Integrated Calendar

For the purpose of identifying novel absorber materials based on experimental or computational material screening, it is useful to identify the basic ingredients required to make a good solar cell out of the combination of different absorber and contact materials. Figures of merit are needed that quantify whether a certain material is likely to perform well as a solar cell. To answer the question, which parameters are most important, we look into the key properties of good solar cells such as high absorption coefficient, mobility and charge carrier lifetime and study their interdependences and how they determine the efficiency at different thickness of the solar cell. Finally, we study some microscopic parameters such as the effective mass or electron-phonon coupling in a device to identify key microscopic properties that are likely to lead to a combination of high absorption, high mobilities and long lifetimes and thereby high photovoltaic efficiencies

The seminar summarizes three recent studies (1,2,3) since that share some common elements: they concern porous materials and the method used is NMR spectroscopy. Yet, the aims differ. In the first study (1), the unknown is the pore structure. In particular, pore structure in hydrogels is difficult to access as water cannot be removed without affecting the pores and in the presence of water the well-honed gas sorption and mercury porosimetries just do not work. The method we invented to remedy this situation is called size-exclusion quantification (SEQ) NMR and it can be seen as the multiplexed analogue of inverse size exclusion chromatography. In effect, we sample by diffusion NMR the size distribution in a polydisperse polymer solution before and after it had been equilibrated with a porous matrix. Size-dependent polymer ingress reveals the pore structure. The method has several advantages over possible alternatives, not least its speed. In the second study (2), we sample the self-diffusion of neat water and other molecules like dimethyl sulfoxide (DMSO) and their mixtures by NMR diffusion experiments for those fluids imbibed into controlled pore glasses (CPG, pore size range 7.5 to 73 nm). Their highly interconnected structure is scaled by pore size and exhibits pore topology independent of size. Relative to the respective diffusion coefficients obtained in bulk phases, we observe a reduction in the diffusion coefficient that is independent of pore size for the larger pores and becomes stronger toward the smaller pores. Geometric tortuosity governs the behavior at larger pore sizes, while the interaction with pore walls becomes the dominant factor toward smaller pore diameters. Deviation from the trends predicted by the popular Renkin equation and variants (4) indicates that the interaction with the pore wall is not just a simple steric one. In the third study (3), the porous material is hydrated cellulose. In that matrix, we identify by using 2H MAS NMR two different groups of water molecules being in slow exchange with each other. Water molecules in one of the groups exhibit anisotropic molecular motions with a high order parameter. Based on, among other things, the observed behavior with increasing vapor pressure, we argue that this water is an integral structural element of the cellulose fibril, that itself is an aggregate of the basic units, the cellulose nanofibrils.

We use a local magnetic imaging technique, scanning SQUID microscopy, to map the spatial distribution of electronic states near surfaces and interfaces. We track conductivity, superconductivity and magnetism in systems undergoing phase transitions, where the local picture is particularly meaningful. I will describe two measurements: At the superconductor-insulator transition in NbTiN we map superconducting fluctuations and detect a non-trivial behavior near the quantum critical point. Near the metal to insulator transition at the 2D LaAlO3/SrTiO3 interface, we find that the conduction landscape changes dramatically and identify the way different types of defects control the behavior.

Retinal prostheses represent an exciting development in science, engineering, and medicine – an opportunity to exploit our knowledge of neural circuitry and function to restore or even enhance vision. However, although existing retinal prostheses demonstrate proof of principle in treating incurable blindness, they produce limited visual function. Some of the reasons for this can be understood based on the exquisitely precise and specific neural circuitry that mediates visual signaling in the retina. Consideration of this circuitry suggests that future devices may need to operate at single-cell, single-spike resolution in order to mediate naturalistic visual function. I will show large-scale multi-electrode recording and stimulation data from the primate retina indicating that, in some cases, such resolution is possible. I will also discuss cases in which it fails, and propose that we can improve artificial vision in such conditions by incorporating our knowledge of the visual system in bi-directional devices that adapt to the host neural circuity. Finally, I will briefly discuss the potential implications for other neural interfaces of the future.