Thomas Frauenheim (University of Bremen, Germany)
Oleg Prezhdo (University of Southern California, L. A., US)
Sheng Meng (Institute of Physics, CAS Beijing, China)
Johannes Lischner (Imperial College London, UK)
Location: University of Bremen, Germany,
10th until 14th of October 2016
There is enormous interest in understanding and controlling photo-induced charge transfer and chemical reactions for energy storage. These can be due either to water splitting and carbon dioxide reduction or by electron-hole pair separation at hybrid chromophore- or hybrid polymer-solid interfaces in photovoltaic devices, stimulating an increasing number of experimental and theoretical studies. Computational atomistic studies of experimental realistic setups require models that include an inorganic semiconductor nanostructure, acting as a catalyst and organic molecules in solvents. In photovoltaic applications, e.g. one has to consider multi-component systems, involving several chromophores tuned to absorb different wavelengths of light, an acceptor that removes an electron from the chromophores and creates separated electron-hole pairs, as well as electron and hole conducting media. Such models already may involve hundreds to thousands of atoms, extending far beyond the limits of any ab initio calculations. Furthermore, the non-equilibrium processes involved in the photo-induced charge separation and transport require explicit time domain modelling. Relevant processes occur on ultrafast time-scales and in most cases cannot be described by rate expressions. Charge separation, Auger-type energy exchange between electrons and holes, generation of additional charges by Auger mechanisms, energy losses to heat due to charge-phonon interactions, charge and energy transfer, and electron-hole recombination occur in parallel and competition requiring significant efforts in method development and clarification of multiple conceptual problems.
As major scientific objectives of the proposed workshop we have achieved:
- Bringing together researchers from quantum chemistry and computational solid state physics working on photo-catalysis and photovoltaics. We were able to highlight recent progress and discuss challenges and opportunities in the materials aspect of tailor-made nanostructures and hybrid interfaces for highly efficient energy applications.
- We have fostered the exchange of methodological expertise and new developments between scientists working on different aspects of metal oxide photo-catalysis.
- We discussed possibilities for optimizing the materials properties and device design. The interdisciplinary character of the workshop helped finding solutions for overcoming current limitations.
- The workshop stimulated new worldwide interdisciplinary collaborations on computational photo-catalysis and photovoltaics for the mutual benefit of theoretical, experimental and applied researchers.
The program consisted of 30 invited talks of 40 minutes (35+5) each and one poster session presenting 39 posters. In addition, many social events (reception and conference dinner) to allow for informal exchange were held. The invited talks were given by well-established scientists from the different theoretical communities, which acted as platform for interesting cross-/interdisciplinary discussions. The invited talks were followed by a poster session where the younger participants could show their scientific work and exchange of ideas with a broad knowledge in computational chemistry, solid state physics and computational materials science. The organization was very compact with the scientists accommodated in the same hotel fostering exchange and discussion between the participants also outside the meeting room.
Financial support from the DFG, Psi-k Network, Fonds der Chemischen Industrie im Verband der Chemischen Industrie e.V., and the German CECAM node multi-scale modelling from first principles, cecam-mm1p.de and the University Bremen is gratefull acknowledged.
2. Scientific content, main outcome of key presentations, selected discussions
Currently, theoretical studies of light-induced processes at interfaces usually fall in one of two broad categories: i) modelling of the atomic structure and ground state electronic properties of complex interfaces and ii) simulation of light-matter interactions and electronic excited states in relatively simple systems. For example, several talks at the conference addressed the atomic surface structure of photocatalysts, such as titanium dioxide, and discussed the complex interaction of these surfaces with adsorbed atoms and molecules. Other talks addressed excited states of such photocatalysts with high-level methods, such as many-body perturbation theory. For a full understanding of photocatalysis and other light-induced processes at interfaces, it is necessary to combine these two aspects. We therefore expect and hope that in the near future more studies will attempt to bridge and connect these categories, i.e. simulate the interaction of light with matter at realistically complex systems.
