DC conduction mechanism in polyvinyl alcohol films doped with potassium thiocyanate

The current–voltage characteristics of pure polyvinyl alcohol (PVA) films and those doped with potassium thiocyanate (KSCN) are studied as a function of film temperature and dopant concentration. The conduction mechanisms operative in the films in different temperature and voltage ranges are estimated from the behaviour of log I versus V^1/2 plots (I=current, V=voltage). For undoped (pure) films, the conduction mechanism appears to be essentially a Schottky type. On doping, there is considerable influence on the type of conduction mechanism, especially at lower temperatures. At higher temperatures, however, there is no significant effect of doping on the conduction mechanism.     

Diffusion and influence of Cu on properties of CdTe thin films and CdTe/CdS cells

The effective diffusion coefficients of Cu for thermal and photodiffusion in the CdTe films have been estimated from resistivity versus duration of thermal or photoannealing curves. In the temperature range 60–200°C the effective coefficient of thermal diffusion (D_t) and photodiffusion (D_ph) are described as D_t=7.3×10⁻⁷exp(−0.33/kT) and D_ph=4.7×10⁻⁸exp(−0.20/kT).It is found that the diffusion doping of CdTe thin films by Cu at 400°C results in a sharp decrease of resistivity up to 7 orders of magnitude of p-type material, depending on thickness of Cu film. The comparative study of performance of CdTe(Cu)/CdS and CdTe/CdS cells has been studied. It is shown that the diffusion doping of CdTe film by Cu increases efficiency of CdTe(Cu)/CdS cells from 0.9% to 6.8%. The degradation of photovoltaic parameters of CdTe(Cu)/CdS cell, during testing under forward and reverse bias at room temperature, proceeds at a larger rate than those of CdTe/CdS cell without Cu. The degradation of performance of CdTe(Cu)/CdS cells is tentatively assigned to electrodiffusion of Cu in CdTe, resulting in redistribution of concentration of Cu-related centers in CdTe film and heterojunction region.     

Growth, structure and composition of electrodeposited CdTe thin films for solar cells

An organic electrolytic deposition process for preparation of stoichiometric CdTe film for solar cell applications is described. Increase in deposition current modifies crystalline growth phase from hexagonal to cubic and surface microstructure from lateral crystallites to nodular grains. Initial surface morphology affects the etched and annealed microstructure of CdTe. Monocrystalline CdTe film growth occurs at low deposition currents < 0.25 mA/cm² due to low nucleation rate and surface diffusion of adions on the substrate to growth sites. Morphological and structural changes are caused by differential activity of adions at different facets of crystallites in CdTe during growth. Lesser defect density in CdTe enables insitu doping and p — conversion by phosphorous atoms. An upper limit of 20 at% of dopant in a deposition bath is set by nonstoichiometry of CdTe film due to concurrent high deposition of phosphorous resulting in depletion of Cd. P dopants occupy Te sites at concentration < 1 × 10⁻⁵ Molar as inferred by the disappearing of the Auger Te(NOO) line and analysis of the Te(MNN) line shape. Substitutional location of P during growth results in CdTe films of p-type with resistivities 20–30 ohm · cm in the as-deposited state, without OZ annealing. Further reduction in resistivity is prevented by formation of P_CD and P_iP_CD compensating defects in CdTe.     

Thickness dependence of ZnO:In thin films doped with different indium compounds and deposited by chemical spray

Indium-doped zinc oxide thin films were deposited on glass substrates by the chemical spray technique. Hydrated zinc 2,4-pentanedionate was used as zinc source. Four different indium compounds were separately used as dopants (indium nitrate, indium sulfate, indium acetate, and indium chloride). The effect of the thickness on the electrical, structural, morphological and optical characteristics of ZnO:In thin films was studied. Electrical resistivity values corresponding to good conductive electrodes were obtained irrespective of the indium compound used, although a decrease in electrical resistivity is found in all the cases as the film thickness increases, reaching a saturation value of 2×10⁻³ Ω cm. All films were polycrystalline with a wurtzite phase, and exhibited a (1 0 1) preferential orientation almost irrespective of neither the doping source nor thickness. The surface morphology was analyzed by scanning electron microscopy and a strong dependence on the indium compound and thickness was found, since grain geometry variations from rounded to rod-like forms were observed. As the film thickness increases, the film transmittance and the band-gap values decrease. Band-gap energy values were in the range of 3.21 to 3.44 eV.     

