Fabrication and characterization of fullerene/porphyrin bulk heterojunction solar cells

Fullerene/porphyrin bulk heterojunction solar cells were fabricated and, the electronic and optical properties were investigated. Effects of exciton-diffusion blocking layer of perylene derivative on the solar cells between active layer and metal layer were also investigated. Optimized structures with the exciton-diffusion blocking layer improved conversion efficiencies. Energy levels of the molecules were calculated and discussed. Nanostructures of the solar cells were investigated by X-ray and electron diffraction, which indicated formation of fullerene/porphyrin mixed crystals. Electronic structures of the molecules were investigated by molecular orbital calculation, and energy levels of the solar cells were discussed.     

Fabrication and characterization of C60/tetrathiafulvalene solar cells

Fabrication and characterization of C₆₀/tetrathiafulvalene solar cells was carried out. Photovoltaic properties of bulk-hetero and heterojunciotn solar cells were investigated by light-induced current vs. voltage curves and optical absorption. Transmission electron microscopy (TEM) image, X-ray and electron diffraction showed that the bulk-heterojunction film had the microstructure of C₆₀ crystal structure with TTF phase. Heat treatment of the heterojunction film with tetraethylsilane improved the photovoltaic performance, yielding a slight increase of conversion efficiency. This result would be originated in improvement of microstructure around inner interface between the both crystal phases. Mechanisms of the photovoltaic properties were discussed on the basis of the experimental results.     

Fabrication and field emission properties of multi-walled carbon nanotube/silicon nanowire array

Silicon nanowire (SiNW) arrays were fabricated on silicon wafers by the metal-assisted chemical etching method. Varied average diameters of SiNW arrays were realized through further treatment in a mixed agent of HF and HNO₃ of certain concentrations. After the treatment, there were more than 93% SiNWs with diameters smaller than 100 nm. The tip of each SiNW was subsequently wrapped with multi-walled carbon nanotubes (MWCNTs) with chemical vapor deposition method. The as-fabricated MWCNT/SiNW arrays were fabricated into electric field emitters, with turn-on field of 2.0 V/μm (current density: 10 μA/cm²), much lower than that of SiNW array (5.0 V/μm). The turn-on electric field of MWCNT/SiNW array decreased with the decreasing of the average diameter of SiNWs, indicating the performance of the field emission is relative to the morphology of SiNWs. As the SiNW array is uniform in height and easy to fabricate, the MWCNT/SiNW array shows potential applications in flat electric display.     

Structures and photovoltaic properties of copper oxides/fullerene solar cells

Copper oxide (CuO_x) thin films were produced by spin-coating and electrodeposition methods, and their microstructures and photovoltaic properties were investigated. Thin film solar cells based on the Cu₂O/C₆₀ and CuO/C₆₀ heterojunction or bulk heterojunction structures were fabricated on F-doped or In-doped SnO₂, which showed photovoltaic activity under air mass 1.5 simulated sunlight conditions. Microstructures of the CuO_x thin films were examined by X-ray diffraction and transmission electron microscopy, which indicated the presence of Cu₂O and CuO nanoparticles. The energy levels of the present solar cells were also discussed.     

Influence of the exciton blocking layer on the stability of layered organic solar cells

The life-time of multi-layer organic solar cells based on the couple donor acceptor copper phthalocyanine/fullerene is studied as a function of the nature of the exciton blocking layer (EBL). It is shown that organic EBL are more efficient than are the inorganic In₂S₃ EBLs. Moreover among the organic EBL, Alq₃ is the most efficient EBL protecting layer. An organic solar cell’s lifetime depends on oxygen- and water-contamination of the organic materials. The solar cell’s degradation may correspond to bulk or interface phenomena. Using equivalent electrical schemes of solar cell diodes, we show that the structure degradation is mainly related to bulk modification. It is proposed that oxygen- and water-diffusion into the C₆₀ induce a large increase in its resistivity and, therefore an increase in the series resistance, which decreases the solar cell efficiency. In the case of In₂S₃ EBLs, the degradation law predicts that with time two different phenomena will be present. The classical oxygen- and water-diffusion into the organic material, during the first hour of air exposure, leads to a modification in the In₂S₃ EBL/organic interface properties.     

Fabrication and characterization of magnetic nanoporous zeolite templated carbon

The effects of heat-treatment and guest adsorption on the structure and electronic/magnetic properties were investigated for zeolite templated carbon (ZTC) by using potassium, bromine, and helium as guest species. ZTC consists of a curved graphene network, which has regularly arranged nanopores with a periodicity of 1.4 nm. X-ray diffraction and Raman spectra reveal that the regular nanopore structure is preserved during vacuum heat-treatment (<380 °C) and potassium adsorption process. On the other hand, bromine adsorption is found to destroy the nanopore structure. Helium atoms adsorbed in the nanospace participate in the energy dissipation of the radical spins in ZTC through collisions with the spins.     

