photo-=light, volta = voltage vb cb hνhν s*/s + e-e- s/s + i 3 -/i - e-e- cathodeanode * -0.5 v 0...
TRANSCRIPT
photo-=light, Volta = voltage
vb
cb
hν
S*/S+
e-
S/S+
I3-/I-
e-
cathodeanode
*-0.5 V
0 V
0.5 V
1.0 V
Semiconductors and Photovoltaics
Neal M. [email protected]
Three Questions about Solar Electricity
• How much solar electricity could we make?• How do solar cells work?• What materials are cells made of?
Looking Towards Nature“Trying to do what Mother Nature has been doing for thousands of years…only better” - Dr. Raymond Orbach
Can we do this? How?
Source:Berkeley U
Solar Irradiance
A (short) historical perspective
• Photoelectric effect discovered in 1839 by Bequerel– All metals produce a voltage when subjected to light of the
correct wavelength (energy)
• Schokley reports the basis of p-n junctions, 1950
• Not pursued until 1954 by Bell Labs– Very expensive, 6% efficiency
– Reemerged in late 70’s-early 80’s with gas crisisand history repeats itself…
• NASA launches 1 kW array, 1966
• Science drives society and vice-versa
How Much Electricity Do We Make Today?
• 1.8x1012 Watts (continuously)
– 6x109 persons
– 300 Watts/person– 3 100W light bulbs per person
• U.S. – 25% of total– 1,500 Watts/person– 15 100W light bulbs per
person– 36 kWhr/day/person
http://www.eia.doe.gov/oiaf/ieo/electricity.html
http://antwrp.gsfc.nasa.gov/apod/ap030426.html .
The Solar Dream: There’s lots of sun energy!
Area required for all US electricity assuming ~10% efficiency, ~100 x 100 miles
1000 W/m2 at High Noon
• 40,000 EJ of solar energy hits the US each year….more than 400x the total energy consumed per year.
THE SUN. ONE BIG ENERGY SOURCE
Total energy reserves
Uncle Harold says: “It is so darned hot here. We just need some of them solar panels!What is the problem?”
Aunt Susie says: “Golly it is windy downtown. They just need to install some of those windmills”
My dad says: “Boy, you need to figure out how I can fill my car up the garden hose”
What is the problem??
Average Irradiance
• 30 Year Average of “full sun” per year:
City, State Hours of full sun (kWh/m2/yr)
San Diego, CA 2044
Phoenix, AZ 2336
Syracuse, NY 1533
Binghamton, NY 1496
New York City, NY 1642
Seattle, WA 1387
• Meaning: Syracuse gets 65% of the sun Phoenix gets and therefore needs more PV modules to get the same number of kWh
The Nature of Light
Energy hc
wavelength, energy
Solar Irradiance
250 500 750 1000 1250 1500 1750 2000 2250 2500 2750 30000.00
0.25
0.50
0.75
1.00
1.25
1.50
1.75
2.00
ASTM G173-03 Reference Spectra
AM0
AM1.5 global
AM 1.5 direct
Wavelength nm
Spectr
al Ir
radia
nce W
m-2
nm
-1 IR
visuv
Energy
Wavelength distribution• 48% of the extraterrestrial irradiance intensity is in the visible
range of 380–780 nm
• Ultraviolet irradiance (< 380 nm) accounts for 6% of the total intensity
• 45% is given off in the upper infrared.
• Above 3000 nm the irradiance is energy-negligible.
Power Distribution• Total ultraviolet irradiance below 380 nm is about 92.6 W/m2
• The visible area has a total power of 660 W/m2
• The remaining IR has a total irradiance of 1367 W/m2
Where should we be looking?
Where the Energy Goes• Ozone absorbs solar irradiance almost completely
under λ = 290 nm and more weakly to around 700 nm.
• Water vapor absorbs in the infrared, with pronounced absorption bands at 1.0, 1.4 and 1.8 μm.
• Above 2.5 μm almost the entire irradiance is absorbed by CO2 and H2O.
