Nonstoichiometric acid-base reaction as reliable synthetic route to highly stable CH3NH3PbI3 perovskite film

Light-Induced Phase Segregation Evolution of All-Inorganic Mixed Halide Perovskites

Light-induced phase segregation in mixed halide perovskites is a major roadblock for commercialization of optoelectronics utilizing these materials. We investigate the phenomenon in a model material system consisting of only surfaces and the bulk of a single-crystalline-like microplate. We utilize environmental in-situ time-dependent photoluminescence spectroscopy to observe the bandgap evolution of phase segregation under illumination. This enables analysis of the evolution of the iodide-rich phase composition as a function of the environment (i.e., surface defects) and carrier concentration. Our study provides microscopic insights into the relationship among photocarrier generations, surface structural defects, and subsequently iodide ion migrations that result in the complex evolution of phase segregation. We elucidate the significance of surface defects with respect to the evolution of phase segregation, which may provide new perspectives for modulating ion migration by engineering of defects and carrier concentrations.

Ammonia for post-healing of formamidinium-based Perovskite films

Solvents employed for perovskite film fabrication not only play important roles in dissolving the precursors but also participate in crystallization process. High boiling point aprotic solvents with O-donor ligands have been extensively studied, but the formation of a highly uniform halide perovskite film still requires the participation of additives or an additional step to accelerate the nucleation rate. The volatile aliphatic methylamine with both coordinating ligands and hydrogen protons as solvent or post-healing gas facilitates the process of methylamine-based perovskite films with high crystallinity, few defects, and easy large-scale fabrication as well. However, the attempt in formamidinium-containing perovskites is challenged heretofore. Here, we reveal that the degradation of formamidinium-containing perovskites in aliphatic amines environment results from the transimination reaction of formamidinium cation and aliphatic amines along with the formation of ammonia. Based on this mechanism, ammonia is selected as a post-healing gas for a highly uniform, compact formamidinium-based perovskite films. In particular, low temperature is proved to be crucial to enable formamidinium-based perovskite materials to absorb enough ammonia molecules and form a liquid intermediate state which is the key to eliminating voids in raw films. As a result, the champion perovskite solar cell based on ammonia post-healing achieves a power conversion efficiency of 23.21% with excellent reproducibility. Especially the module power conversion efficiency with 14 cm² active area is over 20%. This ammonia post-healing treatment potentially makes it easier to upscale fabrication of highly efficient formamidinium-based devices.

Solvents used for perovskite film fabrication not only dissolve the precursors but also play a role in the crystallization process. Here, authors study the role of transamination reactions in the underlying degradation mechanism of formamidinium-containing perovskites in aliphatic amines environment.

Effect of Stability of Two-Dimensional (2D) Aminoethyl Methacrylate Perovskite Using Lead-Based Materials for Ammonia Gas Sensor Application

Changes in physical properties of (H2C=C(CH3)CO2CH2CH2NH3)2PbI2Cl2 and (H2C=C(CH3)CO2CH2CH2NH3)2Pb(NO3)2Cl2 (2D) perovskite materials from iodide-based (I-AMP) and nitrate-based (N-AMP) leads were investigated at different durations (days) for various storage conditions. UV-Vis spectra of both samples showed an absorption band of around λmax 420 nm due to the transition of n to π* of ethylene (C=C) and amine (NH2). XRD perovskite peaks could be observed at approximately 25.35° (I-AMP) and 23.1° (N-AMP). However, a major shift in I-AMP and dramatic changes in the crystallite size, FHWM and crystallinity percentage highlighted the instability of the iodide-based material. In contrast, N-AMP showed superior stability with 96.76% crystallinity even at D20 under the S condition. Both materials were exposed to ammonia (NH3) gas, and a new XRD peak of ammonium lead iodide (NH4PbI3) with a red-shifted perovskite peak (101) was observed for the case of I-AMP. Based on the FWHM, crystallite size, crystallinity and lattice strain analysis, it can be concluded N-AMP's stability was maintained even after a few days of exposure to the said gases. These novel nitrate-based lead perovskite materials exhibited great potential for stable perovskite 2D materials and recorded less toxicity compared to famous lead iodide (PbI2) material.

