以昇陳

Single-layer phosphorescent organic light-emitting diodes (SL-PhOLEDs) represent ideal devices consisting only of the electrodes and the emissive layer. However, in these simplified devices, the injection/transport/recombination of charges should be ensured by the emissive layer and more particularly by the host material. Herein, the different molecular design strategies are analyzed, which have been used to construct high-efficiency hosts for SL-PhOLED.

Abstract

Thanks to the tremendous effort over the last 20 years, phosphorescent organic light-emitting diodes (PhOLEDs) represent a prevalent technology. In this technology, all the high-efficiency PhOLEDs are multi-layer devices constituting, in addition to the emissive layer (EML), of a stack of functional organic layers. These layers play a crucial role in the device performance as they improve the injection, transport, and recombination of charges within the EML. Single-layer PhOLEDs (SL-PhOLEDs) represent ideal OLEDs, consisting only of the electrodes and the EML. However, reaching high-performance SL-PhOLED is far from easy, as removing the functional layers of an OLED stack dramatically decreases the performance. To achieve high SL-PhOLED efficiency, the efficient injection, transport, and recombination of charges should be insured by the EML, and particularly, by the host material. In the present exhaustive review, the different molecular design strategies are analyzed, which have been used to construct high-efficiency hosts for SL-PhOLED. The impact of the electronic properties (triplet energy, HOMO/LUMO energy, mobility etc.) on the device characteristics (threshold voltage, electroluminescent spectrum, external quantum efficiency, etc.) are discussed. This allows to draw a structure/properties/device performance relationship map of interest for the future design of functional materials for SL-PhOLEDs.

New tetradentate Pt(II) complexes composed of adamantyl spacing group are developed. Without a spacing group, the emission spectrum can be largely changed in solution state or highly doped thin film. However, the Pt(II) complexes with a spacing group retain its own blue emission even in solution state. Highly efficient organic light-emitting diodes with external quantum efficiency of 22.6% and Commission International de L'Eclairage y of 0.122 are reported.

Abstract

Tetradentate Pt(II) complexes are promising emitters for deep blue organic light-emitting diodes (OLEDs) due to their emission energy and high photoluminescence efficiency. However, to obtain a pure blue color, spectral red-shifts, and additional emission peaks at longer wavelengths, originating from strong intermolecular interactions between parallel Pt(II) complexes, must be avoided. Herein, a new class of deep-blue emitting tetradentate Pt(II) complexes consisting of a non-planar ligand and a bulky adamantyl group is reported. The six-membered metallacycle structure renders the Pt(II) complex non-planar. In addition, the bulky adamantyl groups increase intermolecular distances and decrease red-shifts in the emission originating from strong dipole–dipole interactions. Therefore, these Pt(II) complexes exhibit little change in emission color with increasing dopant concentration. OLEDs incorporating these new Pt(II) complexes as emitters exhibit deep blue emission with a Commission International de L'Eclairage (CIE) y under 0.13 and a maximum external quantum efficiency of 22.6%, which is one of the highest observed for deep blue (CIE y < 0.15) phosphorescent OLEDs using Pt(II) complexes. These results provide a new approach for designing Pt(II) complexes for high efficiency deep blue OLEDs.

Nature Chemistry, Published online: 08 April 2021; doi:10.1038/s41557-021-00665-7

Nature Chemistry, Published online: 08 April 2021; doi:10.1038/s41557-021-00665-7

Nature Photonics, Published online: 08 April 2021; doi:10.1038/s41566-021-00786-y

A new series of conjugated molecules with spatially 2D sidechains are designed and utilized as the non-fullerene acceptors in all-small-molecule organic solar cells. The multi-dimensional lamellar packing induced by the orthogonal sidechains is able to tune the morphology as effective as the stacking of conjugated backbones, thus providing an impressive power conversion efficiency of 15.67%.

