Calculation of the Band Properties of a Quantum Dot Intermediate Band Solar Cell with Centrally Located Hydrogenic Impurities

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Levy, Michael Yehuda
Citrin, David S.
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In the quantum dot implementation of an intermediate band solar cell presented in this thesis, the offset of the intermediate band with respect to the conduction band is approximated by the ground state energy of a single electron in a single quantum dot heterojunction. The ground state energy is calculated with the radial Schrodinger equation with a Hamiltonian whose potential is composed from the step-like conduction band offset of the quantum dot heterojunction and the 1/r electrostatic potential of the hydrogenic impurity. The position of the intermediate band is tuned by adjusting the radius of the quantum dots. By assuming that the centrally located impurities are ionized, the location of the Fermi energy is guaranteed to be within the intermediate band. An intermediate band solar cell contains three bands: a conduction band, a valence band; and an intermediate band. The addition of an intermediate band augments the photogeneration of carriers. These additional carriers allow for an increased theoretical efficiency as compared to a conventional homojunction solar cell. The challenges in implementing an intermediate band solar cell involve centering the intermediate band at an energy level matched to the solar spectrum and aligning the Fermi energy within the intermediate band. The latter is necessary to ensure both a supply of electrons capable of photon induced transition to the conduction band as well as a large population of holes that allow photon induced electrons to transition from the valence band to the intermediate band. This thesis presents a novel material system, InPAs quantum dots enveloped in AlGaAs barriers grown on GaAs substrates, with which to implement an optimized QD-IBSC. This novel material system is selected based upon a refined set of design rules that include a requirement that the quantum dot/barrier pair offer a negligible valence band offset. With such a design rule the existence of hole levels is avoided, thus reducing bandgap narrowing at the valence band edge and the existence of minibands below the intermediate band.
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