Spin processes and charge dynamics in organic light emitting diodes

In this thesis work we explore spin dependent processes and charge dynamics in α-NPD/Alq₃ based LEDs. Studies on polymer light emitting diodes based on PFO, which are fabricated by us, are also reported. Our objective is to shed light on the fundamental mechanisms that cause large magnetic field effects in these devices. First, we explore charge dynamics using both steady state and time resolved charge modulation spectroscopy. These techniques are able to monitor variations in the charge density by measuring the absorption induced by an external bias voltage. In spite of the presence of several bands in our charge modulation spectra, no effect of the magnetic field is detected. This is in contrast with the large magneto-electroluminescence we observe in our samples. This discrepancy can be explained by assigning these spectral features to free polarons and not to singlet and triplet excitons. These absorption bands are of strong interest in the field of OLED charge dynamics. In fact, to the best of our knowledge, previous works on charge modulation spectroscopy reported only on polaron absorption in the hole transport layer. In our work instead, there is evidence of bands that can be ascribed to polarons in the emission band. This discovery is important from a technical point of view since it proves that a more complete description of charge dynamics is possible using modulation spectroscopy. Moreover, by way of time resolved measurements, we show that a direct exploration of the absorption time evolution is technically possible. In the second part of the thesis we investigate spin processes using pulsed and continuous wave electroluminescence detected magnetic resonance (pELDMR and cwELDMR) and continuous wave electrically detected magnetic resonance (cwEDMR). As a first result, we confirm that the cwEDMR spectrumis composed by at least two bands, as it was previously hypothesized in the literature. However, this is not the same when we look at the cwELDMR spectrum. Here, the linewidth of the resonance peak is reduced and fits with one of the bands composing the cwEDMR spectrum. This observation suggests that our optical approach can selectively detect one part of the overall phenomenology detected by cwEDMR. Interestingly, we do not observe any dependence of the pELDMR temporal evolution on the bias voltage. This behavior cannot be explained by models that predict an effect of magnetic field on charge mobility. To support this statement, we carry out admittance spectroscopy measurements and electroluminescence frequency dependence measurements to estimate the device reaction time. We find this to be inconsistent with the pELDMR measurements. In addition to that, we also performad hoc numerical simulations, proving that, in a space charge limited regime, a voltage dependence from spin dependent mobility changes has to be expected. On the other hand, spin dependent recombination models are able to describe the pELDMR post pulse decay at different temperatures, and to explain the fast reaction time of our measurements.


Related material