Direct Bandgap Behavior in Rashba-Type Metal Halide Perovskites.

The generation and recombination of charge carriers in semiconductors through photons controls photovoltaic and light-emitting diode operation. Understanding of these processes in hybrid perovskites has advanced, but remains incomplete. Using femtosecond transient absorption and photoluminescence, it is observed that the luminescence signal shows a rise over 2 ps, while initially hot photogenerated carriers cool to the band edge. This indicates that the luminescence from hot carriers is weaker than that of cold carriers, as expected from strongly radiative transitions in direct gap semiconductors. It is concluded that the electrons and holes show a strong overlap in momentum space, despite recent proposals that Rashba splitting leads to a band offset suppressing such an overlap. A number of possible resolutions to this, including lattice dynamics that remove the Rashba splitting at room temperature, and localization of luminescence events to length scales below 10 nm are considered.The authors acknowledge financial support from the Engineering and Physical Sciences Research Council of the UK (EPSRC). J.M.R. thanks the Winton Programme for the Physics of Sustainability (University of Cambridge). J.M.R. thanks the Cambridge Home European Scheme for financial support. B.M. thanks Robinson College and the Cambridge Philosophical Society for a Henslow Research Fellowship. F.D. acknowledges funding from a Herchel Smith Research Fellowship and a Winton Advanced Research Fellowship

Excited state properties of novel solution processed inorganic and hybrid photovoltaic materials

Novel solution processed inorganic and hybrid photovoltaic materials have shown great promise in low-cost energy market and advanced optoelectronic device applications. Materials like colloidal nanocrystals could potentially transform device manufacturing with printing or spraying based processing, however often the excited state properties of these emerging materials are not well understood. The lack of fundamental knowledge of charge carrier formation, trapping and relaxation processes obstructs the development of the materials and devices. Therefore we employ time-resolved femtosecond spectroscopy methods such as differential reflectance, diffusion, interferometry and primarily transient absorption spectroscopy to study the characteristics of a range of novel photovoltaic materials. Different materials and their aspects are analysed in the six data chapters of this thesis with first three chapters concerning inorganic colloidal nanocrystals and final three concentrating on the analysis of metal halide perovskite family in bulk and nanocrystal forms. Iron pyrite nanocrystals, which tend to show poor performance in devices due to active surface states, are examined. In particular, the effects of novel surface passivation approach of coating iron pyrite nanoparticles with shells is analysed using transient absorption and long wavelength range steady state absorption spectroscopy techniques. The coated nanocrystals were found to display significant modulation of the surface state behaviour which was governed by the shell properties while the iron pyrite core remained photoactive. The analysis revealed the effectiveness of the new iron pyrite nanocrystal surface defect modulation approach which could potentially solve the issues associated with iron pyrite performance in devices. Cu₂ZnSnS₄ (CZTS) is another novel semiconductor which suffers from trapping effects due to its tendency to form defects. We investigated CZTS nanocrystal properties using transient absorption along with standard steady state and integrating sphere absorption measurements. Rapid relaxation of band edge carriers to a broad defect distribution was found which limited CZTS carrier lifetimes. In addition, plasmon influence induced by CZTS nanocrystal defects was also resolved. The prominent sub-state state influence on the CZTS excited state dynamics revealed that the defects must be efficiently passivated or their formation must be suppressed for effective material performance. Transient absorption spectroscopy along with steady state property analysis was also used to help characterize the properties of a range of silicon nanocrystals doped with different transition metals. Doping of nanoparticles can open new ways to control their properties but the field is relatively new and doped silicon nanoparticles have not been widely explored. By comparing undoped particle excited stated dynamics to doped nanocrystals we discovered rapid exciton transfer to new dopant introduced states which must be taken into account for the consideration of doped silicon nanocrystal applications. Recently metal halide perovskites emerged as the leading novel photovoltaic material family and while a lot of research efforts have been put into bulk material, perovskite nanocrystals have been less widely researched. We analysed a range of CsPbBr₃ nanocrystals spanning different quantum confinement regimes in relation to the bulk material using transient absorption spectroscopy. Comprehensive analysis of different aspects of quantum confinement influence was carried out considering state focusing, bandgap renormalization, degeneracy and carrier cooling. The results highlighted that quantum confinement effects were mostly significant for the smallest CsPbBr₃ nanocrystals that are currently available (~4 nm) while the larger nanocrystal photophysics was better described as a small perturbation on the free carrier photophysics established for the bulk material. The final two chapters concentrated on the photorefractive metal halide perovskite effects. Transient absorption and a range of differential reflectance measurements were employed to study anomalous metal halide perovskite TA response which obstructed the excited state resolution in previous literature. Here the anomalous features were isolated and attributed to the photorefractive effect and were further analysed using frequency domain interferometry. Photoinduced changes in the refractive index of the metal halide perovskite films and nanocrystals were measured with a femtosecond time resolution. The findings allowed for better understanding of metal halide perovskite excited state dynamics and accurate hot carrier temperature and effective mass determination as well as photoinduced refractive index characterization which is crucial for the design of such applications as optical switching, modulating and recording devices, probing techniques, lasers, single photon emitters and hot carrier solar cells. Collectively, this thesis provides an insight into fundamental properties of a range of novel solution processed inorganic and hybrid photovoltaic materials

