Excitation of guided-mode resonances in thin film silicon solar cells
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Excitation of guided-mode resonances in thin film silicon solar cells F.-J. Haug, K. Söderström, A. Naqavi, C. Ballif Ecole Polytechnique Fédérale de Lausanne (EPFL), Institute of Microengineering (IMT), Photovoltaics and Thin Film Electronics Laboratory, Rue A.-L. Breguet 2, CH-2000 Neuchâtel, Switzerland ABSTRACT Thin film silicon solar cells are attractive for photovoltaics; however, the poor charge transport in this material requires that the devices are thinner than the absorption length. Adequate absorption can nevertheless be achieved by light scattering at textured interfaces because light can get trapped inside the absorber layer if it is scattered into angles above the critical angle of total internal reflection. This situation can be identified with the propagation of a guided mode in a waveguide where silicon plays the role of the high index guiding medium and the interface texture serves to couple the incident light to modes via grating coupling. We present an experimental realization of a solar cell structure on a line grating where the enhanced photocurrent can be clearly related to resonant excitation of waveguide modes. INTRODUCTION Thin film silicon is an interesting option for low cost solar energy production because its manufacturing technology is well mastered and it is an abundant material. Its most fundamental limitation against large scale production is related to its poor electronic quality compared to its crystalline counterpart. Poor charge transport poses several fundamental limitations on the device design. For example, the absorber layer thickness is normally much smaller than the absorption length, particularly for light with energy close to the band gap [1]; therefore, state-of-the-art devices require additional elements that enhance the absorption within the absorber layer. The earliest, and so far experimentally most successful approach employs light scattering at textured interfaces [2]. Light that is scattered beyond the critical angle for total internal reflection is thus trapped into the absorber film. Obviously, the condition of total internal reflection is not perfect in the presence of interface texture; light trapping is therefore an equilibrium between in-coupling which results in the desired absorption enhancement in the silicon film, and the undesired out-coupling. In the terminology of the optics community, the silicon absorber layer can be identified with the high index guiding medium, the metal film at the back that serves as reflector and electric contact translates to the cladding, and the interface texture establishes the grating coupling between the external radiation field and eigen-modes of the multilayer stack. Different from standard waveguides, typical dimensions of a solar cell are likely to support multiple modes, the guiding medium is absorbing and the requirement of broad-band coupling means a spectral range between 600 and 750 nm for amorphous solar cells and an even larger range from 700 to 1000 nm for microcrystalline solar cells. In this contribution,
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