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1
Attygalle, Muthuthanthrige Lilani Chandrawansha. "Theoretical modeling of polycrystalline thin-film photovoltaics". University of Toledo / OhioLINK, 2008. http://rave.ohiolink.edu/etdc/view?acc_num=toledo1204144362.
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Bridge, Chris. "Optical electrical and microstructural characterisation of polycrystalline thin film CdTe/CdS heterojunction solar cells". Thesis, University of Manchester, 1999. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.680179.
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Ede, Anthony. "Studies of crystalline CdZnTe radiation detectors and polycrystalline thin film CdTe for X-ray imaging applications". Thesis, University of Surrey, 2002. http://epubs.surrey.ac.uk/843974/.
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The development of a replacement to the conventional film based X-ray imaging technique is required for many reasons. One possible route for this is the use of a large area film of a suitable semiconductor overlaid on an amorphous silicon readout array. A suitable semiconductor exists in cadmium telluride and its tertiary alloy cadmium zinc telluride. In this thesis the spectroscopic characteristics of commercially available CZT X- and gamma-radiation detectors are established. The electronic, optical, electro-optic, structural and compositional properties of these detectors are then investigated. The attained data is used to infer a greater understanding for the carrier transport in a CZT radiation detector following the interaction of a high energy photon. Following this a method used to fabricate large area films of CdTe on a commercial scale is described. This is cathodic electrodeposition from an aqueous electrolyte. The theory and experimental arrangement for this technique are described in detail with preliminary results from the fabricated films presented. Attention is then turned to the CdS/CdTe films that are produced commercially for the photovoltaic industry. In this case the crystalline nature, surface topography and optical properties are investigated. A conclusion examines the progress that has been made towards the development of a large area flat panel digital imaging technique.
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Archbold, Martin David. "Polycrystalline CdS thin films and their role in CdS/CdTe photovoltaic devices". Thesis, Durham University, 2007. http://etheses.dur.ac.uk/2138/.
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This thesis represents a systematic study of polycrystalline CdS thin films and their role as an n-type window layer in CdS/CdTe photovoltaic devices. This work encompasses the growth of CdS, primarily by the solution deposition method, and the subsequent characterisation of these films in isolation and as part of thin film CdS/CdTe device structures. A novel solution deposition approach was devised in order to grow high quality CdS thin films. Structural, electrical and chemical characterisation methods applied to these have shown that in their as-grown state they are highly oriented (in either the c[l1l] or h[002] direction), possess a small grain size of approximately 10-15 nm, and contain a considerable level of compressive strain. Annealing treatments in the presence of the fluxing agent CdCl(_2) have been shown to strongly modify the properties of these films, they are converted to a polycrystalline hexagonal structure with a significantly reduced level of strain, possess a larger grain size (27-28 nm) and a considerably enhanced crystalline quality. Novel 'hybrid' films comprising two CdS layers grown by different growth methods, one grown directly upon the other, have been studied. It has been shown that there are remarkable differences in morphology between these and films grown by a single growth method alone. Complete CdS/CdTe devices have been fabricated from several types of film grown in this study. Cell efficiencies of 9.80% were attained for a limited batch of devices, suggesting that these films possess good qualities for PV device fabrication. Early results from a novel tubular photovoltaic device concept are presented. This geometry has the potential to reduce manufacturing costs, may open up new routes to enhance the efficiency of CdS/CdTe devices, and is an attractive candidate for PV/solar thermal power generation.
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Inns, Daniel Photovoltaics & Renewable Energy Engineering Faculty of Engineering UNSW. "ALICIA polycrystalline silicon thin-film solar cells". Publisher:University of New South Wales. Photovoltaics & Renewable Energy Engineering, 2007. http://handle.unsw.edu.au/1959.4/43600.
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Thin-film silicon photovoltaics are seen as a good possibility for reducing the cost of solar electricity. The focus of this thesis is the ALICIA cell, a thin-film polycrystalline silicon solar cell made on a glass superstrate. The name ALICIA comes from the fabrication steps - ALuminium Induced Crystallisation, Ion Assisted deposition. The concept is to form a high-quality crystalline silicon layer on glass by Aluminium Induced Crystallisation (AIC). This is then the template from which to epitaxially grow the solar cell structure by Ion Assisted Deposition (IAD). IAD allows high-rate silicon epitaxy at low temperatures compatible with glass. In thin-film solar cells, light trapping is critical to increase the absorption of the solar spectrum. ALICIA cells have been fabricated on textured glass sheets, increasing light absorption due to their anti-reflection nature and light trapping properties. A 1.8 μm thick textured ALICIA cell absorbs 55% of the AM1.5G spectrum without a back-surface reflector, or 76% with an optimal reflector. Experimentally, Pigmented Diffuse Reflectors (PDRs) have been shown to be the best reflector. These highly reflective and optically diffuse materials increase the light-trapping potential and hence the short-circuit currents of ALICIA cells. In textured cells, the current increased by almost 30% compared to using a simple aluminium reflector. Current densities up to 13.7 mA/cm2 were achieved by application of a PDR to the best ALICIA cells. The electronic quality of the absorber layer of ALICIA cells is strongly determined by the epitaxy process. Very high-rate epitaxial growth decreases the crystalline quality of the epitaxial layer, but nevertheless increases the short-circuit current density of the solar cells. This indicates that the diffusion length in the absorber layer of the ALICIA cell is primarily limited by contamination, not crystal quality. Further gains in current density can therefore be achieved by increasing the deposition rate of the absorber layer, or by improving the vacuum quality. Large-area ALICIA cells were then fabricated, and series resistance reduced by using an interdigitated metallisation scheme. The best measured efficiency was 2.65%, with considerable efficiency gains still possible from optimisation of the epitaxial growth and metallisation processes.
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Attygalle, Muthuthanthrige Lilani C. "Theoretical modeling of polycrystalline thin-film photovoltaics /". Connect to full text in OhioLINK ETD Center, 2008. http://rave.ohiolink.edu/etdc/view?acc_num=toledo1204144362.
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Desai, Darshini. "Electrical characterization of thin film CdTe solar cells". Access to citation, abstract and download form provided by ProQuest Information and Learning Company; downloadable PDF file, 320 p, 2007. http://proquest.umi.com/pqdweb?did=1257806491&sid=6&Fmt=2&clientId=8331&RQT=309&VName=PQD.
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Sugimoto, Yoshiharu. "Studies of CdTe electrodeposition". Thesis, University of Southampton, 1993. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.241263.
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Mackay, Ian. "Thin film electroluminescence /". Online version of thesis, 1989. http://hdl.handle.net/1850/10551.
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Tetali, Bhaskar Reddy. "Stability studies of CdTe/CdS thin film solar cells". [Tampa, Fla.] : University of South Florida, 2005. http://purl.fcla.edu/fcla/etd/SFE0001135.
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Meakin, Douglas Boyd. "Active devices in polycrystalline silicon". Thesis, University of Liverpool, 1992. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.240344.
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Coxon, Penelope Anne. "Polycrystalline silicon thin-film transistors for large-scale microelectronics". Thesis, University of Cambridge, 1990. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.358595.
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Palekis, Vasilios. "CdTe/CdS Thin Film Solar Cells Fabricated on Flexible Substrates". Scholar Commons, 2011. http://scholarcommons.usf.edu/etd/3280.
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Cadmium Telluride (CdTe) is a leading thin film photovoltaic (PV) material due to its near ideal bandgap of 1.45 eV and its high optical absorption coefficient. The typical CdTe thin film solar cell is of the superstrate configuration where a window layer (CdS), the absorber (CdTe) and a back contact are deposited onto glass coated with a transparent electrode.
Substrate CdTe solar cells where the above listed films are deposited in reverse are not common. In this study substrate CdTe solar cells are fabricated on flexible foils.
The properties of the Molybdenum back contact, Zinc Telluride (ZnTe) interlayer and CdTe absorber on the flexible foils were studied and characterized using X-Ray Diffraction (XRD), and Scanning Electron Microscopy (SEM).
Substrate curvature and film flaking was observed during the fabrication as a result of differences in thermal expansion coefficients between the substrate and the deposited films, and also due to impurity diffusion from the foil into the film stack. In order to overcome this problem diffusion barriers where used to eliminate contamination. Silicon dioxide (SiO2), silicon nitride (Si3N4) and molybdenum nitride (MoxNy) were used as such barriers.
Electrical characterization of completed devices was carried out by Current-Voltage (J-V), Capacitance-Voltage (C-V) and Spectral Response (SR) measurements.
Roll-over was observed in the first quadrant of J-V curves indicating the existence of a back barrier due to a Schottky back contact. The formation of non-rectifying contact to p-CdTe thin-film is one of the major and critical challenges associated with the fabrication of efficient and stable solar cells.
Several materials (ZnTe, Cu, Cu2Te, and Te) were studied as potential candidates for the formation of an effective back contact.
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Lisco, Fabiana. "High rate deposition processes for thin film CdTe solar cells". Thesis, Loughborough University, 2015. https://dspace.lboro.ac.uk/2134/17965.
