Thermally Stable Zinc Disalphen Macrocycles Showing Solid-State and Aggregation-Induced Enhanced Emission

In order to investigate the solid-state light emission of zinc salphen macrocycle complexes, 7 dinuclear zinc salphen macrocycle complexes (<b>1–7</b>), with acetate or hexanoate coligands, are synthesized. The complexes are stable in air up to 300 °C, as shown via thermogravimetric analysis (TGA), and exhibit green to orange-red emission in solution (λ_em = 550–600 nm, PLQE ≤ 1%) and slightly enhanced yellow to orange-red emission in the solid state (λ_em = 570–625 nm, PLQE = 1–5%). Complexes <b>1</b>, <b>2</b>, <b>4</b>, <b>5</b>, and <b>7</b> also display aggregation-induced enhanced emission (AIEE) when hexane (a nonsolvent) is added to a chloroform solution of the complexes, with complex <b>4</b> displaying a 75-fold increase in peak emission intensity upon aggregation (in 0.25:0.75 chloroform:hexane mixture)

Confined Surface Plasmon–Polariton Amplifiers

We demonstrate the realization of confined surface plasmon polariton amplifiers using a thin layer of the organic gain medium 4-dicyanomethylene-2-methyl-6-(<i>p</i>-dimethylaminostyryl)-4<i>H</i>-pyran dispersed in a tris­(8-hydroxy-quinolinato)aluminum matrix. Complete loss compensation, which occurs at a pump fluence of approximately 200 μJ/cm², is directly observed in the time domain and studied for a range of waveguide lengths. The power dependence is also reported, and a significant net gain of 93 dB/mm is observed at the highest fluence

Thickness Effect of Bulk Heterojunction Layers on the Performance and Stability of Polymer:Fullerene Solar Cells with Alkylthiothiophene-Containing Polymer

We report a pronounced thickness effect of bulk heterojunction (BHJ) layers on the performance and stability of inverted polymer solar cells with the BHJ layers of poly­[(4,8-bis­(5-(octylthio)­thiophen-2-yl)­benzo­[1,2-b:4,5-b′]­dithiophene-<i>co</i>-3-fluorothieno­[3,4- b]­thiophene-2-carboxylate] (PBDT-TS1) and [6,6]-phenyl-C₇₁-butyric acid methyl ester (PC₇₁BM). The thickness of the BHJ layers was varied from 40 to 120 nm by changing solution concentrations and spin-coating speeds. The results showed that the film thickness considerably affected the performance and stability of devices. The power conversion efficiency reached ca. 9% at the thickness of 80 nm by the optimized nanoscale phase separation between donor and acceptor components. However, the devices with 120 nm-thick BHJ layers showed better device stability under continuous illumination with a simulated solar light due to the well-maintained surface morphology and nanostructure in addition to the improved morphological volume stability

Efficient Deep Red Light-Sensing All-Polymer Phototransistors with <i>p</i>‑type/<i>n</i>-type Conjugated Polymer Bulk Heterojunction Layers

Here we demonstrate deep red light-sensing all-polymer phototransistors with bulk heterojunction layers of poly­[4,8-bis­[(2-ethylhexyl)-oxy]­benzo­[1,2-b:4,5-b′]­dithiophene-2,6-diyl]­[3-fluoro-2-[(2-ethylhexyl)­carbonyl]­thieno­[3,4-<i>b</i>]-thiophenediyl] (PTB7) and poly­[[<i>N</i>,<i>N</i>′-bis­(2-octyldodecyl)-naphthalene-1,4,5,8-bis­(dicarboximide)-2,6-diyl]-<i>alt</i>-5,5′-(2,2′-bithiophene)] (P­(NDI2OD-T2)). The device performances were investigated by varying the incident light intensity of the deep red light (675 nm), while the signal amplification capability was examined by changing the gate and drain voltages. The result showed that the present all-polymer phototransistors exhibited higher photoresponsivity (∼14 A/W) and better on/off photoswitching characteristics than the devices with the pristine polymers under illumination with the deep red light. The enhanced phototransistor performances were attributed to the well-aligned nanofiber-like morphology and nanocrystalline P­(NDI2OD-T2) domains in the blend films, which are beneficial for charge separation and charge transport in the in-plane direction

Edmund Spenser

Supplemento al quotidiano la Repubblic

Dependence of Charge Separation Efficiency on Film Microstructure in Poly(3-hexylthiophene-2,5-diyl):[6,6]-Phenyl-C₆₁ Butyric Acid Methyl Ester Blend Films

