Biomedical Engineering Reference
In-Depth Information
properties.of.nanoscale.metal.oxide.structures.to.gold.surfaces.for.electrolyteless.biosen-
sitized.solar.cells.
22.4.1.1 Dimension Solid-State Matrices
The. TiO
2
. nanoparticle. network. in. the. Grätzel. cell. is. the. recipient. of. injected. electrons.
from.optically.excited.dye.molecules.adsorbed.onto.its.surface..Indeed,.as.a.result.of.the.
SMO's.small.particle.size.(~20.nm),.the.surface.area.is.more.than.a.thousand.times.that.of.
a.lat.electrode.of.the.same.size..The.SMO.network.then.provides.the.conductive.pathway.
from.the.site.of.electron.injection.to.the.transparent.collecting.electrode..Lastly,.it.provides.
the interconnected.network.of.pores.through.which.the.electrolyte.percolates,.allowing.the.
redox.carrier.(e.g.,.I
3
-
).to.be.reduced.by.the.collected.electrons.at.the.counterelectrode. The.
combination.of.low.cost,.chemical.stability,.and.reasonably.high.solar-to-electrical.energy.
conversion.eficiency.(~11%).has.made.the.anatase.form.of.TiO
2
.the.SMO.material.of.choice.
for.most.DSSC.designs.
Here,.consideration.is.directed.toward.replacing.costly.ruthenium-based.dyes.with.the.
environmentally. friendly. and. lower-cost. light-harvesting. bR. protein.
This. requires. that.
several.requirements.be.satisied..The.irst.is.to.ensure.that.the.SMO.surface.is.receptive.to.
bR.dye.adhesion.and.offers.eficient.charge.transfer.from.dye.to.SMO,.but.not.from.SMO.to.
dye.or.electrolyte..A.second.objective.is.to.improve.the.conductive.SMO.pathway,.limited.
by. electron. diffusivities. several. orders. of. magnitude. lower. than. those. in. single. crystal.
anatase.. A. third. objective. is. to. create. a. hierarchical. porous. 3D. SMO. network. offering. a.
high.surface.area.for.dye.attachment,.reduced.electron.pathways,.and.the.opportunity.for.
tandem.cell.operation..These.are.discussed.below.
22.4.1.2 Charge Transfer between Dye and SMO
The.success.of.the.DSSC.is.the.ability.of.the.dye.to.absorb.in.the.visible.portion.of.the.solar.
spectrum.and.transfer.the.excited.electron.to.the.SMO.conduction.band.while.trapping.
the.hole.until.it.can.be.used.to.oxidize.an.ionic.species.in.the.electrolyte..Given.the.small.
number.of.suitable.SMOs.so.far.identiied.for.DSSC.operation,.the.options.for.optimiz-
ing.cell.performance.by.replacing.the.dye.remain.limited..As.a.consequence,.means.for.
tuning.the.properties.of.suitable.SMOs.to.achieve.higher.DSSC.performance.need.to.be.
investigated.
A.high.charge-injection.eficiency.requires.that.the.distribution.of.dye.excited.states.lie.
above.the.SMO.conduction.band.edge..Raising.the.energies.of.the.dye's.excited.states.is.
likely.to.reduce.its.light-harvesting.eficiency..A.better.option.is.to.shift.the.SMO.conduc-
tion.band.edge.energy.to.lower.values..The.core-shell.approach,.as.discussed.by.Diamant.
et.al.,.offers.some.promise.
38
In.the.core-shell.approach,.the.TiO
2
.network.is.dipped.in.precursor.solutions.of.the.can-
didate.oxides.(e.g.,.SrTiO
3
,.Nb
2
O
5
,.SnO,.ZnO,.etc.),.followed.by.sintering.at.temperatures.
of. ~400-500°C.. For. example,. an. SrTiO
3.
shell. can. be. applied. onto. the. TiO
2
. core. for. shift-
ing.the.TiO
2
.conduction.band.in.the.negative.(upward).direction.to.achieve.an.increased.
open-circuit.photovoltage..On.the.other.hand,.a.shell.of.SnO
2
.can.be.applied.to.induce.a.
positive.shift.in.the.conduction.band.to.enable.the.use.of.dyes.with.excited.states.too.low.
for.eficient.coupling.to.bare.TiO
2
..If.recombination.of.electrons.with.the.oxidized.dye.or.
redox.mediator.should.become.a.problem,.then.a.shell.material.with.electron.afinity.more.
negative.than.that.of.the.core.(e.g.,.Nb
2
O
5.
on.TiO
2
).could.be.applied.to.generate.an.energy.
barrier.for.the.reaction.
