Faculté des sciences

Properties of interfaces in amorphous / crystalline silicon heterojunctions

Olibet, Sara ; Ballif, Christophe (Dir.)

Thèse de doctorat : Université de Neuchâtel, 2008 ; Th. 2066.

The main focus of this work is the study of interfaces in amorphous/crystalline silicon (a-Si:H/c-Si) heterostructures, especially the investigation of the a-Si:H/c-Si heterointerface's electronic quality and its effect on the consecutive a-Si:H/c-Si heterojunction (HJ) solar cell fabrication. c-Si based solar cells have the potential for achieving high conversion efficiencies, but the standard... Plus

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    Summary
    The main focus of this work is the study of interfaces in amorphous/crystalline silicon (a-Si:H/c-Si) heterostructures, especially the investigation of the a-Si:H/c-Si heterointerface's electronic quality and its effect on the consecutive a-Si:H/c-Si heterojunction (HJ) solar cell fabrication. c-Si based solar cells have the potential for achieving high conversion efficiencies, but the standard simple fabrication processes lead to medium module efficiencies. Thin-film Si based technologies offer the prospect of low-cost fabrication but yield lower efficiencies. a-Si:H/c-Si HJ solar cells combine the advantages of both technologies, i.e., the high efficiency potential of c-Si and the low fabrication cost of a-Si:H. In this way the c-Si cost becomes reasonable because it is possible to use very thin wafers to produce highly efficient solar cells. The electronic quality of the heterostructure interface was evaluated experimentally with photogenerated carrier lifetime measurements. In this study, carrier recombination at the interface is the step limiting photogenerated carriers' lifetime. In the theoretical part of this work, c-Si surface recombination is modeled by considering for the first time the amphoteric nature of Si dangling bonds. For this, a model previously established for bulk a-Si:H recombination, is extended to the description of the c-Si surface recombination through amphoteric defects. Its differences and similarities compared to existing interface recombination models are discussed. This new model is currently the simplest that allows an understanding of the largest set of experimentally observed behaviors of passivation layers on c-Si. The potential of the model's applicability to passivation by silicon dioxide (SiO) and silicon nitride (SiNx) layers is also demonstrated. The passivation performances of a-Si:H on c-Si are examined by growing symmetrical layers and layer stacks (intrinsic, microdoped, intrinsic plus doped) by very high frequency plasma enhanced chemical vapor deposition (VHF-PECVD, at 70 MHz). Lifetime measurements in combination with numerical modeling, incorporating our new amphoteric interface recombination model, reveal the microscopic passivation mechanism of a-Si:H on c-Si. The growth of intrinsic (i) a-Si:H efficiently reduces the c-Si dangling bond density at the crystallographic interface. It further decreases the interface recombination rate when set (by the wafer's doping level and type or by an outer potential) in a neutral state (with the smallest free carrier capture cross-sections). Furthermore, the magnitude of an additional field-effect passivation can be tuned by fixing the i a-Si:H's outer surface potential when capping it with a doped thin-film Si layer. i a-Si:H passivation implies complete devices with very high calculated open-circuit voltages (VOC) over 700 mV on at c-Si of all kinds of doping types and levels. This corresponds to effective record low surface recombination velocities under 5 cm/s (and down to 1 cm/s). The emerging interpretation of lifetime measurements on specific heterostructure test samples allows for a rapid Si HJ solar cell development using a fast device diagnostic procedure, based on a single process step analysis. Individual testing of emitter and back surface field (BSF) layers on c-Si wafers allows for a rapid test of their suitability for Si HJ solar cell fabrication. In order to verify the demonstrated passivation quality and suitability of emitter and BSF layer stacks, Si HJ solar cells are produced. On at n-type c-Si, good results are rapidly achieved, i.e. VOCs up to 715 mV and efficiencies up to 19.1%. On at p-type c-Si, VOCs up to 690 mV and efficiencies up to 16.3% are reached. On textured c-Si, the a-Si:H's passivation capability depends on the wafers' surface morphology. Transmission electron microscopy (TEM) micrographs of the textured thin-film Si/c-Si interface, shown here for the first time, hows local epitaxial growth of i a-Si:H in c-Si valleys. Lifetime measurements make it possible to attribute the cause of an increased interface recombination to these features. Highquality texture achieves the same high implied VOCs by i a-Si:H passivation as at c-Si, but interface recombination is still increased on standard textured c-Si. Decreasing the density of epitaxized i-layers by using a large pyramidal texture, a modified BSF layer growth and an additional surface morphology modification, yields complete textured cells with very high VOC values over 700 mV.