37 Search Results

Surface-pressure versus molecular area isotherms, X-ray reflectivity, and X-ray near-total reflection fluorescence were used to study the properties of 1-(3-methoxycarbonyl)propyl-1-phenyl[6,6]C{sub 61} (PCBM) that was pre-mixed with cesium carbonate and spread as a film at the air-water interface. The pre-mixed PCBM with cesium carbonate demonstrated a strikingly strong effect on the organization of the film. Whereas films formed from pure PCBM solution were rough due to strong inter-molecular interactions, the films formed from the mixture were much smoother. This indicates that the cesium carbonate moderates the inter-molecular interactions among PCBM molecules, hinting that the cesium diffusion observed in inverted organic photovoltaicmore » structures and the likely ensuing ionic Cs-PCBM interaction decrease aggregation tendency of PCBM. This implies that the use of cesium salts affects the morphology of the organic layer and consequently improves the efficiency of these devices.« less

Surface-pressure isotherms, X-ray reflectivity, and X-ray near-total reflection fluorescence were used to study the properties of 1-(3-methoxycarbonyl)propyl-1-phenyl[6,6]C₆₁ (PCBM) that was pre-mixed with cesium carbonate and spread as a film at the air-water interface. The pre-mixed PCBM with cesium carbonate demonstrated a strikingly strong effect on the organization of the film. Whereas films formed from pure PCBM solution were rough due to strong inter-molecular interactions, the films formed from the mixture were much smoother. This indicates that the cesium carbonate moderates the inter-molecular interactions among PCBM molecules, hinting that the cesium diffusion observed in inverted organic photovoltaics and the likely ensuingmore » ionic Cs-PCBM interaction decrease aggregation tendency of PCBM. As a result, this implies that the use of cesium salts affects the morphology of the organic layer and consequently improves the efficiency of these devices.« less

X-ray near-total-reflection fluorescence reveals that in multilayers of the inverted organic solar cell (ITO/CsI/P3HT:PCBM-based) Cs diffuses into the organic layer and iodide diffuses into the ITO. Laser ablation inductively coupled plasma mass spectrometry measurements, which integrate elemental concentration across the whole multilayer structure, indicate that the Cs:I ratio remains 1:1 confirming there is no loss of iodine from the sample. Iodide diffusion to the bulk ITO layer is also found in a similarly prepared ITO/NaI/P3HT:PCBM multilayer structure. Our results are consistent with recent XPS measurements which show that the Cs:I ratio at the ITO/CsI surface exceeds 8:1, and rationalize thismore » observation.« less

Multicolor microcavity ({mu}C) organic light-emitting diode (OLED) arrays were fabricated simply by controlling the hole injection and spacer MoO{sub 3} layer thickness. The normal emission was tunable from {approx}490 to 640 nm and can be further expanded. A compact, integrated spectrometer with two-dimensional combinatorial arrays of {mu}C OLEDs was realized. The MoO{sub 3} yields more efficient and stable devices, revealing a new breakdown mechanism. The pixel current density reaches {approx}4 A/cm{sup 2} and a maximal normal brightness {approx}140 000 Cd/m{sup 2}, which improves photoluminescence-based sensing and absorption measurements.

Intriguing electroluminescence (EL) spikes, following a voltage pulse applied to small molecular OLEDs, are discussed, elucidating carrier and exciton quenching dynamics and their relation to device structure. At low temperatures, all devices exhibit spikes at {approx} 70-300 ns and {mu}s-long tails. At 295 K only those with a hole injection barrier, carrier-trapping guest-host emitting layer, and no strong hole-blocking layer exhibit the spikes. They narrow and appear earlier under post-pulse reverse bias. The spikes and tails are in agreement with a revised model of recombination of correlated charge pairs (CCPs) and initially unpaired charges. Decreased post-pulse field-induced dissociative quenching ofmore » singlet excitons and CCPs, and possibly increased post-pulse current of holes that 'turn back' toward the recombination zone after having drifted beyond it are suspected to cause the spikes amplitude, which exceeds the dc EL.« less

No abstract prepared.

Conjugated polymer/fullerene composite films that exhibit steady-state phototransport properties of a unipolar or bipolar photoconductor, depending on the relative concentration of the components, are described. The observed behavior of the composites, in which each component has its own percolation path but its carrier content is not high enough to quench the carriers in the other component, is shown to be due to the coupling of the recombination processes in the two components.

