[Show abstract][Hide abstract]ABSTRACT: Over the past years, organic light-emitting diodes (OLEDs) have attracted increasing interest because of their great potential for use in high-quality flat-panel displays and solid-state lighting. One of the basic requirements in any emissive device is to provide adequate stability to ensure a sufficiently long lifetime. Recently, it was observed that small molecules migrate toward the ITO anode under a direct driving voltage while retaining their original structures. To prevent this bias-driven migration of small molecules, a chemical structure with a higher steric hindrance could be introduced as a blockade, thus molecular migration could be suppressed and the device half-life increased. In this work, OLED devices with different hosts, including CBP, mCP, SimCP2, and SimCP, with increasing steric hindrances are fabricated. The spatial distribution of the tracking molecules after operation for different lengths of time is examined by using X-ray photoelectron spectroscopy (XPS) with in situ high-energy C60+ and low-energy Ar+ co-sputtering for depth profiling. It is found that the bias-driven molecular migration is suppressed and the device half-life prolonged as the steric hindrance of the host increases.
No preview · Article · Feb 2011 · Organic Electronics
[Show abstract][Hide abstract]ABSTRACT: The nanostructure of the light emissive layer (EL) of polymer light emitting diodes (PLEDs) was investigated using force modulation microscopy (FMM) and scanning time-of-flight secondary ion mass spectrometry (ToF-SIMS) excited with focused Bi(3)(2+) primary beam. Three-dimensional nanostructures were reconstructed from high resolution ToF-SIMS images acquired with different C(60)(+) sputtering times. The observed nanostructure is related to the efficiency of the PLED. In poly(9-vinyl-carbazole) (PVK) based EL, a high processing temperature (60 °C) yielded less nanoscale phase separation than a low processing temperature (30 °C). This nanostructure can be further suppressed by replacing the host polymer with poly[oxy(3-(9H-9-carbazol-9-ilmethyl-2-methyltrimethylene)] (SL74) and poly[3-(carbazol-9-ylmethyl)-3-methyloxetane] (RS12), which have similar chemical structures and energy levels as PVK. The device efficiency increases when the phase separation inside the EL is suppressed. While the spontaneous formation of a bicontinuous nanostructure inside the active layer is known to provide a path for charge carrier transportation and to be the key to highly efficient polymeric solar cells, these nanostructures are less efficient for trapping the carrier inside the EL and thus lower the power conversion efficiency of the PLED devices.
[Show abstract][Hide abstract]ABSTRACT: By using 10 kV C(60)(+) and 200 V Ar(+) ion co-sputtering, a crater was created on the light-emitting layer of phosphorescent polymer light-emitting diodes, which consisted of a poly(9-vinyl carbazole) (PVK) host doped with a 24 wt % iridium(III)bis[(4,6-difluorophenyl)pyridinato-N,C(2)] (FIrpic) guest. A force modulation microscope (FMM) was used to analyze the nanostructure at the flat slope near the edge of the crater. The three-dimensional distribution of PVK and FIrpic was determined based on the difference in their mechanical properties from FMM. It was found that significant phase separation occurred when the luminance layer was spin coated at 30 degrees C, and the phase-separated nanostructure provides a route for electron transportation using the guest-enriched phase. This does not generate excitons on the host, which would produce photons less effectively. On the other hand, a more homogeneous distribution of molecules was observed when the layer was spin coated at 60 degrees C. As a result, a 30% enhancement in device performance was observed.
[Show abstract][Hide abstract]ABSTRACT: Solution processable fullerene and copolymer bulk heterojunctions are widely used as the active layers of solar cells. In this work, scanning time-of-flight secondary ion mass spectrometry (ToF-SIMS) is used to examine the distribution of [6,6]phenyl-C61-butyric acid methyl ester (PCBM) and regio-regular poly(3-hexylthiophene) (rrP3HT) that forms the bulk heterojunction. The planar phase separation of P3HT:PCBM is observed by ToF-SIMS imaging. The depth profile of the fragment distribution that reflects the molecular distribution is achieved by low energy Cs(+) ion sputtering. The depth profile clearly shows a vertical phase separation of P3HT:PCBM before annealing, and hence, the inverted device architecture is beneficial. After annealing, the phase segregation is suppressed, and the device efficiency is dramatically enhanced with a normal device structure. The 3D image is obtained by stacking the 2D ToF-SIMS images acquired at different sputtering times, and 50 nm features are clearly differentiated. The whole imaging process requires less than 2 h, making it both rapid and versatile.