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PHOTOVOLTAICSDirect Conversion of Sunlight to Electricity
David T Britton,
NanoSciences Innovation Centre
University of Cape Town
Photovoltaic effect
Early (simple) solar cells: 1839 –
Heterojunction solar cells: 1946 –
• principle of operation
• commercial solar cells and modules
Distributed junction solar cells
State of the Art?
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PHOTOVOLTAIC EFFECT
PhotochemicalEdmond Becquerel,
Comptes Rendus 9, 561 – 567 (1839)
Solid StateW.G. Adams & R.E. Day,
Proc Roy Soc Lond 25, 113 -117 (1876)
Light generates free charges
electrons and ions
electrons and holes
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Single homogeneous material between two contacts
Diffusion/drift of charge to one electrode gives current
Voltage is low
SIMPLE SOLAR CELLS
Becquerel 1839Adams & Day 1876
illuminated electrode is cathode
(photoresistors)
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First thin film solar cell Charles Fritts 1882
Selenium with thin gold leaf front electrode,
(transparent to green light)
tin plated back electrode
Am J. Sci 26, 465 – 482 (1883)
PV discovered in Fritts cell by W. Siemens
Sitzungsber. Preuss. Akad.Wiss zu Berlin
1885 (I), 147-148
Screen printed CdS, CdSe Y. T Sivhonen &
S.G. Parker 1963, (Texas Instruments)
US 3208022 (1965)
First fully printed silicon cell 2008
SIMPLE SOLAR CELLS
0 200 400 600
0
10
20
30
DARK
LIGHT
Open Circuit Voltage
Pote
ntial [
V]
Time [s]
(photoconductors)
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HETEROJUNCTION SOLAR CELLS(photodiodes)
depletion
layer
P
N
Voc
open circuit
short circuit
electrons & holes produced in the junction
electrons pulled to n-type layer
holes puled to p-type layer
SC
kTeV jejj 100
1lnj
j
e
kTV SCOC
Inbuilt electric field
Anode and Cathode given by diode direction
dar
k
light
Short circuit current jsc is from n to p
Open circuit : charge collection gives Voc
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Standard test conditions (IEC 6125)
1kW/m2 25 0C AM1.5 sunlight spectrum
1 cm2 cell
Cell efficiency : maximum electrical power out/ radiant energy in
Module efficiency: useful power out/radiant energy in EA
VIff
EA
VIocscmpmp
HETEROJUNCTION SOLAR CELLSEquivalent circuit
Quantum Efficiency (QE) = number charge carriers/number of photons
Internal quantum efficiency (IQE): only absorbed photons
External quantum efficiency (EQE): all photons hitting the cell
ocsc
mpmp
VI
VIff
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Modern commercial solar cells
Silicon pn junction cell (Bell Labs) 1941
R.S. Ohls, US Patent 2402662 (1946)
HETEROJUNCTION SOLAR CELLS
First organic PV (OPV) cell
D. Kearns & M, Calvin,
J Chem. Phys 29, 950-951 (1958)
Crystalline Thin film
Thin film OPV, C.W. Tang, Appl Phys Lett 42, 183 – 185 (1985)
GaAS solar cells 1970
Thin film a-Si:H (RCA 1976)
Other geometries include tandem cells and graded junctions
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DISTRIBUTED JUNCTION SOLAR CELLS
Only one layer of mixed material
More light reaches the junction
Larger junction
shorter distance for charge to travel
Dye Sensitised Solar Cell (Photochemical)
B. O’Regan & M. Grätzel, Nature 353, 737-740 (1991)
Polymer – fullerine blends (OPV)
G. Yu, J. Gao, J.C. Humelen, F. Wudl, and A.J. Heeger
Alan Heeger & Michael
Grätzel are founders of
Konarka
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2010/10/07
NREL/Spire
42.3%
STATE OF THE ART?
Other factors to discuss:
Industrial scalbility
price per Wp
cost per kWhAre they green?
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2010/10/07 NREL Spire 42.3% triple junction tandem cell.
Different technologies: c-si, thin film, OPV, photochemical,
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