Non-stoichiometric Precursor Research Articles

Multi-year study of environmental stability of Ti3C2Tx MXene films

MXenes are a family of two-dimensional (2D) carbides and nitrides with extraordinary electrical, optical, chemical, and electrochemical properties. There is a perception that MXenes are unstable and degrade quickly, limiting potential applications and requiring specific storage conditions to last for a long time. It primarily comes from studies on delaminated MXenes flakes in dilute dispersions and samples from defective or non-stoichiometric precursors when MXene research was in its infancy. In the years since then, significant developments in synthesis, processing, and understanding of their chemistry have led to dramatic increases in the environmental stability of many MXenes, especially the widely studied Ti3C2Tx. However, previous studies focused primarily on MXene dispersion, while in the majority of applications, MXenes are processed into films soon after synthesis. Herein, we analyze Ti3C2Tx free-standing films aged from 4 to 10 years through structural and morphological characterization along with electrical conductivity measurements to reveal the effect, or lack thereof, of prolonged storage under ambient conditions. Further, we show that the decrease in electronic conductivity over time is caused mainly by water uptake by the hydrophilic surfaces of MXenes, which can be removed by vacuum annealing. As a result, the conductivity of the films can be partially or almost completely recovered.

One-Pot Synthesis of All-Inorganic CsPbClBr2 Blue Perovskite Quantum Dots via Stoichiometric Precursor.

The synthesis of perovskite-based blue light-emitting particles is valuable for several applications as the excellent optical properties and performances of the constituting materials associated with multi-exciton generation can be exploited. However, the preparation of perovskite precursors requires high temperatures, resulting in a complex manufacturing process. This paper proposes a one-pot method to synthesize CsPbClBr2 blue light-emitting quantum dots (QDs). In the case of nonstoichiometric precursor synthesis, the CsPbClBr2 QDs coexisted with additional products. The solvent for synthesizing mixed perovskite nanoparticles (containing chloride) was selected by mixing dimethylformamide (DMF) and/or dimethyl sulfoxide (DMSO) in different ratios. When only DMF was used with the stoichiometric CsBr and PbX2 (X = Cl, Br) ratio, the quantum yield was 70.55%, and superior optical properties were achieved. Moreover, no discoloration was observed for 400 h, and a high photoluminescence intensity was maintained. When deionized water was added to form a double layer with hexane, the luminescence was maintained for 15 days. In other words, the perovskite did not easily decompose even when in contact with water, which suppressed the release of Pb2+, which are heavy metal atoms in the structure. Overall, the proposed one-pot method for all-inorganic-based perovskite QDs provides a platform for synthesizing superior blue light-emitting materials.

Varied performance of printable mesoscopic perovskite solar cells by the non-stoichiometric precursor

Halide perovskite semiconductors such as MAPbI3 always fluctuate around their stoichiometric compositions due to the weighing or annealing, which causes performance variation of related devices. Here, we investigate the comprehensive impact of non-stoichiometric MAPbI3 precursor on the performance including power conversion efficiency (PCE), hysteresis, and stability of printable mesoscopic perovskite solar cells (p-MPSCs). Excess MAI improves PCE, suppresses hysteresis, and enhances stability by promoting hole transport and passivating defects evaluated by a designed mesoporous space-charge-limited-current device, while excess PbI2 impairs device performance. Replacing excess MAI with other monovalent cation halide salts including guanidinium iodide (GUAI), 2-phenylethylamine hydroiodide (PEAI) and rubidium iodide (RbI) further improves the device PCE to 17.02% and minimizes the hysteresis. Our work reveals that making monovalent cation halide salts such as MAI rather than PbI2 excessive is positive for the comprehensive performance enhancement of PSCs and developing more suitable salt substitution will benefit industrializing PSCs.

