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Tecnologie, tecniche impiantistiche e mercato del fotovoltaico

15 Ottobre 2012

Mondello (PA)

Outline

• Thin film module technology

• Amorphous silicon (a-Si:H) and microcrystalline (µc-Si)

• Tandem and multiple junction solar cells

• Enhancement of light absorption in thin film Si

• Development of TCO front and back electrodes

• Next steps on light trapping

• Thin film silicon outlook

• TF PV flexible application for smart systems

A New PV Joint Venture: 3SUNA New PV Joint Venture: 3SUN

ENEL GREEN POWER, ENEL Group Company,

dedicated to the development and

management of activities related to energy

production from renewable sources at an

international level, which operates in Europe

and the American Continent. It is a leading

Company in this sector at global level.

SHARP CORPORATION, a Japanese

Company, which operates at global level in

the manufacturing and distribution of

consumer products (LCD TV, LED TV, ecc). A

leading company at global level in the

photovoltaic sector (Solar Cells, and

Electronic Devices).

STMICROELECTRONICS, is one of the

largest manufacturers of semiconductors

in the world with customers in all

electronics segments. The Corporate

headquarter is in Geneva, advanced

research and development centers in 10

countries, 14 main manufacturing sites

and sales offices all around the world.

3SUN 3SUN –– Thin Film MultiThin Film Multi--Junction Modules Junction Modules FabFab

Numbers:

• 240 000 m2 surface area

• 60 000 m2 Fab area

• 300 employees

• 160 MW/y (2011)

•240MW/y, …possible extension

The biggest PV Italian fab destined to

compete with the most important players

of the sector

Thin film multi-junctions modules are

manufactured in the innovative plant M6

built in Catania

Large area modules: 1m × 1.4 m

Why TF Solar Cells?Solar cell Si raw material Efficiency Peak power Peak power

c-Si 1200-1300 g/m2 16% 160W/m2 0.13W/g

TF-Si 5 g/m2 10% 100W/m2 20W/g

Large area

multi-junction / glass

Amorphous or tandem / flex

Amorphous / glass

Technology options: thin film Technology options: thin film vsvs Si wafersSi wafers

BULK Si SOLAR CELLS

Series connection of individual solar

cells

Mature technology but needs

a lot of Si

THIN FILMS

Monolithic integration

(series connection by lasering)

CVD on very large areas

Potential for ultra low costs

Processing of wafers

Large area modules on glassLarge area modules on glass

Altomonte (CS - Italy): 8,2MW. 11 Millions of kWh. It can satisfy the needs of 4.000 families

Other than Ground PV Plants…Other than Ground PV Plants…

Parking area Roof

Easy installation and no particular

maintenance or cleaning required. No

specific accurate angle to the sun. Perfect

integration with the environment.

Residential, Commercial and Industrial

Roof

Various Applications Innovative Future Solutions

Roof

Installation following the roof profile and

good performance at any slope of the

roof. Nice appearance integrated to the

building design.

Deserts and hot climate Countries

Supplying high performance even at

50~60°C thanks to the low temperature

coefficient (-0,24%/°C). Good performance

even when the panels would be partially

covered by dust and sand thanks to the

feature to produce energy with diffuse

light.

Integration on Building Design

Building front designed with Glass/Glass

Frameless Thin-Film PV Modules

Car, Truck and Trailer PV

Roof

Stand-alone Applications

powered by PV panels

E.g. Water sweetening kit

Thin film PV on flexible substrate

Substrate and superstrate configurations

BC TCOBack electrode (Ag, Al, white pvb)

Opaque sealing

Front TCO

a-Si:H, uc-Si:H or multiJ

Transparent sealing

Thin film deposition at low temperatures on large area substrates

glassFront TCO

a-Si:H, uc-Si:H or multiJ

BC TCO

Metal, plastic..

a-Si:H, uc-Si:H or multiJ

Back electrodeBC TCO

Superstrate Substrate

11Glass with

TCO

Edge seaming

Cleaning

Laser scribe P1

TCO

Laser edge

deletion

Bus bars and

wires

connection

Lamination

In line solar

simulator (IV)

Laser scribe

Isolation P4

•The modules are fabricatedmonolithically on a glasssubstrate during front end process

