Researchers say rubidium Doping & 2D Capping assure perovskite stability

In a new paper entitled ‘Unraveling the compositional heterogeneity and carrier dynamics of alkali cation doped 3D/2D perovskites with improved stability’, first published on 11th December 2020 in the journal Materials Advances, scientists at the U.S. National Institute of Standards and Technology offer a new method- rubidium doping & 2D capping- to enhance the stability of perovskite solar cell technologies.

Named after the 19th Century Russian mineralogist and nobleman Count Lev Perovski, the Perovskite solar cell has emerged as an exciting area for research in the last few years for scientists who are working to increase the efficiency and stability of this technology. Federal grants have also been extended to such endeavours, one example of which is the $40 million allocation of funds by the Biden administration for the research and development of Perovskite materials, announced this March. The perovskite crystal is a photovoltaic material with promising potential for capturing solar energy. Unlike silicon wafers currently in use for making solar panels, perovskites are lighter, smaller, and cheaper, and also physically flexible, with the manufacturing process having smaller environmental footprint. While commercial applications of perovskite capable of exhibiting long-term high performance do not seem far away, whether their stability will be top-notch is a recurring question that many researchers are looking answers for.

The group of NIST scientists who worked on the paper in question, have presented a solution- doping and capping- in their research to overcome the stability issue. As part of their experiment, they first inserted the alkali metal rubidium into a three-dimensional perovskite film, and then grew a two-dimensional “Ruddlesden–Popper” perovskite material on top of it. Then they carried out a comprehensive analysis on the impact of doping photovoltaic perovskites and found that a 5% concentration of rubidium provided the best performance for the perovskite solar cells they studied. They also examined the impact of the increasingly popular practice of capping 3D perovskites with a protective 2D perovskite layer. While the NIST researchers aren’t pioneers in experimenting with doping and capping, they are perhaps the first to really understand the implications of this discovery in a photovoltaic device.

There are many advantages to these two approaches over control devices produced without incorporating them, as the scientists’ work demonstrated. The two-dimensional perovskite provides a capping layer, which protects the more sensitive perovskite cell material beneath from moisture. Even after 60 days of exposure to ambient air at 50% relative humidity without an encapsulant, the device maintained 81% of its initial efficiency. On the other hand, a control sample of the 3D perovskite lost 65% of its initial conversion efficiency in the same period. The paper also details how doping the cell with rubidium, as well as structuring the device in 2D/3D bilayers, reduced the energy needed for electrons to move through the layers, further improving the device’s performance.

It is significant that the 19% performance loss incurred in this technique happened without an encapsulant, which increases hopes for the two new approaches to pave way for further investigations, especially into how much rubidium should be used. The scientists concluded their paper by saying, “This work provides the perovskite community with a facile strategy and fundamental understanding to rationally design a 3D/2D bi-layer architecture with significant guidelines toward high-performance perovskite photovoltaics with improved operational stability.”

Besides Christina A. Hacker, the authors of the paper are Ming-Chun Tang, Siyuan Zhang, Timothy J. Magnanelli, Nhan V. Nguyen, Edwin J. Heilweil, Thomas D. Anthopoulos and Christina A. Hacker.