以昇陳

Asymmetry and halogenation are employed to design a fused-ring non-fullerene electron acceptor, and demonstrate the synergistic effect of tuning optoelectronic properties and enhancing molecular stacking, leading to the highest device efficiency.

Abstract

Fused-ring non-fullerene electron acceptors (NFAs) boost the power conversion efficiencies (PCEs) of organic solar cells (OSCs). Asymmetric and halogenated NFAs have drawn increasing attention in recent years due to their unique optoelectronic properties. Starting from the symmetric NFA ITCC-M, this work systematically designs and synthesizes an asymmetric counterpart ITCC-M-2F, halogenated counterpart ITCC-Cl, and asymmetric and halogenated counterpart IDTT-Cl-2F. Among these NFAs, IDTT-Cl-2F shows the shallowest lowest unoccupied molecular orbital energy level, broader absorption range, and the tightest molecular packing. As a result, when blended with the donor PBDB-T-2Cl, IDTT-Cl-2F-based OSCs yield the highest PCE of 13.3% with an open-circuit voltage of 0.96 V, short-circuit current of 19.20 mA cm^–2, and fill factor of 71.1%, which is the highest PCE of OSCs employing 2-(2-chloro-6-oxo-5,6-dihydro-4H-cyclopenta[b]thiophen-4-ylidene) malononitrile (ClIC) unit terminated NFA. The results demonstrate the synergistic effect of asymmetry and halogenation toward tuning of the optoelectronic properties of NFAs for high performance OSCs.

All thermally activated delayed fluorescence (TADF) single-emitting-layer white organic light-emitting diodes are developed by using a high-efficiency orange–red TADF fluorophor doped in a blue TADF fluorophor. Singlet and triplet excitons in the devices can be well shared and captured in two emitters, resulting in state-of-the-art performances with maximum external quantum efficiencies of over 30%.

Abstract

While monochrome organic light-emitting diodes (OLEDs) based on thermally activated delayed fluorescence (TADF) emitters have achieved over 30% external quantum efficiencies (EQEs), all-TADF white OLEDs (WOLEDs) are still lagging behind. Herein, a simple system based on two color-complementary TADF emitters is exploited to realize high-performance WOLEDs. By doping a high-performance orange–red TADF fluorophor (BPPZ-DPXZ) into a blue TADF host (DBFCz-Trz), energy transfer, and triplet-to-singlet conversion in the host-dopant system can be optimized to simultaneously achieve full exciton utilization and color balance. With this design, all-TADF single-emitting-layer WOLEDs with a maximum EQE up to 32.8% are demonstrated. This high efficiency surpasses EQEs of reported WOLEDs based on both TADF as well as phosphorescence. It is expected that this finding can provide new insight for designing highly efficient all-TADF WOLEDs.

Two simple pure organic molecules (namely p-NN-Br and m-NN-Br) display ultralong organic phosphorescence (UOP), and exhibit dual excited state emission properties. They are found to display distinct responses toward multiple external stimuli including temperature (T), excitation light intensity (I) and pressure (P), and show multicolor emission (orange-yellow to blue (including the white light, CIE = 0.33, 0.34).

Abstract

A simple bromine and cyanogen substituents positional isomeric 9-phenyl-9H-carbazole (PhCz) donor–acceptor system (namely p-NN-Br and m-NN-Br) displays ultralong organic phosphorescence, and exhibits unique dual emission properties (fluorescence and phosphorescence). These molecules are found to display distinct responses toward multiple external stimuli including temperature (T), excitation light intensity (I) and pressure (P), and show multicolor tunable behaviors (including the white light, the Commission Internationale d'Eclairage (CIE) = 0.33, 0.34). The unique stimuli-triggered proportion between singlet and triplet excitons for p-NN-Br and m-NN-Br is demonstrated systematically by investigating the photophysical spectrum, scanning electron microscope (SEM) imaging and X-ray analysis, coupled with theoretical calculations. They reveal that the simultaneous introduction of halogens (Br) and pseudohalogens (CN) to the PhCz skeleton can improve the intermolecular interaction and thermal stability. Single crystal analysis shows that there are many more types of dimers and J-aggregates, thereby stabilizing the excitation of triplet states. Moreover, these isomers have “latent” fingerprint recognition and anti-background interference performance, which is expected to provide a new method for fingerprint identification. All in all, this strategy paves the way to a multifunctional platform for the development of multi-stimuli responsive, multicolor regulation and smart luminescent materials with long-lived emission at room temperature.

