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Synthesis of CdS and CdTe Through A Novel Solution Process for Application in Thin Film Solar Cells
Abstract
Cadmium sulfide (CdS) and cadmium telluride (CdTe) have been recognized as two of the most utilized cadmium (Cd)-based chalcogenide materials for thin-film solar cell applications that have begun to challenge the domination of the silicon-based photovoltaic market. However, there is lot of scopes to enhance the performance and reduce the manufacturing costs of CdS/CdTe based solar cells that at present involves very costly fabrication process. As an alternative solution-processing have been considered as a technique for low cost, large scale manufacturing of thin film high efficiency solar cells.
In this thesis, a novel solution processing of CdS and CdTe thin-films and CdS/CdTe based solar cells has been proposed. The entire works of synthesis, characterizations and their application in the solar cell have been carried out in three phases.
In the first phase, a solution of CdS ink has been successfully prepared from CdS powder in novel thiol-amine co-solvent with the help of Triton X-100 (TX-100) surfactant at room temperature. Thin-films of CdS have then been prepared using the CdS ink by spin coating method on to glass substrate followed by a thermal annealing process in a simple glass-protected air environment to avoid oxidation. Afterward, the effect of TX-100 surfactant on the spin-coated and vacuum-annealed CdS thin-films on glass substrate using the thiol-amine co-solvent has been explored in detail. Also, the effect of thickness of the spin-coated CdS thin-films has been investigated in the range from 100 to 300 nm.
In the second phase, the CdTe ink in the thiol-amine co-solvent has been successfully prepared from CdTe powder via TX-100 surfactant at 50 °C. The CdTe thin-films have been synthesized using the spin coating method too.
The microstructure, surface morphology, crystallography, optical, and electrical properties of CdS, and CdTe thin-films have been studied in details through a variety of thin-film characterization techniques including X-ray diffraction (XRD) analysis, energy-dispersive X-ray (EDX) analysis, scanning electron microscopy (SEM), Raman, and Fourier-transform infrared spectroscopy (FTIR), ultraviolet-visible (UV-vis) spectroscopy, Hall measurement etc.
Finally, in the third phase, simulation of the solar cell with CdS as a window layer and CdTe as the absorber layer has been performed. The simulation has been done using the one-dimensional solar cell capacitance simulator (SCAPS-1D) using all the experimentally obtained parameters of the synthesized CdS and CdTe thin films. The simulative power conversion efficiency (PCE) was found to be around 18.47% with short-circuit current density (JSC)of 25.79 mA/cm2, open-circuit voltage (VOC) of 870 mV, fill factor (FF) of 82.31% and quantum efficiency (QE) of 90.44%.
The findings of this work signify that CdS and CdTe thin-films synthesized using the novel thiol-amine co-solvent in a very simple and low-cost route might be potential candidates for solution-processed high-efficiency CdS/CdTe heterojunction thin-film solar cells.
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Cadmium sulfide (CdS) is considered as a superior buffer/window layer in thin-film solar cells apart from its various applications. The thickness of the buffer/window layer that defines the interface between the absorber layer is very important and needs to be thoroughly investigated for optimized device performance, particularly when a novel synthesis route and deposition technique is used. This manuscript presents the investigation on the various structural, morphological, and optical parameters and their correlation with the thickness of CdS thin film synthesized using a novel thiol-amine cosolvent with the help of Triton-X 100 surfactant. Spin coating technique is used to deposit CdS thin-films having thicknesses ranging from 100 to 300 nm on a glass substrate. X-ray diffraction (XRD) exposed the polycrystalline nature of the CdS films. All the diffraction peak intensities were found to increase with the thickness. Structural parameters like dislocation density, lattice strain, and crystallites per unit area calculated from the XRD data were found superior to those parameters for CdS films synthesized through various chemical routes, and the parameters indicated a positive correlation of the degree of crystallinity on the thickness. Similarly,surface morphology and optical properties of CdS films that were investigated by the scanning electron microscope (SEM) and UV–VIS spectrophotometer, were also dependent on film thickness. These results show that CdS thin film with superior crystallinity can be synthesized using thiol-amine cosolvent along with the Triton-X 100 surfactant and deposited by low-cost spin-coating on glass substrate. Moreover, the thickness variation appeared to be a single tool for obtaining application-specific various properties of CdS thin-film.
Among thin-film photovoltaic technology, cadmium telluride (CdTe) has achieved a truly impressive development that can commercially compete with silicon, which is still the king of the market. Solar cells made on a laboratory scale have reached efficiencies close to 22%, while modules made with fully automated in-line machines show efficiencies above 18%. This success represents the result of over 40 years of research, which led to effective and consolidated production processes. Based on a large literature survey on photovoltaics and on the results of research developed in our laboratories, we present the fabrication processes of both CdTe polycrystalline thin-film solar cells and photovoltaic modules. The most common substrates, the constituent layers, their interaction, the interfaces and the different “tricks” necessary to obtain highly efficient devices will be analyzed. A realistic industrial production process will be analytically described. Moreover, environmental aspects, end-of-life recycling and the life cycle assessment of CdTe-based modules will be deepened and discussed.
The CdTe ink in thiol-amine cosolvents from CdTe powder has been successfully prepared via Triton X-100 surfactant at 50 °C. The CdTe thin films have been synthesized and characterized from the prepared CdTe ink using spin coating method. The SEM micrographs reveal that synthesized films are uniform, smooth and homogeneous with less cracks, voids and pinholes compared to the films deposited from the inks without surfactant. The XRD spectra confirm the mixed hexagonal and cubic structure of the deposited films. The Raman peaks at 142 and 122 cm⁻¹ indicate the CdTe and the pure Te phase, respectively. The FTIR peaks at 1638 and 1384 cm⁻¹ due to C=O and COO- groups originated for the stretching vibration of the ligands of the thiol-capped CdTe, and the absence of vibrational peaks of S–H in the range of 2550–2650 cm⁻¹ confirms the thiol-capped CdTe, respectively. The EDX results reveal the existence of Cd and Te elements in CdTe films. The deposited films exhibit the highest absorbance (~85%) with a band gap of 1.5 eV. Hall studies and electrical analysis indicate the high quality of the CdTe films. Therefore, it can be concluded that the developed surfactant-mediated CdTe ink is highly promising for solution-processed CdTe-based photovoltaics.
