Highly efficient organic light-emitting diodes from delayed fluorescence

TL;DR: A class of metal-free organic electroluminescent molecules in which the energy gap between the singlet and triplet excited states is minimized by design, thereby promoting highly efficient spin up-conversion from non-radiative triplet states to radiative singlet states while maintaining high radiative decay rates.

Abstract: The inherent flexibility afforded by molecular design has accelerated the development of a wide variety of organic semiconductors over the past two decades. In particular, great advances have been made in the development of materials for organic light-emitting diodes (OLEDs), from early devices based on fluorescent molecules to those using phosphorescent molecules. In OLEDs, electrically injected charge carriers recombine to form singlet and triplet excitons in a 1:3 ratio; the use of phosphorescent metal-organic complexes exploits the normally non-radiative triplet excitons and so enhances the overall electroluminescence efficiency. Here we report a class of metal-free organic electroluminescent molecules in which the energy gap between the singlet and triplet excited states is minimized by design, thereby promoting highly efficient spin up-conversion from non-radiative triplet states to radiative singlet states while maintaining high radiative decay rates, of more than 10(6) decays per second. In other words, these molecules harness both singlet and triplet excitons for light emission through fluorescence decay channels, leading to an intrinsic fluorescence efficiency in excess of 90 per cent and a very high external electroluminescence efficiency, of more than 19 per cent, which is comparable to that achieved in high-efficiency phosphorescence-based OLEDs.

Summary

INTRODUCTION

• The material combination of silicon (Si) and silica (SiOx) has an exceptionally diverse application area including electronics, photonics and medicine.
• This leads to further losses and malfunctions of the applications.
• Therefore, decreasing the defect-level densities in SiOx/Si is essential to further improve SiOx/Si-containing devices.
• Many hybrid materials, such as metal-Si interconnects, degrade at high temperatures and lose their properties.
• LT post-annealing after ALD activates another type of the interface passivation: the field-effect passivation.

EXPERIMENTAL SECTION

• Measurements were in part performed in an ultrahigh-vacuum (UHV) multi- chamber system, which allows preparation and characterization of small wafer pieces, 6 mm x 12 mm, and in-situ ALD growth using a prototype instrument of the University of Turku.
• STS current-voltage (I-V) curves were measured with the grid mode.
• A different UHV system was used to perform 4-inch wafer treatments after the RCA chemical cleaning with a final HF dip.
• A shadow mask was used to deposit the Au/Cr metal contacts by sputtering on top of HfO2/p-Si(111).
• The post heating tests were performed on commercial photodiode chips, which were small enough for the surface-science vacuum instrument.

RESULTS AND DISCUSSION

• Figure 1 presents a scheme of the investigated LT-UHV approaches, which can be divided into the two categories: pre-treatment (Fig. 1a) and post-treatment (Fig. 1b) methods.
• Furthermore their preliminary tests for oxidation of Si(111) wafer pieces (see Fig. S3) suggest that the crystalline oxidation at LT is not limited to Si(100) surfaces, but might be a more general property at well-defined conditions.
• This issue might have been underestimated previously, if the (1x1) diffraction pattern has been taken as prove of crystal quality of Si sample.
• Then the wafers were thermally oxidized to protect them during wafer transfer via air.
• The LT-UHV post-treatment can decrease the amount of defect-induced gap levels at the detector sidewall region by re-forming their surface-oxide structure.

CONCLUSIONS

• The authors have investigated the issue, whether it is possible to decrease defect densities of the widely used SiOx/Si interfaces by means of a low-temperature (LT) ultrahigh-vacuum (UHV) approach, because in many stages of the manufacturing processes of SiOx/Si-containing applications, the beneficial high temperature treatment (> 700oC) cannot be utilized to decrease defect-induced losses and malfunctions.
• The considered LT-UHV treatments can be combined with the current processing technology in two complementary ways: as a pre-treatment before the insulator growth (e.g. ALD) and as a post-treatment for ready components.
• This approach leads to a decrease in the defect-level density (Dit) as compared to the state-of-the-art chemical oxide reference.
• The decreased Dit is consistent with the found decrease in the leakage currents due to the proper LT-UHV treatment, and also with the finding of hitherto not reported crystalline SiOx, which forms at surprisingly low temperatures in proper oxidation conditions.

