On the role played by polaron pairs in photophysical processes in semiconducting polymers Original Research Article

In this contribution, we consider the main manifestations of polaron pairs in: (i) Generation of the free charge carriers, (ii) delayed luminescence; (iii) intra-chain triplet exciton generation; (iv) recombination of free charge carriers; (v) persistence of the photoconductivity; (vi) displacement current in the transient photoconductivity experiments; (vii) spin-dependent reactions. New results on the photoluminescence of films of poly( p-phenylene vinylene) (PPV), and changes of its intensity under conditions of electron spin resonance as a function of temperature and light intensity are described. Models describing the connection between the resonant transitions and changes of experimentally measured parameters are discussed. These models are: The fluorescence quenching, ground state repopulation, free-polaron fusion, triplet exciton annihilation. Experimental results permitted to conclude that Coulomb bound polaron pairs are produced in PPV under photoexcitation. Microwave induced resonant transitions in triplet polaron pairs change the rate of formation of triplet intra-chain excitons at geminate recombination of these pairs. Those excitons annihilate in the second order reaction and show themselves in the intensity of the photoluminescence. The mechanism suggested was checked on PPV samples converted from the precursor polymer at different temperatures. Results showed that energy levels of the lowest polaron pair state in PPV, situated below that of singlet intra-chain exciton can act as a sink for the excitation energy influencing the quantum yields of the photoluminescence, electroluminescence and photoconductivity.