Alkali fluorides, mostly LiF and CsF, are well-known to improve electron injection/extraction in organic light-emitting diodes (OLEDs) and organic solar cells (OSCs). They are also utilized, though to a lesser extent, for hole injection in OLEDs. Here we demonstrate a new role for such fluorides in enhancing OSCs' hole extraction. We show that an ultrathin air-plasma-treated alkali fluoride layer between the indium tin oxide (ITO) anode and the active layer in copper phthalocyanine (CuPc)/C70-based OSCs increases the short circuit current by up to ∼ 17% for cells with LiF and ∼ 7% for cells with NaF or CsF. The effects of the fluoride layer thickness and treatment duration were evaluated, as were OSCs with oxidized and plasma-treated Li and UV-ozone treated LiF. Measurements included current voltage, absorption, external quantum efficiency (EQE), atomic force microscopy, and x-ray photoelectron spectroscopy, which showed the presence of alkali atoms F and O at the treated ITO/fluoride surface. The EQE of optimized devices with LiF increased at wavelengths >560 nm, exceeding the absorption increase. Overall, the results indicate that the improved performance is due largely to enhanced hole extraction, possibly related to improved energy-level alignment at the fluorinated ITO/CuPc interface, reduced OSC series resistance, and in the case of LiF, improved absorption.
Recent electroluminescence (EL) detected magnetic resonance and transient EL studies reveal the presence and role of holes that drift beyond the recombination zone and approach the cathode in small molecular organic light-emitting diodes (OLEDs) with specific materials and structures. In particular, these studies suggest that these holes are responsible for trion (i.e., a bipolaron stabilized by a counterpolaron on an adjacent molecule) formation in the electron transport layer, and may contribute to EL spikes observed at the end of a bias pulse. The significance of these holes to overall OLED performance is discussed.
The properties of phosphorescent fac tris(2-phenylpyridine) iridium [Ir(ppy3)]-doped poly(N-vinyl carbazole)
(PVK)/4,7-diphenyl-1,10-phenanthroline (Bphen) polymer/small molecular hybrid OLEDs are described. For optimal
BPhen thickness, the power efficiency of the devices exceeds 30 lm/W. The low-temperature electroluminescence-
detected magnetic resonance (ELDMR) exhibits the well-known negative spin 1/2 resonance attributed to
enhanced formation of trions, but the positive spin 1/2 resonance, typically observed at low temperature or at high
current density, is not observed. The OLEDs' performance and the ELDMR results are discussed in relation to the
nature of the defects and their density in these devices.
The electroluminescence (EL)-detected magnetic resonance (ELDMR) of abrupt junction and mixed layer
N,N'-diphenyl-N,N'-bis(1-naphthylphenyl)-1,1'-biphenyl-4,4'-diamine (NPB) / tris(quinolinolate) Al (Alq3) OLEDs was
measured at room temperature and at 20K, at current densities 0.83 ≤ J ≤ 83 mA/cm2. The abrupt junction and
mixed layer devices were indium tin oxide (ITO) / 5 nm copper phthalocyanine (CuPc) / 50 nm NPB / 40 nm Alq3 /
1 nm CsF / Al, and ITO / 5 nm CuPc / 40 nm NPB / 20 nm 1:1 NPB:Alq3 / 30 nm Alq3 / 1 nm CsF / Al, respectively.
As expected, the devices exhibited a positive (EL-enhancing) spin 1/2 resonance at T = 20 K and a negative (ELquenching)
spin 1/2 resonance at room temperature. It was found that the positive and negative resonance was
stronger in the abrupt junction and in the mixed layer devices, respectively. The results are discussed in relation to
the mechanisms responsible for these resonances, namely reduced quenching of singlet excitons by polarons and
triplet excitons, and enhanced quenching by trions, respectively.
A comprehensive photoluminescence (PL)-detected magnetic resonance (PLDMR) study of various vacuum-sealed
rubrene films and powders is described. Three PLDMR features are observed and analyzed:
(i) A negative (PL-quenching) triplet exciton (TE) resonance at T > 50K, due to reduced spin-dependent fusion of
geminate TE pairs to singlet excitons (SEs).
(ii) A positive (PL-enhancing) triplet resonance at T < 50K. This resonance is suspected to result from reduced
quenching of SEs by a reduced population of polarons and nongeminate TEs, the latter due to the spin-dependent
annihilation of TEs by polarons.
(iii) A negative spin 1/2 (polaron) resonance, believed to be due to enhanced formation of trions at oxygen centers.
As single crystal thin films of oxygen-doped rubrene exhibit exceptionally high room-temperature carrier mobility,
the relation of this positive resonance to these transport properties is also discussed.
Some recent photoluminescence (PL)- and electroluminescence (EL)-detected magnetic resonance (PLDMR and
ELDMR, respectively) studies of the negative (PL- and EL-quenching) spin 1/2 resonance are reviewed. These include
the resonances in poly[2-(N-carbazolyl)-5-(2'-ethyl)-hexoxy-1,4-phenylenevinylene] (CzEh-PPV) films, rubrene films,
tris(quinolinolate) Al (Alq3) OLEDs, rubrene-doped Alq3 OLEDs, and fac tris(2-phenylpyridine) iridium [Ir(ppy3)]-doped poly(N-vinyl carbazole) (PVK) polymer LEDs (PLEDs). The resonances are all assigned to quenching of SEs by
trions, which are bipolarons stabilized by a counterpolaron or counterion. As bipolarons are spinless, their formation
from two like-charged polarons is spin-dependent, and hence enhanced at resonance. This enhanced formation, and the
resulting enhanced quenching of SEs, yields the negative spin 1/2 PLDMR and ELDMR. As previously shown, since
trion formation also reduces the mobility of the trapped carriers, this process also results in a negative spin 1/2 electrical
current-detected magnetic resonance (EDMR). Importantly, since the counterpolaron is usually trapped, e.g., at
organic/cathode or organic/organic interfaces, or at impurity sites such the oxygen center in rubrene, it is suspected that
the trions might be responsible for the long term degradation of OLEDs and PLEDs associated with abrupt junctions or
impurities.
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