Hole Blocking Layer Research Articles

Recent Advances of Epitaxial BiVO4 Thin Film: Preparation and Physical and Photoelectrochemical Properties

Semiconductor photocatalysis has two important applications: environmental remediation and H2 production. Bismuth vanadate has attracted great interests as an ideal candidate for use as high-performance photocatalysts for visible light-driven water splitting and photoanode in photoelectrochemical cells. Single crystal film is invaluable to deepen the fundamental understanding of materials. The object of this article was to provide a brief review on the advances in the preparation of epitaxial BiVO4 thin film, the physical and photoelectrochemical properties. The epitaxial BiVO4 thin film could be fabricated on (001) yttria-stabilized cubic zirconia or SrTiO3(001) by molecular beam epitaxy, chemical vapor deposition, and, pulsed laser deposition. Three crystal structures, namely, monoclinic scheelite, monoclinic clinobisvanite, and orthorhombic, were mentioned in the literatures. The optical band gap was reported to be 2.5~2.7 eV for a direct transition. The highest photocurrent of 4.83 mA cm−2at 1.23 V vs. the RHE was obtained at the sample with Lu2O3 as a hole blocking layer.

Suppression of Leakage Current in Proton-Conducting BaZr0.8Y0.2O3−δ Electrolyte by Forming Hole-Blocking Layer

Solid oxide fuel cells (SOFCs) with proton-conducting solid electrolyte, instead of the oxide-ion conducting solid electrolyte have attracted attentions because of their high potential to reduce operating temperatures and to enhance the electrical efficiencies of SOFCs. In addition, the proton-conducting SOFCs with multistage electrochemical oxidation configuration will be promising technology for critically-high electric efficiencies. However, it is known that there are non-negligible charge -carriers other than protons in typical proton-conducting solid oxide electrolytes at relatively high temperatures. The existence of the partial conductivities of holes and/or electrons will cause the internal leakage current that consumes fuel but never generates any electrical power output. The higher ratio of the leakage current to external current will more deteriorate the electrical efficiency. In this study, the effects of blocking -layers formed on the air side surface of base electrolyte layer consisting of BaZr0.8Y0.2O3−δ (BZY82) for suppressing the leakage current have been investigated by using electrochemical parameters of the partial conduction of the materials. The chemical potential profile and leakage current showed large dependence on the material of the blocking-layer. Lanthanum tungstate was found to play a role as unique and strong blocking-layer against the leakage current.

Study of hole-blocking and electron-blocking layers in a InaS/GaAs multiple quantum-well solar cell

In this work, a GaAs-based quantum well solar cell with a 25-layer InAs/GaAs intermediate layer is simulated in Silvaco Atlas TCAD software. In order to reduce the recombination caused by the presence of the quantum layers and increase the absorption of photons, electron blocking layers (EBLs) and hole blocking layers (HBLs) have been added to the solar cell in an In0.5(Al0.7Ga0.3)0.5P semiconductor. The results show that the efficiency of the proposed solar cell increases 17.38% by obtaining impurity the thickness and doping of the EBL and HBL layers. It can be concluded that the use of the In0.5(Al0.7Ga 0.3)0.5P semiconductor with EBL and HBL layers decreases the open circuit voltage (Voc) caused in the quantum wells. The efficiency of the proposed solar cell with EBL and HBL layers was found to be 44.65%.

Bright and fast-response perovskite light-emitting diodes with an ICBA:modified-C60 nanocomposite electrical confinement layer.

Bright and fast-response CH3NH3PbBr3 perovskite light-emitting diodes (PeLEDs) are realized by using ICBA:modified C60 (MC60) nanocomposites as the hole blocking layer (HBL) and electron transport layer (ETL). The photoluminescence spectrum shows that the use of hydrophilic MC60 in the ETL helps the surface passivation of the perovskite layer. In addition, the photoelectron spectra and water-droplet contact angle images show that the use of the ICBA:MC60 nanocomposite ETL can simultaneously confine the electrons and holes in the perovskite layer, which boosts the injected electron-hole radiative recombination efficiency and thereby increases the electroluminescence from 1 cd m-2 to 2080 cd m-2 at 6 V when the ICBA:3,5OEC60 nanocomposite ETL is used. In addition, the operational frequency of the optimal PeLED is up to 1.5 MHz.

