Charge Blocking Layer Research Articles

High‐Detectivity UV–Visible–NIR Broadband Polymer Photodetector with Polymer Charge Blocking Layer Cross‐Linked by Organic Photocrosslinker

AbstractUltraviolet (UV), visible, and near‐infrared (NIR) broadband organic photodetectors are fabricated by sequential solution‐based thin film coatings of a polymer electron blocking layer (EBL) and a polymer photoactive layer. To avoid damage to a preceding polymer EBL during a subsequent solution‐based film coating of a polymer photoactive layer due to lack of solvent orthogonality, 2‐(((4‐azido‐2,3,5,6‐tetrafluorobenzoyl)oxy)methyl)−2‐ethylpropane‐1,3‐diyl bis(4‐azido‐2,3,5,6‐tetrafluorobenzoate) (FPA‐3F) is used as a novel organic cross‐linking agent activated by UV irradiation with a wavelength of 254 nm. Solution‐processed poly[N,N′‐bis(4‐butylphenyl)‐N,N′‐bis(phenyl)‐benzidine] (poly‐TPD) films, which are cross‐linked with a FPA‐3F photocrosslinker, are used for a preceding polymer EBL. A ternary blend film composed of PTB7‐Th, COi8DFIC, and PC71BM is used as a NIR‐sensitive organic photoactive layer with strong photosensitivity in multispectral (UV–visible–NIR) wavelengths of 300–1,050 nm. Poly‐TPD films are successfully cross‐linked even with a very small amount of 1 wt% FPA‐3F. Small amounts of FPA‐3F have little detrimental effect on the electrical and optoelectronic properties of the cross‐linked poly‐TPD EBL. Finally, organic NIR photodetectors with a poly‐TPD EBL cross‐linked by the small addition of FPA‐3F (1 wt%) show the detectivity values higher than 1 × 1012 Jones for the entire UV–visible–NIR wavelengths from 300 nm to 1050 nm, and the maximum detectivity values of 1.41 × 1013 Jones and 8.90 × 1012 Jones at the NIR wavelengths of 900 and 1000 nm, respectively.

Organic- inorganic nanoparticle composite as an electron injection/hole blocking layer in organic light emitting diodes for large area lighting applications

The charge carrier balance in organic light-emitting diodes (OLEDs) is crucial for optimizing external quantum efficiency (EQE). This is typically achieved by using charge injection and blocking layers deposited by vacuum deposition, which increases the fabrication time and cost. This study investigates the potential of polyethylenimine ethoxylated (PEIE), Zinc oxide (ZnO) nanoparticles and a PEIE-ZnO nanocomposite as functional layers for OLEDs with improved electron injection and hole blocking properties fabricated by solution processing. The PEIE-ZnO nanocomposite improves OLED performance: (a) a 28% increase in current density and enhanced electron mobility, (b) two times increase in electron mobility of the electron injection/transport layer, and (c) a reduction in surface and interface trap density. Consequently, the PEIE-ZnO nanocomposite shows an 84% increase in electroluminescence, 52.5% increase in luminous efficacy, 35.5% increment in external quantum efficiency, and increased stability as measured by degradation studies. Employing these interlayers fabricated by the novel and roll-to-roll compatible Spray-on-Screen deposition technique show a comparable OLED performance as that of spin coated and vacuum deposited electron injection layers, which makes the large area fabrication of optoelectronic devices with high efficiencies, high output, low cost, and long lifetime a possible reality.

Artificial Synaptic InGaZnO Thin-Film Transistor With Long Retention Behavior Using Al2O3/SiO2 Gate Insulator

We present a coplanar amorphous InGaZnO (a-IGZO) synaptic thin film transistor (TFT) with counterclockwise hysteresis using an aluminum oxide (Al2O3)/SiO2 gate insulator (GI). The excitatory postsynaptic current (EPSC), paired-pulse facilitation (PPF), short-term plasticity (STP), and long-term plasticity, which are biological synaptic characteristics, are demonstrated by positive charge trapping in the Al2O3 layer. Long retention time can be possible by surface charge in the SiO2, a charge-blocking layer. As a result, multi-level channel conductance can be modulated with positive and negative gate pulses. These results prove synaptic a-InGaZnO TFT with Al2O3/SiO2 stack GI for high-density neuromorphic devices.