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This thesis describes the development of a fast rate method for the deposition of high quality CdS and CdTe thin films. The technique uses Pulsed DC Magnetron Sputtering (PDCMS). Surprisingly, the technique produces highly stable process conditions. CREST is the first laboratory worldwide to show that pulsed DC power may be used to deposit CdS and CdTe thin films. This is a very promising process technology with potential for eventual industrial deployment. The major advantage is that the process produces high deposition rates suitable for use in solar module manufacturing. These rates are over an order of magnitude faster than those obtained by RF sputtering. In common with other applications it has also been found that the energetics of the pulsed DC process produce excellent thin film properties and the power supply configuration avoids the need for complex matching circuits. Conventional deposition methodologies for CdS, Chemical Bath Deposition (CBD) and CdTe thin films, Electrodeposition (ED), have been chosen as baselines to compare film properties with Pulsed DC Magnetron Sputtering (PDCMS). One of the issues encountered with the deposition of CdS thin films (window layers) was the presence of pinholes. A Plasma cleaning process of FTO-coated glass prior to the deposition of the CdS/CdTe solar cell has been developed. It strongly modifies and activates the TCO surface, and improves the density and compactness of the deposited CdS thin film. This, in turn, improves the optical and morphological properties of the deposited CdS thin films, resulting in a higher refractive index. The pinhole removal and the increased density allows the use of a much thinner CdS layer, and this reduces absorption of blue spectrum photons and thereby increases the photocurrent and the efficiency of the thin film CdTe cell. Replacing the conventional magnetic stirrer with an ultrasonic probe in the chemical bath (sonoCBD) was found to result in CdS films with higher optical density, higher refractive index, pinhole and void-free, more compact and uniform along the surface and through the thickness of the deposited material. PDCMS at 150 kHz, 500 W, 2.5 μs, 2 s, results in a highly stable process with no plasma arcing. It allows close control of film thickness using time only. The CdS films exhibited a high level of texture in the <001> direction. The grain size was typically ~50 nm. Pinholes and voids could be avoided by reducing the working gas pressure using gas flows ii below 20 sccm. The deposition rate was measured to be 1.33 nm/s on a rotating substrate holder. The equivalent deposition rate for a static substrate is 8.66 nm/s, which is high and much faster than can be achieved using a chemical bath deposition or RF magnetron sputtering. The transmission of CdS can be improved by engineering the band gap of the CdS layer. It has been shown that by adding oxygen to the working gas pressure in an RF sputtering deposition process it is possible to deposit an oxygenated CdS (CdS:O) layer with an improved band gap. In this thesis, oxygenated CdS films for CdTe TF-PV applications have been successfully deposited by using pulsed DC magnetron sputtering. The process is highly stable using a pulse frequency of 150 kHz and a 2.5 μs pulse reverse time. No plasma arcing was detected. A range of CdS:O films were deposited by using O2 flows from 1 sccm to 10 sccm during the deposition process. The deposition rates achieved using pulsed DC magnetron sputtering with only 500 W of power to the magnetron target were in the range ~1.49 nm/s ~2.44 nm/s, depending on the oxygen flow rate used. The properties of CdS thin films deposited by pulsed DC magnetron sputtering and chemical bath deposition have been studied and compared. The pulsed DC magnetron sputtering process produced CdS thin films with the preferred hexagonal <001> oriented crystalline structure with a columnar grain growth, while sonoCBD deposited films were polycrystalline with a cubic structure and small grainy crystallites throughout the thickness of the films. Examination of the PDCMS deposited CdS films confirmed the increased grain size, increased density, and higher crystallinity compared to the sonoCBD CdS films. The deposition rate for CdS obtained using pulsed DC magnetron sputtering was 2.86 nm/s using only 500 W power on a six inch circular target compared to the much slower (0.027 nm/s) for the sonoChemical bath deposited layers. CdTe thin films were grown on CdS films prepared by sonoCBD and Pulsed DC magnetron sputtering. The results showed that the deposition technique used for the CdS layer affected the growth and properties of the CdTe film and also determined the deposition rate of CdTe, being 3 times faster on the sputtered CdS. PDCMS CdTe layers were deposited at ambient temperature, 500 W, 2.9 μs, 10 s, 150 kHz, with a thickness of approximately 2 μm on CdS/TEC10 coated glass. The layers appear iii uniform and smooth with a grain size less than 100 nm, highly compact with the morphology dominated by columnar grain growth. Stress analysis was performed on the CdTe layers deposited at room temperature using different gas flows. Magnetron sputtered thin films deposited under low gas pressure are often subject to compressive stress due to the high mobility of the atoms during the deposition process. A possible way to reduce the stress in the film is the post-deposition annealing treatment. As the lattice parameter increased; the stress in the film is relieved. Also, a changing the deposition substrate temperature had an effect on the microstructure of CdTe thin films. Increasing the deposition temperature increased the grain size, up to ~600 nm. CdTe thin films with low stress have been deposited on CdS/TEC10 coated glass by setting the deposition substrate temperature at ~200°C and using high argon flows ~ 70 sccm Ar. Finally, broadband multilayer ARCs using alternate high and low refractive index dielectric thin films have been developed to improve the light transmission into solar cell devices by reducing the reflection of the glass in the extended wavelength range utilised by thin-film CdTe devices. A four-layer multilayer stack has been designed and tested, which operates across the wavelength range used by thin-film CdTe PV devices (400 850 nm). Optical modelling predicts that the MAR coating reduces the WAR (400-850 nm) from the glass surface from 4.22% down to 1.22%. The application of the MAR coating on a thin-film CdTe solar cell increased the efficiency from 10.55% to 10.93% or by 0.38% in absolute terms. This is a useful 3.6% relative increase in efficiency. The increased light transmission leads to improvement of the short-circuit current density produced by the cell by 0.65 mA/cm2. The MAR sputtering process developed in this work is capable of scaling to an industrial level.
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Golego, Nickolay. "Thin-film polycrystalline titanium-oxygen semiconductors prepared by spray pyrolysis". Thesis, National Library of Canada = Bibliothèque nationale du Canada, 1998. http://www.collectionscanada.ca/obj/s4/f2/dsk2/ftp02/NQ33300.pdf.
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Chow, Thomas. "A conduction model for intrinsic polycrystalline silicon thin film transistor based on discrete grains /". View abstract or full-text, 2009. http://library.ust.hk/cgi/db/thesis.pl?ECED%202009%20CHOW.
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Alfadhili, Fadhil K. "Development of Back Contacts for CdTe Thin Films Solar Cells". University of Toledo / OhioLINK, 2020. http://rave.ohiolink.edu/etdc/view?acc_num=toledo1588962981116943.
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Bapanapalli, Srilatha. "Cds/cdte thin film solar cells with zinc stannate buffer layer". [Tampa, Fla.] : University of South Florida, 2005. http://purl.fcla.edu/fcla/etd/SFE0001004.
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Muthuswamy, Gokul. "Numerical modeling of CdS/CdTe thin film solar cell using MEDICI". [Tampa, Fla.] : University of South Florida, 2005. http://purl.fcla.edu/fcla/etd/SFE0001360.
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Babar, Shumaila. "Characterisation of surface treated CdZnTe and thin film CdTe based devices". Thesis, University of Surrey, 2016. http://epubs.surrey.ac.uk/810655/.
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Semiconductor materials have a vast range of applications varying from basic electronic products to astronomy and their semiconducting properties can be altered through the growth of binary or ternary compound materials. CdZnTe and CdTe are prominent materials in radiation detector and photovoltaic solar cell applications. For radiation detectors, in the fabrication process, surface preparation (chemical polishing and passivation) and contact deposition are key to the detector performance. This thesis investigates the effect of these two processing steps on CdZnTe detectors through varying the passivation procedures and gold contact configurations. The surface composition, layer thickness and non-uniformity resulting from the passivation treatments have been investigated using X-ray Photoelectron Spectroscopy (XPS), Scanning Transmission Electron Microscopy (STEM), Energy Dispersive X-ray spectroscopy (EDX) and other materials characterisation techniques. The device electrical and spectroscopic responses were measured using the I-V characteristics and alpha spectroscopy respectively. Passivation using 30 % H2O2 and 5% NaClO treatments develops a very thin oxide layer of up to ~2 nm, while NH4F/H2O2 and KOH+NH4F/H2O2 treatments yield oxide layers of varying thickness (30 – 142 nm) and metal oxides comprising of Te2/Te3, CdO and ZnO. Devices were fabricated in metal-semiconductor (MS) and metal-insulator-semiconductor (MIS) configurations. The MIS configuration improves the mobility-lifetime product, partial charge collection and leakage current of a CdZnTe device. The MIS device barrier heights were calculated to be 0.83 ± 0.02 eV and 0.86 ± 0.02 eV for very thin and thick oxide layers respectively. For ultra-thin (0.5 µm CdTe layer in) CdTe/CdZnS solar cells, XPS and X-ray diffraction (XRD) were employed to study the effect of varying the CdCl2 processing step. Increasing the degree of CdCl2 activation and annealing treatment was found to increase sulphur diffusion into the CdTe layer (up to a concentration of ~ 2 at.%). Cell performance measurements showed that the increase in S concentration is directly related to the open-circuit voltage (Voc), and increasing the degree of CdCl2 treatment gives higher Voc values.