Full text

九州大学学術情報リポジトリ
Kyushu University Institutional Repository
Highly efficient organic light-emitting diodes
from delayed fluorescence
Uoyama, Hiroki
Center for Organic Photonics and Electronics Research, Kyushu University
Goushi, Kenichi
International Institute for Carbon Neutral Energy Research (WPI-I2CNER), Kyushu University |
Center for Organic Photonics and Electronics Research, Kyushu University
Shizu, Katsuyuki
Center for Organic Photonics and Electronics Research, Kyushu University
Nomura, Hiroko
Center for Organic Photonics and Electronics Research, Kyushu University
他
http://hdl.handle.net/2324/25887
出版情報：Nature. 492 (7428), pp.234-238, 2012-12-12. Nature Publishing Group
バージョン：
権利関係：(C) 2012 Macmillan Publishers Limited.

1
Third Generation Organic LED by Hyper-Fluorescence
Hiroki Uoyama, Kenichi Goushi, Katsuyuki Shizu, Hiroko Nomura, and
Chihaya Adachi
Center for Organic Photonics and Electronics Research (OPERA), Kyushu University
744 Motooka, Nishi, Fukuoka 819-0395, Japan

2
Although typical organic molecules are simply composed of carbon (C),
hydrogen (H), nitrogen (N) and oxygen (O) atoms, carbon’s unique bonding
manners based on sp
3
, sp
2
and sp hybrid orbitals enable very complicated
molecular architectures, leading to amazing functions in a wide variety of
creatures and industrial products. In the last two decades, the allure of unlimited
freedom of design with organic molecules has shifted a significant part of the
research effort on electronics from inorganic into organic materials. In particular,
great progress has been achieved in the development of organic light-emitting
diodes (OLEDs). The successive progress of 1st generation OLEDs using
fluorescent molecules and 2nd generation OLEDs using phosphorescent molecules
solidified organic materials as a very attractive system for practical electronics. In
this study, we designed new advanced electroluminescent (EL) molecules composed
of only conventional CHN atoms without any precious metals. With proper
molecular design, the energy gap between the two excited states, i.e., singlet (S
1
)
and triplet (T
1
) excited states, are minimized, promoting very efficient spin
up-conversion from T
1
to S
1
states (reverse intersystem crossing (ISC)) while
maintaining a rather high radiative decay rate of >10
6
/s, leading to a high
fluorescence efficiency of >90%. Using these unique molecules, we realized a very

3
high external EL efficiency of over 19% that is comparable with those of
high-efficiency phosphorescence-based OLEDs. Thus, these molecules harvest both
singlet and triplet excitons for light emission under electrical excitation through
fluorescence decay channels. We call this new luminescence concept
“Hyper-fluorescence”.
The recombination of holes and electrons can produce light that is referred as
electroluminescence (EL). EL in organic materials was first discovered by M. Pope et al.
in 1963 using an anthracene single crystal connected to high-field carrier injection
electrodes
1
. Carriers of both signs were injected into the organic layers, and the
subsequent carrier transport and recombination produced blue EL that originates from
singlet excitons, i.e., fluorescence. In principle, carrier recombination is expected,
according to spin statistics, to produce both singlet and triplet excitons in the ratio of
1:3
2,3
, and this relationship has been well demonstrated for many cases
4,5
. The produced
singlet excitons decay promptly, yielding prompt EL (fluorescence), while two triplet
excitons can fuse to form a singlet exciton through triplet-triplet annihilation, yielding
delayed EL (delayed fluorescence). On the other hand, while the direct radiative decay
of triplet excitons results in phosphorescence, it usually occurs only at very low
temperatures in conventional organic aromatic compounds. In fact, in 1990, one of

4
authors, C. A., reported the first demonstration of phosphorescent EL using
keto-coumarin derivatives
6
. However, the very faint EL was observed at 77 K with
difficulty and was assumed to be virtually useless in most cases, even if including rare
earth complexes
7
. In 1999, Forrest and Thompson’s group first demonstrated efficient
electrophosphorescence using iridium phenylpyridine complexes that promote an
efficient radiative decay rate of ~10
6
/s by taking advantage of a heavy metal effect,
strong spin-orbital coupling
8
. Nearly 100% internal EL efficiency was demonstrated
9
,
providing convincing evidence that OLED technology could be useful for display and
lighting applications.
In this report, we achieved a novel pathway to reach the ultimate EL efficiency by
inventing simple aromatic compounds displaying efficient thermally-activated delayed
fluorescence (TADF) with high photoluminescence (PL) efficiency. Figure 1 (a) shows
the energy diagram of a conventional organic molecule, depicting singlet (S
1
) and triplet
(T
1
) excited states with a ground state (S
0
). While we had previously assumed that the
S
1
level should be significantly higher than the T
1
level, i.e., 0.5~1.0 eV higher, due to
the presence of electron exchange energy, we found that the proper design of organic
molecules can lead to a small energy gap (E
ST
) between them
10,11
. Relatedly, a
molecule displaying efficient TADF requires a very small E
ST
between its S
1
and T
1

Frequently asked questions

Q1. What is the driving force for the efficient reverse ISC?

