Spectroscopic ellipsometry has been applied to a wide variety of material and device characterizations in solar cell research fields. In particular, device performance analyses using exact optical constants of component layers and direct analyses of complex solar cell structures are unique features of advanced ellipsometry methods. This second volume of Spectroscopic Ellipsometry for Photovoltaics presents various applications of the ellipsometry technique for device analyses, including optical/recombination loss analyses, real-time control and on-line monitoring of solar cell structures, and large-area structural mapping. Furthermore, this book describes the optical constants of 148 solar cell component layers, covering a broad range of materials from semiconductor light absorbers (inorganic, organic and hybrid perovskite semiconductors) to transparent conductive oxides and metals. The tabulated and completely parameterized optical constants described in this book are the most current resource that is vital for device simulations and solar cell structural analyses.

Spectroscopic ellipsometry has been applied to a wide variety of material and device characterizations in solar cell research fields. In particular, device performance analyses using exact optical constants of component layers and direct analyses of complex solar cell structures are unique features of advanced ellipsometry methods. This second volume of Spectroscopic Ellipsometry for Photovoltaics presents various applications of the ellipsometry technique for device analyses, including optical/recombination loss analyses, real-time control and on-line monitoring of solar cell structures, and large-area structural mapping. Furthermore, this book describes the optical constants of 148 solar cell component layers, covering a broad range of materials from semiconductor light absorbers (inorganic, organic and hybrid perovskite semiconductors) to transparent conductive oxides and metals. The tabulated and completely parameterized optical constants described in this book are the most current resource that is vital for device simulations and solar cell structural analyses.

This dissertation describes a collection of studies that demonstrate and expand the many configurations in which spectroscopic ellipsometry (SE) can be applied to material characterization, primarily for thin films. The materials investigated each have relevance to photovoltaics, but the methods described herein can be applicable to the study of materials used in virtually any application. In aggregate, the measurement and data modeling techniques represent a broad set of tools that can be used to study the optoelectronic and structural characteristics of amorphous, polycrystalline, nanostructured, or inhomogeneous layers within thin film solar cell devices. The capabilities for SE to determine properties of a sample of interest well beyond simple optical response functions are demonstrated. In particular, SE is used in a real-time, in situ configuration where a series of measurements are taken continually during the deposition of all layers in complete hydrogenated amorphous silicon (a-Si:H) solar cells. Thus, the application of real-time SE (RTSE) to the entire process of solar cell fabrication is realized. The optical response and thicknesses of each layer are obtained and are used to interpret variation in the measured electrical performance between different devices. The SE-derived results are then used as inputs to a simulation of the expected current generated by the devices, the results of which were successful in identifying damage to a transparent conducting layer resulting from exposure to plasma during sample fabrication as the source of performance losses. The study of full a-Si:H-based solar cells revealed the presence of subtle optical property gradients within individual layers. The ability to characterize slight inhomogeneity using RTSE was further developed using measurements collected for a-Si:H films deposited under various conditions and on various substrates. In particular, this work systematically examines a range of modeling configurations in the virtual interface analysis (VIA) technique used to extract the results. Through testing the influence of two specific modeling parameters on the overall error associated with each model, the optimum analysis models are identified, enabling the extraction of the most accurate results. Ultimately, the evolution of an optical broadening parameter is extracted for aSi:H films grown on various substrates and under various conditions. Limitations of the VIA technique are also identified and discussed. Next, the effects of varying deposition and processing conditions on the optical response of resultant films is studied through ex situ SE measurements of a series of oxygenated cadmium sulfide (CdS:O) films. A custom parametric description of the optical response of these films applicable to polycrystalline semiconductors is developed that makes use of physically realistic descriptions of the optical features over the full measured spectral range. From the resulting optical properties, increasing oxygen presence during deposition is shown to suppress absorption in the films and modify the band gap energy for as-deposited films. Additionally, annealing is shown to revert all CdS:O band gap energies to that of pure cadmium sulfide (2.4 eV) and improve crystallographic order. Since CdS:O is used in high performance thin film photovoltaics, these optical results contribute to an explanation of what the role of this material is specifically in improved device performance, specifically the decreased optical absorption at ultraviolet photon energies. Finally, the benefits of extending SE into the THz frequency regime is investigated. Since THz SE is an emerging subfield of ellipsometry unique THz-specific considerations are investigated and discussed. Limitations imposed by the instrument are identified and efforts to identify the minimum measurement resolution and range that will still produce acceptable results are presented as a means for decreasing total measurement time. Then, the results of THz SE applied to single bulk crystals is presented where sensitivity to carrier concentrations as low as 1014 cm-3 is demonstrated. Finally, a study of the effects of doping on a single walled carbon nanotube (SWCNT) thin film is reported. More specifically, the optical response of the SWCNT film over a wide spectral range spanning the THz to the ultraviolet and uniaxially anisotropic electrical properties are determined for the film in two doping states.

