Hole Injection Process Research Articles

Molecular engineering of inorganic halide perovskites and HTMs for photovoltaic applications

AbstractA comprehensive study was performed for the design of ABX3 perovskites, (A = Li, K, Na, B = Ge, Sn, Pb, X = F, Cl, Br, I) and organic hole transfer materials, HTMs (Fu‐2a, Fu‐2b, Fu‐2c, and Dm‐Q) for efficient perovskite solar cells (PSCs) through quantum chemistry calculations. Photovoltaic characteristics of the investigated perovskites are strongly affected by the halide anions. The results reveal that reducing the exciton binding energy of perovskites enhances the rate of the formation/dissociation of holes and electrons so F‐based perovskites are superior from this viewpoint. Additionally, the electron and hole injection processes are more favorable in the case of the F‐based perovskites in comparison with other studied perovskites. Moreover, spectroscopic properties of the perovskites demonstrate that KSnCl3, NaSnCl3, and F‐based perovskites exhibit a greater ability of the light‐harvesting and incident photon to current conversion efficiency. Ultimately, based on diverse analyses, F‐based perovskites, KSnCl3 and NaSnCl3 are the preferred candidates to be applied in the PSCs due to an excellent incident photon to current conversion efficiency, light‐harvesting efficiency, short circuit current, and solar cell final efficiency.

Coreactant-free and Near-Infrared Electrochemiluminescence Immunoassay with n-Type Au Nanocrystals as Luminophores.

The electrochemiluminescence (ECL) bioassay is prominently carried out with the involvement of the coreactant. To remove the detrimental effects of the coreactant on the ECL of luminophores, herein, a promising ECL immunoassay strategy with biocompatible nanoparticles as the luminophore is proposed, which involves directly and electrochemically oxidizing the luminophore methionine-capped Au (Met@Au) nanocrystals (NCs) without the participation of any coreactant. Met@Au NCs are a kind of n-type nanoparticles, and they can be electrochemically injected with valence band (VB) holes around +0.80 and +1.10 V (vs Ag/AgCl). The electrochemically injected exogenous VB hole can recombine with the endogenous conduction band electron of Met@Au NCs and eventually bring out two coreactant-free and near-infrared ECL processes around 0.80 V (ECL-1) and 1.10 V (ECL-2). The intensity of coreactant-free ECL is primarily determined by the electrochemical oxidation-induced hole-injection process. ECL-2 is considerably stronger than ECL-1 and can be exploited for determining the carcinoembryonic antigen (CEA) in a sandwich immunoassay procedure with a linear range from 0.1 to 50 pg/mL as well as a limit of detection of 0.03 pg/mL (S/N = 3).

Unraveling the effect of shell thickness on charge injection in blue quantum-dot light-emitting diodes

Although huge progress has been made for quantum-dot light-emitting diodes (QLEDs), blue devices remain a short slab limiting the development of QLED based applications. For blue devices, a fundamental and key question—how the shell thickness affects the charge injection—remains unclear by far. Herein, we show a comprehensive investigation on the effect of shell thickness on the charge injection by employing blue CdxZn1−xSeyS1−y/ZnS core/shell QDs with various shell thicknesses as the emission layers. We demonstrate that, during the electroluminescence onset processes, the fast and slow components of the rising edges characterized by transient electroluminescence spectra are strongly associated with the electron and hole injection processes, respectively. We find that the shell thickness influences the injection of electrons more dramatically than that of holes. A thick shell suppresses the electron injection and consequently reaches a more balanced charge injection into QDs, hence leading to an enhanced efficiency.

Electrochemistry and Electrochemiluminescence of Coumarin Derivative Microrods: Mechanism Insights.

Organic molecules and related nanomaterials have attracted extensive attention in the realm of electrochemiluminescence (ECL). Herein, a well-known electroluminescence (EL) dopant 2,3,6,7-tetrahydro-1,1,7,7,-tetramethyl-1H,5H,11H-10-(2-benzothiazolyl)quinolizino-[9,9a,1gh] coumarin (C545T) is selected as a new ECL illuminant, which shows a high photoluminescence quantum yield of nearly 100% and excellent ECL performance in the organic phase. For utilizing C545T to achieve ECL detection in aqueous solution, organic microrods of C545T (C545T MRs) were synthesized by a precipitation method. Cyclic voltammetry and differential pulse voltammetry of C545T and C545T MRs in acetonitrile or phosphate buffer showed one reduction and multiple oxidation peaks, suggesting that the multiple charge states of C545T could be produced by continuous electron- or hole-injection processes. The annihilated ECL emission of C545T and C545T MRs was observed using ECL transient technology. In the presence of triethanolamine (TEOA) or potassium persulfate (K2S2O8), C545T MRs can also give bright anodic and cathodic ECL emission at the GCE/water interface. The proposed ECL system not only has multichannel ECL emission but also shows intense yellow emission (569 nm) with a relative ECL efficiency of 0.81 when TEOA was used as a coreactant. Benefiting from the strong ECL emission of the C545T MRs/TEOA system and the quenching effect of dopamine (DA) on ECL, a convenient sensor for DA was developed with high selectivity and sensitivity.

