High‐Performance Perovskite Solar Cells with Enhanced Environmental Stability Based on a (p‐FC₆H₄C₂H₄NH₃)₂[PbI₄] Capping Layer

Abstract

lengths, low exciton binding energy, and ease of solution processability, which promise the perovskite-based photovoltaic devices both high efficiency and low manufacture cost. The intense effort to improve power conversion efficiencies (PCEs) has resulted in PCEs of perovskite solar cells (PSCs) increasing from 3.8% in 2009 [5] to over 23% in 9 years. [6,7] Changes in material composition (e.g., using mixed cations and halides together into a 3D lead−halide perovskites, such as [FA/MA]Pb[I/Br] 3 , [8] [FA/Cs]Pb[I/Br] 3 , [9,10] or [Cs/FA/MA]Pb[I/Br] 3[11] ) have played essential role to improve the performances and structural stability. However, the intrinsic instability of the 3D lead−halide perovskites with regard to moisture, heat, light, and oxygen remains to be entirely circumvented. [12,13] Two main measures have been taken to increase the stability of PSCs: providing sufficient protection to the perovskite material [14] and increasing the intrinsic stability of the perovskite material. [15] More importantly, techniques that can improve device stability without sacrificing the efficiency are critical for the future of PSCs. In this regard, strategies called multidimensional perovskite (MDP) using Ruddlesden-Popper type perovskite (Class I) or low dimensional polymorphs passivated 3D perovskites (Class II) were brought into being increasing the intrinsic stability of the perovskite material. [16] Ruddlesden-Popper phase layered perovskites ((RNH 3 ) 2 (MA) n−1 Pb n X 3n+1 , n = 1, 2, 3, 4, 5, …) [17][18][19][20][21] which was introduced by Smith et al. showed albeit low PCE the superior ambient stability, [18] where RNH 3 are large aliphatic or aromatic ammonium cations represented by n-butylammonium (BA) and 2-phenylethylammonium (PEA). (BA) 2 (MA) 2 Pb 3 I 10 as a light absorber retained its performance after exposure to a highhumidity environment for 60 d. [20] So far the highest reported PCE of Ruddlesden-Popper type perovskite-based solar cell is 15.42%. [22] Very recently, Zhang et al. utilized synchrotron source based in situ measurement to disclose the phase transition kinetics during the crystallization of Ruddlesden-Popper type perovskite. This study provided insight into the relationship between phase purity, quantum well orientation, and photovoltaic performance. [23] On the other hand, the approaches that involve low dimensional polymorphs passivated 3D perovskites, or 3D-2D perovskite stacked structures attracted extensive interest in recent years. This approach features not only the Supported by the density functional theory (DFT) calculations, for the first time, a fluorinated aromatic cation, 2-(4-fluorophenyl)ethyl ammonium iodide (FPEAI), is introduced to grow in situ a low dimensional perovskite layer atop 3D perovskite film with excess PbI 2 . The resulted (p-FC 6 H 4 C 2 H 4 NH 3 ) 2 [PbI 4 ] perovskite functions as a protective capping layer to protect the 3D perovskite from moisture. In the meantime, the thin layer facilitates charge transfer at the interfaces, thereby reducing the...

Efficient Perovskite Solar Cells by Reducing Interface‐Mediated Recombination: a Bulky Amine Approach

“…Absorbance spectra and Tauc‐plots in Figure S8, Supporting Information, display little change in the absorption edges before and after NMAI treatment, which is consistent with results from XRD measurement. [ 45 ] Given the fact that typical 2D/3D stacked perovskite film configuration usually induces absorbance edge change, [ 21,23,46 ] we can infer that the quantity of formed low‐dimensional perovskite after NMAI treatment is indeed so negligible that it cannot affect the bandgap.…”

“…[ 28 ] However, after NMAI treatment, three new peaks appear at around 331, 507, and 555 nm, the intensity of which is increasing with the concertation of NMAI (Figure S9, Supporting Information). The weak emission at 331 nm can be assigned to NMAI, while emissions at 507 and 555 nm can be ascribed to the 2D phase (NMA) 2 PbI 4 [ 21,23 ] and other low‐n quasi‐2D phases, respectively. [ 25,38,47 ] The new emission peaks originated from new phases after NMAI treatment is consistent with the XRD measurement.…”

“…The blue‐shift of the PL from the pure (NMA) 2 PbI 4 can be explained by the quantum confinement effect of the in situ formed very small amount of (NMA) 2 PbI 4 . [ 21,23 ] When the films were excited at a wavelength of 405 nm (Figure 1D), much stronger emission (770 nm) from the bottom 3D perovskite than that from 2D perovskite (507 nm) appeared. Similar to the cases of treatment by other bulky ammonium salts, [ 21,23 ] the PL spectra by excitation from both sides of the perovskite film suggested that the low‐dimensional perovskites formed only on the top‐side of the film.…”

“…For example, excess PbI 2 [ 4,12–14 ] was believed to passivate the grain boundaries and interface of electron transport layer with perovskite; conjugated polymers [ 15 ] and insulating poly(methyl methacrylate) [ 16,17 ] were also used to passivate the interface of perovskite/HTL. Recently, various ammonium halide compounds, [ 8,18,19 ] especially bulky organic ammonium halide salts were adopted to passivate the surface of perovskite, such as 1,8‐octanediammonium iodide, [ 3 ] adamantylammonium hydroiodide, [ 7 ] (fluorine‐)phenylethylammonium iodide (F‐)PEAI, [ 20–23 ] n ‐hexyl trimethyl ammonium bromide, [ 24 ] etc. They could interact with the undercoordinated ions or form low‐dimensional perovskite phases to passivate the surface/interface of bulk perovskite.…”

“…The stability of devices was also usually improved due to the suppression of ionic migration by the packed organic moiety or the formation of hydrophobic low‐dimensional perovskites. [ 7,23,25 ] A very recent work by Jiang et al found exceptionally that the insulating PEAI salt, rather than the 2D layered PEA 2 PbI 4 perovskite, served as a more effective passivation additive for a 3D perovskite. [ 22 ] But this operating state is not stable due to the high reactivity of PEAI.…”