KNbO3 photoelectrode for DSSC: a structural, optical and electrical approach.

In this work, we present the potassium niobate (KNbO3) nanoparticles as a suitable mesoporous photoelectrode for dye-sensitized solar cells (DSSCs). The KNbO3 particles were synthesized by the microwave-assisted hydrothermal method using mild conditions and characterized by SEM, XRD, Raman, and UV-Vis diffuse reflectance. The particles presented a pyramidal tower-like shape with an orthorhombic structure and an indirect bandgap of (3.0 ± 0.1) eV. Dye-sensitized solar cells were assembled using the synthesized KNbO3 nanoparticles, which were deposited as a photoelectrode on a TiO2 recombination charge blocking layer. It is noticeable that the synergistic operation of the TiO2 blocking layer and KNbO3 photoelectrode is essential to achieve photovoltaic behaviour in our solar cells. The short-circuit current density of Jsc = 2.82 mA, open-circuit voltage Voc = 669 mV, fill factor FF = 0.62, and a power conversion efficiency PCE = 1.17%, reports elevated parameters if compared to other DSSCs alternative materials, becoming potassium niobate suitable as photoelectrode.

Enhanced High-Temperature Capacitive Performance of a Bilayer-Structured Composite Film Employing a Charge Blocking Layer

The great development potential of polymer dielectric capacitors in harsh environments urgently requires enhancing capacitive performance at high temperatures. However, the exponentially increased conduction loss at high temperature and high field results in a drastic drop in energy density and charge-discharge efficiency. Here, a bilayer-structured polyimide (PI) composite film containing a wide-band gap inorganic layer as a charge blocking layer is designed. The inorganic layer improves the charge trapping ability and regulates the charge mobility at the electrode/dielectric interface. The charge injection mechanism in the interface-optimized PI/boron nitride nanosheet (BNNS) composite films is investigated by finite element simulation, and the effect of the BNNS layer on high temperature conduction is further understood. An appropriate thickness of the charge blocking layer establishes an effective energy barrier. Therefore, the composite films exhibit significantly suppressed conduction loss and excellent capacitive performance at a high temperature. A high energy density of 4.37 J cm-3 with efficiency of 92% is obtained at 200 °C and 500 MV m-1, which is superior to reported high-temperature dielectric polymers and their composite films. This work provides a promising approach to improve the energy storage performance of polymer materials at high temperatures.

Piezoelectric composites from sandwiched polydimethylsiloxane sponges

A novel elastomer-based composite material with enhanced piezoelectric performances is proposed in this paper, which is composed of the top and bottom polytetrafluoroethylene (PTFE) solid films with the middle PTFE nanoparticle–polydimethylsiloxane (PDMS) sponge layer. To enhance the charge retention capability of elastomers, PTFE nanoparticles are introduced to form PTFE–PDMS interfaces, which can trap charges with longevity. Besides, PTFE solid films take on the role of the charge blocking layers to further improve the piezoelectric performances. As a result, the PTFE–PDMS sandwich structure shows the advantages of remarkable sensitivity (1053 pC/N), high stability, and flexibility. After a 6 h of annealing treatment at the temperature of 100 °C, no significant deterioration of the piezoelectric properties can be observed, which reveals the great thermal stability of the sandwich structure. In addition, the sandwich structure can be immersed in water for 24 h without any loss of piezoelectric activity. Finally, the experiment of lighting one LED by hand pressing successfully demonstrates that the sandwich structure has good applicability in the field of energy harvesting. Considering the excellent electrical and mechanical features, the PTFE–PDMS sandwich structure has promising applications in sensing, energy harvesting, and actuation.

