Bimolecular recombination in regiorandom poly(3-hexylthiophene)

Using cw and transient photoinduced absorption (PA) techniques we study the photoexcitation dynamics in the ms time regime, in thin films of regiorandom poly(3-hexylthiophene). The PA spectrum shows a high energy band (HE) peaked at 1.46 eV and a low energy band (LE) at 0.5 eV. The PA dependence on pump intensity and modulation frequency is modeled by analytically solving the full bimolecular rate equation using harmonic analysis. As an important result of these calculations we found that in the steady-state limit (i.e., omegatau(B) < 1), the out-of-phase PA signal saturates and becomes independent of pump intensity. This gives rise to a new possibility of directly estimating the bimolecular recombination coefficient beta from the saturation value of the out-of-phase PA signal at high pump intensities. The transient PA decay. of the HE band shows two components having different relaxation times. The PA decay of the HE band is mainly dominated by the fast component, which is attributed to a bimolecular recombination process in good agreement with the result from the cw measurements, together yielding a bimolecular recombination constant beta = 1.6 +/- 1.4 x 10(-14) cm(3)/s. (C) 2002 Elsevier Science B.V. All rights reserved.