Conventional CdS Research Articles

Optimization of (Zn,Sn)O buffer layer in Cu(In,Ga)Se2 based solar cells

The conventional CdS layer in Cu(In1−x,Gax)Se2 based solar cells replaced by (Zn1−y,Sny)O as a buffer layer and the solar performance analyzed. An initial model based on an experimental device has been established. The dependence of solar cells performance on the change of Ga concentrations in CIGS absorber and Sn concentrations ZnSnO (band gap of ZnSnO) buffer layer was investigated. The optimum values of Ga and Sn concentrations were found at x = 1 and y = 0.2, leading to a conversion efficiency of 25.36%.

Band alignment of Cd-free (Zn, Mg)O layer with Cu2ZnSn(S,Se)4 and its effect on the photovoltaic properties

Cu2ZnSn(S,Se)4 (CZTSSe) is an interesting absorber material for thin film solar cells. However, one of the key challenges for the kesterite-based solar cells is to improve the open-circuit voltage (Voc) deficit, which is resultant of recombination at the interface of buffer/absorber. In this work, Cd-free n-type buffer layers with two different Mg-doped ZnO layers (Mg0.26Zn0.74O, Mg0.30Zn0.70O) have been examined using ultraviolet photoelectron spectroscopy. The most important electronic properties which are essential for the band offset study, ie. fermi level location, valence and conduction band offsets at the interface in the CZTSSe substrate, have been determined. The conduction band offset values for Mg0.26Zn0.74O, Mg0.30Zn0.70O buffer layers has been calculated experimentally. We have also established the correlation between device parameters and performances for dual ion beam sputtered ZnO buffer/CZTSSe-based heterojunction solar cells as a function of conduction band offset and the energy distribution of interface defects, to gain deeper understanding about the Voc-deficit behavior from a high recombination rate at the buffer/kesterite interface using simulation study. From the simulation study, the values of the solar cell efficiency with Mg0.26Zn0.74O and Mg0.30Zn0.70O buffer layers are 10.18 and 10.25%, respectively, which are higher in comparison to those obtained by using conventional CdS buffer layer.

Atomic layer deposition of Zn(O,S) buffer layers for Cu(In,Ga)Se2 solar cells with KF post-deposition treatment

We investigate the possibility to combine Zn(O,S) buffer layers grown by atomic layer deposition (ALD) with KF post-deposition treated Cu(In,Ga)Se2 (CIGS-KF) in solar cells. It is shown that the beneficial effect on open-circuit voltage from the post-deposition treatment is essentially independent of buffer layer material. However, a wet-chemical surface treatment is required prior to ALD in order to achieve competitive fill factor values. A water rinse is sufficient to create an absorber surface similar to the one formed during a conventional CdS chemical bath deposition process. However, it is observed that CIGS-KF/Zn(O,S) devices made with water-rinsed absorbers systematically result in lower fill factor values than for the corresponding CIGS-KF/CdS references. This effect can be mitigated by decreasing the H2S:H2O precursor ratio during ALD initiation, indicating that the fill factor limitation is linked to the initial Zn(O,S) growth on the modified CIGS-KF surface. The best CIGS-KF/Zn(O,S) devices were fabricated by etching away the KF-modified surface layer prior to ALD, followed by a low-temperature anneal. The thermal treatment step is needed to increase the open-circuit voltage close to the value of the CdS devices. The results presented in this contribution indicate that the main beneficial effects from KF-PDT in our devices are neither associated with the CdS CBD process nor due to the formation of a K-In-Se rich phase on the CIGS surface.

Thermal annealing evolution to physical properties of ZnS thin films as buffer layer for solar cell applications

The conventional CdS window layer in solar cells is found to be hazardous for the environment due to toxic nature of the cadmium. Therefore, in order to seek an alternative, a study on effect of post-annealing treatment on physical properties of e-beam evaporated ZnS thin films has been carried out where films of thickness 150 nm were deposited on glass and indium tin oxide (ITO) substrates. The post annealing treatment was performed in air atmosphere within the temperature range from 100 °C to 500 °C. X-ray diffraction analysis reveals that the films on glass substrate are found to be amorphous at low temperature annealing (≤300 °C) while have α-ZnS hexagonal phase (wurtzite structure) at higher annealing. The patterns also show that the possibility of oxidation is increased significantly at temperature 500 °C which leads to decrease in direct band gap from 3.28 eV to 3.18 eV except films annealed at 300 °C (i.e. 3.39 eV). The maximum transmittance is found about 95% as a result of Doppler blue shift while electrical analysis indicated almost ohmic behavior between current and voltage and surface roughness is increased with post-annealing treatment.