In this article, the effect of Triton X-100 surfactant on the spin coated and vacuum annealed CdS thin films on glass sub-strates using thiol-amine cosolvents have been explored in details. The SEM study reveals that the surfactant-assisted CdS films are uniform, homogeneous and smooth having less cracks, voids, pinholes compared to CdS films synthesized
without surfactant. The X-ray diffraction spectra indicate the hexagonal structure of the surfactant-added films with (002) and (100) planes having relatively high intensity, whereas a weak hexagonal (002) peak appears for the films deposited without surfactant. Two prominent Raman peaks of CdS thin films are observed at wavenumber 207 and 212 cm−1 due to the multiphonon vibration of hexagonal CdS. Relatively strong and broad FTIR peaks at 1647 and 670 cm−1 were observed due to the Cd-S bending and stretching vibration in the surfactant-mediated films. Surfactant-induced CdS films are nearly stoichiometric as manifested by the energy dispersive X-ray report. Finally, higher optical transmission and band gap have been achieved for the CdS thin films deposited with Triton X-100 surfactant. This study reveals that
surfactant-mediated CdS thin films with thiol-amine cosolvents can be exploited effectively in higher effciency thin film solar cell technologies with low-cost.
Cadmium sulfide (CdS) buffer layer that decouples the absorber layer and window layer in thin-film solar cells was synthesized by two different chemical bath deposition (CBD) techniques with varying deposition parameters. X-ray diffraction (XRD) revealed that the CdS thin film crystallizes in a stable hexagonal wurtzite structure having a preferential orientation along (002) reflection plane with a crystallite size varying from 20 to 40 nm. First longitudinal optical phonon mode was identified at Raman shift of 305 cm⁻¹. Uniform, granular, continuous, and smooth surface with an average grain sizes (< 100 nm) as well as small roughness (< 9 nm) was observed by scanning electron microscopy (SEM) and atomic force microscopy (AFM), respectively. The symmetric composition of cadmium and sulfur along with larger grains (20 nm) was observed at higher deposition temperatures and times. The optical band gap of CdS samples obtained from process one was in the range of 2.3–2.35 eV, while the band gap by the second CBD process lay in between 2.49 and 2.65 eV, showing the most stable compound of CdS. The presence of a green emission band in photoluminescence spectra (PL) demonstrated that the CdS material has better crystallinity with minimum defect density. Hall effect studies revealed the n-type conductivity of CdS thin films with a carrier concentration values in the order of 10¹⁶ cm⁻³. Furthermore, CdS thin films fabricated by CBD process exposed better quality that might be more suitable material as a buffer layer for thin-film solar cells.
We demonstrate the direct synthesis of CdS thin films by spin coating method with thiol-amine co-solvents system. Anneal-ing of the films at various temperatures has been performed in the air using simple glass protector. The XRD patterns show a strong peak along (110) plane related to cubic lattice while two weak peaks at (002) and (100) planes indicate the hexagonal symmetry for the CdS thin films. The Raman peak at 305 cm −1 also confirms the formation of crystalline CdS thin films. The FTIR study also reveals the formation of CdS thin films. The SEM images reveal the surface uniformity and homogeneity of the CdS thin films. The EDX results indicate nearly stoichiometric CdS thin films. The optical band gap of CdS thin films is ~ 2.4 eV when coated at 2000 rpm and annealed at 300 °C for 5 min. These findings indicate that synthesized CdS films are potential candidates for solution-processed CdTe/CdS solar cells.
Ultrasonically cleaned glass slides are used as substrates for receiving the different thickness of Zinc selenide (ZnSe) films. The deposition processes of our investigated films were done at room temperature using physical thermal evaporation mechanism under vacuum ≈2 × 10 ⁵ mbar. We investigated the optical and structural parameters of ZnSe thin films in correlation with film thickness (200–650 nm). Various techniques such as UV–vis-NIR spectrophotometer, x-ray diffraction lines and field emission scanning electron microscope were used to investigate aforementioned parameters. Structural analysis indicate that the films exhibited cubic preferred orientation along the plane (111) and the crystallinity and crystallite size of films increases linearly with film thickness. The optical band gap ranges from 2.69 to 2.81 eV and it is founded that it increases with film thickness. According to the applied Swanepoel’s approach, it is possible to estimate the optical parameters and average thickness of the ZnSe thin films of different thicknesses with higher accuracy.
Conventional copper indium gallium diselenide (CIGS)-based solar cells offer higher efficiency than other second-generation technologies such as hydrogenated amorphous silicon (a-Si:H)- or cadmium telluride (CdTe)-based solar cells, but higher manufacturing cost due to the use of the rare metals indium and gallium. The purpose of the work presented herein is to improve the efficiency of such devices by using cheaper materials. Accordingly, a back-surface field layer made of low-cost and widely available barium silicide (BaSi2) with a thickness of 0.3 µm is introduced for the first time into the basic CIGS solar cell structure consisting of Al/ZnO/CdS/CIGS/Mo, resulting in the alternative structure of Al/FTO/CdS/CIGS/BaSi2/Mo, with fluorine-doped tin oxide (FTO) as the window layer. One-dimensional simulations of the solar cell capacitance are employed to study the photovoltaic parameters such as the power conversion efficiency, short-circuit current density, open-circuit voltage, fill factor, and quantum efficiency of the devices. The thickness of the CIGS absorber layer is varied from 0.1 to 3 µm to optimize the device. Besides, the effects of the acceptor ion and bulk defect densities in the CIGS absorber layer, cell resistances, and operating temperature on the overall performance are also investigated. The proposed structure offers an efficiency of 26.24% with a thin CIGS layer of only 0.8 µm. In addition to reduced CIGS thickness and cost, the presented approach results in CIGS solar cells with enhanced performance compared with previously reported conventional designs.
CdS is an important II-VI semiconductor material for devices with possible applications such as solar cells and many optoelectronic devices. The wide band gap of CdS has made it promising and it has widely been used in modern technology and industry. It has versatile use in electronics, optoelectronics, microelectronics, catalysis, coating and energy generation. In this study, various optical properties of CdS have been investigated such as refractive index, extinction coefficient, optical band gap, Urbach energy, steepness parameter, skin depth, optical density, and electron-phonon interaction and a simulation program was developed. Famous computing language "MATLAB" was chosen as the coding language as it is highly capable of generating ideal graph. The above stated optical properties were studied as a function of wavelength in the range of 300nm-1100nm. The dielectric properties of the CdS thin film have also been studied. The imaginary dielectric constant was increased with the increase of incident photon wavelength as unexpected for semiconducting materials. The theory used for determination of refractive index and extinction coefficient was Sellmeier model. This study concludes that Sellmeier model is not best suited for CdS thin film. It is effective for wider band gap materials.
