Biomedical Engineering Reference
In-Depth Information
22.4.1.3 Conductive SMO Pathway
Under.operating.conditions,.the.electrons.are.required.to.diffuse.several.microns.in.the.
SMO.surrounded.by.electron.acceptors.(dye,.electrolyte).only.several.nanometers.away..
Fortuitously,. recombination. is. slow,. but. overall. eficiency. remains. limited. by. the. slow.
effective. diffusivity. of. electrons. within,. e.g.,. the. TiO
2
. conductive. pathways.. These. dif-
fusivities.of.~10
-5
.to.10
-4
.cm
2
/s.are.at.least.several.orders.of.magnitude.lower.than.those.
reported.for.single.crystal.anatase..The.low.D.values.are.attributed.to.multiple.trapping.
of.carriers,.with.the.trap.states.exhibiting.an.exponential.energy.distribution..While.the.
traps. are. suspected. to. be. due. to. charged. defects,. grain. boundaries,. and. surface. states,.
only.limited.evidence.supports.these.choices..Because.the.SMO.networks.are.composed.
of.arrays.of.nano-sized.particles.connected.either.three.dimensionally.or.in.the.form.of.
pseudo.1D.wires,.each.electron.must.traverse.large.numbers.of.grain.boundaries.before.
reaching.the.transparent.electrode..Small.changes.in.grain.boundary.chemistry.or.struc-
ture. can. have. a. large. impact. on. the. density. and. distribution. of. traps. at. the. boundar-
ies..Correlations.between.boundary.chemistry/structure.and.trap.therefore.need.further.
investigation.
22.4.1.4 Hierarchical Porous 3D SMO Network
Electron.transport.dynamics.modeled.using.simulated.mesoporous.random
nanoparticle.
TiO
2
. ilms. by. random-walk. diffusion. ind. that. (1). the. average. number. of. particles. vis-
ited.by.electrons,.and.as.a.result.the.pathway,.increases.signiicantly.with.higher.porosity,.
and. (2). ilms. composed. of. ordered. particles. with. similar. porosity. show. a. reduced. path.
length.
Indeed,.the.use.of.TiO
2
.nanoibers.promises.a.directed.electron.transport.medium.as.well.
as. enhanced. light. scattering.
39
. Electrospinning. is. a. cost-effective. and. scalable. technique.
for. the. fabrication. of. 1D. nanostructures,. with. reported. eficiencies. of. electrospun. DSSCs.
as. high. as. 5.8%.
40
. Tuller. and. colleagues. have. reported. the. fabrication. of. electrospun. TiO
2
.
ibers.with.a.hierarchical.porous.structure.achieved.by.controlling.the.ratio.of.the.polyvinyl.
acetate.(PVAc)/TiO
2
.precursor.solution.used.in.spinning.(see.Figure 22.5).
41
.Here.one.obtains.
iber-like.structures.controllable.at.several.length.scales,.e.g.,.a.bundle.structure.composed.of.
sheaths.of.200.to.500.nm.and.their.cores.illed.with.aligned.10.nm.ibrils.
22.4.2 Gold Surfaces
The.present.scheme.of.BSSCs.relies.on.the.SMO.to.provide.a.conducting.medium.that.will.
keep.the.excited.electrons.from.recombining.with.charged.holes.in.the.dye..The.electrons.
will.travel.to.the.cathode,.where.they.reduce.the.electrolyte,.which.in.turn.diffuses.back.
through.the.separator.membrane.and.resupplies. electrons. to.the.photoactive.adsorbent..
Recent. experiments. with. metal-semiconductor. Schottky. barrier. thin. ilms. suggest. that.
the. liquid. electrolyte. can. be. replaced. by. thin. ilms. of. gold. on. the. metal. oxide.
42
. It. has.
been. shown. for. photoactive. dye. adsorbed. onto. 10-50. nm. of. Au. coated. onto. 200. nm. of.
TiO
2
.that.the.photoexcited.electrons.have.suficient.ballistic.momentum.to.transverse.the.
gold.layer,.into.the.TiO
2
.portion,.before.being.backscattered.to.the.dye.or.stopped.at.the.
Schottky.barrier.formed.at.the.metal-semiconductor.junction..Once.within.the.SMO.mate-
rial,.the.electrons.conduct.as.they.would.be.in.the.traditional.DSSC.scheme,.where.they.are.