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Yilmaz, Sibel. "Thin film CDTE solar cells deposited by pulsed DC magnetron sputtering". Thesis, Loughborough University, 2017. https://dspace.lboro.ac.uk/2134/31838.
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Thin film cadmium telluride (CdTe) technology is the most important competitor for silicon (Si) based solar cells. Pulsed direct current (DC) magnetron sputtering is a new technique has been developed for thin film CdTe deposition. This technique is industrially scalable and provides uniform coating. It is also possible to deposit thin films at low substrate temperatures. A series of experiments are presented for the optimisation of the cadmium chloride (CdCl2) activation process. Thin film CdTe solar cells require CdCl2 activation process to improve conversion efficiencies. The role of this activation process is to increase the grain size by recrystallisation and to remove stacking faults. Compaan and Bohn [1] used the radio-frequency (RF) sputtering technique for CdTe solar cell deposition and they observed small blisters on CdTe layer surface. They reported that blistering occurred after the CdCl2 treatment during the annealing process. Moreover, void formation was observed in the CdTe layer after the CdCl2 activation process. Voids at the cadmium sulphide (CdS)/CdTe junction caused delamination hence quality of the junction is poor. This issue has been known for more than two decades but the mechanisms of the blister formation have not been understood. One reason may be the stress formation during CdTe solar cells deposition or during the CdCl2 treatment. Therefore, the stress analysis was performed to remove the defects observed after the CdCl2 treatment. This was followed by the rapid thermal annealing to isolate the CdCl2 effect by simply annealing. Small bubbles observed in the CdTe layer which is the first step of the blister formation. Using high resolution transmission electron microscopy (HR-TEM), it has been discovered that argon (Ar) working gas trapped during the deposition process diffuses in the lattice which merge and form the bubbles during the annealing process and grow agglomeration mainly at interfaces and grain boundaries (GBs). Blister and void formation were observed in the CdTe devices after the CdCl2 treatment. Therefore, krypton (Kr), neon (Ne) gases were used as the magnetron working gas during the deposition of CdTe layer. The results presented in this thesis indicated that blister and void formation were still existing with the use of Kr an Ne. Xe, which has a higher atomic mass than Kr, Ne, Ar, Cd and Te, was used as the magnetron working gas and it resulted in surface blister and void free devices.
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Diso, Dahiru Garba. "Research and development of CdTe based thin film PV solar cells". Thesis, Sheffield Hallam University, 2011. http://shura.shu.ac.uk/4941/.
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The motivation behind this research is to bring cheap, low-cost and clean energy technologies to the society. Colossal use of fossil fuel has created noticeable pollution problems contributing to climate change and health hazards. Silicon based solar cells have dominated the market but it is cost is high due to the manufacturing process. Therefore, the way forward is to develop thin films solar cells using low-cost attractive materials, grown by cheaper, scalable and manufacturable techniques. The aim and objectives of this work is to develop low-cost, high efficiency solar cell using electrodeposition (ED) technique. The material layers include CdS and ZnTe as the window materials, while the absorber material is CdTe. Fabricating a suitable devices for solar energy conversion (i.e. glass/conducting glass/window material/absorber material/metal) structure. Traditional way of fabricating this structure is to grow window material (CdS) using chemical bath deposition (CBD) and absorber material (CdTe) using electrodeposition. However, CBD is a batch process and therefore creates large volumes of Cd-containing waste solutions each time adding high cost in manufacturing process. This research programme is therefore on development of an "All ED-solar cells" structure. Material studies were carried out using photoelectrochemical (PEC) studies, UV-Vis spectrophotometry, X-ray diffraction (XRD), X-ray fluorescence (XRF), scanning electron microscopy (SEM), atomic force microscopy (AFM), Raman spectroscopy and X-ray photoelectron spectroscopy (XPS). Furthermore, the electrical characterisation of fully fabricated devices was performed using current-voltage (I-V) and capacitance-voltage (C-V) measurements. This research programme has demonstrated that CdS and ZnTe window materials can be electrodeposited and used in thin film solar cell devices. The CdS electrolytic bath can be used for a period of 7 months without discarding it like in the CBD process which usually has life-time of 2-3 days. Further work should be carried out to increase the life-time of this bath, so that there can be used continuously minimising waste solution production in a manufacturing line. An efficiencies showing up to 7% was achieved for complete devices. However, the consistency and reproducibility remains un-resolved due to production of efficiencies between (2 - 7)% efficient devices varying from batch to batch. One of the reasons has been identified as the growth of CdS nano-rods with spacing between them. This is the first observation of CdS nano-rods and could open up many applications in nano-devices area. In order to improve the consistency of the solar cell efficiency, CdS layers should be grown with nano-rods aligned perpendicular to the glass surface and with tight packing without gaps, or with uniform coverage of CdS over the conducting glass surface. The possibility of growth of CdTe absorber layers with n- and p-type electrical conduction using change of stoichiometry was confirmed using the results presented in this thesis. This is a key finding, important to form multi-layer solar cell structures in the future.
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Hesp, David. "Surface characterisation of contact materials for thin film CdTe solar cells". Thesis, University of Liverpool, 2015. http://livrepository.liverpool.ac.uk/2020780/.
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The deposition of tellurium onto the low index faces of copper single crystals induces a range of structures such as the (2√3×2√3)R30° surface substitutional alloy on the Cu(111) surface. These structures have been studied using a combination of scanning tunnelling microscopy (STM), low energy electron diffraction (LEED), X-ray photoemission (XPS) and ultraviolet photoemission (UPS). The deposition of tellurium on to the Cu(111) surface produced several structures including (2√3×2√3)R30° phase for a coverage of 0.17ML, For a coverage of 0.33ML a (√3×√3)R30° surface alloy was found. For coverages greater than 0.66ML a bulk alloy was formed which is consistent with the Cu3Te2 phase. The deposition of tellurium onto the Cu(110) surface produced a c(2×2) structure for a coverage of 0.5ML. Further deposition causes uniaxial compression of the over-layer as indicated by the LEED patterns. STM of this surface revealed two hexagonal domains rotated 30° with respect to each other. The deposition of tellurium on to the Cu(100) surface produced a p(2×2) over−layer for a coverage of 0.25ML. Further deposition lead to a series of coincidence lattices showing co-existing structures including split c(2×2) spots in the LEED pattern. A heavily streaked split c(2×2) LEED pattern was observed for tellurium coverages over 1ML dosage where STM images revealed a heavily striped surface with two domains perpendicular to each other. The larger structures observed suggest 3D growth of these stripes. On all 3 surfaces the work function was found to increase upon tellurium deposition. The electronic structure of indium oxide thin films was investigated after different treatments in ultra high vacuum using XPS and UPS. The (111) surface was investigated using a combination of hard and soft X-rays showing the lower portion of the valence band to be dominated by oxygen orbitals while the higher portion of the valence band indium orbitals. The valence band was then probed and the work function measured after different annealing treatments. For all 3 samples, (111) (110) and (100) the work function was found to increase when annealing in oxygen when compared to annealing in vacuum. This is attributed to the movement of the Fermi level as the carrier concentration at the surface is altered. The lowering of observed gap states when annealing in oxygen also suggests they may originate from oxygen vacancies within the surface.
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Hodges, Deidra Ranel. "Development of CdTe thin film solar cells on flexible foil substrates". [Tampa, Fla] : University of South Florida, 2009. http://purl.fcla.edu/usf/dc/et/SFE0003210.
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Moffett, Christina. "Characterization of Tellurium Back Contact Layer for CdTe Thin Film Devices". Thesis, Colorado State University, 2018. http://pqdtopen.proquest.com/#viewpdf?dispub=10826197.
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Cadmium Telluride (CdTe) thin film photovoltaic technology has shown favorable progress due to inexpensive and efficient processing techniques. However, efficiencies have yet to reach the overall projected CdTe device efficiency, with the back contact being a main source of CdTe performance limitations. Tellurium (Te) applied as a back contact has led to significant increases in fill factor and an overall progress in device efficiency. Devices deposited with Te show significant improvement in uniformity, even without intentional Cu doping, when compared to devices without Te. In current - density measurements, Te shows stability even at low temperatures, which is indicative of a low barrier developed at the CdTe/Te interface. X-ray and ultra-violet photoelectron spectroscopy were carried out to examine the valence band offset at the CdTe/Te back contact interface. The valence band offset was shown to be highly dependent on the Te thickness and was largely affected by oxidation and contamination at the surface. Capacitance measurements were carried out to study the effect Te has on the absorber depletion width. Data indicate a decreased depletion width with Te applied at the back of thin film CdTe devices, which agrees with increased device performance. Te thickness was varied in all studies to understand the effect of application thickness on device performance and material characteristics. With a thicker Te layer leading to overall improvement in device performance and favorable device characteristics.
