Episode 2: Optoelectronic properties of perovskites tunable due to reversible surface relaxation

Research on perovskites has progressed rapidly for PV and LEDs, with new solar-cell efficiency records being set at a regular pace. There are hints of the first commercial products reaching the market by 2020, just a decade since perovskite photovoltaics were first discovered. MRS Bulletin presents the impact of a recent advance in this burgeoning field.

Read the abstract in Nature Materials (doi:10.1038/s41563-018-0164-8).

Transcript
Welcome to MRS Bulletin’s Materials News Podcast, providing breakthrough news & interviews with researchers on the hot topics of 3D bioprinting, artificial intelligence and machine learning, bioelectronics, perovskites, quantum materials, robotics, and synthetic biology. My name is Bob Braughler. 

The layered nature of Ruddlesden–Popper perovskites means that the materials can be shaved down to a single layer or just a few layers. The properties of any material at the molecular level are different from those at larger scales. Kian Ping Loh, at the National University of Singapore, and his colleagues have revealed what makes the properties of two-dimensional perovskite differ at molecularly thin dimensions. 

The researchers made centimeter-sized crystals of a specific perovskite with four compositions with increasing number of atoms and exfoliated 20–100-micron-thick monolayer sheets from the material. They measured the optical properties of the bulk and monolayer flakes using photoluminescence and optical absorption measurements. To keep the flakes from decomposing under laser irradiation used for these studies, they encapsulated the flakes with a transparent 2D hexagonal boron nitride layer.

The researchers studied the photoresponsivity of the single-crystal 2D perovskites as a function of thickness and discovered that excitons—which are joint states of an electron and a positively charged hole—tunnel across the material interlayers to dissociate at the electrodes, leading to efficient photocurrent generation. With increasing composition – or number of atoms - the luminescence of the materials shifted toward longer, redder wavelengths. The redshift also happened when the material was exposed to the laser for a long time, because thermal fluctuations reoriented the surface organic cations in the monolayer perovskite. The color shift can be reversed by exposing the sample to higher power laser annealing under vacuum. This cycle could be repeated tens of times.

The disordering of the organic cations also creates defects that trap only positively charged carriers, allowing electrons to circulate longer. To test this, the researchers made a photodetector with the monolayer perovskites. The detector had a low current in the dark, but the current increased linearly with laser power because under illumination, excitons tunneled across the interlayers, creating a highly conductive state.

This work was published in a recent issue of Nature Materials. My name is Bob Braughler from the Materials Research Society.

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