Energy Barrier Layer Research Articles

Implementation of energy barrier layers for 1550 nm high-power laser diodes

The influence of a thin AlInAs energy barrier on the efficiency of 1550 nm high-power semiconductor lasers efficiency has been experimentally studied. It was shown that the position and number of barriers in an asymmetric laser heterostructure based on a 1.8–1.9 μm thick waveguide has a significant effect on the output optical power. It is shown that in a barrierless structure, the main reason for radiative efficiency decrease is internal quantum yield drop due to the absence of an energy barrier for the type-II heterojunction at the waveguide-p-cladding interface and electron leakage to the p-emitter layer. It is demonstrated that the implementation of single AlInAs energy barrier layer on waveguide-p-cladding heterojunction allows significantly increase laser diode maximum output power. 2 W maximum CW optical power has been achieved from 40 μm aperture laser diode at heatsink temperature 25 °C.

Resonant plasmonic detection of terahertz radiation in field-effect transistors with the graphene channel and the black-As_xP_{1-x} gate layer

We propose the terahertz (THz) detectors based on field-effect transistors (FETs) with the graphene channel (GC) and the black-Arsenic (b-As) black-Phosphorus (b-P), or black-Arsenic-Phosphorus (b-As_xP_{1-x}) gate barrier layer. The operation of the GC-FET detectors is associated with the carrier heating in the GC by the THz electric field resonantly excited by incoming radiation leading to an increase in the rectified current between the channel and the gate over the b-As_xP_{1-x} energy barrier layer (BLs). The specific feature of the GC-FETs under consideration is relatively low energy BLs and the possibility to optimize the device characteristics by choosing the barriers containing a necessary number of the b-As_xP_{1-x} atomic layers and a proper gate voltage. The excitation of the plasma oscillations in the GC-FETs leads to the resonant reinforcement of the carrier heating and the enhancement of the detector responsivity. The room temperature responsivity can exceed the values of 10^3 A/W. The speed of the GC-FET detector’s response to the modulated THz radiation is determined by the processes of carrier heating. As shown, the modulation frequency can be in the range of several GHz at room temperatures.

UV-ozone induced surface passivation to enhance the performance of Cu2ZnSnS4 solar cells

Interface property has been considered one of the most critical factors affecting the performance of semiconductor devices. In this work, we demonstrate an efficient surface passivation for the interface between Cu2ZnSnS4 (CZTS) and CdS buffer layer by using UV-ozone treatment at room temperature. The passivation led to a significant enhancement of short circuit current density (Jsc) of the device from 11.70 mA/cm2 to 18.34 mA/cm2 and thus efficiency of the CZTS solar cells from 3.18% to 5.55%. The study of surface chemistry has revealed that the UV-ozone exposure led to formation of a Sn–O rich surface on CZTS, which passivates the dangling bonds and forms an ultra-thin energy barrier layer at the interface of CZTS/CdS. The barrier is considered to be responsible for the reduction of non-radiative recombination loss in the solar cells as confirmed by photoluminescence (PL) measurement. The elongated lifetime of minority carriers in the CZTS solar cells by time-resolved PL has further verified the interface passivation effect induced by UV-ozone treatment. This work provides a fast, simple yet very effective approach for surface passivation of CZTS film to boost the performance of CZTS solar cells.

Luminescent Ta doped WO3 thin films as a probable candidate for excitonic solar cell applications

Post-annealed undoped and Ta doped (doping concentration 1, 3, 5 and 7 wt%) WO3 thin films are prepared using RF magnetron sputtering technique. All the films are in monoclinic phase with 〈1 0 0〉 as the preferred crystal growth direction and show a nanostructured nature. The fabricated films have a smooth surface with densely packed grains. All the films show good optical transparency in the wavelength region 400−900 nm. Optical constants of the films are calculated from the transmittance and reflectance spectra. All the films show a broad blue emission corresponds to the indirect band to band transition in WO3 under UV excitation. The smooth pin hole free surface, good crystalline quality, moderate band gap energy and bright luminescence of the Ta doped WO3 thin films suggest the possibility of using this material as an energy barrier layer for performance enhancement in excitonic solar cells.

Silver nanoparticles-incorporated Nb2O5 surface passivation layer for efficiency enhancement in dye-sensitized solar cells

Guiding and capturing photons at the nanoscale by means of metal nanoparticles and interfacial engineering for preventing back-electron transfer are well documented techniques for performance enhancement in excitonic solar cells. Drifting from the conventional route, we propose a simple one-step process to integrate both metal nanoparticles and surface passivation layer in the porous photoanode matrix of a dye-sensitized solar cell. Silver nanoparticles and Nb2O5 surface passivation layer are simultaneously deposited on the surface of a highly porous nanocrystalline TiO2 photoanode, facilitating an absorption enhancement in the 465 nm and 570 nm wavelength region and a reduction in back-electron transfer in the fabricated dye-sensitized solar cells together. The TiO2 photoanodes were prepared by spray pyrolysis deposition method from a colloidal solution of TiO2 nanoparticles. An impressive 43% enhancement in device performance was accomplished in photoanodes having an Ag-incorporated Nb2O5 passivation layer as against a cell without Ag nanoparticles. By introducing this idea, we were able to record two benefits - the metal nanoparticles function as the absorption enhancement agent, and the Nb2O5 layer as surface passivation for TiO2 nanoparticles and as an energy barrier layer for preventing back-electron transfer - in a single step.

