Native and Radiation-Induced Defects in III-V Solar Cells and Photodiodes

Photodiodes made of III-V materials are ubiquitous with applications for telecommunications, photonics, consumer electronics, and spectroscopy. The III-V solar cell, specifically, is a large-area photodiode that is used by the satellite industry for power conversion due to its unrivaled efficiency and wide range of available materials. As a device driven by its minority carrier diffusion length (MCDL), the performance of a photodiode is sensitive to crystallographic defects that create states in the forbidden energy gap. Defects commonly arise during growth of the crystal and during device fabrication, and they accumulate slowly over time when deployed into the damaging environment of space. Defect-assisted carrier recombination leads to lower MCDL, higher dark current, reduced sensitivity and signal-to-noise ratio, and, in the case of solar cells, reduced power conversion efficiency. Consequently, the development of photodiode technology requires techniques for detection, characterization, and mitigation of defects and the inter-bandgap states they create. In this work, III-V material defects are addressed across a variety of materials and devices. The first half of the work makes use of deep-level transient spectroscopy (DLTS) to deduce the energy level, cross-section, and density of traps the InAlAs, InAlAsSb, and InGaAs lattice-matched to InP. An in situ DLTS system that can monitor defects immediately after irradiation was developed and applied to InGaAs photodiodes irradiated by protons. Evidence of trap annealing was found to occur as low as 150 K. The second half begins with development of GaSb solar cells grown by molecular beam epitaxy on GaAs substrate intended for use in lower-cost monolithic multi-junction cells. Defect analysis by microscopy, dark lock-in thermography, and dark current measurement, among others, was performed. The best GaSb-on-GaAs cell achieved state-of-the-art 4.5% efficiency under concentrated solar spectrum. Finally, light management in III-V photodiodes was explored as a possible route for defect mitigation. Textures, diffraction gratings, metallic mirrors, and Bragg reflectors were simulated by finite difference time domain for single- and multi-junction GaAs-based cells with the aim of reducing the amount of absorber material required and to simultaneously reduce MCDL requirements by generating carriers closer to the junction. The results were inputted into a device simulator to predict efficiency. A backside reflective pyramidal-textured grating was simulated to allow a GaAs cell to be thinned by a factor of \u3e30 compared to a conventional cell

Proceedings of the 19th Space Photovoltaic Research and Technology Conference

The 19th Space Photovoltaic Research and Technology Conference (SPRAT XIX) was held September 20 to 22, 2005, at the Ohio Aerospace Institute (OAI) in Brook Park, Ohio. The SPRAT Conference, hosted by the Photovoltaic and Space Environments Branch of the NASA Glenn Research Center, brought together representatives of the space photovoltaic community from around the world to share the latest advances in space solar cell technology. This year's conference continued to build on many of the trends shown in SPRAT XVIII-the continued advances of thin-film and multijunction solar cell technologies and the new issues required to qualify those types of cells for space applications

Quantum Dot-Based Thin-Film III–V Solar Cells

In this work, we report our recent results in the development of thin-film III–V solar cells fabricated by epitaxial lift-off (ELO) combining quantum dots (QD) and light management structures. Possible paths to overcome two of the most relevant issues posed by quantum dot solar cells (QDSC), namely, the degradation of open circuit voltage and the weak photon harvesting by QDs, are evaluated both theoretically and experimentally. High open circuit voltage QDSCs grown by molecular beam epitaxy are demonstrated, both in wafer-based and ELO thin-film configuration. This paves the way to the implementation in the genuine thin-film structure of advanced photon management approaches to enhance the QD photocurrent and to further optimize the photovoltage. We show that the use of light trapping is essential to attain high-efficiency QDSCs. Based on transport and rigorous electromagnetic simulations, we derive design guidelines towards light-trapping enhanced thin-film QDSCs with efficiency higher than 28% under unconcentrated light, ambient temperature. If photon recycling can be fully exploited, 30% efficiency is deemed to be feasible. Towards this goal, results on the development and integration of optimized planar and micro-patterned mirrors, diffractive gratings and broadband antireflection coatings are presented

