Development of High Efficiency III/V Photovoltaic Devices

Developments of photovoltaic (PV) devices are driven by increasing needs for economically competitive renewable energy conversion. To improve the efficiency of PV devices for outdoor applications, the concept of intermediate band solar cell (IBSC) has been proposed to boost the conversation efficiency to 63% under concentrated suns illumination, which requires two-step photon absorption (TSPA) dominates among other competing processes: carrier thermal escape, tunneling and recombination. To optimize the design of III-V QD-IBSCs, first, the effect of electric field on band structure and carrier dynamics and device performances were quantitative investigated via simulation and experiments. Second, to experimentally increase TSPA at room temperature, novel QD systems related QD-IBSCs were designed, fabricated and characterized. The InAs/Al0.3GaAs QD-IBSC shows high TSPA working temperature towards 110K, promising for a room temperature IBSC under concentrated sunlight. Alternative QD systems including GaSb/GaAs and type II InP/InGaP were also investigated via band structure simulations. Meanwhile, developments of PV devices under indoor low intensity light (0.1 µW/cm2-1 mW/cm2) illumination not only enable long lifetime radio-isotope based batteries, but also, more important for the daily life, have the potential to promote an emerging market of internet of things by efficiently powering wireless sensors. Single junction InGaP PV devices were optimized for low intensity light sources using via simulations and statistical control. To reduce the dark current and increase the absorption at longer wavelengths (\u3e550 nm), several parameters including doping and thickness were evaluated. The experimental results on the devices show higher conversion efficiencies than other commercial PVs under varied indoor light sources: 29% under 1µW/cm2 phosphor spectrum and over 30% efficiency under LEDs illumination. In addition, the work includes developments of InAs nanowires epi-growth for PV applications. Several marks for selective area growth were successfully made

Development and Life Cycle Assessment of Advanced-Concept III-V Multijunction Photovoltaics

III-V semiconductors make for highly efficient solar cells, but are expensive to manufacture. However, there are many mechanisms for improving III-V photovoltaics in order to make them more competitive with other photovoltaic (PV) technologies. One possible method is to design cells for high efficiency under concentrated sunlight, effectively trading expensive III-V material for cheaper materials such as glass lenses. Another approach is to reduce the amount of III-V material necessary for the same power output, which can be achieved by removing the substrate and installing a reflector on the back of the cell, while also adding quantum structures to the cell to permit absorption of a greater portion of the solar spectrum. Regarding the first approach, this dissertation focused on the development of an InAlAsSb material for a mulitjunction design with the potential of achieving 52.8% efficiency under 500 suns. First, development of a single-junction InAlAs cell lattice-matched to InP was executed as a preliminary step. The InAlAs cell design was optimized via simulation, then grown via metal organic vapor phase epitaxy (MOVPE) and fabricated resulting in 17.9% efficiency under 1-sun AM1.5, which was unprecedented for the InAlAs material. Identical InAlAs cells were grown using alternative MOVPE precursors to study the effects of necessary precursors for InAlAsSb. Fits to experimental device results showed longer lifetimes when grown with the alternative aluminum precursor. InAlAsSb grown using these alternative precursors targeted a 1.8 eV bandgap required for the multijunction design. Ultimately, InAlAsSb material with the desired bandgap was confirmed by photoreflectance spectroscopy. For the second approach, this dissertation studied the integration of InAs quantum dots (QDs) in a GaAs solar cell in conjunction a back surface reflector (BSR). A quantum dot solar cell (QDSC) with a BSR has the potential to increase short-circuit current by 2.5 mA/cm2 and also increase open-circuit voltage due to photon recycling. In this study, multiple textured BSRs were fabricated by growing inverted QDSCs on epitaxial lift-off templates and then texturing the rear surface before removing the device from the substrate. Identical cells with a flat BSR served as controls. Optimization of inverted QDSC growth conditions was also performed via a cell design study. Device results showed increased open-circuit voltage with increasing optical path length, and the greatest improvement in sub-band current over a flat BSR control device was 40%. In the final chapter, a life cycle assessment (LCA) of these technologies was performed to identify the hypothetical optimum at which energy investments in cell performance (such as the two described above) no longer correspond to improvements in the overall life cycle performance of the PV system. Four cell designs with sequentially increasing efficiencies were compared using a functional unit of 1 kWp. The first is a commercially available and has been studied in previous LCAs. The second is the design containing InAlAsSb mentioned above. The third represents the most material-intensive option, which bonds two substrates to create a five-junction cell. The fourth is a six-junction cell that uses a metamorphic grade between subcells and represents the most energy-intensive option. A thorough literature review of existing LCAs of high-concentration photovoltaic (HCPV) systems was performed, which obviated the need for data on the manufacture of MOVPE precursors and substrates. LCAs for the most common III-V substrate (GaAs) and precursors were executed prior to conducting the HCPV system LCA, due to the absence of detailed information on the life cycle impacts of these compounds in literature. Ultimately, both the cumulative energy demand and greenhouse gas emissions of the HCPV system decreased proportionally with increasing cell efficiency, even for the most energy and material-intensive cell designs. It was found that the substrates and precursors corresponded to less than 2% of system impacts. This implies that current mechanisms to increase cell efficiency are environmentally viable in HCPV applications without the need for material reduction, and would make III-V HCPV more environmentally competitive with dominant silicon PV technologies

