Highly Efficient,Reproducible, Uniform (CH3NH3)PbI3 Layer by Processing Additive Dripping for Solution‐Processed Planar Heterojunction Perovskite Solar Cells

A processing additive dripping (PAD) approach to forming highly efficient (CH₃NH₃)PbI₃ (MAPbI₃) perovskite layers was investigated. A MAPbI₃(CB/DIO) perovskite film fabricated by this approach, which included briefly dripping chlorobenzene incorporating a small amount of diiodooctane (DIO) during casting of a MAPbI₃ perovskite precursor dissolved in dimethylformamide, exhibited superior smooth, uniform morphologies with high crystallinity and large grains and revealed completely homogeneous surface coverage. The surface coverage and morphology of the substrate significantly affected the photovoltaic performance of planar heterojunction (PHJ) perovskite solar cells (PrSCs), resulting in a power conversion efficiency of 11.45 % with high open‐circuit voltage of 0.91 V and the highest fill factor of 80.87 %. Moreover, the PAD approach could effectively provide efficient MAPbI₃(CB/DIO) perovskite layers for highly efficient, reproducible, uniform PHJ PrSC devices without performance loss or variation even over larger active areas.     

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.     

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.     

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.     

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.     

Preparation and Characterization of Thin-Film Solar Cells with Ag/C60/MAPbI3/CZTSe/Mo/FTO Multilayered Structures

New solar cells with Ag/C₆₀/MAPbI₃/Cu₂ZnSnSe₄ (CZTSe)/Mo/FTO multilayered structures on glass substrates have been prepared and investigated in this study. The electron-transport layer, active photovoltaic layer, and hole-transport layer were made of C₆₀, CH₃NH₃PbI₃ (MAPbI₃) perovskite, and CZTSe, respectively. The CZTSe hole-transport layers were deposited by magnetic sputtering, with the various thermal annealing temperatures at 300 °C, 400 °C, and 500 °C, and the film thickness was also varied at 50~300 nm The active photovoltaic MAPbI₃ films were prepared using a two-step spin-coating method on the CZTSe hole-transport layers. It has been revealed that the crystalline structure and domain size of the MAPbI₃ perovskite films could be substantially improved. Finally, n-type C₆₀ was vacuum-evaporated to be the electronic transport layer. The 50 nm C₆₀ thin film, in conjunction with 100 nm Ag electrode layer, provided adequate electron current transport in the multilayered structures. The solar cell current density–voltage characteristics were evaluated and compared with the thin-film microstructures. The photo-electronic power-conversion efficiency could be improved to 14.2% when the annealing temperature was 500 °C and the film thickness was 200 nm. The thin-film solar cell characteristics of open-circuit voltage, short-circuit current density, fill factor, series-resistance, and Pmax were found to be 1.07 V, 19.69 mA/cm², 67.39%, 18.5 Ω and 1.42 mW, respectively.     

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.     

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.     

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.     

A Modified Sequential Deposition Route for High-Performance Carbon-Based Perovskite Solar Cells under Atmosphere Condition

Carbon-based hole transport material (HTM)-free perovskite solar cells have exhibited a promising commercialization prospect, attributed to their outstanding stability and low manufacturing cost. However, the serious charge recombination at the interface of the carbon counter electrode and titanium dioxide (TiO₂) suppresses the improvement in the carbon-based perovskite solar cells’ performance. Here, we propose a modified sequential deposition process in air, which introduces a mixed solvent to improve the morphology of lead iodide (PbI₂) film. Combined with ethanol treatment, the preferred crystallization orientation of the PbI₂ film is generated. This new deposition strategy can prepare a thick and compact methylammonium lead halide (MAPbI₃) film under high-humidity conditions, which acts as a natural active layer that separates the carbon counter electrode and TiO₂. Meanwhile, the modified sequential deposition method provides a simple way to facilitate the conversion of the ultrathick PbI₂ capping layer to MAPbI₃, as the light absorption layer. By adjusting the thickness of the MAPbI₃ capping layer, we achieved a power conversation efficiency (PCE) of 12.5% for the carbon-based 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.     

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.     

Effect of Growth Temperature on the Characteristics of CsPbI3-Quantum Dots Doped Perovskite Film

In this study, adding CsPbI₃ quantum dots to organic perovskite methylamine lead triiodide (CH₃NH₃PbI₃) to form a doped perovskite film filmed by different temperatures was found to effectively reduce the formation of unsaturated metal Pb. Doping a small amount of CsPbI₃ quantum dots could enhance thermal stability and improve surface defects. The electron mobility of the doped film was 2.5 times higher than the pristine film. This was a major breakthrough for inorganic quantum dot doped organic perovskite thin films.     

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.     

A Rutile TiO2 Electron Transport Layer for the Enhancement of Charge Collection for Efficient Perovskite Solar Cells

Interfacial charge collection efficiency has demonstrated significant effects on the power conversion efficiency (PCE) of perovskite solar cells (PSCs). Herein, crystalline phase‐dependent charge collection is investigated by using rutile and anatase TiO₂ electron transport layer (ETL) to fabricate PSCs. The results show that rutile TiO₂ ETL enhances the extraction and transportation of electrons to FTO and reduces the recombination, thanks to its better conductivity and improved interface with the CH₃NH₃PbI₃ (MAPbI₃) layer. Moreover, this may be also attributed to the fact that rutile TiO₂ has better match with perovskite grains, and less trap density. As a result, comparing with anatase TiO₂ ETL, MAPbI₃ PSCs with rutile TiO₂ ETL delivers significantly enhanced performance with a champion PCE of 20.9 % and a large open circuit voltage (V_OC) of 1.17 V.     

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.     

First-Principles Study of Aziridinium Lead Iodide Perovskite for Photovoltaics

The long-term stability remains one of the main challenges for the commercialization of the rapidly developing hybrid organic-inorganic perovskite solar cells. Herein, we investigate the electronic and optical properties of the recently reported hybrid halide perovskite (CH₂)₂NH₂PbI₃ (AZPbI₃), which exhibits a much better stability than the popular halide perovskites CH₃NH₃PbI₃ and HC(NH₂)₂PbI₃, by using density functional theory (DFT). We find that AZPbI₃ possesses a band gap of 1.31 eV, ideal for single-junction solar cells, and its optical absorption is comparable with those of the popular CH₃NH₃PbI₃ and HC(NH₂)₂PbI₃ materials in the whole visible-light region. In addition, the conductivity of AZPbI₃ can be tuned from efficient p-type to n-type, depending on the growth conditions. Besides, the charge-carrier mobilities and lifetimes are unlikely hampered by deep transition energy levels, which have higher formation energies in AZPbI₃ according to our calculations. Overall, we suggest that the perovskite AZPbI₃ is an excellent candidate as a stable high-performance photovoltaic absorber material.     

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.     

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.     

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.