Small angle x-ray scattering (SAXS), IR spectroscopy, and deuterium secondary ion mass spectrometry (DSIMS) were used to study the microstructure and hydrogen dynamics of undoped and boron-doped rf-sputter-deposited (RFS) and electron cyclotron resonance (ECR)-deposited hydrogenated amorphous silicon carbides (a-Si{sub 1{minus}x}C{sub x}:H) with x {le} 19 at.%. The SAXS measurements indicated residual columnar-like features and roughly spherical nanovoids of total content C{sub nV} {le} 1.0 vol.%. The growth of C{sub nV} with annealing was due largely to an increase in the average nanovoid radius. It was noticeably smaller than in RFS a-Si:H films. The IR spectra demonstrated H transfer by annealingmore » from mostly bulk-like Si-H groups to C-bonds. The H diffusion and its temperature dependence in undoped films resembled those of a-Si:H and were consistent with the SAXS and IR data. Suppression of long-range motion of most of the H atoms, consistent with increased C{sub nV}, was observed in B-doped ECR films. However, a small fraction of the H atoms appeared to undergo fast diffusion, reminiscent of the fast diffusion in doped a-Si:H. The results are consistent with impeded relaxation processes of the Si network, caused by the presence of C atoms, and H trapping at C-H bonds.« less

Long-range atomic H motion in hot-wire deposited (HW) a-Si:H is compared directly to that in glow-discharge deposited (GD) a-Si:H by monitoring the deuterium secondary ion mass spectrometry (DSIMS) profiles in [GD a-Si:H]/[GD a-Si:(H,D)]/[HW a-Si:H] multilayers vs annealing temperature and time. While the profiles in the GD layer are in excellent agreement with complenetary error-function behavior and previous studies, the profiles in the HW layer suggest that the multiple-trapping motion of the H and D atoms is much slower, possibly due to an interface layer of defects. However, an exponential tail of D atoms extends deep into the HW layer, probablymore » due to a long diffusion length of mobile D atoms, consistent with the established release times of H and D from the GD layer and H loss typical during growth of HW films. The results are also discussed in terms of the H exchange model and compared to previous NMR studies of HW a-Si:H, which suggest that most of the hydrogen in the HW layer is concentrated in H-rich clusters dispersed in a network of very low H content.« less

Small angle x-ray scattering (SAXS) and deuterium secondary-ion-mass spectrometry (DSIMS) studies of the microstructure and hydrogen dynamics in undoped rf-sputter-deposited (RFS) and undoped and boron-doped electron-cyclotron-resonance-deposited (ECR) hydrogenated amorphous silicon carbides (a-Si{sub 1-x}C{sub x}:H) are described. In the RFS carbides with x{<=}19 at. %, the SAXS indicated that the films contained elongated features larger than 20 nm with preferred orientation, consistent with a residual columnarlike growth of the films. In addition, the SAXS also included a clear nanostructural component consistent with roughly spherical nanovoids {approx}1.1 nm in diameter, of total content 0.5{<=}C{sub nV}{<=}1.0 vol. %. C{sub nV} increased by {approx}100%more » after isochronal 1-h annealing at 300, 350, and 375 degree sign C, followed by further annealing for 2-15 hours at 375 degree sign C. The growth of C{sub nV} was apparently due largely to a {approx}20% increase in the average void diameter. This growth was noticeably weaker than in similarly fabricated a-Si:H. In RFS carbides with x{<=}3 at. %, the DSIMS yielded power-law time dependent H diffusion constants D(t)=D{sub 00}({omega}t){sup -{alpha}}, where the dispersion parameter {alpha} varied from 0 to {approx}0.5{+-}0.1 among the samples, but was temperature independent at 350 degree sign {<=}T{<=}475 degree sign C. The moderate values of {alpha} are consistent with the moderate initial nanovoid contents C{sub nV}{<=}1.0 vol. % determined by SAXS. The weak dependence of {alpha} on T is consistent with the weaker growth of the SAXS with annealing as compared to a-Si:H. The values of activation energy E{sub a}(1000 Aa ) for a diffusion length L=1000 Aa among the different films were {approx}1.7, {approx}1.4, and {approx}0.65 eV. While the first two values are similar to those found in a-Si:H, the nature of the anomalously low value of {approx}0.65 eV is not clear. In undoped ECR a-Si{sub 0.86}C{sub 0.14}:H, D(t) exhibited a similar power-law time dependence, but {alpha} decreased from {approx}0.3 at 350 degree sign C and 400 degree sign C to {approx}0.1 at 450 degree sign C, also consistent with a low C{sub nV}. Thus, in spite of the high-C content, the behavior of {alpha} was similar to that of typical a-Si:H at lower temperatures, where it decreases at T{<=}350 degree sign C. However, E{sub a}(1000 Aa ) was an anomalously low {approx}1.0 eV. The evolution of the infrared (IR) spectra of both the RFS and ECR films showed that during annealing the Si-bonded H content decreases relative to the C-bonded H content, consistent with a transfer of H from Si- to C-bonded sites or hydrogen evolution. In addition, the reduction in the 2000-cm{sup -1} band characteristic of bulk-like Si-H group was much greater than the reduction of the 2100-cm{sup -1} band characteristic of surface Si-H, O-Si-H, and C-Si-H groups. Boron doping of the ECR carbides also reduced the bulklike Si-bonded H content, suggesting that it induces nanovoids, consistent with the observed suppression of long-range motion of most of the H and D atoms. However, a small fraction of the H atoms appeared to undergo fast diffusion, reminiscent of the fast diffusion in B-doped a-Si:H. (c) 1999 The American Physical Society.« less