Does a foreign entity doing sales in Lithuania only through the online web store have a permanent establishment?

Šis baigiamasis magistrinis darbas aktualus tuo, kad sparčiai vystantis technologijoms keičiasi ir verslas. Atsiradusi elektroninė komercija sukelia daug mokesčių klausimų ir problemų. Pirmoje darbo dalyje aptariama mokestinio rezidento samprata ir nuolatinės buveinės sąvoka tiek tarptautiniu lygiu, tiek ir Lietuvos teisės aktais. Vadovaujantis nustatytais nuolatinės buveinės kriterijais antroje darbo dalyje analizuojama jos vieta elektroninėje komercijoje ir pateikiamos šiuolaikiniam kontekstui nebetinkančios taisyklės bei jų sprendimo būdai. Po 2018 metų JAV bylos buvo nustatyta teisinė taisyklė, kad valstybės gali apmokestinti užsienio vienetą, net jei pardavėjas fiziškai nedalyvauja apmokestinančioje valstybėje ir tai nustatė ekonominio ryšio atsiradimą. Trečioje darbo dalyje pristatomi nauji verslo modeliai bei jiems naujai siūlomi skaitmeninės ekonomikos apmokestinimo alternatyvūs ir trumpalaikiai sprendimo būdai. Taip pat pabrėžiami svarbiausi Europos Sąjungos iškelti tikslai, kurie turi padėti pasiekti naują, modernią ir stabilią skaitmeninės ekonomikos mokesčių sistemą. Analizuojamas Europos Komisijos paskelbtas laikinas pasiūlymas - skaitmeninis paslaugų mokestis. Taip pat, aptariamas EBPO projektas, kuris yra skirtas spręsti ekonomikos skaitmeninimo mokesčių problemas ir yra būsimo susitarimo pagrindas įgyvendinant tarptautinę mokesčių reformą. Šiame darbe siekiama ištirti ar užsienio vienetas, vykdantis pardavimus tik per internetinę parduotuvę, turi nuolatinę buveinę Lietuvoje. Šiam klausimui atsakyti buvo pasitelkti istorinis, aprašomasis, analizės metodas, taip pat loginis ir lyginamasis metodas. Svarbiausia darbe padaryta išvada yra, kad užsienio vientas, kuris vykdo pardavimus Lietuvoje tik per internetinę parduotuvę, kol kas neturi nuolatinės buveinės, bet ateityje, internetinės parduotuvės, kurios neturi fizinių vietų, vis tiek turės nuolatines buveines, nes fizinis buvimas nėra būtinas norint sukurti esminį ryšį tarp verslo ir rinkos jurisdikcijos.This final master's thesis is relevant in the fact that with the rapid development of technologies, business is also changing. States face a new challenge in how to tax revenue from e-commerce. The first part of the paper discusses the concept of a tax resident and the concept of permanent establishment both at the international level and in Lithuanian legislation. In accordance with the established criteria of permanent establishment, which are adapted and effective in the physical space, the second part of the work analyzes the place of permanent establishment in e-commerce and presents rules that are no longer suitable for the modern context and their solutions. The third part presents new examples of business models and newly proposed alternative and short-term solutions for the taxation of the digital economy. The OECD project, which is the basis for a future agreement on international tax reform, is also discussed. The aim of this work is to investigate whether a foreign entity that sells only through an online store has a permanent establishment in Lithuania. The most important conclusion of the work is that a foreign entity that sells in Lithuania only through an online store does not have a permanent establishment yet, but in the future, online stores that do not have physical locations will still have permanent establishments, because physical presence is not necessary to create a substantial the nexus between business and market jurisdiction.Teisės fakulteta