High performance electron blocking layer-free InGaN/GaN nanowire white-light-emitting diodes.

We investigated the effect of coupled quantum wells to reduce electron overflow in InGaN/GaN dot-in-a-wire phosphor-free white color light-emitting diodes (white LEDs) and to improve the device performance. The light output power and external quantum efficiency (EQE) of the white LEDs with coupled quantum wells were increased and indicated that the efficiency droop was reduced. The improved output power and EQE of LEDs with the coupled quantum wells were attributed to the significant reduction of electron overflow primarily responsible for efficiency degradation through the near-surface GaN region. Compared to the commonly used AlGaN electron blocking layer between the device active region and p-GaN, the incorporation of a suitable InGaN quantum well between the n-GaN and the active region does not adversely affect the hole injection process. Moreover, the electron transport to the device active region can be further controlled by optimizing the thickness and bandgap energy of this InGaN quantum well. In addition, a blue-emitting InGaN quantum well is incorporated between the quantum dot active region and the p-GaN, wherein electrons escaping from the device active region can recombine with holes and contribute to white-light emission. The resulting device exhibits high internal quantum efficiency of 58.5% with highly stable emission characteristics and virtually no efficiency droop.

Tetraalkyl-substituted zinc phthalocyanines used as anode buffer layers for organic light-emitting diodes**Project supported by the Shenzhen Personal Maker Project, China (Grant No. GRCK2017082316173208), the Shenzhen Overseas High-level Talents Innovation Plan of Technical Innovation, China (Grant No. KQJSCX20180323140712012), and the Special Funds for the Development of Strategic Emerging Industries in Shenzhen, China (Grant No. JCJY20170818154457845).

Two soluble tetraalkyl-substituted zinc phthalocyanines (ZnPcs) for use as anode buffer layer materials in tris(8-hydroxyquinoline)aluminum (Alq3)-based organic light-emitting diodes (OLEDs) are presented in this work. The hole-blocking properties of these ZnPc layers slowed the hole injection process into the Alq3 emissive layer greatly and thus reduced the production of unstable cationic Alq3 () species. This led to the enhanced brightness and efficiency when compared with the corresponding properties of OLEDs based on the popular poly-(3, 4-ethylenedioxythiophene): poly(styrene sulfonate) (PEDOT: PSS) buffer layer. Furthermore, because of the high thermal and chemical stabilities of these ZnPcs, a nonaqueous film fabrication process was realized together with improved charge balance in the OLEDs and enhanced OLED lifetimes.

Moisture proof hole transport layers based on CISe quantum dots for highly stable and large active area perovskite solar cells

Extensive studies have been performed to improve the environmental stability of perovskite solar cells (PSCs) with the use of inorganic charge transport layers (CTLs). However, for n-i-p structures, it is difficult to deposit p-type inorganic nanocrystals onto perovskites to form the CTLs because they are usually prepared in polar solvents. In this regard, hydrophobic nanoparticles dispersed in nonpolar solvents would be beneficial for their deposition onto the perovskites, thus leading to the formation of a hole transport layer (HTL). In this work, we report on the preparation of monodispersed CuIn1.5Se3 (CISe) quantum dots (QDs) (diameter = 4 nm) for the design of PSCs based on all-inorganic CTLs. By means of efficient hole injection and transfer process through the CISe-HTLs, impressive power conversion efficiencies (PCEs) of 13.72% and 12.19% for active areas of 0.12 cm2 and 1.0 cm2 are achieved, respectively, and the devices exhibit hysteresis-less behaviors. Furthermore, the devices show excellent PCE retentions of 89.2% and 74.9% after 30 d relative to their initial values at relative humidity of 25% and 50%, respectively. The hydrophobic QDs effectively suppress the penetration of moisture such that the device maintains its stability in humid environments.

