Spin-dependent charge transfer state design rules in organic photovoltaics.

Wendi Chang,David P Mcmahon,Marc A Baldo,Sebastian Reineke,Daniel N Congreve,Troy Van Voorhis,Tony C Wu,Vladimir Bulović,Eric Hontz,Matthias E Bahlke

doi:10.1038/ncomms7415

Wendi Chang, David P Mcmahon + Show 8 more

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https://doi.org/10.1038/ncomms7415

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Abstract

Charge transfer states play a crucial role in organic photovoltaics, mediating both photocurrent generation and recombination losses. In this work, we examine recombination losses as a function of the electron-hole spacing in fluorescent charge transfer states, including direct monitoring of both singlet and triplet charge transfer state dynamics. Here we demonstrate that large donor-acceptor separations minimize back transfer from the charge transfer state to a low-lying triplet exciton 'drain' or the ground state by utilizing external pressure to modulate molecular spacing. The triplet drain quenches triplet charge transfer states that would otherwise be spin protected against recombination, and switches the most efficient origin of the photocurrent from triplet to singlet charge transfer states. Future organic solar cell designs should focus on raising the energy of triplet excitons to better utilize triplet charge transfer mediated photocurrent generation or increasing the donor-acceptor spacing to minimize recombination losses.

Highlights

Charge transfer states play a crucial role in organic photovoltaics, mediating both photocurrent generation and recombination losses
We find that when a triplet drain is present, singlet Charge transfer (CT) states are responsible for the efficient generation of photocurrent, but in the absence of a triplet drain, photocurrent is more efficiently generated from the triplet CT states
We find that the CT states of conventional organic photovoltaics (OPVs) should be designed to be as physically large as possible, to minimize the back transfer from triplet CT states to triplet excitons, and with minimal sources of spin mixing within the CT manifold, to prevent intersystem crossing from the original singlet CT state

Summary

Results

We fit a single exponential to each of these time windows and compare the pressure-dependent transient dynamics of m-MTDATA:t-Bu-PBD and m-MTDATA:3TPYMB This reveals the effect of the low-lying t-Bu-PBD triplet exciton drain on the dynamics of singlet and triplet CT states. Recombination processes back to localized states increase—most notably fluorescence from the singlet CT state due to an increase in the transition dipole moment We expect both the forward and reverse intersystem crossing rates to slow for CT states localized on neighbouring molecules; the exchange splitting is proportional to the overlap of the donor HOMO and the acceptor LUMO and will increase under pressure, and to the first order, the mixing between triplet and singlet states is inversely proportional to the exchange splitting.

10 No drain

Discussion

Methods