•The back end is dedicated toadd electrical connection,

Thin Film Module process flow Thin Film Module process flow

PECVD

deposition

Laser Scribe cell

P2

PVD deposition

Back contact

Laser scribe

back contact P3

Lamination

with PVB and

back glass

J-box

connection

2nd in line solar

simulator (IV)

Packaging

add electrical connection, protection layers, frame and junction box

•Typical process flow tandem modules

•1 x 1.4 m2

ThinThin film on film on glassglass: FEOL : FEOL processprocess Glass with TCO Layer

Cleaning

Laser Scribe P1

PECVD Depositiona-Si:H -pin

SOIR

µc-Si:H - pin

Laser ScribeP2

TCO Deposition

Laser ScribeBack Contact

P3

Cleaning

Scheme of thin film moduleScheme of thin film module

load

- +

TCO

glass

cell

back contact

PECVD

High

automation

Glass Size Matters for Thin Film

PECVD

deposition

TCO

depositionlarge area and high

throughput is needed to

achieve low cost/Wp

TF Silicon Costs breakdown

3%

5%6%

15%6%14%

5%3%

46%

TCO Gas/Chem Target Back glass Encapsulant Terminal Box Silver Paste/ Bus Bar/ Packing/Other Lead Wire / MultiFrame J

Amorphous siliconAmorphous Si: a-Si:H layers were first deposited by R. Chittick

(1969) experimenting with SiH4 in a plasma reactor.

First systematic study by Spear et al Phil Magaz, 33, 935 (1976)

Tetrahedrally

bonded

c-Si structure

Amorphous Si:

absence of

Long range order

Distribution of density of allowed energystates for electrons

17

due to the disorder direct

optical transitions are not

forbidden in amorphous Si

Eg = ~1.8 eV

better light absorption

than c-Si

Amorphous Si for thin film PV• Deposited by plasma-enhanced CVD of SiH4 at 150-300°C. Low gas utilization (10-30%). Heavilyhydrogenated 1-10 at.% H.

• PN (PIN) junctions formed through boron or phosphorous containing gases.

• Total thicknesses in some cases below 1 µm (100 timesthinner than c-Si).

• Multiple junction devices with two or three junctionsgrown one upon the other and current matched.

The BIG three challenges

• Improve efficiency from 6-8% up to 12-15%;

• Minimize or eliminate the self-limited degradation

• Increase deposition rate

StaeblerStaebler--WronskiWronski EffectEffect

L

• Exposure to light induced degradation, which stabilizes with time

• Typically after 1000h of continuous light soaking at 1 Sun AM1.5G

• New dangling bonds (from 1e15 to 1E17 cm-3) are created under light exposure

• Degradation is recovered after annealing at T<150C

Typically 10-13 % of degradation

For a-Si:H of 150-300nm

Limitation on the thickness

annealing at T<150C

Amorphous a-Si:H: p-i-n

Drift charge transport – p-i-n junction

ip n

20

• Carriers are photogenerated in the

intrinsic region and collected by drift

Amorphous and Microcrystalline silicon

Two materials with

the same process

21

the same process

PECVD

a-Si:H Eg=1.8eV

µµµµc-Si:H Eg=1.1 eVColumnar microstructure

Amorphous

Eg=1.8eV

«High» absorption

in the green-blue

Microcrystalline

Eg=1.1eV

«High» absorption in

the red-near I.R.

Enhanced absorption: double junction/tandemEnhanced absorption: double junction/tandemLi

ght i

nten

sity

(kW

/m2 µ

m) “spectrum splitting.”

Wavelength (nm)

Ligh

t int

ensi

ty (

kW/m

Micromorph cell efficiency 11-14%

Micromorph module efficiency 8.5-10.8%

Tandem configuration: Top Tandem configuration: Top aa--Si:HSi:H, Bottom , Bottom µµµµµµµµcc--Si:HSi:H

TCO

0.40

0.50

0.60

0.70

0.80

0.90

1.00

EX

TE

RN

AL Q

UA

NT

UM

EF

FIC

IEN

CY

a-Si:H

23

Multiple junction devices with two junctions grown one upon the other

and current matched spectrum splitting enables higher absorption

and higher efficiency

0.00

0.10

0.20

0.30

250 350 450 550 650 750 850 950 1050 1150

EX

TE

RN

AL Q

UA

NT

UM

EF

FIC

IEN

CY

Wavelength (nm)

a-Si:H

µµµµc-Si:H

FromFrom Single Single toto Multiple Multiple junctionsjunctions

glass

• Single Junction–aSi:H cell with enhanced light trapping – TCO and Texturing

• Double Junction / Tandem cell–highest efficiency: combination of absorber materials having band gap 1.8 eV and 1.1 eV for the top and bottom cell..

