One of the most critical concerns of our times is the generation of electricity from renewable sources. A popular solution is the use of photovoltaics (from the Greek phōs, meaning light, and volt, the unit of electromotive force), which convert the energy radiated by a source such as the Sun to electricity, using a selected material integrated in a device. Although progress to date has been noteworthy, particularly in the case of silicon-based devices, we are still a long way from the desired performance and cost effectiveness relation of these units. Therefore: how can we harvest light more efficiently?
The possibilities are multiple yet should always be sustainable. A few decades ago, scientists began their quest for alternatives to silicon, whose manufacture is both a monetary and environmental burden. One of the most promising candidates for the next-generation of photovoltaics are the so-called hybrid organic-inorganic perovskites. Methylammonium lead iodide (MAPbI3) is a prime example, consisting of a charged organic molecule, methylammonium, surrounded by lead and iodine atoms. MAPbI3 stands out as it is a cheap, easy to process and efficient light-to-electricity converter. These properties emerge from the structure of its atomic (and molecular) constituents. Thus, knowing their arrangement and the mechanisms behind the physical properties would allow scientists to understand MAPbI3 better and could potentially make room for future tailoring of the properties of the material.
To do this, a group of researchers explored the vibrational dynamics of the methylammonium cations in MAPbI3 at low temperatures using the TOSCA beamline at ISIS and validated them by computer simulations. TOSCA is superbly sensitive to the quantized motions of the organic cation, and accurate theoretical calculations can predict the vibrational dynamics at low-energy transfers, so combining them brings a unique opportunity to compare several structural models for MAPbI3. Furthermore, as thermophysical properties such as heat capacity are also linked to the atomic arrangement of any crystal through lattice vibrations, these experiments provide an excellent probe of the structure at temperatures where other contributions can be neglected.
The comprehensive analysis of TOSCA data, heat capacity experiments and simulations led the researchers to choose between a model for MAPbI3 deduced solely from diffraction experiments and an alternative candidate. Eventually they chose the latter, due to its more accurate ability to predict both neutron and thermophysical properties.
Researcher Pelayo Marín Villa explains: “This candidate model was first proposed in an earlier work using TOSCA, which already highlighted the importance of addressing the dynamical features of the organic molecule in order to understand the structure of MAPbI3. Most importantly, we anticipate that the methodology developed for this work will pave the road towards a better understanding of the atomic structures of extremely soft solids like MAPbI3 and other alike. And, in that way, we would hope to be an inch closer to having photovoltaic materials matching the ever-growing energetic needs of our times."
The recent study on MAPbI3 can be found at: DOI: 10.1021/acs.jpclett.2c02208.
Related works linked to the use of TOSCA for the study of MAPbI3 and other hybrid perovskites can be found at DOI: 10.1021/acs.jpclett.6b01822 and DOI: 10.1021/acs.jpclett.1c00616.