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Hsu, Chih-An. "Absorber and Window Study – CdSexTe1-x/CdTe Thin Film Solar Cells". Scholar Commons, 2019. https://scholarcommons.usf.edu/etd/7813.
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CdTe an II-VI semiconductor has been a leading thin film photovoltaic material due to its near ideal bandgap and high absorption coefficient [1]. The typical thin film CdTe solar cells have been of the superstrate configuration with CdS (Eg-2.42eV) as the n-type heterojunction partner. Due to the relatively narrow bandgap of CdS, a wider bandgap n-type window layer has recently emerged as a promising substitute: alloys of MgyZn1-yO have been successfully used as the emitter or window layer. The benefits in the usage of MgyZn1-yO (MZO) are its tunable bandgap and wide optical spectrum on optoelectronic devices. Due to an increasing bandgap of the window layer, the carrier collection can be improved in the short wavelength range (<500 nm). In addition alloys of CdSexTe1-x (CST) have also been used in the absorber layer (i.e., CST/CdTe) for the fabrication of CdTe devices to improve the carrier collection and lifetime [2]. The lower bandgap of the CST alloy can lead to higher short-circuit current (JSC), but it can also result in lower open circuit voltage (VOC). Another critical aspect of the CdTe solar cell is the use of copper as a p-type dopant, which is typically incorporated in the cell during the fabrication of the back contact. The most challenging issue related to further advancing the CdTe solar cell efficiency is the relatively low level of p-type doping, which limits the VOC. Efforts to dope CdTe with group V dopants are yet to produce the desired results.
ZnO has been used as an effective high resistivity transparent. When CdTe is deposited directly on sputtered ZnO, VOC of typically 500-600 mV is produced. Band alignment measurements indicate that a negative conduction band offset with CdS exists; alloying with MgO to produce MgyZn1-yO with a composition of y = 0.15 can produce a flat conduction band alignment with CdS. This material has an additional benefit for improving the energy bandgap of the MZO for better UV light transmission in the short wavelengths. By changing the magnesium content from y = 0 to 0.30 allowed researchers to make the tunable conduction band offset from a “cliff” to a “spike,” with both increased open-circuit voltage and fill factor as increasing magnesium compositions [3] — the bandgap gains as expected with increased magnesium composition. The large compositions (y > 0.30) of MgyZn1-yO cause the enormous spike result in S-kink in the IV measurement so that the FF decreases. Besides, due to the instability of MZO material, the fabrication process has to proceed carefully.
The properties of CST films and cells were investigated as a function of Se composition (x), substrate temperature (TSUB), and ambient used during the CSS deposition. The higher ratio of Se in CST alloy causes the smaller grain structures and lower bandgap, which profoundly detrimental to the device performance (VOC). However, the CST can be deposited in various substrate temperatures and different inert ambient gas to improve the grain structure by utilizing the especial Close Space Sublimation (CSS) deposition system. Therefore, despite the fact that the CST (25% Se) has the optical bandgap (1.37eV), the improvement of grain structure can slightly increase the doping concentration and decrease the grain boundary (GBs) due to increased alloys grain size 3X larger, which is contributed to improving the VOC [4]. The study of higher ratio Se of CST alloy is significant to achieve the high efficiency polycrystalline CST/CdTe photovoltaic devices.
The effect of Cu doping back contact in CdSexTe1-x (CST)/CdTe solar cells with varying amounts of Se (x) has been investigated. The Cu-based back contact was annealed at different thermal temperatures in order to vary the amount of Cu in-diffusion. Net p-type doping was found to increase as the back-contact annealing temperature increased. All cells exhibited a decrease in VOC with increased annealing temperature (i.e., higher Cu concertation), presumably due to a degradation of the lifetime with increased amounts of Cu [5]. However, cells with the highest Se composition appeared to exhibit a higher degree of tolerance to the amount of Cu – i.e., they exhibited a smaller loss in VOC with the increased amount of Cu.
Extrinsic p-type doping of CdSeTe can be fabricated using two different experimental processes. Firstly, by using group I elements such as, Cu to substitute Cd, which is promising during the back contact process. Secondly, using group V (P, As, Sb) elements to substitute Te, and this is suitable for Cd-rich of intrinsic CdTe. Intrinsic CST alloy has lower hole density concentration as higher Se composition with limitation of the VOC. Thus, in order to increase the p-type net doping up to 1016 cm-3 the extrinsic P or As doping have been widely investigated recently. The research studies show the CST/CdTe devices lead to improve VOC up to 850 mV with higher hole density in higher Se compositions of As doped CST alloys. Nevertheless, the group V doped CdTe still cause the formation of compensating defects limits the upper boundary of dupability on the CdTe thin film solar cells. Even if a high hole density concentration is achieved for intrinsically-doped p-type CST/CdTe, it is believed the poor carrier lifetime in the CdTe side would still limit the VOC.
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Maniscalco, Bianca. "Microstructure and performance of CdTe solar devices". Thesis, Loughborough University, 2015. https://dspace.lboro.ac.uk/2134/16754.
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One of the most critical processes in CdTe device production is the activation process induced by cadmium chloride (CdCl2). In this thesis, the CdCl2 treatment has been optimized using both wet and thermal evaporation methods for close-spaced sublimated (CSS) devices. Maximum cell efficiencies of η=7.24% and η=9.37% respectively have been measured without the use of copper in the back contact. A clear link has been established between treatment conditions, electrical measurements and microstructure, where parameters such as the dwell annealing temperature for evaporated CdCl2 and the concentration of the solution for the wet treatment are varied. It has been shown that a certain concentration of chlorine is necessary to remove high densities of planar defects present in the as-deposited material. The CSS CdTe is deposited in a dual layer structure with smaller grains at the CdS interface and with larger grains developing towards the surface. The defects are initially removed in the smaller grains at the CdS interface. When the temperature and concentration increase, more grains recrystallize with the total removal of stacking faults. At a critical temperature and Cl concentration, the entire CdTe film recrystallizes into large grains with no stacking faults. The CdS grains and the interface with the CdTe also changes with sulphur migration into the CdTe. The results indicate that the recrystallization actually initiates at the CdS/CdTe junction. This has been observed clearly for both sputtered and electrodeposited CdTe. The recrystallization process gradually propagates towards the surface as the concentration of the CdCl2 solution in methanol is increased. This observation is not intuitive because the solution is initially in contact with the outer surface of the CdTe. Finally, the use of different chlorine containing compounds has been used as an alternative to CdCl2 and to further understand the role of chlorine in the process. All the samples treated with Cl containing compounds have shown the elimination of the dual layer structure and the recrystallization of the small grains at the interface. Tellurium tetrachloride (TeCl4) and zinc chloride (ZnCl2) have shown the most promising increase in conversion efficiency. The maximum efficiencies measured using these two solutions were 4.58% and 5.05% respectively. TeCl4 has shown an encouraging open circuit voltage of 594 mV, while the open circuit voltage using ZnCl2 was 494 mV. However, TeCl4 has shunting issues and low current density (17.9 mA/cm2), whereas ZnCl2 has the promising current density of 20.8 mA/cm2. This work has shown that alternatives to CdCl2 treatment exist, however further work is required to optimize the performance of these treatments to enable them to be competitive. Advanced materials characterization techniques are essential to understand and then enhance photovoltaic cell and module performance. New and improved tools are being developed to deliver fast, accurate and non-destructive characterization. One of these tools is coherence correlation interferometry (CCI) which has been developed by Taylor Hobson Ltd. This is a particular variant of scanning white light interferometry used for surface metrology with a high vertical resolution. In this thesis, it has been shown that the capability of the CCI can be extended to perform accurate thin film thickness measurements using the Helix Complex Field (HFC) function. The main attraction of this technique for thin film PV applications is that it allows surface metrology and thin film thickness measurements to be obtained simultaneously from the same area of the sample in the same system. The results obtained from CCI on a variety of materials, used in thin film PV, correlate very well the results obtained from other techniques such as ellipsometry, electron microscopy and atomic force microscopy. The CCI has also been used in the optimization of a new one-step interconnect process (OSI) for thin film PV module interconnects.
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Bhatti, Muhammad Tariq. "A novel method of production of CdS/CdTe thin film solar cells". Thesis, Northumbria University, 1993. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.356984.
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Bayhan, Murat. "Preparation and characterisation of n-CdS/p-CdTe thin film solar cells". Thesis, Durham University, 1994. http://etheses.dur.ac.uk/1697/.
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Samantilleke, Anura Priyajith. "Electrodeposition of ZnSe, CdSe and CdTe thin film materials and optoelectronic devices". Thesis, Sheffield Hallam University, 1998. http://shura.shu.ac.uk/20319/.
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Zhao, Hehong. "Impurity and back contact effects on CdTe/CdS thin film solar cells". [Tampa, Fla] : University of South Florida, 2008. http://purl.fcla.edu/usf/dc/et/SFE0002377.
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Viswanathan, Vijay. "Study Of Cu free back contacts to thin film CdTe solar cells". [Tampa, Fla.] : University of South Florida, 2004. http://purl.fcla.edu/fcla/etd/SFE0000348.
