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Mechanical Pressure-induced Solid State Solvation in Organics Thin Films

• Authors: W. Chang, V. Bulovic
• Sponsorship: EFRC Center for Excitonics

Significant technological progress of organic semiconducting structures has led to their commercialization in the form of organic LED displays, solid state lighting, and thin film photovoltaics. A guest:host doping system is often employed, and dicyanomethylene (DCM) class dyes are used extensively as the dopant molecules in both fundamental studies and applications^[1],^[2]. The solvation effect describes the physical reorientation of surrounding solvent molecules due to a change in solute dipole moment during transition, affecting the excitonic energy states of the emitting solute dye molecules. This local dielectric effect has been extensively studied in liquid state^[3] and has been reported in molecular doping studies in solid state^[4],^[5]. In this work we demonstrate and numerically quantify a new method of probing dielectric-dependent excitonic-energy-shifts though application of pressure. Thin films of polystyrene (PS) and Alq₃ doped with laser dye DCM(II) showed bathochromic shift in photoluminescence (PL) under increasing pressure, as shown in Figure 1. Films of 5% doped PS:DCM(II) showed peak energy shifts up to 40 meV under pressure change of 0.4 GPa, which when fitted to solid state solvation theory (Figure 2)^[5],^[6], suggests a PS elastic modulus of 1.2±0.6 GPa. Similarly, films of 1% doped Alq3:DCM(II) under pressure showed peak energy shifts up to 50meV, suggesting an Alq₃ elastic modulus of 0.9±0.4 GPa. The results demonstrate the first numerically quantified solid-state solvation effect due to pressure. This ability to tune exciton energy using an external parameter creates a novel method to explore applications such as controlling exciton diffusion and optimizing optoelectronic device.



Figure 1: Spectral shift in PL in response to pressure applied on a thin film with pressure, indicated in legend, of: (a) .5% doping of DCM(II) in PS and (b) 1% doping of DCM(II) in Alq3. The black line indicates a typical Gaussian fit of the peak PL of PS:DCM(II) film under no pressure. The maximum of the peak wavelength as computed from fitting is indicated for each PL as a corresponding dotted line.



Figure 2: Fitting experimental results of PS;DCM(II) peak energy shift as function of changed molecular packing density or dielectric susceptibility. Red dashed line indicate the OLM theory of DCM(II) with initial dielectric constant of 2.44 (x0 = 1.44) in polystyrene, and blue points indicate the fitted peak wavelength from the pressure experiment using elastic modulus as a fitting parameter. The plot shows good agreement for a PS elastic modulus of 1.2 0.6GPa.




1. M. Swoboda, Organic Microcavity Laser Dynamics: Fundamentals and Experiments on a DCM:Alq3 Guest: Host-composite System: VDM Verlag, 2008. [↩]
2. Z. Guo, W. Zhu, and H. Tian, “Dicyanomethylene-4H-pyran chromophores for OLED emitters, logic gates and optical chemosensors,” Chemical communications (Cambridge, England), vol. 48, no. 49, pp. 6073–84, June 2012. [↩]
3. C. Reichardt, Solvents and Solvation Effects in Organic Chemistry: VCH, 1990. [↩]
4. V. Bulovic, R. Deshpande, M. E. Thompson, and S. R. Forrest, “Tuning the color emission of thin film molecular organic light emitting devices by the solid state solvation effect,” Chemical Physics Letters, no. 308, pp. 317–322, 1999. [↩]
5. C. F. Madigan and V. Bulović, “Solid state solvation in amorphous organic thin films,” Physical Review Letters, vol. 91, no. 24, p. 247403, 2003. [↩] [↩]
6. L. Onsager, “Electric Moments of Molecules in Liquids,” Journal of the American Chemical Society, vol. 58, pp. 1486–1493, 1936. [↩]