Structure-Property Relations of Methylamine Vapor Treated Hybrid Perovskite CH3NH3PbI3 Films and Solar Cells

Self-Healing and Light-Soaking in MAPbI3 : The Effect of H2 O

The future of halide perovskites (HaPs) is beclouded by limited understanding of their long-term stability. While HaPs can be altered by radiation that induces multiple processes, they can also return to their original state by "self-healing." Here two-photon (2P) absorption is used to effect light-induced modifications within MAPbI₃ single crystals. Then the changes in the photodamaged region are followed by measuring the photoluminescence, from 2P absorption with 2.5 orders of magnitude lower intensity than that used for photodamaging the MAPbI₃ . After photodamage, two brightening and one darkening process are found, all of which recover but on different timescales. The first two are attributed to trap-filling (the fastest) and to proton-amine-related chemistry (the slowest), while photodamage is attributed to the lead-iodide sublattice. Surprisingly, while after 2P-irradiation of crystals that are stored in dry, inert ambient, photobrightening (or "light-soaking") occurs, mostly photodarkening is seen after photodamage in humid ambient, showing an important connection between the self-healing of a HaP and the presence of H₂ O, for long-term steady-state illumination, practically no difference remains between samples kept in dry or humid environments. This result suggests that photobrightening requires a chemical-reservoir that is sensitive to the presence of H₂ O, or possibly other proton-related, particularly amine, chemistry.

The pursuit of stability in halide perovskites: the monovalent cation and the key for surface and bulk self-healing

We find significant differences between degradation and healing at the surface or in the bulk for each of the different APbBr₃ single crystals (A = CH₃NH₃⁺, methylammonium (MA); HC(NH₂)₂⁺, formamidinium (FA); and cesium, Cs⁺). Using 1- and 2-photon microscopy and photobleaching we conclude that kinetics dominate the surface and thermodynamics the bulk stability. Fluorescence-lifetime imaging microscopy, as well as results from several other methods, relate the (damaged) state of the halide perovskite (HaP) after photobleaching to its modified optical and electronic properties. The A cation type strongly influences both the kinetics and the thermodynamics of recovery and degradation: FA heals best the bulk material with faster self-healing; Cs⁺ protects the surface best, being the least volatile of the A cations and possibly through O-passivation; MA passivates defects via methylamine from photo-dissociation, which binds to Pb²⁺. DFT simulations provide insight into the passivating role of MA, and also indicate the importance of the Br₃^- defect as well as predicts its stability. The occurrence and rate of self-healing are suggested to explain the low effective defect density in the HaPs and through this, their excellent performance. These results rationalize the use of mixed A-cation materials for optimizing both solar cell stability and overall performance of HaP-based devices, and provide a basis for designing new HaP variants.

Methylamine-assisted growth of uniaxial-oriented perovskite thin films with millimeter-sized grains

Defects from grain interiors and boundaries of perovskite films cause significant nonradiative recombination energy loss, and thus perovskite films with controlled crystallinity and large grains is critical for improvement of both photovoltaic performance and stability for perovskite-based solar cells. Here, a methylamine (MA⁰) gas-assisted crystallization method is developed for fabrication of methylammonium lead iodide (MAPbI₃) perovskite films. In the process, the perovskite film is formed via controlled release of MA0 gas molecules from a liquid intermediate phase MAPbI₃·xMA⁰. The resulting perovskite film comprises millimeter-sized grains with (110)-uniaxial crystallographic orientation, exhibiting much low trap density, long carrier lifetime, and excellent environmental stability. The corresponding perovskite solar cell exhibits a power conversion efficiency (PCE) of ~ 21.36%, which is among the highest reported for MAPbI₃-based devices. This method provides important progress towards the fabrication of high-quality perovskite thin films for low-cost, highly efficient and stable perovskite solar cells.

Suppressing grain boundaries and defects in perovskite solar cells remains a quest to address the efficiency and stability issues. Here, the authors use methylamine gas for assisting the growth of uniaxial-oriented perovskite thin films with millimeter-sized grains.

Rising from the Ashes: Gaseous Therapy for Robust and Large-Area Perovskite Solar Cells

Scalable fabrication of perovskite solar cells (PSCs) with high reliability is one of the most pivotal concerns that must be addressed before they get into the photovoltaic (PV) market. Scaling large-area high-quality perovskite films is of great importance in this process. Here, gaseous therapy has been proposed for the post-treatment of perovskite films with high scalability and low cost. An inspiring evolvement from poor perovskite films to high quality ones is demonstrated under a joint treatment of methylamine gas and hot solvent vapors. The perovskite films are completely reconstructed and repaired regardless of the morphology of the original films. As a consequence, small-area (0.09 cm²) and large-area (4 cm²) PSCs based on the healed MAPbI₃ films can afford J-V scanned efficiencies of 19.2 and 16.5% under a reverse sweep, respectively. Furthermore, stabilized power outputs of 18.5 and 15.2% are obtained from the small one and large one under continuous maximum power point tracking.

