Electron-withdrawing organic ligands “freeze” the surface of tin-based perovskite

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Electron-withdrawing organic ligands “freeze” the surface of tin-based perovskite
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The development of renewable energy source plays an important role for the sustainable development of the world. Among all kinds of renewable energies, photovoltaic is the only one that can meet the demand of human being. In recent years, metal halide perovskite is emerging as a promising material to prompt the efficiency improvement of low cost photovoltaic technology.  

Currently, the efficiency of single-junction metal halide perovskite solar cells has exceeded 26%, gradually approaching the Shockley-Queisser (S-Q) limit. However, researchers aim not to stop there. A novel type of all-perovskite tandem solar cell appears to redefine our understanding of solar cell efficiency. Specifically, these cells are composed of a wide-bandgap (WBG, 1.7~1.9eV) top cell and a narrow bandgap (NBG, 1.1~1.3eV) tin-lead mixed bottom cell, interconnected by intermediate recombination layers. Each sub-cell absorbs different solar spectrum bands, and calculations suggest that the theoretical maximum efficiency of these all-perovskite tandem cells could reach as high as 42% [Nature, 2022, 603, 73].

From a technical perspective, the fabrication of all-perovskite tandem solar cells is a complex process, where each layer significantly impacts the device's performance. A critical issue of concern is the intrinsic oxidization-prone nature of Sn2+ in the narrow bandgap bottom cells. Oxidation of Sn2+ can lead to vacancy defects and unfavorable surface p-type structures, severely impacting the efficiency and stability of the tandem devices.

In the field of all-perovskite, many excellent studies addressing these tin-lead mixed perovskite issues have been published in prestigious journals. Here are some representative works from this field:

Professor Hairen Tan's team at Nanjing University, through a hybrid evaporation–solution-processing method, first co-evaporated PbI2 and CsBr on the surface of the tin-lead mixed perovskite, followed by spin-coating organic salts to form a 3D/3D heterojunction structure. This type II energy level structure optimized carrier transport, significantly improving the open-circuit voltage (Voc) of the tin-lead perovskite thin film. The final all-perovskite tandem devices achieved an energy conversion efficiency of 28.5% [Nature 620, 994-1000 (2023)].

Professor Dewei Zhao of Sichuan University and colleagues designed a donor–acceptor-type molecule, MPA2FPh-BT-BA (2F), as a hole-selective contact. This molecule can be applied to both wide and narrow bandgap cells. In wide bandgap cells, the 2F molecule effectively passivates perovskite surface defects and optimizes energy level alignment. In narrow bandgap cells, it passivates surface defects and appropriately regulates the crystallization process. Using this versatile molecule, the prepared 2-T all-perovskite tandem solar cells achieved an energy conversion efficiency of 27% [Nature 618, 80-86 (2023)].

Our research group previously surface-passivated tin-lead perovskite using TEASCN molecules. Specifically, TEASCN, compared to the traditional surface passivation agent TEAI, tends to form an n=2 low-dimensional structure on the surface. This n=2 structure has less impact on the device's short-circuit current than the n=1 structure. Ultimately, without sacrificing the short-circuit current, we effectively improved the open-circuit voltage of the tin-lead mixed perovskite, reaching an efficiency of 21.26% [Angewandte Chemie International Edition 61 (2022)].

Building on this work, our team recently proposed using an electron-withdrawing ligand, chloromethylphosphonic acid (CMP), in the narrow bandgap tin-lead mixed bottom cell. Due to its strong electron accepting ability, CMP downshifts the energy level of perovskite surface, and elevates the redox potential of Sn complex. oxidation. In a figurative sense, it's like "freezing" the perovskite film in its appearance.

Figure 1. The illustration of antioxidation at grain boundaries enabled by CMP.

Experimental results show that CMP addition effectively reduces the tin oxidation, leading to reduced defect density and enhanced carrier mobility. The prepared devices, compared to the control group, exhibited significantly improved fill factor (FF) and open-circuit voltage (Voc).

Furthermore, we also fabricated 2-T all-perovskite tandem devices, connecting CMP-containing tin-lead mixed thin films with wide bandgap films (1.79eV) using appropriate processes. The devices achieved an efficiency of 27.3% (certified at 26.9%). Additionally, the 2-T all-perovskite tandem devices maintained 80% of their original efficiency after over 370 hours of illumination.

The substitution of electron-withdrawing ligand in chloromethylphosphonic acid molecules effectively passivate tin perovskite surface. This opens possibilities for developing various electron-withdrawing ligands to inhibit Sn2+ oxidation and enhance device performance.

However, besides the issue of Sn2+ oxidation in narrow bandgap tin-lead mixed perovskites, all-perovskite tandem solar cells still face many other challenges that need to be addressed, such as phase segregation in wide bandgap cells, parasitic absorption in interlayers, and simplification of fabrication processes.

The title of this work is "Electron-withdrawing organic ligand for high efficiency all-perovskite tandem solar cells" published in the latest issue of Nature Energy (DOI: [10.1038/s41560-023-01441-2]).

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