Session: Physical Chemistry Poster:

This presentation will describe the experimental observation of Marcus-like photoinduced electron transfer (PET) in sensitized films of a crystalline organic semiconductor (PC₆₀BM) using time-resolved microwave conductivity. It has been proposed that the Marcus inverted region should not exist in crystalline semiconductors (e.g. fullerenes) due to a sizeable density of states (DOS) available for efficient PET. This DOS argument stems from large electronic coupling over delocalized states in these crystallites that is hypothesized to facilitate efficient charge separation despite unfavorable energetics estimated from molecular redox properties. However, the blend concentrations at which these studies were done leave open the possibility for other contributing bulk effects, which obfuscate those conclusions. In this work, we show that carefully sensitized films of PC₆₀BM with a series of small molecule donors enables the observation of an entire Marcus curve for PET – including the inverted regime. This will begin by describing the design of our sensitized films in which we achieved an appreciable range of driving forces (ΔG, 0 – 0.7 eV) calculated from cyclic voltammetry measurements. We find a relative yield of free electrons of ~60% at the optimum (ΔG, ~ 0.3 eV) and a drop to ~10% at the largest driving force (ΔG, ~ 0.7 eV). We propose that high electronic coupling of the host material does not invalidate Marcus Theory like behavior as we observe an inverted region, however, there appears to be a weakening or asymmetry in the inverted region.

Garry Rumbles

National Renewable Energy Laboratory

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