Fullerene Network Research Articles

Bulk Heterojunction Solar Cells: Impact of Minor Structural Modifications to the Polymer Backbone on the Polymer–Fullerene Mixing and Packing and on the Fullerene–Fullerene Connecting Network

AbstractThe morphology of the active layer of a bulk heterojunction solar cell, made of a blend of an electron‐donating polymer and an electron‐accepting fullerene derivative, is known to play a determining role in device performance. Here, a combination of molecular dynamics simulations and long‐range corrected density functional theory calculations is used to elucidate the molecular‐scale effects that even minor structural changes to the polymer backbone can have on the “local” morphology; this study focuses on the extent of polymer–fullerene mixing, on their packing, and on the characteristics of the fullerene–fullerene connecting network in the mixed regions, aspects that are difficult to access experimentally. Three representative polymer donors are investigated: (i) poly[(5,6‐difluoro‐2,1,3‐benzothiadiazol‐4,7‐diyl)‐alt‐(3,3′″‐di(2‐octyldodecyl)‐2,2′;5′,2″;5″,2′″‐quaterthiophen‐5,5′″‐diyl)] (PffBT4T‐2OD); (ii) poly[(2,1,3‐benzothiadiazol‐4,7‐diyl)‐alt‐(3,3′″‐di(2‐octyldodecyl)‐2,2′;5′,2″;5″,2′″‐quaterthiophen‐5,5′″‐diyl)] (PBT4T‐2OD), where the fluorine atoms in the benzothiadiazole moieties of PffBT4T‐2OD are replaced with hydrogen atoms; and (iii) poly[(2,2′‐bithiophene)‐alt‐(4,7‐bis((2‐decyltetradecyl)thiophen‐2‐yl)‐5,6‐difluoro‐2‐propyl‐2H‐benzo[d][1,2,3]triazole)] (PT2‐FTAZ), where the sulfur atoms in the benzothiadiazole moieties of PffBT4T‐2OD are replaced with nitrogen atoms carrying a linear C3H7 side‐chain; these polymers are mixed with the phenyl‐C71‐butyric acid methyl ester (PC71BM) acceptor. This study also discusses the nature of the charge‐transfer electronic states appearing at the donor–acceptor interfaces, the electronic couplings relevant for the charge‐recombination process, and the electron‐transfer features between neighboring PC71BM molecules.

Hydrogenation of Substituted Fullerenes – a DFT Study

Hydrogen storage by carbon materials is a topic of current interest. In order to exploit fullerenes as one of the new forms of carbon for hydrogen storage, it is shown that an activator for hydrogen is necessary in the fullerene network. Even though one can generate stoichiometric hydrides the formation of such hydrides have to be established. In this present study we have examined what type of species on carbon surfaces may be able to activate hydrogen molecule and lead to hydride formation. The Density Functional Theory calculations have been carried out on some typical model systems wherein the fullerene molecule is substituted in the network with heteroatoms like N, P and S and the reduction in the dissociation energy of hydrogen molecule is considered as a measure of the ability to hydride the carbon materials. On the basis of the reduction in the dissociation energy for the hydrogen molecule it was shown that heteroatom substitution in the fullerene net work may be suitable for the activation and dissociation of hydrogen molecule.

A new fullerene network phase obtained from C70 at high-pressure and high-temperature

Fullerene networks are an exciting class of materials that may display exceptional physical properties. A new C70-fullerene network phase, synthesized at high-pressure, 7 GPa, and high-temperature, 600 °C, is presented. Its structure, determined by Rietveld analysis of the X-ray diffraction data combined with density functional theory modeling, consists of puckered polymerized layers where each molecule is bonded to three neighbors through 2 + 2 cycloaddition bonds, a new polymeric configuration unseen in other fullerene networks. This new C70-fullerene network structure adds to the previous zigzag structures, demonstrating the potential of C70 molecules as building blocks to synthesize new carbon phases. (© 2015 WILEY-VCH Verlag GmbH &Co. KGaA, Weinheim)

Understanding Local and Macroscopic Electron Mobilities in the Fullerene Network of Conjugated Polymer‐based Solar Cells: Time‐Resolved Microwave Conductivity and Theory

