Thin‐Film Transformation of NH4PbI3 to CH3NH3PbI3 Perovskite: A Methylamine‐Induced Conversion–Healing Process

Methylamine‐induced thin‐film transformation at room‐temperature is discovered, where a porous, rough, polycrystalline NH₄PbI₃ non‐perovskite thin film converts stepwise into a dense, ultrasmooth, textured CH₃NH₃PbI₃ perovskite thin film. Owing to the beneficial phase/structural development of the thin film, its photovoltaic properties undergo dramatic enhancement during this NH₄PbI₃‐to‐CH₃NH₃PbI₃ transformation process. The chemical origins of this transformation are studied at various length scales.     

The domination of ionic conductivity in tetragonal phase of the organometal halide perovskite CH3NH3PbI3-xClx

Organometal trihalide perovskites have recently gained extreme attention due to their high solar energy conversion in photovoltaic cells. Here, we investigate the contribution of iodide ions to a total conductivity of the mixed lead halide perovskite CH₃NH₃PbI_3−xCl_x with a use of the modified DC Hebb–Wagner polarization method. It has been identified that an ionic conductivity dominates in tetragonal phase which is associated with room temperature. The obtained activation energy for this type of hopping mechanism is equal to (0.87 ± 0.02) eV, which is in a good agreement with previous literature reports. The high contribution of ionic conductivity at room temperature might be a reason of the observed hysteresis in halide perovskite solar cells.     

Methylamine‐Gas‐Induced Defect‐Healing Behavior of CH3NH3PbI3 Thin Films for Perovskite Solar Cells

We report herein the discovery of methylamine (CH₃NH₂) induced defect‐healing (MIDH) of CH₃NH₃PbI₃ perovskite thin films based on their ultrafast (seconds), reversible chemical reaction with CH₃NH₂ gas at room temperature. The key to this healing behavior is the formation and spreading of an intermediate CH₃NH₃PbI₃?xCH₃NH₂ liquid phase during this unusual perovskite–gas interaction. We demonstrate the versatility and scalability of the MIDH process, and show dramatic enhancement in the performance of perovskite solar cells (PSCs) with MIDH. This study represents a new direction in the formation of defect‐free films of hybrid perovskites.     

Mixed‐Halide CH3NH3PbI3−xXx (X=Cl,Br, I) Perovskites: Vapor‐Assisted Solution Deposition and Application as Solar Cell Absorbers

There have been recent reports on the formation of single‐halide perovskites, CH₃NH₃PbX₃ (X=Cl, Br, I), by means of vapor‐assisted solution processing. Herein, the successful formation of mixed‐halide perovskites (CH₃NH₃PbI_3?xX_x) by means of a vapor‐assisted solution method at ambient atmosphere is reported. The perovskite films are synthesized by exposing PbI₂ film to CH₃NH₃X (X=I, Br, or Cl) vapor. The prepared perovskite films have uniform surfaces with good coverage, as confirmed by SEM images. The inclusion of chlorine and bromine into the structure leads to a lower temperature and shorter reaction time for optimum perovskite film formation. In the case of CH₃NH₃PbI_3?xCl_x, the optimum reaction temperature is reduced to 100 °C, and the resulting phases are CH₃NH₃PbI₃ (with trace Cl) and CH₃NH₃PbCl₃ with a ratio of about 2:1. In the case of CH₃NH₃PbI_3?xBr_x, single‐phase CH₃NH₃PbI₂Br is formed in a considerably shorter reaction time than that of CH₃NH₃PbI₃. The mesostructured perovskite solar cells based on CH₃NH₃PbI₃ films show the best optimal power conversion efficiency of 13.5 %, whereas for CH₃NH₃PbI_3?xCl_x and CH₃NH₃PbI_3?xBr_x the best recorded efficiencies are 11.6 and 10.5 %, respectively.     

Pseudohalide‐Induced Moisture Tolerance in Perovskite CH3NH3Pb(SCN)2I Thin Films

Two pseudohalide thiocyanate ions (SCN^?) have been used to replace two iodides in CH₃NH₃PbI₃, and the resulting perovskite material was used as the active material in solar cells. In accelerated stability tests, the CH₃NH₃Pb(SCN)₂I perovskite films were shown to be superior to the conventional CH₃NH₃PbI₃ films as no significant degradation was observed after the film had been exposed to air with a relative humidity of 95 % for over four hours, whereas CH₃NH₃PbI₃ films degraded in less than 1.5 hours. Solar cells based on CH₃NH₃Pb(SCN)₂I thin films exhibited an efficiency of 8.3 %, which is comparable to that of CH₃NH₃PbI₃ based cells fabricated in the same way.     

