Eletrophosphorescence from Organic Materials Excitons generated by charge recombination in organic LEDs Spin statistics says the ratio of singlet : triplet, 1 P* : 3 P*= 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 P + ‧ + 2 P - ‧ 1 P* + 3 P* Singlet :electroluminescenceTriplet: electrophosphorescence

Eletrophosphorescence from Organic Materials The external quantum efficiency (η ext ) is given by η ext = η int η ph = (γ η ex φ p )η ph η ph = light out-coupling from device η ex = fraction of total excitons formed which result in radiative transitions (~0.25 from fluoresent polymers) γ = ratio of electrons to holes injected from opposite contacts φ p = intrinsic quantum efficiency for radiative decay If only singlets are radiative as in fluorescent materials, η ext is limited to ~ 5%, assuming η ph ~ 1/2n 2 ~ 20 % for a glass substrate (n=1.5) By using high efficiency phosphorescent materials, η int can approach 100 %, in which case we can anitcipate η ph ~ 20 %

All emission colors possible by using appropriate phosphorescent molecules Maximum EQE Blue emittersGreen emittersRed emitters 7.5 ± 0.8 %15.4 ± 0.2 % 7 ± 0.5% Nature, 2000, 403, 750APL 2003, 82, 2422APL, 2001, 78, 1622 From S. R. Forrest Group (EE, Princeton University) 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

As DCM2 acts as a filter that removes singlet Alq 3 excitons, the only possible origin of the PtOEP luminescence is Alq 3 triplet states that have diffused through the DCM2 and intervening Alq3 layers.

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.

Emissive Materials in PLEDs Blue emitters Green emitters Red emitters White emitters ~436nm (0.15,0.22) ~546 nm (0.15,0.60) ~700nm (0.65,0.35) (0.33,0.33) cover all visible region

Synthesis of Fluorene-Acceptor Alternating Copolymers Fluorene-Acceptor Alternating Copolymers : Acceptor strength: Q < TP < BT Effects of “acceptor strength” on optoelectronic properties Polymer, 47, (2006)

Absorption Spectra & Optical Band Gaps – Alternating Copolymers Optical Band Gaps: E g opt = 2.95 eV E g opt = 2.64 eV E g opt = 2.34 eV E g opt = 1.82 eV Acceptor Strength: Q PFQ > PFBT > PFTP Coplanar Conformation of Backbone → Exceptional Low Optical Band Gap of PFTP Calculated band gaps (eV): PF > P(F-Q) > P(F-BT) > P(F-TP) → good agreement !!

CV & Electronic Structures – Alternating Copolymers Electronic Structures: HOMO: eV LUMO: eV* HOMO: eV LUMO: eV HOMO: eV LUMO: eV HOMO: eV LUMO: eV HOMO: almost the same; LUMO: PF > PFQ > PFBT > PFTP Incorporation of Acceptor → LUMO ↓ Calculated LUMO (eV): PF > P(F-Q) > P(F-BT) > P(F-TP) → good agreement !!

PL Spectra & Emissive Colors – Alternating Copolymers Emission Maximum: λ max PL = 412 nm Blue λ max PL = 488 nm Green λ max PL = 532 nm Yellow λ max PL = 646 nm Red Emission Maximum: PF < PFQ < PFBT < PFTP Emissive Color: Blue → Green → Yellow → Red Cover Entire Visible Region!!! PL Efficiencies (%): PF (56.6) > PFQ (22.4) > PFBT (18.5) > PFTP (2.1) → due to intramolecular charge transfer and heavy-atom effect

EL Spectra & Emissive Colors – Alternating Copolymers λ max EL = 425 nm (0.22, 0.26) → Sky Blue λ max EL = 480 nm (0.23, 0.40) → Blue-Green λ max EL = 540 nm (0.43, 0.56) → Yellow Emission Maximum & CIE: Emission Maximum: PF < PFQ < PFBT Emissive Color: Blue → Green → Yellow EQE (%): PF (0.18) PFBT (0.13) → due to LUMO decrement fluorescence quenching

Synthesis of Fluorene-Acceptor Random Copolymers PFTP Random Copolymers: Effects of “acceptor content” on optoelectronic properties Polymer, 47, (2006)

Absorption Spectra & Optical Band Gaps – PFTP Random Copolymers Optical Band Gaps: 2.95 eV 1.82 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 TP Content ↑ → Optical Band Gap↓ TP Content ↑ → Intensity of long-wavelength peak ↑

PL Spectra & Emissive Colors – PFTP Random Copolymers 1. PF peak↓, PFTP↑ with TP content ↑ → increasing energy transfer with increasing TP content 2.Complete energy transfer from PF to TP segments as TP content > 25%. 3.Additional peaks at 439 and 508 nm as TP > 35% due to inter-chain interaction of PF and excimer formation. 4.PL efficiencies decrease with increasing TP content. → due to intramolecular charge transfer and heavy-atom effect

EL Spectra & Emissive Colors – PFTP Random Copolymers 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 Emission Maximum & CIE: Complete energy transfer from PF to TP segments with only 1% of TP in the backbone (PL needs >25%). → “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).

Synthesis of Fluorene-Acceptor Random Copolymers for WLEDs PFQTP and PFBTTP Random Copolymers: Realization of “white emission” through composition control Macromol. Chem. Phys., 207, (2006)

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

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)

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).

Absorption and PL Spectra Good overlap between donor’s emission peak and acceptor’s absorption peak → efficient Förster energy transfer

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.

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 levelsfluorescence quenching

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.

EL of Ternary Blends White EL from TQ1 and TBT1White 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.