Preliminary photovoltaic cells with single crystal CIS substrates

Using the Bridgman method, ingots of CuInSe₂ have been grown, which are microcrackfree, void-free and adhesion-free. From these, p-type substrates have been obtained for the fabrication of preliminary CIS/CdS/ZnO and CIS/CdS/CdO photovoltaic cells, where the window layers were deposited, respectively, by rf sputtering from a ZnO target and by dc reactive sputtering from a Cd target and where the CdS buffer layer was deposited by a chemical bath method. These cells have yielded approximate illuminated jj_sc, V_oc,η and FF values, respectively, up to 28 mA/cm², 0.42 V, 5% and 0.41 for effective areas of 7 to 22 mm².     

Photoluminescence characteristics of CdS layers deposited in a chemical bath and their correlation to CdS/CdTe solar cell performance

In this work, we study CdS films processed by chemical bath deposition (CBD) using different thiourea concentrations in the bath solution with post-thermal treatments using CdCl₂. We study the effects of the thiourea concentration on the photovoltaic performance of the CdS/CdTe solar cells, by the analysis of the I–V curve, for S/Cd ratios in the CBD solution from 3 to 8. In this range of S/Cd ratios the CdS/CdTe solar cells show variations of the open circuit voltage (V_oc), the short circuit current (J_sc) and the fill factor (FF). Other experimental data such as the optical transmittance and photoluminescence were obtained in order to correlate to the I–V characteristics of the solar cells. The best performance of CdS–CdTe solar cells made with CdS films obtained with a S/Cd ratio of 6 is explained in terms of the sulfur vacancies to sulfur interstitials ratio in the CBD–CdS layers.     

A suitable deposition method of CdS for high performance CdS-sensitized ZnO electrodes: Sequential chemical bath deposition

A suitable deposition method of CdS is necessary for the high performance CdS-sensitized ZnO electrodes. In this paper, chemical bath deposition (CBD) and sequential chemical bath deposition (S-CBD) methods were used to deposit CdS on ZnO mesoporous films for ZnO/CdS electrodes. The analysis results of XRD patterns and UV-vis spectroscopy indicated that CBD deposition method leaded to the dissolving of ZnO mesoporous films in deposition solution and thickness reduction of ZnO/CdS electrodes. Absorption in visible region by the ZnO/CdS electrodes with CdS deposition by S-CBD was enhanced as deposition cycles increased due to the stability of ZnO mesoporous films in the S-CBD deposition solutions. The results of photocurrent-voltage (I-V) measurement showed that the performance of ZnO/CdS electrodes with CdS deposition by CBD first increased and then decreased as deposition time increased, and the greatest short-circuit current (J_sc) was obtained at the deposition time of 4 min. The performance of ZnO/CdS electrodes with CdS deposition by S-CBD increased as deposition cycles increased, and both open-circuit voltage (V_oc) and J_sc were greater than those electrodes with CdS deposition by CBD when the deposition cycles of S-CBD were 10 or greater. These results indicated that S-CBD is a more suitable method for high performance ZnO/CdS electrodes.     

Design issues in the fabrication of CdS–CdTe solar cells on molybdenum foil substrates

Investigations on CdTe–CdS solar cells on molybdenum foil substrates revealed that the depletion layer spans the entire CdS and CdTe film thickness and the cell should be conceived as a single junction device instead of the three separate junctions (Mo–CdTe, CdTe–CdS, CdS–TCO). Higher open circuit voltages were achieved when two CdS layers (separated by an air anneal) were used instead of a single CdS layer. The high series resistance of this solar cell continues to be the limiting factor in cell performance. Modeling and design issues for improving cell performance are presented.     

Photocatalytic hydrogen evolution from CdS–ZnO–CdO systems under visible light irradiation: Effect of thermal treatment and presence of Pt and Ru cocatalysts

A CdS–CdO–ZnO mixture annealed at different temperatures and loaded with Ru or Pt cocatalysts has been investigated in the production of hydrogen from aqueous solutions containing SO₃²⁻ + S²⁻ as sacrificial reagents under visible light. The physicochemical characterization of the CdS–CdO–ZnO catalyst revealed significant changes in the crystalline structure and visible light absorption capacity as a result of thermal treatments. Catalytic activity was found to be strongly dependent on physicochemical changes associated with thermal annealing. Hydrogen evolution over the CdS–CdO–ZnO catalyst was enhanced in the sample annealed at 773 K by the better contact between the CdS and CdO–ZnO phases, which improved physical charge separation. CdS–CdO–ZnO catalyst activity was significantly improved by the addition of Pt or Ru cocatalysts. Among the noble metals studied, activity promotion was higher for the sample loaded with Ru. The enhancement of activity associated with Ru loading is linked to a good interaction between Ru oxide particles and CdS, which reduces the possibility of electron–hole recombination, thus resulting in more efficient water splitting.     