Theoretical Treatment of CH3NH3PbI3 Perovskite Solar Cells

Hybrid halide perovskite solar cells (PSCs) giving over 22 % power conversion efficiencies (PCEs) have attracted considerable attention. Although perovskite plays a significant role in the operation of PSCs, the fundamental theories associated with perovskites have not been resolved in spite of the increase in research. In this Minireview, we assess the current understanding, based on the first‐principles calculations, of structural and electronic properties, defects, ionic diffusion, and shift current for CH₃NH₃PbI₃ perovskite, and the effect of ionic transport on the hysteresis of current–voltage curves in PSCs. The shift current connected to the possible presence of ferroelectricity is also discussed. The current state‐of‐the‐art and some open questions regarding PSCs are also highlighted, and the benefits, challenges, and potentials of perovskite for use in PSCs are stressed.     

Effect of the heat treatment of CH3NH3PbI3 perovskite on its electrical and photoelectric properties

The effect of the annealing of CH₃NH₃PbI₃ perovskite on its electrical, photoelectric and optical properties has been estimated. The annealing leads to a two-phase structure consisting of perovskite and lead iodide, whose relative concentrations depend on the annealing temperature. The formation of a PbI₂ phase in a perovskite film upon heating leads to a decrease in the conductivity and photoconductivity of two-phase material, which contradicts the assumption of a decrease in recombination associated with PbI₂, obtained by measuring the parameters of a solar cell.     

Control of Charge Transport in the Perovskite CH3NH3PbI3 Thin Film

Carrier density and transport properties in the CH₃NH₃PbI₃ thin film have been investigated. It is found that the carrier density, the depletion field, and the charge collection and transport properties in the CH₃NH₃PbI₃ absorber film can be controlled effectively by different concentrations of reactants. That is, the carrier properties and the self‐doping characteristics in CH₃NH₃PbI₃ films are strongly influenced by the reaction thermodynamic and kinetic processes. Furthermore, by employing mixed solvents with ethanol and isopropanol to deposit the CH₃NH₃PbI₃ film, the charge collection and transport efficiencies are improved significantly, thereby yielding an overall enhanced cell performance.     

Similar Structural Dynamics for the Degradation of CH3NH3PbI3 in Air and in Vacuum

We investigate the degradation path of MAPbI₃ (MA=methylammonium) films over flat TiO₂ substrates at room temperature by means of X‐ray diffraction, spectroscopic ellipsometry, X‐ray photoelectron spectroscopy, and high‐resolution transmission electron microscopy. The degradation dynamics is found to be similar in air and under vacuum conditions, which leads to the conclusion that the occurrence of intrinsic thermodynamic mechanisms is not necessarily linked to humidity. The process has an early stage, which drives the starting tetragonal lattice in the direction of a cubic atomic arrangement. This early stage is followed by a phase change towards PbI₂. We describe how this degradation product is structurally coupled with the original MAPbI₃ lattice through the orientation of its constituent PbI₆ octahedra. Our results suggest a slight octahedral rearrangement after volatilization of HI+CH₃NH₂ or MAI, with a relatively low energy cost. Our experiments also clarify why reducing the interfaces and internal defects in the perovskite lattice enhances the stability of the material.     

Ultrafast Photogenerated Hole Extraction/Transport Behavior in a CH3NH3PbI3/Carbon Nanocomposite and Its Application in a Metal‐Electrode‐Free Solar Cell

Aligned and flexible electrospun carbon nanomaterials are used to synthesize carbon/perovskite nanocomposites. The free‐electron diffusion length in the CH₃NH₃PbI₃ phase of the CH₃NH₃PbI₃/carbon nanocomposite is almost twice that of bare CH₃NH₃PbI₃, and nearly 95 % of the photogenerated free holes can be injected from the CH₃NH₃PbI₃ phase into the carbon nanomaterial. The exciton binding energy of the composite is estimated to be 23 meV by utilizing temperature‐dependent optical absorption spectroscopy. The calculated free carriers increase with increasing total photoexcitation density, and this broadens the potential of this material for a broad range of optoelectronics applications. A metal‐electrode‐free perovskite solar cell (power conversion efficiency: 13.0 %) is fabricated with this perovskite/carbon composite, which shows great potential for the fabrication of efficient, large‐scale, low‐cost, and metal‐electrode‐free perovskite solar cells.     

