ARTICLE pubs.acs.org/JPCC
Optical Spectroscopy of Trap States in Amorphous Perylene Derivative Films Harald Graaf,* Frank Friedriszik,‡ Christian Wagner, and Christian von Borczyskowski Center of Nanostructured Materials and Analytics (NanoMA), Chemnitz University of Technology, 09107 Chemnitz, Germany
bS Supporting Information ABSTRACT: Emission spectra of amorphous thin films of chlorinated N,N0 dimethyl-perylene-tetra-carboxylic-acid-diimide (Cl4MePTCDI) are characterized by two features: A small monomer emission and a further dominating and broad red-shifted peak. Temperature-dependent fluorescence and lifetime studies of the different excited states were carried out and led to the conclusion that the new state responsible for the red-shifted emission is populated via the excited monomeric state and vice versa. This new state can be understood as a trap caused by irregularities of the material, which can influence, for example, the twisting of the perylene core and therefore the energetics of the molecule.
’ INTRODUCTION Perylene derivates are typical n-type organic semiconductors that have also been investigated in detail by optical methods.1 Special interest has been drawn on the perylene-tetra-carboxylicacid-dianhydrid (PTCDA) and the N,N0 -dimethyl-perylene-tetracarboxylic-acid-diimide (MePTCDI). They are known to form defined crystalline structures.2,3 The optical properties of these materials in thin films were investigated in detail.4,5 Theoretical calculations have shown that the absorption spectrum of MePTCDI is dominated by the mixing between Frenkel states and Charge Transfer states having a large influence on the line shape of the optical response.6,7 Such excitonic states can also be detected in the luminescence of crystalline perylene derivatives. Time-resolved measurements of PTCDA indicated the existence of different excitons beside several excimer states.8 The excitons, partially characterized as self-trapped or Frenkel-type ones, are always determined by a lower energy compared to the monomer.9 Two different excimers (E and Y) have been found in highly crystalline materials like perylene,10,11 MePTCDI12 and recently also for ordered films of the 1,6,7,12-tetra-chloro derivative of the MePTCDI (Cl4MePTCDI)13 at low temperatures. The Cl4MePTCDI is known to show smaller intermolecular interactions due to the twist of the aromatic core14 caused by the chlorination in the bay position.15 Generally, the substitution of the aromatic system in perylene derivatives is a well-known procedure to improve different properties. In crystalline systems of tetrachlorinated perylene derivatives, for example high charge carrier r 2011 American Chemical Society
mobilities up to 0.28 cm2V1s1 have been reported.16 The twisted aromatic chromophores enable additionally close contacts of the core in 2D within the crystal, which is different from the 1D columns found for planar perylene systems.17 The chlorination in the bay position leads also to a dramatic increase in charge carrier lifetime as it has been shown for liquid crystalline perylene derivatives.18 Generally, bay-substituted perylene molecules are also useful as optical probes for various systems.1921 However, the chlorinated derivative chosen here as a model substance is known to form amorphous films when prepared on substrates at room temperature. These films show a high fluorescence yield14 but a rather small electron mobility, which is about 2 orders of magnitude smaller compared to the highly ordered unchlorinated derivative.22 As known from previous investigations, a further weak and red-shifted emission for the amorphous phase can be seen for thicker films (100 nm).13 Within this contribution, we use different optical techniques to investigate and understand the nature of this red-shifted emission and discuss the results in relation to previous obtained electrical data.
’ EXPERIMENTAL SECTION Cl4MePTCDI (for chemical structure, Figure 1) was synthesized as reported earlier15 and purified by column chromatography Received: December 21, 2010 Revised: March 19, 2011 Published: April 01, 2011 8150
dx.doi.org/10.1021/jp1121323 | J. Phys. Chem. C 2011, 115, 8150–8154
The Journal of Physical Chemistry C
ARTICLE
Figure 1. Geometry-optimized structure of Cl4MePTCDI. The different atoms are marked by their atom symbol, gray balls are carbon atoms.
and three-zone-sublimation. Samples were prepared on glass substrates (cleaned by acetone and ethanol) by physical vapor deposition. A constant film thickness of all samples of each batch (5 substrates) was realized by a rotating sample holder. The evaporation rate was about 0.5 to 0.6 nm per minute at a base pressure of 104 Pa within the chamber. For each batch, the film thickness was adjusted to (95 ( 10) nm and checked by ellipsometry. The samples were investigated in absorption (Varian Cary 100), emission and excitation (Varian Cary Eclipse Spectrophotometer), temperature-dependent luminescence (home-build laser scanning confocal microscope described elsewhere),23 and the time-resolved photoluminescence (excitation with a 465 nm Laser diode from PicoQuant (PDL 800-D with LDHP-C-470) and detection by time correlated single photon counting with a time resolution of about 50 ps (Hamamatsu R3809U-5X-Series)).
’ RESULTS Figure 2 shows the normalized absorption, emission and excitation spectra of a typical 100 nm thick film of Cl4MePTCDI. The absorption spectrum is very close to that reported earlier for pristine amorphous films of this material (for comparison, Figure S1 of the Supporting Information),13 with main absorptions at 533 nm (2.326 eV) and 493 nm (2.515 eV, energy separation of (189 ( 7) meV). These absorptions correspond to the S0 f S1 transition and the related vibronic satellite. The smaller absorption peak at 429 nm (2.890 eV) can be attributed to the S0 f S2 transition.15 The shift of the two main absorption peaks of 60 to 65 meV to lower energies compared to the spectrum in solution can be explained by changes in the dielectric environment (solvent molecules in the solution and other molecules in the film).13 It should be noted that the peak positions did not change for thinner amorphous films. Nevertheless, the intensity relation between the main absorptions changed slightly (the emission of the vibronic satellite increases in its intensity relative to the main absorption), which is explained by a saturation due to the large absorbance.13 The emission spectrum differs from the one reported earlier.13 The spectrum is dominated by an emission maximum at 644 nm (1.925 eV) accompanied by two shoulders, a blue-shifted one at 575 nm (2.156 eV) with a related energy difference of (230 ( 15) meV and a red-shifted one at 704 nm (1.761 eV, energy difference of (165 ( 15) meV). This red-shifted line probably relates to the vibronic sideband as the energetic shift is close to the energetic difference of 188 meV between the two main absorption peaks. As these two absorption peaks are the S0 f S1 transition with its vibronic satellite, it can be concluded that also
Figure 2. Normalized absorption (blue stars), excitation (red triangles, detection at 645 nm) and luminescence (black squares) spectra of a pristine Cl4MePTCDI film on a glass substrate. The corresponding solution spectra (in chlorobenzene and normalized to 0.8) are given by dashed lines.
the peak at 704 nm in the emission can be attributed to the vibronic satellite of the 645 nm peak. However, comparing the emission spectrum with those reported recently13 the energetic positions of the emission maxima are identical but their intensity relation differs significantly. The blue-shifted emission at 575 nm can therefore be attributed to the known direct emission from the monomer, which was found to dominate the spectra of thin and thicker films reported earlier.13,14 It is further reasonable to assume that also the emission at 575 nm will have a vibronic satellite at around 625 nm, which cannot be seen due to its intensity is to small compared to the dominant 644 nm peak. In the excitation spectrum, the same peaks as in the absorption spectrum are found. An additional slightly red-shifted excitation at around (545 ( 2) nm ((2.27 ( 0.04) eV) was detected. This is a clear hint for an additional state. The concentration of this state must be small (
