Grain‐Boundary “Patches” by In Situ Conversion to Enhance Perovskite Solar Cells Stability

The power conversion efficiency of organic–inorganic hybrid perovskite solar cells has increased rapidly, but the device stability remains a big challenge. Previous studies show the grain boundary (GB) can facilitate ion migration and initiate device degradation. Herein, methimazole (MMI) is employed for the first time to construct a surface “patch” by in situ converting residual PbI₂ at GBs. The resultant MMI–PbI₂ complex can effectively suppress ion migration and inhibit diffusion of the metal electrodes. The origin of the surface “patch” effect and their working mechanisms are investigated experimentally and theoretically at the microscopic level. It hence demonstrates a simple and effective method to prolong the device stability in the context of GB engineering, which could be extensively applied to perovskite‐based optoelectronics.     

Effects of porosity on ferroelectric properties of Pb(Zr0.2Ti0.8)O3 films

The sol-gel deposition method has been successfully applied to obtain Pb(Zr_0.2Ti_0.8)O₃ thin films on platinized silicon wafers. Addition of different amounts (7-15 wt.%) of organic macromolecular polyvinylpyrrolidone in the precursor solution prior to spin coating proves to be an excellent method for obtaining porous films. The crystal structure of as deposited films was analyzed by X-ray diffraction. The porous films show perovskite phase after annealing at 650 °C. The surface morphology has been studied by Atomic Force Microscopy and Scanning Electron Microscopy. The surface profile indicates a roughness of the film of 5 nm and no microcracks on the surface. The ferroelectric behavior was proved for each film, by hysteresis loops and by the “butterfly” shape of the capacitance-voltage characteristics. The remnant polarization and the coercive field decrease while the amount of added PVP increases.     

The Impact of a Dynamic Two‐Step Solution Process on Film Formation of Cs0.15(MA0.7FA0.3)0.85PbI3 Perovskite and Solar Cell Performance

This paper provides deep understanding of the formation mechanism of perovskite film fabricated by sequential solution‐based methods. It compares two sequential spin‐coating methods for Cs_0.15(MA_0.7FA_0.3)_0.85PbI₃ perovskite. First is the “static process,” with a stoppage between the two spin‐coating steps (1st PbI₂‐CsI‐dimethyl sulfoxide (DMSO)‐dimethylformamide (DMF) and 2nd methylammonium iodide (MAI)‐formamidinium iodide (FAI)‐isopropyl alcohol). Second is the “dynamic process,” where the 2nd precursor is dispensed while the substrate is still spinning from the 1st step. For the first time, such a dynamic process is used for Cs_0.15(MA_0.7FA_0.3)_0.85PbI₃ perovskite. Characterizations reveal improved film formation with the dynamic process due to the “retainment” of DMSO‐complex necessary for the intermediate phase which i) promotes intercalation between precursors and ii) slows down perovskite crystallization for full conversion. The comparison on as‐deposited perovskite before annealing indicates a more ordered film using this dynamic process. This results in a thicker, more uniform film with higher degree of preferred crystal orientation and higher carrier lifetime after annealing. Therefore, dynamic‐processed devices present better performance repeatability, achieving a higher average efficiency of 17.0% compared to static ones (15.0%). The new insights provided by this work are important for perovskite solar cells processed sequentially as the process has greater flexibility in resolving solvent incompatibility, allowing separate optimizations and allowing different deposition methods.     

Ligand-Modulated Excess PbI2 Nanosheets for Highly Efficient and Stable Perovskite Solar Cells

Excess lead iodide (PbI₂), as a defect passivation material in perovskite films, contributes to the longer carrier lifetime and reduced halide vacancies for high-efficiency perovskite solar cells. However, the random distribution of excess PbI₂ also leads to accelerated degradation of the perovskite layer. Inspired by nanocrystal synthesis, here, a universal ligand-modulation technology is developed to modulate the shape and distribution of excess PbI₂ in perovskite films. By adding certain ligands, perovskite films with vertically distributed PbI₂ nanosheets between the grain boundaries are successfully achieved, which reduces the nonradiative recombination and trap density of the perovskite layer. Thus, the power conversion efficiency of the modulated device increases from 20% to 22% compared to the control device. In addition, benefiting from the vertical distribution of excess PbI₂ and the hydrophobic nature of the surface ligands, the modulated devices exhibit much longer stability, retaining 72% of their initial efficiency after 360 h constant illumination under maximum power point tracking measurement.     