Excited state properties of novel solution processed inorganic and hybrid photovoltaic materials

Novel solution processed inorganic and hybrid photovoltaic materials have shown great promise in low-cost energy market and advanced optoelectronic device applications. Materials like colloidal nanocrystals could potentially transform device manufacturing with printing or spraying based processing, however often the excited state properties of these emerging materials are not well understood. The lack of fundamental knowledge of charge carrier formation, trapping and relaxation processes obstructs the development of the materials and devices. Therefore we employ time-resolved femtosecond spectroscopy methods such as differential reflectance, diffusion, interferometry and primarily transient absorption spectroscopy to study the characteristics of a range of novel photovoltaic materials. Different materials and their aspects are analysed in the six data chapters of this thesis with first three chapters concerning inorganic colloidal nanocrystals and final three concentrating on the analysis of metal halide perovskite family in bulk and nanocrystal forms. Iron pyrite nanocrystals, which tend to show poor performance in devices due to active surface states, are examined. In particular, the effects of novel surface passivation approach of coating iron pyrite nanoparticles with shells is analysed using transient absorption and long wavelength range steady state absorption spectroscopy techniques. The coated nanocrystals were found to display significant modulation of the surface state behaviour which was governed by the shell properties while the iron pyrite core remained photoactive. The analysis revealed the effectiveness of the new iron pyrite nanocrystal surface defect modulation approach which could potentially solve the issues associated with iron pyrite performance in devices. Cu₂ZnSnS₄ (CZTS) is another novel semiconductor which suffers from trapping effects due to its tendency to form defects. We investigated CZTS nanocrystal properties using transient absorption along with standard steady state and integrating sphere absorption measurements. Rapid relaxation of band edge carriers to a broad defect distribution was found which limited CZTS carrier lifetimes. In addition, plasmon influence induced by CZTS nanocrystal defects was also resolved. The prominent sub-state state influence on the CZTS excited state dynamics revealed that the defects must be efficiently passivated or their formation must be suppressed for effective material performance. Transient absorption spectroscopy along with steady state property analysis was also used to help characterize the properties of a range of silicon nanocrystals doped with different transition metals. Doping of nanoparticles can open new ways to control their properties but the field is relatively new and doped silicon nanoparticles have not been widely explored. By comparing undoped particle excited stated dynamics to doped nanocrystals we discovered rapid exciton transfer to new dopant introduced states which must be taken into account for the consideration of doped silicon nanocrystal applications. Recently metal halide perovskites emerged as the leading novel photovoltaic material family and while a lot of research efforts have been put into bulk material, perovskite nanocrystals have been less widely researched. We analysed a range of CsPbBr₃ nanocrystals spanning different quantum confinement regimes in relation to the bulk material using transient absorption spectroscopy. Comprehensive analysis of different aspects of quantum confinement influence was carried out considering state focusing, bandgap renormalization, degeneracy and carrier cooling. The results highlighted that quantum confinement effects were mostly significant for the smallest CsPbBr₃ nanocrystals that are currently available (~4 nm) while the larger nanocrystal photophysics was better described as a small perturbation on the free carrier photophysics established for the bulk material. The final two chapters concentrated on the photorefractive metal halide perovskite effects. Transient absorption and a range of differential reflectance measurements were employed to study anomalous metal halide perovskite TA response which obstructed the excited state resolution in previous literature. Here the anomalous features were isolated and attributed to the photorefractive effect and were further analysed using frequency domain interferometry. Photoinduced changes in the refractive index of the metal halide perovskite films and nanocrystals were measured with a femtosecond time resolution. The findings allowed for better understanding of metal halide perovskite excited state dynamics and accurate hot carrier temperature and effective mass determination as well as photoinduced refractive index characterization which is crucial for the design of such applications as optical switching, modulating and recording devices, probing techniques, lasers, single photon emitters and hot carrier solar cells. Collectively, this thesis provides an insight into fundamental properties of a range of novel solution processed inorganic and hybrid photovoltaic materials

