RESUMO
We report the magnetic field dependence of the magneto-photocurrent (MPC) in organic light emitting diodes made of homo-polymer organic layers and compare it to the measured magneto-conductance (MC) in the same diodes. We find that the response MPC(B) is very different from MC(B) in at least two respects. (a) The low field (B < 50 mT) response of MPC(B) is narrower by a factor of â¼5 from that of MC(B). (b) At high fields (B > 4 T), MPC(B) has a stronger dependence on B, d(MPC)/dB â¼ 5d(MC)/dB. We attribute these differences to a unique feature of charge transfer excimers that are responsible for MPC: sub-ns fast fusion back to singlet excitons and slow (ns to µs) dissociation to free charges. In contrast, MC(B) is determined by long lived (>10 ns) polaron pairs having singlet and triplet dissociation rates of the same order.
RESUMO
We measured magnetoconductance (MC) response in a number of unipolar and bipolar organic diodes based on π-conjugated polymers and small molecules at fields |B|<100 mT and various bias voltages and temperatures. Similar to magneto-electroluminescence, the MC(B) response in bipolar diodes shows a sign reversal at ultrasmall |B|<1-2 mT due to interplay of hyperfine and Zeeman interactions in opposite-charge polaron pairs. Surprisingly, similar MC(B) response was also measured in unipolar devices, indicating the existence of like-charge polaron pairs, however, with a clear difference between the hyperfine interaction constants of electron polaron and hole polaron.
RESUMO
We describe a method for obtaining the polaron spin-lattice relaxation time T{SL} in pi-conjugated polymers by measuring the optically detected magnetic resonance (ODMR) dynamics as a function of microwave power and laser intensity. The peculiar ODMR dynamics is well described by a spin dependent recombination model where both recombination and spin relaxation rates determine together the response dynamics. We apply this method to the spin 1/2 ODMR in films of pristine 2-methoxy-5-(2{'}-ethylhexyloxy) phenylene vinylene [MEH-PPV] polymer, as well as MEH-PPV doped with various concentrations of radical impurities. We obtained T{SL} approximately 30 micros in pristine MEH-PPV, but substantially shorter when the magnetic impurities are added.
RESUMO
A silver-halide infrared optical fiber is used in a double-beam spectrometer, demonstrating the ability to guide small infrared signals efficiently. We show that a fractional transmission change of less than 1% is easily measured. Use of the fiber may obviate the necessity for the unfriendly optics currently used in spectrometry systems to probe remote sample zones. As a specific example, we were able to guide with high efficiency an infrared signal to and from a sample that was mounted in a cryostat. Using this setup, we measured the photoinduced absorption due to the e1 ? e2 intersubband transition in a GaAs-GaAlAs multiple quantum-well structure.