Characterization of functional nanoparticles

Treballs Finals de Grau de Física, Facultat de Física, Universitat de Barcelona, Curs: 2017, Tutors: Sònia Estradé Albiol i Pau Torruella BesaIn the present work, we focus on the study of nanoparticles of the Ag-Au-S ternary system. The nanoscopic dimensions recquire the use of powerful instruments and effcient techniques, so High Resolution Transmission Electron Microscopy (HRTEM) is performed. Using the Gatan DigitalMicrograph and the CaRIne Crystallography softwares, the nanostructures are identified and characterized

TEM characterization of advanced kesterite structures

Màster en Nanociència i Nanotecnologia, Facultat de Física, Universitat de Barcelona, Curs: 2017-2018. Tutors: Sònia Estradé Albiol, Víctor Izquierdo Roca and Javier Blanco PortalsIn the present work, we study a kesterite Cu2ZnSnSe4 solar cell at the nanoscale level in order to analyze the defects that lower its energy conversion efficiency. First of all, current density{voltage characteristics, quan- tum effciency, photoluminiscence, and Raman spectroscopy are studied to obtain macroscopic and microscopic information about the device. Then, High Resolution Transmission Electron Microscopy (HRTEM) is performed to observe the cross-section and, for the first time, the front interface of the absorber with nanoscopic detai

2-step process for 5.4% CuGaSe2 solar cell using fluorine doped tin oxide transparent back contacts

As single-junction solar cells are approaching theoretical limits, multijunction solar cells are becoming increasingly relevant, and low-cost wider bandgap light harvesters in tandem with silicon are the next frontier in thin film photovoltaic research. Cu-based chalcogenide compounds have achieved great success as standard absorbers, but performance for bandgaps above 1.5¿eV is still lacking. Additionally, the use of transparent back contacts remains challenging for this class of materials. In this work, we report on the fabrication of wide bandgap CuGaSe2 absorbers by a combination of metallic sputtering and reactive thermal annealing grown on transparent fluorine-doped tin oxide-coated glass substrate. The annealing temperature is carefully tuned in regard to material and photovoltaic device properties. The introduction of an ultrathin Mo interlayer at the CuGaSe2/back interface favors a higher contact's ohmicity and results in an important improvement of all figures of merit. A record conversion efficiency of 5.4% is obtained, which is the highest value reported for this class of absorber on transparent back contact. Fundamental material characterization of the as-grown CuGaSe2 films reveals a better homogeneity in Cu distribution throughout the absorber's thickness when using a Mo interlayer, along with an enhanced crystalline quality. The sub-bandgap transparency of the final device remains perfectible, and improvement pathways are proposed using transfer matrix-based optical modeling, suggesting to use more specular interfaces to enhance optical transmission.Peer ReviewedPostprint (published version

Characterization of the stability of indium tin oxide and functional layers for semitransparent back-contact applications on Cu(in,Ga)Se2 solar cells

Herein, a detailed study of the stability of different ITO-based back-contact configurations (including bare ITO contacts and contacts functionalized with nanometric Mo, MoSe2, and MoS2 layers) under the coevaporation processes developed for the synthesis of high-efficiency Cu(In,Ga)Se2 (CIGSe) solar cells is reported. The results show that bare ITO layers can be used as efficient back contacts for coevaporation process temperatures of 480¿ºC. However, higher temperatures produce an amorphous In–Se phase at the ITO surface that reduces the contacts transparency in the visible region. This is accompanied by degradation of the solar cells’ efficiency. Inclusion of a Mo functional layer leads to the formation of a MoSe2 interfacial phase during the coevaporation process, which improves the cells’ efficiency, achieving device efficiencies similar to those obtained with reference solar cells fabricated with standard Mo back contacts. Optimization of the initial Mo layer thickness improves the contact transparency, achieving contacts with an optical transparency of 50% in the visible region. This is accompanied by a relevant decrease in back reflectivity in the CIGSe devices, confirming the potential of these contact configurations for the development of semitransparent CIGSe devices with improved optical aesthetic quality without compromising the device performance.Peer ReviewedPostprint (published version

Effects of ITO based back contacts on Cu(In,Ga)Se2 thin films, solar cells, and mini-modules relevant for semi-transparent building integrated photovoltaics

