Thin Films and Solar Cells Based on Semiconducting Two-Dimensional Ruddlesden–Popper (CH₃(CH₂)₃NH₃)₂(CH₃NH₃)_n−1Sn_nI_3n+1 Perovskites

Abstract

Low electrical resistivity (high dark carrier concentration) of CH 3 NH 3 SnI 3 often leads to short-circuiting in solar cells, and appropriate thin-film modifications are required to ensure functional devices. The longterm durability of organic−inorganic perovskite solar cells necessitates the protection of perovskite thin films from moisture to prevent material decomposition. Herein, we report that the electrical resistivity and the moisture stability of two-dimensional (2D) Ruddlesden−Popper (CH 3 (CH 2 ) 3 NH 3 ) 2 (CH 3 NH 3 ) n−1 Sn n I 3n+1 perovskites are considerably improved compared to those of the three-dimensional (3D) CH 3 NH 3 SnI 3 perovskite and subsequently show the solar cell fabrication using a simple one-step spin-coating method. These 2D perovskites are semiconductors with optical band gaps progressively decreasing from 1.83 eV (n = 1) to 1.20 eV (n = ∞). The n = 3 and n = 4 members with optimal band gaps of 1.50 and 1.42 eV for solar cells, respectively, were thus chosen for in-depth studies. We demonstrate that thin films of 2D perovskites orient the {(CH 3 NH 3 ) n−1 Sn n I 3n+1 } 2− slabs parallel to the substrate when dimethyl sulfoxide solvent is used for deposition, and this orientation can be flipped to perpendicular when N,Ndimethylformamide solvent is used. We find that high-purity, single-phase films can be grown only by using precursor solutions of "pre-synthesized" single-phase bulk perovskite materials. We introduce for the first time the use of triethylphosphine as an effective antioxidant, which suppresses the doping level of the 2D films and improves film morphology. The resulting semiconducting 2D Sn-based iodide perovskite films were incorporated in solar cells yielding a power conversion efficiency of 2.5% from the Sn 4 I 13 member. From the temporal stability standpoint, the 2D Sn perovskite solar cells outperform their 3D analogs.

Highly Reproducible Sn‐Based Hybrid Perovskite Solar Cells with 9% Efficiency

“…[23][24][25] Unlike the comprehensive studies on lead-based perovskite, only two papers about tin-based HPSCs using lowdimensional perovskite such as (CH 3 (CH 2 ) 3 NH 3 ) 2 (CH 3 NH 3 ) n-1 Sn n I 3n+1 and PEA 2 FA n−1 Sn n I 3n+1 (n is the number of the inorganic SnI 6 octahedra layers encapsulated by the PEA + (PEA = C 6 H 5 (CH 2 ) 2 NH 3 + ) double layer, the increase (decrease) in n value means increase (decrease) in the dimension; n = ∞ 3D perovskite, n = 1 2D perovskite) were published during the preparation of this manuscript. [26,27] In both papers, the PCE of the tin-based HPSCs are still lower than 6%. Cao et al reported a PCE of 2.5% by using (CH 3 (CH 2 ) 3 NH 3 ) 2 (CH 3 NH 3 ) 3 Sn 4 I 13 (n = 4) as light harvesting layer.…”

“…Cao et al reported a PCE of 2.5% by using (CH 3 (CH 2 ) 3 NH 3 ) 2 (CH 3 NH 3 ) 3 Sn 4 I 13 (n = 4) as light harvesting layer. [26] Ning and co-workers reported a PCE of 5.9% using PEA 2 FA 8 Sn 9 I 28 (n = 9) as light harvesting layer. [27] For the low-dimensional tin-based perovskite family (CH 3 (CH 2 ) 3 NH 3 ) 2 (CH 3 NH 3 ) n-1 Sn n I 3n+1 and PEA 2 FA n−1 Sn n I 3n+1 , how the device using tin perovskite with lower content of bulkier organic cations (∞ >n > 5 for CH 3 (CH 2 ) 3 NH 3 ) 2 (CH 3 N H 3 ) n−1 Sn n I 3n+1 , ∞ > n > 9 for PEA 2 FA n−1 Sn n I 3n+1 ) as light harvesting layer behaves, remains an open question.…”

Lead-free hybrid perovskites for photovoltaics

“…A series of 2D (CH 3 (CH 2 ) 3 NH 3 ) 2 (CH 3 NH 3 ) n −1 Sn n I 3 n +1 perovskites was recently introduced as stable and promising alternatives of 3D ASnX 3 HPs for photovoltaic applications [75]. The 2D HPs revealed semiconductor properties with a bandgap decreasing from 1.83 eV for n = 1 to 1.2 eV at n → ∞ (Fig.…”

“…The Sn 4 I 13 “isomer” with a “close-to-ideal” E g of 1.42 eV was suggested as an optimal light harvester, displaying a promising PCE of 2.5% (Table 1) [75]. …”

Scaling law for excitons in 2D perovskite quantum wells

“…Recent breakthrough in the synthesis of phase-pure (a single n -value) RPPs with higher values 3–5 of n , up to n equals to 5, has inspired their use as low-cost semiconductors in optoelectronics 5–8 as an alternative to three-dimensional (3D) perovskites due to their technologically relevant intrinsic photo- and chemical-stability 5–10 . However, key fundamental questions remain unanswered in RPPs with n greater than 1, such as the nature of optical transitions, as well as the behavior of Coulomb interactions especially with increasing quantum well thickness.…”