Growth and characterization of two-dimensional crystals for communication and energy applications

This review article covers the growth and characterization of two-dimensional (2D) crystals of transition metal chalcogenides, h-BN, graphene, etc. The chemical vapor transport method for bulk single crystal growth is discussed in detail. Top-down methods like mechanical and liquid exfoliation and bottom-up methods like chemical vapor deposition and molecular beam epitaxy for mono/few-layer growth are described. The optimal characterization techniques such as optical, atomic force, scanning electron, and Raman spectroscopy for identification of mono/few-layer(s) of the 2D crystals are discussed. In addition, a survey was done for the application of 2D crystals for both creation and deterministic transfer of single-photon sources and photovoltaic systems. Finally, the application of plasmonic nanoantenna was proposed for enhanced solar-to-electrical energy conversion and faster/brighter quantum communication devices.     

Improvement in electrical and photovoltaic properties of a-Si/c-Si heterojunction with slanted nano-columnar amorphous silicon thin films for photovoltaic applications

The flat a-Si and slanted nanocolumnar (S-nC) a-Si thin films were prepared on c-Si and corning glass substrates by e-beam physical vapor deposition (EB-PVD) technique. The structural properties of all the grown thin films were determined by X-Ray Diffraction (XRD) analysis and Raman spectroscopy. Surface and cross-sectional morphology of a-Si/c-Si and S-nC a-Si/c-Si heterojunctions were investigated by Field Emission Scanning Electron Microscopy (FE-SEM). Sculptured thin films demonstrate potential for significant nanoscale applications in the area of thin film technology. The electrical and photovoltaic properties of these heterojunctions have been investigated by means of dc current–voltage (I–V) measurements at room temperature in dark and light conditions. The S-nC STFs' performance has been found to be improvable on changing the morphology of the thin film. We have found that, the porous morphology of this structure improves the photosensitivity features in photovoltaic devices and solar cell technology. We gained a high open voltage value, such as 900 mV in S-nC a-Si/c-Si thin film, without any doping process.     

Synthesis of the wheat-like CdSe/CdTe thin film heterojunction and their photovoltaic applications

We report on the fabrication of wheat-like CdSe/CdTe thin film heterojunction solar cells made using a simple electrochemical deposition method and close-spaced sublimation technology on indium tin oxide (ITO) substrates. Structural, optical, and electrical properties of the wheat-like CdSe/CdTe thin film junctions were characterized by X-ray diffraction (XRD), field emission scanning electron microscope (FESEM), energy dispersive spectrometry (EDS), ultraviolet–visible (UV–vis) absorption spectrum and Keithley 2400 analysis. A significant red-shift of the absorption edge is observed in this heterojunction. The heterostructure is composed of the wheat-like CdSe array and CdTe thin film, showing optical properties typical of type II heterostructures that are suited for photovoltaic applications. A photocurrent density of 8.34 mA/cm² has been obtained under visible light illumination of 100 mW/cm². This study demonstrates that the electrochemical deposition and the close-spaced sublimation technology, which are easily adapted to other chemical systems, are promising techniques for large-scale fabrication of low-cost heterojunction solar cells.     

Synthesis and characterization of novel conjugated polymers based on 3-cyclobutene-1,2-dione moiety

Three novel conjugated polymers bearing 3,4-bis（4-hexylthiophen-2-yl）-3-cyclobutene-1,2-dione unit in their main chain have been synthesized successfully in good yields through Suzuki or Stille coupling reaction.Their molecular structures have been confirmed by FT-IR,¹H NMR and ¹³C NMR.All these copolymers exhibit broad and strong absorption bands in UV-vis region,and their optical band gaps are calculated to be 1.6-2.0 eV.suggesting that they have good coverage with the solar spectrum.These polymers have good thermostability and solubility in common organic solvents.Moreover,all these objective macromolecules possess high electron affinity of～3.8 eV determined from cyclic voltammetry measurement,implying that they are potential n-type polymeric photovoltaic materials.     

Synthesis and characterization of novel conjugated polymers based on 3-cyclobutene-l,2-dione moiety

Three novel conjugated polymers bearing 3,4-bis(4-hexylthiophen-2-yl)-3-cyclobutene-1,2-dione unit in their main chain have been synthesized successfully in good yields through Suzuki or Stille coupling reaction.Their molecular structures have been confirmed by FT-IR,~1H NMR and ~(13)C NMR.All these copolymers exhibit broad and strong absorption bands in UV-vis region,and their optical band gaps are calculated to be 1.6-2.0 eV.suggesting that they have good coverage with the solar spectrum.These polymers...     

Electro-photo modulation of the fermi level in WSe2/graphene van der Waals heterojunction

We report an electro-photo double modulation of the fermi level in a WSe₂/graphene heterojunction. The heterojunction exhibits high I_ON/I_OFF ratio (~10³) in transfer characteristic in dark and distinct rectification behavior in output characteristic under light illumination, respectively. Time-dependent photoresponse reveals that the heterojunction has a considerable potential in the application of photodetection. Interestingly, an exotic current peak is observed in transfer characteristic under light illumination. This novel behavior is attributed to the tunable fermi level at the WSe₂/graphene heterojunction by electro-photo double modulation. The results may be helpful to develop tunable photovoltaic optoelectronics based on van der Waals heterojunctions.     

Impact of average photon-energy coefficient of solar spectrum on the short circuit current of photovoltaic modules

The output energy of photovoltaic (PV) modules is influenced by the spectral irradiance distribution of the solar spectrum under outdoor conditions. To rate the precise output energy of PV modules, the correction of short circuit current (I_SC) based on actual environmental conditions is needed, because I_SC significantly depends on the shape of the spectral irradiance distribution. The average photon energy (APE) is a zero-dimensional index for spectral irradiance distribution, and APE value uniquely describes the shape of a solar spectrum. Thus, APE has an impact on I_SC of PV modules. In this contribution, the relationship between APE coefficient and I_SC of the multi-crystalline silicon, single-crystalline silicon, heterojunction intrinsic thin-layer, back contact, copper indium selenide and cadmium telluride PV modules has explored. It is revealed that APE value changes the I_SC of PV modules which appeared to have immense possibilities of I_SC correction using APE coefficient. This new approach can be very effective for precise rating the output energy of PV modules under actual outdoor conditions.     

Photovoltaic Cells Involving the Nonconjugated Conductive Polymer Iodine-Doped Styrene-Butadiene-Rubber (SBR)

Novel photovoltaic cells involving a nonconjugated conductive polymer have been fabricated using titanium dioxide/doped styrene-butadiene-rubber/carbon on ITO coated PET substrates. Photocurrents and photo-voltages for different intensities of light (emission at 300–700 nm) have been measured. These cells have shown significantly higher photocurrents and photo-voltages compared to previous reports. A photocurrent density of about 0.25 mA/cm² and a photo-voltage of 0.74 V have been measured for a light intensity of ～4 mW/cm².