.

Photovoltaic Energy Conversion Status and Research Directions

Siva Sivoththaman

Centre for Advanced Photovoltaic Devices and SystemsElectrical and Computer Engineering

University of Waterloo

.

Outline of the Talk

Energy consumption and environmentImpact of PV technologyTechno-economics of PVGrowth predictionsCurrent Applications (space, terrestrial)Fa

cts

& F

igs

Bulk crystals for PVThin film PV materialsDevice architecturesLosses and Theoretical Limitations

New technologiesAdvanced conceptsNano-structured materialsNew device architectures

Centre for Advanced Photovoltaic Devices and SystemsResearch strategyCurrent Research

Tech

nolo

gyR

esea

rch

UW

CA

PD

S

.

1973 2005

4700

Mtoe

7912

Mtoe

68%

1973 2005

27136

15661

Mt

Mt

73%

13.1%

48.2%

14.3%

13.5%9.3%

1.6%

8.3%

43.4%

15.6%

12.9%

16.3%

3.5%

coal

oil

gas

waste

electricity

other

increase in world energy use CO2 emission

1973 2005

World energy consumption and the environment

OECD OECD

nonOECD

nonOECD

700

445

Qua

drill

ion

BTU

s

world energy usage prediction

2004 2030

200

400

600

800

1980 1990 2000 2010 2020 2030

Qua

drilli

on B

tu

year

634

773high growth

low growthCO2 capture and storage (CSS) activities of future

post combustion CSSpre combustion CSSoxyfuel combustionindustrial processes

-electricity sector can take the highest share of CSS

Wind Biomass Geo PV

59 GW 44 GW 9.3 6.2 GW

Hydro 800GW

Solar thermal 0.4 GWOcean power 0.3 GW

Sustainable Energy - Existing electricity generation capacity ( 2005)

(num

bers

IEA

)

.

Photovoltaics today: facts and figures

0 0.5 1 1.5 2 2.5 3

1995

1996

1997

1998

1999

2000

2001

2002

2003

2004

2005

2006

Annual PV production

Gigawatts

2005

2006 com m ercial grid res idential grid

res idential grid

rem ote

rem otecom m ercial grid consum er

25% 56% 15% 4%

43% 42% 13% 2%

major application areas of PV

crys tallines ilicon

thinfilm s

7%

93%34%

56%

3%

4%

2%

1%

single crystalline Si

multi crystalline Si

Si ribbons

Siliconthin films

CdTethin films

CIGSthin films

Base semiconductors for PV – 2005 market share

0 1 2 3 4 5 6 7 8 9

1995

1996

1997

1998

1999

2000

2001

2002

2003

2004

2005

2006

PV priceevolution

$ / Watt

.

Photovoltaics tomorrow: growth predictions

0

2

4

6

8

10

12

2006 2007 2008 2009 2010 2011

conservative

accelerated

year

Gig

awat

tspredicted PV growth scenarios

55%

10%

9%

9%

17%

50%

20%

30%

PV module

back-endelectronics

installation costs

BOS

other

silicon material

cell processing

module processing

PV cost breakdownCurrent Technology (crystalline silicon)

SYSTEM MODULE

0.E+00

1.E+08

2.E+08

3.E+08

4.E+08

5.E+08

6.E+08

7.E+08

2005 2010 2015 2020 2025

MWh

year0.E+00

5.E+05

1.E+06

2.E+06

2.E+06

3.E+06

3.E+06

4.E+06

2005 2010 2015 2020 2025

jobs

year

estimated PV electricity production

estimated PV jobs

Eur

opea

n C

omm

issi

on, A

Vis

ion

for P

V 2

005)

.

Photovoltaics tomorrow: growth predictions

(Ger

man

Adv

isor

y C

ounc

il on

Glo

bal C

hang

e)

.

Photovoltaics tomorrow: environmental impact

energy used in PV production

300 MJ/kgdirect metallurgic process

500 MJ/kgfluidized bed reactor process

1070 MJ/kgimproved Siemens process

process energy used in poly-silicon production

energy input: process energy, material consumption

energy embedded in materialslaminates, frames, mg-Si..

poly-Si process energy is nearly 30% of total primary energy input for a multicrystalline Si PV module

ingot growing, wafering..

overhead facilitiesprocess area climate control, de-ionized water plant, compressed air,..

