Suna Choi

Thermally activated delayed fluorescence blue dopants and hosts: from the design strategy to organic light-emitting diode applications

Thermally activated delayed fluorescence (TADF) materials have attracted much attention in the field of organic light-emitting diodes (OLEDs), with their state-of-the-art performance in terms of external quantum efficiencies (EQEs), turn-on voltages, and color coordinates. TADF materials exhibited EQEs above 25% due to harvesting both singlet and triplet excitons via reverse intersystem crossing (RISC). A small singlet–triplet energy gap (ΔEST) is essential for TADF materials to exhibit efficient upconversion from the lowest triplet excited state (T1) to the lowest singlet excited state (S1, T1 → S1). Moreover, these materials are purely organic and thus not costly. Therefore, the TADF approach provides the best alternative to conventional fluorescent and phosphorescent OLEDs, regarding device efficiency and cost. On the other hand, blue light-emitting devices are facing several issues related to their stability and efficiency, making their development quite challenging for researchers. Herein, we review the recent advances in the use of blue TADF dopants and hosts in OLEDs.

Perylene diimide isomers containing a simple sp3-core for non-fullerene-based polymer solar cells

In order to investigate the effect of the geometries of perylene diimide (PDI)-based small molecules, five different isomers were synthesized by using a cyclohexane core as a simple sp3-σ core. Diaminocylohexane is such an effective core for the systematic development of many kinds of isomers via geometric tuning as well as for reducing the self-aggregation tendency of PDIs. Depending on the anchoring position of the PDI units on the cyclohexane core (ortho-, meta- and para-), isomers exhibited differences in solubility and crystallinity. Among the studied isomers, ortho-substituted t-OCP was found to have a highly twisted molecular structure which minimizes the strong tendency towards crystallization due to individual PDI moieties. The unique geometrical nature of the t-OCP isomer led to the highest power conversion efficiency (PCE = 6.23%) of bulk heterojunction (BHJ) polymer solar cells (PSCs) with a higher short-circuit current density (Jsc) and fill factor (FF). It is mainly ascribed to the formation of a nanophase interpenetrating network with well-balanced carrier mobility in the blend film.

Optimized structure of silane-core containing host materials for highly efficient blue TADF OLEDs

Three new derivatives containing silane cores, viz. 9,9′,9′′-(((4-(pyridin-3-yl)phenyl)silanetriyl)tris(benzene-4,1-diyl))tris(9H-carbazole) (SiCz3Py1), bis(4-(9H-carbazol-9-yl)phenyl)bis(4-(pyridin-3-yl)phenyl)silane (SiCz2Py2), and 9-(4-(tris(4-(pyridin-3-yl)phenyl)silyl)phenyl)-9H-carbazole (SiCz1Py3), were designed and synthesized. Carbazole as a donor and pyridine as an acceptor were tethered to tetraphenylsilane at different mole ratios. All three host materials showed high glass transition temperatures between 118 and 164 °C, which are different from those of the previous silane-based host materials (e.g., diphenyldi-o-tolylsilane (UGH-1), 1,4-bis(triphenylsilyl)benzene (UGH-2), and 1,3-bis(triphenylsilyl)benzene (UGH-3)). The triplet energies of these three hosts are observed at 2.85–2.90 eV, which is high enough for them to act as blue host materials in thermally activated delayed fluorescence organic light emitting diodes (TADF OLEDs). In particular, SiCz2Py2 and SiCz1Py3 hosted-TADF OLEDs demonstrated excellent performances, with a maximum external quantum efficiency (EQEmax) of 18.7 and 18.8%, respectively. Such good performances of SiCz2Py2 and SiCz1Py3 are originated by suppressing the non-radiative triplet decay and high reverse intersystem crossing (RISC) rate constant for efficient triplet to singlet up-conversion. This work demonstrates that tetraphenylsilane is a promising non-conjugate (i.e., sp3-hybridized) linkage core for developing a variety of high Tg host materials, particularly for blue TADF OLEDs.

