Molecular Photovoltaics and Perovskite Solar Cells
Ecole Polytechnique Fédérale de Lausanne, Switzerland
ABSTRACT: Molecular photovoltaics has emerged as credible contender to conventional p-n junction photovoltaics. Mimicking light harvesting and charge carrier generation in natural photosynthesis, dye sensitized solar cells (DSCs) were the first to use three-dimensional mesoscopic junctions for solar electricity production, reaching currently a power conversion efficiency of close to 15 % in standard air mass 1.5 sunlight and 32% in ambient light. By now, large-scale DSC production and commercial sales have been launched for applications in building integrated PV and light-weight flexible power sources. Recently, the DSC has engendered perovskite solar cells (PSCs) whose meteoric rise has stunned the scientific community. The certified power conversion efficiency currently attains 25.2 %, exceeding that of the market leader polycrystalline silicon. PSCs produce high photovoltages rendering them attractive for applications in tandem cells and for the generation of fuels from sunlight. Recent progress in attaining long term operational stability with PSCs employing inorganic hole conductors as hole selective layer will be presented.
BIOGRAPHY: Michael Graetzel is a Professor at the School of Chemical Science & Engineering of the Ecole Polytechnique Fédérale de Lausanne (EPFL). He received his PhD from the Technical University in Berlin in 1971. After a postdoctoral training at the University of Notre Dame Indiana, USA, he joined EPFL since 1977. He published over 1500 peer-reviewed scientific papers regarding the conversion of solar energy to electricity and chemical fuels and storage of electricity in batteries. He pioneered research on energy and electron transfer reactions in mesoscopic systems and their use to generate electricity and fuels from sunlight. He is credited with moving the solar cell field beyond the principle of light absorption via diodes to the molecular level exploiting the sensitization of 3-dimensional networks of colloidal semiconductor oxides nanoparticles by dyes, pigments or quantum dots for light energy harvesting. His dye-sensitized solar cells engendered the advent of perovskite solar cells the most exciting break-through in the recent history of photovoltaics. His current research focuses on dye sensitized and perovskite solar cells as well as the photoelectrochemical generation of hydrogen and reduction of carbon dioxide semiconductor. Recent Honors and awards include the Zewail Prize for Molecular Science the Global Energy Prize, the Millennium Technology Grand Prize, the Marcel Benoist Prize, the King Faisal International Science Prize, the Albert Einstein World award of Science and the Balzan Prize. Elected member of several learned societies he holds eleven honorary doctor’s degrees from European and Asian Universities. His publications received over 270’000 citations, h = 239 (Web of Science) demonstrating the strong impact of his scientific work
A Revolution in Miniature Optical Clocks and Frequency Synthesizers
Kerry J. Vahala
California Institute of Technology, USA
ABSTRACT: Over the last few decades, the distinctly different strengths of photonics and electronics have been harnessed in entirely new ways by the optical frequency comb. As a bi-directional and coherent link, it has enabled sharing of performance attributes previously unique to these two worlds. The end result has been transformative for time keeping, frequency metrology, precision spectroscopy, microwave-generation, ranging and other technologies. Over the same span of time, resonantly enhanced parametric gain and four-wave mixing in high-Q microcavities were observed. And by cascading the four-wave mixing process to many orders a miniature frequency comb or `microcomb’ was demonstrated. Even more recently, the demonstration of coherently-pumped soliton mode locking in fiber resonators and in micro-resonators has been a major advance for these miniature frequency comb devices. As mode-locked optical oscillators, soliton operation provides stable repetition rates and reproducible waveforms, which are essential ingredients in all comb applications. Referred to as both `cavity solitons’ and `dissipative Kerr’ solitons, they have opened new perspectives on optical soliton physics and integrated comb systems on-a-chip. The latter includes system-level demonstration of massively-parallel WDM channel generation for coherent communication, C-band optical frequency synthesis with Hertz-level absolute accuracy, and optical clocks. In this talk the physics of soliton generation in high-Q micro-cavities that underlies these systems is reviewed. Finally, details on the integrated photonic synthesizer and its closely-related converse, the optical clock, are described.
BIOGRAPHY: Kerry Vahala is the Jenkins Professor and Professor of Applied Physics at Caltech. He has pioneered nonlinear optics in high-Q optical micro resonators. His research group has launched many of the areas of study in this field and invented optical resonators that hold the record for highest optical Q on a semiconductor chip. Vahala has applied these devices to a wide range of nonlinear phenomena and applications. This includes the first demonstration of parametric oscillation and cascaded four-wave mixing in a micro cavity - the central regeneration mechanisms for frequency micro combs; electro-optical frequency division - used in the most stable commercial K-band oscillators; and the first observation of dynamic back action in cavity optomechanical systems. His micro-resonator devices are used at the National Institute of Standards and Technology (NIST) in chip-based optical clocks and frequency synthesizers. They have also been used at the Keck II observatory in Hawaii as miniature astrocombs in the search for exoplanets. Vahala's current research is focused on the application of high-Q optical micro resonators to miniature precision metrology systems as well as monolithic optical gyroscopes. Professor Vahala was involved in the early effort to develop quantum-well lasers for optical communications and received the IEEE Sarnoff Award for his research on quantum-well laser dynamics. He has also received an Alexander von Humboldt Award for his work on ultra-high-Q optical microcavities, a NASA achievement award for application of frequency combs to exoplanet detection and is a fellow of the IEEE, the IEEE Photonics Society and the Optical Society of America.
Terahertz Frequency Topological Switches
Stanford University, USA
ABSTRACT: In this talk I will describe femtosecond-resolution crystallographic measurements probing dynamic switching responses driven by terahertz light pulses in topological Weyl semimetals, focusing on the 2D transition metal dichalcogenide WTe2. I will show that one can induce large amplitude interlayer shear oscillations with ~1% strain amplitudes, leading to a topologically distinct metastable phase not found under equilibrium conditions. Separate nonlinear optical measurements confirm that this transition is associated with a symmetry change and therefore corresponds to a transition to a topologically trivial phase. We further show that such shear strain serves as an ultrafast, energy-efficient means to induce more robust, well-separated Weyl points or to annihilate all Weyl points of opposite chirality. I will also discuss initial efforts investigating other means for manipulating the topological and ferroelectric phase diagram of this material through the application of pure electric fields and through doping/intercalation, showing a new low energy ferroelectric switching mechanism through interlayer gliding and the observation of controllable topological transport via a structural phase transition. This work defines new possibilities for ultrafast manipulation of the topological and ferroelectric properties of solids and for a topological switch operating at THz frequencies.
BIOGRAPHY: Aaron M. Lindenberg is an Associate Professor at Stanford University with joint appointments in the Department of Materials Science and Engineering and the Department of Photon Science. He received his B.A. from Columbia University in 1996 and his Ph.D. in Physics from the University of California, Berkeley in 2001. He was named a Faculty Fellow at Berkeley from 2001-2003 and then became a staff scientist at the SLAC National Accelerator Laboratory from 2003-2007. He is a winner of the DARPA Young Faculty Award, the Department of Energy Outstanding Mentor Award, the Alfred Moritz Michaelis Prize, and was named a Terman Fellow and a Chambers Faculty Scholar at Stanford and an I.I. Rabi Scholar at Columbia.