No abstract

I nitially, the role of an alkyl chain on a conjugated polymer was to confer solubility of the rigid-rod-like structure in common organic solvents. It is now apparent that the choice of solubilizing chain can vastly affect the properties of the polymer, as it can be used to tune a materials' crystallinity in the solid state as well as its miscibility with other materials in thin film blends. The latter of these is of extreme importance in the field of OPV where the extent of phase separation between a donor conjugated polymer and a fullerene acceptor is a critical factor in determining solar cell performance. 1À5 Yang et al. studied the effects of side chain variation on a naphtho[2,1-b:3,4-b 0 ]dithiophene-co-4,7-di(thiophen-2-yl)benzothiadiazole system 6 and observed differences in the V oc and J sc . These were quantitatively correlated with a pre-exponential dark current term, which they claimed accounts for the intermolecular interactions in the polymer/fullerene blend. Biniek et al. studied the effect of side chain variation and site of attachment on a benzo[1,2-b:4,5-b 0 ] dithiophene-co-thieno[3,4-b]thiophene polymer. 7 They found an increased solar cell efficiency using ethylhexyl chains over linear dodecyl chains. Szarko et al. very recently reported on the effect of side chain variation on thieno[3,-b]thiophene-co-benzodithiophene polymer systems, which have given record breaking (∼8%) PCEs. 8 They found that varying the degree of alkyl chain branching affected the πÀπ stacking distance which they suggested allowed a varying amount of fullerene intercalation to occur. A study on the variation of the side chain on the acceptor portion of a Nalkylthieno[3,4-c]pyrrole-4,6-dione polymer demonstrated that the highest solar cell efficiency was achieved with a linear chain. 9 Recently, we reported the synthesis and high hole mobility of an indacenodithiophene-co-benzothiadiazole (IDTÀBT) copolymer with hexadecyl alkyl chains P4. 10 The long linear alkyl chains were attributed to impart some degree of crystallinity in thin film. Chen et al. demonstrated the high performance of a similar IDTÀBT copolymer bearing phenylhexyl side chains in solar cell devices. 11 Replacement of benzothiadiazole on this system with alternative acceptors has also yielded efficient solar cells. 12 We were interested in investigating the role the solubilizing chains play in the performance of this low-band-gap polymer in both TFT and OPV devices. The effect of varying the alkyl chain from short and branched (methylbutyl) to long and linear (hexadecyl) on the optical and electronic properties is presented here.Copolymers P1ÀP4 were synthesized via Suzuki couplings using previously reported conditions. 10 It was not possible to achieve high molecular weights with the methylbutyl side chains P1 due to precipitation of the polymer during polymerization, a consequence of its lower solubility. The remaining polymers were synthesized in high molecular weights, as expected, due to the enhanced solubilizing effect of the longer alkyl chains...

No abstract

GaAs nanowires were epitaxially grown on Si(001) and Si(111) substrates by using Au-catalyzed vapor-liquid-solid (VLS) growth in a solid source molecular beam epitaxy system. Scanning electron microscopy analysis revealed that almost all the GaAs nanowires were grown along <111> directions on both Si substrates for growth conditions investigated. The GaAs nanowires had a very uniform diameter along the growth direction. X-ray diffraction data and transmission electron microscopy analysis revealed that the GaAs<111> nanowires had a mixed crystal structure of the hexagonal wurtzite and the cubic zinc-blende. Current-voltage characteristics of junctions formed by the epitaxially grown GaAs nanowires and the Si substrate were investigated by using a current-sensing atomic force microscopy.

There remains a critical need for large‐area imaging technologies that operate in the shortwave infrared spectral region. Upconversion imagers that combine photo‐sensing and display in a compact structure are attractive since they avoid the costly and complex process of pixilation. However, upconversion device research is primarily focused on the optical output, while electronic signals from the imager remain underutilized. Here, an organic upconversion imager that is efficient in both optical and electronic readouts, extending the capability of human and machine vision to 1400 nm, is designed and demonstrated. The imager structure incorporates interfacial layers to suppress non‐radiative recombination and provide enhanced optical upconversion efficiency and electronic detectivity. The photoresponse is comparable to state‐of‐the‐art organic infrared photodiodes exhibiting a high external quantum efficiency of ≤35% at a low bias of ≤3 V and 3 dB bandwidth of 10 kHz. The large active area of 2 cm2 enables demonstrations such as object inspection, imaging through smog, and concurrent recording of blood vessel location and blood flow pulses. These examples showcase the potential of the authors’ dual‐readout imager to directly upconvert infrared light for human visual perception and simultaneously yield electronic signals for automated monitoring applications.