Effects of alkali and transition metal-doped TiO2 hole blocking layers on the perovskite solar cells obtained by a two-step sequential deposition method in air and under vacuum.

Planar perovskite solar cells (PPSCs) have received great attention in recent years due to their intriguing properties, which make them a good choice for photovoltaic applications. In this work, the effect of alkali and transition metal-doped TiO2 (cesium-doped TiO2 (Cs-TiO2) and yttrium-doped TiO2 (Y-TiO2)) compact layers on the optical, structural and the photovoltaic performance of the PPSCs have been investigated. The perovskite layer syntheses were carried out by depositing a lead iodide (PbI2) layer via spin-coating; converting PbI2 into methyl ammonium iodide (CH3NH3PbI3) by chemical vapor deposition (CVD) and spin-coating at 60 min and 60 s conversion times respectively. The as-deposited PPSCs were studied layer-by-layer using an X-ray diffractometer, scanning electron microscope, and UV-vis diffuse reflectance, transmittance and absorbance. The power conversion efficiency for stable processed perovskite solar cells were 3.61% and 12.89% for air and vacuum processed, respectively.

Solution-processed TiO2 as a hole blocking layer in PEDOT:PSS/n-Si heterojunction solar cells

The junction properties at the solution-processed titanium dioxide (TiO2)/n-type crystalline Si(n-Si) interface were studied for poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS)/n-Si heterojunction solar cells by the steady-state photovoltaic performance and transient reverse recovery characterizations. The power conversion efficiency could be increased from 11.23% to 13.08% by adjusting the layer thickness of TiO2 together with increasing open-circuit voltage and suppressed dark saturation current density. These findings originate from the enhancement of the carrier collection efficiency at the n-Si/cathode interface. The transient reverse recovery characterization revealed that the surface recombination velocity S was ∼375 cm/s for double TiO2 interlayer of ∼2 nm thickness. This value was almost the same as that determined by microwave photoconductance decay measurement. These findings suggest that solution-processed TiO2 has potential as a hole blocking layer for the crystalline Si photovoltaics.

Origins of the s-shape characteristic in J–V curve of inverted-type perovskite solar cells

Fullerene derivative thin films have been widely used in inverted-type perovskite solar cells as the electron transport layer (ETL) and hole blocking layer. However, the smooth contact at the interface between the hydrophobic [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) and hydrophilic CH3NH3PbI3 (MAPbI3) thin film has not yet been completely understood. The contact at the PCBM/MAPbI3 interface strongly influences the photovoltaic performance. The photovoltaic devices were characterized by measuring the light intensity-dependent current density–voltage (J–V) curves and impedance spectra, which show that the contact at the PCBM/MAPbI3 interface simultaneously influences the shunt resistance (carrier recombination) and series resistance (interfacial contact). In addition, x-ray diffraction patterns, atomic force microscopic images, absorbance spectra and photoluminescence spectra were used to explore the contact at the PCBM/MAPbI3 interface. The experimental results show that the flat MAPbI3 thin film cannot be completely covered by a PCBM thin film and thereby results in the s-shape characteristic in the J–V curve of the resultant solar cells. The s-shaped J–V curve can be suppressed by increasing the crystallinity and surface roughness of the MAPbI3 thin film. With the use of an interface modification layer in between the PCBM thin film and Ag cathode, the power conversion efficiency of MAPbI3 solar cells can be increased from 10.50% to 13.71%.

