Effect of substrate temperature on the structure and optical properties of CdTe thin film

The present work deals with the preparation, structure and optical characterization of cadmium telluride (CdTe) thin films. These films are formed by vacuum evaporation on the well-cleaned glass substrates. The compositional analyses are made by energy dispersive analysis by X-ray. The thicknesses of the samples are measured by multiple beam interferometry. The samples are prepared at different substrate temperatures. The X-ray diffraction has been employed to study the structure of the film. The structures of the samples are found to be crystalline and the crystallite size increases with the increase of substrate temperature. The d-spacing and lattice parameters of the samples are calculated and the results are also discussed. Optical characteristics of the CdTe samples have been analyzed using spectrophotometer in the wavelength range of 400–800 nm. The transmittance is found to decrease with the increase of film thickness. The transmittance falls steeply with decreasing wavelength. It reveals that CdTe films are having considerable absorption throughout the wavelength region (400–800 nm). The optical band gap energy has been evaluated from the plot of α² vs. hν. Two possible direct transitions have been observed for all the CdTe films in visible region. The observed allowed transition may be attributed due to the spin orbit splitting of the valence band.     

Optoelectronic properties of chlorine- and oxygen-doped CdTe thin films

The influence of oxygen codopant upon the optoelectronic properties of chlorine-doped CdTe films is being investigated. It is shown that a small quantity of oxygen decreases the resistivity of films, whereas at higher concentrations oxygen codopant increases the resistivity of films up to 6 orders of magnitude. A subsequent annealing in tellurium vapor pressure decreases the resistivity of films. It is supposed that an anomalous resistivity drop around 0.22 kPa is caused by shallow acceptor complexes that oxygen forms with group I impurities like copper and silver. At higher concentrations oxygen forms isoelectronic complexes with cadmium vacancies, which cause a high resistivity of films. Te annealing extracts oxygen from the films as Te forms with dissolved oxygen tellurium oxide TeO₂ which easily sublimates. Photoconductivity of the oxygen and chlorine-doped CdTe films is poor, or is not detected.     

Spectroscopic ellipsometry investigation of optical and interface properties of CdTe films deposited on metal foils

Optical and interface properties of the CdTe films electrodeposited on Molybdenum and Stainless Steel substrates were investigated using variable angle spectroscopic ellipsometer measurement and multilayer optical analysis. The refractive index of CdTe film obtained from the multilayer optical modeling is found to be lower than single crystal data. The Bruggeman effective medium analysis shows that the films consist of nearly 11% void due to poor crystallinity resulting in the lower refractive index. The multilayer optical model also indicates the presence of a Te rich interface between CdTe and substrate, which can be associated to the kinetics of CdTe electrodeposition that starts from nucleating Te on substrate surface followed by the formation of CdTe.     

Characterization of CdTe thin film—dependence of structural and optical properties on temperature and thickness

The X-ray diffraction analysis of vacuum-evaporated cadmium telluride (CdTe) films reveals that the structure of the films is polycrystalline in nature for the samples prepared at higher substrate temperatures. The crystallite size (D), dislocation density (δ) and strain () were calculated. The composition analysis was made by the energy dispersive X-ray analysis. It confirmed the equal distribution of Cd and Te elements in the CdTe films. The fundamental optical parameters like band gap and extinction coefficient are calculated from the transmission spectra. The possible optical transition in these films is found to be direct and allowed. The charge transport phenomenon appears to be space charge limited conduction. Various electrical parameters were determined from the I–V analysis.     

Optical properties of electrodeposited CdTe thin films

The absorption coefficient spectra of the elctrodeposited CdTe thin films were analyzed and compared with that of the single crystal. Pinhole-free thin films facilitated the analysis of the high-energy regions of the absorption coefficient spectra. The various allowed direct and indirect transitions were detected successively by subtracting the extrapolated values of the lower-energy transitions. The effect of heat treatment on the optical transitions were analyzed with films annealed at 300°C in air, argon and CdCl₂.The direct band gap of the electrodeposited films decreased with increasing film thickness and approaches the value of the single crystal. The films annealed at different environments show slightly lower value for the band gap. Annealing in argon caused significant change in the optical transition spectra.     

Development of CdTe thin films on flexible substrates—a review

In the recent years there has been an increased interest in photovoltaic structures on lightweight flexible substrates. Photovoltaic structures on lightweight substrates have several advantages over the heavy glass-based structures in both terrestrial and space applications. CdTe is one of the leading candidates for the solar cells due to its optimum band gap and the variety of film preparation methods. The development of CdTe thin films on flexible substrates is discussed. The film growth and characterization are reviewed.     

