Advanced Functional Antireflection Coatings for Broadband Multijunction Solar Cells

Tähänastisesti korkein aurinkokennolla saavutettu hyötysuhde on tuotettu III‒V puolijohteisiin perustuvilla korkean hyötysuhteen moniliitoskennoilla. Nämä uusiin materiaaleihin pohjautuvat aurinkokennot mahdollistavat yli neljän kennoliitoksen rakenteet, joilla todennäköisesti ylitetään 50 %:n hyötysuhde vuosikymmenen loppuun mennessä. Tällöin koko aurinkokennon pitää olla loppuun asti optimoitu ja valmistettu, jotta vältytään ylimääräisiltä optisilta ja sähköisiltä häviöiltä. Tavoitteen saavuttaminen vaatii soveltuvia laajakaistaisia heijastuksenestopinnoiteita erilaisille moniliitosrakenteille estämään hyödyllisen auringonvalon heijastuminen kennon pinnalta. Tässä työssä on keskitytty kehittämään heijastuksenestopinnoitteita korkean hyötysuhteen moniliitoskennoille. Mitä leveämpi kaista auringonvaloa hyödynnetään, sitä vaikeammaksi tasomaisten pinnoitteiden optimointi käy. Halutun pinnoitteen täytyisi laajakaistaisuuden lisäksi olla yksinkertainen valmistaa toistettavasti erilaisille uusille aurinkokennoratkaisuille. Potentiaalisena ratkaisuna vaatimuksille on käyttää tasomaisten kerrosten lisäksi nanokuvioitua pintaa, jota hyödyntämällä voidaan suhteellisen yksinkertaisesti minimoida heijastus laajalta auringonvalon kaistalta. Työssä tutkittiin pinnoitteiden kerrosmateriaaleina matalan taitekertoimen magnesiumfluoridia ja korkean taitekertoimen tantaalipentoksidia. Näiden lisäksi tutkittiin uuden nanopinnoitusmenetelmän soveltamista monikerroksisiin heijastuksenestopinnoitteisiin. Menetelmässä alumiinioksidikerroksesta muokataan satunnainen nanokuvioitu pinta de-ionisoidussa vesihauteessa. Pinnoitetta käytettiin hilasovitetuille III‒V moniliitoskennoille, sen toimivuutta tarkasteltiin normaaleissa käyttöolosuhteissa, ja nanokuvion säilyvyyttä tutkittiin syklisen jäädyttämisen alaisena. Nanorakenteen kestävyyden parantamiseksi testattiin myös hydrofobisuuskäsittelyä päällystämällä pinnoite ohuella fluoropolymeeri-kerroksella.High efficiency III‒V semiconductor multijunction solar cells hold the record of the highest achieved conversion efficiency. Solar cells based on new materials enabling more than 4-junction architectures will most likely push the highest efficiency above 50% within the next decade. To be able to achieve this goal, every aspect of the solar cell structure needs to be designed and fabricated spot on, minimizing any possible optical and electrical losses. To this end, broadband antireflection coatings are instrumental for suppressing the amount of reflected light from the surface of the solar cell. This work contributes to the development of broadband antireflection coatings for primary use in connection with high efficiency multijunction solar cells. As the bandwidth of the utilized solar irradiation is getting increasingly wider, the antireflection coatings based on standard planar structures become harder to optimize, requiring fabrication of more complex films. On the other hand, there is a need to deploy simple and cost effective fabrication techniques to enable economical deployment of new photovoltaic technologies. This work focuses on developing multilayer antireflection coatings that utilize a nanostructured top layer to surpass the limitations of the conventional planar structures. As a first strand of work, material properties and their relation to the fabrication processes are investigated for low refractive index MgF2 films deposited by electron beam evaporation and the high refractive index Ta2O5 films deposited by ion beam sputtering. The second major part is related to the investigation of a novel technique to fabricate nanostructures with antireflective properties employing a simple de-ionized water treatment. The process is applied to form randomly distributed nanostructures on thin planar amorphous Al2O3 layer. A key result introduced in this work is the novel integration of the alumina nanostructuring with an underlying multilayer antireflection coating, specifically aimed to be used in lattice-matched III‒V semiconductor multijunction solar cells. The performance of the nanostructured coating was assessed in practical III-V multijunction solar cells, revealing its suitability for practical applaiction. Finally, the stability and durability of the nanostructrure has been improved using a hydrophobicity treatment based on fluoropolymerization, and evaluated under atmospheric icing conditions