A red emissive poly(dendrimer) for solution processed organic light-emitting diodes

Abstract A red-emissive poly(dendrimer) comprised of a norbornenyl-derived polymer backbone with a phosphorescent dendrimer side-chain has been prepared by ring-opening methathesis polymerization with a number average molecular weight of 164 kDa and a dispersity of 2.4. The dendrimer side-chain had a heteroleptic iridium(III) complex core containing two 2-phenylpyridyl ligands and one 2-(thiophen-2-yl)quinoline ligand. The 2-phenylpyridyl ligands had first generation fluorenylcarbazole dendrons with n-propyl surface groups attached and the 2-(thiophen-2-yl)quinoline ligand was linked to the polymer backbone. In spite of the presence of the two 2-phenylpyridine ligands that would normally give rise to green emission when complexed with iridium(III), the 2-(thiophen-2-yl)quinoline was responsible for the observed saturated red emission with CIE coordinates of (0.67, 0.33). The poly(dendrimer) had a high photoluminescence quantum yield (PLQY) for a red emissive species at 72 ± 7%, which was maintained when blended with 4,4′-bis(carbazol-9-yl)biphenyl (CBP) at a concentration of 10 wt % (PLQY = 74 ± 8%). Simple two-layer devices containing the blend emissive layer and an electron transporting hole blocking layer had a maximum external quantum efficiency of almost 11%.

Rational Design of 3D Hierarchical Ternary SnO2/TiO2/BiVO4 Arrays Photoanode toward Efficient Photoelectrochemical Performance.

BiVO4 as a promising semiconductor absorber is widely investigated as photoanode in photoelectrochemical water splitting. Herein, the rational design of 3D hierarchical ternary SnO2/TiO2/BiVO4 arrays is reported as photoanode for photoelectrochemical application, in which the SnO2 hierarchically hollow microspheres core/nanosheets shell arrays act as conductive skeletons, while the sandwiched TiO2 and surface BiVO4 are working as hole blocking layer and light absorber, respectively. Arising to the hierarchically ordered structure and synergistic effect between each component in the composite, the ternary SnO2/TiO2/BiVO4 photoanode enables high light harvesting efficiency as well as enhanced charge transport and separation efficiency, yielding a maximum photocurrent density of ≈5.03 mA cm−2 for sulfite oxidation and ≈3.1 mA cm−2 for water oxidation, respectively, measured at 1.23 V versus reversible hydrogen electrode under simulated air mass (AM) 1.5 solar light illumination. The results reveal that electrode design and interface engineering play important roles on the overall PEC performance.

Enhancing the Open-Circuit Voltage of Perovskite Solar Cells by Embedding Molecular Dipoles within Their Hole-Blocking Layer.

Engineering the energetics of perovskite photovoltaic devices through deliberate introduction of dipoles to control the built-in potential of the devices offers an opportunity to enhance their performance without the need to modify the active layer itself. In this work, we demonstrate how the incorporation of molecular dipoles into the bathocuproine (BCP) hole-blocking layer of inverted perovskite solar cells improves the device open-circuit voltage (VOC) and, consequently, their performance. We explore a series of four thiaazulenic derivatives that exhibit increasing dipole moments and demonstrate that these molecules can be introduced into the solution-processed BCP layer to effectively increase the built-in potential within the device without altering any of the other device layers. As a result, the VOC of the devices is enhanced by up to 130 mV, with larger dipoles resulting in higher VOC. To investigate the limitations of this approach, we employ numerical device simulations that demonstrate that the highest dipole derivatives used in this work eliminate all limitations on the VOC stemming from the built-in potential of the device.

An efficient CsPbBr3 perovskite light-emitting diode by employing 1,3,5-tri(m-pyrid-3-yl-phenyl)benzene as a hole and exciton blocking layer

An efficient perovkite light-emitting diode (PeLED) is fabricated by incorporating 1,3,5-tri (m-pyrid-3-yl-phenyl)benzene (TmPyPB) between the emitting layer (EML) and the electron transporting layer (ETL), where TmPyPB plays an indispensable blocking effect on both excitons and holes. It shows a maximum current efficiency of 9.37 cd/A, and the corresponding external quantum efficiency (EQE) of 2.42% with a saturated green color Commission International de I'Eclairage (CIE1993) coordinated of (0.1279, 0.7951) under a driving voltage of 6 V. Meanwhile, the operational lifetime of the TmPyPB-based PeLED is improved to about 2.6-fold to that of the one with only the traditional ETL of aluminum 8-hydroxyquinolinate (Alq3). The enhancements of electroluminescence (EL) performance are mainly attributed to better confinement of both the holes and the excitons in the EML, and more balanced carrier transportation in the TmPyPB-based PeLED. This work offers a feasible strategy to develop high performance optoelectronic devices.