Modification and characterization of (energetic) nanomaterials

Nanomaterials are a topic of increased interest, since they have properties which differ from their macroscopic counterparts. Many applications nowadays take advantage of the new functionalities which natural and manufactured nanoparticles possess. Based on these developments, also the research on energetic nanomaterials is receiving more and more attention. Apart from the synthesis of energetic nanomaterials, another area of interest is the coating of energetic (nano)powders, in order to be able to modify their properties or to add new functionalities to these particles. (Modified) energetic materials find applications in explosives, gun and rocket propellants and pyrotechnic devices. The modified energetic materials are expected to yield enhanced properties, e.g. a lower vulnerability towards shock initiation, enhanced blast, enhanced shelf-life and environmentally friendly replacements of currently used materials.An experimental set-up for the coating of existing powders has been designed and constructed. The experimental technique is based on a special plasma application which, contrary to more general plasmas, can be operated at relatively low temperatures and ambient pressure. This allows the handling of heat-sensitive materials, which would otherwise readily decompose or react at higher temperatures. The facility used for the coating of energetic powders in the lower micron range is based on a fluidized bed reactor in which the powder circulates.In this paper, the experimental technique will be described and experimental results will be shown of CuO powders that have been coated with a very thin, nanoscale deposit of a SiO-containing layer.     

Impact of film thickness on threshold voltage of polysilicon TFTs

The threshold voltage is a key parameter in the silicon MOSFETS design and operation. In this paper, we study the factors that contribute to the changes of threshold voltage of thin-film LPCVD polysilicon transistors when varying the thickness of the active layer.The results show that the threshold voltage depends strongly on the film thickness. For high thicknesses, the threshold voltage is shown to be determined by the trapped holes at grain boundaries. The variation of this parameter with film thickness can be attributed to inter-granular trap states density variation in the film.For low thicknesses, a simple electrostatic model of the study structure, associated with a numerical method of solving 2D-Poisson's equations, shows that the changes of threshold voltage of polysilicon TFT depends on grain-boundary properties and charge-coupling between the front and back gates. Based on this consideration, the usual threshold voltage expression is modified. The results so obtained are compared with the available experimental data, which show a satisfactory match thus justifying the validity of the proposed relation.     

Synthesis and characterization of ZnO thin films by thermal evaporation

ZnO thin films have been successfully synthesized by thermal evaporation of pure zinc at 900 °C under the flow of different percentages of argon and oxygen gases. The films were characterized by X-ray diffraction (XRD), variable pressure scanning electron microscopy (VPSEM), energy dispersive X-ray spectroscopy (EDS) and UV–vis spectroscopy. The aim of this paper is to study the influence of the oxygen percentage on the structural and morphological properties of the ZnO films. VPSEM results show that very thick needle structures were produced at high oxygen percentages. EDS results revealed that only Zn and O are present in the sample, indicating a composition of pure ZnO. XRD results showed that the ZnO synthesized under different quantities of oxygen were crystalline with the hexagonal wurtzite structure. UV–vis spectroscopy results indicated that the optical band gap energies from the transmission spectrum are between 3.62 and 3.69 eV for ZnO thin films.     

Nanotribological behavior of ZnO films prepared by atomic layer deposition

We used atomic layer deposition to form ZnO thin-film coatings on Si substrates and then evaluate the effect of pile-up using the nanoscratch technique under a ramped mode. The wear volume decreased with increasing annealing temperature from room temperature to 400 °C for a given load. Elastic-to-plastic deformation occurred during sliding scratch processing between the groove and film for loading penetration of 30 nm. The onset of non-elastic behavior and greater contact pressure were evident for loading penetration of 150 nm; thus, full plastic deformation occurred as a result of a substrate effect. We suspect that elastic–plastic failure events were related to edge bulging between the groove and film, with elastic–plastic deformation attributable to adhesion discontinuities and/or cohesion failure of the ZnO films.     