The efficiency of bulk heterojunction (BHJ) organic photovoltaics is sensitive to the morphology of the fullerene network that transports electrons through the device. This sensitivity makes it difficult to distinguish the contrasting roles of local electron mobility (how easily electrons can transfer between neighboring fullerene molecules) and macroscopic electron mobility (how well‐connected is the fullerene network on device length scales) in solar cell performance. In this work, a combination of density functional theory (DFT) calculations, flash‐photolysis time‐resolved microwave conductivity (TRMC) experiments, and space‐charge‐limit current (SCLC) mobility estimates are used to examine the roles of local and macroscopic electron mobility in conjugated polymer/fullerene BHJ photovoltaics. The local mobility of different pentaaryl fullerene derivatives (so‐called ‘shuttlecock’ molecules) is similar, so that differences in solar cell efficiency and SCLC mobilities result directly from the different propensities of these molecules to self‐assemble on macroscopic length scales. These experiments and calculations also demonstrate that the local mobility of phenyl‐C60 butyl methyl ester (PCBM) is an order of magnitude higher than that of other fullerene derivatives, explaining why PCBM has been the acceptor of choice for conjugated polymer BHJ devices even though it does not form an optimal macroscopic network. The DFT calculations indicate that PCBM's superior local mobility comes from the near‐spherical nature of its molecular orbitals, which allow strong electronic coupling between adjacent molecules. In combination, DFT and TRMC techniques provide a tool for screening new fullerene derivatives for good local mobility when designing new molecules that can improve on the macroscopic electron mobility offered by PCBM.

Moving through the Phase Diagram: Morphology Formation in Solution Cast Polymer–Fullerene Blend Films for Organic Solar Cells

The efficiency of organic bulk heterojunction solar cells strongly depends on the multiscale morphology of the interpenetrating polymer-fullerene network. Understanding the molecular assembly and the identification of influencing parameters is essential for a systematic optimization of such devices. Here, we investigate the molecular ordering during the drying of doctor-bladed polymer-fullerene blends on PEDOT:PSS-coated substrates simultaneously using in situ grazing incidence X-ray diffraction (GIXD) and laser reflectometry. In the process of blend crystallization, we observe the nucleation of well-aligned P3HT crystallites in edge-on orientation at the interface at the instant when P3HT solubility is crossed. A comparison of the real-time GIXD study at ternary blends with the binary phase diagrams of the drying blend film gives evidence of strong polymer-fullerene interactions that impede the crystal growth of PCBM, resulting in the aggregation of PCBM in the final drying stage. A systematic dependence of the film roughness on the drying time after crossing P3HT solubility has been shown. The highest efficiencies have been observed for slow drying at low temperatures which showed the strongest P3HT interchain π-π-ordering along the substrate surface. By adding the "unfriendly" solvent cyclohexanone to a chlorobenzene solution of P3HT:PCBM, the solubility can be crossed prior to the drying process. Such solutions exhibit randomly orientated crystalline structures in the freshly cast film which results in a large crystalline orientation distribution in the dry film that has been shown to be beneficial for solar cell performance.

The role of alkane dithiols in controlling polymer crystallization in small band gap polymer:Fullerene solar cells

AbstractThe effect of the addition of 1,8‐octanedithiol (ODT) during processing on the microstructure of blend films of poly[2,6‐(4,4‐bis‐(2‐ethylhexyl)‐4H‐cyclopenta[2,1‐b;3,4‐b′]dithiophene)‐alt‐4,7(2,1,3‐benzothiadiazole)] (PCPDTBT) and [6,6]‐phenyl‐C71 butyric acid methyl ester ([70]PCBM) is studied. Grazing incidence X‐ray diffraction and absorption spectroscopy show that the crystalline order of PCPDTBT increases when ODT is introduced in the solution phase either to neat polymer systems or to blends with [70]PCBM. The increased crystalline order is accompanied by less dispersive hole transport in the polymer, and leads to a more efficient formation of a percolating fullerene network within the blend. This contributes to an increase in photocurrent generation. However, the bimolecular recombination rate as determined from photovoltage transients increases upon addition of ODT, limiting the power conversion efficiency to values well below those expected from the energy levels of PCPDTBT. We propose some explanations for this increase in bimolecular recombination, based also on variable angle spectroscopic ellipsometry measurements. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2011

Effect of multiple adduct fullerenes on charge generation and transport in photovoltaic blends with poly(3‐hexylthiophene‐2,5‐diyl)

