High Photoluminescence Quantum Efficiency Research Articles

Alkali Chlorides for the Suppression of the Interfacial Recombination in Inverted Planar Perovskite Solar Cells

AbstractIn this work, significant suppression of the interfacial recombination by facile alkali chloride interface modification of the NiOx hole transport layer in inverted planar perovskite solar cells is achieved. Experimental and theoretical results reveal that the alkali chloride interface modification results in improved ordering of the perovskite films, which in turn reduces defect/trap density, causing reduced interfacial recombination. This leads to a significant improvement in the open‐circuit voltage from 1.07 eV for pristine NiOx to 1.15 eV for KCl‐treated NiOx, resulting in a power conversion efficiency approaching 21%. Furthermore, the suppression of the ion diffusion in the devices is observed, as evidenced by stable photoluminescence (PL) under illumination and high PL quantum efficiency with alkali chloride treatment, as opposed to the luminescence enhancement and low PL quantum efficiency observed for perovskite on pristine NiOx. The suppressed ion diffusion is also consistent with improved stability of the devices with KCl‐treated NiOx. Thus, it is demonstrated that a simple interfacial modification is an effective method to not only suppress interfacial recombination but also to suppress ion migration in the layers deposited on the modified interface due to improved interface ordering and reduced defect density.

Modulation of Excited State Property Based on Benzo[a, c]phenazine Acceptor: Three Typical Excited States and Electroluminescence Performance.

Throwing light upon the structure-property relationship of the excited state properties for next-generation fluorescent materials is crucial for the organic light emitting diode (OLED) field. Herein, we designed and synthesized three donor-acceptor (D-A) structure compounds based on a strong spin orbit coupling (SOC) acceptor benzo[a, c]phenazine (DPPZ) to research on the three typical types of excited states, namely, the locally-excited (LE) dominated excited state (CZP-DPPZ), the hybridized local and charge-transfer (HLCT) state (TPA-DPPZ), and the charge-transfer (CT) dominated state with TADF characteristics (PXZ-DPPZ). A theoretical combined experimental research was adopted for the excited state properties and their regulation methods of the three compounds. Benefiting from the HLCT character, TPA-DPPZ achieves the best non-doped device performance with maximum brightness of 61,951 cd m−2 and maximum external quantum efficiency of 3.42%, with both high photoluminescence quantum efficiency of 40.2% and high exciton utilization of 42.8%. Additionally, for the doped OLED, PXZ-DPPZ can achieve a max EQE of 9.35%, due to a suppressed triplet quenching and an enhanced SOC.

How β-Phase Content Moderates Chain Conjugation and Energy Transfer in Polyfluorene Films.

Poly(9,9-dioctylfluorene) (PFO) is a blue-light-emitting polymer exhibiting two distinct phases, namely, the disordered "glassy" phase and a more ordered β-phase. We investigate how a systematic increase in the fraction of β-phase present in PFO films controls chain conformation, photoluminescence quantum efficiency (PLQE), and the resonant energy transfer from the glassy to the β-phase. All films are prepared by the same technique, using paraffin oil as an additive to the spin-coating solution, allowing systematic tuning of the β-phase fraction. The PFO films exhibit high PLQE with values increasing to 0.72 for increasing fractions of β-phase present, with the β-phase chain conformation becoming more planar and including more repeat units. Differences in Förster radii calculated from the overlap of steady-state absorptance and emission spectra and from time-resolved ultrafast photoluminescence transients indicate that exciton diffusion within the glassy phase plays an important role in the energy transfer process.

Understanding the atomic and electronic structures origin of defect luminescence of CdSe quantum dots in glass matrix

AbstractWell‐defined density functional theory (DFT) calculations are performed as the first exploratory study for the atomic and electronic mechanism of defect mediated morphology and optical properties of CdnSen (n = 3, 10, 13, and 33) quantum dots (QDs) in inorganic amorphous matrix. The intrinsic defects of pristine CdSe QDs, and the interfacial defects between the QDs and surrounding amorphous matrix, were systematically studied. The calculated electronic structure suggested that the pristine CdSe QDs capped by the structural modifiers or non‐bridging oxygen in the amorphous matrix gave rise to the structure reconstruction and paired defect states at the edge of the valence and conduction bands. The orbital analysis elucidated that the redistribution of the majority of HOMO and LUMO electron density was localized over the bonds formed by capping atoms and QDs. These changes in the electronic structures were further demonstrated by CdSe QDs embedded sodium silicate glasses. It turned out that Se atoms at QDs/glass interface were much more active than those found on the surface of organically passivated CdSe QDs. The results serve as a new paradigm in materials research to explore structural origins of defect emission from QDs and a new strategy to develop glasses containing QDs with high photoluminescence quantum efficiency.

