Molecular Insights of Non-fused Ring Acceptors for High-Performance Non-fullerene Organic Solar Cells.

Non-fullerene acceptors with fused-ring structures have rapidly improved the performance of organic solar cells over the past five years, but they still suffer from synthetic complexity and thus high material costs, one of the major obstacles of hindering their commercialization process. The construction of non-fused ring acceptors (NFRAs) has recently been regarded as a feasible solution due to their facile synthesis and satisfactory device performances. Thus in this concept, we highlight the important progress of NFRAs in recent years, and discuss the key relationship between molecular design strategies and device performance. Finally, we provide some potential molecular insights for the future design of high-performance NFRAs.

A Simple Dithieno[3,2-b:2',3'-d]pyrrol-Rhodanine Molecular Third Component Enables Over 16.7% Efficiency and Stable Organic Solar Cells.

Organic solar cells (OSCs) can achieve greatly improved power conversion efficiency (PCE) by incorporating suitable additives in active layers. Their structure design often faces the challenge of operation generality for more binary blends. Herein, a simple dithieno[3,2-b:2',3'-d]pyrrole-rhodanine molecule (DR8) featuring high compatibility with polymer donor PM6 is developed as a cost-effective third component. By employing classic ITIC-like ITC6-4Cl and Y6 as model nonfullerene acceptors (NFAs) in PM6-based binary blends, DR8 added PM6:ITC6-4Cl blends exhibit significantly promoted energy transfer and exciton dissociation. The PM6:ITC6-4Cl:DR8 (1:1:0.1, weight ratio) OSCs contribute an exciting PCE of 14.94% in comparison to host binary devices (13.52%), while PM6:Y6:DR8 (1:1.2:0.1) blends enable 16.73% PCE with all simultaneously improved photovoltaic parameters. To the best of the knowledge, this performance is among the best for ternary OSCs with simple small molecular third components in the literature. More importantly, DR8-added ternary OSCs exhibit much improved device stability against thermal aging and light soaking over binary ones. This work provides new insight on the design of efficient third components for OSCs.

Unfused Nonfullerene Acceptors Based on Simple Dipolar Merocyanines.

Merocyanine (MC) dyes exhibit facile synthesis and attractive optical properties, making them widely studied as the donor materials in organic solar cells (OSCs). In this study, for the first time, simple indole-based MCs are successfully designed as unfused nonfullerene acceptors (NFAs) for OSCs by forming dimers with A-D-π-D-A structure, which possess enhanced photostability compared to the well-known ITIC acceptor and high electron mobility in blend films. When blended with P3HT donor, one of the dimers, i. e. Z2, shows a good cell efficiency of 3.53 %, which outperforms the performance of most of P3HT/NFA blends, particularly for unfused systems, and thus indicates good potential of simple MCs as NFAs.

Selenium-substituted polymers for improved photovoltaic performance.

Four isostructural donor-acceptor alternating polymers of benzodithiophene (BDT)/naphthodifuran (NDF) and benzoselenadiazole (BSe)/benzothiadiazole (BT) have been developed and evaluated for organic photovoltaics. The substitution of one-atom (Se for S) in the accepting units exerts remarkable impact on the optoelectronic properties of polymers. Extended absorption, narrowed bandgap and higher HOMO energy levels were observed for Se-containing polymers in comparison to their S-containing counterparts. Theoretical calculations confirmed the measurable effect on energy levels as found in experimental studies. Bulk-heterojuction solar cells based on the BDT-BSe copolymer and [6,6]-phenyl-C71-butyric acid methyl ester (1 : 2, w/w) blend films deliver the best PCE of 5.40%. BSe-based polymers showed enhanced photovoltaic performances than BT-based polymers. The device performance is found to be strongly dependent on the processing conditions and morphology of the active layers.

Design and photovoltaic characterization of dialkylthio benzo[1,2-b:4,5-b']dithiophene polymers with different accepting units.

