Youyou Bian

The pressure to move towards renewable energy has inspired researchers to look for ideas in photovoltaics that may lead to a major breakthrough. Recently the use of perovskites as a light harvester has lead to stunning progress. The power conversion efficiency of perovskite solar cells is now approaching parity (>22 %) with that of the established technology which took decades to reach this level of performance. The use of a hole transport material (HTM) remains indispensable in perovskite solar cells. Perovskites can conduct holes, but they are present at low levels, and for efficient charge extraction a HTM layer is a prerequisite. Herein we provide an overview of the diverse types of HTM available, from organic to inorganic, in the hope of encouraging further research and the optimization of these materials.

Hole for whole: Semiconductor hole-transport materials (HTMs) are an essential component for perovskite solar cells. The three classes of materials available, inorganic, polymeric, and small molecule HTMs are reviewed, particularly the optoelectrical properties of molecular HTMs, which seem to be the most effective materials.

Y. Tsuchiya, F. Ito, C. Adachi, and co-workers present page 6703 that full-color-tunable OLEDs can be obtained by using a single emitter and a single emissive layer. Color tuning is achieved by controlling aggregation of the emitter and its aggregation-induced exciplex formation. The approach is not only promising for OLEDs, but all systems that require emission property management.

Efficiency roll-off in blue organic light-emitting diodes especially at high brightness still remains a vital issue for which the excitons density-dependent mechanism of host materials takes most responsibility. Additionally, the efficiency roll-off leads to high power consumption and reduces the operating lifetime because higher driving voltage and current are required. Here, by subtly modifying the triphenylamine to oxygen-bridged quasi-planar structure, a novel thermally activated delayed fluorescence type blue host Tri-o-2PO is successfully developed. Efficiency roll-off based on Tri-o-2PO is ultralow with external quantum efficiency (EQE) just dropping by around 2% in the high luminance range from 1000 cd m⁻² to 10 000 cd m⁻². As expected, low turn-on voltage (≈2.9 V) of device is also achieved, which is close to the theory limit value (≈2.62 V). Super-high power efficiency (≈60 lm W⁻¹) and EQE (>22%) are also achieved when utilizing Tri-o-2PO as host. Furthermore, two-color warm-white light with CIE of (0.45, 0.43) and correlated color temperature of 2921 K is also fabricated and a champion EQE of 21% is delivered. These excellent performances prove the strategy of bridging the triphenylamine to reduce ΔE_st is validated and suggest the great potential of this novel skeleton.

A blue thermally activated delayed fluorescence material, Tri-o-2PO, is developed and implemented as the host for organic light-emitting diodes (OLEDs). Efficiency roll-off based on Tri-o-2PO is ultralow with external quantum efficiency (EQE) dropping by only around 2% in the high luminance range (1000 to  10 000 cd m⁻²). Additionally, low turn-on voltages (≈2.9 V) and high power efficiency (≈60 lm W⁻¹) are achieved simutaneously.

The excited state properties of organic fluorescent materials are crucial for their photoelectronic performance. Here, a study on the highly efficient electrofluorescent material 4-(2-(4′-(diphenylamino)-[1,1′-biphenyl]-4-yl)-1H-phenanthro[9,10-d]imidazol-1-yl)benzonitrile (TBPMCN) is carried out, focusing mainly on its crystal structure and photophysical properties under pressure stimulation. The special triangular-cone (TC) configuration of triphenylamine group in TBPMCN crystal exhibits charge-transfer (CT)-dominated excited state property in TBPMCN, which gives rise to a blue-shifted emission in the crystal. Theoretical calculations prove that the TC conformation is a dynamically metastable state, which is higher in energy than the three-blade-propeller (TBP) configuration. In a further piezochromic experiment, a unique rehybridization-induced emission enhancement phenomenon is found in this crystal, which is essentially different from the aggregation-induced emission enhancement (AIEE) mechanism. It can be assigned to the change of excited state property from a CT-dominated state to a hybridized locally excited and charge-transfer state, as a result of the rehybridization of nitrogen atom upon the increased external pressure. This work provides deep insight into the relationship between molecular structure and excited state properties in crystal by means of the pressure stimulation and further enriches the AIEE mechanism. Additionally, the large red-shifted piezochromic phenomenon of this CT material is stressed.

