An international and multidisciplinary collaboration has brought together expertise in materials synthesis, spectroscopy, and theoretical modeling to describe a new method that interrogates the interaction between the organic and inorganic features of hybrid perovskites, providing new insights that could guide the future development of dynamic electronic materials, including photovoltaics and next-generation lighting.
by Daniel Morton
Spearheaded by lead author Nathanial Gallop, this work, that was reported in Nature Materials in late 2023, began when the Bakulin group were exploring whether they could use vibrational stimulation to control the properties of materials – there were some unusual signals observed during control experiments. Seeking to understand this phenomenon, the Bakulin group collaborated with scientists from across the world. According to Artem Bakulin “the collaboration evolved naturally. Along the way we tested many hypotheses based on the materials we looked at. Collaborating with theoreticians (The Rappe Group, IMOD member), helped us understand and interpret the results”.
Collaboration was critical in the development of this new approach and understanding the underlying principles. This included materials synthesis and device fabrication (Vaynzof Group), emissive hybrid perovskite nanoparticle synthesis (Kovalenko Group), to expanded IR spectroscopy capabilities (Tahara Group), and the atomistic perspective of the behavior of molecules and materials systems (Rappe Group, IMOD member), were essential to the success of this project.
The multidisciplinary nature of the work means that there are multiple aspects to the impact of these results. We already know that structural dynamics of molecules drive chemical reactions, and vibrations are elementary building blocks of structural dynamics. Photo switching of molecules, where one shines light on a molecule and it causes a rotation, or a vibration, are well established, but can you do the opposite? Artem admits that as a physicist “[he is] a little obsessed with exploring the vibrational control of materials,” so the Bakulin Group set out to explore whether they could answer this question and use these vibrational building blocks to selectively drive certain types of chemical reactions.
To do this, a new spectroscopic tool was developed by synergistically combining several advanced spectroscopic techniques. Using a vibrational approach is challenging; when something is vibrationally excited, it also heats up, and it can be very hard to separate out the thermal effects from the specific vibrational motions. The presence of vibrational-electronic coupling was identified using the previously reported VIPER (vibrationally promoted electronic resonance) methodology, and high sensitivity was achieved using an ‘optical control’ approach with action detection, which facilitates background-free measurements via either the materials photoluminescence or photovoltaic response. Excitingly, this combination of techniques enabled the team to perform experiments directly on operational photovoltaic devices, making their findings relevant for practical applications.
The underlying theory of how these experiments work is best summed up by Andrew M. Rappe, IMOD member and lead of the theoretical team that helped develop understanding around these processes. In his explanation, Andrew says “Suppose I ask you to climb to the roof of a house so you can sit there and have a nice afternoon, but I give you a ladder that is too short. You dutifully climb up the ladder, but it doesn’t reach the roof. If there is someone at the upper floor window who hands you a small stepladder you can climb the rest of the way and get to the roof. That is what they did with this technique; they excited with light that does not have enough energy to excite electrons on its own, but because they first excited the organic molecule with an IR pulse, that initial IR energy can be “stolen” by the electron on its way up to the excited state.” Achieving this “two-stage” approach was supported through collaboration with the Tahara group in Japan, who have advanced capabilities in IR spectroscopy. The energy of the ‘second step’, the visible pulse, can be tuned so that the combination of the two stages will bring the material to an electronically excited state – but only if the stimulated vibrational mode is coupled to the material’s electronic dynamics. In the case of hybrid organic-inorganic perovskites this sensitivity to vibrational-electronic coupling, and the possibility to use it to control the optical properties of the material provides information about the interactions between the organic and inorganic parts of the perovskite.
The team explored several different materials but were not satisfied with the results, until they investigated the perovskite-based materials from the Vaynzof group. Artem explains “We were quite lucky in looking at perovskites, because of the structure of hybrid organic-inorganic systems, the organic molecule is, in a sense, isolated.” This was the first time they were able to see vibrational control. To build on this initial observation and get a more complete picture, focus then turned to emissive materials. High-quality nanoparticle samples, with very controlled parameters were required, which is where the Kovalenko Group fit into the collaboration. Using their observations from these two material classes, two signals were developed, Photoluminescence (PL)-VIPER and the Photocurrent (PC)-VIPER.
