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We report the preparation of high-quality, solvent-free [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) large single crystals (size up to 0.5 mm) by slow drying of a chlorobenzene solution at room temperature. The monoclinic structure containing four PCBM molecules per unit cell was successfully solved (R-factor = 0.0512) via micro-focused X-ray diffraction and employed as a reliable experimental model for further molecular dynamics simulations. We find that the first peak of the simulated fullerene–fullerene radial distribution function is centred at 10.05 Å, giving a nearest neighbour coordination number of 7.0. The work reported herein provides the structural basis for a fundamental understanding of charge transport in this important functional material that is particularly relevant to organic solar cells. [ABSTRACT FROM AUTHOR]

The ability to manipulate the local molecular environment is crucial to access fundamentally new classes of organic functional materials with unprecedented properties and performance. Despite the progress of the last ten years in the area of organic semiconductors for electronics and optoelectronics, much remains to be done to achieve control at the nanoscale of the local environment of the functional molecules, both regarding interaction with other molecular units or species and with the electrodes or other inorganic components of the ‘device’. [Copyright &y& Elsevier]

Oxygen-plasma treatment of indium-tin oxide (ITO) anodes is now widely used as one of the most effective ways to improve the device performance of organic light-emitting diodes (LEDs). However, the role of oxygen-plasma treatment has not been clearly understood. We have performed detailed studies of the surface and bulk of the ITO thin films exposed to oxygen-plasma. We employed a multitude of experimental techniques, including X-ray and ultraviolet photoelectron spectroscopies, atomic force microscopy, dynamic contact angle measurement, four-point probe and Hall measurements to investigate the changes induced by the plasma. We have also analyzed the device characteristics of polymer LEDs fabricated with these anodes. We found significant modifications of the physico-chemical, morphological, transport and optical properties of the oxygen-plasma treated ITO. Although oxygen-plasma does not show any measurable etching effect, it induces considerable changes leading to an increase in work function, electron carrier concentration and conductivity. It also increases the surface energy and polarity. We relate these modifications to enhancement of the device performance, such as electroluminescence efficiency and lifetime, through their effects on hole injection, and interface structure and stability. Finally, we show that even in the presence of a hole-transport layer such as a poly(styrene sulphonate)-doped poly(3,4-ethylene dioxythiophene) (PEDOT:PSS) inserted between the anode and the emissive polymer layer, oxygen-plasma treatment of the ITO anodes is still beneficial for the devices. [Copyright &y& Elsevier]

Two new soluble block copolymers are reported in which chromophores and hexa(ethylene oxide) units alternate along the polymer backbone. In polymer 1 the chromophore was the distyrylbenzene unit. The polymer was synthesised via the Wittig reaction and the ionisation potential of 5.4±0.2 eV was measured by cyclic voltammetry and photoelectron spectroscopy. Polymer 1 showed a high solid-state photoluminescence efficiency (34%) and was used to make efficient (0.5 cd/A) light emitting diodes (LEDs). Polymer 1 was also used in light emitting cells; these showed luminescence in reverse bias and a reduced turn-on voltage compared to the LEDs. Polymer 2, in which the chromophore was dodecafluoro-distryrylbenzene, was prepared via the Horner–Wittig reaction and showed an ionisation potential of 6.25±0.15 eV and a solid-state photoluminescence efficiency of 17%. It was used as electron-conducting layer in a LED but failed to give significant electroluminescence. The optical energy gap for both polymers was 3.0 eV. [Copyright &y& Elsevier]

There has been a surge in the interest for (semi)transparent photovoltaics (sTPVs) in recent years, since the more traditional, opaque, devices are not ideally suited for a variety of innovative applications spanning from smart and self-powered windows for buildings to those for vehicle integration. Additional requirements for these photovoltaic applications are a high conversion efficiency (despite the necessary compromise to achieve a degree of transparency) and an aesthetically pleasing design. One potential realm to explore in the attempt to meet such challenges is the biological world, where evolution has led to highly efficient and fascinating light-management structures. In this mini-review, we explore some of the biomimetic approaches that can be used to improve both transparent and semi-transparent photovoltaic cells, such as moth-eye inspired structures for improved performance and stability or tunable, coloured, and semi-transparent devices inspired by beetles' cuticles. Lastly, we briefly discuss possible future developments for bio-inspired and potentially bio-compatible sTPVs. [ABSTRACT FROM AUTHOR]

