Thin Film Silicon Solar Cell

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Thin Film Silicon Solar Cells

Comparison between crystalline and amorphous silicon

a-Si:H (and related materials) solar cells

Transparent conducting Oxides (TCO)

Multi-junction solar cells

Solar modules and applications

Thin film crystalline solar cells and heterojunctions


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Photovoltaic Technologies

Ribbon c-Si 2.2%

a-Si/ucSi 5.2%

CdTe 4.7%

Ribbon c-Si 2.2%

CIS 0.5%

Others 0.1%

multi c-Si 45.2%

mono c-Si 42.2%


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Comparison between the Photovoltaic Technologies

DyeOrganic

Thin

Films

III-V

Si


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c-Si : intrinsic better conversion efficiency

a-Si:H : potential cost reduction

Solar Cells : Comparison c-Si / a-Si:H


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a-Si:H solar cell cannot be

based onp n junction !

The space-charge width W can be obtained from the Poisson equation with the diffusion length :

L = (D)1/2 and the mobility : = eD/kT(D : diffusion coefficient).

Diffusion effects are negligible dans a-Si:H (weak mobility). Carrier collection takes place within

the space-charge region : the increase of this region is required.

Comparison c-Si / a-Si:H

c-Si a-Si:H

diffusion length (m) 10 - 200 0.1 - 2electron mobility (cm2/V.s) 500 - 1000 0.05 - 1

conductivity (S/cm) 10-4 - 104 10-13 - 102

doping efficiency 1 10-3 - 10-2

pn junction asymetrical ohmic


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p-i-n junction :

Technological issues :

Absorption of high energy photons

(penetration length : 12-20 nm) Red light absorption (thickness limited by the

space-charge width : 0.5 - 1 m)

Band diagram of a p-i-n junction

a-Si:H Cell Structure

Width of the depletion region :

a-Si:H i W 1m

a-Si:H p,n W ~10-20 nm

p et n regions are used to set-up the internal electric

field but do not significantly contribute to the carrier

collection (increase of defect densities in doping

regions).


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The Physics of a-Si:H Solar Cell

Space charge regions and internal

electric fieldE(x), without applied

external voltage.


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SiH4

PH3

NH3

B H62

Pumping

RF electrode

Plasma

Substrate

Flexible process allowing the growth of various Si

structures (amorphous, microcrystalline), Sidoping and Si alloys : SiC, SiGe

-1

0

1

2

3

4

0.8

Log

alpha

(cm -1

)

1 1.2 1.4 1.6 1.8 2

a-Ge:H

a-SiGe:H

a-Si:H

Log

(

cm-1)

Plasma Deposition (PECVD)

Band gap tuning (opposite behaviorwith SiC)

Energy (eV)


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The increase of the crystalline

volume fraction in c-Si leads to

optical properties close to c-Si.

Band gap variation between 1.0 and 2.5 eV, from

gaseous mixture (SiH4 with CH4 or GeH4).

Low y values are generally used because of

defect density increase with alloying

Optical properties of Si structures and alloys


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Window layers of a-Si:H solar cells

Objective : the decrease of the blue light absorption in thep layer

SiC is generally used for p doped window layer in a-Si:H solar cells


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Metastability in Si thin film solar cells

Advantage of partially crystallized Si thin films : significant reduction of mtastability

effect (Staebler-Wronski)


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a-Si:H solar cells

Ag

ZnO

n

ia-Si:H

p

ITO

substrate

Jsc = 14,36 mA/cm2

Voc= 0.965 V

FF = 0.672

Stabilized efficiency = 9.3%

Optimized performances

(Sa= 0.25 cm 2

)

simplepin structure


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General Trends of Si Thin Film Cells

Same deposition process (PECVD)

typical efficiency ~ 8 - 10 % whatever the silicon structure

a-Si:H

drawbacks : instability and weak deposition rate (0.1 0.2 nm/s)

pm-Si : optical properties close of a-Si:H (Eg, Voc)

advantage : high rate (~1 nm/s) induced by growth from crystallites

nc-Si and c-Si : optical properties close of c-Si

advantage : complementarity with the other materials

possibility to combine within tandem structures


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Electrode window layer Two simultaneous requirements : conducting (3eV) with degeneratedn-type doping

(Fermi level inside conduction band)

Free carier absorption(intraband) and reflexionInterband absorption

Transmission de ZnO:Al

conc. Al

Act ive mater ial

TCO

Back reflector

0

0,1

0,2

0,3

0,4

0,5

0,6

0,7

0,8

0,9

300 400 500 600 700 800 900 1000 1100

J(mA/cm2)

