Eletrophosphorescence from Organic Materials

1
Eletrophosphorescence from Organic Materials
Excitons generated by charge recombination in
organic LEDs
2P? 2P-? 1P 3P
Singlet electroluminescence
Triplet electrophosphorescence
Spin statistics says the ratio of singlet
triplet, 1P 3P 1 3
To obtain the maximum efficiency from an organic
LED, one should harness both the singlet and
triplet excitations that result from electrical
pumping
2
Eletrophosphorescence from Organic Materials
The external quantum efficiency (?ext) is given
by
?ext ?int ?ph (? ?ex fp )?ph
?ph light out-coupling from device
?ex fraction of total excitons formed which
result in radiative transitions
(0.25 from fluoresent polymers)
? ratio of electrons to holes injected from
opposite contacts
fp intrinsic quantum efficiency for radiative
decay
If only singlets are radiative as in fluorescent
materials, ?ext is limited to 5, assuming ?ph
1/2n2 20 for a glass substrate (n1.5)
By using high efficiency phosphorescent
materials, ?int can approach 100 , in which
case we can anitcipate ?ph 20
3
High Efficiency LEDs from Eletrophosphorescence
Organometallic compounds which introduce
spin-orbit coupling due to the central heavy atom
show a relatively high ligand based
phosphorescence efficiency even at room
temperature
All emission colors possible by using appropriate
phosphorescent molecules
From S. R. Forrest Group (EE, Princeton
University)
Maximum EQE
Blue emitters
Green emitters
Red emitters
7.5 0.8
15.4 0.2
7 0.5
Nature, 2000, 403, 750
APL 2003, 82, 2422
APL, 2001, 78, 1622
4
http//www.cibasc.com/pic-ind-pc-tech-protection-l
ightstabilization2.jpg
As DCM2 acts as a filter that removes singlet
Alq3 excitons, the only possible origin of the
PtOEP luminescence is Alq3 triplet states that
have diffused through the DCM2 and intervening
Alq3 layers.
5

The phosphorescent sensitizer acts as a donor
(sensitize the energy transfer from the host) to
excite the fluorescence dye and such energy
transfer significantly enhances the luminescence
efficiency. Baldo and Forrest, Nature 2000, 403,
750.
6
(No Transcript)
7
(No Transcript)
8
(No Transcript)
9
Emissive Materials in PLEDs
Blue emitters
White emitters
436nm (0.15,0.22)
Green emitters
546 nm (0.15,0.60)
Red emitters
(0.33,0.33) cover all visible region
700nm (0.65,0.35)
10
Synthesis of Fluorene-Acceptor Alternating
Copolymers
Fluorene-Acceptor Alternating Copolymers
Acceptor strength Q lt TP lt BT Effects of
acceptor strength on optoelectronic properties
Polymer, 47, 527-538(2006)
11
Absorption Spectra Optical Band Gaps
Alternating Copolymers
Optical Band Gaps
Egopt 2.95 eV
Egopt 2.64 eV
Egopt 2.34 eV
Egopt 1.82 eV
Acceptor Strength Q lt TP lt BT ? Optical Band
Gap PF gt PFQ gt PFBT gt PFTP
Coplanar Conformation of Backbone ? Exceptional
Low Optical Band Gap of PFTP
Calculated band gaps (eV) PF gt P(F-Q) gt P(F-BT)
gt P(F-TP) ? good agreement !!
12
CV Electronic Structures Alternating
Copolymers
Electronic Structures
HOMO -5.39 eV LUMO -2.44 eV
HOMO -5.51 eV LUMO -2.65 eV
HOMO -5.49 eV LUMO -3.14 eV
HOMO -5.33 eV LUMO -3.33 eV
HOMO almost the same LUMO PF gt PFQ gt PFBT gt
PFTP
Incorporation of Acceptor ? LUMO ?
Calculated LUMO (eV) PF gt P(F-Q) gt P(F-BT) gt
P(F-TP) ? good agreement !!
13
PL Spectra Emissive Colors Alternating
Copolymers
Emission Maximum
?maxPL 412 nm Blue
?maxPL 488 nm Green
?maxPL 532 nm Yellow
?maxPL 646 nm Red
Emission Maximum PF lt PFQ lt PFBT lt PFTP
Emissive Color Blue ? Green ? Yellow ? Red
Cover Entire Visible Region!!!
PL Efficiencies () PF (56.6) gt PFQ (22.4) gt
PFBT (18.5) gt PFTP (2.1) ? due to intramolecular
charge transfer and heavy-atom effect
14
EL Spectra Emissive Colors Alternating
Copolymers
Emission Maximum CIE
?maxEL 425 nm(0.22, 0.26) ? Sky Blue
?maxEL 480 nm (0.23, 0.40) ? Blue-Green
?maxEL 540 nm (0.43, 0.56) ? Yellow
Emission Maximum PF lt PFQ lt PFBT Emissive
Color Blue ? Green ? Yellow
EQE () PF (0.18) lt PFQ (0.20) gt PFBT (0.13) ?
due to LUMO decrement fluorescence quenching
15
Synthesis of Fluorene-Acceptor Random Copolymers
PFTP Random Copolymers
Effects of acceptor content on optoelectronic
properties Polymer, 47, 527-538(2006)
16
Absorption Spectra Optical Band Gaps PFTP
Random Copolymers
Optical Band Gaps
2.95 eV
PFTP0.5 2.95 eV PFTP01 2.02 eV PFTP05 1.98
eV PFTP15 1.94 eV PFTP25 1.90 eV PFTP35
1.82 eV
1.82 eV
TP Content ? ? Intensity of long-wavelength peak ?
TP Content ? ? Optical Band Gap?
17
PL Spectra Emissive Colors PFTP Random
Copolymers

