(Digital Presentation) Angle-Resolved Photoluminescence Measurements of Rubrene Single Crystals to Study the Exciton Energy Dispersion
Authors: Kobayashi Shunki , Nakayama Yasuo , Hosokai Takuya 
Published: July 2022

Angle-resolved spectroscopic measurements provide information on the electronic structure of single crystals in reciprocal space. So far the single crystal of rubrene (RubSC), a representative of p-type organic semiconductor, has been investigated by using photoelectron spectroscopies and the highest occupied molecular orbital band energy dispersion of ~ 0.4 eV was characterized [1]. The degree of the dispersion was depended on the crystal axes [2]. The formation of energy band dispersion means that the hole carrier can be delocalized in certain crystal axes. An anisotropic hole carrier mobility is anticipated for RubSC and is indeed demonstrated in organic filed-effect transistors [3]. Considering the anisotropy of electronic structure and hole delocalization, it is questioned whether excitons can be also delocalized in RubSC. Delocalization of molecular excitons is relevant for organic devices, such as organic light-emitting diodes and organic photovoltaic cells, and is of fundamental interest. To investigate the possibility of such an exciton delocalization, we investigated angle-resolved photoluminescence (AR-PL) measurement RubSC using a newly built AR-PL apparatus.
RubSC samples were prepared by using a physical vapor transport technique in a purified nitrogen stream, where “sublimed grade” (99.99% purity) source materials purchased from Sigma-Aldrich were used as received. AR-PL spectra were measured by a home-made apparatus consisting of a picosecond pulsed laser light source and some optical elements bought by Thorlabs, Inc. The excitation wavelength was 532 nm (the second harmonics of Nd:YAG laser; Ekspla, PL-2211) and the laser power was 4.9 mW with a (Gaussian) beam diameter of ~ 2 mm and a repetition rate of 1 kHz. The p-polarized light was prepared by using a half-wave plate and a beam splitter. The p-polarized light was focused on the rubrene single crystals using a convex lens (f=600 mm) at an incident angle of 34 degree (°) relative to the surface [(100) plane] normal of the RubSC. The emitted PL spectra were recorded by using a photonic multichannel spectrometer (PMA-12, Hamamatsu) connecting an optical bundled fiber (light-receiving area Φ 1mm), which was placed at a PL emission angle of 70° with a distance of 5.9 cm between RubSC and the fiber edge. To measure AR-PL spectra, the fiber mount was rotated step-by-step at a rotation center, which is a surface of rubrene single crystal. The resolution of the detected angle was estimated to be ±1degree. The alignment of the apparatus, namely, setting the in-plane and out-of-plane rotation center of the system, was done by using a line laser maker and digital cameras, which were used to check the light irradiation position on the crystals. All the measurements were done at 295 K in air.
Figure 1(a) shows the results of AR-PL spectra of RubSC measured at an azimuthal angle (α, see in Fig.1(a)) of 200 °and 290°. For two spectra, the strongest peak was found at around 655 nm and the second strongest one is at around 705 nm. These peaks are consistent with those reported ones of RubSc; the largest one is the main PL band, and the second one is its vibronic progression. Note that photo-oxidization of the RubSC can possibly occur at the surface of the SC and, if in that case, its PL band would be seen at around 643 nm. However, our spectra do not show such a band and there was almost no irradiation time dependence, indicating the detected PL originates mostly from the bulk region of the RubSC. At the different α conditions, the main band shifted apparently, while the second one stayed unchanged. To more clarify the angle dependency, the second derivatives of all the data set of the AR-PL spectra were calculated and the results are shown in Fig. 1(b) as an intensity color map. It is clear that the first band shifts zigzag by every 90±10 °, while the second largest one is unchanged. The crystal structure of our RubSC grown by our way is orthorhombic and the angle between a and b axes of the RubSC is 90°. The periodicity of the turning point of the first band, i.e. 90°, is coincidently matched with the ab angle of the RubSC. This implies that the zigzag shift of the first PL band is related to the crystal structure of RubSC, whispering the detection of an energy dispersion of excitons of RubSC.
Ref.[1] S. Machida, et al., Phys. Rev. Lett. 104, 156401 (2010). [2] Y. Nakayama, et al., Appl. Phys. Express 5,111601 (2012). [3] C. Reese and Z. Bao, Adv. Mater. 19, 4535 (2007).

Figure 1