CdS Layer Thickness Research Articles

Structural and physical characteristics optimization of a dual junction CGS/CIGS solar cell: A numerical simulation

The structure and performance parameters of a dual junction CGS/CIGS solar cell including a CdS antireflection top layer are studied and optimized using Silvaco ATLAS software to obtain maximum possible value of efficiency. To gain a optimized design, the affects of CIGS bottom cell band gap and n-type doping concentration on the efficiency of the cell were examined and the optimum values of these parameters were obtained. In continue, the maximum JSC which is occurs when the current values of the two sub-cells are matched was obtained by varying the thickness of CdS layer, where an optimum efficiency of 43.05% is achieved.

An optimized efficient dual junction InGaN/CIGS solar cell: A numerical simulation

The photovoltaic performance of an efficient double junction InGaN/CIGS solar cell including a CdS antireflector top cover layer is studied using Silvaco ATLAS software. In this study, to gain a desired structure, the different design parameters, including the CIGS various band gaps, the doping concentration and the thickness of CdS layer are optimized. The simulation indicates that under current matching condition, an optimum efficiency of 40.42% is achieved.

Robust surface passivation of trap sites in PbS q-dots by controlling the thickness of CdS layers in PbS/CdS quantum dot solar cells

The PbS quantum dot sensitized solar cells were prepared on 3-mercaptoproponic acid (MPA) anchored mesoporous TiO2 films by a successive ionic layer adsorption and reaction (SILAR) process and the fabricated PbS q-dots were passivated by deposition of a CdS layer. In SILAR process, MPA solution was mixed with sulfide precursor solution and used it in every SILAR cycle for rapid conversion of Pb and Cd into their respective sulfides form. Performances of surface passivated and non-passivated PbS q-dot solar cells were compared where solar cells fabricated with q-dots of PbS, PbS/CdS, CdS showed best efficiencies (η) of 1.0, 5.7, 0.9% with current densities (Jsc) of 10.1, 22.8, 2.7mAcm−2 and open circuit voltages (Voc) of 280, 501, 498mV respectively. The enhanced efficiency of PbS/CdS q-dot solar cell was mainly due to increase in Jsc and Voc compared to PbS q-dot solar cells. The outstanding performance of PbS/CdS sensitized solar cell was found to be due to reduction of trap states in PbS particle by the surface passivation effect of CdS q-dots and the surface passivation effect of CdS in PbS/CdS sensitized solar cell is reported by EIS and voltage decay investigations.

On the Applications of Newly Architectured CdTe Nanostructures from Ionic Liquid Medium

The 2D nanostructures are gaining lot of interest in the field of electronics and solar photovoltaics. We have recently developed new shape and architecture of CdTe nanostructures on low cost F:SnO2 (FTO) substrate by electrodeposition from ionic liquid medium. It was observed that the ionic liquid medium facilitate the fabrication of nanostructured thin film directly on the substrate. By using two different substrates, FTO and CdS coated FTO it was possible to fabricate Schottky and p-n junction solar cells out of developed CdTe nanostructured films. It was interesting to observe that the Schottky junction solar cells of such nanostructured films result in higher Voc of 805 mV which is a quite high value for such solar cells. However, low connectivity between nanostructures resulted in lower Jsc values. On the other hand we found that the growth of semiconductor films from ionic liquid medium is highly substrate dependent. It was found that stoichiometric CdTe films can only be deposited on 50 nm thick film of CdS. Increasing the thickness of CdS layer always lead to non stoichiometric CdTe films which no longer can be utilized in solar cells. The first ever successful deposition of stoichiometric CdTe film on CdS buffer layer resulted in a solar cell with Voc of 150 mV, Jsc of 500 μA/cm2, FF of 27% and efficiency of 0.02%. This initial study will be helpful in understanding the growth of binary alloy on different substrates from ionic liquid medium. Figure 1

Inverted quantum-dot solar cells with depleted heterojunction structure employing CdS as the electron acceptor

Colloidal quantum-dot (QD) solar cells have emerged as one of the most promising photovoltaic techniques. Herein, we report an inverted PbS QD solar cells employing solution-processed CdS as the electron acceptor materials. Chemical bath deposition – one featuring ease of fabrication and compatibility with a low-temperature process – was employed to prepare CdS film on FTO glass, followed by deposition of PbS QDs as absorbing layer in a layer-by-layer fashion, thereby forming a depleted heterojunction structure. The results suggested that the thickness of CdS layer can be a key factor for determining the photovoltaic performance. After systematical optimization, an inverted FTO/CdS/PbS QDs/MoO3/Ag structure with an ultra-thin CdS film of 40nm delivered a decent power conversion efficiency of 5.22%. The maximum temperature is ~85°C via device fabrication demonstrates potential large-scale production process.

