Ab Initio and DFT‐Based Assignment of the Vibrational Spectra of Polymerized Fullerenes

Abstract

Several high‐level quantum chemical methods are used to calculate the vibrational properties of fullerenes, and the PBE/tz2p approach is shown to be the optimal one. The model to simulate the vibrational spectra of fullerenes polymers from the quantum chemical calculations of their finite fragments was analyzed. The complete interpretation of the spectra of 1D and 2D polymerized fullerenes is fulfilled.     

Fine-tuning the fluorescent properties of Li and Na intercalated C60 with hydrogen

In this work we demonstrate how hydrogen can be utilized to fine tune the emission gap of C₆₀ through the formation of direct CH bonds in sodium and lithium intercalated systems (M₆C₆₀H_x). Upon hydrogenation, a shift in the emission spectrum to shorter wavelengths (higher energy) is observed relative to alkali metal free fulleranes (C₆₀H_x) and pure C₆₀. This is attributed to the higher degree of hydrogenation of C₆₀ that can be achieved upon intercalation with alkali metals which increases the sp³ hybridization of the system (decreases conjugation). Quantum yields of the sodium and lithium intercalated fulleranes are 1.3% and 1.8% respectively which are similar to those of alkali metal free fulleranes (1.4%). We also show that polymethyl methacrylate (PMMA) can be infused with the metal intercalated fulleranes to produce a fluorescent polymer with excellent transparency over the visible spectrum. This could potentially lead to further use of these materials in luminescent down shifting applications.     

Nanoscale Morphology of Conjugated Polymer/Fullerene‐Based Bulk‐ Heterojunction Solar Cells

The relation between the nanoscale morphology and associated device properties in conjugated polymer/fullerene bulk‐heterojunction “plastic solar cells” is investigated. We perform complementary measurements on solid‐state blends of poly[2‐methoxy‐5‐(3,7‐dimethyloctyloxy)]‐1,4‐phenylenevinylene (MDMO‐PPV) and the soluble fullerene C₆₀ derivative 1‐(3‐methoxycarbonyl) propyl‐1‐phenyl [6,6]C₆₁ (PCBM), spin‐cast from either toluene or chlorobenzene solutions. The characterization of the nanomorphology is carried out via scanning electron microscopy (SEM) and atomic force microscopy (AFM), while solar‐cell devices were characterized by means of current–voltage (I–V) and spectral photocurrent measurements. In addition, the morphology is manipulated via annealing, to increase the extent of phase separation in the thin‐film blends and to identify the distribution of materials. Photoluminescence measurements confirm the demixing of the materials under thermal treatment. Furthermore the photoluminescence of PCBM clusters with sizes of up to a few hundred nanometers indicates a photocurrent loss in films of the coarser phase‐separated blends cast from toluene. For toluene‐cast films the scale of phase separation depends strongly on the ratio of MDMO‐PPV to PCBM, as well as on the total concentration of the casting solution. Finally we observe small beads of 20–30 nm diameter, attributed to MDMO‐PPV, in blend films cast from both toluene and chlorobenzene.     

Ultrafast Spectroscopic Study of Photoinduced Electron Transfer in an Oligo(thienylenevinylene):Fullerene Composite

Photoinduced electron transfer and competing processes have been studied in composites of an oligo(thienylenevinylene) (OTV), comprised of ten dibuthoxyl‐thiophene units separated by vinylene units, and a C₆₀ derivative, [6,6]‐phenyl‐C₆₁ butyric acid methyl ester (PCBM), by using femtosecond transient absorption spectroscopy and sub‐nanosecond transient photoconductivity. We find that in OTV:PCBM the photoexcitations decay primarily via intrachain relaxation rather than photoinduced electron transfer from OTV to PCBM. The electron‐transfer process requires ca. 14 ps; larger by more than two orders of magnitude than the required time observed in conjugated‐polymer:C₆₀ composites, and also larger than the 0.6 ps singlet‐state lifetime in OTV. These observations indicate that the quantum efficiency for photoinduced electron transfer in OTV:PCBM is less than 5 %.     

Cover Picture: Tuning the Dimensions of C60‐Based Needlelike Crystals in Blended Thin Films (Adv. Funct. Mater. 6/2006)

A new ordered structure of the C₆₀ derivative PCBM is obtained in thin films based on the blend PCBM:P3HT, as detailed by Swinnen, Manca, and co‐workers on p. 760. Needlelike crystalline PCBM structures, whose dimensions and spatial distribution ca be tuned by adjusting the blend ratio and annealing conditions, are formed. In typical solar‐cell applications of these blended films, these results indicate that during long‐term operation under normal conditions (50–70 °C) morphology changes and a decrease in cell performance could occur. A new ordered structure of the C₆₀ derivative PCBM ([6‐6]‐phenyl C₆₁‐butyric acid methyl ester) is obtained in thin films based on the blend PCBM:regioregular P3HT (poly(3‐hexylthiophene)). Rapid formation of needlelike crystalline PCBM structures of a few micrometers up to 100 μm in size is demonstrated by submitting the blended thin films to an appropriate thermal treatment. These structures can grow out to a 2D network of PCBM needles and, in specific cases, to spectacular PCBM fans. Key parameters to tune the dimensions and spatial distribution of the PCBM needles are blend ratio and annealing conditions. The as‐obtained blended films and crystals are probed using atomic force microscopy, transmission electron microscopy, selected area electron diffraction, optical microscopy, and confocal fluorescence microscopy. Based on the analytical results, the growth mechanism of the PCBM structures within the film is described in terms of diffusion of PCBM towards the PCBM crystals, leaving highly crystalline P3HT behind in the surrounding matrix.