Lanthanide-doped Mn2+-based perovskite-like single crystals: Switching on highly thermal-stable near-infrared emission and LED device

Metal halide perovskites (MHPs) have sparked ongoing research interest due to their high-performance optoelectronic properties. However, blue-light excitable near-infrared (NIR) emitting MHPs is still inaccessible and the achievement of robust thermal-quenching resistance so far remains a huge challenge. In this work, we report on the synthesis of lead-free all-inorganic Mn2+-based perovskite-like single crystals using the designed nonstoichiometric precursor ratio. The special crystal structure endows Mn2+ with efficient blue light excitation and red emission, which enables the capabilities of a good matching with commercial blue LED chips and an efficient sensitization for Ln3+ emitters. The incorporations of Yb3+, Er3+, and Ho3+ functionalize the CsMnCl3 single crystals with multiple NIR emissions by virtue of feeding the energy from Mn2+ to Ln3+ via multi-channels. Most remarkable is the achievement of the robust thermal-quenching resistance, exhibiting (near-) zero-thermal-quenching and even anti-thermal quenching, of the Ln3+ NIR emissions above room temperature. Finally, as a proof-of-concept study, a prototype of an NIR-LED device was fabricated. This work not only provides a general strategy to unlock the blue-light excitable NIR emission from f-f transitions of Ln3+ ions, and a fundamental understanding of the sensitization-activation mechanisms in Ln3+-functionalized manganese (Ⅱ)-based perovskite-like phosphor, but also endows the MHPs with optical functionalities for the future high-potential applications, such as NIR phosphor-converted LEDs, and optical telecommunication.

Excess Ion-Induced Efficiency Roll-Off in High-Efficiency Perovskite Light-Emitting Diodes.

Applying extensively excess ammonium halides in forming perovskites is a widely used approach to achieve high-performance perovskite light-emitting diodes (PeLEDs). However, most of these PeLEDs suffer from severe external quantum efficiency (EQE) roll-off at high current densities, thereby restricting the realization of high-brightness PeLEDs and laser diodes. In this work, we explore the underlying mechanism of the EQE roll-off in high-efficiency formamidinium lead iodide (FAPbI3)-based PeLEDs. By combining voltage-dependent electrical stress measurements and ex situ ion distribution analysis of PeLEDs, we found that the electric field-driven migration and local segregation of excess iodide ions, originated from nonstoichiometric precursors, trigger the EQE roll-off via promoting imbalanced charge injection. Based on this discovery, we introduced a simple wash-off treatment with chloroform to remove the excess iodides from the perovskite surface and demonstrated that the treatment is highly effective in suppressing the roll-off behavior. By combining the treatment and the use of an ultrathin poly(methyl methacrylate) (PMMA) interlayer, we achieved a high-brightness PeLED with an EQEmax of 19.6%, a critical current density of 1550 mA cm-2, and a radiancemax of 875 W sr-1 m-2. The study reveals the double-edge sword effect of precursor nonstoichiometry and highlights the importance of managing excess ions in perovskite films.

High-Intensity CsPbBr3 Perovskite LED using Poly(bis(4-phenyl)(2,4,6-trimethylphenyl)amine) as Hole Transport and Electron-Blocking Layer

The majority of highly efficient perovskite light-emitting diodes (PeLED) contain PEDOT:PSS (poly(3,4-ethylenedioxythiophene):polystyrenesulfonate) as hole transport layer (HTL). However, the hygroscopic and acidic nature of PEDOT:PSS may lead to deterioration of PeLED performance. Moreover, due to its inferior electron-blocking properties, an additional electron-blocking layer (EBL) is required to establish charge balance and consequently obtain superior emission characteristics in typically electron-rich PeLED structures. In this work, PTAA (poly(bis(4-phenyl)(2,4,6-trimethylphenyl)amine)) serving both as HTL and EBL is employed to substitute PEDOT:PSS in PeLED. The perovskite CsPbBr3 is chosen as emissive layer (EML) material due to its high color purity and photoluminescence (PL) quantum yield. Dense CsPbBr3 films are fabricated on PTAA-coated ITO substrates by employing a one-step spin-coating approach based on nonstoichiometric perovskite precursor solutions. To suppress non-radiative recombination, a small amount of methylammonium bromide (MABr) is incorporated in the CsPbBr3 lattice. The resulting films exhibit excellent coverage and PL intensity. PeLED containing pure CsPbBr3 films as EML show a green emission with a peak at 520 nm, maximum luminance of 11,000 cd/m2, an external quantum efficiency (EQE) of 3.3 % and a current efficiency (CE) of 10.3 cd/A. Further enhancement to 21,000 cd/m2, 7.5 % and 27.0 cd/A is demonstrated by PeLED with MABr-doped CsPbBr3 layers.