Three dialkylthio benzo[1,2-b:4,5-b']dithiophene (S-BDT) based polymers have been developed using different accepting units to tune their bandgaps. The polymer:PC71BM solar cells achieved the highest power conversion efficiency (PCE) of 4.51% without any post-treatment (such as annealing and solvent additive) in conventional single-cell devices. Joint photophysical, electrical and computational studies on the polymer based solar cells revealed the considerable impact of molecular planarity on polymer design. The polymer:PC71BM devices processed with 1,8-diiodooctane for improving their morphology afforded an improved PCE value of 5.63%, with a Voc of 0.83, a Jsc of 10.24 mA cm(-2) and a FF of 66.3%.

Design and photovoltaic characterization of dithieno[3,2-b:2',3'-d]silole copolymers with positioning phenyl groups.

A two-dimensional (2D) low bandgap polymer () based on dithieno[3,2-b:2',3'-d]silole (DTS) with phenyl substitution on the bridging silicon atom and thiazolo[5,4-d]thiazole (TTz) was designed and synthesized for photovoltaic applications. The impact of conjugated side chains on the optical, electrochemical and energy levels of the polymer was studied. The phenyl substituted DTS polymer exhibited a 0.16 eV down-shifted highest occupied molecular orbital (HOMO) energy level and ca. 0.1 eV narrowed bandgap in comparison to the corresponding polymers with alkyl substitution on the silicon bridge. The influence of the blend weight ratio, the PFN layer, mixed solvent, THF exposure and polar solvent treatment and thermal annealing on the performance of :PC71BM devices was studied. : PC71BM (1 : 1, weight ratio) devices delivered the highest power conversion efficiency of 2.14% by using the PFN layer and THF annealing. Thermal annealing was found to exert a negative effect on the device performance. The morphology evolution of blend films processed with different solvents explained the difference in device performance. The results indicate that phenyl substitution is an effective way to tune the HOMO and bandgap of polymer donors for enhanced photovoltaic performance with the as-demonstrated 2D-conjugated DTS structure.

Tailoring the Buried Interface by Dipolar Halogen-Substituted Arylamine for Efficient and Stable Perovskite Solar Cells.

Improving the quality of the buried interface is decisive for achieving stable and high-efficiency perovskite solar cells. Herein, we report the interface engineering by using dipolar 2,4-difluoro-3,5-dichloroaniline (DDE) as the adhesive between titanium dioxide (TiO2) and MAPbI3. By manipulation of the anchoring groups of DDE, this molecule not only passivated defects of TiO2 but also optimized the energy level alignment. Furthermore, the perovskite film on the modified TiO2 surface showed improved crystallinity, released residual stress, and reduced trap states. Therefore, these benefits directly contribute to achieving a power conversion efficiency of up to 22.10%. The unencapsulated device retained 90% of initial power conversion efficiencies (PCE) after continuous light illumination for 1000 h and 93% of initial PCE after exposure to air with a relative humidity of 30-40% for over 3000 h. Moreover, the performance of PSCs based on FA0.15MA0.85PbI3 has also increased from 20.48 to 23.51%. Our results demonstrate the effectiveness and universality of dipolar halogen-substituted arylamine (DDE) for enhancing PSC performance.

Multifunctional Thiophene Cascading SnO2/Perovskite Interfaces for Efficient and Stable MAPbI3 Photovoltaics.

The power conversion efficiency (PCE) and stability of n-i-p perovskite solar cells (PSCs) are significantly affected by inherent defects of SnO2 and perovskite layers. In this work, we incorporate 2-bromo-3-thiophenic acid (BrThCOOH) as a multifunctional passivant to simultaneously passivate the defects of SnO2 surface and perovskite layer. BrThCOOH permeates evenly into the MAPbI3 and coordinates with Pb2+ and iodine vacancies (VI+) to reduce surface defect density and inhibit the decomposition of MAPbI3. Carboxylic acid effectively passives the oxygen vacancy on the surface of SnO2 through coordination bonds, reducing the probability of electron capture by SnO2 surface defects, thus contributing to electron transport in device. The interaction of BrThCOOH with MAPbI3 and SnO2 surfaces leads to an upward shift in energy levels, reducing energy loss during charge migration. The optimal MAPbI3 device with BrThCOOH-modified SnO2 (T-SnO2) reveals an improved PCE of 21.12%, much higher than that of the control one (19.12%). The hydrophobicity of BrThCOOH-modified MAPbI3 is also improved, which is beneficial to the durability of the device. After 100 h of storage in the environment, the generated PSCs maintain their initial PCE of 75%, demonstrating excellent long-term stability without any encapsulation.