The Role of Quantum Dot Size on the Performance of Intermediate Band Solar Cells

The goal of this thesis is to understand possible mechanisms for the reported decrease of the open circuit voltage and solar cell efficiency in quantum dot (QD) intermediate band solar cells (IBSCs). More specifically, the effect of indium arsenide (InAs) QD height on the open circuit voltage and solar cell efficiency was studied in a systematic way. To explore this effect in QD solar cells, several solar cells (SCs) were grown with varying InAs QD heights. All experimental characteristics of the QD solar cells were compared to a reference structure without QDs. All samples were grown by Molecular Beam Epitaxy (MBE), and self-assembled InAs QDs were formed using the Stranski-Krastanov (SK) growth method. Using a QD truncation technique, the height of the QDs was accurately varied between 2 nm and 5 nm, while maintaining both lateral size and areal density of the QDs. The intermediate band (IB) of each solar cell was constructed from 10 layers of InAs QDs of the same size and density. All samples were fabricated as solar cell devices using standard optical photolithography, for electrical characterization and solar cell efficiency studies. Optical and structural characterization was done for all samples. The following characterizations were performed: Transmission Electron Microscopy (TEM), Low Temperature Photoluminescence (PL), Power Dependent PL, External Quantum Efficiency (EQE), Temperature Dependent Solar Power Conversion Efficiency, and Current-Voltage measurements. The efficiency measurements demonstrate the critical role of QD size on the performance of QD IBSCs. The EQE measurement indicates a change in the position of the band edge, due to carrier confinement, consistent with a QD size variation as verified by TEM and PL. Measurements demonstrate that the EQE in the NIR range of the spectrum is enhanced in the QD IBSCs devices due to light absorption by the QDs. This work also demonstrates that open circuit voltage (Voc) decreases with an increase of the QD height, which leads to significant degradation of the solar cell conversion efficiency for QD sizes above 3 nm. In addition, for samples with QD heights of 4 nm and above, the EQE spectra in the GaAs region decreases, indicating a loss of photocurrent, most likely due to traps introduced by the large QDs. These experimental results suggest that the open circuit voltage in QD IBSCs degrades with the increase of QD height as a result of (i) a decrease of the effective band gap of the absorber media and (ii) enhanced Shockley-Read-Hall recombination in the presence of traps in the solar cell space charge region

Mid-IR type-II InAs/GaSb nanoscale superlattice sensors

The detection of mid-wavelength infrared radiation (MWIR) is very important for many military, industrial and biomedical applications. Present-day commercially available uncooled IR sensors operating in MWIR region (2-5μm) use microbolometric detectors which are inherently slow. Available photon detectors (mercury cadmium telluride (MCT), bulk InSb and quantum well infrared detectors (QWIPs))overcome this limitation. However, there are some fundamental issues decreasing their performance and ability for high temperature operation, including fast Auger recombination rates and high thermal generation rate. These detectors operate at low temperatures (77K-200K) in order to obtain high signal to noise ratio. The requirement of cooling limits the lifetime, increases the weight and the total cost, as well as the power budget, of the whole infrared system. In recent years, InAs/GaSb superlattice based detectors have appeared as an interesting alternative to the present-day IR detector systems. These heterostructures have a type-II band alignment such that the conduction band of InAs layer is lower than the valence band of GaSb layer. The effective bandgap of these structures can be adjusted from 0.4 eV to values below 0.1 eV by varying the thickness of constituent layers leading to an enormous range of detector cutoff wavelengths (3-30μm). The InAs/GaSb SLs have a higher degree of uniformity than the MCT alloys, making them attractive for large area focal plane arrays. They provide a smaller leakage current due to larger effective electron mass, which suppresses tunneling. This material system is also characterized by high operating temperatures and long Auger recombination rates. This suggests the potential for using the SLs technology for realizing high operating temperature devices. This work is focused on the development of mid-IR InAs/GaSb SLs sensors with high-operating temperature. Contributions of this thesis include 1) development of growth and processing procedure for the n-on-p and p-on-n design of SL detectors leading to improved detector performance, 2) careful evaluation of characteristics of SL detectors, 3) methods of reduction of surface component of dark current passivation techniques)

