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abstract: Infant mortality rate of field deployed photovoltaic (PV) modules may be expected to be higher than that estimated by standard qualification tests. The reason for increased failure rates may be attributed to the high system voltages. High voltages (HV) in grid connected modules induce additional stress factors that cause new degradation mechanisms. These new degradation mechanisms are not recognized by qualification stress tests. To study and model the effect of high system voltages, recently, potential induced degradation (PID) test method has been introduced. Using PID studies, it has been reported that high voltage failure rates are essentially due to increased leakage currents from active semiconducting layer to the grounded module frame, through encapsulant and/or glass. This project involved designing and commissioning of a new PID test bed at Photovoltaic Reliability Laboratory (PRL) of Arizona State University (ASU) to study the mechanisms of HV induced degradation. In this study, PID stress tests have been performed on accelerated stress modules, in addition to fresh modules of crystalline silicon technology. Accelerated stressing includes thermal cycling (TC200 cycles) and damp heat (1000 hours) tests as per IEC 61215. Failure rates in field deployed modules that are exposed to long term weather conditions are better simulated by conducting HV tests on prior accelerated stress tested modules. The PID testing was performed in 3 phases on a set of 5 mono crystalline silicon modules. In Phase-I of PID test, a positive bias of +600 V was applied, between shorted leads and frame of each module, on 3 modules with conducting carbon coating on glass superstrate. The 3 module set was comprised of: 1 fresh control, TC200 and DH1000. The PID test was conducted in an environmental chamber by stressing the modules at 85°C, for 35 hours with an intermittent evaluation for Arrhenius effects. In the Phase-II, a negative bias of -600 V was applied on a set of 3 modules in the chamber as defined above. The 3 module set in phase-II was comprised of: control module from phase-I, TC200 and DH1000. In the Phase-III, the same set of 3 modules which were used in the phase-II again subjected to +600 V bias to observe the recovery of lost power during the Phase-II. Electrical performance, infrared (IR) and electroluminescence (EL) were done prior and post PID testing. It was observed that high voltage positive bias in the first phase resulted in little/no power loss, high voltage negative bias in the second phase caused significant power loss and the high voltage positive bias in the third phase resulted in major recovery of lost power. / Dissertation/Thesis / M.S. Engineering 2011

abstract: Photovoltaic (PV) modules are typically rated at three test conditions: STC (standard test conditions), NOCT (nominal operating cell temperature) and Low E (low irradiance). The current thesis deals with the power rating of PV modules at twenty-three test conditions as per the recent International Electrotechnical Commission (IEC) standard of IEC 61853 – 1. In the current research, an automation software tool developed by a previous researcher of ASU – PRL (ASU Photovoltaic Reliability Laboratory) is validated at various stages. Also in the current research, the power rating of PV modules for four different manufacturers is carried out according to IEC 61853 – 1 standard using a new outdoor test method. The new outdoor method described in this thesis is very different from the one reported by a previous researcher of ASU – PRL. The new method was designed to reduce the labor hours in collecting the current-voltage ( I – V) curves at various temperatures and irradiance levels. The power matrices for all the four manufacturers were generated using the I – V data generated at different temperatures and irradiance levels and the translation procedures described in IEC 60891 standard. All the measurements were carried out on both clear and cloudy days using an automated 2 – axis tracker located at ASU – PRL, Mesa, Arizona. The modules were left on the 2 – axis tracker for 12 continuous days and the data was continuously and automatically collected for every two minutes from 6 am to 6 pm. In order to obtain the I – V data at wide range of temperatures and irradiance levels, four identical (or nearly identical) modules were simultaneously installed on the 2 – axis tracker with and without thermal insulators on the back of the modules and with and without mesh screens on the front of the modules. Several issues related to the automation software were uncovered and the required improvement in the software has been suggested. The power matrices for four manufacturers have been successfully generated using the new outdoor test method developed in this work. The data generated in this work has been extensively analyzed for accuracy and for performance efficiency comparison at various temperatures and irradiance levels. / Dissertation/Thesis / M.S.Tech Technology 2012