Ultra-deep-blue aggregation-induced delayed fluorescence emitters (TB-tCz and TB-tPCz) bearing organoboron-based core and carbazole derivatives are developed. Solution-processed nondoped organic light-emitting diodes (OLEDs) based on TB-tPCz exhibit a record external quantum efficiency of 15.8% with Commission international de l'éclairage color coordinates of (0.16, 0.05). Furthermore, both emitters also exhibit excellent device performances in solution-processed doped OLEDs.

Abstract

Ultra-deep-blue aggregation-induced delayed fluorescence (AIDF) emitters (TB-tCz and TB-tPCz) bearing organoboron-based cores as acceptors and 3,6-substituted carbazoles as donors are presented. The thermally activated delayed fluorescence (TADF) properties of the two emitters are confirmed by theoretical calculations and time-resolved photoluminescence experiments. TB-tCz and TB-tPCz exhibit fast reverse intersystem crossing rate constants owing to efficient spin–orbit coupling between the singlet and triplet states. When applied in solution-processed organic light-emitting diodes (OLEDs), the TB-tCz- and TB-tPCz-based nondoped devices exhibit ultra-deep-blue emissions of 416–428 nm and high color purity owing to their narrow bandwidths of 42.2–44.4 nm, corresponding to the Commission International de l´Eclairage color coordinates of (x = 0.16–0.17, y = 0.05–0.06). They show a maximum external quantum efficiency (EQE_max) of 8.21% and 15.8%, respectively, exhibiting an unprecedented high performance in solution-processed deep-blue TADF-OLEDs. Furthermore, both emitters exhibit excellent device performances (EQE_max = 14.1–15.9%) and color purity in solution-processed doped OLEDs. The current study provides an AIDF emitter design strategy to implement high-efficiency deep-blue OLEDs in the future.

Newly synthesized hexabenzocoronene (HBC)–osmapentalyne complexes that combine fragments of graphene and metalla-aromatics are emerging as cathode interlayer materials. Further extending the d_π –p_π conjugated systems of osmapentalynes, the most successful complex, in this work, HBC-S is found to boost the efficiency of non-fullerene solar cells to over 18%.

Abstract

Interface engineering is a critical method by which to efficiently enhance the photovoltaic performance of nonfullerene solar cells (NFSC). Herein, a series of metal–nanographene-containing large transition metal involving d_π –p_π conjugated systems by way of the addition reactions of osmapentalynes and p-diethynyl-hexabenzocoronenes is reported. Conjugated extensions are engineered to optimize the π-conjugation of these metal–nanographene molecules, which serve as alcohol-soluble cathode interlayer (CIL) materials. Upon extension of the π-conjugation, the power conversion efficiency (PCE) of PM6:BTP-eC9-based NFSCs increases from 16% to over 18%, giving the highest recorded PCE. It is deduced by X-ray crystallographic analysis, interfacial contact methods, morphology characterization, and carrier dynamics that modification of hexabenzocoronenes-styryl can effectively improve the short-circuit current density (J _sc) and fill factor of organic solar cells (OSCs), mainly due to the strong and ordered charge transfer, more matching energy level alignments, better interfacial contacts between the active layer and the electrodes, and regulated morphology. Consequently, the carrier transport is largely facilitated, and the carrier recombination is simultaneously impeded. These new CIL materials are broadly able to enhance the photovoltaic properties of OSCs in other systems, which provides a promising potential to serve as CILs for higher-quality OSCs.

