Search

Funding: China Scholarship Council - 2019 06250199, 2020 06250026; Engineering and Physical Sciences Research Council - EP/L017008, EP/P010482/1. Multiresonant thermally activated delayed fluorescence (MR-TADF) compounds are attractive as emitters for organic light-emitting diodes (OLEDs) as they can simultaneously harvest both singlet and triplet excitons to produce light in the device and show very narrow emission spectra, which translates to excellent color purity. Here, we report the first example of an MR-TADF emitter (DOBDiKTa) that fuses together fragments from the two major classes of MR-TADF compounds, those containing boron (DOBNA) and those containing carbonyl groups (DiKTa) as acceptor fragments in the MR-TADF skeleton. The resulting molecular design, this compound shows desirable narrowband pure blue emission and efficient TADF character. The co-host OLED with DOBDiKTa as the emitter showed a maximum external quantum efficiency (EQEmax) of 17.4 %, an efficiency roll-off of 32 % at 100 cd m−2, and Commission Internationale de l’Éclairage (CIE) coordinates of (0.14, 0.12). Compared to DOBNA and DiKTa, DOBDiKTa shows higher device efficiency with reduced efficiency roll-off while maintaining a high color purity, which demonstrates the promise of the proposed molecular design. Publisher PDF

Financial support from the National Council of Science and Technology (Conacyt) Mexico. Funding from the Foundation of Polish Science (First TEAM/2017-3/30). Perylene diimide derivatives with different functional groups (OR) in the bay position were synthesised (PDI-1, OR = OC6H4OMe; PDI-2, OR = OC6H4CH2CH2NHBoc; PDI-3, OR = OC6H4CO2Me) and their optoelectronical properties were characterised. The derivatives were applied as alternative electron transport materials (ETMs) to replace the commonly used PCBM in inverted perovskite solar cells (PSCs). Devices with the structure ITO/PTAA/Cs0.04(MA0.17FA0.83)0.96Pb(I0.83Br0.17)3/ETM/Ag (ETM = PCBM or PDI-1, -2 or -3) were fabricated through solution processing techniques. A competitive power conversion efficiency (PCE) of 16.8% was obtained for the PDI-3-based device, which was comparable to the PCBM-based device with PCE of 17.3%. It was found that the electronic nature of the functional groups plays an important role in the charge extraction and band alignment of these small molecular semiconductors. Publisher PDF

Three new 1,2,4,5-tetraphenylimidazole derivatives, 9,9-dimethyl-10-(4-(2,4,5-triphenyl-1H-imidazol-1-yl)phenyl)-9,10-dihydroacridine (DMAC-TPI), 10-(4-(2,4,5-triphenyl-1H-imidazol-1-yl)phenyl)-10H-phenoxazine (PXZ-TPI), and 10-(4-(2,4,5-triphenyl-1H-imidazol-1-yl)phenyl)-10H-phenothiazine (PTZ-TPI), bearing different electron donors at the N1 position of the imidazole were synthesised and characterised. DMAC-TPI and PXZ-TPI showed narrow, deep blue emission at λPL of 388 and 418 nm in toluene, and in doped films in Zeonex polymer (1 wt.%) at λPL 381 and 407 nm, respectively, with a full width at half maximum (FWHM) ranging 0.42-0.44 eV. DMAC-TPI and PXZ-TPI are predicted to show very low oscillator strength for the low-energy transitions, which aligns to the observed low photoluminescence quantum yields. Both molecules showed a singlet-triplet energy gap (EST of around 1.2 eV) that is much too large to enable reverse intersystem crossing and thermally activated delayed fluorescence. Connecting a donor group to TPI at the N1 position can lead to room temperature phosphorescence (RTP), as the example of PTZ-TPI showed.

