Bipolar polaron pair recombination in P3HT/PCBM solar cells

The unique properties of organic semiconductors make them versatile base materials for many applications ranging from light emitting diodes to transistors. The low spin-orbit coupling typical for carbon-based materials and the resulting long spin lifetimes give rise to a large influence of the electron spin on charge transport which can be exploited in spintronic devices or to improve solar cell efficiencies. Magnetic resonance techniques are particularly helpful to elucidate the microscopic structure of paramagnetic states in semiconductors as well as the transport processes they are involved in. However, in organic devices the nature of the dominant spin-dependent processes is still subject to considerable debate. Using multi-frequency pulsed electrically detected magnetic resonance (pEDMR), we show that the spin-dependent response of P3HT/PCBM solar cells at low temperatures is governed by bipolar polaron pair recombination involving the positive and negative polarons in P3HT and PCBM, respectively, thus excluding a unipolar bipolaron formation as the main contribution to the spin-dependent charge transfer in this temperature regime. Moreover the polaron-polaron coupling strength and the recombination times of polaron pairs with parallel and antiparallel spins are determined. Our results demonstrate that the pEDMR pulse sequences recently developed for inorganic semiconductor devices can very successfully be transferred to the study of spin and charge transport in organic semiconductors, in particular when the different polarons can be distinguished spectrally

Disturbances of C-fibre-mediated sensibility in lumbosacral disc disease.

In nine patients with chronic lumbosacral disc disease and radicular symptoms clearly restricted to one leg, C-fibre-mediated sensibility was measured by determination of the thresholds for heat pain and warmth on the foot, ipsi- and contralaterally to the nerve root compression. The thresholds were compared with the values for 19 healthy subjects. In the patients the warmth threshold was increased in the ipsilateral dermatome and normal in the contralateral dermatome. In contrast, the heat pain threshold was near normal ipsilaterally but was clearly decreased contralaterally. These findings are discussed with respect to a possible pain sensitisation resulting from nerve root compression

Effects of chronic back pain on the perception of experimental heat pain

19 patients between 23 and 65 yr. of age whose chronic back pain was caused by lumbosacral disk disease and 19 healthy volunteers matched for age and sex were studied. Pain thresholds under phasic and tonic heat stimulation and thermal thresholds for warmth and cold were measured on the right hand. The patients rated any current back pain on a visual analog scale. There was a significant negative correlation between current back pain and the threshold for tonic pain, but there was no correlation between current back pain and either the threshold for phasic pain or temperature sensitivity. Hence, current back pain and experimental tonic pain seem to have an additive effect on pain perception because perceptual qualities are similar. Reduced somatosensory perception of chronic back pain patients could be demonstrated for temperature sensitivity and to a lesser degree for phasic pain, but as a consequence of the “opposing” effect of current back pain, not for tonic pain perception. </jats:p