J. Anders

Steady state photoconductive response of C60/C70 films

Abstract We report steady state photoconductivity in C60/C70 films. Measuring the action spectrum and the temperature dependence of the photocurrent, we could separate a low and a high temperature regime showing different transport mechanisms similar to those found in amorphous silicon.

Fullerenes in the highly excited state

Under high intensity illumination, the optical and electronic properties of fullerenes are seen to undergo dramatic, nonlinear changes. The photoluminescence emission is seen to increase with approximately the third power of the input intensity above an apparent threshold intensity. Associated with this nonlinear increase is the emergence of a long lifetime emission component and a redshifting of the emission spectrum. Investigations of the photo-transport properties associate the observed behaviour with a phase transition in the highly excited state. Above an intensity which coincides with the onset of the nonlinear emission, the photoconductive response increases with approximately the cube of the input power. In the highly excited state, the photoconductive response becomes relatively temperature independent compared to the thermally activated behaviour observed at low intensities. The characteristics of the temperature dependence are associated with a high electron mobility phase in the highly excited state and therefore an optically driven insulator to metal transition is proposed as a description of the observed phenomena.

Broadband electroluminescent emission from fullerene crystals

The observation of electroluminescence from crystalline fullerenes is described and discussed for the first time. Using gold and aluminium electrodes as contacts, a broad band emission spectrum, extending from 400 nm to 1100 nm is observed. The spectrum has a primary maximum at 920 nm and a weaker feature centered on 420 nm. The spectral characteristics are independent of the applied field and the longer wavelength region is identical to that measured in the high excitation density photoluminescence spectrum. In addition, the electroluminescent output intensity increases with the cube of the injection current, strengthening the association to the nonlinear phenomena observed in the highly excited state of fullerenes.

Photoconductivity of C60/C70 films

We report results on the steady-state photoconductivity of solid C60/C70 films including a comparison between the optical absorption and the photocondutivity action spectra, and the photocurrent temperature dependence at different wavelengths. We observed photoconductivity in the visible range of the spectrum, the temperature dependence of which indicated two distinct transport regions. The mechanisms involved are discussed and compared with the behaviour observed in hydrogenised amorphous silicon.

p-type doping of C60 films

Abstract The influence of antimony pentafluoride vapour on the electrical and optical properties of C 60 films is investigated. Exposure at room temperature is seen to have no influence, whereas a visible colour change is observed at elevated temperatures. The optical absorption spectrum is dominated by the features of the untreated sample above the band edge, but, in the region of the edge, a significant loss in oscillator strength is observed. The d.c. conductivity of the treated sample is three orders of magnitude higher than that of the C 60 film and is seen to increase by a further eight orders on heating to 400 K. The temperature dependence is, however, nonmetallic and is nonreversible. This initial heat cycle is interpreted as an annealing process resulting in further indiffusion of the p-dopant.

Many-body effects in the highly excited state of fullerenes

The highly excited state of fullerenes is characterised by a luminescence output which is dependent on the cube of the input intensity. This nonlinear emission is red shifted from the low-level emission and has a long, intensity dependent lifetime. Under similar irradiation conditions, the photoconductive response is seen to increase with the cube of the input intensity and the photocurrent in the highly excited state is observed to be largely independent of temperature, contrasting sharply with the thermally activated behaviour at low excitation densities. The degree of nonlinearity of the observed phenomena exclude an interpretation in terms of intra-molecular processes and the temperature dependence of the photoconductive response is suggestive of a Mott-like transition. The nonlinear behaviour is compared to that of indirect band-gap semiconductors in which the origin of similar nonlinear phenomena in the highly excited state luminescence and photoconductivity are explained in terms of electron-hole droplet formation. The similarities of the behaviours leads to a consideration of exchange and correlation energies in fullerenes, which are calculated according to a phenomenological model. Estimates of the contributions are consistent with a Mott-like transition at high excitation densities and an excess exchange/correlation energy in the highly excited state of ∼150 meV.

Conducting Polymers for Molecular Electronics

Basic concepts of molecular switching devices are reviewed and discussed. The applicability of simple molecular systems such as push-pull polyenes to these concepts is considered. Optically excited states are probed by transient spectroscopy and the influence of chemical modification on the lifetime of the excited state is described for bianthrone based systems.

Photophysical and photochemical processes in fullerenes under high-intensity illumination

Abstract The origins and structure of molecular and solid-state fullerenes are reviewed. Comparison of the optical properties of solution and solid state indicates strongly that the molecular nature is preserved in the solid state. Picosecond time resolved photoluminescence, photoconductivity and resonant Raman measurements are performed to investigate the influence of high-intensity illumination on the properties of Fullerene single crystals. A highly non-linear dependence of the luminescence emission efficiency and lifetime is observed on increasing the intensity. This non-linear increase is associated with a dramatic shift to the red of the emission maximum. Under similar conditions, the photoconductive response of the fullerenes is also seen to increase non-linearly with input intensity. Temperature-dependent measurements indicate that the non-linear processes are associated with an insulator-metal phase transition in the material. The transition is reversible and the observed photophysical changes coincide with a reversible shifting of the characteristic fullerene Raman lines to lower energies. At room temperature, in many samples, the shifting becomes irreversible, and a high molecular weight, insoluble material is formed. The photochemical process is proposed to be a polymerisation-like reaction of the fullerene molecules in the triplet excited state. This is supported by the observation that the rate of the reaction is reduced greatly in the presence of oxygen, an efficient triplet quencher. In conclusion, the response of Fullerene crystals to light is divided into three categories. At low intensities the photophysical processes are characteristic of those of a molecular insulator, the electronic wavefunctions being molecularly localised. At higher intensities, the material undergoes an optically-induced Mott-like transition to a semiconductor/metal, in which the electrons become delocalised in three dimensions. Thirdly, the material is found to be photochemically unstable under some conditions but analysis of the temperature and intensity dependence of Raman spectroscopy shows that the photodegradation process can be predicted and therefore controlled.

A Molecular Switch Involving Large Conformational Changes. A Theoretical Study

Abstract Ab initio and semi-empirical calculations on a supramolecular structure consisting of the electron acceptor molecule bianthrone covalently linked to the electron donor molecule 2,3-dimethyl-6-alkyl-l,4-dithiafulven are prensented. The molecule may act as a molecular switch and calculations show that a charge transfer state is involved in the initial stage of the switch process. The charge recombination process is calculated to occur early in the switching process. Experimental data supports this interpretation and calculations are used as a guideline for synthetic strategies leading to molecules with improved switching properties.

Excited state transient spectroscopy of anthracene based photochromic systems

Abstract In this study, picosecond time resolved excited state absorption spectroscopy is employed to determine the evolution and lifetime of the photochromic states in anthracene based molecular systems and the contribution of conformational changes. The unsubstituted bianthrone shows a broad transient absorption which peaks in the region of 600 nm. Equivalent measurements on anthracene give similar results indicating that this transition originates from a locally excited state on the anthrone ring system. Upon substitution of the bianthrone, an additional transient absorption at 700 nm evolves within 10 ps after excitation. This is assigned to a twisted state in which the excited state is stabilised by a strong conformational change. The metastable nature of the state is confirmed by time delay measurements.