We present a p- and n-doped nonacene compound, NOBNacene, that represents a rare example of a linearly extended ladder-type multiresonant thermally activated delayed fluorescence (MR-TADF) emitter. This compound shows efficient narrow deep blue emission (PL = 410 nm, FWHM = 38 nm, ФPL = 71%, d = 1.18 ms) in 1.5 wt% TSPO1 thin film. The organic light-emitting diode (OLED) using this compound as the emitter shows a comparable electrolumines-cence spectrum (EL = 409 nm, FWHM = 37 nm) and a maximum external quantum efficiency (EQEmax) of 8.5% at CIE coordinates of (0.173, 0.055). The EQEmax values were increased to 11.2% at 3 wt% doping of the emitter within the emissive layer of the device. At this concentra-tion, the electroluminescence spectrum broadened slightly, leading to CIE coordinates of (0.176, 0.068).

We show how borylation of an acceptor-donor-acceptor (A-D-A) thermally activated delayed fluorescence (TADF) emitter, DIDOBNA-N, transforms the compound into a multi-resonant TADF (MR-TADF) emitter, MesB-DIDOBNA-N. DIDOBNA-N emits bright blue light (𝜆PL = 444 nm, FWHM = 64 nm, 𝛷PL = 81%, 𝜏d = 23 𝜇s, 1.5 wt% in TSPO1). The deep blue organic light-emitting diode (OLED) based on this compound shows a very high maximum external quantum efficiency (EQEmax) of 15.3% for a device with CIEy of 0.073. The MR-TADF emitter, MesB-DIDOBNA-N shows efficient and narrowband violet emission (𝜆PL = 402 nm, FWHM = 19 nm, 𝛷PL = 74.7%, 𝜏d = 133 𝜇s, 1.5 wt% in TSPO1). The OLED with MesB-DIDOBNA-N shows out-standing efficiency for a violet OLED at 9.3% and CIEy = 0.044, which is the bluest EL reported for a MR-TADF OLED to date. Noteworthy are the CIE coordinates of (0.166, 0.045), which are very close to the Rec.2020 standard for blue (0.131, 0.046). The EQEmax values were improved from 9.3% to 13.6% by increasing the concentration of the emitter in the host from 1.5 wt% to 5 wt%.

JMS acknowledges the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior – Brasil (CAPES) – Finance Code 001 for PhD funding. LKJ acknowledges support from the Marie Skłodowska-Curie Individual Fellowship (European Commission) (MCIF: no. 745776) and funding from UKRI-FLF through MR/T022094/1. GC thanks the EPSRC (EP/E036244/1) for funding. BODIPY-based materials are well known for their outstanding chemical and photo-stability as well as their ease of synthesis and tunability of their frontier molecular orbitals. These are attractive features for hole transporting materials (HTMs) for perovskite solar cells (PSCs) that could help improve device stability and cost. In this paper, we report the straightforward synthesis of a new BODIPY small molecule, PTZ-PTZ-BDP , functionalised with phenothiazine moieties in both the meso and α positions giving rise to a Y-shaped structure. As estimated by DFT calculations, and confirmed by electrochemical and ambient photoemission spectroscopy studies, PTZ-PTZ-BDP presents appropriate energy levels suitable for its use as a HTM in PSCs. Electrochemical measurements also reveal several redox processes with excellent reversibility. Systematic evaluation of its performance as HTM in n–i–p PSC with and without dopants was conducted and the device parameters compared with commonly used HTMs of spiro-OMeTAD and PTAA. The CH3NH3PbI3 based PSCs incorporating simple solution processed PTZ-PTZ-BDP as HTM demonstrated a champion power conversion efficiency of 14.6%, matched in performance and shelf-life stability to complex and expensive state-of-the-art HTMs, showing that BODIPY based HTMs are a promising direction for perovskite solar cells. Publisher PDF