In Situ Characterization of Interfaces Relevant for Efficient Photoinduced Reactions

Solar energy conversion and photoinduced bioactive sensors are representing topical scientific fields, where interfaces play a decisive role for efficient applications. The key to specifically tune these interfaces is a precise knowledge of interfacial structures and their formation on the microscopic, preferably atomic scale. Gaining thorough insight into interfacial reactions, however, is particularly challenging in relevant complex chemical environment. This review introduces a spectrum of material systems with corresponding interfaces significant for efficient applications in energy conversion and sensor technologies. It highlights appropriate analysis techniques capable of monitoring critical physicochemical reactions in situ during non-vacuum preparation and photoactivity studies including well-defined inorganic epitaxial reference surfaces, buried interfaces, and low-defect nucleation of disjunct epitaxial materials that are analyzed during preparation in chemical vapor environment. Their surfaces are then modified and functionalized in gaseous and liquid environment. Finally, even more complex coupling of inorganic stable photoactive materials with responsive soft matter for bioactivity is reviewed. Interface formation, structure, and/or artificial photochemical interfacial reactions are scrutinized down to the atomic scale in real time, also accounting for equilibrium versus non-equilibrium, kinetically driven processes, in order to accelerate progresses in the realization of efficient energy materials and in the exploitation of photoinduced processes at interfaces

Molecular beam epitaxy and characterisation of GaAsBi for photovoltaic applications

GaAsBi is a promising candidate material for a 1 eV junction for multi-junction photovoltaics, as well as having many other potential applications in areas such as telecommunications and spintronics. The growth of GaAsBi has proven problematic due to the large size and low electronegativity of the Bi atom, and this has hindered its development. In this thesis, the growth and material characterisation of GaAsBi is presented. A systematic series of bulk GaAsBi samples were grown by molecular beam epitaxy to investigate the effects of growth temperature, As flux and As species on the bismuth content and optical quality of the samples. Two growth regimes became apparent: a temperature limited regime in which the Bi content is limited by the miscibility of GaAs and GaBi, and a Bi flux limited regime in which the Bi incorporation coefficient approaches unity. The production of good quality GaAsBi was shown to require near a near stoichiometric Ga:As atomic flux ratio. The dependence of Bi content on As species was explained by considering the results of Foxon and Joyce, which show the necessary desorption of 50 % of the incident As4 flux during GaAs growth. The optical quality of GaAsBi was shown to have no dependence on the As species used during growth. Using the expertise gained from the growth of bulk GaAsBi, a series of GaAsBi/GaAs multiple quantum well p-i-n diodes was grown and characterised. Preliminary results showed that good quality structures were grown with photoluminescence peaks at around 1050 nm. The samples containing a large number of quantum wells showed signs of strain relaxation and a redshift and attenuation of their photoluminescence spectra. Calculations of the effects of strain relaxation and loss of quantum confinement on the photoluminescence emission wavelength, suggest that both factors contribute to the observed redshift. The onset of strain relaxation in these samples appeared to occur at a similar average strain to InGaAs/GaAs samples reported in the literature. These results suggest that GaAsBi could provide a competitive alternative to InGaAs for high efficiency multi-junction photovoltaics

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%

Space Photovoltaic Research and Technology 1986. High Efficiency, Space Environment, and Array Technology

The conference provided a forum to assess the progress made, the problems remaining, and the strategy for the future of photovoltaic research. Cell research and technology, space environmental effects, array technology and applications were discussed