Robust room‐temperature ferroelectricity and magnetoelectric coupling effect in epitaxial CaTiO3/SmFeO3 thin films

AbstractSmFeO3 has received the increasing scientific attention as a promising room‐temperature single‐phase multiferroic material with great application potential in the next‐generation spintronic devices. However, the ferroelectricity of SmFeO3 has remained as a controversial issue. In the present work, high resistivity incipient ferroelectric CaTiO3 was introduced as a charge blocking layer on an SmFeO3 thin film to suppress the leakage current. CaTiO3/SmFeO3/Nb–SrTiO3 (1 0 0) thin films were prepared by a PLD (pulsed laser deposition) process, and the epitaxial growth of thin films was confirmed by XRD (X‐ray diffraction) and HRTEM (high resolution transmission electron microscope) images. The interface strain resulted in the polar non‐centrosymmetry in space group P42/mc for SmFeO3, and subsequently the room‐temperature ferroelectricity was achieved with a robust remanent polarization of about 4.9 μC/cm2. The 180° switchable domain structure was confirmed by PFM (piezoelectric force microscope). Meanwhile, strong magnetoelectric coupling with magnetoelectric coefficient (αME) of about 2.5 V/(cm Oe) was observed. The present work has revealed the structural origin of ferroelectricity in an SmFeO3 thin film and provided the great reference for its room temperature multiferroic applications.

Effects of charge mobilities and exciton blocking capabilities of charge blocking layers on the performance of organic light-emitting diodes

Blue thermally activated delayed fluorescence organic light-emitting diodes (OLEDs) have been fabricated using engineered charge blocking layers. It is found that the device efficiency increases with increasing the exciton blocking capability of electron blocking layer. But the device stability decreases concomitantly, mostly because the hole mobility of electron blocking layer decreases with exciton blocking capability increasing, thereby leading to the smaller width of exciton formation zone. In addition, the device stability decreases with the electron mobility of hole blocking layer decreasing, despite nearly same exciton blocking capabilities, mainly because the width of exciton formation zone decreases with the electron mobility of hole blocking layer decreasing. The width of exciton formation zone is understood based on the transit times for holes from anode to exciton formation zone and for electrons from cathode to exciton formation zone. We provide some in-depth insight useful for the delicate design of fluorescent, phosphorescent, and thermally activated delayed fluorescence OLEDs.

Unraveling the Origin of Dark Current in Organic Bulk Heterojunction Photodiodes for Achieving High Near-Infrared Detectivity

Organic infrared materials are attractive due to their high absorption coefficients and facile tuning of the absorption spectrum beyond that of silicon. Although bulk heterojunctions (BHJs) are widely used in organic photodetectors (OPDs), achieving a high signal-to-noise ratio in the near-infrared spectrum is often challenging due to the inherently large dark currents in low-bandgap polymers and their blending morphology effect. Herein, we investigate the origin of dark currents in near-infrared OPDs and reveal a strategy to improve the detectivity in terms of two aspects: (1) donor–acceptor blending morphology and (2) effects of carrier injection from outer electrodes. To do so, a series of random terpolymers were synthesized in a similar band structure and their blending morphology with the PC71BM acceptor was gradually controlled. As the phase separation was smaller, the dark current gradually reduced while the responsivity increased, leading to a higher detectivity. Complex charge-transporting paths formed under the fine percolating network account for the dark current reduction, while the increased heterojunction area facilitates the dissociation of the photogenerated excitons. From our thermal admittance spectroscopy, deeper sub-bandgap states were found under the smaller phase separation, further contributing to the dark current reduction. Second, the carrier injection effect was revealed by inserting a charge-blocking layer at the active layer/electrode interface. While manipulating both parameters could yield a high near-infrared detectivity up to 1012 Jones at −0.5 V, the blending morphology effect turns out to be more dominant over the carrier injection effect in suppressing the dark current and achieving higher detectivity in BHJ OPDs.

