Device spectroscopy of magnetic field effects in a polyfluorene organic light-emitting diode

Abstract

We perform charge-induced absorption and electroluminescence spectroscopy in a polyfluorene organic magnetoresistive device. Our experiments allow us to measure the singlet exciton, triplet exciton and polaron densities in a live device under an applied magnetic field, and to distinguish between three different models that were proposed to explain organic magnetoresistance. These models are based on different spin-dependent interactions, namely exciton formation, triplet excitonpolaron quenching and bipolaron formation. We show that the singlet exciton, triplet exciton and polaron densities and conductivity all increase with increasing magnetic field. Our data are inconsistent with the exciton formation and triplet-exciton polaron quenching models.

Magnetic field effects on the conductivity of organic bipolar and unipolar devices at room temperature

“…Thus, in the hole-only device U should be higher than in electron-only devices, meaning that probably we have not reached V high enough to see the transition from positive to negative MR in the hole-only device. Our findings in the electron-only device are in good agreement with the results by Nguyen et al [14]. In the case of hole-only devices Wang et al [15] performed similar measurements in MEH-PPV based devices.…”

“…A decrease in the MR may indicate that rubrene has a higher U than Alq 3 , and thus decrease the population of positively charged bipolarons quenching the MFE. In Alq 3 -based OLEDs, positively charged polarons, should dominate the MFEs in the conductivity [14].…”

Spectroscopic evidence for trap-dominated magnetic field effects in organic semiconductors

“…Independent of the exact mechanism, a decrease of the T formation should be accompanied by an increase of the S formation and vice versa. However, all previous attempts to measure these S and T exciton densities suggest that both increase upon application of a magnetic field [19,20], which is in direct conflict with the model.…”

Coherent Spin Manipulation in Molecular Semiconductors: Getting a Handle on Organic Spintronics

“…[142] However, direct measurements of the exciton population clearly reveal that magnetic field effects in the current and electroluminescence do not appear with changes in the singlet/triplet exciton ratio. [12,155,156] Even at intermediate to large fields, orders of magnitude larger than local hyperfine fields, the effect of the hyperfine fields on spin-dependent transport is still critically important. [11] …”