Biomedical Engineering Reference
In-Depth Information
Reflection
Front
contact
-
n-region
I
-
+
-
+
Charge
separation
Charge
separation
p-region
-
+
Recombination
+
Backside contact
Transmission
FIGURE 22.2
Current. generation. upon. photonic. absorption. at. the. p-n. junction.. (From. http://fourier.eng.hmc.edu/e84/lec-
tures/ch4/node1.html.)
energy.and.environmental.requirements.warrant.cheap,.technically.lexible,.and.benign.
solar-voltaic.technologies.
22.2.2 Dye-Sensitized Solar Cells
An.important.milestone.toward.this.goal.was.the.invention.of.excitonic.solar.cells,.com-
monly.refered.to.as.dye-sensitized.solar.cells.(DSSCs).
4
.DSSCs.are.currently.the.most.efi-
cient.third-generation.solar.cell.technology.(as.high.as.11%),.while.promising.low.cost.and.
ease.of.manufacture..Instead.of.the.potentially.costly.and.toxic.semiconductors.used.in.
other.thin-ilm.technologies,.DSSCs.utilize.nanocrystalline.semiconducting.metal.oxides.
(SMOs).such.as.TiO
2
,.the.low-cost,.inert.pigment.used.in.paint..In.DSSCs,.light.active.syn-
thetic.dyes.(organic.or.organometallic).(Figure.22.3).are.bound.to.nanostructured.wide-
band-gap.metal.oxide.semiconductors.and.used.as.photosensitizers.to.harvest.the.solar.
energy. and. generate. excitons,. thus. fulilling. the. irst. and. second. requirements. of. the.
photovoltaic.scheme.(see.Figure 22.4)..Operationally,.photon.excitation.of.the.dye.results.
in.transfer.of.electrons.from.the.dye.to.the.semiconductor,.with.the.holes.remaining.in.
the.dye..Although.it.is.energetically.possible.for.electrons.to.recombine.with.holes.in.the.
dye,.the.rate.at.which.this.occurs.is.very.slow.compared.to.the.rate.that.the.dye.regains.
an.electron.from.the.surrounding.electrolyte..This.fulills.the.charge.separation.compo-
nent,.the.physical.mechanism.by.which.electrons.are.driven.to.differ.from.the.p-n.junc-
tion,.and.will.be.further.described.in.Section.22.4..The.oxidized.species.in.the.electrolyte.
are. then. reduced. at. the. counterelectrode,. with. electrons. supplied,. via. the. outer. circuit,.
TABLE 22.1
Common.By-Products.of.PV-Grade.Silicon.Treatment
Material
Used
in
Si
Solar
Cell
Manufacturing
EnvironmentalandHumanHealthEffects
Silicon.tetrachloride
An.extremely.toxic.substance.that.reacts.with.water,.causes.
skin.burns,.and.is.a.respiratory,.skin,.and.eye.irritant
Sulfur.hexaluoride
Used.to.clean.reactors.in.silicon.production;.known.to.be.
the.most.potent.greenhouse.gas.molecule
Waste.silicon.dust
Created.by.sawing.Si.wafers;.causes.inhalation.problems
