Large magnetic field effects in electrochemically doped organic light-emitting diodes

Abstract

Large negative magnetoconductance (MC) of ∼12% is observed in electrochemically doped polymer light-emitting diodes at sub-band-gap bias voltages (V bias ). Simultaneously, a positive magnetoefficiency (Mη) of 9% is observed at V bias = 2 V. At higher bias voltages, both the MC and Mη diminish while a negative magnetoelectroluminescence (MEL) appears. The negative MEL effect is rationalized by triplet-triplet annihilation that leads to delayed fluorescence, whereas the positive Mη effect is related to competition between spin mixing and exciton formation leading to an enhanced singlet:triplet ratio at nonzero magnetic field. The resultant reduction in triplet exciton density is argued to reduce detrapping of polarons in the recombination zone at low-bias voltages, explaining the observed negative MC. Regarding organic magnetoresistance, this study provides experimental data to verify existing models describing magnetic field effects in organic semiconductors, which contribute to better understanding hereof. Furthermore, we present indications of strong magnetic field effects related to interactions between trapped carriers and excitons, which specifically can be studied in electrochemically doped organic light-emitting diodes (OLEDs). Regarding light-emitting electrochemical cells (LECs), this work shows that delayed fluorescence from triplet-triplet annihilation substantially contributes to the electroluminescence and the device efficiency.

Investigating the influence of traps on organic magnetoresistance by molecular doping

“…Multiple mechanisms have been proposed for OMAR and recent work shows these can all become dominant under specific conditions [23]. Additionally, there is a growing awareness of the role of defects and polaron traps in the explanation of the large reported OMAR values [12,14,[20][21][22]24]. In this manuscript, we investigate the influence of traps on OMAR more closely with molecular dopants known from previous trap related research.…”

Anisotropic magnetoconductance in polymer thin films

“…Large changes in the current and luminescence with magnetic field have been observed in organic semiconducting devices during the last decade [1][2][3][4][5][6][7][8][9][10][11]. Such effects have been investigated in organic light emitting devices (OLEDs) [1][2][3][4], donor acceptor blends [5,6], field effect transistors [7,8], polymer light-emitting electrochemical cells [9,10], and molecular wires [11].…”

“…Such effects have been investigated in organic light emitting devices (OLEDs) [1][2][3][4], donor acceptor blends [5,6], field effect transistors [7,8], polymer light-emitting electrochemical cells [9,10], and molecular wires [11]. These magnetic field effects (MFEs) manifest themselves at room temperature and small magnetic fields of only a few millitesla, which has led to the suggestion of cheap plastic sensor applications [12].…”

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

“…We recently exploited such an approach to describe magnetic field effects in polymerfullerene blends [14] and electrochemical cells [25]. This device model can easily incorporate bimolecular Langevin recombination, trap-assisted SRH recombination, and exciton formation.…”

“…We found that, as long as the triplet-polaron coefficient is relatively small, tripletpolaron reactions do not significantly influence the outcome. We should note that triplet-polaron reactions are generally used to explain high field effects [14,25] and might therefore be required in a complete physical picture of the magnetic field effects.…”