Efficient hydrogen generation of indium doped BaTiO3 decorated with CdSe quantum dots: Novel understanding of the effect of doping strategy

Doping has been widely utilized as an effective method of semiconductor modification to enhance photocatalytic efficiency. Most researches focus only on the effect of doping on decreasing band gap and inhibiting carriers recombination, however, the proximity of conduction band minimum (CBM) and valence band maximum (VBM) indicates the decrease of energy level and redox potential. Therefore, to further investigate the effect of doping on photocatalytic activity, the composite structure consisted of CdSe quantum dots (QDs) and indium doped BaTiO₃ is synthesized by plain hydrothermal and chemical bath means. Under the condition of maintaining CdSe QDs optimal load ratio of 7.5 mol.%, doping 2.5% indium maximizes hydrogen production (0.437 mmol g^–1 h^–1) compared to undoped composite structure (0.258 mmol g^–1 h^–1), which originates from the increase of photo-induced electron reductive activity. The results suggest that we can use doping in reverse thinking to provide a new tactics for the design of semiconductor photocatalytic materials.     

Back contact formation using Cu2Te as a Cu-doping source and as an electrode in CdTe solar cells

Cu₂Te was utilized as a Cu source for p⁺ doping in CdTe and as a primary back contact material in CdTe solar cells. A 60 nm-thick Cu₂Te layer was deposited on CdTe film by evaporating Cu₂Te and the samples were annealed at various temperatures. An amorphous layer was found at the Cu₂Te/CdTe interface, while the Cu₂Te has both orthorhombic and hexagonal phases. Annealing at 200°C completely crystallized the amorphous interlayer and enhanced the transformation of orthorhombic phase into hexagonal phase that has a coherent interface with CdTe. A good p⁺ contact was formed at 180°C annealing, where the series resistance of CdTe cells was a minimum of 0.5 Ω·cm² and the fill factor and open-circuit voltage were significantly improved. With the good p⁺ contact, it is possible to determine the exact dopant profile at the CdS/CdTe junction.     

RF PLANAR MAGNETRON SPUTTERING OF POLYCRYSTALLINE CdTe THIN-FILM SOLAR CELLS

Polycrystalline CdTe thin films have been prepared (using radio frequency (rf) planar magnetron sputtering), characterized, and used to fabricate CdS/CdTe solar cells. The performance characteristics of the best device (as measured under standard reporting conditions) are a conversion efficiency of 8.2% and 0.68 V and 21.7mA/cm2 for open-circuit voltage (Voc) and short-circuit: current density (J), respectively. Photoluminescence emission from the polycrystalline CdTe films (after the high-temperature heat treatment) showed that this material is dominated by four defects, including two donor levels Dl and D2 (at 120 and 180 meV below the conduction band) and two acceptor levels Al and A2 (at 30 and 65 meV above the valence band [VB]). Capacitance-voltage (C-V) measurements have indicated the formation of an n-i-p structure dominated by a high density of interface states. Deep level transient spectroscopy (DLTS) data also confirmed a minority electron trap at about 1.212 meV below the conduction band (CB) dominating the CdTe device. The trap level detected in a Schottky barrier, fabricated on a CdTe film without heating at high temperature, is deeper and higher in density than that in the heterojunction. This explains the enhancement of the Voc of the heterojunction devices after annealing at 400°C or higher. Although the device performance is modest by comparison with state-of-the-art devices, these results support the potential of sputter depositing CdTe films as a production technique. Considerable work must still be done to improve the material quality and reduce the density of defect levels.     

