Tuning Perovskite Surface Polarity via Dipole Moment Engineering for Efficient Hole-Transport-Layer-Free Sn–Pb Mixed-Perovskite Solar Cells

Abstract

Post-treatment has been recognized as one of the effective methods for passivating the underlying defects in perovskite solar cells (PSCs), but little attention has been paid to how to pick suitable passivation agents with diverse isomers for efficient PSCs, particularly for the tin−lead (Sn−Pb) mixed PSCs. Here, we introduce the dependence of the power conversion efficiency (PCE) on a dipole moment for surface passivator screening, in which we chose three trifluoromethyl-phenylethylamine hydroiodide (CF 3 -PEAI) isomers as surface-treatment materials for hole-transport-layer-free (HTL-free) Sn−Pb mixed PSCs. The different positions of the −CF 3 group for the CF 3 -PEAI isomer result in different dipole moments, which influences the interaction between CF 3 -PEAI and lead iodide. The para position CF 3 with the highest dipole moment exhibits a higher PCE than the ortho-position with a lower dipole moment, which is attributed to the large dipole moment on the surface that could tune the surface polarity from p-type to n-type, facilitating electron charge transport in the HTL-free Sn−Pb mixed PSCs. An ultrathin 2D layer is formed on the perovskite surface to passivate the surface defects, which is responsible for the enhancement of the PCE and stability of the PSCs. As a result, the open-circuit voltage (V OC ) of the device is improved from 0.775 to 0.824 V, yielding a champion PCE of 20.17%, which is one of the highest PCEs among the reported HTL-free Sn−Pb mixed PSCs. The device also shows improved stability with remaining 75% of its initial PCEs after storage in N 2 for 700 h.

Dielectric Engineering of 2D Organic–Inorganic Hybrid Perovskites

“…Owing to the high electronegativity of halides and their strong electron-withdrawing ability, the strength of the dipole moment inside the organic spacers as well as the resulting OIHPs can be fine-tuned. According to first-principles calculation based on the density functional theory, F -PEA + presents a higher electric dipole moment in comparison to that of the weakly polar PEA + . − Among these reconstructive cations, F -PEA especially diminishes the dielectric constant contrast in ( F -PEA) 2 MA n –1 Pb n I 3 n +1 , leading to improved charge transport and elongated charge carrier lifetime in comparison to the pristine (PEA) 2 MA n –1 Pb n I 3 n +1 . ( F -PEA) 2 PbI 4 showed a 7-fold enhancement in mobility compared to the nonfluorinated analogue, as determined by time-resolved microwave conductivity measurements …”

Narrow Bandgap Metal Halide Perovskites for All-Perovskite Tandem Photovoltaics

“…The newly-formed interfaces can block the invasion of air into the perovskite layer, and desired chemical bonds built during the process may also increase the intrinsic resistance of the material. The most commonly employed chemicals for post-treatment in the metal halide PSCs community are ammonium salt, e.g., phenethylammonium (PEA + )-based materials. , Similar to its effects in neat Pb PSCs (Figure a-i), post-coated PEA + can also improve the performance of mixed Sn−Pb PSCs. − Due to the hydrophobicity and superior A-site binding capacity of PEA + , the modified films tend to be more robust with reduced defect states and decreased Sn­(IV) concentrations at the surface. Besides A-site substitutes, strong X-site binding species, e.g., pseudohalide acetates (Ac − ), are also effective in inhibiting the oxidation of Sn­(II) and suppressing ionic migration, as their coordinating energy with Sn­(II)/Pb­(II) is larger than that for the I − anion .…”

Advancing Efficiency and Stability of Lead, Tin, and Lead/Tin Perovskite Solar Cells: Strategies and Perspectives

“…Similar to Pb-HPSCs, there are also a number of exciting results that employ a 2D passivation strategy in Sn-Pb-HPSCs. [234,[256][257][258] Using an ultra-thin 2D perovskite formed with PEAI molecule resulted in an improvement of PCE from 17.90 to 19.40% along with better operational stability of Sn-Pb-HPSCs. [256] Similarly, trifluoromethyl-phenylethylamine hydroiodide (CF 3 -PEAI) isomers were used for forming an ultrathin 2D layer on the surface of Sn-Pb-HP films passivating surface defects.…”

“…[ 256 ] Similarly, trifluoromethyl‐phenylethylamine hydroiodide (CF 3 ‐PEAI) isomers were used for forming an ultrathin 2D layer on the surface of Sn–Pb‐HP films passivating surface defects. [ 257 ] It improved device PCE as high as 20.17% and retained device stability ≈75% of initial PCE after 700 h. It is reported to be a consequence of passivating 2D layer alongside selective tuning of the perovskite surface polarity by dipole moment engineering, which facilitates electron transport in the HTL‐free Sn–Pb‐HPSCs. A report by Hao and co‐workers documented a regulation of the crystallization growth of Sn–Pb‐ HP on the 2D perovskite (4‐AMP)PbI 4 .…”