Enhanced Thermally Activated Delayed Fluorescence by Sole Coordination: From an Organic Molecule to Its Zinc Complex.

A BPAPTPyC organic molecule containing a sandwich structural chromophore is designed and synthesized to produce blue thermally activated delayed fluorescence (TADF). The chromophore is composed of two di(4-tert-butylphenyl)amino donors and one inserted terpyridyl acceptor hitched at positions 1, 8, and 9 of a single carbazole via the p-phenylene group, in which the multiple space π-π interactions between the donor and acceptor enable the molecule to possess the TADF feature with a high energy emission at 470 nm but a low photoluminescence quantum yield (PLQY) and a small proportion of the delayed component. In contrast, the corresponding Zn(BPAPTPyC)Cl2 complex has a high PLQY and a short lifetime with a red-shifted emission due to the enhanced rigidity and electron accepting ability of the terpyridyl group from coordination. A solution-processed organic light-emitting diode (OLED) based on the complex achieves a maximum external quantum efficiency (EQE) of 17.9% with an emission peak at 585 nm, while an OLED of the organic molecule produces blue emission with a maximum EQE of 2.7%.

Improved Hole-Selective Contact Enables Highly Efficient and Stable FAPbBr3 Perovskite Solar Cells and Semitransparent Modules.

Self-assembled monolayers (SAMs) as the hole-selective contact have achieved remarkable success in iodine-based perovskite solar cells (PSCs), while their impact on bromine-based PSCs is limited due to the poor perovskite crystallization behavior and mismatched energy level alignment. Here, a highly efficient SAM of (2-(3,6-diiodo-9H-carbazol-9-yl)ethyl)phosphonic acid (I-2PACz) is employed to address these challenges in FAPbBr3-based PSCs. The incorporation of I atoms into I-2PACz not only releases tensile stress within FAPbBr3 perovskite, promoting oriented crystallization and minimizing defects through halogen-halogen bond, but also optimizes the energy levels alignment at hole-selective interface for enhanced hole extraction. Ultimately, a power conversion efficiency (PCE) of 11.14% is achieved, which stands among the highest reported value for FAPbBr3 PSCs. Furthermore, the semitransparent devices/modules exhibit impressive PCEs of 8.19% and 6.23% with average visible transmittance of 41.98% and 38.99%. Remarkably, after operating at maximum power point for 1000 h, the encapsulated device maintains 93% of its initial PCE. These results demonstrate an effective strategy for achieving high-performance bromine-based PSCs toward further applications.

Enhancing Stability and Performance in Tin-Based Perovskite Field-Effect Transistors Through Hydrogen Bond Suppression of Organic Cation Migration.

Ion migration poses a substantial challenge in perovskite transistors, exerting detrimental effects on hysteresis and operational stability. This study focuses on elucidating the influence of ion migration on the performance of tin-based perovskite field-effect transistors (FETs). It is revealed that the high background carrier density in FASnI3 FETs arises not only from the oxidation of Sn2+ but also from the migration of FA+ ions. The formation of hydrogen bonding between FA+ and F- ions efficiently inhibits ion migration, leading to a reduction in background carrier density and an improvement in the operational stability of the transistors. The strategy of hydrogen bond is extended to fluorine-substituted additives to improve device performance. The incorporation of 4-fluorophenethylammonium iodide additives into FETs significantly minimizes the shift of turn-on voltage during cyclic measurements. Notably, an effective mobility of up to 30 cm2 V-1 s-1 with an Ion/off ratio of 107 is achieved. These findings hold promising potential for advancing tin-based perovskite technology in the field of electronics.

Multi-Functional Spirobifluorene Phosphonate Based Exciplex Interface Enables Voc Reaching 95% of Theoretical Limit for Perovskite Solar Cells.

Metal halide perovskite solar cells (PSCs) show significant advancements in power conversion efficiency (PCE). However, the open-circuit voltage (VOC) of PSCs is limited by interfacial factors such as defect-induced recombination, energy band mismatch, and non-intimate interface contact. Here, an exciplex interface is first developed based on the strategically designed and synthesized two spirobifluorene phosphonate molecules to mitigate VOC loss in PSCs. The exciplex interface constructed by the intimate contact between the multi-functional molecules and hole transport layer takes the roles to promote the hole extraction by donor-acceptor interaction, passivate coordination-unsaturated Pb2+ defects by equipped phosphonate groups, and optimize the energy level alignment. As a result, a record VOC of 1.26 V with a perovskite bandgap of 1.61 eV is achieved, representing over 95% of theoretical limit. This advancement leads to an increase in PCE from 21.29% to 24.12% and improved stability. The exciplex interface paves the way for addressing the long-standing challenge of VOC loss and promotes the wider application of PSCs.

