We have performed fluorescence extended X-ray absorption fine structure (EXAFS) measurements on the Cd K-edge of partial electrolyte (PE) treated Cu(In[subscript 0.7]Ga[subscript 0.3])Se��� (CIGS) thin film samples using synchrotron X-ray radiation. This data was compared to the EXAFS spectra of CdSe and CdO standards. Cd local structure models were constructed and used for the least square analysis of the spectra. The first model employed implantation of a cadmium atom and a single oxygen atom into the CIGS lattice. Specifically, an oxygen atom was introduced in the tetrahedral bonded Cd-Se local structure. Employing FEFF8 with WinXAS software package, experimental data was theoretically fitted to the first shell single-scattering paths of the Cd atom in the (PE) treated Cu(In[subscript 0.7]Ga[subscript 0.3])Se��� thin film samples. The main peak observed in the data represents the Cd-Se bonds and the shoulder corresponds to the Cd-O bond. However, the number of total nearest neighbors is not consistent with this model. A two-phase model that includes both Cd-Se tetrahedron and Cd-O octahedron were then reconstructed. Again, a least-agrees very well with the experimental data, and the total first nearest neighbor number is consistent with the two phase model at NN=4.2. This study indicates the surface of Cd partial electrolyte treated Cu(In[subscript 0.7]Ga[subscript 0.3])Se��� thin films contains both CdSe and CdO. / Graduation date: 2004

Ternary chalcopyrite compounds are the semiconductors with suitable properties to be used as absorber materials in thin film solar cells. AgInSe2 is a promising candidate with its several advantages over the widely used CuInSe2. The purpose of this study was to optimize the physical properties of the Ag-In-Se (AIS) thin films that were deposited by e-beam evaporation of Ag3In5Se9 single crystal powder for solar cell applications by means of post-annealing process under nitrogen atmosphere. The as-grown AIS thin films were annealed at 200, 300 and 400oC and their structural, optical, electrical and photoelectrical properties were examined to observe the effects of post-annealing process. Structural characterization of the films was performed by X-ray diffraction (XRD), energy dispersive X-ray spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM) analyses. Optical properties of the films were investigated by optical transmittance measurements. Electrical and photoelectrical properties of the films were examined by temperature dependent conductivity, photoconductivity under different illumination intensities and spectral photoresponse measurements. It was discovered that the annealing of AIS thin films at 200oC resulted in the best physical properties for solar cell applications. The obtained films were polycrystalline with mixed binary and ternary crystalline phases, such as Ag3In5Se9, AgInSe2 and InSe, and showed n-type conductivity with room temperature conductivity value of 2.3x10-6 (Ohm&shy / cm)-1. The band gap energy of the 200oC-annealed films was determined as 1.68 eV from spectral photoresponse measurements. The results of the study revealed that the inadequate Ag incorporation and segregation and/or reevaporation of Se atoms at high annealing temperatures were the major problems encountered in producing single phase polycrystalline AgInSe2 thin films. The required stoichiometry of thin films should be maintained during the growth of the films by means of an alternative deposition procedure and the films should be selenized during post-annealing process.

Includes vita. / Title from PDF of title page. / Document formatted into pages; contains 204 pages. / Thesis (Ph.D.)--University of South Florida, 2003. / Includes bibliographical references. / Text (Electronic thesis) in PDF format. / ABSTRACT: The goal of this research project was to contribute to the understanding of CuGaSe2/CdS photovoltaic devices, and to improve the performance of these devices. The initial part of the research dealt with the optimization of a Sequential Deposition process for CuIn(Ga)Se2 absorber formation. As an extension of this, a recipe (Type I Process) for CuGaSe2 absorber layer fabrication was developed, and the deposition parameters were optimized. Electrical characterization of the thin films and completed devices was carried out using techniques such as Two-Probe and Three-Probe Current-Voltage, Capacitance-Frequency, Capacitance-Voltage, and Spectral Response measurements. Structural/chemical characterization was done using XRD and EDS analysis. Current densities of up to 15.2 mA/cm2, and Fill Factors of up to 58% were obtained using the Type I CuGaSe2 Process. VOC's, however, were limited to less than 700 mV. / ABSTRACT: Several process variations, such as changes in the rate/order/temperature of depositions and changes in the thickness of layers, resulted in little improvement. With the aim of breaking through this VOC performance ceiling, a new absorber recipe (Type II Process) was developed. VOC's of up to 735 mV without annealing, and those of up to 775 mV after annealing, were observed. Fill Factors were comparable to those obtained with Type I Process, whereas the Current Densities were found to be reduced (typically, 10-12 mA/cm2, with the best value of 12.6 mA/cm2). This performance of Type II devices was correlated to a better intermixing of the elements during the absorber formation. To gain an understanding of the performance limitations, two simulation techniques, viz. SCAPS and AMPS, were used to model our devices. / ABSTRACT: Several processing experiments and SCAPS modeling indicate that a defective interface between CuGaSe2 and CdS, and perhaps a defective absorber layer, are the cause of the VOC limitation. AMPS simulation studies, on the other hand, suggest that the back contact is limiting the performance. Attempts to change the physical back contact, by changes in the absorber processing, were unsuccessful. Processing experiments and simulations also suggest that the CuGaSe2/CdS solar cell involves a true heterojunction between these two layers. / System requirements: World Wide Web browser and PDF reader. / Mode of access: World Wide Web.

