Designing transparent nanophotonic gratings for ultra-thin solar cells.

Integration of a rear surface nanophotonic grating can increase photocurrent in ultra-thin solar cells. Transparent gratings formed of dielectric materials and high bandgap semiconductors can offer efficient diffraction with lower parasitic absorption than more widely studied metal/dielectric equivalents. In these systems, the maximum photocurrent which can be obtained for a grating made of a given combination of materials is shown to follow a simple empirical model based on the optical constants of these materials and independent of grating dimensions. The grating dimensions still require optimization in order to maximize the photocurrent for a given active layer thickness by balancing the effects of diffraction outside the front surface escape cone and the tuning of waveguide modes in long wavelength regions which are poorly absorbed in an ultra-thin film. The optimal grating pitch is shown to be of particular relevance for both effects, changing nonmonotonically as the absorber gets thicker in order to track favourable waveguide mode resonances at wavelengths near the absorber bandgap. These trends together with the empirical model for material selection drastically reduce the design space for highly efficient light trapping with transparent gratings.

Blue-Green Color Tunable Solution Processable Organolead Chloride-Bromide Mixed Halide Perovskites for Optoelectronic Applications.

Solution-processed organo-lead halide perovskites are produced with sharp, color-pure electroluminescence that can be tuned from blue to green region of visible spectrum (425-570 nm). This was accomplished by controlling the halide composition of CH3NH3Pb(BrxCl1-x)3 [0 ≤ x ≤ 1] perovskites. The bandgap and lattice parameters change monotonically with composition. The films possess remarkably sharp band edges and a clean bandgap, with a single optically active phase. These chloride-bromide perovskites can potentially be used in optoelectronic devices like solar cells and light emitting diodes (LEDs). Here we demonstrate high color-purity, tunable LEDs with narrow emission full width at half maxima (FWHM) and low turn on voltages using thin-films of these perovskite materials, including a blue CH3NH3PbCl3 perovskite LED with a narrow emission FWHM of 5 nm.