Abstract
The n-type organic semiconductor, β-phase single crystalline dichloro naphthalene diimide, Cl2–NDI, is investigated in a broad frequency range via optical spectroscopy. The temperature-dependent absorbance spectra reveal the appearance of new molecular vibration modes; several of them exhibit a pronounced splitting and abnormal red shift. In the visible spectral range, we observe a splitting of the J-band upon cooling, which we attributed to self-trapped excitons via coupling between free excitons and molecular vibrations or trapping by disorder induced Lévy states. Additionally, we discover a strong birefringence of these crystals by pronounced polarization beat over a wide frequency range.
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