Credit: NAOKI OHTANI

Although the value of spinach as part of a healthy diet is well-known, its potential role in the optoelectronics industry is only now being understood. Naoki Ohtani and co-workers from Doshisha University in Kyoto, Japan, have successfully fabricated organic light-emitting diodes (OLEDs) that contain chlorophylls extracted from spinach as their active ingredient (Jpn. J. Appl. Phys. 50, 01BC08; 2011). The researchers first spin-coated a 100-nm-thick film containing chlorophylls a and b onto a glass substrate coated with the transparent electrode material indium tin oxide. They then vacuum-deposited aluminium on top of the organic layer to form the top electrode. No photoluminescence was seen from the sample, which the team attributed to concentration quenching from the chlorophylls. By repeating their experiments using a blend of chlorophyll a and the conductive polymer poly(phenylene vinylene), the team successfully created OLEDs that emitted both blue–green and red light when electrically pumped. The blue–green emission peaking at around 500 nm was attributed to the polymer, whereas the longer-wavelength red emission peaking at around 680 nm was attributed to the chlorophyll. The researchers comment that the extraction and fabrication method are important for achieving good results, with chlorophyll prepared in the form of a fat-soluble solution performing much better than that of an acetone or methanol solution. “The OLEDs fabricated using the fat-soluble solution emitted electroluminescent signals for more than one minute, whereas the OLEDs fabricated using the methanol solution worked for less than five seconds,” comment the researchers. “The long operation times of OLEDs fabricated using a fat-soluble solution can be attributed to the antioxidant activities of carotenoids.”