Organic semiconductors, organic spintronics, spin detection, spin generation, spin relaxation, spin transport Perspectives of Organic and Perovskite-Based Spintronics (Advanced Optical Materials 14/2021) Keywords: organic semiconductors; organic spintronics; spin detection; spin generation; spin relaxation; spin transport EN organic semiconductors organic spintronics spin detection spin generation spin relaxation spin transport 1 1 1 07/12/21 20210710 NES 210710 In Perspective number 2100215, Alberto Privitera and co-workers demonstrate how the use of two emerging optoelectronic materials, namely organic semiconductors and halide perovskites, will offer exciting ways to manipulate the spin, charges and photons in new spin/opto-electronic devices. [Extracted from the article]

In the initially published version of this article, in Table the "Osmium complex" class identifier is missing. Furthermore, in the initially published version of this article, the "ELECTRON-DONORS" and "ELECTRON-ACCEPTORS" labels in Figure 9 are switched. [Extracted from the article]

Subsurface mapping is crucial to understanding many biological systems as well as structured thin films for (opto)electronic or photonic applications. A non‐invasive method is presented to map subsurface nanostructures from scanning near‐field optical microscopy images. The Bethe–Bouwkamp model is used to simulate imaging of buried nano‐objects or subsurface slanted planar interfaces, and it is shown how to determine their depth and size, or the interface inclination, from just one image. It is shown that the steep optical field gradient makes near‐field microscopy a particularly sensitive depth probe for thin films. [ABSTRACT FROM AUTHOR]

We investigate the dependence of the characteristics of polymer photovoltaic cells on the concentration of doping of the active layers by heavy-metal complexes. We wish to exploit the strong spin-orbit coupling of such complexes to induce a relatively fast exciton intersystem crossing from the singlet to the longer-lived triplet manifold, to increase in turn the lifetime of the exciton, its diffusion length, and thus the probability of dissociation and the overall device conversion efficiency. We combine current-voltage curves, photocurrent spectra, and atomic force micrographs to show that there is an optimum doping load above which the device performance severely deteriorates. We find, for an Ir-pyridine dopant, a maximum load of approximately 5%. [ABSTRACT FROM AUTHOR]

Abstract: Fluoropolymers are characterized by high chemical inertness and, when in solid state, by superficial dipoles due to the C–F bond where the charge density is strongly displaced. These two characteristics are exploited here for fine control of charge balance in organic light-emitting devices and for preventing electrochemical interaction between heterogeneous layers. The insertion of a thin layer of polytetrafluoroethylene, PTFE, at the interface between poly(ethylene dioxythiophene):poly(styrene sulfonic acid), PEDOT:PSS, and an electroluminescent polymer leads to improved device efficiency and longevity. The presence of the superficial dipole increases the effective work function of the anode and improves the charge balance which enhances the external quantum efficiency, EQE, of the devices by up to a factor of two without significant effects on the luminance levels. The insertion of the PTFE layer reduces the photoluminescence quenching at the PEDOT:PSS/polymer interface, however we show that the EQE enhancement is mainly due to a better confinement of minority carrier electrons in the active layer. The lifetime of the devices shows a remarkable increase correlated with the insertion of the PTFE layer. Such improvements are ascribed to the reduced electrochemical interaction between the electroluminescent polymer and PEDOT:PSS due to the chemically inert nature of PTFE. The PTFE acts as a chemical zipper of two heterogeneous media with the added functionality of control over the charge balance. [Copyright &y& Elsevier]

The design, fabrication, and characterization of optically switchable organic light‐emitting diodes (OSOLEDs) based on the combination of the commercially available light‐emitting polymer poly(9,9′‐dioctylfluorene‐alt‐benzothiadiazole), F8BT, doped with a diarylethene derivative is reported. The photochromic activity of the dopant in the solid state has been investigated both via UV/vis absorption and photoluminescence spectroscopy, whereas the morphology of different blends is investigated via atomic force microscopy. OSOLEDs embedding dopant loadings of 1, 5, and 10 wt% exhibit optical responsivity with a maximum reversible optical threshold voltage shift of 4 V. The best performing devices containing 5 wt% dopant show a maximum current density and luminance ON/OFF ratio of ≈20 and ≈90, respectively. For the first time, the impact of the diarylethene isomerization on hole and electron transport has been decoupled and directly investigated, via the design, fabrication, and characterization of single‐carrier switchable devices based on the same blends. Not only do these results confirm the photo‐responsive trapping activity of the diarylethenes on both charge carriers, but they also demonstrate its asymmetry, with a predominant effect on electron transport that is over 3.4 times larger as compared to hole transport. [ABSTRACT FROM AUTHOR]