Wearable Tin-Based Perovskite Solar Cells Achieved by a Crystallographic Size Effect

Tin-based perovskite solar cells (PSCs) demonstrate a potential application in wearable electronics due to its hypotoxicity. However, poor crystal quality is still the bottleneck for achieving high-performance flexible devices. In this work, graphite phase-C₃ N₄ (g-C₃ N₄ ) is applied into tin-based perovskite as a crystalline template, which delays crystallization via a size-effect and passivates defects simultaneously. The double hydrogen bond between g-C₃ N₄ and formamidine cation can optimize lattice matching and passivation. Moreover, the two-dimensional network structure of g-C₃ N₄ can fit on the crystals, resulting an enhanced hydrophobicity and oxidation resistance. Therefore, the flexible tin-based PSCs with g-C₃ N₄ realize a stabilized power conversion efficiency (PCE) of 8.56 % with negligible hysteresis. In addition, the PSCs can maintain 91 % of the initial PCE after 1000 h under N₂ environment and keep 92 % of their original PCE after 600 cycles at a curvature radius of 3 mm.

Design of Chemoresponsive Soft Matter Using Hydrogen-Bonded Liquid Crystals

Soft matter that undergoes programmed macroscopic responses to molecular analytes has potential utility in a range of health and safety-related contexts. In this study, we report the design of a nematic liquid crystal (LC) composition that forms through dimerization of carboxylic acids and responds to the presence of vapors of organoamines by undergoing a visually distinct phase transition to an isotropic phase. Specifically, we screened mixtures of two carboxylic acids, 4-butylbenzoic acid and trans-4-pentylcyclohexanecarboxylic acid, and found select compositions that exhibited a nematic phase from 30.6 to 111.7 °C during heating and 110.6 to 3.1 °C during cooling. The metastable nematic phase formed at ambient temperatures was found to be long-lived (>5 days), thus enabling the use of the LC as a chemoresponsive optical material. By comparing experimental infrared (IR) spectra of the LC phase with vibrational frequencies calculated using density functional theory (DFT), we show that it is possible to distinguish between the presence of monomers, homodimers and heterodimers in the mixture, leading us to conclude that a one-to-one heterodimer is the dominant species within this LC composition. Further support for this conclusion is obtained by using differential scanning calorimetry. Exposure of the LC to 12 ppm triethylamine (TEA) triggers a phase transition to an isotropic phase, which we show by IR spectroscopy to be driven by an acid-base reaction, leading to the formation of ammonium carboxylate salts. We characterized the dynamics of the phase transition and found that it proceeds via a characteristic spatiotemporal pathway involving the nucleation, growth, and coalescence of isotropic domains, thus amplifying the atomic-scale acid-base reaction into an information-rich optical output. In contrast to TEA, we determined via both experiment and computation that neither hydrogen bonding donor or acceptor molecules, such as water, dimethyl methylphosphonate, ethylene oxide or formaldehyde, disrupt the heterodimers formed in the LC, hinting that the phase transition (including spatial-temporal characteristics of the pathway) induced in this class of hydrogen bonded LC may offer the basis of a facile and chemically selective way of reporting the presence of volatile amines. This proposal is supported by exploratory experiments in which we show that it is possible to trigger a phase transition in the LC by exposure to volatile amines emitted from rotting fish. Overall, these results provide new principles for the design of chemoresponsive soft matter based on hydrogen bonded LCs that may find use as the basis of low-cost visual indicators of chemical environments.

Perovskite Granular Wire Photodetectors with Ultrahigh Photodetectivity

Control over the morphology and crystallinity of metal halide perovskite materials is of key importance to enable high-performance optoelectronics. Here, a simple yet effective template-free self-assembly synthesis of perovskite granular wires with ultrahigh photodetectivity (3.17 × 10¹⁵ Jones) is reported. The 1D self-assembly of perovskite grains is driven by differences in the surface interaction energies of the granular facets. The superb photodetecting performance originates from extremely low dark current engendered by energetic barriers featuring unique band-edge modulation along the long axis of wire. Flexible photodetector arrays, fabricated by selectively placing perovskite granular wires onto pre-patterned electrode arrays on a transparent polymer substrate, show independently addressable photonic signal mapping with remarkably high detectivity, photoconductive gain, and responsivity. The "self-assembled nanograin engineering" strategy developed in this study provides a viable method for the development of high-performance perovskite photodetectors and can be extended to other integrated optoelectronic systems.

