Determinant Factors of Light Enhancement Spectra Caused by Surface Plasmon Scattering in Organic Light Emitting Materials

Takahiro Inui College of Science and Engineering, Ritsumeikan University, Kusatsu, Shiga, Japan Yuta Mizoguchi College of Science and Engineering, Ritsumeikan University, Kusatsu, Shiga, Japan Takuya Yoneda College of Science and Engineering, Ritsumeikan University, Kusatsu, Shiga, Japan Hiroto Kawase College of Science and Engineering, Ritsumeikan University, Kusatsu, Shiga, Japan Kenichi Kasahara College of Science and Engineering, Ritsumeikan University, Kusatsu, Shiga, Japan Naoki Ikeda Nanotechnology Innovation Center, National Institute for Materials Science (NIMS), Tsukuba, Ibaraki, Japan Hirotaka Oosato Nanotechnology Innovation Center, National Institute for Materials Science (NIMS), Tsukuba, Ibaraki, Japan Yoshimasa Sugimoto Nanotechnology Innovation Center, National Institute for Materials Science (NIMS), Tsukuba, Ibaraki, Japan

For increasing the light extraction efficiency of light-emitting diodes, it is important to understand the mechanism of surface plasmon (SP) absorption and its scattering by nanometer-scale irregularities which are naturally formed on the surface of cathode metals [1]. It may have once been believed that the enhancement caused by the SP scattering grows with angular frequency, ω, as it approaches the plasma wavelength, λp, of a metal. Such a belief, however, isn’t the case [2], and the reason is not fully understood. To this end, we have investigated the enhancement spectra of MEH-PPV on Ag. The peak emission wavelength of MEH-PPV is considerably distant from the λp of Ag. To make a thin film, we precisely applied specific amounts of dilute p-xylene solutions of MEH-PPV (0.5 wt. %) using a pipette on circular apertures formed on an Ag/glass (λ/20) and glass substrate. After evaporating the solvent, we were able to prepare 20~80 nm MEH-PPV films. Peak enhancement wavelengths obtained from the PL spectra were around 500 nm, situated between the λp (375 nm) and original PL peak (590 nm), and the enhancement properties were never expressed by curves monotonously increasing towards the λp. The maximum enhancement was observed with a 60-nm film. The enhancement diminished after reaching 500 nm, yet lasted far beyond 700 nm. The observed results could be interpreted by the wavelength dependence of the density of state (DOS), trap states presumably created inside a part of MEH-PPV material adjacent to Ag, and the penetration depth of SPs. DOS is proportional to dkSP/dω (kSP: wavenumber of SP). However, trap states, when they exist near a metal, partly capture excitions generated in the organic layer, and block SP creation. In particular, when the original emission peak is far from the λp, an enhancement property deviating from a prediction based solely on DOS should be expected. Furthermore, one must also take the dependence of the penetration depth on wavelength into consideration; more SPs are created with increased wavelength.

References

[1] T.D. Neal et al., Opt. Exp. 13, 5522 (2005).

[2] S. Kawasaki et al., SPP6, Ottawa, Canada, Tu-28-P-57 (2013).

kasahara@se.ritsumei.ac.jp









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