Comparison of 'shallow' and 'deep' junction architectures for MBE-grown InAs/GaAs quantum dot solar cells

We report on the fabrication of InAs/GaAs quantum dot solar cells with high open circuit voltage by molecular beam epitaxy. `Shallow' and `deep' junction architectures were compared. The highest open circuit voltage of 0.94 V was obtained for the `shallow' junction configuration. The open circuit voltage of InAs quantum dot solar cells decreased only by ~40 mV compared to GaAs reference cells for both junction architectures indicating high quality quantum dots. The open circuit voltage of InAs/GaAs quantum dot solar cells was also found to be dependent on the size of quantum dots

Back Reflector with Diffractive Gratings for Light-Trapping in Thin-Film III-V Solar Cells

We report on the development of light-Trapping architectures applied to thin-film solar cells. In particular, we focus on enhancing the absorption at 1-eV spectral range for dilute nitride and quantum dot materials and report on the influence of planar back reflectors on the photovoltaic properties. Moreover, we discuss the properties of polymer diffraction gratings with enhanced light-Trapping capability pointing to advantageous properties of pyramidal gratings. In order to understand the suitability of these polymer grating architectures for space applications, we have performed an electron irradiation study (1 MeV) revealing the absence of reflectance changes up to doses of 1×1015 e-/cm

Thin-film InAs/GaAs quantum dot solar cell with planar and pyramidal back reflectors

Quantum dot solar cells are promising for next-generation photovoltaics owing to their potential for improved device efficiency related to bandgap tailoring and quantum confinement of charge carriers. Yet implementing effective photon management to increase the absorptivity of the quantum dots is instrumental. To this end, the performance of thin-film InAs/GaAs quantum dot solar cells with planar and structured back reflectors is reported. The experimental thin-film solar cells with planar reflectors exhibited a bandgap-voltage offset of 0.3 V with an open circuit voltage of 0.884 V, which is one of the highest values reported for quantum dot solar cells grown by molecular beam epitaxy to our knowledge. Using measured external quantum efficiency and current-voltage characteristics, we parametrize a simulation model that was used to design an advanced reflector with diffractive pyramidal gratings revealing a 12-fold increase of the photocurrent generation in the quantum dot layers

Comparison of metal/polymer back reflectors with half-sphere, blazed, and pyramid gratings for light trapping in III-V solar cells

We report on the fabrication of diffraction gratings for application as back contact reflectors. The gratings are designed for thin-film solar cells incorporating absorbers with bandgap slightly lower than GaAs, i.e. InAs quantum dot or GaInNAs solar cells. Light trapping in the solar cells enables the increase of the absorption leading to higher short circuit current densities and higher efficiencies. We study metal/polymer back reflectors with half-sphere, blazed, and pyramid gratings, which were fabricated either by photolithography or by nanoimprint lithography. The gratings are compared in terms of the total and the specular reflectance, which determine their diffraction capabilities, i.e. the feature responsible for increasing the absorption. The pyramid grating showed the highest diffuse reflection of light compared to the half-sphere structure and the blazed grating. The diffraction efficiency measurements were in agreement with the numerical simulations. The validated model enables designing such metal/polymer back reflectors for other type of solar cells by refining the optimal dimensions of the gratings for different wavelength ranges

