Organic Luminescent Molecule with Energetically
Equivalent Singlet and Triplet Excited States for
Organic Light-Emitting Diodes (OLED)
Keigo Sato, Katsuyuki Shizu, Kazuaki Yoshimura, Atsushi Kawada, Hiroshi Miyazaki,
and Chihaya Adachi
Phys. Rev. Lett. 110, 247401 (2013)
Published June 10, 2013
Speaker：陳永祈
Report Date：10月24日
http://dx.doi.org/10.1103/PhysRevLett.110.247401
Outline
•
•
•
•
Introduction
Experiment method
Result and discussion
summary
Introduction
Organic luminescent materials are widely used in various consumer
products such as highlighters, detergents .
In addition to their use as chemical products, recen t development
of organic materials from a new perspective has opened up a novel
possibility for their use in optoelectronic devices . ex：display
有機基板顯示器
螢光筆
zh.wikipedia.org
In particular, the luminescence and unique semiconducting
characteristics of organic materials can be combined, leading to the
rapid development of organic semiconductor devices such as organic
light emitting diodes (OLEDs)
OLED電視
LED
可撓性OLED
zh.wikipedia.org
OLEDs possess attractive features such as
1. high EL efficiency .
2. high contrast .
3. low weight and low cost .
ηEQE≈20%
OLED無論是顏色對比度或者是發光強度
都比LCD來得好
4. It possess the flexible
of charactertistic .
cck1616tw.pixnet.net
• 分子放光的方式可以有兩種：
電致發光
光致發光
After extensive study of OLEDs for the past 20 years,
electroluminescent(EL, 電致發光) materials have usually been
classified by one of two mechanisms, fluorescence(螢光) or
phosphorescence(磷光), where radiative decay occurs from singlet
or triplet excited states, respectively .
Electroluminescent(電致發光)：
當電子和電洞在發光層內“相遇
“時，就會產生激子，因而放光
e：電荷量
d：發光層的厚度
J：電流密度
kS：單重激子的輻射速率
η S：單重激子數與總激子數的比值
η out：內部產生的激子數與外部流動的電子數之比值
Spin multiplicity ：2S＋1
T1
S1
2S+1=3
S=1
2S+1=1
S=0
χ00=1/√2(|↑↓> ― |↓↑>)
χ11=|↑↑>
χ10=1/√2(|↑↓> + |↓↑>)
χ1-1=|↓↓>
S1：Singlet excited state (單重激發態)
T1：Triplet excited state (三重激發態)
＊激子也可以叫做是激發態!!!
電洞注入
電子注入
τ ＝ 10-6~101 s
單重態激子
τ ＝ 10-9~10-8 s
螢光
τ ＝ 10-6~101 s
磷光
kr ：the fluorescence decay rate
kp： the phosphorescence decay rate
knp： the nonradiative decay rate from the T1 excited state
kISC ：the intersystem crossing rate
三重態激子
Photoluminescent(PL, 光致發光)：
Photoluminescent materials have usually been classified by one of
two mechanisms, fluorescence or phosphorescence , where radiative
decay occurs from singlet or triplet excited states, respectively .
光致發光
Intersystem
crossing
螢光
S1
S1
磷光
T1
S0
T1
S0
Energy State(能階)
電子的躍遷是否發生， 取
決於state與state之間的偶合
程度
<S0∣u∣S1>
S1
LUMO
HOMO
S0
S0：Singlet ground state
S1：Singlet excited state
T1：Triplet excited state
HOMO ：highest occupied molecular orbital
(最高電子佔據軌域)
LUMO ：lowest unoccupied molecular orbital
(最低電子未佔據軌域)
S0
S1
2S+1=1
S=0
T1
2S+1=3
S=1
τ ＝ 10-6~101 s
受光激發
螢光
磷光
τ ＝ 10-9~10-8 s
τ ＝ 10-6~101 s
kr ：the fluorescence decay rate
kp： the phosphorescence decay rate
knp： the nonradiative decay rate from the T1 excited state
kISC ：the intersystem crossing rate
螢光和磷光最大的不同就是輻射生命期的長短：
• 螢光輻射生命期：10-9~10-8 s
• 磷光輻射生命期：10-6~101 s
奎寧水因吸收紫外光之後而放出螢光
夜明珠所放出的磷光
zh.wikipedia.org/zh-tw
The use of phosphorescent iridium 銥(Ir) complexes overcame the limitation of
exciton production efficiency through spin-orbit coupling, resulting in devices
with very high external EL efficiency (EQE)of over 20% . In fact, the high EQE of
such phosphorescent complexes prompted commercialization of OLEDs.