In this article, semiconducting Barium Silicide (BaSi2) absorber based Al/SnO2:F/CdS/BaSi2:B/Cu novel heterostructure thin-film solar cell (TFSC) has been studied in details. The solar cell has been numerically simulated and intensely analyzed by Solar cell Capacitance Simulator (SCAPS). Layer thickness was varied from 100-3000 nm for p+-BaSi2 absorber, 20-200 nm for both n-CdS buffer, and n+-SnO2:F window layers to optimize the device. Hitherwards, the impurities concentration for acceptor (NA) and donor (ND) ions was optimized for each layer through ample variation. The influence of single-donor and acceptor type bulk defect densities has been investigated thoroughly in p-BaSi2 and n-CdS materials, respectively. An efficiency >30% is achievable ideally with a 2 µm thick BaSi2 absorber without incorporating defects whereas it reduces to 26.32% with only 1.2 µm thick absorber including certain amount of defects. Cell thermal stability and alteration of cell parameters were studied under cell operating temperature from 273°K to 473°K. Finally, the effect of series (Rs) and shunt (Rsh) resistances on proposed cell has been investigated meticulously. This newly designed solar cell structure proclaims the chance of fabricating a resourceful, low cost, and highly efficient TFSC near future.
CdTe-based solar technology has achieved one of the lowest levelized costs of electricity among all energy sources as well as state-of-the-art field stability. Yet, there is still ample headroom to improve. For decades, mainstream technology has combined fast CdTe deposition with a CdCl2 anneal and Cu doping. The resulting defect chemistry is strongly compensated and limits the useful hole density to ~10¹⁴ cm⁻³, creating a ceiling for fill factor, photovoltage and efficiency. In addition, Cu easily changes energy states and diffuses spatially, creating a risk of instabilities that must be managed with care. Here, we demonstrate a significant shift by doping polycrystalline CdSexTe1 − x and CdTe films with As while removing Cu entirely from the solar cell. The absorber majority-carrier density is increased by orders of magnitude to 10¹⁶–10¹⁷ cm⁻³ without compromising the lifetime, and is coupled with a high photocurrent greater than 30 mA cm⁻². We demonstrate pathways for fast dopant incorporation in polycrystalline thin films, improved stability and 20.8% solar cell efficiency.
In this article, simulation results of novel and facilitated heterostructures of the Second Generation (2G) Thin-film Solar Cells (TFSCs): hydrogenated amorphous Silicon (a-Si:H), Cadmium Telluride (CdTe), and Copper Indium Gallium di-Selenide (Cu(In,Ga)Se2 or CIGS) have been presented to compare their performances. The solar cells have been modeled and analyzed for investigating optimized structure with higher stabilized efficiency. Entire simulations have been accomplished using Analysis of Microelectronic and Photonic Structures – 1 Dimensional (AMPS-1D) device simulator. The thickness of the absorber layer was varied from 50 nm to 1400 nm for a-Si:H and from 50 nm to 3 μm for both CdTe and CIGS cells to realize its impact on cell performance. The utmost efficiency, η of 9.134%, 20.776%, and 23.03% were achieved at AM 1.5 (1000 W/m²) for a-Si:H, CdTe, and CIGS material cells, respectively. Lastly, the operating temperature of the three cells was varied from 280°K to 328°K to realize its effect on the cell PV performances.
NiFe nanocrystalline films were formed onto Au film via pulsed electrolyte deposition with variable time of relaxation between the sequential pulses. Analysis of anomalous non-liner changes in the NiFe film structure showed the implementation of three mechanisms, resulting in “layer-by-layer”, “layer-plus-island”, or “island” growth. The change of the interpulse relaxation time provides the possibility to realize all three different mechanisms. The grown NiFe films may have different morphology. The ultrathin NiFe films have nanosized grains and exhibit high uniformity of structure, the films may be also combined of less that 10 nm nanoscale grains and their conglomerates, typically, about 50 nm in size, and, additionally, the film structure, consisting of isolated magnetic islands can be realized. The phenomenological explanation of the different mechanisms was obtained using atomic-force-microscopy (AFM) approach. It has been demonstrated that the growth mechanism can be controlled by nanocrystallites conglomeration, which is accelerated with increasing the relaxation time. The origin of the conglomeration process is mainly associated with high surface energy of nanosized grains.
Cadmium sulphide (CdS) thin-films have been electrodeposited using two electrode system to be used as the hole back diffusion barrier (hbdb) layer for graded bandgap solar cells with p-type windows. Cadmium acetate dihydrate [Cd(CH3COO)2·2H2O] and ammonium thiosulphate [(NH4)2S2O3] have been used as the cadmium (Cd) and sulphur (S) precursors respectively. In this work, CdS layers have been grown on glass/FTO (fluorine doped tin oxide) substrates at cathodic potentials ranging from 1300 to 1460 mV in order to find the best growth voltage. N-type conductivity is observed for all the layers and band-gap ranged between ~ 2.36 and ~ 2.40 eV for as-deposited layers and ~ 2.31 and ~ 2.36 eV for air-annealed layers. X-ray diffraction (XRD) analysis revealed cubic/hexagonal mixed crystallinity for the as-grown layers which indicates a tendency of transiting towards hexagonal structure upon annealing. Compositional and morphological characteristics of the layers have been investigated with energy dispersive X-ray (EDX) and scanning electron microscopy (SEM) respectively.
Abstract— Simulation results of a novel and simplified heterostructure of CdTe and CdS as a thin-film solar cell have been presented in this article. All the simulations have been done using Analysis of Microelectronic and Photonic Structures – 1 Dimensional (AMPS-1D) simulator. The thickness of the absorber and window layers was varied from 500nm to 3μm and 30nm to 100nm respectively to realize its effect on cell performance. The highest light conversion efficiency, η of ~25% with short circuit current density, Jsc of 25.88mA/cm2, open circuit voltage, Voc of 1.077V and fill factor, FF of 0.888 (88.8%) were obtained with 1μm (1000nm) p-type CdTe absorber layer and 0.04μm (40nm) n-type CdS window layer. Lastly cell performance was investigated by varying cell temperature from 280°K (7°C) to 333°K (60°C).