For the light-matter interaction, there has been an increasing number of studies using the real-time formalism. These studies give important insights into the kinetics of light-induced processes at interfaces. However, the increased numerical effort of these simulations usually necessitates the use of approximate theories, such as time-dependent density-functional theory with its well-known limitations. Conversely, higher-level methods, such as quantum chemical wavefunction approaches or the Bethe-Salpeter equation, can only be applied with a linear-response framework. We expect that the next 2-3 years will see the transfer of high-level methods from the frequency-domain to the real-time domain. This would open up the description of exciton dynamics in heterogeneous systems which are highly relevant to photocatalysis and photovoltaics.
3. Assessment of the results and impact on future direction of the field
A major obstacle to the accurate description of light-induced processes at interfaces is the intrinsic interdisciplinarity of the subject. The study of such processes requires knowledge of physics, chemistry, materials science and even biology. Therefore, advancing our understanding of photocatalysis and photovoltaics necessitates a joint effort from experts in different fields. To enable such collaborations, it is of crucial importance to organize interdisciplinary workshops like ours to act as platforms for exchanging ideas and for bringing together researchers from different subject areas who work on different aspects of the same topic. In the future, we will try to continue organizing workshop to achieve this goal.
The workshop became a forum to discuss about possible solutions of improving the quality of hybrid interfaces for studying electron dynamics and charge transfer reactions and correlating experiment and theory on a highly predictive level. We have been able to achieve the following key objectives:
- In the discussions we have identified the major problems in our current understanding of novel photoactive materials with a focus on lateral and multi-layer stacking effects (moiré structures, novel electronic states), transport (heat and charge carrier), contacts, quantum confinement, and doping. To this end we brought together experiment and different theory communities seeking for predictive power and general understanding of electronic properties of novel photoactive materials. Invited overview talks by highly recognised experimentalists from different parts of the field (transport and STM of organic and inorganic layered materials, topological insulators, biological sensors, supercapacitors, solar cells, and lithium ion batteries) and related computational talks have contributed to the general understanding.
- We have summarised the major achievements from communities working on different time-dependent approaches to study the coupled electron-ion dynamics in organic and hybrid materials, and have identified common problems. The workshop has stimulated knowledge exchange across the boundaries of formerly rather separate communities.
- In the discussions the main advantages and shortcomings of currently available theoretical techniques to model and understand the electronic, electrical and charge transfer properties of novel photoactive materials have been specified. The techniques considered and discussed comprise (but not be limited to): density functional theory (LDA, GGA, LDA+U, etc), TD-DFT, GW and BSE quasi-particle methods, quantum transport techniques (Landauer Büttiker, Kubo formula), multi-scale approaches, quantum lattice models, many-body theory and quantum optics.
- During the period of the workshop possible solutions in optimizing the quality and properties of novel photoactive materials and fabricating new devices have been outlined. The mutual exchange between researchers from both experiments and computational materials science helped a lot to better understand current problems in synthesis and application of photoactive materials in photocatalysis and photovoltaics, and determined the priority target of questions to be addressed by state-of-the-art first-principle methods.
4. Infrastructure requirements to make advances in the field
As discussed above, the advancement of theories of light-induced processes at interfaces requires the development of novel theories and codes which can i) capture the inherent complexity of realistic interfaces and ii) contain sufficiently accurate description of physico-chemical processes, including photon-electron interactions, electron-hole coupling, electron-phonon coupling, etc. The development of such theories and the resulting computer software will benefit the broad community of theoretical researchers, but also have important impacts on experimental studies and industry. However, to achieve this, a continued investment is required, as method and code development usually occur on a longer time scale compared to the study of applications. This also requires the training of masters and PhD students not only in physics, materials science or biology, but also in computer programming (including parallelization of software) and use of high-performance computing resources.
5. Impact to address the need of industry in driving economic growth
Progress in the field of many body physics, time-dependent electron dynamics and wave function based correlated quantum chemistry is fundamental to many European industries connected to high-tech materials design and device applications. Examples are
- Advanced hybrid photovoltaics
- Photo-catalytic processes in energy storage and pollutant degradation
- Hybrid nano/bio-systems for medical applications
- Single-defect-based quantum optical and spintronic devices
Such directions can be strengthened by focused research projects for the development of new materials and devices in key enabling technologies. The field of nanodevices is currently opening to new materials, especially 2D. The EU flagship on graphene and 2D materials is indeed expected with the aid of computational predictions to produce several new outcomes. However, technological innovation is not limited to these materials.
November 25th 2016