Open PhD-Positions (scholarships available)
Research Areas
1. Nanofabrication
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| Figure 1. self-assembled lines of gold nanoparticles (30nm) on a silicon substrate (atomic force microscopic picture) |
Current micro and nanofabrication methods (e.g. silicon chip production) are based on top-down approaches using photolithographic methods. Alternatively devices can be assembled through bottom-up approaches using nanoscale building blocks of metals, semiconductors and insulators. Inorganic chemistry offers a wide range of techniques to synthesise such nanoparticles with excellent control over their size and shape. Single nanoparticles and even single atoms can be manipulated on surfaces using techniques such as atomic force or surface tunnelling microscopy. However these methods are inherently slow and will never be a viable option for the mass production of nanoscale devices. Our group aims to develop methods that rely on molecular self-recognition to direct the assembly of nanoparticle building blocks to allow the formation of any desired nanostructure. Short synthetic strands of DNA selectively bind to their complementary counterparts through Watson-Crick base-pairing. We will use this highly selective recognition behaviour to direct the assembly of particles to larger functional structures with unique optical, electrical and mechanical properties. Our project is carried out in collaboration with Professor Paul Alivisatos at the University of California Berkeley as well as the Molecular Foundry at the Lawrence Berkeley National Laboratory (USA).
2. Dye-Sensitized Solar Cells
Dye-sensitized solar cells have become a viable alternative to conventional silicon solar cells with confirmed conversion efficiencies of 11%. Synthetic dyes take over the function of the light absorber in these devices, similar to the chlorophyll molecule in the natural photosynthesis process. In our group we are focusing on the fabrication of dye-sensitized solar cells on flexible substrates using solid-state and ionic liquid electrolytes. Click here to visit our Dye-sensitized Solar Cell Group.
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Figure 2. Schematic of operation of a dye-sensitized electrochemical photovoltaic cell. The photoanode, made of a mesoporous dye-sensitized semiconductor, receives electrons from the photo-excited dye (S*) which is thereby oxidized, and which in turn oxidizes the mediator, a redox species dissolved in the electrolyte. The mediator is regenerated by reduction at the cathode by the electrons circulated through the external circuit. Figure courtesy of EPFL - LPI. |
3. Third Generation Photovoltaics
The existing generations of solar cells are slowly reaching their theoretical and practical cost and efficiency limits. This project looks beyond the horizon to the materials and technologies required to develop the next generation of photovoltaic devices that will be cheaper, cleaner and more efficient to supply our energy needs for the future. It will focus on the use of nanostructured electromaterials and molecular functional materials as building blocks for a new generation of organic hybrid solar cells. Combining these materials and assembly strategies with novel concepts developed in the area of conventional photovoltaics (e.g. tandem solar cells, upconversion of photons) will enable us to realize plastic solar cells with efficiencies beyond the theoretical limitations of conventional single junction solar cells. Our current focus is on the study of dye-sensitized hole-injection into p-type semiconductors for the development of dye-sensitized tandem solar cells.
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Figure 3. Scheme of a dye-sensitized tandem solar cell. The relative energy levels of the semiconductor (valence and conduction band), the dye (highest and lowest molecular orbital) and the redox mediator are shown. Photoexcitation of dye 1 results in ultrafast injection of an electron into the conduction band of the n-type semiconductor (e.g. TiO2). The ground state of the thereby photooxidized dye is regenerated by hole transfer to a neighbouring redox mediator in the electrolyte. Photoexcitation of dye 2, absorbed to the mesoporous p-type semiconductor electrode of the photocathode, results in the injection of a positive charge into the valence band of the p-type semiconductor. The remaining negative charge on the dye is then transferred to a redox mediator in solution (e.g. iodine/iodide). The absorption of two photons thereby gives rise to one high energy electron hole pair that can be collected at the backcontacts of the two electrodes. Each hole transfer is equivalent to an electron transfer in the opposite direction .The two are distinguished for illustrative purposes only. | 
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