Magnetoelectroluminescence of organic heterostructures: Analytical theory and spectrally resolved measurements

Abstract

The effect of a magnetic field on the electroluminescence of organic light emitting devices originates from the hyperfine interaction between the electron/hole polarons and the hydrogen nuclei of the host molecules. In this paper, we present an analytical theory of magneto-electroluminescence for organic semiconductors. To be specific, we focus on bilayer heterostructure devices. In the case we are considering, light generation at the interface of the donor and acceptor layers results from the formation and recombination of exciplexes. The spin physics is described by a stochastic Liouville equation for the electron/hole spin density matrix. By finding the steady-state analytical solution using Bloch-Wangsness-Redfield theory, we explore how the singlet/triplet exciplex ratio is affected by the hyperfine interaction strength and by the external magnetic field. To validate the theory, spectrally-resolved electroluminescence experiments on BPhen/m-MTDATA devices are analyzed. With increasing emission wavelength, the width of the magnetic field modulation curve of the electroluminescence increases while its depth decreases. These observations are consistent with the model.

Current–voltage characteristics of organic heterostructure devices with insulating spacer layers

“…Charge carriers separated by larger distances are assumed to be independent electron and hole polarons. This physical picture, which includes three classes of states: independent electron and hole polarons, weakly bound polaron pairs, and strongly bound states (exciplexes or excitons), has been extensively used to describe the magneto-luminescence properties of organic semiconductor structures [19][20][21][22][23][24]. Exciplex states recombine to the ground state through radiative or nonradiative processes.…”

Abnormal temperature dependence of triplet-triplet annihilation in excimer-based devices

“…因为PP态中电子与 空穴之间的距离较远, 所以PP 1 与PP 3 之间的自旋交换 作用较弱, 从而PP 1 很容易在超精细相互作用下通过 ISC过程变成PP 3 [11] . 当加入外磁场时, 如果外磁场强度 大于内部磁相互作用, 则外磁场能增强三重态能级的 Zeeman分裂, 从而抑制PP 1 向PP 3 的转化, 即抑制ISC过 程 [16] . 因为ISC过程被外磁场抑制, 所以PP 1 的数量增加, 从而引起S 1 的数量增加以及器件的电致发光强度增强, 导致低场部分快速上升 [16] .…”

Manipulation of the electroluminescence of organic light-emitting diodes via fringe fields from patterned magnetic domains

“…We assume k S Շ k T . 21 Alternatively, if k S տ k T (not shown), the excitonic DEL/EL changes sign, but still does not reach the magnitude seen for exciplexes. Figure 3 shows the figure of merit when HF is included as well.…”

“…6,19 Early studies of exciplexes have assumed a very similar picture to that of excitons where exciplexes simply play the role of exciton. [20][21][22][23] Thus, we call them exciton models. Spin does not evolve in the exciplex state since the exchange splitting, though smaller than in excitons, is still as large as and more often larger than room temperature.…”