Heijastamattomat pinnoitteet moniliitosaurinkokennoille

This thesis studied anti-reflective (AR) coatings for multi-junction solar cells (MJSC). The focus was on fabrication and characterization of dielectric thin films for AR coatings. In particular, the work focused on MgF2-based coatings and studying the effects of substrate temperature on the refractive index and mechanical properties of MgF2 films deposited by electron beam evaporation. Similarly, we studied the process parameters of nanoporous SiO2 deposited by plasma-enhanced chemical vapor deposition at different substrate temperatures and precursor gas ratios. Then for the two different spinnable siloxane coating, we studied parameters including spinning speed and lid position. The study revealed that MgF2 refractive index increases with substrate temperature until temperature of over 250 oC. For SiO2 the decrease in temperature and altered gas ratio generated porous structure that lowered the refractive index. The characterization results were used to simulate and optimize four different AR coatings for triple-junction InGaP/GaAs/GaInNAsSb solar cell using Essential Macleod design program. The coatings were MgF2/TiO2, nanoporous SiO2 with TiO2, siloxane layer with TiO2 and a triple layer coating of MgF2/Al2O3/TiO2. The coatings were compared to the conventional SiO2/TiO2 AR coating. The coated cells were tested with solar simulator under AM1.5D spectrum and all coating designs showed proper functionality, thus promising more efficient coating designs with these materials

Ice resistance of hydrophobic fluoropolymerized nanostructured alumina films for antireflective coatings

The functionality and durability of nanostructured alumina coatings exposed to atmospheric icing has been assessed to probe their usability in practical applications and to estimate the need for further development of the coatings. In particular, the changes in surface microstructure and in optical performance as well as in the wetting characteristics of the surfaces are reported. Without a hydrophobicity treatment the alumina nanostructures are superhydrophilic and do not endure large environmental changes. Hydrophobicity treated fluoropolymerized nanostructured alumina provides characteristics with partial anti-icing capabilities, enhanced durability, and excellent transmission levels of >95%, but the performance degrades in cyclic icing/de-icing. However, the hydrophobic nanostructured alumina outperforms both the nanostructured and planar alumina coatings and possesses increased durability and stability even under harsh conditions, such as the atmospheric icing. This indicates a clear need to use a hydrophobicity treatment for the nanostructured alumina antireflection coatings to be used in any environments. Therefore, its utilization in applications where little or occasional exposure to icing or other humidity and temperature changes is favorable over standard planar coatings. Further process optimization of the hydrophobicity treatment is still needed for better durability for cyclical icing exposure.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

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

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

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

Epitaxial lift-off process for GaAs solar cells controlled by InGaAs internal sacrificial stressor layers and a PMMA surface stressor

Epitaxial lift-off (ELO) techniques enable the development of thin-film III–V solar cell devices that are flexible and lightweight. To this end, we report an ELO process employing an internal sacrificial stressor layer (ISSL) and a surface polymer (PMMA) stressor layer. The combined action enhances the lateral etching rate and promotes a more controllable release process. The ISSL consists of quantum well-like GaInAs heterostructures that enable an accurate control of the stress required to enhance the lateral etching of the sacrificial layer, and hence the release of the thin film. More specifically, the use of the ISSL results in about 5-fold faster etch rate of the AlAs sacrificial layer. The ISSL layers can be etched away after the lift-off. Likewise, the PMMA surface stressor, which serves also as a sacrificial intermediate transfer layer, can be easily removed. The proof-of-concept device demonstration of the enhanced ELO technique was made by fabricating single-junction GaAs solar cells. The solar cell performance was evaluated under AM1.5d illumination and by external quantum efficiency measurements. Modelling based analysis shows that although the GaAs solar cell would require improvement of the front contact, yet the novel release process was successfully validated.publishedVersionPeer reviewe

Heijastamattomat pinnoitteet moniliitosaurinkokennoille

This thesis studied anti-reflective (AR) coatings for multi-junction solar cells (MJSC). The focus was on fabrication and characterization of dielectric thin films for AR coatings. In particular, the work focused on MgF2-based coatings and studying the effects of substrate temperature on the refractive index and mechanical properties of MgF2 films deposited by electron beam evaporation. Similarly, we studied the process parameters of nanoporous SiO2 deposited by plasma-enhanced chemical vapor deposition at different substrate temperatures and precursor gas ratios. Then for the two different spinnable siloxane coating, we studied parameters including spinning speed and lid position. The study revealed that MgF2 refractive index increases with substrate temperature until temperature of over 250 oC. For SiO2 the decrease in temperature and altered gas ratio generated porous structure that lowered the refractive index. The characterization results were used to simulate and optimize four different AR coatings for triple-junction InGaP/GaAs/GaInNAsSb solar cell using Essential Macleod design program. The coatings were MgF2/TiO2, nanoporous SiO2 with TiO2, siloxane layer with TiO2 and a triple layer coating of MgF2/Al2O3/TiO2. The coatings were compared to the conventional SiO2/TiO2 AR coating. The coated cells were tested with solar simulator under AM1.5D spectrum and all coating designs showed proper functionality, thus promising more efficient coating designs with these materials