A CdTe/PMeT photovoltaic structure formed by electrodeposition and processing

CdTe thin films were electrodeposited from an ethylene-glyco-based bath by the galvanostatic method. As-deposited and tellurized films were characterized by structural, optoelectronic and photoelectrochemical methods. The film stoichiometry improved after tellurization of the film at 300°C by a technique called chemical vapor transport by Gas (CVTG) in a tubular furnace. Tellurized films showed near stoichiometry with p-type conductivity in the bulk and n-type surface conductivity. Schottky barrier type photovoltaic junctions were obtained using a heavily doped PMeT (poly-3(methylthiophene), prepared by electropolymerization, displaying nearly metallic behavior, and CdTe obtained by electrodeposition. A solar to electrical conversion efficiency of the order of 1% was obtained in the case of PMeT/CdTe junction.     

Early stage growth mechanisms of CdTe thin films deposited by close space sublimation for solar cells

The early stage growth mechanisms of sublimation-grown thin-film polycrystalline CdTe are evaluated by growth interrupts and ex-situ SEM/AFM analysis for growth under 100 Torr of inert gas. Development of island size, density and coverage demonstrates that growth proceeds by island nucleation, island growth and density increase, followed by coalescence, channel formation and secondary nucleation. Addition of material to the islands occurs partly by the ‘step-flow’ mechanism. Grains in the completed films are considered to arise from individual nuclei. Nucleation and coalescence models are used to explain the correlation between increased substrate temperature and increased CdTe grain size in sublimation deposited CdTe films.     

Diamond/CdTe: a new inverted heterojunction CdTe thin film solar cell

Recently the concept of the inverted photovoltaic cell has become more attainable as a practical cell. This thin film cell consists of a p–n heterojunction in which the window layer is p-type and the absorber layer is n-type. The feasibility of a new inverted p–n heterojunction p-diamond/n-CdTe solar cell has been demonstrated. The non-optimized solar cell structure grown on semi-transparent p-diamond yielded an open circuit voltage of 0.23 V and a short circuit current of 1.54 mA/cm² when illuminated.     

Structural properties of CdTe thin films on different substrates

Cadmium telluride (CdTe) thin films were grown by thermal evaporation method on glass and thin metallic foils such as stainless steel (SS) and molybdenum (Mo). The as-deposited film structure was studied by X-ray diffractometer. Films coated on glass for lower thickness shows amorphous structure whereas films at higher thickness exhibit crystalline nature. The structure exhibits a mixture of both cubic and hexagonal phases for films on metallic substrates. The analysis shows that the preferential orientation and grain size depend on the type of substrate used and its surface smoothness. The lattice constant and the structural parameters viz., crystallite size (D), dislocation density (δ), and strain (ε) were calculated and the influence of the substrates was demonstrated in this paper.     

Physical properties of chemical bath deposited CdS thin films

Cadmium sulfide films of different thicknesses were deposited by chemical bath deposition (CBD) from a bath containing cadmium chloride, ammonium chloride, ammonium hydroxide and thiourea. The XRD patterns show that the films have a hexagonal phase with a preferential (0 0 2) orientation. The photoluminescence spectra show a defect structure, characteristics of the CdS films obtained by CBD. The electrical behavior in dark and under illumination, the optical properties and the band gap value reported in this work is in agreement with that reported in the literature.     

Structural, photoluminescent and electrical properties of CdTe films with different compositions fabricated by CMBD

CdTe films with different compositions (Cd-rich, Te-rich and stoichiometric) were fabricated by a novel and low cost chemical molecular beam deposition method (CMBD) in atmospheric pressure hydrogen flow. Cd and Te granules were used as precursors. The films were deposited on ceramic (SiO₂:Al₂O₃) substrates at 580 °C and 600 °C. The growth rate was varied in the range of 9-30 Å/s. The composition (Cd/Te) of the samples was changed by controlling the molecular beam intensity (MBI) ratio. Three samples fabricated at MBI ratios Cd/Te = 0.5, 1.0 and 1.16 were investigated by XRD, AFM, EDX, SEM, photoluminescence (PL) and Hall methods.     

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.     

Physical properties of Bi doped CdTe thin films grown by the CSVT method

A study of the physical properties of CdTe thin films doped with Bi is presented. CdTe:Bi thin films were deposited by the close space vapor transport (CSVT) technique using powdered CdTe:Bi crystals grown by the vertical Bridgman method. CdTe:Bi crystals were obtained with nominal Bi doping concentrations varying in the 1×10¹⁷–8×10¹⁸ cm⁻³ range. The physical properties of CdTe:Bi thin films were studied performing photoluminescence, X-ray, SEM, photoacoustic spectroscopy and resistivity measurements. We observed a decrease of the resistivity values of CdTe:Bi films with the Bi content as low as 6×10⁵ Ω-cm for Bi concentrations of 8×10¹⁸ cm⁻³. These are meaningful results for CdTe-based solar cells.     

