Fitting the magnetoresponses of the OLED using polaron pair model to obtain spin-pair dynamics and local hyperfine fields

Abstract

Organic light-emitting diode (OLED) displays a sign reversal magnetic field effect (MFE) when the applied magnetic field range is reduced to the sub-milliTesla range and the Polaron Pair Model has been successful in explaining the ultra-small MFE. Here, we obtained high resolution (~ 1 µT) magnetoconductance (MC) and magnetoelectroluminescence (MEL) of a tris-(8-hydroxyquinoline)aluminium-based (Alq3) OLED within the magnetic field range of ± 500 µT with the earth magnetic field components cancelled. A clear “W” shaped MC with a dip position of ± 250 µT and a monotonic MEL were observed. We demonstrate a fitting technique using the polaron pair model to the experimentally obtained MC and MEL. The fitting process extracts physically significant parameters within a working OLED: the local hyperfine fields for electron and hole in Alq3: Bhf1 = (0.63 ± 0.01) mT (electron), Bhf2 = (0.24 ± 0.01) mT (hole); the separation rates for singlet and triplet polaron pairs: kS,s = (44.59 ± 0.01) MHz, kT,s = (43.97 ± 0.01) MHz, and the recombination rate for singlet polaron pair kS,r = (88 ± 6) MHz. The yielded parameters are highly reproducible across different OLEDs and are in broad agreement with density functional theory (DFT) calculations and reported experimental observations. This demonstrates the feasibility of this fitting technique to approach any working OLED for obtaining significant microscopic parameters.

Spontaneous exciton dissociation enables spin state interconversion in delayed fluorescence organic semiconductors

“…from the crystal value of 0.35 to 0.7 nm (Supplementary Figs. [25][26][27][28][29][30][31][32][33][34]. In Fig.…”

“…Additional information about the HFI-ISC processes occurring in inter-CT states can be obtained by investigating the response of the PLDMR signal to increased microwave intensities, where B 1 is perpendicular to B 0 . When B 1 exceeds the hyperfine field (B HF , typically ~1 mT in organic radicals 20,[29][30][31] ), the rate of the HFI-mediated inter-1 CT 0 /inter-3 CT 0 transitions is reduced as the radical spins begin to precess around B 1 with the same frequency, negating their HFI-induced spin precession frequency difference 25,32,33 . In the absence of an effective spin-mixing process, the spin population now becomes trapped in the initially generated inter-1 CT 0 , leading to spin-locking (Fig.…”

Disorder‐Induced Dispersive Magneto‐Electroluminescence of Blue Emitters in Organic Light Emitting Diodes

“…Xin Pan, Ohyun Kwon,* Dipak Raj Khanal, Byoungki Choi, and Zeev Valy Vardeny* DOI: 10.1002/adom.202101334 rates can be tuned by an external magnetic field B, which modulates the EL emission intensity giving rise to magneto-EL (MEL) of which response is an efficient tool for investigating the e-h pair dynamics in the active layer of the device. [2][3][4][5][6][7][8][9][10][11][12][13][14] The MEL(B) response requires spinmixing activity among the four spin sublevels of the spin-entangled e-h pairs, namely intersystem-crossing between singlet and triplet spin configurations; which is dominated by various spinrelated interactions. [2][3][4][5][6]10,11,[16][17][18] These include the hyperfine interaction (HFI), spin orbit coupling (SOC), and also the difference, Δg of the Landé g-factors of the electron and hole constituents of the e-h species, known as the "Δg mechanism".…”

“…[2][3][4][5][6][7][8][9][10][11][12][13][14] The MEL(B) response requires spinmixing activity among the four spin sublevels of the spin-entangled e-h pairs, namely intersystem-crossing between singlet and triplet spin configurations; which is dominated by various spinrelated interactions. [2][3][4][5][6]10,11,[16][17][18] These include the hyperfine interaction (HFI), spin orbit coupling (SOC), and also the difference, Δg of the Landé g-factors of the electron and hole constituents of the e-h species, known as the "Δg mechanism". [19] In the following discussion we will focus on the "Δg mechanism".…”

“…The magnetic field effect has been proven to be a powerful tool for investigating the spin-dependent processes associated with spin-pair species in organic-based devices, especially in organic light emitting diodes (OLEDs). [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18] Upon carriers' injection into the organic active layer of the device, the principal process that counts is the formation of spin-entangled electron and hole (e-h) pairs which generate singlet and triplet excitons; or, alternatively dissociate into free carries. These OLED processes may be studied indirectly by measuring the device electro-luminescence (EL) versus bias voltage V (namely EL-V) and current density, J versus V (J-V response).…”