AbstractThe effect of replacing [6,6]‐phenyl‐C61 butyric acid methyl ester (PCBM) by its multiadduct analogs (bis‐PCBM and tris‐PCBM) in bulk heterojunction organic solar cells with poly(3‐hexylthiophene‐2,5‐diyl) (P3HT) is studied in terms of blend film microstructure, photophysics, electron transport properties, and device performance. Although the power conversion efficiency of the blend with bis‐PCBM is similar to the blend with PCBM, the performance of the devices with tris‐PCBM is considerably lower as a result of small photocurrent. Despite the lower electron affinity of the fullerene multiadducts, μs‐ms transient absorption measurements show that the charge generation efficiency is similar for all three fullerenes. The annealed blend films with multiadducts show a lower degree of fullerene aggregation and lower P3HT crystallinity than the annealed blend films with PCBM. We conclude that the reduction in performance is due largely to poorer electron transport in the blend films from higher adducts, due to the poorer fullerene network formation as well as the slower electron transport within the fullerene phase, confirmed here by field effect transistor measurements. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2010

Electronic structure and localization properties ofC60Tanclusters(n=1,3): A first-principles study

The electronic structure and the electronic localization properties of the exohedrally doped fullerene ${\text{C}}_{60}\text{Ta}$, ${\text{C}}_{60}{\text{Ta}}_{2}$, and ${\text{C}}_{60}{\text{Ta}}_{3}$ systems are studied in the framework of density functional theory calculations. The effect of doping the fullerene network with Ta impurities results in modifications of the Kohn--Sham energy levels spectrum in the highest occupied molecular orbital--lowest unoccupied molecular orbital (HOMO-LUMO) region and a drastic HOMO-LUMO band-gap reduction. In the vicinity of the HOMO, most of the occupied electronic states are Ta-like for ${\text{C}}_{60}\text{Ta}$ and ${\text{C}}_{60}{\text{Ta}}_{2}$, while C-like states or mixed C-Ta-like states are predominant for the case of ${\text{C}}_{60}{\text{Ta}}_{3}$. In all cases, we observe a conspicuous charge transfer from the Ta to the neighboring C atoms, Mulliken charges are positive on the Ta atoms and equal to 2.12 $({\text{C}}_{60}\text{Ta})$, 1.77/1.80 $({\text{C}}_{60}{\text{Ta}}_{2})$, and 1.61/1.62 $({\text{C}}_{60}{\text{Ta}}_{3})$. The values of the valence charges on the Ta atoms reflect their coordination environment and are the largest in ${\text{C}}_{60}{\text{Ta}}_{3}$ (3.00/3.25). This is compatible with the existence of three nearest neighbors (two Ta and one C) for each one of the Ta atoms in ${\text{C}}_{60}{\text{Ta}}_{3}$. We provide an insight into the physical nature of bonding by means of an accurate electronic structure analysis in terms of the electron localization function and the maximally localized Wannier orbitals. Among the Ta valence electrons, the most localized ones are those not involved in bond formation, charge transfer effects concurring to the establishment of ionic-covalent bonds between the Ta and the neighboring C atoms.

Endohedral nitrogen storage in carbon fullerene structures: Physisorption to chemisorption transition with increasing gas pressure

We present extensive pseudopotential density functional theory (DFT) calculations in order to analyze the structural properties and chemical reactivity of nitrogen molecules confined in spheroidal (C(82)) and tubelike (C(110)) carbon fullerene structures. For a small number of encapsulated nitrogens, the N(2) species exist in a nonbonded state within the cavities and form well defined molecular conformations such as linear chains, zigzag arrays, as well as both spheroidal and tubular configurations. However, with increasing the number of stored molecules, the interaction among the confined nitrogens as well as between the N(2) species and the fullerene wall is not always mainly repulsive. Actually, at high densities of the encapsulated gas, we found both adsorption of N(2) to the inner carbon surface together with the formation of (N(2))(m) molecular clusters. Total energy DFT calculations reveal that the shape of the interaction potential of a test molecule moving within the carbon cavities strongly varies with the number and proximity of the coadsorbed N(2) from being purely repulsive to having short-range attractive contributions close to the inner wall. In particular, the latter are always found when a group of closely spaced nitrogens is located near the carbon cage (a fact that will naturally occur at high densities of the encapsulated gas), inducing the formation of covalent bonds between the N(2) and the fullerene network. Interestingly, in some cases, the previous nitrogen adsorption to the inner surface is reversible by reducing the gas pressure. The calculated average density of states of our considered carbon compounds reveals the appearance of well defined features that clearly reflect the occurring structural changes and modifications in the adsorption properties in the systems. Our results clearly underline the crucial role played by confinement effects on the reactivity of our endohedral compounds, define this kind of materials as nonideal nanocontainers for high density nitrogen storage applications, and must be taken into account when analyzing the diffusion properties of the encapsulated species.

