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제목 : Research highlight, NPG Asia Materials, 29 June (2009)

원문보기 : http://www.natureasia.com/asia-materials/highlight.php?id=474

Research highlight, NPG Asia Materials, 29 June (2009)

Organic electronics: Promising solutions

Published online 29 June 2009
 

Fig. 1: Atomic force microscopy images of films

deposited by solution processing from toluene and chlorobenzene (CB), and by vacuum deposition.

 

Organic light-emitting diodes (OLEDs) constructed from small molecules are promising for various commercial applications. However, the most common method of fabricating these devices — vacuum deposition — is expensive and wasteful. An alternative would be to exploit solution-processing techniques, which not only offer increased efficiency and lower production costs, but also allow the fabrication of large-area panels using methods such as inkjet printing.

However, there are few data suitable for comparing the characteristics of devices prepared using these two approaches. Tae-Woo Lee at the Pohang University of Science and Technology, Jong-Jin Park from the Samsung Advanced Institute of Technology and colleagues1 now report the results of such a comparative study. They found that films fabricated using the two techniques differ in density and degree of aggregation, as well as in electronic properties. The researchers suggest methods for improving the performance of solution-processed devices, and further show that very efficient devices can be prepared by this method.

Lee and colleagues performed the comparative study using an anthracene-based compound that can be deposited using both methods. They found that this compound undergoes some aggregation when solution-cast from toluene, but not when deposited from chlorobenzene or in a vacuum (Fig. 1).

The researchers showed that OLEDs fabricated from the spin-cast small-molecule films had higher current densities than those prepared from vacuum-deposited films, although both types had similar turn-on voltages. The higher current density for the solution-processed device was attributed to higher charge-carrier mobility. 

Through measurement of emission from devices over long periods under application of a pulsed voltage, charge carriers were found to remain longer in the solution-deposited films, implying that more charge traps are present in these films as a result of the inclusion of solvent-derived impurities.

Lee explains that small-molecule OLEDs processed from solution could be very efficient. “High luminous efficiency from solution-processed blue-light-emitting diodes — higher than the vacuum-deposited versions — was achieved, even with a simpler structure,” he says. “We also found that the lower density of the solution-processed films (compared to the vacuum-deposited ones) can be the main reason for the short lifetimes commonly observed in the devices.” 

He adds that, “Our results can be used as a guide for improving the performance of other organic devices based on solution printing, including memories, thin-film transistors and photovoltaic cells.”

Reference

  1. Lee, T.-W.,1* Noh, T.,2 Shin, H.-W.,3 Kwon, O.,2 Park, J.-J.,2* Choi, B.-K.,2 Kim, M.-S.,2 Shin, D.W.2 & Kim, Y.-R.3 Characteristics of solution-processed small molecule organic films and light-emitting diodes compared with their vacuum-deposited counterparts. Adv. Funct. Mater. 19, 1625–1630 (2009). | article |

Author affiliation

1. Department of Materials Science and Engineering, Pohang University of Science and Technology, San 31 Hyoja-dong, Nam-gu, Pohang Gyeongbuk 790-784, Korea
2. Display Device and Processing Laboratory, Samsung Advanced Institute of Technology, Mt. 14-1, Nongseo-dong, Giheung-gu, Yongin-si, Gyeonggi-do 446-712, Korea
3. Photon Applied Functional Molecule Research Laboratory, Department of Chemistry, Yonsei University, Seoul 120-749, Korea
*E-mail: twlee@postech.ac.krjongjin00.park@samsung.com

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