A Three‐Step Method for the Deposition of Large Cuboids of Organic–Inorganic Perovskite and Application in Solar Cells

A three‐step method for the deposition of CH₃NH₃PbI₃ perovskite films with a high crystalline structure and large cuboid overlayer morphology is reported. The method includes PbI₂ deposition, which is followed by dipping into a solution of C₄H₉NH₃I (BAI) and (BA)₂PbI₄ perovskite formation. In the final step, the poorly thermodynamically stable (BA)₂PbI₄ phase converts into the more stable CH₃NH₃PbI₃ perovskite by dipping into a solution of CH₃NH₃I. The final product is characterized by XRD, SEM, UV/Vis, and photoluminescence analysis methods. The experimental results indicate that the prepared perovskite has cuboids with high crystallinity and large sizes (up to 1 μm), as confirmed by XRD and SEM data. Photovoltaic investigations show that the three‐step method results in higher solar cell efficiency (15 % enhancement in efficiency) with a better reproducibility than the conventional two‐step deposition method.     

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.     

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.     

A Layered Hybrid Perovskite Solar‐Cell Absorber with Enhanced Moisture Stability

Two‐dimensional hybrid perovskites are used as absorbers in solar cells. Our first‐generation devices containing (PEA)₂(MA)₂[Pb₃I₁₀] ( 1 ; PEA=C₆H₅(CH₂)₂NH₃⁺, MA=CH₃NH₃⁺) show an open‐circuit voltage of 1.18 V and a power conversion efficiency of 4.73 %. The layered structure allows for high‐quality films to be deposited through spin coating and high‐temperature annealing is not required for device fabrication. The 3D perovskite (MA)[PbI₃] ( 2 ) has recently been identified as a promising absorber for solar cells. However, its instability to moisture requires anhydrous processing and operating conditions. Films of 1 are more moisture resistant than films of 2 and devices containing 1 can be fabricated under ambient humidity levels. The larger bandgap of the 2D structure is also suitable as the higher bandgap absorber in a dual‐absorber tandem device. Compared to 2 , the layered perovskite structure may offer greater tunability at the molecular level for material optimization.     

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.     

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.     

A Purified,Solvent‐Intercalated Precursor Complex for Wide‐Process‐Window Fabrication of Efficient Perovskite Solar Cells and Modules

A high‐purity methylammonium lead iodide complex with intercalated dimethylformamide (DMF) molecules, CH₃NH₃PbI₃?DMF, is introduced as an effective precursor material for fabricating high‐quality solution‐processed perovskite layers. Spin‐coated films of the solvent‐intercalated complex dissolved in pure dimethyl sulfoxide (DMSO) yielded thick, dense perovskite layers after thermal annealing. The low volatility of the pure DMSO solvent extended the allowable time for low‐speed spin programs and considerably relaxed the precision needed for the antisolvent addition step. An optimized, reliable fabrication method was devised to take advantage of this extended process window and resulted in highly consistent performance of perovskite solar cell devices, with up to 19.8 % power‐conversion efficiency (PCE). The optimized method was also used to fabricate a 22.0 cm², eight‐cell module with 14.2 % PCE (active area) and 8.64 V output (1.08 V/cell).     

A Purified,Solvent‐Intercalated Precursor Complex for Wide‐Process‐Window Fabrication of Efficient Perovskite Solar Cells and Modules

A high‐purity methylammonium lead iodide complex with intercalated dimethylformamide (DMF) molecules, CH₃NH₃PbI₃?DMF, is introduced as an effective precursor material for fabricating high‐quality solution‐processed perovskite layers. Spin‐coated films of the solvent‐intercalated complex dissolved in pure dimethyl sulfoxide (DMSO) yielded thick, dense perovskite layers after thermal annealing. The low volatility of the pure DMSO solvent extended the allowable time for low‐speed spin programs and considerably relaxed the precision needed for the antisolvent addition step. An optimized, reliable fabrication method was devised to take advantage of this extended process window and resulted in highly consistent performance of perovskite solar cell devices, with up to 19.8 % power‐conversion efficiency (PCE). The optimized method was also used to fabricate a 22.0 cm², eight‐cell module with 14.2 % PCE (active area) and 8.64 V output (1.08 V/cell).     

Bulk heterojunction perovskite solar cells incorporated with solution-processed TiOx nanoparticles as the electron acceptors

In the past ten years, perovskite solar cells were rapidly developed, but the intrinsic unbalanced charge carrier diffusion lengths within perovskite materials were not fully addressed by either a planar heterojunction or meso-superstructured perovskite solar cells. In this study, we report bulk heterojunction perovskite solar cells, where perovskite materials CH₃NH₃PbI₃ is blended with solution-processed n-type TiO_x nanoparticles as the photoactive layer. Studies indicate that one-step solution-processed CH₃NH₃PbI₃:TiO_x bulk-heterojunction thin film possesses enhanced and balanced charge carrier mobilities, superior film morphology with enlarged crystal sizes, and suppressed trap-induced charge recombination. Thus, bulk heterojunction perovskite solar cells by CH₃NH₃PbI₃ mixed with 5 wt% of TiO_x, which is processed by one-step method rather than typical two-step method, show a short-circuit current density of 20.93 mA/cm², an open-circuit voltage of 0.90 V, a fill factor of 80% and with a corresponding power conversion efficiency of 14.91%, which is more than 30% enhancement as compared with that of perovskite solar cells with a planar heterojunction device structure. Moreover, bulk heterojunction perovskite solar cells possess enhanced device stability. All these results demonstrate that perovskite solar cells with a bulk heterojunction device structure are one of apparent approaches to boost device performance.     