Highly Sensitive UV–Vis‐to‐Near‐Infrared Organic Photodetectors Employing ZnO: Polyethylenimine Ethoxylated Composite as Hole‐Blocking Layer

Significantly suppressed leakage current and reduced shot noise in organic photodetectors (OPDs) are achieved by employing charge blocking layers, which have led to tremendous advances in highly sensitive devices with photoresponse covering from the ultraviolet to near‐infrared regions. However, trap‐assisted charge carrier injection through tunneling can significantly contribute to the sources of leakage current upon the use of charge blocking layers. Herein, it is shown that leakage current in organic photodetectors can be effectively reduced to an intrinsic lower limit by using a composite hole blocking layer (HBL) that consists of zinc oxide (ZnO) blended with different weight concentration of polymer polyethylenimine ethoxylated (PEIE). The best device shows an ultralow dark current density down to 0.18 nA cm−2, which translates to high specific detectivity (D*) over 1 × 1013 Jones in broad response range from 340 to 1100 nm (with peak value of 4.2 × 1013 Jones at 940 nm and 3.5 × 1013 Jones at 1050 nm), approaching to the intrinsic dark current‐limited detectivity value. It is found that the incorporated N atoms fill the deficient site of oxygen in ZnO, thus giving rise to the reduced proportion of emission via defects.

Ultralow dark current in near-infrared perovskite photodiodes by reducing charge injection and interfacial charge generation

Metal halide perovskite photodiodes (PPDs) offer high responsivity and broad spectral sensitivity, making them attractive for low-cost visible and near-infrared sensing. A significant challenge in achieving high detectivity in PPDs is lowering the dark current density (JD) and noise current (in). This is commonly accomplished using charge-blocking layers to reduce charge injection. By analyzing the temperature dependence of JD for lead-tin based PPDs with different bandgaps and electron-blocking layers (EBL), we demonstrate that while EBLs eliminate electron injection, they facilitate undesired thermal charge generation at the EBL-perovskite interface. The interfacial energy offset between the EBL and the perovskite determines the magnitude and activation energy of JD. By increasing this offset we realized a PPD with ultralow JD and in of 5 × 10−8 mA cm−2 and 2 × 10−14 A Hz−1/2, respectively, and wavelength sensitivity up to 1050 nm, establishing a new design principle to maximize detectivity in perovskite photodiodes.

Electro-Optic Activity in Excess of 1000 pm V-1 Achieved via Theory-Guided Organic Chromophore Design.

High performance organic electro-optic (OEO) materials enable ultrahigh bandwidth, small footprint, and extremely low drive voltage in silicon-organic hybrid and plasmonic-organic hybrid photonic devices. However, practical OEO materials under device-relevant conditions are generally limited to performance of ≈300 pm V-1 (10× the EO response of lithium niobate). By means of theory-guided design, a new series of OEO chromophores is demonstrated, based on strong bis(4-dialkylaminophenyl)phenylamino electron donating groups, capable of EO coefficients (r33 ) in excess of 1000 pm V-1 . Density functional theory modeling and hyper-Rayleigh scattering measurements are performed and confirm the large improvement in hyperpolarizability due to the stronger donor. The EO performance of the exemplar chromophore in the series, BAY1, is evaluated neat and at various concentrations in polymer host and shows a nearly linear increase in r33 and poling efficiency (r33 /Ep , Ep is poling field) with increasing chromophore concentration. 25 wt% BAY1/polymer composite shows a higher poling efficiency (3.9± 0.1 nm2 V-2 ) than state-of-the-art neat chromophores. Using a high-ε charge blocking layer with BAY1, a record-high r33 (1100± 100 pm V-1 ) and poling efficiency (17.8± 0.8nm2 V-2 ) at 1310nm are achieved. This is the first reported OEO material with electro-optic response larger than thin-film barium titanate.