>Magister Scientiae - MSc / The efficiency of organic photovoltaics is influenced by the structure of the polymer, the morphology of the film, the interfaces between the layers, the choice of electron acceptor material and the ratio between the electron acceptor material and the polymer. In this project, we have used regioregular poly (3-hexylthiophene) (rr-P3HT) as the electron donor material and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM), a derivative of the C60 fullerene, as the acceptor material. Different weight-ratios of rr-P3HT to PCBM were prepared by stirring for several hours in a chloroform solution and subsequently spin coated onto crystalline silicon and transparent conductive oxide/glass substrates. The effect of the PCBM concentration on the photo-induced optical emission and absorption properties of rr-P3HT was investigated by photoluminescence and ultraviolet-visible spectroscopy, respectively. Changes in the structural properties, as a function of the weight-ratio, were probed by high-resolution transmission electron microscopy, x-ray diffraction, Fourier transform infrared spectroscopy and Raman spectroscopy. Results show that the structural integrity and crystallinity of rr-P3HT is compromised with the addition of excessive amounts of PCBM, which has a negative impact on the optical absorption of rr-P3HT and the photo-induced charge transfer mechanism between the rr-P3HT and PCBM. This work illustrates that blending rr-P3HT with an equal weight of PCBM results in an optimum configuration for improved performance in organic photovoltaic devices.

En vue de limiter la dégradation par l’humidité et l’oxygène des dispositifs photovoltaïquesorganiques flexibles, les cellules solaires sont encapsulées entre des films barrières aux gaz.Malgré l’importance des procédés d’encapsulation et leur potentiel impact sur les performancesinitiales et lors du vieillissement des dispositifs, ils sont rarement étudiés dans la littérature. Enoutre, plusieurs études de vieillissement sur le terrain ont montré que la détérioration mécaniquelimitait la durée de vie des échantillons bien avant que leur stabilité photochimique ne soit miseen cause. L’adhésion entre les différentes couches composant les cellules est donc un facteurcritique afin d’obtenir des dispositifs flexibles fiables après leur mise en oeuvre et lors de leurutilisation.Dans ce travail, deux procédés d’encapsulation ont été étudiés : la lamination à rouleauxd’un adhésif sensible à la pression et la lamination sous vide d’un thermoplastique. Afin dequantifier l’adhésion de chacune des interfaces comprises dans les échantillons, la technique decaractérisation mécanique par pelage à 180° a été adaptée et ensuite appliquée aux dispositifsflexibles. De plus, des techniques de caractérisation des dispositifs par imagerie non-destructiveont été développées : la cartographie du courant induit par faisceau laser (plus courammentconnu sous l’acronyme anglais « LBIC ») et l’imagerie de luminescence sous excitation optiqueet électrique. Grace à ces techniques, l’hypothèse d’une dégradation mécanique des dispositifsdurant le procédé d’encapsulation par lamination à rouleaux a été émise. Des solutions permettantl’amélioration des interfaces identifiées comme mécaniquement faibles ont été recherchées etensuite évaluées par rapport aux performances photovoltaïques des dispositifs de référence.Les techniques d’imagerie développées précédemment ont également été appliquées durant levieillissement en condition accélérées des cellules encapsulées. Un mécanisme a été proposé,qui permet d’expliquer la localisation spatiale de la dégradation mais également le type dedégradation, optique ou électrique, survenu à chaque étape du vieillissement. / In order to limit the flexible organic photovoltaic devices degradation induced by moisture andoxygen, the solar cells are encapsulated between two gas-barrier films. Despite the importanceof the encapsulation processes and their potential influence on the initial performances as well asduring aging of the devices, they are scarcely described in the literature. Furthermore, severalfield aging studies showed that mechanical degradation could limit the devices lifetime beforetheir photo-chemical stability became an issue. Thus, adhesion between the different layerscomposing the devices is a critical factor in order to develop flexible OPV devices reliable aftertheir manufacturing and during their use.In this work, two encapsulation protocols were studied: the roll-to-roll lamination of apressure sensitive adhesive and the vacuum lamination of a hotmelt thermoplastic. In order toquantify the adhesion strength of every interface comprised in the samples, the 180° peelingtest mechanical characterization was adapted for and then applied to the flexible devices. Inaddition, non-destructive imaging characterization techniques were developed: the laser-beaminduced-current mapping and the luminescence emission imaging under optical and electricalexcitation. Thank to these techniques, the hypothesis of a mechanical degradation occurringduring the roll-to-roll lamination process was made. Answers allowing for the improvement ofthe interfaces identified as weak were searched for and evaluated with respect to the photovoltaicperformances of the reference devices. The imaging techniques previously developed were alsoapplied along the accelerated aging of encapsulated cells. A mechanism was proposed, whichallows one to explain the localization of the degradation but also the failure type, either opticalor electrical, occurring during each aging step.