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Echendu, Obi Kingsley. "Thin film solar cells using all-electrodeposited ZnS, CdS and CdTe materials". Thesis, Sheffield Hallam University, 2014. http://shura.shu.ac.uk/19597/.
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The urgent global need for affordable alternative and clean energy supply has triggered extensive research on the development of thin-film solar cells since the past few decades. This has necessitated the search for low-cost, scalable and manufacturable thin-film semiconductor deposition techniques which in turn has led to the research on electrodeposition technique as a possible candidate for the deposition of semiconductor materials and the fabrication of thin-film solar cells using these materials. Electronic quality ZnS, CdS, and CdTe thin layers have been successfully electrodeposited from aqueous solutions on glass/fluorine-doped tin oxide (FTO) substrates, using simplified two-electrode system instead of the conventional three-electrode system. This process was also carried out in a normal physical chemistry laboratory instead of the conventional cleanroom that is very expensive to maintain. The electrodeposited materials were characterised for their structural, optical, electrical, morphological and compositional properties using x-ray diffraction, optical absorption, photoelectrochemical cell, current-voltage, scanning electron microscopy and energy dispersive x-ray techniques respectively. The results show that amorphous n-type and p-type ZnS layers were deposited by varying the concentrations of Zn[2+] and S[2-] in the deposition electrolyte. The CdS layers show hexagonal structure with n-type electrical conduction while CdTe layers show cubic structure with n-type electrical conduction, in the cathodic deposition potential range explored. Using CdTe as the main absorber material, fully fabricated solar cell structures of the n-n hetero-junction + large Schottky barrier type were fabricated instead of the conventional p-n junction type structure. Conventional post-deposition CdCl[2] treatment of CdTe rather carried out with a mixture of CdCl[2] and CdF[2], resulted in pronounced improvement of all the device parameters. Characterisation of the fully fabricated solar cells was done using current-voltage and capacitance-voltage techniques. Promising device parameters were obtained for the best devices, with barrier heights greater than (1.00 - 1.13) eV, short-circuit current densities of (20 - 48) mAcm[-2], open-circuit voltages of (500 - 670) mV, fill factors of (0.33 - 0.47) and overall conversion efficiencies of (5.0 - 12.0)%. Remarkably, the two highest efficiency figures of 10.4% and 12.0% came up for solar cells involving ZnS as buffer layer and window layer with the structures, glass/FTO/n-ZnS/n-CdS/n-CdTe/Au and glass/FTO/n-ZnS/n-CdTe/Au, respectively. At present, the reproducibility and consistency of these devices is poor, but these results demonstrate that these devices structures have the potential to achieve efficiency values over 20% when fully optimised.
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Bittau, Francesco. "Analysis and optimisation of window layers for thin film CDTE solar cells". Thesis, Loughborough University, 2017. https://dspace.lboro.ac.uk/2134/32642.
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The work presented in this thesis focuses on the investigation and improvement of the window stack of layers for thin film CdTe solar cells fabricated in the Center for Renewable Energy Systems Technology (CREST) laboratories. In particular the aim was to change the standard structure including TCO, high resistive transparent (HRT)layer and CdS which is limited by the low transparency of the CdS layer, to a better performing one. The first result chapter of the thesis describes the study of ZnO HRT layers. ZnO thin films were deposited by radio frequency (RF) magnetron sputtering with different structural, optical and electrical properties which were characterized by X-ray diffraction, electron microscopy, spectrophotometry, Hall Effect method and 4-point probe. ZnO films were then incorporated in CdTe solar cells with the structure: FTO/ZnO/CdS/CdTe/Au back contact and the performance of these devices were compared with the film properties to single out trends and identify optimal film characteristics. By varying the deposition pressure of ZnO films, it was possible to increase their transparency and significantly increase their resistivity. While better transparency positively affected the solar cell current density output and efficiency, the resistivity of ZnO films did not show any clear impact on device efficiency. By increasing the deposition temperature the ZnO film grain size was increased. Increased FF was observed in devices incorporating ZnO layers with bigger grains, although this gain was partially counterbalanced by the Voc degradation, leading to a limited efficiency improvement. Finally the addition of oxygen had the main effect of increasing the resistivity of ZnO films, similarly to what happened with the increase of the sputtering pressure. In this case however, an improvement of FF, Jsc and efficiency was observed, especially at an O2/Ar ratio of 1%. By simulating the solar cells behavior with SCAPS-1D, it was found that these performance change can be explained by the variation of interface properties, precisely the amount of interface defects, rather than by bulk properties. The study presented in the second result chapter focuses on magnesium-doped zinc oxide (MZO) and the variation of its energy band structure. MZO was initially used as the HRT layer within a solar cell structure: FTO/MZO/CdS/CdTe/Au back contact. Sputtering MZO films with a target containing MgO 11 weight% and ZnO 89 weight% allowed for and increased band gap from 3.3 eV of intrinsic ZnO to 3.65 eV for MZO deposited at room temperature. Increasing the superstrate deposition temperature allowed for a further band gap increase up to 3.95 eV at 400 °C due mainly to an conduction band minimum upward shift. It was highlighted the importance to create a positive conduction band offset with the MZO layer conduction band slightly above the CdS conduction band, with an optimum found in this case to be 0.3 eV (efficiency 10.6 %). By creating a positive conduction band offset all the performance parameters (Voc, FF, Jsc, efficiency) significantly increased. One of the reasons for this improvement was found to be a diminished interface recombination due to a more ideal MZO/CdS band alignment. In the second part of this investigation the MZO was used as a replacement for the CdS in a simplified structure: FTO/MZO/CdTe/Au back contact. The concepts used to optimise the performance of these devices also involved tuning the conduction band alignment between MZO/CdTe and efficiencies of 12.5 % were achieved with a at conduction band offset. The efficiency increase was achieved mainly thanks to a better transparency of the MZO layer and a higher Jsc output, compared to devices using a CdS buffer layer. The MZO buffers have been tested in combination with different TCOs. Results are presented in the third result chapter and showed that AZO is a good alternative to FTO working effectively in combination with MZO. AZO/MZO efficiency thin film CdTe solar cells (12.6%, compared to 12.5% with FTO). It was found that increasing the IR transparency of the TCOs leads to a potentially higher Jsc. Achieving a better transparency was obtained by using TCOs with high mobility and lower carrier concentration (AZO and ITiO) and also by using a boro-aluminosilicate glass with low iron content. ITiO yielded the best opto-electrical properties among all the TCO materials. Devices incorporating ITiO however, showed lower performance then those using FTO and AZO. ITO/MZO windows also yielded poor performance. In addition, the ITO films deposited had a high carrier concentration leading to a high NIR absorption by plasma resonance and resulted not ideal for application in thin film CdTe PV.
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Zhao, Hehong. "Impurity and Back Contact Effects on CdTe/CdS Thin Film Solar Cells". Scholar Commons, 2007. https://scholarcommons.usf.edu/etd/580.
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CdTe/CdS thin film solar cells are the most promising cost-effective solar cells. The goal of this project is to improve the performance for CdS/CdTe devices by improving the open circuit voltage Voc and current density Jsc. Efforts focused on increasing the Voc, which include increasing the doping concentration by introducing Phosphorus and Antimony, finding and testing new back contact materials, and varying the ambient of CSS CdTe. In addition, the effect of Zn2SnO4 on the cells' performance was also studied.
Electrical characterization of the thin films and completed devices were carried out by Current-Voltage (J-V), Capacitance-Voltage (C-V), and Spectral Response (SR) measurements. Structural/chemical characterization was done by SEM, XRD and EDS analysis.
The ambient of CSS CdTe affects the growth rate, the grain size and electronic properties of CdTe. The N2/O2 mixture with varied ratio (N2/O2=9/1, 7/3, 5/5 and 1/9) was used in this study. The cells' performance and the net carrier concentration were studied as a function of the N2/O2 ratio. The net carrier concentration increases with the increasing O2 concentration.
The extrinsic impurities (P and Sb) were incorporated into CdTe layer. Phosphorus was directly introduced into CSS CdTe source. The Sb was incorporated into CdTe by a diffusion process. The effects of the annealing parameters, the excess Sb on CdTe surface, the CdCl2 treatment and the depth of Sb in CdTe were studied. Higher doping concentration up to 1016 cm-3 has been achieved, however, Voc is still in the range of 830 mV.
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Kunz, Oliver Photovoltaics & Renewable Energy Engineering Faculty of Engineering UNSW. "Evaporated solid-phase crystallised poly-silicon thin film solar cells on glass". Publisher:University of New South Wales. Photovoltaics & Renewable Energy Engineering, 2009. http://handle.unsw.edu.au/1959.4/43644.