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.     

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.     

Correlation between Interfacial Structures and Device Performance: The Double-Edged Sword Effect of Lead Iodide in Perovskite Solar Cells

The interfacial electronic structure of perovskite layers and transport layers is critical for the performance and stability of perovskite solar cells (PSCs). The device performance of PSCs can generally be improved by adding a slight excess of lead iodide (PbI₂) to the precursor solution. However, its underlying working mechanism is controversial. Here, we performed a comprehensive study of the electronic structures at the interface between CH₃NH₃PbI₃ and C₆₀ with and without the modification of PbI₂ using in situ photoemission spectroscopy measurements. The correlation between the interfacial structures and the device performance was explored based on performance and stability tests. We found that there is an interfacial dipole reversal, and the downward band bending is larger at the CH₃NH₃PbI₃/C₆₀ interface with the modification of PbI₂ as compared to that without PbI₂. Therefore, PSCs with PbI₂ modification exhibit faster charge carrier transport and slower carrier recombination. Nevertheless, the modification of PbI₂ undermines the device stability due to aggravated iodide migration. Our findings provide a fundamental understanding of the CH₃NH₃PbI₃/C₆₀ interfacial structure from the perspective of the atomic layer and insight into the double-edged sword effect of PbI₂ as an additive.     

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.     

The Effect of Humidity upon the Crystallization Process of Two‐Step Spin‐Coated Organic–Inorganic Perovskites

Moisture is shown to activate the reaction between PbI₂ and methylammonium halides. In addition, two activating mechanisms are proposed for the formation of CH₃NH₃PbI₃ and CH₃NH₃PbI_3?xCl_x films from a series of carefully controlled experiments. When these rapidly formed perovskite films are directly fabricated into the devices, poor photovoltaic properties are found, due to heavy surface charge recombination. However, the cell performance can be significantly enhanced to 13.63 % and to over 12 % in the steady state for CH₃NH₃PbI₃ and to 15.50 % and over 14 % in the steady state for CH₃NH₃PbI_3?xCl_x, if the rapidly formed perovskite film is annealed. Thus, it is believed that moisture (below 60 % RH) is not a problem for the fabrication of highly efficient perovskite solar cells.     

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.     

A Lead Iodide Perovskite Based on a Large Organic Cation for Solar Cell Applications

Methylammonium (CH₃NH₃⁺) and formamidinium ((NH₂)₂CH⁺) based lead iodide perovskites are currently the two commonly used organic–inorganic lead iodide perovskites. There are still no alternative organic cations that can produce perovskites with band gaps spanning the visible spectrum (that is, <1.7 eV) for solar cell applications. Now, a new perovskite using large propane‐1,3‐diammonium cation (1,3‐Pr(NH₃)₂²⁺) with a chemical structure of (1,3‐Pr(NH₃)₂)_0.5PbI₃ is demonstrated. X‐ray diffraction (XRD) shows that the new perovskite exhibits a three‐dimensional tetragonal phase. The band gap of the new perovskite is about 1.6 eV, which is desirable for photovoltaic applications. A (1,3‐Pr(NH₃)₂)_0.5PbI₃ perovskite solar cell (PSC) yields a power conversion efficiency (PCE) of 5.1 %. More importantly, this perovskite is composed of a large hydrophobic cation that provides better moisture resistance compared to CH₃NH₃PbI₃ perovskite.     

The Role of Oxygen in the Degradation of Methylammonium Lead Trihalide Perovskite Photoactive Layers

In this paper we report on the influence of light and oxygen on the stability of CH₃NH₃PbI₃ perovskite‐based photoactive layers. When exposed to both light and dry air the mp‐Al₂O₃/CH₃NH₃PbI₃ photoactive layers rapidly decompose yielding methylamine, PbI₂, and I₂ as products. We show that this degradation is initiated by the reaction of superoxide (O₂^?) with the methylammonium moiety of the perovskite absorber. Fluorescent molecular probe studies indicate that the O₂^? species is generated by the reaction of photoexcited electrons in the perovskite and molecular oxygen. We show that the yield of O₂^? generation is significantly reduced when the mp‐Al₂O₃ film is replaced with an mp‐TiO₂ electron extraction and transport layer. The present findings suggest that replacing the methylammonium component in CH₃NH₃PbI₃ to a species without acid protons could improve tolerance to oxygen and enhance stability.     

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.     

Synthesis of hybrid organic-inorganic perovskite platelets by vacuum impregnation

In perovskite solar cells and optoelectronics, perovskite film morphology controls the performance of the device. Various methods have been developed to control the morphology and coverage of the perovskite films. In this article platelet type perovskite morphlogy was synthesized using low temperature vacuum impregnation of the perovskite solution CH₃NH₃PbI₃ resulting in complete coverage on TiO₂ film. Vacuum impregnation synthesis of perovskites has the advantage of low cost and low temperature which faciliates application in flexible electronics and solar cells.