The tunable sensitivity of this method was used to explore formamidinium lead bromide (FAPbBr3), a hybrid perovskite, which provided some interesting results. To effectively rationalize these results required a close collaboration between the spectroscopists and theoreticians. “The Rappe group were not the first theoretical team [they] approached. [they] talked to couple of teams who said that they could do some of the basic modelling, but getting to the level of information needed to address this problem was not possible using their techniques. We were lucky to find a paper from the Rappe group that included some very similar calculations that included one of our longtime collaborators, David Cahen, who introduced us”. This collaboration ultimately proved to be the best fit because of some prior experience by the Rappe Group. Aaron Schankler (Lead IMOD Graduate student, U Penn, who worked closely with U Penn graduate student Zhenbang Dai, now a postdoc at UT Austin, to directly model this unique vibrational coupling) explains “Our group has done some molecular dynamics of hybrid perovskites in the past, so this is something we have a track record in, but vibrationally excited states are hard to model. [however] after some back and forth we were able to figure out how to include the vibrational component, not just in an equilibrium simulation, but with some driving of the excited vibrational mode that gave us a way to see the impact of the excited vibrations”. Extension of their existing computational techniques enabled the Rappe group to explore new structural factors to help rationalize these newly observed processes. “This was a really enjoyable collaboration. Members of Artem’s group have some experience with computational modeling, so we could speak each other’s language, even if we approached the problem from different perspectives, and that really helped with our discussions and back-and-forth”, says Aaron.
“Perovskite structures are quite soft, so their vibrational motions and in particular the motions of the organic cations can have significant impacts on their observed properties. That can make things complicated. The spectroscopists could see something was happening, but it is hard to see what exactly is happening microscopically. We came at the problem from the opposite direction and built a model from the bottom up”, says Aaron. By employing an atomistic approach, the team were able to build a model of how the coupling happens between the organic and inorganic features of the hybrid perovskite. The experimental observations showed that only some vibrational modes of the organic formamidinium cation are coupled to the electronic states of the perovskite material, and that coupling lasts for an extremely short span of time (~ 0.3 ps). “In the models we were able to show that the coupling between the organic and inorganic sublattices is mediated by the hydrogen bonding between the organic cation and specifically the bromine atoms of the inorganic sublattice” explains Aaron. The lifetime of the coupling between the organic and inorganic features is proposed to be governed by the rotation of cations, not necessarily the lifetime of the molecular vibration. You could stimulate the cations with the pulse, and they could be vibrating, but if the hydrogen bonding stayed intact, then the coupling was observed. As soon as the cation rotated due to thermal fluctuations, the hydrogen-bonding is broken, and the coupling is lost. This proposed model supported the observed signals. The signal is much shorter than the lifetime of the vibrational modes and corresponds to the timescale of rotational reorientation of organic molecules in hybrid perovskites.
Artem is already thinking about the next steps “This is a very preliminary report in this space, and there are other groups working in this area. I am excited to see more exploration in this space, investigating other materials and classes, to understand better how general these phenomena are, and how we can use vibrational control to impact physical properties.” Validation of the general applicability of these principles could lead to their use in the design of materials that reduce non-radiative recombination, which means losing less of the input energy before it becomes light. “We would like our devices to always emit the same color of light, and this work is very good at seeing whether, as the molecule rearranges itself, will the absorption, and therefore by extension, the emission, be different as a function of time, and what is the range of photon energies, or colors, that we are going to be averaging over? We could then guide the design of molecules that have good properties but rotate less, or rotate differently,” predicted Andrew. This new approach, along with the understanding of interactions between the organic and inorganic features of hybrid perovskites, offers the exciting prospect of an additional parameter to optimize in the design of dynamic electronic materials, including optoelectronics, photovoltaics, and next-generation lighting.
