Biomedical Engineering Reference
In-Depth Information
proton.pumping.of.bR..Any.structural.modiications.or.analogs.that.elevate.the.height.
of. the. isomerization. barrier. slow. down. the. isomerization. and,. as. a. consequence,. fur-
ther. photocycle. steps.. For. the. same. reasons,. the. slowed. rate. of. retinal. isomerization.
may. have. a. positive. effect. on. the. electron. injection. to. TiO
2
. in. the. bR-sensitized. solar.
cell.. In. summary,. the. faster. retinal. isomerization. provides. the. higher. eficiency. of. the.
whole.photocycle..In.order.for.TiO
2
.sensitization.to.occur,.the.retinal.chromophore.needs.
to.absorb.light.and.inject.an.excited.electron.in.the.semiconductor.surface;.bR.photocycle.
steps,.including.chromophore.isomerization,.seem.to.be.competitive.side.steps.in.regard.
to.the.bR-to-TiO
2
.electron.injection..Hence,.it.can.be.stated.that.to.improve.the.dynam-
ics. of. the. electron. injection. to. the. electrode,. structural. modiications. of. the. retinal. mol-
ecule.or.its.surroundings,.to.allow.slowing.down.of.retinal.photoisomerization,.could.be.
considered.
22.4 Design,Characterization,andOptimizationof
theSemiconductingMetalOxideNetwork
Referring.back.to.the.four.essential.steps.of.photovoltage.generation,.a.functioning.solar.
cell.must.be.capable.of.performing.the.following.actions:
.
1.. Absorption.of.photons
.
2.. Excitation.of.electronic.states
.
3.. Charge.separation.and.transport.through.a.load
.
4.. Returning.of.electron.to.the.original.donor
We.have.already.discussed.how.photoactive.proteins.are.capable.of.absorbing.photons.
to. generate. excited. electronic. states,. thus. fulilling. the. irst. two. required. procedures..
In. this. section. we. will. describe. the. solid-state. substrate. onto. which. these. proteins. are.
adsorbed—these.materials.are.largely.responsible.for.step.3:.accepting.the.newly.excited.
electron,.keeping.it.separated.from.the.electron's.hole.counterpart,.and.transporting.the.
charge.through.the.load.circuit..In.another.major.departure.from.the.silicon-based.solar.
paradigm,.the.electrons.are.returned.to.their.donors.via.an.electrolyte.that.shuttles.nega-
tively.charged.species.from.the.cathode.to.anode,.thus.fulilling.the.fourth.requirement.
of.photovoltaic.devices..A.novel.method.of.using.gold.as.a.solid-state.replacement.for.the.
aqueous.electrolyte.process.will.be.discussed.here.as.well.
22.4.1 Current Generation via Conduction Band Energy Gradient
Electronic.properties.are.closely.related.to.the.relative.energy.levels.of.the.highest.occu-
pied.molecular.orbitals.(HOMOs).and.lowest.unoccupied.molecular.orbitals.(LUMOs),.or.
the. work. functions. required. to. liberate. an. electron. from. a. valence. shell.. In. the. context.
of. repeating. atomic. conigurations—as. is. the. case. of. solid. crystalline. materials—these.
energy.states.combine.to.form.continuous.bands,.known.as.the.valence.and.conduction.
bands.respective.to.the.HOMO.and.LUMO.energy.states..The.energy.differences.between.
the.valence.and.conduction.bands.are.what.classify.most.materials.as.conductors,.semicon-
ductors,.or.insulators,.as.they.dictate.electron.concentration.and.mobility..These.electrical.
