Optical Properties of Thin Polycrystalline Films

In contrast to the low resolution found for the absorption spectra within most of the literature, Fichou et al. [145] succeeded in a much better resolution of the vibronic structure for thin T films. From the spectra it became quite clear that the electronic origin of the optical transition for 6T films must be located lower than 18 500cm (Figs 11 and 14). The spectral features for different oligomers (n = 4-6) were foimd to be very similar as expected from the spectra in matrix-isolation [76], although the vibronic fine structures could not be fully assigned [145] at that time. 

For the very unusual bandshape of the lowest transition band (Fig. 11) different interpretations were offered. By applying the excitonic molecule model [146] (a two-dimensional dipole picture), Oelkrug and his coworkers pointed first to an excitonic band structure [64, 67] with a width of 10000cm where the ft = 0 states correspond to the highest energy levels within the excitonic band, whereas the bottom of the band is located at the zone boundary with ft 5 0. The apparent blue-shift in the spectra would then correspond to the width of the excitonic band. A very similar interpretation was published later by Kanemitsu et al. [147], again neglecting the 

In addition to the absorption, PL spectra were also analysed to disentangle the nature of the lowest excited state. The general luminescence properties of thin T6 films were quite poor and a typical example is shown in Fig. 12 [149, 150]. By going from solution to thin films the PL quantum yields decreases by three orders of magnitude [84] and apparently broad emission lines dominate the spectra. Even at very low temperatures the resolution of the optical spectra is rather poor (several 100 cm ) and spectroscopic details are smeared out. In most cases a considerable red-shift between the absorption and PL onsets and multiple different PL-components could be found within the spectra (Fig.l2) [149, 151]. The main radiative decay channels were attributed to deep trap levels [149] or aggregates [8], which are strongly depending on the preparation conditions and film thicknesses (Fig. 12) [149]. By site selective PL-spectroscopy the positions of at least three trap levels could be located which are up to 2 000 cm lower than the absorption onset 

F1gm 12. PI and absorption spectra of 6T films on glass substrates at T lOK. The lowest absorption band is located around 19000cm. The PL in thick films ( 100 nm) is mainly based on a deep trap level (labeled which is located at 173S0cm [11. (b) With decreasing film thickness a second component (arrows, bo located at 17 900cm ) increases in the relative intensity with respect to component a. Both components are redshifted with respect to the absorption onset and show the same vibronic progression [149]. 

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