17 September 2009

Solar cells go 3-D

High-resolution 3-D images of the inside of a polymer solar cell can give new insights in the nanoscale structure of polymer solar cells and its effect on the performance.

The new view shed light on the operational principles of polymer solar cells, according to researchers from the Eindhoven University of Technology and the University of Ulm.

This is a 3-D electron tomography image of a polymer-metal oxide solar cell. The 3-D nanoscopic morphology shows the interpenetrating metal oxide network in yellow.

These solar cells do not have the high efficiencies of their silicon counterparts yet. But polymer cells can print on roll-to-roll processes, at very high speeds, which makes the technology potentially very cost-effective. Added to that, polymer cells are flexible and lightweight, and therefore suitable for use on vehicles or clothing or able to incorporate into the design of objects.

In these hybrid solar cells, a mixture of two different materials, a polymer and a metal oxide, is able to create charges at their interface when sunlight hits the mixture. The degree of mixing of the two materials is essential for its efficiency. Intimate mixing enhances the area of the interface where charges form but at the same time obstructs charge transport because it leads to long and winding roads for the charges to travel. Larger domains do exactly the opposite. The vastly different chemical nature of polymers and metal oxides generally makes it very difficult to control the nanoscale structure.

Eindhoven researchers have been able to largely circumvent this problem by using a precursor compound that mixes with the polymer and is able to convert into the metal oxide after incorporating in the photoactive layer. This allows better mixing and enables extracting up to 50% of the absorbed photons as charges in an external circuit.

The importance of the degree of mixing became clear via visualization of the structure of these blends in three dimensions. Traditionally, such visualization has been extremely challenging, but by using 3-D electron tomography, the team has been able to resolve the mixing with greater detail on a nanoscale. From these images, the researchers at the Institute of Stochastics in Ulm have been able to extract typical distances between the two components, relating to the efficiency of charge generation, and analyze the percolation pathways—how much of each component connects to the electrode. These quantitative analyses of the structure matched perfectly with the observed performance of the solar cells in sunlight.

Even though these hybrid polymer solar cells are among the most efficient reported to date for this class, the power conversion efficiency of 2% in sunlight must get better to make them useful. Researchers will be able to realize greater efficiency through improved control over the morphology of the photoactive blend, for example by creating polymers that can interact with the metal oxide and by developing polymers or molecules that absorb a larger part of the solar spectrum. At such point, the intrinsic advantages of hybrid polymer solar cells in terms of low cost and thermal stability of the nanoscale structure could be greater.

For related information, go to www.isa.org/manufacturing_automation.