Highly stable and efficient copper(I) sensitizer for narrowband red organic light-emitting diodes with an operational lifetime (LT95) of up to 3689 h at 1000 cd m-2

Abstract

Copper-based organic light-emitting diodes (OLEDs) are low-cost alternatives to precious metal-based OLEDs, but currently no such OLEDs can meet the practical requirements for high colour purity, device efficiency, and operational stability. Carbene-Cu(I)-amide emitters reported here exhibited thermally activated delayed fluorescent emission with quantum efficiencies up to 0.90 and radiative decay rates of 2.7 × 10⁶ s^-1. These enable blue to near-infrared Cu(I)-OLEDs with high brightness (265,000 cd m^-2) and extended LT₉₅ lifetime (3582 hours at 1000 cd m^-2). Deuteriation and π-extension of carbazole significantly enhance OLED stability. Cu(I)-sensitized fluorescence OLEDs showed efficient narrowband electroluminescence (λ_max 612-614 nm; full-width half maximum of 33-38 nm; maximum external quantum efficiencies reach 21.9%) and prolonged LT₉₅ lifetime (up to 3689 h at 1000 cd m^-2). This work highlights earth-abundant metal-based sensitized-OLEDs that exhibit high colour purity and long device lifetime comparable to the best non-iridium metal-based OLEDs.

Fig. 1. Selected examples of Cu(I)-based (TSF/TST-)OLEDs.

a Representative examples of Cu(I)-OLEDs and their performance during 25 years’ development; Φ_em, k_r, λ_EL, EQE, L_max, LT_x refer to the emission quantum yield, radiative decay rate constant, electroluminescence peak maxima, external quantum efficiencies, maximum luminance, and operational lifetime at x% of initial luminance, respectively. b Schematic mechanism of TSF-OLEDs; S₀, S₁ and T₁ refer to the ground state, singlet excited state, and triplet excited state, respectively, of the Cu(I) sensitizer (denoted as [Cu]) or the fluorescence emitter (denoted as [emitter]). c Examples of TSF/TST-OLEDs using CMA(Cu) sensitizers and organic fluorescence or TADF emitters.

Fig. 2. Chemical structures, photophysical measurements and theoretical calculations of CMA(Cu) emitters 1–7.

a Chemical structures of CMA(Cu) emitters in this work. b Emission spectra of emitters 1–7 doped in 1,3-Bis(N-carbazolyl)benzene (mCP) thin film (in a concentration of 2 wt%) by drop-cast method. c Emission spectra of 1-D, 2-D, 3-D, 7-D (solid line) and 1-H, 2-H, 3-H, 7-H (dashed line) measured in degassed methylcyclohexane (MeCy), toluene (Tol), or 2-methyltetrahydrofuran (MeTHF). Emission spectra of 1-H, 2-H and 7-H have been reported previously. d Calculated hole and electron distribution, the overlap between hole and electron wavefunctions (O_h,e) and the distance between centroids of hole and electron (Δr) of coplanar structures of 2-H and 3-H in the S₁ excited state; blue and green isosurfaces represent hole and electron distributions, respectively. e Reorganization energy distribution of S₁ → S₀ vibrational relaxation for 1-H and 1-D, red lines represent major vibrational modes localized on the carbazole ligand, blue lines represent other major vibrational modes, and black lines represent minor vibrational modes; f Major vibrational modes 1-5 localized on the carbazole ligand, the corresponding vibrational frequencies (ω_j) for 1-H and 1-D are labeled.

Fig. 3. OLED characteristics and operational lifetime measurement.

a Normalized EL spectra and b EQE-luminance characteristics of OLEDs based on CMA(Cu) emitter 1–6 in RH host and 7-D in SiCzCz:SiTrzCz₂ co-host. c Normalized electroluminescence (EL) spectra and d EQE-luminance characteristics of OLEDs based on emitter 6 in RH host; inset of c displays the electroluminescence image of OLED containing 2 wt% 6. e Operational lifetime measurement of OLEDs with emitters 1–6 in RH host and 7-D in SiCzCz:SiTrzCz₂ co-host at L₀ of: 11500 cd m⁻² for 1-D (4 wt%), 20000 cd m⁻² for 2-D (6 wt%), 8500 cd m⁻² for 3-H (8 wt%), 20000 cd m⁻² for 3-D (2 wt%), 10000 cd m⁻² for 4 (4 wt%), 7000 cd m⁻² for 5 (2 wt%), 900 cd m⁻² for 6 (2 wt%), and 10000 cd m⁻² for 7-D (8 wt%).

Fig. 4. TSF-OLED characteristics and operational lifetime measurement.

a Chemical structures of CMA(Cu) sensitizers and α-NAICZ emitter. b Absorption spectrum of α-NAICZ in DCM and normalized photoluminescence (PL) spectra of binary system of CMA(Cu) sensitizers in RH and α-NAICZ emitter in RH, and ternary system of CMA(Cu) sensitizers and α-NAICZ emitter in RH; inset displays the PL decay of 2-D/α-NAICZ/RH ternary system, 2-D/RH or α-NAICZ/RH binary systems. c Normalized electroluminescence (EL) spectra, d EQE-luminance characteristics, e luminance-voltage (left) and current density-voltage (right) characteristics of TSF-OLED with CMA(Cu) complex: α-NAICZ and OLEDs with CMA(Cu) complexes or α-NAICZ only; insets of c display the electroluminescence images of OLEDs containing 2 wt% 3-H only (upper) and TSF-C (down). f Operational lifetime measurement of TSF-OLEDs; L₀ refer to the initial luminance. Composition of TSF-OLEDs: TSF-A contains 1-D (6 wt%):α-NAICZ (0.2 wt%); TSF-B contains 2-D (8 wt%):α-NAICZ (0.4 wt%); TSF-C contains 3-H (8 wt%):α-NAICZ (0.3 wt%); TSF-D contains 3-D (6 wt%): α-NAICZ (0.3 wt%).