Polycrystalline CdS thin films comprising of hexagonal crystal structure are successfully fabricated by pulse electrodeposition using
a two-electrode system at room temperature. The method utilizes tartaric acid as a complexing agent for the first time that minimizes
the formation of colloidal sulfur while optimizing the deposition potential and avoids conventional post-deposition heat-treatment
step. Additionally, deposition time is varied to obtain the desired thickness of CdS films suitable for application in thin films solar
cells. Pulse electrodeposited CdS films are comprehensively characterized using SEM, XRD andmicro-Raman analyses unveiling the
stoichiometric polycrystalline hexagonal CdS films. The bandgap is determined to be around 2.4 eV from optical studies. Flat-band
potential and carrier density are inferred to be −0.82 V vs SCE and 8.51 × 1018 cm−3, respectively, from the Mott-Schottky analysis.
Photoelectrochemical studies show a photocurrent density of about 0.2 mA/cm2 at 0.2 V vs SCE for the optimized films, which is
competent with previously reported values. These pulse electrodeposited CdS films prepared without any high-temperature treatment are of high quality with desirable properties and are suitable for application in thin films solar cells and photoelectrochemical cells.
© 2018 The Electrochemical Society. [DOI: 10.1149/2.0261808jss]
In this work, we have developed one-pot aqueous synthesis of glutathione (GSH) binding CdTe quantum dots (QDs) for cell imaging. UV-Vis absorption spectrum, Fourier transform infrared spectrum, photoluminescence spectrum, and high-resolution transmission electron microscopy are applied to characterize the physical and chemical properties of the nanocomposites. The bioimaging efficiency of the GSH-capped CdTe QDs is further evaluated on Hela cells. The groups on the surface of QDs are able to bind to basic proteins, which are abundant in cell nuclei, enabling the application of QDs for direct cell imaging. Experimental results also indicate the GSH layer on the surface of QDs is able to reduce the cytotoxicity significantly. In conclusion, the as-prepared GSH-capped QDs are highly promising fluorescent probes for cell imaging in the near future.
Mercaptopropionic Acid (MPA) capped CdTe QDs with crystallite size is 0.8 nm were successfully synthesized in aqueous medium by reflux method. The HRSEM demonstrates the spherical shape with varying sizes in the nanometer scales. From the electrochemical studies, the prepared QDs improve the electron transfer between electrode and H2O2. The sensor exhibits a linear range from 0.67-8.04 μM with a sensitivity of 0.2833 mA mM-1cm-2with a linear coefficient of 0.9877. The sensor shows detection limit of 6.7 μM with a rapid amperometric response time of 5s and possesses a good reproducibility. It shows the selectivity toward H2O2 and interference free phenomenon for other electro active species like oxalic acid and ascorbic acid. MPA capped CdTe QDs sensor was used in rat and human serum samples and cell labeling.
Manganese selenide (MnSe) crystalline thin film was produced with chemical bath deposition on substrates (commercial glass). Transmittance, absorption, optical band gap and refractive index were investigated by UV/VIS spectrum. The hexagonal form was observed in the structural properties in XRD. The structural and optical properties of the MnSe thin films were analyzed at different pHs. SEM and EDX analysis were used for the surface analysis in the films. The films had the best crystalline at pH: 9. At pHs of 11 and 10 the MnSeO4 structure was observed. The films with the lowest film thickness were found in baths prepared with pH: 11. The highest refractive index was observed in films produced with a pH of 10 at a film density of 1.96. The grain size of MnSe thin films has a higher value at pH: 9.
Electrodeposition (ED) has been recognized as a low cost and scalable technique available for fabrication of CdS/CdTe solar cells. Photovoltaic activity of these electrodeposited semiconductor materials drastically depends on the ED growth parameters namely; electrodeposition potential, concentrations and ratios of concentrations of precursors used to prepare the bath electrolyte, pH of the electrolyte, deposition temperature and rate of stirring of the electrolyte. In order to grow thin films with good photovoltaic properties, it is essential to maintain these variables at their optimum ranges of values during electrodepositions. Hence, this study was conducted to investigate the dependence of the properties of electrodeposited CdTe thin film material on the rate of stirring of the bath electrolyte. The CdTe material was grown on glass/FTO (2 × 3 cm²) and glass/FTO/CdS (2 × 3 cm²) surfaces in bath electrolytes containing 1.0 mol/L CdSO4 and 1.0 mmol/L TeO2 solutions at different rates of stirring within the range of 0–350 rpm while keeping the values of pH of the electrolyte, deposition temperature and cathodic deposition potential with respect to the saturated calomel electrode at 2.3, 65 °C and 650 mV respectively. After the heat treatment at 400 °C in air atmosphere, the deposited samples with a good visual appearance were selected and evaluated based on their morphological, elemental, structural, optical and electrical properties in order to identify the optimum range of rate of stirring for electrodeposition of CdTe thin film semiconductors. Results revealed that, rates of stirring in the range of 60–85 rpm in a 100 mL volume of electrolyte containing the substrate and the counter electrodes in the center of the bath with a separation of 2.0 cm between them can electrodeposit CdTe layers exhibiting required levels of morphological, structural, optical and electrical properties on both glass/FTO and glass/FTO/CdS surfaces.
Thin films of CdTe were obtained by vapor phase condensation, namely by open vacuum evaporation, using different technological factors, in particular, different thickness (different time of deposition t) d = (540 - 2835) nm, deposition temperature Tv = 200°C and evaporator temperature Te (500 - 600)°C. The films were deposited on silicon substrates. The morphology of thin film condensates is determined on the basis of ASM and SEM studies analysis. Were received dependences of avarage roughness and root mean square roughness from the material of substrate and film thickness. It was stablished that the growth of surface nanostructures is determined by Strankі-Krastanov mechanism.
