The concept of 3D photonic intermediate reflectors for micromorph silicon tandem solar cells has been investigated. In thin‐film silicon tandem solar cells consisting of amorphous and microcrystalline silicon with two junctions of a‐Si/μc‐Si, efficiency enhancements can be achieved by increasing the current density in the a‐Si top cell. It is one goal to provide an optimized current matching at high current densities. For an ideal photon‐management between top and bottom cell, a spectrally selective intermediate reflective layer (IRL) is necessary, which is less dependent of the angle of incidence than state‐of‐the‐art thickness dependent massive interlayers. The design, preparation and characterization of a 3D photonic thin‐film filter device for this purpose has been pursued straight forward in simulation and experimental realization. The inverted opal is capable of providing a suitable optical band stop with high reflectance and the necessary long wavelength transmittance as well and provides further options for improved light trapping. We have determined numerically the relative efficiency enhancement of an a‐Si/μc‐Si tandem solar cell using a conductive 3D‐photonic crystal. We have further fabricated such structures by ZnO‐replication of polymeric opals using chemical vapour deposition and atomic layer deposition techniques and present the results of their characterization. Thin film photonic IRL have been prepared at the rear side of a‐Si solar cells. Completed with a back contact, this is the first step to integrate this novel technology into an a‐Si/μc‐Si tandem solar cell process. The spectral response of the cell is presented and compared with reference cells. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Directional and energy selective optical surface structures attached to solar cells may result in both, enhancement or deterioration of photovoltaic performance of a solar cell. On the one hand, restricting the cell acceptance to the small incidence angle of direct and circumsolar irradiation enhances the maximum path length of the light in a solar cell with Lambertian surfaces even above the Yablonovitch limit (ultra-light-trapping). On the other hand, restrictions to small acceptance angles imply losses of diffuse sunlight, even for perfectly tracked cells. Using temporally resolved solar irradiation spectra, we simulate the enhancement and loss in the annual energy yield of silicon solar cells of various thicknesses. We assume an idealized angular and energy selective filter on top of the Lambertian surface of the absorber and compare the results to a Lambertian surface only. We find a maximum annual gain in the energy density of 32.5% for 1 mu m, and of similar to 10% for 10 mu m and 100 mu m thick perfectly tracked crystalline silicon solar cells. The simulation implies various tracking modes and two different locations. Finally, we introduce two possible realizations of such a filter; a Ru-gate stack and inverted opals. In experimental measurements, we could verify the absorptance enhancement by such a structure applied on top of a thin silicon wafer

A three‐dimensional photonic crystal intermediate reflector for enhanced light trapping in tandem solar cells is presented. The intermediate reflector consists of a transparent and conductive ZnO:Al inverted opal sandwiched in between the top amorphous silicon and bottom microcrystalline silicon cell.

Abstract:We review several examples of how spectrally-selective photonic structures may be used to improve solar cell systems. Firstly, we introduce different spectrally-selective structures that are based on interference effects. Examples shown include Rugate filter, edge filter and 3D photonic crystals such as artificial opals. In the second part, we discuss several examples of photovoltaic (PV) concepts that utilize spectral selectivity such as fluorescence collectors, upconversion systems, spectrum splitting concepts and the intermediate reflector concept. The potential of spectrally selective filters in the context of solar cells is discussed.