Admittance spectroscopy study of polymer diodes in small magnetic fields

We performed a systematic study of bipolar and unipolar organic diodes based on the p-conjugated polymer, 2-methoxy-5-(20-ethylhexyloxy) (MEH-PPV), using electronic and magneto-transport measurements with magnetic field in the range 0-180mT and admittance spectroscopy in the frequency range 1 Hz<f<10MHz. The admittance spectra of bipolar devices reveal two relaxation processes with distinct time scales that are influenced by the magnetic field. The slower process, which dominates the device capacitance at f<10 Hz, is attributed to the trap-assisted monomolecular recombination. The faster process is attributed to the electron-hole bimolecular recombination kinetics. We found that the time scale of this process, s2, decreases exponentially with the bias voltage. Application of magnetic field, B¼30mT decreases s2 by ~30%. We also found that the magneto-conductance, DGðx; BÞ=Gðx; 0Þ, has a characteristic cutoff frequency that shifts to higher frequencies with increasing bias voltage. In particular, the magnetoconductance at 10MHz in a bipolar device was measured to be 4.5% at B¼30mT. For bipolar devices we found that the frequency-dependent response of the device admittance to the small magnetic field is identical to the response of the admittance to a small increase in the bias voltage in zero magnetic field. We also found that the admittance of unipolar diodes does not depend on magnetic field.