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Currently conventional sources like coal, petroleum and natural gas meet the energy requirements of developing and undeveloped countries. Over a period of time there is high risk of these energy sources getting depleted. Hence an alternate source of energy i.e. renewable energy is the need of the hour. The advantages of renewable energy like higher sustainability, lesser maintenance, low cost of operation, and minimal impact on the environment make the role of renewable energy sources significant. Out of the various renewable energy sources like solar energy, wind energy, hydropower, biogas, tidal and geothermal, usage of solar energy is gradually increasing. Among various solar energy sources, Photovoltaics has dominated over the past two decades since it is free clean energy and availability of abundant sunlight on earth. Over the past few decades, thin film solar cells (TFSC) have gained considerable interest as an economically feasible alternative to conventional silicon (Si) photovoltaic devices. TFSCs have the potential to be as efficient as Si solar cells both in terms of conversion efficiency as well as cost. The advantages of TFSC are that they are easy to prepare, lesser thickness, requires lesser materials, light weight, low cost and opto-electronic properties can be tuned by varying the process parameters. The present study is focused on the fabrication of AgInS2/ZnS heterojunction thin film solar cell. AgInS2 absorber layer is deposited using both vacuum (sputtering/sulfurization) and non-vacuum (ultrasonic spray pyrolysis) techniques. ZnS window layer is prepared using thermal evaporation technique, detailed experimental investigation has been conducted and the results have been reported in this work. The thesis is divided into 6 chapters. Chapter 1 gives general introduction about solar cells and working principle of solar cell. It also discusses thin film solar cell technology and its advantages. Layers of thin film solar cell structure, Significance of each layers and possible materials to be used are emphasized. A detailed overview of the available literature on both AgInS2 absorber layer and ZnS window layer has been presented. Based on the literature review, objectives of the present work are defined. Chapter 2 explains the theory and experimental details of deposition techniques used for the growth of AgInS2 and ZnS films. Details of characterization techniques to study film properties are described in detail. Chapter 3 presents a systematic study of AgInS2 thin films deposited by sulfurization of sputtered Ag-In metallic precursors. Initially, AgInS2 films are deposited by varying the substrate temperature and properties of as-deposited films are characterized. Structural, morphological, electrical and optical properties of AgInS2 films are explained. From these studies, samples with better properties at particular substrate temperature are optimized. By fixing the substrate temperature, deposition time of silver is varied by keeping other deposition conditions same and the properties of films are discussed. It was observed that deposition time of silver doesn’t have much impact on structural properties of AgInS2 films. However, opto-electric properties of AgInS2 films are enhanced. Based on characterization studies, deposition time of silver is optimized. Deposition time of indium is varied by keeping substrate temperature and silver deposition to optimized value. The properties of as-deposited films are discussed. Based on the above studies, the optimized p type films have a band gap of 1.64 eV, carrier concentration of 1013 ions/cm3 and Resistivity of order 103 Ω-cm. Chapter 4 presents a systematic study of AgInS2 thin films deposited by ultrasonic spray pyrolysis. AgInS2 films are deposited by varying the substrate temperature and properties of as deposited films are characterized. Structural, morphological, electrical and optical properties of AgInS2 films are explained. From these studies, samples with better properties at particular substrate temperature are optimized. By fixing the substrate temperature, concentration of silver molarity in the precursor solution is varied by keeping other deposition conditions same and the properties of films are discussed. Structural, optical and electrical properties of AgInS2 films are enhanced with the increase in silver concentration. Based on characterization studies, concentration of silver is optimized. Similarly concentration of indium molarity in the precursor solution is varied and the properties of as-deposited films are discussed. Finally, sulfur molarity in the precursor solution is varied and properties of films are discussed. It was observed that increasing sulfur after certain limit does not have any effect on the properties of the films. Based on the above studies, this method resulted in the films with resistivity of 103 Ω-cm and band gap of 1.64 eV. These films showed a carrier concentration of 1013 ions/cm3. Chapter 5 describes the growth of ZnS films using thermal evaporation technique. Influence of thickness on the properties of ZnS films is explained. Samples with good crystallinity, high transmission, and wider gap are selected for device fabrication. This p type layer showed a band gap of 3.52 eV. Solar cells have been fabricated using the AgInS2 films developed by both sputtering and ultrasonic spray pyrolysis techniques. A maximum cell efficiency of 0.92 percent has been achieved for the cell with 0.950 µm thick sputtered AgInS2 layer and thermally evaporated 42 nm thick ZnS layer. In comparison, the ultrasonic spray pyrolysis deposited films gave an efficiency of 0.54 percent. These values are comparable to those mentioned in a couple of reports earlier. Chapter 6 summarizes the conclusions drawn from the present investigations and scope of future work is suggested.

