Fabrication of vacuum-evaporated SnS/CdS heterojunction for PV applications

SnS/CdS heterojunction is a promising system for the fabrication of thin film solar cells. In our work, thin film SnS/CdS heterojunction was prepared by evaporating CdS and SnS films. The photovoltaic properties of the heterojunction were investigated with posttreatment of the window material treatment by CdCl₂ for grain size enlargement. I–V characteristics in dark and at light were taken and figures of merit were evaluated. The efficiency with and without window layer treatment were about 0.08% and 0.05%, respectively, under 100 mW/cm² intensity. To the best of our knowledge so far there has been no report on vacuum-evaporated SnS-based heterojunction with window material treatment by CdCl₂.     

Photovoltaic cells based on chemically deposited p-type SnS

The aim of this work is to attract the attention of the scientific workers in the field of PV conversion of solar energy to SnS polycrystalline thin film as a candidate for construction of cheap solar cells, since it posseses similar photoelectric properties as polycrystalline silicon, but it can be produced on any kind of substrate, by simple, economic and environmentally approved technique. By the use of the method of chemical deposition from two separate solutions, complete preparation of three types of cells was done. All of them use SnS as base absorbing layer, with a difference in the window layer electrode. The first one has CdO, the second one has Cd₂SnO₄ thin film window electrode, both prepared by the chemical deposition method. The third cell was purely Schottky barrier cell in which the window electrode was SnO₂:F, prepared by spray pyrolysis. The I–V, C–V and spectral characteristics were registered and the conclusion was drawn that the best performances has shown the cells with Cd₂SnO₄ film as a window electrode.     

A new approach to thin-film SnS PV using MOCVD

SnS thin films were deposited by an inline metal organic chemical vapour deposition process using tetramethyltin and diethyldisulfide as precursors. A N₂/H₂ carrier was used with pre-mixing of the precursors before overhead injection into the deposition chamber. NSG AB soda lime glass was used as the substrate with area of 50 × 50 mm². The resulting SnS films had calculated band gaps between 1.3 and 1.5 eV. Scanning electron microscopy showed relatively large grains ranging from 0.5 to 1 μm across for a SnS film sample deposited at 556–558 °C. X-ray diffraction confirmed the films to be SnS, but with small concentrations of impure phases such as Sn₂S₃. Post-growth annealing treatment in a N₂ atmosphere at 435 °C using SnCl₂/MeOH solution at different molar concentrations only showed changes to the film at 0.05 M. The 0.05 M SnCl₂/MeOH treatment was aggressive with blistering and etching occurring.     

Investigations on the parameters limiting the performance of CdS/SnS solar cell

Debutant analysis of the parameters impeding the efficiency of the CdS/SnS‐based photovoltaic device is the chief novelty of the present report. We have developed thin‐film heterojunction solar cells with the stacking sequence: glass/Al‐doped ZnO/CdS/SnS/In. The two crucial issues, band offsets and cell studies, are discussed in detail. The band offsets at the CdS/SnS interface have been systematically evaluated by semidirect X‐ray photoelectron spectroscopy. The calculated valance band offset (ΔE_v) and conduction band offsets (ΔE_c) are found to be 1.46 and ?0.36 eV, respectively. The negative value of conduction band offset indicates that the junction formed is of type‐II (staggered‐type heterojunction). Electrical studies revealed power conversion efficiency of 0.32% with V_OC, J_SC, and fill factor as 170.61 mV, 7.26 mA/cm², and 0.26, respectively. The impact of the offset values on the cell studies is clearly elucidated. The reasons for the low efficiency are spotlighted. Collectively, this article gives the overview of the systematic approach undertaken to get obvious picture about the barriers that limit the conversion efficiency of the CdS/SnS‐based solar cell and the measurements required for enhancing the efficiency of the SnS‐based solar device.     

A study on the improved growth rate and morphology of chemically deposited ZnS thin film buffer layer for thin film solar cells in acidic medium

Zinc sulfide (ZnS) thin films have been prepared by chemical bath deposition method with improving growth rate and morphology using the mixed complexing agents of ethylenediamine tetra-acetate disodium salt (Na₂EDTA) and hexamethylenetetramine (HMTA). The effects of HMTA quantity on the morphological, compositional, optical, structural and electrical properties of ZnS thin films with fixed Na₂EDTA concentration have been investigated. ZnS thin films were deposited on glass substrates using aqueous solutions containing zinc acetate dehydrate and thioacetamide in acidic medium (pH 4). Field emission scanning electron microscopy results show that the morphology of a deposited ZnS thin film using HMTA as a complexing agent is rough. However, very uniform and smooth ZnS thin films are obtained using mixed complexing agents of Na₂EDTA and HMTA. The growth rate and root mean square of ZnS thin films are improved with increasing HMTA quantities. X-ray diffraction patterns show that all the ZnS thin films are grown as a hexagonal structure without secondary phase (ZnO) regardless of HMTA quantity. Optical band gap energy of ZnS thin films deposited using mixed complexing agents increase from 3.75 to 3.87 eV with increasing quantity of HMTA.     

