Enhancement Of Quantum Efficiency Research Articles

The postsynthetic anion exchange of CsPbI3 nanocrystals for photoluminescence tuning and enhanced quantum efficiency

We propose a postsynthetic anion exchange method via ZnBr2 addition to enhance photoluminescence quantum yield and stability of CsPbI3 perovskite nanocrystals.

Reflective Back Contacts for Ultrathin Cu(In,Ga)Se2-Based Solar Cells

We report on the development of highly reflective back contacts (RBCs) made of multilayer stacks for ultrathin CIGS solar cells. Two architectures are compared: they are made of a silver mirror coated either with a single layer of In2O3:Sn (ITO) or with a bilayer of ZnO:Al/ITO. Due to the improvement of CIGS rear reflectance, both back contacts result in a significant external quantum efficiency enhancement, in agreement with optical simulations. However, solar cells fabricated with Ag/ITO back contacts exhibit a strong shunting behavior. The key role of the ZnO:Al layer to control the morphology of the top ITO layer and to avoid silver diffusion through the back contact is highlighted. For a 500-nm-thick CIGS layer, this optimized RBC leads to a best cell with a short-circuit current of 27.8 mA/cm2 (+2.2 mA/cm2 as compared to a Mo back contact) and a 12.2%-efficiency (+2.5% absolute).

A synergetic enhancement of localized surface plasmon resonance and photo-induced effect for graphene/GaAs photodetector

Photodetectors based on graphene/GaAs heterostructure were fabricated and demonstrated for application in self-powered photodetection. Then, Si quantum dots (QDs) were spin-coated onto the surface of the devices to enhance the built-in field by photo-induced doping, because of the tunable Fermi level (EF) of graphene and shallow junction of the heterojunction. Additionally, Au nanoparticles working as a light trapping structure were used to the enhance quantum efficiency of the Si QDs and the optical absorption of the heterojunction, benefitting from localized surface plasmon resonance. Therefore, a large-area photodetector under self-powered conditions achieved a high performance i.e. responsivity (1.81 × 105 V W−1), detectivity (2.0 × 1012 Jones), fast response speed (<0.04 ms), and on–off ratio (6 × 103). The high voltage responsivity opens a promising pathway to ultra-weak light detection, and facilities the development of novel sensors.

Regulating Vertical Domain Distribution in Ruddlesden-Popper Perovskites for Electroluminescence Devices.

Efficient perovskite light-emitting diodes are designed by employing an ordered vertical domain distribution in quasi-two-dimensional (quasi-2D) perovskites to induce better electron flow to the emitting domains. Dimethyl sulfoxide is added to the precursor solution to tune the crystallization rate and promote the formation of high-m domains near the substrate surface via the one-step deposition method. Optimized deposition conditions yielding a film with favorable energetic landscape for both carrier injection and confinement results in a 4-fold external quantum efficiency (EQE) enhancement with maximum EQE of 5.79%. Better carrier injection is further supported by a turn-on voltage value that is comparable to the band gap of the emitter material (∼2.25 eV).

Quantum Efficiency Enhancement of Bialkali Photocathodes by an Atomically Thin Layer on Substrates

We report quantum efficiency (QE) enhancements in accelerator technology relevant antimonide photocathodes (K2CsSb) by interfacing them with atomically thin two-dimensional (2D) crystal layers. The enhancement occurs in a reflection mode, when a 2D crystal is placed in between the photocathodes and optically reflective substrates. Specifically, the peak QE at 405 nm (3.1 eV) increases by a relative 10 percent, while the long wavelength response at 633 nm (2.0 eV) increases by a relative 36 percent on average and up to 80 percent at localized hot spot regions when photocathodes are deposited onto graphene coated stainless steel. There is a similar effect for photocathodes deposited on hexagonal boron nitride monolayer coatings using nickel substrates. The enhancement does not occur when reflective substrates are replaced with optically transparent sapphire. Optical transmission, X-ray diffraction (XRD) and X-ray fluorescence (XRF) revealed that thickness, crystal orientation, quality and elemental stoichiometry of photocathodes do not appreciably change due to 2D crystal coatings. These results suggest optical interactions are responsible for the QE enhancements when 2D crystal sublayers are present on reflective substrates, and provide a pathway toward a simple method of QE enhancement in semiconductor photocathodes by an atomically thin 2D crystal on substrates.