Novel Ir(III) complexes ligated with 2-(2,6-difluoropyridin-3-yl)benzo[d]thiazole for highly efficient OLEDs with mild efficiency roll-off

2-phenylbenzothiazole-based Ir(III) complex [(bt)2Ir(acac)] is a class of typical yellow phosphorescence material, which has been widely used in OLEDs. However, (bt)2Ir(acac) exhibited only mild photoluminescence quantum efficiency (PLQY) of 0.26–0.28. To further improve the luminous efficiency of benzothiazole-based Ir(III) complex and then boost the corresponding electroluminescent performance, (fpbt)2Ir(pic) and (fpbt)2Ir(tftp) were successfully synthesized, where 2-(2,6-difluoropyridin-3-yl)benzo[d]thiazole (fpbt) was applied as C∩N main ligand, and 2-pyridine carboxylic acid (pic), 2-(3-(trifluoromethyl)-1H-1,2,4-triazol-5-yl)pyridine (tftp) acted as different auxiliary ligands. Both (fpbt)2Ir(pic) and (fpbt)2Ir(tftp) presented excellent PLQYs (0.93–0.95) and high thermostability (330–360 °C). The vacuum-deposited devices following the architecture of ITO/TAPC/CBP: dopant/TPBi/Liq/Al demonstrated prominent performances. Of them, (fpbt)2Ir(pic)-doped OLEDs exhibited better efficiencies of 20.6%, 68.8 cd A−1, and 40.8 l m W−1 while maintaining very mild efficiency roll-off. Such outstanding electrophosphorescent performances were further investigated, which was probably attributed to the following factors: 1) high PLQY; 2) good electron mobility; 3) uniform film-forming property.

Synthesis, Photophysical Properties and Two-Photon Absorption Study of Tetraazachrysene-based N-Heteroacenes.

Three novel N-heteroacene molecules (SDNU-1, SDNU-2 and SDNU-3) based on tetraazachrysene units as cores have been designed, synthesized and fully characterized. Their photophysical, electrochemical and fluorescence properties were investigated, and they exhibited blue to green emission in the solid state. Interestingly, SDNU-2 exhibited high solid photoluminescence quantum efficiencies (75.3 %), which is the highest value of N-heteroacenes derivatives to date. Two-photon absorption studies have been conducted by using the open and close aperture Z-san technique. SDNU-3 showed a significant enhancement in the two-photon absorption cross-section with magnitudes as high as about 700 GM (1 GM=1×10-50 cm4 s/photon) when excited with 800 nm light, which is the largest value based on a heteroacene system measured by using a Z-scan experiment so far. We attribute the outcome to sufficient electronic coupling between the strong charge transfer of quadrupolar substituents and the tetraazachrysene core. Our result would provide a new guideline to design novel efficient two-photon materials based on N-heteroacene cores.

Improving performance of Cs-based perovskite light-emitting diodes by dual additives consisting of polar polymer and n-type small molecule

Abstract Organic-inorganic halide perovskites were emerging as promising light emitters due to the high photoluminescence quantum efficiency (PLQE), narrow color spectrum, and facile bandgap tunability. However, the morphological issues of this class of materials remain as a bottleneck in generating high-performance light-emitting devices (LED). In this work, we describe a facile and cost-effective method to improve the morphology of an all-inorganic-perovskite emissive layer by using a dual-additive blending strategy. A polar polymer, polyethylene oxide (PEO), and an n-type small molecule, TPBi, are blended with CsPbBr3 to provide synchronous effect in passivizing the surface defects and reducing the exciton quenching, as evidenced by the formation of smaller uniform grains and improved radiative recombination lifetime. Consequently, the optimized CsPbBr3/PEO/TPBi LED reaches maximum luminance, current efficiency, and external quantum efficiency of 6807 cd m−2 (at 6 V), 0.86 cd A−1 (at 5.5 V) and 0.25% (at 5.5 V), far surpassing the performance of the control CsPbBr3 device. On this basis, we further demonstrated a proof-of-concept LED device with touch-responsivity by using a flexible composite electrode. The merit of dual-additive strategy has been clearly manifested and will facilitate the future development of cost-effective solution processable perovskite LEDs.