• Triple junction–aSiGe:H middle absorber more than 12% on largeareas

• Best stabilized efficiencies above 12%

glass

textured TCO

a-Si:H top absorber

a-SiGe:Hmiddle absorber

µµµµc-Si:H bottom absorber

ZnOAg

• Higher efficiencies (from 12 to 20%) are possible with additional junctions

• But so far :

•Reduced throughput:~ 30% lower for triple Junction

•Costs ~ 20% higher than tandem

•Despite the lower efficiency of tandem technology higher throughput in MW/years

Issues limiting a-Si:H and µc-Si:Hefficiencies

• a-Si:H : Voc too low 0.9V instead of 1.4V (bandtails contacts)

25

• µc-Si:H: Low Jsc. Improve absorption, light trapping

Light trapping increases the absorption because increases the optical thickness

Light can be captured in the desired parts of a solar cell (absorber layers)

and can be confined in it.

26

The cell current can be enhanced by increasing the effective

optical path in the absorber layer (a-Si:H or µµµµcSi:H)

300nm a-Si:H

M. Zeman, J ELECTRICAL ENG, VOL. 61, NO. 5, 2010, 271–276

Light trapping

p-i-n a-Si:H

p-i-n uc-Si:H

TCO

glass

light~700nm

~250nm

~1.6µm

Asahi VU (SnO2:F) Asahi W

ZnO:B -MOCVD W text ZnOp-i-n uc-Si:H

TCOBack reflector

~1.6µm

~50nm

• Improvement: about 50 % reduction of the deposition time

• (today limiting process step).

Figures of merit for TCO 28

• A good TCO must have a high figure of merit (conductivity/absorbtion coefficient)

• Rs is the sheet resistivity

• R is the reflectance

R.G. Gordon MRS Bulletin 2000

BAND GAP BAND GAP Engineering: Engineering: Impact of work function on solar cell conductivityImpact of work function on solar cell conductivity

TCO / aSi:H (n doped)

barrier

TCO / aSi:H (p doped)

barrier

electrons rich area

barrier

TCO objective: WF < 4,3eV

Hole rich area

barrier

TCO objective: WF > 5eV

Haze impact

• Apart from the transmittance and the low sheet resistance (~7-10 Ω/) a TCO must have:

• reduced reflection due to refractive index grading (1<n<3); this effect applies to

100

Index grading at the

TCO/p interface

(whole spectral range)

Light trapping and index

Grading at the back reflector

(red spectral range)

λλλλ/n

H=Tdiffused/Ttotal

(1<n<3); this effect applies to the whole wavelength range of the spectral response

• light scattering and subsequent trapping in the silicon absorber; this applies more to the weakly absorbed light that penetrates up to the back contact400 500 600 700 800

0

20

40

60

80

Cel

l ref

lect

ivity

(%

)

Wavelength (nm)

low haze

high haze

Impact of texturing and TCO material

low haze TCO high haze TCO

Haze: ratio between diffusely scattered and total

intensity

0

10

20

30

40

50

60

70

80

90

100

200 300 400 500 600 700 800 900 1000 1100

Tran

smitt

ance

(%)

Wavelength (nm)

UV-typeANX10OE_B_TDOE_C_TDANX10

Total T

Diffused T

SnO2:F

ZnO:B

TCO

a-Si:H

µµµµc-Si:H

BR

• Texturing (increases optical path) can improve the currents generated in the top and bottom cells

intensity

• θ1 direction incident beam

• θ2 direction scattering beam

• exponent: 2<β<3

−−−==β

ϑϑλπ

2211 coscos2

exp1 nnT

TH

total

diffused

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0.80

0.90

1.00

200 300 400 500 600 700 800 900 1000 1100 1200

EX

TE

RN

AL

QU

AN

TU

M E

FF

ICIE

NC

Y

Wavelength (nm)