Unfortunately, rare metals such as Ir and Pt are not the ideal choice for OLED
emitters. They are unevenly distributed resources, expensive, rather low
solubility and produce toxic waste .
spin-orbit coupling → <L · S>＝value
<L · S>的值越大，就越有可能放出磷光
iridium 銥(Ir) complexes
To produce OLED emitters without these drawbacks, we
recently prepared novel light-emitting materials that realize
high efficiency without using rare metal complexes by taking
advantage of very efficient thermally activated delayed
fluorescence (TADF) .
受光激發
螢光
磷光
電洞注入
電子注入
螢光
磷光
Although some molecules are already known to exhibit TADF, their
efficiencies are generally quite low .
In the last few years, they have surveyed various candidates as TADF
emitters and found the molecules that exhibit significant TADF .
SnF2OEP
ΔEST ＝0.24eV
CC2TA
ηEQE ＝0.1%
ΔEST ＝0.06eV
ηEQE ＝11%
PIC-TRZ
ACRFLCN
ΔEST ＝0.1eV
ηEQE ＝5.3% >2%
considering only the photoluminescence (PL)
efficiency of ΦPL ＝39% related to
conventional fluorescence
ΔEST ＝0.1eV
ηEQE ＝10%
J. Chem. Theory Comput. 2013, 9, 3872−3877
Recently, they found an advanced molecule with an ultimately small
ΔEST ≈ 0eV, virtually zero gap. They believe that this is the first report
on the molecules having zero-gap between the singlet and triplet
excited states.
An OLED containing this molecule exhibited a high ηEQE of 14% ±1%
S1
S0
T1
△E≈0 eV
In their successive design of TADF molecules, taking advantage of
the backbone of PIC-TRZ, they systematically changed the number
and position of an indolocarbazole unit through quantum
mechanism calculation .
Indolocarbazole unit
PIC-TRZ
PIC-TRZ2
Experiment method
• 計算方法：DFT/PBE0/6-31G(d)
DFT(Density Function Theoy)
PBE0：
Setting up of parameter during
calculation
6-31G(d) ：A kind of basis set
Φi ： electron basis function
ci ： coefficient
Ψ：the molecular orbital of
wavefunction
氫原子的分子軌域
HOMO
PIC-TRZ
HOMO and LUMO are separated incompletely
ΔEcalc＝0.08eV
HOMO
PIC-TRZ2
LUMO
LUMO
HOMO and LUMO are completely separated
ΔEcalc＝0.003eV
Their goal is to achieve high TADF performance in a solid film,
aimed for application in OLEDs, so they focused on host-guest
systems .
guest
PIC-TRZ2
Dope(摻
雜)
host
Figures 3(a) and 3(b) show transient PL decay and fluorescence
(black line) and delayed fluorescence , 延遲螢光(red line) spectra of
6 wt %-PIC-TRZ2 doped(摻雜) into a 1,3-bis(carbazol-9-yl)benzene
(mCP) host layer at T = 5K.
λ ＝475nm
(S1 ＝2.61 eV)
Spectra corresponded well with each other, indicate that the
delayed component is caused by TADF .
Result and discussion
In the case of PIC-TRZ, although we clearly observed the
appearance of phosphorescence at low temperature, demonstrating
a characteristic emission spectrum with clear vibrational peaks ,
no such emission spectrum was observed for PIC-TRZ2.
