ETD Collection

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Browsing ETD Collection by Author "Abdallah Ali, Ibrahim Omer"

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    A First-Principles Study on Phase Stability, Electronic Structure and optical properties of some halide perovskites for photovoltaic applications
    (2019) Abdallah Ali, Ibrahim Omer
    Hybrid and all-inorganic halide perovskites have recently emerged as new materials for solar cell applications leading to a new class of semiconductor photovoltaic cells. The power conversion e ciencies of perovskite based solar cells have been improved signi cantly to over 20%. This performance is due to the exceptional properties of hybrid halide perovskites displaying high absorption coe cients, high carrier mobility, direct and tunable band gaps and long di usion lengths. However, poor long-term stability of organic-inorganic halide perovskite solar cells in oxygen and moisture remains the biggest challenge for realistic implementation of perovskite solar cells. In this thesis, we present a detailed ab-initio investigation of the structural, mechanical, dynamical stabilities, electronic, lattice thermal conductivities, transport and optical properties of the orthorhombic perovskites of hybrid methylammonium lead triiodide (CH3NH3PbI3), hybrid methylammonium tin triiodide (CH3NH3SnI3), and all-inorganic caesium lead triiodide ( CsPbI3). Our theoretical calculations of the structural, mechanical, dynamical stabilities, electronic, lattice thermal conductivities, transport and optical properties are based on density functional theory (DFT) and many-body perturbation theory (MBPT) calculations. The structural properties (e.g. equilibrium lattice parameters and cohesive energy) were extracted from the calculated energy-volume equation of state (EOS). Mechanical and dynamical stabilities were tested on elastic constants and phonon dispersion relation, respectively. The lattice thermal conductivities were calculated within the single-mode relaxation-time (SMRT) approximation and linearized phonon Boltzmann equation as implemented in phono3py code. The calculations of the electronic transport properties of the optimized structure were performed using the Boltzmann transport theory within the constant relaxation time approximation as implemented in the BoltzWann code. In order to investigate the optical spectra, we carry out Bethe-Salpeter equation (BSE) calculations on top of non-self-consistent G0W0 calculations. Using the cohesive energy, we found that all the studied structures are energetically stable. The calculations of the elastic constants and phonon dispersions at zero pressure showed that CH3NH3PbI3 and CsPbI3 in the orthorhombic structures remained stable, while the orthorhombic CH3NH3SnI3 is only stable at 0.7 GPa. The most energetically stable con gurations were used to investigate electronic properties through total and partial density of states (PDOS) and band structure analysis. The calculated lattice thermal conductivities for CH3NH3PbI3 and CsPbI3 were found to be very low and anisotropic. Despite low thermal conductivity, CH3NH3PbI3 in the orthorhombic phase at low temperature exhibits a very small gure of merit. This suggest that the orthorhombic CH3NH3PbI3 is a poor candidate for thermoelectric applications. Finally, with the help of the frequency-dependent microscopic dielectric tensor we derived all the desired frequency-dependent optical spectra such as absorption coe cient, refractive index, reectivity and energy-loss spectrum of the stable con gurations. The average optical band gap of CH3NH3PbI3, CsPbI3 and CH3NH3SnI3 was found to be 2.01 eV, 2.49 eV and 1.28 eV, respectively. This suggest that these materials in the orthorhombic phase may be potential material for solar photovoltaic applications. Using the Shockley-Queisser model we estimated the solar cell e ciency of the compounds. Our obtained results were discussed within the employed theoretical methods and compared with experiment and with previous theoretical studies. Moreover, we report for the rst time rst theoretical attempt to study the mechanical and dynamical stabilities of lead-free tin halide perovskite under pressure. We hope that the present theoretical investigation would be helpful in providing a better theoretical knowledge of these materials.