ABSTRACT
Electron transport in semiconducting polymers is usually inferior to hole transport, which is ascribed to charge trapping on isolated defect sites situated within the energy bandgap. However, a general understanding of the origin of these omnipresent charge traps, as well as their energetic position, distribution and concentration, is lacking. Here we investigate electron transport in a wide range of semiconducting polymers by current-voltage measurements of single-carrier devices. We observe for this materials class that electron transport is limited by traps that exhibit a gaussian energy distribution in the bandgap. Remarkably, the electron-trap distribution is identical for all polymers considered: the number of traps amounts to 3 × 10(23) traps per m(3) centred at an energy of ~3.6 eV below the vacuum level, with a typical distribution width of ~0.1 eV. This indicates that the electron traps have a common origin that, we suggest, is most likely related to hydrated oxygen complexes. A consequence of this finding is that the trap-limited electron current can be predicted for any polymer.
ABSTRACT
The trap-assisted recombination of electrons and holes in organic semiconductors is investigated. The extracted capture coefficients of the trap-assisted recombination process are thermally activated with an identical activation energy as measured for the hole mobility µ(p). We demonstrate that the rate limiting step for this mechanism is the diffusion of free holes towards trapped electrons in their mutual Coulomb field, with the capture coefficient given by (q/ε)µ(p). As a result, both the bimolecular and trap-assisted recombination processes in organic semiconductors are governed by the charge carrier mobilities, allowing predictive modeling of organic light-emitting diodes.
ABSTRACT
Aim of the investigation was to study the frequency and prognosis of patellar chondropathy in general practice. As far as the frequency was concerned a secondary analysis was done of data from the Nijmegen Continuous Morbidity Registration (4 general practices, 12,000 patients). The prognosis was studied in patients in these practices with at least one episode of patellar chondropathy in the years 1985-1988. The GPs were asked about the diagnostic criteria they used. The registration showed an average of 6 new cases of patellar chondropathy per 1000 patient-years; in 7% of the cases the GPs referred a patient for specialist care. To study the prognosis patients were asked to fill in a questionnaire. Response was 84%, 178 questionnaires were analysed. The female-male ratio of the respondents was 2, 60% were younger than 60 years. In 44% of the cases the complaints subsided within 6 months. Interventions such as X-ray and physiotherapy depended on severity, duration and frequency of the knee complaints.
