Effects of trap sites on magnetic-field dependent electric conductance and recombination of carriers photogenerated in a dye-doped organic semiconductor film device

Photogenerated radical ion pairs are important intermediates that govern the optoelectronic and magnetic responses of organic materials. We have succeeded in observing a singlet-born radical pair with a microsecond-order lifetime in a photoelectric device comprising a perylenediimide-doped poly(N-vinylcarbazole) film at room temperature. The simultaneously detected transient optical absorption signal due to the electron and photocurrent signal due to the hole are enhanced by an applied magnetic field of 250 mT, which is explained by the quantum spin dynamics of the radical pair. Detailed analysis of the magnetic field effect and its temperature dependence has revealed that the photogenerated holes that comprise the radical pair are captured in either a shallow trap site of a few meV depth or a deep trap site of 100-200 meV depth, from which detrapping-limited (∼100 ns) and tunneling recombination (a few microseconds) occur, respectively. In contrast to the nearly temperature-independent recombination dynamics, the hole drift mobility exhibits large activation energies of over 100 meV because the hole transport in the micrometer range is limited by detrapping from the deep trap site. The present study demonstrates the importance of trap sites in disordered materials, which can be utilized as reservoirs for long-lived radical pairs.