Automatic coil selection for streak artifact reduction in radial MRI.

In radial MR imaging, streaking artifacts contaminating the entire field of view can arise from regions at the outer edges of the prescribed field of view. This can occur even when the Nyquist criterion is satisfied within the desired field of view. These artifacts become exacerbated when parts of the object lie in the superior/inferior regions of the scanner where the gradient strengths become weakened. When multiple coil arrays are used for signal reception, coils at the outer edges can be disabled before data acquisition to reduce the artifact levels. However, as the weakened gradient strengths near the edges often distort the object, causing the signal to become highly concentrated into a small region, the streaks are often not completely removed. Data from certain coils can also be excluded during reconstruction by visually inspecting the individual coil images, but this is impractical for routine use. In this work, a postprocessing method is proposed to automatically identify those coils whose images contain high levels of streaking for subsequent exclusion during reconstruction. The proposed method was demonstrated in vivo dynamic contrast enhanced MRI datasets acquired using a three-dimensional hybrid radial sequence. The results demonstrate that the proposed strategy substantially improves the image quality and show excellent agreement with images reconstructed with manually determined coil selection.

Comparison of lung T2* during free-breathing at 1.5 T and 3.0 T with ultrashort echo time imaging.

Assessment of lung effective transverse relaxation time (T(2)*) may play an important role in the detection of structural and functional changes caused by lung diseases such as emphysema and chronic bronchitis. While T(2)* measurements have been conducted in both animals and humans at 1.5 T, studies on human lung at 3.0 T have not yet been reported. In this work, ultrashort echo time imaging technique was applied for the measurement and comparison of T(2)* values in normal human lungs at 1.5 T and 3.0 T. A 2D ultrashort echo time pulse sequence was implemented and evaluated in phantom experiments, in which an eraser served as a homogeneous short T(2)* sample. For the in vivo study, five normal human subjects were imaged at both field strengths and the results compared. The average T(2)* values measured during free-breathing were 2.11(±0.27) ms at 1.5 T and 0.74(±0.1) ms at 3.0 T, respectively, resulting in a 3.0 T/1.5 T ratio of 2.9. Furthermore, comparison of the relaxation values at end-expiration and end-inspiration, accomplished through self-gating, showed that during normal breathing, differences in T(2)* between the two phases may be negligible.

An unfused-ring acceptor with high side-chain economy enabling 11.17% as-cast organic solar cells.

Side-chain engineering on nonfullerene acceptors (NFAs) is crucial for modulating their solubility and crystallinity as well as packing behaviours in active layers to pursue high-performance organic solar cells (OSCs). High weight ratios of side chains are generally used by NFAs for the desired device efficiencies. Side-chain economy has seldom been discussed despite increased cost and difficulties in synthesis when optimizing the molecular design. Herein, we introduce 7H-dibenzo[c,g]carbazole (DCB) as an electron-donating core to design unfused-ring acceptors (UFAs) with a dramatically low weight ratio of side chains. DCB-4F has thus been designed and compared with the carbazole cored analogue (CB-4F). The unique conformation of the DCB core endows DCB-4F with higher solubility (8.2 mg mL-1 in chloroform) compared to CB-4F (2.2 mg mL-1) when using the same side chains. Featuring a lowest unoccupied molecular orbital (LUMO) level of -3.86 eV and an optical bandgap of 1.55 eV, the DCB-4F film exhibits an absorption profile (maximum 667 nm) complementary to polymer donor PM6. The PM6:DCB-4F as-cast OSCs deliver a power conversion efficiency (PCE) of 9.56% with a high open-circuit voltage (VOC) of 1.00 V. By adding 10 wt% PC71BM into the casting solutions, a greatly improved PCE of 11.17% is readily achieved, which is one of the highest PCEs for as-cast single-junction UFA-based devices. The PM6:DCB-4F based blends show homogeneous nano-fiberous morphology and higher hydrophobicity. The design of conformation-tuned NFAs using sterically hindered DCB-like cores is promising to achieve highly efficient as-cast OSCs.