We designed and synthesized two new ionic thermally activated delayed fluorescent (TADF) emitters that are charged analogues of a known multiresonant TADF (MR-TADF) compound, DiKTa. The emission of the charged derivatives is red-shifted compared to the parent com-pound. For instance, DiKTa-OBuIm emits in the green (λPL = 499 nm, 1 wt% in mCP) while DiKTa-DPA-OBuIm emits in the red (λPL = 577 nm, 1 wt% in mCP). In 1 wt% mCP films, both emitters showed good photoluminescence quantum yields of 71% and 61%, and delayed life-times of 316.6 μs and 241.7 μs, respectively, for DiKTa-OBuIm and DiKTa-DPA-OBuIm, leading to reverse intersystem crossing rates of 2.85 × 103 s-1 and 3.04 × 103 s-1. Light-emitting electrochemical cells were prepared using both DiKTa-OBuIm and DiKTa-DPA-OBuIm as ac-tive emitters showing green (λmax = 534 nm) and red (λmax = 656 nm) emission, respectively.

Examples of multi-resonant thermally activated delayed fluorescence (MR-TADF) emitters that emit at longer wavelengths remain rare. To reach that goal, we decorate different numbers and types of electron-donors about a central MR-TADF core, DiKTa. Depending on the identity and number of donor groups, the excited state either remains short range charge transfer (SRCT) and thus characteristic of an MR-TADF emitter or becomes a long-range charge transfer (CT) that is typically observed in donor-acceptor TADF emitters. The impact is that in three examples, Cz-DiKTa, Cz-Ph-DiKTa and 3Cz -DiKTa, which emit from a SRCT state, the emission remains narrow, while in four examples, TMCz-DiKTa, DMAC-DiKTa, 3TMCz-DiKTa and 3DMAC-DiKTa, which emit via a CT state, the emission broadens significantly. Through this strategy, the organic light-emitting diodes fabricated with the three MR-TADF emitters show maximum electroluminescence emission wavelengths, EL, of 511, 492 and 547 nm with moderate full-width at half maxima (FWHM) of 62, 61 and 54 nm, respectively. Importantly, each of these devices show high maximum external quantum efficiencies (EQEmax) of 24.4%, 23.0% and 24.4%, which are amongst the highest reported with ketone-based MR-TADF emitters. OLEDs with D-A type emitters, DMAC-DiKTa and TMCz-DiKTa, also show high efficiencies, with EQEmax of 23.8% and 20.2%, but accompanied by broad emission at EL of 549 and 527 nm, respectively. Noteworthy is that the DMAC-DiKTa-based OLED shows very small efficiency roll-off, and its EQE remains 18.5% at 1000 cd m-2. Therefore, this work demonstrates that manipulating the nature and numbers of donor groups decorating a central MR-TADF core is a promising strategy for both red-shifting the emission and improving the performance of the OLEDs.

We have designed a highly twisted small TADF emitter PXZ-α-DK based on an -diketone (α-DK) as a strong acceptor and phenoxazine (PXZ) as a strong donor to obtain red-shifted emission in comparison to the equivalent -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.

We report a novel molecular engineering approach to organic materials for LEDs based on dendrimers. The molecules consist of a conjugated core, conjugated linkers, and surface groups, as illustrated schematically in figure 1. The conjugated core and linkers control the electronic properties, whilst the surface groups control the processing properties. This is a very attractive feature as it should enable electronic and processing properties to be optimised independently, in contrast to linear polymers where substitution can affect both.

The predominant class of molecular systems considered so-far towards applications in the realm of quadratic nonlinear optics has been derived from the all-pervading "molecular diode" template as exemplified by paranitroaniline-like molecules. The underlying basic paradigm consists in the dipolar anchoring of an interacting couple of electron donor and acceptor groups to a conjugated π electron linkage(1). The virtue of such a configuration is to provide a significant electronic charge displacement in the ground state which is further enhanced upon directional optical excitation towards the charge-transfer level. This basic mechanism has been confirmed by nearly two decades of experiments in solutions, crystals and polymer media with the two-level quantum model providing solid theoretical support(2). Both the Electric Field Induced Second Harmonic (EFISH) experiment and the current poled polymer technology essentially depend on the existence and magnitude of a strong ground state dipole μ contributing to the μ.E coupling potential between individual molecules and the externally applied de poling field E.