修饰ZnO对钙钛矿电池性能的影响

ZnO表面的羟基会与CH₃NH₃PbI₃分子中的甲铵离子发生路易斯酸碱反应,因此本工作将针对ZnO对钙钛矿太阳能电池性能的影响及如何抑制CH₃NH₃PbI₃的分解进行研究。结果表明:Sn元素的掺杂能够有效减少ZnO表面羟基的量,从而抑制钙钛矿层的分解,起到保护作用。通过石墨烯对ZnO的包覆,减少了外露羟基的数量,避免其表面的羟基与CH₃NH₃PbI₃直接接触,从而减少路易斯酸碱反应的发生,起到保护钙钛矿层的作用。     

Highly Efficient 2D/3D Hybrid Perovskite Solar Cells via Low‐Pressure Vapor‐Assisted Solution Process

The fabrication of multidimensional organometallic halide perovskite via a low‐pressure vapor‐assisted solution process is demonstrated for the first time. Phenyl ethyl‐ammonium iodide (PEAI)‐doped lead iodide (PbI₂) is first spin‐coated onto the substrate and subsequently reacts with methyl‐ammonium iodide (MAI) vapor in a low‐pressure heating oven. The doping ratio of PEAI in MAI‐vapor‐treated perovskite has significant impact on the crystalline structure, surface morphology, grain size, UV–vis absorption and photoluminescence spectra, and the resultant device performance. Multiple photoluminescence spectra are observed in the perovskite film starting with high PEAI/PbI₂ ratio, which suggests the coexistence of low‐dimensional perovskite (PEA₂MA_n?1Pb_nI_3n+1) with various values of n after vapor reaction. The dimensionality of the as‐fabricated perovskite film reveals an evolution from 2D, hybrid 2D/3D to 3D structure when the doping level of PEAI/PbI₂ ratio varies from 2 to 0. Scanning electron microscopy images and Kelvin probe force microscopy mapping show that the PEAI‐containing perovskite grain is presumably formed around the MAPbI₃ perovskite grain to benefit MAPbI₃ grain growth. The device employing perovskite with PEAI/PbI₂ = 0.05 achieves a champion power conversion efficiency of 19.10% with an open‐circuit voltage of 1.08 V, a current density of 21.91 mA cm^?2, and a remarkable fill factor of 80.36%.     

Ultrasonic-assisted organic–inorganic multilayer thin film synthesis and enhanced visible-light phototropy based on PVP /PMoA

Novel organic–inorganic layer-by-layer (LbL) multilayers for visible-light phototropy, comprised of polyvinylpyrrolidone (PVP) and phosphomolybdic acid (PMoA), were fabricated via layer-by-layer (LbL) assembly with ultrasonic pretreatment. The characterizations revealed that the Keggin spatial geometry of the PMoA was still maintained in the final ultrasonic-assisted LbL thin films, which promoted halogenesis between the PVP and PMoA. Intriguingly, instead of destroying the PVP thin film, the ultrasonic pretreatment process successfully dispersed the PMoA in the PVP thin film with a median size of approximately 3.6 nm. Thanks to the dispersion of the PMoA particles, the photochromic capabilities of the ultrasonic-assisted PVP/PMoA LbL thin films gained remarkable improvement. Moreover, decreasing the size of the PMoA particles to quantum dots (10–3.4 nm) induces an up-conversion and dielectric confinement effect, which increases the intensity of the absorption peak (0.235 increased to 0.45) while narrowing the emission energy (2.51 eV narrowed to 2.23 eV). At the same time, the size reduction also extended the range of the absorption wavelength, increases the characteristic absorption intensity, and accelerates the coloration rate over the same illumination period.

