Decoupling engineering of formamidinium–cesium perovskites for efficient photovoltaics

Metal halide perovskites (ABX₃) have emerged as promising candidates for various optoelectronic applications due to their excellent optoelectronic properties and low-cost fabrication. At present, the light-absorbing layer of the highest-efficiency single-junction perovskite solar cells (PSCs) is almost all based on FAPbI₃ perovskite, achieving power conversion efficiency (PCE) that is comparable to commercial crystalline silicon cells.

However, the photoactive black-phase FAPbI₃ readily transforms to a photo-inactive yellow phase under humid conditions. Composition engineering such as A/X-site alloying has been developed to stabilize the black-phase FAPbI₃.

Notably, alloying FA⁺ with Cs⁺ to formpure-iodide FA-Cs perovskite (FA_1-xCs_xPbI₃) is an ideal approach to obtain PSCs with high efficiency and stability. However, due to the complex crystallization kinetics between FAPbI₃ and CsPbI₃, FA_1-xCs_xPbI₃ perovskite prepared by typical one-step (1S) crystallization exhibits poor compositional homogeneity and high trap density, which limits the device performance and long-term stability.

To tackle this challenge, Professor Yixin Zhao from Shanghai Jiao Tong University and co-workers recently developed a strategy of sequential cesium incorporation (SCI) to decouple the crystallization of FA-Cs triiodide perovskite with highly efficient and stable PSCs achieved.

In this work, cesium formate (HCOOCs) as a cesium source is sequentially introduced into high-quality FA precursor film. By cooperating with Professor Feng Gao from Linköping University, a new stabilization mechanism for Cs doping to stabilize FAPbI₃ is also revealed. This research article is published in National Science Review.

In their work, high-quality FA_1-xCs_xPbI₃ (x=0.05-0.16) perovskites are obtained by the SCI method. The ratio of FA to Cs in these SCI-FA_1-xCs_xPbI₃ perovskites can be facilely tuned by adjusting the content of the cesium source.

Compared with the conventional one-step-prepared 1S-FA_1-xCs_xPbI₃ perovskites, SCI-FA_1-xCs_xPbI₃ perovskites have demonstrated a much more uniform Cs distribution. "The uniform composition distribution of Cs is the key to the enhancement of device performance," Zhao says, while the PSCs based on SCI-FA_0.91Cs_0.09PbI₃ films achieved a PCE of 24.7% (certified 23.8%), which is the highest value among the FA-Cs triiodide PSCs reported so far.

Moreover, the collaboration with Gao's group further revealed a new stabilization mechanism for this Cs doping. The incorporation of Cs into FAPbI₃ significantly reduces the electron-phonon coupling strength and lattice fluctuation, thereby suppressing ionic migration and the formation of iodide-rich clusters. As a result, the stability of FA-Cs based devices has been greatly improved.

Overall, this work opens up new possibilities to strategically develop high-quality mixed-cation perovskites with good control over the crystallization kinetics, presenting a milestone towards the rational construction of highly efficient and stable perovskite-based optoelectronic applications, including but not limited to solar cells, light-emitting diodes, and lasers.