Enhanced photovoltaic and carrier collection efficiency of dye sensitized solar cell: Nb2O5 coated TiO2 photoanode

ABSTRACTDye sensitized solar cells (DSSCs) were fabricated using Nb2O5 energy barrier layer is coated on the TiO2 photoanode. This energy barrier (Nb2O5 layer) reduces the recombination rate of the photoinjected electrons with their counter holes. A monoclinic Nb2O5 energy barrier layer can be coated 1 and 3 times on a TiO2 photoanode using a dip-coating method. The Nb2O5/TiO2 photoanode exhibits enhanced photovoltaic performance, which is better than the bare TiO2 photoanode. The results presented in this study show that this Nb2O5 layer forms an inherent energy barrier at the electrode-electrolyte interface. The electron transport and charge recombination behaviors of DSSCs were investigated by electrochemical impedance spectra (EIS) and the results illustrated that the DSSCs showed the lowest charge transport resistance and the longest electron lifetime. The electron transport and recombination were studied by stepped light- induced transient measurements of photocurrent and voltage (SLIM-PCV) method and shown that the electron diffusion length is increasing with the Nb2O5 coating.

Improving the efficiency of cadmium sulfide-sensitized titanium dioxide/indium tin oxide glass photoelectrodes using silver sulfide as an energy barrier layer and a light absorber.

Cadmium sulfide (CdS) and silver sulfide (Ag2S) nanocrystals are deposited on the titanium dioxide (TiO2) nanocrystalline film on indium tin oxide (ITO) substrate to prepare CdS/Ag2S/TiO2/ITO photoelectrodes through a new method known as the molecular precursor decomposition method. The Ag2S is interposed between the TiO2 nanocrystal film and CdS nanocrystals as an energy barrier layer and a light absorber. As a consequence, the energy conversion efficiency of the CdS/Ag2S/TiO2/ITO electrodes is significantly improved. Under AM 1.5 G sunlight irradiation, the maximum efficiency achieved for the CdS(4)/Ag2S/TiO2/ITO electrode is 3.46%, corresponding to an increase of about 150% as compared to the CdS(4)/TiO2/ITO electrode without the Ag2S layer. Our experimental results show that the improved efficiency is mainly due to the formation of Ag2S layer that may increase the light absorbance and reduce the recombination of photogenerated electrons with redox ions from the electrolyte.

Photovoltaic performance of bithiazole-bridged dyes-sensitized solar cells employing semiconducting quantum dot CuInS2 as barrier layer material

In this work, the quantum dot CuInS2 layer was deposited on TiO2 film using successive ionic layer absorption and reaction (SILAR) method, and then two bithiazole-bridged dyes (BTF and BTB) were sensitized on the CuInS2/TiO2 films to form dye/CuInS2/TiO2 photoanodes for DSSCs. It was found that the quantum dots CuInS2 as an energy barrier layer not only could effectively improve open-circuit voltage (Voc) of solar cell, but also increase short-circuit photocurrent (Jsc) compared to the large decrease in Jsc of ZnO as energy barrier layer. The electrochemical impedance spectroscopy (EIS) measurement showed that the CuInS2 formed a barrier layer to suppress the recombination from injection electron to the electrolyte and improve open-circuit voltage. Finally, the open-circuit voltage increased about 22 and 27mV for BTF and BTB-/CuInS2/TiO2-based cells, the overall conversion efficiencies also reached to 7.20% and 6.74%, respectively.

Significant influence of nano-SiO2 on the performance of dye-sensitized solar cells based on P25

The present paper describes a new type of composite photo-anode of SiO2/TiO2 that was prepared by adding nano-sized SiO2 into the TiO2 (P25) paste during the fabrication of dye-sensitized solar cells (DSSCs). The effects of the added SiO2 on the performance of DSSCs were investigated and the preparative procedures of DSSCs were optimized. It was found that certain amount of SiO2 added in the P25 powder was crucial to reduce the agglomeration and enhance the dispersion of P25 powders, which helps to achieve a better connection and thus higher electron transfer efficiency not only between the TiO2 particles but also between the TiO2 particles and the FTO substrate. Under the same standard air mass 1.5 illumination (100mWcm−2), the DSSCs constructed by SiO2/P25 composite photo-anodes show higher solar energy-to-electricity conversion efficiency (e.g., η=8.4%) compared to those by bare P25 electrodes (typically η=5.8%). The higher η mainly results from its higher short-circuit photocurrent density (Jsc=18.7mAcm−2) while the open-circuit voltage (Voc) remains unchanged and was found to be ∼780mV. The fill factor (FF) was 0.57. The great enhancement in Jsc of DSSCs is ascribed to the SiO2 energy barrier layer which increases the separation of injected electrons and oxidized dyes/redox couple as well as retards the recombination reactions in the resulting DSSCs.

