In this paper we report on systematic studies conducted for the improvement in both the device structure and the materials quality of perovskite based solar cells (PSCs). We have incorporated TiO2 nanorods, of length around 350-400 nm, in the device structure. Such structures were grown by solvothermal technique directly on the glass/FTO substrates. Characterization by femtosecond transient absorption (fs-TA) spectroscopy indicates that the incorporation of TiO2- nanorod array (NA) greatly enhances the collection efficiency of the photo-generated carriers due to substantial reduction in carrier diffusion distance. To improve the crystallinity of the perovskite films we performed systematic studies on cryoassisted growth of the material. The technique eliminates the need for environmentally harmful anti-solvents and enables decoupling of the nucleation and crystallization phases by inhibiting chemical reactions in the precursor films rapidly cooled by immersion in liquid nitrogen. Furthermore, the technique leads to uniform precipitation of precursors due to the supersaturation condition in the residual solvents at cryogenic temperature resulting in highly uniform coverage of the films. Systematic characterization of the films by low-frequency noise and photothermal deflection technique indicate significant in the trap density of the films which is attributed as the main underlying reason for the observed improvement in the power conversion efficiency of the device. A high efficiency of 21.4% is achieved for our champion device.
We present investigations on the growth of high quality CH3NH3PbI3 (MAPI) thin films using both vapor and solution techniques. Recent work on perovskite film growth indicates critical dependencies of the film quality on the nucleation and crystallization steps requiring: i.) uniform distribution of nucleation sites; and ii.) optimal crystallization rate that facilitates the growth of a compact, continuous film with low density of pinholes. Our work shows that the hybrid chemical vapor deposition technique (HCVD) technique is well suited for the deposition of evenly distributed nucleation sites and the optimization of the crystallization rate of the film through detailed monitoring of the thermal profile of the growth process. Signficant reduction in the defect states is recorded by annealing the perovskite films in O2. The results are consistent with theoretical studies by Yin et al. 1 on O and Cl passivation of the shallow states at the grain boundary of MAPI. Their work provides the theoretical basis for our experimental observations on the passivation of shallow states by oxygen annealing. High quality films were achieved through detailed management of the carrier gas composition and the thermal profile of the nucleation and crystallization steps.
Hybrid organic-inorganic perovskite solar cells have attracted lots of attention in recent years. Growth and properties of perovskite layer and its relationship to photovoltaic performance have been extensively studied. Comparably less attention was devoted to the research of the influence of electron transporting layer (ETL). Conventionally, TiO2 is selected as ETL. However, photocatalytic property of this transparent conductive metal oxide reduces the stability of perovskite solar cells under illumination. To realize the commercialization, the stability of perovskite solar cell must be improved. In this study, we replace TiO2 by In2O3, which is not only transparent and conductive, but also has little photocatalytic effect and it has higher electron mobility than TiO2. Investigation on different solution process methods of In2O3 as ETL is demonstrated.
Zinc oxide (ZnO) is a material of great interest for short-wavelength optoelectronic applications due to its wide band gap
(3.37 eV) and high exciton binding energy (60 meV). Due to the difficulty in stable p-type doping of ZnO, other p-type
materials such as gallium nitride (GaN) have been used to form heterojunctions with ZnO. p-GaN/n-ZnO heterojunction
devices, in particular light-emitting diodes (LED) have been extensively studied. There was a huge variety of electronic
properties and emission colors on the reported devices. It is due to the different energy alignment at the interface caused
by different properties of the GaN layer and ZnO counterpart in the junction. Attempts have been made on modifying the
heterojunction by various methods, such as introducing a dielectric interlayer and post-growth surface treatment, and
changing the growth methods of ZnO. In this study, heterojunction LED devices with p-GaN and ZnO nanorods array
are demonstrated. The ZnO nanorods were grown by a solution method. The ZnO nanorods were exposed to different
kinds of plasma treatments (such as nitrogen and oxygen) after the growth. It was found that the treatment could cause
significant change on the optical properties of the ZnO nanorods, as well as the electronic properties and light emissions
of the resultant LED devices.