ZnO - H=20%

ZnO - H=20%

ZnO - H=20%

SnO2-H=10%

SnO2-H=10%

SnO2-H=10%

a-Si:H

µc-Si:H

SnO2:F

ZnO:B

Impact of TCO on the cell performances

• (1) ZnO:B 20% Haze higher Jsc

• (2) SnO2:F 10% Haze

• Φ < Φ

32

8

10

12

14

16

Cu

rre

nt D

en

sis

ty

(m

A/cm

2)

η=11.5%

η=12.5%(1)

(2)

• ΦZnO< ΦSnO2

• Difference of Workfunctions

differences in the Voc

0

2

4

6

8

0 0.5 1 1.5

Cu

rre

nt D

en

sis

ty

(m

A/cm

2)

Voltage (V)

Increase the efficiency: intermediate reflecting layer

• Currents (Jsc) matching (top cell –bottom cell

• IRL refractive index between 1 and 3

• Filters low energy photons

• Reflects high energy photons

• SOIR is obtained in the same PECVD chamber used for a-Si and µµµµc-Si

silicon oxide based

intermediate

reflector layer

(SOIR)

A. Feltrin et al, MRS 2009

Texturing of ZnO on front and on backside• Use of LPCVD or MOCVD ZnO

controlling texturing on front and backside

• Increased light path in a-Si and µc-Si

• Reduced absorber thickness of~ 50%

• Increased efficiency

Front TCO

Glass

Front TCO

Back TCO

a-Si:H

µc-Si:H

White pvb

EQ

E

Wavelength (nm)

Thin cell /Effect of white sheet reflection on microcrystalline

6.00E-01

7.00E-01

8.00E-01

9.00E-01

1.00E+00TOP (ZnO no white paper)

BOTTOM (ZnO no white paper)

SUM (ZnO no white paper)

TOP(ZnO with white paper)

BOTTOM(ZnO with white

White polymer can be used instead of expensive Ag for reflection

Textured thick TCO as back contact

Glass

White PVB contributes significantly to the reflection especially in the bottom

cell (µc-Si:H)

0.00E+00

1.00E-01

2.00E-01

3.00E-01

4.00E-01

5.00E-01

250 350 450 550 650 750 850 950 1050 1150

EQ

E (

%)

wavelength (nm)

BOTTOM(ZnO with white

paper)

SUM(ZnO with white paper)

a-Si:H

µc-Si:H

White pvb

Back TCO

3D Structures3D TCO

• 3D architectures by using TCO 3D patterning

– Higher efficiency 3D structures obtained by using TCO 3D

templates

– To increase light trapping and orthogonalize light

absorption and photocarrier collection

W. Soppe eta al 26th PVSEC 2011

• Planar waveguides with disordered pores to enhance the absorption of the light

(Anderson localization effects )

Riboli et al Optics Letter, 36, 127, 2011

Plasmonic enhancement effects by metal layers and nanoparticles

Plasmonics

Waveguide modesScattering Near-field enhancement

37

Waveguide modesScattering Near-field enhancement

V.E. Ferry, et.al., APL 95 183503 (2009)

• Developing cell architectures with silicon wires

in order to orthogonalize light absorption and photocarrier collection

Silicon wires and quantum dots

p-type

n-type

Traditional planar, single junction solar

cell

~L

ħω1/α

Idealized radial junctionwire solar cell

~L

1/α

• Quantum dot based heterojunction solar cells

38

cell

B.M. Kayes, et.al., J. APPL PHYS 97 114302 (2005).

Substrate chuck in atmospheric pressure

Liquidinjection head

Substrate spin chuck

host precursorc:Si synthesis

+

Colloidalnanocrystals

TCO precursor

Substrate chuck in atmospheric pressure

Liquidinjection head

Substrate spin chuck

host precursorc:Si synthesis

+

Colloidalnanocrystals

TCO precursor

Substrate chuck in atmospheric pressure

Liquidinjection head

Substrate chuck in atmospheric pressure

Liquidinjection head

Substrate spin chuckSubstrate spin chuck

host precursorc:Si synthesis

+

Colloidalnanocrystals

TCO precursor

host precursorc:Si synthesis

+

Colloidalnanocrystals

TCO precursor

Applied on Si Thin Film for efficiency > 20%

Si PV Thin film outlookThin film technology addresses low cost/Wp by using large area high throughput (e.g. PECVD with high dep, rate low T)e quipments with very high level of automation

To achieve the target material costs, especially th e front glass with TCO, need to be low .