Graphene/p-AlGaN/p-GaN electron tunnelling light emitting diodes with high external quantum efficiency

The realization of p-type conduction in GaN and commercialized GaN light-emitting diodes (LED) is accepted as an advance in the development of human society. Conventionally, LED is based on PN junction and the quantum wells are introduced for confining electrons and holes, which improves the external quantum efficiency (EQE). Herein, we realized a graphene/AlGaN/GaN LED emitting double peaks at 430 nm and 560 nm. The blue peak at 430 nm is originated from the recombination process between holes injected from graphene/AlGaN heterostructure and electron produced from the tunnelling and avalanche multiplication process in p-type AlGaN/p-type GaN interface. Compared with the bare p-type AlGaN/GaN heterostructure without blue light emission, the graphene/p-AlGaN enhances the hole injection process through the band gap alignment and the mediate thickness of p-AlGaN is important taking the hole transport in AlGaN into consideration. The AlGaN/GaN interface confines the produced electrons and thus result in a high EQE of 55.8% under 5 mA current after intercalating an Al2O3 barrier at the interface between graphene and AlGaN/GaN. Moreover, the tunable graphene/AlGaN/GaN LED with different intensity of 430 nm has been demonstrated through changing the Fermi level of graphene by gating effect. The graphene/AlGaN heterostructure induced confinement effect can bring a novel type of LED, which can be optimized for special applications.

Identifying the acceptor state in NiO hole collection layers: direct observation of exciton dissociation and interfacial hole transfer across a Fe2O3/NiO heterojunction.

NiO is widely utilized as a hole transport layer in solar energy devices where light absorption in a photoactive layer is followed by charge separation and hole injection into a NiO collection layer. Due to the complex electronic structure of the hybridized valence band in NiO, the chemical nature of the hole acceptor state has remained an open question, despite the fact that hole localization in this material significantly influences device efficiency. To comment on this, we present results of ultrafast charge carrier dynamics in a NiO based model heterojunction (Fe2O3/NiO) using extreme ultraviolet reflection-absorption (XUV-RA) spectroscopy. Element specific XUV-RA spectroscopy demonstrates the formation of transient Ni3+ within 10 ps following selective photoexcitation of the underlying Fe2O3 substrate. This indicates that hole transfer in this system occurs to NiO valence band states composed of significant Ni 3d character. Additionally, we show that this hole injection process proceeds via a two-step sequential mechanism where fast, field-driven exciton dissociation occurs in Fe2O3 in 680 ± 60 fs, followed by subsequent hole injection to NiO in 9.2 ± 2.9 ps. These results reveal the chemical nature of the hole acceptor state in widely used NiO hole transport layers and provides a direct observation of exciton dissociation and interfacial hole transfer in this model system.

InGaN/GaN light-emitting diode having direct hole injection plugs and its high-current operation.

The light-emitting diode (LED) with an improved hole injection and straightforward process integration is proposed. p-type GaN direct hole injection plugs (DHIPs) are formed on locally etched multiple-quantum wells (MQWs) by epitaxial lateral overgrowth (ELO) method. We confirm that the optical output power is increased up to 23.2% at an operating current density of 100 A/cm2. Furthermore, in order to identify the origin of improvement in optical performance, the transient light decay time and light intensity distribution characteristics were analyzed on the DHIP LED devices. Through the calculation of the electroluminescence (EL) decay time, internal quantum efficiency (IQE) is extracted along with the recombination parameters, which reveals that the DHIPs have a significant effect on enhancement of radiative recombination and reduction of efficiency droop. Furthermore, the mapping PL reveals that the DHIP LED also has a potential to improve the light extraction efficiency by hexagonal pyramid shaped DHIPs.

Generation of Long-Lived Redox Equivalents in Self-Assembled Bilayer Structures on Metal Oxide Electrodes

We report on the synthesis and photophysical properties of a photocathode consisting of a molecular bilayer structure self-assembled on p-type NiO nanostructured films. The resulting photocathode and its nanostructured indium–tin oxide analog absorb visible light and convert it into injected holes with injection yields of ∼30%, measured at the first observation time by nanosecond transient absorption spectroscopy, and long-lived reducing equivalents that last for several milliseconds without applied bias. An initial quantum yield of 15% was achieved for photogeneration of the reduced dye on the p-NiO electrode. Nanosecond transient absorption experiments and detailed analyses of the underlying electron transfer steps demonstrate that the overall efficiency of the cell is limited by hole injection and charge recombination processes. Compared with the highly doped indium–tin oxide photocathode, the NiO photocathode shows superior photoconversion efficiencies for generating reducing equivalents and longer li...