(D)n–σ–(A)m type partially conjugated block copolymer and its performance in single-component polymer solar cells

We synthesized two types of novel poly(3-alkylthiophene)-free (D)n-b-(A)m conjugated block copolymers: PTQI-block-PNDISs. PTQI-b-PNDIS is the fully conjugated block copolymer, in which the donor block is connected to the acceptor block via a π-conjugated unit. In addition, the donor block was connected to an acceptor block through a non-conjugated alkylene spacer, yielding PTQI-b-PNDISL, which is a partially conjugated block copolymer. The power conversion efficiency (PCE) was 1.54% with an open-circuit voltage of 0.79 V in the single-component polymer solar cell (PSC) based on PTQI-b-PNDISL, better performance than that of the PSC containing PTQI-b-PNDIS (PCE ∼ 0.36%). These results reveal the potential for improving the photovoltaic performance in PSCs by controlling the internal morphology with exploitation of the self-segregating behavior of p- and n-blocks in the film state.

Solution-processed thermally activated delayed fluorescence organic light-emitting diodes using a new polymeric emitter containing non-conjugated cyclohexane units

The importance of thermally activated delayed fluorescence (TADF) materials in organic light-emitting diode (OLED) applications continues to grow as a consequence of their unique properties and excellent performance. Herein, a new green-emitting TADF polymer, P(DMTRZ-Cp), for solution-processable OLEDs was designed and synthesized based on the structure of DMAC-TRZ that shows typical TADF characteristics as a small molecule. The 1,1-diphenylcyclohexane (Cp) moiety introduced into the structure of the polymeric emitter not only acts as a linker between the conjugated monomeric units, but also helps enhancing solubility while disconnecting conjugation along the polymer backbones. P(DMTRZ-Cp) exhibits obvious TADF features such as a small energy gap (0.023 eV) between its lowest singlet and triplet excited states, and obvious delayed photoluminescence (PL) decay behavior. Moreover, this new polymeric emitter exhibits high PL quantum yield over 90% in the film state. By applying solution-processed TADF-OLED devices, P(DMTRZ-Cp) exhibited the maximum external quantum efficiency of up to 15.4% with green emission. As far as we know, this is the first report to introduce a non-conjugated linker, Cp, in the main-chain type polymeric emitter structure that presents TADF characteristics. The Cp linker is considered to be a promising moiety for the development of solution-processable polymeric emitters with high efficiencies.

A new n-type semiconducting molecule with an asymmetric indenothiophene core for a high-performing non-fullerene type organic solar cell

Herein, a new asymmetric n-type semiconducting molecule (PhITBD) with an indenothiophene core was designed, synthesized using tethering 2-(benzo[c][1,2,5]-thiadiazol-4-ylmethylene)-malononitrile (BM) as terminal groups, and applied to polymer solar cells (PSCs). The PSCs with asymmetric PhITBD displayed improved power conversion efficiencies (PCE) of 6.57% as compared to those with the symmetric molecule IDT-2BM. The higher PCE value of the PhITBD-based PSC was mainly attributed to the enhanced photovoltaic properties, the Voc, Jsc, FF induced by complementary light absorption (550–600 nm range), morphological improvement, and balanced charge carrier transport in the active layer. Due to the effective morphological control and absorption enhancement, asymmetric structured n-type-conjugated molecules are potential candidates for improving the performance of bulk heterojunction non-fullerene type PSCs.

Effect of a methyl thiophene-3-carboxylate bridge in an indacenodithiophene-based acceptor–donor–acceptor-type molecule on the performance of non-fullerene polymer solar cells

