Combinatorial solar cell libraries for the investigation of different metal back contacts for TiO2-Cu2O hetero-junction solar cells.

Here we present a comprehensive investigation of TiO2-Cu2O hetero-junction solar cells with different back contacts (Au, ITO, Cu or Ag). Combinatorial hetero-junction libraries consisting of a linear TiO2 thickness gradient produced by spray pyrolysis and a bell shaped Cu2O profile synthesized by pulsed laser deposition were chosen to investigate the impact of the two metal oxide layer thicknesses. The back contacts were deposited as round patches onto a grid of 13 × 13 points, 169 contacts for each contact material, forming a library containing 4 × 13 × 13 = 676 back contacts. Each back contact represented a solar cell with an individual TiO2 and Cu2O thickness. I-V measurements show that all four materials provide an ohmic contact and that the open circuit voltage of ∼300 mV is rather independent of both layer thicknesses and contact material. The size of the Cu2O crystals drastically decreases with distance from the center of deposition, which leads to a drastic increase of series resistance when the crystal size is <50 nm.

Polarization, microscopic origin, and mode structure of luminescence and lasing from single ZnO nanowires.

The emission spectrum of individual single crystalline ZnO nanowires shows three regimes characterized by distinct polarization properties and spatial emission patterns along the length of the wire. In the visible range below 2.9 eV, emission is polarized along the long axis of the wire, along the c-axis (E//c). In the second regime between 2.9 and 3.22 eV, Fabry-Pérot guided modes polarized perpendicular to the wire (E perpendicularc) prevail. From their dispersion, it is clear that these modes signify strong coupling between the B-exciton and linearly polarized transverse electric modes that are guided by the wire and trapped between the wire end facets. The third regime is characterized by uniform emission along the wire and a marked dip in the polarization at around the electronic band gap at 3.3 eV. Lasing is observed only in the second regime in which strong light-matter interaction prevails.

Nature of sub-band gap luminescent eigenmodes in a ZnO nanowire.

The emission spectrum of individual high-quality ZnO nanowires consists of a series of Fabry-Pérot-like eigenmodes that extend far below the band gap of ZnO. Spatially resolved luminescence spectroscopy shows that light is emitted predominantly at both wire ends, with identical spectra reflecting standing wave polariton eigenmodes. The intensity of the modes increases supralinearly with the excitation intensity, indicating that the mode population is governed by scattering among polaritons. Due to strong light-matter interaction, light emission from a ZnO nanowire is not dictated by the electronic band diagram of ZnO but depends also on the wire geometry and the excitation intensity. Delocalized polaritons provide a natural explanation for the pronounced subwavelength guiding in ZnO wires that has been reported previously.