Deposition of cadmium telluride (CdTe) from cadmium chloride (CdCl2) and tellurium oxide has been achieved by electroplating technique using two-electrode configuration. Cyclic voltammetry shows that near-stoichiometric CdTe is achievable between 1330 and 1400 mV deposition voltage range. The layers grown were characterised using X-ray diffraction (XRD), UV–Visible spectrophotometry, scanning electron microscopy (SEM), energy-dispersive X-ray analysis (EDX), photoelectrochemical (PEC) cell and DC conductivity measurements. The XRD shows that the electrodeposited CdTe layer is polycrystalline in nature. The UV–Visible spectrophotometry shows that the bandgap of both as-deposited and heat-treated CdTe films are in the range of (1.44–1.46) eV. The SEM shows grain growth after CdCl2 treatment, while, the EDX shows the effect of growth voltage on the atomic composition of CdTe layers. The PEC results show that both p- and n-type CdTe can be electrodeposited and the DC conductivity reveals that the h
The stabilized pure CdSnanoparticles were synthesized using chemical precipitation method in aqueous medium using cadmium acetate and sodium sulphide with (CTAB) as a capping agent and their effect on surface morphology of nanoparticles. The obtained particles were characterized using X-ray diffraction studies (XRD and FTIR. The X-ray diffraction pattern revealed that the synthesized cadmium sulphide nanoparticles were polycrystalline nature. The temperature and frequency dependence of dielectric constant, dielectric loss and AC conductivities were studied over a range of the frequency 75Hz to 6MHz.
Synthesis of CdS thin films were carried out onto glass substrates by chemical bath deposition (CBD) method using CdCl2asCd 2+ and thiourea as S 2-ion source with ammonia as a complexing agent. Influence of bath temperature on structural, morphology and optical properties has been systematically and carefully investigated. XRD analysis revealed that the synthesized CdS films are nanocrystalline having hexagonal structure with (002) preferential orientation. Estimated crystallite size was found in the range 16-32 nm. The UV-Visible spectroscopy analysis showed that the films have high transmission (> 70 %) in visible and NIR region of solar spectrum. Optical band gap was found > 2.3 eV over the entire range of bath temperature studied. The Raman spectra for the CdS films deposited at various bath temperatures shows a continuous shift of 1 LO phonon peak towards higher frequency which suggest the improvement of structural order with increase in bath temperature. The increase in the intensity ratio, 2LO/1LO with bath temperature indicates enhancement of crystallinity of CdS films with increase in bath temperature. The SEM analysis showed that CdS films deposited at various bath temperatures are smooth, homogeneous, and nearly uniform with randomly oriented spherical nanocrystallites.
In this work, we have studied the influence of RF power on structural and optical properties of CdTe thin films deposited by indigenously designed locally fabricated RF magnetron sputtering. Films were analyzed by using variety of techniques such as low angle X- ray diffraction, UV-Visible spectroscopy, Raman spectroscopy etc. to study its structural and optical properties. Low angle XRD analysis showed that CdTe films are polycrystalline and has cubic structure with preferred orientation in (111) direction. Raman scattering studies revealed the presence of CdTephase over the entire range of RF power studied. The UV-Visible spectroscopy analysis showed that the band gap decreases with increase in RF power. However, CdTe films deposited at higher RF power has optimum band gap values (1.44-1.60eV). Such optimum band gap CdTe can be use as absorber material in CdS/CdTe and ZnO/CdTe solar cells.
This study focuses on aiming higher stabilized sunlight to electricity conversion efficiency for Cadmium Telluride (CdTe) absorber based solar cell. Common CdTe solar cell with Au back or Zinc Telluride (ZnTe) back surface field (BSF) with Cu back metal has voltages <1 V. The reason behind this low voltage has been investigated in details. In this report, simulation yielding of MgF2/glass/SnO2:Sb/CdS/CdTe/M (M = Cu, Ni, Au, Ir, Pd, and Pt) thin-film solar cell (TFSC) has been presented. The solar cell has been modelled and analyzed using Analysis of Microelectronic and Photonic Structures – 1 Dimensional (AMPS-1D) device simulator for various back metals with work function, Φm ≥ 5 eV. The thickness was varied from 100-1800 nm, and from 15-100 nm for the active p-CdTe absorber, and n-Cadmium Sulphide (CdS) window layers, respectively, to optimize the device depth. Meanwhile, the donor and acceptor concentration was varied to outline their effects on photovoltaic (PV) parameters. The utmost efficiency (η) of ~30% along with open circuit voltage (Voc) of 1.194 V, short circuit current density (Jsc) of 28.6 mA/cm^2, and fill factor (FF) of 89.3% has been achieved ideally with Platinum (Pt) back contact at (111) plane under standard test conditions (STC) of 298°K and 1 Sun illumination (1000 W/m^2) at air mass (AM) 1.5. Lastly, the cell operating temperature was varied from 273°K to 373°K to verify the thermal stability of the proposed structure. Present work provides a guideline to select back metal in CdTe solar cell. Our proposed solar cell offers the prospect of being a highly efficient cost effective second generation (2G) TFSC.
This article presents the role of Vanadium Pentoxide (V2O5) as electron blocking-hole transport layer (EB-HTL) in Cadmium Sulfide (CdS) and Cadmium Telluride (CdTe) based heterojunction solar cells using simulation employing experimental data. Noble solvents Thiolamine and methanol with diethaylamine were used to synthesis of CdS and V2O5 respectively using sol-gel method. At first, the optical and electrical properties of CdS, V2O5 spin coated thin films deposited on glass substrate were investigated experimentally. Then, these experimental data of optical and electrical properties were used to simulate FTO/CdS/CdTe/Al and FTO/CdS/CdTe/V2O5/Al heterojunction solar cells. The simulation was carried out using SCAPS-1D simulator, where data of CdTe absorber layer were taken from reported work. Simulations were performed at a range of concentration, thickness, defect density and temperature to conjecture the better cell efficiency and efficacy of V2O5 as EB-HTL. At carrier concentration of 1×1017 cm-3, 1×1016 cm-3 and 1×1018 cm-3 for CdS, CdTe and V2O5 respectively, the optimum thickness was found to be 0.100 μm, 1.00 μm and 0.100 μm for the corresponding material layers respectively with interface defect density (donor and acceptor) of 2.5×1017 cm-3 at a junction temperature of 300 ºC (573 K). Simulation results reveal that, the designed FTO/CdS/CdTe/V2O5/Al heterojunction solar cell shows the highest efficiency of 23.50% with enhanced open circuit voltage of 0.708 V, current density of 44.41 mA/cm2, fill factor of 75 % compared to those of FTO/CdS/CdTe/Al heterostructure (without EB-HTL) counterparts. Therefore, it can be envisage that, solution processed V2O5 as EB-HTL is a potential and flair material for CdS/CdTe heterojunction solar cells.
Consolidated tables showing an extensive listing of the highest independently confirmed efficiencies for solar cells and modules are presented. Guidelines for inclusion of results into these tables are outlined, and new entries since January 2019 are reviewed.
