Dye-sensitized solar cells (DSSCs) are a promising alternative to fossil fuels. Researchers at Tampere University of Technology (TUT; Tampere, Finland) have demonstrated that the titanium dioxide conventionally used in DSSCs could be replaced with more affordable and environmentally friendly zinc oxide. The results were recently published in Royal Society Open Science.
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First reported by Michael Grätzel in the 1990s, DSSCs generated a new wave of solar cell research. The invention earned Grätzel the 2010 Millennium Technology Prize. While DSSCs are traditionally built on a substrate made of titanium dioxide nanoparticles, other materials have also been studied. Zinc oxide is among the most promising due to its high natural abundance and consequent low cost. The solar cell photoanodes are easy to manufacture with environmentally friendly wet chemical methods, which can be scaled up to meet commercial production requirements.
"Titanium dioxide is still the mainstream material in these solar cell applications. Despite its promising properties, zinc oxide has performed less efficiently than titanium dioxide. We wanted to find out why," says professor Nikolai Tkachenko from the Laboratory of Chemistry and Bioengineering at TUT.
"With ultrafast transient absorption spectroscopy we can compare these two materials and possibly find optimisation methods for ZnO-based cells to equal or even outperform TiO2-based cells," he says.
As the photochemical reactions inside a solar cell are extremely fast, ultrafast spectroscopy is needed to track the fates of photogenerated electrons.
"We found out that the photoelectrons escape from the semiconductor-sensitizing dye interface into the semiconductor bulk faster in titanium dioxide than in zinc oxide. This is in line with several other studies reported globally," says PhD student Kirsi Virkki, who is currently writing her dissertation on the topic.
In a working solar cell, the electrons need to travel through the semiconductor material to reach the external circuit. The longer the mean free path of the electrons, the higher the chance to get them to generate electricity. If the electrons are trapped inside the semiconductor material, they will eventually recombine with the organic dye, and no electric current will be produced. "To our surprise, the electrons in titanium dioxide samples recombined faster with the organic dye than in zinc oxide nanorods", says Virkki.
One possible reason for the faster recombination in the titanium dioxide samples lies in the morphology differences between the two sample sets. The zinc oxide samples consist of long, single-crystal nanorods, whereas the titanium dioxide samples are composed of an interconnected network of independent nanoparticles. Thus, there are additional interfaces between each titanium dioxide nanoparticle. These interfaces may cause the electrons to "get stuck" inside one nanoparticle and recombine with the organic dye instead of contributing to the photocurrent.
"Although the electrons do have a longer lifetime in zinc oxide, practice has shown that their lifetime in TiO2 nanoparticle systems is sufficient for efficient current generation. Researchers have more experience of TiO2-based systems, but with careful planning of the photoanode morphology, I don't see a reason why we couldn't build very efficient solar cells using zinc oxide," Virkki says.
SOURCE: Tampere University of Technology (TUT); http://www.tut.fi/en/about-tut/news-and-events/arti?art_id=394