We report investigation of SnS van der Waals epitaxies (vdWEs) grown by molecular beam epitaxy (MBE) technique.
Experimental results demonstrate an indirect bandgap of ~1 eV and a direct bandgap of ~1.25 eV. Substantial
improvement in the crystallinity for the SnS thin films is accomplished by using graphene as the buffer layer. Using this
novel growth technique we observed significant lowering in the rocking curve FWHM of the SnS films. Crystallite size
in the range of 2-3 μm is observed which represents a significant improvement over the existing results. The absorption coefficient, α, is found to be of the order of 104 cm-1 which demonstrates sharp cutoff as a function of energy for films grown using graphene buffer layers indicating low concentration of localized states in the bandgap. Hole mobility as high as 81 cm2V-1s-1 is observed for SnS films on graphene/GaAs(100) substrates. The improvements in the physical properties of the films are attributed to the unique layered structure and chemically saturated bonds at the SnS/graphene interface. As a result, the interaction between the SnS thin films and the graphene buffer layer is dominated by a weak vdW force and structural defects at the interface, such as dangling bonds or dislocations, are substantially reduced.
We investigated the influence of the growth method, growth conditions, and post-growth treatments on the ZnO nanorod
properties and the performance of heterojunction light emitting diodes (LEDs) based on ZnO nanorods. Due to small
lattice mismatch between GaN and ZnO, we will mainly consider p-GaN/n-ZnO nanorod heterojunctions. The influence
of p-GaN substrate and the influence of growth method and growth conditions used for ZnO nanorods on the LED
performance will be discussed.
In this paper we report the growth and characterization of Cu2ZnSnS4 (CZTS) nanostructures by co-electrodeposition
technique using CuCl2, SnCl2 and ZnCl2 as sources and choline-based ionic liquid (IL) as the electrolyte. X-ray
diffraction analysis of CZTS thin films grown by this technique indicated that the films have a kesterite structure with
preferred grain orientation along (112). It is found that the energy bandgap of the material is about 1.49eV and the
optical absorption coefficient is in the order of 104cm-1. Anodized aluminum oxide (AAO) was used as the growth mask
for the growth of the nanostructures. Anodization of the aluminum foil was carried out in phosphoric acid solution at
1°C and a potential of 40 to 100V was applied. Sulfurization of the rods was performed in elemental sulfur vapor at
450°C for four hours using N2 as the ambient gas. Experimental results show that nanotubes were formed using the
technique and the diameter can be well controlled by varying the applied potential in the anodization process. Electron
diffraction experiments show that a mixture of single- and poly-crystalline nanostructures was found.
In this paper we report systematic reliability studies of GaN UV detectors exposed to high power UV radiation. GaN
epitaxial layers are deposited by rf plasma-assisted molecular beam epitaxy (MBE) utilizing a double buffer layer
structure. Our studies show that the optimal buffer layer structure consists of a conventional AlN high-temperature
buffer layer (HTBL) and an 800 nm thick GaN intermediate temperature buffer layer (ITBL) deposited at 690°C. Two
types of devices are being investigated. Type I devices were fabricated on the optimal double buffer layer structure. Type
II devices have only a conventional AlN buffer layer. Flicker noise measurement is used to monitor the degradation of
the device due to optical stress. In addition, I-V and responsivity measurements were also performed. The experimental
results are consistent with each other which show that the degradation of the devices arises from the generation of
crystalline defects at the Schottky junction due to the exposure of the devices to the high power UV radiation. Both types
of devices demonstrate degradation in their optoelectronic properties. However, while type I devices general exhibit
gradual and slow degradations type II devices exhibit catastrophic breakdowns in the device characteristics. Our
experimental data show that visible-blind UV detectors fabricated on the optimized double buffer layer structure indicate
significant improvements in the radiation hardness of the devices.
