R&D R&D sul sul fotovoltaico fotovoltaico in STM in STM - - PowerPoint PPT Presentation

R&D R&D sul sul fotovoltaico fotovoltaico in STM in STM

Marina Foti IMS R&D STMicroelctronics STMicroelctronics Convegno su 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: 3SUN A 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

• f

consumer products (LCD TV, LED TV, ecc). A leading company at global level in the photovoltaic sector (Solar Cells, and Electronic Devices). STMICROELECTRONICS, is

• ne
• f

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 Multi Thin Film Multi-Junction Modules Junction Modules Fab Fab

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

• f 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 vs vs Si wafers Si 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 glass Large 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 TCO Back 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 glass

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

BC TCO

Metal, plastic..

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

BC TCO

Superstrate Substrate

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Glass 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 fabricated

monolithically on a glass substrate during front end process

• The back end is dedicated to

add 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

Thin Thin film on film on glass glass: FEOL : FEOL process process

Glass with TCO Layer Cleaning Laser Scribe P1 PECVD Deposition a-Si:H -pin SOIR cSi:H pin Laser Scribe P2 TCO Deposition Laser Scribe Back Contact P3 Cleaning

Scheme of thin film module Scheme of thin film module

load

• +

TCO

glass

cell

back contact

PECVD High automation

Glass Size Matters for Thin Film

PECVD deposition TCO deposition large 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 silicon

Amorphous 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 energy states for electrons

17

due to the disorder direct

• ptical 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-
• 300C. Low gas utilization (10-30%). Heavily

hydrogenated 1-10 at.% H.

• PN (PIN) junctions formed through boron or

phosphorous containing gases.

• Total thicknesses in some cases below 1 µm (100 times

thinner than c-Si).

• Multiple junction devices with two or three junctions

grown 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

Staebler Staebler-Wronski Wronski Effect Effect

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 i p n

20

• Carriers are photogenerated in the

intrinsic region and collected by drift

Amorphous and Microcrystalline silicon

Two materials with the same process

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the same process PECVD

a-Si:H Eg=1.8eV µ µ µ µc-Si:H Eg=1.1 eV

Columnar 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/tandem Enhanced absorption: double junction/tandem

nsity (kW/m2µm) “spectrum splitting.” Wavelength (nm) Light intens

Micromorph cell efficiency 11-14% Micromorph module efficiency 8.5-10.8%

Tandem configuration: Top Tandem configuration: Top a-Si:H Si:H, Bottom , Bottom µ µ µ µ µ µ µ µc-Si:H Si:H

TCO

0.40 0.50 0.60 0.70 0.80 0.90 1.00 AL QUANTUM EFFICIENCY

a-Si:H

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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 EXTERNAL Q

Wavelength (nm)

a-Si:H µ µ µ µc-Si:H

From From Single Single to to Multiple Multiple junctions junctions

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 large areas

• Best stabilized efficiencies above 12%

glass textured TCO a-Si:H top absorber a-SiGe:H middle absorber µ µ µ µc-Si:H bottom absorber ZnO Ag

• 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:H efficiencies

• 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

• ptical path in the absorber layer (a-Si:H or µ

µ µ µcSi: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 ZnO p-i-n uc-Si:H TCO Back 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 conductivity Impact 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)

λ λ λ λ/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 contact

400 500 600 700 800 20 40 60 80

Cell reflectivity (%) 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

10 20 30 40 50 60 70 80 90 100 200 300 400 500 600 700 800 900 1000 1100

Transmittance (%) Wavelength (nm)

UV-type ANX10 OE_B_TD OE_C_TD ANX10

Total T Diffused T

SnO2:F ZnO:B

TCO a-Si:H µ µ µ µc-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

ϑ ϑ λ π

2 2 1 1

cos cos 2 exp 1 n n T T H

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 EXTERNAL QUANTUM EFFICIENCY 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 y (mA/cm2)

η=11.5% η=12.5% (1) (2)

• ΦZnO< ΦSnO2
• Difference of

Workfunctions differences in the Voc

2 4 6 8 0.5 1 1.5 Current Densisty (m 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 µ µ µ µc-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 EQE 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+00 TOP (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 EQE (%) wavelength (nm) BOTTOM(ZnO with white paper) SUM(ZnO with white paper)

a-Si:H µc-Si:H White pvb

Back TCO

3D Structures

3D 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 modes Scattering Near-field enhancement

37

Waveguide modes Scattering 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

ħω

1/α Idealized radial junction wire 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 Liquid injection head Substrate spin chuck

host precursor c:Si synthesis

+

Colloidal nanocrystals TCO precursor Substrate chuck in atmospheric pressure Liquid injection head Substrate spin chuck

host precursor c:Si synthesis

+

Colloidal nanocrystals TCO precursor Substrate chuck in atmospheric pressure Liquid injection head Substrate chuck in atmospheric pressure Liquid injection head Substrate spin chuck Substrate spin chuck

host precursor c:Si synthesis

+

Colloidal nanocrystals TCO precursor

host precursor c:Si synthesis

+

Colloidal nanocrystals TCO precursor

Applied on Si Thin Film for efficiency > 20%

Si PV Thin film outlook

Thin film technology addresses low cost/Wp by using large area high throughput (e.g. PECVD with high dep, rate low T)equipments with very high level of automation To achieve the target material costs, especially the 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 absorber thickness is necessary necessary Multiple-junctions solar cells are necessary to increase the efficiency but to date they are still characterized by low throughput and higher costs Silicon TF solar cells are expected to achieve a much higher conversion efficiency (up to ~20%) than other TF technologies (CdTe, CIGS,..), which today are strong rival to Si, by exploiting new materials and by applying multi-junction structures. Bring together the experience and know-how of researchers in applied physics to speed up the development of materials and 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 Power Energy 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

r (W) nt (A)

Efficiency @ 300 Lux: 8% - 9%

AM1.5G F12 Spectral density (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

Power (W Current ( Voltage (V)

Jsc (µA) 43.60 Voc (V) 8.42 Pmax (µW) 238.52 eff (%) 7.98

Fluorescence lamp spectrum 300 lux ~ 1W/m2

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)

5 10 15 max power Voc Isc

tio 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

200 400 600 800 Number of bending

variatio

46

3.9cm 2.9cm 1.9cm 1.5cm 1.2cm 0.95cm

• 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