High-Quality Concentrated Precursor Solution in N,N-Dimethylformamide for Thick Methylammonium Triiodoplumbate Layer in Solar Cells

A high-quality precursor solution is essential for the fabrication of hybrid perovskite solar cells. This article reports a simple and efficient method for preparing a high-quality concentrated solution of methylammonium triiodoplumbate (MAPbI₃) in N,N-dimethylformamide (DMF) by using MAPbI₃ crystals instead of conventional lead iodine and methylammonium iodine blend. The MAPbI₃ concentration of the precursor solution is easily and accurately adjusted from 0 up to 1.64 M. An investigation of the dissolution process of the MAPbI₃ crystals reveals that the concentrated solution of MAPbI₃ in DMF is metastable, and the transition from the concentrated solution to solvated intermediate MAPbI₃·DMF determines the solubility of MAPbI₃ in DMF. The high purity and precise stoichiometric ratio of the crystals eliminate the possible impurities that initialize the transition to MAPbI₃·DMF and consequently suppress the transition and increase the stability of the concentrated solution. MAPbI₃ films with different thicknesses up to 800 nm are prepared with the conventional film fabrication technique, and the highest power conversion efficiency of 20.7% is achieved on corresponding solar cells. This newly developed method for preparing a concentrated precursor solution can be easily combined with other fabrication techniques for further development of industrial-scale manufacture of solar cells.

Unveiling the Effects of Hydrolysis-Derived DMAI/DMAPbI x Intermediate Compound on the Performance of CsPbI3 Solar Cells

Introducing hydroiodic acid (HI) as a hydrolysis-derived precursor of the intermediate compounds has become an increasingly important issue for fabricating high quality and stable CsPbI3 perovskite solar cells (PSCs). However, the materials composition of the intermediate compounds and their effects on the device performance remain unclear. Here, a series of high-quality intermediate compounds are prepared and it is shown that they consist of DMAI/DMAPbI x . Further characterization of the products show that the main component of this system is still CsPbI3. Most of the dimethylammonium (DMA+) organic component is lost during annealing. Only an ultrasmall amount of DMA+ is doped into the CsPbI3 and its structure is stabilized. Meanwhile, excessive DMA+ forms Lewis acid-base adducts and interactions with Pb2+ on the CsPbI3 surface. This process passivates the CsPbI3 film and decreases the recombination rate. Finally, CsPbI3 film is fabricated with high crystalline, uniform morphology, and excellent stability. Its corresponding PSC exhibits stable property and improved power conversion efficiency (PCE) up to 17.3%.

New Acidic Precursor and Acetone-Based Solvent for Fast Perovskite Processing via Proton-Exchange Reaction with Methylamine

A new solvent system for PbI₂ based on HI solution in acetone with a low boiling point is proposed. High solubility of PbI₂ is caused by the formation of iodoplumbate complexes, and reaches a concentration of 1.6 M. Upon its crystallization metastable solvate phases PbI₂∙HI∙n{(CH₃)₂CO} are formed. The latter allows for their easy deposition on substrates in a form of smooth and uniform thin films by spin-coating. Through a fast acid-base reaction with a gaseous amine, the films of the intermediate phase can be completely converted to single-phase perovskite films. The developed method allows one to form smooth perovskite films with high crystallinity with a thickness up to 1 μm. Due to easy and fast processing, the developed method can be promising for perovskite technology upscaling.

Strategies to Improve Luminescence Efficiency of Metal-Halide Perovskites and Light-Emitting Diodes

Metal-halide perovskites (MHPs) are well suited to be vivid natural color emitters due to their superior optical and electrical properties, such as narrow emission linewidths, easily and widely tunable emission wavelengths, low material cost, and high charge carrier mobility. Since the first development of MHP light-emitting diodes (PeLEDs) in 2014, many researchers have tried to understand the properties of MHP emitters and the limitations to luminescence efficiency (LE) of PeLEDs, and have devoted efforts to increase the LE of MHP emitters and PeLEDs. Within three and half years, PeLEDs have shown rapidly increased LE from external quantum efficiency ≈0.1% to ≈14.36%. Herein, the factors that limit the LE of PeLEDs are reviewed; the factors are characterized into the following groups: i) photophysical properties of MHP crystals, ii) morphological factors of MHP layers, and iii) problems caused by device architectures. Then, the strategies to overcome those luminescence-limiting factors in MHP emitters and PeLEDs are critically evaluated. Finally, research directions to further increase the LE of MHP emitters and the potential of MHPs as a core component in next-generation displays and solid-state lightings are suggested.

