CdS Heterojunction Research Articles

Room temperature liquefied petroleum gas sensing using Cu 2 SnS 3 /CdS heterojunction

Abstract In the present work, for the first time, we report the fabrication of Cu2SnS3 (CTS)/CdS heterojunction for room temperature (27 °C) liquefied petroleum gas (LPG) sensing. The heterojunction is formed between sputter deposited CTS thin films and chemically bath deposited CdS thin films. The structural, morphological and optical properties of the thin films are evaluated using X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM) and UV–visible spectroscopy techniques, respectively. The fabricated heterojunction quality is evaluated from the forward biased current-voltage (I-V) study and the influence of annealing treatment of p-CTS thin film on LPG sensing properties of the heterojunction has been investigated. The CTS/CdS heterojunction exhibits maximum LPG response of 56% at room temperature under exposure of 780 ppm LPG concentration with 31s and 56s response and recovery time, respectively. The device retained 95% gas sensing stability after time period of 60 days suggesting that the fabricated CTS/CdS heterojunction is reliable and promising for LPG sensing. The factors affecting LPG sensing performance and plausible gas sensing mechanism are discussed in this study.

Enhanced photocatalytic properties of CdS nanoparticles decorated α-Fe2O3 nanopillar arrays under visible light

CdS nanoparticles (NPs) decorated α-Fe2O3 nanopillar arrays (NPAs) were fabricated through several steps. Fe NPAs were firstly fabricated by glancing angle deposition technique and oxidized in air to gain α-Fe2O3 NPAs. Then these NPAs were decorated by CdS NPs through successive ion layer adsorption and reaction (SILAR). Here we have tested photodegredation of methylene blue (MB) and photoelectrochemical properties under visible light. Especially, when SILAR cycle number reaches to 10, it shows the highest degradation efficiency (94% in 75min on MB) which improves 72% comparing with pure one and the highest photocurrent density (2.0mAcm−2 at 0.4V vs Ag/AgCl electrode). α-Fe2O3/CdS hetero-junctions could greatly enhance photocatalytic performance, which can help to accomplish sufficient usage of solar energy and be exploited on pollution abatement in future.

Fabrication of novel Z-scheme InVO4/CdS heterojunctions with efficiently enhanced visible light photocatalytic activity

Novel Z-scheme InVO4/CdS heterojunction photocatalysts have been successfully synthesized for the first time via a microwave-assisted process, followed by a mild hydrothermal method. The crystal structures, morphologies and sizes, chemical compositions and optical properties of the prepared photocatalysts were characterized via X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), high-resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectroscopy (XPS), UV-vis diffuse reflectance spectroscopy (DRS), and photocurrent measurements. Results showed that the prepared InVO4/CdS heterojunctions were composed of about 15 nm InVO4 nanoparticles and 1.5 μm CdS microspheres, and all heterojunctions exhibited good photoabsorption in the visible light region. The photocatalytic activity of the obtained samples was carefully evaluated via the degradation of rhodamine B (RhB) and ciprofloxacin (CIP) under visible light irradiation (λ > 420 nm). Compared to that of bare InVO4 and CdS, the InVO4/CdS heterojunctions exhibited significantly enhanced photocatalytic activity for RhB and CIP degradation. Moreover, the 40 wt% InVO4-coupled CdS composite displayed the highest catalytic efficiency for RhB photodegradation, which is about 59.4 and 4.8 times higher than that of pure InVO4 and CdS, respectively. In addition, the active species trapping experiment and electron spin resonance (ESR) measurement demonstrated that h+ and ˙O2− radicals were the predominant active species in the photocatalytic reaction process. Furthermore, the possible enhanced photocatalytic mechanism of InVO4/CdS heterojunctions was also proposed based on the band position measurements and ESR analysis.