A triangular-cone confirmation caused by a special stacking pattern is found in the crystal of blue-emissive material 4-(2-(4′-(diphenylamino)-[1,1′-biphenyl]-4-yl)-1H-phenanthro[9,10-d]imidazol-1-yl)benzonitrile. Under increasing external pressure, a unique rehybridization-induced emission enhancement phenomenon that can be assigned to the pressure-induced N atom rehybridization of the triphenylamine group is observed.

A series of twisted D–π–A type emitters based on the acridine donor unit and CN-substituted pyridine, pyrimidine, and benzene acceptor units are studied. They not only allow one to systematically probe the influence of different acceptor strengths, but also permit one to intriguingly probe the influence of tunable conformations (twist angles) within the acceptor moieties through controlling the orientation of asymmetric heteroaromatic ring relative to the donor component. Intramolecular charge-transfer transitions are observed in all these compounds and emission wavelengths are widely tunable from deep blue to yellow not only by the general acceptor strength due to the characters of heteroarene and CN-substitution pattern but also by the subtle control of in-acceptor conformation (twist angles). Small triplet-to-singlet energy gaps (ΔE_ST) and significant thermally activated delayed fluorescence (TADF) characteristics are obtained in a series of D–π–A compounds with sufficient acceptor strengths and tunable in-acceptor conformation, yielding a series of efficient blue-green to yellow TADF emitters with promisingly high photoluminescence quantum yields of 90%–100%. Highly efficient blue-green to yellow TADF organic light-emitting diodes (OLEDs) having external quantum efficiencies of up to 23.1%–31.3% are achieved using these efficient TADF emitters, which are among the most efficient TADF OLEDs ever reported.

Efficient blue-green to yellow thermally activated delayed fluorescence emitters capable of generating 23%–31% electroluminescence external quantum efficiencies are developed adopting the acridine donor unit and cyano (CN)-substituted pyridine and pyrimidine acceptor units. They permit systematic probing of influences of acceptor strengths and tunable conformations (twist angles) within the acceptor moieties through controlling the orientation of asymmetric heteroaromatic ring.

Well-defined small molecule (SM) donors can be used as alternatives to π-conjugated polymers in bulk-heterojunction (BHJ) solar cells with fullerene acceptors (e.g., PC₆₁/₇₁BM). Taking advantage of their synthetic tunability, combinations of various donor and acceptor motifs can lead to a wide range of optical, electronic, and self-assembling properties that, in turn, may impact material performance in BHJ solar cells. In this report, it is shown that changing the sequence of donor and acceptor units along the π-extended backbone of benzo[1,2-b:4,5-b′]dithiophene–6,7-difluoroquinoxaline SM donors critically impacts (i) molecular packing, (ii) propensity to order and preferential aggregate orientations in thin-films, and (iii) charge transport in BHJ solar cells. In these systems (SM1-3), it is found that 6,7-difluoroquinoxaline ([2F]Q) motifs directly appended to the central benzo[1,2-b:4,5-b′]dithiophene (BDT) unit yield a lower-bandgap analogue (SM1) with favorable molecular packing and aggregation patterns in thin films, and optimized BHJ solar cell efficiencies of ≈6.6%. ¹H-¹H DQ-SQ NMR analyses indicate that SM1 and its counterpart with [2F]Q motifs substituted as end-group SM3 possess distinct self-assembly patterns, correlating with the significant charge transport and BHJ device efficiency differences observed for the two analogous SM donors (avg. 6.3% vs 2.0%, respectively).

Changing the sequence of donor and acceptor units along the π-extended backbone of benzo[1,2-b:4,5-b′]dithiophene–6,7-difluoroquinoxaline small molecule (SM) donors critically impacts (i) molecular packing, (ii) propensity to order and preferential aggregate orientations in thin-films, and (iii) charge transport in bulk-heterojunction (BHJ) solar cells. The lower-bandgap analogue (SM1) achieves distinct local packing and aggregation patterns in thin films, and optimized BHJ solar cell efficiencies of ≈6.6%.

A new type of organic light-emitting diode (OLED) has emerged that shows enhanced operational stability and large internal quantum efficiency approaching 100%, which is based on thermally activated delayed fluorescence (TADF) compounds doped with fluorescent emitters. Magneto-electroluminescence (MEL) in such TADF-based OLEDs and magneto-photoluminescence (MPL) in thin films based on donor–acceptor (D–A) exciplexes doped with fluorescent emitters with various concentrations are investigated. It has been found that both MEL and MPL responses are thermally activated with substantially lower activation energy compared to that in the pristine undoped D–A exciplex host blend. In addition, both MPL and MEL steeply decrease with the emitter's concentration. This indicates the existence of a loss mechanism, whereby the triplet charge-transfer state in the exciplex host blend may directly decay to the lowest, nonemissive triplet state of the fluorescent emitter molecules.

