EPFL researchers enhance perovskite solar cell performance via rubidium

Researchers at the Ecole Polytechnique Federale de Lausanne (EPFL) in Switzerland have recently led efforts to improve perovskite optoelectronic performance using small-radius rubidium ion (Rb⁺) chemistry. In two recent studies, targeting two different structural locations and operating through distinct physical mechanisms, the researchers described methods using Rb⁺ in perovskite films to boost the stability of perovskite solar cells in the lab.

In the most recent study, “In-situ boundary bridging unlocks multi-grain-domain carrier diffusion in polycrystalline metal halide perovskites,” published in Nature Communications, the researchers unraveled the “remarkable” effects of Rb⁺ cations when selectively introduced at the grain domain boundaries (GDBs) of polycrystalline perovskite films.

“By using a crown-ether complex to precisely deliver Rb⁺ into the perovskite film, we observed remarkable improvements in carrier diffusion length and carrier lifetime,” corresponding author Michael Grätzel told pv magazine.

“Together with our collaborators in Dalian, we further demonstrated that Rb⁺ facilitates cross-grain-boundary charge transport through the formation of a one-dimensional RbPbI₃ phase,” he added.

“These boundary-localized cations effectively bridge neighboring grains and promote carrier transport across multiple grain domains,” corresponding author Likai Zheng told pv magazine.

Indeed, noting that Rb⁺ cations cannot reside in A cation site in normal bandgap perovskites due to their small size, the researchers proposed a “universal post-treatment strategy,” based on a supramolecular crown ether-assisted slow release with “precise delivery of Rb⁺ cations to GDBs where they form in situ one-dimensional (1D) Rb-based non-perovskite phase bridge that facilitates defect passivation and carrier diffusion.”

Perovskite solar cells made with the modified film had a certified champion efficiency of 25.77%, with the result being validated by China’s Fujian Metrology Institute (FMI) and the National Photovoltaic Industry Measurement and Testing Center (NPVM).

Moreover, a “remarkable” stability was noted where 99.2% of initial efficiency was retained after 1,300 h of continuous one-sun illumination under maximum power point tracking based on the International Summit on Organic Solar Cells Stability protocol ISOS-L-1I.

Researchers from China’s Dalian University of Technology collaborated, as well as teams from Chinese Academy of Sciences, Lanzhou University, and Hong Kong University of Science and Technology (Guangzhou).

In an earlier study, “Strain-induced rubidium incorporation into wide-bandgap perovskites reduces photovoltage loss,” published in April in Science, the team developed a lattice strain approach to incorporate Rb⁺ in 1.67 eV wide-bandgap (WBG) perovskite films targeting greater solar cell stability.

“In our study, we found that Rb⁺ can occupy the A-site in the perovskite lattice, and that its incorporation depends on the triple-halide composition, and it is enabled by the lattice strain,” said Zheng.

The researchers noted that the method, which included using chloride to facilitate the incorporation of Rb⁺ into the perovskite lattice, enabled a “marked suppression of halide phase segregation, which is a well-known source of instability in mixed-halide WBG perovskites.”

The team demonstrated the film’s properties in a triple-halide perovskite solar cell (PSC) with a power conversion efficiency (PCE) of 20.65% and 1.30 V open circuit voltage (Voc). These results correspond to 93.5% of the radiative Voc limit, “representing the lowest photovoltage loss relative to the theoretical limit observed in WBG perovskites,” according to the research.

The results were attributed to a “substantial improvement in stability of lattice structure” to keep Rb locked into the perovskite lattice.

The perovskite solar cells were based on perovskite material containing cesium (Cs), methylammonium (MA), formamidinium (FA) in a stack as follows: indium tin oxide (ITO) on glass substrate, tin oxide (SnO₂) electron transport layer, perovskite absorber, spiro-OMeTAD hole transport layer, gold contacts, with spiro-OMeTAD short for 2,2′,7,7′-tetrakis (N,N-di-p-methoxyphenylamino)-9,9′-spirobifluorene.

The research was co-led by a team from Nanjing University of Aeronautics and Astronautics (NUAA) with participation from teams at the National University of Singapore, the University of Ioannina, and Politecnico di Milano.

Looking ahead, the researchers said that they are continuing to focus on “fully understanding and leveraging the multifaceted benefits of Rb⁺ in perovskite solar cells.”

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