emissions in PV productionCO2 emission in silica reductionfluorinated gases – cell processingNOx gasesvolatile organic compounds

energy pay-back time (EPBT) =energy input

energy saved / year(years)

energy input: manufacturing, installation, operation, decommissioning(for whole life-cycle)

energy saved: annual energy savings due to generated PV electricity

energy return factor (ERF) = energy input(energy saved / year) * lifetime

8.713.83.102.02Los Angeles

7.09.33.742.9Oslo

7.711.93.442.32Vancouver

8.813.13.062.13Ottawa

10.117.92.711.59Perth

FacadeRoof-topFacadeRoof-top

ERFEPBT (years)

0.E+00

5.E+07

1.E+08

2.E+08

2.E+08

3.E+08

3.E+08

4.E+08

4.E+08

2006 2010 2014 2018 2022

tons

year

Predicted CO2 emission reduction by PV

(EP

IA, S

elec

ted

envi

ronm

enta

l ind

icat

ors)

.

photovoltaic cells: basic operation

hν1 hν2 hν3photons

e- h+

e- h+

p-type semiconductor

n-type

valence band

conduction band

EG

E1

E2

photon absorption

valence band

conduction band

EG

E1

E2

photon absorption

phonon absorption

emission

photon absorption in semiconductors

direct bandgap indirect bandgap

(over)simplified schematic of a solar cell

voltage

curre

nt

VOC

JSC

JP, VP

maximumpower point

rs

rshJph

d1 d2V

Iin

tern

al q

uant

um e

ffici

ency

, IQ

E (%

)

wavelength, λ

efficiency (η) = JSC * VOC *FF

∑ P(λ) dλ

IQE ≈JSC(λ)

q [1 – R(λ)] f(λ) [exp(-αλ)WOPT] – 1]

Internal quantum efficiency

emitter

base

electrodes

.

photovoltaic cells and interconnection

glass

encapsulant

solar cell

back layer

frame

interconnector

cellmodule

.

Technology: bulk crystals for photovoltaics

0.E+00

2.E+04

4.E+04

6.E+04

8.E+04

1.E+05

1.E+05

2005 2006 2007 2008 2009 2010

PV indus try

Electronic Indus try

tons

polysilicon feedstockSiO2 or quartzite

metallurgical grade silicon

chlorosilanes

high purity SiHCl3

polycrystalline silicon

SiO2 + C Si + CO2

99% pure

Si + HCl SiHxCly

reduction with H2

(≈ $2 / kg)

(≈ $50 / kg)

separation and purification

process sequence for semiconductor grade silicon

99.999999999% pure

PV requires “6N” pure (>99.9999%)

material purity and PV device performance

(J R Davis et al, IEEE Trans. Elect. Dev., 1980)

.

Technology: bulk crystals for photovoltaics

seed

crystal ingot

neck

Si melt

d1

d2

V2

V1

crucible

V2 d22 = V1 d1

2

melt

d1

d2

seed

V1

V2

RF coil

crystal ingot

poly-siliconfeed rod

V2 d22 = V1 d1

2

Czochralski Process Float Zone Process

growth speed: 1- 2 mm / minute

uses crucible

long heat-up and cool-down duration

high oxygen content ( > 1018 / cm3)

high carbon content (> 1017 / cm3)

carrier lifetime in microseconds range

ingot diameter up to 200 mm

Si feed can be in any form.

growth speed: 3- 5 mm / minute

no crucible

relatively short heat-up and cool-down

low oxygen content ( < 1016 / cm3)

low carbon content (< 1016 / cm3)

carrier lifetime in milliseconds range

ingot diameter up to 150 mm

Si feed must be crack-free rod.

PV devices based on FZ-Si result in 10% - 20% superior performance compared to Cz Si.

Direct Cast Process – multicrystalline silicon

melt

ingot

Random, columnar grains

Dislocations, impurities

Simplicity compared to Cz or FZ

Relatively lower cost process

Mainly used for PV applications

Performance inferior to FZ, Cz

Czochralski reactorat UW-CAPDS

.

Technology: bulk crystals for photovoltaics

molten silicon

crucible

silicon ribbon

meniscus capillary slot

die

dendrite seed

button

silicon melt

webbounding dendrite

bounding dendrite

substrateribbon transport

silicon melt

silicon ribbon

casting die crystalgrowth

Dendritic web technique (WEB) Edge-defined film-fed growth (EFG)

Ribbon growth on substrate (RGS)

When lowered into the

melt, the seed spreads to

form the button.

Secondary dendrites

propagate from the ends

of the button, forming the

frame.

slotted graphite die controls the geometry of the ribbon.silicon is fed via capillary action and gets in touch with the seed crystal.

direction of crystallization and growth are perpendicularAfter cooling, the Si foil is separated from the substrate, which is re-usable.

