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Transparent displays move closer to reality
RESEARCH NEWS
Transparent displays move closer to reality ELECTRONIC MATERIALS
Until now, fully transparent computer displays have been the stuff of science fiction. Researchers from the Technische Universität Braunschweig in Germany have now taken a step toward realizing such devices. Thomas Riedl and coworkers have demonstrated the first fully transparent organic light-emitting diode (OLED) pixels [Görrn et al., Adv. Mater. (2006) 18, 738]. The use of such pixels could enable the realization of active-matrix transparent displays. The pixels consist of transparent OLEDs integrated on top of transparent thin-film transistors (TTFTs). The TTFTs are based on zinc tin oxide, (ZnO)x(SnO2)1-x, or ZTO, which offers a viable alternative to conventional, opaque α-Si:H or poly-Si TFT backplanes. “Our metaloxide TFTs transport current ten times better than their established predecessors made from α-Si,” claims Riedl. The ZTO TFTs are fabricated using oxygenplasma-assisted pulsed laser deposition (PAPLD), which only requires process temperatures of ~150°C. Such low process temperatures mean that plastic substrates can even be used, leading to the possible development of flexible displays in the future. The devices have an average transmittance of >70% in the visible part of the spectrum, which could be sufficient for some automotive applications. A luminescence of 700 cd/m2 is achieved at a gate voltage of 5 V. “In the future, large-area, high-resolution, transparent displays could be made from millions of these pixels,”
ELECTRONIC MATERIALS
The technique is simple, fast, and highly reproducible. It has advantages over other related techniques, such as microcontact printing, in that there is no need to ‘ink’ the self-assembled monolayers (SAMs), which can lead to the introduction of defects through smearing or diffusion. This ‘dry’ printing technique could be easily scaled-up to large areas and various substrates.
Researchers from Oregon State University (OSU) and Hewlett-Packard have fabricated highly transparent integrated circuits [Presley, R. E., et al., Solid-State Electron. (2006), doi: 10.1016/j.sse.2006.02.004]. The inverter and five-state ring oscillator devices are based on transparent thin-film transistors (TTFTs) fabricated using indium gallium oxide (IGO) as the active channel material. Standard photolithography is used to fabricate the TTFTs on glass substrates in a seven-stage process. This comprises: (i) definition of gate electrodes using photolithographic patterning and an HCl wet etch; (ii) fabrication of SiO2 gate dielectric by plasma-enhanced chemical vapor deposition; (iii) contact to gate electrodes with vias by photolithography and reactive ion etching; (iv) deposition of IGO channel layer by rf magnetron sputtering; (v) patterning of the channel with HCl wet etch; (vi) 500°C furnace anneal; and (vii) deposition of indium-tin-oxide source/drain contacts by rf magnetron sputtering. The n-channel IGO TTFTs show a peak mobility of ~7 cm2/Vs and a turn-on voltage of ~2 V. The five-stage ring oscillators show an oscillation frequency of ~2.2 kHz when the gate and drain of the load transistor are biased at 30 V and a maximum oscillation frequency of ~9.5 kHz at 80 V. Most importantly, the ring oscillators exhibit ~75% optical transmittance in the visible portion of the spectrum. “[This is] proof that transparent transistors can be used to create an integrated circuit, [it] tells us quite a bit about the speeds we may be able to achieve, and shows that we can make transparent circuits with conventional photolithographic techniques,” says John F. Wager, OSU.
Cordelia Sealy
Cordelia Sealy
Active pixel on a glass substrate in the on-state with the seal of the Technische Universität Braunschweig clearly visible underneath. (© 2006 Wiley-VCH Verlag.)
says Riedl. See-through displays open up a large variety of novel applications, he believes, from car windshields to surgery. “You [can] conceive of dozens of novel applications,” he says, “[which] may go far beyond fancy, [bringing] new design elements in consumer electronics.” Within the next two years, prototypes of such transparent OLED displays should be possible. Cordelia Sealy
Simple pattern for improved organic semiconductors ELECTRONIC MATERIALS The patterning of organic semiconductors is a necessary condition for the fabrication of devices and complex circuits. Conventional lithography tends to degrade the performance of organic semiconductor devices and requires elaborate, time-consuming, and expensive systems. These limitations have led to the development of a host of alternative, low-cost, simple patterning techniques. Researchers from the University of California-Los Angeles, Stanford University, and California Institute of Technology have developed a technique for the patterning of organic semiconductors and conducting polymers into functional devices from solution [Briseno et al., J. Am. Chem. Soc. (2006) 128, 3880]. The technique enables transfer of unreacted low molecular weight (LMW) siloxane oligomers from ‘dry’ poly(dimethylsiloxane) (PDMS) stamps. The technique can pattern features down to 1 µm. Actual transistors fabricated using the technique show mobilities as high as 0.07 cm2/Vs. The researchers also used the technique to fabricate a flexible transistor based on PEDOT source-drain electrodes.
10
MAY 2006 | VOLUME 9 | NUMBER 5
Clear future for transistors
A flexible organic TFT fabricated from patterned PEDOT electrodes. The PDMS-stamped patterning of the electrodes was accomplished in as little as 20 s by dipcoating. The organic TFT active layer is P3HT. (© 2006 American Chemical Society.)
