Facet-Dependent Passivation for Efficient Perovskite Solar Cells.

Understanding the interplay between the surface structure and the passivation materials and their effects associated with surface structure modification is of fundamental importance; however, it remains an unsolved problem in the perovskite passivation field. Here, we report a surface passivation principle for efficient perovskite solar cells via a facet-dependent passivation phenomenon. The passivation process selectively occurs on facets, which is observed with various post-treatment materials with different functionality, and the atomic arrangements of the facets determine the alignments of the passivation layers. The profound understanding of facet-dependent passivation leads to the finding of 2-amidinopyridine hydroiodide as the material for a uniform and effective passivation on both (100) and (111) facets. Consequently, we achieved perovskite solar cells with an efficiency of 25.10% and enhanced stability. The concept of facet-dependent passivation can provide an important clue on unidentified passivation principles for perovskite materials and a novel means to enhance the performance and stability of perovskite-based devices.

Unveiling facet-dependent degradation and facet engineering for stable perovskite solar cells.

A myriad of studies and strategies have already been devoted to improving the stability of perovskite films; however, the role of the different perovskite crystal facets in stability is still unknown. Here, we reveal the underlying mechanisms of facet-dependent degradation of formamidinium lead iodide (FAPbI3) films. We show that the (100) facet is substantially more vulnerable to moisture-induced degradation than the (111) facet. With combined experimental and theoretical studies, the degradation mechanisms are revealed; a strong water adhesion following an elongated lead-iodine (Pb-I) bond distance is observed, which leads to a δ-phase transition on the (100) facet. Through engineering, a higher surface fraction of the (111) facet can be achieved, and the (111)-dominated crystalline FAPbI3 films show exceptional stability against moisture. Our findings elucidate unknown facet-dependent degradation mechanisms and kinetics.

Antiseptic Povidone-Iodine Heals the Grain Boundary of Perovskite Solar Cells.

Povidone, also know as polyvinylpyrrolidone (PVP), is used as a reservoir for iodine, and the povidone-iodine (PVP-I) complex has antiseptic properties for wound healing by releasing iodine. In this report, we utilized this unique characteristic of PVP-I to heal the photovoltaic parameters of perovskite solar cells (PSCs). PVP-I was added in the perovskite precursor solution, where the effect of the PVP-I concentration on the photovoltaic performance was investigated. The power conversion efficiency (PCE) of PSC was enhanced from 20.73% to 22.59% by addition of 0.1 mg/mL PVP-I, mainly due to an improved fill factor from 0.76 to 0.80 together with a slight increase in current density. Scanning electron microscopy revealed that the grain boundaries were passivated by PVP-I. Conductive atomic force microscopy combined with time-resolved photoluminesence and space charge-limited current studies showed that the addition of PVP-I decreased the defect density of the perovskite film together and enhanced the film conductivity. Furthermore, better stability was observed from the PVP-I-treated PSCs than the control device without the additive, which is probably owing to the grain boundary healing effect.

Organic-Inorganic Charge Transfer Complex with Charge Modulation after Electrical Pre-biasing.

Several breakthroughs in organic optoelectronic devices with new applications and performance improvement have been made recently by exploiting novel properties of charge transfer complexes (CTCs). In this work, a CTC film formed by coevaporating molybdenum(VI) oxide and pentacene (MoO3:pentacene) shows a strong dipole of 2.4 eV with direction controllability via pre-biasing with an external voltage. While CTCs are most widely known for their much red-shifted energy gaps, there is so far no report on their controllable dipoles. By controlling this dipole with an electrical pre-bias in a MoO3:pentacene CTC based device, current changes over 2 orders of magnitude can be achieved. Kelvin probe force microscopy further confirms that surface potential of the MoO3:pentacene film can be modulated by an external electric field. It is shown for the first time that a dipole of controllable direction can be set up inside a CTC layer by pre-biasing. This concept is further tested by incorporating the CTC layer in organic photovoltaic (OPV) devices. It was demonstrated that by pre-biasing the OPV devices in different directions, their open circuit voltages (Voc) can be significantly tuned via the built-in potentials.

FA-Assistant Iodide Coordination in Organic-Inorganic Wide-Bandgap Perovskite with Mixed Halides.