Herein we address the factors controlling photocurrent generation in P3HT:PCBM blend films as a function of blend composition and annealing treatment. Absorption, photoluminescence, and transient absorption spectroscopy are used to distinguish the role of exciton dissociation, charge pair separation, and charge collection. Variations in blend film microstructure with composition and annealing treatment are studied using X-ray diffraction. While the trend in photocurrent generation with composition and annealing [Muller, et al., <i>Adv. Mater.</i> <b>2008</b>, <i>20</i>, 3510] does not follow the trend in exciton dissociation, it closely follows the trend in charge pair generation. Moreover, charge pair generation efficiency is positively correlated to the degree of polymer crystallization and the appearance of large domains of both polymer and fullerene phases. We argue that larger domains assist charge pair separation by increasing the probability of escape from the P3HT:PCBM interface, thus reducing geminate charge recombination

Competition between the Charge Transfer State and the Singlet States of Donor or Acceptor Limiting the Efficiency in Polymer:Fullerene Solar Cells

We study the appearance and energy of the charge transfer (CT) state using measurements of electroluminescence (EL) and photoluminescence (PL) in blend films of high-performance polymers with fullerene acceptors. EL spectroscopy provides a direct probe of the energy of the interfacial states without the need to rely on the LUMO and HOMO energies as estimated in pristine materials. For each polymer, we use different fullerenes with varying LUMO levels as electron acceptors, in order to vary the energy of the CT state relative to the blend with [6,6]-phenyl C61-butyric acid methyl ester (PCBM). As the energy of the CT state emission approaches the absorption onset of the blend component with the smaller optical bandgap, <i>E</i>_opt,min ≡ min­{<i>E</i>_opt,donor; <i>E</i>_opt,acceptor}, we observe a transition in the EL spectrum from CT emission to singlet emission from the component with the smaller bandgap. The appearance of component singlet emission coincides with reduced photocurrent and fill factor. We conclude that the open circuit voltage <i>V</i>_OC is limited by the smaller bandgap of the two blend components. From the losses of the studied materials, we derive an empirical limit for the open circuit voltage: <i>V</i>_OC ≲ <i>E</i>_opt,min/<i>e</i> – (0.66 ± 0.08)­eV

Steric-Hindrance-Functionalized Polydiarylfluorenes: Conformational Behavior, Stabilized Blue Electroluminescence, and Efficient Amplified Spontaneous Emission

Control of the hierarchical molecular organization of polydiarylfluorenes by synthetic strategies is significant for optimizing photophysical properties as well as the performance of light-emitting devices. Herein, for the suppression of molecular aggregation and enhancement of luminescence efficiency, a series of steric units were introduced into polydiarylfluorenes by copolymerization, with the aim of integrating the advantages of the steric-hindrance effect and of the β-phase. Optical and Raman spectroscopies revealed a β-phase conformation for a polymer copolymerized with spiro­[fluorene-9,9′-xanthene] (SFX), with photoluminescence (PL) peaks at 454, 482, and 517 nm. Moreover, the morphological stability and electroluminescence (EL) stability were also improved without compromising the performance of the polymer light-emitting diodes (PLEDs). Furthermore, three steric-hindrance-functionalized copolymers showed significantly decreased thresholds for amplified spontaneous emission (<i>E</i>_th^ASE) and enhanced stability following thermal annealing treatment. These results indicate that steric-hindrance functionalization is a superior approach to improve the overall stability and optoelectronic properties for blue-light-emitting π-conjugated polymers

Photophysical and Fluorescence Anisotropic Behavior of Polyfluorene β‑Conformation Films

We demonstrate a systematic visualization of the unique photophysical and fluorescence anisotropic properties of polyfluorene coplanar conformation (β-conformation) using time-resolved scanning confocal fluorescence imaging (FLIM) and fluorescence anisotropy imaging microscopy (FAIM) measurements. We observe inhomogeneous morphologies and fluorescence decay profiles at various micrometer-sized regions within all types of polyfluorene β-conformational spin-coated films. Poly­(9,9-dioctylfluorene-2,7-diyl) (PFO) and poly­[4-(octyloxy)-9,9-diphenylfluoren-2,7-diyl]-<i>co</i>-[5-(octyloxy)-9,9-diphenylfluoren-2,7-diyl] (PODPF) β-domains both have shorter lifetime than those of the glassy conformation for the longer effective conjugated length and rigid chain structures. Besides, β-conformational regions have larger fluorescence anisotropy for the low molecular rotational motion and high chain orientation, while the low anisotropy in glassy conformational regions shows more rotational freedom of the chain and efficient energy migration from amorphous regions to β-conformation as a whole. Finally, ultrastable ASE threshold in the PODPF β-conformational films also confirms its potential application in organic lasers. In this regard, FLIM and FAIM measurements provide an effective platform to explore the fundamental photophysical process of conformational transitions in conjugated polymer