Transparent and conductive electrodes
The development of highly conductive and transparent interface materials and their application in organic optoelectronic devices is an important research topic. One challenge is to develop new materials that compete or outperform the commonly used ITO in terms of costs, transparency, electrical resistance and resistance to cracking when bent. Another challenge is that thin films of such materials can be deposited at low temperatures by large-area compatible coating methods on flexible substrates. The development of highly conductive and transparent materials with improved properties will increase the device performance and lower the industrial production costs. Such materials are interesting for organic solar cells, displays and light-emitting devices, and for transparent electronics in general.
Together with our industrial partner Sefar AG, we have introduced a flexible, stable and transparent electrode for OPV cells consisting of polymer fibres and current-collecting metal wires woven into a mesh. To provide a stable support for the thin OPV active film (~ 150 nm), the holes in the mesh structure (~ 0.07 mm2) were filled with a curable transparent polymer emulsion in such a way that one side of the fabric was fully embedded in the polymer, while metal wires were exposed by ~ 10-15 micrometers to air at the other side. We fabricated woven fabric/PEDOT:PSS(~ 1-1.5 micrometers)/P3HT:PCBM/Al solar cells with an efficiency = 2.2%, and demonstrated that the photocurrent can be transported practically loss-free over a distance of several cm at least, a relevant distance for a large-area OPV cell applications. We then showed that the OPV performance using the woven fabric electrode can be considerably improved (efficiency = 3.1%) by further decreasing the distance (~ 5 micrometers) between the protruding metal wires and the plane of the polymer-filled holes. For reduced metal wire exposure, thinner but still covering PEDOT:PSS layers could be used; this resulted in increased light transmittance reaching the active layer, and translated in an associated increase in the short-circuit current density and efficiency. The production of the woven substrate electrode is fully integrated into a technological R2R process (fabric width 120 cm, 10 meters per minute) at Sefar AG. This substrate electrode was further developed and used in the EU project TREASORES.
In the CCEM-CH project (CONNECT-PV) we developed flexible and transparent electrodes that can be applied by lamination. We used the lamination process of the top electrode for perovskite solar cells. The laminate electrode consisted of a transparent and conductive plastic/metal mesh substrate, coated with an adhesive mixture of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate), PEDOT:PSS, and sorbitol. The laminate electrode showed a high degree of transparency of 85%. Best cell performance was achieved for laminate electrodes prepared with a sorbitol concentration of ~30 wt% per milliliter PEDOT:PSS dispersion, and using a pre-annealing temperature of 120 degrees for 10 minutes before lamination. Thereby, perovskite solar cells with stabilized power conversion efficiencies of (7.6 ± 1.0)% were obtained which corresponds to 80% of the reference devices with reflective opaque gold electrodes.
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