We describe resonant infrared pulsed laser deposition (RIR-PLD) of cyclic olefin copolymer, a barrier and protective
layer; for comparison, we describe RIR-PLD of polystyrene and poly(ethylene dioxythiophene) about which we already
have significant knowledge. Film deposition based on resonant infrared laser ablation is a low-temperature process leading
to evaporation and deposition of intact molecules. In this paper, we focus on deposition of this model barrier and
protective material that is potentially useful in the fabrication of organic light emitting diodes. The films were characterized
by scanning electron microscopy and Fourier-transform infrared spectroscopy. We also compared the properties of
films deposited by a free electron laser and a picosecond optical parametric oscillator.
We investigate the fundamental mechanisms of resonant-infrared laser ablation of polymers using polystyrene as
a model material. Time-resolved plume shadowgraphy coupled with laser-induced temperature-rise calculations
indicate that spinodal decomposition of a superheated surface layer is the primary mechanism for the initial stages
of material removal. The majority of the ablated material is then released by way of recoil-induced ejection of
liquid which proceeds for some tens of microseconds following a ~μs laser pulse excitation. The recoil-induced
ejection of liquid material as the dominant ablation mechanism helps to explain previous observations of laser
deposition of intact polymeric material.
Thin films of a conducting polymer have been grown by resonant infrared matrix-assisted pulsed-laser evaporation
(RIR-MAPLE). Properties of the thin films such as surface morphology and electrical conductivity have
been investigated as a function of laser wavelength, fluence, and pulse structure. Using a free-electron laser
whose wavelength is continuously tunable throughout the mid-infrared region (2-10 μm), we are able to deposit
polymer films from various liquid matrices by resonantly exciting selective vibrational modes of the solvent. An
Er:YAG laser operating at 2.94 μm is used to study the effects of different laser pulse durations. In the case of
poly(3,4 ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS), it is found that only specific excitation
wavelengths and pulse durations lead to the deposition of smooth and functional polymer films.
Multi-layered polymer light-emitting diodes (PLEDs) have been fabricated in a vacuum environment by resonant
infrared pulsed-laser deposition of the polymer layers. The light emitter used was poly[2-methoxy-5-(2-
ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV), and in some cases a layer of the hole-transport polymer
poly(3,4 etylenedioxythiophene:polystyrenesulfonate) (PEDOT:PSS) was also laser deposited, resulting in a device
structure of ITO/PEDOT:PSS/MEH-PPV/Al. Fourier transform infrared (FTIR) spectroscopy confirmed
that neither of the laser-deposited polymers was significantly altered by the deposition process. Laser-fabricated
devices displayed electroluminescent spectra similar to those of conventional spin-coated devices, but the differences
in electrical characteristics and device efficiency were substantial. These discrepancies can probably
be attributed to surface roughness of the deposited polymer layers. With the appropriate refinement of the
deposition protocols, however, we believe that this process can be improved to a level that is suitable for routine
fabrication of organic electronic components.
Thin films of the conducting polymer poly(3,4-ethylenedioxy-thiophene):poly(styrenesulfonate) (PEDOT:PSS)
were deposited by resonant infrared laser ablation. The PEDOT:PSS was frozen in various matrix solutions and
deposited using a tunable, mid-infrared free-electron laser (FEL). The films so produced exhibited morphologies
and conductivities that were highly dependent on the solvent matrix and laser irradiation wavelength used.
When deposited from a native solution (5% by weight in water), as in matrix-assisted pulsed laser evaporation
(MAPLE), films were rough and electrically insulating. When the matrix included other organic "co-matrices"
that were doped into the solution prior to freezing, however, the resulting films were smooth and exhibited good
electrical conductivity (0.2 S/cm), but only when the ablation was carried out at certain wavelengths. These
results highlight the importance of the matrix/solute and matrix/laser interactions in the ablation process.
Polymer light emitting diodes (PLEDs) have been fabricated in a vacuum environment by resonant infrared laser
ablation of the light emitting layer. The light emitting polymer used was poly[2-methoxy-5-(2-ethylhexyloxy)-
1,4-phenylenevinylene] (MEH-PPV) and was deposited into the device structure ITO/MEH-PPV/Al. Fourier
transform infrared (FTIR) spectroscopy confirmed that the laser-deposited polymer was not drastically altered by
the deposition process. Laser-fabricated devices displayed similar properties such as electroluminescence spectra
and IV characteristics as conventional spin-coated devices. The dependence of these device properties on laser
fluence was investigated, and showed no strong dependency. Peak emission wavelengths of electroluminescence
spectra were all within 10 nm of electroluminescence spectra of spin coated devices and showed only slight peak
broadening. These results are technologically important in that shadow mask technology can be incorporated
into this method to arbitrarily pattern substrates with light emitting polymers.
Experiments on pulsed laser vaporization of many different kinds of polymers have demonstrated that it is possible to
eject intact polymers into the ambient, whether air or vacuum, by resonant pulsed laser excitation, using both neat and
matrix targets. Two recent studies of resonant infrared ablation - one on polystyrene, the other on poly(amic acid), the
precursor for the thermoset polyimide - show moreover that the ablation process is both wavelength selective and
surprisingly non-energetic, especially compared to ultraviolet laser ablation. We propose a wavelength-selective photothermal
mechanism involving breaking of intermolecular hydrogen bonds that is consistent with these observations.