≈ 12 cmup to 1000 cm/minRGS

≈ 12 cmup to 2 cm/minEFG

≈ 8 cmup to 2 cm/minWEB

WidthPull speed

.

Technology: thin films for photovoltaics

amorphous silicon

Van

ecek

et a

l., J

. Non

-Cry

stal

line

Sol

ids,

199

8.

photon energy (eV)

abso

rptio

n co

effic

ient

(cm

-1 )

amorphous silicon deposition methods

high deposition rates, uniformity not excellent50 angs / secondμ-wave PECVD

high deposition rates, uniformity not excellent50 angs / secondHotwire CVD

high quality, uniform films, slow1 angs / secondPhoto CVD

high deposition rates, uniformity not excellent15 angs / secondVHF PECVD

high quality, uniform films, slow3 angs / secondRF PECVD

high quality, uniform films, slow3 angs / secondDC PECVD

very poor quality, slow3 angs / secondSputtering

Generally observed propertiesTypical maximum deposition rates

RF

film is deposited by de-composition of silane

Typical RF-PECVD system

optical absorption property of a-Si

1.72 eV

Donor-like tail states

Acceptor-like tail states

density of gap states in a-Si

VHF and HW CVD have potential for future, high throughput manufacture for PV.

.

Technology: thin films for photovoltaics

copper indium (gallium) selenide, Cu(InGa)Se2 cadmium teluride (CdTe)

deposition methods

A uniform film is first obtained by low temperature deposition of Cu, In, and Ga.The film is then annealed in a Se atmosphere, in the 400 - 600ºC range.Cheaper options available.

Two-step process(selenization)

The constituents, Cu, In, Ga, Se, etc., are co-evaporated onto the substrate.Substrate temoerature: 400 - 600ºCType of evaporation: thermalThis can described as a single step deposition process.

co-evaporation

energy gap ≈ 1.02 eV

high absorption coefficient: 105/cm at 1.4 eV

reported mobilities: 200 cm2/Vs for

Cu(InGa)Se2 with about 1017/cm3 hole

concentration; electron mobilities in single

crystals 90 -900 cm2/V-s

some properties

some deposition methods

From CdTe target, 10-4 torr, substrate temperature 200ºCRate: 0.1 μm/min

Sputter deposition

evaporation of stoichiometric CdTe results in a stoichiometric composition of the vapor. High quality material can be deposited at very high rates(1-5 μm/min) at substrate temperatures of 450–600 ºC.

Close spaced sublimation (CSS)

some properties

optical bandgap: 1.5 eV +/- 0.01 eVabsorption coefficient: 6 x 104 cm-1 at 600 nmelectron mobility: 500 – 1000 cm2/V-shole mobility: 50-80 cm2/V-s

CSS process

.

Technology: materials and performance

(Goe

tsbe

rger

, Mat

. Sci

. Eng

., 20

03)

Effi

cien

cy (%

)

Semiconductor bandgap, eV

Semiconductor properties and PV device performance

bandgap

type of semiconductor: direct or indirect

solar spectrum (AM 1.5)

controllability (defects, doping,..)

device design

technological feasibility

9 – 12%17%CdTe

32%GaInP/GaAs/Ge multi junction

9% - 13%19%CuIn(Ga)Se2

6% - 7%Amorphous Silicon (single junction)

9% - 10%13%Amorphous silicon (multi-junction)

13% - 17%24.7%Single crystalline silicon

Efficiency – industrialEfficiency - LaboratoryPV material

some typical numbers on current technology

terre

stria

l

space

material availabilitytoxicity issues

large scale manufacturabilitylong term stability

.

(M. G

reen

, UN

SW

)

high efficiency devices based on single crystalline silicon

“passivated emitter and rear cell”(PERC)

“passivated emitter, rear locally diffused” cell(PERL)

locally diffused back surface fieldminimal metal contact area

deeper emitter under metal contactsshallow junctions

inverted pyramid structure for optical confinementoxide passivation to reduce surface recombination

Technology: photovoltaic device architectures

.

Technology: photovoltaic device architectures

amorphous silicon / crystalline silicon heterojunctions

high efficiency devices with amorphous/crystalline Si heterojunction

(Tag

uchi

et a

l., P

rog.

In P

V, 2

000)

low temperature process

good interfacial passivation can be obtained

emitter and back surface field are formed in non-diffusion

processes

a-Si layers are thin, contribute little to the power generation, so

stability problems (Staebler Wronski effect) are minimal.