     

Effect of crystallite size and calcination temperature on the thermal stability of single nanocrystalline chromium oxide: expressed by novel correlation

   The thermal reaction of chromium acetylacetonate in various organic solvents at 300 °C for 2 h yielded an amorphous product. Single nanocrystalline chromium oxide was obtained after being calcined at 300 °C for 1 h. The crystallite size of product is in the range of 16–26 nm. In this work, the thermal stability of product was given by BET/BET₀. It was found that the crystals of large crystallite size show higher thermal stability than the crystals of small crystallite size. Thermal stability of chromium oxide can be presented by the correlation of the BET surface area after calcination, crystallite size of as-synthesized product and calcination temperature (500–900 °C) as shown below.

Suppressed Voltage Deficit and Degradation of Perovskite Solar Cells by Regulating the Mineralization of Lead Iodide

Both the uncoordinated Pb²⁺ and excess PbI₂ in perovskite film will create defects and perturb carrier collection, thus leading to the open-circuit voltage (V_OC) loss and inducing rapid performance degradation of perovskite solar cells (PSCs). Herein, an additive of 3-aminothiophene-2-carboxamide (3-AzTca) that contains amide and amino and features a large molecular size is introduced to improve the quality of perovskite film. The interplay of size effect and adequate bonding strength between 3-AzTca and uncoordinated Pb²⁺ regulates the mineralization of PbI₂ and generates low-dimensional PbI₂ phase, thereby boosting the crystallization of perovskite. The decreased defect states result in suppressed nonradiative recombination and reduced V_OC loss. The power conversion efficiency (PCE) of modified PSC is improved to 22.79% with a high V_OC of 1.22 V. Moreover, the decomposition of PbI₂ and perovskite films is also retarded, yielding enhanced device stability. This study provides an effective method to minimize the concentration of uncoordinated Pb²⁺ and improve the PCE and stability of PSCs.     

Interfacial Molecular Doping of Metal Halide Perovskites for Highly Efficient Solar Cells

Tailoring the doping of semiconductors in heterojunction solar cells shows tremendous success in enhancing the performance of many types of inorganic solar cells, while it is found challenging in perovskite solar cells because of the difficulty in doping perovskites in a controllable way. Here, a small molecule of 4,4′,4″,4″′-(pyrazine-2,3,5,6-tetrayl) tetrakis (N,N-bis(4-methoxyphenyl) aniline) (PT-TPA) which can effectively p-dope the surface of FA_xMA_1−xPbI₃ (FA: HC(NH₂)₂; MA: CH₃NH₃) perovskite films is reported. The intermolecular charge transfer property of PT-TPA forms a stabilized resonance structure to accept electrons from perovskites. The doping effect increases perovskite dark conductivity and carrier concentration by up to 4737 times. Computation shows that electrons in the first two layers of octahedral cages in perovskites are transferred to PT-TPA. After applying PT-TPA into perovskite solar cells, the doping-induced band bending in perovskite effectively facilitates hole extraction to hole transport layer and expels electrons toward cathode side, which reduces the charge recombination there. The optimized devices demonstrate an increased photovoltage from 1.12 to 1.17 V and an efficiency of 23.4% from photocurrent scanning with a stabilized efficiency of 22.9%. The findings demonstrate that molecular doping is an effective route to control the interfacial charge recombination in perovskite solar cells which is in complimentary to broadly applied defect passivation techniques.     

Design of a Synthetic Strategy to Achieve Enhanced Fluorescent Carbon Dots with Sulfur and Nitrogen Codoping and Its Multifunctional Applications

Here, a rational strategy to achieve multifunctional N, S codoped carbon dots (N, S-CDs) is reported, aiming to improve the photoluminescence quantum yields (PLQYs) of the CDs. The synthesized N, S-CDs have excellent stability and emission properties independent of excitation wavelength. Through the introduction of S element doping, the fluorescence emission of CDs is red-shifted from 430 to 545 nm, and the corresponding PLQYs can be greatly enhanced from 11.2% to 65.1%. It is found that the doping of S elements causes an increase in the size of CDs and an elevated graphite N content, which may be the key factors to cause the redshift of fluorescence emission. Furthermore, the introduction of S element also serves to suppress the nonradiative transitions, which may be responsible for the elevated PLQYs. Besides, the synthesized N, S-CDs have certain solvent effect and can be applied to detect water content in organic solvents, and have strong sensitivity to alkaline environment. More importantly, the N, S-CDs can be used to achieve an “on–off–on” dual detection mode between Zr⁴⁺ and NO₂⁻. In addition, N, S-CDs combinedwith polyvinylpyrrolidone (PVP) can also be utilized as fluorescent inks for anti-counterfeiting applications.     