• Some reflection and scattering
SHUTTLING ELECTRONS
http://www.jimhillmedia.com/mb/images/upload/Van-de-Graaf-Generator-web.jpg
Making Electricity from Light: The Photoelectric Effect
Light in (frequency ν)
Cathode Anode
i
Vacuum tube
Electrons out
Einstein’s Explanation of the Photoelectric Effect
EnergyGap
BluePhoton
Electrons in the Cathode
Elec
tron
Ene
rgy Vacuum
RedPhoton
Ephoton = hν
h – Max Planck’s constant
The Science• Solar energy comes in the form of photons• The photovoltaic effect:
E = energyh = Planck’s constant = 6.634 x 10-34 Jsc = speed of light = 3 x 108 m/s λ = wavelength of light
• Likewise, E = mc2 → energy, mass, and wavelength are related
• Atoms are composed of…
E hc
Bands
• Energy states transition from discrete to “smeared” progressing from atom molecule solid
• Electrons fill from the bottom up• Highest filled band is the “valence” band• Lowest filled is the “conduction” band
Ionization boundary
atom Diatomic atom Triatomic atom n atomsE1
E2
E3
E∞
Conduction
• Materials can be separated as insulators, semiconductors, or conductors
• Based on size of VB-CB transition (bandgap)
VB VB VB
Eg* > 5.0 eV
Eg* < 5.0 eV
Eg* ≈ 0 eV
1 eV = 1240 nm = 1.6 x 10-19 J
CBCB
CB
Forbidden bandForbidden band
http://upload.wikimedia.org/wikipedia/commons/3/3f/BandGap-Comparison-withfermi-E.PNG
Solar cells: Photons in, Electrons out
i
Photonsin
Electrons out
SiliconCrystal
+--
--
-
-
-
++
+
+
+
+
silicon wafers
Solar Cells: Photoelectric Effect in a Semiconductor
BandGap
Conduction Band
Elec
tron
Ene
rgy
Valence Band
GreenPhoton
InfraredPhoton
free electron
free hole
= Cell Voltage
Mechanism of Electron Generationa Goldilocks problem
• Photons with an energy >Eg collide with the material• Energy is conserved and electrons are excited from the VB to
the CB• CB electrons travel through a circuit, powering a device
- - - - - - - - - -
-
-
-
-
Valence Band
Conduction Band
Eg too smallEg too largeEg just right
How do we get the right bandgap?
Doping• Increases conductivity (lowers VB-CB
threshold) by adding electrons or holes• Adding electrons: n-type (negative); P, As, Sb• Adding holes: p-type (positive); B, Al
The p-n junction
• Electrons diffuse to border of p-type region• Holes diffuse to border of n-type region
+
-
++
+
+
+
+
+
+
+
+ - --
-
-
-
-
-- -
-
Space charge regionp region n region
Solar Cell Processes
• Charge separation• Reflection• Transmission
RecombinationCharge
separation
Reflection
n-region-
+-+
p-regionTransmission
The Magic in the Panel
• Photons in sunlight hit the solar panel and are absorbed creating a dc source (a battery)
• An array of solar panels converts solar energy into usable DC electricity. Inverters convert the DC to 60 Hz AC to feed the grid.
n-layer
p-layer
back contact
anti-reflective coating
front contact
Cover glass
e-
Anatomy of PV cell
n-layer
p-layer
back contact
anti-reflective coating
front contact
Cover glass
e-
Electron Generation and Movement
FLAVORS OF PHOTOVOLTAICS
Photovoltaic types and benefits
• Silicon– Single crystal silicon (c-Si)– Multicrystalline silicon (mc-Si)– Amorphous silicon (a-Si)
• Thin-film– Silicon– Cadmium telluride, CdTe– Copper indium gallium diselenide , CIGS
• Very efficient in diffuse light conditions
• Dye-sensitized
Efficiency: How high?c-
Silico
n
mc-
silico
n
GaAs InP
CIGS
CdTe a-Si
nc-S
i
DSSC
GaIn
P/Ga
As/G
e
0
10
20
30
Cell type
% E
ffici
ency
Maximum measured efficiencies under lab conditions as of 2008
Limits to Ideal Solar Cell Efficiencies
0 1 2 3 40
500
1000
Po
we
r (W
/m2 )
Bandgap Energy (eV)
AbsorbedSunlight
CellOutput
33%
William Shockley
Assumed that recombination is “radiative”
• Recall:– 37% of sunlight is in the visible, 400-700 nm– 32 % of sunlight is in the low-IR, 700-1200 nm– Silicon does not convert photons to electrons above
~1200– Most of the energy above the bandgap (low
wavelengths) is converted to heat
Limits to Solar Cell Efficiency
Single Crystal Silicon
• First commercial solar cell• High efficiency (Theoretical 27 %)
– Practical ~10-15 %• Expensive to produce
– Cleanroom environment, ultrahigh purity required
= 1.12 eV = 1100 nm
Max efficiency = 27 %
Silicon – what PV is made of (for now)
• Silicon is the dominant materials in PV production
• 26% of the Earth’s crust, second most abundant element by weight (oxygen is #1)
• Melting point: 1410 C• Production of pure PV-grade silicon
– not easy!
