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Charge Generation in Non-Fullerene Donor–Acceptor Blends for Organic Solar Cells
journal contribution
posted on 2017-08-21, 19:14 authored by Nasim Zarrabi, Dani M. Stoltzfus, Paul L. Burn, Paul E. ShawThe
power conversion efficiencies of solar cells incorporating
non-fullerene donor–acceptor blends now approach those of fullerene-based devices in the best performing
examples. However, the lack of clear structure–property relationships
means that the poor device performance of many novel non-fullerene
systems cannot be readily explained. We report a series of non-fullerene
acceptors, comprising essentially the same chromophore, that differ
in terms of their shape and number of chromophores. To understand
the impact of these structural differences on charge generation, we
have employed transient absorption spectroscopy to investigate the
photophysical properties of the acceptors in blends with the conjugated
polymer PTB7. To minimize the impact of morphology, we employed a
broad range of acceptor concentrations and compared the results with
those of blends containing the fullerene derivative PC70BM. In terms
of singlet exciton harvesting, the non-fullerene acceptors exhibited
similar performance to PC70BM in blends with PTB7. The rate of singlet
exciton quenching as a function of acceptor concentration was consistent
with exciton diffusion mediated quenching with a diffusion length
of 4–5 nm for the PTB7 singlet exciton. The polaron generation
efficiency of the non-fullerene acceptors was comparable to that of
PC70BM although the fraction of polarons that subsequently underwent
geminate recombination was much greater in the non-fullerene blends.
Furthermore, the photophysical properties of the non-fullerene acceptors
were not influenced by the shape of the acceptor, the chromophore
number, or the donor–acceptor ratio, which indicates that the
higher geminate recombination is related to the structure of the acceptor
chromophore and the interface formed with the donor. The implication
of these results is that despite appropriate energetics and optical
absorption, the performance of some non-fullerene donor–acceptor
blends will be intrinsically limited by the choice of acceptor chromophore
and the nature of the interface it forms with the donor material.