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The cost of photovoltaic electricity needs to be significantly reduced in order to achieve a high electricity market penetration. Thin-film solar cells have good potential to achieve such cost savings though (i) large-area deposition onto low-cost foreign substrates, (ii) more streamlined processing, (iii) monolithic cell interconnection, and very efficient use of the expensive semiconductor material. Polycrystalline silicon (poly-Si) on glass is a promising technology for the cost-effective large volume production of PV modules since it (i) makes use of an abundant raw material, (ii) is non-toxic, (iii) does not suffer from light-induced degradation, and (iv) does not rely on TCO layers. Usually plasma enhanced chemical vapour deposition (PECVD) is used for the layer formation. This thesis explores the use of e-beam evaporation as deposition method since it is potentially much faster and cheaper than PECVD. The resulting solar cells are referred to as EVA (from EVAporation). Two inherent shunting mechanisms in EVA cells are demonstrated to be shunting through sub-micron sized pinholes when the back electrode is deposited and shunting between the emitter and the absorber layer at the glass-side electrode. Through the improved understanding of these shunting mechanisms it was possible to develop a suitable metallisation scheme for EVA cells using an aligned deposition of emitter and back surface field line contacts and a specially developed shunt mitigation etching technique. For the first time appreciable efficiencies of up to 5.2% were demonstrated on this material. It was also shown that only very lightly doped absorber layers can lead to the required high short-circuit currents in EVA cells. The resulting cells are currently completely limited by space charge region recombination occurring with comparatively low ideality factors of only ~ 1.4 This thesis also demonstrates the usefulness of Jsc-Suns measurements and investigates optical loss mechanisms in the current devices. Advanced modelling of distributed series resistance effects, influencing Suns-Voc, m-Voc and Jsc-Suns curves, is employed. PC1D modelling is used to extract relevant device parameters. In this work it was found that the diffusion length in the best EVA cells is longer than the absorber layer and that insufficient light trapping is currently the major hurdle to higher cell efficiencies. From the obtained results it can be concluded that EVA solar cells are promising candidates for the low-cost and high-volume production of solar modules.
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Straub, Axel Electrical Engineering & Telecommunications Faculty of Engineering UNSW. "Polycrystalline silicon thin-film solar cells on glass by ion-assisted deposition". Awarded by:University of New South Wales. School of Electrical Engineering and Telecommunications, 2005. http://handle.unsw.edu.au/1959.4/22435.
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Polycrystalline silicon (pc-Si, grain size > 1??m, no amorphous tissue) on glass is an interesting material for thin-film solar cells due to the low costs, the abundance and the non-toxic character of Si, and the properties of pc-Si like long-term stability and lateral conductance. Glass as supporting material significantly complicates the fabrication process as it limits the thermal budget and the maximum temperature. In this work, the feasibility of forming large-grained pc-Si thin-film solar cells on glass by ion-assisted deposition (IAD) on aluminium-induced crystallisation (AIC) seed layers (ALICIA solar cells) is investigated. IAD allows epitaxial growth at high rate, and being based on evaporation, is of low cost (high source material usage, no toxic gases involved). High-quality epitaxy on (100)-oriented Si wafer substrates is demonstrated in a non{UHV environment, to further increase its industrial appli- cability. High{rate growth and a sacrificial protective layer control contamination problems associated with the non-UHV environment. The process is then trans- ferred to AIC-seeded glass and optimised, with particular focus on the influence of the glass. Using high-temperature rapid thermal annealing and hydrogenation as post-deposition treatments, ALICIA solar cells with a 1-Sun open-circuit voltage of 420 mV are achieved. Moreover, two novel characterisation techniques are presented. One allows the fast and non-destructive assessment of the structural quality of pc-Si films using opti- cal measurements. Furthermore, `impedance analysis', a novel capacitance-voltage measurement technique based on impedance spectroscopy, is presented. It allows the reliable determination of the absorber layer doping density and the built{in potential of non-ideal p-n junction solar cells. The latter is used to investigate the influence of post{deposition treatments on the n-type absorber layer doping of ALICIA solar cells. It is found, using temperature dependent impedance analysis, that unintentional doping and defects have a strong influence on the absorber layer doping. A maximum in the short-circuit current density of ALICIA solar cells is found for phosphorus concentrations in the absorber of 1??1017 cm??3. For such ALI- CIA cells a base difusion length in the range 600 - 950nm, a short{circuit current density in the range 10 - 13.5 mA/cm2 and an energy conversion efficiency of 2.2% are obtained.
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He, Song Photovoltaics & Renewable Energy Engineering Faculty of Engineering UNSW. "Evaporated polycrystalline silicon thin-film solar cells by aluminium-induced crytallization solid-phase epitaxy". Awarded By:University of New South Wales. Photovoltaics & Renewable Energy Engineering, 2009. http://handle.unsw.edu.au/1959.4/44888.
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Polycrystalline silicon (poly-Si) thin-film solar cells are receiving attention by many researchers in recent times. The focus of this thesis is the evaporated ALICE solar cell, a thin-film poly-Si solar cell fabricated on a glass superstrate by e-beam evaporation. The acronymn ALICE comes from - ALuminium Induced Crystallization Solid Phase Epitaxy. The concept is first to form a high-quality crystalline silicon layer on glass by Aluminium Induced Crystallization (AIC). The AIC seed layer (grain size>20 ??m) acts as the template from which the crystalline information is transferred into the silicon over-layer by solid-phase epitaxy (SPE). As a result, the ALICE solar cells have much larger grain size compared to the poly-Si thin-film solar cell (2~3 ??m) by random nucleation and growth (RNG). This leads to the minimized grain boundary recombination and hence potential improved conversion efficiency. The temperature of 200??C is found to be optimal for the deposition of amorphous Si (a-Si) precursor thin films. The epitaxy process of the ALICE cell is successful, proving the feasibility and reliability of the deposition and post-treatment processes. The ALICE cell is successfully metallized using a bifacial interdigitated scheme. Wet etching using KOH is introduced to realize the uniform Si etching, and phosphoric acid etching is introduced to remove the local shunts in the ALICE cell. The results show that the material quality of ALICE solar cells are much worse than that of the AIC seed layer, which is related to the poor epitaxy quality on (111) planes grown from the AIC seed layer. Additional experiments show that the fraction of (100) oriented grains in AIC is the main factor in determining the material quality and the resulting solar cell performance, rather than grain size. Therefore, both a high fraction of (100) oriented grains and large grain size are required for AIC seed layers to achieve the ALICE solar cells with superior performance. Comparison of the ALICE cells prepared at different base pressures and deposition rates show that the base pressure is much less important than the deposition rate. Therefore, the capital cost of the evaporator system can be reduced and hence potentially the manufacturing cost of solar cells. The densification anneal was introduced to improve the crystal quality of poly-Si thin films by SPE. It is shown that the cause is the structural relaxation induced into the a-Si film, instead of the prevention of the oxygen percolation. The crystal quality of c-Si films obtained from low-rate (50 nm/min) evaporated a-Si is considerably improved by densification anneal, whereas densification has no beneficial effect on c-Si films obtained from high-rate (300 nm/min) evaporated a-Si. However, the densification anneal has no improvement on the electrical performance of ALICE solar cell. The ALICE solar cell performances are strongly related to the doping level in the absorber layer. The optimal doping density needs to be determined to achieve the best performance. The highest Voc and Jsc are simultaneously achieved when the minimum phosphorous doping density of ~5.5??1015 cm-3 (unintentionally doped) is applied for the evaporated ALICE solar cells. Since silicon is a weak absorber and ALICE solar cell has only ~1.5 ??m thickness, light trapping is applied to enhance the light absorption of the visible and the red light. Three different approaches are applied: ALICE cells on textured glass sheet, back surface reflector and thicker Si film. The ALICE cells on textured glass suffer from a significant loss of performance. The only successful approach to improve the light trapping in this thesis is to apply white paint as back surface reflector, which increases the Jsc drastically (~60%) compared to a planar sample. Analysis of the optical properties of poly-Si thin films is important as it assists the design of the thin-film solar cells. It is found that there is enhanced absorption in the visible wavelengths. This is mainly attributed to defected a-Si material at the grain boundaries. The hydrogenation process does not affect this enhanced absorption. The optical analysis proves that large grain size is desired to obtain high performance poly-Si thin-film solar cell, e.g. ALICE solar cell. At the end of this research, ALICE cells with η~3.83%, Voc~485 mV, Jsc~17.75 mA/cm2 have been achieved.
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Shi, Lei Photovoltaics & Renewable Energy Engineering Faculty of Engineering UNSW. "Contact resistance study on polycrystalline silicon thin-film solar cells on glass". Publisher:University of New South Wales. Photovoltaics & Renewable Energy Engineering, 2008. http://handle.unsw.edu.au/1959.4/41425.
