Recent research efforts on solution-processed semitransparent organic solar cells (OSCs) are presented. Essential properties of organic donor:acceptor bulk heterojunction blends and electrode materials, required for the combination of simultaneous high power conversion efficiency (PCE) and average visible transmittance of photovoltaic devices, are presented from the materials science and device engineering points of view. Aspects of optical perception, charge generation-recombination, and extraction processes relevant for semitransparent OSCs are also discussed in detail. Furthermore, the theoretical limits of PCE for fully transparent OSCs, compared to the performance of the best reported semitransparent OSCs, and options for further optimization are discussed.

Solution-Processed Semitransparent Organic Photovoltaics: From Molecular Design to Device Performance

In the last decade, perovskite solar cells have been considered a promising and burgeoning technology for solar energy conversion with a power conversion efficiency currently exceeding 24%. However, although perovskite solar cells have achieved high power conversion efficiency, there are still several challenges limiting their industrial realization. The actual bottleneck for real uptake in the market still remains the cost-ineffective components and instability, to which doping-induced degradation of charge selective layers may contribute significantly. This article overviews the highest performance molecular and polymeric doped and dopant-free HTMs, showing how small changes in the molecular structure such as different atoms and different functional groups and changes in substitution positions or the length of the π-conjugated systems can affect photovoltaic performance and long-term stability of perovskite solar cells.

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Efficiency vs. stability: dopant-free hole transporting materials towards stabilized perovskite solar cells

Purely organic TADF emitters have been rapidly developed in recent years and their TADF has become one of the most promising achievement towards highly efficient OLEDs. Apart from the advantages such as stable structure, low-cost and high internal quantum efficiency of these emitters that have been found, the demand for high colour purity has become a new challenge. There have been many reports that show possibilities of designing narrowband TADF emitters using totally different methods. Recently, TADF emitters have exhibited a narrow emission with a FWHM of below 20 nm. Therefore, broad emission spectra and bad colour purity shall not always be considered as fatal defects of TADF emitters. Herein, a review of narrowband TADF emitters is presented with a focus on the molecular design, photophysical properties and performance of the corresponding OLED devices. Besides, this review will exhibit the common characteristics of emitters for the purpose of further designs.

Recent progress of narrowband TADF emitters and their applications in OLEDs

Purely Organic Materials with Aggregation-Induced Delayed Fluorescence for Efficient Nondoped OLEDs

Organic light emitting diodes (OLEDs) have been well known for their potential usage in the lighting and display industry. The device efficiency and lifetime have improved considerably in the last three decades. However, for commercial applications, operational lifetime still lies as one of the looming challenges. In this review paper, an in-depth description of the various factors which affect OLED lifetime, and the related solutions is attempted to be consolidated. Notably, all the known intrinsic and extrinsic degradation phenomena and failure mechanisms, which include the presence of dark spot, high heat during device operation, substrate fracture, downgrading luminance, moisture attack, oxidation, corrosion, electron induced migrations, photochemical degradation, electrochemical degradation, electric breakdown, thermomechanical failures, thermal breakdown/degradation, and presence of impurities within the materials and evaporator chamber are reviewed. Light is also shed on the materials and device structures which are developed in order to obtain along with developed materials and device structures to obtain stable devices. It is believed that the theme of this report, summarizing the knowledge of mechanisms allied with OLED degradation, would be contributory in developing better-quality OLED materials and, accordingly, longer lifespan devices.

https://onlinelibrary.wiley.com/doi/pdf/10.1002/advs.202002254

Approaches for Long Lifetime Organic Light Emitting Diodes.

Hole‐Transporting Materials Incorporating Carbazole into Spiro‐Core for Highly Efficient Perovskite Solar Cells