Enhancing violet photoluminescence of 2D perovskite thin films via swift cation doping and grain size reduction

Perovskite materials are emerging as suitable materials for low-cost, high efficiency optoelectronics. In this study, we reported the surface modification of 2D (PEA)2PbBr4 for violet emission using PEG doping followed by a newly developed step called swift cation doping (SCD) where phenylethylammonium cations (PEA+) in isopropanol (IPA) are abruptly applied during crystallization. 2D Perovskite doped with PEG using SCD resulted in smaller grain, smoother surface, higher film density, higher photoluminescence quantum efficiency, and longer average PL lifetime. These features are highly beneficial for photoluminescence application and can be easily applied to enhance other perovskite LED thin films.

Highly-Efficient Doped and Nondoped Organic Light-Emitting Diodes with External Quantum Efficiencies over 20% from a Multifunctional Green Thermally Activated Delayed Fluorescence Emitter

Thermally activated delayed fluorescence (TADF) emitters integrated with aggregation-induced emission (AIE) property are expected to enable nondoped organic light-emitting diodes (OLEDs) with merits of high efficiency and simple fabrication process. We previously proposed a novel design strategy of combining AIE and TADF features to develop AIE–TADF emitters with asymmetric structure, leading to an AIE–TADF emitter 4,4-CzSPz exhibiting mechanoluminescence. Herein, photophysical, thermal, and electroluminescence properties of neat 4,4-CzSPz films have been systematically investigated. It was found that 4,4-CzSPz film exhibits a high photoluminescence quantum efficiency of 97.3% and excellent thermal properties. Significantly, 4,4-CzSPz afforded its doped and nondoped OLEDs with very high external quantum efficiencies of 26.2% and 20.7%, respectively, which are rarely reported among TADF-based OLEDs. The results demonstrate universal applications of 4,4-CzSPz in OLEDs with different device configurations fo...

Control of Barrier Width in Perovskite Multiple Quantum Wells for High Performance Green Light–Emitting Diodes

AbstractSolution‐processed, self‐organized multiple quantum well (MQW) perovskites possess good film coverage and high photoluminescence quantum efficiency, which are promising for high performance light‐emitting diodes (LEDs). However, due to the inclusion of insulating large organic cation as barrier layer, the charge transport in MQW perovskites is not as efficient as 3D perovskites, which limits the improvement of power conversion efficiency of MQW perovskite LEDs. Here, it is demonstrated that by molecular engineering, the conductivity of MQW perovskite film can be effectively increased by reducing the barrier width in QWs, thus leading to enhanced device performance. By controlling the constitution of the narrow‐barrier‐width MQW perovskites, one can achieve green LEDs with a high luminance of 30 000 cd m−2 at a low voltage of 6 V and a peak external quantum efficiency of 7.7%. Moreover, the green perovskite LEDs show a lifetime of 63 min with initial luminance of 1330 cd m−2, representing one of the best performing green perovskite LEDs. Here, a promising strategy is provided to further boost the efficiency, brightness, and stability of MQW perovskite LEDs.

High-efficiency colloidal quantum dot infrared light-emitting diodes via engineering at the supra-nanocrystalline level.

Colloidal quantum dot (CQD) light-emitting diodes (LEDs) deliver a compelling performance in the visible, yet infrared CQD LEDs underperform their visible-emitting counterparts, largely due to their low photoluminescence quantum efficiency. Here we employ a ternary blend of CQD thin film that comprises a binary host matrix that serves to electronically passivate as well as to cater for an efficient and balanced carrier supply to the emitting quantum dot species. In doing so, we report infrared PbS CQD LEDs with an external quantum efficiency of ~7.9% and a power conversion efficiency of ~9.3%, thanks to their very low density of trap states, on the order of 1014 cm-3, and very high photoluminescence quantum efficiency in electrically conductive quantum dot solids of more than 60%. When these blend devices operate as solar cells they deliver an open circuit voltage that approaches their radiative limit thanks to the synergistic effect of the reduced trap-state density and the density of state modification in the nanocomposite.

