New Generation Silicon Solar Cells 10.08.2015New Generation Silicon Solar Cells By Sarah Lindner...

New Generation Silicon Solar Cells

19.04.23New Generation Silicon Solar Cells

By Sarah LindnerEngineering Physics

TUM

Table of contents

1. Introduction - Photovoltaics on the world market

2. Semiconductor

2.1 Electronic band structure

2.2 Metal – Isolator – Semiconductor

2.3 Definition

2.4 Doping

2.5 Intrinsic/Extrinsic

2.6 Conductivity

2.7 Direct/indirect band gap

2.8 Absorptioncoefficient

3. Solar cell – functionality

3.1 pn-junction

3.2 pn-junction under radiation

3.3 Solar cell characteristics

3.4 Equivalent circuit

3.5 Generation and recombination


Table of contents

3.6 Diffusion length

4. Solar cell – efficiency

4.1 Dilemma

4.2 Solar basics

4.3 Losses

4.4 Efficiency values

5. How to optimize silicon solar cells

5.1 Why still silicon?

5.2 Surface passivation

5.3 Reflection

5.4 Laser operations

5.5 Solar cell contacts

5.6 OECO-cell

5.7 Further prospects

6. Bibliography


• „In 2007 the photovoltaic market grew over 40% with ~ 2.3 GW of newly installed capacity“ (EPIA)

• Germany has the first position on the world market with 50% global market share• power Installed by region: 80% Europe 16% North America 4% Asia• Most dynamic market is Spain• Seven Countries hosting the majority of large photovoltaic power plants: RoW, Italy, Japan, Korea, USA, Spain, Germany• the cumulative power quadrupled

• Installed PV world wide 7300MWP

• Annual growth predicted ~ 25%• Turnover by modules (2030) ~100billion €/a• By 2030 a worldwide contribution of 1% is reached

1. Introduction - Photovoltaics on the world market

Annual installed power grew significantly from 2004

2.1 Electronic band structure


One single atom discrete energy levels

Bring atoms close together , e.g. crystall lattice Interaction of the electrons Energy levels split up band structure

Band structure of silicon E(k)Band structure of Mg with potential well

Discrete energy levels



Metal: • either the conduction band is partly filled• or no seperate conduction and valence band exist• electrons can move freely• T ↑ resistivity ↑• electrons give their energy to the phonons very fast ~ 10-12sIsolator:

• at T = 0 the conduction band is empty very high resistivity• band gap EG > 3eV• no conductivity despite doping possible

Band structure



Semiconductor:• isolator for deep temperatures (T = 0)• conduction band at low temperatures as good as empty, valence band almost full • band gap 0,1eV < EG < 3eV•

• T ↑ resistivity ↓• Electrons can stay in the conduction band for about 10-3s

(intrinsic semiconductor)

Band structure

2.3 Definition


A semiconductor is a material that has electrical conductivity between that of a conductor and

that of an insulator

Its resistivity decreases with increasing temperature

and therefore its conductivity increases.

2.4 Doping


Donor - doping

• add an extra electron• number of e- > number valence e-

• n – type dopant• ED right under conduction band EC

Acceptor - doping

• add an extra hole• number of e- < number valence e-

• p – type dopant• EA right above valence band EV

Doping: Change in carrier concentration change in electrical properties

n-type doping p-type doping

2.4 Doping




Intrinsic Extrinsic

pure semiconductor doped semiconductor

n = p n ≠ p

self conduction Self conduction + conduction because of doping

Conductivity depends on T Conductivity depends on T and on additional charge carriers

(dopant)

Change in EF

At thermal equilibrium

T>0


Fermi-level for a) T = 0K and b) T > =K Fermi- level for n-doped semiconductor and T > 0K

Intrinsic caseExtrinsic case



Switch of the Fermi level with increasing temperature a) n-doped b) p-doped

2.6 Conductivity


σi depends strongly on the temperature and the charge carrier densities

extrinsic conductivity depends additionaly on excitation of dopants into the conduction band.

kT

ETeCen G

heheii 2exp)()( 2

3

2.7 Direct/indirect band gap


Indirect:• need a photon, a phonon, and a charge carrier happens more seldom longer absorption length • recombination at grain boundarys and point defects

Direct:• need just the right photon for band transition• higher transition probability

Material

c-Si a-Si:H GaAs

Band gap

1,12 eV (indirekt)

1,8 eV(„direct“)

1, 43 eV (direct)

Absorption coefficient(hν = 2,2) [cm-1]

