Unraveling the Influence of Solvent on Side Reactions between Formamidinium Lead Triiodide and Methylammonium Cations.

Formamidinium lead triiodide (FAPbI3) perovskite thin films are commonly deposited through a solution process, often incorporating a specific amount of methylammonium halide to stabilize the α-phase or enhance their crystallinity. The precursor solution for such coatings significantly influences the fabrication of perovskite solar cells (PSCs), involving time-dependent aging and byproduct formation. The chemical principle underlying this behavior is believed to be related to the deprotonation of methylamine cations (MA+) and subsequent chemical reactions with FA+ to generate N-methylformamidinium. Nevertheless, the role of the solvent in the side reactions between these organic cations remains unclear. This work systematically investigates the reaction reactivity in three protic solvents and three aprotic solvents. We uncover the hidden role of dimethylamine from the hydrolysis products of N,N-dimethylformamide, promoting the reaction between FA+ and MA+. Additionally, we elucidate the impact of environmental factors, such as water and oxygen, in stabilizing precursor solutions. This work establishes a basic concept and scientific direction for rationalizing high-efficiency, reproducible, and long-term-stable PSCs.

Flexible Thin-Film Speaker Integrated with an Array of Quantum-Dot Light-Emitting Diodes for the Interactive Audiovisual Display of Multi-functional Sensor Signals.

One of the core technologies for wearable electronics is the use of an interactive display device that is attached to the body or clothes to transmit various bio-signals and environmental stimuli to the user. In this study, we report a flexible audiovisual display device consisting of a polyvinylidene difluoride (PVDF) thin-film speaker stacked on an 8 × 8 array of quantum-dot light-emitting diodes (QD-LEDs) and a multi-functional sensor consisting of temperature and ultraviolet (UV) sensors connected to a pressure sensor, allowing the body temperature and UV exposure to be displayed both visually and acoustically. Polydimethylsiloxane is employed as an insulator between the carbon nanotube (CNT)/polyaniline temperature sensor and the ZnO/CNT UV sensor to form a capacitor-type pressure sensor. With the use of a stretchable polymer substrate, liquid metal Galinstan interconnections, and the flexible Au-grid electrodes, both the PVDF speaker and the QD-LED array are stable under repeated cycles of bending deformation with a bending radius of 7.5 mm. By connecting the audiovisual display device to the skin-attached multi-functional sensor, changes in the body temperature and UV exposure are displayed as LED patterns with accompanying acoustic alarms. This study demonstrates the significant potential of our proposed audiovisual monitoring device and multi-functional sensor for use in health-monitoring applications, especially for the elderly and infants requiring prompt care.

Origins of High Performance and Degradation in the Mixed Perovskite Solar Cells.

The origins of the high device performance and degradation in the air are the greatest issues for commercialization of perovskite solar cells. Here this study investigates the possible origins of the mixed perovskite cells by monitoring defect states and compositional changes of the perovskite layer over the time. The results of deep-level transient spectroscopy analysis reveal that a newly identified defect formed by Br atoms exists at deep levels of the mixed perovskite film, and its defect state shifts when the film is aged in the air. The change of the defect state is originated from loss of the methylammonium molecules of the perovskite layer, which results in decreased JSC , deterioration of the power conversion efficiency and long-term stability of perovskite solar cells. The results provide a powerful strategy to diagnose and manage the efficiency and stability of perovskite solar cells.

Efficient Planar Perovskite Solar Cells Using Passivated Tin Oxide as an Electron Transport Layer.

Planar perovskite solar cells using low-temperature atomic layer deposition (ALD) of the SnO2 electron transporting layer (ETL), with excellent electron extraction and hole-blocking ability, offer significant advantages compared with high-temperature deposition methods. The optical, chemical, and electrical properties of the ALD SnO2 layer and its influence on the device performance are investigated. It is found that surface passivation of SnO2 is essential to reduce charge recombination at the perovskite and ETL interface and show that the fabricated planar perovskite solar cells exhibit high reproducibility, stability, and power conversion efficiency of 20%.

Dopant-Free Hole-Transporting Materials for Stable and Efficient Perovskite Solar Cells.

