An Introduction to Solar Cells
Or
How to Make Green Electrons
Eric A. Schiff
Dept. of Physics
Syracuse University
Three Questions about Solar Electricity
 How much solar electricity could we make?
 How do solar cells work?
 What materials can we make cells from?
How Much Electricity Do We Make Today?
 1.8x1012 Watts
(continuously)
o 6x109 persons
o 300 Watts/person
 U.S. – 25% of total
o 1,500 Watts/person
o 36 kWhr/day/person
http://www.eia.doe.gov/oiaf/ieo/electricity.html
Where Does U.S. Electricity Come From?
Burning of Coal,
Oil, Gas, etc.
Nuclear
Hydroelectric
Wind, solar,
Geothermal, etc.
0
200
400
600
Billions of Watts
http://www.eia.doe.gov/oiaf/aeo/aeoref_tab.html (Table 9)
800
The Solar Dream:
There’s Lots of Sunlight - 1000 W/m2 at High Noon
160-200 W/m2
200-240
280-320
240-280
Area required for
all US electricity?
Power (averaged over 1 year)
The Solar Dream:
There’s Lots of Sunlight - 1000 W/m2 at High Noon
Area required for all
US electricity (100 x 100 miles)
Making Electricity from Light:
The Photoelectric Effect:
Light in
(frequency ν)
Cathode
Electrons out
Vacuum tube
i
Anode
Photoelectric Effect & Stopping Potential
Wavelength λ (nm)
0
-10
-20
-30
0.0
0.5
1.0
Electric Potential V (Volts)
800 600
Stopping potential (V)
Current Density J (mA/cm2)
Vstop (ν)
400
3
2
1
0
500
1000
Frequency ν (1012 s-1)
νλ=c
Electron Energy
Einstein’s Explanation of the Photoelectric Effect
Vacuum
Blue
Photon
Red
Photon
Electrons in the Cathode
Ephoton ∝ ν
Energy
Gap
Electron Energy
Einstein’s Explanation of the Photoelectric Effect
Vacuum
Blue
Photon
Red
Photon
Energy
Gap
Electrons in the Cathode
Ephoton = hν
h – Max Planck’s constant
Vacuum
Energy Blue
Gap Photon
Electrons in the Cathode
K.E. = hν - Egap
Current Density J (mA/cm2)
Photoelectric Effect & Stopping Potential
eVstop = hν - Egap
0
-10
-20
-30
0.0
0.5
1.0
Electric Potential V (Volts)
-Jsat/e ~ flux of blue photons
Solar cells: Photons in, Electrons out
slices of silicon
-
Photons
in
-
+
Silicon
Crystal
i
+
+
+
+
+
+
Electrons
out
Solar Cells:
Photoelectric Effect in a Semiconductor
Electron Energy
free electron
Conduction Band
Green
Photon
Infrared
Photon
Energy
Gap
Valence Band
free hole
2
Current Density J (mA/cm )
Semiconductors as Solar Batteries
generation
thermalization
Energy
Gap



hν
recombination
eVOC = EG – δ
δ determined by
recombination
δ = -kT ln(G/bNcNv)

0
VOC
-10
-20
-30
maximum
power
-Jsc
0.0 0.2 0.4 0.6 0.8
Electric Potential V (Volts)
JSC/e = solar photon flux
(hν > EG)
Limits to Ideal Solar Cell Efficiencies
Absorbed
Sunlight
2
Power (W/m )
1000
500
William Shockley

Assumed that recombination
is “radiative”
33%
0
Cell
Output
0
1
2
3
4
Bandgap Energy (eV)
Beyond Crystalline Silicon #1:
Amorphous and Nanocrystalline Silicon


Based on “plasma”
decomposition of silane
(SiH4)
Synergy with LCD TV & LCD
displays
o


Applied Materials
MUCH thinner than crystal
silicon
Less efficient too…
Beyond Crystal Silicon #2:
Dye-sensitized (Grätzel) solar cells
Pt-coated electrode
Liquid electrolyte
(I3-/I-)
Sintered TiO2
nanocrystals
(dye stained)
O’Regan and Grätzel
École Polytechnique Féderalé de Lausanne
Transparent
electrode (SnO2)
Output voltages for solar cells
Open-circuit voltage VOC
[V]
1.2
1.0
VOC = EG/e
0.8
c-Si
0.6
nc-Si:H
0.4
a-SiGe:H
0.0
1.2
1.4
1.6
1.8
Bandgap EG (eV)
Thickness [um]
10.0
Open-circuit voltage VOC
[V]
Cell thickness:
the second shoe
100.0
1.0
0.1
1.2
1.0
VOC = EG/e
0.8
c-Si
0.6
nc-Si:H
0.4
a-SiGe:H
0.0
1.2
1.4
1.6
1.8
Bandgap EG (eV)
Beyond Crystal Silicon for Solar Cells
Thin-film silicon
United Solar Ovonic
Thin-film CdTe
First Solar
III-V semiconductor films
Wakonda Technologies
Dye-sensitized solar cells
G24 Innovations (Wales)
CIGS nanoparticle inks
Nanosolar
Si nanoparticle inks
Innovalight
Plastic solar cells
Konarka
Average Selling Price ($ - 2006)
The Mysterious Learning Curve:
“If you build it, they will come …”
100
10
1
0.1
1
10
100
1000 10000
Cumulative power module shipments (MWp)
W. G. J. H. M. van Sark, E. A. Alsema, H. M. Junginger, H. H. C. de Moor, and G. J. Schaeffer,
Prog. in Photoovolt.: Res. and Appl. (2007).
Research Directions in Solar Cells

New materials –
Paints?
o
o
o
o

Organics
Polymers
Nanoparticle inks
Self-assembled mesoporous
materials
Middle-aged materials
o
o
o
Amorphous silicon &
its brethren
CIGS
CdTe

Optics
o
o

Concentrating cells
o
o

GaAs …
Very high efficiency
3rd Generation Ideas
o

Photonic crystals
Plasmonics
Multiexciton …
(your idea here)