Cd and Cu Interdiffusion in Cu(In, Ga)Se2/CdS Hetero-Interfaces

We report a detailed characterization of an industry-like prepared Cu(In, Ga)Se2 (CIGS)/CdS heterojunction by scanning transmission electron microscopy and photoluminescence (PL). Energy dispersive X-ray spectroscopy shows the presence of several regions in the CIGS layer that are Cu deprived and Cd enriched, suggesting the segregation of Cd–Se. Concurrently, the CdS layer shows Cd-deprived regions with the presence of Cu, suggesting a segregation of Cu–S. The two types of segregations are always found together, which, to the best of our knowledge, is observed for the first time. The results indicate that there is a diffusion process that replaces Cu with Cd in the CIGS layer and Cd with Cu in the CdS layer. Using a combinatorial approach, we identified that this effect is independent of focused-ion beam sample preparation and of the transmission electron microscopy grid. Furthermore, PL measurements before and after an HCl etch indicate a lower degree of defects in the postetch sample, compatible with the segregates removal. We hypothesize that Cu $_{2-x}$ Se nanodomains react during the chemical bath process to form these segregates since the chemical reaction that dominates this process is thermodynamically favorable. These results provide important additional information about the formation of the CIGS/CdS interface.

Interesting structural transformation of CdS from zinc blende into wurtzite during minuscule loading of magnetic nanoparticles: emergence of heterojunctions with enhanced photocatalytic performance

Herein, we present morphologically modified CdS and nanoferrite MFe2O4 (M = Zn, Co and Ni) heterojunctions with efficacious visible-light-driven photocatalytic properties.

Ultrahigh sensitivity and gain white light photodetector based on GaTe/Sn : CdS nanoflake/nanowire heterostructures

Optoelectronic diode based on PN heterostructure is one of the most fundamental device building blocks with extensive applications. Here we reported the fabrication and optoelectronic properties of GaTe/Sn : CdS nanoflake/nanowire PN heterojunction photodetectors. With high quality contacts between metal electrodes and Sn : CdS or GaTe, the electrical measurement of GaTe/Sn : CdS hybrid heterojunction under dark condition demonstrates an excellent diode characteristic with well-defined current rectification behavior. The photocurrent increases drastically under LED white light as well as red, green, UV illumination. The on-off ratio of current is about 100 for forward bias and 3000 for reverse bias, which clearly indicates the ultrahigh sensitivity of the heterostructure photodetector to white light. The responsivity and optical gain are determined to be 607 A W−1 and (1.06–2.16) × 105%, which is higher than previous reports of single GaTe or CdS nanostructures. Combination the Ids–Vds curves under different illumination power with energy band diagrams, we assign that both the light modulation effect under forward and reverse bias and the surface molecular oxygen adsorption/desorption mechanism are dominant to the electrical transport behavior of GaTe/Sn : CdS heterojunction. This heterostructure photodetector also shows good stability and fast response speed. Both the high photosensibility and fast response time described in the present study suggest strongly that the GaTe/Sn : CdS hybrid heterostructure is a promising candidate for photodetection, optical sensing and switching devices.

Preparation and characterization of the hybrid ZnS(en)0.5–CdS heterojunction

The ZnS(en)0.5–CdS heterojunction semiconductor with different CdS contents were synthesized in a mixture of solvents (water–ethylenediamine) at moderate temperature. The results indicate that the presence of ethylenediamine (en) in the heterojunction structure is associated with the ZnS contents and the optical and electronic properties of heterojunctions are modified by the presence of CdS semiconductor. This kind of the heterojunctions are formed by plates overlapped showing Moire patterns. The interaction between both hybrid ZnS(en)0.5 and CdS semiconductor improved the photocatalytic activity in the Cr(VI) reduction in acid media (pH 3.2).

Chemical reactions at CdS heterojunctions with CuInSe2

The stability of the CdS/CuInSe2 (CIS) heterojunction is critical to understanding the projected lifetime of CIS devices and the effect of processing conditions on the nanoscale chemistry of the heterojunction. This article reports the results of annealing heterojunctions between CdS deposited by chemical bath deposition and single crystal and polycrystalline CIS films between 200 and 500 °C for 10 to 150 min. No atomic movement was observed by secondary ion mass spectrometry at temperatures of 300 °C and below. At 400 °C even for the shortest time studied, Cu and In were found throughout the region initially consisting of CdS only and Cd was found to have moved into the CIS. In the polycrystal, annealing at 500 °C resulted in movement of Cd throughout the CIS layer. No time dependence was observed in the 400 and 500 °C anneals indicating that a reaction had occurred forming a compound that was in thermodynamic equilibrium with the remaining CIS. Diffusion turns on rapidly between 300 and 400 °C, indicating a high activation energy for atomic movement (∼2.4 eV). The onset of diffusion is consistent with the onset of Cu diffusion in CIS.

