High Band Gap Semiconductor Research Articles

Effect of the TiO2 electron transport layer thickness on charge transfer processes in perovskite solar cells

Dense TiO2 thin films are widely used as electron transport layers in perovskite solar cells. TiO2 is a high band gap semiconductor with n-type conductivity showing excellent compatibility with metal-halide perovskites in terms of energy alignment providing efficient charge separation and less energy loss of electrons during electron transfer. In this work, TiO2 layers were deposited from TiO2 sol-gel solution by spin-coating methods. The optimization of TiO2 ETL thickness is a necessary procedure to reduce ETL resistance and minimize charge recombination. The thickness reduction leads to a decrease in resistance, however, also enhances charge recombination due to the formation of pin-holes and cracks. We found that TiO2 thickness reduction also causes the TiO2 defects density changes. By studying luminescence spectra of TiO2 with various thicknesses we found that the thickness reduction leads to the increase in the interstitial Ti+3 defects density, while the oxygen vacancies density decreases. Ti+3 species form deep levels trapping free electrons and as result increasing TiO2 resistance. TiO2 films with thicknesses in the range of 40–120 nm were deposited and used as ETL for PSCs with ITO/TiO2/CH3NH3I3PbClx/CuPc/MoOx/Ag structure. Our study revealed two competitive charge transport processes taking place in TiO2: electron transport through the TiO2 conduction band and electron trapping by deep trap levels formed by Ti+3 species. Therefore according to the IV and IS studies, the optimal TiO2 layer thickness at which there is a balance between the competitive charge transport processes is about 60 nm for our deposition condition.

WO3-NP-activated WS2 layered heterostructures for efficient broadband (254 nm-940 nm) photodetection.

Broadband photodetection including deep UV using Si is technically challenging due to its negligible optical absorption at 254 nm and the requirement of heterogeneous integration with very high bandgap photoactive materials. However, monolithic integration of high-bandgap semiconductors on Si is not possible due to CMOS fabrication incompatibility. Comprehensive experimental studies to achieve broadband photodetection including deep UV on Si are lacking in the literature. Here for the first time we have investigated 2D/0D heterojunctions of WS2/WO3 on a Si platform both experimentally and theoretically and established the charge transfer mechanism between them. Transient photocarrier decay experiments demonstrate effective quenching of excited photocarriers generated in WO3/WS2, signifying its utility in facilitating carrier transport, which is further evidenced by charge density calculation from DFT simulation. Our designed vertically aligned p-Si/WS2/WO3 heterojunction-based photodetector exhibits an excellent photosensitivity performance with a broad spectral response ranging from deep ultraviolet (254 nm) to near infrared (940 nm) wavelengths, and it not only provides a peak responsivity of 251 A W-1 and a specific detectivity of 1.89 × 014 Jones, but also possesses a rapid response speed with a rise/fall time of 0.64/0.48 s at 365 nm with a bias of 2 volt.

Optoelectronic properties and ultrafast carrier dynamics of copper iodide thin films

As a promising high mobility p-type wide bandgap semiconductor, copper iodide has received increasing attention in recent years. However, the defect physics/evolution are still controversial, and particularly the ultrafast carrier and exciton dynamics in copper iodide has rarely been investigated. Here, we study these fundamental properties for copper iodide thin films by a synergistic approach employing a combination of analytical techniques. Steady-state photoluminescence spectra reveal that the emission at ~420 nm arises from the recombination of electrons with neutral copper vacancies. The photogenerated carrier density dependent ultrafast physical processes are elucidated with using the femtosecond transient absorption spectroscopy. Both the effects of hot-phonon bottleneck and the Auger heating significantly slow down the cooling rate of hot-carriers in the case of high excitation density. The effect of defects on the carrier recombination and the two-photon induced ultrafast carrier dynamics are also investigated. These findings are crucial to the optoelectronic applications of copper iodide.

Graphene nanopattern as a universal epitaxy platform for single-crystal membrane production and defect reduction.

