Abstract:
New photovoltaic concepts such as intermediate band solar cells or hot-carrier solar cells require long lifetimes of hot photoexcited carriers. Collection of their extra energy with respect to the band edges could then succeed. Under this aspect, the electronic structure and the electron relaxation in semiconductors were investigated focusing on the material's geometry and its surface. Both alter the electronic structure and thus can affect the electron-phonon interaction that is the main mechanism for carrier cooling. The role of the geometry was studied with time-resolved two-photon photoemission spectroscopy (tr-2PPE) on CdSe nanostructures, in particular 0D quantum dots and 2D quantum well nanoplatelets. For quantum dots it was shown that electron-phonon scattering via bulk states is not the dominant energy loss mechanism. Instead, the cooling rate depends notably on the surrounding capping or shell. Scattering via surface states is considered as an alternative relaxation pathway. In contrast, the nanoplatelets reveal fast relaxation rates, independent of thickness, ligands or a CdS shell. The corresponding energy loss rate is described with a model for LO phonon scattering in quantum wells. The effect of surface states on the relaxation of bulk electrons is studied exemplarily on III-V semiconductors, prepared by metal organic vapor phase epitaxy. Studies with 2PPE on GaP(100) reveal several surface states for two different surface reconstructions. Their corresponding energies can be used to explain prominent features in reflectance anisotropy spectra of the same surfaces. Tr-2PPE measurements on InP(100) were performed to analyze the electron scattering between 3D bulk states and a particular surface state (C2). The scattering rates were determined and the cooling of bulk electrons in the presence and absence of C2 was compared. All of the results indicate fast carrier relaxation. Therefore, a photovoltaic concept is presented that is based on intersubband transitions and that employs fast separation of the hot carriers to counteract the short lifetimes. This design was realized as a tandem solar cell, combining an InP pin-junction and a photovoltaic intersubband absorber of InGaAs/InAlAs/InAs quantum wells. Proof-of-principle for the operation of this concept is provided experimentally and the basic characteristics of the device are explained with an equivalent circuit design.