Tris(8-hydroxyquinoline) aluminum (Alq3) is one of the most commonly used electron transporting and luminescent materials for organic light emitting diodes (OLEDs). It is thermally and morphologically stable to evaporate into thin films and it is a good green emitter. Due to its importance in OLEDs, the properties of Alq3 have been extensively studied. Most of the studies, however, were concentrated on the single crystals, powder or thin films of Alq3. Recently, synthesis of Alq3 nanostructures, such as nanoparticles and nanowires, has been reported. Nanostructures have been attracting increasing attention because they may have new optical, electronic, magnetic, and mechanical properties compared with those of bulk materials. In this work, we reported synthesis of Alq3 nanowires by heating Alq3 powder in a gas flow. The nanowires were grown on glass substrates which were located in the downstream. The obtained nanostructures were characterized by scanning electron microscopy (SEM) and photoluminescence (PL). The effect of substrate temperatures, fabrication system geometry (i.e. source to substrate distance), the choice of gas, and gas flow rate on the resulting nanostructures were investigated. It is found that the synthesis conditions had significant effect on the morphologies of the resulting nanostructures, but the PL showed no significant dependence on the morphology.
We report systematic investigations on hot-electron degradation in GaN-based HEMTs with different gate recess depths, d r , fabricated by reactive ion etching. The experimental data stipulate two different mechanisms underlying the hot-electron degradations of the devices. During the initial phase of hot-electron injection significant changes were observed in the dc characteristics of the devices and the flicker noise power spectral density, SV(f).The degradations were partially recovered by annealing the devices at 100°C for 20 minutes. It is shown that for stress time ts≤25 hours the reverse bias gate current, IG, decreases systematically with ts, whereas SV(f) fluctuates randomly. Detailed analyses of SV(f) measured over a wide range temperatures show that the initial degradations originate from the percolation of carriers in the 2DEG. The significant increase in the flicker noise during the initial phase of high-voltage stress is due to the generation of H+ at the AlGaN/GaN interface. The fluctuations in the magnitudes of SV(f) for ts≤25 hours originate from the motion of the H+ in the direction of the electric field. This results in the modulation of the percolation path leading to significant variations in SV(f) as a function of ts. For ts>25 hours both IGand SV(f) are stabilized resulting from the drifting of the H+ away from the gate region. Further stressing beyond 25 hours indicate strong dependencies of the device lifetimes on dr suggesting significant material degradation due to the reactive ion etching process for the fabrication of the gate recesses. Detailed characterization of the noise show that the final irreversible degradation is due to the generation of traps at the AlGaN/GaN interface.
In this work, we report a simple method for mass production of ZnO tetrapod nanorods. A mixture of Zn and graphite powders (ratio 2:1) was placed in a quartz tube. The quartz tube was placed in a horizontal tube furnace and heated up to 950°C. The tube was then removed from the furnance and quenched to room temperature. Fluffy products white in color were formed on the walls of the tube. Obtained products were characterized by X-ray diffraction, scanning electron microscopy (SEM), transmission electron microscopy (TEM) and photoluminescence. SEM images showed tetrapod-like ZnO nanorods. The four tetrapod legs were approximately equal in length, and the length of tetrapod legs was in the range ~1-3 μm. We investigated influence of the growth temperature (in the range from 700°C to 1100°C) and Zn to catalyst ratio to the properties of obtained products. Fabrication is different atmospheres (air, argon, nitrogen, humid argon, and humid nitrogen) was also performed. The influence of growth conditions (temperature, atmosphere, and catalyst concentration) to the formation and properties of ZnO nanorods is discussed.
We report the growth of high-mobility Si-doped GaN epilayers
utilizing unique double buffer layer (DBL) structures, which
consist of a thin buffer layer (TBL) and a thick GaN intermediate-temperature buffer layer (ITBL). In this study, three types of DBL were investigated: (i) thin low-temperature GaN buffer layer (LTBL)/GaN ITBL (type I); (ii) nitridated Ga metal film/GaN ITBL (type II); and (iii) AlN high-temperature buffer layer (HTBL)/GaN ITBL (type III). It is found that the electron mobilities of the GaN films are substantially improved with the use of DBLs. The sample grown using type II DBL has the highest room temperature electron mobility of 450 cm2V-1s-1, yet the sample grown with type III DBL exhibit the highest overall electron mobility of 520 cm2V-1s-1 at around 200K. The PL results show that the sample deposited on type III DBL exhibit the highest compressive stress at room temperature. The data suggest that the use of type III DBL leads to the relaxation tensile stress at the growth temperature
resulting in the improvements in the crystallinity and defect
properties. Due to the mismatch in the coefficients of expansion between GaN and sapphire the sample with the highest compressive stress at room temperature corresponds to the one with the lowest tensile stress at the growth temperature. It was shown that low tensile stress facilitates two-dimensional growth leading to improvements in the crystallinity and defect properties. This is supported by the experimental results of low-frequency noise measurements, which indicated substantial reduction in the flicker noise level as well as the elimination of deep-levels with the use of type III DBL.