Improving the efficiency of silicon solar cells using in situ fabricated perovskite quantum dots as luminescence downshifting materials

Abstract Luminescence downshifting (LDS) layer integration has been proven to be an efficient way to ameliorate the poor UV-blue spectral response and improve the power conversion efficiency (PCE) for solar cells (SCs). By employing an in situ fabricated CH3NH3PbBr3 (CH3NH3 = methylammonium, MAPbBr3) quantum dot/polyacrylonitrile (PAN) composite film as the LDS layer, we observed a clear enhancement in the external quantum efficiency (EQE) for silicon SCs, predominantly in the UV-blue region. With a theoretically calculated intrinsic LDS efficiency (η LDS) of up to 72%, silicon SCs with the LDS layer exhibited an absolute value of 1% for PCE improvement in comparison to those without the LDS layer. The combination of easy fabrication and low cost makes it a practical way to achieve photovoltaic enhancement of Si-based SCs.

Temperature dependence of quantum efficiency enhancement using plasmonic gratings on nBn detectors with thin absorbers

For diffusion-limited nBn detectors, using an absorption layer much thinner than the optical attenuation length and minority carrier diffusion length can improve the dark current to provide greater sensitivity or higher temperature operation. However, if the quantum efficiency (QE) also decreases with absorber thickness, the advantage of reduced dark current is eliminated. We discuss the use of a metallic grating to couple the incident light into laterally propagating surface plasmon polariton (SPP) modes and increase the effective absorption length. We fabricate the gratings using a deposited Ge layer, which provides a uniform profile without increasing the dark current. Using this process in conjunction with a 0.5-μm-thick InAsSb absorber lattice-matched to GaSb, we demonstrate an external QE of 34% for T = 78 to 240 K. An nBn structure with an InAs0.8Sb0.2 absorber that is grown metamorphically on GaSb using a step-graded InGaSb buffer has a peak external QE of 39% at 100 K, which decreases to 32% by 240 K. Finally, we demonstrate that a grating with SPP resonance near the bandgap extends the absorption band and can potentially reduce the dark current by another factor of 3 to 8 times in addition to the 5 × reduction due to the thinner absorber.

Blue Phosphorescence with High Quantum Efficiency Engaging the Trifluoromethylsulfonyl Group to Iridium Phenylpyridine Complexes.

Incorporation of an electron-withdrawing -SO2CF3 substituent to cyclometalating C^N-phenylpyridine (ppy) ligand resulted in an expected blue-shifted phosphorescence in the corresponding homoleptic Ir(ppySCF3)3 complex, showing the emission of λem = 464 nm at 300 K. One of its heteroleptic derivatives, modified by a pyrazolyl borate LX ligand, Ir(ppySCF3)2(bor), exhibited further blue-shifted phosphorescence of λem = 460 nm at 300 K. Cyclic voltammograms (CVs) and density-functional theory (DFT) calculations supported the efficacy of the electron-withdrawing capability of the SO2CF3 substituent lowering HOMO energy and obtained widened bandgaps and resumed blue emissions for all of the iridium complexes studied. The homoleptic complexes of both substituents, Ir(ppySCF3)3 and Ir(ppySF)3, reached the higher quantum yields (ΦPL) of (0.89 and 0.72), respectively. Similarly, emission quantum yields (ΦPL) of the heteroleptic derivatives were reported to be (0.75, 0.83, and 0.87) for Ir(ppySCF3)2(acac), Ir(ppySCF3)2(bor), and Ir(ppySCF3)2(pic), respectively. Emission kinetics support the enhanced quantum efficiency when kr and knr values are compared between Ir(ppySCF3)3 and Ir(ppySF)3, and both values favorably contribute to attaining a higher quantum efficiency for Ir(ppySCF3)3. Among solution-processed multilayered devices having an ITO/PEDOT:PSS/TCTA:Ir dopant (10:1, w/w)/TmPyPB/Liq/Al structure, a heteroleptic dopant, Ir(ppySCF3)2(bor), exhibited better device performance, reporting an external quantum efficiency (EQE) of 1.14%, current efficiency (CE) of 2.31 cd A-1, and power efficiency (PE) of 1.21 lm W-1, together with blue chromaticity of CIEx,y = (0.16, 0.32).

