Scientists drive solar cells forward

17 Jun 2002

Self-assembling photovoltaics could lead the way to harnessing the Sun's rays cheaply and efficiently.

Lukas Schmidt-Mende and colleagues from the UK-based Cambridge University and Germany's Max-Planck Institute of Polymer Research have created a cheap organic photovoltaic cell that converts up to 34% of incident blue-green light into electricity. The device, based on a blend of liquid crystal and organic dye, "instinctively" organizes itself into a two-layered structure, ready for energy conversion.

Solar cells consist of a semiconductor sandwiched between two electrodes. When light hits the semiconductor, photogenerated electron-hole pairs, or excitons, are created. The key to any photovoltaic process is to first split these excitons and then whisk away each separate charge to the electrodes, thus creating electricity.

In the past, scientists have combined two materials with different electron affinities to create an electric field and split the excitons. Each material then provides a path to carry the separated charges in opposite directions to the electrode. However, these attempts have been dogged by time-consuming fabrication and high costs. Poor material separation has also led to devices with low quantum efficiencies.

To combat these problems Schmidt-Mende and his colleagues have blended a hexabenzocoronene liquid crystal with a perylene dye. They discovered that if they spin-coated the blend onto a silicon substrate, it self-assembled directly out of solution to create a two-layered photovoltaic thin film. A closer look revealed a lower layer of disc-shaped liquid-crystal molecules that had aligned into columns perpendicular to the substrate, with the needle-shaped molecules of the dye forming a top layer.

By adding photodiodes to the films Schmidt-Mende and his colleagues confirmed that the layers separated the charges and efficiently transported them to the electrodes. The scientists believe that the needle-like molecules and disc-shaped molecules of each layer make excellent charge-carrying channels, while the needle-shaped dye molecules are good light absorbers.

"This development brings together the simplicity of single-step solution processing with an efficient combination of molecular materials that can spontaneously develop a multilayer transport structure," said Schmidt-Mende. "Further optimization could be the way toward a commercially viable organic photovoltaic system."