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Thin-film solar cells are widely recognised to have the potential to compete with fossil fuels in the electricity market due to their low cost per peak Watt. The Thin-Film Group at the University of New South Wales (UNSW) is engaged in developing polycrystalline silicon (poly-Si) thin-film solar cells on glass using e-beam evaporation technology. We believe our solar cells have the potential of significantly lowering the manufacturing cost compared to conventional, PECVD-fabricated thin-film solar cells. After years of materials research, the focus of the Group??s work is now moving to the metallisation of evaporated solar cells. Minimising various kinds of losses is the main challenge of the cell metallisation procedure, within which the contact resistance is always a big issue. In this thesis, the contact resistance of aluminium contacts on poly-Si thin-film solar cells on glass is investigated. To the best of the author??s knowledge, this is the first ever contact resistance investigation of Al contacts on evaporated poly-Si material for photovoltaic applications. Various transmission line models (TLM) are employed to measure the contact resistance. An improved TLM model is developed to increase the measurement precision and, simultaneously, to simplify the TLM pattern fabrication process. In order to accommodate the particular requirements of poly-Si coated glass substrates, a TLM pattern fabrication process using photolithography is established. Furthermore, a Kelvin sense tester is set up to ensure an accurate measurement of the contact resistance. After establishment of the TLM technique at UNSW, it is successfully tested on singlecrystalline silicon wafer samples. The thermal annealing process of the contacts is also optimised. Then, the general behaviour of Al contacts on uniformly doped poly-Si films (i.e., no p-n junction) is investigated using the verified TLM technique. The long-term stability of the contacts is also studied. This is followed by an investigation of the contact resistance of the back surface field and emitter layers of different types of poly-Si thin-film solar cells. Finally, a novel contact resistance measurement model is proposed that is believed to be able to overcome the measurement bottleneck of the transmission line models.
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Pohl, John E. T. "The electrical properties of bulk polycrystalline and thin film high temperature superconductors". Thesis, University of Nottingham, 1992. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.315041.
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Ricciardo, Rebecca Ann. "Chemical, Magnetic, and Orbital Order of Polycrystalline and Thin film Double Perovskites". The Ohio State University, 2009. http://rave.ohiolink.edu/etdc/view?acc_num=osu1250277883.
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Lee, Jinwoo. "Metastability of copper indium gallium diselenide polycrystalline thin film solar cell devices /". Connect to title online (Scholars' Bank) Connect to title online (ProQuest), 2008. http://hdl.handle.net/1794/8588.
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Thesis (Ph. D.)--University of Oregon, 2008.Typescript. Includes vita and abstract. Includes bibliographical references (leaves 112-117). Also available online in Scholars' Bank; and in ProQuest, free to University of Oregon users.
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Lee, Jinwoo 1973. "Metastability of copper indium gallium diselenide polycrystalline thin film solar cell devices". Thesis, University of Oregon, 2008. http://hdl.handle.net/1794/8588.
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xvi, 117 p. ; ill. (some col.) A print copy of this thesis is available through the UO Libraries. Search the library catalog for the location and call number.High efficiency thin film solar cells have the potential for being a world energy solution because of their cost-effectiveness. Looking to the future of solar energy, there is the opportunity and challenge for thin film solar cells. The main theme of this research is to develop a detailed understanding of electronically active defect states and their role in limiting device performance in copper indium gallium diselenide (CIGS) solar cells. Metastability in the CIGS is a good tool to manipulate electronic defect density and thus identify its effect on the device performance. Especially, this approach keeps many device parameters constant, including the chemical composition, grain size, and interface layers. Understanding metastability is likely to lead to the improvement of CIGS solar cells. We observed systematic changes in CIGS device properties as a result of the metastable changes, such as increases in sub-bandgap defect densities and decreases in hole carrier mobilities. Metastable changes were characterized using high frequency admittance spectroscopy, drive-level capacitance profiling (DLCP), and current-voltage measurements. We found two distinctive capacitance steps in the high frequency admittance spectra that correspond to (1) the thermal activation of hole carriers into/out of acceptor defect and (2) a temperature-independent dielectric relaxation freeze-out process and an equivalent circuit analysis was employed to deduce the dielectric relaxation time. Finally, hole carrier mobility was deduced once hole carrier density was determined by DLCP method. We found that metastable defect creation in CIGS films can be made either by light-soaking or with forward bias current injection. The deep acceptor density and the hole carrier density were observed to increase in a 1:1 ratio, which seems to be consistent with the theoretical model of V Cu -V Se defect complex suggested by Lany and Zunger. Metastable defect creation kinetics follows a sub-linear power law in time and intensity. Numerical simulation using SCAPS-1D strongly supports a compensated donor- acceptor conversion model for the experimentally observed metastable changes in CIGS. This detailed numerical modeling yielded qualitative and quantitative agreement even for a specially fabricated bifacial CIGS solar cell. Finally, the influence of reduced hole carrier mobility and its role in limiting device performance was investigated.
Adviser: J. David Cohen
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Kang, Jun. "Thin film CdTe as high energy x-ray detector material for medical applications". Connect to full text in OhioLINK ETD Center, 2008. http://rave.ohiolink.edu/etdc/view?acc_num=toledo1228060515.
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Muftah, G. E. A. "Research and development of CuInTe2 and CdTe based thin film PV solar cells". Thesis, Sheffield Hallam University, 2010. http://shura.shu.ac.uk/20098/.
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The aim of this work was to make low cost thin film solar cells by electrodeposition, using two different materials; copper indium ditelluride (CuInTe[2]) and cadmium telluride (CdTe). CuInTe[2] was chosen to see if it could eliminate the problems associated with the more commonly used materials CuInSe[2] and CuInGaSe[2]. The CdTe was grown by a novel two electrode system and compared with material grown by the conventional three electrode system. Also the chlorine treatment of CdTe was investigated. CuInTe[2] films were electrochemically deposited from aqueous solutions and cyclic voltametry was used to determine suitable deposition parameters. X-ray diffraction, optical absorption and scanning electron microscopy were used to investigate the bulk structure, the band gap energy and the surface morphology of the materials respectively. It was found that layers deposited for 3 hours were ~1.5 mum thick and had a polycrystalline chalcopyrite structure with the band gap varying between 1.05 and 1.30 eV. Current-voltage characteristics of the CuInTe[2]/electrolyte, solid/liquid junctions were measured under dark and illuminated conditions, the layers were found to be photo active and p-type in electrical conduction. CdTe films were electrochemically deposited from aqueous solutions using both two and three electrode systems. Different preparative parameters such as growth voltage and heat treatment conditions were optimised by measuring the I-V characteristic of glass/FTO/CdS/CdTe/Au devices. The device efficiency was high at a growth voltage of 695 mV vs SCE for the 3 electrode system and 1570 mV for the two electrode system. The optimum annealing conditions were 350°C for 20 minutes in air. It is established that treating CdTe with chlorine has a beneficial effect on the solar cell device. For comparison, untreated and treated CdTe were characterised by various techniques. XRD analysis showed a cubic structure, with a significant increase in intensity from the (111) reflection after chlorine treatment. Optical absorption showed a decrease in the band gap energy after chlorine treatment. A considerable change in the morphology between untreated and treated CdTe was observed by SEM. I-V measurements of completed chlorine treated glass/FTO/CdS/CdTe/Au devices showed a significant improvement in efficiency. Furthermore, a comparison was made between the two and three electrode systems; the two electrode system produced a better quality material with higher efficiencies than that obtained by the three electrode system. The best efficiency obtained from the two electrode system was 8.7% while the best efficiency obtained from the three electrode system was 8.4%.
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Kung, Kenneth Ting-Yuan. "Polycrystalline Si thin films and devices : I. Seed selection through ion channeling II. Thin-film transistors". Thesis, Massachusetts Institute of Technology, 1988. http://hdl.handle.net/1721.1/69699.
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Garza, Ezra. "Pulsed Laser Deposition of Thin Film Heterostructures". ScholarWorks@UNO, 2011. http://scholarworks.uno.edu/td/459.
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Thin films of Strontium Ruthenate have been grown on Strontium Titanate and Lanthanum Aluminate (100) substrates by pulsed laser deposition. X-ray diffraction results show that the films grown on the Strontium Titanate are amorphous and polycrystalline on the Lanthanum Aluminate. Resistances versus temperature measurements show that the films exhibit semiconducting characteristics. In addition to the growth of Strontium Ruthenate thin films, multilayer heterostructures of Terfenol-D thin films on polycrystalline Lead Titanate thin films were grown by pulsed laser deposition. By using a novel experimental technique called magnetic field assisted piezoelectric force microscopy it is possible to investigate the magnetoelectric coupling between the electrostrictive Lead Titanate and magnetostrictive Terfenol-D thin film. Upon examination of the produced thin films the phase and amplitude components of the piezoelectric signal experience changes in response to an applied in-plane magnetic field. These changes provide experimental evidence of a magnetoelectric coupling between the Terfenol-D and Lead Titanate layers.
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RIMMAUDO, Ivan. "Study of structure and electronic properties of high performance CdTe solar cells by electrical investigation". Doctoral thesis, 2013. http://hdl.handle.net/11562/546550.