• PF peak?, PFTP? with TP content ? ? increasing
  energy transfer with increasing TP content
• Complete energy transfer from PF to TP segments
  as TP content gt 25.
• Additional peaks at 439 and 508 nm as TP gt 35
  due to inter-chain interaction of PF and excimer
  formation.
• PL efficiencies decrease with increasing TP
  content. ? due to intramolecular charge
  transfer and heavy-atom effect

18
EL Spectra Emissive Colors PFTP Random
Copolymers
Emission Maximum CIE
PFTP0.5 632 nm(0.55, 0.30) ? Purple PFTP01
638 nm(0.66, 0.31) ? Deep Red PFTP05 656 nm
(0.66, 0.32) ? Deep Red PFTP15 662 nm
(0.66, 0.32) ? Deep Red PFTP25 667 nm
(0.70, 0.30) ? Deep Red

• Complete energy transfer from PF to TP segments
  with only 1 of TP in the backbone (PL needs
  gt25). ? Charge Trapping mechanism
• The optimum EQE is 0.48 (PFTP01).
• The emissive color of PFTP01 is almost identical
  to the standard red demanded by the NTSC (0.66,
  0.34).

19
Synthesis of Fluorene-Acceptor Random Copolymers
for WLEDs
PFQTP and PFBTTP Random Copolymers
Realization of white emission through
composition control
Macromol. Chem. Phys., 207, 1131-1138 (2006)
20
PL Spectra Emissive Colors PFQTP and PFBTTP
Random Copolymers

• Efficient Förster energy transfer from PF to Q
  (or BT) and from Q (or BT) to TP.
• PL efficiencies decrease with increasing TP
  content ? due to intramolecular charge transfer
  and heavy-atom effect

21
EL Spectra Emissive Colors PFQTP and PFBTTP
Random Copolymers

• More efficient energy transfer than PL ? charge
  trapping mechanism
• Simultaneous emission from three units ?
  white-light emission
• Stand white emission (0.33, 0.33) ? PFQTP1 (0.34,
  0.33) PFBTTP1 (0.33, 0.34)

22
PF-Based Polymer Blends for Light-Emitting
Applications PF-Based Polymer Blends

• Binary Blends
• BQ PF PFQ
• BBT PF PFBT
• Ternary Blends
• TQ PF PFQ PFTP
• TBT PF PFBT PFTP

Förster energy transfer Effects of acceptor
structure and content
White-Light Emission Incomplete energy transfer
J. Polym. Sci. B Polym. Phys., 45, 67-78(2007).
23
Absorption and PL Spectra

• Good overlap between donors emission peak and
  acceptors absorption peak? efficient Förster
  energy transfer

24
PL Spectra of Binary Blends

• Complete energy transfer from PF to PFQ (or PFBT)
  at the acceptor content as low as 5 .
• PL efficiencies decrease with dopant contents.
• Binary blends with more efficient PL? feasible
  approach for color tuning without sacrificing PL
  efficiencies.

25
EL of Binary Blends

• Efficient energy transfer from PF to PFQ (or
  PFBT)
• Binary blends with higher EQE? feasible approach
  for color tuning without sacrificing EL
  efficiencies.
• Optimum composition at 10 LUMO
  levels fluorescence quenching

26
PL Spectra of Ternary Blends

• Cascade energy transfer from PF to PFQ (or PFBT)
  then from PFQ (or PFBT) to PFTP
• More efficient Förster energy transfer from PFBT
  to PFTP
• PL efficiencies decrease with dopant contents.
• Precise control of composition results in
  incomplete energy transfer and white-light
  emission.

27
EL of Ternary Blends

• White EL from TQ1 and TBT1 White PL from TQ6 and
  TBT6
• The difference in the composition between TQ1 and
  TBT1 is attributed to (1) more efficient energy
  transfer from PFBT to PFTP(2) PFBT is a better
  electron trap than PFQ(3) different emissive
  colors of PFQ and PFBT
• EQE?with PFTP content?? due to low efficiency of
  PFTP
• Bright and efficient white EL from TQ1 and TBT1.