Sequential Passivation for Lead-Free Tin Perovskite Solar Cells with High Efficiency.

Lead-free tin perovskite solar cells (PKSCs) have attracted tremendous interest as a replacement for toxic lead-based PKSCs. Nevertheless, the efficiency is significantly low due to the rough surface morphology and high number of defects, which are caused by the fast crystallization and easy oxidization. In this study, a facile and universal posttreatment strategy of sequential passivation with acetylacetone (ACAC) and ethylenediamine (EDA) is proposed. The results show that ACAC can reduce the trap density and enlarge the grain size (short-circuit current (Jsc ) enhancement), while EDA can bond the undercoordinated tin and regulate the energy level (open-circuit voltage (Voc ) enhancement). A promising 13 % efficiency is achieved with better stability. In addition, other combinations of diketones or amines are selected, with similar effects. This study provides a universal strategy to enhance the crystallinity and passivate defects while fabricating stable PKSCs with high efficiency.

Indent-Free Vapor-Assisted Surface Passivation Strategy toward Tin Halide Perovskite Solar Cells.

Sn halide perovskite solar cells (PKSCs) are the most promising competitors to conventional lead PKSCs. Nevertheless, defects at the surfaces and grain boundaries hinder the improvement of the PKSCs' performance. Liquid surface passivation on the perovskite layer is commonly used to decrease these defects. In the case of tin perovskite solar cells, the liquid passivation improved the open-circuit voltage (Voc). However, this decreased the short-circuit current density (Jsc). We found that this Jsc loss is brought about by the thickness loss after the liquid passivation because tin perovskite layers are partially soluble in common solvents, and the calculated impact pressure was up to 155.4 kPa. Here, we introduce new vapor passivation including solvent and passivation molecules and report efficiency enhancement without decreasing Jsc. The vapor-passivated film showed longer time-resolved photoluminescence decay, smoother morphology, and lower defect densities. Most importantly, the vapor passivation method significantly enhanced the efficiency from 9.41 to 11.29% with Jsc increasing from 22.82 to 24.05 mA·cm-2. On the contrary, the corresponding liquid passivation method gave an efficiency of 10.90% with a decreased Jsc from 22.82 to 22.38 mA·cm-2. A commonly used and simple indent-free surface passivation strategy is proposed to enhance the efficiency and stability of PKSCs.

Enhancing the Electronic Properties and Stability of High-Efficiency Tin-Lead Mixed Halide Perovskite Solar Cells via Doping Engineering.

Overcoming Voc loss to increase the efficiency of perovskite solar cells (PSCs) has been aggressively studied. In this work, we introduce and compare rubidium iodide (RbI) and potassium iodide (KI) alkali metal halides (AMHs) as dopants in a tin-lead (SnPb)-based perovskite system to improve the performance of PSCs by enhancing their Voc. Improvement in terms of surface morphology, crystallinity, charge transfer, and carrier transport in the SnPb perovskites was observed with the addition of AMH dopants. Significant power conversion efficiency improvement has been achieved with the incorporation of either dopant, and the highest efficiency was 21.04% in SnPb mixed halide PSCs when the RbI dopant was employed. In conclusion, we can outline the enhancement strategy that yields a remarkable efficiency of >20% with a smaller Voc loss and improved storage, light, and thermal stability in SnPb PSCs via doping engineering.

Cesium Acetate-Induced Interfacial Compositional Change and Graded Band Level in MAPbI3 Perovskite Solar Cells.

Compositional engineering and interfacial modifications have played pivotal roles in the accomplishment of high-efficiency perovskite solar cells (PSCs). Different interfaces in the PSCs influence the performance remarkably either by altering the crystallization of the active material or shifting the energy levels or improving the electrical contact. This work reports how a thin layer of cesium acetate on the TiO2 electron transport layer (ETL) induces generation of a PbI2-rich methylammonium lead iodide (MAPbI3) composition at the ETL/MAPbI3 interface, which downshifts the conduction band level of MAPbI3 to create an energy level gradient favorable for carrier collection, resulting in higher photocurrent, fill factor, and overall power conversion efficiency.

VOC Over 1.4 V for Amorphous Tin-Oxide-Based Dopant-Free CsPbI2Br Perovskite Solar Cells.

CsPbI2Br perovskite solar cells have attracted much attention because of the rapid development in their efficiency and their great potential as a top cell of tandem solar cells. However, the VOC outputs observed so far in most cases are far from that desired for a top cell. Up to now, with various kinds of treatments, the reported champion VOC is only 1.32 V, with a VOC deficit of 0.60 V. In this work, we found that aging of the SnCl2 precursor solution for the electron-transporting layer can promote the VOC of CsPbI2Br solar cells by employing a dopant-free-polymer hole transport material (HTM) over 1.40 V and efficiency over 15.5% with high reproducibility. With the champion VOC of 1.43 V, the VOC deficit was reduced to <0.50 V, which is achieved for the first time. This simple technique of SnCl2 solution aging forms a uniform and smooth amorphous SnOx film with pure Sn4+, elevates the conduction band of SnOx, and reduces the interfacial gaps and the trap state density of the device, resulting in enhancement in average VOC from ∼1.2 V in the nonaged case to ∼1.4 V in the aged case. Furthermore, the device using an aged SnCl2 solution also exhibits a much better long-term stability than that made of the fresh solution. These achievements in dopant/additive-free CsPbI2Br solar cells can be useful for future research on CsPbI2Br and tandem solar cells.

Enhanced Device Performance with Passivation of the TiO2 Surface Using a Carboxylic Acid Fullerene Monolayer for a SnPb Perovskite Solar Cell with a Normal Planar Structure.