.

Technology: photovoltaic device architectures

amorphous silicon thin film cells

glass

p-type a-SiC:HSnO2:F

intrinsic a-Si:H

n-type a-Si:HTCO (ZnO or ITO)Al or Ag

simple p-i-n cell structure

+ -few hundred nm

p

i

n+

-

EF

p i n

schematic band diagram

a-Si:H

(i)

a-Si:H/a-SiGe:H

(i)

a-SiGe:H

(i)

p p pn n n

ITO Zn

O

Ag

triple junction device

tunneljunction

tunneljunction

hν

hν

1.8 eV 1.6 eV 1.4 eV

Rec

h, A

ppl.

Phy

s. A

, 199

9)

.

Technology: photovoltaic device architectures

copper indium (gallium) selenide devices

cadmium teluride (CdTe) devices

high lab efficiencies approaching 19% have been reported.

commercial modules have reached 13% efficiency.

heterojunction is achieved by chemical deposition of a CdSlayer from a solution containing Cd-ions

due to the intermixing of the CdS and the CdTe layer, not an abrupt heterojunction but a graded gap structure is formed.

Goe

tzbe

rger

, Mat

. Sci

. Eng

. 200

3

.

Technology: photovoltaic device architectures

space cells, high conversion efficiency

( tend to reach 40% conversion efficiencies under concentrated light)

http://www.spectrolab.com/prd/prd.asp

current 1-Sun efficiency ≈ 31%

.

Photovoltaic devices: performance limits

thermallization

transmission loss

junction loss

recombination

contact loss

loss mechanisms in a single junction device

(Noz

ik, A

nn. R

ev. P

hys.

Che

m.,

2001

)

carrier relaxation/cooling dynamics

*h

*e

Ge

mmEhE

+

−=Δ

1

ν ( ) eGh EEhE Δ−−=Δ ν

future advanced research approach tend to minimize the losses, make maximal usage of available photons, and, to break what has so far been considered as theoretical performance limits of the technology.

.

crystalline silicon based photovoltaics and thin films

the market share of these technologies will continue for the foreseeable period

R&D on PV-specific bulk crystals continue to be carried out successfully (MG-grade, feedstock refinement, ribbons)

high efficiency device designs and advanced fabrication technologies will contribute to enhanced device performance

Improved a-Si based techniques and CIGS technology will also continue to grow

future R&D approaches for ultra-high performance

theoretically proven concepts, many yet to be demonstrated

considered to be topics of medium to long term research

research efforts are still small and definitely need growth

some examples are:

intermediate band photovoltaic devicesquantum dot devicesspectrally engineered devicesmultiple junction approaches

Photovoltaic devices: approaches for increased cost performance

.

Advanced approaches: intermediate band photovoltaic devices

conduction band

valence band

intermediateband

the intermediate band concept exploits the two-step absorption of subbandgap photons via a half filled intermediate band (IB) located within the semiconductor gap.

electrons can be excited by a single high energy photon, or in 2 steps using two lower energy photons.

theoretical efficiency limit is > 80% !

semiconductor base with intermediate band

p-emittern-emitter

metal contacts

Practical implementation is challenging.

Use of quantum dots is a possibility. Quantum dots can produce an

electron level within the host semiconductor, therefore a quantum

dot superlattice can lead to intermediated band.

schematic of a semiconductor base with intermediate band and absorption processes.

the intermediate band base material should be placed between two “regular” semiconductors, one n-doped, the other p-doped.

n pbase

.

Advanced approaches: quantum dot photovoltaic devices

(Noz

ik, P

hysi

caE

, 200

2)

one photon creates two electron hole pairs

impact ionization

theoretical efficiency limit >80%

example configuration

.

Advanced approaches: spectral engineering

λ1λ2

λ2

λ3λ4

λ3 λ3 > λ4

λ2 > λ1solar cell

solar cell

down conversion layer

isolation

metal contact

isolation

up conversion layer

metal contact orbifacial structure

“peripheral approach” - without changing much the core structure of the device

broaden the range of harvestable photons in the solar spectrum.

down conversion

up conversion

quantum dots

silicon nanostructures, nanowires

erbium-doped up-converters

nanostructured materials doped with ions

transition metal-based

organic materials

(strumpel et al., Sol. Mat. Sol. Cells., 2007)

.