Nature Energy, Published online: 07 June 2021; doi:10.1038/s41560-021-00843-4

A “two-in-one” strategy is applied to form an acceptor alloy for fine-tuning the donor/acceptor energy alignment and blend morphology. Enhanced hole transfer and suppressed charge recombination in the alloy acceptor consisting of AQx-3 and Y6 enable a power conversion efficiency of over 18%, which is the highest documented for ternary organic solar cells utilizing two nonfullerene acceptors.

Abstract

The trade-off between the open-circuit voltage (V _oc) and short-circuit current density (J _sc) has become the core of current organic photovoltaic research, and realizing the minimum energy offsets that can guarantee effective charge generation is strongly desired for high-performance systems. Herein, a high-performance ternary solar cell with a power conversion efficiency of over 18% using a large-bandgap polymer donor, PM6, and a small-bandgap alloy acceptor containing two structurally similar nonfullerene acceptors (Y6 and AQx-3) is reported. This system can take full advantage of solar irradiation and forms a favorable morphology. By varying the ratio of the two acceptors, delicate regulation of the energy levels of the alloy acceptor is achieved, thereby affecting the charge dynamics in the devices. The optimal ternary device exhibits more efficient hole transfer and exciton separation than the PM6:AQx-3-based system and reduced energy loss compared with the PM6:Y6-based system, contributing to better performance. Such a “two-in-one” alloy strategy, which synergizes two highly compatible acceptors, provides a promising path for boosting the photovoltaic performance of devices.

The involvement of guanidinium in perovskite bulk film and CH₃O-PEABr passivation on the perovskite surface synergistically suppresses the trap states. The charge carrier lifetimes of perovskite films increase by tenfold and fivefold to 981 ns and 8.02 µs at the crystal surface and in its bulk, respectively. The decreased nonradiative recombination loss translates to a record efficiency of 40.1%.

Abstract

Perovskite solar cells exhibit not only high efficiency under full AM1.5 sunlight, but also have great potential for applications in low-light environments, such as indoors, cloudy conditions, early morning, late evening, etc. Unfortunately, their performance still suffers from severe trap-induced nonradiative recombination, particularly under low-light conditions. Here, a holistic passivation strategy is developed to reduce traps both on the surface and in the bulk of micrometer-thick perovskite film, leading to a record efficiency of 40.1% under 301.6 µW cm⁻² warm light-emitting diode (LED) light for low-light solar-cell applications. The involvement of guanidinium into the perovskite bulk film and 2-(4-methoxyphenyl)ethylamine hydrobromide (CH₃O-PEABr) passivation on the perovskite surface synergistically suppresses the trap states. The charge carrier lifetimes of the perovskite film increase by tenfold and fivefold to 981 ns and 8.02 µs at the crystal surface and in its bulk, respectively. The decreased nonradiative recombination loss translates to a high open-circuit voltage (V _oc) of 1.00 V, a high short-circuit current (J _sc) of 152.10 µA cm⁻², and a fill factor (FF) of 79.52%. Note that this performance also stands as the highest among all photovoltaics measured under indoor light illumination. This work of trap passivation for micrometer-thick perovskite film paves a way for high-performance, self-powered IoT devices.

Narrowband near-infrared organic photodiodes are reported based on a dipolar squaraine dye. J-type coupling in the solid state allows SQ-H thin films to combine favorable NIR absorption at 1040 nm and charge carrier mobility. The bulk-heterojunction with PC₆₁BM yields an organic photodiode with external quantum efficiency of 12.3% at 1050 nm with a full-width half-maximum of 85 nm under short-circuit condition.