The effects of temperature and liquid-phase iodination on the electrical property of spin coated CuI thin films have been investigated in details. The XRD study indicates that CuI thin films are polycrystalline in nature and I-doping enhances the crystal quality and size of the films. The SEM images show that the surface uniformity of the CuI thin films increases due to I-doping. The doping of iodine increases the conductivity as well as carrier concentration and mobility of the films as confirmed by Hall study. The temperature dependent resistivity of CuI film shows a sharp fall of resistivity at ~80 °C for un-doped films whereas this behavior disappears for I-doped films. The optical transmittance and band gap of the I-doped films also increases indicating high degeneracy of the films. These findings imply that I-doped spin coated CuI thin films are potential candidates for the solution-processed CuI/n-Si solar cells.
We have carried out a detailed study of the morphological, structural, optical and magnetic properties of Cr doped TiO2 nanocrystals with doping concentrations varying from 3 to 12 atomic weight%. The results obtained from transmission electron microscopy analysis, size-strain plots of all the Cr-doped samples and crystallite size estimation reveal the particle size of the prepared nanocrystals to be well below 10 nm, which is observed to exhibit a decreasing trend with an increase in the Cr dopant concentration. All the samples crystallize in the anatase tetragonal phase of TiO2, which is confirmed from the Rietveld refinement of the X-ray diffraction patterns and the different modes present in the Raman spectra. The Eg(1) mode shows a clear red shift and broadening with increase in the Cr concentration, which indicates the replacement of Ti ions with Cr ions in the TiO2 lattice. The possibility of the presence of different functional groups present is verified by Fourier transform infra-red spectroscopy. The presence of Cr3+ and Ti4+ is confirmed from the X-ray photoelectron spectroscopy (XPS) results suggesting the formation of oxygen vacancies to compensate for the charge neutrality. The XPS results validate the Cr3+ existence in the Cr:TiO2 system and corroborate with a slight peak shift towards lower diffraction angle and further confirm the substitutional doping in the present case. Enhanced visible range optical absorption and a clear red shift associated with the absorption edge also suggest the incorporation of Cr3+ ions into the host system. The estimated band-gap of Cr-doped TiO2 nanocrystals reveals a decreasing trend with increasing Cr concentration. The Urbach energy associated with all the Cr-doped samples signifies the presence of oxygen vacancy related defects in the present system, which is further verified using photoluminescence (PL) spectra, and the deconvolution of the PL spectra provides an insight into the oxygen vacancy defects associated with the system. Paramagnetic (PM) behaviour is observed with an increase in the PM moment, suggesting the increase in isolated Cr ions with increase in the Cr concentration, which is further explained using a bound magnetic polaron (BMP) model. Isolated BMP formation could be the reason for the observed PM behavior of the present system, where the trapping of 3d electrons associated with Cr3+ in the vacancy sites could ultimately lead to fewer overlapped BMPs, yielding a net PM moment. The present Cr:TiO2 system could be modified with tailored optical and magnetic properties for functional applications such as magneto-optics and optoelectronic devices.
Ion flotation is an efficient method to remove or recovery of heavy metals, rare and precious metal ions, and organic pollutants from aqueous solution. The added surfactants served as collectors and frother mainly dominate the indexes of ion flotation. Therefore, the review summarized the surfactants (chemical synthetic surfactants, biosurfactants, and nanoparticle surfactants) used in ion flotation. The advantage, disadvantage, and outlook of the three types of surfactants were also discussed.
Colloidal quantum dots have garnered significant interest in optoelectronics, particularly in quantum dot solar cells (QDSCs). Here we report QDSCs fabricated using a ligand that is modified, following film formation, such that it becomes an efficient hole transport layer. The ligand, O-((9H-fluoren-9-yl)methyl) S-(2-mercaptoethyl) carbonothioate (FMT), contains the surface ligand 1,2-ethanedithiol (EDT) protected at one end using fluorenylmethyloxycarbonyl (Fmoc). The strategy enables deprotection following colloidal deposition, producing films containing quantum dots whose surfaces are more thoroughly covered with the remaining EDT molecules. To compare fabrication methods, we deposited CQDs onto the active layer: in one case, the traditional EDT-PbS/EDT-PbS is used, while in the other EDT-PbS/FMT-PbS is used. The devices based on the new EDT/FMT match the PCE values of EDT/EDT controls, and maintain a higher PCE over an 18 day storage interval, a finding we attribute to an increased thiol coverage using the FMT protocol.
CdS thin films (CBD-CdS) and Triton X-100 (TX-100)-treated CdS thin films (TX100-CdS) were successfully deposited on fluorine-doped tin oxide (FTO) conducting substrates using the chemical bath deposition (CBD) method at bath temperatures of 40°C, 60°C and 80°C. The two types of films were characterized for their structural, optical, morphological and electrical properties. The introduction of TX-100 was found to effectively improve the structural, optical, morphological and electrical properties of the CdS thin films at all temperatures studied and thereby the photo-activity.
Earth–abundant and nontoxic kesterite compounds such as Cu2ZnSnS4 (CZTS), Cu2ZnSnSe4 (CZTSe) and Cu2ZnSn(S,Se)4 (CZTSSe) have made tremendous progress and become one of the hottest topics in optoelectronic materials research for power generation by capturing solar energy. These compounds can be considered a possible alternative to conventional, toxic and expensive absorber layers such as crystalline silicon (c–Si), Cu(In,Ga)Se2 (CIGS) and CdTe. The p–type semiconductor kesterites (CZTS, CZTSe CZTSSe) are most promising light–absorber for solar cell application and reaching record efficiency upto 12.7 % due to its unique optical properties such as variable band gap energy ranging from 1.0 eV (Cu2ZnSnSe4) to 1.5 eV (Cu2ZnSnS4), large optical absorption coefficient 104 cm–1 making kesterite a feasible option for photovoltaic technologies scalable to terawatt. Additionally, kesterite absorbers do not contain any rare metals or toxic materials; one can expect the low-cost solar cells in the near future. This review aims to present an overview of the most recent research activity on the state–of–art kesterite-based absorbers with a brief introduction to kesterite, deposition methods and improved device performance by doping of alkali, germanium and other metal elements either during the kesterite synthesis or afterwards the growth as a post–deposition treatment. As the key component for improving power conversion efficacy of kesterite absorber, doping of alkali, germanium amd other metal elements have attracted great interest and is vital to deliver the promising efficiency. Also, we will summarize what is known about doping, their impact on structural, optical and electrical properties and solar cell performance of kesterite compound. Finally, an effort to summarize important achievements that have been made in this field so far and further directions for improvement of solar cell efficiency of kesterite–based absorber have been focused.