Improved technique for measurement of regional fractional ventilation by hyperpolarized 3He MRI.

Quantitative measurement of regional lung ventilation is of great significance in assessment of lung function in many obstructive and restrictive pulmonary diseases. A new technique for regional measurement of fractional ventilation using hyperpolarized 3He MRI is proposed, addressing the shortcomings of an earlier approach that limited its use to small animals. The new approach allows for the acquisition of similar quantitative maps over a shortened period and requires substantially less 3He gas. This technique is therefore a better platform for implementation in large species, including humans. The measurements using the two approaches were comparable to a great degree, as verified in a healthy rat lung, and are very reproducible. Preliminary validation is performed in a lung phantom system. Volume dependency of measurements was assessed both in vivo and in vitro. A scheme for selecting an optimum flip angle is proposed. In addition, a dead space modeling approach is proposed to yield more accurate measurements of regional fractional ventilation using either method. Finally, sensitivity of the new technique to model parameters, noise, and number of included images were assessed numerically. As a prelude to application in humans, the technique was implemented in a large animal study successfully.

Over 15% Efficiency in Ternary Organic Solar Cells by Enhanced Charge Transport and Reduced Energy Loss.

In this study, an efficient ternary bulk-heterojunction (BHJ) organic solar cell (OSC) is demonstrated by incorporating two acceptors, PC61BM and ITC6-4F, with a polymer donor (PM6). This reveals that the addition of PC61BM not only enhances the electron mobility of the derived BHJ blend but also facilitates exciton dissociation, resulting in a more balanced charge transport alongside with reduced trap-assisted charge recombination. Consequently, as compared to the pristine PM6/ITC6-4F device, the optimal ternary OSC is revealed to deliver an improved power conversion efficiency (PCE) of 15.11% with a boosted JSC, VOC, and fill factor (FF) simultaneously. The resultant VOC and FF are among the highest values recorded in the literature for the ternary OSCs with a PCE exceeding 15%. This result thus suggests that besides improving the charge transport characteristics in devices, incorporating a fullerene derivative as part of the acceptor can also improve the resultant VOC, which can reduce the energy loss to realize efficient organic photovoltaics.

Simultaneous measurement of pulmonary partial pressure of oxygen and apparent diffusion coefficient by hyperpolarized 3He MRI.

Hyperpolarized (3)He (HP (3)He) MRI shows promise to assess structural and functional pulmonary parameters in a sensitive, regional, and noninvasive way. Structural HP (3)He MRI has applied the apparent diffusion coefficient (ADC) for the detection of disease-induced lung microstructure changes at the alveolar level, and HP (3)He pulmonary partial pressure of oxygen (pO(2)) imaging measures the oxygen transfer efficiency between the lung and blood stream. Although both parameters are affected in chronic obstructive pulmonary disease (COPD), a quantitative assessment of the regional correlation of the two parameters has not been reported in the literature. In this work, a single acquisition technique for the simultaneous measurement of ADC and pO(2) is presented. This technique is based on the multiple regression method, in which a general linear estimator is used to retrieve the values of ADC and pO(2) from a series of measurements. The measurement uncertainties are also analytically derived and used to find an optimal measurement scheme. The technique was first tested on a phantom model, and then on an in vivo normal pig experiment. A case study was performed on a COPD patient, which showed that in a region of interest ADC was 29% higher while oxygen depletion rate was 61% lower than the corresponding global average values.

Early changes of lung function and structure in an elastase model of emphysema--a hyperpolarized 3He MRI study.