Optoelectronic devices on AlGaN/GaN HEMT platform

Integration of a photon source into the AlGaN/GaN high election mobility transistor (HEMT) platform will realize the functionality of on‐chip optical pumping of deep electron traps which suppress the dynamic performances of power HEMTs. Here, we report a Schottky‐on‐heterojunction light‐emitting diode (SoH‐LED) realized on the p‐doping‐free lateral AlGaN/GaN heterostructure. A physical mode based on hot electron induced surface states impact ionization was proposed to explain the hole generation and injection processes in this p‐doping‐free SoH‐LED. Since the SoH‐LED shares identical epitaxial structures with HEMT, integration of SoH‐LED and HEMT requires no additional epi‐layers during the wafer growth and minimum process modification during device fabrication. The SoH‐LED structure was seamlessly integrated into the HEMT platform as an on‐chip photon source. Experiment results showed that the SoH‐LED photons can effectively assist the electron de‐trapping processes from both of the surface and bulk deep traps, demonstrating the feasibility of using on‐chip generated photons to improve the dynamic performances of AlGaN/GaN power HEMTs.

Correction: Corrigendum: The work mechanism and sub-bandgap-voltage electroluminescence in inverted quantum dot light-emitting diodes

Through introducing a probe layer of bis(4,6-difluorophenylpyridinato-N,C2)picolinatoiridium (FIrpic) between QD emission layer and 4, 4-N, N- dicarbazole-biphenyl (CBP) hole transport layer, we successfully demonstrate that the electroluminescence (EL) mechanism of the inverted quantum dot light-emitting diodes (QD-LEDs) with a ZnO nanoparticle electron injection/transport layer should be direct charge-injection from charge transport layers into the QDs. Further, the EL from QD-LEDs at sub-bandgap drive voltages is achieved, which is in contrast to the general device in which the turn-on voltage is generally equal to or greater than its bandgap voltage (the bandgap energy divided by the electron charge). This sub-bandgap EL is attributed to the Auger-assisted energy up-conversion hole-injection process at the QDs/organic interface. The high energy holes induced by Auger-assisted processes can be injected into the QDs at sub-bandgap applied voltages. These results are of important significance to deeply understand the EL mechanism in QD-LEDs and to further improve device performance.

Structure and energy level alignment at the dye-electrode interface in p-type DSSCs: new hints on the role of anchoring modes from ab initio calculations.

p-type dye-sensitized solar cells (DSSCs) represent the complementary photocathodes to the well-studied n-type DSSCs (Grätzel cells), but their low performances have hindered the development of convenient tandem solar cells based on cost-effective n- and p-type DSSCs. Because of their low efficiencies, experimental investigations highlighted the role of hole-electron transport processes at the dye-electrode interface. However, the effects of the dye anchoring groups on interfacial electronic features are still unclear. We report here a first principles study of a benchmark p-type DSSC model, namely the widely used Coumarin-based dye C343 adsorbed on the p-NiO surface. Together with the original carboxylic acid, we test the alternative phosphonic acid as the anchoring group. We investigate binding energies, structural features and electronic energy level alignments: our results highlight that these properties are highly sensitive to the binding modes. In particular, both the chemical nature of the anchoring group and the binding mode strongly affect the thermodynamic driving force for the dye-electrode hole injection process. From analysis of the electronic densities, we find that favorable driving forces are correlated with small values of the interfacial electrostatic dipole that is formed upon dye adsorption. From our results, we derive new hints for improving open circuit potential and the hole injection process in p-type DSSCs based on NiO electrodes.

Performance Improvement of P3HT:CdSe Hybrid Solar Cell by Modifying Hole Injection Layer

Hybrid inorganic-organic solar cells based on bulk heterojunction structures are promising devices in the field of clean energy. Employing quantum dot nanocrystals such as CdS, CdSe, and CdTe have shown great potential in improving their photovoltaic performance because of their stability and tunability. Numerical study of these devices can provide more modifications in their structures. In this work, the performance dependency of ITO/PEDOT:PSS/ P3HT:CdSe/Al hybrid solar cell on hole conducting layer properties is investigated. First, hybrid cell with mentioned structure is simulated by solving Poisson's equations and using Poole- Frenkel mobility and Langevin recombination models. Then, in order to improve the hole injection process, the work function of hole injection layer (PEDOT:PSS) is changed by doping the layer. Calculated photovoltaic performance shows that by decreasing the work function from 5 .1eV to 4.9eV, the short circuit current (Jsc) and fill factor (FF) are enhanced, and lead power conversion efficiency increases. The achieved improvement is because of the more efficient hole injection between highest occupied molecular orbital (HOMO) level of P3HT and PEDOT:PSS Fermi level.