Spray deposition of CdTe–Te thin films using ethylene-diamine-tetra-acetic acid as a complexing agent in the precursor solution

CdTe thin films have been deposited by spray pyrolysis technique using varying concentration of Ethylene-diamine-tetra-acetic acid (EDTA) as a complexing agent (CA) in the precursor solution. Scanning electron micrographs show the films deposited with EDTA have a more homogeneous distribution of fine grains. X-ray diffraction patterns show that the films consist of a mixture of CdTe and Te, the Te peaks intensity increasing with EDTA concentration. X-ray photoelectron spectroscopy study of the surface reveals that there is a change-over in chemical environment of Te from −2 to +4 state after using CA. A broad photoluminescence peak at 1.40 eV is observed in the films without CA, which can be resolved into two peaks 1.43 and 1.40 eV. The intensities of both peaks are increased with EDTA due to formation of elemental Te and Cd vacancies. Optical study indicates that an increment in optical reflection due to Te metallic cluster formation in the film. Thus, the stoichiometry of the CdTe films can be controlled by varying EDTA concentration.     

Characterization of electrical properties and photosensitivity of SnS thin films prepared by the electrochemical deposition method

Using the electrochemical deposition (ECD) method, we prepared tin sulfide thin films, which are suitable for the absorption layer in solar cells because of its bandgap energy (1 eV). We first optimized pulse-form biasing for ECD by characterizing deposited samples with scanning electron microscope, Auger electron spectroscopy and X-ray diffraction measurements. Then, we investigated the electrical properties of deposited SnS thin films and the properties of contacts with several different metals. Furthermore, we observed the photoconductivity of the films by means of photoelectrochemical measurements. From these results, we confirmed that the SnS thin films show p-type conduction.     

Enhanced UV-sensitivity of vis-responsive anatase thin films fabricated by using precursor solutions involving Ti complexes

Fabrication of vis-responsive anatase thin films with enhanced UV-sensitivity was attained on an ITO pre-coated glass substrate by applying two precursor solutions involving Ti complexes of oxalic acid and EDTA. The transparent and crack-free thin films were characterized by XRD, XPS, UV–vis and FE-SEM observation. The highest sensitivity to UV light of the vis-responsive film, whose photocatalytic activity was measured by the decomposition rate of methylene blue, was four times as compared with that formed by a sol–gel method under the same conditions. The vis-responsive films showed a characteristic absorption band at around 480 nm.     

Studies on Cu2ZnSnS4 (CZTS) absorber layer using different stacking orders in precursor thin films

Cu₂ZnSnS₄ (CZTS) thin films were deposited by sputtering on glass substrates using stacked precursors. The stacked precursor thin films were prepared from Cu, SnS₂ and ZnS targets at room temperature with different stacking orders of Cu/SnS₂/ZnS/glass (A), ZnS/Cu/SnS₂/glass (B) and SnS₂/ZnS/Cu/glass (C). The stacked precursor thin films were sulfurized using a tubular rapid thermal annealing system in a mixed N₂ (95%)+H₂S (5%) atmosphere at 550 °C for 10 min. The effects of the stacking order in the precursor thin films on the structural, morphological, chemical, electrical and optical properties of the CZTS thin films were investigated. X-ray diffraction, Raman spectroscopy and X-ray photoelectron spectroscopy studies showed that the annealed CZTS thin film using a stacking order A had a single kesterite crystal structure without secondary phases, whereas stacking orders B and C have a kesterite phase with secondary phases, such as Cu_2−xS, SnS₂ and SnS. The annealed CZTS thin film using stacking order A showed a very dense morphology without voids. On the other hand, the annealed CZTS thin films using stacking orders B and C contained the volcano shape voids (B) and Sn-based secondary phases (C) on the surface of the annealed thin films. The direct band gap energies of the CZTS thin films were approximately 1.45 eV (A), 1.35 eV (B) and 1.1 eV (C).     

Cathodic electrodeposition of Cu2S thin film for solar energy conversion

Cathodic electrodeposition in the presence of EDTA in aqueous solution was used to prepare Cu₂S thin film deposited on Ti substrate. The effect of deposition potential, concentration and deposition time was studied to determine the optimum condition for electro-deposition process. Cyclic voltammetry was performed to elucidate the electrodic processes that occur while potentials for electrodeposition were applied to determine the optimum potential for electrodeposition. The thin films are characterised by X-ray diffractometry. The photoactivity of the deposited films and their conduction types were evaluated using photoelectrochemical technique. The band gap energy and type of optical transitions were determined from optical absorbance data.     

