Reconciling the Driving Force and the Barrier to Charge Separation in Donor–Nonfullerene Acceptor Films

Abstract

We investigate the dependence of charge yields on the driving force for photoinduced electron transfer in a series of all-small-molecule, semiconducting films made from indacenodithiophene nonfullerene acceptors (IDT NFAs). In contrast to reports of efficient, barrierless charge separation at near zero driving force for NFA-containing organic photovoltaics, we find that barrierless charge separation occurs only if the driving force is sufficient to overcome the Coulomb potential, ≥300 meV, based on time-resolved microwave conductivity and PL quenching measurements of 5 mmolal sensitized and 700 mmolal blended films comprising IDT NFAs paired with three different donors. This discrepancy with recent literature is caused by a difference in the way that we calculate driving force. Far from being semantic, the driving force calculation is crucial because its value controls the mechanisms needed to explain experimental observations. We provide a candid assessment of the uncertainties for our methods and popular ones used in the literature and emphasize the importance of standardizing methods across this field.

Ferroelectric Polymer Drives Performance Enhancement of Non‐fullerene Organic Solar Cells

“…Apart from providing the power for CTEs dissociation, built‐in electric field is also responsible for the excitons diffusion and carriers transport to the corresponding electrode for collection and to avoid recombination [17] . However, in the OSCs, built‐in electric field is too small to provide sufficient driving for SEs and CTEs to split completely, or for the free charges generated by separation to migrate quickly, resulting in the charges captured in the traps or accumulated at the interfaces [18] . Such suboptimal charge dynamics normally causes a pronounced non‐radiative recombination of charges or excitons, which has been identified as the main energy loss ( E loss ) limiting the photovoltaic efficiency of OSCs [19, 20] .…”

“…[17] However, in the OSCs, built-in electric field is too small to provide sufficient driving for SEs and CTEs to split completely, or for the free charges generated by separation to migrate quickly, resulting in the charges captured in the traps or accumulated at the interfaces. [18] Such suboptimal charge dynamics normally causes a pronounced non-radiative recombination of charges or excitons, which has been identified as the main energy loss (E loss ) limiting the photovoltaic efficiency of OSCs. [19,20] Nevertheless, strengthening the built-in electric field to promote exciton separation and transport, and reduce the nonradiative recombination, is of great significance to improve the PCE of the OSCs.…”

Ferroelectric Polymer Drives Performance Enhancement of Non‐fullerene Organic Solar Cells

“…Apart from providing the power for CTEs dissociation, built‐in electric field is also responsible for the excitons diffusion and carriers transport to the corresponding electrode for collection and to avoid recombination [17] . However, in the OSCs, built‐in electric field is too small to provide sufficient driving for SEs and CTEs to split completely, or for the free charges generated by separation to migrate quickly, resulting in the charges captured in the traps or accumulated at the interfaces [18] . Such suboptimal charge dynamics normally causes a pronounced non‐radiative recombination of charges or excitons, which has been identified as the main energy loss ( E loss ) limiting the photovoltaic efficiency of OSCs [19, 20] .…”

Tuning Excited-State Energy Transfer for Light Energy Conversion: A Virtual Issue

“…These studies revealed that the barrierless charge separation occurs only if the driving force is sufficient to overcome the Coulomb potential. 26 The recent advancements in utilizing the excitedstate energy transfer for enhanced light energy conversion demonstrate its potential as an emergent strategy to realize artificial photosynthetic mimics.…”

“…Reid, Rumbles, and co-workers investigated the dependence of charge yields on the driving force for photoinduced electron transfer in indaceno­dithiophene NFA-based semiconducting films. These studies revealed that the barrierless charge separation occurs only if the driving force is sufficient to overcome the Coulomb potential . The recent advancements in utilizing the excited-state energy transfer for enhanced light energy conversion demonstrate its potential as an emergent strategy to realize artificial photosynthetic mimics.…”