The exploitation of universal spiro-type hosts suitable for full-color phosphors is still a bottleneck for fabricating highly efficient phosphorescent organic light-emitting diodes (PHOLEDs) with a similar device structure. In this work, a novel spiro-configured host 3,6-di(9H-carbazol-9-yl)spiro[indolo[3,2,1-de]acridine-8,9′-thioxanthene]10′,10′-dioxide (TCZSO2) incorporating 9′H-9,3′:6′,9′′-tercarbazole and 9H-thioxanthene 10,10-dioxide moieties is designed and synthesized. Utilizing 9′H-9,3′:6′,9′′-tercarbazole as the building-block in designing new spiro-type host materials in organic light-emitting diodes (OLEDs) is seldom reported. In particular, the three color-based devices based on TCZSO2/bis(4,6-(difluorophenyl)pyridinato-N,C2′)picolinate (FIrpic), TCZSO2/iridium(III) bis(2-phenylpyridinato-N,C2′)acetylacetonate (Ir(ppy)2(acac)) and TCZSO2/iridium(III) bis(2-methyldibenzo[f,h]quinoxaline)acetylacetonate (Ir(MDQ)2(acac)) exhibit respectable external quantum efficiencies (EQEs) of 22.8%, 24.0% and 21.0%, respectively. More encouragingly, blue and green PHOLED devices have quite low efficiency roll-off (EQE: 22.7%/24.0%, FIrpic/Ir(ppy)2(acac)) at a high brightness of 1000 cd m−2. At a higher brightness of 5000 cd m−2, there is only 5.3% and 4.6% decrease in EQEs from their maximum values, indicating a very low efficiency roll-off. These excellent results confirm a novel spiro-type host design strategy for incorporating reported electron donors with high triplet energy (ET) to achieve high-performance RGB PHOLEDs.

Incorporating a tercarbazole donor in a spiro-type host material for efficient RGB phosphorescent organic light-emitting diodes

All‐Fluorescence White Organic Light‐Emitting Diodes Exceeding 20% EQEs by Rational Manipulation of Singlet and Triplet Excitons

A new thermally activated delayed fluorescence (TADF) emitter, 3-(9,9′-spirobi[fluoren]-6-yl)-9-(4-(4,6-diphenyl-1,3,5-triazin-2-yl)phenyl)-9H-carbazole (SFCC), was developed for blue organic light-emitting diodes (OLEDs), and its thermal, photophysical and electrochemical properties were systematically investigated Upon introducing a 9,9′-spiro-bifluorene group through its C3 position to the C3 position of carbazole, the resulting new backbone had a more delocalized highest occupied molecular orbital (HOMO) than carbazole itself which led to a higher photoluminescence quantum yield (PLQY) compared with that of the control material, 3-bromo-9-(4-(4,6-diphenyl-1,3,5-triazin-2-yl)phenyl)-9H-carbazole (TRTZ) The OLED based on SFCC as the blue TADF emitter has a maximum external quantum efficiency (EQE) of 1059%, with Commission Internationale de L'Eclairage (CIE) coordinates of (017, 021), significantly surpassing the performance of the reference emitter TRTZ, indicating that the introduction of the 9,9′-spiro-bifluorene unit is a promising strategy for the modification of TADF materials

A blue thermally activated delayed fluorescence emitter developed by appending a fluorene moiety to a carbazole donor with meta-linkage for high-efficiency OLEDs

Two thermally activated delayed fluorescence (TADF) emitters 9′-(4-(4,6-diphenyl-1,3,5-triazin-2-yl)-2-fluorophenyl)-9′H-9,3′:6′,9′′-tercarbazole (TCTZ-F) and 9′-(4-(4,6-diphenyl-1,3,5-triazin-2-yl)-2,6-difluorophenyl)-9′H-9,3′:6′,9′′-tercarbazole (TCTZ-2F) were synthesized and compared to the previously reported molecule 9′-(4-(4,6-diphenyl-1,3,5-triazin-2-yl)phenyl)-9′H-9,3′:6′,9′′-tercarbazole (TCTZ) to explore the possible effects of the fluorine atom(s) on the molecular conformation, electronic coupling between the donor and acceptor groups, and consequently the photophysical behavior of the emitters. Specifically, the TCTZ-F molecule was calculated to have a smaller dihedral angle than TCTZ-2F and a smaller S1–T1 gap than TCTZ, which led it to have the highest photoluminescence quantum yield (PLQY) among this series of compounds. An OLED fabricated with TCTZ-F as the emitter achieved the highest external quantum efficiencies (22.5%) of this series, indicating that fluorination can play a useful role in TADF emitters.

Yun Li

Papers

Hole‐Transporting Materials Incorporating Carbazole into Spiro‐Core for Highly Efficient Perovskite Solar Cells

Incorporating a tercarbazole donor in a spiro-type host material for efficient RGB phosphorescent organic light-emitting diodes

All‐Fluorescence White Organic Light‐Emitting Diodes Exceeding 20% EQEs by Rational Manipulation of Singlet and Triplet Excitons

A blue thermally activated delayed fluorescence emitter developed by appending a fluorene moiety to a carbazole donor with meta-linkage for high-efficiency OLEDs

The role of fluorine-substitution on the π-bridge in constructing effective thermally activated delayed fluorescence molecules