Abstract

A highly sensitive short-wave infrared (SWIR, λ > 1000 nm) organic photodiode (OPD) is described based on a well-organized nanocrystalline bulk-heterojunction (BHJ) active layer composed of a dicyanovinyl-functionalized squaraine dye (SQ-H) donor material in combination with PC₆₁BM. Through thermal annealing, dipolar SQ-H chromophores self-assemble in a nanoscale structure with intermolecular charge transfer mediated coupling, resulting in a redshifted and narrow absorption band at 1040 nm as well as enhanced charge carrier mobility. The optimized OPD exhibits an external quantum efficiency (EQE) of 12.3% and a full-width at half-maximum of only 85 nm (815 cm⁻¹) at 1050 nm under 0 V, which is the first efficient SWIR OPD based on J-type aggregates. Photoplethysmography application for heart-rate monitoring is successfully demonstrated on flexible substrates without applying reverse bias, indicating the potential of OPDs based on short-range coupled dye aggregates for low-power operating wearable applications.

Since 2000, researchers at University of Washington have significantly contributed to the study of organic semiconductors, providing new scientific insights and breaking records. The ideas and experiences that have been gained along the way are summarized and reflected upon, and through this reflection insights are offered into future directions.

Abstract

Research at the University of Washington regarding organic semiconductors is reviewed, covering four major topics: electro-optics, organic light emitting diodes, organic field-effect transistors, and organic solar cells. Underlying principles of materials design are demonstrated along with efforts toward unlocking the full potential of organic semiconductors. Finally, opinions on future research directions are presented, with a focus on commercial competency, environmental sustainability, and scalability of organic-semiconductor-based devices.

The rate of charge transfer (CT) state dissociation in cyanine/fullerene solar cells strongly depends on the electric field but is temperature independent. CT states dissociate via a temperature-independent electron tunneling mechanism through a thin, high-energy potential barrier at the donor–acceptor interface. The results support a new mechanism for charge generation in organic solar cells via carrier tunneling from CT states.

Abstract

Charge transfer (CT) states play a key role in the functioning of organic solar cells; however, understanding the mechanism by which CT states dissociate efficiently into free charges remain a conceptual challenge. Here, the electric field dependent dynamics of charge generation in planar cyanine/fullerene photovoltaic cells is probed over a wide temperature range using time-resolved Stark effect experiments, transient absorption, and photocurrent measurements. Results indicate that dissociation of thermalized CT states is the rate-limiting step for all temperatures. The dissociation rate strongly depends on the field, but is temperature independent. The results also suggest that the yield of generated charges is temperature independent. Model electrostatic calculations illustrate that specific orientations of the cyanine crystal relative to C₆₀ create a repulsive potential for an electron near the interface that is largely due to the quadrupole moment of the unit cell. In combination with the electron-hole coulomb attraction and the electric field-induced barrier lowering, a high-energy potential barrier forms with a narrow width of a few nanometers. It is proposed that charge separation occurs via a field-dependent electron tunneling mechanism through that barrier, which is temperature independent. The results support a thus far overlooked pathway for CT state dissociation via carrier tunneling.

This work demonstrates a versatile approach by peripheral decoration of multi-resonance molecules, which enables emission spectra red-shift while remains narrowband emission. Fully decorating the parent BNCz molecules with diphenylamine gives the first narrowband yellow emissive MR emitter with an emission maxima of 562 nm. Electroluminescence devices employing these MR emitters reveal narrowband emission with high external quantum efficiency of over 24%.

Abstract

High device efficiency and color-purity are the two essentials for high-quality organic light-emitting diodes (OLEDs). Multi-resonance (MR) molecules show great potentials for high color-purity OLEDs due to their sharp emission bands. However, most MR molecules exhibit emission limited from deep-blue to green spectral region. Herein, through peripherally decorating MR emitter with electron donors, a new approach enabling the emission spectra of MR emitters red-shift while retaining narrowband emission is demonstrated. By manipulating the numbers and electron-donating abilities of the peripheries, the first narrowband yellow emitter with emission maxima of 562 nm and a full-width at half-maximum (FWHM) of 30 nm is realized. Highly efficient OLEDs with an external quantum efficiency of over 24% and excellent color purity are fabricated by employing these newly developed MR molecules as emitters.