Molecularly engineered novel dopant-free hole-transporting materials for perovskite solar cells (PSCs) combined with mixed-perovskite (FAPbI3 )0.85 (MAPbBr3 )0.15 (MA: CH3 NH3+ , FA: NH=CHNH3+ ) that exhibit an excellent power conversion efficiency of 18.9% under AM 1.5 conditions are investigated. The mobilities of FA-CN, and TPA-CN are determined to be 1.2 × 10-4 cm2 V-1 s-1 and 1.1 × 10-4 cm2 V-1 s-1 , respectively. Exceptional stability up to 500 h is measured with the PSC based on FA-CN. Additionally, it is found that the maximum power output collected after 1300 h remained 65% of its initial value. This opens up new avenue for efficient and stable PSCs exploring new materials as alternatives to Spiro-OMeTAD.

A Strategy to Produce High Efficiency, High Stability Perovskite Solar Cells Using Functionalized Ionic Liquid-Dopants.

Functionalized imidazolium iodide salts (ionic liquids) modified with CH2 CHCH2 , CH2 CCH, or CH2 CN groups are applied as dopants in the synthesis of CH3 NH3 PbI3 -type perovskites together with a fumigation step. Notably, a solar cell device prepared from the perovskite film doped with the salt containing the CH2 CHCH2 side-chain has a power conversion efficiency of 19.21%, which is the highest efficiency reported for perovskite solar cells involving a fumigation step. However, doping with the imidazolium salts with the CH2 CCH and CH2 CN groups result in perovskite layers that lead to solar cell devices with similar or lower power conversion efficiencies than the dopant-free cell.

Femtosecond Charge-Injection Dynamics at Hybrid Perovskite Interfaces.

With a power conversion efficiency (PCE) exceeding 22 %, perovskite solar cells (PSCs) have thrilled photovoltaic research. However, the interface behavior is still not understood and is a hot topic of research: different processes occur over a hierarchy of timescales, from femtoseconds to seconds, which makes perovskite interface physics intriguing. Herein, through femtosecond transient absorption spectroscopy with spectral coverage extending into the crucial IR region, the ultrafast interface-specific processes at standard and newly molecularly engineered perovskite interfaces in state-of-the-art PSCs are interrogated. Ultrafast interfacial charge injection occurs with a time constant of 100 fs, resulting in hot transfer from energetic charges and setting the timescale for the first step involved in the complex charge-transfer process. This is also true for 20 % efficient devices measured under real operation, for which the femtosecond injection is followed by a slower picosecond component. These findings provide compelling evidence for the femtosecond interfacial charge-injection step and demonstrate a robust method for the straightforward identification of interfacial non-equilibrium processes on the ultrafast timescale.

Intrinsic Halide Segregation at Nanometer Scale Determines the High Efficiency of Mixed Cation/Mixed Halide Perovskite Solar Cells.

Compositional engineering of a mixed cation/mixed halide perovskite in the form of (FAPbI3)0.85(MAPbBr3)0.15 is one of the most effective strategies to obtain record-efficiency perovskite solar cells. However, the perovskite self-organization upon crystallization and the final elemental distribution, which are paramount for device optimization, are still poorly understood. Here we map the nanoscale charge carrier and elemental distribution of mixed perovskite films yielding 20% efficient devices. Combining a novel in-house-developed high-resolution helium ion microscope coupled with a secondary ion mass spectrometer (HIM-SIMS) with Kelvin probe force microscopy (KPFM), we demonstrate that part of the mixed perovskite film intrinsically segregates into iodide-rich perovskite nanodomains on a length scale of up to a few hundred nanometers. Thus, the homogeneity of the film is disrupted, leading to a variation in the optical properties at the micrometer scale. Our results provide unprecedented understanding of the nanoscale perovskite composition.

PbI2-HMPA Complex Pretreatment for Highly Reproducible and Efficient CH3NH3PbI3 Perovskite Solar Cells.