An experimental and theoretical study of the effect of Ce doping in ZnO/CNT composite thin film with enhanced visible light photo-catalysis

Ce-doped ZnO/CNT composite thin film was fabricated successfully on soda-lime-silica glass substrate by sol-gel drop coating method. The structure and morphology of nanocrystalline Ce-doped ZnO/CNT thin film were characterized by X-ray Diffraction (XRD), X-ray photo-electron spectroscopy (XPS), Field Emission Scanning Electron Microscope (FESEM) and UV–Visible spectroscopy. The photocatalytic activity of Ce-doped ZnO/CNT thin film was evaluated by photocatalytic degradation of methylene blue (MB) in aqueous solution as a model pollutant under visible light irradiation. The synthesized Ce-doped ZnO/CNT composite thin film showed 76.71% photocatalytic efficiency whereas bare ZnO thin film showed that of only 25.30%. It has been reported that improved photocatalytic efficiency of composite is due to the synergistic effect of Ce doping and insertion of CNTs into ZnO matrix. The experimental photodegradation data were well fitted to first-order kinetics. The photocatalytic activity of the prepared thin film can be regenerated, which implies that the photocatalytic degradation process could be operated at a relatively low cost. The results suggest that Ce-doped ZnO/CNT composite thin film prepared by sol-gel drop coating method can be developed as an economically feasible and environmentally friendly method to degrade dye containing wastewater using visible light. Furthermore, atomic models for Ce doping in ZnO cluster was used to investigate the effect of doping on electronic structure of ZnO through density functional theory calculations. The computational study suggested a significant narrowing of the band gap and change of the maximum absorption bands towards higher wavelength. These all support the experimental results.     

Molecular dynamics simulations of Li transport between cathode crystals

The molecular dynamics (MD) computer simulation technique has been used to study the effect of an amorphous intergranular film (IGF) present in a polycrystalline cathode on Li transport. The solid electrolyte is a model lithium silicate glass while the cathode is a nanocrystalline vanadia with an amorphous V₂O₅ IGF separating the crystals. Thin (1 to a few nanometer thick) IGFs are known to be present in most polycrystalline oxide materials. However, the role of such a film on Li transport in oxide cathodes has not been addressed. Current scanning probe microscopy (SPM) studies have shown that the orientation of the layered nanocrystalline vanadia crystals near the cathode/solid electrolyte interface is not optimized for Li ion transport. While the precise structure of the material between the crystals has not been identified, initially it can be initially considered as likely to be a thin non-crystalline (amorphous) film. This is based on the ubiquitous presence of such a structure in other polycrystalline oxides. Also, and with more relevance to the materials used in thin film batteries, an amorphous film can be expected to form between nanocrystals that crystallized from an amorphous matrix, as would be the case in a deposited thin film cathode. Consistent with simulations of Li transport in amorphous vanadia, the current simulations show that Li ions diffuse more rapidly into the amorphous intergranular thin film than into the layered vanadia with the (0 0 1) planes parallel to the cathode/electrolyte interface.     

Coupling effects of water content,temperature, oxygen density,and polytetrafluoroethylene loading on oxygen transport through ionomer thin film on platinum surface in catalyst layer of proton exchange membrane fuel cell

In this work, coupling effects of water content, temperature, oxygen density, and polytetrafluoroethylene (PTFE) loading on oxygen transport through an ionomer thin film on a platinum surface in a catalyst layer of a proton exchange membrane (PEM) fuel cell are investigated using molecular dynamics approach. Taguchi orthogonal algorithm is employed to comprehensively analyze the coupling effects in a limited number of cases. It is found that the effect of operation temperature is the weakest among the four factors, which has the smallest effect index 14.4. Coupling effects including the PTFE loadings on the oxygen transfer through the ionomer thin film is uncovered. Less PTFE loadings should be beneficial for the oxygen transfer. The chemical potential gradient is considered as the major driven force for the oxygen transport through the ionomer thin film, and oxygen density is the dominating factor, significantly affecting the chemical potential in the thin film.     

SnS thin films fabricated by pulsed and normal electrochemical deposition

SnS thin films were deposited onto Sn-doped In₂O₃-coated glass substrates by pulsed and normal electrochemical deposition (ECD) from aqueous solutions containing SnSO₄ and Na₂S₂O₃. Deposited SnS was polycrystalline and had orthorhombic structure, and its composition was S-rich. The films deposited by pulsed ECD were much denser and had smaller surface roughness than the films deposited by normal ECD.     

