A figure of merit for efficiency roll-off in TADF-based organic LEDs

Funding: We are grateful to the Engineering and Physical Sciences Research Council of the UK for financial support through grants EP/R035164/1 and EP/P010482/1.Organic light-emitting diodes (OLEDs) are a revolutionary light-emitting display technology that has been successfully commercialized in mobile phones and TVs1,2. The injected charges form both singlet and triplet excitons, and for high efficiency it is important to enable triplets as well as singlets to emit light. Currently materials that harvest triplets by thermally activated delayed fluorescence (TADF) are a very active field of research as an alternative to phosphorescent emitters which usually employ heavy metal atoms3,4. Whilst excellent progress has been made, there is a severe decrease of efficiency as the drive current is increased, i.e. efficiency roll-off, in most TADF OLEDs. At present much of the literature suggests that efficiency roll-off should be reduced by minimising the energy difference between singlet and triplet excited states (EST) in order to maximise the rate of conversion of triplets to singlets via reverse intersystem crossing (kRISC) 5-20. We analyse the efficiency roll-off in a wide range of TADF OLEDs and find that neither of these parameters fully accounts for the reported efficiency roll-off. By considering the dynamic equilibrium between singlets and triplets in TADF materials, we propose a figure of merit (FOM) for materials design to reduce efficiency roll-off and discuss its correlation with reported data of TADF OLEDs. Our new FOM will guide the design and development of TADF materials that can reduce efficiency roll-off. It will help improve the efficiency of TADF OLEDs at realistic display operating conditions and expand the use of TADF materials to applications that require high brightness, such as lighting, augmented reality, and lasing.Peer reviewe

Highly efficient organic light-emitting diodes and light-emitting electrochemical cells employing multiresonant thermally activated delayed fluorescent emitters with bulky donor or acceptor peripheral groups

Jingxiang Wang thanks the China Scholarship Council (202006250026). We thank the Engineering and Physical Sciences Research Council (EP/R035164/1, EP/W015137/1, and EP/W007517/1) for support. Ludvig Edman and Shi Tang acknowledge financial support from the Swedish Research Council (2019-02345 and 2021–04778), the Swedish Energy Agency (50779-1 and P2021-00032), the Wallenberg Initiative Materials Science for Sustainability (WISE) funded by the Knut and Alice Wallenberg Foundation (WISE-AP01-D02), and the European Research Council for an ERC Advanced Grant (Project 101096650).Multiresonant thermally activated delayed fluorescence (MR-TADF) emitters have been the focus of extensive design efforts as they are recognized to show bright, narrowband emission, which makes them very appealing for display applications. However, the planar geometry and relatively large singlet?triplet energy gap lead to, respectively, severe aggregation-caused quenching (ACQ) and slow reverse intersystem crossing (RISC). Here, a design strategy is proposed to address both issues. Two MR-TADF emitters triphenylphosphine oxide (TPPO)-tBu-DiKTa and triphenylamine (TPA)-tBu-DiKTa have been synthesized. Twisted ortho-substituted groups help increase the intermolecular distance and largely suppress the ACQ. In addition, the contributions from intermolecular charge transfer states in the case of TPA-tBu-DiKTa help to accelerate RISC. The organic light-emitting diodes (OLEDs) with TPPO-tBu-DiKTa and TPA-tBu-DiKTa exhibit high maximum external quantum efficiencies (EQEmax) of 24.4% and 31.0%, respectively. Notably, the device with 25 wt% TPA-tBu-DiKTa showed both high EQEmax of 28.0% and reduced efficiency roll-off (19.9% EQE at 1000 cd m?2) compared to the device with 5 wt% emitter (31.0% EQEmax and 11.0% EQE at 1000 cd m?2). The new emitters were also introduced into single-layer light-emitting electrochemical cells (LECs), equipped with air-stable electrodes. The LEC containing TPA-tBu-DiKTa dispersed at 0.5 wt% in a matrix comprising a mobility-balanced blend-host and an ionic liquid electrolyte delivered blue luminance with an EQEmax of 2.6% at 425 cd m?2. The high efficiencies of the OLEDs and LECs with TPA-tBu-DiKTa illustrate the potential for improving device performance when the DiKTa core is decorated with twisted bulky donors.Peer reviewe