15.1% Highly efficient thin film CdS/CdTe solar cell

High-efficiency CdS/CdTe solar cells with thin CdS film have recently been developed. Semiconductive layers of CdS via the CVD method and of CdTe via the CSS method were deposited on an ITO/#7059 substrate. Cell performance depends primarily on the thickness of CdS film, and the conversion efficiency is highest for a CdS film thickness of around 60 nm. Since the CdS film thickness decreases by about 30% during deposition of the CdTe layer, a thickness of 95 nm is required to obtain a 60 nm-thick CdS film after deposition of a CdTe layer. By observing the CdS film during the CdTe deposition process, a decrease was detected before CdTe layer completely covers the surface of the CdS film. By optimizing the thickness of CdS film, an efficiency of 15.12% for the best cell under AM 1.5 verified at JQA was obtained. This fabrication process has good reproducibility; 92.5% of 1 cm² solar cells fabricated under the same conditions have efficiencies above 14%.     

THE INFLUENCE OF DEPOSITION AND PROCESSING CONDITIONS ON THE PROPERTIES OF EVAPORATED CdTe/ CdS LAYERS AND SOLAR CELLS

The influence of the deposition and CdCl₂ doping conditions on the properties of CdTe layers and on the performance of CdTe/CdS/ITO/glass solar cells is reported. Relatively high deposition temperatures (300-350°C) were found to enhance the reproducibility of the optical quality of the CdTe. Oxidation of CdTe layers during air annealing was observed and monitored by XRD. Conventional wet dip and CdCl₂ vapour doping of CdTe are compared. Methods for reducing the incidence of pinholes in the CdTe are described, the junction uniformity having been monitored by EBIC. The best solar cell made in this work had an efficiency of 9.87%. (V _oc = 0.696V, J _sc = 24.1 mA/cm², FF = 59%).     

Structural and optical characterizations of CdTe on CdS grown by hot-wall vacuum evaporation

We report on characterizations of polycrystalline CdTe on CdS grown by hot-wall vacuum evaporation. The CdTe film grown on CdS/SnO₂/glass was compared with the other two CdTe films, which were grown directly on #7059 glass and on SnO₂-coated glass. The grown CdTe/CdS film is composed of a grain size 3–15 μm and have a close-packed structure compared to other CdTe films. A weak excitonic peak at 1.59 eV as well as two kinds of donor–acceptor pair emission bands has been observed in PL spectrum of CdTe/CdS film. On the other hand, the excitonic peak cannot be detected in other CdTe films.     

BEHAVIOR OF SOLUTION GROWN CdS FOR THIN-FILM SOLAR CELL TECHNOLOGIES†

For efficient polycrystalline thin-film photovoltaic devices, such as CIS and CdTe, cadmium sulfide is the commonly used window material, and an inexpensive method for preparing thin films of CdS is by solution growth. In research at Martin Marietta, thin films of CdS were deposited onto glass and indium tin oxide-coated glass substrates using an optimized chemical immersion technique which minimizes chemical waste. Solubility equilibria and film-growth rate constants were determined for cadmium salts in NH₄CI and NH₄OH buffered systems to characterize and optimize process conditions for stable solution-grown cadmium sulfide. Results will be presented which relate process variables to cadmium sulfide film quality.     