Ambipolar Transport in Methylammonium Lead Iodide Thin Film Transistors

We report clear room temperature ambipolar transport in ambient-air processed methylammonium lead iodide (MAPbI3) thin-film transistors (TFTs) with aluminum oxide gate-insulators and indium-zinc-oxide source/drain electrodes. The high ionicity of the MAPbI3 leads to p-type and n-type self-doping, and depending on the applied bias we show that simultaneous or selective transport of electrons and/or holes is possible in a single MAPbI3 TFT. The electron transport (n-type), however, is slightly more pronounced than the hole transport (p-type), and the respective channel resistances range from 5–11 and 44–55 MΩ/μm. Both p-type and n-type TFTs show good on-state characteristics for low driving voltages. It is also shown here that the on-state current of the n-type and p-type TFTs is highest in the slightly PbI2-rich and MAI-rich films, respectively, suggesting controllable n-type or p-type transport by varying precursor ratio.

Hydrophobic polythiophene hole-transport layers to address the moisture-induced decomposition problem of perovskite solar cells

Perovskite solar cells have emerged as one of the most promising next-generation photovoltaic technologies and have achieved a record power conversion efficiency of 22.7%. The technology meets industrial demands for cost effectiveness and scalability; however, the instability of lead halide perovskites toward moisture is a major barrier to their commercial development. Previous studies have revealed that the use of hydrophobic hole-transport layers (e.g., poly(3-hexylthiophene), P3HT) can slow the ingress of water vapor and improve the lifetime of the underlying perovskite, suggesting a route to longer lived devices. In this work, we report the synthesis of a variety of poly(3-alkoxythiophenes) with different side chains. The side chains range from hydrophilic (triethylene glycol methyl ether) to extremely hydrophobic (highly fluorinated hexyloxy). We evaluated the polymers, alongside commercially available P3HT, for their ability to stabilize methylammonium lead iodide films at high relative humidities. The fluorinated polythiophenes were able to substantially improve the perovskite lifetime, suggesting that more hydrophobic hole-transport layers may be a route to more stable perovskite solar cells.

Unveiling Property of Hydrolysis-Derived DMAPbI3 for Perovskite Devices: Composition Engineering, Defect Mitigation, and Stability Optimization

Additive engineering has become increasingly important for making high-quality perovskite solar cells (PSCs), with a recent example involving acid during fabrication of cesium-based perovskites. Lately, it has been suggested that this process would introduce dimethylammonium ((CH₃)₂NH₂⁺, DMA⁺) through hydrolysis of the organic solvent. However, material composition of the hydrolyzed product and its effect on the device performance remain to be understood. Here, we present an in-depth investigation of the hydrolysis-derived material (i.e., DMAPbI₃) and detailed analysis of its role in producing high-quality PSCs. By varying the ratio of CsI/DMAPbI₃ in the precursor, we achieve high-quality Cs_xDMA_1-xPbI₃ perovskite films with uniform morphology, low density of trap states, and good stability, leading to optimized power conversion efficiency up to 14.3%, with over 85% of the initial efficiency retained after ∼20 days in air without encapsulation. Our findings offer new insights into producing high-quality Cs-based perovskite materials.

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Dissolving PbI₂ and HI in DMF is confirmed not to produce the “mythical” HPbI₃

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Detailed composition analyses show that DMAPbI₃ is the hydrolysis product instead

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Performance of devices can be optimized by tuning the CsI:DMAPbI₃ ratio

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The Cs_xDMA_1-xPbI₃ films remain stable in air for more than 20 days

Degradation Mechanism and Relative Stability of Methylammonium Halide Based Perovskites Analyzed on the Basis of Acid-Base Theory

The correct identification of all gases released during hybrid perovskite degradation is of great significance to develop strategies to extend the lifespan of any device based on this semiconductor. CH₃X (X = Br/I) is a released degradation gas/low boiling point liquid arising from methylammonium (MA⁺) based perovskites, which has been largely overlooked in the literature focusing on stability of perovskite solar cells. Herein, we present an unambiguous identification of CH₃I release using microwave (rotational) spectroscopy. An experimental back-reaction test demonstrates that the well-known CH₃NH₂/HX degradation route may not be the ultimate degradation pathway of MAPbX₃ in thermodynamic closed systems. Meanwhile, the CH₃X/NH₃ route cannot back-react selectively to MAX formation as occurred for the former back-reaction. Metadynamics calculations uncover the X halide effect on energy barriers for both degradation reactions showing a better stability of Br based perovskite ascribed to two aspects: (i) lower Brönsted-Lowry acidity of HBr compared to HI and (ii) higher nucleophilic character of CH₃NH₂ compared to NH₃. The latter property makes CH₃NH₂ molecules stay preferentially attached on the electrophilic perovskite surface (Pb²⁺) during the dynamic simulation instead of being detached as observed for the NH₃ molecule.