Stable photocatalytic hydrogen evolution from water over ZnO–CdS core–shell nanorods

Stability and efficiency are important to realize the practical applications of photocatalysts for photocatalytic hydrogen evolution from water splitting. ZnO–CdS core–shell nanorods with a wide absorption range were designed and synthesized by a two-step route. The ZnO–CdS core–shell nanorods exhibit stable and high photocatalytic activity for water splitting into hydrogen in the presence of S²⁻ and SO₃²⁻ as sacrificial reagents. Furthermore, the photocatalytic activity and stability of ZnO–CdS core–shell nanorods/RuO₂ co-catalyst is superior to that of ZnO–CdS core–shell nanorods/Pt co-catalyst. The merits of stable ZnO and CdS, core–shell and nanorod structures employed are considered to contribute to the favorable photocatalytic hydrogen evolution of ZnO–CdS core–shell nanorods.     

Photocatalytic hydrogen generation with simultaneous organic degradation by composite CdS–ZnS nanoparticles under visible light

A visible light-driven CdS–ZnS photocatalyst in the form of nanoparticles with a heterogeneous structure was synthesized using the stepped microemulsion method. The composite CdS–ZnS was capable of simultaneous photocatalytic hydrogen production and organic degradation under visible light. The ZnS deposition on CdS helped to suppress the recombination of electron/hole pairs generated on the more reactive CdS, leading to faster hydrogen production and improved stability of the CdS–ZnS in comparison to the bare CdS catalyst. Deposition of Ru on the catalyst surface further increased its photo-reactivity by about 4 times for hydrogen production. The heterostructured nanoparticles were effective in photocatalytic hydrogen production together with the degradation of model organic substances, including formic acid, methanol, and ethanol. The highest hydrogen production rate was achieved by the (CdS–ZnS)/Ru catalyst at 266 mmol/m²-h in the formic acid solution with an energy conversion efficiency of 3.05% in visible light, and the corresponding organic degradation rate in terms of the removal of chemical oxygen demand (COD) was estimated at 4272 mg COD/m²-h.     

Quality control and characterization of Cu(In,Ga)Se2-based thin-film solar cells by surface photovoltage spectroscopy

Surface photovoltage spectroscopy (SPS) has been used for quality control of ZnO/CdS/ Cu(In,Ga)Se₂ (CIGS) thin-film solar cells. The results show that SPS makes it possible to detect “hard failures” following CIGS deposition, and both “hard” and “soft” failures following CdS deposition and following ZnO deposition. In addition, a semi-quantitative screening of CdS/CIGS and ZnO/CdS/CIGS samples is possible. Hence, SPS is suggested as a useful tool for in-line monitoring of CIGS-based solar cell production lines. Moreover, SPS is shown to yield important new information regarding CIGS-based solar cells: (a) A deep gap state is found in samples of superior performance. (b) As opposed to the CdS/CIGS structure, a marked decrease in the open-circuit voltage upon Na contamination in ZnO/CIGS structures is found.     

CIS/CdS/ZnO/ZnO:Al modified photocathode for enhanced photoelectrochemical behavior under visible irradiation: Effects of pH and concentration of electrolyte solution

In this paper, the CuInS₂ films were firstly modified with CdS and CdS/ZnO/ZnO:Al/Au layers in order to improve the photoelectrochemical (PEC) water splitting efficiency. The CuInS₂ photoelectrode was synthesized by electrodeposition method as a facial and green method, on the FTO substrate. The effects of pH and concentration of Na₂S electrolyte solution on the photocurrent density of photoelectrode samples were studied. As a p-n junction photocathode, the CIS/CdS/ZnO/ZnO:Al/Au photoelectrode indicates the enhanced PEC activity. The photocurrent density of CIS/CdS/ZnO/ZnO:Al/Au photoelectrode reaches to 1.91 mA/cm², while is about 2.5 times higher than that for CuInS₂ film at pH = 8 (−0.6 V vs Ag/AgCl). The formation of a p-n junction at the CuInS₂ photoelectrode surface not only reduces the recombination of electron-hole pairs but also increases the PEC response and water splitting performance of the as-prepared CIS/CdS/ZnO/ZnO:Al/Au photoelectrode.     

