
developer of polymer light-emitting diodes.
Significance
Light-emitting dendrimers provided a breakthrough in the efficiency of
organic light emitting diode (OLED) materials deposited from solution.
This enabled the convenience of solution-processing to be combined with
high efficiency, and enabled solution-processed materials to compete with
evaporated materials.
Beneficiaries
CDT, display manufacturers around the world and display users.
Attribution
The research was performed by Professor Samuel in collaboration with
Professor Burn of the University of Oxford.
Reach
Materials based on light-emitting dendrimers are manufactured by Sumitomo
Chemical in Japan and supplied to global displays manufacturers.
Underpinning research
Light-emitting dendrimers were a major part of the extensive research
programme on organic optoelectronic materials and devices led by Professor
Samuel at the University of St Andrews since 2000. The research was
seeking to understand how optical and electrical properties of organic
semiconductors relate to their structure, and then apply this
understanding to developing improved materials for organic light-emitting
diodes (OLEDs). The idea of light-emitting dendrimers was developed
jointly with Professor Paul Burn (University of Oxford.) This technology
was an alternative to light-emitting polymers and evaporated small
molecules that were the dominant OLED materials at the time. A dendrimer
is a highly branched molecule, and the idea was to have a core which would
define key electronic properties such as the colour of light emission,
dendrons to keep the cores apart, and surface groups around the outside of
the macromolecule to confer solubility and favourable processing
properties. By using such a modular molecular architecture, it was hoped
that better OLED materials could be made.
By 2000, we had made some light-emitting dendrimers, but the efficiency
of OLEDs made from them was very low (around 0.1%) whilst polymer LEDs had
efficiencies of a few percent. The research programme identified the
reasons for the limited efficiency — low photoluminescence quantum yield,
hole-dominated charge transport and losses to the formation of triplet
states. We then set about developing materials to overcome these problems.
Our focus, like much of the UK was on solution-processed materials i.e.
materials that can be deposited from solution — a simpler process than
evaporation in high vacuum. Although it was known that phosphorescence
could potentially increase the efficiency, there were not successful
examples of efficient solution-processed phosphorescent OLEDs.
We therefore explored whether dendrimers could address this challenge by
incorporating a phosphorescent chromophore into the core and using the
dendrons and surface groups to make it solution-processable. Accordingly
we developed the first phosphorescent light dendrimer OLED [R1], though
its efficiency was also low. Subsequently, we developed an iridium-cored
dendrimer which gave a dramatic leap in the external efficiency of devices
to 8% [R2]. This was a world record by a large margin for a
solution-processed LED.
The first Ir-cored dendrimer OLEDs were very efficient but we realised
the charge transport was dominated by holes, so that the efficiency could
be further improved by improving the charge balance. This was achieved by
introducing an electron-transporting layer and led to an outstanding
external quantum efficiency of 16% [R3] — three times higher than
light-emitting polymers at the time.
Subsequent light-emitting dendrimer research aimed at simplifying the
device structure by making "host-free" materials, and making a full range
of colours for displays [R4]. Hence we adjusted the core to give red light
emission [R5], and then worked on the very challenging problem of deep
blue phosphorescence. We made considerable progress at understanding
chromophore design for deep blue and then showed how this could give
efficient deep-blue phosphorescence [R6],
This research led to approximately 70 refereed journal papers on
light-emitting dendrimers, and a string of patent applications (more than
10) for efficient light-emitting dendrimers in the period 2000-2010. The
light-emitting dendrimer research was recognised by Prof Samuel winning
the Ben Sturgeon award of the Society for Information Display in 2008,
showing the quality of the research and its influence on the user
community. The dendrimer research was a substantial part of the research
portfolio recognised by the award of the Beilby Medal to Prof Samuel and
the Academic R&D prize at Printed Electronics USA, the world's largest
printed electronics meeting.
Personnel
Key PHYESTA researchers involved were Professor Ifor Samuel (2000 —
Present), Dr Ebinazar Namdas (PDRA 2001-2004), Dr Thomas Anthopoulos (PDRA
2001-2003) and Dr Ruth Harding (PDRA 2004-2007).