Phosphate ester functionalized fluorene-benzothiadiazole alternating copolymer/hydroxylated g-C3N4 heterojunctions for efficient hydrogen evolution under visible-light irradiation.

It is highly desirable to explore functionalized polymer semiconductor/g-C3N4 heterojunction photocatalysts with the tight interfacial connection for promoting the photogenerated electron-hole pair separation, improving the hydrophilicity, extending the visible light response and achieving the efficient visible light-driven H2 evolution. Herein, we synthesized novel poly[9,9-bis(3-ethyl phosphate propyl)fluorene-alt-benzothiadiazole] (PPFBT) with a phosphate ester on every repeating unit by the Suzuki polymerization and then fabricated PPFBT/hydroxylated g-C3N4 (PPFBT/CN-OH) heterojunctions via a surface hydroxyl-induced assembly process. The ratio-optimized 5PPFBT/CN-OH shows the hydrogen evolution activity of 2662.4 µmol·g-1·h-1, an 11.1-time enhancement compared to CN-OH. The improved photocatalytic activity is mainly attributed to the enhanced electron-hole pair separation due to the tight interfacial connection by hydrogen bond (P=O…H-O) and N…S interactions between PPFBT and CN-OH. It is verified that abundant phosphate ester groups of PPFBT improve the hydrophilicity and form coordination bonds with platinum (P=O:Pt) as a cocatalyst to facilitate water splitting for H2 evolution. It is also confirmed that the enhanced electron-hole pair separation is mainly dependent on the excited high-energy level electron transfer from CN-OH to PPFBT. This work provides a rational molecular design strategy for constructing efficient functionalized polymer semiconductor/g-C3N4 heterojunctions for sunlight-driven H2 evolution.

Room-Temperature Continuous-Wave Microcavity Lasers from Solution-Processed Smooth Quasi-2D Perovskite Films with Low Thresholds.

Continuous-wave (CW) lasing in quasi-two-dimensional (2D) perovskite-based distributed feedback cavities has been achieved at room temperature; however, CW microcavity lasers comprising distributed Bragg reflectors (DBRs) have rarely been prepared using solution-processed quasi-2D perovskite films because the roughness of perovskite films significantly increases intersurface scattering loss in the microcavity. Herein, high-quality spin-coated quasi-2D perovskite gain films were prepared using an antisolvent to reduce roughness. The highly reflective top DBR mirrors were deposited via room-temperature e-beam evaporation to protect the perovskite gain layer. Lasing emission of the prepared quasi-2D perovskite microcavity lasers under CW optical pumping was clearly observed at room temperature, featuring a low threshold of ∼1.4 W cm-2 and beam divergence of ∼3.5°. It was concluded that these lasers originated from weakly coupled excitons. These results elucidate the importance of controlling the roughness of quasi-2D films to achieve CW lasing, thus facilitating the design of electrically pumped perovskite microcavity lasers.

Enhanced Circularly Polarized Photoluminescence of Chiral Perovskite Films by Surface Passivation with Chiral Amines.

Hybrid organic-inorganic perovskites have shown promise in circularly polarized light source applications when chirality has been introduced. Circularly polarized photoluminescence (CPL) is a significant tool for investigating the chiroptical properties of perovskites. However, further research is still urgently needed, especially with regard to optimization. Here we demonstrate that chiral ligands can influence the electronic structure of perovskites, increasing the asymmetry and emitting circularly polarized photons in photoluminescence. After the modification of chiral amines, the defects of films are passivated, leading to enhanced radiation recombination for which more circularly polarized photons are emitted. Meanwhile, the modification increases the asymmetry in the electronic structure of perovskites, manifested by an increase in the magnetic dipole moment from 0.166 to 0.257 µB and an enhanced CPL signal. This approach offers the possibility of fabricating and refining circularly polarized light-emitting diodes.