This study presents CdTe/CdS:Mn core–shell nanocrystals (NCs) prepared in aqueous media, using thioglycolic acid as capping agent. Firstly, CdTe NCs were synthesized, and then, Mn-doped CdS shell were deposited on the top of the CdTe core under Ar atmosphere. The NCs were structurally and optically characterized by transmission electron microscopy (TEM), X-ray diffraction (XRD), energy-dispersive X-ray (EDX), X-ray photoelectron (XPS), photoluminescence (PL), and Fourier transform infrared (FTIR) spectroscopy. The results obtained from XRD, XPS, PL, and EDX showed that the Mn ions were successfully introduced into the nanocrystalline shells. Moreover, the effect of Mn concentrations on the optical properties of the synthesized core-shell NCs was investigated. The effective band gap of the sample is in an indirect relationship with the Mn: Cd molar ratio, confirmed by PL analysis, and PL emissions can be measured at various wavelengths. The PL spectra showed 990 and 170% enhancement in the emission intensity of CdTe/CdS:Mn core/shell NCs (Mn: Cd 2.5%) compared to CdTe NCs and CdTe/CdS core/shell NCs, respectively. Consequently, the introduction of Mn dopants into the core-shell structures not only diminishes the density of quenching centers, but also reduces the effective band gap energies.
In this study, CrSe thin films were produced by using a new variation of chemical bath deposition (CBD) on glass substrates at different pH values, namely pH: 8, 9, 10 and 11. Some optical properties of these films including T% (transmittance), R% (reflectance), n (refractive index), k (extinction coefficient) and ϵ 1 and ϵ 2 (real and imaginary parts of dielectric constants) for pH: 8, 9, 10 and 11 were found as (82.89, 80.00, 78.54, 79.05%), (3.76, 5.32, 4.08, 5.44), (1.48, 1.59, 1.48, 1.60), (0.055, 0.079, 0.127, 0.116) and (1.33, 1.47, 1.21,1.52) and (0.87, 0.33, 0.12, 0.14) at 550 nm wavelength. Surface morphologies of thin films were examined by SEM. Film thicknesses were measured to be 34, 44, 50 and 104 nm by AFM, and the optical band gap (Egap) were calculated as 3.14; 3.72; 3.71 and 3.74 eV for pH: 8, 9, 10 and 11, respectively. Optical conductivity variation of CrSe thin films were calculated as a function of frequency for pH: 8, 9, 10 and 11, respectively. The usage areas of these films in optoelectronic applications were discussed.
BiSe thin films have been grown on substrates as PMMA, ITO, glass and Si wafer by using chemical bath deposition (CBD) method. Deposition temperature and time and pH are kept to be constant during the production of the thin films. The thickness of BiSe thin films, which are produced on ITO, glass, PMMA and Si wafer substrate are 513, 468, 1039 and 260 nm, respectively. According to GAXRD results, the films, which are grown on glass and PMMA substrate, have amorphous structure, but, the films, which are grown on ITO and Si wafer substrate, have peaks of Bi2Se3 crystal. Grain sizes, crystallization number per unit area and dislocation density for ITO and Si wafer substrate are calculated as 112.40 nm and 43.04 nm; 2.25 × 10⁻⁵ and 7.91 × 10⁻⁵ (1/nm²); 40.11 × 10⁻⁵ (1/nm²) and 53.96 × 10⁻⁵ (1/nm²), respectively. The contact angles and critical surface tension of distilled water, ethylene glycol, formamide and diiodamethane liquids for thin films grown on glass, ITO, PMMA and Si wafers were obtained by the Zisman method. The % transmittance and % reflectance values of thin films grown on glass, ITO, PMMA are calculated as % T: 79.90, 92.76 and 67.37; % R: 6.18, 2.07 and 10.59, respectively. Eg values of thin films grown on glass, ITO, PMMA are calculated as Eg = 1.92; 2.18; 1.60 eV. The extinction coefficients, refractive indexes and relative dielectric constants of thin films grown on glass, ITO, PMMA are calculated as k = 0.007; 0.002 and 0.012; n = 1.65; 1.34 and 1.96; ϵ 1 = 0.271; 0.083 and 0.528 respectively. Sheet resistance, hall mobility, sheet carrier densities, bulk carrier densities and conductivity types for glass, ITO, PMMA and Si are 6.52 × 10⁷, 6.65 × 10¹, 1.09 × 10⁸ and 6.45 × 10² (Ω/cm²); 2.38, 1.21 × 10⁻¹, 5.34 and 1.52 (cm²/V.s); 4.01 × 10¹⁰, 7.71 × 10¹⁷, 1.06 × 10¹⁰ and 6.34 × 10¹⁵ (cm⁻²); 4.58 × 10¹⁴, 1.50 × 10²², 1.02 × 10¹⁴ and 2.89 × 10²⁰ (cm⁻³); p, n, p and p, respectively. In addition, I-V characteristics and changes of magnetoresistance values versus magnetic field of the thin films are obtained by Van der Pauw method and HEMS.
In this study, tellurium oxide thin film (TOTF) was deposited on amorphous glass using chemical bath deposition method. Structural properties of the film were analyzed through X-ray diffraction. As a result of these analyses, average grain size was found to be 48 nm. Film thickness was measured using an atomic force microscopy at 1200 nm, whereas its optical properties were examined through UV–Vis spectroscopic technique. TOTF’s optical properties, which are already known, were compared with the related data in the literature, and in addition, unknown optical properties of TOTF were investigated. Transmittance of the film was found to be 45%, whereas its refractive index was found to be 2.63 at 550 nm wavelengths. The thin film of tellurium oxide is transparent, its optical band gap was graphically estimated to be 1.57 eV, and its surface energy was calculated to be 32.44 mN/m. Distilled water, which is used as one of the test liquids, showed that TOTF is hydrophobic. Structural properties of the films were found to be in line with the literature.
