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See detailOptical and vibrational properties of layered materials
Wirtz, Ludger UL

Presentation (2012, June 13)

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See detailOptical and vibrational properties of layered MoS2
Molina-Sanchez, Alejandro UL; Wirtz, Ludger UL

Scientific Conference (2014, January 24)

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See detailOptical and vibrational properties of M0S2
Wirtz, Ludger UL

Presentation (2012, November 26)

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See detailOptical and x-ray evidence of the de vries sm-A*-sm-C* transition in a non-layer shrinkage ferroelectric liquid crystal with very weak interlayer tilt correlation
Lagerwall, Jan UL; Giesselmann, Frank; Radcliffe, M. D.

in Physical Review. E. (2002), 66(3), 031703

A non-layer-shrinkage fluorinated ferroelectric liquid crystal compound, 8422@2F3\#, has been characterized by means of optical, x-ray, and calorimetric methods. The orientational distribution within ... [more ▼]

A non-layer-shrinkage fluorinated ferroelectric liquid crystal compound, 8422@2F3\#, has been characterized by means of optical, x-ray, and calorimetric methods. The orientational distribution within macroscopic volumes determined through wide-angle x-ray scattering and birefringence measurements, was found to be identical in the Sm-A* and helical Sm-C* phases. Together with the absence of layer shrinkage, this constitutes strong evidence that the second-order Sm-A*–Sm-C* transition in this material is well described by the diffuse cone model of de Vries. The absolute values of the layer spacing show that the molecules aggregate to antiparallel pairs. The molecular interaction across the layer boundaries will then occur only between fluorine atoms, leading to unusually weak interlayer tilt direction correlation. This explains the experimental observations of a very easily disturbed Sm-C* helix and a peculiar surface-stabilized texture. Tilt angle and birefringence values as a function of field and temperature have been evaluated in the Sm-A* and Sm-C* phases and the results corroborate the conclusions from the x-ray investigations. [less ▲]

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See detailOptical Characterization of Cu2ZnSnSe4 Thin Films
Sendler, Jan Michael UL

Doctoral thesis (2017)

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See detailOptical Communications in a Legal Vacuum?
Hofmann, Mahulena UL

in Congress of the International Astronautical Federation (2013), 2012

The use of optical communication has many technical advantages. The thesis that it stands outside of any legal framework can, however, be hardly defended. Despite of the fact that the intensity of legal ... [more ▼]

The use of optical communication has many technical advantages. The thesis that it stands outside of any legal framework can, however, be hardly defended. Despite of the fact that the intensity of legal rules regulating the use of optical communication is lower that eg the use of radio frequences, both ITU basic texts and the UN space treaties contain binding rules which have to be respected. [less ▲]

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See detailOptical conductivity of triple point fermions
Habibi, Alireza UL; Farajollahpour, Tohid; Jafari, S A

in Journal of Physics: Condensed Matter (2021), 33

As a low-energy effective theory on non-symmorphic lattices, we consider a generic triple point fermion Hamiltonian, which is parameterized by an angular parameter λ. We find strong λ dependence in both ... [more ▼]

As a low-energy effective theory on non-symmorphic lattices, we consider a generic triple point fermion Hamiltonian, which is parameterized by an angular parameter λ. We find strong λ dependence in both Drude and interband optical absorption of these systems. The deviation of the T2 coefficient of the Drude weight from Dirac/Weyl fermions can be used as a quick way to optically distinguish the triple point degeneracies from the Dirac/Weyl degeneracies. At the particular λ = π/6 point, we find that the 'helicity' reversal optical transition matrix element is identically zero. Nevertheless, deviating from this point, the helicity reversal emerges as an absorption channel. [less ▲]

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See detailOptical control of polarization in ferroelectric heterostructures
Tao, Li; Lipatov, Alexej; Lu, Haidong et al

in Nature Communications (2018), 9

In the ferroelectric devices, polarization control is usually accomplished by application of an electric field. In this paper, we demonstrate optically induced polarization switching in BaTiO3-based ... [more ▼]

