comprised of a conjugated polymer as the electron donor and a fullerene derivative as the electron acceptor. [2,3] Conventional fullerene acceptors like [6,6]-phenyl-C61 butyric acid methyl ester (PCBM) may be unfavorable for practical application, due to the inefficient absorption in visible and near-infrared (IR) regions, fixed molecular structure, complex purification process, and other complications well-documented in the literature. [4][5][6][7][8] In addition, the morphology of polymer-fullerene blends is very sensitive to thermal annealing, solvent additive, film thickness, and especially D:A ratio, resulting in significantly different device performance. [9][10][11] Recently, people have shown that the integration of either polymeric or molecular acceptor into photovoltaic devices would be advantageous. The chemical structures of nonfullerene acceptor can be easily adjusted to tune their energy levels, and the conjugated molecule or polymer acceptor demonstrates enhanced absorption at long wavelengths relative to fullerenes. [12] Thus nonfullerene photovoltaics can have improved harvest of solar radiation, enhanced thermal and mechanical stability, and reduced open-circuit voltage loss. [13] To date, solution-processed nonfullerene solar cells based on polymer-polymer (all-polymer) and polymer-small molecule blend have achieved power conversion efficiencies (PCEs) of 10% and 13%, respectively. [14][15][16][17][18][19] Exciton dissociation and charge transport are at the core of organic photovoltaics, which is strongly affected by the BHJ blend morphology. [20][21][22][23] In fullerene-based systems, the morphology of thin films is critical to the device performance and has been extensively studied. [24] How the blend morphology of nonfullerene blend affects device efficiency and stability is of growing interest as the PCEs of many nonfullerene solar cells now exceed the best fullerene devices. It is well known that the morphology has been largely influenced by the D:A blend ratio. Early studies on fullerene-based devices observed a drastic change in film morphology when fabricating the devices with increased acceptor content. [25][26][27][28] However, for nonfullerene solar cells, recent efforts mainly focus on materials synthesis and device optimization. To the best of our knowledge, systematical study on the effect of blend ratio on the device morphology, performance, and stability has not been reported, while it is of great importance to help achieve in-depth Tuning the blend composition is an essential step to optimize the power conversion efficiency (PCE) of organic bulk heterojunction (BHJ) solar cells. PCEs from devices of unoptimized donor:acceptor (D:A) weight ratio are generally significantly lower than optimized devices. Here, two high-performance organic nonfullerene BHJ blends PBDB-T:ITIC and PBDB-T:N2200 are adopted to investigate the effect of blend ratio on device performance. It is found that the PCEs of polymer-polymer (PBDB-T:N2200) blend are more tolerant to composition changes, relati...

SummaryBackgroundIncreased mortality rates associated with weekend hospital admission (the so-called weekend effect) have been attributed to suboptimum staffing levels of specialist consultants. However, evidence for a causal association is elusive, and the magnitude of the weekend specialist deficit remains unquantified. This uncertainty could hamper efforts by national health systems to introduce 7 day health services. We aimed to examine preliminary associations between specialist intensity and weekend admission mortality across the English National Health Service.MethodsEligible hospital trusts were those in England receiving unselected emergency admissions. On Sunday June 15 and Wednesday June 18, 2014, we undertook a point prevalence survey of hospital specialists (consultants) to obtain data relating to the care of patients admitted as emergencies. We defined specialist intensity at each trust as the self-reported estimated number of specialist hours per ten emergency admissions between 0800 h and 2000 h on Sunday and Wednesday. With use of data for all adult emergency admissions for financial year 2013–14, we compared weekend to weekday admission risk of mortality with the Sunday to Wednesday specialist intensity ratio within each trust. We stratified trusts by size quintile.Findings127 of 141 eligible acute hospital trusts agreed to participate; 115 (91%) trusts contributed data to the point prevalence survey. Of 34 350 clinicians surveyed, 15 537 (45%) responded. Substantially fewer specialists were present providing care to emergency admissions on Sunday (1667 [11%]) than on Wednesday (6105 [42%]). Specialists present on Sunday spent 40% more time caring for emergency patients than did those present on Wednesday (mean 5·74 h [SD 3·39] vs 3·97 h [3·31]); however, the median specialist intensity on Sunday was only 48% (IQR 40–58) of that on Wednesday. The Sunday to Wednesday intensity ratio was less than 0·7 in 104 (90%) of the contributing trusts. Mortality risk among patients admitted at weekends was higher than among those admitted on weekdays (adjusted odds ratio 1·10, 95% CI 1·08–1·11; p<0·0001). There was no significant association between Sunday to Wednesday specialist intensity ratios and weekend to weekday mortality ratios (r −0·042; p=0·654).InterpretationThis cross-sectional analysis did not detect a correlation between weekend staffing of hospital specialists and mortality risk for emergency admissions. Further investigation is needed to evaluate whole-system secular change during the implementation of 7 day services. Policy makers should exercise caution before attributing the weekend effect mainly to differences in specialist staffing.FundingNational Institute for Health Research Health Services and Delivery Research Programme.

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By the introduction of different building blocks and side‐chains, a series of donor–acceptor type polymer acceptors containing naphthalene diimide have been successfully prepared. The theoretical and experimental results show that the molecular design effectively tunes the energy levels, solubility, and coplanarity of the acceptor polymers. The intermolecular packing, which has been considered as a key factor in the bulk heterojunction morphology, has been adjusted by changing the coplanarity. As a result of improved morphology and fine‐tuned energy levels, a power conversion efficiency of 6.0% has been demonstrated for the optimized devices, which is among the highest‐efficiencies for reported all‐polymer solar cells. The improved device performance may be attributed to the resemble crystallinity of the donor/acceptor polymers, which can lead to the optimal phase separation morphology balancing both charge transfer and transport.

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