Synthesis, characterization, and performance of oligothiophene cyanoacrylic acid derivatives for solar cell applications

New dye sensitizers based on an oligothiophene cyanoacrylic acid derivative were synthesized and characterized for solar cell applications. The structures of the new dyes prepared as sensitizers based on oligothiophenes, namely5,5''-di-2-cyanoacrylic acid [2,2':5',2''-terthiophene] (dye1), [2,2':5',2''-terthiophene]-5-cyanoacrylic acid(dye2), and [2,2':5',2'':5'',2'''-quaterthiophene]-5-cyanoacrylic acid(dye3) were confirmed by elemental analysis, mass spectrometry, and 1H-NMR spectral data. The P3HT/dye2/nc-TiO2 solar cell produced the highest efficiency of 0.05% with an open circuit voltage of 0.65V compared to dyes 1 and 3 solar cells. That may have been attributed to the dyes’ molecular structure, which had different chain lengths and numbers of groups of cyanoacrylic connected to the dyes’ thiophene moiety The dark current suppressed in the P3HT/dye2/nc-TiO2 solar cells indicated the formation of the charge blocking layer, which produced an enhanced open-circuit voltage accompanied by a high onset voltage.

Graphene oxide charge blocking layer with high K TiO2 nanowire for improved capacitive memory

Glancing angle deposition technique was adopted to fabricate TiO2 nanowire (NW) over spin coated graphene oxide (GO) thin-film (TF) for capacitive memory application. The formation of hybrid structure was studied using field emission scanning electron microscope. High accumulation capacitance of 0.994 nF/cm2 at 7 MHz frequency was obtained for fabricated Au/TiO2 NW/GO TF/Si device as compared to Au/TiO2 NW/Si device (0.4014 nF/cm2). A theoretical study using delta depletion model was done, which further validated the relative permeability and flat band voltage of the devices at higher frequency. The memory window of Au/TiO2 NW/Si was found to be 1.36 V at±10 V, which increased to 3.72 V for Au/TiO2 NW/GO TF/Si hybrid device. Also, a large charge storage capacity with good retention (105 s) and endurance (1000 cycles) was observed for the hybrid device. Moreover, Au/TiO2 NW/GO TF/Si device exhibited low leakage current density of 1.6 µA/cm2 at +1 V as compared to Au/TiO2 NW/Si device (11.3 µA/cm2 at +1 V). The GO TF layer at the bottom increased the potential difference between TiO2 and GO, which acted as a charge trapping layer and thus demonstrated as a good candidate for non-volatile memory application.

Low voltage operating organic light emitting transistors with efficient charge blocking layer

Charge injection/blocking layers play important roles in the performances of organic electronic devices. Their incorporation into organic light emitting transistors has been limitted, due to generally high operating voltages (above 60 V) of these devices. In this work, two hole blocking molecules are integrated into tris-(8-hydroxyquinoline) aluminum (Alq 3 ) based light emitting transistors under operating voltage as low as 5 V. The effects of hole blocking and electron injection are decoupled through the differences in the energy levels. Significantly improved optical performance is achieved with the molecule of suitable energy level for electron injection. Surprisingly, a decreased performance is observed in the case of another hole blocking molecule evidencing that charge injection overweighs charge blocking in this device architecture. • Low Voltage (5 V) operating OLETs are reported. • Hole blocking and electron injection mechanisms are decoupled. • Significantly improved OLET performance is achieved with suitable hole blocking layer.

Ba‐based complex perovskite ceramics with superior energy storage characteristics

AbstractLinear dielectrics are widely used to create high power capacitors, where it is a big challenge to achieve high energy storage density in such dielectrics. Here, Ba‐based complex perovskite ceramics with high dielectric strength, medium dielectric constant, and ultra‐low dielectric loss are proposed as the candidates for high energy storage density dielectric materials, and the significant effects of 1:2 B‐site ordering and ordering domain structure are systematically investigated. In Ba(Mg1/3Nb2/3)O3 ceramics, high dielectric strength of 1452 kV cm−1 combined with high energy storage density of 3.31 J cm−3 are achieved in the samples after post‐densification annealing, and they are 28% and 57%, respectively, higher than those in the as‐sintered samples. The significant enhancement of energy storage performance could be attributed to the increased B‐site ordering degree, and the uniform ordering domain structure. Furthermore, amorphous alumina thin films are introduced as the charge blocking layers, which significantly enhance the energy storage density to 5.09 J cm−3. The present work provides a new approach to develop the dielectric ceramics with high energy storage density.