Wavelength (nm)

5900K

Other applications :

Flat pannel displays

Architectural glasses for thermal insulation

Transparent Conducting Oxides (TCOs)


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Deposition methods

Sputtering Reactive sputtering (O2) MOCVD

LPCVD

Technology Cell application Advantages / Drawbacks

Indium Tin

Oxide (ITO)

HIT cells, a-Si:H

(back reflector)

Performant

Indium : expensive

(limited resources)

Non-textured

Tin Oxide

(SnO2:F)

a-Si:H, CdTe Less expensive

Textured during growth

High temprature process

Zinc Oxide(ZnO:Al) a/c-Si:H, CIGS Less expensiveResistant against H-

plasma

Texturation in/ex-situ

Complex process

Typical materialsTexturation

TCO texturation favors light

trapping in the cell can naturally appears duringdeposition (SnO2) or created aftergrowth (ZnO)

TCO technologies


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Sputtering (and reactive sputtering)


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M lti l j ti S l C ll


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The theoretical best efficiency of over 20%occurs with a combination of 1.8 eV in the

top cell (a-Si:H) and 1.2 eV in the bottom.

Multiple-junction Solar Cells

Possible tandem

structure

Tandem can have two or three electrical

terminals. The introduction of an internal TCO

layer is incompatible with epitaxy


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Thin film Multi-junctions

a Si:H and c Si/a Si:H Cells


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10 nmP(a-SiC:H)

I (a-Si:H)~0.3 m

N(a-Si:H)

Contact Al

SnO2

Glass substrate

pin Structure

400 600 800 100

Spectral

respo

nse[a.u.]

Wavelength [nm]

a-Si:H

c-Si:H

Micromorph

0

5

10

15

0 0.5 1 1.5

Current(mA)

Voltage (V)

1 cm2 Hybrid cell

AM 1.5, 25 oC

(KANEKA double-light

source simulator)

Jsc: 14.4 mA/cm2

Voc: 1.41 V

F.F. : 0.719

Eff: 14.5%

0

5

10

15

0 0.5 1 1.5

Current(mA)

Voltage (V)

0

5

10

15

0 0.5 1 1.5

Current(mA)

Voltage (V)

1 cm2 Hybrid cell

AM 1.5, 25 oC


source simulator)

Jsc: 14.4 mA/cm2

Voc: 1.41 V

F.F. : 0.719

Eff: 14.5%

0

5

10

15

0 0.5 1 1.5

Current(mA)

Voltage (V)

0

5

10

15

0 0.5 1 1.5

Current(mA)

Voltage (V)

1 cm2 Hybrid cell

AM 1.5, 25 oC


source simulator)

Jsc: 14.4 mA/cm2

Voc: 1.41 V

F.F. : 0.719

Eff: 14.5%

0

5

10

15

0 0.5 1 1.5

Current(mA)

Voltage (V)

0

5

10

15

0 0.5 1 1.5

Current(mA)

Voltage (V)

1 cm2 Hybrid cell

AM 1.5, 25 oC


source simulator)

Jsc: 14.4 mA/cm2

Voc: 1.41 V

F.F. : 0.719

Eff: 14.5%

0

5

10

15

0 0.5 1 1.5

Current(mA)

Voltage (V)

0

5

10

15

0 0.5 1 1.5

Current(mA)

Voltage (V)

1 cm2 Hybrid cell

AM 1.5, 25 oC


source simulator)

Jsc: 14.4 mA/cm2

Voc: 1.41 V

F.F. : 0.719

Eff: 14.5%light

Gl ass

TCO

c-Si :H

(Bottom cell )

a-Si :H(Top cell)

Back

contacts

pin/pin Tandem

a-Si:H and c-Si/a-Si:H Cells

c-Si less sensitive to light-degradation effects


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Thin Film Solar Cells Based on a-Si:H


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Schematic representation of light

trapping in a-Si:H single junction

and tandem cell ( micromorph ).

Time deposition of the Si layers is a

technological issue in practical

applications

An anti-reflection coating (n=1.2)can be added on top of the glass

substrate.

Thin Film Solar Cells Based on a Si:H

Si Thin Film Modules


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Typical cell interconnection systems and packaging (EVA : ethylene-vynil-acetate polymer foil).

Cells can be manufactured on large area substrates (6-8 m2) on 3 mm thick float glass.

Series connection needed for pactical applications (Vused= 12 30 V). Laser scribing is used to

subdivide TCO and Si layers into parallel stripes. The slight offset between scribes is required for serie

connection.

w : photo-inactive interconnection width.

