Biomedical Engineering Reference
In-Depth Information
-3.8 eV
LUMO
-4.3 eV
A1
E C
-4.2 eV
bR
-4.7 eV
ITO
TiO 2
-5.4 eV
HOMO
-7.4 eV
E V??
FIGURE 22.9
Fermi. level. diagram. (chemical. potentials). of. the. sensitizer,. substrate,. and. conductive. electrode. contact.. An.
energy.gradient.creates.a.voltage.that.drives.the.movement.of.delocalized.charge.carriers.
properties. and. energy. band. gaps. become. especially. pertinent. to. solar. cell. technology.
because. they. are. directly. responsible. for. their. optical. properties,. thereby. dictating. how.
light.affects.the.materials'.electronic.states.
This.is.why.semiconducting.materials.are.a.fundamental.element.in.solid-state.solar.cell.
technologies.today..As.was.discussed.earlier,.doped,.semiconducting.silicon.is.the.mate-
rial.of.choice.of.commercial.applications.of.high-output.photovoltaics.because.its.band.gap.
energies.are.suitable.for.operation.under.the.sun's.visible.spectrum.
In. BSSC. and. DSSC. applications,. photoactive. proteins. or. dye. take. the. responsibility.
of. photon. absorption. and. electron. generation,. and. the. substrate. they. are. absorbed. onto.
assumes. the. task. of. maintaining. charge. separation. and. charge. carrier. transport. to. the.
circuit.electrodes..Because.it.is.no.longer.responsible.for.light.adsorption,.this.solid-state.
substrate.can.be.chosen.from.a.much.wider.variety.of.materials.that.do.not.exhibit.electron.
excitation.under.the.visible.light.spectrum..They.still,.however,.need.to.be.semiconduct-
ing;.again,.this.is.directly.attributed.to.the.band.gap.energies.inherent.in.conducting.vs..
semiconducting.materials..In.excitonic.solar.cells,.these.band.gap.differences.between.the.
charge-producing.proteins.and.dyes.and.the.charge-accepting.substrates.are.what.dictate.
electron.low,.and.therefore.current.
FigureĀ  22.9. is. representative. of. the. energy. differences. between. bacteriorhodopsin,. its.
TiO 2 .substrate,.and.an.indium.tin.oxide.(ITO).conductive.electrode.
The.HOMO.energy.state.of.bR.is.representative.of.the.preexcited.electron.at.-5.4.electron.
volts. and. -3.8. eV. volts. after. photon. absorption.. When. paired. with. TiO 2 . with. a. conduc-
tion.band.energy.at.-4.2.eV,.the.excited.electron.will.naturally.seek.out.the.state.of.lower.
energy.in.the.metal.oxide.conduction.band..Furthermore,.the.valence.band.of.the.metal.
oxide.semiconductor.is.illed,.disallowing.the.free.electrons.in.the.conduction.band.to.fall.
into. the. valence. states.. Finally,. when. placed. in. contact. with. the. ITO. electrode,. electron.
movement. is. generated. by. this. stepwise. gradient. of. energy. states.. This. is. in. contrast. to.
silicon-based.solid-state.solar.cell,.which.relies.on.a.p-n.junction.to.create.an.electric.ield.
to.move.electrons.
Understanding.these.intricate.energy.level.relationships.reveals.the.importance.of.the.
substrate.design..The.foremost.advantage.that.this.excitonic.solar.cell.design.demonstrates.
is. that. it. is. no. longer. reliant. on. a. lat. p-n. junction,. as. is. the. case. in. conventional. solar.
cells..Because.of.this,.the.substrate.can.be.created.into.unique.3D.architectures.that.maxi-
mize.surface.area,.and.thus.transferred.electrons.per.unit.volume.of.material..This.section.
will. review. elements. of. the. solid-state. substrate. design,. discussing. the. charge. transfer.
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