Photoluminescence, radioluminescence and optically stimulated luminescence in nanoparticle and bulk KMgF3(Eu)

Photoluminescence (PL) and radioluminescence (RL) were observed in both bulk polycrystalline and nanoparticle KMgF3(Eu). Eu2+ and Eu3+ emissions were detected in the PL of both samples. The nanoparticles had more Eu3+, more distorted Eu3+ sites, and a higher PL quantum efficiency when compared to the bulk compound. X-ray-induced valence conversion of Eu2+ to Eu3+ was observed in the bulk, while Eu3+ to Eu2+ was found for nanoparticles. The valence conversion was reversed for the bulk compound by UV bleaching (254 nm) and partially reversed in the nanoparticles by 330 nm stimulation. F2 and F3-centre PL was observed in the bulk after X-ray irradiation but was absent in the nanoparticles. PL from other point defects occurred in both samples that is likely to be oxygen related. The PL from one of these defects in the nanoparticles changed with X-ray irradiation or UV illumination and the PL slowly recovered at room temperature. We propose this may be due to charge-transfer in O2--F+ pairs where this defect could potentially be used in a solar UV dosimeter. Optically stimulated luminescence was observed only from Eu2+ in the bulk material that can be used for dose monitoring after irradiation. It was absent in the nanoparticles, which may be due to the absence of fluorine related point defects. The bulk RL from Eu2+ and Eu3+ had significant dose-history dependence that could be corrected for. The nanoparticles had RL that was independent of dose history after ~ 4 kGy. Thus, bulk and nanoparticle KMgF3(Eu) may have potential application as an RL dosimeter.

Self-Activated Low-Dimensional Metal Halide Phosphors and Scintillators

We report theoretical studies of exciton excitation and emission in low-dimensional metal halides, (C4N2H14X)4SnX6 (X = Br, I), Cs4PbBr6, (Ph4P)2SbCl5 (Ph4P = tetraphenylphosphonium), (C9NH20)2SnBr4, and (C4N2H14)PbCl4. These are bulk crystalline materials, in which isolated metal halide clusters/wires are separated from each other by large inorganic or organic countercations, forming 0D/1D structures. For example, in (C9NH20)2SnBr4, the anionic (SnBr4)2- clusters are separated from each other by (C9NH20)+ molecular countercations. The metal halides shown above all have 0D structures except (C4N2H14)PbCl4, which has a 1D wire structure. High photoluminescence quantum efficiencies (PLQE) (close to 100% in some cases) have been reported for the 0D metal halides. We calculated exciton excitation and emission energies in these low-dimensional metal halides; the results are in excellent agreement with the experimental values. This enables us to study the luminescence mechanisms and suggest new compounds for synthesis. We show that exciton self-trapping in the metal-halide cluster/wire leads to localized excitons, which is important for efficient radiative recombination. The relatively low PLQE (6% - 18%) in 1D (C4N2H14)PbCl4 is likely due to the exciton diffusion along the wire and the subsequent nonradiative recombination at defects. The large organic countercations in hybrid organic-inorganic 0D halides plays the important role of decoupling metal halide clusters (which are luminescent centers), leading to highly immobile excitons and high PLQE. On the other hand, in all inorganic 0D Cs4PbBr6, the significant electronic coupling between PbBr6 clusters and the small Stokes shift should enable resonant transfer of excitation energy and the subsequent energy loss at defects, which explains the strong thermal quenching of luminescence in Cs4PbBr6. We further suggest that Cs4EuX6 (X = Br,I), which are analogs to Cs4PbBr6, should exhibit improved luminescent properties compared to Cs4PbBr6 due to stronger exciton localization at Eu2+ than at Pb2+. Indeed, recent synthesis and characterization of these compounds show high light yields under gamma-ray irradiation at room temperature. In particular, the light yield of Cs4EuBr6 (78,000 photons/MeV) is the highest among the self-activated scintillators.