Influence of side chain symmetry on the performance of poly(2,5-dialkoxy- p-phenylenevinylene): fullerene blend solar cells

We report on studies of poly-(2,5-dihexyloxy- p-phenylenevinylene) (PDHeOPV), a symmetric side-chain polymer, as a potential new donor material for polymer:fullerene blend solar cells. We study the surface morphology of blend films of PDHeOPV with PCBM, the transport properties of the blend films, and the performance of photovoltaic devices made from such blend films, all as a function of PCBM content. In each case, results are compared with those obtained using the asymmetric side chain polymer, poly[2-methoxy-5-(3,7-dimethyloctyloxy)-1,4-phenylenevinylene] (MDMO-PPV), in order to investigate the influence of polymer side chain symmetry on solar cell performance. AFM images show that large PCBM aggregates appear at lower PCBM content (50 wt.% PCBM) for PDHeOPV:PCBM than for MDMO-PPV:PCBM (67 wt.% PCBM) blend films. Time-of-Flight (ToF) mobility measurements show that charge mobilities depend more weakly on PCBM content in PDHeOPV:PCBM than in MDMO:PPV:PCBM, with the result that at high PCBM content the mobilities in PDHeOPV:PCBM are significantly lower than in MDMO:PPV:PCBM blend films, despite the higher mobilities in pristine PDHeOPV compared to pristine MDMO-PPV. Photovoltaic devices show significantly lower power conversion efficiency (∼0.93%) for PDHeOPV:PCBM (80 wt.% PCBM) blend films than for MDMO-PPV:PCBM (2.2% at 80 wt.% PCBM) blends. This is attributed to the relatively poor transport properties of the PDHeOPV:PCBM blend, which limit the optimum thickness of the photoactive layer in PDHeOPV:PCBM blend devices. The behaviour is tentatively attributed to a higher tendency for the symmetric side-chain polymer chains to aggregate, resulting in poorer interaction with the fullerene and poorer network formation for charge transport.

Room to Improve Conjugated Polymer-Based Solar Cells: Understanding How Thermal Annealing Affects the Fullerene Component of a Bulk Heterojunction Photovoltaic Device

We examine how thermal annealing affects the fullerene network in conjugated polymer bulk heterojunction (BHJ) solar cells. We begin by creating electron-only devices with a BHJ geometry by blending the fullerene derivative [6,6]-phenyl-C61-butyric-acid-methyl-ester (PCBM) with polystyrene (PS). These electron-only PS:PCBM diodes function even with a poly(ethylenedioxithiophene):poly(styrenesulfonate) (PEDOT:PSS) layer, indicating that PEDOT:PSS films do not serve as electron blocking layers. Atomic force microscopy shows that the degree of phase segregation in the PS:PCBM blend films is similar to that in the active layer of blends of PCBM with poly(3-hexylthiophene-2,5-diyl) (P3HT), so that the PS:PCBM blends provide a good model for the fullerene part of the BHJ network in P3HT:PCBM solar cells. We find that thermal annealing dramatically decreases the electron current that flows in the PS:PCBM diodes, suggesting that annealing leads to increased phase segregation that lowers the electron mobility on fullerenes in the BHJ geometry. We also find that annealing increases the photoluminesence of P3HT:PCBM blend films, indicating that thermal treatment produces increased phase segregation that leads to decreased exciton harvesting. The fact that annealing decreases both exciton harvesting and electron mobility implies that there is significant room to further improve polymer/fullerene photovoltaics by controlling the amount of phase segregation.

Long-lived charge separated state in molecular films containing porphyrin–fullerene dyad

A photoinduced interlayer charge separation in bilayered molecular film, constructed by the Langmuir–Blodgett technique, was demonstrated. The lifetime of the primary charge separated state in a monolayer of porphyrin–fullerene dyad, P– F, was found to be in a microsecond time domain, and was increased due to the secondary electron transfer from polythiophene, PHT, to the porphyrin cation of the dyad. In the final state positive charges are located at the PHT layer and negative charges in the fullerene network, yielding the long lifetime, about 150 μs, in the final interlayer charge separated state.

