Excitonic Photoluminescence in Pentacene Single Crystal

Abstract

Photoluminescence of pentacene single crystals is studied in the temperature range of 7 K to 200 K under excitation with He-Ne laser light. Photoluminescence spectra consist of four broad bands, L1 to L4. The highest energy band, L1, located close to the lowest exciton absorption band mainly appears for //b-polarization. The intensity of the second one, L2, with Stokes-shift of about 1500 cm -1, decreases as temperature rises above 30 K and disappears at 100 K. The bands, L3 and L4, which are located at lower energy, are observed at higher temperatures up to 200 K. Based on their energy positions, the band L2 is assigned to a shallow self-trapped exciton luminescence band, and the bands L3 and L4 to deep self-trapped exciton luminescence bands. By comparing this result with reported result on tetracene crystals, self-trapped excitons are considered to be more stable in pentacene.

Photoluminescence spectroscopy of pure pentacene, perfluoropentacene, and mixed thin films

“…In good agreement with Ref. 7, we observe a maximum for bands A 3 and A 4 at approximately 50 K. Both peaks grow in parallel exhibiting a constant intensity ratio. Another band arises at 1.66 eV (A 2 ) for temperatures below 100 K. Below 200 K we observe the bands A 5 (2.14 eV) and A 6 (2.29 eV).…”

“…A 1 is known in the literature as the free exciton band 7,8 , originating from the S 1 → S 0 transition. Its position varies between 1.76 eV in thicker thin films 9 and 1.88 eV in monolayers with a parabolically increasing intensity for small film thicknesses 22 .…”

“…[1][2][3][4][5] The coupling between these compounds including the energy level alignment and possible charge transfer (CT) states are not well understood, but they are crucially important, since they determine to a large extent the resulting electronic and optical properties including absorption and emission cross sections. One of the fundamental properties is photoluminescence (PL), which is also an important way of testing the interactions and the effective energy levels [6][7][8][9] . In order to maximize the potential for coupling effects, good intermixing of the respective two compounds is desirable, for which structural compatibility is beneficial.…”

Franck-Condon processes in pentacene monolayers revealed in resonance Raman scattering

“…The comparison between the fits reveals that the DY state prompts a weak Raman resonance that interferes with the two strong FE resonances by slightly distorting the resonance profiles around its energy position. The observed quantum coherence of the FE and DY doublet states, which correspond to the different polarizations along the a and b axes in the monolayer structures, 11,12,17 indicates that the orientation of pentacene molecules in the a-b plane is non-uniform. Table I summarizes the values of adjustable parameters obtained from fitting the RRS enhancement profiles.…”

“…4,11,12 This unexpected effect reveals that within the short exciton lifetime there is quantum coherence of the two states. [13][14][15][16] For bulk pentacene the Davydov doublet corresponds to exciton bands with polarizations b and ⊥ b axis 17 (the a and b axes form an angle γ = 85 o ). In pentacene monolayers the a and b axes are almost perpendicular (γ = 90 ± 0.2 o ).…”