Crystalline organic semiconductors constitute the new frontier in the field of organic light-emitting materials. These materials are characterized by a reduced amount of structural defects, compared to the amorphous polymer thin films commonly employed in optoelectronic applications, that determines, for instance, high charge transport mobility. In collaboration with the University of Milan-Bicocca (Department of Materials Science), we are carrying out a detailed characterization of the emission properties of different classes of organic single crystals, including acridines, tetracene, rubrene, and quaterthiophene (4T) and exploring the possibility of using these materials as active layers in organic-based laser devices.
In particular, fluorinated molecules are largely studied as they constitute good candidates to obtain n-type semiconductors, which are required for the implementation of organic materials in commercial devices. Among the possible candidates, we focused our attention on tetrafluoro-acridines, particularly the 1,2,3,4–tetrafluoro–7–(N,N)dimethyl–amino–acridine (DMAA). We have studied the photoluminescence (PL) properties of these organic crystals, by analyzing the emission spectra as a function of temperature [1]. In Figure 1 the PL spectra of DMAA crystals are displayed as a function of the sample temperature, showing the presence of a broad emission band. This band is attributed to the self-trapped (ST) exciton originating from the lowest molecular state. Indeed, after absorption the strong coupling of the exciton to the lattice is expected to cause lattice distortion, and a ST state is formed at lower energy than the corresponding free (F) exciton. Moreover, a high energy tail is visible in Fig. 1 upon increasing the temperature, attributable to the emission from the F state observed if the temperature is high enough for the system to overcome the potential barrier separating F and ST states. The emission properties are consistent with the absorption data measured as a function of temperature, that allows to determine the origin of the emission form DMAA crystals [1].
    Another widely studied molecule is tetracene. This molecule has the first molecular electronic transition polarized along the short molecular M axis and split in the crystal into two transitions, whose directions of polarization are usually assumed parallel and perpendicular to the shorter unit cell a axis in the ab crystal plane (Fig. 2a). Polarized photoluminescence spectra (Fig 2b) show that the emission is mainly polarized along the a crystal axis and occur mainly along the normal to the crystal surface (Fig. 2c).

 

 

 

 

We are currently studying the superradiance properties of tetracene crystals, that should be at the origin of the observed (0-0) intensity increase for temperatures below 50 K (Fig. 2d), a result that we are interpreting taking into account optical anisotropy and exciton-phonon coupling.
Rubrene is a derivative of tetracene with phenyl substituents attached to the side of the tetracene backbone. It has recently been gaining much attention since it exhibits interesting physical properties, such as one of the highest reported electronic mobility at room temperature (up to 20 cm2V-1s-1). We studied the emission properties of rubrene single crystals (Fig. 3a) observing emission mainly from the crystal edge, which we proved to be c-polarized and waveguided inside the crystal, and which is particularly interesting for the design of devices based on rubrene crystals (Fig. 3b-d). In agreement with the experimental results, a detailed modelling study of the light propagation inside the crystal also predicts self-waveguiding of the emitted radiation, on the basis of the dielectric tensor deduced by generalized ellipsometry measurements [2-3].
Finally, we have studied the emission properties of 4T single crystals in the visible range under both cw and pulsed laser excitation for temperatures in the range 15-300 K. Fluorescence microscope images (Fig. 4a) show that the material is characterized by self-waveguiding of the emitted light. This phenomenon could be fully described by considering the propagation of emitted photons inside the crystal, taking into account the polarization of the corresponding excitonic transition and the material dielectric tensor [4]. Under pulsed UV excitation, 4T crystals exhibit a line-narrowing of the emission due to ASE: a peak at 2.25 eV (FWHM 30 meV) becomes more and more intense upon increasing the pumping fluence from 0.5 mJ/cm2 to 3 mJ/cm2 (Fig. 4b). The ASE is strongly influenced by the angle of incidence, being maximized around 30° due to the arrangement of the 4T molecules inside the crystal. The ASE emission and the internal losses due to self absorption have been fully characterized and interpreted on the basis of the involved excitonic transitions. In conclusion, our analysis opens the way for the employment of these organic crystals as active media of laser devices.
 

 

For more information, please contact: Dr. Andrea Camposeo ( This e-mail address is being protected from spam bots, you need JavaScript enabled to view it )



Publications:
[1] S. Tavazzi,  L. Miozzo, A. Papagni, L. Raimondo,  L. Silvestri, P. Spearman, A. Camposeo, M. Polo, D. Pisignano, J. Chem. Phys. 126, Art. No. 234501 (2007).

[2] S. Tavazzi, A. Borghesi, A. Papagni, P. Spearman, L. Silvestri, A. Yassar, A. Camposeo, M. Polo, D. Pisignano, Phys. Rev. B 75, Art. No. 245416 (2007).
 
[3] S. Tavazzi, L. Silvestri, M. Campione,  A. Borghesi, A. Papagni, P. Spearman, , A. Yassar, A. Camposeo, D. Pisignano, J. Appl. Phys. 102, Art. No. 023107 (2007).

[4] S. Tavazzi, P. Spearman, L. Silvestri, L. Raimondo, A. Camposeo, D. Pisignano, Organic Electronics 7, 561-567 (2006).