In the ferroelectric devices, polarization control is usually accomplished by application of an electric field. In this paper, we demonstrate optically induced polarization switching in BaTiO3-based ferroelectric heterostructures utilizing a two-dimensional narrow-gap semiconductor MoS2 as a top electrode. This effect is attributed to the redistribution of the photo-generated carriers and screening charges at the MoS2/BaTiO3 interface. Specifically, a two-step process, which involves formation of intra-layer excitons during light absorption followed by their decay into inter-layer excitons, results in the positive charge accumulation at the interface forcing the polarization reversal from the upward to the downward direction. Theoretical modeling of the MoS2 optical absorption spectra with and without the applied electric field provides quantitative support for the proposed mechanism. It is suggested that the discovered effect is of general nature and should be observable in any heterostructure comprising a ferroelectric and a narrow gap semiconductor. [less ▲]

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See detailOPTICAL DEFECT SPECTROSCOPY IN CUINS2 THIN FILMS AND SOLAR CELLS
Lomuscio, Alberto UL

Doctoral thesis (2020)

Pure-sulphide Cu(In,Ga)S2 solar cells have reached certified power conversion efficiency as high as 15.5 %. While this record performance has been achieved by growing the semiconducting absorber at very ... [more ▼]

Pure-sulphide Cu(In,Ga)S2 solar cells have reached certified power conversion efficiency as high as 15.5 %. While this record performance has been achieved by growing the semiconducting absorber at very high temperature with a copper deficient composition, all other previous records were based on chalcopyrite films deposited under Cu excess. Still, this world record is far from the theoretical power conversion achievable in single junction solar cell for this semiconductor (about 30 %), which has a tunable band gap between 1.5 and 2.4 eV. This thesis aims to gain insight into the optoelectronic properties of this semiconductor, particularly CuInS2, looking at their variation as a function of the deposition temperature and of the absorber composition. The investigations are carried out mainly by photoluminescence (PL) spectroscopy, which allows to measure the quasi Fermi level splitting (QFLS), that is an upper limit of the maximum open circuit voltage (VOC) an absorber is capable of. PL spectroscopy is used to get insights onto the electronic defects as well, both the shallow ones, which contribute to the doping, and the deep ones, which enhance non-radiative recombination. By increasing the Cu content in the as-grown compositions, the morphology and microstructure of the thin films improve, as they show larger grains and less structural defects than films deposited with Cu deficiency. The composition affects the QFLS as well, which is significantly higher for sample deposited under Cu excess, in contrast to the observations in selenide chalcopyrite. The increment of the process temperature leads to an improvement of the QFLS too, although absorbers grown in Cu deficiency are less influenced, likely because of a lower sodium content in the high-temperature glass used as substrate. The QFLS increase correlates with the lowering of a deep defect related band, which manifests itself with a peak maximum at around 0.8 eV in room temperature PL spectra. In literature, the low efficiencies exhibited by Cu(In,Ga)S2–based solar cells are often attributed to interface problems at the p-n junction, i.e. at the absorber-buffer layer interface. In this work, the comparison of the QFLS and VOC of pure sulphides CIGS with those measured on selenides clearly points out that the lower efficiencies exhibited by the former are caused also by the intrinsic lower optoelectronic quality of Cu(In,Ga)S2 films. To shed light on the electronic structure, high quality CuInS2 films are deeply investigated by means of low temperature PL. Four shallow defects are detected: one shallow donor at about 30 meV from the conduction band and three shallow acceptors at about 105, 145 and 170 meV from the valence band. The first of these acceptors dominates the band edge luminescence of sample grown with composition close to the stoichiometry, whereas the second deeper acceptor is characteristic of absorbers deposited in Cu rich regime. The deepest of these acceptors seems to be present over a wide range of compositions, although its luminescence is observable only for slight Cu-poor samples with sodium incorporation during the deposition. The quality of the examined films allows the observations of phonon coupling of these shallow defects for the first time in this semiconductor. All these observations on shallow defects and their phonon coupling behaviour allowed to revise the defect model for this semiconductor. The findings of this thesis reveal the strong similarity of the shallow defects structure with selenium based compounds. On the other hand, the presence of deep defects in CuInS2 strongly limits the optoelectronic quality of the bulk material, causing the gap in power conversion efficiencies compared to low-band gap Cu(In,Ga)Se2 solar cells, which show efficiencies above 23%. [less ▲]

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See detailOptical Detection of Deep Defects in Cu(In,Ga)Se2
Spindler, Conrad UL

Doctoral thesis (2018)

The aim of this thesis is to shed light on the deep defect structure in Cu(In,Ga)Se2 by photoluminescence measurements and to propose a possible conclusive defect model by attributing experimental ... [more ▼]