Effect of cobalt ion contamination on proton conduction of ultrathin Nafion film

With the wide use of platinum alloy electrocatalysts proton exchange membrane fuel cells, the dissolution of transition metal in the acid environment of catalyst layer becomes a major concern due to the decreased catalytic activity during long-term operation. Although great efforts have been done, few attention is paid to the effect of transition metal ion contamination of Nafion ionomer that wrapped around catalyst particles within catalyst layer which might greatly influence the proton conduction. To elucidate such effect, ultrathin Nafion film on SiO₂ model substrate is prepared via self-assembly method to represent the ionomer in fuel-cell catalyst layers, and the ultrathin film with specific thickness is further equilibrated in Co(NO₃)₂/HNO₃ solutions with different Co²⁺ concentration to achieve different doping level. Conductivity measurements demonstrate that the non-precious metal contamination severely worsens proton conduction of Nafion ionomer, but it is always ignored before. In addition, when the occupation of H⁺ in Nafion ionomer is less than 50%, the activation energy shows a sharp increase, indicating a possible change in proton conduction mechanism.     

EFFECTS OF PROCESSING ON CdTe/CdS MATERIALS AND DEVICES

By analyzing CdTe/CdS devices fabricated by vacuum evaporation, a self consistent picture of the effects of processing on the evolution of CdTe cells is developed which can be applied to other fabrication methods. In fabricating CdTe/CdS solar cells by evaporation, a 400°C CdCI₂ heat treatment is used which recrystallizes the CdTe and interdiffuses the CdS and CdTe layers. The interdiffuson can change the bandgap of both the CdTe and CdS which modifies the spectral response of the solar cell. After this heat treatment a contacting/doping procedure is used which converts the CdTe conductivity to p-type by diffusion from Cu from the contact. Finally, the cell is treated with Br₂CH₃,OH which improves both Voc and FF. Analogous process steps are used in most fabrication processes for CdTe/CdS solar cells.     

A new catalyst of AlCu@ZnO for hydrogen evolution reaction

Thin films of undoped ZnO, Al-doped ZnO, Cu-doped ZnO, and AlCu@ZnO deposited on indium tin oxide were performed by the sol-gel spin coating method. The prepared ZnO thin films were investigated for their structural and electrical properties after annealing at 500 °C for 1 h. ZnO thin films were characterized by electrochemical impedance spectroscopy, linear sweep voltammetry, scanning electron microscopy, Fourier transform infrared spectroscopy and Mott Schottky. According to the results obtained from the Nyquist diagrams of the ZnO thin films, the resistance value was found to decrease with binary doping and the resistance value was found to be lowest in AlCu@ZnO doped thin film containing 0.01 M Al and 0.1 M Cu. As ZnO thin films go to cathodic potentials, it is seen that the cathodic current value of ZnO with undoped is the lowest. It has been found that only Al and Cu doping showed less cathodic current than double doping.     

Recent advances in alternative material photovoltaics

Abstract

Alternative material photovoltaics (PVs) have started gaining more attention recently. Although the field is not new, it just started growing a few years ago. The PV market has been dominated by various silicon technologies, besides a few other popular thin films, such as CdTe, copper–indium–galium–selenide varieties and some III–V materials. This has been reflected in research as well. Successful developments of efficient solar cells using alternative absorbers will significantly enrich the PV industry and reduce the market gap with other energy sources. Hence, in this review, recent advances and trends to develop PVs using alternative materials are presented and discussed. The focus will be mainly on binary as well as environmentally friendly compounds and thin film devices. Nonetheless, some other more complex materials and structures will be briefly addressed.     

Foil mounted thin-film solar cells for space and terrestrial applications

The developing photovoltaic market for civilian communications satellites has caused a reassessment of the traditional single-crystal cells used in space. Thin-film cells mounted on metal foil substrates offer considerable weight, manufacturing, and radiation resistance advantages. These same cells can solve the manufacturing problems of glass breakage and temperature uniformity in cells intended for terrestrial applications. This paper describes work on a backwall CdS/CdTe cell on molybdenum foils. Large grained adherent films of CdTe are formed. A thin interlayer of Cu and Te between the CdTe and the molybdenum formed a low resistance (less than 1 Ω cm²) ohmic contact. The cell is completed with a sputtered indium tin oxide top contact layer and a V_oc of 580 mV was obtained.     