Eppur si Muove: Proton Diffusion in Halide Perovskite Single Crystals

Ion diffusion affects the optoelectronic properties of halide-perovskites (HaPs). Until now, the fastest diffusion has been attributed to the movement of the halides, largely neglecting the contribution of protons, on the basis of computed density estimates. Here, the process of proton diffusion inside HaPs, following deuterium-hydrogen exchange and migration in MAPbI₃ , MAPbBr₃ , and FAPbBr₃ single crystals, is proven through D/H NMR quantification, Raman spectroscopy, and elastic recoil detection analysis, challenging the original assumption of halide-dominated diffusion. The results are confirmed by impedance spectroscopy, where MAPbBr₃ - and CsPbBr₃ -based solar cells respond at very different frequencies. Water plays a key role in allowing the migration of protons as deuteration is not detected in its absence. The water contribution is modeled to explain and forecast its effect as a function of its concentration in the perovskite structure. These findings are of great importance as they evidence how unexpected, water-dependent proton diffusion can be at the basis of the ≈7 orders of magnitude spread of diffusion (attributed to I^- and Br^- ) coefficient values, reported in the literature. The reported enhancement of the optoelectronic properties of HaP when exposed to small amounts of water may be related to the finding.

Simultaneous heteroepitaxial growth of SrO (001) and SrO (111) during strontium-assisted deoxidation of the Si (001) surface

Epitaxial integration of transition-metal oxides with silicon brings a variety of functional properties to the well-established platform of electronic components. In this process, deoxidation and passivation of the silicon surface are one of the most important steps, which in our study were controlled by an ultra-thin layer of SrO and monitored by using transmission electron microscopy (TEM), electron energy-loss spectroscopy (EELS), synchrotron X-ray diffraction (XRD) and reflection high energy electron diffraction (RHEED) methods. Results revealed that an insufficient amount of SrO leads to uneven deoxidation of the silicon surface i.e. formation of pits and islands, whereas the composition of the as-formed heterostructure gradually changes from strontium silicide at the interface with silicon, to strontium silicate and SrO in the topmost layer. Epitaxial ordering of SrO, occurring simultaneously with silicon deoxidation, was observed. RHEED analysis has identified that SrO is epitaxially aligned with the (001) Si substrate both with SrO (001) and SrO (111) out-of-plane directions. This observation was discussed from the point of view of SrO desorption, SrO-induced deoxidation of the Si (001) surface and other interfacial reactions as well as structural ordering of deposited SrO. Results of the study present an important milestone in understanding subsequent epitaxial integration of functional oxides with silicon using SrO.

Controlling Crystal Growth via an Autonomously Longitudinal Scaffold for Planar Perovskite Solar Cells

Sequential deposition is certified as an effective technology to obtain high-performance perovskite solar cells (PVSCs), which can be derivatized into large-scale industrial production. However, dense lead iodide (PbI₂ ) causes incomplete reaction and unsatisfactory solution utilization of perovskite in planar PVSCs without mesoporous titanium dioxide as a support. Here, a novel autonomously longitudinal scaffold constructed by the interspersion of in situ self-polymerized methyl methacrylate (sMMA) in PbI₂ is introduced to fabricate efficient PVSCs with excellent flexural endurance and environmental adaptability. By this strategy perovskite solution can be confined within an organic scaffold with vertical crystal growth promoted, effectively inhibiting exciton accumulation and recombination at grain boundaries. Additionally, sMMA cross-linked perovskite network can release mechanical stress and occupy the main channels for ion migration and water/oxygen permeation to significantly improve operational stability, which opens up a new strategy for the commercial development of large-area PVSCs in flexible electronics.

In Situ Observation of Vapor-Assisted 2D-3D Heterostructure Formation for Stable and Efficient Perovskite Solar Cells

The heterogeneous stacking of a thin two-dimensional (2D) perovskite layer over the three-dimensional (3D) perovskite film creates a sophisticated architecture for perovskite solar cells (PSCs). It combines the remarkable thermal and environmental stabilities of 2D perovskites with the superior optoelectronic properties of 3D materials which resolves the chronic stability issue with no compromise on efficiency. Herein, we propose the vapor-assisted growth strategy to fabricate high-quality 2D/3D heterostructured perovskite films by introducing long-chain organoamine gases in which the 2D layers have a uniform and tunable thickness. The 3D to 2D transformation of the widely adopted MAPbI₃ (MA = methylammonium) film is initiated by the butylamine vapor and monitored through the in situ grazing-incidence X-ray diffraction technique. A variety of 2D species are observed and rationalized by the different collapsing and reconstruction models of the Pb-I octahedra. The PSC devices based on the optimized 2D/3D heterostructures show significant improvements in photovoltaic performances, owing to better energy level alignments, longer carrier lifetimes, and less defects as compared to their 3D analogues. In addition, both the butylamine vapor-treated perovskite films and the derived PSC devices demonstrate exceptional long-term stabilities.