This thesis presents a study of hybrid solar cells, specifically looking at various methods which can be employed in order to increase the power conversion efficiency of these devices. The experiments and results contained herein also present a very accurate picture of how rapidly the field of hybrid solar cells has progressed within the past three years. Chapters 1 and 2 present the background and motivation for the investigations undertaken, as well as the relevant theory underpinning solar cell operation. Chapter 2 also gives a brief review of the literature pertinent to the main types of devices investigated in this thesis; dye-sensitised solar cells, semiconductor sensitized solar cells and perovskite solar cells. Descriptions of the synthetic procedures, as well as the details of device fabrication and any measurement techniques used are outlined in Chapter 3. The first set of experimental results is presented in Chapter 4. This chapter outlines the synthesis of mesoporous single crystals (MSCs) of anatase TiO₂ as well as an investigation of its electronic properties. Having shown that this material has superior electronic properties to the conventionally used nanoparticle films, they were then integrated into low temperature processed dye-sensitised solar cells and achieved power conversion efficiencies of > 3%, exhibiting electron transport rates which were orders of magnitude higher than those obtained for the high temperature processed control films. Chapter 5 further investigates the use of MSCs in photovoltaic devices, this time utilising a more strongly absorbing inorganic sensitiser, Sb₂S₃. Utilising the readily tunable pore size of MSCs, these Sb₂S₃ devices showed an increase in voltage and fill factor which can be attributed to a decrease in recombination within these devices. This chapter also presents the use of Sb₂S₃ in the meso-superstructured configuration. This device architecture showed consistently higher voltages suggesting that in this architecture, charge transport occurs through the absorber and not the mesoporous scaffold. Chapters 6 and 7 focus on the use of hybrid organic-inorganic perovskites in photovoltaic devices. In Chapter 6 the mixed halide, lead-based perovskite, CH₃NH₃PbI_3-xCl_x is employed in a planar heterojunction device architecture. The effects of Lewis base passivation on this material are investigated by determining the photoluminescence (PL) lifetimes and quantum efficiencies of treated and untreated films. It is found that passivating films of this material using Lewis bases causes an increase in the PLQE at low fluences as well as increasing the PL lifetime. By globally fitting these results to a model the trap densities are extracted and it is found that using these surface treatments decreases the trap density of the perovskite films. Finally, these treatments are used in complete solar cells resulting in increased power conversion efficiencies and an improvement in the stabilised power output of the devices. Chapter 7 describes the materials synthesis and characterisation of the tin-based perovskite CH₃NH₃SnI₃ and presents the first operational, lead-free perovskite solar cell. The work presented in this thesis describes significant advances in the field of hybrid solar cells, specifically with regards to improvements made to the nanostructured electrode, and the development and implementation of more highly absorbing sensitizers. The improvements discussed here will prove to be quite important in the drive towards exploiting solar power as a clean, affordable source of energy.

