Nanorod deposition method maximises solar-cell absorption

Scientists in Ireland have developed a cost-effective method for depositing semiconductor material in the form of nanorods to maximise absorption in solar cells.

The team of researchers from the Materials and Surface Science Institute (MSSI) at Limerick University said its method is purely solution-based, simply using electrophoresis to line up the all-important nanorods.

‘There are two components to what we’ve done — we’ve managed to synthesise the rods and develop technology to assemble them so they’re all packed together and perpendicular,’ Dr Kevin Ryan of Limerick told The Engineer.

Current silicon-based commercial solar cells are costly to process, whereas alternative technologies rely on toxic materials such as cadmium or tellurium.

The most sought-after materials currently for solar energy conversion are copper-indium-galium-disulfide (CIGS) and di-copper-zinc-tin-tetrasulphide (CZTS).

By combining the four elements in each material in the correct combination, very high conversion of solar energy to electricity can be obtained. CIGS cells have shown the highest-possible efficiencies whereas CZTS, although marginally lower, are regarded in the industry as being equally attractive for large-scale commercialisation as each of the elements are in high natural abundance.

The barrier to application is that current methods of making these materials requires very expensive vacuum-based technologies.

Ryan’s team developed a low-cost laboratory method for forming CIGS and CZTS in nanorod form, which can maximise solar absorption. He explained that 250 billion of these rods fit on the head of a pin and the technology essentially allows them to process these as an ink to form densely packed forests over very large areas.

‘With that arrangement, when you’re absorbing light you have essentially all your absorbers orientated in the direction that maximises solar absorption —  you’re standing up essentially, you’re getting a total absorption that’s defined by rod length. When you generate your excitons within your material you want them to be separated to generate your current.’

Because the entire process is done in solution, Ryan claimed it is readily scaleable and so will be suitable for large-area solar-cell deployment.

The research project recently received further government support through a €700,000 (£548,000) Science Foundation Ireland Grant.