Inkjet Printing and Instant Chemical Transformation of a CH3NH3PbI3/Nanocarbon Electrode and Interface for Planar Perovskite Solar Cells

A planar perovskite solar cell that incorporates a nanocarbon hole‐extraction layer is demonstrated for the first time by an inkjet printing technique with a precisely controlled pattern and interface. By designing the carbon plus CH₃NH₃I ink to transform PbI₂ in situ to CH₃NH₃PbI₃, an interpenetrating seamless interface between the CH₃NH₃PbI₃ active layer and the carbon hole‐extraction electrode was instantly constructed, with a markedly reduced charge recombination compared to that with the carbon ink alone. As a result, a considerably higher power conversion efficiency up to 11.60 % was delivered by the corresponding solar cell. This method provides a major step towards the fabrication of low‐cost, large‐scale, metal‐electrode‐free but still highly efficient perovskite solar cells.     

CH3NH3PbI3 perovskite single crystals: surface photophysics and their interaction with the environment

Here we identify structural inhomogeneity on a micrometer scale across the surface of a CH₃NH₃PbI₃ perovskite single crystal. At the crystal edge a local distortion of the crystal lattice is responsible for a widening of the optical bandgap and faster photo-carrier recombination. These effects are inherently present at the edge of the crystal, and further enhanced upon water intercalation, as a preliminary step in the hydration of the perovskite material.     

Controllable Growth of Perovskite Films by Room‐Temperature Air Exposure for Efficient Planar Heterojunction Photovoltaic Cells

A two‐step solution processing approach has been established to grow void‐free perovskite films for low‐cost high‐performance planar heterojunction photovoltaic devices. A high‐temperature thermal annealing treatment was applied to drive the diffusion of CH₃NH₃I precursor molecules into a compact PbI₂ layer to form perovskite films. However, thermal annealing for extended periods led to degraded device performance owing to the defects generated by decomposition of perovskite into PbI₂. A controllable layer‐by‐layer spin‐coating method was used to grow “bilayer” CH₃NH₃I/PbI₂ films, and then drive the interdiffusion between PbI₂ and CH₃NH₃I layers by a simple air exposure at room temperature for making well‐oriented, highly crystalline perovskite films without thermal annealing. This high degree of crystallinity resulted in a carrier diffusion length of ca. 800 nm and a high device efficiency of 15.6 %, which is comparable to values reported for thermally annealed perovskite films.     

Stable and Low‐Cost Mesoscopic CH3NH3PbI2Br Perovskite Solar Cells by using a Thin Poly(3‐hexylthiophene) Layer as a Hole Transporter

Mesoscopic perovskite solar cells using stable CH₃NH₃PbI₂Br as a light absorber and low‐cost poly(3‐hexylthiophene) (P3HT) as hole‐transporting layer were fabricated, and a power conversion efficiency of 6.64 % was achieved. The partial substitution of iodine with bromine in the perovskite led to remarkably prolonged charge carrier lifetime. Meanwhile, the replacement of conventional thick spiro‐MeOTAD layer with a thin P3HT layer has significantly reduced the fabrication cost. The solar cells retained their photovoltaic performance well when they were exposed to air without any encapsulation, presenting a favorable stability. The combination of CH₃NH₃PbI₂Br and P3HT may render a practical and cost‐effective solid‐state photovoltaic system. The superior stability of CH₃NH₃PbI₂Br is also promising for other photoconversion applications.     

Role of the Iodide–Methylammonium Interaction in the Ferroelectricity of CH3NH3PbI3

Excellent conversion efficiencies of over 20 % and facile cell production have placed hybrid perovskites at the forefront of novel solar cell materials, with CH₃NH₃PbI₃ being an archetypal compound. The question why CH₃NH₃PbI₃ has such extraordinary characteristics, particularly a very efficient power conversion from absorbed light to electrical power, is hotly debated, with ferroelectricity being a promising candidate. This does, however, require the crystal structure to be non‐centrosymmetric and we herein present crystallographic evidence as to how the symmetry breaking occurs on a crystallographic and, therefore, long‐range level. Although the molecular cation CH₃NH₃⁺ is intrinsically polar, it is heavily disordered and this cannot be the sole reason for the ferroelectricity. We show that it, nonetheless, plays an important role, as it distorts the neighboring iodide positions from their centrosymmetric positions.     

Hybrid Bilayer WSe2–CH3NH3PbI3 Organolead Halide Perovskite as a High‐Performance Photodetector

A high‐performance 2D photodetector based on a bilayer structure comprising a WSe₂ monolayer and CH₃NH₃PbI₃ organolead halide perovskite is reported. High performance is realized by modification of the WSe₂ monolayer with laser healing and perovskite functionalization. After modification, the output of the device was three orders of magnitude better than the pristine device; the performance is superior to that of most of the 2D photodetectors based on transition‐metal‐dichalcogenides (TMDs). This result indicates that combinatory TMDs–halide perovskite hybrids can be promising building blocks in optoelectronics.     