xv, 132 p. : ill. A print copy of this title is available from the UO Libraries, under the call number: SCIENCE TK7871.15.F5 H325 2007 / The effects of sulfur alloying on the electronic properties of CuIn(SeS) 2 and CuInGa(SeS) 2 materials has been investigated using sophisticated junction capacitance techniques including drive-level capacitance profiling and transient photocapacitance and photocurrent spectroscopies. CISSe and CIGSSe materials are used as absorber layers in thin-film photovoltaic devices. By characterizing the electronic properties of these materials we hope to understand how these materials can be improved to make thin-film devices with better conversion efficiencies. Sulfur widens the bandgap of these materials by moving the valence band to lower energies and the conduction band to higher energies. This significantly affects the electronic structure of these devices by increasing the activation energies of dominant acceptor levels and lowering room temperature free hole carrier densities. Using optical spectroscopies we observe a large, broad defect that also changes its apparent energetic depth with sulfur alloying. The occupation of this defect was controlled both optically and thermally, and showed a striking temperature dependence. This temperature dependence was measured by recording the relative defect signal, the ratio of the TPC signal in the defect regime to the above bandgap regime, as a function of temperature. As the temperature of the measurement was decreased, steps in the relative defect signal were observed, indicating the turning off of the thermal pathway that emptied trapped charge from the defect. Remarkably, such steps were seen at the same temperature in CISSe and CIGSSe devices with similar sulfur content. In addition, no steps were seen in CMS devices. This points to a defect state specific to the incorporation of sulfur in the absorber material. We hope that a better understanding of the electronic structure of these materials will assist in the creation of improved wide-bandgap thin-film photovoltaic devices. / Adviser: J. David Cohen

xiii, 84 p. ; ill. / This dissertation describes the deposition and characterization of an amorphous thin film with a composition near that of CuInSe 2 (CIS). The creation of an amorphous intermediate leads to a crystalline film at low annealing temperatures. Thin films were deposited from elemental sources in a custom built high vacuum chamber. Copper-selenium and indium-selenium binary layered samples were investigated to identify interfacial reactions that would form undesired binary intermediate compounds resulting in the need for high temperature annealing. Although the indium-selenium system did not form interfacial compounds on deposit, indium crystallized when the indium layer thickness exceeded 15 angstroms, disrupting the continuity of the elemental layers. Copper-selenium elemental layers with a repeat thickness of over 30 angstroms or compositions with less than 63% selenium formed CuSe on deposit. Several deposition schemes were investigated to identify the proper deposition pattern and thicknesses to form the CIS amorphous film. Simple co-deposition resulted in the nucleation of CIS. A simple stacking of the three elements in the older Se-In-Cu at a repeat thickness of 60 angstroms resulted in the nucleation of CuSe and sometimes CIS. The CIS most likely formed due to the disruption of the elemental layers by the growth of the CuSe. Reduction of the repeat thickness to 20 angstroms eliminated the nucleation of CuSe, as predicted by the study of the binary Cu-Se layered samples, but resulted in the nucleation of CIS, similar to the co-deposited samples. To eliminate both the thick Cu-Se region, and prevent the intermixing of all three elements, a more complex deposition pattern was initiated. The copper and selenium repeat thicknesses were reduced into a Se-Cu-Se-Cu-Se pattern followed by deposition of the indium layer at a total repeat thickness of 60 angstroms. At a Se:Cu ratio of 2:1 and the small repeat thickness, no Cu-Se phases nucleated. Additionally, the Cu-In interface was eliminated. For this deposition scheme, films with a selenium rich composition relative to CuInSez were generally amorphous. Those that were Cu-In rich always nucleated CIS on deposit. Annealing of all samples produced crystalline CIS. / Adviser: David C. Johnson

