Semiconductors and Photovoltaics S*/S+ -­‐0.5 V cb 0 V * e-­‐ e-­‐ hν I3-­‐/I-­‐ 0.5 V 1.0 V vb anode S/S+ cathode photo-­‐=light, Volta = voltage Neal M. Abrams nmabrams@esf.edu Three QuesJons 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 be9er” -­‐ Dr. Raymond Orbach Source:Berkeley U Can we do this? How? Solar Irradiance A (short) historical perspecJve •  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 juncJons, 1950 •  Not pursued unJl 1954 by Bell Labs –  Very expensive, 6% efficiency –  Reemerged in late 70’s-­‐early 80’s with gas crisis and history repeats itself… •  NASA launches 1 kW array, 1966 •  Science drives society and vice-­‐versa How Much Electricity Do We Make Today? •  1.8x1012 Wahs (conJnuously) –  6x109 persons –  300 Wahs/person –  3 100W light bulbs per person •  U.S. – 25% of total –  1,500 Wahs/person –  15 100W light bulbs per person –  36 kWhr/day/person hhp://antwrp.gsfc.nasa.gov/apod/ap030426.html . http://www.eia.doe.gov/oiaf/ieo/electricity.html
The Solar Dream: There’s lots of sun energy! 1000 W/m2 at High Noon Area required for all US electricity
assuming ~10% efficiency, ~100 x
100 miles
•  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 Seahle, 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 =
h⋅ c
λ
↓ wavelength, ↑ energy Solar Irradiance ASTM G173-03 Reference Spectra
uv vis 2.00
AM0
IR Spectral Irradiance W m-2 nm -1
1.75
1.50
AM1.5 global
AM 1.5 direct
1.25
1.00
0.75
0.50
0.25
0.00
250
500
750
1000
1250
1500
1750
Wavelength nm
2000
2250
2500
2750
Energy 3000
Wavelength distribuJon •  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 DistribuJon •  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 absorpJon bands at 1.0, 1.4 and 1.8 μm. •  Above 2.5 μm almost the enJre irradiance is absorbed by CO2 and H2O. •  Some reflecJon and scahering SHUTTLING ELECTRONS hhp://www.jimhillmedia.com/mb/images/upload/Van-­‐de-­‐Graaf-­‐Generator-­‐web.jpg Making Electricity from Light: The Photoelectric Effect Light in (frequency ν) Cathode Electrons out Vacuum tube i Anode Electron Energy Einstein’s ExplanaJon of the Photoelectric Effect Vacuum Blue Red Photon Photon Electrons in the Cathode Ephoton = hν h – Max Planck’s constant Energy Gap The Science •  Solar energy comes in the form of photons •  The photovoltaic effect: hc
E = energy E
=
h = Planck’s constant = 6.634 x 10-­‐34 Js λ
c = 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… Bands E∞ IonizaJon boundary E3 E2 E1 atom Diatomic atom Triatomic atom n atoms •  Energy states transiJon from discrete to “smeared” progressing from atom à molecule à solid •  Electrons fill from the bohom à up •  Highest filled band is the “valence” band •  Lowest filled is the “conducJon” band ConducJon •  Materials can be separated as insulators, semiconductors, or conductors •  Based on size of VB-­‐CB transiJon (bandgap) CB CB Forbidden band Eg* > 5.0 eV Eg* ≈ 0 eV CB VB VB hhp://upload.wikimedia.org/wikipedia/commons/3/3f/BandGap-­‐Comparison-­‐withfermi-­‐E.PNG Forbidden band Eg* < 5.0 eV VB 1 eV = 1240 nm = 1.6 x 10-­‐19 J Solar cells: Photons in, Electrons out silicon wafers
-
Photons in -
+
+
+
+
-
+
+
+
Silicon
Crystal
i Electrons out Solar Cells: Photoelectric Effect in a Semiconductor Electron Energy free electron ConducJon Band Green Infrared Photon Photon Band = Cell Voltage Gap Valence Band free hole Mechanism of Electron GeneraJon a 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 ConducJon Band -­‐ -­‐ Eg too small -­‐ -­‐ -­‐ -­‐ Eg too large Eg just right -­‐ -­‐ -­‐ -­‐ Valence Band -­‐ -­‐ -­‐ -­‐ How do we get the right bandgap? Doping •  Increases conducJvity (lowers VB-­‐CB threshold) by adding electrons or holes •  Adding electrons: n-­‐type (negaJve); P, As, Sb •  Adding holes: p-­‐type (posiJve); B, Al The p-­‐n juncJon p region + + + + -­‐ + -­‐ + -­‐ + -­‐ + + + n region Space charge region + -­‐ -­‐ -­‐ -­‐ •  Electrons diffuse to border of p-­‐type region •  Holes diffuse to border of n-­‐type region -­‐ -­‐ -­‐ -­‐ Solar Cell Processes •  Charge separaJon •  ReflecJon •  Transmission ReflecJon n-­‐region + Charge separaJon -­‐ RecombinaJon + Transmission -­‐ p-­‐region The Magic in the Panel •  Photons in sunlight hit the solar panel and are absorbed creaJng a dc source (a bahery) •  An array of solar panels