| Description |
We performed a systematic study of bipolar and unipolar diodes based on the π-conjugated polymer, 2-methoxy-5-(2'-ethylhexyloxy) (MEH-PPV), using electronic and magneto-transport measurements with magnetic field in the range 0 mT - 180 mT and admittance spectroscopy in the frequencies varying from 1 Hz to 10 MHz. 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 frequencies less than 10 Hz, is attributed to the trap-assisted monomolecular recombination. The second faster process is attributed to the electron-hole bimolecular recombination kinetics. When magnetic field of magnitude 30 mT is applied, T2 decreases by approximately 30 %. We observed that bipolar devices have strong divergent contribution to the device differential capacitance at low frequencies. It is positive at low biases voltages, turns negative at intermediate biases, and becomes positive again at stronger biases. In addition, by carefully selecting bias voltage, we were able to tune some bipolar diodes from the state with the negative capacitance to the state with the positive capacitance just by applying magnetic field. The magneto-conductance has a characteristic cutoff frequency that shifts to higher frequencies with increasing bias voltages. In particular, the magneto-conductance at 10 MHz in a bipolar device was measured to be 4.5 % in the magnetic field of magnitude 30 mT. For bipolar devices, the frequency-dependent response of the device admittance to the small magnetic field is identical to the response of the admittance to the small increase in the bias voltage in zero magnetic field. We found that the response of the admittance on the magnetic field is consistent with the polaron-polaron model of the organic magnetoresistance. The admittance of unipolar diodes did not reveal any magnetic field. |
