Abstract
The fast growing market of organic electronics, including organic photovoltaics (OPV), stimulates the development of
versatile technologies for structuring thin-film materials. Ultraviolet lasers have proven their full potential for patterning
single organic layers, but in a multilayer organic device the obtained layer selectivity is limited as all organic layers show
high UV absorption. In this paper, we introduce mid-infrared (IR) resonant ablation as an alternative approach, in which
a short pulse mid-infrared laser can be wavelength tuned to one of the molecular vibrational transitions of the organic
material to be ablated. As a result, the technique is selective in respect of processing a diversity of organics, which
usually have different infrared absorption bands. Mid-IR resonant ablation is demonstrated for a variety of organic thin
films, employing both nanosecond (15 ns) and picosecond (250 ps) laser pulses tunable between 3 and 4 microns. The
nanosecond experimental set-up is based on a commercial laser at 1064 nm pumping a singly resonant Optical
Parametric Oscillator (OPO) built around a Periodically-Poled Lithium Niobate (PPLN) crystal with several Quasi-Phase
Matching (QPM) periods, delivering more than 0.3 W of mid-IR power, corresponding to 15 μJ pulses. The picosecond
laser set-up is based on Optical Parametric Amplification (OPA) in a similar crystal, allowing for a comparison between
both pulse length regimes.
The wavelength of the mid-infrared laser can be tuned to one of the molecular vibrational transitions of the organic
material to be ablated. For that reason, the IR absorption spectra of the organic materials used in a typical OPV device
were characterized in the wavelength region that can be reached by the laser setups. Focus was on OPV substrate
materials, transparent conductive materials, hole transport materials, and absorber materials. The process has been
successfully demonstrated for selective thin film patterning, and the influence of the various laser parameters is
discussed.