Highly twisted α-diketone-based thermally activated delayed fluorescence emitters and their use in organic light-emitting diodes

AKG is grateful to the Royal Society for Newton International Fellowship NF171163. We acknowledge support from the Engineering and Physical Sciences Research Council of the UK (grants EP/P010482/1 and EP/L017008/1). We are also grateful for financial support from the University of St Andrews Restarting Research and Restarting Interdisciplinary Research Funding Schemes (SARRF and SARIRF) which are funded through the Scottish Funding Council grant reference SFC/AN/08/020.We have designed a highly twisted small TADF emitter PXZ-α-DK based on an a-diketone (α-DK) as a strong acceptor and phenoxazine (PXZ) as a strong donor to obtain red-shifted emission in comparison to the equivalent a-diketone linked to 9,9-dimethyl-9,10-dihydroacridine (DMAC). The PXZ-α-DK shows emission at 586 nm and DMAC-α-DK shows emission at 548 nm in 1,3-bis(N-carbazolyl)benzene (mCP) host at 1.5 wt% doping of the emitter, with short-delayed lifetimes of 6.9 μs for PXZ-α-DK and 7.6 μs for DMAC-α-DK. OLEDs fabricated using these emitters show green electroluminescence at 555 nm for DMAC-α-DK, with a maximum external quantum efficiency, EQEmax, of 6.3%, and orange electroluminescence at 585 nm for PXZ-α-DK, with an EQEmax of 0.8%. We corroborate the optoelectronic properties of these emitters with DFT calculations.PostprintPeer reviewe

Near-infrared fluorescence of silicon phthalocyanine carboxylate esters

EZ-C acknowledges the University of St Andrews for financial support. We are grateful to the EPSRC for financial support from grant EP/M02105X/1 and the European Research Council for financial support from grant 321305. I.D.W.S. acknowledges a Royal Society Wolfson Research Merit Award.Seven silicon(IV) phthalocyanine carboxylate esters (SiPcs, 1–7 ) with non-, partially- and per-fluorinated aliphatic (linear or branched at the alpha-carbon) and aromatic ester groups have been synthesized, their solid-state structures determined and their optoelectronic properties characterized. The SiPcs exhibit quasi-reversible oxidation waves (vs. Fc+/Fc) at 0.58–0.75 V and reduction waves at −0.97 to −1.16 V centered on the phthalocyanine ring with a narrow redox gap range of 1.70–1.75 V. Strong absorbance in the near-infrared (NIR) region is observed for 1–7 with the lowest-energy absorption maximum (Q band) varying little as a function of ester between 682 and 691 nm. SiPcs 1–7 fluorescence in the near-infrared with emission maxima at 691–700 nm. The photoluminescence quantum yields range from 40 to 52%. As a function of esterification, the SiPcs 1–7 exhibit moderate-to-good solubility in chlorinated solvents, such as 1,2-dichlorobenzene and chloroform.Publisher PDFPeer reviewe

Solubilised bright blue-emitting iridium complexes for solution processed OLEDs

EZ-C acknowledges the University of St Andrews for financial support. IDWS and AKB acknowledge support from EPSRC (EP/J01771X). The authors would like to thank the Engineering and Physical Sciences Research Council for financial support for Adam Henwood: EPSRC DTG Grants: EP/J500549/1; EP/K503162/1; EP/L505097/1.Combining a sterically bulky, electron-deficient 2-(2,4-difluorophenyl)-4-(2,4,6- trimethylphenyl)pyridine (dFMesppy) cyclometalating C^N ligand with an electron rich, highly rigidified 1,1’-(α,α’-o-xylylene)-2,2’-biimidazole (o-xylbiim) N^N ligand gives an iridium complex, [Ir(dFMesppy)2(o-Xylbiim)](PF6), that achieves extraordinarily bright blue emission (ΦPL = 90%; λmax = 459 nm in MeCN) for a cationic iridium complex. This complex is compared with two reference complexes bearing 4,4’-di-tert-butyl-2,2’- bipyridine, and solution-processed organic light emitting diodes (OLEDs) have been fabricated from these materials.Publisher PDFPeer reviewe

