Laimeiden typpeä sisältävien korkean hyötysuhteen moniliitosaurinkokennojen spektri- ja lämpötiladynamiikka

Moniliitosaurinkokennojen ja uusien materiaaliratkaisujen avulla on mahdollista saavuttaa erittäin korkeita hyötysuhteita aurinkokennojen eri sovelluksissa. Käyttöolosuhteilla on merkittävä vaikutus kennojen toimintaan, minkä vuoksi kennojen toiminnan syvällinen ymmärtäminen on tärkeää kehitettäessä moniliitoskennoja, joilla pyritään entistä tehokkaampaan energiantuotantoon. Tässä työssä tutkittiin valaisun ja lämpötilan vaikutusta III–V-puolijohteista valmistettujen kolmiliitoskennojen toimintaan. Tutkitut kennot valmistettiin plasma-avusteisella molekyylisuihkuepitaksialla. Kennojen rakenteessa on käytetty osakennoja, jotka sisältävät pienen määrän typpeä. Kennot karakterisoitiin erilaisissa olosuhteissa virta-jännitemittauksin, joita varten rakennettiin kolmesta valonlähteestä koostuva aurinkosimulaattori. Simulaattori osoittautui tarkaksi ja hyödylliseksi työkaluksi kennojen karakterisoinnissa, ja sen tuottaman valotehon ajalliseksi stabiiliudeksi määritettiin ±2 %. Virta-jännitemittausten lisäksi parhaan moniliitoskennon toimintaa tutkittiin ulkoisen kvanttihyötysuhteen avulla. Huoneenlämmössä parhaan kolmiliitoskennon hyötysuhteeksi mitattiin 27 % AM0-spektrillä ja 28 % AM1.5-spektrillä. Typpeä sisältävät osakennot ylituottivat virtaa kaikissa kolmiliitoskennoissa, kun taas GaInP-liitoksen havaittiin rajoittavan kennojen virtaa. Kennojen toimintaa tutkittiin myös lämpötilariippuvilla mittauksilla. Kennojen virrantuotto kasvoi ja hyötysuhde laski lämpötilan noustessa. Lämpötilariippuvista mittauksista määritettiin myös lämpötilavakiot kennojen toimintaa kuvaaville suureille. Lämpötilavakioiden arvot ovat yhteneviä kirjallisuudessa esitettyjen vakioiden kanssa

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%

31% European InGaP/GaAs/InGaAs Solar Cells for Space Application

We report a triple junction InGaP/GaAs/InGaNAs solar cell with efficiency of ~31% at AM0, 25 °C fabricated using a combined molecular beam epitaxy (MBE) and metal-organic chemical vapour deposition (MOCVD) processes. The prototype cells comprise of InGaNAs (Indium Gallium Nitride Arsenide) bottom junction grown on a GaAs (Gallium Arsenide) substrate by MBE and middle and top junctions deposited by MOCVD. Repeatable cell characteristics and uniform efficiency pattern over 4-inch wafers were obtained. Combining the advantages offered by MBE and MOCVD opens a new perspective for fabrication of high-efficiency space tandem solar cells with three or more junctions. Results of radiation resistance of the sub-cells are also presented and critically evaluated to achieve high efficiency in EOL conditions

Performace of Dilute Nitride Triple Junction Space Solar Cell Grown by MBE

Dilute nitride arsenide antimonide compounds offer widely tailorable band-gaps, ranging from 0.8 eV to 1.4 eV, for the development of lattice-matched multijunction solar cells with three or more junctions. Here we report on the performance of GaInP/GaAs/GaInNAsSb solar cell grown by molecular beam epitaxy. An efficiency of 27% under AM0 conditions is demonstrated. In addition, the cell was measured at different temperatures. The short circuit current density exhibited a temperature coefficient of 0.006 mA/cm2/°C while the corresponding slope for the open circuit voltage was −6.8 mV/°C. Further efficiency improvement, up to 32%, is projected by better current balancing and structural optimization

Optimization of reactive ion beam sputtered Ta2O5 for III–V compounds

We report the optimization of the process parameters used in ion beam sputtering of dielectric Ta2O5 thin films on III–V semiconductor surfaces, with an aim of minimizing the deterioration of semiconductor surfaces and their opto-electric performance. We demonstrate that linear tuning of the three main sputtering parameters, namely, the primary source radiofrequency power, the ion beam current, and the ion beam voltage, allows optimizing the deposition conditions of Ta2O5 minimizing the damage to the III–V surfaces. The effect of parametrization is evaluated by deposition of a Ta2O5 antireflection coating on GaAs-based multijunction solar cells employing AlGaAs and AlInP window layers. Numerical study reveals that the main source of damage is the scattered primary ions, in this case argon ions, that have not contributed to the sputtering process of the Ta2O5 at the target. Moreover, it is likely that the reactive oxygen atmosphere oxidizes the semiconductor surfaces in the initial phase of the deposition process. A similar optimization procedure should be employed for any other thin film directly deposited by reactive ion beam sputtering on III–V surfaces and optoelectronics devices to avoid deposition induced damage.publishedVersionPeer reviewe

