9.
3 Photovoltaic solar cells
of Solar Energy Converter
Principle of photovoltaic converters
e–
excited excited
state state Means of charge
Sustained separation
charge separation
ant energy
cal potential
h! Eg ΔEex
motes
Solid or molecular
(excited ground state
intrinsic absorber
ground state h+
eparated
an energy Solar thermal converter:
• The radiant energy absorbed is converted
1) Electronic transition leading to local charge separation (e–...h+
mainly into internal energy and raised the
collected 2)
andSustained charge separation
temperature ofthrough various possible mechanism
the cell.
uit and do • It ofoperates
3) Diffusion as and
charge carriers a heat engine
collection in metaland does work.
electrodes
• It utilizes the full range of solar wavelengths. 89
nly from • It is thermally insulated from the ambient.
Mechanisms of sustained charge separation
×
E a) b) e–
e–
hν
hν
h+
h+
×
load load
Sustained charge separation requires some built-in driving force
a) Light-induced spatial gradient of the quasi-Fermi levels of electrons
and hole
b) Light absorbing material is connected by paths of different resistance.
One has much lower resistance for electrons and the other for holes.
This requires usually a heterojunction between an electron-transporting
material and a hole-transporter. 90
s
Schottky-barrier photovoltaic cells
Vacuum
χs
φs cb
φm2
EF e–
φm1
EF
φB EF,n*
EC
EF Eg EF ∆U ∙ F
EF
hν
vb EF,p*
h+
EV
n-type SC under
semiconductor Metal SC Metal illumination Metal
Schottky junction Schottky junction
Metal
Metal
with large work
with low work
function φm1
function φm2
(Cu, Ag, Au, Pt,
(Al, Zn, Pb, ...
for example) 91
for example)
Schottky-barrier photovoltaic cells
Light
Pt wire vitrous Se
Pb wire
W. G. Adams & R. E. Day,
+ – Proc. Royal Soc. 1877, A25, 113
glass tube
+
Light Au foil Light
glass plate Cu2O, Ti2S,
or Se
– 25 µm
Cu2O Zn plate
Pb coil
Cu ribbon –
+
L. O. Grondahl,
Rev. Modern Phys. 1933, 5, 141
92
Solid state p-n homojunction photovoltaic cells
Vacuum
e–
cb cb
p EC EF,n*
EF Eg hν ∆U
Eg Eg EF
EF n EF,p*
vb vb h+
EV EV
p-doped n-doped
Evolution of Silicon Sola
p-n junction under
semiconductor semiconductor p-n junction illumination
metal nger anti-re ective coating
Space si
n-type in the ea
C. Fuller, D. Chapin, G. Pearson,
standard
(AT&T Bell Labs)
J. Appl. Phys. 1954, 25, 676
p-type
metal contact
93
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fl
Drawbacks of 1st generation solar cells
Monocrystalline Si Polycrystalline Si Amorphous Si
Silicon is an indirect bandgap material with low absorption constant
(α = 5·102 cm–1 at 1.5 eV). A thick layer is thus required for light harvesting
Because the material ensures also carriers transport, purity of the material is a key issue
Major drawbacks – Cost
– Extended energy payback time (up to 4 years)
– Ef ciency drops at low light intensity (trap states)
– Degradation of performances due to partial
shading of series-parallel multi-cell solar panels
94
:
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95
Thin- lm (2nd generation) p-n heterojunction solar cells
Direct bandgap semiconductor material. Absorption constant increased by >103.
CuInGaSe2 (CIGS) CdTe (α = 5·105 cm–1 @ 1.5 eV)
95
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Best research cell power conversion ef ciencies
96
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3rd generation solar cells
Third generation solar cells are potentially able to overcome the Shockley–Queisser limit
of 31-41% power ef ciency. This includes a range of alternatives to the 1st generation
photovoltaics, which are made of semiconducting p-n junctions and 2nd generation solar
cells based on thin lm technologies.
The 3rd generation is
$ 0.1/ Wp $ 0.2/ Wp $ 0.5/ Wp somewhat ambiguous in
100 the technologies that it
encompasses, though
80 generally it tends to
Power conv. ef ciency [%]
Ultimate thermodynamic
limit at one sun include, non-semicon-
ductor technologies
60
$ 1.0/ Wp (including dye-sensitized
solar cells and organic
40 Shockley-Queisser limi photovoltaics), quantum
III (single bandgap) dot, tandem/multi-
20 junction cells, inter-
I $ 3.5/ Wp mediate band solar cells,
II hot-carrier cells, and up-
0 conversion and down-
0 100 200 300 400 500 conversion tech-
Cost [ US$/ m2 ] nologies.
97
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Photo-electrochemical (liquid junction) solar cells
Antoine
Edmondand Edmond(1839):
Becquerel Becquerel (1839)
Discovery
Discoveryofof
thethe
"Photo-electrochemical
“Photoelectrochemicaleffect"
effect”
Antoine
Edmond C. Becquere
Becquerel
e- + e– – 1/2 X– 1788-1878
e– 1820-1891
1/2 X3– e–
X–
h+
– e– + 1/2 X– E. Becquerel
A. Edmond Becquere
Ag
Ag AgX
AgX aqueous electrolyte Ag
Ag Comptes Rendus 1839, 9, 561
1820-1891
A. E. Becquerel,
Comptes Rendus 1839, 9, 561-567
98
Moser dye-sensitized solar cells
First report of a dye-sensitized photo-electrochemical solar cell.
Dye-sensitization of AgX
photographic plates
H. W. Vogel
Ber. Dtsch. Chem. Ges. 1873, 6, 1730
Dye-sensitization of Becquerel's
photo-electrochemical cell
J. Moser
Monatsh. Chem. 1887, 8, 373
99
Liquid junction photogalvanic cells
Heinz Gerischer(1968)
Heinz Gerischer (1968):
Dye-sensitized photogalvanic
Dye-sensitized photogalvanic cell cell
A
(S+/S*)
e-
Heinz Gerische
(D+/D) e- 1919-1994
× (S+/S)
Zn
Zn ZnO
ZnO aqueous electrolyte Pt
Pt
H. Gerischer & H. Tributsch
Ber. Bunsenges. Phys. Chem. 1968, 72, 437
100
:
Light absorption by a dye monolayer
Typical optical molar decadic extinction coef cient ε = 104 - 2·105 mol–1· l· cm–1
(ε) and absorption cross-section (σ) for dyes with σ = 4·10–3 - 10–1 nm2
fully-allowed electronic transition:
Typical geometrical area of a dye molecule: S = 1 - 2 nm2
A dye monolayer absorbs at most a few percent of incident light. Ef cient light-harvesting
thus requires a 3-D structure. Photoanode of Dye-sensitized Solar Cells
L i g h t - h a r v e s t i n g i n n a t u r a l Wide band gap semiconductors,
Nanostructured such as TiO2, SnO2, ZnO, Nb2O5,
semiconductor
photosynthesis: Stacks of thylakoids SrTiO3, SnS2, ZnSnO 3, Zn2SnO4, NiO, etc., with different
in arti cial solar energy conversion
morphologies have been studied.
in chloroplast's granna 101
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