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.
Residual levels of O2 in OLEDs and their relation to device performance were evaluated by measuring (i) the
photoluminescence (PL) decay time (following pulsed UV LED excitation) of the O2 sensing dye Pd octaethylporphyrin
(PdOEP) doped in the active OLED layer poly(N-vinyl carbazole) (PVK) and (ii) the electroluminescence (EL) decay
time (following a bias pulse) of glass/ITO/PEDOT:PSS/6 wt.% PdOEP:PVK/CsF/Al OLEDs. The active layer was
prepared under various conditions of exposure to controlled O2 levels and relative humidity. PdOEP was used
successfully for monitoring exposure of PdOEP:PVK to low levels of oxygen and shortened device PL decay times often
indicated device deterioration. The PL decay time at various applied voltages and the EL decay time at various current
densities were monitored to evaluate degradation processes related to oxygen and other bimolecular quenching
Microcavity tandem organic light-emitting diodes (OLEDs) are simulated and compared to experimental results. The simulations are based on two complementary techniques: rigorous finite element solutions of Maxwell's equations and Fourier space scattering matrix solutions. A narrowing and blue shift of the emission spectrum relative to the noncavity single unit OLED is obtained both theoretically and experimentally. In the simulations, a distribution of emitting sources is placed near the interface of the electron transport layer tris(8-hydroxyquinoline) Al (Alq3) and the hole transport layer (N,N′-bis(naphthalen-1-yl)-N,N′-bis(phenyl)benzidine (α-NPB). Far-field electric field intensities are simulated. The simulated widths of the emission peaks also agree with the experimental results. The simulations of the 2-unit tandem OLEDs shifted the emission to shorter wavelength, in agreement with experimental measurements. The emission spectra's dependence on individual layer thicknesses also agreed well with measurements. Approaches to simulate and improve the light emission intensity from these OLEDs, in particular for white OLEDs, are discussed.
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.
Typical guest-host small molecular OLEDs (SMOLEDs) exhibit an emission spike at 100 - 200 ns and a tail that
extends over several μs following a bias pulse. The spike and tail are attributed to recombination of correlated
charge pairs and detrapped charges (mostly from the host shallow states), respectively. They may also be associated
with other OLED layers and other phenomena, e.g., triplet-triplet annihilation. The implications of the spike and tail
for OLED-based, photoluminescent oxygen sensors operated in the time domain are evaluated and compared to the
behavior observed when using undoped OLEDs or inorganic LEDs as the excitation sources.