Synergies between Unsaturated Zn/Cu Doping Sites in Carbon Dots Provide New Pathways for Photocatalytic Oxidation

Unsaturated metal species (UMS) confined in nanomaterials play important roles for electron transfer in a wide range of catalytic reactions. However, the limited fabrication methods of UMS restrict their wider catalytic applications. Here, we report on the synergy of unsaturated Zn and Cu dopants confined in carbon dots (ZnCu-CDs) to produce enhanced electron transfer and photooxidation processes in the doped CDs. The Zn/Cu species chelate with the carbon matrix mainly through Cu–O(N)–Zn–O(N)–Cu complexes. Within this structure, Cu2+ acts as a mild oxidizer that facilely increases the unsaturated Zn content and also precisely tunes the unsaturated Zn valence state to Znd+, where d is between 1 and 2, instead of Zn0. With the help of UMS, electron-transfer pathways are produced, enhancing both the electron-donating (7.0 times) and -accepting (5.3 times) abilities relative to conventional CDs. Because of these synergistic effects, the photocatalytic efficiency of CDs in photooxidation reactions is shown to ...

Direct one pot synthesis of blue luminescent polymeric carbon dots gel and their application for selective detection of Ag+ ions

ABSTRACTHere, we report, the direct synthesis of fluorescent carbon dots in gel form by hydrothermal reaction of 3-aminopropyltriethoxysilane in the presence of aqueous nitric acid at 300°C. The chemical structure, morphology, rheological and adsorption properties of the silicon containing carbon dots (SiCDs) gel is studied using spectroscopic and microscopic characterization methods. The average fluorescent lifetime of the SiCDs gel was approximately 11 ns, and such high lifetime value is entailed in photo-catalytic and energy conversion applications. Nitrogen adsorption studies revealed that the gel has high nitrogen adsorption tendency, approximately two orders of magnitude higher than that of the conventional CDs. Further, SiCDs gel reduces silver ions (Ag+) to Ag nano-particles at ambient temperature without additional reducing agents. The luminescence of CDs was quenched by Ag nano-particles formed on the surface, which is proportional to the concentration of Ag+ ions. The sensor shows high selectivity towards Ag+ with a sensitivity and dynamic range of 85 nM and 0.4 µM– 0.1 M, respectively.

Exploring Cd-Zn-O-S alloys for improved buffer layers in thin-film photovoltaics

To compete with existing and more mature solar cell technologies such as crystalline Si, thin-film photovoltaics require optimization of every aspect in the device heterostructure to reach maximum efficiencies and cost effectiveness. For absorbers like CdTe, ${\mathrm{Cu}(\mathrm{In},\mathrm{Ga})\mathrm{Se}}_{2}$ (CIGSe), and ${\mathrm{Cu}}_{2}{\mathrm{ZnSn}(\mathrm{S},\mathrm{Se})}_{4}$ (CZTSSe), improving the $n$-type buffer layer partner beyond conventional CdS is one avenue that can reduce photocurrent losses and improve overall performance. Here, we use first-principles calculations based on hybrid functionals to explore alloys spanning the Cd-, Zn-, O-, and S-containing phase space to identify compositions that may be superior to common buffers like pure CdS or Zn(O,S). We address issues highly correlated with device performance such as lattice-matching for improved buffer-absorber epitaxy and interface quality, dopability, the band gap for reduced absorption losses in the buffer, and the conduction-band offsets shown to facilitate improved charge separation from photoexcited carriers. We supplement our analysis with device-level simulations as parameterized from our calculations and real devices to assess our conclusions of low-Zn and O content buffers showing improved performance with respect to CdS buffers.

