Controlled synthesis of the electroluminescent polymer PFO-MEH-PPV (poly[(9,9-dioctyl-2,7-divinylenefluorenylene)-alt-co-(2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylene)]) provided samples of varying molecular weight (Mw) in the range 20–360 kDa, as determined by gel-permeation chromatography and light scattering. The samples were used as the active layers in organic light-emitting diodes (OLEDs), and the performance of the devices was examined as a function of Mw. Turn-on voltages fell in the range 1.92–2.78 V, luminances varied from 231 to 5826 cd/m2, and luminous efficacies ranged from 0.06 to 0.90 lm/W. The emitted colour was found to vary from green to yellow as Mw increases. Optimal performance was attained by using PFO-MEH-PPV with Mw = 100 kDa. To help reveal how Mw determines the performance of OLEDs, relative quantum yields of photoluminescence in solutions and films were measured, and films were characterized by atomic force microscopy and transmission electron microscopy.

Conjugated polymers are widely used in thin-film organic photovoltaic devices to absorb light and serve as electron donors or acceptors. Small molecular analogues are attractive substitutes because they have fully defined structures, can be purified rigorously, and are typically more soluble and volatile. However, producing active films composed primarily of small molecules remains challenging. We have devised bulk heterojunction solar cells in which poly(3-hexylthiophene-2,5-diyl) and poly[[9-(1-octylnonyl)-9H-carbazole-2,7-diyl]-2,5-thiophenediyl-2,1,3-benzothiadiazole-4,7-diyl-2,5-thiophenediyl] are used as matrices to prepare films containing low-bandgap push–pull molecules as electron donors and (6,6)-phenyl-C61-butyric acid methyl ester or (6,6)-phenyl-C71-butyric acid methyl ester as electron acceptors. Compared with reference devices devoid of push–pull molecular additives, increases in power conversion efficiencies up to 30.4% were measured.

The performance of molecule-based thin-film devices such as organic light-emitting diodes, photovoltaic cells, and thin-film transistors depends on the electronic properties of the individual molecular components, as well as on their association to form complex morphologies. Transmission electron microscopy (TEM) can be used to image the morphologies and help reveal how the devices work and can be improved. We have examined the suitability of various ways to prepare samples of thin molecular films for imaging by TEM. Specifically, we have used focused ion beams to mill cross sections of complete devices that have been glued together with epoxy adhesives. In addition, thin films of the type used as active layers in molecule-based devices can be deposited on disks of NaCl, which can then be dissolved in water to release free-standing films that can be imaged by TEM, without loss of nanostructural details. Films of this type can also be deposited on Si wafers, which can then be fractured to expose sections of film that overhang edges of fragments and can be imaged conveniently by TEM. This allows TEM to be used as a quick method for screening samples and monitoring the purification of active materials.

The effect of tuning molecular weight (Mn) in poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine] (PTAA) to increase both mechanical properties of the film and electrical properties of perovskite solar cells is reported. Perovskite solar cell devices are fabricated to investigate the effect of Mn on power conversion efficiency. Moisture stability for various Mn is also studied in PTAA films exposed to mechanical loads in humid environments. Furthermore, cohesion and tensile tests are employed to determine the mechanical properties of PTAA, where higher Mn leads to more robust films. To elucidate the effect of Mn on the debonding kinetics, a viscoelastic fracture kinetic model is proposed as a function of Mn, and the debonding mechanism is found to be dependent on Mn. Finally, the effect of small-molecule-based dopants on the mechanical stability of PTAA is investigated.

The n-type organic semiconductor phenyl-C-61-butyric acid methyl ester (PCBM), a soluble fullerene derivative well investigated for organic solar cells and transistors, can undergo several successive reversible, diffusion-controlled, one-electron reduction processes. We exploited such processes to shed light on the correlation between electron transfer properties, ionic and electronic transport as well as device performance in ionic liquid (IL)-gated transistors. Two ILs were considered, based on bis(trifluoromethylsulfonyl)imide [TFSI] as the anion and 1-ethyl-3-methylimidazolium [EMIM] or 1-butyl-1-methylpyrrolidinium [PYR14] as the cation. The aromatic structure of [EMIM] and its lower steric hindrance with respect to [PYR14] favor a 3D (bulk) electrochemical doping. As opposed to this, for [PYR14] the doping seems to be 2D (surface-confined). If the n-doping of the PCBM is pursued beyond the first electrochemical process, the transistor current vs. gate-source voltage plots in [PYR14][TFSI] feature a maximum that points to the presence of finite windows of high conductivity in IL-gated PCBM transistors.

Active layers in many thin-film organic photovoltaic devices (OPVs) contain light-absorbing polymers that serve as electron donors, mixed with appropriate electron acceptors. In principle, the polymers can be replaced by small molecules with suitable bandgaps, which offer multiple advantages, including well-defined structures and methods of synthesis and purification that provide uniform samples. However, such materials often undergo separation of phases and crystallization, so making long-lived films that remain smooth, homogeneous, flexible, and transparent is not easy. We have found that effective OPVs can be made by dispersing mixtures of low-bandgap push-pull small molecules as electron donors and [6,6]-phenyl-C-71-butyric acid methyl ester (PC71BM) as electron acceptor in matrices of optoelectronically passive conventional polymers, including polystyrene, poly(methyl methacrylate), poly(vinyl chloride), poly(ethylene glycol), and poly(dimethylsiloxane). By varying the identity of the matrix, its molecular weight, the loading of active components, and the conditions of annealing, we have produced efficient OPVs from components that would otherwise have undergone phase separation and crystallization, leading to poor performance. Layers with up to 35% matrix were found to be effective and could be fabricated at room temperature by simple processes. To probe the role of the polymers as dispersants, morphologies of composite films were examined by atomic force microscopy and electron microscopy. (c) 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017, 55, 1479-1492

Organic thin-film optoelectronic devices, unlike inorganic analogues, offer the attractive prospect of large, flexible, and inexpensive arrays made by simple procedures such as roll-to-roll printing. In current organic thin-film devices, layers of tin-doped indium oxide (ITO) are widely used as electrodes. Motivated by the increasing price of indium and the high cost of ITO-coated substrates, we have examined ways to recover and recycle ITO substrates in typical devices by environmentally benign methods. A process using only water yields recovered ITO substrates that can be reused at least 10 times to prepare new devices without loss of efficiency.

The aggregation in conjugated polyelectrolytes (CPs) can be effectively reduced by the formation of CP/nanoparticle assemblies. The photophysical properties of various nanoassemblies were studied by means of UV-visible and fluorescence spectroscopy in solution and as thin films. The dissociation of the polymer chains is caused by favorable electrostatic interactions between the cationic substituents of the CPs and the anionic charges present on the surface of the nanoparticles. Such an efficient displacement of pi-stacking by competitive positive interactions constitutes the first example of positive aggregation modulation.