Early changes of lung function and structure were studied in the presence of an elastase-induced model of emphysema in 35 Sprague-Dawley rats at mild (5 U/100 g) and moderate (10 U/100 g) severities. Lung ventilation was measured on a regional basis (at a planar resolution of 3.2 mm) by hyperpolarized 3He MRI at 5 and 10 wk after model induction. Subsequent to imaging, average alveolar diameter was measured from histological slices taken from the centers of each lobe. Changes of mean fractional ventilation, mean linear intercept, and intrasubject heterogeneity of ventilation were studied during disease progression. Mean fractional ventilation was significantly different between healthy controls (0.23 +/- 0.04) and emphysematous animals at both time points in the 10-unit group (0.06 +/- 0.02 and 0.12 +/- 0.05, respectively). Changes in average alveolar diameter were not statistically observable until the 10th wk between healthy (37 +/- 10 microm) and emphysematous rats (73 +/- 25 and 95 +/- 31 microm, for 5 and 10 units, respectively). Assessment of function-structure correlation suggested that the majority of the decline in fractional ventilation occurred in the first 5 wk, while enlargement of alveolar diameters appeared primarily between the 5th and 10th wk. A thresholding metric, based on the 20th percentile of fractional ventilation over the entire lung, was utilized to detect the onset of the disease with confidence, independent of whether the regional ventilation measurements were normalized with respect to the delivered tidal volume and estimated functional residual capacity of each individual rat.

Measurement of pulmonary partial pressure of oxygen and oxygen depletion rate with hyperpolarized helium-3 MRI: a preliminary reproducibility study on pig model.

RATIONAL AND OBJECTIVES: Pulmonary partial pressure of oxygen (pO(2)) and oxygen depletion rate (R) are two important parameters of lung function. The dependence of hyperpolarized (3)He (HP (3)He) T(1) on local oxygen concentration provides the basis for high-resolution mapping of the regional distributions of pO(2) and R in the lung. Although the oxygen-sensitive HP (3)He magnetic resonance imaging technique has been applied in human subjects and several animal species, reproducibility studies are rarely reported in the literature. This work presents a preliminary reproducibility study on a pig model. In this study, important scan parameters, such as measurement timing and flip angle, are optimized to minimize the noise-induced measurement uncertainty. MATERIALS AND METHODS: In the in vivo study, five normal pigs and one diseased pig with simulated pulmonary emboli were scanned with a small flip angle gradient echo sequence. The pulmonary oxygen measurement was repeated two to four times in each pig. In each measurement, a series of six images were acquired with optimal timing and flip angle. The parametric maps were generated using a bin-based data processing procedure that applied the multiple regression fitting method to extract the pO(2) and R. Variations of global mean, percentiles, and regions of interest were calculated from the maps to analyze reproducibility. RESULTS: The global statistical analyses show that average variation of global mean is 10.7% for pO(2) and 23.8% for R, and that the average variation of percentiles (10th, 25th, 50th, 75th, and 90th) and interquartile range is 14.8% for pO(2) and 30.4% for R. The region-of-interest analysis on the manually selected regions shows that the average variation of mean is 12.6% for pO(2) and 21.9% for R. CONCLUSION: In this work, a preliminary study on the reproducibility of measuring pO(2) and R with HP (3)He magnetic resonance imaging on a pig model is presented.

A robust method for estimating regional pulmonary parameters in presence of noise.

RATIONALE AND OBJECTIVES: Estimation of regional lung function parameters from hyperpolarized gas magnetic resonance images can be very sensitive to presence of noise. Clustering pixels and averaging over the resulting groups is an effective method for reducing the effects of noise in these images, commonly performed by grouping proximal pixels together, thus creating large groups called "bins." This method has several drawbacks, primarily that it can group dissimilar pixels together, and it degrades spatial resolution. This study presents an improved approach to simplifying data via principal component analysis (PCA) when noise level prohibits a pixel-by-pixel treatment of data, by clustering them based on similarity to one another rather than spatial proximity. The application to this technique is demonstrated in measurements of regional lung oxygen tension using hyperpolarized (3)He magnetic resonance imaging (MRI). MATERIALS AND METHODS: A synthetic dataset was generated from an experimental set of oxygen tension measurements by treating the experimentally derived parameters as "true" values, and then solving backwards to generate "noiseless" images. Artificial noise was added to the synthetic data, and both traditional binning and PCA-based clustering were performed. For both methods, the root-mean-square (RMS) error between each pixel's "estimated" and "true" parameters was computed and the resulting effects were compared. RESULTS: At high signal-to-noise ratios (SNRs), clustering did not enhance accuracy. Clustering did, however, improve parameter estimations for moderate SNR values (below 100). For SNR values between 100 and 20, the PCA-based K-means clustering analysis yielded greater accuracy than Cartesian binning. In extreme cases (SNR<5), Cartesian binning can be more accurate. CONCLUSIONS: The reliability of parameters estimation in imaging-based regional functional measurements can be improved in the presence of noise by utilizing principal component analysis-based clustering without sacrificing spatial resolution compared to Cartesian binning. Results suggest that this approach has a great potential for robust grouping of pixels in hyperpolarized (3)He MRI maps of lung oxygen tension.

