Herein,
we investigated a series of fullerene-free organic solar
cells (OSCs) based on six different donor:acceptor (D:A) blends with
varied highest occupied molecular orbital (HOMO) offsets from −0.05
to 0.21 eV. First, to verify the energetic compatibility of a specific
D:A pair, especially for HOMO offsets, we established a simple method
to estimate the hole transfer tendencies between D and A by using
bilayer hole-only devices. It reveals that the asymmetrical diode
effect of the bilayer hole-only devices can correlate with the FF
and J
sc of the relevant OSCs. Second,
to find out whether HOMO offset is the main restriction of hole transfer,
we measured transient absorption spectra and examined the hole transfer
behavior in the blends, revealing that the occurrence of hole transfer
is independent of the HOMO offsets and ultrafast in the time scale
of ≤4.6 ps for those blends with ≥0 eV HOMO offsets.
In contrast, a negative HOMO offset can significantly slow down the
hole transfer with a half-time of ∼400 ps. Furthermore, we
compare the device parameters under varied light intensities and discover
that the bimolecular recombination should be one of the main restrictions
for high device performance. Surprisingly, small HOMO offsets of 0
and 0.06 eV can also enable high PCEs of 10.42% and 11.75% for blend
2 (PTQ10:HC-PCIC) and blend 3 (PBDB-TF:HC-PCIC), respectively. Overall,
our work demonstrates not only the validity of high-performance OSCs
operating at the near zero HOMO offsets but also the charge dynamic
insights of these blends, which will help gain understanding on the
further improvement of OSCs.
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