Mixed-halide wide-bandgap perovskites are key components for the development of high-efficiency tandem structured devices. However, mixed-halide perovskites usually suffer from phase-impurity and high defect density issues, where the causes are still unclear. By using in situ photoluminescence (PL) spectroscopy, it is found that in methylammonium (MA+ )-based mixed-halide perovskites, MAPb(I0.6 Br0.4 )3 , the halide composition of the spin-coated perovskite films is preferentially dominated by the bromide ions (Br- ). Additional thermal energy is required to initiate the insertion of iodide ions (I- ) to achieve the stoichiometric balance. Notably, by incorporating a small amount of formamidinium ions (FA+ ) in the precursor solution, it can effectively facilitate the I- coordination in the perovskite framework during the spin-coating and improve the composition homogeneity of the initial small particles. The aggregation of these homogenous small particles is found to be essential to achieve uniform and high-crystallinity perovskite film with high Br- content. As a result, high-quality MA0.9 FA0.1 Pb(I0.6 Br0.4 )3 perovskite film with a bandgap (Eg ) of 1.81 eV is achieved, along with an encouraging power-conversion-efficiency of 17.1% and open-circuit voltage (Voc ) of 1.21 V. This work also demonstrates the in situ PL can provide a direct observation of the dynamic of ion coordination during the perovskite crystallization.

Effects of Hydrogen Bonds between Polymeric Hole-Transporting Material and Organic Cation Spacer on Morphology of Quasi-Two-Dimensional Perovskite Grains and Their Performance in Light-Emitting Diodes.

Perovskite is emerging as a novel emitter in solution-processed light-emitting diodes (LEDs). In these LEDs, morphology, especially the grain size of perovskite, plays a key role in determining electroluminescence performance. Several studies have shown that sizes of the perovskite grains can be controlled by the contact angle between the perovskite solution and the substrate. In this work, we found that in the quasi-two-dimensional (2D) system, the perovskite grain size can be substantially refined when there are hydrogen bonding between the perovskite's organic spacer and the substrates. In fact, for quasi-2D perovskite, with the presence of such hydrogen bond, its effects on the perovskite grain size overshadow the contact angle's effect. We demonstrated that perovskite with refined grains can form amine- or carbazole-based polymers which can form N···H hydrogen bonding with the perovskite's organic spacer. Using these polymers as hole-transporting layers on poly(3,4-ethylenedioxythiophene):polystyrene sulfonate, external quantum efficiency of CsPbBr3-based LEDs can be enhanced from 1.5 to 10.0% without passivation treatment. This work suggests that bonding between perovskite precursors and the substrate can have significant influence on the morphology of the final perovskite grains and their optoelectronic performance.

Synergistic Effect of Pseudo-Halide Thiocyanate Anion and Cesium Cation on Realizing High-Performance Pinhole-Free MA-Based Wide-Band Gap Perovskites.

The performance of wide-band gap perovskite solar cells has a profound impact on the multijunction tandem device efficiency. However, once bromide (Br-) has been adopted to substitute the iodide (I-) in the MAPbI3 framework, it becomes very challenging to achieve uniform and high crystalline perovskite films. Here, a synergistic effect of pseudo-halide anion thiocyanate (SCN-) and inorganic cation cesium (Cs+) on the crystallization and film formation of MA-based wide-band gap perovskite is reported. It is found that the intrinsic ability of SCN- for increasing the perovskite crystal size can make the crystallization process more tolerable to the different affinity of the initial inhomogeneous small particles. However, the introduction of SCN- usually comes along with undesired large PbI2 aggregates. By further incorporating Cs+ in the precursor solution to improve the solubility of the halide/pseudo-halide coordination to Pb2+, the formation of the aggregated PbI2 particles is successfully inhibited. As a result, uniform pinhole-free MA0.9Cs0.1PbI2Br(SCN)0.08 perovskites with a wide band gap of 1.77 eV can be achieved. The corresponding photovoltaic device exhibits a record-high fill-factor over 80% and a promising power conversion efficiency of 16.3%.

Porous and Intercrossed PbI2-CsI Nanorod Scaffold for Inverted Planar FA-Cs Mixed-Cation Perovskite Solar Cells.

