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Posted: Nov 27th, 2007
Low-cost inkjet printing of nanocrystal photodetectors
(Nanowerk Spotlight) In October, Sony introduced the world's first OLED (organic light-emitting diode) television set. This The 11-inch OLED TV - although quite expensive at $1,700 - realizes an astonishing 3mm-thinness (at thinnest point) and unparalleled image quality. What makes OLEDs so attractive is that they do not require a backlight to function and therefore they draw far less power and, when powered from a battery, can operate longer on the same charge. OLED devices can be made thinner and lighter than comparable LED devices. Another major leap forward compared to current display technologies is that OLEDs can be printed onto almost any substrate with inkjet printer technology, making new applications like displays embedded in clothes or roll-up displays possible. Unfortunately there are also drawbacks to this technology, in particular its currently high manufacturing cost. A possible solution could lie in cost efficient mass production methods using printing or spraying techniques. These methods use semiconductor materials that are processed in the form of a dispersion, comprising soluble conjugated polymers or precursor molecules as well as colloidal, organic or inorganic nanoparticles. Inkjet-printing represents a powerful, economic tool for accurate deposition of liquids which is not only useful for graphics applications, but has also enormous potential for the direct writing of electronic devices. Researchers in Austria have now shown for the first time the highly reproducible ink-jet printing of semiconducting nanocrystals for the fabrication of optoelectronic devices.
"We succeeded in demonstrating nanocrystal based photodetectors operating at room temperature in the visible as well as in the infrared are demonstrated" Dr. Wolfgang Heiss explains to Nanowerk. " The devices are fabricated by ink-jet printing of dispersions containing the semiconducting nanocrystals. Ink-jet printing of colloidal nanocrystals is material effective and highly reproducible, and can be applied not only for the photosensitive materials but also for fabrication of the electrodes (see our Spotlight: En route to inkjet-printing transparent electronics and thin film solar cells), opening up prospects for low cost, all ink-jet printed photodetector devices."
Heiss is an Associate Professor at the Institute of Semiconductor and Solid State Physics at the University Linz in Austria. This new approach, developed by his group in collaboration with the group of Prof. Emil List from the Graz University of Technology (Austria), to make use of ink-jet printing to fabricate an electronic device which is based on colloidal nanocrystals has been published in a paper titled "Inkjet-Printed Nanocrystal Photodetectors Operating up to 3 µm Wavelengths" in the November 5, 2007 online edition of Advanced Materials.
"The motivation for our work was to show that with colloidal nanocrystals cost effective and simple technologies can be applied to achieve electronic devices which can have an as good performance as devices based on high effort technologies like molecular beam epitaxy" says Heiss.
The achieved long wavelength limit of the presented photodetector – up to 3 µm – is the longest wavelength ever obtained with a solution processed semiconductor device (so far the longest wavelength obtained with nanocrystal based devices was at 1.8 µm). This is significant with regard to fabricating higher performance photovoltaic devices. For instance, researchers are attempting to push the maximum wavelength of solar cells further to the infrared. Unfortunately, this has been resulting in the degradation of their performance.
The photodetectors consist of two gold electrodes with an interdigitate finger structure and the nanocrystals are ink-jet printed in stripes perpendicular across the electrode fingers. Photocurrents are extracted by a bias voltage of 10 V applied across the meander shaped gap between the electrodes with a width of 20 µm. (Image: Dr. Heiss/University Linz)
The Austrian researchers controlled the printing of patterns of their mercury telluride (HgTe) nanocrystal solution on glass substrates by adjusting the printing frequency at a fixed stage speed and substrate temperature. At a printing frequency of 15 Hz separated dots with a diameter of 125 µm are formed.
"Each deposited droplet consists of a semitransparent, homogeneously covered area in the interior surrounded by a dark ring containing densely packed nanocrystals" Heiss describes the results. "This coffee-stain effect, known also from other colloidal liquids, can also be suppressed for nanocrystal solutions by using solvents providing deposited droplets with a high contact angle" (as was demonstrated by Tekin et al.).
Increasing the printing frequency to 40 Hz results in the individual dots merging to undulated stripes and at 100 Hz stripes with smooth borders and a stripe width of about 200 µm are obtained.
Heiss points out that, in view of the little efforts required to make these devices, they exhibit perfect performances. "At room temperature, the detctivity, the figure of merit for photodetectors, reaches a value of D*=3.2*10∧10 cmHz½W-1 for the telecommunication wavelength region, which is considerably better than what is achieved with epitaxially grown quantum dot photodetectors. The ink-jet printing is ideal for the preparation of such devices because it allows excellent control over the amount of deposited material and also about the location of the material after deposition" he says. "Therefore, only very little amounts of the semiconducting materials are required for the fabrication of the devices and the waste is kept to a minimum. This is a big advantage compared to the other technologies used so far for the preparation of nanocrystal devices like spin-casting, drop-casting or dip coating where most of the semiconducting material is placed in locations where the material is not needed. Furthermore, the cut-off wavelength of the devices is just determined by the size of the chosen nanocrystals."
Apart from the pure economic effects, using as little as possible material is also beneficial considering that the photosensitive material that Heiss and his team used – hydrophobic mercury telluride – is a potentially highly toxic substance. This also poses a challenge for scaling this technique up to industrial scale: researchers need to come up with a synthetic route that can deliver this material without creating a health risk for manufacturer and end user.