Fabrication of Transparent Carbon Nanotube Films for OLED Applications

This process produces single-wall carbon nanotube (SWCNT) dispersion solutions that are stable enough to be stored and used repeatedly.

June 1, 2011

Air Force Office of Scientific Research, Arlington, Virginia

The process to prepare single-wall carbon nanotube (SWCNT) solutions with a good degree of dispersion of individually isolated SWCNTs and very thin SWCNT bundles was optimized. The SWCNT-dispersion solutions are so stable that they can be stored and used repeatedly for some period of time.

Typical AFM images of **SWCNT Films on Substrates**: (a) single spin-coating using dispersion, and after repeating spin-coating 100 times on (b) glass and (c) PET. The scale bar is 1 mm.

The thin-film casting by spin-coating resulted in SWCNT films with uniform sheet resistance and visible-light transmittance. With the control of the spin-coating conditions, the researchers were able to control the thickness, sheet resistance, and visible-light transmittance with the general trend that the reduction of sheet resistance requires the sacrifice of visible transmittance. Further reduction of the sheet resistance with a limited sacrifice of visible-light transmittance was achieved through the treatment of SWCNT films with nitric-acid. For example, the sheet resistance of 85 W/sq and the transmittance of 80% (@ l = 550 nm) were measured from the 40-nm-thick SWCNT on a polyethylene terephtalate (PET) substrate. Moreover, the repetition of the HNO₃ treatment and thermal annealing has the extra benefit of the improved adhesion of SWCNT films to substrates.

Another process that was developed for more efficient reduction of the sheet resistance was the combination treatment of SWCNT films with HNO₃ and SOCl₂. This new process turned out to be more stable against undesired increase of the sheet resistance due to the inevitable heating that the SWCNT would experience during the organic light emitting diode (OLED) fabrication. It appears that the doping and dedoping effects are more prominent in the combination treatment with HNO₃ and SOCl2, as evidenced by the suppression and recovery of the van Hove singularities in response to the combination treatment and heating.

More evidence for the anodic doping (electron withdrawal) due to the combination treatment was found from the resonance Raman scattering. The distinct Raman modes of some semiconducting SWCNTs that appeared strong because of the resonance of the excitation energy (1.15 eV) with the S22 transition became substantially weak after the HNO₃-SOCl₂ combination treatment.

One of the key process steps for OLED fabrication on SWCNT electrodes is the anode patterning. Through systematic investigation of photolithography and oxygen plasma etching processes, process parameters were optimized to pattern the SWCNT films into desired anode patterns without the contamination, delamination, and sheet-resistance increase.

Another important issue related to the SWCNT anode is how to overcome the surface roughness; in particular, very large peak-to-valley height variation. This problem was solved by using a PEDOT:PSS hole-injection layer for the planarization. OLEDs with the structure of SWCNT-anode/PEDOT:PSS(90nm)/ NPB(200nm)/Alq3(40nm)/LiF(1nm)/Al(100nm) were successfully fabricated. The operation of such a device showed that the performance of a green-emitting OLED based on the SWCNT anode is comparable to that of more conventional green-emitting OLEDs fabricated on ITO anodes.

SWCNT-network films were cast on either glass or flexible PET substrates using a spin coater. The number of spin-coatings was one of the important process parameters to control the sheet resistance and the visible transmittance of SWCNT films. To cast SWCNT films on PET substrates, proper-size PET pieces were attached to glass backing plates using double-sided adhesive tapes to prevent bending and sliding of PET substrates during spin-coating operation. In the case of casting on glass substrates, 3-aminopropyl triethoxysilane was used to improve the adhesion between SWCNT films and the glass substrates. First, 1 wt% solution of 3-aminopropyl triethoxysilane was dropped on the glass substrate and left set for 5 minutes. Next, the excess 3-aminopropyl triethoxysilane solution was removed by spinning and thoroughly rinsing the substrates prior to the SWCNT-film casting.

The conventional photolithography technique was used to form patterned SWCNT anodes. The exposed areas of the SWCNT films were easily removed by using a radio-frequency oxygen-plasma (40 s, 100 mTorr, 100mW), whereas the PR-coated areas remained intact. Acetone-rinsing to remove the protective PR after the reactive ion etching induced no degradation to the SWCNT anodes. For OLED fabrication, a PEDOT:PSS layer was deposited by spin coating, and then NPB, Alq3, LiF, and Al layers were evaporated at the pressure of 110-6 torr. All the fabrication steps were performed in a glovebox with a thermal evaporator directly connected to it. The fabricated OLEDs were hermetically sealed by glass encapsulations with getters before being taken out from the glovebox for current-luminance voltage measurements.

Selection of dispersion medium and ultrasonication conditions, such as power, frequency, and time, are important process parameters to make stable, good-quality SWCNT dispersion solutions. Preparation of good CNT dispersion soluions, without cutting CNTs too short due to excessive ultrasonication, is essential for the successful fabrication of CNT electrodes with low sheet resistance and high visible-light transmission. Through an extensive search for optimal process conditions, dispersion solutions were created that contained less bundled and individually separated micrometer-scale SWCNTs.

This work was done by Soon Il Lee of Ajou University, South Korea, for the Asian Office of Aerospace Research and Development. AFRL-0155