Heterogeneous integration of single-crystal materials offers great opportunities for advanced device platforms and functional systems1. Although substantial efforts have been made to co-integrate active device layers by heteroepitaxy, the mismatch in lattice polarity and lattice constants has been limiting the quality of the grown materials2. Layer transfer methods as an alternative approach, on the other hand, suffer from the limited availability of transferrable materials and transfer-process-related obstacles3. Here, we introduce graphene nanopatterns as an advanced heterointegration platform that allows the creation of a broad spectrum of freestanding single-crystalline membranes with substantially reduced defects, ranging from non-polar materials to polar materials and from low-bandgap to high-bandgap semiconductors. Additionally, we unveil unique mechanisms to substantially reduce crystallographic defects such as misfit dislocations, threading dislocations and antiphase boundaries in lattice- and polarity-mismatched heteroepitaxial systems, owing to the flexibility and chemical inertness of graphene nanopatterns. More importantly, we develop a comprehensive mechanics theory to precisely guide cracks through the graphene layer, and demonstrate the successful exfoliation of any epitaxial overlayers grown on the graphene nanopatterns. Thus, this approach has the potential to revolutionize the heterogeneous integration of dissimilar materials by widening the choice of materials and offering flexibility in designing heterointegrated systems.

Designing transparent nanophotonic gratings for ultra-thin solar cells.

Integration of a rear surface nanophotonic grating can increase photocurrent in ultra-thin solar cells. Transparent gratings formed of dielectric materials and high bandgap semiconductors can offer efficient diffraction with lower parasitic absorption than more widely studied metal/dielectric equivalents. In these systems, the maximum photocurrent which can be obtained for a grating made of a given combination of materials is shown to follow a simple empirical model based on the optical constants of these materials and independent of grating dimensions. The grating dimensions still require optimization in order to maximize the photocurrent for a given active layer thickness by balancing the effects of diffraction outside the front surface escape cone and the tuning of waveguide modes in long wavelength regions which are poorly absorbed in an ultra-thin film. The optimal grating pitch is shown to be of particular relevance for both effects, changing nonmonotonically as the absorber gets thicker in order to track favourable waveguide mode resonances at wavelengths near the absorber bandgap. These trends together with the empirical model for material selection drastically reduce the design space for highly efficient light trapping with transparent gratings.

The Regularity of the Decrease in the Quantum Yield of Quantum-Wells LEDs at the Long-Term Current Flow from the ABC Model Position

The works on degradation of light emitting diodes (LEDs) with quantum wells (QW) were analysed. The calculation model of the relation between LED luminous flux and duration of LED current flow and current density was proposed. It allows us to forecast service life of such radiators with the pre-set electric modes and temperature. It is demonstrated that: – Reduction of quantum yield of LED with QW based on high-bandgap semiconductors with longterm flow of forward current occurs due to generation of point defects in the QW areas; – The dot defects occur as a result of interaction between hot electrons and semiconductor atoms caused by subthreshold displacement out of the lattice dots; – The dot defects create non-radiative recombination centres with particular concentration in the energy gap of a semiconductor, as a result of which intensities of recombination flows in QWs and in barriers between QWs redistribute towards the non-radiative component of the ABC model.

Aluminum to germanium inversion in mullite‐type RAlGeO5: Characterization of a rare phenomenon for R = Y, Sm–Lu

AbstractMullite‐type RMn2O5 (R = Y, rare‐earth element) ceramics are of ongoing research attentions because of their interesting crystal‐chemical, physical, and thermal properties. We report a detailed structural, spectroscopic and thermal analysis of the series of mullite‐type RAlGeO5 (R = Y, Sm‐Lu) phases. Polycrystalline samples are prepared by solid‐state synthesis methods. Each sample is characterized by X‐ray powder diffraction followed by Rietveld refinements, showing that they are isotypic and crystallize in the space group Pbam. The change of the metric parameters is explained in term of the lanthanide contraction effect. A rare inversion of Al/Ge between octahedral and pyramidal sites have been observed for these mullite‐type so called O10 compounds, and the inversion parameter found to be between 0.22(1) and 0.30(1) for different R‐cations. The <Al/Ge–O> bond distances and their bond valence sums (BVSs) support the respective inversions. Density functional theory (DFT) calculated phonon density of states (PDOS) and electronic band structures are compared for the vibrational and electronic band gap features respectively. Analysis of UV/Vis absorption spectra using both derivation of absorption spectra fitting (DASF) and Tauc's methods demonstrates that each of the RAlGeO5 O10 compounds is high bandgap semiconductor, possessing direct transition between 4.1(1) and 5.4(1) eV. Both Raman and Fourier transform infrared spectra show clear red shift (quasi‐harmonic) of the vibrational wavenumbers with respect to the ionic radii of the R‐cations. Selective Raman bands at higher wavenumber region further complement the inversion of Al/Ge between two coordination sites. The higher decomposition temperature of the RAlGeO5 compounds, compared to those of RMn2O5 phases, is explained in terms of higher bond strength of Al/Ge‐O than those of Mn‐O. Irrespective to the inversion between Al‐ and Ge‐sites, the decomposition temperature also depends on the type of R‐cation in RAlGeO5.

