Photogeneration of polarons in sexithiophene oligomers

Summary form only given. We have studied films of neutral 6T in order to compare the electronic structure of its photoexcited state with that of the radical cations and dications, on one hand, and with the photogenerated defects observed for the neutral polythiophene. Using the photoinduced absorption spectroscopy, we have found two strong absorption bands very similar in energy (0.80 and 1.54 eV) and shape (both have distinct vibronic shoulders) to those found for the radical cation 6T/sup middot/+/. In addition, there is a very weak separate band (at 1.1 eV) close in energy to the position of the dication related band (1.2 eV). Photoinduced infrared active vibrations were also observed, confirming that these defects are charged. Good agreement with the calculated transitions of the oligothiophene radical cations is obtained. We conclude from our measurements that in the time domain of our steady state measurements (I msec), most of the photogenerated defects are polarons (radical cations). We further conclude that, at least in the 6T oligomer, the characteristic absorption of the polaron contains two strong transitions, whereas that of bipolaron (dication) contains only a single band.