Tunable Plasmon Resonance and Fluorescence of Au/ZnS/CdS Core/Shell Nanorods

Au/ZnS/CdS core/shell nanorods were synthesized via layer-by-layer over-growing sulfide semiconductors ZnS and CdS onto Au-core nanorods, and the thickness of ZnS and CdS layers was tuned by adjusting the amounts of the added metal (Zn/Cd) salts. Owing to efficient interaction between the metal and semiconductor components in the multi-core/shell nanorods, we observed redshift of the surface plasmon resonance (SPR) band, enhancement of the SPR intensity, and generation of a new SPR band. Furthermore, the spectral redshift and enhancement were also observed in the excitonic fluorescence of the Au/ZnS/CdS core/shell nanorods, which is caused by the interaction between activated excitons in the CdS shell and the transverse SPR of the Au nanorod and can be tuned by adjusting the thickness of the middle shell (ZnS). These observations have prospective applications in active plasmonic nanodevices.

Construction of efficient CdS–TiO2 heterojunction for enhanced photocurrent, photostability, and photoelectron lifetimes

The photoefficiency of CdS/TiO2 electrodes can be enhanced by employing efficient method of CdS sensitization from which, the contact area, thickness of CdS layer, and the recombination of photoelectrons with electrolyte can be controlled. Here, we demonstrate a simple solvothermal approach of CdS quantum dots (QDs) sensitization on TiO2 nanoparticle (NP) film coated on FTO. Our new approach prevents the clogging of CdS QDs and promotes uniform deposition of QDs throughout the mesoporous TiO2 NP film. The sensitization of CdS can be controlled by the reaction time and the concentration of the precursors. The solvothermally sensitized photoanodes exhibit enhanced photocurrents and fill factors and improved photostability in aqueous solution compared to the one prepared by a conventional SILAR method. Open-circuit potential decay measurement under shutting off illumination shows that the lifetime of photoelectron is extended with solvothermally prepared CdS layer, indicating efficient suppression of recombination of the accumulated electron in TiO2 to the electrolyte. This methodology can be applied in making more efficient heterojunctions consisting of CdS and other wide band gap oxide semiconductors which could improve charge separation and mitigate charge recombination for photoelectrochemical applications.

Dependence of efficiency of thin-film CdS/CdTe solar cell on optical and recombination losses

Thin-film CdS/CdTe solar cells fabricated on glass substrates have been considered as one of the most promising candidates for large-scale applications in the field of photovoltaic energy conversion. The recorded experimental efficiency of these cells is about 16%–17% and the corresponding theoretical values are more than 28%. The main causes of efficiency loss are due to optical and recombination losses. Most of the theoretical literatures either study the effect of recombination or optical losses on the CdS/CdTe solar cell efficiency. The present work studies the effect of both the optical and recombination losses on the current density and hence the solar cell efficiency. Calculations of optical losses have been carried out based on the optical constants (refractive index and extinction coefficient) of materials used and the thickness of ITO and CdS layers. Calculation of recombination losses has been based on the values of width of space-charge region and the absorption coefficient for CdTe. It has been found that the reflection losses of the interfaces air-glass, glass-ITO, ITO-CdS, and CdS-CdTe decrease the short-circuit current (JSC) from 31.24 to 28.2 mA/cm2 (9%). The absorption losses in ITO and CdS layers decrease JSC to 22.2 (20%). The recombination losses decrease JSC to 19.7 mA/cm2 (8%). The optical and recombination losses yield efficiency of CdS/CdTe solar cells in the range of 12%–16% at thickness 100 nm of each layer of ITO and CdS. According to these results, there is a good agreement between experimental and theoretical studies and this is the real start to develop the solar cells efficiency in the future studies.

Generation-dependent charge carrier transport in Cu(In,Ga)Se2/CdS/ZnO thin-film solar-cells

Cross section electron-beam induced current (EBIC) and illumination-dependent current voltage (IV) measurements show that charge carrier transport in Cu(In,Ga)Se2 (CIGSe)/CdS/ZnO solar-cells is generation-dependent. We perform a detailed analysis of CIGSe solar cells with different CdS layer thicknesses and varying Ga-content in the absorber layer. In conjunction with numerical simulations, EBIC and IV data are used to develop a consistent model for charge and defect distributions with a focus on the heterojunction region. The best model to explain our experimental data is based on a p+ layer at the CIGSe/CdS interface leading to generation-dependent transport in EBIC at room temperature. Acceptor-type defect states at the CdS/ZnO interface cause a significant reduction of the photocurrent in the red-light illuminated IV characteristics at low temperatures (red kink effect). Shallow donor-type defect states at the p+ layer/CdS interface of some grains of the absorber layer are responsible for grain specific, i.e., spatially inhomogeneous, charge carrier transport observed in EBIC.