Analytical Modeling of Line-Tunneling TFETs Based on Low-Bandgap Semiconductors

The combination of two techniques: low-bandgap semiconductor and line-tunneling structure is an effective way to achieve the highest on-current in TFETs. In this paper, design of low-bandgap line-tunneling TEFT and its analytical modeling of drain current equation is proposed. The previously suggested drain current equation for the low-bandgap line-tunneling TEFT has been explained in a relatively complex form based on the minimum tunnel path that is an effective factor in determining band-to-band tunneling (BTBT). It has been simplified in this paper and reformulated based on gate-to-source voltage. Important design factors such as source doping concentration, material and thickness of the gate-insulator were examined by simulation and numerical calculations based on the minimum tunnel path for two low-bandgap In0.88Ga0.12As and relatively high-bandgap GaSb semiconductors. The comparison of the results obtained from simulations with the proposed analytical drain current model show a good agreement. Drain doping concentration, is an effective factor on the off-state current of low-bandgap TFET. This factor was examined in order to reduce the off-current.

Structural, Electronic and Mechanical Properties of Few-layer Porous Nanosheet from Spheroidal Cage-like ZnO Polymorph

The low-dimensional II-VI group semiconductors have recently emerged as interesting candidate materials for the tailoring of two dimensional (2D) layered structures. Herein, a series of the cage-like nanoporous composed of spheroidal hollow cages (ZnO)12, cutting from the high symmetrical cubic SOD cage-like polymorph as building block, is proposed. We have performed the density-functional tight binding (DFTB+) calculations on the structural, electronic and mechanical properties of this few-layer SOD-cage-block nanosheet series, to investigate the effects of structural modification and sheet thickness on their structural, electronic, and mechanical properties. Optimized geometries, formation energy, phonon spectra, electronic band structure, and elastic tensor calculation has ensured the energetically, dynamical and mechanical stability for the sheets. Furthermore, the theoretically found nanosheet series possess an intrinsic wide direct band gap preserving from wurtzite tetragonal-based bonding. This high symmetry wide bandgap semiconductor nanosheet series and their derivatives are expected to have broad applications in photocatalysis, and biomedicine.

Novel cage-like nanoporous ZnO polymorphs with cubic lattice frameworks

Two cage-like nanoporous polymorphs composed of spheroidal cages (ZnO)24 as building block in high symmetrical cubic lattice frameworks are proposed. By using this cluster, instead of atom, as primary building block we studied various scheme of its assembling via bottom up cluster-cluster coalescence schemes. First principles and density-functional tight binding (DFTB+) calculations confirm that the new polymorphs are energetically, dynamically, and mechanically stable. The two newly found nanoporous phases possess an intrinsic wide direct band gap preserving from wurtzite tetragonal-based bonding. The high cubic symmetry wide bandgap semiconductor and its derivatives are expected to have broad applications in photocatalysis, and biomedicine.

Design and fabrication of Ni(OH)2/BiVO4 heterostructured nano-photocatalyst for high-efficient removal of organic dyes

Abstract Configuring new nanostructured photocatalyst with high efficient photocatalystic capabilities is meaningfully for environmental pollution control. Herein, a p-n heterojunction nanostructured photocatalyst of Ni(OH)2/BiVO4 was well designed and successfully fabricated via a facile solvothermal method, in which Ni(OH)2 is a wide band-gap semiconductor. Being applied for photo-degradation of anion organic dye, it is found such Ni(OH)2/BiVO4 heterostructures can not only increase the specific surface area and enhance the light response, but also effectively facilitate the photo-generated charge separation and transfer. As a result, 0.5-Ni(OH)2/BiVO4 can possess a photocatalytic capability of 96% and a reaction rate constant 1.9572 h−1 within 90 min irradiation, being 6 and 18 times higher than that of pure BiVO4 (16%, 0.1098 h−1). This study presents a new means to design a kind of high efficient wide band-gap semiconductor constructed BiVO4-based photocatalysts.