Characterization of interface between CuInSe2 and In2O3

CuInSe₂/In₂O₃ structures were formed by depositing CuInSe₂ films by stepwise flash evaporation onto In₂O₃ films, which were grown by DC reactive sputtering of In target in presence of (Ar+O₂) gas mixture. Phase purity of the CuInSe₂ and In₂O₃ films was confirmed by Transmission Electron Microscopy (TEM) studies. X-ray diffraction (XRD) results on CuInSe₂/In₂O₃/glass structures showed sharp peaks corresponding to (112) plane of CuInSe₂ and (222) plane of In₂O_3. Rutherford Backscattering Spectrometry (RBS) investigations were carried out on CuInSe₂/In₂O₃/Si structures in order to characterize the interface between In₂O₃ and CuInSe₂. The results show that the CuInSe₂ films were near stoichoimetric and In₂O₃ films had oxygen deficient composition. CuInSe₂/In₂O₃ interface was found to include a ∼20 nm thick region consisting of copper, indium and oxygen. Also, the In₂O₃/Si interface showed the formation of ∼20 nm thick region consisting of silicon, indium and oxygen. The results are explained on the basis of diffusion/reaction taking place at the respective interfaces.     

The effects of exciton-diffusion blocking layers on pentacene/C60 bulk heterojunction solar cells

Organic thin film solar cells, with structures based on pentacene/C₆₀ bulk heterojunctions, have been fabricated and characterized. The effect of introducing an exciton-diffusion blocking layer (EBL) of 3,4,9,10-perylenetetracarboxylic dianhydride, between the active layer and the metal layer in the solar cell, was investigated. It was shown that optimized structures containing EBLs resulted in an improvement in solar cell conversion efficiencies. The energy levels of the molecules were calculated and are subsequently discussed.     

Synthesis and characterization of water-soluble gold colloids stabilized with aminoresorcinarene

An aminoresorcinarene (TOMR) with eight amino groups as a novel ligand was synthesized and the fabrication of gold hydrosol stabilized with TOMR was reported in this paper. The TOMR-capped gold nanoparticles were characterized and analyzed by the ultraviolet visible (UV-vis) spectroscopy, Fourier transmission infrared spectra (FT-IR), X-ray diffraction (XRD) and transmission electron microscopy (TEM), respectively. The experiment showed that nearly 10 times of the usual amount of hydrate hydrazine as a reducing agent needed to be used, and the obtained TOMR-stabilized gold hydrosol had a higher level of stability at room temperature, which might be related to the structure of TOMR molecule and the formation of hydrophilic double layer structure of TOMR on the surface of gold core.     

Changes in the optical properties of as-deposited plasma polymerized 2,6-diethylaniline thin films by iodine doping

In this study, plasma polymerized 2, 6-diethylaniline (PPDEA) thin films of different thicknesses were synthesized using a glow discharge plasma polymerization method. Scanning electron microscopy showed that the surface morphology of an as-deposited PPDEA thin film was comparatively smooth after iodine doping. The iodine-doped PPDEA was found to be thermally stable up to ca about 560 K, which was slightly lower than that observed for as-deposited PPDEA. Ultraviolet-visible spectroscopic analyses demonstrated that iodine doping resulted in a significant decrease in the optical energy gap. As the doping period increased, the direct optical transition energy gap was reduced from 3.56 to 2.79 eV and the indirect optical transition energy gap was decreased from 2.23 to 1.97 eV. Thus it is observed that, the optical parameters of as-deposited PPDEA thin films with different thicknesses can be modified with different iodine doping periods.     

Determination of electrical and solar cell parameters of FTO/CuPc/Al Schottky devices

A Schottky structure is fabricated using CuPc sandwiched between fluorinated tin oxide (FTO) and aluminium electrodes. The electrical properties of the device are measured at room temperature. Permittivity of the device is calculated from capacitance measurements. The saturation current density, , diode ideality factor, n=3.02 and barrier height, are determined for the Schottky juction. Reverse bias versus is interpreted in terms of Schottky emission. Solar cell parameters are determined from the J-V characteristics. Power conversion efficiency, η of 0.0024% is obtained for the cell. Band gap energy of the material is determined from UV-visible absorption spectrum.     

Effect of nickel incorporation on the optical properties of diamond-like carbon (DLC) matrix

The present study investigates the optical behavior of composite nanostructured DLC based films and functional coatings. Diamond-like carbon (DLC) thin films were synthesized by electrodeposition method onto SnO₂-coated glass substrates using an electrolyte of a mixture of acetic acid and water. Nanoparticles of nickel were then introduced into the DLC matrix. Morphology of the metal incorporated thin films and distribution of nanoparticles were studied by SEM; continuous homogeneous distribution of the particles was observed. Raman spectroscopy showed additional peaks in addition to the peaks due to DLC matrix. FTIR spectra revealed new peaks in the lower wave number region due to metal inclusion. UV-vis transmittance studies were performed to calculate the band gap of the samples. The estimated band gap from the Tauc relation was found to vary from 2.63 eV for the virgin DLC to 1.48 eV for the metal incorporated DLC.     