xxi, 134 p. : ill. (some col.) / Hydrogenated nanocrystalline silicon (nc-Si:H) is a semiconducting material that is very useful as a thin film photovoltaic. A mixture of amorphous and crystalline silicon components, nc-Si:H shows good carrier mobilities, enhanced infrared response, and high resilience to light-induced degradation of its electronic properties, a thermally reversible degenerative phenomenon known as the Staebler-Wronski Effect (SWE). However, production of nc-Si:H is difficult in part because the structural and electronic properties of this material are not well understood. For example, its electronic properties have even been observed by some authors to improve upon prolonged light exposure, in direct opposition to the SWE observed in purely amorphous thin film silicon. We used several junction capacitance based measurements together with characterization methods such as Raman spectroscopy and secondary ion mass spectroscopy to better understand the structure/function relationships present in nc-Si:H. Drive level capacitance profiling (DLCP) was used to determine densities, spatial distributions, and energies of deep-gap defects. Transient photocapacitance (TPC) and transient photocurrent (TPI) were used to characterize optical transitions and the degree of minority carrier collection. Materials had crystallite volume fractions between 20% and 80% and were deposited using RF and modified VHF glow discharge (PECVD) processes at United Solar Ovonic, LLC. Measurements were made as a function of metastable state: annealed states were produced by exposing the material to temperatures above 370K for 0.5h and the lightsoaked state was produced by exposure to 200mW/cm 2 610nm long-pass filtered light from an ELH halogen source for 100h. We identified two deep defects in nc-Si:H. A primary defect appearing throughout the material at an electronic transition energy of roughly 0.7eV below the conduction band, and a second defect 0.4eV below the conduction band which was localized near the p/i junction interface. Results suggested that the deeper defect is related to the presence of oxygen and is located in grain boundary regions. The energy depth of this defect appears also to be somewhat dependent on metastable state. This phenomenon, and the universal decrease in minority carrier collection upon lightsoaking are accounted for in a model of electronic behavior we have developed over the course of this study. / Committee in charge: Dr. Miriam Deutsch, Chairperson; Dr. J. David Cohen, Advisor; Dr. Roger Haydock, Member; Dr. Heiner Linke, Member Dr. Mark Lonergan Outside Member

abstract: GaAs single-junction solar cells have been studied extensively in recent years, and have reached over 28 % efficiency. Further improvement requires an optically thick but physically thin absorber to provide both large short-circuit current and high open-circuit voltage. By detailed simulation, it is concluded that ultra-thin GaAs cells with hundreds of nanometers thickness and reflective back scattering can potentially offer efficiencies greater than 30 %. The 300 nm GaAs solar cell with AlInP/Au reflective back scattering is carefully designed and demonstrates an efficiency of 19.1 %. The device performance is analyzed using the semi-analytical model with Phong distribution implemented to account for non-Lambertian scattering. A Phong exponent m of ~12, a non-radiative lifetime of 130 ns, and a specific series resistivity of 1.2 Ω·cm2 are determined. Thin-film CdTe solar cells have also attracted lots of attention due to the continuous improvements in their device performance. To address the issue of the lower efficiency record compared to detailed-balance limit, the single-crystalline Cd(Zn)Te/MgCdTe double heterostructures (DH) grown on InSb (100) substrates by molecular beam epitaxy (MBE) are carefully studied. The Cd0.9946Zn0.0054Te alloy lattice-matched to InSb has been demonstrated with a carrier lifetime of 0.34 µs observed in a 3 µm thick Cd0.9946Zn0.0054Te/MgCdTe DH sample. The substantial improvement of lifetime is due to the reduction in misfit dislocation density. The recombination lifetime and interface recombination velocity (IRV) of CdTe/MgxCd1-xTe DHs are investigated. The IRV is found to be dependent on both the MgCdTe barrier height and width due to the thermionic emission and tunneling processes. A record-long carrier lifetime of 2.7 µs and a record-low IRV of close to zero have been confirmed experimentally. The MgCdTe/Si tandem solar cell is proposed to address the issue of high manufacturing costs and poor performance of thin-film solar cells. The MBE grown MgxCd1-xTe/MgyCd1-yTe DHs have demonstrated the required bandgap energy of 1.7 eV, a carrier lifetime of 11 ns, and an effective IRV of (1.869 ± 0.007) × 103 cm/s. The large IRV is attributed to thermionic-emission induced interface recombination. These understandings can be applied to fabricating the high-efficiency low-cost MgCdTe/Si tandem solar cell. / Dissertation/Thesis / Doctoral Dissertation Electrical Engineering 2015