Photoelectrochemical water splitting by engineered multilayer TiO2/GQDs photoanode with cascade charge transfer structure

Herein, for the first time, an efficient photoanode engineered with the cascade structure of FTO|c-TiO2|few graphene layers|TiO2/GQDs|Ni(OH)2 assembly (Ni(OH)2 photoanode) is designed. This photoanode exhibited much lower electron–hole recombination, fast charge transport, higher visible light harvesting, and excellent performance with respect to FTO|c-TiO2|TiO2 assembly (TiO2 photoanode) in the photoelectrocatalytic oxygen evolution process. The photocurrent density of Ni(OH)2 photoanode is 7 times (0.35 mA cm−2 at 1.23 V vs. RHE) greater than that of TiO2 photoanode (0.045 mA cm−2 at 1.23 V vs. RHE). The compact TiO2 (c-TiO2) layer in Ni(OH)2 photoanode plays a role of an effective hole-blocking layer. Few-layer graphene layer could speed up the transport of the photogenerated electrons from the conduction band of the TiO2/GQDs to FTO. Ni(OH)2 layer could transfer rapidly holes into electrolyte solution.

Spin-coated copper(I) thiocyanate as a hole transport layer for perovskite solar cells

Application of a low-cost and efficient p-type inorganic hole-transporting material, copper thiocyanate (CuSCN), on mesoporous n-i-p-configurated perovskite-based devices was conducted in this study. Diethylsulfide was chosen for the preparation of precursor solution in order to deposit CuSCN layer on perovskite without degrading it. Topographical, elemental, and electrical characterizations of spin-coated CuSCN layers were performed using XRD, AFM, SEM, XPS, UPS, and UV-Vis studies. A power conversion efficiency exceeding 11.02% with an open-circuit voltage of 0.83 V was succeeded in the perovskite solar cells under full sun illumination. Low-temperature solution process used for the deposition of CuSCN and a fast solvent removal method allowed the creation of compact, highly conformal CuSCN layers that facilitate rapid carrier extraction and collection. The differences in series and recombination resistances for CuSCN-free and CuSCN-containing cells were also determined using impedance spectroscopy (IS) analysis. Moreover, the effect of TiO2 layer thickness on the cell performance was studied where these TiO2 layers were used not only for electron extraction and transportation, but also as hole blocking layer in perovskite solar cells. The impedance spectroscopy results were also consistent with the differently configurated cell performances. This work shows a well-defined n-i-p perovskite cell with optimized layers which utilize low-cost and abundant materials for photovoltaic applications.

Unveiling the Root Cause of the Efficiency-Lifetime Trade-Off in Blue Fluorescent Organic Light-Emitting Diodes

The origin of efficiency-lifetime trade-off in triplet–triplet fusion (TTF) type blue fluorescent organic light-emitting diodes (OLEDs) was investigated and the device structure to resolve the issue was developed. The efficiency and lifetime were simultaneously improved in the blue OLEDs by developing a multilayer hole transport stack which can adjust carrier densities and recombination zone in the emitting layer (EML). It was found that electron leakage from EML and high spatial density of excitons in the vicinity of the electron blocking layer for high TTF rates by narrow recombination zone are the detrimental factors for efficiency-lifetime trade-off. A multilayer hole transport stack employing a deep highest occupied molecular orbital hole transport layer and an electron blocking layer combined with an appropriate hole blocking layer simultaneously improved the power efficiency by 16% at 500 cd/m2 and lifetime by almost 100% (from 73 h up to 145 h). In addition, the low efficiency in the low luminance region was also completely controlled, resulting in negligible efficiency variation in the entire luminance range.