Excited state properties of novel solution processed inorganic and hybrid photovoltaic materials

Novel solution processed inorganic and hybrid photovoltaic materials have shown great promise in low-cost energy market and advanced optoelectronic device applications. Materials like colloidal nanocrystals could potentially transform device manufacturing with printing or spraying based processing, however often the excited state properties of these emerging materials are not well understood. The lack of fundamental knowledge of charge carrier formation, trapping and relaxation processes obstructs the development of the materials and devices. Therefore we employ time-resolved femtosecond spectroscopy methods such as differential reflectance, diffusion, interferometry and primarily transient absorption spectroscopy to study the characteristics of a range of novel photovoltaic materials. Different materials and their aspects are analysed in the six data chapters of this thesis with first three chapters concerning inorganic colloidal nanocrystals and final three concentrating on the analysis of metal halide perovskite family in bulk and nanocrystal forms.  Iron pyrite nanocrystals, which tend to show poor performance in devices due to active surface states, are examined. In particular, the effects of novel surface passivation approach of coating iron pyrite nanoparticles with shells is analysed using transient absorption and long wavelength range steady state absorption spectroscopy techniques. The coated nanocrystals were found to display significant modulation of the surface state behaviour which was governed by the shell properties while the iron pyrite core remained photoactive. The analysis revealed the effectiveness of the new iron pyrite nanocrystal surface defect modulation approach which could potentially solve the issues associated with iron pyrite performance in devices.  Cu₂ZnSnS₄ (CZTS) is another novel semiconductor which suffers from trapping effects due to its tendency to form defects. We investigated CZTS nanocrystal properties using transient absorption along with standard steady state and integrating sphere absorption measurements. Rapid relaxation of band edge carriers to a broad defect distribution was found which limited CZTS carrier lifetimes. In addition, plasmon influence induced by CZTS nanocrystal defects was also resolved. The prominent sub-state state influence on the CZTS excited state dynamics revealed that the defects must be efficiently passivated or their formation must be suppressed for effective material performance.  Transient absorption spectroscopy along with steady state property analysis was also used to help characterize the properties of a range of silicon nanocrystals doped with different transition metals. Doping of nanoparticles can open new ways to control their properties but the field is relatively new and doped silicon nanoparticles have not been widely explored. By comparing undoped particle excited stated dynamics to doped nanocrystals we discovered rapid exciton transfer to new dopant introduced states which must be taken into account for the consideration of doped silicon nanocrystal applications.  Recently metal halide perovskites emerged as the leading novel photovoltaic material family and while a lot of research efforts have been put into bulk material, perovskite nanocrystals have been less widely researched. We analysed a range of CsPbBr₃ nanocrystals spanning different quantum confinement regimes in relation to the bulk material using transient absorption spectroscopy. Comprehensive analysis of different aspects of quantum confinement influence was carried out considering state focusing, bandgap renormalization, degeneracy and carrier cooling. The results highlighted that quantum confinement effects were mostly significant for the smallest CsPbBr₃ nanocrystals that are currently available (~4 nm) while the larger nanocrystal photophysics was better described as a small perturbation on the free carrier photophysics established for the bulk material.  The final two chapters concentrated on the photorefractive metal halide perovskite effects. Transient absorption and a range of differential reflectance measurements were employed to study anomalous metal halide perovskite TA response which obstructed the excited state resolution in previous literature. Here the anomalous features were isolated and attributed to the photorefractive effect and were further analysed using frequency domain interferometry. Photoinduced changes in the refractive index of the metal halide perovskite films and nanocrystals were measured with a femtosecond time resolution. The findings allowed for better understanding of metal halide perovskite excited state dynamics and accurate hot carrier temperature and effective mass determination as well as photoinduced refractive index characterization which is crucial for the design of such applications as optical switching, modulating and recording devices, probing techniques, lasers, single photon emitters and hot carrier solar cells.  Collectively, this thesis provides an insight into fundamental properties of a range of novel solution processed inorganic and hybrid photovoltaic materials.</p