Narrow Bandgap (0.7–0.9 eV) Dilute Nitride Materials for Advanced Multijunction Solar Cells

Aurinkosähköllä on merkittävä rooli maailmanlaajuisessa siirtymässä kohti kestävää energiantuotantoa, sillä aurinkopaneelit tuottavat vihreää sähköä suoraan auringonvalosta. Yksi aurinkosähkön avainteknologioista on III–V puolijohteisiin perustuvat moniliitosaurinkokennot, joiden avulla on saavutettu korkeimmat hyötysuhteet sekä maanpäällisessä energiantuotannossa että avaruussovelluksissa. Moniliitosaurinkokennoilla onkin saavutettu jopa 47,6 %:n hyötysuhde käyttäen keskitettyä valoa, mutta ponnisteluista huolimatta 50 %:n rajaa ei ole vielä saavutettu. Näin korkeiden hyötysuhteiden saavuttaminen edellyttää auringon spektrin erittäin tehokasta hyödyntämistä, mikä käytännössä vaatii viiden tai useamman liitoksen käyttämistä rakenteissa, mikä puolestaan edellyttää uusien alikennojen ja materiaalien kehitystyötä. Etenkin hilasovitettuja moniliitoskennoja ajatellen uusien materiaalien kehittäminen on tärkeää hilasovitettujen materiaalien määrän rajallisuuden vuoksi. Tämä väitöskirjatyö keskittyy hilasovitettujen kapean energia-aukon omaavien laimeiden typpiyhdisteiden ja niihin pohjautuvien moniliitosaurinkokennojen kehitykseen, viimekädessä tähdäten 50 %:n hyötysuhteen saavuttamiseen. Ensimmäisenä askeleena kohti tätä tavoitetta kehitettiin neliliitosaurinkokennoja, jotka sisältävät kaksi laimeisiin typpiyhdisteisiin perustuvaa alikennoa. Näissä rakenteissa pohjaliitoksen energia-aukkoa siirrettiin kohti 0,9 eV:n energiaa. Kokeellisilla neliliitoskennoilla saavutettiin 39 %:n hyötysuhde keskitetyn valon alla. Lisäkehitystyöllä kyseisillä rakenteilla olisi mahdollista saavuttaa yli 46 % hyötysuhde. Merkittävä osa tämän väitöskirjan kokeellisesta työstä liittyi 6–8 % typpeä sisältävien kapean energia-aukon GaInNAsSb-alikennojen valmistukseen, joiden avulla voidaan paremmin kattaa energiakaista germaniumin ja vakiintuneiden hilasovitettujen materiaalien välillä. Tässä työssä esitellään kehitystyötä ensimmäisistä kapean energia-aukon GaInNAsSb-liitoksista kohti korkean suorituskyvyn alikennoja rakenteellisten ja valmistusteknisten kehitysaskelten avulla. Kapean energia-aukon (0,8 eV) GaInNAsSb-kennojen toiminnassa saatiin aikaan merkittäviä parannuksia takapeilin avulla sekä molekyylisuihkuepitaksia-prosessin optimoinnilla. Parhailla työssä esitetyllä kapean energia-aukon alikennolla onkin mahdollista saavuttaa virtasovitus seuraavan sukupolven moniliitoskennoissa, joiden avulla yli 50 %:n hyötysuhde voitaisiin saavuttaa.A prominent role in the worldwide transition towards sustainable energy production is played by photovoltaics that is used to convert sunlight directly into green electricity. One of the key photovoltaic technologies is multijunction solar cell architecture based on III–V compound semiconductors, which provides the highest conversion efficiencies to date in terrestrial and space applications of solar cells. Currently, up to 47.6% conversion efficiency has been achieved under concentrated illumination with this approach. Still, despite major efforts, the milestone efficiency of 50% has not been realized. Reaching this efficiency level practically requires implementation of five or more junctions into multijunction solar cell devices, which allows more efficient utilization of the solar spectrum. In turn, this requires the development of new sub-cells and related materials. This is especially true for lattice-matched multijunction architecture, where the library of materials is more strictly limited. To this end, the thesis focuses on the development of narrow bandgap dilute nitrides and related multijunction solar cells lattice-matched to GaAs, ultimately targeting at 50% conversion efficiencies. As the initial steps towards realization of this, four-junction solar cells employing two dilute nitride subcells were demonstrated. To this end, the bandgap of the bottom junction was shifted towards 0.9 eV. The experimental four-junction devices yielded efficiencies of up to 39% under concentration, yet with fine-tuning and higher concentration factors over 46% could be attainable. A major part of the experimental work in this thesis involved fabrication of narrow bandgap GaInNAsSb subcells with 6–8% nitrogen concentrations for bridging the gap to Ge with lattice-matched materials. The thesis covers the progress from the first proof-of-concept narrow-gap GaInNAsSb junctions towards high performance subcells enabled by structural and epitaxial developments. Significant improvements for the performance of 0.8 eV GaInNAsSb solar cells were obtained by employing a back reflector behind the dilute nitride junction, and by optimizing the molecular beam epitaxy growth of the narrow-gap materials. The best narrow bandgap subcells presented in this work would already enable current-matching in next-generation multijunction devices with projected efficiencies exceeding 50%

