Broadband enhancement of light harvesting in luminescent solar concentrator

Luminescent solar concentrator (LSC) can absorb large-area incident sunlight, then emit luminescence with high quantum efficiency, which finally be collected by a small photovoltaic (PV) system. The light-harvesting area of the PV system is much smaller than that of the LSC system, potentially improving the efficiency and reducing the cost of solar cells. Here, based on Fermi-golden rule, we present a theoretical description of the luminescent process in nanoscale LSCs where the conventional ray-optics model is no longer applicable. As an example calculated with this new model, we demonstrate that a slot waveguide consisting of a nanometer-sized low-index slot region sandwiched by two high-index regions provides a broadband enhancement of light harvesting by the luminescent centers in the slot region. This is because the slot waveguide can (1) greatly enhance the spontaneous emission due to the Purcell effect, (2) dramatically increase the effective absorption cross-section of luminescent centers, and (3) strongly improve the quantum efficiency of luminescent centers. It is found that about 80% solar photons can be ultimately converted to waveguide-coupled luminescent photons even for a low luminescent quantum efficiency of 0.5. This LSC is potential to construct a tandem structure which can absorb nearly full-spectrum solar photons, and also may be of special interest for building integrated nano-PV applications

Two energy scales and close relationship between the pseudogap and superconductivity in underdoped cuprate superconductors

By measuring the low temperature specific heat, the low energy quasi-particle excitation has been derived and analyzed in systematically doped La$_{2-x}$Sr$_{x}$CuO$_{4}$ single crystals. The Volovik's relation predicted for a d-wave superconductor has been well demonstrated in wide doping regime, showing a robust evidence for the d-wave pairing symmetry. Furthermore the nodal gap slope $v_\Delta$ of the superconducting gap is derived and is found to follow the same doping dependence of the pseudogap obtained from ARPES and tunnelling measurement. This strongly suggests a close relationship between the pseudogap and superconductivity. Taking the entropy conservation into account, we argue that the ground state of the pseudogap phase should have Fermi arcs with finite density of states at zero K, and the transport data show that it behaves like an insulator due to probably weak localization. A nodal metal picture for the pseudogap phase cannot interpret the data. Based on the Fermi arc picture for the pseudogap phase it is found that the superconducting energy scale or $T_c$ in underdoped regime is governed by both the maximum gap and the spectral weight from the Fermi arcs. This suggests that there are two energy scales: superconducting energy scale and the pseudogap. The superconductivity may be formed by the condensation of Fermi arc quasiparticles through pairing by exchanging virtue bosons.Comment: 4 pages, 5 figure