Three-dimensional macroporous photonic crystal enhanced photon collection for quantum dot-based luminescent solar concentrator

A luminescent solar concentrator (LSC) is a photon managing device that can harvest, direct and concentrate solar light to small areas, enabling subsequent coupling to photovoltaic devices (PVs) for enhanced solar energy conversion. However, the intrinsic photon loss through the so-called escape cone of the LSCs significantly limits their light harvesting and concentrating performance. In this work, we introduce a facile and low-cost approach for the fabrication of a three-dimensional (3D) macroporous photonic crystal (PC) filter as an efficient photon reflector, which can be coated onto quantum dot (QD) based LSC devices. We demonstrate that by controlling the PC reflection band to match the emission profile of the QD emitters, the light trapping efficiency of the PC coated LSC (PC-LSC) can be significantly improved from 73.3% to 95.1% as compared to the conventional PC-free LSC due to the reduced escape cone photon loss. In addition, we have developed a simulation model that considers the PC reflector effect. Both experimental and simulation results show that the enhancement in LSC device performance induced by the PC reflector increases with increasing dimension. In fact, simulation data predicts a maximum of 13.3-fold enhancement in external quantum efficiency (EQE) and concentration factor (C factor) of the PC-LSC under more ideal conditions. Moreover, the simulation result offers insight into the relationship between photon output efficiencies and the geometric design of the PC-LSC. Our study sheds light on future design and fabrication of LSC devices with enhanced photon collection and concentrating efficiencies through novel and wavelength-selective photon reflectors.

Quantum Efficiency Enhancement of Bialkali Photocathodes by an Atomically Thin Layer on Substrates

Quantum efficiency (QE) enhancement in accelerator technology relevant to antimonide photocathodes (K2CsSb) is achieved by interfacing them with atomically thin 2D crystal layers. The enhancement occurs in a reflection mode, when a 2D crystal is placed in between the photocathodes and optically reflective substrates. Specifically, the peak QE at 405 nm (3.1 eV) increases by a relative 10%, whereas the long wavelength response at 633 nm (2.0 eV) increases by a relative 36% on average and up to 80% at localized “hot spot” regions when photocathodes are deposited onto graphene‐coated stainless steel. There is a similar effect for photocathodes deposited on hexagonal boron nitride monolayer coatings using nickel substrates. The enhancement does not occur when reflective substrates are replaced with optically transparent sapphire. Optical transmission, X‐ray diffraction (XRD), and X‐ray fluorescence (XRF) revealed that thickness, crystal orientation, quality, and elemental stoichiometry of photocathodes do not appreciably change due to 2D crystal coatings. These results suggest that optical interactions are responsible for the QE enhancements when 2D crystal sublayers are present on reflective substrates, and provide a pathway toward a simple method of QE enhancement in semiconductor photocathodes by an atomically thin 2D crystal on substrates.

Boosting the Photocatalytic Ability of g-C3N4 for Hydrogen Production by Ti3C2 MXene Quantum Dots.

The big challenging issues in photocatalytic H2 evolution are efficient separation of the photoinduced carriers, the stability of the catalyst, enhancing quantum efficiency, and requiring photoinduced electrons to enrich on photocatalysts' surface. Herein, Ti3C2 MXene quantum dots (QDs) possess the activity of Pt as a co-catalyst in promoting the photocatalytic H2 evolution to form heterostructures with g-C3N4 nanosheets (NSs) (denoted g-C3N4@Ti3C2 QDs). The photocatalytic H2 evolution rate of g-C3N4@Ti3C2 QD composites with an optimized Ti3C2 QD loading amounts (100 mL) is nearly 26, 3 and 10 times higher than pristine g-C3N4 NSs, Pt/g-C3N4, and Ti3C2 MXene sheet/g-C3N4, respectively. The Ti3C2 QDs increase the specific surface area of g-C3N4 and boost the density of the active site. Besides, metallic Ti3C2 QDs possess excellent electronic conductivity, causing the improvement of carrier transfer efficiency.

Highly solid-luminescent graphitic C3N4 nanotubes for white light-emitting diodes

Graphitic C3N4 (g-C3N4) has seldom been used for white light-emitting diode (LED) phosphors due to its low solid-state fluorescence quantum yield. Here, we report a facile thermal condensation path for the synthesis of g-C3N4 nanotubes stuffed with quantum dots. The g-C3N4 nanotubes have a high fluorescence quantum yield of 30.92% in the solid state. Nitrogen-rich and graphitic carbon-free features are responsible for the enhanced quantum efficiency. The photoluminescence quantum yield can be controlled and improved by the reaction temperature. We finally demonstrate white light emission by coating the highly solid-luminescent g-C3N4 nanotubes as phosphors onto a 370 nm ultraviolet LED.

