Extremely Thin Absorber Layers in Solid State Solar Cells
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EXTREMELY THIN ABSORBER LAYERS IN SOLID STATE SOLAR CELLS A. Bellaidi1, K. Ernst1 and R. Könenkamp2 1 Hahn-Meitner Instut Berlin, Glienicker Strasse 100, 14109 Berlin, Germany 2 Physics Department Portland State University Portland, OR. 97201, USA ABSTRACT We report on recent progress in fabricating an inorganic solid state solar cell based on a nano-structured substrate with an extremely thin absorber, the so-called eta-cell. The cell uses HgCdTe as an absorber layer and TiO2 as a substrate. By adjusting the Hg/Cd ratio the conduction band edges in these two materials can be brought into alignment. This tuning provides an ideal tool to optimize the optical and electrical properties of the cell. In qualitative agreement with earlier work we find a band offset of 0.25 eV to give maximum transfer rates across the interface. The optimized cell has a short circuit current of 15 mA/cm2 and an open-circuit voltage of 0.6 V. We discuss the optical and electrical properties of the cell, and outline ideas to further improve the performance. SOLAR CELL CONCEPT Over the last years considerable effort has been devoted to the fabrication of deeply structured solid state solar cells with very thin absorber layers (1-3). The potential advantages of this type of cell lie in the possibility to obtain very short electronic transport paths in the absorber layer, to have very strong optical scattering at the extended interfaces, and to allow simple and inexpensive fabrication processes due to less stringent electronic quality requirements. Fig. 1 shows a schematic diagram for a deeply structured cell based on a porous substrate. The device consists of a sandwich arrangement with a very thin absorber layer embedded between two transparent transport layers and two highly conductive contact layers. Only the interfaces between the absorber and the transport layers are structured; the interface between the transport layers and the contact layers is planar. The contact layer on the light entry side is necessarily transparent, while the back contact layer is reflective. When the absorbance of the cell is to remain approximately constant, the local absorber layer thickness can be decreased in inverse proportion to the surface enlargement at the substrate/absorber interface. This implies a similar reduction of the collection path for excited electrons and holes in the absorber, which in turn allows the use of lower quality semiconductor materials and less elaborate processing. It is immediately apparent, however, that these potential advantages come at the cost of enlarged interface areas. When strong optical and electrical losses at the interfaces occur, this trade-off may indeed outweigh the advantages of a short transport path. On the other hand, when bulk recombination is the predominant loss mechanism, a reduced transport path length may be advantageous and the non-planar geometry may open new avenues for device fabrication. When the total device thickness is to remain in the micrometer regime, it is clear that the structuring leng
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