Preparation and characterization of ZnS/CdS bi-layer for CdTe solar cell application

In this work, bilayer ZnS/CdS film was prepared as an improved window layer of CdTe solar cell. TEM was used to observe the cross section of the bilayer structure. The total thickness of ZnS/CdS film was about 60 nm, which could allow more photons to pass through it and contribute to the photocurrent. Optical properties of the bilayers were investigated using UV–vis spectroscopy. Compared with poor transmission of standard CdS film in the short wavelength range of 350–550 nm, the transmission of ZnS/CdS was improved and reached above 50%. The ZnS/CdS was annealed with CdCl₂. X-ray photoelectron spectroscopy (XPS) was used to investigate its chemical properties. A possible diffusion between CdS and ZnS was observed after annealing. The efficiency of standard CdS/CdTe solar cell was 9.53%. The device based on ZnS/CdS window layer had a poor 6% efficiency. With annealing treatment on ZnS/CdS layer, the performance was improved and reached 10.3%. In addition, the homogeneity of solar cell performance was improved using ZnS/CdS window layer. A thin ZnS layer was quite effective to reduce the possible shunt paths and short parts of window layer and consequently contributed to fabrication of a homogeneous CdTe solar cell.     

Synthesis and characterization of MCM-41 decorated with CuO particles

CuO particles have decorated on the external surface of MCM-41 by in situ introducing cupric nitrate during the hydrothermal synthesis followed by the calcination. The textural and structural properties of CuO/MCM-41 are compared with those of pure MCM-41. The results show that CuO particles are about 40 nm in size and are not agglomerated. The addition of cupric nitrate to the synthesis gel leads to materials with somewhat reduced quality as evidenced from X-ray diffraction patterns and nitrogen adsorption measurements. CuO/MCM-41 is less ordered relative to pure MCM-41 and there are inter-aggregate pores resulting in a higher average pore diameter in the material. The formation of CuO particles on the external surface of MCM-41 and the possible reason for the less ordered structure of CuO/MCM-41 are also discussed in the present paper.     

Structure and properties of DLC–MoS2 thin films synthesized by BTIBD method

Diamond-like carbon (DLC)–MoS₂ composite thin films were synthesized using a biased target ion beam deposition (BTIBD) technique in which MoS₂ was produced by sputtering a MoS₂ target using Ar ion beams while DLC was deposited by ion beam deposition with CH₄ gas as carbon source. The structure and properties of the synthesized films were characterized by X-ray diffraction, X-ray absorption near edge structure (XANES), Raman spectroscopy, nanoindentation, ball-on-disk testing, and corrosion testing. The effect of MoS₂ target bias voltage, ranging from −200 to −800 V, on the structure and properties of the DLC–MoS₂ films was further investigated. The results showed that the hardness decreases from 9.1 GPa to 7 GPa, the Young?s modulus decreases from 100 GPa to 78 GPa, the coefficient of friction (COF) increases from 0.02 to 0.17, and the specific wear rate coefficient (k) increases from 5×10⁻⁷ to 5×10⁻⁶ mm³ N⁻¹ m⁻¹, with increasing the biasing voltage from 200 V to 800 V. Also, the corrosion resistance of the DLC–MoS₂ films decreased with the raise of biasing voltage. Comparing with the pure DLC and pure MoS₂ films, the DLC–MoS₂ films deposited at low biasing voltages showed better tribological properties including lower COF and k in ambient air environment.     

Composition and morphology of partially selenized CuIn1−XGaXSe2 thin films prepared using diethylselenide (DESe) as selenium source

CuIn_1−xGa_xSe₂ (CIGS) thin films are being prepared by selenization of Cu-In-Ga precursors using diethylselenide, (C₂H₅)₂Se, (DESe) as selenium source in place of H₂Se gas because of lower toxicity and ease of handling. Rough estimates indicate that selenization process using DESe would cost approximately same or slightly less compared to that using H₂Se. Price of DESe per mole is approximately five times that of H₂Se. However, partial pressure of DESe, which reflects source material consumption, is approximately three to four times less than that of H₂Se, due to higher decomposition rate of DESe compared to that of H₂Se. The actual DESe consumption would be four to ten times less compared to that of H₂Se. A selenization set-up using DESe as selenium source has been designed, fabricated and installed at FSEC Photovoltaic Materials Lab. Initial characterization of CIGS thin films have been carried out using electron probe microanalysis (EPMA), X-ray diffraction (XRD), scanning electron microscopy, secondary ion mass spectroscopy and Auger electron spectroscopy. EPMA showed elemental ratios of film to be near stoichiometric composition CuInSe₂ with very low gallium content mainly because of tendency of gallium to diffuse towards back contact. XRD data shows formation of high crystalline CuInSe₂ phase consistent with the EPMA data.