One-stage pulsed laser deposition of conductive zinc oxysulfide layers

Zinc oxysulfide – Zn(O,S) is one of the prospective materials for substitution of conventional CdS buffer layer in complete optoelectronic devices due to its optimal bandgap and low toxicity. In this work Zn(O,S) thin films have been prepared by one-step pulsed laser deposition technique. The films with a thickness of 650nm were deposited onto the FTO/glass substrates at different substrate temperatures from room temperature to 400°C. Zn(O,S) layers were characterized by means of scanning electron microscopy, energy dispersive spectroscopy, Raman, X-ray diffraction, UV–vis spectroscopy and Van der Pauw technique. It was found, that obtained Zn(O,S) layers are mainly polycrystalline, highly uniform, transparent, electrically conductive and demonstrate good adhesion to the FTO/glass substrates. In addition, we show that elemental composition of PLD Zn(O,S) films depends on the substrate temperature. For the first time high quality single phase conductive Zn(O,S) layers were prepared by one stage PLD in high vacuum at relatively low temperature 200°C without any post treatment. The properties of prepared Zn(O,S) films suggest that these films can be applied as buffer layer in optoelectronic devices.

Lattice-matched Cu2ZnSnS4/CeO2 solar cell with open circuit voltage boost

We report a reproducible enhancement of the open circuit voltage in Cu2ZnSnS4 solar cells by introduction of a very thin CeO2 interlayer between the Cu2ZnSnS4 absorber and the conventional CdS buffer. CeO2, a non-toxic earth-abundant compound, has a nearly optimal band alignment with Cu2ZnSnS4 and the two materials are lattice-matched within 0.4%. This makes it possible to achieve an epitaxial interface when growing CeO2 by chemical bath deposition at temperatures as low as 50 °C. The open circuit voltage improvement is then attributed to a decrease in the interface recombination rate through formation of a high-quality heterointerface.

Recovery of rectifying behavior in Cu2ZnSn(S,Se)4/Zn(O,S) thin-film solar cells by in-situ nitrogen doping of buffer layers

The possibility of using N-doping to reduce the carrier concentration of zinc oxysulfide (Zn(O,S)) thin films grown by atomic layer deposition (ALD) was investigated. The effect of N-doping on the structural, chemical, and electrical properties of the Zn(O,S) films was investigated using X-ray diffraction, X-ray photoelectron spectroscopy, and Hall measurements. The Zn(O,S) films were applied as alternative n-type buffers replacing conventional CdS for earth-abundant Cu2ZnSn(S,Se)4 thin-film solar cells. With increased O/(O+S) ratio up to ∼0.87–0.90, the fabricated cells suffered severe drops in efficiency, mainly due to the increased shunt conductance coming from excessive carrier concentration of ZnO-like Zn(O,S). Recovery of the rectifying diode behavior and cell efficiency was demonstrated by applying N-doping to the conductive Zn(O,S) buffer layer, further supported by current–voltage and external quantum efficiency measurements.

Improvement of optical proximity-effect correction model accuracy by hybrid optical proximity-effect correction modeling and shrink correction technique for 10-nm node process

The model accuracy of optical proximity-effect correction (OPC) was investigated by two modeling methods for a 10-nm node process. The first method is to use contours of two-dimensional structures extracted from critical dimension-scanning electron microscope (CD-SEM) images combined with conventional CDs of one-dimensional structures. The accuracy of this hybrid OPC model was compared with that of a conventional OPC model, which was created with only CD data, in terms of root-mean-square (RMS) error for metal and contact layers of 10-nm node logic devices. Results showed improvement of model accuracy with the use of hybrid OPC modeling by 23% for contact layer and 18% for metal layer, respectively. The second method is to apply a correction technique for resist shrinkage caused by CD-SEM measurement to extracted contours for improving OPC model accuracy. The accuracy of OPC model with shrink correction was compared with that without shrink correction, and total RMS error was decreased by 12% by using the shrink correction technique. It can be concluded that the use of CD-SEM contours and the shrink correction of contours are effective to improve the accuracy of OPC model for the 10-nm node process.