Magnetic properties of nanocarbon

Our experimental results provide the evidence that ferromagnetic properties of polymerized fullerenes are of non-trivial origin. Comparison of magnetic measurements, impurity analysis and Raman spectroscopy proves that in certain cases the sample structure affects the magnetic ordering. We provide the explanation of the results obtained of fullerenes within the model of charged fullerenes, where the charge comes from various defects and impurities. It is important that the impurities do not have to be magnetic: even carbon impurity in the all-carbon fullerene network can act as a magnetic trigger.

Nonorthogonal tight-binding study 0f Si 60 cluster

The lowest-energy structure of Si 60 cluster is obtained by using a genetic algorithm (GA) based on nonorthogonal tight-binding (NTB) model. The atoms are organized into two shells. The outer shell forms a distorted cage consisting of 50 atoms and the inner shell with 10 atoms. The inner shell is looser than the outer shell. The density of states (DOS) and the HOMO-LUMO gap are also given. The Si 60 fullerene cage structure and network structure are also studied using molecular dynamics annealing based on NTB model. It is found that the cage structure of Si 60 is not stable, the network structure is stable, although it is not the lowest-energy structure. The single point energies on the different structures are also calculated by using ab initio method at B3LYP/LANL2DZ level and Kwon's tight binding model. Although the total energy differences obtained by these two methods are not same, both of them confirm that the energy of the structure obtained by NTB-GA is the lowest.

Thermal and Tribological Properties of Fullerene‐Containing Composite Systems. Part 2. Formation of Tribo‐Polymer Films during Boundary Sliding Friction in the Presence of Fullerene C60

The techniques of mass‐spectrometric thermal analysis, wide‐angle X‐ray diffraction, scanning and transmission electron microscopy, light microscopy, and hardness and tribological measurements were used for study of formation and properties of protective—anti‐wear and friction‐reducing—tribo‐polymer film forming on metal surfaces during sliding boundary friction, in the presence of fullerene C60 used as an additive to some mineral lubricating oils and greases. A possible mechanism of formation of tribo‐polymer film was proposed based on the idea of attachment of paraffin and naphthenic oligomers of the lubricants to C60 molecules, which in turn are polymerized and form a hard fullerene network with attached polyolefin chains. A fullerene network provides a high wear resistance while polyolefin chains provide some decrease in the friction coefficient and widening of the working intervals of friction joints.

Boron‐Substituted Fullerenes—Can They be One of the Options for Hydrogen Storage?

The need for an activator for hydrogenation of carbon nanomaterials is outlined. The positions at which boron atoms are substituted in the fullerene network have been identified for easy hydrogenation.

Superconducting three-dimensional networks in a magnetic field: Frustrated systems

Making use of the de Gennes-Alexander network equation, we have investigated the transition temperature of three-dimensional superconducting networks in a magnetic field. For the magnetic field H(0.75<Ha 2 /Φ 0 < 2.25), a superconducting tetrahedron has a nonsuperconducting vertex because of the frustration. In contrast to this, the C60 fullerene network shows antifluxons when the magnetic field is normal to one of the pentagons and hexagons, because of the frustration that comes from the rotational symmetry around these polygons.

