No abstract

Organic semiconductors have garnered substantial interest in optoelectronics, but their device performances exhibit strong dependencies on material crystallinity and packing. In an effort to understand the interactions dictating the morphological and photophysical properties of a high-performing photovoltaic polymer, PTB7, a series of short oligomers and low molecular weight polymers of PTB7 were synthesized. Chain-length dependent optical studies of these oligomers demonstrate that PTB7's low-energy visible absorption is largely due to self-aggregation-induced ordering, rather than inchain charge transfer, as previously thought. By examining molecular weight and concentration dependent optical properties, supplemented by molecular dynamics simulations, we attribute polymeric PTB7's unique midgap fluorescence and concentration independent absorption spectrum to an interplay between low molecular weight unaggregated strands and high-molecular weight self-aggregated (folded) strands. Specifically, we propose that the onset of PTB7 self-folding occurs between 7 and 13 repeat units, but the aggregates characteristic of polymeric PTB7 only develop at lengths of ∼30 repeat units. Atomistic molecular dynamics simulations of PTB7 corroborate these conclusions, and a simple relation is proposed which quantifies the free-energy of conjugated polymer folding. This study provides detailed guidance in the design of intra-and interchain contributions to the photophysical and morphological properties of polymeric semiconductors.

Labeled Faces in the Wild (LFW) database has been widely utilized as the benchmark of unconstrained face verification and due to big data driven machine learning methods, the performance on the database approaches nearly 100%. However, we argue that this accuracy may be too optimistic because of some limiting factors. Besides different poses, illuminations, occlusions and expressions, crossage face is another challenge in face recognition. Different ages of the same person result in large intra-class variations and aging process is unavoidable in real world face verification. However, LFW does not pay much attention on it. Thereby we construct a Cross-Age LFW (CALFW) which deliberately searches and selects 3,000 positive face pairs with age gaps to add aging process intra-class variance. Negative pairs with same gender and race are also selected to reduce the influence of attribute difference between positive/negative pairs and achieve face verification instead of attributes classification. We evaluate several metric learning and deep learning methods on the new database. Compared to the accuracy on LFW, the accuracy drops about 10%-17% on CALFW.

Synthesis and Photovoltaic Effect in Dithieno[2,3-d :2 ′ ,3 ′ -d ′ ] Benzo[1,2-b :4,5-b ′ ]dithiophene-Based Conjugated PolymersThe recent surge of enthusiasm in bulk-heterojunction (BHJ) organic photovoltaics (OPVs) is driven by their potential for fabricating fl exible and light-weight solar cells via facile, lowcost solution processing techniques. [ 1 ] New materials are crucial in order for OPVs to mature fully from research and development into cost effective products. The power conversion effi ciency (PCE) of large-area OPV solar cells is still inferior to the corresponding inorganic devices and should be continuously improved through major advances in new materials and enhancing our understanding of structure-property relationships. [2][3][4][5][6] Our recent work [ 7 ] showed that when the bandgap, the energy levels, and the morphology were all optimized, a large local dipole-moment change between the ground and excited states, Δ μ eg , played a crucial role in the performance of OPV BHJ cells. To our surprise, the initial relative population ratio of the charge-separated state that leads to free charge carriers and the charge-transfer state that is a strongly bound hole and electron pair in isolated polymer chains is linearly proportional to the power conversion effi ciency (PCE) of the device, which is related to the Δ μ eg values. [ 8 ] In a BHJ fi lm, as shown in Figure S1 in the Supporting Information, upon photoexcitation, a large Δ μ eg ( D ) in the polymer leads to a large displacement in the electron-density distribution in the lowest occupied molecular orbital (LUMO) away from that in the highest occupied molecular orbital (HOMO). Consequently, a polarized exciton is produced with the positive charge concentrated on the benzo[1,2b :4,5b ′ ]dithiophene (BDT) moiety (red + in Figure S1) and the negative charge on the electron-defi cient ester group on thieno[3,4b ]thio phene (TT) moiety from which the electron density will transfer to PCBM (curved red arrow in Figure S1). The intrinsic local electron-affi nity gradient due to the displacement and the distance between the negative and positive charge densities reduces Coulombic interactions between the hole and electron inside the initial charge-transfer complex, making further charge separation/charge-carrier transport easier and resulting in an enhancement of the solar-cell effi ciency. Although exciton delocalization can extend beyond a repeating unit, as seen from a series of calculations, [ 9 ] correlations between the local dipole change of the monomer with the device PCE have been observed in several copolymers with charge-transfer character. [ 8,9 ] Theoretical bases of this phenomenological correlation are being investigated. The observed linear correlation between the local dipole change and the PCE suggests two ways to enhance the device PCE in developing new donor polymers: 1) increasing the local dipole moment further to produce even more polar excitons, which will further reduce the exciton binding energy and the probability of formin...