Blue-to-green emitting neutral Ir(III) complexes bearing pentafluorosulfanyl groups : a combined experimental and theoretical study

EZ-C acknowledges the University of St Andrews for financial support. The authors are grateful to the EPSRC for financial support from grant EP/M02105X/1, DTG grants: EP/J500549/1, EP/K503162/1, EP/L505097/1. The authors are also grateful to the European Research Council (321305) for support. IDWS is a Royal Society Wolfon Research Merit Award holder.A structure-property relationship study of neutral heteroleptic ( 1 and 2 , [Ir(C^N)2(L^X)]) and homoleptic ( 3 and 4 , fac-[Ir(C^N)3]) Ir(III) complexes [where L^X = anionic 2,2,6,6-tetramethylheptane-3,5-dionato-κO3,κO6 (thd) and C^N = a cyclometalating ligand bearing a pentafluorosulfanyl (-SF5) electron-withdrawing group (EWG) at C4 (H L1 ) and C3 (H L2 ) positions of the phenyl moiety] is presented. These complexes have been fully structurally characterised, including by single crystal X-ray diffraction, and their electrochemical and optical properties have also been extensively studied. While complexes 1 ([Ir( L1 )2(thd)]), 3 (Ir( L1 )3) and 4 (Ir( L2 )3) exhibit irreversible first reduction waves based on the pentafluorosulfanyl substituent in the range of -1.71 V to -1.88 V (vs. SCE), complex 2 ([Ir( L2 )2(thd)]) exhibits a quasi-reversible pyridineC^N-based first reduction wave that is anodically-shifted at -1.38 V. The metal+C^N ligand oxidation waves are all quasi-reversible in the range of 1.08-1.54 V (vs. SCE). The optical gap, determined from the lowest energy absorption maxima, decreases from 4 to 2 to 3 to 1 and this trend is consistent with the Hammett behaviour (σm/σp with respect to the metal-carbon bond) of the –SF5 EWG. In degassed acetonitrile, for complexes 2 - 4 , introduction of the -SF5 group produced a blue-shifted emission (λem = 484-506 nm) compared to reference complexes [Ir(ppy)2(acac)], R1 (where acac = acetylacetonato) (λem = 528 nm in MeCN), [Ir(CF3-ppy)(acac)], R3 (where CF3-ppy = 2-(4-(trifluoromethyl)phenyl)pyridine) (λem = 522 nm in DCM) and [Ir(CF3-ppy)3], R8 (λem = 507 nm in MeCN). The emission of complex 1 , by contrast, was modestly red-shifted (λem = 534 nm). Complexes 2 and 4 , where the –SF5 EWG is substituted para to the Ir-Cppy bond are efficient phosphorescent emitters, with high photoluminescence quantum yields (ΦPL = 58-79% in degassed MeCN solution) and microsecond emission lifetimes (τε = 1.35-3.02 μs). Theoretical and experimental observations point towards excited states that are principally ligand-centered (3LC) in nature, but with a minor metal-to-ligand charge-transfer (3MLCT) transition component, as a function of the regiochemistry of the pentafluorosulfanyl group. The 3LC character is predominant over the mixed 3CT character for complexes 1 , 2 and 4 while in complex 3 , there is exclusive 3LC character as demonstrated by unrestricted Density Functional Theory (DFT) calculations. The short emission lifetimes and reasonable ΦPL values in doped thin film (5 wt% in PMMA), particularly for 4 , suggest that these neutral complexes would be attractive candidate emitters in organic light-emitting diodes.Publisher PDFPeer reviewe

(Deep) blue through-space conjugated TADF emitters based on [2.2]paracyclophanes

E. S. and S. B. acknowledge funding from DFG in the frame of SFB1176 (projects A4, B3 and C6) and KSOP (fellowship to E. S.). E. Z.-C. and I. D. W. S. thank the Engineering and Physical Sciences Research Council (EPSRC) (No. EP/P010482/1) for financial support. I.D.W.S. also acknowledges support from a Royal Society Wolfson Research Merit Award.The first examples of through-space conjugated thermally activated delayed fluorescence (TADF) emitters based on a [2.2]para- cyclophane (PCP) skeleton with stacked (coplanar) donor-acceptor groups have been synthesized. The optoelectronic properties are studied by the relative configuration, cis (pseudo-geminal) and trans (pseudo-para), of the donor and acceptor groups.PostprintPeer reviewe