Depth-dependent growth of perovskite crystals remains challenging for high-performance perovskite solar cells made by a two-step spin-coating method. Effective morphology engineering approaches that enable depth-independent perovskite crystals growth and facile characterization technique to monitor subtle yet influential accompanying changes are urgently required. Here, a porous and intercrossed PbI2-(CsI)0.15 nanorods scaffold is prepared by integrating CsI incorporation with toluene dripping in ambient air, and the underlying mechanism is uncovered. With this porous scaffold and moisture-assisted thermal annealing, depth-independent growth of FA0.85Cs0.15PbI3 is achieved, as evidenced in the photoluminescent (PL) spectra acquired by exciting the perovskite film from the top and bottom individually. It is of broad interest that PL spectroscopy is demonstrated as a sensitive technique to monitor the depth-dependent growth of perovskite. Moreover, the resulting inverted planar FA0.85Cs0.15PbI3 perovskite solar cells deliver an efficiency of 16.85%, along with superior thermal and photostability. By incorporating 2% large-sized diammonium cation, propane-1,3-diammonium, the efficiency is further increased to 17.74%. Our work not only proposes a unique porous PbI2-(CsI)0.15 nanorods scaffold to achieve high-quality perovskite films in a two-step method but also highlights the distinctive advantage of PL spectroscopy in monitoring the depth-dependent quality of perovskite films.

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

Methylammonium (CH3 NH3+ ) and formamidinium ((NH2 )2 CH+ ) 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 )0.5 PbI3 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 )0.5 PbI3 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 CH3 NH3 PbI3 perovskite.

Stabilization of organometallic halide perovskite nanocrystals in aqueous solutions and their applications in copper ion detection.

Stabilization of two-dimensional (2D) PEA2PbI4 (PEA is phenethylammonium) perovskite nanocrystals (PNCs) in water is achieved. By inhibiting the desorption process, the PNCs show exceptional stability for more than 2 months in PEA+ aqueous solutions. Stabilized PNCs are successfully applied for probing Cu2+ in aqueous solution.

Magnetic-field-induced dielectric behaviors and magneto-electrical coupling of multiferroic compounds containing cobalt ferrite/barium calcium titanate composite fibers.

Multiferroics have broad application prospects in various fields such as multi-layer ceramic capacitors and multifunctional devices owing to their high dielectric constants and coupled magnetic and ferroelectric properties at room temperature. In this study, cobalt ferrite (CFO)/barium calcium titanate (BCT) composite fibers are prepared from BCT and CFO sols by an electrospinning method, and are then oriented by magnetic fields and sintered at high temperatures. The effects of magnetic fields and CFO contents on the nanostructures and magnetoelectric properties of the composites are investigated. Strong coupling between magnetic and ferroelectric properties occurs in CFO/BCT composites with magnetic orientation. More interestingly, the dielectric constants of CFO/BCT composites with magnetic orientation are found to be enhanced (by ∼1.5-3.5 times) as compared with those of BCT and CFO/BCT without magnetic orientation. The boost of dielectric constants of magnetic-field orientated CFO/BCT is attributed to the magneto-electrical coupling between CFO and BCT, where the polar domains of BCT are pinned by the orientated CFO. Therefore, this work not only provides a novel and effective approach in enhancing the dielectric constants of ceramic ferroelectrics, which is of tremendous value for industrial applications, but also elucidates the interaction mechanisms between ferromagnetic phase and ferroelectric phase in multiferroic compounds.

2D Perovskites with Short Interlayer Distance for High-Performance Solar Cell Application.

2D perovskites have emerged as one of the most promising photovoltaic materials owing to their excellent stability compared with their 3D counterparts. However, in typical 2D perovskites, the highly conductive inorganic layers are isolated by large organic cations leading to quantum confinement and thus inferior electrical conductivity across layers. To address this issue, the large organic cations are replaced with small propane-1,3-diammonium (PDA) cations to reduce distance between the inorganic perovskite layers. As shown by optical characterizations, quantum confinement is no longer dominating in the PDA-based 2D perovskites. This leads to considerable enhancement of charge transport as confirmed with electrochemical impedance spectroscopy, time-resolved photoluminescence, and mobility measurements. The improved electric properties of the interlayer-engineered 2D perovskites yield a power conversion efficiency of 13.0%. Furthermore, environmental stabilities of the PDA-based 2D perovskites are improved. PDA-based 2D perovskite solar cells (PSCs) with encapsulation can retain over 90% of their efficiency upon storage for over 1000 h, and PSCs without encapsulation can maintain their initial efficiency at 70 °C for over 100 h, which exhibit promising stabilities. These results reveal excellent optoelectronic properties and intrinsic stabilities of the layered perovskites with reduced interlayer distance.

Heat Treatment for Regenerating Degraded Low-Dimensional Perovskite Solar Cells.