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Vedasi abstract in lingua ingleseElectrical characterization is a powerful investigation method for semiconductor devices. Compared to other types of characterization, its main advantage consists in the possibility to analyze the finished devices. For many kinds of technologies this issue is mandatory to understand deeply the device structure, its operation mechanism and how the materials can change during the fabrication process. Therefore, electrical characterization techniques represent probably the most important feedback in the device development. In this thesis a possible methodology to investigate thin films solar cells by means of some electrical characterization techniques will be described. Moreover I will show how these methodologies have been used to extract important information for CdS/CdTe solar cells fabricated in our laboratories. This information has been very useful to develop and optimize a low temperature (< 450 °C) production process so to be able to achieve CdTe solar cells with efficiency exceeding 14 % (best cases over 15 %). In general, the investigations which constitute the chapters of this thesis, have been approached changing in reasonable manner important process parameters and, then, analyzing the resulting effects on the electronic and structural properties of the materials and consequently on the devices. It is known that CdTe needs a special “activation treatment” to perform high efficiency devices. This treatment has been studied to further assess the “magic” benefits on the CdTe semiconductor properties. Two different activation treatments have been optimized in our labs. The first is based on a mixture of gases Ar and difluorochlorometane (Freon®), already used by other researchers, who demonstrated its effectiveness on CdTe cells fabricated at high temperature. The second is based on the deposition of a liquid solution of CdCl2 in methanol and a subsequent annealing in air. Solar cells with CdTe treated in these two different ways were fabricated and analyzed also by means of electrical characterization. Results were compared also with cells fabricated at high temperature kindly provided by Parma University. CdCl2 treatment was able to recrystallize the low temperature deposited CdTe also by improving the electrical properties, while the gaseous treatment was demonstrated to be weak in increasing the carriers concentration but, at the same time, too invasive in affecting the intermixed layer at the CdS/CdTe interface, with an excess of sulfur diffusion. The treatment based on liquid CdCl2 has been further investigated modulating its effectiveness. Under-treated, sub-optimum, optimum and over-treated samples were prepared and analyzed, in order to address the changes involved by the activation treatment on the films composing the devices. It has been demonstrated that a strong connection between the treatment effectiveness and the defect concentration in CdTe polycrystals is present. The CdTe carrier concentration increases as treatment increases but at the same time recombination is also enhanced by the deep defects close to the junction, the optimum treatment represent the best tradeoff between this two phenomena. Another important issue in thin film devices is the absorber thickness, which is desired to be as small as possible. Unfortunately the scaling is usually challenging. By preparing several numbers of cells with different CdTe thickness, it has been demonstrated that the problems connected with thickness are not only light absorption and films homogeneity, but most important they are mainly generated by different materials composition and different transport mechanism. Within this study solar cells with 1.5 µm of CdTe and efficiency exceeding 10 % have been fabricated. Finally, the performance degradation of CdTe solar cells with Cu/Au back contact has also been investigated by electrical characterization. Identical samples were stressed for long time in different condition of light, temperature and electrical bias. Different Cu migration has been observed for the different kinds of stresses, excluding the hypothesis that at different stresses it corresponds just only a different diffusion speed. Moreover we conclude that probably Cu in our samples does not migrate as positively ionized like it has been proposed by other authors, but in negative or neutral configuration generating middle band defects, which enhance recombination.
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Zan, Hsiao Wen y 冉曉雯. "Unified Characterization of Polycrystalline Silicon Thin-Film Transistors and Novel Structures of Polycrystalline Silicon Thin-Film Transistors". Thesis, 2003. http://ndltd.ncl.edu.tw/handle/57597955664005905124.
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博士國立交通大學
電子工程系
91
In this dissertation, the dimensional effects of polycrystalline silicon thin-film transistors (poly-Si TFTs) are studied. By fabricating and characterizing poly-Si TFTs with various channel geometry, different channel thickness, and different grain size. Short channel effects and narrow width effects are investigated and discussed carefully. A physically-based model describing the impact ionization effect is also established and compared with experiment results. In order to suppress the severe short channel effects of poly-Si TFTs, two kinds of novel structures are also proposed and demonstrated. They are found to exhibit better immunity to the kink effects and better performances than their conventional counterparts. Important characteristics such as parasitic effects and reliability issues of these novel poly-Si TFTs are also carefully investigated. First, we focus on studying the short channel effects of poly-Si TFTs. By extracting parameters such as threshold voltage, subthreshold swing, and field-effect mobility, typical short channel effects including threshold voltage roll-off and impact ionization effects under high electric field are discussed. The single transistor latch phenomenon is also observed in short-channel devices under high drain field, the underlying floating body mechanism and its hysteresis phenomenon can be suppressed for devices with narrow channel width. Moreover, the substrate current of devices with various channel length is measured directly from the body contact. Excluding the parasitic band-to-band tunneling current generated from the junction between inversion layer and body region, the electric-field dependent impact ionization current is observed and characterized. A physically-based model considering the vertical-field scattering effect is established to explain the behavior of impact ionization current. Good agreements are found between simulated and measured data through a wide range of gate voltage at various drain voltages. Then, the narrow channel effects are studied by using multichannel structures. The influences of side channel, RIE-induced edge defects and channel trap density are separately discussed by different groups of test devices. The RIE-induced edge defects are found to degrade devices performance when the channel width is wide. The trap density reduction with decreasing channel width, however, compensates this degradation and starts to enhance device performances when the channel width is scaled down to be comparable with the grain size. A novel poly-Si TFTs with ultrathin channel and tungsten-clad source/drain is then fabricated and demonstrated. Channel thickness of 30 nm is used to suppress the floating body effect by reducing body neutral region. To prevent the enormous parasitic source/drain resistance of ultrathin film from degrading device performances, tungsten film is deposited on the source/drain and gate regions by selectively deposition technology. The resulting parasitic resistance of proposed devices is then extracted by analyzing characteristics of devices with various channel length. It is compared with that of conventional ultrathin-channel devices and also the thick channel devices. It is found that the cladding tungsten film effectively reduce the source/drain resistance as expected. Then, a physically-based model describing the linear region transconductance including the parasitic resistance effects is derived. Simulated data are compared with measured data for devices with short channel length. Good agreements are obtained to verify the behavior of parasitic-resistance dominant transconductance as well as the parasitic-resistance dominant field effect mobility. Finally, the output resistance and voltage gain of proposed devices operated under high voltages are also investigated. Good performances are found to ensure the capability of applying the proposed TFTs on both high-speed and high-gain circuitry. Finally, a novel W-spacer short channel poly-Si TFT is fabricated. The W spacer is also formed by selectively W deposition at low temperature. No additional masks or RIE process is needed, leading to reduced production cost and less plasma damage. The spacer thickness can be controlled by changing deposition time. Generally, 600-nm-thick spacer is achievable since W deposition has a very long incubation time on oxide film. While compared with conventional TFTs, small-dimensional W-spacer TFTs have lower leakage current and comparable driving ability. This is because the LDD regions under W-spacers reduce the drain electric field and therefore lower the leakage current. When devices operated under ON state, the series resistance in the LDD region does not degrade the performance of W-spacer TFTs obviously since the W-spacer acts as a part of gate electrode to induce channel. To further study W-spacer TFTs, devices with different channel thickness, spacer thickness and LDD dopant density are fabricated and compared. It is found that W-spacer TFTs with thinner channel thickness have lower leakage current and less pronounced kink effect. This can be explained by the reduced leakage cross sectional area and smaller floating body region in thinner channel. When spacer thickness increases, the turn on current decreases slightly while the kink effect efficiently suppressed by the wider LDD region. The dopant density of LDD regions also influences the kink effect obviously. More lightly doped LDD region reduces drain electric field more efficiently. During plasma passivation process, it is also found that W film will block NH3 molecules from entering the channel region. Therefore, the passivaiton effect will be varied with different spacer thickness and various channel geometries. For devices with small dimension and thinner spacer thickness, devices exhibit better performance since they have better passivation condition. Finally, the hot-carrier reliability of W-spacer TFTs is also investigated by introducing hot-carrier stress. It is found that W-spacer TFTs have better reliability than conventional ones because they have lower drain electric field, which suppresses the impact ionization effect during hot-carrier stress.
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Lien, Wen-Hung y 連文宏. "Study of polycrystalline silicon thin film transistors". Thesis, 1995. http://ndltd.ncl.edu.tw/handle/57694513232941617495.
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碩士國立交通大學
應用化學系
83
The solid pases recrystallization of amorphous silicon ( α-Si ) films deposited by a low-pressure chemical vapor deposition system using disilane (Si2H6) gas at various annealing conditions are investigated. The grain sizes of recrystallized films formed from Si2H6 are larger than that formed from SiH4. The maximum grain size is obtained at the deposition temperature of 475℃, where the nucleation rate is minimum due to the maximum structural disorder of the Si network. The structural disorder increases not only by lowering the substrate temperature but also by increasing the deposition rate. A significant improvements concerning electrical characteristics are achieved due to the great grain size. A new method which combinates low-temperature furnace annealing and high-temperature rapid thermal annealing leads to obtained high- quality poly-Si films and to reduce the long annealing time for solid phase crystallization(SPC) of amorphous silicon films. The low temperature oxide films deposited at 200℃ by photochemical- assisted vapor deposition process can be used to combinate the recrystallized poly-Si films to achieve the low temperature fabrication of Poly-Si TFTs.We also obtaine excellent electrical characteristics, even though the maximum temperature during the TFT fabrication process is only 600 ℃.