Polysulfide shuttling has been the primary cause of failure in lithium-sulfur (Li-S) battery cycling. Here, we demonstrate an uncleophilic substitution reaction between polysulfides and binder functional groups can unexpectedly immobilizes the polysulfides. The substitution reaction is verified by UV-visible spectra and X-ray photoelectron spectra. The immobilization of polysulfide is in situ monitored by synchrotron based sulfur K-edge X-ray absorption spectra. The resulting electrodes exhibite initial capacity up to 20.4 mAh/cm 2 , corresponding to 1199.1 mAh/g based on a micron-sulfur mass loading of 17.0 mg/cm 2 . The micron size sulfur transformed into nano layer coating on the cathode binder during cycling.Directly usage of nano-size sulfur promotes higher capacity of 33.7 mAh/cm 2 , which is the highest areal capacity reported in Li-S battery. This enhance performance is due to the reduced shuttle effect by covalently binding of the polysulfide with the polymer binder.2

Perylene diimide (PDI) derivatives functionalized at the ortho-position (αPPID, αPBDT) were synthesized and used as electron acceptors in non-fullerene organic photovoltaic cells. Because of the good planarity and strong πstacking of ortho-functionalized PDI, the αPPID and αPBDT exhibit a strong tendency to form aggregates, which endow the materials with high electron mobility. The inverted OPVs employing αPDI-based compounds as the acceptors and PBT7-Th as the donor give the highest power conversion efficiency (PCE) values: 4.92% for αPBDT-based devices and 3.61% for αPPID-based devices, which are, respectively, 39% and 4% higher than that of their β-substituted counterparts βPBDT and βPPID. Charge separation studies show more efficient exciton dissociation at interfaces between αPDI-based compounds and PTB7-Th. The results suggest that α-substituted PDI derivatives are more promising electron acceptors for organic photovoltaic (OPV) components than β-isomers.

Bulk heterojunction (BHJ) polymer solar cells (PSCs) are a popular research subject currently pursued by many groups around the world. The state-of-the-art PSCs are composed of a polymer donor and a fullerene acceptor as the active layer, and their overall photovoltaic performance is dependent on many factors, such as the electrical and optical properties of donor polymers, device architectures, and interfacial layers used. Among them, the nature of donor polymer is without doubt one of the determining factors in performance of PSCs. In this work, we report for the first time the study of the influence of polymer dispersity (Đ) on the performance of PSCs composed of PTB7 and PC 71 BM as the active layer materials. It was found that polymers exhibiting large Đ contained structural defects that played the role of energy transfer and charge trapping/recombination centers. The power conversion efficiency of PTB7 devices decreased from 7.59% to 2.55% with increased Đ. The results highlighted the importance of controlling Đ of donor polymers for PSCs.

A series of ladder-type thienoacenes based on benzo[1,2-b:4,5-b']dithiophene (BDT) have been synthesized and characterized. They were shown to be p-type semiconductors with wide band gaps and able to support multiple stable cationic states. As the conjugation lengthens, these oligomers become more emissive, showing high quantum yields. They were shown to be good two-photon absorbers, exhibiting high two-photon absorption coefficients.