Polycrystalline Silicon
• Lower cost• Lower efficiency
– Grain boundaries cause electron-hole recombination
• Easier to produce• Also amenable to thin film or
multicrystalline cells+-
Grain boundary
Czochralski method for obtaining single crystal silicon from polycrystalline
• Goal: Turn high-purity polycrystalline into high-purity single-crystal
• Small single-crystal seed is produced
• Used to grow remaining single crystal silicon
http://en.wikipedia.org/wiki/File:Czochralski_Process.svg
Thin-film/heterojunctions
• Direct-bandgap semiconductors (silicon is indirect)• Very thin layers of high-efficiency PV material
– Silicon cells need to be 87.5x thicker to absorb same amount of light
– Lower manufacturing costs, less purity• Multiple bandgaps possible (solar lasagna)• Issues with junctions between layers (grain boundaries,
current limiting)• Examples: GaInAs, CuInGaSe2(CIGS), CdTe• Materials tend to be toxic (or just not good)
Dye cells
• Use molecular dyes as light absorber• Inject electrons into a semiconductor• Inexpensive, flexible materials• Relatively low efficiency (8-12 %)
400 800 1200 1600 20000.00
0.02
0.04
0.06
0.08
0.10
0.00.20.40.60.81.01.21.41.61.82.0
Ab
sro
ba
nce
(a
.u.)
Wavelength (nm)
Irra
dia
nce
(W
/m2 /n
m)
AM1.5N719
Ruthenium 535-bisTBA (N719)
Dye Cells
TiO2 particles(13 nm)
*Kalyanasundaram, K.; Grätzel, M. Coord. Chem. Rev. 1998, 77, 347.
vb
cbhν
S*/S+
e-
S/S+
I3-/I-
e-
cathodeanode
*I3
-
I-
Dye
e- dye
dye*/dye+
~10μm
e-
e- e-
e-
load
Ptcounter
transparent conductive oxide (TCO)
13nm
-0.5 V
0 V
0.5 V
1.0 V
Maximum Solar Cell Efficiencies
National Renewable Energy Lab (NREL)
EVALUATING PV CELLS
http://www.udel.edu/iec/NREL_IBC_SHJ_IV_Curve.gif
Specifications for PV modulesAbb. Term MeaningVoc Open circuit voltage max voltage with no load
Vmax Voltage at maximum max voltage at max power
Isc Short circuit current max current with no load
Imax Current at maximum max current at max power
P Maximum power P = Imax x Vmax
Revisiting bandgaps
• Extra energy leaves as heat
- - - - - - - - - -
-
-
-
-
Valence Band
Conduction Band
Eg too small
Eg just right
heat
The Heat Problem in Silicon
Efficiency (%)
Spectral Range
White100 mW/cm2
Transmitted (visible)100 mw/cm2
Reflected (NIR)100 mw/cm2
Efficiency 19.5 12.5 23.7
0.63
0.635
0.64
0.645
0.65
0.655
0.66
0.665
0.67
0 100 200 300 400 500 600Time (s)
Op
en
circu
it vo
ltag
e (
V)
White light
Visible light
NIR light
Voc decreases 2.3 mV/°C for silicon
Voc vs. time for a Si cell at ~7x white light concentration, with wavelength-selective mirrors placed in the beam path.
Voc losses are lowest using NIR light - less power is thermalized
• Heat increases level of valence band electrons• Decreases band gap; distance between Ec and Ev is smaller• Lowers cell voltage
Why might this be?
e- e-
e-e-e- e-
e- e-
e-
e- e- e- e- e-e-e-e-e-
e- e- e-
e- e- e-e-
e- e-e-
e-e-
e-e-e-
e- e- e-e- e-
e-
Eg
EV
EC
e- e- e- e-e- e- e- e- e-
heat
e- e-
e-e-e- e-
e- e-
e-
e- e- e- e- e-e-e-e-e-
e- e- e-
e- e- e-e-
e- e-e-
e-e-
e-e-e-
e- e- e-e- e-
e-
Eg
EV
EC
The Heat Problem – a real example
• Voc decreases 136.8 mV/°C
• Solar arrays typically put out ~40 DCV
• Arrays can heat to 65% above ambient– 90 °F day 140 °F panel (60 °C)
• Voc at 25 °C = 40 V, now 35.2 V– 12% loss in power (assuming no
change in current) • Take home message:
Cooling is very important
• Passive works well
The Heat Problem isn’t a problem…(sometimes)
• Example: Operating temperature of 10 °F = -12 °C
• Then, with Voc decreasing 136.8 mV/°C– a 40 VDC cell could produce 45V, or 13% increase
over standard conditions • When and where might this happen?
