Episode 3: Surface defects control bulk properties of lead halide perovskites

In this podcast episode, MRS Bulletin’s Laura Leay interviews David Cahen from the Weizmann Institute of Science, Israel, about the impact surface defects have on bulk properties, specifically in the case of lead halide perovskites. In a perspective he co-authored, Cahen connected numerous experimental data from other researchers that exposed this phenomenon. By understanding how surface defects control the material’s electronic behavior, researchers can pursue new materials for the development of long-lasting devices. This work was published in a recent issue of Advanced Materials. 

Transcript

LAURA LEAY: Welcome to MRS Bulletin’s Materials News Podcast, providing breakthrough news & interviews with researchers on hot topics in materials research. My name is Laura Leay. In the course of conducting research, it’s sometimes easy to get a sense that underlying or unknown phenomena might be happening. Eventually, you manage to make the connection. For Professor David Cahen from the Weizmann Institute of Science in Israel, this lightbulb moment came when he was considering the effects of a surface interaction on the semiconductor copper indium selenide when it was exposed to air. This surface interaction gives control over the bulk properties, and is most easily seen in lead halide perovskites. The idea going through his mind was that it’s the defects in the material that impart a given property. Although this is usually controlled by doping there are also defects on the surface. Some simple mathematics occurred to him.

DAVID CAHEN: The calculation I made was: if I have a grain, what is the surface area and how much of the surface has to be such an imperfection so that all of the electrons – the number of which I want to control for controlling the material’s electronic behavior – that the number of electrons involved in that imperfection is the same as if I had a certain density of imperfections in the whole grain?

LAURA LEAY: This is a simple calculation, but often, the simplest explanation is usually the most appropriate.

DAVID CAHEN: The moment you see that the surface area of a small enough grain is large enough to contain the imperfections that you otherwise would have spread around inside the grain in the bulk of the material, you are home free.

LAURA LEAY: The original work on the copper indium selenide took place in the 1980s but other research has pointed to this phenomenon, which has been drawn together in a recently published perspective article.

DAVID CAHEN: We take a lot of experimental data from others and show that this is the explanation. This simplifies by bringing together a lot of data and information. The big satisfaction is that: oh, yes, we can take a lot of data that seems to be somewhat disconnected and connect them.

LAURA LEAY: Experimental evidence comes from well-known sensors that are produced commercially and reversible phenomena where water or adsorbed organics can be driven off from the surface. Previous computational work on nanoparticles also demonstrated the same principle. In lead halide perovskites, the phenomenon is most easily seen as it’s not overshadowed by uncontrolled imperfections in the bulk. This perspective is somewhat controversial. Co-authors of the perspective article helped ensure that focus was maintained and ensured that confirmation bias didn’t influence how data were analyzed. Bringing attention to this concept means that the development of new devices could become easier and quicker. It also means that new devices could last longer.

DAVID CAHEN: It’s all based on these two processes of self-healing and defect tolerance. And those are properties that many people, including me and my colleagues are now deeply involved in, because we want to try to figure out what the basis conditions for these phenomena to occur so that we can find other materials. And why is that important? That’s important for sustainability. If we have materials that last longer, and do their work – their function – longer, we can make devices that last longer.

LAURA LEAY: This work was published in a recent issue of Advanced Materials. My name is Laura Leay from the Materials Research Society. For more news, log onto the MRS Bulletin website at mrsbulletin.org and follow us on X, @MRSBulletin. Don’t miss the next episode of MRS Bulletin Materials News – subscribe now. Thank you for listening.