We developed a three-step colloidal synthesis of near-infrared active Cu-In-Se (CISe)/ZnS core/shell nanocrystals (NCs) via a sequential partial cation exchange. In the first step binary highly copper deficient Cu2‒xSe NCs were synthesized, followed by a partial cation exchange of copper to indium ions yielding CISe NCs. In order to enhance the stability and the photoluminescence (PL) properties of the NCs, a subsequent ZnS shell was grown, resulting in CISe/ZnS core/shell NCs. These core/shell hetero-NCs exhibited a dramatic increase in size and a restructuring to trigonal pyramidal particles. The reaction parameters, e.g. the Cu:Se-ratio, the temperature and the time were carefully tuned enabling a distinct control over the size and the composition of the NCs. By varying only the size of the CISe/ZnS NCs (from 9 to 18 nm) the PL spectra could be tuned covering a wide range with maxima from 990 nm to 1210 nm. Thus, in these experiments we demonstrate a clear dependence of the optical properties of these materials on their size and extend the PL range of CISe-based nanoparticles further to the infrared part of the spectrum. Furthermore, the relatively large size of these NCs allows their detailed structural analysis via electron microscopy techniques, which is particularly challenging in the case of small particles and especially important to relate the size, composition and crystal structure to their optoelectronic properties.

Indiana University-Purdue University Indianapolis (IUPUI) / In this research thesis, copper indium gallium selenium (CIGS) nanoparticles were synthesized from metal chlorides, functionalized to disperse in water, and further used in layer by layer (LbL) nanoassembly of CIGS films. CIGS nanoparticles were synthesized through the colloidal precipitation in an organic solvent. The peak and average sizes of the synthesized particles were measured to be 68 nm and 75 nm in chloroform, and 30 nm and 115 nm in water, respectively. Two methods were used to disperse the particle in water. In the first method the stabilizing agent oleylamine (OLA) was removed through multiple cleaning processes, and in the second method ligand exchange was performed with polystyrene sulfonate (PSS). Zeta potential of CIGS nanoparticles dispersed in water was measured to be +61 mV. The surface charge of the nanoparticles was reversed by raising the pH of the solution, which was measured to be −43.3 mV at 10.5 pH. In a separate process, the CIGS nanoparticles dispersed in water were coated with PSS. The resulting dispersion was observed to be stable and the surface charge was measured to be −56.9 mV. The LbL deposition process of CIGS nanoparticles was characterized by depositing thin films on quartz crystal microbalance (QCM). LbL depositions was conducted using (i) oppositely charged CIGS nanoparticles, (ii) positively charged CIGS nanoparticles and PSS, and (iii) PSS-coated CIGS (CIGS-PSS) and polyethyleneimine (PEI). The average thickness of each bi-layer of the above mentioned depositions were measured to be 2.2 nm, 1.37 nm, and 10.12 nm, respectively. The results from the QCM have been observed to be consistent with the film thickness results obtained from atomic force microscopy (AFM). Various immersion times versus thickness of the film were also studied. For electrical characterization, the CIGS films were deposited on indium tindioxide (ITO)-coated glass substrates. Current versus voltage (I/V) measurements were carried out for each of the films using the Keithley semiconductor characterization instruments and micromanipulator probing station. It was observed that the conductivity of the films was increased with the deposition of each additional layer. The I/V characteristics were also measured under the light illumination and after annealing to study the photovoltaic and annealing effects. It was observed that under light illumination, the resistivity of a 12-layer CIGS film decreased by 93% to 0.54 MΩ.m, and that of the same number of layers of PSS-coated CIGS and PEI film decreased by 60% to 0.97 MΩ.m under illumination. The resistivity of an 8-layer CIGS and PSS film decreased by 76.4% to 0.1 MΩ.m, and that of the same layers of PSS-coated CIGS and PEI decreased by 87% to 0.07 MΩ.m after annealing. The functionalized nanoparticles and the LbL CIGS films were implemented in the solar cell devices. Several configurations of CIGS films (p-type), and ZnO and CdS films (n-type) were considered. Poly(3,4-ethylenedioxythiophene) (PEDOT), molybdenum (Mo), and ITO were used as back contacts and ITO was used as front contact for all the devices. The devices were characterized the Keithley semiconductor characterization instruments and micromanipulator probing station. For a CIGS and n-ZnO films device with PEDOT as back contact and ITO as front contact, the current