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.

Hybrid CPbX 3 (C:C s, CH 3 NH 3 ;X :B r, I) perovskites possess excellent photovoltaic properties but are highly toxic, which hinders their practical application. Unfortunately,a ll Pb-free alternatives based on Sn and Ge are extremely unstable. Although stable and non-toxic C 2 ABX 6 double perovskites based on alternating corner-shared AX 6 and BX 6 octahedra (A = Ag, Cu;B = Bi, Sb) are possible, they have indirect and wide band gaps of over 2eV. However,i si tn ecessary to keep the corner-shared perovskite structure to retain good photovoltaic properties? Here, we demonstrate another family of photovoltaic halides based on edge-shared AX 6 and BX 6 octahedra with the general formula A a B b X x (x = a + 3 b)s uch as Ag 3 BiI 6 ,A g 2 BiI 5 ,A gBiI 4 , AgBi 2 I 7 .A sp erovskites were named after their prototype oxide CaTiO 3 discovered by Lev Perovski, we propose to name these new ABX halidesa sr udorffitesa fter Walter Rüdorff,w ho discoveredt heir prototype oxide NaVO 2 .W es tudied structural and optoelectronic properties of severalh ighly stable and promising Ag-Bi-I photovoltaic rudorffites that feature direct band gaps in the range of 1.79- Photovoltaic (PV) hybrid lead halide perovskites were first reported by Kojimae tal. [1] in 2006 with power conversion efficiency (PCE) of 2.2 %i nadye-sensitized solarc ell (DSSC) device configuration. However,t hese materials gained considerable attention only 6years later after two incremental improvements of their PCE to 3.8 %b yK ojimae tal. [2] and to 6.2 %b yI me tal. [3] Substitution of the liquid electrolyte with an efficient polymer hole-extraction layer by Lee et al. [4] in 2012 increased PCE to 10.9 %and was the turning point in perovskite photovoltaics that openedaway towardh ighly efficient and stable perovskite PV devices.S ince 2012 many researchers, mainly from the dye-sensitized and organic PV fields, joined the exciting research on perovskite solar cells. As ar esult, the PCE of the perovskite solarc ells showed as teep sigmoidal growth thatl ed to the contemporary efficiency of over 22 %. [5] Although this PCE is on par with other highly efficient thin-film PV technologies based on cadmium telluride (CdTe) and copper-indium-gallium selenide (CIGS), lead halide perovskites have the significant advantage of being solution processable, whicho fferss ubstantial cost reduction. Unfortunately,t he relianceo nh ighly toxic Pb hinders the commercial potentialo ft his technology. The toxicity of Pb is very high. The 50 %l ethal dose of lead [LD 50 (Pb)] is less than 5mgp er kg of body weight. In contrast to CdTe, which has excellent stability and negligible solubility in water with as olubility constant of K SP = 10 À34 ,P b-based halide perovskites can easily degrade and Pb can escape from ab roken PV module owing to the moderate solubility of PbI 2 (K SP = 4.4 10 À9 ). Despite various attempts to quantify the impact of potential pollution andi ntroduce life-cycle business modelst hat include integrity monitoring and recycling of perovskite PV modules, ...

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.

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.