This study presents the results of the development of semi-transparent Cu(In,Ga)Se2 (CIGSe) mini-modules for the application in building integrated photovoltaics (BIPV). Applying in-situ X-ray diffraction in real-time during CIGSe growth we find that the bulk of indium-tin-oxide (ITO), acting as the transparent back contact, is chemically stable in CIGSe processing. CIGSe layers grown on reactively sputtered ITO (Ar/O2 flux ratio = 60:1) or on ITO annealed in ambient air have a pro-portionally higher (220/204) orientation compared to CIGSe layers grown on as fabricated ITO sputtered solely by Ar. However, independent from the fabrication and annealing state of the ITO back contact, after CIGSe deposition at high substrate temperatures >= 600 degrees C accumulation of Ga at the CIGSe/ITO back contact interface combined with reduced solar cell efficiency is observed. This Ga accumulation visible in elemental depth profiles is attributed to the formation of gallium -oxide (GaOx). Applying a very thin (approximate to 10-30 nm) functional molybdenum layer in between CIGSe and the ITO back contact inhibits the formation of GaOx. Based on this Mo/ITO back contact configuration semi-transparent 10 x 10 cm2 mini-modules with 14 cells interconnected in series have been fabricated. Module parameters resulted in a fill factor of 63% and >12% in efficiency. The solar active coverage of the modules amounts to approximate to 70%, and the average visible transmittance (in the range 380-780 nm) of the transparent sections was 27.6% (9.6% for the total area of the device). Optimisation of the Mo/ITO contact allows increasing this transparency to values > 50%. Long-term outdoor testing of a semi-transparent module prototype reveals no degradation in electric output power for 3 months, demonstrating the device stability under changing climatic conditions

A new approach for alkali incorporation in Cu2ZnSnS4 solar cells

The addition of alkali elements has become mandatory for boosting solar cell performance in chalcogenide thin films based on kesterites (Cu2ZnSnS4, CZTS). A novel doping process is presented here, that consists in the incorporation of sodium or lithium during the deposition of the CdS buffer layer, followed by a post-deposition annealing (PDA). As the doping route leads to more efficient devices in comparison with the undoped reference sample, the influence of PDA temperature was also investigated. Compositional profiling techniques, time-of-flight secondary ion mass spectrometry (TOF-SIMS) and glow discharge optical mission spectroscopy (GDOES), revealed a dependence of the alkaline distribution in kesterites with the PDA temperature. Although the doping process is effective in that it increases the alkaline concentration compared to the undoped sample, the compositional profiles indicate that a significant proportion of Li and Na remains 'trapped' within the CdS layer. In the 200 °C–300 °C range the alkali profiles registered the higher concentration inside the kesterite. Despite this, an additional alkali accumulation close to the molybdenum/fluorine doped tin oxide substrate was found for all the samples, which is frequently related to alkali segregation at interfaces. The addition of both, lithium and sodium, improves the photovoltaic response compared to the undoped reference device. This is mainly explained by a substantial improvement in the open-circuit potential (Voc) of the cells, with best devices achieving efficiencies of 4.5% and 3% for lithium and sodium, respectively. Scanning-electron microscopy images depicted a 'bilayer structure' with larger grains at the top and small grains at the bottom in all samples. Moreover, the calculated bandgap energies of the CZTS films account for changes in the crystallographic order-disorder of the kesterites, more related to the PDA treatment rather than alkali incorporation. Even if further optimization of the absorber synthesis and doping process will be required, this investigation allowed the evaluation of a novel strategy for alkali incorporation in kesterite based solar cells.Peer ReviewedPostprint (published version

UV‐selective optically transparent Zn(O,S)‐based solar cells

This work reports experimental evidence of a photovoltaic effect in transparent UV‐selective Zn(O,S)‐based heterojunctions. Zn(O,S) has a strong interest for the development of UV‐selective solar cells with high transparency in the visible region, required for the development of nonintrusive building‐integrated photovoltaic (BIPV) elements as transparent solar windows and glass‐based solar façades. By anion alloying, Zn(O,S) mixed crystal absorbers can be fabricated with different sulfur content across the whole compositional range. This allows adjustment of the bandgap of the absorbers in the 2.7-2.9 eV region, maximizing absorption in the UV, while keeping a high level of transparency. Zn(O,S) alloys with composition corresponding to S/(S + O) content ratios of 0.6 are successfully grown by sputtering deposition, and first glass/FTO/NiO/Zn(O,S)/ITO device prototypes are produced. The resulting devices present an average visible transmittance (AVT) of 75% and present photovoltaic effect. By introducing a thin C60 film as electron transport layer (ETL), charge extraction is enhanced, and devices show an efficiency of 0.5% and an AVT > 69%. The transparency of these devices can potentially allow for their ubiquitous installation in glazing systems as part of nonintrusive BIPV elements or to power Internet of Things (IoT) devices and sensors as an integrated transparent component