Efficient and Stable CsPbI3 Solar Cells via Regulating Lattice Distortion with Surface Organic Terminal Groups

All‐inorganic cesium lead iodide perovskites (CsPbI₃) are promising wide‐bandgap materials for use in the perovskite/silicon tandem solar cells, but they easily undergo a phase transition from a cubic black phase to an orthorhombic yellow phase under ambient conditions. It is shown that this phase transition is triggered by moisture that causes distortion of the corner‐sharing octahedral framework ([PbI₆]^4?). Here, a novel strategy to suppress the octahedral tilting of [PbI₆]^4? units in cubic CsPbI₃ by systematically controlling the steric hindrance of surface organic terminal groups is provided. This steric hindrance effectively prevents the lattice distortion and thus increases the energy barrier for phase transition. This mechanism is verified by X‐ray diffraction measurements and density functional theory calculations. Meanwhile, the formation of an organic capping layer can also passivate the surface electronic trap states of perovskite absorber. These modifications contribute to a stable power conversion efficiency (PCE) of 13.2% for the inverted planar perovskite solar cells (PSCs), which is the highest efficiency achieved by the inverted‐structure inorganic PSCs. More importantly, the optimized devices retained 85% of their initial PCE after aging under ambient conditions for 30 days.     

Luminescence characteristic of YVO4:Eu thin film phosphors by Li doping

YVO₄:Eu³⁺ and Li-doped YVO₄:Eu³⁺ thin film phosphors have been deposited on Al₂O₃ (0001) substrate using a pulsed laser deposition technique. The Li⁺ ions concentration was varied from 0 to 3 wt.% and Li⁺ doping influenced crystallinity and surface morphology of YVO₄:Eu³⁺ films.. As Li⁺ content increases from 0 wt.% to 2 wt.%, not only crystallinity was improved, but also the shape of grains was rounded. However, Li⁺ content, increases further to 3 wt.% the shape of grains was changed to elliptical. The emitted radiation was dominated by a red emission peak at 619 nm radiated from the ⁵D₀-⁷F₂ transition of Eu³⁺ ions. In particular, the incorporation of Li⁺ ions into YVO₄ lattice could induce an increase of photoluminescence. The enhanced luminescence results not only from the improved crystallinity but also from the enhanced surface roughness. The luminescent intensity and surface roughness exhibited similar behavior as a function of Li⁺ ions concentration.     

Polyvinylpyrrolidone macromolecules function as a diffusion barrier during dealloying

The inhibition effect of polyvinylprrolidone (PVP) during dealloying on the formation of nanoporous Cu from a Ti₆₀Cu₄₀ amorphous alloy in hydrofluoric acids (HF) was investigated. A bicontinuous nanoporous Cu structure formed on Ti₆₀Cu₄₀ after dealloying. The pore size of nanoporous Cu formed in HF solution was 71 nm, but this decreased to 12 nm and 11 nm after dealloying for the same period of dealloying time of 43.2 ks when 0.01 M and 0.03 M PVP, respectively, was added into the 0.03 M HF base solution. The surface diffusivity was estimated to decrease from 2.5 × 10⁻¹⁸ m² s⁻¹ in 0.03 M HF solution to 1.84 × 10⁻²¹ m² s⁻¹ when 0.01 M PVP was added, and to 1.42 × 10⁻²¹ m² s⁻¹ when 0.03 M PVP was added. More PVP macromolecules were adsorbed onto the nanoporous Cu surface in the 0.03 M HF solution with the addition of 0.03 M PVP than when 0.01 M PVP was added to the solution, which resulted in the formation of smaller nanopores. The suppressed diffusion of Cu adatoms due to the PVP macromolecule resulted in the formation of finer Cu ligaments than that formed in 0.03 M HF solution. This long chain organic molecule was shown to act as a diffusion barrier for the diffusion of metal adatoms during dealloying and to elaborate the nanoporous structure.     

