Organic Semiconductor Thin Films Deposited by Resonant Infrared Matrix-Assisted Pulsed Laser Evaporation: A Fundamental
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Organic Semiconductor Thin Films Deposited by Resonant Infrared Matrix-Assisted Pulsed Laser Evaporation: A Fundamental Study of the Emulsion Target Yuankai Liu1, 2，Ayomide Atewologun1, 2 and Adrienne D. Stiff-Roberts1, 2* 1
Department of Electrical and Computer Engineering, Duke University, Durham NC 27708,
U.S.A. 2
NSF Research Triangle Materials Research Science & Engineering Center, Duke University,
Durham, NC 27708, U.S.A. ABSTRACT Poly (3-hexylthiophene) (P3HT) thin films were deposited using emulsion-based, resonant infrared matrix-assisted pulsed laser evaporation (RIR-MAPLE) from emulsions containing different solvents and different alcohols, to investigate the impact of emulsion on film morphology. The atomic force microscopy (AFM) and grazing-incidence, wide angle x-ray scattering (GIWAXS) results show that surface morphology of RIR-MAPLE as-deposited films can be varied from rough to smooth and the microcrystalline domain orientations with respect to the substrate can be tuned from randomly oriented to preferentially oriented in the vertical direction. The demonstrated ability to tune the structural characteristics of polymer thin films by controlling the target emulsion is important for the application of organic optoelectronic devices deposited by RIR-MAPLE. INTRODUCTION Emulsion-based RIR-MAPLE is an enabling organic thin-film deposition technique for two primary reasons: i) the ability to control and tune many aspects of nanoscale morphology [1-4], and ii) the ability to deposit multi-layered films regardless of the constituent material solubility [5,6]. However, better understanding of RIR-MAPLE growth mechanisms is required in order for this deposition technique to become a mainstream approach for polymer-based optoelectronic devices and other organic-based thin films. Previous studies have investigated the impact of laser-target absorption depth [3], solvent quality [7], and laser fluence [8] on the surface roughness of MAPLE-deposited films, yet, a full accounting of the growth mechanisms related to the emulsion target is still incomplete. The RIR-MAPLE emulsion target typically comprises guest organic material, primary solvent, secondary solvent, deionized (DI) water, and surfactant. The purpose of the surfactant, which has been investigated previously, is to stabilize the emulsion for flash freezing. The purpose of the DI water is to enrich the hydroxyl (OH) bond concentration which is resonant with the laser energy to facilitate the deposition. However, water vapor degrades organic solar cells due to adsorption onto the Poly (3, 4-ethylenedioxythiophene) poly (styrenesulfonate) (PEDOT: PSS) [9] layer. It is not yet clear if water vapor resulting from
RIR-MAPLE has any direct impact on organic semiconductor films. Therefore, emulsion host matrix alternatives, such as alcohols, will be explored. The purpose of the low vapor pressure secondary solvent (e.g., phenol) is to provide additional OH bonds and to stabilize the target. The purpose of the primary solvent is to dissolve the guest mat
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