The aim of this thesis is to shed light on the deep defect structure in Cu(In,Ga)Se2 by photoluminescence measurements and to propose a possible conclusive defect model by attributing experimental findings to a literature review of defect calculations from first principles. Epitaxial films are grown on GaAs by metal organic vapor phase epitaxy and characterized by photoluminescence at room or low temperature. In CuGaSe2, deep defect bands at ca. 1.1 eV and 1.23 eV are resolved. A model for the power law behavior in excitation dependent measurements of the peak intensities is derived, which leads to the experimental finding of two deep donor-like defects as a result. In Cu(In,Ga)Se2, the deeper band at around 1.1 eV remains constant in energy when more and more gallium is replaced by indium in the solid solution. For decreasing Ga-contents, the band gap is mainly lowered by a decrease of the conduction band energy. From fitting models for electron-phonon coupling, the dominating deep donor-like defect is determined at 1.3 eV above the valence band maximum. This level is proposed to be crucial for high Ga-contents when it is deep inside the band gap and most likely acts as a recombination center. At low Ga-contents it is resonant with the conduction band. The larger open circuit voltage deficits for high Ga-contents are proposed to stem at least partly from this defect which is qualitatively supported by simulations. Additionally another defect band at around 0.7 eV is observed for high Ga-contents at low temperatures and at 0.8 eV for low Ga-contents. The intensity of the 0.8 eV band seems to disappear in a sample with Cu-deficiency. In general, deep luminescence is always observed with similar energies in all Cu-rich compositions, independent of the Ga-content. The deep defect involved could explain inferior efficiencies of Cu-rich devices which show increased non-radiative recombination in general. It is further discussed that the same deep defect could be the origin of a level at 0.8 eV which is observed in several photo-capacitance measurements in literature. Based on the literature review for intrinsic defect calculations by hybrid-functionals, a possible defect model for shallow and deep defects is derived with a focus on those results, where different authors using different methods agree. By comparing the experimental results in the scope of this thesis, the deep defect found at 1.3 eV above the valence band is attributed to the GaCu antisites. The single (0/-1) charge transition of CuIn and CuGa is proposed to be the main shallow acceptor in the near-band-edge luminescence of Cu-rich compositions at 60 - 100 meV, whereas the second (-1/-2) charge transition is attributed to the deep 0.8 eV defect band. The present findings could be useful for the improvement of Cu(In,Ga)Se2 solar cells with stochiometric absorber compositions (Cu-rich growth) or with high band gaps (high Ga-content). Furthermore, the results show a very good agreement of experiment and recent theoretical calculations of defects, which can be seen as a promising relation between photoluminescence spectroscopy and predictions from theory for other complex materials. [less ▲]

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See detailThe Optical Diode Ideality Factor Enables Fast Screening of Semiconductors for Solar Cells
Babbe, Finn UL; Choubrac, Leo; Siebentritt, Susanne UL

in Solar RRL (2018), 1800248

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See detailOptical Feeder Links Study towards Future Generation MEO VHTS Systems
Mengali, Alberto UL; Lyras, Nikos K.; Shankar, Bhavani UL et al

in Proceedings of the 35th AIAA International Communications Satellite Systems Conference (ICSSC) (2017, October)

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See detailOptical frequency comb based on nonlinear spectral broadening of a phase modulated comb source driven by dual offset locked carriers
Nagarjun, K.P.; Vikram, B. S.; Prakash, Roopa et al

in Optics Letters (2020)

We demonstrate a versatile technique to generate a broadband optical frequency comb source in the C-band. This is accomplished by nonlinear spectral broadening of a phase modulated comb source driven by ... [more ▼]

We demonstrate a versatile technique to generate a broadband optical frequency comb source in the C-band. This is accomplished by nonlinear spectral broadening of a phase modulated comb source driven by dual frequency offset locked carriers. The locking is achieved by setting up a heterodyne optical frequency locked loop to lock two phase modulated electro-optic 25 GHz frequency combs sourced from individual seed carriers offset by 100 GHz, to within 6.7 MHz of each other.We realize spectral broadening in highly nonlinear fiber after suitable amplification to obtain an equalized, nonlinearly broadened frequency comb.We obtain ~86 lines in a 20 dB band spanning over 2 THz. [less ▲]

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See detailOptical investigation of voltage losses in high-efficiency Cu(In,Ga)Se2 thin-film solar cells
Wolter, Max UL

Doctoral thesis (2019)