It is generally accepted that the introduction of the spin-orbit coupling that is provided by heavy atoms is indispensable for both efficient ISC and reverse ISC. 

Q2. What is the critical point of the molecular design?

11 The critical point of the molecular design is the compatibility of a small EST ~ 0 eV and a reasonable radiative decay rate of over 10 6 /s that overcomes competitive non-radiative decay paths, leading to highly luminescent TADF materials. 

Q3. What is the effect of the EL design on the fluorescence efficiency of organic materials?

With proper molecular design, the energy gap between the two excited states, i.e., singlet (S1) and triplet (T1) excited states, are minimized, promoting very efficient spin up-conversion from T1 to S1 states (reverse intersystem crossing (ISC)) while maintaining a rather high radiative decay rate of >10 6 /s, leading to a high fluorescence efficiency of >90%. 

Q4. What is the simplest way to obtain a CDCB?

The aromatic nucleophilic substitution reaction (SNAr) of an anion of carbazole, generated by treatment with NaH and dicyanobenzene at room temperature, yielded CDCBs. 

Q5. What is the cost advantage of CDCBs?

CDCBs were synthesized through a one-step-only reaction from commerciallyavailable starting materials without the addition of palladium or other rare metal catalysts, indicating that CDCBs also have cost advantages. 

Q6. What is the effect of the inhibition of large geometry change on CDCBs?

The change in the geometry of CDCBs between S0 and S1 states occurs not all over the molecule but only in the central dicyanobenzene unit, and the inhibition of large geometry change leads to a high quantum efficiency. 

Q7. What is the effect of iridium phenylpyridine on EL?

In 1999, Forrest and Thompson’s group first demonstrated efficient electrophosphorescence using iridium phenylpyridine complexes that promote an efficient radiative decay rate of ~10 6 /s by taking advantage of a heavy metal effect, strong spin-orbital coupling 8 . 

Q8. What is the PL spectra of CDCBs in toluene?

In addition, the authors note that while the oscillator strength for the ground states of CDCBs estimated by TD-DFT is a rather small value of less than 0.1, the peculiar geometric characteristics discussed in this paragraph are consistent with the suppression of the non-radiative decay, leading to the high PLQY. 

Q9. What is the reason for the small geometry relaxation of 4CzIPN?

The lack of the quinoid-type deformation accounts for the small geometry relaxation of 4CzIPN compared with those of 4CzTPN and 4CzPN. 

Q10. What is the effect of the reverse ISC process on the delayed component?

On the other hand, the delayed component monotonically decreases with a decrease in temperature, since the reverse ISC process becomes the rate-determining step, similar to the temperature dependence of tin(IV) fluoride-porphyrin complexes, which are typical TADF emitters 10 . 

Q11. What is the kRISC of the orange and sky-blue OLEDs?

In addition, the orange and sky-blue OLEDs show higher external EL quantum efficiency of 11.2±1% and 8.0±1%, respectively, compared to those of conventional fluorescence-based OLEDs. 

Q12. What was the reaction reaction of a carbazolyl anion and a dicyan?

CDCBs were synthesized by reaction of a carbazolyl anion and a fluorinated dicyanobenzene at room temperature for 10 h under a nitrogen atmosphere. 

Q13. What is the simplest way to achieve a high PL efficiency?

Since very small orbital overlapping generally results in virtually no emission as is shown in benzophenone derivatives, one assumes that high PL efficiency could never be obtained with molecules having small EST; however, the authors have overcome this issue. 

Q14. How much efficiency can be achieved with a high-efficiency phosphorescent OLED?

Using these unique molecules, the authors realized a very3high external EL efficiency of over 19% that is comparable with those of high-efficiency phosphorescence-based OLEDs. 

Q15. Why is the first-order mixing coefficient inversely proportional to the EST?

This is because the first-order mixing coefficient between singlet and triplet states () is inversely proportional to the EST as described by 18 :STSOEH (2)where HSO is the spin-orbital interaction.