Varun Sivaram | University of Oxford (original) (raw)
Address: san francisco, CA
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Papers by Varun Sivaram
Mesoporous ceramics and semiconductors enable low-cost solar power, solar fuel, (photo)catalyst a... more Mesoporous ceramics and semiconductors enable low-cost solar
power, solar fuel, (photo)catalyst and electrical energy storage
technologies1. State-of-the-art, printable high-surface-area electrodes
are fabricated from thermally sintered pre-formed nanocrystals2–
5. Mesoporosity provides the desired highly accessible surfaces
but many applications also demand long-range electronic connectivity
and structural coherence6. A mesoporous single-crystal
(MSC) semiconductor can meet both criteria. Here we demonstrate
a general synthetic method of growing semiconductor MSCs of
anatase TiO2 based on seeded nucleation and growth inside a mesoporous
template immersed in a dilute reaction solution. We show
that both isolatedMSCs and ensembles incorporated into filmshave
substantially higher conductivities and electron mobilities than
does nanocrystalline TiO2. Conventional nanocrystals, unlike
MSCs, require in-filmthermal sintering to reinforce electronic contact
between particles, thus increasing fabrication cost, limiting the
use of flexible substrates and precluding, for instance, multijunction
solar cell processing. UsingMSCfilms processed entirely below
150 6C, we have fabricated all-solid-state, low-temperature sensitized
solar cells that have 7.3 per cent efficiency, the highest efficiency
yet reported. These high-surface-area anatase single crystals
will find application in many different technologies, and this
generic synthetic strategy extends the possibility of mesoporous
single-crystal growth to a range of functional ceramics and
semiconductors.
We have prepared single crystalline SnO2 and ZnO nanowires and polycrystalline TiO2 nanotubes (1D... more We have prepared single crystalline SnO2 and ZnO nanowires and polycrystalline TiO2 nanotubes (1D
networks) as well as nanoparticle-based films (3D networks) from the same materials to be used as
photoanodes for solid-state dye-sensitized solar cells. In general, superior photovoltaic performance can
be achieved from devices based on 3-dimensional networks, mostly due to their higher short circuit
currents. To further characterize the fabricated devices, the electronic properties of the different
networks were measured via the transient photocurrent and photovoltage decay techniques. Nanowirebased
devices exhibit extremely high, light independent electron transport rates while recombination
dynamics remain unchanged. This indicates, contrary to expectations, a decoupling of transport and
recombination dynamics. For typical nanoparticle-based photoanodes, the devices are usually
considered electron-limited due to the poor electron transport through nanocrystalline titania networks.
In the case of the nanowire-based devices, the system becomes limited by the organic hole transporter
used. In the case of polycrystalline TiO2 nanotube-based devices, we observe lower transport rates and
higher recombination dynamics than their nanoparticle-based counterparts, suggesting that in order to
improve the electron transport properties of solid-state dye-sensitized solar cells, single crystalline
structures should be used. These findings should aid future design of photoanodes based on nanowires
or porous semiconductors with extended crystallinity to be used in dye-sensitized solar cells.
Mesoporous ceramics and semiconductors enable low-cost solar power, solar fuel, (photo)catalyst a... more Mesoporous ceramics and semiconductors enable low-cost solar
power, solar fuel, (photo)catalyst and electrical energy storage
technologies1. State-of-the-art, printable high-surface-area electrodes
are fabricated from thermally sintered pre-formed nanocrystals2–
5. Mesoporosity provides the desired highly accessible surfaces
but many applications also demand long-range electronic connectivity
and structural coherence6. A mesoporous single-crystal
(MSC) semiconductor can meet both criteria. Here we demonstrate
a general synthetic method of growing semiconductor MSCs of
anatase TiO2 based on seeded nucleation and growth inside a mesoporous
template immersed in a dilute reaction solution. We show
that both isolatedMSCs and ensembles incorporated into filmshave
substantially higher conductivities and electron mobilities than
does nanocrystalline TiO2. Conventional nanocrystals, unlike
MSCs, require in-filmthermal sintering to reinforce electronic contact
between particles, thus increasing fabrication cost, limiting the
use of flexible substrates and precluding, for instance, multijunction
solar cell processing. UsingMSCfilms processed entirely below
150 6C, we have fabricated all-solid-state, low-temperature sensitized
solar cells that have 7.3 per cent efficiency, the highest efficiency
yet reported. These high-surface-area anatase single crystals
will find application in many different technologies, and this
generic synthetic strategy extends the possibility of mesoporous
single-crystal growth to a range of functional ceramics and
semiconductors.
We have prepared single crystalline SnO2 and ZnO nanowires and polycrystalline TiO2 nanotubes (1D... more We have prepared single crystalline SnO2 and ZnO nanowires and polycrystalline TiO2 nanotubes (1D
networks) as well as nanoparticle-based films (3D networks) from the same materials to be used as
photoanodes for solid-state dye-sensitized solar cells. In general, superior photovoltaic performance can
be achieved from devices based on 3-dimensional networks, mostly due to their higher short circuit
currents. To further characterize the fabricated devices, the electronic properties of the different
networks were measured via the transient photocurrent and photovoltage decay techniques. Nanowirebased
devices exhibit extremely high, light independent electron transport rates while recombination
dynamics remain unchanged. This indicates, contrary to expectations, a decoupling of transport and
recombination dynamics. For typical nanoparticle-based photoanodes, the devices are usually
considered electron-limited due to the poor electron transport through nanocrystalline titania networks.
In the case of the nanowire-based devices, the system becomes limited by the organic hole transporter
used. In the case of polycrystalline TiO2 nanotube-based devices, we observe lower transport rates and
higher recombination dynamics than their nanoparticle-based counterparts, suggesting that in order to
improve the electron transport properties of solid-state dye-sensitized solar cells, single crystalline
structures should be used. These findings should aid future design of photoanodes based on nanowires
or porous semiconductors with extended crystallinity to be used in dye-sensitized solar cells.