Michael Krüger

Bulk-heterojunction hybrid solar cells based on colloidal nanocrystals and conjugated polymers

Emerging alternative photovoltaic technologies such as dye sensitized solar cells (DSSCs) and organic solar cells (OSCs) have recently gained much attention as well as maturity and are on the step of being commercialized. Bulk heterojunction hybrid solar cells containing inorganic nanoparticles and semiconducting polymers are still lagging behind in respect of device performance although they have theoretically the potential to exhibit better performances than devices containing solely organic compounds. In this article we review the recent state of the art development of bulk heterojunction hybrid solar cells. Critical factors limiting the solar cell device performance are highlighted and strategies for further device improvement are demonstrated by giving recent examples from literature.

Improved efficiency of hybrid solar cells based on non-ligand-exchanged CdSe quantum dots and poly(3-hexylthiophene)

We report on bulk-heterojunction hybrid solar cells based on blends of non-ligand-exchanged CdSe quantum dots (QDs) and the conjugated polymer poly(3-hexylthiophene) with improved power conversion efficiencies of about 2% under AM1.5G illumination after spectral mismatch correction. This is the highest reported value for a spherical CdSe QD based photovoltaic device. After synthesis, the CdSe QDs are treated by a simple and fast acid-assisted washing procedure, which has been identified as a crucial factor in enhancing the device performance. A simple model of a reduced ligand sphere is proposed explaining the power conversion efficiency improvement.

Synthesis, Stabilization, Functionalization and, DFT Calculations of Gold Nanoparticles in Fluorous Phases (PTFE and Ionic Liquids)

Gold nanoparticles (Au-NPs) were reproducibly obtained by thermal, photolytic, or microwave-assisted decomposition/reduction under argon from Au(CO)Cl or KAuCl(4) in the presence of n-butylimidazol dispersed in the ionic liquids (ILs) BMIm(+)BF(4)(-), BMIm(+)OTf(-), or BtMA(+)NTf(2)(-) (BMIm(+) = n-butylmethylimidazolium, BtMA(+) = n-butyltrimethylammonium, OTf(-) = (-)O(3)SCF(3), NTf(2)(-) = (-)N(O(2)SCF(3))(2)). The ultra small and uniform nanoparticles of about 1-2 nm diameter were produced in BMIm(+)BF(4)(-) and increased in size with the molecular volume of the ionic liquid anion used in BMIm(+)OTf(-) and BtMA(+)NTf(2)(-). Under argon the Au-NP/IL dispersion is stable without any additional stabilizers or capping molecules. From the ionic liquids, the gold nanoparticles can be functionalized with organic thiol ligands, transferred, and stabilized in different polar and nonpolar organic solvents. Au-NPs can also be brought onto and stabilized by interaction with a polytetrafluoroethylene (PTFE, Teflon) surface. Density functional theory (DFT) calculations favor interactions between IL anions instead of IL cations. This suggests a AuF interaction and anionic Au(n) stabilization in fluorine-containing ILs. The (19)F NMR signal in BMIm(+)BF(4)(-) shows a small Au-NP concentration-dependent shift. Characterization of the dispersed and deposited gold nanoparticles was done by transmission electron microscopy (TEM/HRTEM), transmission electron diffraction (TED), dynamic light scattering (DLS), UV/Vis absorbance spectroscopy, scanning electron microscopy (SEM), electron spin resonance (ESR), and electron probe micro analyses (EPM, SEM/EDX).

Fabrication and characterization of buckypaper-based nanostructured electrodes as a novel material for biofuel cell applications

The fabrication process of buckypapers (BPs) made from stable suspensions of as-received or functionalized multi-walled carbon nanotubes (MWCNTs) with high purity (97.5 wt%, Baytubes), their characterization and their utilization towards novel biofuel cell electrode applications are reported. The BPs can vary in thickness between 1 μm and 200 μm, are mechanically robust, flexible, stable in solvents, possess high meso-porosities as well as high apparent electrical conductivities of up to 2500 S m(-1). Potentiodynamic measurements of biocathodes based on bilirubin oxidase (BOD)-decorated BPs for the oxygen reduction reaction (ORR) in neutral media (phosphate buffer solution) containing glucose indicate that BP electrodes based on functionalized MWCNTs (fBPs) perform better than BP electrodes of as-received MWCNTs and have high potential as an effective electrode material in biofuel cells and biosensors.

A facile synthesis method to silica coated CdSe/ZnS nanocomposites with tuneable size and optical properties

A facile method for the preparation of single Quantum Dots (QDs) in silica spheres with tunable size and optical property has been developed. A time interval addition of silica precursor method has been used to increase the size of the silica shell and in parallel tuning the photoluminescence property. The resulting CdSe/ZnS/SiO(2) nanocomposites were characterized by transmission electron microscopy, photoluminescence spectroscopy and zeta-potential measurements. Since for different applications of silica coated core-shell nanoparticles, control over the thickness of the shell in parallel with tuning on the optical property of the final product required, these study could pave the way for the application of this nanocomposites in different fields like photonic crystals, biolabeling, etc.

