Biomedical Engineering Reference
In-Depth Information
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p-type
n-type
Junction
Current
FIGURE 22.1
A.p-n.junction.and.creation.of.a.depletion.zone..The.charge.imbalance.creates.an.electric.ield.that.drives.elec-
trons.only.after.they.have.obtained.suficient.energy.to.exceed.the.energy.barrier.
excitation.. To. generate. an. electric. ield,. an. n-type. material. is. introduced. to. the. p-type.
one..Then,.the.excess.electrons.and.holes,.in.their.respective.n-.and.p-type.doped.silicons.
(Figure.22.1),.migrate.toward.the.oppositely.charged.material.(i.e.,.electrons.from.n-type.
will. occupy. adjacent. regions. of. the. p-type. material,. and. vice. versa),. creating. a. charge.
gradient.at.this.junction.
This. depleted. zone. at. the. p-n. junction. creates. an. electrical. bias. that. moves. liberated.
electrons.away.from.their.positively.generated.hole.counterparts,.fulilling.the.third.pho-
tovoltaic.requirement.of.preventing.charge.recombination..The.fourth.and.inal.part.of.the.
cycle.occurs.after.the.electron.low.passes.through.the.circuit.and.returns.the.electrons.to.
the.p-type.material.to.be.excited.and.start.the.cycle.anew.(Figure.22.2).
Because. this. singular. doped. semiconductor. material. is. responsible. for. all. four. of. the.
listed. fundamental. functions,. a. huge. challenge. in. solid-state. solar. cell. technology. is.
inding. the. right. materials. that. will. best. fulill. these. conditions.. These. constraints. limit.
the. kinds. of. materials. that. can. be. used. and. the. impurity. tolerances.. Furthermore,. the.
manufacturing.processes.necessary.for.photovoltaic.applications.are.energetically.intense.
and.environmentally.harmful.(Table 22.1).
While. there. are. systems. in. place. to. mitigate. the. release. of. harmful. materials. into. the.
environment. and. populace,. the. management. of. these. waste. materials. will. always. be. a.
concern,. especially. considering. that. solar. cells. are. posed. to. be. a. signiicant. source. of.
alternative.energy..Currently.the.expected.lifetime.for.a.commercial.solid-state.solar.cell.
is.approximately.25.years.. But.not.all.applications.require.such.long.lifetimes,.and.high.
