A Quasi-Quantum Well Sensitized Solar Cell with Accelerated
Charge Separation and Collection
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Abstract
Semiconductor-sensitized solar cell
(SSSC) represents a new generation
of device aiming to achieve easy fabrication and cost-effective performance.
However, the power of the semiconductor sensitizers has not been fully
demonstrated in SSSC, making it actually overshadowed by dye-sensitized
solar cell (DSSC). At least part of the problem is related to the
inefficient charge separation and severe recombination with the current
technologies, which calls on rethinking about how to better engineer
the semiconductor sensitizer structure in order to enhance the power
conversion efficiency (PCE). Herein we report on using for the first
time a quasi-quantum well (QW) structure (ZnSe/CdSe/ZnSe) as the sensitizer,
which is quasi-epitaxially deposited on ZnO tetrapods. Such a novel
photoanode architecture has attained 6.20% PCE, among the highest
reported to date for this type of SSSCs. Impedance spectra have revealed
that the ZnSe/CdSe/ZnSe QW structure has a transport resistance only
a quarter that of, but a recombination resistance twice that of the
ZnSe/CdSe heterojunction (HJ) structure, yielding much longer electron
diffusion length, consistent with the resulting higher photovoltage,
photocurrent, and fill factor. Time-resolved photoluminescence spectroscopy
indicates dramatically reduced electron transfer from ZnO to the QW
sensitizer, a feature which is conducive to charge separation and
collection. This study together with the impedance spectra and intensity
modulated photocurrent spectroscopies supports a core/shell two-channel
transport mechanism in this type of solar cells and further suggests
that the electron transport along sensitizer can be considerably accelerated
by the QW structure employed