One-step P2 scribing of organometal halide perovskite solar cells by picosecond laser of visible wavelength

Perovskite solar cells (PSC) have attracted attention by their unprecedented rise in device efficiency. However, the PSCs were mostly tested for the unit cells of small area, and it is important to upscale to solar modules. Laser scribing has great potential to this end. However, the key challenge identified in one-step P2 laser scribing is to remove film stack including the hole block layer (HBL) without damaging transparent conducting oxide (TCO) based bottom contact and thus to provide interconnecting paths with minimized contact resistance.In this study, we evaluate one-step P2 scribing performance of picosecond laser of 532 nm wavelength for mesoscopic perovskite architecture with the c-TiO2 (compact TiO2) as the HBL and fluorine doped tin oxide (FTO) as bottom contact. Despite anticipated challenge of low absorption selectivity, c-TiO2 layer could be efficiently removed without FTO damage over wide processing window enabling a self-terminating mechanism. Detailed parametric studies elucidate relevant scribing mechanisms such as layer-by-layer ablation mechanism or lift-off mechanism depending laser illumination direction. It is also revealed that the removal of c-TiO2 is triggered by ablation of upper layers, in particular, the mesoporous TiO2. Module level test is under way in conjunction with investigation of further scribing mechanisms.

Cooperative Catalytic Behavior of SnO2 and NiWO4 over BiVO4 Photoanodes for Enhanced Photoelectrochemical Water Splitting Performance

n-BiVO4 is a favorable photoelectrode candidate for a photoelectrochemical (PEC) water splitting reaction owing to its suitable energy level edge locations for an oxygen evolution reaction. On the other hand, the sluggish water oxidation kinetics of BiVO4 photoanodes when used individually make it necessary to use a hole blocking layer as well as water oxidation catalysts to overcome the high kinetic barrier for the PEC water oxidation reaction. Here, we describe a very simple synthetic strategy to fabricate nanocomposite photoanodes that synergistically address both of these critical limitations. In particular, we examine the effect of a SnO2 buffer layer over BiVO4 films and further modify the photoanode surface with a crystalline nickel tungstate (NiWO4) nanoparticle film to boost PEC water oxidation. When NiWO4 is incorporated over BiVO4/SnO2 films, the PEC performance of the resultant triple-layer NiWO4/BiVO4/SnO2 films for the oxygen evolution reaction (OER) is further improved. The enhanced performance for the PEC OER is credited to the synergetic effect of the individual layers and the introduction of a SnO2 buffer layer over the BiVO4 film. The optimized NiWO4/BiVO4/SnO2 electrode demonstrated both enriched visible light absorption and achieves charge separation and transfer efficiencies of 23% and 30%, respectively. The photoanodic current density for the OER on optimized NiWO4/BiVO4/SnO2 photoanode shows a maximum photocurrent of 0.93 mA/cm2 at 1.23 V vs. RHE in a phosphate buffer solution (pH~7.5) under an AM1.5G solar simulator, which is an incredible five-fold and two-fold enhancement compared to its parent BiVO4 photoanode and BiVO4/SnO2 photoanodes, respectively. Further, the incorporation of the NiWO4 co-catalyst over the BiVO4/SnO2 film increases the interfacial electron transfer rate across the composite/solution interface.

Organic LED based light sensor for detection of ovarian cancer

Organic devices possess an interdisciplinary facet that can be utilized in the different fields; communication, memory devices, bio-degradable technology and sensor application owing to their robustness, light weight and low power requirements. The proposed work is focused on the development of a sensor application based on the organic LEDs for the diagnosis of ‘Ovarian Cancer’. Two organic LEDs: Multilayered OLED and triple hole block layer OLED are analyzed for utilization as light detector and light source in the sensing device for cancer detection. Multilayered OLED depicts excellent light detection qualities owing to lower electron hole recombination and it is six times better as compared to the triple hole block layer OLED. Therefore, it is used as the detector element in the sensing device. On the other hand, triple hole block layer OLED, is used as light source due to its high luminescence characteristics of 25,285 cd/m2. Further, a dual gate OTFT is used to drive the triple hole block layer OLED, which is utilized as the light source. DG OTFT in dual gate mode is 18% better compared to single gate mode. Thus DG-OTFT in dual gate mode is able to generate 18 V at the terminals of the triple HBL OLED necessary for its operation. Thereafter, light detection is performed utilizing the OLED. Multilayered OLED depicted excellent light detecting capabilities. It is able to generate a cathode current of 29 mA and 13 mA at an incident wavelength of 420 and 440 nm, respectively, an essential requirement for present sensor application. Therefore, it presents a possibility to fabricate a portable fully flexible device for the screening and diagnosis of the ovarian cancer.

