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Stamped polymer top layer provides opportunity for more-efficient hybrid solar cells

Stamped polymer top layer provides opportunity for more-efficient hybrid solar cells

30-Apr-12
Some serious limitations are being faced for improving the efficiency of bulk hybrid solar cell, such as excessive recombination of free carriers, decay of excitons within an exciton diffusion length and inefficient carrier transport inside the polymer, as per Syed Abdul Moiz, a research professor at Hanyang University. These limitations can be minimized by incorporating nanowires inside the optimum thickness of polymer close to 70 nm. The incorporation of nanowires inside polymer as a bulk hybrid solar cell has launched new opportunities for the development of next-generation systems. One-dimensional inorganic nanowires inside polymer provide direct paths for efficient charge transport, and scientists have responded with a range of designs. As per nanotechweb.org, researchers from the SNPL (Semiconductor Nanoprocessing Lab) at Hanyang University have proposed a stamped hybrid PEDOT: PSS�SiNW approach. A novel stamped hybrid solar cell was proposed using the stamping transfer technique by stamping an active PEDOT: PSS thin layer onto the top of silicon nanowires (SiNWs). Compared to a bulk-type counterpart that fully embeds SiNWs inside PEDOT: PSS, an increase in the photovoltaic efficiency was observed by a factor of ~4.6, along with improvements in both electrical and optical responses for the stamped hybrid cell. Such improvements for hybrid cells was due to the formation of well-connected and linearly aligned active PEDOT:PSS channels at the top ends of the nanowires after the stamping process. These stamped channels facilitated not only to improve the charge transport, light absorption, but also to decrease the free carriers as well as exciton recombination losses for stamped hybrid solar cells. To fabricate this type of solar cell, a thin layer of PEDOT:PSS was successfully transferred using a simple stamping process onto the top of a SiNW array. As a result of the stamping transfer, conductive channels consisting of nanowire tips interlinked via agglomerated PEDOT:PSS were linearly formed on the top side of the SiNW array. Compared with a bulk-type structure consisting of nanowires fully embedded in polymer, the stamped hybrid cell revealed a greater enhancement of the power conversion efficiency (PCE) due to improvements in major photovoltaic parameters such as the short-circuit current, open-circuit voltage, series resistance, shunt resistance, diode ideality factor and optical reflection. The optimum thickness of polymer is close to 70 nm, but unfortunately such a type of hybrid solar cell with nanowire lengths below this optimum thickness becomes much less practical. The team is hopeful for the future of the stamped PEDOT: PSS-SiNW hybrid solar cell, and believes that the design could have important implications for device fabrication.

Another new development in the field of solar technology is tinted solar panels capable of converting 8% of its collected light energy into electricity. German startup Heliatek has created semi-transparent solar panels that imitate the materials of OLED displays. The tinted windows could be incorporated into buildings and ultimately generate electricity. While traditional solar panels are polymer-based, the panels are made of small, organic molecules called oligomers. Oligomers are more stable than polymers and are able to be deposited via a vacuum-deposition process, which allows for uniformity and thickness control in the films. This is important because uniformity enhances efficiency. These panels are capable of converting 8% of its collected light energy into electricity, where polymer panels are only about 3-5% efficient. Silicon solar panels are about 14-15% efficient, but these panels make up through ability to collect energy in high heat or low light, unlike traditional panels. Currently, the cost of these solar panels will be higher than that of traditional solar panels, but in about 4-5 years, the cost will drop 40-50 cents/watt. This new development is expected to evoke interest in builders as they are cheaper to place in a building since they don't require hardware to be anchored to the roof.

Heliateck�s new, small molecule, absorber material is called HDR14™. It offers high and broad absorption as well as good thermal and photo stability. HDR14™ has been key to Heliatek setting a series of world breaking efficiency records for organic PV cells, with the most recent being 9.8% last November (independently certified by Fraunhofer ISE CalLab). The company�s solar technology offers many advantages over traditional panels and opens up new application areas thanks to its key features, namely ultra-light, ultra-thin, flexible and transparent. The solar cells keep working efficiently in real world conditions as they maintain the same efficiency in low lighting and in hot conditions. The technology is essentially the reverse of organic LEDs (OLED) - they take in electricity and emit light while we use light to create electricity. Heliatek is the only commercial player specializing in vacuum deposition of small molecules in organic photovoltaics, following the successful manufacturing approach of all major OLED display companies. There are many clear advantages for using this approach to create our patented tandem cell structure such as the excellent uniformity of the roll-to-roll vapor deposition process and scalability. As per the company, this is the first truly green solar technology as very little energy and material are required in our non-toxic production process, so the energy payback time of our cells is much shorter, under 6 months, than the one of silicon cells. This innovative organic solar film technology that promises to deliver a lighter, greener and more versatile alternative to traditional solar panels.
 
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