OPTOIONICS
A New Research Field on Light-Matter Interaction
What is Optoionics?
New technologies are the driving force for progress and innovation. At the same time, they form the supporting pillars of the energy transition: Photovoltaics (PV) laid the foundation for the use of renewable energies in the 1970s; today, electrochemical energy storage systems are considered a central component of sustainable electromobility. All key technologies are the result of decades of basic research and are inextricably linked to material systems in which they are realized: While optoelectronics in semiconductor systems forms the basis of PV, battery research is based on solid-state ionics, a subfield of electrochemistry that requires conductive solids with mobile ions.
At the system level, solar batteries represent the integration of solar cells and batteries. From the perspective of basic research, they mean the convergence of optoelectronics and solid-state ionics into a new interdisciplinary science—namely, optoionics. This research direction, recently established at the MPI for Solid State Research and the TUM, generally deals with the control of ions through light and goes far beyond the development of light storage devices such as solar batteries. It opens up unforeseen perspectives for novel optoionic technologies at the interface between photovoltaics, photocatalysis, and electrochemical energy storage.
Examples include innovative light storage technologies such as photoconductors and solar redox flow batteries. In addition, optoionic effects can be specifically used for the development of the next generation of solid-state batteries by accelerating the charging process and increasing interface stability. Optoionic effects can also be used as “probes” for the diagnostics and cycle prediction of a solid-state battery. On the other hand, light-induced material transformations with built-in “memory effect” are made possible, represented by “dark photocatalysis.”
This allows light and dark reactions in photocatalysis to be temporally separated, following the model of natural photosynthesis and enabling the production of solar fuels like hydrogen even in the dark. There is also enormous potential in the fields of diagnostics, sensor technology, and neural data storage and processing (e.g., photomemristors), as well as in the development of scalable light-supported synthesis and processing methods. In the long term, this could make large-scale industrially relevant processes of material production significantly more CO2-neutral.
Solar Batteries: A Novel Bridge Technology
Renewable energies are the backbone of the energy transition; the globally available solar energy exceeds the global demand many times over. At the same time, the fluctuating availability confronts us with enormous challenges in feeding it into the power grid. To counteract the resulting over- and underproduction, energy storage systems are needed that can balance not only seasonal (summer-winter) and day-night fluctuations, but also short-term weather-related fluctuations on the timescale of hours and minutes.
In addition to the use of pumped storage power plants or chemical storage technologies such as Power-to-X, hybrid bridge technologies are particularly needed, which combine energy conversion and storage and are thus able to buffer short-term fluctuations. Solar batteries are such “solar buffers” that integrate solar cells and batteries hybridly in one component. They can be charged directly with light, without the detour of conversion into electricity. This optoionic mechanism can not only lead to significant increases in energy efficiency compared to conventional storage systems, but could also accelerate conventional charging processes by additional “light charging” (i.e., additional light-induced photocurrent).
The SolBat Challenge
At the molecular level, the charging of a battery means the diffusion of ions from one electrode to the other. In a conventional Li-ion battery, the driving force for this motion comes from the externally applied bias. In a solar battery, it comes from the absorption of light. Solar batteries therefore need to be optimized toward a dual functionality: light absorption and ion mobility and storage. This constitutes an intertwined research challenge both at the level of the involved materials and at the level of the employed device geometry– and thus rationalizes the holistic vertical approach taken within the SolBat Center: It can only be tackled in a corresponding integrated research center that spans fundamental materials science up to first battery prototypes.