Red Organic Light-Emitting Diode with External Quantum Efficiency beyond 20% Based on a Novel Thermally Activated Delayed Fluorescence Emitter.

A novel thermally activated delayed fluorescence (TADF) emitter 12,15‐di(10H‐phenoxazin‐10‐yl)dibenzo[a,c]dipyrido[3,2‐h:2′,3′‐j]phenazine (DPXZ‐BPPZ) is developed for a highly efficient red organic light‐emitting diode (OLED). With rigid and planar constituent groups and evident steric hindrance between electron‐donor (D) and electron‐acceptor (A) segments, DPXZ‐BPPZ realizes extremely high rigidity to suppress the internal conversion process. Meanwhile, the highly twisted structure between D and A segments will also lead to an extremely small singlet–triplet energy split to DPXZ‐BPPZ. Therefore, DPXZ‐BPPZ successfully realizes an efficient fluorescent radiation transition and reverse intersystem crossing process, and possesses an extremely high photoluminescence quantum efficiency of 97.1 ± 1.1% under oxygen‐free conditions. The OLED based on DPXZ‐BPPZ shows red emission with a peak at 612 nm and a Commission Internationale de L'Eclairage (CIE) coordinate of (0.60, 0.40), and it achieves high maximum forward‐viewing efficiencies of 20.1 ± 0.2% (external quantum efficiency), 30.2 ± 0.6 cd A−1 (current efficiency), and 30.9 ± 1.3 lm W−1 (power efficiency). The prepared OLED has the best performance among the reported red TADF OLEDs. These results prove that DPXZ‐BPPZ is an ideal candidate for red TADF emitters, and the designing approach is valuable for highly efficient red TADF emitters.

Charge carrier recombination and ion migration in metal-halide perovskite nanoparticle films for efficient light-emitting diodes

Metal-halide perovskite (MHP) nanoparticles (NPs) have attracted considerable attention as a promising light emitter for efficient light-emitting diodes (LEDs) due to their high color-purity and photoluminescence quantum efficiency (PLQE). In contrast to MHP polycrystalline (PC) bulk films, charge carrier recombination as well as ion migration dynamics in MHP NP films have not been studied yet despite the importance to achieve high electroluminescence efficiency in LEDs. Here, we use steady-state and transient photoluminescence measurements, and photo-responsivity analysis to study the charge carrier recombination and ion migration dynamics in MHP NP films and then, compare these with those of MHP PC films. Results show that MHP NP films do not undergo the severe ion migration and have dominant radiative recombination of excitons by efficiently confining electron-hole pairs. As a result, MHP NP films have high photo-stability, photo-responsibility, and PLQE (> 60%). Our findings provide insight into charge carrier and ion dynamics in MHP NP films, and confirm the potential of NP films for use in efficient LEDs.

Photonics and Optoelectronics of 2D Metal-Halide Perovskites.

In the growing list of 2D semiconductors as potential successors to silicon in future devices, metal-halide perovskites have recently joined the family. Unlike other conversional 2D covalent semiconductors such as graphene, transition metal dichalcogenides, black phosphorus, etc., 2D perovskites are ionic materials, affording many distinct properties of their own, including high photoluminescence quantum efficiency, balanced large exciton binding energy and oscillator strength, and long carrier diffusion length. These unique properties make 2D perovskites potential candidates for optoelectronic and photonic devices such as solar cells, light-emitting diodes, photodetectors, nanolasers, waveguides, modulators, and so on, which represent a relatively new but exciting and rapidly expanding area of research. In this Review, the recent advances in emerging 2D metal-halide perovskites and their applications in the fields of optoelectronics and photonics are summarized and insights into the future direction of these fields are offered.