Stable structure and effects of sulfur in CdTe/CdS heterojunctions

Structural and electronic properties of S in the CdTe/CdS(0001) interface are studied using the density functional theory. The interstitial S atom may induce the inversion of the surface Cd atoms and sublayer Te atoms of the Cd‐terminated surface, while S atoms may adsorb at the top sites, substitute Te atoms or accumulate at the voids inside the Te‐terminated (111) CdTe surface. Isovalent substituting S for Te in the CdTe(111)/CdS(0001) heterojunctions can reduce the strain arising from lattice mismatch and cause a reduction of interface states, so it may be better for solar cells. Copyright © 2011 John Wiley & Sons, Ltd.

Highly efficient CuIn 1− xGa xSe 2− yS y/CdS thin-film solar cells by using diethylselenide as selenium precursor

Conventional furnace selenization process was optimized to achieve effective selenization using diethylselenide (C 2H 5) 2Se (DESe). An optimized quantity of Na was added to improve V oc, FF and morphology. Sputter-deposited CuGa and In metallic precursors were homogenized in an inert atmosphere prior to the introduction of DESe followed by rapidly heating to the maximum process temperature to avoid formation of detrimental binary phases. Selenization was carried out in the temperature range 475–515 °C followed by sulfurization in dilute H 2S. Solar cells were completed by depositing CdS heterojunction partner layer, i:ZnO/ZnO:Al window-bilayer and metallic contact fingers. PV conversion efficiency of 13.7% with a V oc of 540 mV, J sc of 38.3 mA/cm 2 and FF of 66.3% were obtained on 0.442 cm 2 cell areas. The process can be easily scaled-up for economic large-scale manufacture.

Different roles of cadmium‐ and sulfur (selenium)‐terminated crystal facets in the formation of a photovoltaic response from hybrid organic/inorganic CdS (CdSe) heterojunctions

AbstractPhotovoltaic (PV) response of hybrid organic/inorganic bilayer heterostructures based on organic polymers or small molecules on the one hand, such as poly(3‐hexylthiophene), polyaniline doped with poly(styrenesulfonic acid), poly(ethylene‐3,4‐dioxythiophene) doped with poly(styrenesulfonic acid) (PEDOT:PSS), phthalocyanine, and inorganic CdS or CdSe single crystals on the other hand, has been studied depending on the crystal facet used. The effect of the surface‐terminated layer of the crystal on the PV performance of hybrid heterojunctions has been observed. It was found that the different crystal facets could render both quantitative and qualitative effect on PV properties of the hybrid heterojunctions.Photocurrent spectra of CdS/PEDOT:PSS heterojunctions based on the Cd‐terminated (solid curve) and S‐terminated (dashed curve) crystal facet.magnified image

Comparative study of Cu(In,Ga)Se 2/(PVD)In 2S 3 and Cu(In,Ga)Se 2/(CBD)CdS heterojunction based solar cells by admittance spectroscopy, current–voltage and spectral response measurements

Co-evaporated Cu(In,Ga)Se 2 (CIGSe) based solar cells with Physical Vapour Deposited (PVD) Indium Sulphide (In 2S 3) as buffer layer have been studied by admittance spectroscopy and current-voltage characteristics measurements. The results have been compared to those obtained with a reference CBD-CdS/CIGSe device. In darkness, the PVD-In 2S 3 buffer layer devices exhibit higher densities of trapping defects and low values of shunt resistance. However, under illumination we have observed an important improvement of the In 2S 3/CIGSe electronic transport properties. This behavior seems to be linked to the presence of a metastable defect with activation energy of 0.3 eV.

