Transient spectroscopy of excitons and polarons in C60 films from femtoseconds to milliseconds

The role of spin in the degradation of organic photovoltaics

Stability is now a critical factor in the commercialization of organic photovoltaic (OPV) devices. Both extrinsic stability to oxygen and water and intrinsic stability to light and heat in inert conditions must be achieved. Triplet states are known to be problematic in both cases, leading to singlet oxygen production or fullerene dimerization. The latter is thought to proceed from unquenched singlet excitons that have undergone intersystem crossing (ISC). Instead, we show that in bulk heterojunction (BHJ) solar cells the photo-degradation of C60 via photo-oligomerization occurs primarily via back-hole transfer (BHT) from a charge-transfer state to a C60 excited triplet state. We demonstrate this to be the principal pathway from a combination of steady-state optoelectronic measurements, time-resolved electron paramagnetic resonance, and temperature-dependent transient absorption spectroscopy on model systems. BHT is a much more serious concern than ISC because it cannot be mitigated by improved exciton quenching, obtained for example by a finer BHJ morphology. As BHT is not specific to fullerenes, our results suggest that the role of electron and hole back transfer in the degradation of BHJs should also be carefully considered when designing stable OPV devices.

Femtosecond Dynamics of Photoexcited C60 Films

The well known organic semiconductor C₆₀ is attracting renewed attention due to its centimeter-long electron diffusion length and high performance of solar cells containing 95% fullerene, yet its photophysical properties remain poorly understood. We elucidate the dynamics of Frenkel and intermolecular (inter-C₆₀) charge-transfer (CT) excitons in neat and diluted C₆₀ films from high-quality femtosecond transient absorption (TA) measurements performed at low fluences and free from oxygen or pump-induced photodimerization. We find from preferential excitation of either species that the CT excitons give rise to a strong electro-absorption (EA) signal but are extremely short-lived. The Frenkel exciton relaxation and triplet yield strongly depend on the C₆₀ aggregation. Finally, TA measurements on full devices with applied electric field allow us to optically monitor the dissociation of CT excitons into free charges for the first time and to demonstrate the influence of cluster size on the spectral signature of the C₆₀ anion.

Direct observation of photocarrier electron dynamics in C60 films on graphite by time-resolved two-photon photoemission

Time-resolved two-photon photoemission (TR-2PPE) spectroscopy is employed to probe the electronic states of a C₆₀ fullerene film formed on highly oriented pyrolytic graphite (HOPG), acting as a model two-dimensional (2D) material for multi-layered graphene. Owing to the in-plane sp²-hybridized nature of the HOPG, the TR-2PPE spectra reveal the energetics and dynamics of photocarriers in the C₆₀ film: after hot excitons are nascently formed in C₆₀ via intramolecular excitation by a pump photon, they dissociate into photocarriers of free electrons and the corresponding holes, and the electrons are subsequently detected by a probe photon as photoelectrons. The decay rate of photocarriers from the C₆₀ film into the HOPG is evaluated to be 1.31 × 10¹² s⁻¹, suggesting a weak van der Waals interaction at the interface, where the photocarriers tentatively occupy the lowest unoccupied molecular orbital (LUMO) of C₆₀. The photocarrier electron dynamics following the hot exciton dissociation in the organic thin films has not been realized for any metallic substrates exhibiting strong interactions with the overlayer. Furthermore, the thickness dependence of the electron lifetime in the LUMO reveals that the electron hopping rate in C₆₀ layers is 3.3 ± 1.2 × 10¹³ s⁻¹.

Nonvolatile analog memory transistor based on carbon nanotubes and C60 molecules

A nonvolatile analog memory transistor is demonstrated by integrating C60 molecules as charge storage molecules in the transistor gate, and carbon nanotubes (CNTs) in the transistor channel. The currents through the CNT channel can be tuned quantitatively and reversibly to analog values by controlling the number of electrons trapped in the C60 molecules. After tuning, the electrons trapped in the C60 molecules in the gate, and the current through the CNT channel, can be preserved in a nonvolatile manner, indicating the characteristics of the nonvolatile analog memory.