Research on tin-lead (SnPb) perovskite solar cells (PSCs) has gained popularity in recent years because of their low band gap, which could be applied to tandem solar cells. However, most of the work is based on inverted PSCs using PEDOT:PSS as the hole-transport layer as normal-structure PSCs show lower efficiency. In this work, the reason behind the low efficiency of normal-structure SnPb PSCs is elucidated and surface passivation has been tested as a method to overcome the problem. In the case of normal PSCs, at the interface between the titania layer and SnPb perovskite, there are many carrier traps observed originating from Ti-O-Sn bonds. In order to avoid the direct contact between titania and the SnPb perovskite layer, the titania surface is passivated with carboxylic acid C60 resulting in an efficiency increase from 5.14 to 7.91%. This will provide a direction of enhancing the efficiency of the normal-structure SnPb PSCs through heterojunction engineering.

Relationship between Lattice Strain and Efficiency for Sn-Perovskite Solar Cells.

In the composition of Q0.1(FA0.75MA0.25)0.9SnI3, Q is replaced with Na+, K+, Cs+, ethylammonium+ (EA+), and butylammonium+ (BA+), respectively, and the relationship between actually measured lattice strain and photovoltaic performances is discussed. The lattice strain evaluated by the Williamson-hall plot of X-ray diffraction data decreased as the tolerance factor was close to one. The efficiency of the Sn-perovskite solar cell was enhanced as the lattice strain decreased. Among them, EA0.1(FA0.75MA0.25)0.9SnI3 having lowest lattice strain gave the best result of 5.41%. Because the carrier mobility increased with a decrease in the lattice strain, these lattice strains would disturb carrier mobility and decrease the solar cell efficiency. Finally, the results that the efficiency of the SnGe-perovskite solar cells was gradually enhanced from 6.42 to 7.60% during storage, was explained by the lattice strain relaxation during the storage.

Suppression of Charge Carrier Recombination in Lead-Free Tin Halide Perovskite via Lewis Base Post-treatment.

Lead-free tin perovskite solar cells (PSCs) show the most promise to replace the more toxic lead-based perovskite solar cells. However, the efficiency is significantly less than that of lead-based PSCs as a result of low open-circuit voltage. This is due to the tendency of Sn2+ to oxidize into Sn4+ in the presence of air together with the formation of defects and traps caused by the fast crystallization of tin perovskite materials. Here, post-treatment of the tin perovskite layer with edamine Lewis base to suppress the recombination reaction in tin halide PSCs results in efficiencies higher than 10%, which is the highest reported efficiency to date for pure tin halide PSCs. The X-ray photoelectron spectroscopy data suggest that the recombination reaction originates from the nonstoichiometric Sn:I ratio rather than the Sn4+:Sn2+ ratio. The amine group in edamine bonded the undercoordinated tin, passivating the dangling bonds and defects, resulting in suppressed charge carrier recombination.

Passivation of Grain Boundary by Squaraine Zwitterions for Defect Passivation and Efficient Perovskite Solar Cells.

Unavoidable defects in grain boundaries (GBs) are detrimental and critically influence the organometal halide perovskite performance and stability. To address this issue, semiconducting molecules have been employed to passivate traps along perovskite GBs. Here, we designed and synthesized three squaraine molecules (SQ) with zwitterionic structure to interact with under-coordinated Pb2+ and passivate Pb-I antisite defects. Density functional theory calculation shows symmetric O atoms could coordinate with perovskite grains simultaneously, resulting in continuous charge distribution at the SQ-perovskite interface. The energetic traps distribution in CH3NH3PbI3 perovskite is influenced significantly by the interaction between SQ and perovskite as analyzed by thermally stimulated current, in which the deep-level defects are considerably reduced due to efficient SQ passivation. In addition, we explore how SQ molecules with different energy offset affect the charge extraction, which is suggested to facilitate exciton separation at the perovskite-SQ interface. These benefits lead to enhanced perovskite efficiency from 15.77 to 18.83% with the fill factor approaching 80%, which is among the highest efficiency reported for MAPbI3 solar cells fabricated in an ambient environment at 60% relative humidity (RH). Considerable retardation of perovskite device degradation was achieved, retaining 90% of initial efficiency when kept 600 h at 60 ± 5% RH.

Interfacial Sulfur Functionalization Anchoring SnO2 and CH3 NH3 PbI3 for Enhanced Stability and Trap Passivation in Perovskite Solar Cells.

Trap states at the interface or in bulk perovskite materials critically influence perovskite solar cells performance and long-term stability. Here, a strategy for efficiently passivating charge traps and mitigating interfacial recombination by SnO2 surface sulfur functionalization is reported, which utilizes xanthate decomposition on the SnO2 surface at low temperature. The results show that functionalized sulfur atoms can coordinate with under-coordinated Pb2+ ions near the interface. After device fabrication under more than 60 % humidity in ambient air, the efficiency of methylammonium lead iodide (MAPbI3 ) perovskite solar cells based on sulfur-functionalized SnO2 increased from 16.56 % to 18.41 % with suppressed hysteresis, which resulted from the accelerated interfacial charge transport kinetics and decreased traps in bulk perovskite by interfacial sulfur functionalization. Additionally, thermally stimulated current studies show the decreased trap density in the shallow trap area after interfacial sulfur functionalization. The interfacial sulfur functionalized solar cells without sealing also exhibited considerable retardation of solar cell degradation with only 10 % degradation after 70 days air storage. This work demonstrates a facile sulfur functionalization strategy by using xanthate decomposition on SnO2 surfaces to obtain highly efficient perovskite solar cells.