—promising 3rd-gen solar cells utilizing a nanocrystal ink deposition that could make traditional expensive silicon-based solar panels a thing of the past. While progress on organic thin-film photovoltaics is rapidly growing, inorganic devices still hold the record for highest efficiencies which is in part due to their broad spectral absorption and excellent electronic properties.
A major disadvantage compared to organics, however, is that inorganic materials are difficult to deposit from solution. To overcome this, Townsend synthesized materials on the nanoscale. Inorganic nanocrystals encased in an organic ligand shell are soluble in organic solvents and can be deposited from solution (i.e., spin-, dip-, spray-coat) whereas traditional inorganic materials require a high temperature vacuum chamber. The solar devices are fabricated from nanoscale particle inks of the light absorbing layers, cadmium telluride/cadmium selenide, and metallic inks above and below. This way, the entire electronic device can be built on non-conductive glass substrates using equipment you can find in your kitchen.
The outstanding challenge facing the (3-5 nm) inorganic nanocrystals is that they must be annealed or heated to form larger ‘bulk scale’ grains (100 nm to 1 μm) in order to produce working devices. Townsend recently teamed with Navy researchers to explore this process.
“When you spray on these nanocrystals, you have to heat them to make them work,” explained Townsend, “but you can’t just heat the crystals by themselves, you have to add a sintering agent and that, for the last 40 years, has been cadmium chloride, a toxic salt used in commercial thin-film devices. No one has tested non-toxic alternatives for nanoscale ink devices, and we wanted to explore the mechanism of the sintering process to be able to implement safer salts.”
In the study, ammonium chloride was found to be a non-toxic, inexpensive viable alternative to cadmium chloride for nanocrystal solar cells. This discovery came after testing several different salts. Devices made using ammonium chloride (which is commonly used in bread making) had comparable device characteristics to those made with cadmium chloride, and the move away from cadmium salt treatments alleviates concerns about the environmental health and safety of current processing methods.
The team also discovered that the role of the salt treatment involves crucial ligand removal reactions. This is unique to inorganic nanocrystals and is not observed for bulk-scale vacuum deposition methods. Because nanomaterials are at the forefront of emerging new properties compared to their bulk counterpart, the study is important to the future of electronic device fabrication. http://www.newswise.com/articles/new-research-puts-us-closer-to-diy-spray-on-solar-cell-technology
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