An affordable method to produce CuInS2 ‘mechano-targets’ for film deposition

We report on the room temperature, simple and cheap preparation by mechano-synthesis of polycrystalline targets of complex sulphide semiconductors, namely CuInS2 (CIS), to be used for film deposition in physical vacuum techniques such as PLD, sputtering or LT-PED. The sulphur (S)-based targets usually require expensive high pressure equipment to compensate the high S vapour pressure; nevertheless they are indispensable for the deposition of high band-gap semiconductor materials. The comparison of CIS films grown by mechano-synthesis and by commercial targets demonstrates similar material quality, making the mechano-synthesis a viable solution in the fabrication of high band-gap, S-based targets of complex chalcogenides.

Limiting Efficiency of Heterojunction Solar Cells

A heterojunction solar cell consists of a p-n junction between a high-bandgap and a low-bandgap semiconductor. As the cell is made up of two different semiconductors, the fundamental question that arises is whether the limiting efficiency of the heterojunction solar cell is determined by the low-bandgap semiconductor, the high-bandgap semiconductor, some value in-between or, perhaps, even some other value exceeding the limiting efficiency of single gap solar cells. In this respect, in this paper, we demonstrate that the limiting efficiency lies in-between, without the absolute limiting efficiency of single gap solar cells being exceeded. However, in spite of this result, we find that the open-circuit voltage of heterojunction solar cells can exceed the bandgap (divided by the electron charge) of the low-bandgap semiconductor, something that is not possible in single gap solar cells.

A Study on Fastening the Switching Speed for Wide Bandgap Semiconductor Based Super Cascode

The Super Cascode is a series connected structure with a normally-off low voltage Si-MOSFET and multiple normally-on wide bandgap semiconductors. It has low switching losses compared with silicon based bipolar devices, and low on-resistance and low cost compared with other single high voltage normally-off wide bandgap semiconductor devices. In practice, however, there are inevitable parasitic inductances, which result in the increase of switching losses. The method is proposed to eliminate the common-source inductances (CSIs), such as using stack-die configuration with each device and adding an additional inductance in the gate loop of Si-MOSFET. It is numerically shown that the rise and fall times of the proposed method were 33.5% and 7.2% faster than the conventional one, respectively.

Mn-Doped Semiconductor Nanocrystals: 25 Years and Beyond.

Doping Mn(II) in high bandgap semiconductor hosts is widely known for its yellow-orange emission, typically centered within 580–600 nm. The unique feature of this emission that identifies its origin from Mn(II) is its longer excited-state decay lifetime. As an optically active dopant in nanocrystalline materials, it has been studied for 25 years and achieved several milestones. The doping was first performed with different chalcogenide materials, and it has also been extended to recently emerged perovskite nanocrystals. Analyzing the entire journey of this Mn(II) dopant in different nanocrystal hosts, this Viewpoint provides several achievements obtained, from fundamentals to applied research, and also quotes important findings of some of the leading researchers.

Effect of Cu incorporation on morphology and optical band gap properties of nano-porous lithium magneso-silicate (LMS) thin films

A high bandgap semiconductor of Cu doped LMS (Cu-LMS) thin films incorporation in nano-porous lithium magneso-silicate (LMS) thin films as in the formula (10 Li2O: 15 MgO: (75-x) SiO2: (x = 0, 2, 5 and 8) Cu) has been successfully prepared by a facile acidic sol-gel method, spin-coating on glass substrate and then calcined at 450 °C for 5h in air. The x-ray diffraction (XRD) pattern indicated the formation of nanocrystalline anorthic phase for pure (LMS) and Cu doped (Cu-LMS). The surface morphological analysis through the scanning electron microscopy (SEM) approved the formation of nano-size (LMS) with good periodicity and great adherence. The optical measurements revealed an increase in both the refractive index values and the band gap of the nano-porous LMS with increasing Cu content. These results suggest that nano-porous Cu-LMS films can be a good candidate for various optical applications such as sensors, waveguide, and solar cells.

Impact of Surfaces on Photoinduced Halide Segregation in Mixed-Halide Perovskites

Photoinduced halide segregation currently limits the perovskite chemistries available for use in high-bandgap semiconductors needed for tandem solar cells. Here, we study the impact of post-deposition surface modifications on photoinduced halide segregation in methylammonium lead mixed-halide perovskites. By coating a perovskite surface with the electron-donating ligand trioctylphosphine oxide (TOPO), we are able to both reduce nonradiative recombination and dramatically slow the onset of halide segregation in CH3NH3PbI2Br films. This result, coupled with the observation that the rate of halide segregation can be tuned by varying the selective contact, provides a direct link between surface modifications and photoinduced trap formation. On the basis of these observations, we discuss possible mechanisms for photoinduced halide segregation supported by drift-diffusion simulations. This work suggests that improved understanding and control of perovskite surfaces provides a pathway toward stable and high-perf...