Interfacial engineering of the meso-TiO2 surface through a modified sequential deposition procedure involving a novel PbI2-HMPA complex pretreatment is conducted as a reproducible method for preparing MAPbI3 based perovskite solar cells providing the highest efficiencies yet reported with the polymer HTM layer. Grazing-incidence X-ray diffraction depth profiling confirms the formation of a perovskite film with a PbI2-rich region close to the electron transport layer (ETL) due to the strong interaction of HMPA with PbI2, which successfully retarded the dissolution of the PbI2 phase when depositing the perovskite layer on top. These results are further confirmed by energy-dispersive X-ray spectroscopy performed in a scanning transmission electron microscope, which reveals that the I/Pb ratio in samples treated with the complex is indeed reduced in the vicinity of the ETL contact when compared to samples without the treatment. The engineered interface leads to an average power conversion efficiency of 19.2% (reverse scan, standard deviation SD < 0.2) over 30 cells (best cell at 19.5% with high FF of 0.80).

Beneficial Role of Reduced Graphene Oxide for Electron Extraction in Highly Efficient Perovskite Solar Cells.

In this work we systematically investigated the role of reduced graphene oxide (rGO) in hybrid perovskite solar cells (PSCs). By mixing rGO within the mesoporous TiO2 (m-TiO2 ) matrix, highly efficient solar cells with power conversion efficiency values up to 19.54 % were realized. In addition, the boosted beneficial role of rGO with and without Li-treated m-TiO2 is highlighted, improving transport and injection of photoexcited electrons. This combined system may pave the way for further development and optimization of electron transport and collection in high efficiency PSCs.

Correlation of common carotid artery intima media thickness, intracranial arterial stenosis and post-stroke cognitive impairment.

BACKGROUND AND PURPOSE: Atherosclerosis of the intracranial arteries is a well-recognized cause of ischemic stroke in Asians, and extracranial carotid artery disease is more often seen in western countries. The relationship of common carotid artery intima-media thickness (CCA-IMT), intracranial arteries stenosis (ICS) and vascular cognitive impairment (VCI) after ischemic stroke has not been fully elucidated. In this study, we investigated the relationship between CCA-IMT and the severity of ICS and VCI. METHODS: We recruited patients from December 2004, to June 2005, with the inclusion criteria: (1) first-ever ischemic stroke, (2) admission within 3 days of stroke onset, (3) under 80 years old, and (4) no previous dementia history. We excluded patients with stroke scores greater than an NIHSS of 15; those with recurrent stroke, and those with extracranial internal carotid artery stenosis > 50%. All the patients underwent brain MR angiography, carotid ultrasonography and neuropsychological testing during hospitalization and at 3 months after stroke. We defined the percent of ICS using the method of Warfarin-Aspirin Symptomatic Intracranial Disease. Measurement of CCA-IMT was made on the far wall of the common carotid artery, 1.5 cm proximal to the bifurcation at a point free of plaques. Cognitive performance was assessed using the Cognition Assessment State Instrument (CASI). RESULTS: Thirty patients (21M/9F, mean age 65.97 +/- 10.33 years) were studied. The initial CCA-IMT was 1.04 +/- 0.59 mm and the initial CASI was 64.73 +/- 14.75. The ICS was 70 +/- 26%. At 3 months after stroke, the CCA-IMT was 1.06 +/- 0.59 mm; and CASI was 70.07 +/- 18.50. Compared with patients with CCA-IMT > 0.87 mm, those with CCA-IMT < or = 0.87 mm had lower ICS (57 +/- 23% vs. 81 +/- 24%, p = 0.013), but similar initial CASI score (67.92 +/- 13.52 vs. 61.93 +/- 16.64, p = 0.28). The improvement of CASI score at 3 months was significantly higher in patients with CCA-IMT < or = 0.87 mm (67.92 +/- 13.52 vs. 77.36 +/- 14.12, p = 0.001), than those with CCA-IMT > 0.87 mm (61.93 +/- 16.64 vs. 63.69 +/- 19.89, p = 0.612). CONCLUSIONS: CCA-IMT might be associated with the severity of ICS and VCI at 3 months after the first-ever ischemic stroke. The patients with lower CCA-IMT had a better CASI evaluation at 3 months after stroke. A larger scale of study to explore the association of CCA-IMT, VCI and ICS at 3 months after stroke might help farther delineation of these relationships.