Bulk Heterojunction Quasi-Two-Dimensional Perovskite Solar Cell with 1.18 V High Photovoltage

Multicomponent quasi-two-dimensional perovskites (Q-2DPs) have efficient luminescence and improved stability, which are highly desirable for light-emitting diode and perovskite solar cell (PSC). However, the lack of radiative recombination at room temperature is still not well understood and the performance of PSC is not good enough as well. The open-circuit voltage ( V_OC) is even lower than that of three-dimensional (3D) PSC with a narrower band gap. In this work, we study the energy transfer of excitons between their multiple components by time-resolved photoluminescence and find that charge transfer from high-energy states to low-energy state is gradually suppressed during elevating temperature owing to trap-mediated recombination. This may reveal the bottleneck of luminescence at room temperature in Q-2DPs, leading to large photovoltage loss in 2D PSC. Therefore, we develop a p-i-n bulk heterojunction (BHJ) structure to reduce the nonradiative recombination and obtain high V_OC of 1.18 V for (PMA)₂MA₄Pb₅I₁₅Cl (33.3% PMA) BHJ device, much higher than that of the planar counterparts. The enhanced efficiency is attributed to the improved exciton dissociation via BHJ interface. Our results provide an important step toward high V_OC and stable 2D PSCs, which could be used for tandem solar cell and colorful photovoltaic windows.

Direct synthesis of cubic phase CsPbI3 nanowires

One-dimensional all-inorganic halide perovskites have emerged as one of the most prominent materials in the application of optoelectronic devices due to their remarkable properties such as a low number of defects, morphological anisotropy, mechanical flexibility and fast charge transfer capability.

Controlled crystal facet of MAPbI3 perovskite for highly efficient and stable solar cell via nucleation modulation

The crystallization of MAPbI3 perovskite films was purposefully engineered to investigate the governing factors which determine their morphological properties and moisture stability. By modulating nucleation, we obtained a single layer perovskite film with controlled crystal facet orientation and grain size. The lack of perovskite nucleation sites during crystallization allowed us to tailor the resulting crystallization phase. Theoretical calculations indicated that the nucleation sites for perovskite growth are related to the electron density around the oxygen atom (C[double bond, length as m-dash]O and S[double bond, length as m-dash]O) in a Lewis base. A single layer of micrometer-sized and (110)-oriented perovskite crystals was achieved in the optimized MAPbI3 films via suppressing the formation of nucleation sites. We fabricated inverted perovskite solar cells with the structure of glass/ITO/PEDOT:PSS/MAPbI3/PC61BM/Al which exhibited a high power conversion efficiency of 17.5% and a high fill factor over 83%. In addition, a study of the moisture stability indicated that the (110) facet orientation of the perovskite grains plays a more important role in film degradation than grain size.

General Nondestructive Passivation by 4-Fluoroaniline for Perovskite Solar Cells with Improved Performance and Stability

Hybrid perovskite thin films are prone to producing surface vacancies during the film formation, which degrade the stability and photovoltaic performance. Passivation via post-treatment can heal these defects, but present methods are slightly destructive to the bulk of 3D perovskite due to the solvent effect, which hinders fabrication reproducibility. Herein, nondestructive surface/interface passivation using 4-fluoroaniline (FAL) is established. FAL is not only an effective antisolvent candidate for surface modification, but also a large dipole molecule (2.84 Debye) with directional field for charge separation. Density functional theory calculation reveals that the nondestructive properties are attributed to both the conjugated amine in aromatic ring and the para-fluoro-substituent. A hot vapor assisted colloidal process is employed for the post-treatment. The molecular passivation yields an ultrathin protection layer with a hydrophobic fluoro-substituent tail and thus enhances the stability and optoelectronic properties. FAL post-treated perovskite solar cell (PSC) delivers a 20.48% power conversion efficiency under ambient conditions. Micro-photoluminescence reveals that passivation activates the dark defective state at the surface and interface, delivering the impact picture of boundary on the local carriers. This work demonstrates a generic nondestructive chemical approach for improving the performance and stability of PSCs.