Cuinse<SUB>2</SUB> Solar Cells Efficiency Optimization

In the present paper calculation of CuInSe2(CIS)/CdS solar cell efficiency is presented. The influence of the thickness and the optical band gap of CdS and CIS layers on the solar cell performances are investigated. The thickness of CdS buffer layer, which is generally neglected is taken into account. The solar cells performances calculations are based on the calculation, by means of developed software written with Matlab, of photocurrent from the resolution of the well known three coupling equations: continuity equation for holes in the CdS (n-region) and for electrons in the CIS (p- region) and Poisson equation. The obtained results indicated that the solar cell efficiency can be improved by reducing the CdS thickness or by increasing the CIS thickness. The efficiency increasing rates are 0.01 %/ nm and 0.5 %/nm for CdS and CIS layer thickness respectively. The CdS layer alters the shorter wavelength of the solar spectrum, while the CIS layer alters the longer wavelength. CIS layer optical band gap is the most crucial parameter by comparison to the optical gap of CdS layer.

Optimization of CdS layer on ZnO nanorod arrays for efficient CdS/CdSe co-sensitized solar cell

Vertically aligned one-dimensional (1D) ZnO nanorods (NRs) were applied in fabricating CdS/CdSe co-sensitized quantum dot sensitized solar cells (QDSSCs) by chemical bath deposition (CBD) method. It was found that the optimum thickness of CdS layer on ZnO NRs was quite different from that on TiO2 electrode: the optimized CdS layer was acquired by 9 cycles of CBD process, whereas 3 cycles were suitable for TiO2. The optimum thickness of CdS was 18nm, and the ZnO/CdS(9)/CdSe(4)/ZnS(1) cell exhibited the photovoltaic conversion efficiency (η) of 1.33%. In the TiO2/CdS(3), the CdS QDs deposited on the surface of TiO2 is a particle-like structure and only 35% of the TiO2 surface was covered with CdS QDs, but, in the ZnO/CdS(3) and ZnO/CdS(9), CdS was uniformly coated on the ZnO surface with coverage of 91% and 99%, respectively. Presumably, thick and uniform CdS layer will have advantage in protecting the ZnO NR from the chemical attack by polysulfide electrolytes.

Investigation of the photovoltaic performance of n-ZnO:Al/i-ZnO/n-CdS/p-Cu2ZnSnS4 solar cell

The semiconducting Cu2ZnSnS4 (CZTS) thin film with high absorption coefficient has long been recognized as a novel solar cell material. In this work, the performances of n-ZnO:Al/i-ZnO/n-CdS/p-CZTS solar cells are analyzed by using semiconductor theory. The influences of doping concentration, thickness, and defect states of CZTS layer and the doping concentration and thickness of CdS layer on the performances of the solar cells and the temperature characteristics are investigated. The calculated results show that the CZTS layer is a main absorption layer in the solar cell. The changes in doping concentration and thickness of CZTS layer have significant influence on the conversion efficiency of the solar cell. The density of defect states in CZTS can sharply degrade the photovoltaic performances. The influences of the doping concentration and thickness of CdS layer can be neglected. The calculated results show that the optimal n-ZnO:Al/i-ZnO/n-CdS/p-CZTS structure has open-circuit voltage of 1.127 V, short circuit current density of 27.39 mA/cm2, fill factor of 87.5%, and conversion efficiency of 27.02%. In addition, the temperature gradient of conversion efficiency is -0.14%/K. These results reveal the promising photovoltaic characteristics of CZTS thin film serving as a solar cell absorber.

Photoemission studies on undoped SnO2 buffer layers for CdTe thin film solar cells

Intermediate highly resistive layers of transparent undoped oxide materials are known to improve the characteristics of thin film solar cells, as observed for SnO₂ used in CdTe solar cells with reduced CdS layer thickness. Comparative photoemission spectroscopy studies were carried out on commercial undoped and fluorine doped SnO₂ prepared by spray-pyrolysis, revealing that the Fermi level positions of the surfaces of both commercial samples are identical and degenerate. The commercial SnO₂ was compared to SnO₂ prepared by RF magnetron sputtering under different oxidation conditions. The sputtered samples were only weakly n-doped. Additionally, the influence of different sample treatments was investigated.