Organolead halide perovskite devices are reported to be susceptible to thermal degradation, which results from heat-induced fast ion diffusion and structural decomposition. In this work, it is found that the performances of degraded low-dimensional perovskite solar cells can be considerably improved (e.g., power conversion efficiency shows ∼10% increase over the fresh device) by a short-time heat treatment (85 °C, 3 min). Capacitance-frequency, X-ray diffraction, and ionic diffusion calculation results suggest that heat treatment can enhance the crystallinity of the degraded low-dimensional perovskite and minimize the detrimental effects caused by water molecules, leading to improved performances. Our results indicate that the heat treatment does not necessarily lead to the accelerated degradation but can also regenerate the degraded low-dimensional perovskite.

Effects of Small Polar Molecules (MA+ and H2O) on Degradation Processes of Perovskite Solar Cells.

Degradation mechanisms of methylammonium lead halide perovskite solar cells (PSCs) have drawn much attention recently. Herein, the bulk and surface degradation processes of the perovskite were differentiated for the first time by employing combinational studies using electrochemical impedance spectroscopy (EIS), capacitance frequency (CF), and X-ray diffraction (XRD) studies with particular attention on the roles of small polar molecules (MA+ and H2O). CF study shows that short-circuit current density of the PSCs is increased by H2O at the beginning of the degradation process coupled with an increased surface capacitance. On the basis of EIS and XRD analysis, we show that the bulk degradation of PSCs involves a lattice expansion process, which facilitates MA+ ion diffusion by creating more efficient channels. These results provide a better understanding of the roles of small polar molecules on degradation processes in the bulk and on the surface of the perovskite film.

Clusterin silencing sensitizes pancreatic cancer MIA-PaCa-2 cells to gmcitabine via regulation of NF-kB/Bcl-2 signaling.

Clusterin (CLU) is known as a multifunctional protein involved in a variety of physiological processes including lipid transport, epithelial cell differentiation, tumorigenesis, and apoptosis. Our recent study has demonstrated that knockdown of clusterin sensitizes pancreatic cancer cell lines to gmcitabine treatment. However the details of this survival mechanism remain undefined. Of the various downstream targets of CLU, we examined activation of the NF-kB transcription factor and subsequent transcriptional regulation of BCL-2 gene in pancreatic cancer cell MIA-PaCa-2. The MIA-PaCa-2 cells were transfected with an antisense oligonucleotide (ASO) against clusterin, which led to a decreased protein level of the antiapoptotic gene BCL-2. Furthermore, inhibition of CLU decreased the function of NF-kB, which is capable of transcriptional regulation of the BCL-2 gene. Inhibiting this pathway increased the apoptotic effect of gmcitabine chemotherapy. Re-activated NF-kB resulted in attenuation of ASO-induced effects, followed by the bcl-2 upregulation, and bcl-2 re-inhibition resulted in attenuation of Re-activated NF-kB -induced effects. Animals injected with ASO CLU in MIA-PaCa-2 cells combined with gmcitabine treatment had fewer tumors than gmcitabine or ASO CLU alone. These findings suggest that knockdown of CLU sensitized MIA-PaCa-2 cells to gmcitabine chemotherapy through modulating NF-Kb/bcl-2 pathway.

Protective effect of hydrogen sulfide on hyperbaric hyperoxia-induced lung injury in a rat model.

Hyperbaric oxygen therapy is one of the most widely used clinical interventions to counteract insufficient pulmonary oxygen delivery in patients with severe lung injury. However, prolonged exposure to hyperoxia leads to inflammation and acute lung injury. This study aimed to investigate the protective effect of hydrogen sulfide on hyperbaric hyperoxia-induced lung injury. Rats were intraperitoneally treated with sodium hydrosulphide (NaHS) at 28 µmol/kg immediately before hyperoxia exposure and then exposed to pure oxygen at 2.5 atmospheres absolute (atm abs) with continuous ventilation for six hours, Immediately after hyperoxia exposure, rats were sacrificed via anesthesia. The bronchoalveolar lavage fluid (BALF) was harvested for the detection of protein concentration and IL-1 content, and the lungs were collected for HE staining, TUNEL staining and detection of wet/dry weight ratio. Our results showed hyperbaric hyperoixa exposure could significantly damage the lung (HE staining), increase the protein and IL-13 in the BALF, elevate the wet/dry Weight ratio and raise the TUNEL positive cells. However, pre-treatment with hydrogen sulfide improved the lung morphology, reduced the TUNEL positive cells and attenuated the lung inflammation (reduction in IL-13 of BALF and HE staining). Taken together, our findings indicate that hydrogen sulfide pretreatment may exert protective effects on hyperbaric hyperoxia-induced lung injury.