Insight into Diphenyl Phosphine Oxygen-Based Molecular Additives as Defect Passivators toward Efficient Quasi-2D Perovskite Light-Emitting Diodes.

The introduction of additives has become an important method for enhancing the device performance of quasi-two-dimensional perovskite light-emitting diodes. In this work, we systematically studied the electronic and spatial effects of molecular additives on defect passivation abilities using the methyl, hydrogen, and hydroxyl groups substituted three diphenyl phosphine oxygen additives. The electron-donating conjugation effect of the hydroxyl group on diphenylphosphinic acid (OH-DPPO) leads to a more electron-rich region in OH-DPPO, and the hydroxyl group has a moderate steric hindrance. All these factors endow it with best passivation ability than the other two additives. Furthermore, ion migration was suppressed due to hydrogen bonding between the hydroxyl group and Br. Ultimately, the OH-DPPO passivated devices achieved an external quantum efficiency of 22.44% and a 6-fold improvement in lifetime. These findings provide guidance for developing multifunctional additives in the field of perovskite optoelectronics.

A cyano-based electron-accepting building block to design n-type conjugated polymers with absorption wavelength of >1000 nm.

There are much fewer n-type conjugated polymers than p-type counterparts because of the lack of strong electron-accepting building blocks. We report a novel strong electron-accepting unit based on a dicyanomethylene unit. The resulting polymers exhibit LUMO energy levels of as low as -4.04 eV and narrow optical gaps with an onset absorption wavelength of 1050 nm. Moreover, the polymers exhibit near-infrared light absorption with good photostability because of their low-lying LUMO/HOMO energy levels.

Comprehensive Passivation for High-Performance Quasi-2D Perovskite LEDs.

Quasi-2D perovskites have demonstrated great application potential in light-emitting diodes (LEDs). Defect passivation with chemicals plays a critical role to achieve high efficiency. However, there are still challenges in comprehensively passivating the defects distributed at surface, bulk, and buried interface of quasi-2D perovskite emitting films, hindering the further improvement of device performance. Herein, 9,9-substituted fluorene derivatives with different terminal functional groups are developed tactfully to realize comprehensive passivation, which greatly contributes to reducing nonradiative recombination at surface, suppressing ion migration in bulk, and filling interfacial charge traps at buried interface, respectively. Eventually, quasi-2D perovskite LEDs have an increased external quantum efficiency from 18.2% to 23.2%, improved operation lifetime by more than six times and lower turn-on voltage simultaneously. Here the importance of comprehensive passivation is highlighted and guidelines for the design and application of passivators for perovskite optoelectronics are provided.

Suppressing High-Order Phase for Efficient Pure Red Quasi-2D Perovskite Light-Emitting Diodes.

Quasi-two-dimensional (quasi-2D) perovskites are promising for the realization of spectrally stable pure red perovskite light-emitting diodes (PeLEDs) with a single iodide component, because they avoid the halide separation that red three-dimensional perovskites of mixed halides have faced. However, the distribution of high-order phases in solution-processed quasi-2D perovskite films causes the spectral shift away from the pure red region. Here, we introduced a simple approach of adding excessive ligand combinations to redistribute the phase distribution of quasi-2D perovskite and to inhibit the high-order phase. Appropriate excess organic ligands will not affect charge injection but will keep the efficient energy funneling and passivate the defect. The narrowed phase distribution reduced the band tail state and restrained reverse charge transfer, resulting in enhanced radiation recombination. We obtained efficient and spectrally stable pure red PeLEDs at 638 nm (approaching the Rec. 2020 specification) with a peak EQE of 11.8% and maximum luminance of 1688 cd/cm2. This study provides guidance for future developments of highly efficient pure red PeLEDs.

Suppressing thermal quenching via defect passivation for efficient quasi-2D perovskite light-emitting diodes.