Charge-transfer versus energy-transfer in quasi-2D perovskite light-emitting diodes

“Ruddlesden-Popper” perovskites with multiple quantum wells (MQWs) layered structure are extremely attractive for light-emitting diodes (LEDs) due to their effective energy confinement and high device stability properties. Two important processes of energy transfer (ET) and charge transfer (CT) are responsible for the effective energy confinement in MQWs. At present stage, the ET mechanism has been widely accepted as the main origin of high efficiency in MQWs. However, the high dielectric constant (ε) of hybrid perovskites should significantly limit the ET process, because the ET rate is inversely proportional to the ε. Therefore, the detailed contribution of CT mechanism, especially under electrical excitation, should be carefully considered. Herein, we report a high quality quasi-2D perovskite film of PA2(CsPbBr3)n-1PbBr4 MQWs through a simple solution process, with highly-dense and smooth morphology, and a high photoluminescence (PL) quantum efficiency. Then, the PL and transient PL spectra were measured under electrical injection in a hole-only and electron-only devices, respectively, to verify the CT mechanism. Furthermore, the photo-excitation intensity dependent PL spectra reveal that the CT mechanism is more important than ET mechanism for the electroluminescence of PA2(CsPbBr3)n-1PbBr4 perovskite LEDs (PeLEDs), resulting in a pure cyan emission at ~ 505 nm and maximal external quantum efficiency up to 3.6%, representing one of the best results in reported blue PeLEDs.

Hybrid perovskite films approaching the radiative limit with over 90% photoluminescence quantum efficiency

Reducing non-radiative recombination in semiconducting materials is a prerequisite for achieving the highest performance in light-emitting and photovoltaic applications. Here, we characterize both external and internal photoluminescence quantum efficiency and quasi-Fermi-level splitting of surface-treated hybrid perovskite (CH3NH3PbI3) thin films. With respect to the material bandgap, these passivated films exhibit the highest quasi-Fermi-level splitting measured to date, reaching 97.1 ± 0.7% of the radiative limit, approaching that of the highest performing GaAs solar cells. We confirm these values with independent measurements of internal photoluminescence quantum efficiency of 91.9 ± 2.7% under 1 Sun illumination intensity, setting a new benchmark for these materials. These results suggest hybrid perovskite solar cells are inherently capable of further increases in power conversion efficiency if surface passivation can be combined with optimized charge carrier selective interfaces. Surface treatment is shown to yield passivated perovskite films with very high quasi-Fermi level splitting and internal photoluminescence quantum efficiency, indicating that further improvements in the performance of perovskite optoelectronics should be feasible.

Highly Efficient Spectrally Stable Red Perovskite Light-Emitting Diodes.

Perovskite light-emitting diodes (LEDs) have recently attracted great research interest for their narrow emissions and solution processability. Remarkable progress has been achieved in green perovskite LEDs in recent years, but not blue or red ones. Here, highly efficient and spectrally stable red perovskite LEDs with quasi-2D perovskite/poly(ethylene oxide) (PEO) composite thin films as the light-emitting layer are reported. By controlling the molar ratios of organic salt (benzylammonium iodide) to inorganic salts (cesium iodide and lead iodide), luminescent quasi-2D perovskite thin films are obtained with tunable emission colors from red to deep red. The perovskite/polymer composite approach enables quasi-2D perovskite/PEO composite thin films to possess much higher photoluminescence quantum efficiencies and smoothness than their neat quasi-2D perovskite counterparts. Electrically driven LEDs with emissions peaked at 638, 664, 680, and 690 nm have been fabricated to exhibit high brightness and external quantum efficiencies (EQEs). For instance, the perovskite LED with an emission peaked at 680 nm exhibits a brightness of 1392 cd m-2 and an EQE of 6.23%. Moreover, exceptional electroluminescence spectral stability under continuous device operation has been achieved for these red perovskite LEDs.

Monocrystalline 1.7-eV-Bandgap MgCdTe Solar Cell With 11.2% Efficiency

This work demonstrates a monocrystalline 1.7 eV Mg0.13Cd0.87Te solar cell with an open-circuit voltage of 1.176 V and an active-area efficiency of 11.2%. The absorber layer is clad in wider bandgap passivation layers that effectively confine electrons and holes via the resulting band offsets. The potential barriers cladding the absorber are generated using higher magnesium compositions than the absorber and provide excellent carrier confinement. This ultimately leads to long minority carrier lifetimes (>500 ns) and high photoluminescence quantum efficiencies yielding an implied open-circuit voltage of 1.3 V. However, the same barriers at the heterointerfaces reduce fill factor by impeding transport; this is apparent as series resistance losses that can be overcome with operation at higher temperatures. The photocurrent loss mechanisms are simulated and analyzed, laying out the pathway for further improvements in current generation and, thus, efficiency.