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 CdCl 2 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/cm 2 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 CdCl 2.

Preparation and photocatalytic activity of TiOxNy∕CdS heterojunctions

Nitrogen doped titanium oxide (TiOxNy) thin films were deposited by facing-target sputtering on glass substrates at different nitrogen flow rates (NR) of 0, 5, 10, 15, and 20SCCM (SCCM denotes cubic centimeter per minute at STP) at a fixed sputtering pressure of 0.8Pa to identify the optimum dopant concentration for the efficient photocatalytic activity. X-ray diffraction experiments revealed that the films have anatase phase until NR of 10SCCM and become amorphous with further increase in NR. Higher level nitrogen doping is inhibited due to the difficulty in the replacement of oxygen. CdS thin films were deposited by chemical bath deposition method using the CdCl2 and thiourea as the precursors, and NH3NO4 and KOH as the complexing agent have the optical band gaps of 2.38–2.5eV for the different thiourea solution concentrations and hexagonal structures. The photocatalytic decomposition of methanol proceeds via formaldehyde and CO as the intermediate species and CO2 is produced after the complete decomposition. The optimum NR for the nitrogen doping in TiO2 films were found be 5SCCM. The properties of the films, including the crystallinity, surface morphology, and light absorption capability, are influenced by the nitrogen flow rate, whereas the photocatalytic reaction on TiOxNy thin films is very sensitive to crystallinity and its surface structure.

Sulphurization of single-phase Cu 11In 9 precursors for CuInS 2 solar cells

Using Rutherford backscattering (RBS), X-ray diffraction (XRD), and scanning electron microscopy (SEM), the sulphurization of single-phase Cu 11In 9 precursors to be employed as light absorbing CuInS 2 (CIS) layers in CIS–CdS heterojunction thin-film solar cells has been investigated. The Cu 11In 9 precursor films were produced by DC-sputtering from a single-phase Cu 11In 9 target. The sulphurization at 500 or 300 °C was performed by adding different amounts of elemental sulphur with heating rate and sulphurization time as additional parameters. During sulphurization at 500 °C, up to 50% of the indium initially present in the precursor is lost. We relate the In-loss to the volatile In 2S compound, the formation of which is favoured by the phase transition of Cu 11In 9 to Cu 16In 9 at 307 °C. Consequently, the In-loss can be suppressed by employing a sulphurization temperature of 300 °C. At this temperature, a prolonged sulphurization time and a large sulphur excess are necessary in order to obtain stoichiometric CIS beneath a CuS x surface phase.

Photoemission study and band alignment of the CuInSe2(001)/CdS heterojunction

The contact formation of thin-film epitaxial CuInSe2(001) with a physical-vapor-deposited CdS layer is presented in this work. Synchrotron-excited photoelectron spectroscopy was used for this investigation. The epitaxial CuInSe2 films contain a surface layer of reduced Cu stoichiometry similar to the ordered defect compound CuIn3Se5. A valence band offset of 0.79±0.15 eV has been determined for this heterojunction. The comparison to literature data indicates that neither surface orientation nor surface copper content have a major impact on the valence band offset of CuIn3Se5, respectively, CuInSe2 with CdS.

The wet-topotaxial process of junction formation and surface treatments of Cu 2S–CdS thin-film solar cells

The particular properties of Cu 2S–CdS heterojunctions are revealed. The effects of lattice mismatch on epitaxy as well as wet- and dry-topotaxy are discussed and preconditions for successful application of topotaxy are elaborated. The influence of surface treatments of the cells and of additional semiconducting or metallic layers of monolayer-range thicknesses at the surface is demonstrated. The junction formation technology and the surface treatments appear to be relevant also to other photovoltaic devices.

