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. Abramsnmabrams@esf.edu

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

12:00 AM 3:00 AM 6:00 AM 9:00 AM 12:00 PM 3:00 PM 6:00 PM 9:00 PM 12:00 AM0.0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

8.0

9.0

10.0

Time

Ener

gy (k

Wh)

Science in Practice

12:00 AM 3:00 AM 6:00 AM 9:00 AM 12:00 PM 3:00 PM 6:00 PM 9:00 PM 12:00 AM0.0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

8.0

9.0

10.0

Time

Energ

y (

kW

h)

• ESF PV array on Walters

Science into Practice

• ESF PV array on Walters

date

9/2/08

9/4/08

9/6/08

9/8/08

9/10/08

9/12/08

9/14/08

9/16/08

9/18/08

9/20/08

9/22/08

9/24/08

9/26/08

9/28/08

9/30/08

10/2/08

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10/8/08

10/10/08

10/12/08

10/14/08

10/16/08

10/18/08

10/20/08

10/22/08

10/24/08

10/26/08

10/28/08

10/30/080

10

20

30

40

50

60

70

80

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/