多晶硅薄膜太阳电池的研究与进展

从薄膜的沉积方式和沉积温度以及衬底材料等几方面综合分析了多晶硅薄膜制备工艺的特点及多晶硅薄膜太阳电池的最新研究进展。并以颗粒硅带(SSP)为衬底，采用快热化学气相沉积(RTCVD)法制备了多晶硅薄膜，随后制得的多晶硅薄膜太阳电池的效率达到6．05％。     

PH值对化学沉积制备CdS薄膜性质的影响

实验研究了pH值对化学沉积制备CdS薄膜性质的影响。表明：在柠檬酸钠作为络合剂的体系中随着溶液中氨水浓度的提高CdS薄膜会发生相变，从立方相变为六方相，即当氨水浓度为0．31M时，得到立方相的CdS薄膜；而当氨水的浓度大于0．51M时，得到六方相的CdS薄膜。氨水浓度的提高也使得CdS薄膜的形貌有了很大的改善，且制备得到的CdS薄膜从富CA变为富S，但是薄膜仍是n型。氨水浓度对CdS薄膜的光学性质也有很大的影响，随着氨水浓度的提高所得到的CdS薄膜的禁带宽度增大。     

Evaporation heat transfer characteristics of a grooved heat pipe with micro-trapezoidal grooves

A detailed mathematical model predicting the effect of contact angle on the meniscus radius, thin film profile and heat flux distribution occurring in the micro-trapezoidal grooves of a heat pipe has been presented. The model can be used to determine the maximum evaporating heat transfer rate in the evaporator including the effects of disjoining pressure and surface tension. The equation of meniscus radii calculation in the evaporator at given heat load based on the liquid wicks configuration has been put forward. The numerical results show that while the capillary limitation governs the maximum heat transport capability in a grooved heat pipe, the thin film evaporation determines the effective thermal conductivity in a grooved heat pipe. The ratio of the heat transfer through the thin film region to the total heat transfer through the wall to the vapor phase decreases when the contact angle increases. The superheat effects on the heat flux distribution in the thin film region also have been conducted and the results show that the disjoining pressure plays an important role in this region. The current investigation will result in a better understanding of thin film evaporation and its effect on the effective thermal conductivity in a grooved heat pipe.     

Calculation of band offsets at the CdS/SnS heterojunction

SnS is a promising material for heterojunction solar cells, but the energy band alignment is not known for SnS-based heterojunctions. In this study, the energy band offset at the CdS/SnS heterojunction is calculated using the first principle, density-functional, pseudopotential method. A procedure analogous to that used in the core-level photoemission spectroscopy is adopted to calculate the band offset. The 4d core-level difference between Cd and Sn was estimated from the energy calculation of a superstructure consisting of zincblende CdS and rock-salt or zincblende SnS. The calculated valence-band offset is 0.1 eV when the rock-salt SnS is assumed and 0.84 eV when the zincblende SnS is assumed.     

Conduction based calibration of handmade platinum thin film heat transfer gauges for transient measurements

Heat transfer measurement using thin film gauges (TFG) is the most prevalently used technique for determination of surface heat flux. They are best suited for short duration transient surface temperature measurements and typically used in the applications where convection is a dominant mode of heat transfer such as gas turbine engines, high speed flights etc. However, in few interdisciplinary research areas, there are practical issues and difficulties in exposing the gauges for convection based measurements. These present investigations are aimed at exploring the possibility of using thin film gauges for short duration conduction based transient measurements with pure conduction mode of heat transfer. A simple calibration set-up has been used to supply known heat flux of different magnitudes to the thin film gauges that are fabricated in-house with platinum as sensing element and pyrex as an insulating substrate. Experimentally recorded temperature signals from the gauges are compared with simulated temperature histories obtained through finite element analysis. Convoluted integral of one-dimensional heat conduction equation is used to predict the surface heat flux and compared with input heat loads. The presently developed calibration setup is seen to be very useful for conduction based measurements of thin film gauges.     

Transport phenomena in the thin-film region of a micro-channel

A mathematical model to predict the flow and heat transfer characteristics for a thin film region of a micro-channel is proposed. Gradient of the vapor pressure and the capillary force are considered. The effects of channel height, heat flux and slip boundary condition at the solid-liquid interface are investigated. The length of the thin film region is calculated by comparing the magnitude of the capillary and disjoining pressures. The length and the thickness of the thin film region decrease exponentially with increasing heat flux. The channel height has no effect on the shape of film thickness. In the case of slip condition, the decreased film thickness causes the capillary and disjoining pressures to increase.