Thin Film Module Performance


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odu e e o a ce

Stabilized efficiency of a-Si:H based

PV modules manufactured byvarious companies



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Easy to make thin film solar modules

A solar cell gives about 0.5 volt

Many cells connected together make a solar module

Thin film solar cells are interconnected during the fabrication of the

thin layers - no handling of individual cells as in the conventional

techniques

Encapsulation needed to protect the solar cellsCrystalline Si module Thin film module


Thin Film Modules on Flexible System


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Iowa Thin Film Technologies : roll-to-roll production of a-Si/a-Si tandem PV modules on polyimid

substrates for consumer applications, capacity about 5 10 MWp, stabilised efficiencies are 4 5%.

y

Silicon based thin film deposition possible on flexible substrates : steel or aluminium foils,

polymers compatible with PECVD process (150 C)

Thin Film Modules : Building-Integrated PV


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g g

PV faade of a Bavarian

Ministry (Munich)

Principle and appearance of semi transparent a-Si

semitransparent modules (material removal by laser

scribing)

a-Si:H cells for indoor applications


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The energy gap of a-Si:H is higher, and thus best matched to the spectrum of the indoorlight sources

Due to the much lower indoor irradiance, light-induced degradation is a lesser issue

Applications : calculator, wall mounted sensors, alarm clocks

pp

Thin Film Module : Life cycle analysis


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sdsd

1840 MJ/ module : a significant impact of Front-end line




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Front end impact


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Si Cells : Comparison Bulk crystalline / Thin Films


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c-Si : thick material required to allow the full conversion of the solar spectrum(indirect band gap)

PECVD Si thin films : cost reduction expected

Thin Films Crystalline Silicon Solar Cells (1)


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Using the simple rule (bandedge)-1, the c-Si wafer thickness for sufficient absorption of the solar

spectrum is > 700 m (without light management). Such a large thikness is not desirable for

commercial production.

Calculation ot the maximum achievable current

density (MACD) for a AR-coated c-Si solar cell as a

function of the cell thickness (AM 1.5 incident

spectrum)

At a thickness of 300 m, the current density

is within 5% of the saturation value 300 m

is suitable for fabricating high-efficiency c-Si

cells.

Thin Films Crystalline Silicon Solar Cells (2)


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Thinner wafers conserve material (cost reduction) and also offer a performance advantage by

decreasing the bulk-carrier recombination within the solar cell.

Calculation of the Voc variation of a c-Si front

textued solar cell as a function of thickness for high

and low surface-recombination velocities (front and

back velocities assumed to be equal)

However, as the cell thickness is reduced, the

surface recombination becomes an inceasingly

important component of the total recombination.

Wafer thickness and surface recombination

should be reduced simultaneously.

HIT Heterojunction with intrinsic thin layer


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Device motivations :

Tendancy of thin crystalline Silicon wafers begin to be

incompatible with high temperature process (dopantdiffusion, aluminium annealing): curvature creation

Substitution of the dopant implantation by PECVD growth

a-Si:H used for c-Si surface passivation

HIT process

Thin silicon wafer (100 m)

Cleaning and etching for wafer texturation

PECVD growth of intrinsic and dopped a-

Si:H

Sputtering growth of ITOITO texturationfrom c-Si

passivatedinterfaces




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intrinsic a-Si:H used for c-Si surface

passivation : dangling bondpassivation (surface recombination

becomes crucial when the device thickness

decreases)

doped a-Si:H for diode creation (front

end) and BSF (back contact)

TCO : optimised optical performances

Structure of a HIT cell

Heterojunction (HIT) : band profiles


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Front contact

The vacuum level (that of a free electron at rest outside the solid) is the same for the two Si materials.

The band discontinuity is due to the differences of the electron affinities (and doping) :

ABE = c

Further advantage of the HIT

structure : combines different

band gap materials

Ev favors hole collection

Ec creates a potential barrierfor electrons (crossed by

tunnelling)



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Back contact (BSF)

Evcreates a potential barrier forholes (crossed by tunnelling)

Ec favors BSF effect (electrons

repelling)



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Performances

www.sanyo.com

World record (10 x 10cm) (SANYO)

VOC=0.722V

JSC=38.64mA/cm2

FF=78.8

= 22%

Commercial production

- SANYO (Japan) cells PV modules (205-230 W)

- Year production 400-500 MW/y- Module efficiency : ~20%

- Available in Europe since 2003

Date post:	08-Aug-2018
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Thin Film Silicon Solar Cell

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