Clustering ofH2molecules encapsulated in fullerene structures

We have performed semiempirical (MNDO) as well as ab initio density-functional theory calculations at $T=0$ to analyze the hydrogen storage behavior in spheroidal ${\mathrm{C}}_{60}$ and ${\mathrm{C}}_{82},$ and cylindrical finite-length (5,5) armchair C and BN fullerenes. We have found that, while chemisorption of individual H atoms to the external surface of the fullerenes is observed, hydrogen atoms cannot be attached to the inner wall of the structures and can only exist in a molecular form inside the fullerenes. We further find that, as a function of the symmetry of the encapsulating cavity and a delicate balance between repulsive energies among ${\mathrm{H}}_{2}$ molecules inside the structures and those between ${\mathrm{H}}_{2}$ molecules and the fullerene walls, molecular $({\mathrm{H}}_{2}{)}_{N}$ clusters of well defined shape are formed namely: linear configurations, two-dimensional zig-zag and triangular arrays, and three-dimensional structures such as octahedral and icosahedral clusters, as well as helicoidal cylindrical-shape assemblies. In the cylindrical configurations (C and BN tubes), hydrogen atoms are placed inside the structures up to a bond breakage of the fullerene network, which allow us to estimate the maximum storage capacities of the different configurations. Actually, in our closed nanotubes, we relate the bond breakage mechanism to the development of a nonuniform hydrogen accommodation along the tubes, driven by the both highly anisotropic ${\mathrm{H}}_{2}\ensuremath{-}{\mathrm{H}}_{2}$ and wall-${\mathrm{H}}_{2}$ repulsive interactions. With increasing the number of stored ${\mathrm{H}}_{2},$ tubes are found to be mainly radially deformed, a fact that reduces (up to $\ensuremath{\sim}13%)$ the energy difference between the highest occupied and lowest unoccupied molecular orbitals in the structures. Finally, saturation of the tube ends with molecular terminations results in stable compounds from which a density-controlled storage of hydrogen seems to be possible.

Stability and electronic properties of Si-doped carbon fullerenes

The electronic structure and stability of several Si-doped ${\mathrm{C}}_{60}$ and ${\mathrm{C}}_{70}$ fullerenes (endohedral, exohedral, and substitutional) are investigated within the framework of both semiempirical modified-neglect-of-diatomic-overlap (to determine the most stable geometrical arrangements) and ab initio density-functional theory (calculation of the electronic structure and total energies). For endohedral species, the equilibrium configurations show a tendency of the Si atoms to move from the center of the molecule toward the cage, producing sizable expansions of the carbon bonds closer to the encapsulated atoms. Exohedrally doped structures exhibit similar geometrical deformations, being localized (also around the impurity) and relatively large (expansions in the C---C bond lengths as large as $12%),$ while minor changes in the C---C distances of the cage are obtained in fullerenes with substitutional Si. In agreement with relative abundance spectra, the endohedral silicon-doped carbon fullerenes are found to be less stable when compared to the hollow structures $({\mathrm{C}}_{60}$ and ${\mathrm{C}}_{70}),$ with configurations in which the Si atoms are attached to the outside of the cage, and also with arrangements in which carbon atoms of the fullerene are replaced by silicon atoms. The small energy difference between the highest occupied molecular orbital and lowest unoccupied molecular orbital obtained with the inclusion of Si atoms into the fullerene cage as well as the weak Si---C bonds formed in these kind of arrangements seems to be at the origin of this behavior. On the other hand, substitutionally doped molecules are the ground-state configurations since Si atoms can be viewed as carbonlike atoms in the fullerene network, also due to its four valence electrons, binding thus more strongly to the carbon units of the structure. For some particular cases we analyze the influence of oxygen chemisorption on the electronic spectrum and stability of our Si-doped carbon molecules, and finally we give a qualitative estimation of the energy-barrier height for ${\mathrm{Si}}^{+}$ penetration into a ${\mathrm{C}}_{60}$ fullerene.

Formation mechanism and structural and electronic properties of metal-substituted fullerenesC69M(M=Co,Rh, and Ir)

The formation mechanism and structural and electronic properties of the metal-substituted fullerenes ${\mathrm{C}}_{69}M$ $(M=\mathrm{Co},$ Rh, and Ir) have been studied using the frontier orbitals and density functional theories. According to the symmetry of the ${\mathrm{C}}_{70}$ ${D}_{5h}$ structure, we have studied five kinds of possible substitution site for the M atom and found that it has a tendency to substitute in the site where the substituted carbon atom has the maximum contribution to the lowest unoccupied molecular orbital of ${\mathrm{C}}_{70}.$ The clusters are found to be stable in the cage structure, while the deformation of the fullerene network due to the substituted metal atom is larger than that due to a substituted boron or nitrogen atom. The electronic structure of ${\mathrm{C}}_{69}M$ can be described in terms of defect levels in the host fullerene. Deep defect states due to the metal atom appear in the carbon-derived energy gap, and high chemical reactivity is predicted for the clusters. The magnetic moment of the M atom in ${\mathrm{C}}_{69}M$ is found to be almost completely quenched due to the strong hybridization between the orbitals of the M atom and those of its neighboring carbons.