n-i-p Micromorph Solar Cells on Aluminium Substrates

N-I-P MICROMORPH SOLAR CELLS ON ALUMINIUM SUBSTRATESM.GOETZ, P.TORRES, P.PERNET, J.MEIER, D.FISCHER, H.KEPPNER, A.SHAHInstitut de Microtechnique (IMT) de l'Universite de Neuchâtel, Rue Breguet 2, CH-2000Neuchâtel, Switzerland, goetz@imt.unine.chABSTRACTThe first successful deposition of 'micromorph' silicon tandem solar cells of the n-i-p-n-i-pconfiguration is reported. In order to implement the 'micromorph' solar cell concept, four keyelements had to be prepared: First, the deposition of mid-gap, intrinsic microcrystalline silicon(µc-Si:H) by the 'gas purifier method', second, the amorphous silicon (a-Si:H) n-i-p singlejunction solar cell, third, the microcrystalline silicon n-i-p single junction solar cell and fourth,the ability of depositing on aluminium sheet substrates.All the solar cells presented have been deposited on flat aluminium sheets, using a single layerantireflection coating to couple the light into the cell. It is shown, that this antireflectionconcept- together with a flat substrate- holds for amorphous single junction solar cells, but itreaches its limit with the extended range of spectral response of the 'micromorph' cell.The best initial efficiencies for each category of n-i-p cells on flat substrates were: 8.7% for theamorphous silicon single junction cell, 4.9% for the microcrystalline silicon single junction celland 9.25% for the 'micromorph' tandem cell.INTRODUCTIONThe mixed amorphous / microcrystalline tandem solar cell concept was introduced in 1994 [1]by our group (IMT). These so-called 'micromorph' cells are 'real' tandem cells, employing twomaterials of a different optical gap for the top and the bottom cell (1.7eV and 1eV, respectively[2]). The enhanced infrared absorption and the lack of light-induced degradation [3] of themicrocrystalline silicon are successfully combined with the high V