Measuring Power
• Always less than 100 %
Some definitionsVoc: Open circuit Voltage - Maximum voltage when there is no current draw.
Isc: Short circuit Current - Maximum current when there is no voltage draw.
ff: fill-factor - The ratio between the maximum power and theoretical maximum (A/B). Indicates ‘quality’ of the cell.
0
5
10
15
20
25
30
35
40
45
50
0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70
Voltage (V)
Cu
rre
nt
(mA
)
isc
Voc
BA
% Efficiency Pout
Pin
100
Measuring Power
• PV power dependent on: – Incident energy– Type of module– Module temperature– Angle of incidence
VB
CB
VB
CB
E1 E2 < E1
Voc Jsc ff
Pin
ni.com
Calculating EfficiencyArea 1.44 cm2
Lamp power
176.9 mW/cm2
Voc 0.616 V
Isc 45.7 mA
Unit Calculation
Powermax Vmax x Imax= 21.3 mW
Current density (Jsc)
Isc/area
Fill factor Pmax/(Isc x Voc) = 75.7%
Efficiency (Jsc x Voc x ff)/Irradiance
0
5
10
15
20
25
30
35
40
45
50
0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70
Voltage (V)
Cu
rre
nt
(mA
)
Pmax
Isc x Voc
η = 8.3 %
PV panels, ESF Walters Grid
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Time
Ener
gy (k
Wh)
Science in Practice
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Energ
y (
kW
h)
• ESF PV array on Walters
Science into Practice
• ESF PV array on Walters
date
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Monthly Power Output
Date
Ener
gy (k
Wh)
PV wiring
Using backup power with batteries
Series vs. Parallel
Series – voltage adds, current constantParallel – current adds, voltage constant
High current resistive lossesHigh voltage, but current limited
http://www.sustainableenergy.com/typo3temp/pics/fadf66b23f.jpg
Anatomy of a PV installation
What does PV depend on?
Photovoltaic Power
Distancefrom the sun
Material Angle/Tilt
Temperature
What is next in PV?
New Materials• So-called “3rd generation”
photovoltaics– Thin films– Mixed semiconductors– Organic PVs– Multiple bandgaps
New Architectures• Increase light absorption
– Scattering
• Improve the electron pathway– Inexpensive single crystal
materials• Nanowire arrays• Enhanced absorption and
carrier collection in Si wire arrays for photovoltaic applications, Nature Materials 9, 239 - 244 (2010)
The Ideal Solar CellA multi-wavelength absorber where all energy is absorbed, none is wasted.
AM1.5 Solar Irradiance
0.00
0.20
0.40
0.60
0.80
1.00
1.20
1.40
1.60
0 200 400 600 800 1000 1200 1400Wavelength (nm)
Sp
ect
ral
Irra
dia
nce
Wm
-2n
m-
EG1
EG2EG3
EG4EG5 UV IR
vis
Helios
• Unpiloted prototype aircraft for flight at 30 km (18.5 miles)
The Sun: A periodic (but predictable) energy source
• Energy output is not constant• This needs to be addressed at a system-wide level
• The biggest limit on how much useful energy is panel efficiency.– The energy not converted to electricity is about 85%!
• Cost – think about economy of scale
• Periodic and intermittent nature of sunlight– Storage – batteries, capacitors, water, hydrogen
• Electricity only
• While optimizing the system’s efficiency is important, be aware that it may be less expensive, more aesthetic or more convenient to sacrifice some efficiency.
Limits to Real World PV
Storing Solar Energy
e-
h+e-
h+h+
e-
H2 O2
- +
What to do when the sun goes down?
Solar thermal
capacitorsfuel cells
batteries
direct on grid
solar cell
PEM* Fuel Cell and Electrolyzer*Polymer electrolyte membrane
H
H
H
HH
HH
O
O
O
O
O
OOO
H
H
H
Hydrogen Oxygen
Anod
e
Cath
ode
– +
H2 H2O
Elec
trol
yte
H3O+
H2O
e-’s
H2 O2- +
e-’sYour favorite PV
…but that is for another time
References and Resources
• US DOE, Energy Efficiency and Renewable Energy (EERE)– http://www.eere.energy.gov/
• National Renewable Energy Lab (NREL)– http://www.nrel.gov
• NY State Energy Research and Development Authority (NYSERDA)– http://www.nyserda.org
• School Power Naturally– http://www.powernaturally.org/programs/SchoolPowerNaturally/
default.asp• Handbook of Photovoltaic Science and Engineering, Luque and Hegedus, Eds.• V. Quaschning, “Understanding Renewable Energy Systems”, 2005.• PVCDROM
– http://pvcdrom.pveducation.org/