Femtosecond Dynamics of Photoexcited C₆₀ Films

Abstract

The well known organic semiconductor C 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-C) charge-transfer (CT) excitons in neat and diluted C 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 C 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 C anion.

The role of spin in the degradation of organic photovoltaics

“…The resulting spectra are shown in Supplementary Fig. 10 and match well with published results 14,42 . The superposition of the excited state spectral signatures makes the TA spectral analysis challenging.…”

“…E T1 is the energy of the lowest excited C 60 triplet state. The BHT in neat C 60 is from charges generated from spontaneous dissociation of intermolecular excitons with CT character 14 . Errors in E CT fits are below 3 meV.…”

“…Upon irradiation of neat C 60 films, Raman and mass spectroscopy have shown the formation of C 60 oligomers with up to 20 repeat units. Based on the short C 60 singlet lifetime, the observation that the reaction is quenched in oxygen, and the linear increase in the dimerization rate with illumination intensity, it was suggested that dimerization proceeds via a [2 + 2] cycloaddition mediated by the lowest energy C 60 triplet exciton T 1 [10][11][12][13][14] . The reaction is understood to proceed identically for PC 61 BM and C 70 , although their steric groups and lower symmetry impede the bond-face alignment required for cycloaddition 15,16 .…”

Efficient Charge Generation via Hole Transfer in Dilute Organic Donor–Fullerene Blends

“…Transition rate constants for each pair of dyad states are calculated based on Fermi's golden rule and invoking the harmonic approximation (13), which is presented in detail in Section S4 of the SI and which was widely benchmarked in our previous studies [22][23][24][25] . The rate expression is based upon the energetic and structural difference between minima of the initial and the target state.…”

Terahertz short-range mobilities in neat and intermixed regions of polymer:fullerene blends with controlled phase morphology

“…We have recently shown that significant electric fields must be applied via external electrodes to generate free charges in neat C 60 and PCBM films, so that we exclude here direct generation of charges in the PCBM-rich regions due to the very weak THz probe beam. 36 The charge population derived from TA measurements is shown as dashed lines in Figure 2. We find compelling agreement of THz absorption and the time evolution of charges hinting at no significant change in the effective mobilities.…”