Improved conversion efficiency of CdS quantum dot-sensitized TiO2 nanotube-arrays using CuInS2 as a co-sensitizer and an energy barrier layer

A thin layer of CuInS2 and CdS quantum dots (QDs) is deposited on TiO2 nanotube arrays (TNTs) to form CdS/CuInS2/TNTs photoelectrodes. The CuInS2 layer is prepared by a successive ionic layer absorption and reaction method, and the CdS QDs are deposited by a chemical bath deposition method. The CuInS2 layer acts as both a co-sensitizer and an energy barrier layer between TNTs and CdS QDs. The deposited CuInS2 layer significantly extends the visible-light response of CdS-sensitized TNTs into 500–700 nm wavelength range. As a consequence, the photoelectrochemical response of the CdS/CuInS2/TNTs electrodes is much improved compared with CdS sensitized TNTs. The CdS/CuInS2/TNTs electrodes exhibit a maximum power conversion efficiency of 7.3%, which is a 120% improvement compared with the highest efficiency of 3.3% for CdS/TNTs electrodes in our study. The improved efficiency is mainly due to the increased absorbance and the reduced recombination between the photoinjected electrons and the redox ions from the electrolyte, resulting from the formation of a CuInS2 layer.

Multi-color colloidal quantum dot based light emitting diodes micropatterned on siliconhole transporting layers

We present a colloidal quantum dot based light emitting diode (QD-LED) which utilizesthe p-type silicon substrate as the hole transporting layer. A microcontact printingtechnique was introduced to pattern self-assembled CdSe/ZnS QD films, whichallowed creation of an LED with well-defined geometry suitable for monolithicintegration on silicon substrates. Our QD-LED consists of multi-layers of inorganicmaterials: a combination of Au (thickness: 5 nm) and Ag (12 nm) as the cathode, aZnO:SnO2 mixture (ratio 3:1, 40 nm) as the electron transporting layer, CdSe/ZnS QDs as the light emission layer,1 nm SiO2 as an energy barrier layer, and p-type silicon as the hole transporting layer. These printedQD-LEDs are capable of multi-color emission peaked at wavelengths of 576 nm, 598 nm, and622 nm, corresponding to sizes of the embedded QDs with the diameters of 8.4 nm, 9.0 nm,and 9.8 nm respectively. The optimal thickness of the quantum dot layers needed for lightemission is characterized using atomic force microscopy: for 8.4 nm QDs, the value is 33 nm (± 5 nm)or ∼4 ML (monolayers). Larger turn on voltages were measured (2, 4 and 5 V) for thesmaller average particle diameters (9.8 nm, 9.0 nm and 8.4 nm, respectively). Themixture ratio of Zn and Sn was optimized (40% Zn and 25% Sn) to maintain properhole–electron recombination at the QD layer and avoid the yellowish-white emission fromZnO/SnO2.

Structural and optical properties of ZnMgO nanostructures formed by Mg in-diffused ZnO nanowires

ZnMgO nanostructures with wurtzite phase were prepared by thermal diffusion of Mg into the ZnO nanowires. As ZnO light-emitting devices have been operated by using ZnMgO layers as energy barrier layers to confine the carriers, it is essential to realize the characterization of ZnMgO particularly. In this work, the Mg content in Zn 1 −x Mg x O alloy determined by X-ray diffraction (XRD) and photoluminescence (PL) shows a good coincidence. The variation of lattice constant and the blueshift of near-band-edge emission indicate that Zn 2+ ions are successfully substituted by Mg 2+ ions in the ZnO lattice. In Raman-scattering studies, the change of E 2(high) phonon line shape in ZnO:Mg nanostructures reveals the microscopic substitutional disorder. In addition to the host phonons of ZnO, two additional bands around 383 and 510 cm −1 are presumably attributed to the Mg-related vibrational modes.

UV Electroluminescence Emission from ZnO Light‐Emitting Diodes Grown by High‐Temperature Radiofrequency Sputtering

to improve the structural properties of n- and p-type ZnO compared to previous studies. [7] In addition, a thermal annealing process was carried out to activate the phosphorus dopants in p-type ZnO and improve the electrical and optical properties of the ZnO layers. The LED showed excellent current-rectifying behavior with a threshold voltage of 3.2 V and an EL emission peak at 380 nm at room temperature. The UV EL emission spectrum was in good agreement with the room-temperature photoluminescence (PL) spectrum of the p-type ZnO used in the LED. Furthermore, the near-bandedge emission was increased and the deep-level emission was decreased when (Mg,Zn)O alloy layers were introduced as energy barrier layers between n-type and p-type ZnO films to confine the carrier recombination process to the high-quality n-type ZnO film. A schematic diagram of the p–n homojunction ZnO LED is shown in Figure 1. The gallium-doped n-type ZnO with a thickness of 1.5 lm was grown on a c-Al2O3 substrate. It