HPbI3 as a Bifunctional Additive for Morphology Control and Grain Boundary Passivation toward Efficient Planar Perovskite Solar Cells

One of the key aspects contributing to the rapid development of perovskite solar cells is to prepare high-quality perovskite films via morphology control and interface engineering. Here, we demonstrate that the additive HPbI₃ works effectively on both morphology control and grain boundary passivation of CH₃NH₃PbI_{3- x}Cl _x thin films. By inducing HPbI₃ to the crystal transformation process, high-quality perovskite films consisting of micro-sized grains with boundaries passivated by PbI₂ can be readily produced. The perovskite film obtained with HPbI₃ as the additive achieves a much longer carrier lifetime compared to the pristine perovskite film without the additive. Under the optimal HPbI₃ amount (5.0%), the average power conversion efficiency of the planar-heterojunction solar cells is increased by ∼24% to 17.42% from 14.09% for the device without the additive, and the champion efficiency reaches 18.59%. The devices without any encapsulation show impressive shelf stability, retaining more than 85% of the initial efficiency after being stored in ambient environment for over 40 days.

Lead Halide Perovskite Nanostructures for Dynamic Color Display

Nanoprint-based color display using either extrinsic structural colors or intrinsic emission colors is a rapidly emerging research field for high-density information storage. Nevertheless, advanced applications, e. g., dynamic full-color display and secure information encryption, call for demanding requirements on in situ color change, nonvacuum operation, prompt response, and favorable reusability. By transplanting the concept of electrical/chemical doping in the semiconductor industry, we demonstrate an in situ reversible color nanoprinting paradigm via photon doping, triggered by the interplay of structural colors and photon emission of lead halide perovskite gratings. It solves the aforementioned challenges at one go. By controlling the pumping light, the synergy between interlaced mechanisms enables color tuning over a large range with a transition time on the nanosecond scale in a nonvacuum environment. Our design presents a promising realization of in situ dynamic color nanoprinting and will empower the advances in structural color and classified nanoprinting.

Gas-solid reaction based over one-micrometer thick stable perovskite films for efficient solar cells and modules

Besides high efficiency, the stability and reproducibility of perovskite solar cells (PSCs) are also key for their commercialization. Herein, we report a simple perovskite formation method to fabricate perovskite films with thickness over 1 μm in ambient condition on the basis of the fast gas-solid reaction of chlorine-incorporated hydrogen lead triiodide and methylamine gas. The resultant thick and smooth chlorine-incorporated perovskite films exhibit full coverage, improved crystallinity, low surface roughness and low thickness variation. The resultant PSCs achieve an average power conversion efficiency of 19.1 ± 0.4% with good reproducibility. Meanwhile, this method enables an active area efficiency of 15.3% for 5 cm × 5 cm solar modules. The un-encapsulated PSCs exhibit an excellent T₈₀ lifetime exceeding 1600 h under continuous operation conditions in dry nitrogen environment.

Abnormal Synergetic Effect of Organic and Halide Ions on the Stability and Optoelectronic Properties of a Mixed Perovskite via In Situ Characterizations

The mixed cation lead mixed halide perovskite (MLMP) Cs_x FA_1-x PbI_y Br_3-y is one of the most promising candidates for both single-junction and tandem solar cells due to its high efficiency and remarkable stability. However, the composition effect on thermal stability and photovoltaic performances has not yet been comprehensively investigated. Therefore, the interplay between composition, crystal structure, morphology, and optoelectronic properties under heat stress, is systematically elucidated here through a series of in situ characterizations. It is revealed for the first time that the FA⁺ and Br^- release synchronously at first even under mild annealing. This leads to a serious FA- and Br-deficiency issue, with only 88.3% of Br and 90.2% of FA retained after annealing at 100 °C, which significantly magnifies the hysteresis, phase segregation, and instability issues. Finally, a trace amount of FA⁺ and Br^- is introduced onto the post-annealed MLMP surface to compensate for the deficiency through vacancy filling. The degradation lifetime to 80% of the initial efficiency (t₈₀ ) is improved from 504 to 1056 h and the hysteresis issue is also well resolved. This work highlights the importance of the synergetic composition effect of the organic cation and halide anion on stability and efficiency optimization for long-term applications.

Chelate-Pb Intermediate Engineering for High-Efficiency Perovskite Solar Cells

Crystallization quality and grain size are key factors in fabricating high-performance planar-type perovskite photovoltaics. Herein, we used 1,8-octanedithiol as an effective additive in the [HC(NH₂)₂]_0.95Cs_0.05PbI₃ (FA_0.95Cs_0.05PbI₃) solution to improve the FA_0.95Cs_0.05PbI₃ film quality via solution processing. 1,8-Octanedithiol would coordinate with lead to form the chelate-Pb compound, leading to smaller Gibbs free energy during the perovskite crystallization process, facilitating formation of high-quality perovskite films with larger grains, smoother surfaces, lower electron trap densities, and longer carrier lifetimes compared to the nonadditive ones. As a result, the champion efficiency for devices with 3% 1,8-octanedithiol-doped FA_0.95Cs_0.05PbI₃ is raised to 19.36% from 18.39% of a device without the additive. The new technique is a promising way to fabricate perovskite photovoltaics with high performance.