Use of nanostructured alumina thin films in multilayer anti-reflective coatings

A new method for modification of planar multilayer structures to create nanostructured aluminum oxide anti-reflection coatings is reported. The method is non-toxic and low-cost, being based on treatment of the coating with heated de-ionized water after the deposition of aluminum oxide. The results show that the method provides a viable alternative for attaining a low reflectance ARC. In particular, a low average reflectivity of ∼3.3% is demonstrated in a broadband spectrum extending from 400 nm to 2000 nm for ARCs deposited on GaInP solar-cells, the typical material used as top-junction in solar cell tandem architectures. Moreover, the process is compatible with volume manufacturing technologies used in photovoltaics, such as ion beam sputtering and electron beam evaporation.acceptedVersionPeer reviewe

Optical Performance Assessment of Nanostructured Alumina Multilayer Antireflective Coatings Used in III-V Multijunction Solar Cells

The optical performance of a multilayer antireflective coating incorporating lithography-free nanostructured alumina is assessed. To this end, the performance of single-junction GaInP solar cells and four-junction GaInP/GaAs/GaInNAsSb/GaInNAsSb multijunction solar cells incorporating the nanostructured alumina is compared against the performance of similar solar cells using conventional double-layer antireflective coating. External quantum efficiency measurements for GaInP solar cells with the nanostructured coating demonstrate angle-independent operation, showing only a marginal difference at 60° incident angle. The average reflectance of the nanostructured antireflective coating is 3 percentage points smaller than the reflectance of the double-layer antireflective coating within the operation bandwidth of the GaInP solar cell (280-710 nm), which is equivalent of 0.2 mA/cm2 higher current density at AM1.5D (1000 W/m2). When used in conjunction with the four-junction solar cell, the nanostructured coating provides 0.8 percentage points lower average reflectance over the operation bandwidth from 280 to 1380 nm. However, it is noted that only the reflectance of the bottom GaInNAsSb junction is improved in comparison to the planar coating. In this respect, since in such solar cells the bottom junction typically is limiting the operation, the nanostructured coating would enable increasing the current density 0.6 mA/cm2 in comparison to the standard two-layer coating. The light-biased current-voltage measurements show that the fabrication process for the nanostructured coating does not induce notable recombination or loss mechanisms compared to the established deposition methods. Angle-dependent external quantum efficiency measurements incline that the nanostructured coating excels in oblique angles, and due to low reflectance at a 1000-1800 nm wavelength range, it is very promising for next-generation broadband multijunction solar cells with four or more junctions.publishedVersionPeer reviewe

Data transmission at 1.3 µm using hybrid integrated silicon interposer and GalnNAs/GaAs electroabsorption modulator

Transmission of NRZ data at 12.5 Gbit/s is demonstrated using a hybrid integrated silicon photonics optical interconnect. The interconnect comprises a dilute nitride quantum well electroabsorption modulator on GaAs substrate, which is optically coupled to large core Si waveguide

Optimized molecular beam epitaxy process for lattice-matched narrow-bandgap (0.8 eV) GaInNAsSb solar junctions

High performance narrow-bandgap GaInNAsSb solar cells are instrumental for the development of lattice-matched GaAs-based solar cells with more than four junctions. To this end a comprehensive optimization process including the effects of growth temperature, As/III beam equivalent pressure ratio, and Sb flux on the performance of 0.8 eV GaInNAsSb solar cells grown by molecular beam epitaxy is reported. For this, sets of GaInNAsSb p-i-n solar cell structures with 5–6% nitrogen compositions were fabricated, while varying the key growth parameters. The quantum efficiency and current generation increased significantly when the narrow gap materials were grown at elevated growth temperatures, close to phase separation. A further improvement in the current generation was observed by employing lower As/III beam equivalent pressure ratios. The best GaInNAsSb cell exhibited about 94% peak external quantum efficiency and generated a short-circuit current of 17.7 mA/cm2 with AM1.5D (1000 W/m2) illumination at wavelengths above 900 nm without employing a back surface reflector. Our analysis indicates that the best cell is already close to being absorption limited. While the N composition should be kept as low as possible (i.e., ≲5%) to achieve high performance, increasing the Sb flux generally results in improved the material quality, i.e., leading to a slight improvement for the open-circuit voltages and fill factors. In addition, it was found that the phase separation observed at the growth temperature of 480 °C could effectively be inhibited by employing higher Sb fluxes.publishedVersionPeer reviewe

Data transmission at 1300 nm using optical interposer comprising hybrid integrated silicon waveguide and dilute nitride electroabsorption modulator

High speed back-to-back transmission of NRZ data at 12.5 Gbit/s was achieved over a repeaterless optical network without the use of forward error correction or optical clock recovery using a hybrid integrated silicon photonics optical interconnect. The interconnect comprises an electroabsorption modulator based on dilute nitride multiple quantum well material on GaAs substrate optically coupled to large core silicon waveguide using passive alignment and flip-chip bonding