Polarons inπ-conjugated semiconductors: absorption spectroscopy and spin-dependent recombination

We have studied the properties of polaron charge carriers in a large variety of π-conjugated polymers and oligomers. We first study the optical absorption spectrum of polarons and find that the transition photon energy of the so-called low-energy transition obeys a universal relationship depending only on the oligomer-length in a wide class of oligomer solutions and films. This observation provides us with a sensitive method for measuring the conjugation-length of polymer films. We then study polaron recombination, which is the most fundamental process occurring in organic electroluminescent devices. In fact, the spin-statistics of the polaron recombination step determines the maximum achievable electroluminescent efficiency. We summarize our recent experimental results where we found strong evidence that polaron recombination, and therefore exciton formation, is spin-dependent. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)