有磷光產生
at T = 5K
沒有磷光產生
They conclude that the delayed component is related to
TADF, and up-conversion occurs in PIC-TRZ2 even at T ＝ 5K,
indicating it possesses virtually zero gap .
The transient PL decay characteristics of PIC-TRZ2 at T＝300 K at Figure (c)
Fast PL transient lifetime
τf ＝83 ns
slow PL decay τd ＝2.7 μs
iridium 2-phenylpyridine
They note that the latterτd is comparable to that of conventional Ir
complexes such as iridium 2-phenylpyridine [Ir(ppy)3] derivatives with
τ＝1~5 μs , indicating that PIC-TRZ2 has comparable transient performance
of delayed fluorescence to those of Ir complexes .
They measured the temperature dependence of the radiative decay
rate of the delayed fluorescence(kr(TADF))
＊阿瑞尼士方程式(Arrhenius equation)告訴我們：溫度越高，kr的反應速率就越快 !
To maximize the TADF efficiency of films doped with PIC-TRZ2,
they optimized the host materials.
PLQE ： PL quantum efficiencies
Total PLQE ： 總放光之效率
Prompt PLQE ：放螢光之效率
Delayed PLQE ：放延遲螢光之效率
學者發現說PIC-TRZ摻雜其他不同的host
materials之後，會有不同大小的發光效率
＊總放光效率＝放螢光之效率＋放延遲螢光之效率
Triplet energy
(eV)
ΦPL
TPT1
2.33
18~24%
TAPC
2.87
58~62%
PYD2
2.90
43~47%
mCP
2.91
39~43%
DPEPO
≈3.1
49~53%
UGH2
≈3.5
57~61%
Back energy transfer
energy transfer
PIC-TRZ2
學者猜測三重激子會在TAPAC分子待上一段
很長的時間，而後再將能量釋放出來，並
轉移至PIC-TRZ，使得PIC-TRZ的激子數增
多，
放光效率因而提升。
Transient PL measurements revealed that
Triplet excitation energy ：host materials ＞PIC-TRZ2
Energy transfer
host materials
PIC-TRZ2
→ 延遲螢光會發生
Triplet excitation energy ：host materials ＜PIC-TRZ2
Energy transfer
host materials
PIC-TRZ2
→ 能量會以非輻射(nonradiation)的方式流失，
例如：熱 。因而造成不必要的能量浪費。
當然!這是我們不希望的!!!
if energy transfer occurs readily, we can expect a further
enhancement of EQE, suggesting a theoretical maximum of 11.8%.
They tried to optimize the device parameters to maximize ηEQE
due to imperfect triplet exciton confinement.
(45nm)
(20nm)
(40nm)
device V
device VI
LiF/Al Cathode
LiF/Al Cathode
DPEPO
PYD2：6wt％PIC-TRZ2
TAPAC
ITO Anode
Metal Cathode
Electron Transport Layer
Light Emitting Polymer
Hole Transport Layer
ITO Anode
TPBi
device VII
LiF/Al Cathode
TmPyPBi
PYD2：6wt％PIC-TRZ2
PYD2：6wt％PIC-TRZ2
TAPAC
TAPAC
ITO Anode
ITO Anode
?
Device VII
LiF/Al Cathode
雖然本篇Paper沒有顯示出Device I、Device II、Device III
以及Device IV的結構，但唯一能夠確定的是；無論是效率
或著是穩定性都沒有比右邊的三種Device來得好!
並且效率最大的是Device VII
TmPyPBi
PYD2：6wt％PIC-TRZ2
TAPAC
ITO Anode
Summary ：
1. They expect that improved understanding of molecular structures
will reveal undeveloped functions that will further enhance OLED
performance.
2. While PIC-TRZ2 has nearly zero-gap energy, a rather high f of 60%
was obtained. This is because PIC-TRZ2 has appreciable oscillator
strength (振子強度) → <S0∣u∣S1> , leading to highly efficient TADF.
＊振子強度的數值越大，兩個State之間的躍遷就越容易發生
3. The radiative decay rate of the delayed fluorescence(kr(TADF))
dependence of temperature .