Polymer Solar Cells

Posted by on set 20, 2013 in Sustainability Mission | 0 comments

Polymer Solar Cells

Visibly Transparent Polymer Solar Cells Produced by Solution Processing

Polymer solar cells (PSCs) have drawn intense attention due to their advantages over competing solar cell technologies. Current progress in the power-conversion efficiency (PCE) of PSCs has reached a new record of 10.6% based on tandem architecture, demonstrating the promising future of PSCs as low-cost and efficient photovoltaic (PV) candidates for solar energy harvesting. In addition to the pursuit of high device efficiency, PSCs have also been intensely investigated for their potential in making unique advances for broader applications. Several such applications would be enabled by highperformance visibly transparent PV devices,including building-integrated photovoltaics (BIPV) and integrated PV chargers for portable electronics. Previously, many attempts have been made toward demonstrating visibly transparent or semitransparent PSCs. Transparent conductors, such as thin metal films, metallic grids, metal nanowire networks, metal oxides, conducting polymers, and graphene, have been deposited onto photoactive layers as top electrodes to achieve visibly transparent or semitransparent PSCs. However, these demonstrations often result in low visible light transparency and/or low device efficiency, because suitable polymeric PV materials and efficient transparent conductors were not well deployed in device design and fabrication.
From the PV materials point of view, an ideal photoactive layer material for visibly transparent PSCs needs to harvest most of the photons from ultraviolet (UV) and nearinfrared (NIR) wavelengths in the solar spectrum, while the photons in the visible range should be transmitted. Since high power conversion efficiency is strongly dependent on the fraction of photons absorbed, there is often a compromise between captured photons and polymeric film transparency that limits materials development for visibly transparent PSCs. For example, poly(3-hexylthiophene) (P3HT):[6,6]-phenyl-C61 butyric acid methyl ester (PCBM) blend is the most commonly used photoactive layer material in visibly semitransparent PSCs. However, due to its efficient photon harvesting in the visible wavelength region, P3HT:PCBM (and many other) devices often have low visible transparency. On the other hand, the transparent conductor is another key factor that determines the performance of visibly transparent PSCs. An ideal transparent conductor for visibly transparent PSCs must simultaneously have high transparency and low resistance together with ease of processing and effective charge collecting. However, a trade-off is often found with these electrode materials, as high conductivity often sacrifices transparency. For example, thermally evaporated thin metallic films are commonly used as semitransparent electrodes for PSCs, but the conductivity is significantly compromised by the film transparency. Some recently developed solutionprocessable conductive materials, such as carbon nanotubes, graphene, poly(3,4-ethylenedioxythiophene): poly(styrene sulfonate) (PEDOT:PSS), and silver nanowires (AgNWs),have opened up a new era for transparent conductors. Despite the unique advantages of these candidates, they all have drawbacks limiting their applications in visibly transparent PSCs. For example, one major drawback is the damage caused by these transparent conductors to the underlying, soft polymeric photoactive layers during their deposition. Other issues include the chemical, physical, mechanical, or energetic incompatibility between the polymeric photoactive layer and the transparent conductor that can lead to the low performance or low transparency of the visibly transparent PSCs reported to date.
In this work, we demonstrate a solution to overcome the aforementioned challenges for visibly transparent PSCs. High-performance visibly transparent PSCs are achieved by combining polymeric PV materials sensitive to NIR light but highly transparent to visible light, together with solution-processed highperformance AgNW-based composite transparent conductors. Both visible light transparency and PCE are addressed simultaneously. Finally, a solutionprocessed and highly transparent solar cell is demonstrated with a PCE of 4% and a maximum transmission of ∼66% at 550 nm.

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