Photo-induced charge recombination kinetics in low bandgap PCPDTBT polymer:CdSe quantum dot bulk heterojunction solar cells

The interfacial charge transfer recombination processes under working conditions that limit the device performance in polymer:CdSe quantum dot bulk heterojunction hybrid solar cells have been measured. The recombination lifetimes for electrons and holes in the device show an exponential dependence in a similar way to that observed for other molecular based solar cells such as bulk heterojunction organic solar cells (OSC) and dye sensitized solar cells (DSSC). The implications of this unprecedented observation on the design of novel devices are discussed as well as the relationship between the charge accumulation in these devices under operation and the device open-circuit voltage.

Bright luminescent, colloidal stable silica coated CdSe/ZnS nanocomposite by an in situ, one-pot surface functionalization.

In this article, a systematic study of the design and development of surface-modification schemes for silica coated nanocomposite via an in situ, one-pot way is presented. Silica coated CdSe/ZnS nanoparticles were prepared in a water-in-oil microemulsion and subsequently surface modified via addition of various organosilane reagents to the microemulsion system. The resulting functionalized composite nanoparticles were characterized by different techniques like Transmission Electron Microscopy (TEM), photoluminescence spectroscopy and zeta-potential measurements. The results demonstrate that depending on the sequence of addition of silica precursors and organosilanes the product can show bright luminescence or considerable colloidal stability. The organosilanes molecules which are used here, act both as a stabilizer of the microemulsion system (regarding the charge compensation) and as a functional group the final product on top of silica shell. Using these surface-modification process, silica coated nanoparticles can be more readily conjugated with biomolecules and used as highly luminescent, sensitive, and reproducible labels in bioanalytical applications. Most importantly such surface functionalization could pave the way for controlled multi-mixed nanoparticles encapsulation (for example magnetic and QD nanoparticles).

Atomic layer deposition on phase-shift lithography generated photoresist patterns for 1D nanochannel fabrication.

A versatile, low-cost, and flexible approach is presented for the fabrication of millimeter-long, sub-100 nm wide 1D nanochannels with tunable wall properties (wall thickness and material) over wafer-scale areas on glass, alumina, and silicon surfaces. This approach includes three fabrication steps. First, sub-100 nm photoresist line patterns were generated by near-field contact phase-shift lithography (NFC-PSL) using an inexpensive homemade borosilicate mask (NFC-PSM). Second, various metal oxides were directly coated on the resist patterns with low-temperature atomic layer deposition (ALD). Finally, the remaining photoresist was removed via an acetone dip, and then planar nanochannel arrays were formed on the substrate. In contrast to all the previous fabrication routes, the sub-100 nm photoresist line patterns produced by NFC-PSL are directly employed as a sacrificial layer for the creation of nanochannels. Because both the NFC-PSL and the ALD deposition are highly reproducible processes, the strategy proposed here can be regarded as a general route for nanochannel fabrication in a simplified and reliable manner. In addition, the fabricated nanochannels were used as templates to synthesize various organic and inorganic 1D nanostructures on the substrate surface.

Individual filamentous phage imaged by electron holography

An in-line electron hologram of an individual f1.K phage was recorded with a purpose-built low energy electron point source (LEEPS) microscope. Cryo-microscopic methods were employed to prepare the specimen so that a single phage could be presented to the coherent low energy electrons: An aqueous phage suspension was applied to a thin carbon membrane with micro-machined slits. The membrane was rapidly cooled to freeze the remaining water as an amorphous ice sheet, which was then sublimated at low temperatures and pressures to leave individual free-standing phages suspended across slits. An image of a phage particle, depicted as the amplitude of the object wave, was reconstructed numerically from a digitized record of the hologram, obtained using 88xa0eV coherent electrons. The reconstructed image shows a single phage suspended across a slit in a supporting carbon membrane, magnified by a factor of 100,000. The width and shape in the reconstructed image compared well with a TEM image of the same filament. It is thus possible to record and reconstruct electron holograms of an individual phage. The challenge now is to improve the resolution of reconstructed images obtained by this method and to extend these structural studies to other biological molecules.

Blocked-micropores, surface functionalized, bio-compatible and silica-coated iron oxide nanocomposites as advanced MRI contrast agent

Biocompatible magnetic nanoparticles have been found promising in several biomedical applications for tagging, imaging, sensing and separation in recent years. In this article, a systematic study of the design and development of surface-modification schemes for silica-coated iron oxide nanoparticles (IONP) via a one-pot, in situ method at room temperature is presented. Silica-coated IONP were prepared in a water-in-oil microemulsion, and subsequently the surface was modified via addition of organosilane reagents to the microemulsion system. The structure and the morphology of the as synthesized nanoparticles have been investigated by means of transmission electron microscopy (TEM) and measurement of N2 adsorption–desorption. Electron diffraction and high-resolution transmission electron microscopic (TEM) images of the nanoparticles showed the highly crystalline nature of the IONP structures. Nitrogen adsorption indicates microporous and blocked-microporous structures for the silica-coated and amine functionalized silica-coated IONP, respectively which could prove less cytotoxicity of the functionalized final product. Besides, the colloidal stability of the final product and the presence of the modified functional groups on top of surface layer have been proven by zeta-potential measurements. Owing to the benefit from the inner IONP core and the hydrophilic silica shell, the as-synthesized nanocomposites were exploited as an MRI contrast enhancement agent. Relaxometric results prove that the surface functionalized IONP have also signal enhancement properties. These surface functionalized nanocomposites are not only potential candidates for highly efficient contrast agents for MRI, but could also be used as ultrasensitive biological-magnetic labels, because they are in nanoscale size, having magnetic properties, blocked-microporous and are well dispersible in biological environment.