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Research Associate and first author Nicholas Weadock working in the lab. Newly published research in the department could lead to the development of better hybrid lead halide perovskites. 

Researchers in the Department of Chemical and Biological Engineering and the Materials Science and Engineering Program have published new findings in – a class of materials proposed for use as low-cost, high-efficiency solar cells. We asked first author and materials engineering research associate Nicholas Weadock about the work, his time at ���������� Boulder and more.

Question: How would you describe the work and results of this paper?
A: “This work develops a better understanding of hybrid lead halide perovskites, a class of materials proposed for use as low-cost, high-efficiency solar cells. These materials are made up of lead, halide anions like chlorine, bromine, and iodine, and small organic cations like methylammonium. 

Up to this point, scientists and engineers have been asking ‘how do we make lead halide perovskites’ or ‘why do they work so well for solar cells’ – but the harder question of why they behave the way they do is still unanswered. To solve that question, we investigated the structure-function relationship, which applies scientific concepts to determine the useful properties of a material for solar cells based on the arrangement of atoms. We used a combination of experimental and computational techniques to observe the atomic arrangement of these materials – the first step in determining the structure-function relationship. What we find is that these materials contain two-dimensional ‘pancakes’ which are 10-25 atoms across and only three atoms thick. This is a surprising result as previous studies suggested only three-dimensional arrangements of atoms. The pancakes are likely responsible for the remarkable properties of the lead halide perovskites, giving them their function as solar cell materials.”
  
Q: What are the applications in the real world from this research? Why do we want to investigate these questions?
A: "The two-dimensional, pancake-like atomic arrangements that we found can help us better understand and address some of the unanswered questions and problems associated with these materials. When sunlight hits solar cells made from lead halide perovskites electric current is created. The current must flow out of the solar cell to generate useful power on the grid. In lead halide perovskites the useful current flows farther than in other solar cell materials, but scientists do not understand why. We believe that positively charged methylammonium molecules within the pancakes align in such a way as to create small electric fields which improve current flow. Other solar cell materials, like the silicon found now in most solar cells, do not have this property.

Additionally, an unsolved problem in lead halide perovskites solar cells is the migration of charged atoms (ions) during solar cell operation. This migration causes the material to break down after extended use, which might limit their usefulness. The 2D atomic correlations we found have been demonstrated to slow ion migration in other computational studies. If they could be fixed in place, we could slow ion migration and increase the lifetime of these solar cells."

Q: Was there a particular aspect of this work that was hard to complete?
A: “We’ve been working on this since 2020, and were very lucky that our diffuse scattering experiments were relatively “simple” to perform in that they could be run remotely during the COVID shutdowns. Similarly, our collaborators at the University of Illinois Urbana-Champaign already had molecular dynamics simulations that we, together with TY Sterling and Dmitry Reznik in physics could use to directly calculate diffuse scattering for comparison. What was difficult was analyzing the results to comprehend the atomic structure on the local level and determine whether this structure is static (fixed in place) or dynamic (moving around). To answer this question we needed to perform additional neutron inelastic spectroscopy experiments. It turns out that these two-dimensional “pancakes” have a dynamic component, but there are also static distortions!

Somewhat related to this, I secured, together with professors Mike Toney and Ryan Hayward, $700,000 funding for a new X-ray instrument that was installed recently.

Q: What was it like working with Professor Mike Toney on this project? How have you liked working at ���������� Boulder?
A: “I have really enjoyed my time at ���������� Boulder so far! The university research setting is very exciting, and I have made several research connections and collaborations with other groups local institutions. 

Working with professor Toney here has been a big change from our time together in California, as the group at the SLAC National Accelerator Laboratory was primarily postdoctoral associates and senior graduate students. At ���������� Boulder, the group is much larger and younger. I’ve really been impressed with the progress that the graduate students have made so far, both in their experimental and academic prowess, and overall understanding of the concepts underlying their research projects.”

This work was supported in part by the Center for Hybrid Organic Inorganic Semiconductors for Energy (CHOISE), an Energy Frontier Research Center funded by the Office of Basic Energy Sciences, an office of science within the U.S. Department of Energy; and by the DOE Office of Basic Energy Sciences, Office of Science, under Contract NO. DE-SC0006939. A portion of this work (S(Q) calculations) used the Summit supercomputer, which is supported by the National Science Foundation (awards ACI-1532235 and ACI-1532236), the University of Colorado Boulder, and Colorado State University. The Summit supercomputer is a joint effort of the University of Colorado Boulder and Colorado State University. Other ���������� Boulder authors include Tyler C. Sterling, Dmitry Reznik, and Michael F. Toney.