CdTe thin film solar cells: device and technology issues

Polycrystalline thin film CdTe continues to be a leading material for the development of cost effective and reliable photovoltaics. Thin film CdTe solar cells and modules are typically heterojunctions with CdS being the n-type partner, or window layer. The preferred configuration for CdTe solar cells is the superstrate structure. The cadmium chloride heat treatment, the back contact formation process, and the utilization of resistive, buffer layers in tandem with a thin cadmium sulfide window layer, are important areas of research in thin film CdTe solar cells. This paper reviews work on CdTe thin film solar cells sponsored by the National Renewable Energy Laboratory. Results for a vapor chloride heat treatment with high throughput characteristics, a dry back contact process, and a comparative study of resistive buffer layers and their effect on the performance of CdTe solar cells are presented.     

Electrodeposition and characterization of CdTe thin films on Mo foils using a two voltage technique

Polycrystalline thin film CdTe-based solar cells are one of the most promising candidates for low-cost terrestrial conversion of solar energy because of the optimum energy band gap (E_g=1.44eV) and high absorption coefficient of CdTe. In this work, a two-step electrodeposition technique has been used to prepare CdTe thin films from acidic solutions. In the first step, a thin Te rich CdTe layer was deposited at –300 mV (SCE) on a Mo foil substrate. On top of this film, a Cd rich CdTe layer was deposited at more negative voltages. The resulting films showed good adherence to the substrate and very low contact resistance between the substrate and the p+ Te rich CdTe layer. The composite film was of nearly stoichiometric composition. From X-ray diffraction results, the as-deposited films show very small grain sizes but after annealing the grain size increases considerably showing very well-defined peaks. The morphological, structural and composition results of CdTe thin films obtained by Scanning Electron microscopy, X-ray diffraction, and X-ray fluorescence will be presented. Electrical properties such as conductivity type and contact resistance values for the Mo/CdTe structures will also be presented.     

Pulsed electrodeposition of CdTe thin films: effect of pulse parameters over structure, stoichiometry and optical absorption

CdTe thin films were electrochemically deposited using unipolar current pulses of high magnitude between 2.5 and 30 mA/cm² in an aqueous solution. Parametric study of the effect of periodic current pulse magnitude, average current and ON and OFF duration was undertaken to understand the effect of pulse variables on CdTe film properties. Increasing pulse deposition current modifies crystalline growth phase from single cubic to mixed cubic and hexagonal growth phases. In addition to the modification in CdTe growth phases, there is an increasing tendency of the oxide formation particularly CdTeO₃. Increase in pulse current density or average current yields Cd rich CdTe films. The optical absorption coefficient decreases with the decrease in pulse current density, whereas an increase is observed as the OFF time decreases. The optical energy gap is found to increase with OFF time. A systematic study on the effect of pulse variables over the structure, compositional and optical properties of CdTe film is described.     

Chemically deposited CdS by an ammonia-free process for solar cells window layers

Chemically deposited CdS window layers were studied on two different transparent conductive substrates, namely indium tin oxide (ITO) and fluorine doped tin oxide (FTO), to determine the influence of their properties on CdS/CdTe solar cells performance. Three types of CdS films obtained from different chemical bath deposition (CBD) processes were studied. The three CBD processes employed sodium citrate as the complexing agent in partial or full substitution of ammonia. The CdS films were studied by X-ray diffraction, optical transmission spectroscopy and atomic force microscopy. CdS/CdTe devices were completed by depositing 3 μm thick CdTe absorbent layers by means of the close-spaced vapor transport technique (CSVT). Evaporated Cu-Au was used as the back contact in all the solar cells. Dark and under illumination J-V characteristic and quantum efficiency measurements were done on the CdS/CdTe devices to determine their conversion efficiency and spectral response. The efficiency of the cells depended on the window layer and on the transparent contact with values between 5.7% and 8.7%.