The work mechanism and sub-bandgap-voltage electroluminescence in inverted quantum dot light-emitting diodes.

Through introducing a probe layer of bis(4,6-difluorophenylpyridinato-N,C2)picolinatoiridium (FIrpic) between QD emission layer and 4, 4-N, N- dicarbazole-biphenyl (CBP) hole transport layer, we successfully demonstrate that the electroluminescence (EL) mechanism of the inverted quantum dot light-emitting diodes (QD-LEDs) with a ZnO nanoparticle electron injection/transport layer should be direct charge-injection from charge transport layers into the QDs. Further, the EL from QD-LEDs at sub-bandgap drive voltages is achieved, which is in contrast to the general device in which the turn-on voltage is generally equal to or greater than its bandgap voltage (the bandgap energy divided by the electron charge). This sub-bandgap EL is attributed to the Auger-assisted energy up-conversion hole-injection process at the QDs/organic interface. The high energy holes induced by Auger-assisted processes can be injected into the QDs at sub-bandgap applied voltages. These results are of important significance to deeply understand the EL mechanism in QD-LEDs and to further improve device performance.

Organic Sensitizers with Pyridine Ring Anchoring Group for p-Type Dye-Sensitized Solar Cells

Recently, p-type dye-sensitized solar cells (p-DSSCs) have attracted increasing attention. The widely used carboxylic acid groups for TiO2 based sensitizers may not be the optimal choice for p-DSSCs. Herein new donor-π-acceptor organic sensitizers with pyridine ring as anchoring group are designed and synthesized for p-DSSCs. The detailed investigation demonstrates that carboxylic acid groups may have an effect on the negative shift of the valence band edge of NiO induced by surface protonation, which lowers the hole-injection process and the device photovoltage, while the pyridine ring works effectively without this problem. The p-DSSC based on the new sensitizer shows an overall conversion efficiency of ∼0.16% under full sunlight (AM 1.5G, 100 mW cm–2) irradiation.

Bandgap engineered and high monochromatic electrochemiluminescence from dual-stabilizers-capped CdSe nanocrystals with practical application potential

The development of electrochemiluminescence (ECL) emitters over a broad spectrum of wavelengths is anticipated for the multiplexed ECL sensing and diagnostic application. Herein, a facile dual-stabilizers-capped strategy was developed for synthesizing a series of water-soluble CdSe nanocrystals (NCs) with strong, bandgap engineered and monochromatic ECL emissions in greenish region. The linkage of surface cadmium atoms to the dual stabilizers, mercaptopropionic acid and sodium hexametaphosphate, not only can effectively remove the nonradiative surface state and deep surface trap of CdSe NCs for improved ECL efficiency and monochromaticity, but is also favorable for the electrochemical involved electron and hole injection processes for higher ECL intensity. ECL spectra of dual-stabilizers-capped CdSe NCs are narrow and much close to corresponding photoluminescence spectra, which indicates a series of bandgap engineered and monochromatic ECL emitters. The ECL signal of dual-stabilizers-capped CdSe NCs also shows acceptable accuracy and precision for the detection of dopamine concentration in the practical drug (dopamine hydrochloride injection).

Optical absorption and charging effect in nano-crystalline Ge/SiNx multilayers

Nanocrystalline Ge (nc-Ge)/SiNx multilayers and sandwiched structures were fabricated by thermally annealing amorphous Ge/SiNx layered films at 600°C. The evolution of microstructure before and after annealing was studied by various characterization techniques, which reveals the formation of nc-Ge after annealing. The tunable optical absorption and band gap were observed by changing the grain size of nc-Ge. The study on carrier transport behavior of nc-Ge/SiNx MLs indicated that the transport process was dominated by space-charge limited current mechanism. Furthermore, the charging storage effect in SiNx/nc-Ge/SiNx floating gate structures was demonstrated due to both the electron and hole injection processes.

Probing of Transient Electric Field Distribution in ITO/PI/P3HT/Au By Time-Resolved Optical Second Harmonic Generation Measurement

By using time-resolved optical second harmonic generation (TR-SHG) measurements, we studied carrier behaviors in poly(3-hexylthiophene) (P3HT) metal-insulator-semiconductor (MIS) diodes. TR-SHG measurements probed transients of electric fielddistribution in the P3HT active layer. Results showed that hole injection and removal processes were non-reversal, where the response times were different from each other and the relaxation time of the transient electric field strongly depended on the hole injection process.