Photoexcitation dynamics in films of C60 and Zn phthalocyanine with a layered nanostructure

We elucidate photoexcitation dynamics in C(60) and zinc phthalocyanine (ZnPc) from picoseconds to milliseconds by transient absorption and time-resolved terahertz spectroscopy. Autoionization of C(60) is a precursor to photocarrier generation. Decay of the terahertz signal is due to decreasing photocarrier mobility over the first 20 ps and thereafter reflects recombination dynamics. Singlet diffusion rates in C(60) are determined by modeling the rise of ground state bleaching of ZnPc absorption following C(60) excitation. Recombination dynamics transform from bimolecular to monomolecular as the layer thickness is reduced, revealing a metastable exciplex at the C(60)/ZnPc interface with a lifetime of 150 μs.

Pump-probe spectroscopy in organic semiconductors: monitoring fundamental processes of relevance in optoelectronics

In this review we highlight the contribution of pump-probe spectroscopy to understand elementary processes taking place in organic based optoelectronic devices. The techniques described in this article span from conventional pump-probe spectroscopy to electromodulated pump-probe and the state-of-the-art confocal pump-probe microscopy. The article is structured according to three fundamental processes (optical gain, charge photogeneration and charge transport) and the contribution of these techniques on them. The combination of these tools opens up new perspectives for assessing the role of short-lived excited states on processes lying underneath organic device operation.

Enhanced Photogeneration of Triplet Excitons in an Oligothiophene–Fullerene Blend

Photoinduced and transient absorption spectroscopy is used to study triplet exciton dynamics in thin films of a new thiophene-based oligomer (DCV3T) and blends of DCV3T and fullerene C60. We find enhanced DCV3T triplet exciton generation in the blend layer, which is explained as an excitonic ping-pong effect: singlet energy transfer from DCV3T to C60, followed by immediate intersystem crossing to C60, and triplet exciton back-transfer. Estimations of the rate constants involved show that the ping-pong effect has an overall efficiency close to unity. The singlet-singlet energy transfer from DCV3T to C60 is demonstrated by efficient quenching of DCV3T luminescence in the blend, leading to sensitized emission of C60. We discuss a promising new concept of solar cells with an enlarged active-layer thickness based on potentially long-ranged triplet exciton diffusion in combination with efficient intersystem crossing.

Ring of C60 polymers formed by electron or hole injection from a scanning tunneling microscope tip

Carrier (electron or hole) injection from a scanning tunneling microscope tip causes various surface modifications on the molecular scale. We report that injection into C60 close-packed layers forms a ring-shaped distribution of C60 polymers. This can be explained on the basis of the radial propagation and energy dissipation of carriers. Subsequent electron or hole injections enlarge the ring, showing that both carriers can induce both polymerization and depolymerization. Furthermore, we demonstrate visualization of carrier scattering by injecting carriers into C60 layers with grain boundaries.

Femtosecond pump/probe photoelectron spectroscopy of isolated C60 negative ions

We have measured pump/probe photoelectron spectra of mass-selected, near room temperature C60- in the gas phase. The lifetime of the vibrationally excited B- (2Eg) state at a calculated energy of 1.26 eV was found to be tau = 2.2+/-0.2 ps. The dominant decay process corresponds to intramolecular radiationless transitions into ground state C60-. This is in contrast to C60 for which pumping at the absorption onset (1.95 eV) leads to predominantly intersystem crossing.

Photoinduced transient stark spectroscopy in organic semiconductors: a method for charge mobility determination in the picosecond regime

Subpicosecond photoinduced Stark spectroscopy experiments are carried out for measuring charge carrier mobility in organic semiconductors. The technique is demonstrated in state-of-art devices based on methanofullerene. The transient mobility of photogenerated charge carriers is measured in the picosecond time domain. Electric field dependent mobility is observed from the earliest time scales. In addition, two distinct transport regimes are revealed: a short-lived state, approximately 10 ps, of high mobility and a transient towards the trap limited transport, associated with the mesoscopic structure of the medium.