CdTe thin-film solar cells have become popular due to low manufacturing cost, but this benefit comes at the expense of cell performance. While performance is improving, CdTe is still plagued by open circuit voltage (VOC ) losses. These losses are attributed to interface and bulk recombination, but with current methods, evaluation of these properties is convoluted. This thesis reports on TRPL simulations on a Cd(Se,Te) double heterostructures (DHs), a semiconductor material that acts as the absorber layer in commercially relevant thin-film solar cells. TRPL was simulated on Alumina/Cd(Se,Te)/Alumina DHs, where alumina acts as an excellent surface passivation agent for Cd(Se,Te). Simulations were conducted with COMSOL Multiphysics®, where time-dependent charge transport equations and Poisson's equation were numerically solved using the finite element method. Models were created to investigate the effects of bulk, surface recombination, mobility, and material thickness. These models used single photon excitation (1PE) and two-photon excitation (2PE) to examine the surface and bulk of the material. Results were analyzed to develop a combined experimental and numerical simulation procedure to distinguish and quantify bulk and interface recombination mechanisms in Cd(Se,Te) photovoltaic devices. This general approach can be applied to other thin film solar cells to help determine where significant VOC losses are occurring.

This thesis contains optical investigations on 3,4,9,10 perylene tetracarboxylic dianhydride (PTCDA) and tris (8-hydroxy) quinoline aluminum (Alq3) films as well as on PTCDA/Alq3 multilayers and co-deposited films. The organic layers are grown on Si and Pyrex substrates using the technique of organic molecular beam deposition (OMBD). In temperature dependent (10 - 300 K) transmission measurements the exciton absorption in PTCDA⁄Alq3 multilayers and co-deposited films is shifted to higher energies with respect to the pure PTCDA films. In comparison with a recently developed theoretical model for α PTCDA the observed energy shift is explained by Coulomb screening. In strain dependent photoluminescence (PL) measurements of PTCDA film uniaxial pressure is applied along the molecular stacking direction using a specially designed pressure cell. With increasing pressure exciton emission channels are shifted to lower energy and the integrated PL intensity is quenched. When the pressure is released, the PL spectrum and the total PL intensity partially recover, indicating a reversibility of the strain effects to a large extent. The experimental results are compared with recent total energy calculations. The assignment of different emission channels in PTCDA single crystals and polycrystalline films is validated by photoluminescence excitation studies. From the excitation energy dependence of the spectral position of different emission bands the transition energies of free and self trapped excitons are deduced. The obtained transition energies are compared with theoretically predicted values. The anisotropy of the refractive index in PTCDA waveguides is investigated using the m-line technique. The observed effective refractive index values of TE and TM modes are used to determine the waveguide thickness as well as the inplane and perpendicular bulk refractive index values. The recombination dynamics of excitonic states in thin Alq3 films is studied using temperature and time (open full item for complete abstract)

In this study we used Photoluminescence (PL), Time-resolved photoluminescence (TRPL) and Photoluminescence excitation (PLE) spectroscopy to investigate optical and electronic properties of individual Zincblend GaAs/AlGaAs core-shell and Wurtzite InP nanowires (NWs) at low temperatures (10K). GaAs/AlGaAs core-shell NWs were prepared by using Au catalyst-assisted metal organic chemical vapor deposition (MOCVD) method and a titanium-Sapphire laser was used to excite the nanowire sample. PL emission from single NWs exhibit an exciton peak at ~1.515eV. PL and TRPL spectroscopies exhibit high quantum efficiency and exciton lifetimes of ~1ns, which is equivalent to high quality two-dimensional heterostructures. State filling and many-body interaction effects were observed by increasing the carrier densities using pulsed laser excitation. Further, polarized TRPL spectroscopy was used to study exciton dynamics in these nanowires at 10K. The polarization of the emitted PL was monitored at the exciton emission peak (1.515~eV) as a function of time after excitation by a polarized pulse. With no quantum confinement effects, in thermal equilibrium the density of excitons dipoles parallel and perpendicular to the NW should be equal. This investigation revealed at low excitation intensities the excitons are created out of thermal equilibrium, but relax within several hundred picoseconds (~200 ps). At higher excitation powers, the exciton dipoles relax much more rapidly within a time less than our temporal system response of 80ps. This suggests that exciton dipole relaxation is very sensitive to carrier-carrier scattering. PLE spectroscopy was used to investigate the electronic band structure of wurtzite InP NWs at 10K with nominal diameters of 50 and 100nm, along with PL and TRPL. The NWs were prepared by Au catalyst-assisted MOCVD growth with 420 °C growth temperature and a precursor flow rate (V/III ratio) of 700. PLE spectra show three main peaks for band-to-band tra (open full item for complete abstract)