The increases in power conversion efficiencies up to 23.35 % in thin-film Cu(In,Ga)Se2 (CIGS) solar cells in recent years can mainly be ascribed to the alkali post-deposition treatment (PDT). The latter ... [more ▼]

The increases in power conversion efficiencies up to 23.35 % in thin-film Cu(In,Ga)Se2 (CIGS) solar cells in recent years can mainly be ascribed to the alkali post-deposition treatment (PDT). The latter consists of an additional treatment step after absorber growth where alkali elements, such as sodium (Na) or rubidium (Rb), are injected into the absorber. While the beneficial effects of the alkali PDT, attributed partly to a reduction of voltage losses, are undeniable, it is not yet entirely clear what underlying mechanisms are responsible. To clarify the specific influence of the alkali PDT on the voltage of the CIGS solar cells, photoluminescence (PL) spectroscopy experiments were conducted on state-of-the-art CIGS absorbers having undergone different alkali PDTs. Photoluminescence allows the investigation of possible voltage losses on the absorbers through the analysis of optoelectronic quantities such as the absorption coefficient, the quasi-Fermi level splitting (QFLS), electronic defects, and potential fluctuations. Mainly due to a smooth surface and a band gap minimum inside the bulk, the PL spectra of state-of-the-art CIGS absorbers are distorted by interference fringes. To remove the interference fringes at room temperature, an experimental method, which revolves around the measurement of PL under varying angles, is developed in this thesis. In addition, to enable PL experiments even at low temperatures, an auxiliary polystyrene-based scattering layer is conceptualized and deposited on the surface of the absorbers. With the influence of the interference fringes under control, the quasi-Fermi level splitting can be measured on bare and CdS-covered absorbers. The results reveal an improvement of the QFLS in absorbers that contain Na with an additional increase being recorded in absorbers that also contain Rb. The improvement of the QFLS is present in both bare and CdS-covered absorbers, indicating that the beneficial effect of the alkali PDT is not only occurring on the surface but also inside the bulk. To identify possible origins of the QFLS increase, various PL-based experiments were performed. At room temperature, spatially-resolved PL measurements on the microscopic scale do not reveal any optoelectronic inhomogeneities in state-of-the-art CIGS absorbers. Defect spectroscopy at low temperatures also does not reveal the presence of deep-level trap states. Through temperature- and excitation-dependent PL experiments, a reduction of electrostatic potential fluctuations is observed in absorbers that contain Na with a stronger reduction witnessed in absorbers that contain Rb as well. The extraction of the absorption coefficient through PL measurements at room temperature reveals a reduction of band tails with alkali PDT that empirically correlates to the measured increase in the QFLS. This correlation might indicate that the band tails, through non-radiative recombination, may be the origin of the performance-limiting voltage losses. In combination with reports from literature, it is suggested that the beneficial effect of the light alkali PDT (Na) is mainly a doping effect i.e. an increase in the QFLS through an increase in the hole carrier concentration. The beneficial effect of the heavier alkali PDT (Rb) is attributed partly to a surface effect but mainly to a grain boundary effect, either through a reduction in band bending or a reduction of non-radiative recombination through tail states. Finally, the various voltage losses in state-of-the-art CIGS solar cells are compared to the best crystalline silicon device, revealing almost identical losses. This shows that the alkali PDT enables the fabrication of high-efficiency CIGS solar cells that show, in terms of voltage, identical performance. To bridge the gap between CIGS and the even better performing GaAs, the results of this thesis suggest that grain boundaries are crucial in this endeavour. [less ▲]

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See detailOptical measurement under space conditions
Bremer, Mats UL

Doctoral thesis (2022)

The growing interest in space by governmental and private institutions has increased significantly in recent years. The issue of quality control plays an extremely important role in space travel, as ... [more ▼]

The growing interest in space by governmental and private institutions has increased significantly in recent years. The issue of quality control plays an extremely important role in space travel, as possible defects can cause enormous damage. The present work deals with a possible method to improve already existing quality control procedures for space flight. With the help of a 3D scanner, different components are measured and evaluated under space conditions. In particular, the linear thermal expansions are analyzed. The work has shown that the elaborated procedure works for metallic materials. For composites or joints between different materials, positive approaches were shown, which, however, could not be validated within the scope of this work. Components made of pure carbon fiber material cannot be evaluated with the technical equipment used. [less ▲]

