OPTICAL MATERIALS: Photovoltaic-material sensitivity stretches from UV to near-IR

New research results appear to enhance the optoelectronic properties of organic semiconductors known as conjugated organic oligomers and polymers, and may eventually extend their list of current applications to include highly efficient solar cells, as well as night-vision displays and sensors.

Superior hole-transport properties and low bandgap energies have garnered particular attention for conjugated organic systems based on the ring-shaped thiophene (C₄H₄S) compound. Researchers at Ohio State University (Columbus, OH) and National Taiwan University (Taipei, Taiwan) have collaborated to tune the wavelength sensitivity of oligothiopene rings across a 300 to 800 nm wavelength range, and increase by several orders of magnitude the electronic response of these materials to optical stimulation.¹

The primary enabler is the incorporation of transition metals molybdenum and tungsten for electron sharing (covalent bonding) with the organic ligand, and to add much broader photovoltaic wavelength sensitivity to the colorful photochemistry currently enabled by various metal-ligand systems.

Previous research on metal-thiophene systems has generally involved transition metals such as gold, platinum, ruthenium, and osmium, in which the most sensitive optical absorption bands tend to arise from charge transfer between energy levels within the ligand. Ruthenium and osmium thiophene systems do have sensitive optical absorption bands based on metal-to-ligand charge transfer, but the energy level is still very close to the ligand-to-ligand charge transfer seen with the other metals and responds to optical excitation between 400 and 500 nm. A slight red shift in sensitivity occurs on changing from rubidium to osmium.

Using molybdenum and tungsten instead, the researchers observed photoinduced metal-to-ligand energy transfer at a much higher optical sensitivity, primarily due to intervalence (metal-to-metal) charge transfers between different oxidation states in the metal. In addition, changing the metal from molybdenum to tungsten produced a significant red shift (0.65 eV), which is expected to enable optical sensitivity ranging from the ultraviolet to the near-IR, by combining multiple thiopene rings each with slight variations in metallic content.

At this point, the material is years from commercial development. Malcolm Chisholm, team leader and chair of the Department of Chemistry at Ohio State, characterized the experiment as a proof of concept. “We did not report a solar cell or device but rather the synthesis and properties of an inorganic-organic hybrid polymer that absorbs across the majority of the solar spectrum,” he said. “Also, these new polymers have long-lived photoexcited states, which should allow more efficient separation of electron and holes in the exciton. Both of these are very promising developments in terms of their potential application in photovoltaic devices.” The researchers also noted that photoexcited electrons remained free for much longer time periods–tens of microseconds–in the molybdenum and tungsten systems than with the other transition metals, where time periods were on the order of picoseconds. When distributed within a planar thin-film structure, the length of time that photoexcited electrons remained free increased even further, to on the order of hundreds of microseconds.

REFERENCE

G.T. Burdzinski et al., PNAS 105(40), 15247 (Oct. 7, 2008).