Electrodeposited CdS on CIS pn junctions

We have been investigating the electrochemical deposition of thin films and junctions of cadmium sulfide (CdS) and copper indium diselenide (CIS). We show that it is possible to fabricate pn junctions based on n-type CdS and p-type CIS entirely by electrodeposition. CIS is considered to be one of the best absorber materials for use in polycrystalline thin-film photovoltaic solar cells. CdS provides a closely lattice-matched window layer for CIS. Electrodeposition is a simple and inexpensive method for producing thin-film CdS and CIS. We have produced both p- and n-type CIS thin films, as well as a CdS on CIS pn junction via electrodeposition. Elemental analysis of the CdS and CIS thin films was performed using X-ray photoelectron spectroscopy and energy dispersive spectroscopy. Optical band gaps were determined for these films using optical transmission spectroscopy. Carrier densities of the CIS films as a function of their deposition voltage were determined from capacitance vs. voltage measurements using Al Schottky barriers. Current vs. voltage characteristics were measured for the Al on CIS Schottky barriers and for the CdS on CIS pn junction.     

Thin CdS films prepared by metalorganic chemical vapor deposition

Polycrystalline CdS thin films have been deposited on borosilicate glass substrates coated with ITO film by metalorganic chemical vapor deposition using dimethyl cadmium and diethyl sulfide as source materials. The growth of CdS film occurred at substrate temperatures within the range of 280–360°C. The deposition rate increased with increasing VI/II molar ratio at any substrate temperature and showed a maximum value at the VI/II molar ratio of 4. The grain size of as-deposited CdS film prepared at substrate temperatures from 300°C to 360°C was about 0.1 μm. The CdS films consist of hexagonal form with a preferential orientation of the (0 0 2) plane parallel to the substrate. Thin CdS film with high optical transmittance was prepared at 350°C with the VI/II molar ratio of 4. The CdS film deposited by MOCVD may be used as a window layer for CdS/CdTe solar cell.     

Rough ZnO layers by LP-CVD process and their effect in improving performances of amorphous and microcrystalline silicon solar cells

Doped ZnO layers deposited by low-pressure chemical vapour deposition technique have been studied for their use as transparent contact layers for thin-film silicon solar cells.Surface roughness of these ZnO layers is related to their light-scattering capability; this is shown to be of prime importance to enhance the current generation in thin-film silicon solar cells. Surface roughness has been tuned over a large range of values, by varying thickness and/or doping concentration of the ZnO layers.A method is proposed to optimize the light-scattering capacity of ZnO layers, and the incorporation of these layers as front transparent conductive oxides for p–i–n thin-film microcrystalline silicon solar cells is studied.     

Development of CdS based stable thin film photo electrochemical solar cells

CdS semiconductor films have been prepared from chemical bath deposition from basic solutions. Electron microscopic studies have revealed the presence of polycrystalline mixed cubic and hexagonal phases of CdS in the deposits. When surface treated, these layers show good photo electro chemical behaviour in Ti/CdS/S/C cell configuration where as photo etching improves the stability of CdS photo anode in polysulphide electrolyte to a great extent. Optical absorption studies of these films have been carried out in the wavelength range 350–800 nm.     

XPS analysis of CdS/CuInSe2 heterojunctions

CdS/CuInSe₂ (CIS) heterojunctions were investigated by XPS analysis. An In-excess layer which may form an ordered vacancy compound (OVC) was present at the as-deposited CIS surface and it remained after chemical bath deposition of a CdS layer. The In-excess layer was removed by preferential etching with NH₃ aqueous solution. This result implies that the surface of the as-deposited CIS film was converted from the OVC with n-type conductivity into the CIS with p-type by NH₃ treatment. The conduction band offsets at the CdS/p-CIS and CdS/n-OVC were determined to be 1.0 and 0.3 eV, respectively. The CIS solar cells fabricated with n-OVC surface layer exhibited higher cell efficiencies than those fabricated with p-CIS surface layer.     

Properties of amorphous silicon solar cells fabricated from SiH2Cl2

Chlorinated intrinsic amorphous silicon films [a-Si:H(Cl)] and solar cell i-layers were fabricated using electron cyclotron resonance-assisted chemical vapor deposition (ECR-CVD) and SiH₂Cl₂ source gas. n–i–p solar cells deposited on ZnO–coated SnO₂ substrates had poor photovoltaic performances despite the good electronic properties measured on the a-Si:H(Cl) films. Improved open–circuit voltage (V_oc) of 0.84 V and fill factor (FF) of 54% were observed in n–i–p solar cells by providing an n/i buffer layer and by using Ga-doped ZnO coated glass substrates. However, the FF improvement was still rather poor, which is thought to originate from high interface recombination in the ECR deposited solar cells. The V_oc and the FF showed much stable feature against light soaking.     