The Cesium doping using the nonstoichiometric precursor for improved CH3NH3PbI3 perovskite films and solar cells in ambient air

Organic-inorganic hybrid perovskite solar cells have shown great prospect as a low-cost and high efficiency photovoltaic technology. The quality of the perovskite absorber layer is most critical to the performance of the device, and the stability remains one of the challenging issues. In this paper, the Cesium (Cs) doping perovskite film was prepared from nonstoichiometric precursor solution in ambient and humidity-controlled conditions. The results showed that the crystallinity, uniformity, absorption, Photo-luminescence intensity and the thermal stability of these films can be effectively improved compared with the films fabricated from stoichiometric one, which is attributed to the combined effect of the Cs doping and excess methylammonium cations passivation. Finally, the highest efficiency of the perovskite solar cells fabricated from nonstoichiometric solution reached to 14.1%, which is 12.8% higher than that of the control device from stoichiometric solution (12.5%). Furthermore, the device displayed the high stability and efficiency degradation of only 2% occurring over a period of 5 weeks in ambient air without encapsulation. This reported work provides a pathway for further improving the performance of perovskite solar cells with higher stability.

Quantifying the Composition of Methylammonium Lead Iodide Perovskite Thin Films with Infrared Spectroscopy

Lead halide perovskites are generally formed from a reaction of the lead halide salt with the halide salt of the A cation. The effects of varying film compositions as result of nonstoichiometric precursor ratios on electronic properties of halide perovskites are currently under debate. It is imperative, but experimentally challenging, to determine the chemical composition of thin films as a function of precursor ratio for a full understanding of the effect. Herein we report a precise quantification of the methylammonium (MA) content in differently fabricated films of MAPbI3 via infrared spectroscopy. We compare the thin film data to the first high-quality dielectric function obtained from single crystals with IR ellipsometry. For spin-coated samples, we find that the MAI/PbI2 ratio in solution has an effect on the MA content in the resulting thin films, which is in the range of 77–106% compared to single crystals. For coevaporated samples, we show that a very similar range of MA content variation is accessible. Interestingly, coevaporated films prepared like those for high-efficiency solar cells have a significantly lower MA content than that of single crystals.

Role of novel Mg-coating method on transport properties of MgB2/Fe wires

We report a comparative study which reveals the role of excess magnesium addition technique on transport properties of single-core in-situ MgB2/Fe wires. Transport measurements are performed on MgB2/Fe wires which initially contain different amounts of excess Mg with < 1 wt.%, 1 wt.%, and 5 wt.% of Mg+2B precursor powder. The MgB2/Fe wire with <1 wt.% excess Mg is produced by using the Mg-coating technique, in which an excess amount of Mg is first coated into the initial iron tube by evaporation, then the stoichiometric Mg+2B powder is filled into this Mg-coated iron tube. Transport properties of the Mg-coated wire are compared with those of the wires which are produced directly from the non-stoichiometric precursor powders having 1 wt.% and 5 wt.% excess-Mg. The highest Jc (28 K) of 1.99 × 102A/cm2 and the biggest n-value (28 K) of 4.8 are obtained for the Mg-coated wire sample under B = 7T, despite its lowest initial excess-Mg ratio among the whole wires. We suggest that the results of the present work are expected to contribute to the existing debates on Mg stoichiometry which essentially affect the critical current density of MgB2 wires under low/moderate magnetic fields.

