Tin-lead-metal halide perovskite solar cells with enhanced crystallinity and efficiency by addition of fluorinated long organic cation

Abstract

Highly performing mixed Sn/Pb-metal halide perovskite solar cells (PSCs) are among the most promising options to reduce Pb content in perovskite devices and enable, owing to their reduced bandgap, the fabrication of all-perovskite tandem solar cells. Whereas pure-Pb perovskite devices exhibit efficiency up to 25.5%, alongside a high open-circuit voltage (≈1.2 V), Sn-Pb PSCs still show lower performances (22.2%) due to higher open-circuit voltage losses. Here, we introduced 2,3,4,5,6-pentafluorophenethylammonium cations in a perovskite active layer of composition (FASnI3)0.5(MAPbI3)0.5 to obtain highly oriented films with improved thermal stability. The treated films exhibit merged grains with no evidence of 2D structures, which could help to reduce the trap state density at the surface and grain boundaries. Solar cells fabricated with the fluorinated cation added to the active layer displayed reduced trap-assisted recombination losses and lower background carrier density, which leads to enhanced open-circuit voltages with respect to the reference samples and the active layers incorporating unfluorinated phenethylammonium cations. The best device reached an efficiency of 19.13%, with an open-circuit voltage of 0.84 V, which is substantially improved with respect to the reference sample showing 17.47% efficiency and 0.77 V open-circuit voltage. More importantly, the fluorinated cations' addition is instrumental to improve the device's thermal stability; 90.3% of the solar cell initial efficiency is maintained after 90 min of thermal stress at 85 °C in a nitrogen atmosphere.

Fluorinated 2-Phenylethylammonium Spacer Cations for Improved Stability of Ruddlesden–Popper Pb/Sn Halide Perovskites: Insight from First-Principles Calculations

“…Experiments confirm this property by demonstrating superior humidity and thermal stability for unsealed fluorinated layered perovskite devices . Very recently, partial replacement of Pb with Sn in layers of (FASnI 3 ) 0.5 (MAPbI 3 ) 0.5 capped with 5FPEA cations has shown an efficiency of 19.5%, which is quite encouraging for achieving the goal of Pb-free perovskites …”

Perfluoroarenes: A Versatile Platform for Hybrid Perovskite Photovoltaics

“…On the other hand, the use of layered hybrid perovskites based on perfluoroarenes has already demonstrated potential in stabilizing tin-based perovskite materials, which opens an important perspective for the development of lead-free perovskites that are of particular interest due to their environmental impact. 20 One of the main challenges for clarifying the underlying mechanisms is the lack of appropriate methods for analyzing ionic migration that could identify the nature of the involved ions, such as transient drift measurements. 16 Furthermore, structural characterization at the atomic level in devices during operation, as by solid-state NMR spectroscopy, is yet to be realized.…”

“…Using perfluorinated organic moieties (i.e., perfluoroarenes) has recently demonstrated potential for stabilizing hybrid perovskite materials due to both increased hydrophobicity compared to their nonfluorinated parent compounds as well as the capacity to suppress ion migration. − This Perspective provides an overview of recent developments using perfluoroarenes to control hybrid perovskites for photovoltaics with a particular focus on the underlying mechanisms, while outlining opportunities and challenges for their application.…”

“…Perfluoroarenes can affect the soft perovskite structure through an interplay of different noncovalent (supramolecular) interactions between the organic and inorganic components (Figure , bottom). − While hydrogen bonding has been predominant in the control of perovskite assemblies and the corresponding activation energy for ion migration, , this has also involved van der Waals and π interactions , ,,, as well as others, such as halogen bonding . ,,,, …”

“…Similarly, low-dimensional perovskites based on perfluoroarenes could contribute to the improvements of intrinsic stabilities by suppressing ion migration, , yet this remains to be evidenced unambiguously. On the other hand, the use of layered hybrid perovskites based on perfluoroarenes has already demonstrated potential in stabilizing tin-based perovskite materials, which opens an important perspective for the development of lead-free perovskites that are of particular interest due to their environmental impact . One of the main challenges for clarifying the underlying mechanisms is the lack of appropriate methods for analyzing ionic migration that could identify the nature of the involved ions, such as transient drift measurements .…”