Study the Effect of Thermal Treatment and Variation of CdS Thickness on Structural Properties of CdS/CdTe Heterojunction

A polycrystalline CdS/CdTe heterojunction prepared by thermal evaporation technique at substrate temperature of 373 K and heat treatment for annealing temperature 573 K for different duration times of annealing (ta) 60 and 120 min. under vacuum of (10-5) mbar and different thicknesses of CdS layer of about 1000Å, 1500Å and 2000Å. The X-ray diffraction (XRD) technique shows polycrystalline structure and had a mixture of cubic and hexagonal structure.The recrystallization of films enhanced thermally represented by increasing the grain size. The CdS/CdTe heterojunction with the higher thickness of CdS layer shows better crystal structure

The effect of post-annealing on the conversion efficiency of solar cells sensitized by CdS quantum dots

We investigate the effect of annealing treatment on the power conversion efficiency of solar cells sensitized by CdS quantum dots. The CdS quantum dots were deposited on ZnO nanowire arrays through a chemical bath deposition process. The thickness of CdS layer and the annealing temperature were found to be very crucial for the final performance of CdS/ZnO solar cells. The power conversion efficiency was improved significantly after annealing treatment, which can be attributed to the red shift of the absorption edge of CdS quantum dots and the formation of a diffusion layer between the CdS shell and ZnO core.

Kinetic Studies of CdS Formation for a Better Understanding of Chemical Buffer Layer Deposition

AbstractCdS is an important part of CIGS-based photovoltaics as a buffer layer between the absorber and the window layer. Chemical Bath Deposition (CBD) is the most common way of depositing CdS. Due to the nature of the CBD process CdS forms not only on the absorber layer surface but also in the bulk solution. In order to control the deposition of CdS at the surface and to avoidCdS formation in the bulk solution the kinetics of these reactions must be understood. In this work we study the formation of CdS in the chemical bath by using in situ spectroscopic extinction measurements. By monitoring the absorbance of the solution we are able to observe the bulk phase reaction (chemical formation of CdS in solution). Deposition of CdS (growth of the CdS layer at the surface) is investigated by measuring the CdS layer thickness after the reaction. The kinetics of both reactions were investigated by a systematic variation of temperature, initial reactant concentration, Reynold number as well as time. Finally the results of our study suggest a model of growth behaviour based on molecule-by-molecule deposition, which explains especially the dependency on temperature and hydrodynamics.

Passivation properties of CdS thin films grown by chemical bath deposition on GaSb: the influence of the S/Cd ratio in the solution and of the CdS layer thickness on the surface recombination velocity

Evidences of the passivation effect are given when thin films of CdS are deposited on GaSb crystalline substrates, using a bath chemical deposition method. The passivation process is studied through photoacoustic and photoluminescence experiments. The surface recombination velocity calculated from photoacoustic measurements decreases and the radiative recombination rate as measured from photoluminescence spectra increases when the nominal S/Cd ratio in the layer deposition solution increases. The influence of the CdS layer thickness on the surface passivation of GaSb is also studied.

First‐principles study on atom order and stability of GaInS3(CdS)m natural superlattices

AbstractGaInS3(CdS)m is the layered natural superlattice system made of the combination of II–VI and III–VI group semiconductors. Atom order of this system was investigated by using the first‐principles calculation. It was shown that the CdS layers are located inside the GaInS3 layers when their thickness is small, m < 2, while they are located outside the GaInS3 as their thickness increases, large than m = 2. This change of atom order occurs due to the electric field between GaS and InS layers and causes the band‐gap variation as a function of the CdS layer thickness. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Role of the CdS buffer layer as an active optical element in Cu(In,Ga)Se2 thin‐film solar cells

AbstractZnO/CdS/Cu(In,Ga)Se2 (CIGS) thin‐film heterojunction solar cells with CdS buffer layers of thicknesses between 0 and 85 nm are characterized by current–voltage, quantum efficiency, and optical reflection measurements. We investigate the correlation between the short‐circuit current density and the CdS layer thickness, focusing on the counteracting effects of light absorption and reduced optical reflection induced by the CdS layer. Both effects almost compensate each other for CdS layer thicknesses between 0 and 40 nm. Thus, an optimization of the short‐circuit current density is not achieved by omitting the CdS layer, but rather by replacing the CdS buffer with an alternative buffer material with higher bandgap energy and optical constants similar to those of CdS. Copyright © 2002 John Wiley & Sons, Ltd.

Optimization of CBD CdS process in high-efficiency Cu(In,Ga)Se 2-based solar cells

We present an optimization of the CdS chemical bath deposition process as applied to high-efficiency Cu(In,Ga)Se 2 photovoltaic thin-film absorber materials. Specifically, we investigated deposition time (thickness), bath temperature (65, 80 and 90°C) and a Cd 2+ partial-electrolyte treatment of the chalcopyrite absorber prior to CdS deposition. We found that thinner CdS layers (grown on as-deposited absorbers) allowing more light to reach the junction are not necessarily conducive to higher short-circuit current density. Device performance was found to be dependent on the CdS layer thickness, but rather independent of the growth temperature. On the other hand, devices prepared from absorbers subjected to a Cd 2+ partial electrolyte treatment show that the device performance dependence on CdS thickness is somewhat alleviated, and devices incorporating thinner CdS layers are possible without loss of parameters, such as open-circuit voltage and fill factor.