Using magneto-electroluminesence in organic light-emitting diodes of donor–acceptor exciplex compounds doped with fluorescence emitters, we found that the activation energy of the reversed intersystem crossing process is substantially reduced compared with that of the undoped compound. At lower dopant concentration, the rapid Förster energy transfer enhances the electro-luminesence, whereas at higher dopant concentration Dexter energy transfer interferes, creating a loss mechanism.

Here, a comprehensive photophysical investigation of a the emitter molecule DPTZ-DBTO2, showing thermally activated delayed fluorescence (TADF), with near-orthogonal electron donor (D) and acceptor (A) units is reported. It is shown that DPTZ-DBTO2 has minimal singlet–triplet energy splitting due to its near-rigid molecular geometry. However, the electronic coupling between the local triplet (³LE) and the charge transfer states, singlet and triplet, (¹CT, ³CT), and the effect of dynamic rocking of the D–A units about the orthogonal geometry are crucial for efficient TADF to be achieved. In solvents with low polarity, the guest emissive singlet ¹CT state couples directly to the near-degenerate ³LE, efficiently harvesting the triplet states by a spin orbit coupling charge transfer mechanism (SOCT). However, in solvents with higher polarity the emissive CT state in DPTZ-DBTO2 shifts below (the static) ³LE, leading to decreased TADF efficiencies. The relatively large energy difference between the ¹CT and ³LE states and the extremely low efficiency of the ¹CT to ³CT hyperfine coupling is responsible for the reduction in TADF efficiency. Both the electronic coupling between ¹CT and ³LE, and the (dynamic) orientation of the D–A units are thus critical elements that dictate reverse intersystem crossing processes and thus high efficiency in TADF.

The crucial step in the thermally activated delayed fluorescence (TADF) mechanism is the reverse intersystem crossing that converts triplet to singlet states. Here we show that this is mediated by a spin orbit charge transfer (CT) mechanism between the CT manifold and a local triplet state, and requires dynamic rocking about the D-A bond. This explains how molecular geometry and environment influences TADF and the photophysics of D-A-D molecules.

Luminescent materials consisting of boron clusters, such as carboranes, have attracted immense interest in recent years. In this study, luminescent organic–inorganic conjugated systems based on o-carboranes directly bonded to electron-donating and electron-accepting π-conjugated units were elaborated as novel optoelectronic materials. These o-carborane derivatives simultaneously possessed aggregation-induced emission (AIE) and thermally activated delayed fluorescence (TADF) capabilities, and showed strong yellow-to-red emissions with high photoluminescence quantum efficiencies of up to 97 % in their aggregated states or in solid neat films. Organic light-emitting diodes utilizing these o-carborane derivatives as a nondoped emission layer exhibited maximum external electroluminescence quantum efficiencies as high as 11 %, originating from TADF.

Two sides of the same coin: Organic–inorganic hybrid molecules consisting of an o-carborane tethered with electron donor and acceptor π-conjugated units exhibit efficient photoluminescence and electroluminescence based on aggregation-induced delayed fluorescence (AIDF).

Much effort has been devoted to developing highly efficient organic light-emitting diodes (OLEDs) that function through phosphorescence or thermally activated delayed fluorescence (TADF). However, efficient host materials for blue TADF and phosphorescent guest emitters are limited because of their requirement of high triplet energy levels. Herein, we report the rigid acceptor unit benzimidazobenzothiazole (BID-BT), which is suitable for use in bipolar hosts in blue OLEDs. The designed host materials, based on BID-BT, possess high triplet energy and bipolar carrier transport ability. Both blue TADF and phosphorescent OLEDs containing BID-BT-based derivatives exhibit external quantum efficiencies as high as 20 %, indicating that these hosts allow efficient triplet exciton confinement appropriate for blue TADF and phosphorescent guest emitters.

Gotta catch them all: Over 20 % external quantum efficiencies were achieved in both blue thermally activated delayed fluorescence (TADF) and phosphorescent organic light-emitting diodes (OLEDs) using benzimidazobenzothiazole bipolar host materials.

A new family of thermally activated delayed fluorescence (TADF) emitters based on U-shaped D-A-D architecture with a novel accepting unit has been developed. All investigated compounds have small singlet-triplet energy splitting (ΔE_ST) ranging from 0.02 to 0.20 eV and showed efficient TADF properties. The lowest triplet state of the acceptor unit plays the key role in the TADF mechanism. OLEDs fabricated with these TADF emitters achieved excellent efficiencies up to 16 % external quantum efficiency (EQE).

