Showing papers by "Jan D'Haen published in 2006"

Tuning the dimensions of C60-based needlelike crystals in blended thin films

Abstract: A new ordered structure of the C 60 derivative PCBM ([6-6]-phenyl C 61 -butyric acid methyl ester) is obtained in thin films based on the blend PCRM:regioregular P3HT (poly(3-hexylthiophene)). Rapid formation of needlelike crystalline PCBM structures of a few micrometers up to 100 μm in size is demonstrated by submitting the blended thin films to an appropriate thermal treatment. These structures can grow out to a 2D network of PCBM needles and, in specific cases, to spectacular PCBM fans, Key parameters to tune the dimensions and spatial distribution of the PCBM needles are blend ratio and annealing conditions. The as-obtained blended films and crystals are probed using atomic force microscopy, transmission electron microscopy, selected area electron diffraction, optical microscopy, and conlbcal fluorescence microscopy. Based on the analytical results, the growth mechanism of the PCBM structures within the film is described in tennis of diffusion of PCBM towards the PCBM crystals. leaving highly crystalline P3HT behind in the surrounding matrix.

Seeding, growth and characterization of nanocrystalline diamond films on various substrates

Abstract: By optimizing seeding techniques and plasma parameters for deposition, nanocrystalline diamond (NCD) films were deposited on different sorts of substrates with varying parameters such as temperature and methane content. Results of this optimization process were analysed by SEM, XRD, Raman, and transmission measurements. Experiments with titanium interlayers lead to a better understanding of the nucleation step and can enhance the nucleation density. This research resulted in homogeneously coalesced NCD films of less than 100 nm thickness over 3 inch Si wafers. It is also shown that it is possible to grow on different sorts of glass, using a low temperature process.

Dual crystallization behaviour of polythiophene/fullerene blends

Abstract: The nanoscale morphology of the active layer in bulk-heterojunction solar cells consisting of regioregular poly(3-hexylthiophene-2, 5-diyl) (P3HT) and methanofullerene([6-6]-phenyl C 61 butyric acid methyl ester) (PCBM) was extensively studied using Atomic Force Microscopy (AFM), Transmission Electron Microscopy (TEM), X-Ray Diffraction (XRD) and optical microscopy. Different weight ratios of P3HT:PCBM were investigated as a function of annealing temperature and time revealing the occurrence of crystallization of both components. Firstly, the as-prepared films can be described as a semi-crystalline blend. Secondly, it has been demonstrated that for a short annealing time (5 min) at lower annealing temperatures (75–115 °C) an increased crystallization of P3HT occurs. Thirdly, it has been observed that a prolonged annealing at the given temperature range or a short annealing at higher temperatures ( ≥ 120 °C) leads to the formation of a new ordered crystalline structure of PCBM. These new ordered structures, a few μ m up to 100 μ m in length, form a network of needle-like and even fan-shaped crystals. Key-parameters to “tune” this new ordered structure of PCBM are blend ratio and annealing conditions. The growth mechanism of these new PCBM-structures is described by means of diffusion.

Synthesis of zirconia–alumina and alumina–zirconia core–shell particles via a heterocoagulation mechanism

Abstract: This study describes the synthesis of core–shell particles, consisting of a ZrO 2 or Al 2 O 3 submicron nucleus coated by a nanolayer of Al 2 O 3 or ZrO 2 , respectively. The oxide layers around the cores are deposited via a heterocoagulation route, based on the attraction of oppositely charged core and shell particles. TEM micrographs clearly show a homogeneous Al 2 O 3 shell (originating from boehmite or γ-Al 2 O 3 particles) around the ZrO 2 cores and in the other case, a ZrO 2 layer (originating from hydrothermally prepared ZrO 2 ) around the submicron Al 2 O 3 cores. From PCS measurements, it can also be deduced that the cores are enwrapped by a shell and it is calculated that the thickness of the Al 2 O 3 shell is about 30–35 nm and the ZrO 2 layer is approximately 80 nm. The coated powders are additionally characterized by XRD.

Thin-film polycrystalline-silicon solar cells on high-temperature glass based on aluminum-induced crystallization of amorphous silicon

Abstract: Efficient thin-film polycrystalline-silicon (pc-Si) solar cells on foreign substrates could lower the price of photovoltaic electricity. Aluminum-induced crystallization (AIC) of amorphous silicon (a-Si) followed by epitaxial thickening at high temperatures seems a good way to obtain efficient pc-Si solar cells. Due to its transparency and low cost, glass is well suited as substrate for pc-Si cells. However, most glass substrates do not withstand temperatures around 1000°C that are needed for high-temperature epitaxial growth. In this paper we investigate the use of experimental transparent glass-ceramics with high strain-point temperatures as substrates for pc-Si solar cells. AIC seed layers made on these substrates showed in-plane grain sizes up to 16 µm. Columnar growth was observed on these seed layers during high-temperature epitaxy. First pc-Si solar cells made on glass-ceramic substrates in substrate configuration showed efficiencies up to 4.5%, fill factors up to 75% and open-circuit voltage (Voc) values up to 536 mV. This is the highest Voc reported for pc-Si thin-film solar cells on glass and the best cell efficiency reported for cells made by AIC on glass.