18th Space Photovoltaic Research and Technology Conference

The 18th Space Photovoltaic Research and Technology (SPRAT XVIII) Conference was held September 16 to 18, 2003, at the Ohio Aerospace Institute (OAI) in Brook Park, Ohio. The SPRAT conference, hosted by the Photovoltaic and Space Environments Branch of the NASA Glenn Research Center, brought together representatives of the space photovoltaic community from around the world to share the latest advances in space solar cell technology. This year s conference continued to build on many of the trends shown in SPRAT XVII-the continued advances of thin-film and multijunction solar cell technologies and the new issues required to qualify those types of cells for space applications

Quantum Dot-Based Thin-Film III–V Solar Cells

In this work, we report our recent results in the development of thin-film III–V solar cells fabricated by epitaxial lift-off (ELO) combining quantum dots (QD) and light management structures. Possible paths to overcome two of the most relevant issues posed by quantum dot solar cells (QDSC), namely, the degradation of open circuit voltage and the weak photon harvesting by QDs, are evaluated both theoretically and experimentally. High open circuit voltage QDSCs grown by molecular beam epitaxy are demonstrated, both in wafer-based and ELO thin-film configuration. This paves the way to the implementation in the genuine thin-film structure of advanced photon management approaches to enhance the QD photocurrent and to further optimize the photovoltage. We show that the use of light trapping is essential to attain high-efficiency QDSCs. Based on transport and rigorous electromagnetic simulations, we derive design guidelines towards light-trapping enhanced thin-film QDSCs with efficiency higher than 28% under unconcentrated light, ambient temperature. If photon recycling can be fully exploited, 30% efficiency is deemed to be feasible. Towards this goal, results on the development and integration of optimized planar and micro-patterned mirrors, diffractive gratings and broadband antireflection coatings are presented.acceptedVersionPeer reviewe

Design, Growth, and Characterization of III-Sb and III-N Materials for Photovoltaic Applications

abstract: Photovoltaic (PV) energy has shown tremendous improvements in the past few decades showing great promises for future sustainable energy sources. Among all PV energy sources, III-V-based solar cells have demonstrated the highest efficiencies. This dissertation investigates the two different III-V solar cells with low (III-antimonide) and high (III-nitride) bandgaps. III-antimonide semiconductors, particularly aluminum (indium) gallium antimonide alloys, with relatively low bandgaps, are promising candidates for the absorption of long wavelength photons and thermophotovoltaic applications. GaSb and its alloys can be grown metamorphically on non-native substrates such as GaAs allowing for the understanding of different multijunction solar cell designs. The work in this dissertation presents the molecular beam epitaxy growth, crystal quality, and device performance of AlxGa1−xSb solar cells grown on GaAs substrates. The motivation is on the optimization of the growth of AlxGa1−xSb on GaAs (001) substrates to decrease the threading dislocation density resulting from the significant lattice mismatch between GaSb and GaAs. GaSb, Al0.15Ga0.85Sb, and Al0.5Ga0.5Sb cells grown on GaAs substrates demonstrate open-circuit voltages of 0.16, 0.17, and 0.35 V, respectively. In addition, a detailed study is presented to demonstrate the temperature dependence of (Al)GaSb PV cells. III-nitride semiconductors are promising candidates for high-efficiency solar cells due to their inherent properties and pre-existing infrastructures that can be used as a leverage to improve future nitride-based solar cells. However, to unleash the full potential of III-nitride alloys for PV and PV-thermal (PVT) applications, significant progress in growth, design, and device fabrication are required. In this dissertation, first, the performance of ii InGaN solar cells designed for high temperature application (such as PVT) are presented showing robust cell performance up to 600 ⁰C with no significant degradation. In the final section, extremely low-resistance GaN-based tunnel junctions with different structures are demonstrated showing highly efficient tunneling characteristics with negative differential resistance (NDR). To improve the efficiency of optoelectronic devices such as UV emitters the first AlGaN tunnel diode with Zener characteristic is presented. Finally, enabled by GaN tunnel junction, the first tunnel contacted InGaN solar cell with a high VOC value of 2.22 V is demonstrated.Dissertation/ThesisDoctoral Dissertation Electrical Engineering 201