Perovskite-sensitized solar cell utilizing TiO2 nanograss: Effect of dipping time of CH3NH3PbI3 perovskite

Organic perovskite has potential to replace organic dye as light absorber in solar cell since it possesses better optical absorption in visible region than organic dye. This work is concerned with the investigation of the influence of CH₃NH₃PbI₃ perovskite dipping time on the performance of perovskite-sensitized solar cell (PSSC). The effect of the dipping time on the morphology and photoluminescence of the sample has also been investigated. It was found that the device achieved a power conversion efficiency (PCE) as high as 5.57 ± 0.4% at the optimum dipping time of 4 h. The highest PCE is due to the highest IPCE, lowest R_ct and the longest carrier lifetime (τ).     

Influence of Orientational Disorder on the Optical Absorption Properties of the Hybrid Metal-Halide Perovskite CH3NH3PbI3

An experimental and theoretical investigation is reported to analyze the relation between the structural and absorption properties of CH₃NH₃PbI₃ in the tetragonal phase. More than 3000 geometry optimizations were performed to reveal the structural disorder and identify structures with the lowest energies. The electronic structure calculations provide an averaged band gap of 1.674 eV, which is in excellent agreement with the experimental value of about 1.6 eV. The simulations of the absorption spectrum for three representative structures with lowest energy reproduced the absorption shoulders observed in the experimental spectra. These shoulders are assigned to excitations having similar orbital characters and involving transitions between hybridized 6s(Pb)/5p(I) orbitals and 6p(Pb) orbitals. The geometries of the three structures were analyzed and the effects of the inorganic frame and the CH₃NH₃⁺ cations on the absorption properties were estimated. It was found that both changes in the inorganic frame and the CH₃NH₃⁺ cations orientations impact the absorption spectra, by modifying the transitions energies and intensities. This highlights the role of CH₃NH₃⁺ cation in influencing the absorption properties of CH₃NH₃PbI₃ and demonstrates that CH₃NH₃⁺ cation is one of the key elements explaining the broad and nearly constant absorption spectrum in the visible range.     

In situ Investigations of Interfacial Degradation and Ion Migration at CH3NH3PbI3 Perovskite/Ag Interfaces

Interfacial properties between perovskite layers and metal electrodes play a crucial role in the device performance and the long-term stability of perovskite solar cells. Here, we report a comprehensive study of the interfacial degradation and ion migration at the interface between CH₃NH₃PbI₃ perovskite layer and Ag electrode. Using in situ photoemission spectroscopy measurements, we found that the Ag electrode could induce the degradation of perovskite layers, leading to the formation of PbI₂ and AgI species and the reduction of Pb²⁺ ions to metallic Pb species at the interface. The unconventional enhancement of the intensities of I 3d spectra provides direct experimental evidences for the migration of iodide ions from CH₃NH₃PbI₃ subsurface to Ag electrode. Moreover, the contact of Ag electrode and perovskite layers induces an interfacial dipole of 0.3 eV at CH₃NH₃PbI₃/Ag interfaces, which may further facilitate iodide ion di usion, resulting in the decomposition of perovskite layers and the corrosion of Ag electrode.     

In CH3NH3PbI3 Perovskite Film,the Surface Termination Layer Dominates the Moisture Degradation Pathway

Theoretical studies have shown that surface terminations, such as MAI or PbI layers, greatly affect the environmental stability of organic–inorganic perovskite. However, until now, there has been little effort to experimentally detect the existence of MAI or PbI terminations on MAPbI₃ grains, let alone disclose their effects on the humidity degradation pathway of perovskite solar cell. Here, we successfully modified and detected the surface terminations of MAI and PbI species on polycrystalline MAPbI₃ films. MAI-terminated perovskite film followed the moisture degradation process from MAPbI₃ to hydrate MAPbI₃⋅H₂O and then into PbI₂, with penetration of water molecules being the main driving force leading to the degradation of MAPbI₃ layer by layer. In contrast, for the PbI-terminated perovskite film in a humid atmosphere, a deprotonation degradation pathway was confirmed, in which the film preferentially degraded directly from MAPbI₃ into PbI₂, here the iodine defects played a key role in promoting the dissociation of water molecules into OH⁻ and further catalyzing the decomposition of perovskite.