Emission thermal quenching is commonly observed in quasi-2D perovskite emitters, which causes the severe drop in luminescence efficiency for the quasi-2D perovskite light-emitting diodes (PeLEDs) during practical operations. However, this issue is often neglected and rarely studied, and the root cause of the thermal quenching has not been completely revealed now. Here, we develop a passivation strategy via the 2,7-dibromo-9,9-bis (3'-diethoxylphosphorylpropyl)-fluorene to investigate and suppress the thermal quenching. The agent can effectively passivate coordination-unsaturated Pb2+ defects of both surface and bulk of the film without affecting the perovskite crystallization, which helps to more truly demonstrate the important role of defects in thermal quenching. And our results reveal the root cause that the quenching will be strengthened by the defect-promoted exciton-phonon coupling. Ultimately, the PeLEDs with defect passivation achieve an improved external quantum efficiency (EQE) over 22% and doubled operation lifetime at room temperature, and can maintain about 85% of the initial EQE at 85 °C, much higher than 17% of the control device. These findings provide an important basis for fabricating practical PeLEDs for lighting and displays.

Phosphonate/Phosphine Oxide Dyad Additive for Efficient Perovskite Light-Emitting Diodes.

Additives play a critical role for efficient perovskite light-emitting diodes (PeLEDs). Here, we report a novel phosphonate/phosphine oxide dyad molecular additive (PE-TPPO), with unique dual roles of passivating defects and enhancing carrier radiative recombination, to boost the device efficiency of metal-halide perovskites. In addition to the defect passivation effect of the phosphine oxide group to enhance the photoluminescence intensity and homogeneity of perovskite film, the phosphonate group with strong electron affinity can capture the injected electrons to increase local carrier concentration and accelerate the carrier radiative recombination in the electroluminescence process. Owing to their synergistic enhancement on device efficiency, quasi-two-dimensional green PeLEDs modified by this dyad additive exhibit a maximum external quantum efficiency, current efficiency, and power efficiency of 25.1 %, 100.5 cd A-1 , and 98.7 lm W-1 , respectively, which are among the reported state-of-the-art efficiencies.

Engineering of Annealing and Surface Passivation toward Efficient and Stable Quasi-2D Perovskite Light-Emitting Diodes.

Solution-processed quasi-two-dimensional (quasi-2D) perovskites with self-assembled multiple quantum well (QW) structures exhibit enhanced exciton binding energy, which is ideal for use as light emitters. Here, we have found that postannealing is important to promoting the QWs' composition transfer, and we explored the correlation among the annealing time, the external quantum efficiency (EQE), and the operational stability of the device. During thermal annealing, the low-n QWs will gradually convert to high-n phases, accompanied by an increase in grain size. The EQE and working stability of the device exhibit different annealing-time dependences; that is, with the extension of the annealing time, the EQE gradually decreases while the working stability improves. By introducing trimethylolpropane trimethacrylate (TPTA) to passivate the emitting-region defects, the annealing-time dependence of the EQE was effectively eliminated due to the reduction of the nonradiative recombination rate, wherefore high efficiency and stability can be achieved simultaneously. Our research provides an effective way to develop highly efficiency and stable perovskite light-emitting diodes.

Stable room-temperature continuous-wave lasing in quasi-2D perovskite films.

Organic-inorganic lead halide quasi-two-dimensional (2D) perovskites are promising gain media for lasing applications because of their low cost, tunable colour, excellent stability and solution processability1-3. Optically pumped continuous-wave (CW) lasing is highly desired for practical applications in high-density integrated optoelectronics devices and constitutes a key step towards electrically pumped lasers4-6. However, CW lasing has not yet been realized at room temperature because of the 'lasing death' phenomenon (the abrupt termination of lasing under CW optical pumping), the cause of which remains unknown. Here we study lead halide-based quasi-2D perovskite films with different organic cations and observe that long-lived triplet excitons considerably impede population inversion during amplified spontaneous emission and optically pumped pulsed and CW lasing. Our results indicate that singlet-triplet exciton annihilation is a possible intrinsic mechanism causing lasing death. By using a distributed-feedback cavity with a high quality factor and applying triplet management strategies, we achieve stable green quasi-2D perovskite lasers under CW optical pumping in air at room temperature. We expect that our findings will pave the way to the realization of future current-injection perovskite lasers.

Detrimental Effect of Unreacted PbI2 on the Long-Term Stability of Perovskite Solar Cells.

Excess/unreacted lead iodide (PbI2 ) has been commonly used in perovskite films for the state-of-the-art solar cell applications. However, an understanding of intrinsic degradation mechanisms of perovskite solar cells (PSCs) containing unreacted PbI2 has been still insufficient and, therefore, needs to be clarified for better operational durability. Here, it is shown that degradation of PSCs is hastened by unreacted PbI2 crystals under continuous light illumination. Unreacted PbI2 undergoes photodecomposition under illumination, resulting in the formation of lead and iodine in films. Thus, this photodecomposition of PbI2 is one of the main reasons for accelerated device degradation. Therefore, this work reveals that carefully controlling the formation of unreacted PbI2 crystals in perovskite films is very important to improve device operational stability for diverse opto-electronic applications in the future.

