1,8-Naphthalimides in Phosphorescent Organic LEDs:  The Interplay between Dopant, Exciplex, and Host Emission

Abstract

Four different 1,8-naphthalimide derivatives were examined in phosphorescent organic light emitting diodes (OLEDs), i.e., 1,8-naphthalimide, N-phenyl-1,8-naphthalimide, N-2,6-dibromophenyl-1,8-naphthalimide (niBr), and bis-N,N-1,8-naphthalimide. Photoluminescence from all four naphthalimides have violet-blue fluorescence and phosphorescent bands between 550 and 650 nm (visible at 77 K). While all four compounds gave good glassy films when doped with a phosphorescent dopant, only the niBr films remained glassy for extended periods. OLED studies focused on niBr, with two different architectures. One OLED structure (type 1) had the niBr layer as a doped luminescent layer and an undoped niBr layer to act as a hole-blocking layer. The alternate structure (type 2) utilizes a doped CBP layer as the luminescent layer and the niBr layer is used as a hole-blocking layer only (CBP = 4,4'-N,N'-dicarbazolylbiphenyl). Type 1 and 2 OLEDs were prepared with green, yellow, and red emissive phosphorescent dopants (Irppy, btIr, and btpIr, respectively). The dopants were organometallic Ir complexes, previously shown to give highly efficient OLEDs. Of the three dopants, the btpIr-based OLEDs showed the best device performance in both structures (peak efficiencies for type 2: 3.2% and 2.3 lum/W at 6.3 V; type 1: 1.7% and 1.3 lm/W at 6.1 V). The green and yellow dopants gave very similar performance in both type 1 and 2 devices (peak efficiencies are 0.2-0.3%), which were significantly poorer than the btpIr-based OLEDs. The emission spectrum of the btIr- and btpIr-based devices (type 1 and 2) are the same as the solution photoluminescence spectrum of the dopant alone, while the Irppy device gives a broad red emission line (lambda(max) = 640 nm). The red Irppy.niBr emission line is assigned to an Irppy.niBr exciplex. The type 2 Irppy-based device gave a voltage-dependent spectrum, with the red emission observed at low bias (4-8 V), switching over to strong green emission as the bias was raised. All other devices showed bias-independent spectra. Estimates of HOMO, LUMO, and excited-state energies (dopant, niBr, and exciplex) were used to explain the observed spectral properties of these devices. btpIr-based devices emit efficiently from isolated dopant states (external efficiencies = 3.2 %, 2.3 lum/W). Irppy-based devices emit only from exciplex states, with low efficiency (external efficiency = 0.3%). btIr.niBr films have very similar energies for the dopant, exciplex, and niBr triplet states, such that relaxation can go through any of these states, leading to low device efficiency (external efficiency = 0.4%). High device efficiency is achieved only when dopant emission is the dominant pathway for relaxation, since exciplex and niBr triplet states give either weak or no electroluminescence.

Syntheses and properties of copolymers containing indolocarbazole moiety in the side chain

“…[1][2][3][4][5][6][7] In particular, organic compounds that possess a p-conjugated system, such as pentacene and oligothiophene, have attracted much attention [8][9][10][11][12][13] because of their mechanical properties, low cost and variety of photonic properties obtained by chemical modification. Therefore, significant progress has been made in the study of p-conjugated organic compounds, which might be competitive with amorphous silicon in electronic and photonic devices.…”

Preparation and characterization of nanostructured composite films for organic light emitting diodes

“…the separated energy gap between the lowest unoccupied molecular orbital (LUMO) and the highest occupied molecular orbital (HOMO) of about 3.6 eV [8]. It is transparent and possesses a workfunction higher than that of ITO, so it is often used for the HTL in OLEDs.…”

Blue phosphorescent organic light-emitting devices utilizing cesium–carbonate-doped 2,4,6-tris(2′,4′-difluoro-[1,1′-biphenyl]-4-yl)-1,3,5-triazine

“…The LUMO energy levels were estimated from solution electrochemical reduction potentials. 24 The highest occupied molecular orbital (HOMO) levels were estimated from electrochemical oxidation potentials or the optical bandgap, where electrochemical measurements were not feasible. The triplet energies were obtained from delayed photoluminescence measurements 25 in CH 2 Cl 2 at 77 K. Computations were performed using NWChem, a computational chemistry package for parallel computers, version 5.1 (2007), which was developed by the High Performance Computational Chemistry Group at Pacific Northwest National Laboratory (Richland, WA).…”

“…The first and second reduction potentials of tris-(dFB)Tz are 1.9 and 2.6 V, respectively (DMF, using ferrocene as an internal standard). From this data, the E LUMO of tris-(dFB)Tz is estimated 24 to be −3.5 eV. Attempts to measure the oxidation potential, and thus E HOMO , of tris-(dFB)Tz failed due to the poor solubility of the compound in dichloromethane.…”