N diffusion usually enhances the intermixing in GaAs/AlGaAs quantum well structure. However, Krames et al reported the reduction of layer intermixing in GaAs/AlGaAs quantum well heterostructures by an initial low-temperature 'blocking' Zn diffusion. Zn is commonly used as the dopant of the p-type GaAs substrate. To the best of our knowledge, the effect of Zn diffusion from the Zn-doped GaAs substrate on the intermixing has not been studied. In this work, we report the suppression of GaAs/AlGaAs quantum well intermixing by Zn doping in the GaAs substrate. Three samples with single GaAs/Al0.24Ga0.76As quantum wells were used in the work, all grown together by molecular beam epitaxy, but on three different substrates: Zn-doped p-type GaAs, Si-doped n-type GaAs, and semi-insulating GaAs. The samples were annealed together in a rapid thermal processor at temperatures around 900 degrees C. Photoluminescence measurements were then performed to characterize the samples. The samples with Zn-doped GaAs substrate shows more than 50 percent suppression of quantum well intermixing compared to the other two samples. It is due to Zn diffusion from the substrate into the quantum well, which induces the reduction in the number of group-III vacancies in the quantum well structure.
A small indium flux was used as a surfactant during the growth of gallium nitride films by rf-plasma assisted molecular beam epitaxy. The effects of the indium surfactant on the optical and structural properties of undoped GaN were studied by photoluminescence spectroscopy, high-resolution X-ray diffraction, atomic force microscopy, Rutherford backscattering spectroscopy, and low-frequency noise. Photoluminescence spectra show that the GaN thin films grown in the presence of In surfactant exhibit supressed yellow luminescence compared to films grown under the same experimental conditions but without In surfactant. The X-ray rocking curves demonstrate a 20% decrease in the full width at half maximum value for the films grown with In surfactant. AFM studies show that the root mean squared roughness for films grown with and without In surfactant are 5.86 and 6.99 nm respectively, indicating significant improvement in surface morphology. RBS and ion channeling shows the presence of stacking faults and dislocations and GaN growth with In surfactant and stacking faults in GaN grown without In. For the characterization of defect properties in the films we conducted detailed studies of 1/f and Generation-Recombination (G-R) noise on our samples. About 65% reduction in the Hooge parameter was observed in the film grown with In surfactant, indicative of a corresponding reduction in defect states within the material. For f $GTR 500Hz, G-R noise was the dominating fluctuation process. Detailed characterization of the G-R noise over a wide range of temperatures enabled the determination of the energy levels of the traps responsible for the G-R noise. Three different trap levels were observed using noise measurement technique. For films grown without In surfactant, traps with activation energies 806 meV, 241 meV and 100 meV were observed, whereas for samples grown with In surfactant traps were observed at 666 meV, 208 meV and 90 meV. Such systematic reduction in the energy levels of the traps may arise from the relaxation of strains in the material when grown under the application of In surfactant.
A new four vacuum chamber evaporator for organic devices was proposed and specially designed. The chamber one is for ITO/glass pretreatment, it's have ozone treatment and plasma treatment. The chamber two is used for organic thin film evaporating. There has six sources for organic material, and three power sources can work at the same time, there is a mask storage chamber in the organic evaporation chamber, so for some material research, you can get three kind of doping density of thickness OLED at the same fabrication. The chamber three is a vacuum glove box. It's used for package. The chamber four is used for cathode evaporating, it can evaporates Mg, Ag, LfF, Ca, Al, Au.
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