Spectroscopic ellipsometry (SE) in the mid-infrared to ultraviolet range has been implemented in order to develop and evaluate optimization procedures for CdTe solar cells at the different stages of fabrication. In this dissertation research, real time SE (RT-SE) has been applied during the fabrication of the as-deposited CdS/CdTe solar cell. Two areas of background research were addressed before undertaking the challenging RT-SE analysis procedures. First, optical functions were parameterized versus temperature for the glass substrate and its overlayers, including three different SnO2 layers. This database has applications not only for RT-SE analysis but also for on-line monitoring of the coated glass itself at elevated temperature. Second, post-deposition modifications of substrate have been studied by infrared spectroscopic ellipsometry (IR-SE) prior to the RT-SE analysis in order to evaluate the need for such modification in the analysis. With support from these background studies, RT-SE has been implemented in analyses of the evolution of the thin film structural properties during sputter deposition of polycrystalline CdS/CdTe solar cells on the transparent conducting oxide (TCO) coated glass substrates. The real time optical spectra collected during CdS/CdTe deposition were analyzed using the optical property database for all substrate components as a function of measurement temperature. RT-SE enables characterization of the filling process of the surface roughness modulations on the top-most SnO2 substrate layer, commonly referred to as the high resistivity transparent (HRT) layer. In this filling process, the optical properties of this surface layer are modified in accordance with an effective medium theory. In addition to providing information on interface formation to the substrate during film growth, RT-SE also provides information on the bulk layer CdS growth, its surface roughness evolution, as well as overlying CdTe interface formation and bulk layer growth. Information from RT-SE at a single point during solar cell stack deposition assists in the development of a model that has been used for mapping the properties of the completed cell stack, which can then be correlated with device performance. Independent non-uniformities in the layers over the full area of the cell stack enable optimization of cell performance combinatorially.The polycrystalline CdS/CdTe thin-film solar cell in the superstrate configuration has been studied by SE using glass side illumination whereby the single reflection from the glass/film-stack interface is collected whereas that from the ambient/glass interface and those from multiple glass/film-stack reflections are rejected. The SE data analysis applies an optical model consisting of a multilayer stack with bulk and interface layers. The dielectric functions ¿¿for the solar cell component materials were obtained by variable-angle and in-situ SE. Variability in the properties of the materials are introduced through free parameters in analytical expressions for the dielectric functions. In the SE analysis of the complete cell, a step-wise procedure ranks all free parameters of the model, including thicknesses and those defining the spectra in¿¿, according to their ability to reduce the root-mean-square deviation between simulated and measured SE spectra. The results for the best fit thicknesses compare well with electron microscopy. From the optical model, including all best-fit parameters, the solar cell quantum efficiency (QE) can be simulated without free parameters, and comparisons with QE measurements have enabled the identification of losses. The capabilities have wide applications in off-line photovoltaic module mapping and in-line monitoring of coated glass at intermediate stages of production. Mapping spectroscopic ellipsometry (M-SE) has been applied in this dissertation research as an optimization procedure for polycrystalline CdS/CdTe solar cell fabrication on TCO coated glass superstrates. During fabrication of these solar cells, the structure undergoes key processing steps after the sputter-deposition of the CdS/CdTe. These steps include CdCl2 treatment of the CdTe layer and subsequent deposition of ultrathin Cu. Additional steps involve final metal back contact layer deposition and an anneal for Cu diffusion that completes the device. In this study, we have fabricated cells with variable absorber thicknesses, ranging from 0.5 to 2.5 ¿m, and variable CdCl2 treatment times, ranging from 5 to 30 min. Because both CdS window and Cu back contact layers are critical for determining device performance, the ability to characterize their deposition processes and determine the resulting process-property-performance relationships is important for device optimization. We have applied M-SE to map the effective thickness (volume/area) of the CdS and Cu films over 15 cm x 15 cm substrates prior to the fabrication of 16 x 16 arrays of dot cells. We report correlations of cell performance parameters with the CdCl2 treatment time and with the effective thicknesses from M-SE analysis. We demonstrate that correlations between optical/structural parameters extracted from M-SE analysis and device performance parameters facilitate process optimization. We have explored and applied p-type semiconducting materials as novel back contact materials in CdTe solar cells. Wide band-gap, p-type doped, hydrogenated amorphous silicon-carbon alloy (a-Si1-xCx:H:B) layers deposited by plasma enhanced chemical vapor deposition (PECVD) under conditions that yield efficient hydrogenated amorphous silicon (a Si:H) p-i-n solar cells have been applied as back contacts to sputter-deposited CdTe superstrate solar cells. We report a maximum observed Voc value of 0.78 V and a best initial efficiency of ~ 7.7 % (relative to an ~ 12% standard cell baseline) without the introduction of Cu into the back contact region. Instability of solar cells that incorporate such back contacts have hindered their further development. We also applied copper indium diselenide (CuInSe2) as a novel back contact material in CdTe solar cells in the superstrate configuration. We report a maximum observed Voc value of 0.68 V and a best efficiency of ~ 6.4 % (relative to an ~ 12.6 % standard cell baseline) without the introduction of Cu.

This book provides a basic understanding of spectroscopic ellipsometry, with a focus on characterization methods of a broad range of solar cell materials/devices, from traditional solar cell materials (Si, CuInGaSe2, and CdTe) to more advanced emerging materials (Cu2ZnSnSe4, organics, and hybrid perovskites), fulfilling a critical need in the photovoltaic community. The book describes optical constants of a variety of semiconductor light absorbers, transparent conductive oxides and metals that are vital for the interpretation of solar cell characteristics and device simulations. It is divided into four parts: fundamental principles of ellipsometry; characterization of solar cell materials/structures; ellipsometry applications including optical simulations of solar cell devices and online monitoring of film processing; and the optical constants of solar cell component layers.

The book presents a comprehensive survey about advanced solar cell technologies. Focus is placed on semiconductor materials, solar cell efficiency, improvements in surface recombination velocity, charge density, high ultraviolet (UV) sensitivity, modeling of solar cells etc. The book references 281 original resources with their direct web links for in-depth reading. Keywords: Solar Cells, Thin Film Solar Cells, Solar Cell Efficiency, Semiconductor Materials, Surface Recombination Velocity, Charge Density, High UV Sensitivity, Heavily-doped Silicon Wafers, Amorphous Semiconductors, Nanocrystalline Semiconductors, Field Effect, Ferroelectric Semiconductors, Solar Cell Modelling.