Silver-doped CdS films for PV application

CdS : Ag thin films were deposited by the chemical deposition method (solution growth), on SnO₂ thin transparent electrodes, to obtain n-type window layer for PV-cell. CdS thin films were doped with silver by an ion-exchange process in a neutral 0.025 M thiosulphate Ag-complex solution. Best results were achieved at immersion times of 20–30 s at room temperature. For the sake of comparison, the doping was performed on of the substrate surface, while the remaining part was left undoped. SnS_x thin layer was deposited on the top of such a n-type layer prepared in the same way the p-type layer was selected due to its simplicity of preparation and the possibility for variation of the band gap (E_g), by varying x in the compound. Ohmic contact was produced by graphite paste backelectrodes. Two different types of PV cells were produced on the same test sample, SnO₂/CdS : Ag–SnS_x/C and SnO₂/CdS–SnS_x/C, in order to study the influence of the Ag doping of CdS, on the PV cell parameters. Dark and light I–V characteristics were recorded for the two types of cells at several different light intensities. Considerable enhancement of all cell parameters, efficiency (η), fill factor (FF), diode factor (a), short-circuit current (I_sc), etc., was observed on the CdS : Ag-based sample. Spectral sensitivity in VIS-NIR part of the spectrum, recorded on the two types of cells, showed an improvement on the CdS : Ag-based PV cell.     

Optimisation of CdSTCO bilayers for their application as windows in photovoltaic solar cells

The influence of CdS thickness and TCO deposition condition on the optoelectronic, morphological and structural properties of CdSTCO bilayers has been studied. CdS was deposited by chemical bath, whereas the TCOs (ITO and aluminium-doped ZnO) were prepared by rf-magnetron sputtering. Cadmium sulphide thickness below 0.1 μm have been found to be optimal for avoiding optical absorption losses and conductivity deterioration. Though both types of window coatings, with TCO made at 25 and 200°C, have good quality, the ITO-based samples have shown a higher infrared transmission than those based on ZnO. Additionally, 10 and 70 minutes air-annealing treatments at 200°C have been carried out. Their analysis has demonstrated that the shorter ones are more suitable for enhancing structure maintaining unchanged electro-optical characteristics.     

CIS and CIGS based photovoltaic structures developed from electrodeposited precursors

In the present study we report the electrodeposition and characterization of CIS and CIGS thin films and a post-deposition thermal processing in vacuum to improve the film stoichiometry by incorporating additional In, Ga and Se. Different kinds of analyses showed that CIS as well as CIGS possess a very thin In-rich surface n-layer. The formation and characterization of solar cell structures from the electrodeposited precursor with the configuration glass/Cr/Mo/CIS(CIGS)/CdS/ZnO/MgF₂ is also reported. The optoelectronic properties such as Voc, Isc, FF, η etc. of the cells are presented.     

CIS thin film solar cells with evaporated InSe buffer layers

This paper describes the investigations of CIS-based solar cells with a new In_xSe_y (IS) buffer layer. Studies were concentrated on determining the deposition conditions to get In_xSe_y thin films with adequate properties to be used in substitution of the CdS buffer layer, usually employed in the fabrication of this type of devices. Before the solar cell fabrication, the buffer layers grown by evaporation of the In₂Se₃ compound were characterized through transmittance and X-ray diffraction measurements. It was found that good results can be obtained using indium selenide film as the buffer layer, grown in the In₂Se₃ phase.Solar cells with structure Mo/CIS/In₂Se₃/ZnO were fabricated. The ZnO layer was deposited by reactive evaporation and the absorber CIS layer was grown on Mo by a two-stage process. The preliminary results obtained with this type of solar cells are J_sc=30.8 mA/cm², V_oc=0.445 V, FF≈0.6 and η=8.3% with an irradiance of 100 mW/cm². Solar cells fabricated using a CdS buffer layer deposited by CBD on CIS substrate, prepared under the same conditions used in the fabrication of Mo/CIS/In₂Se₃/ZnO cells, gave the following results: V_oc=0.43 V, J_sc=34 mA/cm², FF≈0.63 and η=9.2%.     

CdTe/CdS Solar cells on flexible molybdenum substrates

Development of CdTe/CdS solar cells on flexible metallic substrates is highly interesting due to the light weight and flexible nature of the solar modules. We have deposited CdTe films onto flexible molybdenum substrates using close-spaced sublimation technique and the CdTe/CdS junction was developed by depositing a thin layer of CdS onto the CdTe substrate from a chemical bath. The devices were characterized by Current–voltage (I–V) and photocurrent spectroscopy techniques. Prior to the deposition of the transparent conducting layer, the devices were annealed in air at different temperatures and found that the devices annealed at 400°C have better photovoltaic parameters. The efficiency of a typical device under 60 mW cm⁻² illumination was estimated as 3.5%.