Optical and Mechanical Investigation of InAs /GaAs Quantum Dots Solar Cells and InAs Nanowires for the Application of Photovoltaic Device

Self-assembled quantum dots (QDs) and nanowires (NWs) are currently the subjects of extensive study due to their promising applications in optoelectronic devices. In order to enhance understanding of the short circuit current improvement in InAs/GaAs quantum dots solar cell (QDSC), the mechanisms of carrier escape by thermal activation and tunneling from InAs quantum dots (QDs) confinements in InAs/GaAs QDSCs are investigated. The fitted activation energy of electrons from temperature dependent photoluminescence (TDPL) is 114 meV. Using this fitted activation energy, calculated thermal escape time and tunneling time of electrons from the ground state of the QDs are 10-12 seconds and 10-6 seconds at 300K, respectively. These results indicate that at room temperature thermal escape is dominant for electrons escape from ground state. At low temperature (8K), tunneling mainly affects the electrons escape from ground state, since thermal energy cannot support electrons to overcome the fitted activation energy (barrier, 114 meV). In addition, in order to describe the new physics and achieve the final success in nanowire device for photovoltaic applications, the first step is to develop high-quality semiconductor nanowires on the selected substrate. Morphological and crystal structure characterizations were performed via SEM and TEM for InAs nanowire samples grown with and without Au seed on GaAs substrate using metal organic vapor phase expitaxy (MOVPE). Several major factors affect the NW growth in terms of shape, density, etc. For nanowire growth with Au seed, its growth direction mainly depends on the substrate, while its uniformity is initially related to the Au seed coverage. III/V ratio affects the NW aspect ratio (length/bottom width), ranged from 12.00 to 38.93. Increasing temperature accelerates the growth rate in both axial and radial directions. NWs grown without Au seed using a pattern mask show no tapering along the growth direction with an average diameter of 26 nm. All defects stop in the buffer layer when InAs nanowires grown with an Au seed, but a mix of ZB and WZ crystal phases were observed along the growth direction of nanowire. InAs NWs grown without Au seeds also show a mixture of different crystal phases along the growth direction. The diameter of InAs nanowire should be further reduced to 3-6 nm as to achieve PL response between 1000~1300 nm

Research on intermediate band solar cells and development of experimental techniques for their characterization under concentrated illumination

