Tin-based halide perovskite materials have been successfully employed in lead-free perovskite solar cells, but the tendency of these materials to form leakage pathways from p-type defect states, mainly Sn and Sn vacancies, causes poor device reproducibility and limits the overall power conversion efficiencies (PCEs). Here, we present an effective process that involves a reducing vapor atmosphere during the preparation of Sn-based halide perovskite solar cells to solve this problem, using MASnI, CsSnI, and CsSnBr as the representative absorbers. This process enables the fabrication of remarkably improved solar cells with PCEs of 3.89%, 1.83%, and 3.04% for MASnI, CsSnI, and CsSnBr, respectively. The reducing vapor atmosphere process results in more than 20% reduction of Sn/Sn ratios, which leads to greatly suppressed carrier recombination, to a level comparable to their lead-based counterparts. These results mark an important step toward a deeper understanding of the intrinsic Sn-based halide perovskite materials, paving the way to the realization of low-cost and lead-free Sn-based halide perovskite solar cells.

The development of Sn-based perovskite solar cells has been challenging because devices often show short-circuit behavior due to poor morphologies and undesired electrical properties of the thin films. A low-temperature vapor-assisted solution process (LT-VASP) has been employed as a novel kinetically controlled gas-solid reaction film fabrication method to prepare lead-free CH3NH3SnI3 thin films. We show that the solid SnI2 substrate temperature is the key parameter in achieving perovskite films with high surface coverage and excellent uniformity. The resulting high-quality CH3NH3SnI3 films allow the successful fabrication of solar cells with drastically improved reproducibility, reaching an efficiency of 1.86%. Furthermore, our Kelvin probe studies show the VASP films have a doping level lower than that of films prepared from the conventional one-step method, effectively lowering the film conductivity. Above all, with (LT)-VASP, the short-circuit behavior often obtained from the conventional one-step-fabricated Sn-based perovskite devices has been overcome. This study facilitates the path to more successful Sn-perovskite photovoltaic research.

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.

Sn-based halide perovskite materials have attracted tremendous attention and have been employed successfully in solar cells. However, their high conductivities resulting from the unstable divalent Sn state in the structure cause poor device performance and poor reproducibility. Herein, we used excess tin iodide (SnI 2 ) in Sn-based halide perovskite solar cells (ASnI 3 , A = Cs, methylammonium, and formamidinium tin iodide as the representative light absorbers) combined with a reducing atmosphere to stabilize the Sn 2+ state. Excess SnI 2 can disperse uniformly into the perovskite films and functions as a compensator as well as a suppressor of Sn 2+ vacancies, thereby effectively reducing the p-type conductivity. This process significantly improved the solar cell performances of all the ASnI 3 materials on mesoporous TiO 2 . Optimized CsSnI 3 devices achieved a maximum power conversion efficiency of 4.81%, which is the highest among all inorganic Pb-free perovskite solar cells to date.

The regioregular narrow band gap (E(g) ~1.5 eV) conjugated polymer PIPCP was designed and synthesized. PIPCP contains a backbone comprised of CPDT-PT-IDT-PT repeat units (CPDT = cyclopentadithiophene, PT = pyridyl[2,1,3]thiadiazole, IDT = indacenodithiophene) and strictly organized PT orientations, such that the pyridyl N-atoms point toward the CPDT fragment. Comparison of PIPCP with the regiorandom counterpart PIPC-RA illustrates that the higher level of molecular order translates to higher power conversion efficiencies (PCEs) when incorporated into bulk heterojunction (BHJ) organic solar cells. Examination of thin films via absorption spectroscopy and grazing incidence wide-angle X-ray diffraction (GIWAXS) experiments provides evidence of higher order within thin films obtained by spin coating. Most significantly, we find that PIPCP:PC61BM blends yield devices with an open circuit voltage (V(oc)) of 0.86 V, while maintaining a PCE of ~6%. Comparison against a wide range of analogous narrow band gap conjugated polymers reveals that this V(oc) value is particularly high for a BHJ system with band gaps in the 1.4-1.5 eV range thereby indicating a very low E(g) - eV(oc) loss.

Tin (Sn)-based perovskites are increasingly attractive because they offer lead-free alternatives in perovskite solar cells. However, depositing high-quality Sn-based perovskite films is still a challenge, particularly for low-temperature planar heterojunction (PHJ) devices. Here, a "multichannel interdiffusion" protocol is demonstrated by annealing stacked layers of aqueous solution deposited formamidinium iodide (FAI)/polymer layer followed with an evaporated SnI layer to create uniform FASnI films. In this protocol, tiny FAI crystals, significantly inhibited by the introduced polymer, can offer multiple interdiffusion pathways for complete reaction with SnI . What is more, water, rather than traditional aprotic organic solvents, is used to dissolve the precursors. The best-performing FASnI PHJ solar cell assembled by this protocol exhibits a power conversion efficiency (PCE) of 3.98%. In addition, a flexible FASnI -based flexible solar cell assembled on a polyethylene naphthalate-indium tin oxide flexible substrate with a PCE of 3.12% is demonstrated. This novel interdiffusion process can help to further boost the performance of lead-free Sn-based perovskites.

A low hole carrier concentration in methylammonium tin halide (MASnX3) perovskite semiconductors is a prerequisite for a nonshorting solar cell device. In-depth film characterizations were performed on MASnI3–x Br x films, fabricated by both a low-temperature vapor-assisted solution process (LT-VASP) and conventional one-step methods, to reveal the origin of the lower hole carrier concentration from films of the former approach. We found that the vaporization of CH3NH3I solid at 150 °C, the temperature at which the LT-VASP occurs, does not supply iodine to the SnX2 (X = Br, I) films. As a result, secondary phases form aside from the desired MASnX3 perovskite; the secondary phases are suggested to be SnO and Sn­(OH)2 via a proposed reaction pathway and are further supported by X-ray photoemission spectroscopy (XPS). These nonperovskite Sn2+ phases are beneficial because they assist in achieving the lower hole-doping levels in LT-VASP films. Remarkably, LT-VASP devices demonstrate improved air stability. Overall, our findings suggest that not only the commonly used SnF2 but also other divalent Sn compounds could serve as Sn vacancy suppressors. Further work on modulating the perovskite film compositions could realize more efficient and stable tin-based perovskite solar cells.

Tin-based halide perovskite materials are promising candidates for lead-free halide perovskite solar cells. However, they suffer from poor device reproducibility and limited overall power conversion efficiencies due to their tendency to become semimetallic from p-type defect states. Herein, we demonstrate an effective approach to address this issue via the addition of piperazine to the precursor solution of tin-based halide perovskite films, to suppress the undesirable p-doping of CsSnI 3 films. Piperazine is found to significantly reduce the conductivity of CsSnI 3 films, improve the film coverage, and at the same time suppress the crystallization of excess SnI 2 . Consequently, shortcircuit behaviors are eliminated, with significantly improved CsSnI 3 solar-cell performance. Moreover, the effects of incorporating SnCl 2 and SnF 2 into the CsSnI 3 devices were investigated in conjunction with addition of piperazine to achieve CsSnI 3 devices with a maximum power conversion efficiency of 3.83%.