Recombination pathways and hole leakage behavior in InGaN/GaN multiple quantum wells with V-shaped pits

Abstract Hole transport in InGaN/GaN multiple quantum wells (MQWs) LEDs has been proposed to be insufficient due to relatively large effective mass and low mobility. Here, for the first time, we have direct observed hole overflow from MQWs and enter into InGaN/GaN superlattices (SLs) layer in V-pits-containing GaN-based green LEDs at room temperature. To suppress the hole leakage, an n-AlGaN hole blocking layer (HBL) is inserted between SLs layer and MQWs. The experimental show an interesting and baffling result is that the HBL-containing sample not only exhibits severe hole leakage in high current density range, but also has higher external quantum efficiency (EQE). Numerical simulation reveals that the HBL can change the transportation path of carriers, forcing more electrons injecting from c-plane into the MQWs instead of from V-pit sidewalls. This will increase the carrier concentration in the MQWs, and eventually lead to higher EQE. Meanwhile, less electrons transported through V-pit allowed more holes injecting into deeper layers via V-pit sidewalls, and electroluminescence emission from the SLs layer became stronger when HBL was inserted.

Perovskite Photovoltaic Devices: Enhancing the Open‐Circuit Voltage of Perovskite Solar Cells by up to 120 mV Using π‐Extended Phosphoniumfluorene Electrolytes as Hole Blocking Layers (Adv. Energy Mater. 33/2019)

Perovskite Photovoltaic Devices: Enhancing the Open‐Circuit Voltage of Perovskite Solar Cells by up to 120 mV Using π‐Extended Phosphoniumfluorene Electrolytes as Hole Blocking Layers (Adv. Energy Mater. 33/2019)

(Invited) The Key Processing Issues and Their Solutions for High-Quality Perovskite Solar Cells

We report the fabrication of cesium-doped mixed cation perovskite solar cells (PSCs) (FTO/c-TiO2/mp-TiO2/Perovskite/spiro-OMeTAD/Au). A few key processing issues and their solutions are introduced. The smooth, shiny, and dark perovskite film is deposited by using the anti-solvent method through spin-coating. First, the deposition of a pinhole-free c-TiO2 layer with a smooth surface is critical. Here, we introduce a new TiO2 solution using titanium isopropoxide (TiO2 99.99%) to tackle the issue the issues related to the conventional TiO2 solutions. Compared to the conventional one, the spin-coating of the new solution provides a pinhole-free, efficient hole-blocking layer without cracks and still cheaper and quicker than the latter one. Highly reproducible PSCs with an average power conversion efficiency (PCE) of 15.4% are fabricated using this solution compared to the conventional solution utilizing both spin-coating and spray pyrolysis with average PCEs of 10.6% and 13.78%, respectively. The anti-solvent method was usually used with chlorobenzene (CBZ), but CBZ cannot dissolve the Li-TFSI particles which may be present on the surface of mp-TiO2 right after Li-doping process. In this case, we should use a solvent that can dissolve the Li-TFSI particles completely. Both diethyl ether (DEE) and the mixed of CBZ and DEE (volume ratio of 4:1) can dissolve the Li-TFSI particles easily. It improves the size of perovskite particles which leads to lower density of defects in the film compared to the one with smaller particles. The overall PCE is enhanced to 15.9% for samples with CBZ:DEE compared to the PCE of 14.76% for samples with CBZ in the same processing conditions. Moreover, a thin layer of silver (Ag) is introduced as the interlayer between the hole transport layer (HTL) and the Au. We deposited a 2-5nm of Ag on top of the HTL followed by 80 nm of Au. By this, the adhesion of the Au layer to the device is improved according to the simple tweezer tool test. A pure Au film can easily come off from the surface of the HTL, however, the Au films with 2-5nm of silver buffer layer show strong adhesion to the device. Interestingly, the employment of 2nm silver film improved the current density and, finally the PCE of the devices by both improving the adhesion of the back electrode onto the device and increasing the light reflection in the PSC. Finally, a reproducible fabrication procedure of PSCs with an average PCE of 16.5%, and a maximum PCE of 17.43% for an active area of 0.08 cm2 is achieved.