High efficiency flexible solar film is robust enough for wearable designs

April 19, 2018 // By Nick Flaherty
Researchers at the Riken technology labs in Japan have developed an ultraflexible, high efficiency organic photovoltaic (OPV) film that is practical enough to use in wearable designs.

The film is just 3μm thick and still achieves a high power conversion efficiency of 10%. The design is stable enough up to 120 °C to fix to fabrics using standard industrial processes such as hot glue without causing performance degradation. This combination allows it to be used as a wearable power source in daily life as part of the Internet of Things (IoT).

Although improvement in thermal stability is essential, simultaneously providing high power conversion efficiency (PCE) and thermal stability in flexible OPVs is a major challenge in maintaining an optimal microstructure of the active layer under thermal stress.

The team successfully fabricated ultraflexible OPVs with initial efficiencies of up to 10% that can endure temperatures of over 100 °C, maintaining 80% of the initial efficiency under accelerated testing conditions for over 500 hours in air. It uses a low-bandgap poly(benzodithiophene-cothieno[3,4-b]thiophene) (PBDTTT) donor polymer that forms a sturdy microstructure when blended with a fullerene acceptor.

The polymer is sandwiched between a 1.3-μm-thick transparent polyimide substrate and 1.36-μm-thick Teflon/parylene double-barrier layer, and the stacks exhibited high uniformity, as shown by cross-section scanning electron microscope (SEM) imaging.

The team uses an inverted device structure to prevent direct contact between indium tin oxide (ITO) and poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS), avoiding proton etching of ITO and its subsequent degradation and extend the lifetime of the cells.

To demonstrate the capability, the team attached the OPV onto textiles using industrially available hot-melt adhesives at 120 °C, which did not cause performance degradation.

The team reported in the Proceedings of the National Academy of the USA (PNAS) at www.pnas.org/content/early/2018/04/10/1801187115

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