Efficient 4,4′,4″‐tris(3‐methylphenylphenylamino)triphenylamine (m‐MTDATA) Hole Transport Layer in Perovskite Solar Cells Enabled by Using the Nonstoichiometric Precursors

AbstractTo overcome the drawbacks of the acidic and hygroscopic nature of the poly(3,4‐ethylenedio‐xythiophene):poly(styrenesulfonate) hole transport layer (HTL), the p‐type polymeric hole transport materials have attracted great attention and have been applied into perovskite solar cells. Here, a starburst amine molecule 4,4′,4″‐tris(3‐methylphenylphenylamino)triphenylamine (m‐MTDATA) without any additive is demonstrated as an effective hole transport material in perovskite solar cells. Meanwhile, considering the different surface affinity of precursor's composition on m‐MTDATA, the influence of the molar ratios between lead iodine (PbI2) and methylammonium iodide in precursor solution on the perovskite characteristics and device performance is investigated in‐depth. Ultimately, an enhanced efficiency of 17.73% and a high fill factor of 79.6% are achieved, which attribute to the strong passivation effect of traps and small resistance loss from appropriate unreacted PbI2 left in the perovskite layer. This work not only provides a remarkable HTL, but also reveals that the adjustment of precursor ratio is necessary for the one‐step solution approach, because the affinities of precursor's composition may be different with the underlying transport layers.

Effect of non-stoichiometric solution chemistry on improving the performance of wide-bandgap perovskite solar cells

A high-efficiency wide-bandgap (WBG) perovskite solar cell is critical for developing perovskite-related (e.g., all-perovskite, perovskite/Si, or perovskite/Cu(In,Ga)Se2) tandem devices. Here, we demonstrate the use of non-stoichiometric precursor chemistry with excess methylammonium halides (MAX; X = I, Br, or Cl) for preparing high-quality ∼1.75-eV FA0.83Cs0.17Pb(I0.6Br0.4)3 perovskite solar cells. Among various methylammonium halides, using excess MABr in the non-stoichiometric precursor exhibits the strongest effect on improving perovskite crystallographic properties and device characteristics without affecting the perovskite composition. In contrast, using excess MAI significantly reduces the bandgap of perovskite due to the replacement of Br with I. Using 40% excess MABr, we demonstrate a single-junction WBG perovskite solar cell with stabilized efficiency of 16.4%. We further demonstrate a 20.3%-efficient 4-terminal tandem device by using a 14.7%-efficient semi-transparent WBG perovskite top cell and an 18.6%-efficient unfiltered (5.6%-efficient filtered) Si bottom cell.

Czochralski growth of LaPd2Al2 single crystals

The present study is focused on the preparation of single crystalline LaPd2Al2 by the Czochralski method. Differential scanning calorimetry (DSC) and energy dispersive X-ray spectroscopy (EDX) analyses reveal that LaPd2Al2 is an incongruently melting phase which causes difficulties for the preparation of single crystalline LaPd2Al2 by the Czochralski method. Therefore several non-stoichiometric polycrystalline samples were studied for its preparation. Finally the successful growth of LaPd2Al2 without foreign phases has been achieved by using a non-stoichiometric precursor with atomic composition 22:39:39 (La:Pd:Al). X-ray powder diffraction, EDX analysis and DSC were used for the characterisation. A single crystalline sample was separated from the ingot prepared by the Czochralski method using the non-stoichiometric precursor. The presented procedure for the preparation of pure single phase LaPd2Al2 could be modified for other incongruently melting phases.

Evaluating the Optoelectronic Quality of Hybrid Perovskites by Conductive Atomic Force Microscopy with Noise Spectroscopy.

