Quantum
Confinement-Tunable Ultrafast Charge Transfer
at the PbS Quantum Dot and Phenyl‑C<sub>61</sub>-butyric Acid
Methyl Ester Interface
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Abstract
Quantum
dot (QD) solar cells have emerged as promising low-cost
alternatives to existing photovoltaic technologies. Here, we investigate
charge transfer and separation at PbS QDs and phenyl-C<sub>61</sub>-butyric acid methyl ester (PCBM) interfaces using a combination
of femtosecond broadband transient absorption (TA) spectroscopy and
steady-state photoluminescence quenching measurements. We analyzed
ultrafast electron injection and charge separation at PbS QD/PCBM
interfaces for four different QD sizes and as a function of PCBM concentration.
The results reveal that the energy band alignment, tuned by the quantum
size effect, is the key element for efficient electron injection and
charge separation processes. More specifically, the steady-state and
time-resolved data demonstrate that only small-sized PbS QDs with
a bandgap larger than 1 eV can transfer electrons to PCBM upon light
absorption. We show that these trends result from the formation of
a type-II interface band alignment, as a consequence of the size distribution
of the QDs. Transient absorption data indicate that electron injection
from photoexcited PbS QDs to PCBM occurs within our temporal resolution
of 120 fs for QDs with bandgaps that achieve type-II alignment, while
virtually all signals observed in smaller bandgap QD samples result
from large bandgap outliers in the size distribution. Taken together,
our results clearly demonstrate that charge transfer rates at QD interfaces
can be tuned by several orders of magnitude by engineering the QD
size distribution. The work presented here will advance both the design
and the understanding of QD interfaces for solar energy conversion