Water Dissociation and Further Hydroxylation of Perfect and Defective Polar ZnO Model Surfaces

ZnO is a high-band gap semiconductor material important for microelectronic and catalytic applications, such as water splitting among others. Although the nonpolar face of ZnO has been well-studied, its polar faces (Zn- and O-terminated) are less studied because of intrinsic difficulties to the model. Here, we combine density functional theory calculations and analytical modelling to determine the thermodynamics of the water molecule interaction with perfect ZnO polar model surfaces, (0001) and (0001̅) surfaces (p-Zn and p-O). Defects (oxygen vacancies, pits, and missing oxygen rows) are also investigated. Adsorption, dissociation, surface migration, and agglomeration are considered. We find that H2O preferentially adsorbs and dissociates on Zn atoms on p-Zn and at defects on p-O. At room temperature, water is found to spontaneously dissociate, except for p-O, in which dissociation is endothermic. After dissociation, the resulting protons either bind to surface oxygen atoms or to zinc atoms to form hydrides. Migration of H and OH is limited on p-Zn with moderate barriers and absent on p-O. Interestingly, further agglomeration or islanding of OH species is inhibited by repulsive OH–OH electrostatic forces. Consequently, although polar surfaces are highly reactive with water, they cannot sustain high OH coverages, unless highly defective. This limitation is one obstacle to ZnO catalytic activity, pointing to the need to tune temperature and pressure conditions.

Nb‐Dopant‐Induced Tuning of Optical and Electrical Property of Anatase TiO2 Nanocrystals

AbstractFor increasing demand of electrochromic smart window, flat panel display synthesis of high quality aliovalent doped wide bandgap semiconductor nanocrystals are gaining interest. In our manuscript we synthesized pure and Nb5+ doped anatase TiO2 nanocrystals by colloidal process. On increasing doping percentage size of TiO2 nanoparticle was found to be decreased. Although pure TiO2 nanocrystals are terminated with thermodynamically stable {101} facets but Nb5+ doped TiO2 was found to be terminated with high energy {103} facet by lowering the surface energy. Successful doping in anatase TiO2 has been characterized by Raman spectroscopy, X‐ray photoelectron spectroscopy (XPS) and electron paramagnetic resonance spectroscopy (EPR) analysis. Aliovalent dopant Nb5+ generated free carriers in conduction band of TiO2. These free electrons results distinct plasmon absorption peak which appeared at wavelength of 1900 nm to 2700 nm. Plasmon peak found to be blue shifted with increase of dopant amount. Broad emission bands correspond to distinct transitions related to intermediate energy levels induced by oxygen vacancy. High colloidaly stable nanocrystals was deposited as crack free transparent conducting oxide thin film where we obtained very low sheet resistance with high optical transparency that leads to good figure of merit of thin film. Temperature driven metal‐insulator transition for the lowest resistance film is explained by electron localization origination from random substitution of Nb into TiO2 nanocrystals.

Al2 O3 Underlayer Prepared by Atomic Layer Deposition for Efficient Perovskite Solar Cells.

Perovskite solar cells, as an emergent technology for solar energy conversion, have attracted much attention in the solar cell community by demonstrating impressive enhancement in power conversion efficiencies. However, the high temperature and manually processed TiO2 underlayer prepared by spray pyrolysis significantly limit the large-scale application and device reproducibility of perovskite solar cells. In this study, lowtemperature atomic layer deposition (ALD) is used to prepare a compact Al2 O3 underlayer for perovskite solar cells. The thickness of the Al2 O3 layer can be controlled well by adjusting the deposition cycles during the ALD process. An optimal Al2 O3 layer effectively blocks electron recombination at the perovskite/fluorine-doped tin oxide interface and sufficiently transports electrons through tunneling. Perovskite solar cells fabricated with an Al2 O3 layer demonstrated a highest efficiency of 16.2 % for the sample with 50 ALD cycles (ca. 5 nm), which is a significant improvement over underlayer-free PSCs, which have a maximum efficiency of 11.0 %. Detailed characterization confirms that the thickness of the Al2 O3 underlayer significantly influences the charge transfer resistance and electron recombination processes in the devices. Furthermore, this work shows the feasibility of using a high band-gap semiconductor such as Al2 O3 as the underlayer in perovskite solar cells and opens up pathways to use ALD Al2 O3 underlayers for flexible solar cells.