Influence of CdCl2 treatment on structural and optical properties of vacuum evaporated CdTe thin films

In this article we have discussed the structural, optical properties of vacuum evaporated CdTe thin films before and after CdCl₂ treatment. The CdTe thin films were prepared by vacuum evaporation. Films were prepared under the vacuum of 10⁻⁶ Torr. The structural studies have been performed by the X-ray diffraction (XRD) technique. The XRD analysis of vacuum evaporated CdTe films reveals that the structure of films is polycrystalline in nature. However, the crystallinity has been improved after the CdCl₂ treatment as shown by an increase of the diffraction peak intensities. This is due to the enhancement in the atomic mobility of CdTe. The optical properties of the CdTe thin films have been studied by the spectrophotometer in the 300–800 nm wavelength range. It is observed that the optical band gap energy is highly dependent on CdCl₂ treatments. The optical transitions in these films are found to be direct and allowed.     

Electrical conduction properties of flash evaporated ZincPhthalocyanine (ZnPc) thin films

Solar energy as the principle source has become very attractive, as it is abundantly available, can be tapped locally, pollution free and commercially viable over a period. Solar energy may be converted to other forms of higher-grade energy through one of several methods such as photothermal, photochemical, photoelectrochemical and photovoltaic (PV). Among these, as noted above the cleanest and most direct and efficient and of converts to electrical power is with the help of PV or solar cell devices. Photovoltaics are one of the hottest areas of research today. Nowadays organic material finds greater importance in the PV cell fabrications. Among the available organic materials ZincPhthalocyanine is a promising candidate for the solar cell applications, because of its easy synthesization and non-toxic to the environment. The major part of incident light in the visible region effectively contributes to photocarrier generation and the excited ZnPc molecules play an important role in PV effect. In this paper, we have reported electrical transport properties of flash evaporated ZincPhthalocyanine thin films have been reported. DC conduction mechanism in these films (Al–ZnPc–Al structure) was studied at different temperatures. The field dependence behaviour on activation energy and possible conduction mechanism in the ZnPc films under DC field has been discussed.     

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.     

Life cycle assessment and energy pay-back time of advanced photovoltaic modules: CdTe and CIS compared to poly-Si

The paper is concerned with the results of a thorough energy and life cycle assessment (LIA) of CdTe and CIS photovoltaic modules. The analysis is based on actual production data, making it one of the very first of its kind to be presented to the scientific community, and therefore especially worthy of attention as a preliminary indication of the future environmental impact that the up-scaling of thin film module production may entail. The analysis is consistent with the recommendations provided by ISO norms 14040 and updates, and makes use of an in-house developed multi-method impact assessment method named SUMMA, which includes resource demand indicators, energy efficiency indicators, and “downstream” environmental impact indicators. A comparative framework is also provided, wherein electricity produced by thin film systems such as the ones under study is set up against electricity from poly-Si systems and the average European electricity mix. Results clearly show an overall very promising picture for thin film technologies, which are found to be characterised by favourable environmental impact indicators (with special reference to CdTe systems), in spite of their still comparatively lower efficiencies.     

The catalytic effect of Fe and Cr on hydrogen and deuterium absorption in Mg thin films

We examined the deuterium absorption and desorption of 55 nm thick Mg films alloyed with Fe and Cr using in-situ neutron reflectometry. The Mg alloy films were covered with bimetallic catalyst layers and could be fully absorbed at room temperature at a pressure of 8 mbar. The NR experiments revealed a deuterium gradient within the Mg alloy layers during absorption and a large deuterium uptake up to a D/M ratio of about 0.45 before the layer started to expand and form magnesium deuteride (MgD₂). Our NR data suggest that the catalytic effect of the Fe–Cr alloy is based on the avoidance of the formation of a blocking MgD₂ layer in the early stages of the absorption process leading to a fast hydrogen absorption kinetics.     

Processing effects on the structure of CdTe, CdS and SnO2 thin films

Thin films of CdTe and CdTe/CdS and SnO₂ used for heterojunction solar cells were deposited on glass substrates. The effects resulting from the processing with thermal heat and CdCl₂ treatments are investigated. The optical properties are determined by photoluminescence (PL) and transmission spectra. The compositional changes within the CdTe film structures are studied by 2 MeV ⁴He⁺ beam using the Rutherford backscattering (RBS) technique. The optical and the RBS data are then correlated to the evolution of high-efficiency solar cells.