Emission layer of F4TCNQ-Doped nanorods for high-efficient red light-emitting diodes

Abstract The emission layer (EML) of nanorods is doped with p-type 2,3,5,6-Tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ) to improve performances of quantum dot light-emitting diodes. The hole-electron balance has been found to enhance in these devices. This has been attributed to increase in hole transport and decrease in electron transport. F4TCNQ also helps to form a more close-packed film and decreases the percentage of voids in EML from 9.5% to 1.3%. It makes good contact between the EML and other functional layers. As a result, the optimized device acquires the 68% enhancement in external quantum efficiency to 11.6% and 85% enhancement in luminance to 16222 cd/m2, respectively.

Non-covalent Methods of Engineering Optical Sensors Based on Single-Walled Carbon Nanotubes.

Optical sensors based on single-walled carbon nanotubes (SWCNTs) demonstrate tradeoffs that limit their use in in vivo and in vitro environments. Sensor characteristics are primarily governed by the non-covalent wrapping used to suspend the hydrophobic SWCNTs in aqueous solutions, and we herein review the advantages and disadvantages of several of these different wrappings. Sensors based on surfactant wrappings can show enhanced quantum efficiency, high stability, scalability, and diminished selectivity. Conversely, sensors based on synthetic and bio-polymer wrappings tend to show lower quantum efficiency, stability, and scalability, while demonstrating improved selectivity. Major efforts have focused on optimizing sensors based on DNA wrappings, which have intermediate properties that can be improved through synthetic modifications. Although SWCNT sensors have, to date, been mainly engineered using empirical approaches, herein we highlight alternative techniques based on iterative screening that offer a more guided approach to tuning sensor properties. These more rational techniques can yield new combinations that incorporate the advantages of the diverse nanotube wrappings available to create high performance optical sensors.

Carbazole-Appended Salen-Indium Conjugate Systems: Synthesis and Enhanced Luminescence Efficiency.

Novel carbazole-conjugated salen-In complexes (Cz1 and Cz2) were prepared and fully characterized by 1H and 13C NMR spectroscopy, elemental analysis, and high-resolution mass spectrometry. The major low-energy absorption bands at λabs = 342 nm for Cz1 and 391 nm for Cz2, respectively, are assigned to typical intramolecular charge transfer (ICT) transitions between the carbazole unit and the salen-In center. The solvatochromism effects in various organic solvents and their large Stokes shift distinctly supported the ICT nature. The photoluminescent spectra of Cz1 and Cz2 showed broad emission bands are centered at 459 nm (blue, λex = 354 nm) and 507 nm (green, λex = 396 nm) in THF, respectively, which are typical feature of CT transitions. In particular, Cz1 showed 8-fold enhanced quantum efficiency relative to that of Cz2, at least 10-fold higher than those of the carbazole-free salen-In complexes. Such enhanced luminescence efficiency of Cz1 originated from efficient radiative decay based on the ICT transition between the salen-In moieties and carbazole parts, as well as its structural rigidity in conversion process between the ground (S0) and excited (S1) states. In other words, Cz2 exhibited low quantum yield due to its structural fluctuation, which is free rotation of both the appended carbazole moieties and bridged phenylene rings in conversion between the S0 and S1 structures. Theoretical calculations clearly supported these intriguing results. In addition, these salen-In complexes exhibited high thermal stability (Td5 = 367 °C for Cz1 and 406 °C for Cz2) and electrochemical stability.

Fluorescence enhancement of cationic styrylcoumarin-cucurbit[7]uril complexes: Enhanced stability and cellular membrane localization

Two red-emitting water-soluble fluorophores bearing a coumarin scaffold were synthesized and their interactions with cucurbit[7]uril [CB7] were investigated. A significant red-shift in the absorption spectra showed the interaction of coumarin dyes with the carbonyl portals of CB[7]. The formation of supramolecular complex is substantiated by the chemical shift changes in 1H-NMR and the thermodynamic parameters were evaluated by isothermal calorimetric titration (ITC). Upon encapsulation with the non-fluorescent CB host, significant enhancement of fluorescence quantum efficiency is observed. The large modulation of the emission properties of fluorophores in the presence of CB[7] indicates the formation of supramolecular complex. The advantageous enhanced quantum efficiency of the CB-dye complexes, is utilized to image live cells revealing localization in cellular membranes with greater photostability. These results based on inclusion complexes of the coumarin fluorophores with a non-toxic host CB[7] could lead to the design of efficient biological probes.