Multipurpose synthesizable SystemVerilog Spi-Bus protocol verification system

The complexity of System-on-a-Chip (SoC) is continuing to increase due to the shrinking die size, increase in the number of sub-modules, power efficiency, performance, higher functionality and used protocols. This has an impact on the verification process related to the overall design process. For the verification process, there are commercial products that can be applied in order to verify and test certain Intellectual Properties (IP) but also platforms that lack these tools. This thesis focuses on the issue where a system has to be constructed that helps the verification and testing process of a data bus protocol used by the Device Under Testing (DUT). The study of the Serial Peripheral Interface (SPI) gives the examples of some issues that can be faced during applying this data bus protocol to any given system. Different kinds of testing and verifying methods are addressed in order to show what the tools can be when applying new DUT to a system or examining the data bus protocol it uses. The flow of a design process is studied by showing the iterations of a particular system that was to be created to meet the need that were introduced while examining the issues relating this subject. This flow can be said to start from ground level and end to the final iteration where the system could be created from. The functionality and structure of a Multipurpose Verification System that was created during this thesis are explained. The proofing process of this system is showed by examining the simulation and synthesis reports. The outcomes and future development ideas are discussed as well. This thesis showed that the study in hand has benefits to Nokia as the applying company and the system could be added to the company tool library after modifying it to be used as a stand-alone IP.Järjestelmäpiirien (SOC) kompleksisuus on jatkuvassa kasvussa johtuen johdinpiirien pienenemisestä, alijärjestelmien määrän kasvusta, vaatimuksista tehonkulutuksessa, suorituskyvyn kasvusta, toiminnallisuuden kasvusta ja käytettävistä protokollista. Näillä attribuuteilla on vaikutusta kokonaissuunnitteluprosessin verifiointiprosessiin. Verifiointiprosessiin on tällä hetkellä saatavilla kaupallisia sovelluksia, joita voidaan hyödyntää kolmannen osapuolen suunnitteleman systeemin testaukseen ja verifioitiin. Samalla on myös olemassa testausalustoja, joista nämä sovellukset puuttuvat. Tämä diplomityö keskittyy tilanteeseen, jossa täytyy rakentaa systeemi joka helpottaa testattavassa laitteessa (DUT) käytettävän tietoväyläprotokollan verifiointi- ja testausprosessia. Serial Peripheral Interface (SPI)-väyläprotokollan tutkimus tuo esiin esimerkkejä, joihin voidaan törmätä, kun kyseistä väyläprotokollaa käytetään missä tahansa sitä hyödyntävässä systeemissä. Diplomityössä tutkitaan erilaisia testaus- ja verifiointimetodeja, jotta voidaan osoittaa mitä erilaisia työkaluja voidaan hyödyntää, kun uusi testattava laite lisätään olemassa olevaan systeemiin tai tutkitaan tämän käyttämää väyläprotokollaa. Kokonaissuunnitteluprosessia on tutkittu esittelemällä tietyn järjestelmän iteraatiovaiheita, joka kehitettiin ratkaisemaan aiemmin tarkasteltuja, tähän aiheeseen liittyviä ongelmia. Suunnitteluprosessin voidaan katsoa alkaneen tilanteesta, jossa mitään konkreettista ei ollut vielä valmiina ja päättyvän tilanteeseen jossa järjestelmän viimeinen iteraatio voitiin alkaa konkretisoimaan. Monikäyttöisen syntetisoituvan verifiointijärjestelmän funktionaalisuus ja rakenne esitellään. Tutkimalla simulointi- ja synteesiraportteja näytetään tämän järjestelmän varmennusprosessi. Diplomityön toteutumista ja tulevaisuuden jatkokehitysideoista keskustellaan. Tämä diplomityö osoittaa, että Nokia soveltajayrityksenä pystyy hyödyntämään tämän tutkielman lopputulemia. Lisäksi työn tulokset voidaan lisätä, modifioinnin jälkeen, yrityksen komponenttikirjastoon toimimaan itsenäisenä instanssina

Metal/Polymer Back Reflectors with Diffraction Gratings for Light Trapping in III-V Solar Cells

We report on the fabrication, characterization and simulation of diffraction gratings for back contact reflectors in III-V solar cells. The gratings are designed for thin-film solar cells incorporating absorbers with bandgap slightly lower than GaAs, such as InAs/GaAs quantum dot or GaInNAs solar cells. Metal/polymer back reflectors with a blazed grating or a pyramid grating were fabricated by nanoimprint lithography. The gratings are compared in terms of diffraction ability, which is the feature responsible for increasing the absorption. The pyramid grating showed higher diffraction of light compared to the blazed grating. The diffraction efficiency measurements were in agreement with the numerical simulations. The model validation enables tailoring the properties of the reflectors for other type of solar cells by adjusting the optimal dimensions of the gratings for different wavelengths.acceptedVersionPeer reviewe