Microstructural control suppresses thermal activation of electron transport at room temperature in polymer transistors

Recent demonstrations of inverted thermal activation of charge mobility in polymer field-effect transistors have excited the interest in transport regimes not limited by thermal barriers. However, rationalization of the limiting factors to access such regimes is still lacking. An improved understanding in this area is critical for development of new materials, establishing processing guidelines, and broadening of the range of applications. Here we show that precise processing of a diketopyrrolopyrrole-tetrafluorobenzene-based electron transporting copolymer results in single crystal-like and voltage-independent mobility with vanishing activation energy above 280 K. Key factors are uniaxial chain alignment and thermal annealing at temperatures within the melting endotherm of films. Experimental and computational evidences converge toward a picture of electrons being delocalized within crystalline domains of increased size. Residual energy barriers introduced by disordered regions are bypassed in the direction of molecular alignment by a more efficient interconnection of the ordered domains following the annealing process

Every Atom Counts: Elucidating the Fundamental Impact of Structural Change in Conjugated Polymers for Organic Photovoltaics

As many conjugated polymer-based organic photovoltaic (OPV) materials provide substantial solar power conversion efficiencies (as high as 13%), it is important to develop a deeper understanding of how the primary repeat unit structures impact device performance. In this work, we have varied the group 14 atom (C, Si, Ge) at the center of a bithiophene fused ring to elucidate the impact of a minimal repeat unit structure change on the optical, transport, and morphological properties, which ultimately control device performance. Careful polymerization and polymer purification produced three “one-atom change” donor–acceptor conjugated alternating copolymers with similar molecular weights and dispersities. DFT calculation, absorption spectroscopy, and high-temperature solution ¹H nuclear magnetic resonance (NMR) results indicate that poly­(dithienosilole-<i>alt</i>-thienopyrrolodione), P­(DTS-TPD), and poly­(dithienogermole-<i>alt</i>-thienopyrrolodione), P­(DTG-TPD) exhibit different rotational conformations when compared to poly­(cyclopentadithiophene-<i>alt</i>-thienopyrrolodione), P­(DTC-TPD). Solid-state ¹H MAS NMR experiments reveal that the greater probability of the anticonformation in P­(DTS-TPD) and P­(DTG-TPD) prevail in the solid phase. The conformational variation seen in solution and solid-state NMR in turn affects the polymer stacking and intermolecular interaction. Two-dimension ¹H-¹H DQ-SQ NMR correlation spectra shows aromatic–aromatic correlations for P­(DTS-TPD) and P­(DTG-TPD), which on the other hand is absent for P­(DTC-TPD). In a thin-film interchain packing study using grazing incidence wide-angle X-ray scattering (GIWAXS), we observe the π-face of the conjugated backbones of P­(DTC-TPD) aligned edge-on to the substrate, whereas in contrast the π-faces of P­(DTS-TPD) and P­(DTG-TPD) align parallel to the surface. These differences in polymer conformations and backbone orientations lead to variations in the OPV performance of blends with the fullerene PC₇₁BM, with the device containing P­(DTC-TPD):PCBM having a lower fill factor and a lower power conversion efficiency. Ultrafast transient absorption spectroscopy shows the P­(DTC-TPD):PCBM blend to have a more pronounced triplet formation from bimolecular recombination of initially separated charges. With a combination of sub-bandgap external quantum efficiency measurements and DFT calculations, we present evidence that the greater charge recombination loss is the result of a lower lying triplet energy level for P­(DTC-TPD), leading to a higher rate of recombination and lower OPV device performance. Importantly, this study ties ultimate photovoltaic performance to morphological features in the active films that are induced from the processing solution and are a result of minimal one-atom differences in polymer repeat unit structure

Congolese kunst voor de Congolezen

status: publishe

Increase in the use of inhaled nitric oxide in neonatal intensive care units in England: a retrospective population study

Objective To describe temporal changes in inhaled nitric oxide (iNO) use in English neonatal units between 2010 and 2015.Design Retrospective analysis using data extracted from the National Neonatal Research Database.Setting All National Health Service neonatal units in England.Patients Infants of all gestational ages born 2010–2015 admitted to a neonatal unit and received intensive care.Main outcome measures Proportion of infants who received iNO; age at initiation and duration of iNO use.Results 4.9% (6346/129 883) of infants received iNO; 31% (1959/6346) were born &lt;29 weeks, 18% (1152/6346) 29–33 weeks and 51% (3235/6346)&gt;34 weeks of gestation. Between epoch 1 (2010–2011) and epoch 3 (2014–2015), there was (1) an increase in the proportion of infants receiving iNO: &lt;29 weeks (4.9% vs 15.9%); 29–33 weeks (1.1% vs 4.8%); &gt;34 weeks (4.5% vs 5.0%), (2) increase in postnatal age at iNO initiation: &lt;29 weeks 10 days vs 18 days; 29–33 weeks 2 days vs 10 days, (iii) reduction in iNO duration: &lt;29 weeks (3 days vs 2 days); 29–33 weeks (2 days vs 1 day).Conclusions Between 2010 and 2015, there was an increase in the use of iNO among infants admitted to English neonatal units. This was most notable among the most premature infants with an almost fourfold increase. Given the cost of iNO therapy, limited evidence of efficacy in preterm infants and potential for harm, we suggest that exposure to iNO should be limited, ideally to infants included in research studies (either observational or randomised placebo-controlled trial) or within a protocolised pathway. Development of consensus guidelines may also help standardise practice