Branching out into solar cells

Electrical contacts made of branched titanium dioxide nanowires developed by developed by researchers from KAUST could improve the efficiency of solar cells.

Titanium dioxide is a white pigment commonly used in paint and a versatile electronic material commonly used in solar cells. It is a cheap and abundant compound and has similar electronic states to those of the light-absorbing compounds used in photovoltaics.

This match ensures that electrical charges are efficiently funnelled away from the active region of a solar cell into the titanium dioxide and from there towards the electrical contacts of the device. “It is the most successful electron transporting material in hybrid organic/inorganic photovoltaics,” explains research leader Aram Amassian.

To ensure good contact between the titanium dioxide and the solar cell material, the titanium dioxide needs a very large surface area for maximum capture of electrical charges. The surface area can be expanded through the use of nanostructured materials such as meshes made from nanowires. These meshes have not been able to effectively transport the electrical charges across the nanowires.

Different architectural structures could help improve charge transport — for instance branched structures would have a much stronger electrical connection. But, established techniques for growing such branched structures is inefficient and produces nanostructures with many impurities and defects.

The researchers have now established a two-stage process for an efficient fabrication of branched titanium dioxide materials. The first step is to deposit nanofibers using an electrospinning technique, where a narrow jet of a titanium dioxide solution is ejected from a needle using electrostatic charges (see image), resulting in a network of electron highways. “Electrospinning of metal oxide nanofibers has emerged as a potentially low cost, rapid and useful technique to grow one-dimensional nanostructures on a variety of substrates,” says Amassian. 

In the second step these structures are heated further which results in the hydrothermal growth of branched nanostructures. The hyperbranched materials perform better than conventional nanofibers in solar cells, which indicates their potential viability in other devices such as batteries, or catalytic applications. 

The team reduced the processing temperature substantially — down to 300 degrees Celsius — but the demand for a high processing temperature remains a practical barrier to the technology, says Amassian. “Future work to halve this temperature could help implement hyperbranched electron transport materials for solar cell fabrication on flexible and stretchable plastic or textile substrates.”