Quantum Efficiency Enhancement Depending on the Thickness of p-GaN Spacer Layer in Localized Surface Plasmon-Enhanced Near-Ultraviolet Light-Emitting Diodes by Using Colloidal Silver Nanoparticles

We demonstrated the dependence on thickness of p-GaN spacer layer in the localized surface plasmons (LSPs)-enhanced near-ultraviolet light-emitting diodes (NUV-LEDs) by pneumatic spray process using colloidal silver (Ag) nanoparticles (NPs). The LSPs-enhanced NUV-LEDs with 10- and 20-nm-thick p-GaN spacer layer showed enhanced internal quantum efficiency (IQE) and reduced effective exciton lifetime by introducing the colloidal Ag NPs. The IQE of LSPs-enhanced NUV-LEDs with 10- and 20-nm-thick p-GaN spacer layer was increased by 18.8% and 24.2%, respectively. These results indicate that the spontaneous emission rate is increased by LSPs-excitons resonant coupling. However, the NUV-LEDs with 40- and 100-nm-thick p-GaN spacer layer showed decreased IQE and extended exciton lifetime due to the evanescent wave property of LSPs field from colloidal Ag NPs.

Enhancement of spectral stability and outcoupling efficiency in organic light-emitting diodes with breath figure patterned microlens array films

We report outcoupling efficiency enhanced organic light-emitting diodes (OLEDs) using microlens array films prepared by a simple solution-processed imprinting technique with breath figure patterns. The films with breath figure micro-patterns (BFMP) effectively extract confined light in the glass substrate, which leads to the greatly improved outcoupling performance of OLEDs. We observe the enhancement of external quantum efficiency of the OLEDs employing the BFMP outcoupling film is about 1.51, corresponding to the enhancement of the power efficiency by a factor of 1.65. Moreover, the device with the outcoupling film shows the limited angle-dependency of emission spectrum over viewing angles. The BFMP films prepared with a low-cost, simple, and solution-processable fabrication process can pave the way to achieving high-performance OLEDs.

Light coupling to quasi-guided modes in nanoimprinted perovskite solar cells

Perovskite photovoltaics has emerged as a promising technology for highly efficient and low-cost solar cells. Further advances in the power conversion efficiencies are awaited by improved light harvesting concepts. One promising route to improve the current-generation in the perovskite solar cells employs nanophotonic perovskite layers. This work reports on a facile route to fabricate nanophotonic perovskite solar cells with enhanced current generation by employing thermal nanoimprint lithography. The nanoimprinted perovskite solar cells show a relative increase in power conversion efficiency by 2% with respect to their planar reference devices. The enhancement in the external quantum efficiency near the bandgap of the nanoimprinted perovskite solar cells by the coupling of the incident light to quasi-guided modes is analyzed in detail. As an outlook, the potential of the nanoimprinting route for semi-transparent perovskite solar cells is discussed via numerical simulations with relative enhancements in short-circuit current density of up to 9%. This increase is particularly promising for nanophotonic perovskite-based multi-junction solar cells.

Interlayer doping with p-type dopant for charge balance in indium phosphide (InP)-based quantum dot light-emitting diodes.

A 2,3,4,6-tetrafluoro-7,7,8,8,-tetracyanoquinodimethane (F4-TCNQ) doping interlayer was developed to improve charge imbalance and the efficiency in indium phosphide (InP)-based quantum dot light-emitting diodes (QLEDs). The doping layer was coated between a hole injecting layer (HIL) and a hole transport layer (HTL) and successfully diffused with thermal annealing. This doping reduces the hole injection barrier and improves the charge balance of InP-based QLEDs, resulting in enhancement of an external quantum efficiency (EQE) of 3.78% (up from 1.6%) and a power efficiency of 6.41 lm/W (up from 2.77 lm/W). This work shows that F4-TCNQ interlayer doping into both HIL and HTL facilitates hole injection and can provide an efficient solution of improving charge balance in QLED for the device efficiency.