solar-generation-cells-62.html
Procedings
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e n e r g y + p r o c e e d i n g s 61 concentration of
solar
energy increases the efficiency of semiconductor pv devices and saves expensive so- lar
cell
s at the expense of cheaper reflectors and lens systems. concentrations up to a 1000 times normal
solar
irradiance (1000 suns) are anticipated for future
solar
farms. compared with a mono c-si
cell
, a multi-junction
cell
harvests and converts a larger part of the
solar
spectrum by stacking several p-n junctions made from different semiconductors. the principle is illustrated in
figure
3. the theoretical pce limit for a multi- junction
cell
under
solar
concentration is 85% (martí 1996). page 86 of 122 concentration of
solar
energy increases the efficiency of semiconductor pv devices and saves expensive
solar
cell
s at the expense of cheaper reflectors and lens systems. concentrations up to a 1000 times normal
solar
irradiance (1000 suns) are anticipated for future
solar
farms. compared with a mono c-si
cell
, a multi-junction
cell
harvests and converts a larger part of the
solar
spectrum by stacking several p-n junctions made from different semiconductors. the principle is illustrated in
figure
3. the theoretical pce limit for a multi-junction
cell
under
solar
concentration is 85% (martí 1996). h ν (ev) 0 1 2 3 4 5 in te n s ity (w m -2 n m -1 ) 0,0 0,5 1,0 1,5 2,0 gainp 1.8 ev ge 0.7 ev gaas 1.4 ev + + + + + + + + + - - - - - - - - - - - - -
figure
3. triple-junction pv made from three semiconductors. different parts of the
solar
spectrum are absorbed by the different layers: photons with energies higher than the band gab of 1.8 ev are absorbed by the gainp layer. photons with energies below 1.8 ev are transmitted through the gainp layer, and visible light from 1.4 to 1.8 ev is subsequently absorbed by the gaas layer. finally, near infrared light is absorbed by the ge layer. the layers are series connected and their generated voltages add up. the
figure
illustrates which parts of the
solar
spectrum each layer converts to electricity.
cell
s from spectrolab inc. show efficiencies up to 41%. at stc, the theoretical optimum efficiency for a single junction
cell
is 31%, whereas the theoretical limit for an infinite-junction
cell
is 65%. adding concentration on top increases the theoretical limit to a 85% efficiency. the most important third generation technologies are dye-sensitized
solar
cell
s (dssc) and polymer
solar
cell
s. the dssc (o'regan & grätzel 1991) relies on oxidation and reduction of chemical species – hence it is also called a photoelectrochemical (pec)
cell
– see
figure
4. the dssc shows ex
cell
ent performances and has reached more than 10% pce. the expectations are that long- term stability through new dye-molecules and the use of solid-state electrolytes may allow for devices better than or comparable to amorphous silicon in price, ease of production and performance.
figure
.3..triple-junction.pv.made.from.three.semiconductors.. different.parts.of.the.
solar
.spectrum.are.absorbed.by.the.differ- ent.layers:.photons.with.energies.higher.than.the.band.gab.of.1.8. ev.are.absorbed.by.the.gainp.layer..photons.with.energies.below. 1.8.ev.are.transmitted.through.the.gainp.layer,.and.visible.light. from.1.4.to.1.8.ev.is.subsequently.absorbed.by.the.gaas.layer.. finally,.near.infrared.light.is.absorbed.by.the.ge.layer..the.layers.are. series.connected.and.their.generated.voltages.add.up..the.
figure
. illustrates.which.parts.of.the.
solar
.spectrum.each.layer.converts.to. electricity..
cell
s.from.spectrolab.inc..show.efficiencies.up.to.41%.. at.stc,.the.theoretical.optimum.efficiency.for.a.single.junction.
cell
. is.31%,.whereas.the.theoretical.limit.for.an.infinite-junction.
cell
.is. 65%..adding.concentration.on.top.increases.the.theoretical.limit.to. a.85%.efficiency. the most important third generation technologies are dye-sensitized
solar
cell
s (dssc) and polymer
solar
cell
s. the dssc (o’regan & grätzel 1991) relies on oxidation and reduction of chemical species – hence it is also called a photoelectrochemical (pec)
cell
– see
figure
4. the dssc shows ex
cell
ent perform- ances and has reached more than 10% pce. the expectations are that long-term stability through new dye-molecules and the use of solid-state electrolytes may allow for devices better than or comparable to amorphous silicon in price, ease of production and performance. page 87 of 122 ito pt i - tio 2 e - i 3 -
figure
4. dye-sensitized
solar
cell
s separate the functions provided by a semiconductor
solar
cell
. absorption of light occurs in a dye absorbed on a nonporous tio 2 -layer. charge separation occurs at the interface between the dye and the electron-conducting tio 2 . the electron is transported to the transparent conducting oxide electrode through the tio 2 . the hole transport occurs by diffusion of iodide to the dye, which extracts electrons from the iodide and oxidizes it to triiodide. the cycle is closed through reduction of triiodide at the pt- electrode, when the generated electron is transferred through an outer circuit. the
cell
is also called a grätzel
cell
after its inventor. polymer pv relies in the principle of a bulk-hetero junction (yu 1995) – see
figure
5. the pce has been the main research focus during the past 15 years and this has led to consistent improvements in pce, and the state of the art of today presents 6.5% pce for a tandem
solar
cell
(kim 2007). the largest advantage of polymer pv is the possibilities for low-temperature solution processing on flexible substrates enabling very fast r2r production methods. most recently research has begun to concentrate on novel materials with a lower band gap (bundgaard 2007), operational lifetimes and stability (jørgensen et al. 2008), and industrial processing of polymer
solar
cell
s. in terms of industrial processing a demonstration of technology transfer, large-scale industrial fabrication and public demonstration took place at the roskilde festival in denmark in 2008 where
solar
hats and plastic
solar
cell
modules were distributed freely – see
figure
6.
figure
.4..dye-sensitized.
solar
.
cell
s.separate.the.functions.provided. by.a.semiconductor.
solar
.
cell
..absorption.of.light.occurs.in.a.dye. absorbed.on.a.nonporous.tio 2 -layer..charge.separation.occurs.at. the.interface.between.the.dye.and.the.electron-conducting.tio 2 .. the.electron.is.transported.to.the.transparent.conducting.oxide. electrode.through.the.tio 2 ..the.hole.transport.occurs.by.diffusion. of.iodide.to.the.dye,.which.extracts.electrons.from.the.iodide.and. oxidizes.it.to.triiodide..the.cycle.is.closed.through.reduction.of.trii- odide.at.the.pt-electrode,.when.the.generated.electron.is.trans- ferred.through.an.outer.circuit..the.
cell
.is.also.called.a.grätzel.
cell
. after.its.inventor. polymer pv relies in the principle of a bulk-hetero junction (yu 1995) – see
figure
5. the pce has been the main research focus during the past 15 years and this has led to consistent improvements in pce, and the state of the art of today presents 6.5% pce for a tandem
solar
cell
(kim 2007). the largest advantage of polymer pv is the possibilities for low-temperature solution processing on flexible substrates enabling very fast r2r production methods. most recently
solar-polymer-electron-64.html