Characterization and reactivity of chromia nanoparticles prepared by urea forced hydrolysis

Chromia (Cr₂O₃) nanoparticles were prepared by urea forced hydrolysis in the presence of chromium (III) nitrate using NaCl as a precipitating agent. The size, distribution, and purity of the particles were evaluated. The necessity of polyvinylpyrrolidone (PVP) as a surfactant to prevent aggregation was also investigated. In the presence of PVP, non-aggregated spherical-like nanoparticles (3 ± 1 nm) were formed, whereas in the absence of PVP, spherical-like weakly agglomerated nanoparticles (85 ± 16 nm) comprised of 10 nm nanoparticle subunits were produced, creating a large surface area. The as-formed hydrated Cr₂O₃ nanoparticles were amorphous, although they could be easily converted into crystalline form by heating to 400 °C for 1 h, with minimal particle aggregation and size reduction. Attenuated total reflectance Fourier transform infrared spectroscopy indicated that preparation methods (surfactant and precipitating agent) influence surface reactivity of the nanoparticles to catechol.     

Broadband Light Absorption and Efficient Charge Separation Using a Light Scattering Layer with Mixed Cavities for High‐Performance Perovskite Photovoltaic Cells with Stability

CH₃NH₃PbI₃ is one of the promising light sensitizers for perovskite photovoltaic cells, but a thick layer is required to enhance light absorption in the long‐wavelength regime ranging from PbI₂ absorption edge (500 nm) to its optical band‐gap edge (780 nm) in visible light. Meanwhile, the thick perovskite layer suppresses visible‐light absorption in the short wavelengths below 500 nm and charge extraction capability of electron–hole pairs produced upon light absorption. Herein, we find that a new light scattering layer with the mixed cavities of sizes in 100 and 200 nm between transparent fluorine‐doped tin oxide and mesoporous titanium dioxide electron transport layer enables full absorption of short‐wavelength photons (λ < 500 nm) to the perovskite along with enhanced absorption of long‐wavelength photons (500 nm < λ < 780 nm). Moreover, the light‐driven electric field is proven to allow efficient charge extraction upon light absorption, thereby leading to the increased photocurrent density as well as the fill factor prompted by the slow recombination rate. Additionally, the photocurrent density of the cell with a light scattering layer of mixed cavities is stabilized due to suppressed charge accumulation. Consequently, this work provides a new route to realize broadband light harvesting of visible light for high‐performance perovskite photovoltaic cells.     

Improved Outcoupling Efficiency and Stability of Perovskite Light‐Emitting Diodes using Thin Emitting Layers

Hybrid organic–inorganic perovskite semiconductors have shown potential to develop into a new generation of light‐emitting diode (LED) technology. Herein, an important design principle for perovskite LEDs is elucidated regarding optimal perovskite thickness. Adopting a thin perovskite layer in the range of 35–40 nm is shown to be critical for both device efficiency and stability improvements. Maximum external quantum efficiencies (EQEs) of 17.6% for Cs_0.2FA_0.8PbI_2.8Br_0.2, 14.3% for CH₃NH₃PbI₃ (MAPbI₃), 10.1% for formamidinium lead iodide (FAPbI₃), and 11.3% for formamidinium lead bromide (FAPbBr₃)‐based LEDs are demonstrated with optimized perovskite layer thickness. Optical simulations show that the improved EQEs source from improved light outcoupling. Furthermore, elevated device temperature caused by Joule heating is shown as an important factor contributing to device degradation, and that thin perovskite emitting layers maintain lower junction temperature during operation and thus demonstrate increased stability.     