Abstract This work is a contribution to the research and development of the intermediate band solar cell (IBSC), a high efficiency photovoltaic concept that features the advantages of both low and high bandgap solar cells. The resemblance with a low bandgap solar cell comes from the fact that the IBSC hosts an electronic energy band -the intermediate band (IB)- within the semiconductor bandgap. This IB allows the collection of sub-bandgap energy photons by means of two-step photon absorption processes, from the valence band (VB) to the IB and from there to the conduction band (CB). The exploitation of these low energy photons implies a more efficient use of the solar spectrum. The resemblance of the IBSC with a high bandgap solar cell is related to the preservation of the voltage: the open-circuit voltage (VOC) of an IBSC is not limited by any of the sub-bandgaps (involving the IB), but only by the fundamental bandgap (defined from the VB to the CB). Nevertheless, the presence of the IB allows new paths for electronic recombination and the performance of the IBSC is degraded at 1 sun operation conditions. A theoretical argument is presented regarding the need for the use of concentrated illumination in order to circumvent the degradation of the voltage derived from the increase in the recombi¬nation. This theory is supported by the experimental verification carried out with our novel characterization technique consisting of the acquisition of photogenerated current (IL)-VOC pairs under low temperature and concentrated light. Besides, at this stage of the IBSC research, several new IB materials are being engineered and our novel character¬ization tool can be very useful to provide feedback on their capability to perform as real IBSCs, verifying or disregarding the fulfillment of the “voltage preservation” principle. An analytical model has also been developed to assess the potential of quantum-dot (QD)-IBSCs. It is based on the calculation of band alignment of III-V alloyed heterojunc-tions, the estimation of the confined energy levels in a QD and the calculation of the de¬tailed balance efficiency. Several potentially useful QD materials have been identified, such as InAs/AlxGa1-xAs, InAs/GaxIn1-xP, InAs1-yNy/AlAsxSb1-x or InAs1-zNz/Alx[GayIn1-y]1-xP. Finally, a model for the analysis of the series resistance of a concentrator solar cell has also been developed to design and fabricate IBSCs adapted to 1,000 suns. Resumen Este trabajo contribuye a la investigación y al desarrollo de la célula solar de banda intermedia (IBSC), un concepto fotovoltaico de alta eficiencia que auna las ventajas de una célula solar de bajo y de alto gap. La IBSC se parece a una célula solar de bajo gap (o banda prohibida) en que la IBSC alberga una banda de energía -la banda intermedia (IB)-en el seno de la banda prohibida. Esta IB permite colectar fotones de energía inferior a la banda prohibida por medio de procesos de absorción de fotones en dos pasos, de la banda de valencia (VB) a la IB y de allí a la banda de conducción (CB). El aprovechamiento de estos fotones de baja energía conlleva un empleo más eficiente del espectro solar. La semejanza antre la IBSC y una célula solar de alto gap está relacionada con la preservación del voltaje: la tensión de circuito abierto (Vbc) de una IBSC no está limitada por ninguna de las fracciones en las que la IB divide a la banda prohibida, sino que está únicamente limitada por el ancho de banda fundamental del semiconductor (definido entre VB y CB). No obstante, la presencia de la IB posibilita nuevos caminos de recombinación electrónica, lo cual degrada el rendimiento de la IBSC a 1 sol. Este trabajo argumenta de forma teórica la necesidad de emplear luz concentrada para evitar compensar el aumento de la recom¬binación de la IBSC y evitar la degradación del voltage. Lo anterior se ha verificado experimentalmente por medio de nuestra novedosa técnica de caracterización consistente en la adquisicin de pares de corriente fotogenerada (IL)-VOG en concentración y a baja temperatura. En esta etapa de la investigación, se están desarrollando nuevos materiales de IB y nuestra herramienta de caracterizacin está siendo empleada para realimentar el proceso de fabricación, comprobando si los materiales tienen capacidad para operar como verdaderas IBSCs por medio de la verificación del principio de preservación del voltaje. También se ha desarrollado un modelo analítico para evaluar el potencial de IBSCs de puntos cuánticos. Dicho modelo está basado en el cálculo del alineamiento de bandas de energía en heterouniones de aleaciones de materiales III-V, en la estimación de la energía de los niveles confinados en un QD y en el cálculo de la eficiencia de balance detallado. Este modelo ha permitido identificar varios materiales de QDs potencialmente útiles como InAs/AlxGai_xAs, InAs/GaxIni_xP, InAsi_yNy/AlAsxSbi_x ó InAsi_zNz/Alx[GayIni_y]i_xP. Finalmente, también se ha desarrollado un modelado teórico para el análisis de la resistencia serie de una célula solar de concentración. Gracias a dicho modelo se han diseñado y fabricado IBSCs adaptadas a 1.000 soles

Solar Power Generation:Technology,New Concepts & Policy

This book offers a global perspective of the current state of affairs in the field of solar power engineering. In four parts, this well-researched volume informs about: Established solar PV (photovoltaic) technologies Third-generation PV technologies based on new materials with potential for low-cost large-scale production Solar cell technology based on new (third-generation) concepts, such as quantum dot solar cells and nano wire solar cells using silicon and compound semiconductors Economic implications and effects, as well as policies and incentives in various countries of the world involved with solar energy implementation In addition to discussing manufacturing facts and implementation issues, this book emphasizes the implications of policy measures in countries with good PV activity, such as Japan, China, India, Germany, Spain, France, Italy, the United States, and Canada. This volume is intended as a reference for a global audience of advanced students and R&D and industry professionals, as well as investors and policy-makers with fundamental knowledge of photovoltaic technology