Conjugated polymers are an important class of chromophores for optoelectronic devices. Understanding and controlling their excited state properties, in particular, radiative and non‐radiative recombination processes are among the greatest challenges that must be overcome. We report the synthesis and characterization of a molecularly encapsulated naphthalene diimide‐based polymer, one of the most successfully used motifs, and explore its structural and optical properties. The molecular encapsulation enables a detailed understanding of the effect of interpolymer interactions. We reveal that the non‐encapsulated analogue P(NDI‐2OD‐T) undergoes aggregation enhanced emission; an effect that is suppressed upon encapsulation due to an increasing π‐interchain stacking distance. This suggests that decreasing π‐stacking distances may be an attractive method to enhance the radiative properties of conjugated polymers in contrast to the current paradigm where it is viewed as a source of optical quenching. [ABSTRACT FROM AUTHOR]

Porous silicon (PSi) is a promising material for future integrated nanophotonics when coupled with guest emitters, still facing challenges in terms of homogenous distribution and nanometric thickness of the emitter coating within the silicon nanostructure. Herein, it is shown that the nanopore surface of a porous silicon oxide (PSiO2) microcavity (MC) can be conformally coated with a uniform nm‐thick layer of a cationic light‐emitting polyelectrolyte, e.g., poly(allylamine hydrochloride) labeled with Rhodamine B (PAH‐RhoB), leveraging the self‐tuned electrostatic interaction of the positively‐charged PAH‐RhoB polymer and negatively‐charged PSiO2 surface. It is found that the emission of PAH‐RhoB in the PSiO2 MC is enhanced (≈2.5×) and narrowed (≈30×) at the resonant wavelength, compared with that of PAH‐RhoB in a non‐resonant PSiO2 reference structure. The time‐resolved photoluminescence analysis highlights a shortening (≈20%) of the PAH‐RhoB emission lifetime in the PSiO2 MC at the resonance versus off‐resonance wavelengths, and with respect to the reference structure, thereby proving a significant variation of the radiative decay rate. Remarkably, an experimental Purcell factor Fp = 2.82 is achieved. This is further confirmed by the enhancement of the photoluminescence quantum yield of the PAH‐RhoB in the PSiO2 MC with respect to the reference structure. Application of the electrostatic nanoassembling approach to other emitting dyes, nanomaterials, and nanophotonic systems is envisaged. [ABSTRACT FROM AUTHOR]

Abstract: Scanning probe techniques such as atomic force microscopy and scanning tunnelling microscopy have been used extensively for the study of surface properties on the nanoscale. A valuable, but less commonly used technique is scanning near-field optical microscopy (SNOM). This scanning probe technique allows the optical properties of supramolecular structures to be probed at resolutions better than allowed by the diffraction limit. This means that resolution better than 100nm is achievable even with visible radiation. However, the geometry of the SNOM probe causes images obtained by this method to contain a mixture of optical and topographical information. Where there is no optical contrast on a surface, topography can introduce contrast to the image, which is obviously undesirable. Additionally, the topographical artifacts can cause apparent distortion or displacement of features in the optical image. To have confidence in results obtained by SNOM, the nature of topographical artifacts must be fully understood. Here we present one-dimensional numerical simulations of SNOM images and an investigation into several observable effects. In addition to the occurrence of topography-induced optical contrast we show that a low topographical feature near a higher one can be visible in an image but with low intensity and can appear displaced and distorted if it is approximately a probe diameter from the larger feature. [Copyright &y& Elsevier]

This work reports the first observation of circularly polarized electroluminescence (CPEL) in thin films of self‐organized oligothiophenes. Four new 1,4‐phenylene and 9H‐carbazole‐based oligothiophenes were ad hoc designed to ensure efficient spontaneous formation of chiral supramolecular order. They were easily synthesized and their chiroptical properties in thin films were measured. Circularly polarized luminescence (CPL) spectra revealed glum in the order of 10−2 on a wide wavelengths range, originating from their self‐organized chiral supramolecular organization. These molecules have reasonable properties as organic semiconductors and for this reason they can constitute the active layer of circularly‐polarized organic light‐emitting diodes (CP‐OLEDs). Thus, we could investigate directly their electroluminescence (EL) and CPEL, without resorting to blends, but rather in a simple multilayer device with basic architecture. This is the first example of a CP‐OLED with active layer made only of a small organic compound. [ABSTRACT FROM AUTHOR]