Organic-inorganic hybrid perovskite solar cells have emerged as promising candidates for next-generation solar cells. To attain high photovoltaic efficiency, reducing the defects in perovskites is crucial along with a uniform coating of the films. Also, evaluating the quality of synthesized perovskites via facile and adequate methods is important as well. Herein, CH3NH3PbI3 perovskites were synthesized by applying second solvent dripping to nonstoichiometric precursors containing excess CH3NH3I. The resulting perovskite films exhibited a larger average grain size with a better crystallinity compared to that from stoichiometric precursors. As a result, the performance of planar perovskite solar cells was significantly improved, achieving an efficiency of 14.3%. Furthermore, perovskite films were effectively analyzed using a conductive AFM and noise spectroscopy, which have been uncommon in the field of perovskite solar cells. Comparing the topography and photocurrent maps, the variation of photocurrents in nanoscale was systematically investigated, and a linear relationship between the grain size and photocurrent was revealed. Also, noise analyses with a conductive probe enabled examination of the defect density of perovskites at specific grain interiors by excluding the grain-boundary effect, and reduced defects were clearly observed for the perovskites using CH3NH3I-rich precursors.

The Influence of Water Vapor on the Stability and Processing of Hybrid Perovskite Solar Cells Made from Non-Stoichiometric Precursor Mixtures.

We investigated the influence of moisture on methylammonium lead iodide perovskite (MAPbI3 ) films and solar cells derived from non-stoichiometric precursor mixtures. We followed both the structural changes under controlled air humidity through in situ X-ray diffraction, and the electronic behavior of devices prepared from these films. A small PbI2 excess in the films improved the stability of the perovskite compared to stoichiometric samples. We assign this to excess PbI2 layers at the perovskite grain boundaries or to the termination of the perovskite crystals with Pb and I. In contrast, the MAI-excess films composed of smaller perovskite crystals showed increased electronic disorder and reduced device performance owing to poor charge collection. Upon exposure to moisture followed by dehydration (so-called solvent annealing), these films recrystallized to form larger, highly oriented crystals with fewer electronic defects and a remarkable improvement in photocurrent and photovoltaic efficiency.

Hydrothermal Synthesis of CoSb2O4: In Situ Powder X-ray Diffraction, Crystal Structure, and Electrochemical Properties

MSb2O4 constitutes a relatively unexplored class of multinary oxides that is traditionally synthesized by high-temperature solid-state methods. Here, we report a facile synthesis of CoSb2O4 under hydrothermal conditions (T = 135–300 °C, 256 bar). Using in situ synchrotron powder X-ray diffraction (PXRD), the formation and growth of CoSb2O4 nanoparticles are followed in real time using different precursor stoichiometries. Phase-pure CoSb2O4 can be formed at 135 °C, although the formation mechanism changes with precursor stoichiometry. The crystallite size can be fine-tuned between 14 and 17.5 nm under nonstoichiometric conditions, but crystallites twice as large are found in the stoichiometric case. An activation energy of 65(12) kJ/mol is obtained for the crystallization from a nonstoichiometric precursor. Modeling of atomic displacement parameters obtained from Rietveld refinement of multi-temperature high-resolution synchrotron PXRD data gives a Debye temperature of 331(11) K. The thermal expansion coef...

Growth and evolution of solution-processed CH3NH3PbI3-xClx layer for highly efficient planar-heterojunction perovskite solar cells

Methylammoniumn lead iodide perovskites have attracted great attention in photovoltaic research community. In this work, we demonstrate the results of studies focusing on the chemical reaction of CH3NH3PbI3-xClx layer formation during the annealing of perovskite precursor films. We identified two kinds of grain morphologies during the formation of perovskite films grown from non-stoichiometric precursor solution. To form single-phase and high absorbance perovskite films, higher concentration of chloride in precursor solution needs longer annealing time and only a very low Cl content could be incorporated in the final CH3NH3PbI3-xClx films. Adding PbCl2–3CH3NH3I to PbI2–CH3NH3I precursor solution can allow a good control of the growth rate and morphology for the final perovskite film, and is beneficial to the photovoltaic performance of perovskite devices. By employing the precursor solutions of PbCl2, PbI2 and CH3NH3I with a mole ratio of 1:1:4 and inserting an ultrathin amino-functionalized polymer interlayer, we achieved planar perovskite solar cell with maximum power conversion efficiency of over 15%.

