Lanthanide Cerium(III) Tris(pyrazolyl)borate Complexes: Efficient Blue Emitters for Doublet Organic Light-Emitting Diodes

Abstract

Organic light-emitting diodes (OLEDs) have had commercial success in displays and lighting. Compared to red and green OLEDs, blue OLEDs are still the bottleneck because the high-energy and long-lived triplet exciton in traditional blue OLEDs causes the short operational lifetime of the device. As a new type emitter, lanthanide complexes with a 5d–4f transition could have short excited-state lifetimes on the order of nanoseconds. To achieve a high-efficiency 5d–4f transition, we systematically tuned the steric and electronic effects of tripodal tris­(pyrazolyl)­borate ligands and drew a full picture of their Ce­(III) complexes. Intriguingly, all of these complexes show bright blue emission with high photoluminescence quantum yields exceeding 95% and short decay lifetimes of 35–73 ns both in the solid powder and in dichloromethane solutions. Using the Ce­(III) complex emitter, we show a blue OLED with a maximum external quantum efficiency of 14.1% and a maximum luminance of 33,160 cd m–2, and the specific electroluminescence mechanism of direct exciton formation on the Ce­(III) ion with a near-unity exciton utilization efficiency is also confirmed. The discovered photoluminescence and electroluminescence property–structure relationships may shed new light on the rational design of highly efficient lanthanide-based blue emitters and their optoelectronic devices such as OLEDs.

Highly Efficient Heteroleptic Cerium(III) Complexes with a Substituted Pyrazole Ancillary Ligand and Their Application in Blue Organic Light-Emitting Diodes

“…In addition, Ce­(Tp Me2 ) 2 (dtfpz) showed two lower-energy absorption bands at 295 and 383 nm, and Ce­(Tp Me2 ) 2 (dmpz) showed three lower energy absorption bands at 280, 330, and 401 nm, while Ce­(Tp Me2 ) 2 (dppz) showed only one lower energy absorption band at 397 nm. All these lower-absorption bands with molar absorption coefficient (ε) of ∼10 2 L mol –1 cm –1 could be attributed to the 4f–5d transitions of Ce 3+ ions, which are similar to the reported Ce­(III) complexes. ,,− , …”

“…All these lower-absorption bands with molar absorption coefficient (ε) of ∼10 2 L mol −1 cm −1 could be attributed to the 4f−5d transitions of Ce 3+ ions, which are similar to the reported Ce(III) complexes. 14,15,[18][19][20][21]23 All three Ce(III) complexes exhibited a strong emission under UV irradiation. Their emission spectra in dichloromethane show a similar broad band with full width at halfmaximum (fwhm) values of ∼90 nm, which originate from the d−f transitions of Ce 3+ ions (Figure 2b, Table 2).…”

“…Organic light-emitting diodes (OLEDs) have developed rapidly over the past few decades. To achieve high efficiency, fluorescence, phosphorescence, − thermally activated delayed fluorescence, − hybridized local and charge-transfer excited-state fluorescence, , radical-based luminescence, − and d-f transition-based luminescence − have been developed sequentially. As a new type of emitter in OLEDs, d-f transition Ce­(III) complexes have significant advantages, such as short excited-state lifetimes in the range of nanoseconds, ,,− up to a 100% theoretical exciton utilization efficiency due to the spin- and parity- allowed d-f transitions, , and tunable emission colors, since the 5d energy levels are sensitive to the external environment. − …”

Rare Earth Complexes with 5d–4f Transition: New Emitters in Organic Light-Emitting Diodes

“…To further study the mechanism of exciton recombination, 3-Me , 4-Pz , and 4- i Pr (Figure ) were selected from a series of cerium­(III) complexes because of their similar low emission energy and large structural differences . The corresponding OLEDs A5, A6, and A7 have the same structure of ITO/MoO 3 (2 nm)/mCP:MoO 3 (20 wt %, 30 nm)/mCP (10 nm)/mCP:cerium­(III) complex (10 wt %, 20 nm)/TmPyPB (40 nm)/LiF (0.7 nm)/Al (100 nm), but their performances are significantly different (Table ).…”

Inorganic Lanthanide Compounds with f–d Transition: From Materials to Electroluminescence Devices

“…For the challenge of unbalanced charge in ILCs, one possible solution could be the host-dopant strategy. By doping the ILCs into organic host materials, charge carrier injection and transportation mainly happen on organic host materials instead of ILCs, thus potentially overcoming the transportation difficulties (Figure b) . Era et al doped CeBr 3 and EuI 2 in the hole transport material CBP (4,4′-N,N′-dicarbazole-biphenyl) and obtained the brightness of 486 cd/m 2 with 0.11% EQE and 377 cd/m 2 with 0.18% EQE, respectively. , They found that the hole-transport material CBP promotes the EL of LICs, while LICs in the electron-transport material BCP did not present EL.…”