Solution Synthesis and Optical Properties of Transition-Metal-Doped Silicon Nanocrystals

A new synthetic method was developed to produce a range of transition-metal (Mn, Ni, and Cu) doped silicon nanocrystals (Si NCs). The synthesis produces monodisperse undoped and doped Si NCs with comparable average sizes as shown by transmission electron microscopy (TEM). Dopant composition was confirmed by EDX (energy dispersive X-ray spectroscopy). The optical properties of undoped and doped were compared and contrasted using absorption (steady-state and transient) and photoluminescence spectroscopy. Doped Si NCs demonstrated unique dopant-dependent optical properties compared to undoped Si NCs such as enhanced subgap absorption, and 40 nm shifts in the emission. Transient absorption (TA) measurements showed that photoexcitations in doped Si NCs relaxed via dopant states not present in undoped Si NCs

Solution Synthesis and Optical Properties of Transition-Metal-Doped Silicon Nanocrystals

A new synthetic method was developed to produce a range of transition-metal (Mn, Ni, and Cu) doped silicon nanocrystals (Si NCs). The synthesis produces monodisperse undoped and doped Si NCs with comparable average sizes as shown by transmission electron microscopy (TEM). Dopant composition was confirmed by EDX (energy dispersive X-ray spectroscopy). The optical properties of undoped and doped were compared and contrasted using absorption (steady-state and transient) and photoluminescence spectroscopy. Doped Si NCs demonstrated unique dopant-dependent optical properties compared to undoped Si NCs such as enhanced subgap absorption, and 40 nm shifts in the emission. Transient absorption (TA) measurements showed that photoexcitations in doped Si NCs relaxed via dopant states not present in undoped Si NCs

Broadband optical phase modulation by colloidal CdSe quantum wells

Two-dimensional (2D) semiconductors are primed to realize a variety of photonic devices that rely on the transient properties of photogenerated charges, yet little is known on the change of the refractive index. The associated optical phase changes can be beneficial or undesired depending on the application, but require proper quantification. Measuring optical phase modulation of dilute 2D materials is, however, not trivial with common methods. Here, we demonstrate that 2D colloidal CdSe quantum wells, a useful model system, can modulate the phase of light across a broad spectrum using a femtosecond interferometry method. Next, we develop a toolbox to calculate the time-dependent refractive index of colloidal 2D materials from widely available transient absorption experiments using a modified effective medium algorithm. Our results show that the excitonic features of 2D materials result in broadband, ultrafast, and sizable phase modulation, even extending to the near infrared because of intraband transitions

Research data supporting "Hot-carrier cooling and photoinduced refractive index changes in organic–inorganic lead halide perovskites"

Figures and Supplementary Figures as embedded objects in a powerpointThis record supports publication and is available at http://dx.doi.org/10.1038/ncomms9420This work was supported by the EPSRC [grant number BR 4869_1-1] and the Deutsche Forschungsgemeinschaft, Winton Programme (Cambridge) for the Physics of Sustainabilit

Research data supporting "Hot-carrier cooling and photoinduced refractive index changes in organic–inorganic lead halide perovskites"

Figures and Supplementary Figures as embedded objects in a powerpointThis record supports publication and is available at http://dx.doi.org/10.1038/ncomms9420This work was supported by the EPSRC [grant number BR 4869_1-1] and the Deutsche Forschungsgemeinschaft, Winton Programme (Cambridge) for the Physics of Sustainabilit