Improving light trapping is fundamental to increase the efficiency (12% on single junction) or reduce costs because lower Si a bsorber thickness is necessarynecessary

Multiple-junctions solar cells are necessary to inc rease the efficiency but to date they are still characterized by low throughp ut and higher costs

Silicon TF solar cells are expected to achieve a mu ch higher conversion efficiency (up to ~20%) than other TF technologies (CdTe, CIGS,..), which today are strong rival to Si, by exploiting new mat erials and by applying multi-junction structures.

Bring together the experience and know-how of resea rchers in applied physics to speed up the development of materials an d devices.

Electronic device integrated energy harvesting with flexible thin film PV

Harvesting in Smart Systems An example in Wireless Sensor Nodes for Automation

Harvesting Device

(PV, Piezo, etc)Low Power RF

Transceiver

Sensors

Ultra Low PowerEnergy Conversion

Integrating Harvesting in Smart

Systems

Energy

Autonomous Wireless Sensor Node

Ultra Low Power

Microcontroller

Energy Conversion

Battery Storage

Enabling wireless sensors for energy

autonomy

Harvesting system with flexible foils

Platform features• PV module collects energy from indoor light

(300lux minimum)

• Harvested energy stored in an micro-

battery (ST-TF)

• Managing of the system energy

• Sensing ambient temperature

• Powering an STM8L15 microprocessor

• Supplying an RF transceiver

• Processed data transmitted to a BST

Flexible PV Modules Modules of 30 cm2

Thin film solar cells are monolithically series

connected (13 cells) 250µW

Back contact

pin a-Si:H

TCO

43

Back contact

Polymeric substrate

4 patents of ST on the subject

13 cells module @ 300 lux F12

1.5E-04

2.0E-04

2.5E-04

3.0E-05

4.0E-05

5.0E-05

Po

we

r (W

)

Cu

rre

nt

(A)

Efficiency @ 300 Lux: 8% - 9%

0.00E+00

5.00E+02

1.00E+03

1.50E+03

2.00E+03

2.50E+03

3.00E+03

AM1.5G

F12

Sp

ect

ral

de

nsi

ty (

a.u

.)

0.0E+00

5.0E-05

1.0E-04

1.5E-04

0.0E+00

1.0E-05

2.0E-05

3.0E-05

0.0 2.0 4.0 6.0 8.0 10.0

Po

we

r (W

)

Cu

rre

nt

(A)

Voltage (V)

Jsc (µA) 43.60

Voc (V) 8.42

Pmax (µW) 238.52

eff (%) 7.98

Fluorescence lamp

spectrum

300 lux ~ 1W/m2

0.00E+00

300 800 1300 1800

wavelenght (nm)

Implementation: contact layer

Sequence SnO2:F/p-type a-Si:H/Mo : to study the interface between the contact layers and the p-type a-Si:H

Plays an important role on the PV cell performances

By C-V and I-V data coupled with modeling we find that the Mo provides a better Schottky contact

(a)

provides a better Schottky contact with p-type a-Si:H compared to SnO2:F.

(b)

M. Foti et al, ECS 2011

G. Cannella et al. JAP 2011

Strong synergy with CNR-IMM (S. Lombardo)

0

5

10

15

max power

Voc

Isc

var

iatio

n (%

)

Flex PV: Benchmark with competitors @ indoor light

Efficiency comparison at indoor

• ST Flex Module 30cm2,

• high robustness,

• less leakage

PV module is very robust

Mechanical stress test: Module is bent with very small

-15

-10

-5

0

0 200 400 600 800

Number of bending

var

iatio

n (%

)

46

3.9cm2.9cm1.9cm1.5cm1.2cm0.95cm

Mechanical stress test: Module is bent with very small

radius, r = 2cm

No significant changes in the electrical characteristics

(Voc, Isc, Max Power)

Roll to roll

polyethylene-naphtalate (PEN)

Roll to roll technology

Si TF development at low deposition T

from 150C to RT using new deposition techniques IC PECVD

48

Marina.foti@st.com

www.st.com

www.3sun.com