High performance from extraordinarily thick organic light-emitting diodes.

Organic light-emitting diode (OLED) technology is promising for applications in next-generation displays and lighting. However, it is difficult-especially in large-area mass production-to cover a large substrate uniformly with organic layers, and variations in thickness cause the formation of shunting paths between electrodes1,2, thereby lowering device production yield. To overcome this issue, thicker organic transport layers are desirable because they can cover particles and residue on substrates, but increasing their thickness increases the driving voltage because of the intrinsically low charge-carrier mobilities of organics. Chemical doping of organic layers increases their electrical conductivity and enables fabrication of thicker OLEDs3,4, but additional absorption bands originating from charge transfer appear5, reducing electroluminescence efficiency because of light absorption. Thick OLEDs made with organic single crystals have been demonstrated6, but are not practical for mass production. Therefore, an alternative method of fabricating thicker OLEDs is needed. Here we show that extraordinarily thick OLEDs can be fabricated by using the organic-inorganic perovskite methylammonium lead chloride, CH3NH3PbCl3 (MAPbCl3), instead of organics as the transport layers. Because MAPbCl3 films have high carrier mobilities and are transparent to visible light, we were able to increase the total thickness of MAPbCl3 transport layers to 2,000 nanometres-more than ten times the thickness of standard OLEDs-without requiring high voltage or reducing either internal electroluminescence quantum efficiency or operational durability. These findings will contribute towards a higher production yield of high-quality OLEDs, which may be used for other organic devices, such as lasers, solar cells, memory devices and sensors.

The Relation of Phase-Transition Effects and Thermal Stability of Planar Perovskite Solar Cells.

A power conversion efficiency of over 20% has been achieved in CH3NH3PbI3-based perovskite solar cells (PSC), however, low thermal stability associated with the presence of a phase transition between tetragonal and cubic structures near room temperature is a major issue that must be overcome for future practical applications. Here, the influence of the phase transition on the thermal stability of PSCs is investigated in detail by comparing four kinds of perovskite films with different compositions of halogen atoms and organic components. Thermally stimulated current measurements reveal that a large number of carrier traps are generated in solar cells with the perovskite CH3NH3PbI3 as a light absorber after operation at 85 °C, which is higher than the phase-transition temperature. Electrochemical impedance spectroscopy measurements further exclude effects of a possible morphology change on the formation of carrier traps. These carrier traps are detrimental to the thermal stability. The thermogravimetric analysis does not show a decomposition for any of the materials in the temperature range relevant for operation. The perovskite alloys do not have this phase transition, resulting in effectively suppressed formation of carrier traps. PSCs with improved thermal stability under the standard thermal cycling test are demonstrated.

Detecting and identifying reversible changes in perovskite solar cells by electrochemical impedance spectroscopy.

The current status of electrochemical impedance spectroscopy (EIS) and related analysis on perovskite solar cells (PSC) is still unsatisfactory. The provided models are still vague and not really helpful for guiding the efforts to develop more efficient and stable devices. Due to the slow and complex dynamics of these devices, the obtained spectra need to be validated, which is hardly ever done. This study may be the first to provide fully validated impedance spectra and presents reproducible EIS time series at open circuit voltage (V OC) for more than 20 hours, with a total of 140 analysed spectra. We conclude that the observed changes stem from a temporary reduction of the electronically active area of the devices, as can be deduced from the inverse behaviour of resistance and capacitance. The changes in these values are almost 100% reversible if the devices are kept in the dark for only one day, while the time constant of the high-frequency process remains unchanged throughout the whole characterization procedure. The tested devices are full PSC devices that have proven to be stable over more than 500 hours, and the non-steady impedance measurements shine a critical light on previously published EIS data. With the results of this study, it can be rationalized that the high-frequency semicircle can serve as a good indicator for ionic migration by monitoring its consequences. The results presented here are helpful to quantify ionic migration on the device level in order to derive new stability criteria and countermeasures against degradation.