converts solar energy into usable DC electricity. Inverters convert the DC to 60 Hz AC to feed the grid. Cover glass e-­‐ anJ-­‐reflecJve coaJng front contact n-­‐layer p-­‐layer back contact Anatomy of PV cell Cover glass e-­‐ anJ-­‐reflecJve coaJng front contact n-­‐layer p-­‐layer back contact Electron GeneraJon and Movement FLAVORS OF PHOTOVOLTAICS Photovoltaic types and benefits •  Silicon –  Single crystal silicon (c-­‐Si) –  MulJcrystalline silicon (mc-­‐Si) –  Amorphous silicon (a-­‐Si) •  Thin-­‐film –  Silicon –  Cadmium telluride, CdTe –  Copper indium gallium diselenide , CIGS •  Very efficient in diffuse light condiJons •  Dye-­‐sensiJzed Efficiency: How high? Maximum measured efficiencies under lab condiJons as of 2008 % Efficiency 30 20 10 0 Cell type Limits to Ideal Solar Cell Efficiencies Absorbed
Sunlight
2
Power (W/m )
1000
William Shockley 
Assumed that recombination is
“radiative”
500
33%
0
Cell
Output
0
1
2
3
4
Bandgap Energy (eV)
Limits to Solar Cell Efficiency •  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 Single Crystal Silicon •  First commercial solar cell •  High efficiency (TheoreJcal 27 %) –  PracJcal ~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 producJon •  26% of the Earth’s crust, second most abundant element by weight (oxygen is #1) •  MelJng point: 1410 C •  ProducJon of pure PV-­‐grade silicon –  not easy! Polycrystalline Silicon •  Lower cost •  Lower efficiency –  Grain boundaries cause electron-­‐
hole recombinaJon •  Easier to produce •  Also amenable to thin film or mulJcrystalline cells -­‐ + Grain boundary Czochralski method for obtaining single crystal silicon from polycrystalline •  Goal: Turn high-­‐purity •  Small single-­‐crystal seed polycrystalline into high-­‐purity is produced single-­‐crystal •  Used to grow remaining single crystal silicon hhp://en.wikipedia.org/wiki/File:Czochralski_Process.svg Thin-­‐film/heterojuncJons •  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 •  MulJple bandgaps possible (solar lasagna) •  Issues with juncJons between layers (grain boundaries, current limiJng) •  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 RelaJvely low efficiency (8-­‐12 %) N719 0.08
AM1.5 0.06
0.04
0.02
0.00
400
800
1200
1600
Wavelength (nm)
2.0
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
2000
2
Absrobance (a.u.)
0.10
Irradiance (W/m /nm)
Ruthenium 535-­‐bisTBA (N719) Dye Cells Dye S*/S+ I3-­‐ dye*/ dye+ -­‐0.5 V I-­‐ cb * e-­‐ hν e-­‐ 0 V dye e-­‐ I3-­‐/I-­‐ 0.5 V 1.0 V Pt counter S/S+ vb 13nm TiO2 parJcles (13 nm) transparent conducJve oxide (TCO) anode *Kalyanasundaram, K.; Grätzel, M. Coord. ~10μm e-­‐ e-­‐ load Chem. Rev. 1998, 77, 347. e-­‐ cathode e-­‐ Maximum Solar Cell Efficiencies National Renewable Energy Lab (NREL)
EVALUATING PV CELLS hhp://www.udel.edu/iec/NREL_IBC_SHJ_IV_Curve.gif SpecificaJons for PV modules Abb. Term Meaning Voc 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 RevisiJng bandgaps •  Extra energy leaves as heat ConducJon Band heat -­‐ -­‐ Eg too small -­‐ -­‐ -­‐ -­‐ Eg just right -­‐ -­‐ -­‐ -­‐ Valence Band -­‐ -­‐ -­‐ -­‐ The Heat Problem in Silicon 0.67
White light
Visible light
NIR light
Open circuit voltage (V)
0.665
0.66
0.655
0.65
Voc decreases 2.3 mV/°C for silicon 0.645
0.64
0.635
0.63
0
100
200
300
Time (s)
400
500
600
Voc vs. Jme for a Si cell at ~7x white light concentraJon, with wavelength-­‐
selecJve mirrors placed in the beam path. Voc losses are lowest using NIR light -­‐ less power is thermalized Efficiency (%)
Spectral
Range
White
100 mW/cm2
Transmitted (visible)
100 mw/cm2
Reflected (NIR)
100 mw/cm2
Efficiency
19.5
12.5
23.7
Why might this be? EC EC heat Eg 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-­‐ •
•
•
EV Eg 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-­‐ e-­‐ e-­‐ e-­‐ e-­‐ e-­‐ e-­‐ e-­‐ e-­‐ e-­‐ Heat increases level of valence band electrons Decreases band gap; distance between Ec and Ev is smaller Lowers cell voltage EV 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 à 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… (someJmes) •  Example: OperaJng 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 condiJons •  When and where might this happen? Measuring Power ⎛ Pout ⎞
% Efficiency = ⎜
⎟ ×100
⎝ Pin ⎠
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.