A Gd-doped HfO2 single film for a charge trapping memory device with a large memory window under a low voltage

In this study, a performance-enhanced charge trapping memory device with a Pt/Gd-doped HfO2/SiO2/Si structure has been investigated, where Gd-doped HfO2 acts as a charge trapping and blocking layer. The device demonstrates a large memory window of 5.4 V under a ±5 V sweeping voltage (360% of the device with pure HfO2), which is extremely attractive in low-power applications. In addition, the device also exhibits good retention characteristics with a 24.5% charge loss after the retention time of 1 × 105 seconds and robust endurance performance with a 1% degradation after 1 × 104 program/erase cycles. It is considered that the high density of defect states and the reduction in the defect energy levels induced by Gd-doping contribute to the performance improvement.

Tuning the charge blocking layer to enhance photomultiplication in organic shortwave infrared photodetectors

This work investigates a series of interfacial materials to understand how charge-blocking layers facilitate trap-assisted photomultiplication in organic infrared detectors.

Organic Photodetectors and their Application in Large Area and Flexible Image Sensors: The Role of Dark Current

AbstractOrganic photodetectors (OPDs) have gained increasing interest as they offer cost‐effective fabrication methods using low temperature processes, making them particularly attractive for large area image detectors on lightweight flexible plastic substrates. Moreover, their photophysical and optoelectronic properties can be tuned both at a material and device level. Visible‐light OPDs are proposed for use in indirect‐conversion X‐ray detectors, fingerprint scanners, and intelligent surfaces for gesture recognition. Near‐infrared OPDs find applications in biomedical imaging and optical communications. For most applications, minimizing the OPD dark current density (Jd) is crucial to improve important figures of merits such as the signal‐to‐noise ratio, the linear dynamic range, and the specific detectivity (D*). Here, a quantitative analysis of the intrinsic dark current processes shows that charge injection from the electrodes is the dominant contribution to Jd in OPDs. Jd reduction is typically addressed by fine‐tuning the active layer energetics and stratification or by using charge blocking layers. Yet, most experimental Jd values are higher than the calculated intrinsic limit. Possible reasons for this deviation are discussed, including extrinsic defects in the photoactive layer and the presence of trap states. This provides the reader with guidelines to improve the OPD performances in view of imaging applications.

Photoconductive properties of polycrystalline selenium based lateral MISIM photodetectors of high quantum efficiency using different dielectrics as the charge blocking layer

We studied the photoconductive performance of polycrystalline selenium (pc-Se) based photodetectors with a lateral metal–insulator-semiconductor-insulator–metal (MISIM) device structure. The insulator layer is a 10 nm-thick Ga2O3, HfO2, or Al2O3 thin film, and used as a charge blocking layer (CBL) to suppress dark current injected from the Al electrodes. The dark current suppression primarily depends on the barrier height of the junctions between the CBLs and electrodes. The Ga2O3 CBL exhibits a poor dark current suppression compared to the HfO2 and the Al2O3 CBLs because of a lower electron barrier at the cathode. The lateral pc-Se photodetectors exhibit a very high internal photocurrent gain due to Fowler–Nordheim tunneling at relatively low applied voltages. The better crystallinity of pc-Se grains formed on the Ga2O3 CBL leads to a higher photoconversion efficiency for the MISIM-Ga2O3 photodetector. Compared with amorphous Se based lateral MISIM photodetectors, the pc-Se photodetectors demonstrate a uniform and much better photoconductive performance over the visible spectrum.