Spin-coated kesterite CZTS thin films for photovoltaic applications

Copper zinc tin-sulfide (CZTS) films have been widely studied over recent years due to the inherent advantages of low cost, high absorption coefficient (≥ 104 cm−1), suitable band gap (∼1.5 eV) and nontoxicity. In this work, spin coating was used to obtain CZTS films of various compositions and degrees of crystallization on glass substrates, which were then annealed at 500 °C in a N2 atmosphere by using a rapid thermal processing (RTP) furnace in order to optimize their structure. Non-stoichiometric precursors and low spinning speeds resulted in a lower sheet resistance, which is considered to be a result of lower degree of crystallization. Furthermore, the non-stoichiometric films had a high optical band gap (> 1.66 eV) whereas the band gap of the stoichiometric films (1.4 to 1.6 eV) was close to the optimal value (1.5 eV) for solar-cell applications.

High efficiency methylammonium lead triiodide perovskite solar cells: the relevance of non-stoichiometric precursors

Improved crystallinity and device performance of CH3NH3PbI3 perovskite solar cells by using a non-stoichiometric PbI2 : CH3NH3I precursor ratio.

Preparing high-efficiency perovskite solar cells

Organometal trihalide perovskites are emerging as a new generation of photovoltaic materials. They are abundant in nature and have rapidly increasing power conversion efficiencies (PCE) of above 15% in common hybrid (mixed organic/inorganic) thin-film-based solar cells.1–3 Up to now, perovskite films have been formed by film deposition approaches such as spin-coating, sequential deposition of the inorganic and organic precursor, and co-evaporation of the precursors.1, 2 Low-temperature solution processes are among the simplest ways to fabricate planar low-cost photovoltaic devices. However, perovskite crystallizes very rapidly, so it is very challenging to form uniform, continuous, and leakage-free perovskite films by solution methods.2 We developed two simple and effective low-temperature solution processes to fabricate high-efficiency perovskite devices (see Figure 1). In the first approach, we studied the influence of the ratio of the precursors methylammonium iodide (CH3NH3I) and lead iodide (PbI2) in a premixed solution. 3 We found that using a non-stoichiometric precursor solution (i.e., not in a simple ratio) was critical to forming stoichiometric (i.e., in a simple ratio) perovskite films. The compositions of the spun perovskite films were very sensitive to the substrate surface, and could be very different from those of the precursor solutions. We varied the composition of the precursor solution, and found a PbI2/CH3NH3Xmolar ratio of 0.6–0.7 gave the best surfacemorphology and electronic properties, such as doping concentration. We used a planar heterojunction device structure of ITO/ PEDOT:PSS/(CH3NH3I)/PCBMor ICBA/C60/BCP/aluminum, where ITO is a substrate of indium tin oxide, PEDOT:PSS is an electron-donating transparent conductive polymer, PCBM and ICBA are fullerene-derived electron acceptors, C60 is buckminsterfullerene (a spherical fullerene molecule), and BCP is 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline, an electrontransporting but hole-blocking layer. Figure 1. Schematic of the prepation of perovskite films by (a) spin coating of pre-mixed precursor of lead iodide (PbI2) and methylammonium iodide (CH3NH3I) and (b) an interdiffusion approach. ITO: Indium tin oxide, a transparent conducting oxide. PEDOT:PSS: A transparent, conductive polymer. MAI: Methylammonium iodide. MAPbI3 Methylammonium lead iodide.

ChemInform Abstract: Synthesis of Complex Nanomaterials via Ostwald Ripening

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