Metal-organic framework-templated synthesis of sulfur-doped core-sheath nanoarrays and nanoporous carbon for flexible all-solid-state asymmetric supercapacitors.

Metal-organic frameworks (MOFs) provide great opportunities for synthesizing advanced electrode materials with hierarchical hollow architectures for energy storage. Herein, we report the facile fabrication of core-sheath nanoarrays (NAs) on carbon cloth (CC@CoO@S-Co3O4) for binder-free electrode materials with MOFs as versatile scaffolds. The hollow S-doped Co3O4 sheath has been facilely prepared using a two-step synthetic protocol, which includes the surface etching of CoO nanowires for synchronous in situ growth of well-aligned ZIF-67 and its following hydrothermal process. The synergistic effect between CC nanofibers and hollow ordered NAs ensures efficient mass and electron transport. The pseudocapacitive NAs present a highest areal specific capacitance of 1013 mF cm-2 at 1 mA cm-2. By assembling the same MOF-derived nanoporous carbons and NAs as the corresponding binder-free anode and cathode, the flexible all-solid-state asymmetric supercapacitors deliver a highest energy density of 0.71 mW h cm-3 at 21.3 mW cm-3 power density, together with 87.9% capacitance retention over 5000 continuous cycles.

Dithieno[3,2-b:2',3'-d]pyrrol Fused Nonfullerene Acceptors Enabling Over 13% Efficiency for Organic Solar Cells.

A new electron-rich central building block, 5,5,12,12-tetrakis(4-hexylphenyl)-indacenobis-(dithieno[3,2-b:2',3'-d]pyrrol) (INP), and two derivative nonfullerene acceptors (INPIC and INPIC-4F) are designed and synthesized. The two molecules reveal broad (600-900 nm) and strong absorption due to the satisfactory electron-donating ability of INP. Compared with its counterpart INPIC, fluorinated nonfullerene acceptor INPIC-4F exhibits a stronger near-infrared absorption with a narrower optical bandgap of 1.39 eV, an improved crystallinity with higher electron mobility, and down-shifted highest occupied molecular orbital and lowest unoccupied molecular orbital energy levels. Organic solar cells (OSCs) based on INPIC-4F exhibit a high power conversion efficiency (PCE) of 13.13% and a relatively low energy loss of 0.54 eV, which is among the highest efficiencies reported for binary OSCs in the literature. The results demonstrate the great potential of the new INP as an electron-donating building block for constructing high-performance nonfullerene acceptors for OSCs.

Advances in magnetic resonance imaging of lung physiology.

This review presents an overview of some recent magnetic resonance imaging (MRI) techniques for measuring aspects of local physiology in the lung. MRI is noninvasive, relatively high resolution, and does not expose subjects to ionizing radiation. Conventional MRI of the lung suffers from low signal intensity caused by the low proton density and the large degree of microscopic field inhomogeneity that degrades the magnetic resonance signal and interferes with image acquisition. However, in recent years, there have been rapid advances in both hardware and software design, allowing these difficulties to be minimized. This review focuses on some newer techniques that measure regional perfusion, ventilation, gas diffusion, ventilation-to-perfusion ratio, partial pressure of oxygen, and lung water. These techniques include contrast-enhanced and arterial spin-labeling techniques for measuring perfusion, hyperpolarized gas techniques for measuring regional ventilation, and apparent diffusion coefficient and multiecho and gradient echo techniques for measuring proton density and lung water. Some of the major advantages and disadvantages of each technique are discussed. In addition, some of the physiological issues associated with making measurements are discussed, along with strategies for understanding large and complex data sets.