density at 0 V and under light illumination was measured to be 60 nA/cm2 and the power density was measured to be 0.018 nW/cm2. For a CIGS and CdS films device with ITO as both back and front contact, the current density at 0 V and under light illumination was measured to be 50 nA/cm2 and the power density was measured to be 0.01 nW/cm2. For a drop-casted CIGS and CdS films device with Mo as back contact and ITO as front contact, the current density of 50 nA/cm2 at 0 V and power density of 0.5 nW/cm2 under light illumination was measured. For the LbL CIGS and chemical bath deposited CdS films device with ITO as both back and front contact, the current density of 0.04 mA/cm2 at 0 V and power density of 1.6 μW/cm2 under light illumination was measured. Comparing to Device-III, an increase by 99% in the power density was observed by using the CIGS LbL film in the device structure. The novel aspects of this research include, (i) functionalization of the CIGS nanoparticles to disperse in water including coating with PSS, (ii) electrostatic LbL deposition of CIGS films using oppositely charged nanoparticles and polymers, and (iii) the utilization of the fabricated LbL CIGS films to develop solar cells. In addition, the n-type cadmium sulfide film (CdS) and zinc oxide (ZnO) buffer layer were also deposited through LbL process after the respective particles were functionalized with PSS coating in separate experiments.

Indiana University-Purdue University Indianapolis (IUPUI) / Fabrication and analysis of Copper Indium Gallium di-Selenide (CIGS) nanoparticles-based thin film solar cells are presented and discussed. This work explores non-traditional fabrication processes, such as spray-coating for the low-cost and highly-scalable production of CIGS-based solar cells. CIGS nanoparticles were synthesized and analyzed, thin CIGS films were spray-deposited using nanoparticle inks, and resulting films were used in low-cost fabrication of a set of CIGS solar cell devices. This synthesis method utilizes a chemical colloidal process resulting in the formation of nanoparticles with tunable band gap and size. Based on theoretical and experimental studies, 100 nm nanoparticles with an associated band gap of 1.33 eV were selected to achieve the desired film characteristics and device performances. Scanning electron microcopy (SEM) and size measurement instruments (Zetasizer) were used to study the size and shape of the nanoparticles. Electron dispersive spectroscopy (EDS) results confirmed the presence of the four elements, Copper (Cu), Indium (In), Gallium (Ga), and Selenium (Se) in the synthesized nanoparticles, while X-ray diffraction (XRD) results confirmed the tetragonal chalcopyrite crystal structure. The ultraviolet-visible-near infra-red (UV-Vis-NIR) spectrophotometry results of the nanoparticles depicted light absorbance characteristics with good overlap against the solar irradiance spectrum. The depositions of the nanoparticles were performed using spray-coating techniques. Nanoparticle ink dispersed in ethanol was sprayed using a simple airbrush tool. The thicknesses of the deposited films were controlled through variations in the deposition steps, substrate to spray-nozzle distance, size of the nozzle, and air pressure. Surface features and topology of the spray-deposited films were analyzed using atomic force microscopy (AFM). The deposited films were observed to be relatively uniform with a minimum thickness of 400 nm. Post-annealing of the films at various temperatures was studied for the photoelectric performance of the deposited films. Current density and voltage (J/V) characteristics were measured under light illumination after annealing at different temperatures. It was observed that the highest photoelectric effect resulted in annealing temperatures of 150-250 degree centigrade under air atmosphere. The developed CIGS films were implemented in solar cell devices that included Cadmium Sulfide (CdS) and Zinc Oxide (ZnO) layers. The CdS film served as the n-type layer to form a pn junction with the p-type CIGS layer. In a typical device, a 300 nm CdS layer was deposited through chemical bath deposition on a 1 $mu$m thick CIGS film. A thin layer of intrinsic ZnO was spray coated on the CdS film to prevent shorting with the top conductor layer, 1.5 μm spray-deposited aluminum doped ZnO layer. A set of fabricated devices were tested using a Keithley semiconductor characterization instrument and micromanipulator probe station. The highest measured device efficiency was 1.49%. The considered solar cell devices were simulated in ADEPT 2.0 solar cell simulator based on the given fabrication and experimental parameters. The simulation module developed was successfully calibrated with the experimental results. This module can be used for future development of the given work.