Spin-OLEDs are multifunctional devices that behave at the same time as organic light emitting diodes and molecular spin valves. Basically the structure of a spin-OLED is alike the configuration of an OLED in which the metallic electrodes are replaced by ferromagnetic metals. Or, seen another way, a spin-OLED is a MSV where transport takes place in the HOMO and LUMO of the molecular semiconductor enabling recombination and light emission.
OLEDs, that were invented in the 1980s, transform the electrical current in visible light using molecular materials. Nowadays they are implemented in most displays and form part of our daily life. OLED devices are built in sandwich-type structures where the two electrodes inject positive (holes) and negative (electrons) charge carriers into one or various organic semiconductors, where recombination of charges and light emission takes place (see figure above).
As standard, the anode is made from indium thin oxide (ITO) that is deposited on a transparent glass substrate allowing light to exit the device through this side. On the other side, the cathode is a low work function metal (hence very reactive), such as Ba, Ca or Li, that is typically capped with Ag or Al for corrosion protection. The low work function cathode injects electrons into the molecular semiconductor thanks to the good alignment between the work function of the metal and the LUMO. Holes are injected from the anode through a hole injection layer (HIL) that is placed between the anode and the emissive layer.
Contrary to OLEDs, the spin polarization of the charge carriers in spin-OLEDs is controlled by an external magnetic field. In particular, the orientation of the magnetization of the electrodes affects the intensity of the light emitted by the device thanks to the dominion of the relative populations of singlets and triplets. The radiative recombination process cascade can start in the same or neighboring molecules. Initially, the electron and hole are attracted and bound coulombically to form a polaron pair, when their wave functions overlap. Then, when the electron and hole are in the same molecule, the exciton is formed which is the first step of recombination and light emission.
In a first approximation, the singlet exciton is the main responsible of light emission because the transition from the singlet to the ground state is allowed. So theoretically, the rate electron to photon conversion or external quantum efficiency (EQE) is limited to 25% in OLEDs. In contrast to OLEDs, the FM electrodes in the spin-OLED inject spins aligned parallel or antiparallel depending on the orientation of the electrodes magnetization. When the electrodes inject charges with opposite spin polarization the singlet ratio is increased since, theoretically, only the S0 and T0 configurations can be formed. Both S0 and T0 have 1/2 probability and thus, the maximum EQE is increased to 50% in a spin-OLED.
Illustration of the singlets and triplets formation in the two states of a spin-OLED. Top panel: In the AP state the electroluminescence quantum efficiency is increased up to a maximum of 50 %. Bottom panel: In the P state, the triplet population is favored.
The polaron pair formed in the spin-OLED will be in a singlet or triplet state depending on the relative orientation of the magnetization of the FM electrodes. In the ideal situation, in the AP state only the T0 and S0 are possible and the singlet-triplet ratio increases to 1:1 with a maximum theoretical EQE of 50 %. Contrary, in the parallel state, where both electrodes are aligned, there is no light emission.
Analogously to the magnetoresistance (MR) curve, the light intensity is registered in a magneto-electroluminescence (MEL) curve. The measurement protocol is analogous to the MR. The magnetic field is varied while a constant voltage is applied or a constant current flows in the device. The electroluminescence intensity changes depending on the magnetic alignment of the electrodes and the singlets and triplets relative populations. In the AP state, the S0 singlet and the T0 triplet can be formed and the light intensity increases because the singlet is the main responsible of light emission. On the contrary, when both electrodes are aligned in the same direction, the light intensity decreases due to an increase of the triplet population. The percentage of MEL effect effect is quantified as: MEL(%) = (ELAP - ELP) / ELP, where ELAP and ELP are the electroluminescence in the antiparallel and parallel states respectively.
Although the origin of the MR is at the scattering that spin carriers encounter when crossing the device and the origin of the MEL effect stems from the variation of singlets and triplets relative populations, they are coupled effects both related to the spin-polarized current. Analogously to MR, the MEL effect can be negative if an interface reverses the spin polarization. Therefore, it should be noted that MR and MEL effect must show the same sign in the same device. That is, if the MR is positive (negative), the light intensity will be higher (smaller) in the AP state generating a positive (negative) MEL response.
REFERENCES
[1] J. Prieto-Ruiz, S. G. Miralles, H. Prima-García, A. Riminucci, P. Graziosi, M. Cinchetti, M. Aeschlimann, V. A. Dediu, and E. Coronado. “Controlling singlet-triplet ratio in OLEDs by spin polarised currents”. arXiv:1612.00633 (2016).
[2] T. D. Nguyen, E. Ehrenfreund, and Z. V. Vardeny. “Spin-polarized light-emitting diode based on an organic bipolar spin valve”. Science 337.6091 (2012), 204– 209.
[2] T. D. Nguyen, E. Ehrenfreund, and Z. V. Vardeny. “Spin-polarized light-emitting diode based on an organic bipolar spin valve”. Science 337.6091 (2012), 204– 209.