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See detailOptical methodology for process monitoring of chalcopyrite photovoltaic technologies: Application to low cost Cu(In,Ga)(S,Se)2 electrodeposition based processes
Oliva, Florian; Kretzschmar, Steffen; Colombara, Diego UL et al

in Solar Energy Materials and Solar Cells (2016)

Non-destructive characterization of both single layers and completed devices are important issues for the development of efficient and low cost Cu(In,Ga)(S,Se)2 (CIGS) modules at high yields. This implies ... [more ▼]

Non-destructive characterization of both single layers and completed devices are important issues for the development of efficient and low cost Cu(In,Ga)(S,Se)2 (CIGS) modules at high yields. This implies for the need of methodologies suitable for the assessment of optical, electrical, and physico-chemical parameters that are relevant for the final device efficiency and that can be used for quality control and process monitoring at different process steps. In these applications, detection of in-homogeneities in the different layers from large area modules is especially relevant, being the presence of these inhomogeneities responsible for the existing gap between the efficiencies achieved in these technologies at cell and module levels. In this context, this work reviews the different optical methodologies that have been developed in the framework of the SCALENANO European project for the advanced assessment of the different layers in high efficiency electrodeposited – based CIGS devices. This has includes different strategies as those based on Raman scattering, Photoluminescence/Electroluminescence (PL/EL) based techniques and new photoelectrochemical based tools and firstly Raman spectroscopy is very sensitive to both composition and crystal quality parameters that are determining for device efficiency. Use of resonant Raman excitation strategies allows achieving a high sensitivity of the Raman spectra to the analysed features in the different regions of the device. This involves selection of the suitable excitation wavelength (in the broad spectral region from UV to IR) for the resonant Raman excitation of the required layer in the device. The strong increase in the intensity of the Raman peaks related to the use of resonant excitation conditions allows also decreasing the measuring time to times compatible with the implementation of these techniques at online process monitoring level. Analysed parameters include the electrical conductivity of the Al-doped ZnO window layer, the thickness of the CdS buffer layer and the chemical composition (S/(S+Se) relative content) and presence of relevant secondary phases as Cu-poor ordered vacancy compounds in the surface region of the absorbers. In addition PL/EL imaging are powerful techniques that provide direct access to the optoelectronic properties of the materials and devices. Whereas EL is performed using complete devices by injecting current in analogy to the operation of a light emitting diode, PL allows the characterization of bare absorber materials without the need for any functional or contacting layers. Moreover, semiconductor photo-electrochemistry (PEC) is a versatile technique that enables many opto-electronic properties of semiconductors to be determined. Essentially, a semiconductor on a conducting substrate placed in a solution containing redox species forms a Schottky barrier junction. The formation of such a diode enables basic semiconductor properties to be measured such as doping type, doping density, band gap and the flat band position versus the vacuum energy scale. In all these cases, quality control indicators suitable for the advanced assessment of these processes have been identified and validated for the electrodeposition-based processes developed at Nexcis Company. [less ▲]

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See detailOptical properties of Cu2ZnSnSe4 thin films and identification of secondary phases by spectroscopic ellipsometry
Demircioglu, Ozden; Salas, Jose Fabio Lopez; Rey, Germain UL et al

in OPTICS EXPRESS (2017), 25(5), 5327-5340

We apply spectroscopic ellipsometry (SE) to identify secondary phases in Cu2ZnSnSe4 (CZTSe) absorbers and to investigate the optical properties of CZTSe. A detailed optical model is used to extract the ... [more ▼]

We apply spectroscopic ellipsometry (SE) to identify secondary phases in Cu2ZnSnSe4 (CZTSe) absorbers and to investigate the optical properties of CZTSe. A detailed optical model is used to extract the optical parameters, such as refractive index and extinction coefficient in order to extrapolate the band gap values of CZTSe samples, and to obtain information about the presence of secondary phases at the front and back sides of the samples. We show that SE can be used as a non-destructive method for detection of the secondary phases ZnSe and MoSe2 and to extrapolate the band gap values of CZTSe phase. (C) 2017 Optical Society of America [less ▲]

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See detailOptical properties of few-layers and bulk MoS2
Molina-Sanchez, Alejandro UL; Wirtz, Ludger UL

Scientific Conference (2013)

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See detailOptical Properties of MoS2: Excitons Beyond the Bandgap
Molina-Sanchez, Alejandro UL

Scientific Conference (2014, August 26)

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