Methodologies toward Highly Efficient Perovskite Solar Cells

A perovskite solar cell (PSC) employing an organic-inorganic lead halide perovskite light harvester, seeded in 2009 with power conversion efficiency (PCE) of 3.8% and grown in 2011 with PCE of 6.5% in dye-sensitized solar cell structure, has received great attention since the breakthrough reports ≈10% efficient solid-state PCSs demonstrating 500 h stability. Developments of device layout and high-quality perovskite film eventually lead to a PCE over 22%. As of October 31, 2017, the highest PCE of 22.7% is listed in an efficiency chart provided by NREL. In this Review, the methodologies to obtain highly efficient PSCs are described in detail. In order to achieve a PCE of over 20% reproducibly, key technologies are disclosed from the viewpoint of precursor solution chemistry, processing for defect-free perovskite films, and passivation of grain boundaries. Understanding chemical species in precursor solution, crystal growth kinetics, light-matter interaction, and controlling defects is expected to give important insights into not only reproducible production of high PCE over 20% but also further enhancement of the PCE of PCSs.

Boosting Light Harvesting in Perovskite Solar Cells by Biomimetic Inverted Hemispherical Architectured Polymer Layer with High Haze Factor as an Antireflective Layer

Biomimetic microarchitectured polymer layers, such as inverted hemispherical architectured (IHSA)-polydimethylsiloxane (PDMS) and hemispherical architectured (HSA)-PDMS layers, were prepared by a simple and cost-effective soft-imprinting lithography method via a hexagonal close-packed polystyrene microsphere array/silicon mold. The IHSA-PDMS/glass possessed superior antireflection (AR) characteristics with the highest/lowest average transmittance/reflectance ( T_avg/ R_avg) values of approximately 89.2%/6.4% compared to the HSA-PDMS/glass, flat-PDMS/glass, and bare glass ( T_avg/ R_avg ∼88.8%/7.5%, 87.5%/7.9%, and 87.3%/8.8%, respectively). In addition, the IHSA-PDMS/glass also exhibited a relatively strong light-scattering property with the higher average haze ratio ( H_avg) of ∼38% than those of the bare glass, flat-PDMS/glass, and HSA-PDMS/glass (i.e., H_avg ≈ 1.1, 1.7, and 34.2%, respectively). At last, to demonstrate the practical feasibility under light control of the solar cells, the IHSA-PDMS was laminated onto the glass substrates of perovskite solar cells (PSCs) as an AR layer, and their device performances were explored. Consequently, the short-circuit current density of the PSCs integrated with the IHSA-PDMS AR layer was improved by ∼17% when compared with the device without AR layer, resulting in the power conversion efficiency (PCE) up to 19%. Therefore, the IHSA-PDMS is expected to be applied as an AR layer for solar cells to enhance their light absorption as well as the PCE.

Alkali Metal Doping for Improved CH3NH3PbI3 Perovskite Solar Cells

Organic-inorganic hybrid halide perovskites are proven to be a promising semiconductor material as the absorber layer of solar cells. However, the perovskite films always suffer from nonuniform coverage or high trap state density due to the polycrystalline characteristics, which degrade the photoelectric properties of thin films. Herein, the alkali metal ions which are stable against oxidation and reduction are used in the perovskite precursor solution to induce the process of crystallization and nucleation, then affect the properties of the perovskite film. It is found that the addition of the alkali metal ions clearly improves the quality of perovskite film: enlarges the grain sizes, reduces the defect state density, passivates the grain boundaries, increases the built-in potential (Vbi), resulting to the enhancement in the power conversion efficiency of perovskite thin film solar cell.