Insights into interface and bulk defects in a high efficiency kesterite-based device

This work provides a detailed analysis of a high efficiency Cu2ZnSnSe4 device using a combination of advanced electron microscopy and spectroscopy techniques. In particular, a full picture of the different defects present at the interfaces of the device and in the bulk of the absorber is achieved through the combination of high resolution electron microscopy techniques with Raman, X-ray fluorescence and Auger spectroscopy measurements at the macro, micro and nano scales. The simultaneous investigation of the bulk and the interfaces allows assessing the impact of the defects found in each part of the device on its performance. Despite a good crystalline quality and homogeneous composition in the bulk, this work reports, for the first time, direct evidence of twinning defects in the bulk, of micro and nano-voids at the back interface and of grain-to-grain non-uniformities and dislocation defects at the front interface. These, together with other issues observed such as strong absorber thickness variations and a bilayer structure with small grains at the bottom, are shown to be the main factors limiting the performance of CZTSe devices. These results open the way to the identification of new solutions to further developing the kesterite technology and pushing it towards higher performances. Moreover, this study provides an example of how the advanced characterization of emergent multilayer-based devices can be employed to elucidate their main limitations.Peer ReviewedPostprint (author's final draft

Over 10% efficient wide bandgap CIGSe solar cells on transparent substrate with Na predeposition treatment

With the recent rise of new photovoltaic applications, it has become necessary to develop specific optoelectronic properties for thin‐film technologies such as Cu(In,Ga)Se2 and to take advantage of their high degree of tunability. The feasibility of efficient wide bandgap absorbers on transparent conductive oxide substrates is, in that context, of critical importance. Using an original approach based on a predeposition sodium treatment, Cu(In,Ga)Se2 absorbers fabricated by sputtering and reactive annealing with a Ga to (Ga + In) content over 0.7 and an optical bandgap above 1.4 eV are deposited on transparent fluorine‐doped tin oxide films, with the insertion of an ultrathin MoSe2 layer preserving the contact's ohmicity. Different material characterizations are carried out, and a thorough Raman analysis of the absorber reveals that the sodium pretreatment significantly enhances the Ga incorporation into the chalcopyrite matrix, along with markedly improving the film's morphology and crystalline quality. This translates to a spectacular boost of the photovoltaic performance for the resulting solar cell as compared with a reference device without Na, specifically in the voltage and fill factor. Eventually, an efficiency exceeding 10% is obtained without antireflection coating, a record value bridging the gap with the state of the art on nontransparent substrates

Composition variations in Cu(In,Ga)(S,Se)2 solar cells: Not a gradient, but an interlaced network of two phases

peer reviewedRecord efficiency in chalcopyrite-based solar cells Cu(In,Ga)(S,Se)2 is achieved using a gallium gradient to increase the bandgap of the absorber toward the back side. Although this structure has successfully reduced recombination at the back contact, we demonstrate that in industrial absorbers grown in the pilot line of Avancis, the back part is a source of non-radiative recombination. Depth-resolved photoluminescence (PL) measurements reveal two main radiative recombination paths at 1.04 eV and 1.5–1.6 eV, attributed to two phases of low and high bandgap material, respectively. Instead of a continuous change in the bandgap throughout the thickness of the absorber, we propose a model where discrete bandgap phases interlace, creating an apparent gradient. Cathodoluminescence and Raman scattering spectroscopy confirm this result. Additionally, deep defects associated with the high gap phase reduce the absorber's performance. Etching away the back part of the absorber leads to an increase of one order of magnitude in the PL intensity, i.e., 60 meV in quasi-Fermi level splitting. Non-radiative voltage losses correlate linearly with the relative contribution of the high energy PL peak, suggesting that reducing the high gap phase could increase the open circuit voltage by up to 180 mV.POLC