Near‐Infrared Light‐Emitting Diodes from Organic Radicals with Charge Control

Abstract

Organic radicals with fluorescence from doublet‐spin energy manifolds circumvent efficiency limits from singlet–triplet photophysics in organic light‐emitting diodes (OLEDs). The singly occupied molecular orbital (SOMO) in radicals enables the higher potential performance. The SOMO also presents substantially lower energy frontier orbitals compared to conventional fluorescent emitters for device operation, which can cause severe electron trapping that limits the performance of radical OLEDs. To improve optoelectronic performance, electron donor–acceptor‐mixed hosts are used to control charge transport for enhanced radical electroluminescence by charge recombination on SOMO and frontier orbitals. The (2‐chloro‐3‐pyridyl)bis(2,4,6‐trichlorophenyl)methyl‐based radical is designed to test the charge‐controlled device architectures in OLEDs by transient analysis and device characterization studies. Efficient radical OLEDs with 4.7% maximum external quantum efficiency are reported—showing substantial advances in performance for OLEDs with peak emission beyond 800 nm. In addition, substantially improved performance at higher current density operation and more than two orders of higher lifetime stability are achieved with mixed hosts. These results enable pathways to infrared‐emitting devices with applications ranging from communications to bioimaging.

A Simple Molecular Design Strategy for Luminescent Radicals to Achieve Near-Infrared Emission

“…Molecular structures and electroluminescence peaks of reported radical emitters and TTM-2PTI in this work. − ,− …”

Stable nitrogen-centered radicals with anti-Kasha emission

“…Stable luminescent radicals have received great attention because their special electronic structure endows them with unique fluorescence, magnetic and electronic properties simultaneously. [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16] As a result, these materials have potential applications in numerous fields, e.g., organic light-emitting diodes (OLEDs), 4,6,7,9,11,15,[17][18][19] fluorescence imagination 20,21 and fluorescent sensors. 22,23 In particular, using luminescent radicals as emitters almost 100% internal quantum efficiency (EQE) of OLEDs may be realized due to the spin-allowed doublet-doublet transition.…”

“…1–16 As a result, these materials have potential applications in numerous fields, e.g. , organic light-emitting diodes (OLEDs), 4,6,7,9,11,15,17–19 fluorescence imagination 20,21 and fluorescent sensors. 22,23 In particular, using luminescent radicals as emitters almost 100% internal quantum efficiency (EQE) of OLEDs may be realized due to the spin-allowed doublet–doublet transition.…”

Efficient and Bright Organic Radical Light‐Emitting Diodes with Low Efficiency Roll‐Off

“…[10][11][12] Doublet fluorescent emission from organic radicals has emerged as a new route to realize more efficient deep-red/NIR lightemitting devices than those using established nonradical organic emitters. [13][14][15][16][17][18][19][20][21][22][23][24][25][26] Organic radicals, incorporating one unpaired electron on the outermost molecular orbital, show a doublet spin manifold, which differs from the singlet and triplet spin states observed in typical organic semiconductors. As the total spin quantum number (S) of both the ground and excited states is maintained at S = ½, the D 1 (the first electronically excited doublet state) → D 0 (the ground electronic doublet state) transition is a spin-allowed process enabling an internal quantum efficiency (IQE) of 100% with nanosecond decay lifetime.…”