Transparent displays move closer to reality ELECTRONIC MATERIALS
Until now, fully transparent computer displays have been the stuff of science fiction. Researchers from the Technische Universität Braunschweig in Germany have now taken a step toward realizing such devices. Thomas Riedl and coworkers have demonstrated the first fully transparent organic light-emitting diode (OLED) pixels [Görrn et al., Adv. Mater. (2006) 18, 738]. The use of such pixels could enable the realization of active-matrix transparent displays. The pixels consist of transparent OLEDs integrated on top of transparent thin-film transistors (TTFTs). The TTFTs are based on zinc tin oxide, (ZnO)x(SnO2)1-x, or ZTO, which offers a viable alternative to conventional, opaque α-Si:H or poly-Si TFT backplanes. “Our metaloxide TFTs transport current ten times better than their established predecessors made from α-Si,” claims Riedl. The ZTO TFTs are fabricated using oxygenplasma-assisted pulsed laser deposition (PAPLD), which only requires process temperatures of ~150°C. Such low process temperatures mean that plastic substrates can even be used, leading to the possible development of flexible displays in the future. The devices have an average transmittance of >70% in the visible part of the spectrum, which could be sufficient for some automotive applications. A luminescence of 700 cd/m2 is achieved at a gate voltage of 5 V. “In the future, large-area, high-resolution, transparent displays could be made from millions of these pixels,”
ELECTRONIC MATERIALS
The technique is simple, fast, and highly reproducible. It has advantages over other related techniques, such as microcontact printing, in that there is no need to ‘ink’ the self-assembled monolayers (SAMs), which can lead to the introduction of defects through smearing or diffusion. This ‘dry’ printing technique could be easily scaled-up to large areas and various substrates.
Researchers from Oregon State University (OSU) and Hewlett-Packard have fabricated highly transparent integrated circuits [Presley, R. E., et al., Solid-State Electron. (2006), doi: 10.1016/j.sse.2006.02.004]. The inverter and five-state ring oscillator devices are based on transparent thin-film transistors (TTFTs) fabricated using indium gallium oxide (IGO) as the active channel material. Standard photolithography is used to fabricate the TTFTs on glass substrates in a seven-stage process. This comprises: (i) definition of gate electrodes using photolithographic patterning and an HCl wet etch; (ii) fabrication of SiO2 gate dielectric by plasma-enhanced chemical vapor deposition; (iii) contact to gate electrodes with vias by photolithography and reactive ion etching; (iv) deposition of IGO channel layer by rf magnetron sputtering; (v) patterning of the channel with HCl wet etch; (vi) 500°C furnace anneal; and (vii) deposition of indium-tin-oxide source/drain contacts by rf magnetron sputtering. The n-channel IGO TTFTs show a peak mobility of ~7 cm2/Vs and a turn-on voltage of ~2 V. The five-stage ring oscillators show an oscillation frequency of ~2.2 kHz when the gate and drain of the load transistor are biased at 30 V and a maximum oscillation frequency of ~9.5 kHz at 80 V. Most importantly, the ring oscillators exhibit ~75% optical transmittance in the visible portion of the spectrum. “[This is] proof that transparent transistors can be used to create an integrated circuit, [it] tells us quite a bit about the speeds we may be able to achieve, and shows that we can make transparent circuits with conventional photolithographic techniques,” says John F. Wager, OSU.
Cordelia Sealy
Cordelia Sealy
Active pixel on a glass substrate in the on-state with the seal of the Technische Universität Braunschweig clearly visible underneath. (© 2006 Wiley-VCH Verlag.)
says Riedl. See-through displays open up a large variety of novel applications, he believes, from car windshields to surgery. “You [can] conceive of dozens of novel applications,” he says, “[which] may go far beyond fancy, [bringing] new design elements in consumer electronics.” Within the next two years, prototypes of such transparent OLED displays should be possible. Cordelia Sealy
Simple pattern for improved organic semiconductors ELECTRONIC MATERIALS The patterning of organic semiconductors is a necessary condition for the fabrication of devices and complex circuits. Conventional lithography tends to degrade the performance of organic semiconductor devices and requires elaborate, time-consuming, and expensive systems. These limitations have led to the development of a host of alternative, low-cost, simple patterning techniques. Researchers from the University of California-Los Angeles, Stanford University, and California Institute of Technology have developed a technique for the patterning of organic semiconductors and conducting polymers into functional devices from solution [Briseno et al., J. Am. Chem. Soc. (2006) 128, 3880]. The technique enables transfer of unreacted low molecular weight (LMW) siloxane oligomers from ‘dry’ poly(dimethylsiloxane) (PDMS) stamps. The technique can pattern features down to 1 µm. Actual transistors fabricated using the technique show mobilities as high as 0.07 cm2/Vs. The researchers also used the technique to fabricate a flexible transistor based on PEDOT source-drain electrodes.
10
MAY 2006 | VOLUME 9 | NUMBER 5
Clear future for transistors
A flexible organic TFT fabricated from patterned PEDOT electrodes. The PDMS-stamped patterning of the electrodes was accomplished in as little as 20 s by dipcoating. The organic TFT active layer is P3HT. (© 2006 American Chemical Society.)