A new A-b-D-b-A-type n-type small molecule, IDT-3MT, was synthesized bearing a weak acceptor thiophene-3-carboxylate bridge (3MT = b) between indacenodithiophene as a donating core and 2-(3-oxo-2,3-dihydroinden-1-ylidene)-malononitrile as the accepting end groups. Compared to IDT-T bearing a neat thiophene bridge, IDT-3MT displayed a red-shifted absorption spectrum in the film state, which is a more effective complementary absorption behavior with PBDB-T as the donor material. The highest occupied molecular orbital and lowest unoccupied molecular orbital levels of IDT-3MT became correspondingly lower. Based on the results of grazing-incidence wide-angle X-ray scattering, the blend film of PBDB-T:IDT-3MT exhibited a more prominent face-on orientation and fine surface morphology compared with PBDB-T:IDT-T, which can facilitate charge transportation in the vertical direction. Among the two acceptors, the polymer solar cell based on a solvent additive-free as-cast PBDB-T:IDT-3MT blend film exhibited the highest power conversion efficiency of 8.40% with a high open-circuit voltage of 0.95 V and a short-circuit current density of 14.43 mA cm−2 due to the more prominent face-on orientation and favorable morphology of the blend film. According to the simple structural modification of the acceptor molecule, the ester group in the thiophene bridge played an important role toward achieving high-performance polymer solar cells.

Unconventional Three-Armed Luminogens Exhibiting Both Aggregation-Induced Emission and Thermally Activated Delayed Fluorescence Resulting in High-Performing Solution-Processed Organic Light-Emitting Diodes

In this work, three-armed luminogens IAcTr-out and IAcTr-in were synthesized and used as emitters bearing triazine and indenoacridine moieties in thermally activated delayed fluorescence organic light-emitting diodes (OLEDs). These molecules could form a uniform thin film via the solution process and also allowed the subsequent deposition of an electron transporting layer either by vacuum deposition or by an all-solution coating method. Intriguingly, the new luminogens displayed aggregation-induced emission (AIE), which is a unique photophysical phenomenon. As a nondoped emitting layer (EML), IAcTr-in showed external quantum efficiencies (EQEs) of 11.8% for the hybrid-solution processed OLED and 10.9% for the all-solution processed OLED with a low efficiency roll-off. This was evident by the higher photoluminescence quantum yield and higher rate constant of reverse intersystem crossing of IAcTr-in, as compared to IAcTr-out. These AIE luminogens were used as dopants and mixed with the well-known host material 1,3-bis( N-carbazolyl)benzene (mCP) to produce a high-efficiency OLED with a two-component EML. The maximum EQE of 17.5% was obtained when using EML with IAcTr-out doping (25 wt %) into mCP, and the OLED with EML bearing IAcTr-in and mCP showed a higher maximum EQE of 18.4% as in the case of the nondoped EML-based device.

Highly efficient bipolar host materials towards solution-processable blue and green thermally activated delayed fluorescence organic light emitting diodes

New versatile host materials with good solubility in common organic solvents are greatly desired to promote solution-processable thermally activated delayed fluorescence organic light-emitting diodes. This paper describes the design and synthesis of two new host materials, namely 3′-(9H-carbazol-9-yl)-[1,1′-biphenyl]-3,5-diylbis(diphenylphosphine oxide) (CDPO) and 3′,5′-di-(9H-carbazol-9-yl)-[1,1′-biphenyl]-3,5-diylbis (diphenylphosphine oxide) (mCPDPO), featuring hole-transporting carbazole and electron-transporting diphenylphosphine oxide (DPO) entities. The donor strength was varied to a constant n-type DPO core to tune the functional properties. The detailed studies proved that the resulting two new materials exhibit excellent solubility in common organic solvents, high triplet energy (>2.80 eV), high glass transition temperature (up to 133 °C), and good bipolar electronic nature. Consequently, both compounds were used as hosts in solution-processable blue and green thermally activated delayed fluorescence (TADF) OLEDs. In particular, the devices using mCPDPO as a host in the emissive layer showed outstanding performance with a maximum current efficiency, power efficiency, and external quantum efficiency of 35.3 cd A−1, 17.0 lm W−1 and 15.4% in the blue-emitting diode, and 61.5 cd A−1, 29.7 lm W−1 and 18.8% in the green-emitting diode, respectively. These results corroborated the potential of carbazole-phosphine oxide derivatives as host materials in solution-processable blue and green TADF OLEDs.