Electronic and Chemical Phenomena at Cu(In, Ga)Se2/CdS Heterojunctions and Their Implications for Photovoltaic Devices

AbstractThe surface chemical composition of Cu(In, Ga)Se2 (CIGS) alloys and CIGS/CdS interfaces are characterized by photoelectron emission spectroscopy. The results show that there is a severe but shallow (1-2 atomic layers) Cu depletion at {112} surfaces of CuInSe2 epitaxial films, consistent with earlier results. The depletion is not due to the existence of ordered defect compounds, but likely a result of the reconstruction of polar CIGS surfaces. Cd was found incorporated into the top 1-2 atomic layers of Cu-poor surfaces of CIGS, which may explain the n-type surface inversion in finished devices. These results and others formed the basis of a numerical model of CIGS solar cells. It is argued from the experimental and simulation results that the performance of devices based on high Ga content alloys may be reduced by failure to dope the surface of CuGaSe2 sufficiently n-type. The effect of observed defect states on the energy gap is considered, in particular with respect to pinning of the Fermi level near the heterojunction. By contrast, devices based on high In-content alloys show strong surface inversion in CIGS/CdS heterojunctions, which is suggestive of an effect of Cd incorporation in the CIGS surface.

Fundamental studies on large area Cu(In,Ga)Se 2 films for high efficiency solar cells

This work reports the growth and characterization of thin films of Cu(In,Ga)Se 2 (CIGS), which were grown by sequential sputtering, electrodeposition, and physical vapor deposition. Photovoltaic cells have been fabricated using these films with CdS heterojunction partners and the performance has been characterized. The effect of annealing conditions (temperature and duration) on the CIGS film microstructure and corresponding device performance has been investigated. Structure-property correlations were made using diffraction studies and Rietveld analysis. SEM studies were carried out to understand the effect of microstructure of the CIGS films on the solar cell efficiency. Cell efficiencies in excess of 10% have been achieved by using optimized annealing conditions. In addition, the optical properties of the sputtered CIGS films were characterized using variable angle spectroscopic ellipsometry and sputtered CIGS films were found to have optimum band gap.

CIGS2 Thin Film Solar Cells on Stainless Steel Foil

ABSTRACTCuIn1−xGaxS2 (CIGS2) thin-film solar cells are of interest for space power applications because of the near optimum bandgap for AM0 solar radiation in space. CuIn1−xGaxSe2−ySy (CIGS) and CIGS2 solar cells are expected to be superior to Si and GaAs solar cells for the space missions especially in terms of the performance at the end of low earth orbit (LEO) mission. Ultra-lightweight thin-film solar cells deposited on flexible stainless steel (SS) foils have a potential for achieving high specific power.Magnetron-sputter-deposition parameters of molybdenum back-contact layer were optimized so as to minimize residual stress. Cu-rich Cu-Ga/In layers were sputter-deposited on unheated Mo-coated SS foils from CuGa(22%) and In targets. Well-adherent, large (3 μm), compact-grain Cu-rich CIGS2 films were obtained by sulfurization in a Ar:H2S 1:0.04 mixture and argon flow rate of 650 sccm, at the maximum temperature of 475° C for 60 minutes with intermediate 30 minute annealing step at 120° C. p-type CIGS2 thin films were obtained by etching away the Cu- rich layer segregated at the surface in a dilute KCN solution. XRD analysis of a CIGS2 film on SS foil revealed growth of chalcopyrite CIGS2 phase having ao= 5.519 Å and co= 11.125 Å and {112} preferred orientation. Positive SIMS depth profile of CIGS2 film showed gallium concentration increasing toward the back contact.Solar cells were completed by deposition of CdS heterojunction partner layer by chemical bath deposition, transparent-conducting ZnO/ZnO:Al window bilayer by RF sputtering, and vacuum deposition of Ni/Al contact fingers through metal mask. PV parameters of a CIGS2 solar cell on SS flexible foil measured under AM 0 conditions at the NASA GRC were: Voc = 802.9 mV, Jsc = 25.07 mA/cm2, FF = 60.06%, and η = 8.84%. For this cell, AM 1.5 PV parameters measured at NREL were: Voc = 788 mV, Jsc = 19.78 mA/cm2, FF = 59.44%, η = 9.26%. Quantum efficiency curve showed a sharp QE cutoff equivalent to CIGS2 bandgap of ∼1.50 eV, fairly close to the optimum value for efficient AM0 PV conversion in the space.