50
isc 45
B 40
A 35
Current (mA)
Some definitions
Voc: Open circuit Voltage Maximum voltage when there is
no current draw.
•  Always less than 100 %
30
25
20
15
10
5
0
0.00
0.10
0.20
0.30
0.40
Voltage (V)
0.50
0.60
Voc
0.70
Measuring Power •  PV power dependent on: –  Incident energy –  Type of module –  Module temperature –  Angle of incidence CB E1 VB Voc ⋅ J sc ⋅ ff
η=
Pin
ni.com CB E2 < E1 VB CalculaJng Efficiency Isc x Voc 50
45
Area
1.44 cm2
40
Pmax Lamp power 176.9 mW/cm2
Voc
0.616 V
Isc
45.7 mA
Current (mA)
35
30
25
20
15
10
5
0
0.00
0.10
0.20
0.30
0.40
0.50
0.60
0.70
Voltage (V)
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
η = 8.3 % PV panels, ESF Walters Grid 10.0 9.0 8.0 Energy (kWh) 7.0 6.0 5.0 4.0 3.0 2.0 1.0 0.0 12:00 AM 3:00 AM 6:00 AM 9:00 AM 12:00 PM Time 3:00 PM 6:00 PM 9:00 PM 12:00 AM Science in PracJce •  ESF PV array on Walters 10.0 9.0 8.0 Energy (kWh) 7.0 6.0 5.0 4.0 3.0 2.0 1.0 0.0 12:00 AM 3:00 AM 6:00 AM 9:00 AM 12:00 PM Time 3:00 PM 6:00 PM 9:00 PM 12:00 AM Science into PracJce •  ESF PV array on Walters Monthly Power Output 80 70 Energy (kWh) 60 50 40 30 20 10 0 Date PV wiring Using backup power with baheries Series vs. Parallel Series – voltage adds, current constant High voltage, but current limited Parallel – current adds, voltage constant High current  resis_ve losses hhp://www.sustainableenergy.com/typo3temp/pics/fadf66b23f.jpg Anatomy of a PV installaJon What does PV depend on? Material Angle/Tilt Distance from the sun Temperature Photovoltaic Power What is next in PV? New Materials •  So-­‐called “3rd generaJon” photovoltaics –
–
–
–
Thin films Mixed semiconductors Organic PVs MulJple bandgaps New Architectures •  Increase light absorpJon –  Scahering •  Improve the electron pathway –  Inexpensive single crystal materials •  Nanowire arrays •  Enhanced absorp_on and carrier collec_on in Si wire arrays for photovoltaic applica_ons, Nature Materials 9, 239 -­‐ 244 (2010) The Ideal Solar Cell A mulJ-­‐wavelength absorber where all energy is absorbed, none is wasted. AM1.5 Solar Irradiance
1.60
1.40
UV -2
Spectral Irradiance Wm nm
EG1 EG2 EG3 EG4 EG5 1.20
IR 1.00
0.80
0.60
0.40
vis 0.20
0.00
0
200
400
600
800
Wavelength (nm)
1000
1200
1400
Helios •  Unpiloted prototype aircra… 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 Limits to Real World PV •  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 intermihent nature of sunlight –  Storage – baheries, capacitors, water, hydrogen •  Electricity only •  While opJmizing the system’s efficiency is important, be aware that it may be less expensive, more aestheJc or more convenient to sacrifice some efficiency. Storing Solar Energy What to do when the sun goes down? H2 -­‐ + O2 solar cell h+ h+ e-­‐ h+ capacitors e-­‐ fuel cells e-­‐ Solar thermal baheries direct on grid PEM Fuel Cell and Electrolyzer *
Polymer electrolyte membrane e-­‐’s e-­‐’s Oxygen
H2 -­‐ + O2 – H H Electrolyte Hydrogen
H H O H O H H H H H H2 + H3O+ O O Cathode Your favorite PV Anode *
O O O O H2O H2O …but that is for another Jme References and Resources •  US DOE, Energy Efficiency and Renewable Energy (EERE) –  hhp://www.eere.energy.gov/ •  NaJonal Renewable Energy Lab (NREL) –  hhp://www.nrel.gov •  NY State Energy Research and Development Authority (NYSERDA) –  hhp://www.nyserda.org •  School Power Naturally –  hhp://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 –  hhp://pvcdrom.pveducaJon.org/