Abstract

Organic semiconductors consist of a conjugated backbone and flexible sidechains. Compared to the meticulous design of backbones, less attention has been paid to the investigation of sidechains, in particular their spatial orientation. Herein, three non-fullerene acceptors, anti-PDFC, syn-PDFC, and PDFC-Ph, are applied in all-small-molecule organic solar cells (ASM-OSCs) to reveal the varied effects of sidechains on morphology and device performance. With spatially orthogonal alkyl chains, anti-PDFC and syn-PDFC show unique bimodal lamellar packing and moderate crystallinity. When blending with an efficient binary BTR-Cl/Y6 system, anti-PDFC as well as syn-PDFC not only form their own crystal phase but also improve the packing order of BTR-Cl, consequently enhancing the power conversion efficiency (PCE) of ternary ASM-OSC to be 14.56%. However, although PDFC-Ph has an identical backbone with anti-PDFC, the alternated sidechains make it relatively amorphous, which is prone to damage the original packing of the host donor/acceptor, and thus deteriorating the device performance. When PC₇₁BM is added to optimize the morphology further, the triple-acceptor device involving anti-PDFC realizes a PCE of 15.67%, which is among the best efficiencies in ASM-OSCs. This study demonstrates that a multi-dimensional sidechain can optimize the morphology of a bulk heterojunction as effective as a conjugated backbone.

The significant advances in efficient photoabsorbent materials have been instrumental in the performance enhancement of semitransparent organic solar cells (ST‐OSCs) from <7% to 12–14% (with good visible transmittance) only in the last 3 years. This study reviews the progress of photoabsorbent materials for ST‐OSCs, and discusses the structure–property relationships and future perspectives for the development of multifunctional ST‐OSCs.

Abstract

Semi‐transparent organic solar cells (ST‐OSCs) have revolutionized the field of photovoltaics (PVs) due to their unique abilities, such as transparency and color tunability, and have transformed normal power‐harvesting OSC devices into multifunctional devices, such as building‐integrated photovoltaics, agrivoltaics, floating photovoltaics, and wearable electronics. Very recently, ST‐OSCs have seen remarkable progress, with a rapid increase in power conversion efficiency from below 7% to 12–14%, with an average visible transparency of 9–25%, especially due to the use of low bandgap semiconductors including polymer donors and non‐fullerene acceptors that exhibit absorption in the near‐infrared region as photoabsorbent materials. From this perspective, the latest developments in ST‐OSCs stemming from the innovations in photovoltaic materials that delivered multifunctional ST‐OSCs with top‐of‐the‐line power conversion efficiencies are discussed to shed light on the structure‐property relationship between molecular design and current challenges in this cutting‐edge research field. Finally, personal perspectives, including research directions for the future use of ST‐OSCs in multifunctional applications, are also proposed.

Publication date: July 2021

Source: Nano Energy, Volume 85

Author(s): Tanya Kumari, Jiyeon Oh, Sang Myeon Lee, Mingyu Jeong, Jungho Lee, Byongkyu Lee, So-Huei Kang, Changduk Yang

Two new orange–red thermally activated delayed fluorescence (TADF) materials, PzTDBA and PzDBA, are developed based on the acceptor–donor–acceptor configuration. The TADF devices fabricated with 5 wt% PzTDBA and PzDBA as emitting dopants show maximum external quantum efficiency (EQE) of 30.3% and 21.8% with extremely low roll‐off of 3.6% and 3.2% at 1000 cd m⁻². The high efficiency and low roll‐off is due to the high photoluminescence quantum yield (PLQY) and short delayed exciton lifetime.

Abstract

Two new orange–red thermally activated delayed fluorescence (TADF) materials, PzTDBA and PzDBA, are reported. These materials are designed based on the acceptor–donor–acceptor (A–D–A) configuration, containing rigid boron acceptors and dihydrophenazine donor moieties. These materials exhibit a small ΔE _ST of 0.05–0.06 eV, photoluminescence quantum yield (PLQY) as high as near unity, and short delayed exciton lifetime (τ_d) of less than 2.63 µs in 5 wt% doped film. Further, these materials show a high reverse intersystem crossing rate (k _risc) on the order of 10⁶ s⁻¹. The TADF devices fabricated with 5 wt% PzTDBA and PzDBA as emitting dopants show maximum EQE of 30.3% and 21.8% with extremely low roll‐off of 3.6% and 3.2% at 1000 cd m⁻² and electroluminescence (EL) maxima at 576 nm and 595 nm, respectively. The low roll‐off character of these materials is analyzed by using a roll‐off model and the exciton annihilation quenching rates are found to be suppressed by the fast k _risc and short delayed exciton lifetime. These devices show operating device lifetimes (LT₅₀) of 159 and 193 h at 1000 cd m⁻² for PzTDBA and PzDBA, respectively. The high efficiency and low roll‐off of these materials are attributed to the good electronic properties originatng from the A–D–A molecular configuration.