We present a joint experimental and computational study of the role of the substrate thermal conductivity on scanning thermochemical lithography (SThL) of thin organic films. We aim this study at lithography of the luminescent conjugated polymer poly(p-phenylene vinylene) (PPV) from its soluble precursor poly(p-xylene tetrahydrothiophenium chloride) (PXT), but our results provide relevant insights into the SThL of thermosensitive polymers in general, and into a wide range of nanoscale thermal and thermochemical processes in thin films. As high thermal conductivity substrates we used gold films on silicon, and indium-tin oxide (ITO) films on glass, successfully patterning PPV on both substrates. We find that a higher probe temperature (>300 °C instead of ≈250 °C) is necessary for lithography of PXT films on ITO compared to those on fused silica (for the same scanning speed and comparable precursor thickness). Surprisingly, however, our experiments show that minimum feature sizes are nearly independent of the underlying substrate. While a lateral resolution (full width at half maximum, FWHM) of 37 nm was achieved previously on fused silica for a 40 nm thick PXT film, we obtain here a FWHM of 36 nm for a 35 nm thick PXT layer on ITO. We compare our experiments with finite element simulations and gain further insight into the possibilities of thermochemical lithography, the necessary minimum probe temperature and the highest attainable resolutions. The model shows that for high thermal conductivity substrates there should be a region of unconverted polymer near the polymer-substrate interface. Our experiments demonstrate that patterned features are able to adhere to the substrate despite this unconverted layer, thus allowing SThL to work on very high thermal conductivity substrates such as gold. Our model builds on this experimental finding and accounts for the experimental lack of dependence of lateral size with substrate conductivity, i.e. it predicts that the minimum feature size increases only slightly for increasing thermal conductivities of the substrates. [ABSTRACT FROM AUTHOR]

The photophysics and electroluminescence (EL) of thin films of unthreaded and cyclodextrin-encapsulated poly(4,4′-diphenylenevinylene) (PDV) with potassium countercations, blended with poly(ethylene oxide) (PEO) are investigated as a function of the PEO concentration. We show that three main factors contribute to increasing the photoluminescence (PL) quantum efficiency as a result of suppressed intermolecular interactions, namely: the high degree of encapsulation of the polyrotaxanes, the relatively large countercation (e.g., compared to lithium), and the complexation of the rotaxanes with PEO. By facilitating cationic transport to the negative electrodes, PEO also leads to devices with enhanced electron injection and improved charge balance, whose operation therefore resembles that of “virtually unipolar” light-emitting electrochemical cells. This effect, together with the enhanced PL efficiency, leads to higher EL efficiency for both polyrotaxanes and unthreaded polymers, upon addition of the PEO. We show that the concurrent exploitation of the various strategies above lead to an overall EL efficiency that is approximately twice the value previously reported for Li-based PDV. A blueshift of the EL spectrum during the devices turn-on is also reported and analyzed in terms of interference and doping effects. [ABSTRACT FROM AUTHOR]

We report on electroabsorption (EA) and built-in voltage (VBI) measurements of polymer light-emitting diodes with the general structure ITO/PEDOT:PSS/emitting polymer/LiF/Ca/Al where ITO is indium tin oxide, PEDOT:PSS is poly(3,4-ethylene dioxythiophene) doped with poly(styrene sulfonate), and the emitting polymer is either poly(9,9-dioctylfluorene), poly(9, 9-dioctyl-fluorene-alt-bis-N,N′-(4-butyl-phenyl)-bis-N, N′-phenyl-1, 4-phenylenediamine), or poly(9,9-dioctylfluorene-alt-N-(4-butylphenyl)-diphenylamine). We find that the EA nulling voltage, i.e., the dc bias at which the EA signal vanishes, depends on the frequency of the ac voltage and on the incident photon wavelength. Such dependence poses a problem for accurate measurement of the built-in voltage (VBI), which is the voltage generated between the electrodes upon equilibration of the Fermi levels through the heterostructure. We find that the EA signal is mixed with a smaller intensity signal which can be ascribed to excited state absorption (ESA). We propose a method for separating the excited state absorption signal and producing accurate VBI measurements. We also demonstrate that in our devices the ESA contribution to the electromodulated (ΔT/T) signal is negligible with respect to the accuracy with which VBI can be determined from the nulling voltage of ΔT/T and can thus be safely ignored. [ABSTRACT FROM AUTHOR]