Julian Steele

Illuminating the future with new perovskite-based optical devices

The on-going expansion of new electronic materials and technologies which source, detect and control light has undeniably reshaped human existence across the globe. As such, the development of new optoelectronic materials will continue to roll out into the foreseeable future. However, for now, the field of optoelectronics (a branch of technology concerned with the combined use of electronics and light) has just experienced an exciting renaissance, reverting back to so-called metal halide perovskite semiconductors – materials which have long been known, though only recently repurposed for high-performance optoelectronic applications.

With modern perovskites research living in the wild since 2009, I was late to the party when in 2016 I joined the perovskites laboratory of Prof. Roeffears and Prof. Hofkens at the KU Leuven; affectionately dubbed the Roeffkens Lab. Since arriving, a rich environment of researchers spanning organic chemists and bioscience-engineers, to solid-state physicists (personal preference) has fostered a rewarding environment to continue my education, and ultimately developed a multifaceted approach to research.

By considering the full landscape of problems in day-to-day life which are tackled by optoelectronic devices – anything from overhead lighting and telecommunication, to solar cells and digital X-ray imaging – it quickly becomes obvious not only how enriched our lives have become, but also how dependent they are on such technologies. It follows that the requirement for low-cost and high-performance devices has reached a premium. These descriptors outline the entry points for perovskite-based devices into the commercial market, with a large field of researchers (quietly) hopeful that many well-established technologies will be irreversibly disrupted.

Perovskite is the general name which is nowadays used for a material consisting of three chemical components A, B and X which are in specific amounts ABX3. It is their common periodic crystal structure that unites them. All perovskites have a crystal structure that resembles natural calcium titanate, or perovskite, named after a Russian mineralogist Lev Perovski. The reason for their wide success is twofold. First, perovskites are very cheap and trivially simple to make (or so I am told). Second, they tend to exhibit the kinds of optoelectronic properties typically reserved for more classical semiconductor materials – silicon (Si) solar cells are a house-hold example – which are made under the most stringent conditions (sounds expensive!). Yet, an empirical arms race to develop ever more performant perovskite-based devices (perovskite-based solar cells are currently on par with commercial Si ones) has rapidly outpaced their fundamental understanding, leaving many important scientific questions wide open.

Within this context, the Roeffkens Lab at KU Leuven have set out to fundamentally explore what makes these materials tick. For this, structure-property relationships of perovskites are key; how do microscopic differences in the materials give rise to dramatic changes in the macro-properties? The facilities in the new Leuven Nanocenter (traditionally equipped to handle optical microscopy studies of living and wet substances) have been effectively adapted for this purpose. This approach has helped reveal the most intimate details of where light emerges and disappears from the belly of the perovskite crystal. Avoiding the (in)famous perovskite solar cell performance race, our work has examined several other interesting applications.

As of last year, for example, perovskite-based X-ray detectors have been brought into our focus. This new research line has emerged after we reported a promising direct X-ray detector design, based on a dual-metal halide perovskite semiconductor, with chemical formula Cs2AgBiBr6. While composed of heavy elements like silver (Ag) and bismuth (Bi), such materials are ideal for direct X-ray conversion: turning high-energy X-ray photons into collectable charges and signal. This is because of their ability to combine both heavy atomic nuclei – for efficient X-ray absorption – with excellent charge formation and transport properties. Our team singled out Cs2AgBiBr6 as a strong candidate because of its high X-ray sensitivity and excellent structural stability. By optimizing the materials and lowering the operating temperature, we achieved a tenfold enhancement in X-ray sensitivity of the device, ultimately peaking near 500 times more sensitive than commercial devices. Such benchmarks will ultimately lower the required dose/exposure of humans to ionizing X-ray radiation.

Within this past year we have made great progress on other perovskite topics too, both fundamentally and applied. With several collaborations underway, and some more in the pipeline, it is my current goal to make the pilgrimage toward the UC Berkeley laboratories (near Silicon Valley) later this year with several promising ideas in hand. With current financial support from FWO (postdoctoral scholarship 2017 – 2020) I feel enthusiastic that many private hours playing in the laboratory will end up impacting the public into the future.