Additive-free Hydrothermal Synthesis of High Aspect Ratio ZnO Particles from Aqueous Solution

Abstract: In this work, a new hydrothermal process is described, in which, for the first time, an aquatic Zn2+ precursor is used for the synthesis of high aspect ratio ZnO particles, without the presence of ...

Record Voc-Values for Thin-Film Polysilicon Solar Cells on Foreign Substrates using a Heterojunction Emitter

Abstract: Thin-film polysilicon solar cells on foreign substrates are often considered as a promising low cost alternative to bulk silicon solar cells. Until now however, the obtained efficiencies and open-circuit voltages are far below those of other technologies. In this paper, we show how the open-circuit voltage can be enhanced significantly using an amorphous silicon ? crystalline silicon heterojunction emitter instead of a diffused homojunction emitter. Open-circuit voltages up to 536 mV were obtained for polysilicon layers with a heterojunction emitter. This is the highest open-circuit voltage obtained for polysilicon solar cells with a p-n structure on foreign substrates.

Cover Picture: Tuning the Dimensions of C60‐Based Needlelike Crystals in Blended Thin Films (Adv. Funct. Mater. 6/2006)

Abstract: A new ordered structure of the C60 derivative PCBM is obtained in thin films based on the blend PCBM:P3HT, as detailed by Swinnen, Manca, and co-workers on p. 760. Needlelike crystalline PCBM structures, whose dimensions and spatial distribution ca be tuned by adjusting the blend ratio and annealing conditions, are formed. In typical solar-cell applications of these blended films, these results indicate that during long-term operation under normal conditions (50–70 °C) morphology changes and a decrease in cell performance could occur. A new ordered structure of the C60 derivative PCBM ([6-6]-phenyl C61-butyric acid methyl ester) is obtained in thin films based on the blend PCBM:regioregular P3HT (poly(3-hexylthiophene)). Rapid formation of needlelike crystalline PCBM structures of a few micrometers up to 100 μm in size is demonstrated by submitting the blended thin films to an appropriate thermal treatment. These structures can grow out to a 2D network of PCBM needles and, in specific cases, to spectacular PCBM fans. Key parameters to tune the dimensions and spatial distribution of the PCBM needles are blend ratio and annealing conditions. The as-obtained blended films and crystals are probed using atomic force microscopy, transmission electron microscopy, selected area electron diffraction, optical microscopy, and confocal fluorescence microscopy. Based on the analytical results, the growth mechanism of the PCBM structures within the film is described in terms of diffusion of PCBM towards the PCBM crystals, leaving highly crystalline P3HT behind in the surrounding matrix.

Stress in Next Generation Interconnects

Abstract: Stress is becoming an increasingly critical parameter for all steps in back‐end‐of‐line integration. As the k‐value of dielectric spacers decreases their mechanical integrity scales accordingly, making the interconnect stack increasingly sensitive to barrier and copper stresses. These stresses need to be studied not only for as‐deposited layers, but also during thermal processing. Stress relaxation of the barrier can for instance occur at elevated temperatures and result in severe dielectric deformation. Copper typically relaxes at elevated temperatures during thermal cycling and then builds‐up thermal stress when cooled back to room temperature. However, the initial stress state can have a strong effect on the final microstructure. An example is shown here where a new growth mode, named ‘super‐secondary‐grain‐growth, is stress induced and leads to grains of many tens of microns.

How stable are polymer:PCBM bulk heterojunction solar cells?

Abstract: This paper investigates the thermal stability of organic bulk heterojunction solar cells, with a special focus on the thermal ageing of both photovoltaic parameters and morphology of the active layer. The photovoltaic parameters of a set of bulk heterojunction solar cells were determined by IV-characterization and their bulk morphology was investigated with transmission electron microscopy (TEM). A link could be made between the degradation of the short circuit current under a thermal treatment and the corresponding change in bulk morphology. A possible improvement of the thermal stability of bulk heterojunction solar cells is presented through the use of a polymer with higher glass transition temperature.

Influence of Grain Orientation on the Microstructural Characterization in Cu During (self)-Anneal using a Surface Acoustic wave Technique

Abstract: This paper describes the influence of microstructure, or more specifically grain orientation and grain size, on the in-line monitoring of copper interconnect properties during (self)-anneal using surface acoustic wave spectroscopy (SAWS). In electroplated Cu after (self)-anneal the SAWS frequency is lower for samples annealed at higher temperature because of the lower porosity induced elasticity. In sputtered Cu, the SAWS frequency shows a clear correlation with grain size, which is induced by a strong re-orientation of the copper film from the as-deposited (111) texture (E=190 GPa) to a strongly (100) textured super grain structure (E=78GPa).