High Phase Stability in CsPbI3 Enabled by Pb–I Octahedra Anchors for Efficient Inorganic Perovskite Photovoltaics

CsPbI₃ inorganic perovskite has exhibited some special properties particularly crystal structure distortion and quantum confinement effect, yet the poor phase stability of CsPbI₃ severely hinders its applications. Herein, the nature of the photoactive CsPbI₃ phase transition from the perspective of PbI₆ octahedra is revealed. A facile method is also developed to stabilize the photoactive phase and to reduce the defect density of CsPbI₃. CsPbI₃ is decorated with multifunctional 4-aminobenzoic acid (ABA), and steric neostigmine bromide (NGBr) is subsequently used to further mediate the thin films' surface (NGBr-CsPbI₃(ABA)). The ABA or NG cation adsorbed onto the grain boundaries/surface of CsPbI₃ anchors the PbI₆ octahedra via increasing the energy barriers of octahedral rotation, which maintains the continuous array of corner-sharing PbI₆ octahedra and kinetically stabilizes the photoactive phase CsPbI₃. Moreover, the added ABA and NGBr not only interact with shallow- or deep-level defects in CsPbI₃ to significantly reduce defect density, but also lead to improved energy-level alignment at the interfaces between the CsPbI₃ and the charge transport layers. Finally, the champion NGBr-CsPbI₃(ABA)-based inorganic perovskite solar cell delivers 18.27% efficiency with excellent stability. Overall, this work demonstrates a promising concept to achieve highly phase-stabilized inorganic perovskite with suppressed defect density for promoting its optoelectronic applications.     

Precise Control of Crystallization and Phase-Transition with Green Anti-Solvent in Wide-Bandgap Perovskite Solar Cells with Open-Circuit Voltage Exceeding 1.25 V

Wide-bandgap perovskite solar cells (PSCs) have attracted a lot of attention due to their application in tandem solar cells. However, the open-circuit voltage (V_OC) of wide-bandgap PSCs is dramatically limited by high defect density existing at the interface and bulk of the perovskite film. Here, an anti-solvent optimized adduct to control perovskite crystallization strategy that reduces nonradiative recombination and minimizes V_OC deficit is proposed. Specifically, an organic solvent with similar dipole moment, isopropanol (IPA) is added into ethyl acetate (EA) anti-solvent, which is beneficial to form PbI₂ adducts with better crystalline orientation and direct formation of α-phase perovskite. As a result, EA-IPA (7-1) based 1.67 eV PSCs deliver a power conversion efficiency of 20.06% and a V_OC of 1.255 V, which is one of the remarkable values for wide-bandgap around 1.67 eV. The findings provide an effective strategy for controlling crystallization to reduce defect density in PSCs.     

Influence of substrate doping on the surface chemistry and morphology of Copper Phthalocyanine ultra thin films on Si (111) substrates

16 nm thick Copper Phthalocyanine (CuPc) films were deposited at room temperature in Ultra High Vacuum onto “n” and “p” type doped Si(111) substrates covered with a native SiO₂ overlayer. Atomic Force Microscopy indicates that the two substrates are both atomically flat (0.15 nm root mean square roughness). Angle dependent X-ray photoemission spectroscopy shows that the thickness of the native SiO₂ over-layer is 0.8 nm (both for the “n” and “p” type Si substrate). Despite the identity of the substrate roughness, of the SiO₂ thickness, and of the CuPc film growth conditions, the organic films (made out of crystallites in the α-phase, as checked with X-ray Diffraction) grown on the “p” and “n” type substrate show clearly different morphological features (determined with Atomic Force Microscopy and Scanning Electron Microscopy measurements). While the CuPc film on “p” Si(111) shows a compact network of densely packed crystallites with sizes (along the substrate plane) ranging from 50 to 100 nm, the CuPc film on “n” Si(111) shows a slightly more open network of larger crystallites (with 75-150 nm size range). Accordingly, the CuPc film roughness is 0.67 nm and 1.15 nm for the “p” and “n” type substrate respectively. Due to the increased surface to volume effects (lower crystallite size) affecting the CuPc film on “p” Si(111), this film exhibits stronger interaction with oxygen and water vapor of the ambient air, as determined by photoemission spectroscopy experiments performed on samples as grown “in situ” and after prolonged (1 year) exposure to air.