Homochiral self-sorted and emissive IrIII metallo-cryptophanes

We thank the EPSRC (DTG award 1238852, EP/K039202/1, EP/M02105X/1, EP/J001325/1), Leverhulme Trust (RPG-2014-148), University of St Andrews, and the MEXT/JSPS Grants in Aid for Scientific Research (JP25102005 and JP25102001) for funding; Simon Barrett for assistance with NMR; Martin Huscroft for assistance with HPLC, and Stephen Boyer for elemental analysis measurements.The racemic ligands (±)-tris(isonicotinoyl)-cyclotriguaiacylene (L1), or (±)-tris(4-pyridyl-methyl)-cyclotriguaiacylene (L2) assemblewith racemic (Λ, Δ)- [Ir(ppy)2(MeCN)2]+, in which ppy = 2-phenylpyridinato to form [{Ir(ppy)2}3(L)2]3+ metallo-cryptophane cages. The crystal structure of [{Ir(ppy)2}3(L1)2]∙3BF4 has MM-ΛΛΛ nd PP-ΔΔΔ isomers, and homochiral self-sorting occurs in solution, a processaccelerated by a chiral guest. Self-recognition between L1 and L2 within cagesdoes not occur, and cages show very slow ligand-exchange. Both cages are phosphorescent,with [{Ir(ppy)2}3(L2)2]3+ havingenhanced and blue-shifted emission when compared with [{Ir(ppy)2}3(L1)2]3+ .PostprintPeer reviewe

Fluorinated dibenzo[a,c]-phenazine-based green to red thermally activated delayed fluorescent OLED emitters

Purely organic thermally activated delayed fluorescence (TADF) emitting materiaLs for organic Light-emitting diodes (OLEDs) enable a facile method to modulate the emission color through judicious choice of donor and acceptor units. Amongst purely organic TADF emitters, the development of TADF molecules that emit at Longer wavelengths and produce high-efficiency devices that show Low efficiency roll-off remains a challenge. We report a modular synthesis route that delivers three structurally related fluorinated dibenzo[a,c]-phenazine-based TADF molecules, each bearing two donor moieties with different electron-donating strengths, namely 3,6-bis(3,6-di-tert-butyl-9H-carbazol-9-yl)-10-fluorodi-benzo[a,c]phenazine (2DTCz-BP-F), 3,6- bis(9,9-dimethylacridin-10(9H)-yl)-10-fluorodibenzo[a,c]-phenazine (2DMAC-BP-F) and 10,10'-(10-fluorodibenzo[a,c]phenazine-3,6-diyl)bis(10H-phenoxazine) (2PXZ-BP-F). They exhibit donor strength-controlled color-tuning over a wide color range from green to deep-red with photoluminescence maxima, lambda(PL), of 505 nm, 589 nm, and 674 nm in toluene solution. OLED devices using these TADF materials showed excellent to moderate performance with an EQE(max) of 21.8% in the case of 2DMAC-BP-F, 12.4% for 2PXZ-BP-F and 2.1% with 2DTCZ-BP-F, and associated electroluminescence (EL) emission maxima, lambda(EL), of 585 nm, 605 nm and 518 nm in an mCBP host, respectively.Peer reviewe

Controlling the emission efficiency of blue-green iridium (iii) phosphorescent emitters and applications in solution-processed organic light-emitting diodes

We show that the emission efficiency of blue-green phosphorescent emitters can be controlled through coupling of the excited state to vibrational modes. We controlled this vibrational coupling through choice of different ligands and as a result, complexes with CF3-groups on the ancillary ligand were essentially non-emissive (ΦPL 50%). Emission of the complexes can be drastically improved (30 times higher ΦPL compared to degassed solution for the CF3-containing complexes) by blending them with an inert solid host such as PMMA, which mitigates metal-ligand vibrations. Solution-processed organic light-emitting diodes made from these materials showed efficiency as high as 6.3%