Present paper describes systematic studies of the functional BaFe12-xDIxO19 (DI = Al³⁺, In³⁺ 0.1 ≤ x ≤ 1.2) solid solutions. Correlation between the fine atomic structure and intrasublattice Fe³⁺ - O²⁻ - Fe³⁺ superexchange interactions were explained using precision neutron powder diffraction and Mossbauer spectroscopy. Critical influence of the diamagnetic Al³⁺ and In³⁺ ions on electromagnetic properties in substituted Ba-hexaferrites (M-type) was discussed. Transmission spectra demonstrated a deep minimum in the frequency range 20–65 GHz due to natural ferromagnetic resonance (NFMR). Increase of the Al³⁺ ions concentration shifted the resonance frequency from 51 GHz to 61 GHz. Increase of the In³⁺ ions shifted the resonance frequency from 50.5 GHz to 27 GHz. Data of the numerical calculations and experimental results correlates well. It has been demonstrated that external magnetic fields critically influence on electromagnetic properties due to increase of magnetic anisotropy. This opens broad perspectives for practical applications.
Cadmium sulphide (CdS) is the n-type, wide band gap II-VI semiconducting material by offering applications in photovoltaics. Among the range of applications offered by CdS thin films, it has found application as a window layer in second generation solar cells by creating photon traps, thereby quantum efficiency. Closed space sublimation (CSS) offers a trivial yet effective approach for the synthesis of thin films for solar cell applications at moderate temperatures. CdS thin films of controlled thicknesses were synthesized by CSS technique by varying exposure time. X-rays diffraction data of these thin films revealed polycrystalline nature with a preferred orientation along (002) direction. The scanning electron microscopy (SEM) based morphological studies showed grain size variation with an increase in thickness of deposited thin films in the range of 300–500 nm. The electrical studies revealed high resistivity of the order of 106 Ω-cm. Spectrophotometric studies performed for CdS thin films concluded with the calculation of the optical parameters such as refractive index, absorption coefficient employing the Swanepoel model, and energy band gap of ∼2.42 eV using the Tauc relation in addition to thin film thickness confirmation. The variations in thickness affect the structure, surface, optical (Khan et al. 2018), electrical properties.
Recently, the CdTe solar cell technology reached a high-tech level able to realize devices showing efficiencies close to 22%. Nowadays, this technology acquires more and more market share, becoming the most promising among the thin film technologies. These important achievements are the result of more than 40 years of studies and researches that have synergically group together the photovoltaic module manufacturing technology with semiconductor and interfaces physics, modeling, characterization and measurements on electronic devices.
This paper shows the main steps of the production process of the thin film CdTe/CdS-based solar cells both from a technological and from a physical point of view.
In particular, the main differences between cells fabricated in superstrate and in substrate configuration will be highlighted. For each of these two structures the fundamental layers, their main alternatives, chemical and thermal treatments, interfaces, the won challenges and the still open-problems will be presented and discussed.
The surface ligands on colloidal nanocrystals (NCs) play an important role in the performance of NCs based optoelectronic devices such as photovoltaic cells, photodetectors and light emitting diodes (LEDs). On one hand, the NC emission depends critically on the passivation of the surface to minimize trap states that can provide non-radiative recombination channels. On the other hand, the electrical properties of NC films are dominated by the ligands that constitute the barriers for charge transport from one NC to its neighbor. Therefore, surface modifications via ligand-exchange have been employed to improve the conductance of NC films. However, in light emitting devices, such surface modifications are more critical due to their possible detrimental effects on the emission properties. In this work, we study the role of surface ligand modifications on the optical and electrical properties of CdSe/CdS dot-in-rods (DiRs) in films, and investigate their performance in all-solution processed LEDs. The DiR films maintain high PLQY, around 40-50 %, and their electroluminescence in the LED preserves the excellent color purity of the PL. In the LEDs, the ligand-exchange boosted the luminance, reaching a four-fold increase from 2200 cd/m2 for native surfactants to 8500 cd/m2 for the exchanged aminoethanethiol ligands. Moreover, the efficiency roll-off, operational stability, and shelf life are significantly improved, and the external quantum efficiency is modestly increased from 5.1 % to 5.4 %. We relate these improvements to the increased conductivity of the emissive layer, and to the better charge balance of the electrically injected carriers. In this respect, we performed ultraviolet photoelectron spectroscopy (UPS) to obtain deeper insight in the band alignment of the LED structure. The UPS data confirms similar flat-band offsets of the emitting layer to the electron- and hole-transport layers, respectively, in the case of aminoethanethiol ligands, which translates to more symmetric barriers for charge injection of electrons and holes. Furthermore, the change in solubility of the nanocrystals induced by the ligand-exchange allows for a layer-by-layer deposition process of the DiR films that yields excellent homogeneity and good thickness control, and enables the fabrication of all the LED layers (except for cathode and anode) by spin-coating.
Despite intensive studies on the improvements of conversion efficiencies in solar cells, many questions regarding the effects of deposition techniques on optical properties and electronic band structures of CH3NH3PbI3 (MAPbI3) remain unresolved. Here, perovskite MAPbI3 films were prepared using different deposition methods and processing techniques. The effects of deposition and processing parameters on dielectric functions and optical absorption were investigated by fitting the reflectance spectra in the photon energy range of 0.5–5.16 eV. It is found that the bandgap (Eg) of the films deposited by two-step spinning (1.591 eV) is larger than that prepared by evaporations (1.514 eV), due to different Pb-I orbital hybridization and spin-orbit coupling. Moreover, the Eg value of the films increases from 1.543 eV to 1.591 eV after toluene solution dripping. Five interband electronic transitions (Ep1, Ep2, Ep3, Ep4, and Ep5) are observed, and the origins of Ep2, Ep3, and Ep4 are assigned to the direct transitions between the highest valence band and the lowest lying conduction band at the R, M, and X symmetry points. Further, the transition energies of the films deposited by evaporation are less than those prepared by two-step spinning. The present results shed light on preparing more reliable and reproducible high performance MAPbI3-based solar cells.
Written for graduate students and R and D scientists and engineers, this text provides a lucid treatment of many facets of materials, technologies, and solid-state physics of thin film solar cell devices. The various types of homo-, hetero-, barrier, and liquid junction solar cells involving amorphous, polycrystalline, and epitaxial semiconductor thin films are all covered. The volume details the basic solid-state physics of junction devices and describes thin film materials and associated preparation, measurement, and analysis techniques, as well as device technology. The authors present a critical comparative analysis of the performance of various types of thin film solar cells in order to focus on the present status of the field and to project future developments.