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Short and Long-Range Electron Transfer Compete to Determine Free-Charge Yield in Organic Semiconductors

Authors

Abstract

Understanding how Frenkel excitons efficiently split to form free-charges in low-dielectric constant organic semiconductors has proven challenging, with many different models proposed in recent years to explain this phenomenon. Here, we present evidence that a simple model invoking a modest amount of charge delocalization, a sum over the available microstates, and the Marcus rate constant for electron transfer can explain many seemingly contradictory phenomena reported in the literature. We use an electron-accepting fullerene host matrix dilutely sensitized with a series of electron donor molecules to test this hypothesis. The donor series enables us to tune the driving force for photoinduced electron transfer over a range of 0.7 eV, mapping out normal, optimal, and inverted regimes for free-charge generation efficiency, as measured by time-resolved microwave conductivity. However, the photoluminescence of the donor is rapidly quenched as the driving force increases, with no evidence for inverted behavior, nor the linear relationship between photoluminescence quenching and charge-generation efficiency one would expect in the absence of additional competing loss pathways. This behavior is self-consistently explained by competitive formation of bound charge-transfer states and long-range or delocalized free-charge states, where both rate constants are described by the Marcus rate equation. Moreover, the model predicts a suppression of the inverted regime for high-concentration blends and efficient ultrafast free-charge generation, providing a mechanistic explanation for why Marcus-inverted-behavior is rarely observed in device studies.

Version notes

Added author initials, corrected one miss-attributed author affiliation, updated discussion of dielectric effects on sensitizer absorption, updated discussion of possible competing photo-physical pathways, and two naming convention errors in the SI.

Content

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Supplementary material

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Supporting information for: Short and Long-Range Electron Transfer Compete to Determine Free-Charge Yield in Organic Semiconductors
Absorption and emission characteristics of all samples and control samples, cyclic voltammograms for all sensitizer and host molecules, microwave conductivity transients for all samples and parameters from the global fits to each data set, femtosecond transient-absorption characterization of every sensitizer molecule including estimated triplet yields and ISC rate-constants, time-resolved photoluminescence characterization of every sensitizer molecule, temperature-dependance of FC yield and CT state spectra calculated using the DRET model, comparison of the DRET model with a sequential intermediate CT-state model, synthesis and characterization of the squaraine donors.