Browsing by Author "Havu, Ville, Dr., Aalto University, Department of Applied Physics, Finland"

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  • Electrical conductivity of functionalised carbon nanotube networks
    (2019) Ketolainen, Tomi
     School of Science | Doctoral dissertation (article-based)
    The fabrication of novel electronic devices requires new kinds of materials. The use of carbon nanotubes (CNTs) in various applications has already been demonstrated and therefore the CNTs are also important carbon materials in addition to graphene and fullerenes. Because the electronic properties of individual CNTs depend on their atomic structures, the individual CNTs are not possibly the best choice for building new electronics. Instead, the new devices could be made using thin films or networks of CNTs. The CNT thin films are transparent, flexible, and conduct electricity. Hence, the CNT thin films are expected to be utilised in a remarkable amount of applications including transistors, touch screens, and solar cells. However, a significant challenge related to the CNT thin films is making a film with both high conductivity and transparency simultaneously. Several methods to improve the conductivity of CNT networks have been studied experimentally. The goal of this thesis is to investigate a few methods to increase the conductivity of CNT networks by using density functional theory combined with the standard Green's function electron transport calculations. In particular, the conductance of junctions of CNTs is examined since the CNT junctions mainly determine the conductivity of the whole network. Two different approaches to improve the electrical conductivity of CNT networks are studied. The conductivity can be enhanced by depositing group 6 transition metal (TM) atoms on the CNT networks because the TM atoms are able to link the CNTs. The four-terminal electron transport calculations show that Cr, Mo, and W linker atoms enhance the conductances of the CNT junctions in a similar way. The increase in the conductance is related to the strong hybridisation between the carbon and TM atom orbitals. The second approach is based on functionalising the CNTs with molecules. The interaction of AuCl4 molecules with CNTs leads to a p-type doping effect. In addition, the doping of CNTs with nitric acid is studied and the NO3 molecules also cause a p-type doping effect in CNTs. Interestingly, the doping effect is larger in semiconducting CNTs than in metallic ones. Moreover, water molecules near the NO3 molecules enhance the doping effect. The electron transport through the CNT junctions can be increased by doping the CNTs with AuCl4 or NO3 molecules and no linker molecule is needed if the concentration of the molecules on the CNTs is high enough. A central result is the pinning of the Fermi level to the van Hove singularities and flat molecular states. The results of our work also improve the understanding of previous experimental studies.
  • Improving Cu(In,Ga)Se2 solar cell absorbers based on atomic-level modeling
    (2019) Malitckaya, Maria
     School of Science | Doctoral dissertation (article-based)
    Cu(In, Ga)Se2 (CIGS)-based solar cells are among the most promising candidates to replace crystalline silicon solar cells, thanks to their high efficiencies and low costs. The defect microstructure of the CIGS light absorber layer influences the optical and electronic properties of CIGS solar cells. The recent progress in their efficiency (up to 23.35 %) is mainly due to the incorporation of different alkali metal atoms into the absorber layer. As efficiency increases towards the Shockley-Queisser limit (31 % for a cell with a band gap of 1.3 eV), it becomes more difficult to improve. Thus, knowledge about native point defects and impurities, as well as about the formation of secondary phases in the CIGS absorber layer, is among essential information for optimizing CIGS solar cell performance. In this thesis, the choice of computational methods and their details strongly affects even the qualitative features of the obtained results. Therefore, the effects of the most important computational parameters are studied carefully in the thesis. Native point defects, native point defect complexes, and alkali metal impurities in the CIGS absorber layer are investigated using first-principles calculations within density functional theory (DFT) in order to understand their effect on the electronic structure. Moreover, based on DFT calculations, a mechanism for secondary phase (e.g. alkali indium selenide) formation is suggested. Calculating defects in CIGS is not straightforward. It is impossible to model defects directly in CIGS because In and Ga randomly occupy the same sites. Therefore all the calculations presented in this thesis are performed for CuInSe2 and CuGaSe2. In this thesis, calculations of native point defect formation energies in CuInSe2 provide information about the abundances of acceptors and donors for materials of different Cu concentrations. Moreover, it is shown that light alkali metal atoms prefer to accumulate on the Cu sublattice as impurities, and incorporation of heavy alkali metal atoms contributes mostly by phase separation. The formation of alkali indium/gallium selenide secondary phases during the post-deposition treatment is predicted by considering possible reactions between CuInSe2/CuGaSe2 and different alkali metal compounds by calculating their formation enthalpies. Interfaces between the secondary phases and the CuInSe2 absorber layer are studied in terms of band offsets. Finally, comparisons between the hard X-ray photoelectron spectroscopy (HAXPES) data and the density of states calculations with potassium post-deposition treatment (PDT) as a case study reveal the formation of the KInSe2 phase on the CIGS absorber surface after heavy potassium post-deposition treatment. In summary, the results in this thesis give information about the energetic characteristics of native point defects, native point defect complexes, alkali metal impurities, and alkali metal secondary phases. The results help to analyse already existing experimental observations of the abundances of point defects, migration mechanisms, and the formation of secondary phases. 
  • Simulating excited-state processes in nanostructures using time-dependent density functional theory-based Ehrenfest dynamics
    (2016) Ojanperä, Ari
     School of Science | Doctoral dissertation (article-based)
    Many phenomena and processes in nature are related to excited electronic states and their time development, for example light absorption, fluorescence and ion-atom collisions. Moreover, many experimental methods such as femtosecond pump-probe laser spectroscopy and photo-emission spectroscopy rely on processes related to excited electronic states. Consequently, a proper description of the excited electron-ion system and the coupled electron-ion dynamics is crucial for the understanding of excited-state processes, interpreting a large amount of experimental data collected with methods that involve electron excitation as well as for developing new technology based on excited-state phenomena. Especially the modeling of nonadiabatic coupled electron-ion motion, which incorporates interactions between the wavefunctions of the electrons and the nuclei, poses an extremely tough challenge. Not only are the wavefunctions themselves cumbersome to model, but simulating the nonadiabatic dynamics of electrons and nuclei to a reasonable accuracy is a highly difficult theoretical and computational task.  During the recent years, a method called Ehrenfest dynamics has been successfully used for simulating nonadiabatic electron-ion dynamics in conjunction with the time-dependent density functional theory (TDDFT). It has been successfully applied to studying, for example, ion bombardment of carbon, gold and aluminium targets, photoexcitation dynamics of biomolecules and excited-state carried dynamics in carbon nanotubes. Typical Ehrenfest dynamics implementations are based on pseudopotentials, which simplify the implementation as compa-red to the projector augmented-wave (PAW) method or localized basis function sets. The PAW method generally increases the accuracy of the calculations as compared to pseudopotential-based calculations.  The main result of this thesis is the development and implementation of Ehrenfest dynamics within the PAW formalism. The formalism has been implemented to the electronic structure program GPAW. The implemented Ehrenfest dynamics method is used for two applications motivated by experimental findings: ion bombardment of graphene sheets and dynamics of a ligand-protected gold cluster after excitation by light. In this thesis Ehrenfest dynamics equations are also derived for the combination of localized basis functions sets and the PAW method. Special focus is given to the rigorous derivation of the quantum-classical forces from the Lagrangian action integral.