Evolution of oxygenated cadmium sulfide (CdS:O) during high-temperature CdTe solar cell fabrication

Oxygenated cadmium sulfide (CdS:O) produced by reactive sputtering has emerged as a promising alternative to conventional CdS for use as the n-type window layer in CdTe solar cells. Here, complementary techniques are used to expose the window layer (CdS or CdS:O) in completed superstrate devices and combined with a suite of materials characterization to elucidate its evolution during high temperature device processing. During device fabrication amorphous CdS:O undergoes significant interdiffusion with CdTe and recrystallization, forming CdS1−yTey nanocrystals whose Te fraction approaches solubility limits. Significant oxygen remains after processing, concentrated in sulfate clusters dispersed among the CdS1−yTey alloy phase, accounting for ~30% of the post-processed window layer based on cross-sectional microscopy. Interdiffusion and recrystallization are observed in devices with un-oxygenated CdS, but to a much lesser extent. Etching experiments suggest that the CdS thickness is minimally changed during processing, but the CdS:O window layer is reduced from 100nm to 60–80nm, which is confirmed by microscopy. Alloying reduces the band gap of the CdS:O window layer to 2.15eV, but reductions in thickness and areal density improve its transmission spectrum, which is well matched to device quantum efficiency. The changes to the window layer in the reactive environments of device fabrication are profoundly different than what occurs by thermal annealing in an inert environment, which produced films with a band gap of 2.4eV for both CdS and CdS:O. These results illustrate for the first time the significant changes that occur to the window layer during processing that are critical to the performance of CdTe solar cells.

Atomic layer deposited zinc oxysulfide n-type buffer layers for Cu2ZnSn(S,Se)4 thin film solar cells

The structural, electrical, chemical, and optical properties of ternary Zn(O,S) thin films formed by atomic layer deposition (ALD) were investigated. It was revealed that the film׳s characteristics were highly influenced by the O/(O+S) ratio. The n-type Zn(O,S) layer was applied to both S-rich and Se-rich Cu2ZnSn(S,Se)4 (CZTSSe) absorbers as an alternative buffer layer to conventional CdS. The device performance relationship to the O/(O+S) ratio was examined. The highest power-conversion efficiency (PCE) of 2.75% and 3.30% was achieved using an actual O/(O+S) ratio of ~0.67 in the buffer layer for S-rich and Se-rich CZTSSe solar cells, and these PCEs correspond to 77% and 67% of the standard CdS-based solar cells, respectively. Further improvement in Se-rich CZTSSe was demonstrated by using NH4OH solution instead of pure H2O as oxygen source during ALD process. The dependence of the solar cell performance on the O/(O+S) ratio was investigated using dark current density–voltage (J–V), external quantum efficiency (EQE), transmission electron microscopy (TEM), and energy-dispersive X-ray spectroscopy (EDX).

Column readout circuit with dual integration CDS for infrared imagers

A column readout circuit with proposed dual integration CDS for low pattern noise infrared imager is presented. By using an extra integration, the dual integration CDS effectively reduces the level of column and row noise (CN and RN) and column fixed pattern noise (CFPN) in an infrared image. In addition, a time flexible integration technique minimizes the penalty of readout time by a dual operation. Simulation of a 0.18 μm CMOS implementation suggests that CN can be reduced by 68%, RN by 71%, and CFPN by 95% compared with a column readout circuit with conventional CDS.

Potential gain in photocurrent generation for Cu(In,Ga)Se2 solar cells by using In2O3 as a transparent conductive oxide layer

AbstractThis study highlights the potential of atomic layer deposited In2O3 as a highly transparent and conductive oxide (TCO) layer in Cu(In,Ga)Se2 (CIGSe) solar cells. It is shown that the efficiency of solar cells which use Zn‐Sn‐O (ZTO) as an alternative buffer layer can be increased by employing In2O3 as a TCO because of a reduction of the parasitic absorption in the window layer structure, resulting in 1.7 mA/cm2 gain in short circuit current density (Jsc). In contrast, a degradation of device properties is observed if the In2O3 TCO is combined with the conventional CdS buffer layer. The estimated improvement for large‐scale modules is discussed. Copyright © 2015 John Wiley & Sons, Ltd.

