Intermediate reflectors for enhanced top cell performance in photovoltaic thin-film tandem cells

We have investigated the impact of three types of intermediate reflectors on the absorption enhancement in the top cell of micromorph tandem solar cells using rigorous diffraction theory. As intermediate reflectors we consider homogenous dielectric thin-films and 1D and 3D photonic crystals. Besides the expected absorption enhancements in cases where photonic band gaps are matched to the absorption edge of the semiconductor, our results distinguish between the impact of zero order Bragg-resonances and diffraction-based enhancement at larger lattice constants of the 3D photonic crystal. Our full-spectrum analysis permits for a quantitative prediction of the photovoltaic conversion efficiency increase of the a-Si:H top cell

Track A Basic Science

Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/138319/1/jia218438.pd

Influence of defects in opal photonic crystals on the optical transmission imaged by near-field scanning optical microscopy

The electric field intensity above the surface of opal photonic crystals (PCs) and its alteration due to 'crystallographic' defects is investigated by using nearfield scanning optical microscopy (NSOM). The photonic crystals are developed by dip coating in a liquid solution with PMMA opals. Highly regular hexagonal planes with lattice constants of about 260 nm grow on the glass substrate. During the drying process several crack lines are formed that correspond to defects in the crystal structure. The transmitted light intensity at wavelengths inside and outside of the stop band of the PC is studied with NSOM using a tapered fiber tip scanning in all three dimensions. By this technique, a 3D image of the electric field intensity can be measured with a resolution better than 100 nm. The results show that the local optical field distribution is strongly dominated by the defect states in all directions in space over a length scale of several mu m. Above the crack lines, the intensity of light is strongly reduced. Beams of light are observed emerging from the edges of the crack lines and propagate in air with heights of more than 3 mu m. In between two different crack lines, periodic repetitions of the beams are observed. These results are interpreted as light diffraction on a microscopic scale

Dispersive elements for spectrum splitting in solar cell applications

Photovoltaic tandem and triple solar cells are currently being developed and produced with reasonable efficiencies at high technological cost. The concept of spectrum splitting has been proposed with the advantage of compatibility to all types of cells. Although additional optical efforts are to be made, external photon management can be achieved to match different solar cell combinations no matter which band gaps involved or how the cells are connected. We present an experimental study comparing optical devices based on either interference or diffraction for tandem and triple cell configurations. Whereas diffractive media such as gratings suffer intrinsically from higher order diffraction losses, devices based on interference such as Bragg filter can yield a significant efficiency increase. For a triple cell configuration consisting of GalnP/GalnAs/GaSb, a net efficiency gain of more than 30 % is shown in a solar cell simulator compared to the best cell in direct light