References to the research
The quality of the underpinning research is best indicated by R2, R3 and
R5 [Number of citations]
| [R1] |
J.M. Lupton, I.D.W. Samuel, M.J. Frampton, R.
Beavington, and P.L. Burn, “Control of electrophosphorescence in
conjugated dendrimer light-emitting diodes”, Advanced Functional
Materials 11, p. 287 (2001), DOI:
10.1002/1616-3028(200108)11:4<287::AID-ADFM287>3.0.CO;2-Z,
URL: tinyurl.com/kjjxm8x, [74] |
| [R2] |
J.P.J. Markham, S-C. Lo, S.W. Magennis, P.L. Burn and I.D.W.
Samuel , “High efficiency green phosphorescence from spin-coated
single-layer dendrimer light-emitting diodes”, Applied Physics
Letters 80, p. 2645, (2002), DOI: 10.1063/1.1469218, URL:
tinyurl.com/kyqmcng, [172] |
| [R3] |
S.C. Lo, N.A.H. Male, J.P.J. Markham, S.W. Magennis, P.L. Burn,
O.V. Salata and I.D.W. Samuel “A green phosphorescent dendrimer for
light-emitting diodes”, Advanced Materials, 14, p. 975 (2002), DOI:
10.1002/1521-4095(20020705)14:13/14<975::AID-ADMA975>3.0.CO;2-D
URL: tinyurl.com/mfqg55v, [250] |
| [R4] |
S.C. Lo, T.D. Anthopoulos, E.B. Namdas, P.L. Burn and I.D.W.
Samuel, “Encapsulated cores: host-free organic light-emitting diodes
based on solution-processible electrophosphorescent dendrimers”,
Advanced Materials 17, p. 1945 (2005), DOI: 10.1002/adma.200500020,
URL: tinyurl.com/l9cm2mx, [96] |
| [R5] |
T.D. Anthopoulos, M.J. Frampton, E.B. Namdas, P.L. Burn and I.D.W.
Samuel , “Solution-processable red phosphorescence dendrimers for
light-emitting device applications”, Advanced Materials, 16, p. 557
(2004), DOI: 10.1002/adma.200306095, URL: tinyurl.com/klefyf2, [130] |
| [R6] |
S.C. Lo, R.E. Harding, C.P. Shipley, S.G. Stevenson, P.L. Burn and
I.D.W. Samuel , “High triplet—energy dendrons: enhancing the
luminescence of deep blue phosphorescent iridium (III) complexes”,
Journal of the American Chemical Society, 131, p. 16681 (2009), DOI:
10.1021/ja903157e, URL: tinyurl.com/k9ahcum, [70] |
Details of the impact
The above research changed the development of OLED materials by showing
that highly efficient solution-processed materials could be made. Several
patents were filed, and the materials were licensed to Cambridge Display
Technology (CDT), the leading developed of light-emitting polymers. In
2007, CDT was purchased by Sumitomo Chemical of Japan for $285M [S1, S2].
This allowed Sumitomo to take a major stake in the OLED business.
The materials have been developed for commercialisation via three
channels: (i) a CDT-funded research programme at St Andrews and Oxford
(ii) an internal research programme at CDT (iii) a joint development
programme with Sumitomo Chemical. The link to Sumitomo Chemical is
extremely important both because of their skills in scaling up materials
production and because of their credibility and proximity to major display
manufacturers. The last point is very important as display manufacture is
capital intensive and the major electronics companies need to be certain
of the reliability of materials supply in order to build the factories to
make the displays.
In 2005 the CEO of CDT explained [S3] "the work on phosphorescent
emission from dendrimers opens up new possibilities for the application
of OLEDs to practical applications and this work complements our work on
fluorescent polymer OLEDs, especially as the technologies potentially
can be combined in one device without any increase in complexity of the
structure." Innovations in display technology are commercially
sensitive and so there is very limited publicly available information
about the subsequent development of the dendrimers and the impact of the
research. Our primary source of information is therefore a
recent letter from [text removed for publication].
Hence the light-emitting dendrimers have had a major influence on one of
the world's largest chemical companies, and its customers who are some of
the world's largest electronics companies.
Sources to corroborate the impact