Low-Temperature Soft-Cover Deposition of Uniform Large-Scale Perovskite Films for High-Performance Solar Cells

Large-scale high-quality perovskite thin films are crucial to produce high-performance perovskite solar cells. However, for perovskite films fabricated by solvent-rich processes, film uniformity can be prevented by convection during thermal evaporation of the solvent. Here, a scalable low-temperature soft-cover deposition (LT-SCD) method is presented, where the thermal convection-induced defects in perovskite films are eliminated through a strategy of surface tension relaxation. Compact, homogeneous, and convection-induced-defects-free perovskite films are obtained on an area of 12 cm² , which enables a power conversion efficiency (PCE) of 15.5% on a solar cell with an area of 5 cm² . This is the highest efficiency at this large cell area. A PCE of 15.3% is also obtained on a flexible perovskite solar cell deposited on the polyethylene terephthalate substrate owing to the advantage of presented low-temperature processing. Hence, the present LT-SCD technology provides a new non-spin-coating route to the deposition of large-area uniform perovskite films for both rigid and flexible perovskite devices.

Controlled Crystal Grain Growth in Mixed Cation-Halide Perovskite by Evaporated Solvent Vapor Recycling Method for High Efficiency Solar Cells

We developed a new and simple solvent vapor-assisted thermal annealing (VA) procedure which can reduce grain boundaries in a perovskite film for fabricating highly efficient perovskite solar cells (PSCs). By recycling of solvent molecules evaporated from an as-prepared perovskite film as a VA vapor source, named the pot-roast VA (PR-VA) method, finely controlled and reproducible device fabrication was achieved for formamidinium (FA) and methylammonium (MA) mixed cation-halide perovskite (FAPbI₃)_0.85(MAPbBr₃)_0.15. The mixed perovskite was crystallized on a low-temperature prepared brookite TiO₂ mesoporous scaffold. When exposed to very dilute solvent vapor, small grains in the perovskite film gradually unified into large grains, resulting in grain boundaries which were highly reduced and improvement of photovoltaic performance in PSC. PR-VA-treated large grain perovskite absorbers exhibited stable photocurrent-voltage performance with high fill factor and suppressed hysteresis, achieving the best conversion efficiency of 18.5% for a 5 × 5 mm² device and 15.2% for a 1.0 × 1.0 cm² device.

Chemical Reduction of Intrinsic Defects in Thicker Heterojunction Planar Perovskite Solar Cells

Minimization of defects in absorber materials is essential for hybrid perovskite solar cells, especially when constructing thick polycrystalline layers in a planar configuration. Here, a simple methylamine solution-based additive is reported to improve film quality with nearly an order of magnitude reduction in intrinsic defect concentration. In the resultant film, an increase in carrier lifetime as a result of a decrease in shallow electronic disorder is observed. This superior crystalline film quality is further evidenced via a doubled spin relaxation time as compared with other reports. Bearing sufficient carrier diffusion length, a thick absorber layer (≈650 nm) is implemented in planar devices to achieve a champion power conversion efficiency of 20.02% with a stabilized output efficiency of 19.01% under one sun illumination. This work demonstrates a simple approach to improve hybrid perovskite film quality by substantial reduction of intrinsic defects for wide applications in optoelectronics.

Enhanced Efficiency and Stability of an Aqueous Lead-Nitrate-Based Organometallic Perovskite Solar Cell

We investigate the stability of an active organometallic perovskite layer prepared from a two-step solution procedure, including spin coating of aqueous lead nitrate (Pb(NO₃)₂) as a Pb²⁺ source and sequential dipping into a methylammonium iodide (CH₃NH₃I) solution. The conversion of CH₃NH₃PbI₃ from a uniform Pb(NO₃)₂ layer generates PbI₂-free and large-grain perovskite crystallites owing to an intermediate ion-exchange reaction step, resulting in improved humidity resistance and, thereby, improved long-term stability with 93% of the initial power conversion efficiency (PCE) after a period of 20 days. The conventional fast-converted PbI₂-dimethylformamide solution system leaves small amounts of intrinsic PbI₂ residue on the resulting perovskite and MAPbI₃ crystallites with uncontrollable sizes. This accelerates the generation of PbI₂ and the decomposition of the perovskite layer, resulting in poor stability with less than 60% of the initial PCE after a period of 20 days.

Enhanced performance of perovskite solar cells by strengthening a self-embedded solvent annealing effect in perovskite precursor films

Smooth perovskite films with large grains are fabricated by strengthening the self-embedded solvent annealing effect in the perovskite precursor film via pre-depositing a protective layer.

Designing of blue, green, and red CsPbX3 perovskite-codoped flexible films with water resistant property and elimination of anion-exchange for tunable white light emission

The blue, green, and red CsPbX₃ QDs-codoped flexible films were prepared. The resulting films are resistant to water, preventing anion exchange and significantly prolonging the lifetime of light emitters under ambient air conditions.