Photophysics: A series of U-shaped donor–acceptor–donor emissive compounds based on the electron-accepting unit dibenzo[a,j]phenazine has been developed. Static and dynamic photophysical investigations of these compounds revealed their detailed thermally activated delayed fluorescence properties. The external quantum efficiency of the organic light-emitting diodes fabricated with the new materials reached values up to 16 %.

The electron positive boron atom usually does not contribute to the frontier orbitals for several lower-lying electronic transitions, and thus is ideal to serve as a hub for the spiro linker of light-emitting molecules, such that the electron donor (HOMO) and acceptor (LUMO) moieties can be spatially separated with orthogonal orientation. On this basis, we prepared a series of novel boron complexes bearing electron deficient pyridyl pyrrolide and electron donating phenylcarbazolyl fragments or triphenylamine. The new boron complexes show strong solvent-polarity dependent charge-transfer emission accompanied by a small, non-negligible normal emission. The slim orbital overlap between HOMO and LUMO and hence the lack of electron correlation lead to a significant reduction of the energy gap between the lowest lying singlet and triplet excited states (ΔE_T-S) and thereby the generation of thermally activated delay fluorescence (TADF).

Reducing the gap: Using a boron atom as the spiro linker between an electron-deficient pyridyl pyrrolide and an electron-donating phenylcarbazolyl or triphenylamine fragment, boron complexes with a narrow HOMO–LUMO orbital overlap, small singlet–triplet energy gap (down to 38 meV), and strong thermally activated delayed fluorescence (TADF) were prepared. For the first time boron-complex-based OLEDs show a significant TADF contribution.

We demonstrate that polymer electron acceptors with excellent all-polymer solar-cell (all-PSC) device performance can be developed from polymer electron donors by using BN units. By alleviating the steric hindrance effect of the bulky pendant moieties on the conjugated polymers that contain BN units, the π–π stacking distance of polymer backbones is decreased and the electron mobility is consequently enhanced by nearly two orders of magnitude. As a result, the power conversion efficiency of all-PSCs with the polymer acting as the electron acceptor is greatly improved from 0.12 % to 5.04 %. This PCE value is comparable to that of the best all-PSCs with state-of-the-art polymer acceptors.

From giver to taker: Incorporation of BN units into polymer electron donors has resulted in a series of polymer electron acceptors. Extending the length of the repeating units of the conjugated polymers alleviates the effect of steric hindrance from the pendant groups and promotes the π–π stacking of the polymer backbones. The all-polymer solar-cell device shows a power conversion efficiency (PCE) exceeding 5.0 %.

Three NIR-emitting neutral Ir^III complexes [Ir(iqbt)₂(dpm)] (1), [Ir(iqbt)₂(tta)] (2), and [Ir(iqbt)₂(dtdk)] (3) based on the 1-(benzo[b]thiophen-2-yl)-isoquinolinate (iqtb) were synthesized and characterized (dpm=2,2,6,6-tetramethyl-3,5-heptanedionate; tta=2-thienoyltrifluoroacetonate; dtdk=1,3-di(thiophen-2-yl)propane-1,3-dionate). The compounds emit between λ=680 and 850 nm with high luminescence quantum yields (up to 16 %). By combining electrochemistry, photophysical measurements, and computational modelling, the relationship between the structure, energy levels, and properties were investigated. NIR-emitting, solution-processed phosphorescent organic light-emitting devices (PHOLEDs) were fabricated using the complexes. The devices show remarkable external quantum efficiencies (above 3 % with 1) with negligible efficiency roll-off values, exceeding the highest reported values for solution-processible NIR emitters.

Let's glow: Heteroleptic benzo[b]thiophenyl isoquinolinate Ir^III complexes with diketonate ancillary ligands of increasing conjugation were prepared. The compounds are NIR emissive with high emission quantum yields (up to 16 %). A solution-processed OLED fabricated with one of the complexes demonstrates a remarkable external quantum efficiency (EQE) of more than 3 % with negligible efficiency roll-off.

High-efficiency all-polymer solar cells with less thickness-dependent behavior are demonstrated by using a low bandgap n-type conjugated polymer N2200 as acceptor and an absorption-complementary difluorobenzotriazole-based medium-bandgap polymer J51 as donor.