Advanced approaches: multiple junction approaches

multi-junction (tandem) solar cells - high efficiencies for spaces and terrestrial applications

over 30% one-sun efficiencies achieved

InGaP/InGaAs/Ge and InGaP/GaAs/InGaAs triple-junction devices

(Yamaguchi, Physica E, 2002)

.

CAPDS Research: Current activities within the group

Semiconductor feedstock, crystal growth, and characterization

Flexible spherical silicon PV technology

Technology for silicon based thin films

Silicon nanostructures and quantum dots for future PV

Theoretical modeling of advanced devices

Photovoltaic device fabrication and testing

.

CAPDS Research: silicon feedstock, crystals, and characterization

Research on low-cost silicon materials for PV

200 μm

Fused silicon from powder Recrystallized silicon

150 μm

piriform silicon granular silicon

0

0.2

0.4

0.6

0.8

1

500 510 520 530 540 550

Wavenumber (1/cm)

Ram

an S

igna

l (a.

u.)

G-Si Cz-SiP-Si G-Sph P-Sph

Raman shift spectrum for different Si particles

0.003

0

0.2

0.4

0.6

0.8

1

B P Al

Con

cent

ratio

n R

atio Ideal

SIMSimpurity concentrations before and after recrystallization

Annealed 925 ºC

P-DiffusedDenudedRecrystalliz-ed

Annealed 850 ºC

Annealed 850 ºC

0

0.5

1

1.5

2

2.5

Life

time

(µs)

crystallization of of powdered silicon

crystal defect analysis

crystal quality

doping

electronic properties – minority carrier lifetime

.

CAPDS Research: flexible spherical silicon PV technology

500 μm500 μmrs

rsh

iph

rsrsh

iph

p

n+

original emitter

p

n+

original emitter

From basic materials to devices

advanced device designsanalysis

testing

Material improvement techniques

Selective emitter devices

Surface-passivated devices

Technological implementation

.

CAPDS Research: silicon thin film technology for photovoltaics

Transmission electron micrograph

Electrical conductivitycrystallinity

Development of low-temperature Si films with high crystal quality

Compatibility with temperaturte-sensitive, low-cost Sisubstrates

Dopant control,and activation linked crystallinity

Technology leads to a new, low temperature process sequence

as-deposited films after rapid opto-thermal processing

New technology for planar photovoltaic devices

.

CAPDS Research: nano-structures for future advanced PV

upright silicon nanopillarsPhotoluminescence – Si nanowires

oxide/nitride coated Si nanowiresCdSe quantum dots

Photoluminescence – CdSe quantum dots

Simple fabrication technologies for silicon nanopillars

Nanowires with diameter is further reduced by post-process opto-thermal oxidation

Nanowires detachable, substrate re-usable

Deployment of nanowires and quantum-dots in photovoltaic devices as photon shifting as well as active components in advanced devices.

.

CAPDS Research: PV device fabrication technologies

Low-T epitaxial pn junctions

Junction formation by RTP-annealed amorphous Si films

Shallow boron diffused junctions by RTP

“pyramidal” texturing for optical confinement pyramidal structures wit upright nanowires

Low-thermal budget techniques for formation of device quality pn junctions

Develop processes that are compatible with low-cost, temperature substrates

Advanced design and optical confinement features for performance enhancement.

Practically abrupt junctions are obtainable by the new low-T epitaxial process.

High process yields possible

.

CAPDS Research: PV device fabrication technologies

New technology

A new technology has been developed recently, resulting in a simple low-temperature process.

The technology employs quasi-epitaxialfilms on silicon substrates.

Compatibility with low-cist siuliconsubstrates.

Technology was demonstrated on low-lifetime multicrystalline silicon substrates and performance independently confirmed.

Process will yield high conversion efficiencies when applied to single crystalline silicon.

.

SUMMARY

An overview of the current world energy situation and the place of photovoltaics as an energy source has been presented.

Future growth predictions has been outlined for photovoltaic technology.

While the current growth rate will be maintained for a foreseeable future, new PV technologies players will be slowly introduced, ultimately changing the market share in the long-run.

Some key areas for further research in PV materials and devices has been presented.

Some current PV research activities at Waterloo has been presented.

.

ACKNOWLEDGEMENT

The contributions of my current and recent-past graduate students are highly appreciated.

Ms. Hua BaiProf. Mahdi Farrokh BaroughiDr. Stella ChangDr. Majid GharghiMr. Hassan El-GoharyMs. Bahareh SadeghimakkiMs. Cherry ChengMr. Roohollah Samadzadeh TarighatMr. Behzad EsfandyarpourMr. Abdulla Bin Ishaq