3D Photonic Crystals as Diffractive and Energy Selective Filter for Tandem Thin Film Solar Cells
- PDF / 521,952 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 105 Downloads / 187 Views
1014-AA05-01
3D Photonic Crystals as Diffractive and Energy Selective Filter for Tandem Thin Film Solar Cells Ralf B. Wehrspohn1, Andreas Bielawny1, Carsten Rockstuhl2, and Falk Lederer2 1 Physics, Martin-Luther-University Halle-Wittenberg, Heinrich-Damerow-Str. 4, Halle, 06120, Germany 2 Physics, University of Jena, Jena, 07743, Germany
ABSTRACT We suggest a photonic structure with energy selective and diffractive properties to be incorporated in thin-film tandem solar cells. Our device enhances the pathway of incident light within a amorphous silicon photovoltaic (PV) top cell in its spectral region of low optical absorption. This leads to an increase in the short-circuit current of the top cell. For a conductive inverted opal structure as intermiediate layer, we numerically determine an current increase of 1.44mA/cm2 for an a-Si:H / µc-Si thin-film tandem cell corresponding to an increase in the absolute efficiency from 11.1% to 12.4%.
INTRODUCTION During the last 20 years, research on single and multi-junction cells advanced basically in the material sector. But it turns out that silicon in its crystalline, polycrystalline or amorphous state will be the dominating material for solar cells in the next decade. Therefore, photon management (PM) inside solar cells is a future challenge for further efficiency enhancement. Photon management consists of two mayor tasks: spatial PM aims at the localization of light - usually referred to as light-trapping - and spectral PM which aims at the improvement of multi-junction efficiency by optimizing the spectral distribution of light among different pn-junctions. Spatial PM so far relies on inverted pyramids for crystalline solar cells with thickness larger than about 10 µm. In thin cells, however inverted pyramids cannot be realized, so isotropic scatters and grating couplers [1-5] have been developed to harvest more light in these cells. The short circuit current has been improved experimentally by about 8% close to the bandgap of amorphous silicon this way [6, 7]. Besides from up- and down coversion systems [8, 9], spectrum splitting PV systems have been made since Moon et al. [10] and experienced several approaches, e.g. with dichroic filters [11-14]. We propose an integrated solution for both, spatial and spectral photon management in thin film tandem solar cells.
EXPERIMENT In this article we present a diffractive and energy-selective optical nanostructure for a thin film silicon tandem cell. Our photonic structure increases the collection efficiency of an amorphous hydrogenated silicon (a-Si:H) the top cell, accompanied by a microcrystalline silicon (µc-Si) bottom cell. In this combined tandem cell the external quantum efficiencies (EQE) of the junctions overlap, as shown in Figure 1. In the spectral region between 580nm and 700nm the top cell suffers from decreasing absorption coefficient of a-Si:H towards longer wavelengths. Due to its higher open circuit voltage (VOC) the a-Si:H top cell delivers higher electrical power for each photon, so one aims at improving
Data Loading...
