Abstract
The behavior of bi- and trilayer coating systems for flexible a-Si:H based solar cells consisting of a barrier, an electrode, and an absorption layer is studied under mechanical load. First, the film morphology, stress, Young’s modulus, and crack onset strain (COS) were analyzed for single film coatings of various thickness on polyethylene terephthalate (PET) substrates. In order to demonstrate the role of the microstructure of a single film on the mechanical behavior of the whole multilayer coating, two sets of InSnOx (indium tin oxide, ITO) conductive coatings were prepared. Whereas a characteristic grain–subgrain structure was observed in ITO-1 films, grain growth was suppressed in ITO-2 films. ITO-1 bilayer coatings showed two-step failure under tensile load with cracks propagating along the ITO-1/a-Si:H-interface, whereas channeling cracks in comparable bi- and trilayers based on amorphous ITO-2 run through all constituent layers. A two-step failure is preferable from an application point of view, as it may lead to only a degradation of the performance instead of the ultimate failure of the device. Hence, the results demonstrate the importance of a fine-tuning of film microstructure not only for excellent electrical properties, but also for a high mechanical performance of flexible devices (e.g., a-Si:H based solar cells) during fabrication in a roll-to-roll process or under service.
Highlights
Silicon-based thin film solar cells on flexible polymer substrates such as polyethylene terephthalate (PET) are of high interest for future low-cost photovoltaic solutions ([1,2,3,4,5], and references therein), as they can be fabricated in a simple and cheap roll-to-roll (R2R) mass production process on large areas and at relatively low temperatures
We demonstrate that multilayer systems with altered microstructure can differ significantly in their failure behavior, which may either lead to degradation or complete failure of the solar cell
The crack onset strain (COS) of films prepared by R2R sputtering increased with increasing film thickness, which was attributed to increasing compressive stress in the films
Summary
Silicon-based thin film solar cells on flexible polymer substrates such as polyethylene terephthalate (PET) are of high interest for future low-cost photovoltaic solutions ([1,2,3,4,5], and references therein), as they can be fabricated in a simple and cheap roll-to-roll (R2R) mass production process on large areas and at relatively low temperatures. Kim et al demonstrate that the electrical resistivity of a ZTO (20 nm)/Ag (10 nm)/ITO (30 nm) multilayer electrode on a PET substrate was hardly affected by cyclic bending, whereas a 60 nm ITO reference film showed a strong resistivity increase after only a few bending cycles [7]. The authors attribute this behavior to the ductile Ag interlayer with higher failure strain. The results of this study directly contribute to the optimization of dimensioning and fabrication of compliant a-Si:H-based thin film solar cells
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