Enhanced Crystallinity of Triple-Cation Perovskite Film via Doping NH4SCN

The trap-state density in perovskite films largely determines the photovoltaic performance of perovskite solar cells (PSCs). Increasing the crystal grain size in perovskite films is an effective method to reduce the trap-state density. Here, we have added NH₄SCN into perovskite precursor solution to obtain perovskite films with an increased crystal grain size. The perovskite with increased crystal grain size shows a much lower trap-state density compared with reference perovskite films, resulting in an improved photovoltaic performance in PSCs. The champion photovoltaic device has achieved a power conversion efficiency of 19.36%. The proposed method may also impact other optoelectronic devices based on perovskite films.

Efficient Methylamine-Containing Antisolvent Strategy to Fabricate High-Efficiency and Stable FA0.85Cs0.15Pb(Br0.15I2.85) Perovskite Solar Cells

Antisolvent and additive strategies are significantly positive to improve the crystal quality, device performance, and long-term stability of perovskite solar cells (PSCs). In addition, the high-quality perovskite thin films could be prepared by the methylamine (MA) gas-induced defect-healing process. However, until now, the research on adding MA into the antisolvent, which may take the advantages of two efficiently modified strategies, is still not systematically studied. Here, we add the MA additive into the chlorobenzene antisolvent to fabricate the FA_0.85Cs_0.15Pb(Br_0.15I_2.85) films and achieve the high-quality light-absorbing layers with the preferred orientation of (101). The use of an antisolvent, with an appropriate amount of MA in chlorobenzene, leads to extremely uniform and dense perovskite layers with a better hydrophobic performance and enables the fabrication of remarkably improved solar cells with a power conversion efficiency (PCE) of 19.6% for a champion cell. This strategy shows an encouraging improvement of more than 13.95% compared with the traditional antisolvent strategy. The high-efficiency devices could maintain more than 95 or 88% of their initial PCEs after 500 h under continuous light soaking or thermal aging in the dark at 85 °C in a N₂-filled glove box, respectively. These results provide an important progress in the realization of highly efficient and stable PSCs.

Perovskite Photovoltaics: The Significant Role of Ligands in Film Formation, Passivation, and Stability

Due to their outstanding optoelectronic properties, metal halide perovskites have been intensively studied in recent years. The latest certificated efficiency of 23.3% recently achieved in perovskite solar cells (PVSCs) enables them to be used as a very promising candidate for next-generation photovoltaics. The morphology, defect density, and water resistance of perovskite films have an enormous impact on the performance and stability of PVSCs. Ligands, with coordinating capability, have been widely developed to improve the quality and stability of perovskite materials significantly. In the first section of this review, the role of ligands in fabricating perovskite films by different methods (one-step, two-step, and postdeposition treatment) is discussed. In the second section, the progress on ligand-passivated perovskites via post-treatment, in situ passivation during perovskite formation, and modifying the substrates before perovskite formation is reviewed. In the third section, a discussion of ligand-stabilized perovskite films from the perspectives of crystal crosslinking, dimensionality engineering, and interfacial modification is presented. Finally, a summary and an outlook are given.

Trash into Treasure: δ-FAPbI3 Polymorph Stabilized MAPbI3 Perovskite with Power Conversion Efficiency beyond 21

Effective passivation and stabilization of both the inside and interface of a perovskite layer are crucial for perovskite solar cells (PSCs), in terms of efficiency, reproducibility, and stability. Here, the first formamidinium lead iodide (δ-FAPbI₃ ) polymorph passivated and stabilized MAPbI₃ PSCs are reported. This novel MAPbI₃ /δ-FAPbI₃ structure is realized via treating a mixed organic cation MA _x FA_1-x PbI₃ perovskite film with methylamine (MA) gas. In addition to the morphology healing, MA gas can also induce the formation of δ-FAPbI₃ phase within the perovskite film. The in situ formed 1D δ-FAPbI₃ polymorph behaves like an organic scaffold that can passivate the trap state, tunnel contact, and restrict organic-cation diffusion. As a result, the device efficiency is easily boosted to 21%. Furthermore, the stability of the MAPbI₃ /δ-FAPbI₃ film is also obviously improved. This δ-FAPbI₃ phase passivation strategy opens up a new direction of perovskite structure modification for further improving stability without sacrificing efficiency.

A Strategy to Produce High Efficiency, High Stability Perovskite Solar Cells Using Functionalized Ionic Liquid-Dopants

Functionalized imidazolium iodide salts (ionic liquids) modified with CH₂ CHCH₂ , CH₂ CCH, or CH₂ CN groups are applied as dopants in the synthesis of CH₃ NH₃ PbI₃ -type perovskites together with a fumigation step. Notably, a solar cell device prepared from the perovskite film doped with the salt containing the CH₂ CHCH₂ side-chain has a power conversion efficiency of 19.21%, which is the highest efficiency reported for perovskite solar cells involving a fumigation step. However, doping with the imidazolium salts with the CH₂ CCH and CH₂ CN groups result in perovskite layers that lead to solar cell devices with similar or lower power conversion efficiencies than the dopant-free cell.