Perovskite solar cells reach 26.61% certified efficiency with cesium-doping strategy

Solar cells, devices that convert sunlight into electricity, are now widely used in many countries. While most existing solar cells are based on silicon, energy engineers have been working on other devices made of so-called perovskites.

Perovskites are materials with a characteristic crystal structure that exhibit outstanding light-absorbing properties and could thus be ideal for the fabrication of photovoltaic technology. A type of perovskite that yields particularly promising solar cell efficiencies are metal halide perovskites, which combine metals, halogens and organic molecules.

Despite their potential, solar cells based on these materials were found to be difficult to fabricate on a large scale and their power-conversion efficiencies can degrade rapidly over time. A key challenge when fabricating these solar cells is stabilizing the so-called α-phase in perovskites, a crystal structure that enables the efficient conversion of sunlight into electricity.

One approach to overcome this limitation entails adding small amounts of other elements to the perovskites that can improve the stability of the perovskite layers, via a process known as doping. Researchers at Nanchang University in China recently designed a new chemical compound that could be used to dope perovskite films for solar cells, improving the cells' efficiencies and ensuring that they perform well for longer.

The newly designed compound, introduced in a paper published in Nature Energy, facilitates the incorporation of positively charged cesium (Cs⁺) ions across perovskite films, ensuring that they are evenly distributed across materials. This stabilizes the materials' crystal structure, which can improve the efficiency with which solar cells convert sunlight into electricity, while also making them more resistant to heat.

Highly efficient and stable cesium-doped solar cells

The new chemical additive designed by the team at Nanchang University was added to perovskite films via a two-step fabrication process. This process offers greater control over the formation of crystals, which can improve the quality and stability of perovskite films.

"We design cesium 4-(diphenylphosphino)benzoate to enable efficient Cs⁺ doping and to homogenize cation distribution, obtaining high-quality perovskite films with improved phase stability," wrote Jiacheng He, Zhao Guo and their colleagues in their paper.

The researchers used films doped with their newly designed compound to create perovskite solar cells. They then evaluated these cells in a series of tests and found that they were more efficient and stable than cells based on non-doped perovskite films.

"The solar cells fabricated via the two-step process achieve an efficiency of 26.91% (certified 26.61%)," wrote the authors. "The devices incorporating a thermally stable charge-transport layer retain 95% of their initial efficiency (23.76%) after continuous operation under 1-sun illumination at the maximum power point tracking and 85°C (ISOS-L-2 protocol) for 1,500 hours."

Contributing to the advancement of solar energy solutions

The insight gathered by the researchers might soon also inform the design of other doping agents that can stabilize the crystal structure of perovskites. These agents could help to overcome some of the issues that have so far limited the overall performance of perovskite solar cells.

"Our study provides insights into the phase transition pathway and the corresponding transition-state structure of FA_0.9Cs_0.1PbI₃ as well as the mechanism of Cs⁺-driven lattice stabilization in perovskites," wrote the team.

Notably, the doping strategy proposed by He, Guo and their colleagues could potentially also be applied to other perovskites with different compositions. In the future, it could help to improve both the efficiency and durability of perovskite-based photovoltaics, potentially contributing to their commercialization and large-scale deployment.

Written for you by our author Ingrid Fadelli, edited by Lisa Lock, and fact-checked and reviewed by Robert Egan—this article is the result of careful human work. We rely on readers like you to keep independent science journalism alive. If this reporting matters to you, please consider a donation (especially monthly). You'll get an ad-free account as a thank-you.