Effects of Ammonium and Alkali Metal Additives on Anisotropic Photoconductivities and Solar Cell Efficiencies of Two-Dimensional Lead Halide Perovskites

Abstract

To address the issue of instability and surface defects in three-dimensional lead halide perovskite solar cells (PSCs), two-dimensional (2D) pseudo-(quasi) perovskites, such as Ruddlesden–Popper (RP) and Dion–Jacobson (DJ) perovskites, have garnered significant attention. Controlling the orientation of their layered crystallites is key to vertical charge transport in 2D PSCs; however, their detailed electronic properties are yet to be fully understood. Herein, we investigated the effect of processing additives (ammonium salts: NH4Cl, NH4Br, NH4Cl, NH4SCN, and CH3NH3SCN with/without alkali metal salts (NaCl, KCl, RbCl, and CsCl) in water) on the crystallographic nature and anisotropic microwave photoconductivity of RP and DJ perovskites. Importantly, a strong correlation (correlation coefficient, r = 0.882) was observed between the power conversion efficiencies (1–11%) of inverted PSCs and anisotropic (both out-of-plane and in-plane to the substrate) photoconductivities for the 34 compositions of additives analyzed herein. In contrast, the one-dimensional photoconductivity and crystallographic degree of orientation exhibited significantly weaker correlations. These results underscore the importance of anisotropic mobility in 2D PSCs and could serve as guidelines for their further development.

Bar Coating Process of Two-Dimensional Lead Iodide Perovskite Solar Cells: Effects of Vertical Orientation, Anisotropic Photoconductivity, and Conversion Time

“…In sharp contrast, the processes with (3) hot air during coating and (4) hot air after coating demonstrated improved PCE values of 10.51% (8.49 ± 1.13%) and 12.15% (10.29 ± 0.83%), respectively (Figure 2c). The latter PCE was even higher than that of the spin-coated film (11.01%), 30 which was obtained using an identical precursor solution and fabricated in a N 2 -filled glovebox. The highest PCE of (4) hot air after coating is the result of improvements in all device parameters: the short-circuit current density (J SC ) of 15.42 mA cm −2 , open-circuit voltage (V OC ) of 1.011 V, and fill factor (FF) of 0.780.…”

“…The average values were calculated over forward and reverse scans in multiple devices. The error values are the standard deviations of multiple devices over the forward and reverse scans. b Hysteresis index (HI) = (forward PCE – reverse PCE)/(forward PCE). c The number of devices. d Taken from ref . …”

“…Anisotropic charge carrier mobilities, which are crucial for the vertical charge transport and device performance of 2D perovskites were investigated by using TRMC measurements. As illustrated in Figure a, the films were placed in a resonant cavity so as to align the film either parallel (in-plane) or perpendicular (out-of-plane) to the microwave electric field. ,, Figure b shows in-plane TRMC transients (φΣμ, where φ is the charge carrier generation yield, and Σμ is the sum of hole and electron mobilities) of four types of bar-coated 2D perovskite films. The low-performing (1) RT and (2) hot-coated films exhibited much larger φΣμ values than high-performing (3) hot air during coating and (4) hot air after coating.…”

“…The colored triangles are PEA 2 MA 3 Pb 4 I 13 films prepared using bar coating. The black and gray circles are reported 2D PSC data (34 points including Ruddlesden–Popper and Dion–Jacobson PSCs with different additives and concentrations) prepared via spin coating (ref ). The black circle corresponds to the same precursor solution (PEA 2 MA 3 Pb 4 I 13 with NH 4 Cl and KCl aq ) as in the present work.…”

“…After the bar coating process, all the films were thermally annealed at 100 °C for 15 min on a hot plate, affording a 200–300 nm thick perovskite layer. A reference spin-coated film was prepared from the identical precursor DMF solution with NH 4 Cl and KCl additives (5000 rpm for 30 s, annealed at 100 °C for 15 min) . PCBM (Frontier Carbon Inc.) was dissolved in chlorobenzene (CB) (20 mg mL –1 ) and deposited via a spin coating at 3000 rpm (30 nm in thickness) in a glovebox.…”