Evaluation of junction quality of buffer-free Zn(O,S):Al/Cu(In,Ga)Se2 thin-film solar cells

Cu(In,Ga)Se2 (CIGS) solar cells without buffer layers, namely, buffer-free solar cells, and those with conventional CdS buffer layers were fabricated. Zn(O,S):Al (AZOS) was used as a transparent conductive oxide layer. The AZOS/CIGS interface in the buffer-free solar cells and the AZOS/CdS/CIGS interface in the CdS-buffered solar cells were investigated by bright-field scanning transmission electron microscopy, high-angle annular dark-field scanning transmission electron microscopy, and fast Fourier transform. It was found that the open-circuit voltage and fill factor of the buffer-free solar cells are lower than those of the CdS-buffered solar cells, which is attributed to the nonepitaxial growth of AZOS on the CIGS layer, forming defects and deteriorating the junction quality of the buffer-free solar cells.

Photocatalytic reduction of CO2to CO utilizing a stable and efficient hetero–homogeneous hybrid system

The combination of the conventional inorganic semiconductor CdS and a homogeneous catalyst could reduce CO2 to CO economically under visible light with high selectivity, stability, and efficiency.

Quantitative Analysis of the Optical Losses in CZTS Thin-Film Semiconductors

For the first time, a quantitative approach was carried out to model the optical losses in several Cu2 ZnSnS <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> (CZTS) based thin-film heterojunction solar cells having different window and buffer layers. The modeling indicates that the heterojunction of glass/FTO/TiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> /In <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> S <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> /CZTS has a lower reflection and higher transmission coefficient than glass/FTO/In <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> S <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> /CZTS and conventional glass/FTO/CdS/CZTS structures. These optical losses at the interfaces (reflection) and inside the thickness (absorption) of stacked layers were calculated based on the known optical constants such as refractive index and extinction coefficient. While TiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> windowed device resulted in a promising high short-circuit current and energy conversion efficiency, the In <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> S <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> windowed and conventional CdS windowed devices showed to be less efficient. We compared our theoretical analysis with the practical reports given for such structures in the literature. Further studies are required to reduce the reflection and absorption losses of CZTS structures in practice. So far, the overall performance parameters (e.g., Jsc and η) are much less than our theoretical estimations. The size of the TiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> nanoparticles on the layer is about 100 nm.

Chemical bath deposition of CdS thin films doped with Zn and Cu

Zn- and Cu-doped CdS thin films were deposited onto glass substrates by the chemical bath technique. ZnCl2 and CuCl2 were incorporated as dopant agents into the conventional CdS chemical bath in order to promote the CdS doping process. The effect of the deposition time and the doping concentration on the physical properties of CdS films were investigated. The morphology, thickness, bandgap energy, crystalline structure and elemental composition of Zn- and Cu-doped CdS films were investigated and compared to the undoped CdS films properties. Both Zn- and Cu-doped CdS films presented a cubic crystalline structure with (1 1 1) as the preferential orientation. Lower values of the bandgap energy were observed for the doped CdS films as compared to those of the undoped CdS films. Zn-doped CdS films presented higher thickness and roughness values than those of Cu-doped CdS films. From the photoluminescence results, it is suggested that the inclusion of Zn and Cu into CdS crystalline structure promotes the formation of acceptor levels above CdS valence band, resulting in lower bandgap energy values for the doped CdS films.

Improved performance of CdS and dye co-sensitized solar cell using a TiO2sol-gel solution

An interfacial layer (IL) between the CdS quantum dot (QD) and dye is necessary in the CdS-dye co-sensitized solar cell (CSC) to prevent the corrosion of CdS and the damage of electrolyte. In this study, the simple method of using a TiO2 sol–gel solution is suggested to fabricate the IL. An air exposure process after dipping into the TiO2 sol–gel solution facilitates formation of the anatase TiO2 thin film. To confirm the effect of TiO2 IL, the performance of CdS-dye CSC is analyzed and compared to the conventional CdS QD sensitized solar cell (QDSC). The formed TiO2 IL effectively prevents the corrosion of CdS and the damage of electrolyte. In addition, this suppresses the electron recombination from the CdS to the electrolyte, resulting in the enhanced electron transport at the TiO2/CdS QD-dye/electrolyte interface. As a result, the overall photovoltaic performance is much increased from 1.33% to 2.77%, compared to the conventional CdS QDSC.