6*103 2*104 5*104

Indirect and direct band gap

2.8 Absorptioncoefficient


3.1 pn-junction


• Equilibrium condition, no bias voltage• diffusion current opposite to the E-field• diffusion voltage V0

with ∆E = eV0

at diffusion force = E-field force

V0 is the electrial voltage at the equlibrium state = diffusion voltage

3.1 pn-junction


a) Band structur for n-doped and p-doped semiconductor before contact

b) Band structure after contact

c) Depletion area

3.2 pn-junction under radiation


Absorption of light:

If Eph < Eg no electron-hole-creation

If Eph > Eg electron-hole-creation drift

and diffusion current and voltage

Band structureSolar cell under radiation



Isc = -Iph for V = 0

for I = 0

phnkT

eU

IeII )1(0

I0 is the saturation currentn is the ideality factork is the Boltzmann`s constantIsc is the short circuit currentVoc is the open circuit voltage

0

1lnI

IIT

e

nkV ph

0

lnI

I

e

nkTV scoc

I-V characteristic of a solar cell



Maximum power point (MMP)

depends on:• Temperature• Irradiance• Solar cell characteristics

Wilson s. 209

Efficency coefficent

Performance of solar cell

Fill factor

3.4 Equivalent circuit


P

Sph

SS

R

IRVI

kTn

IRVeI

kTn

IRVeII

)()1

)((exp)1

)((exp

202

101

Equivalent circuit



n0 n0 + ∆n = n

Recombination and generation processes. Generation processes depend on absorption and on flow of photons

G = R

Life time of minority carriers:

ii R

n

∆n is the surplus concentrationRi is the rate of recombinationn0 is the concentration at equilibriumn is the charge concentrationG is the rate of generation

n

GRGdt

dn 0

dt

dn



Recombination by radiation

Auger-recombination



Recombination by impurityτSRH depends on: Number of impurities Energy level of impurities Cross section of impurities

Recombination on the surface Untreated silicon surfaces S > 106 cm/s Depends strongly on charge carrier injection

and doping


19.04.23

radiation

Auger

SRH

Experimental

1014 1015 1016 1017

105

104

103

102

101

100

τ [µ

s]

p0 [cm-3]

Low innjection, depenence between hole equilibrium concentration and τ

• Low p0 SRH is dominant and τ independet of p0

• High p0 τ ~ p0-2

(Auger recombination)• radiation recombination plays no role for silicon

Normal sunlight radiation the basis of the solar cell is in the are of the SRH recombination



Is the mean free length of path a charge carrier can travel in a volume of a crystall lattice before recombination takes

place.

depends on: The semiconductor material The doping The perfection of the crystall lattice

D is the diffusion constant



Silicon: (10 μm - 100 μm)

λ < 800nm light absorbed within 10μm

λ > 800nm electron-hole generation all over the volume

for an effectiv solar cell the diffusion

length has to be 2-3 times thicker

than the actual solar cell

Multichristall silicon

τeff = 50μs Leff,n

(cm)Leff,p

(cm)

p-type 0,037 0,023

n-type 0,040 0,024

4.1 Dilemma


P = U * I

ideal band gap size, depending on the solar spectrum

The usuall ideal band gap is supposed to be at EG = 1,5eV

A small band gap causesa big short circuit current,because many photons will create electron-hole-couples.

A big band gap causesa larger potential barrierand therefore a larger open circuit voltage.

4.2 Solar basics

19.04.23New Generation Silicon Solar CellsSpectral distribution of solar radiation.

Black body curve 5762K

AM1 solar spectrum

AM0 solar spectrum 1353W/m2

4.2 Solar basics


AM = air mass = degree to which the atmosphere affects the sunlight received at the earth`s surface

The factor behind tells you the length of the way when the light passes through the atmosphere.

Standard Test Conditions (STC):Temperature of 25°; irradiance of 1000W/m2; AM1.5 (air mass

spectrum)

Different air mass numbers

4.3 Losses


1. Reflection: the metall circuit path on the front of a solar

cell reflects the light the solar cell itsself reflects the light

2. ShadowThe metall circuit path obscures the front of the solar cell

3. Recombination On the surface dangling bonds Inside the volume

4. Interaction with phonons

4.3 losses


5. Resistance factors short circuit between the front and the back of the

solar cell transport of the charge carriers through the cables

and contacts

6. Absorption and Transmission Other layers of the solar cell (e.g. ARC) can also

absorb Light can totaly be transmitted trough the solar cell

7. Other factors Dirt on the solar cell No ideal conditions (STC)

4.4 Efficiency values


Material η (laboratory) η (produktion)

Monocrystalline 24,7 14,0 – 18,0

Polycrystalline 19,8 13,0 – 15,5

Amorphous 13,0 8,0

Material Crystalline order

Thickness Wafer

Monocrystalline One ideal lattice 50μm - 300μm One single crystall

Polycristalline Many small crystalls

50μm - 300μm grain (0,1mm – Xcm)

Amorphous No crystalline order;Groups of some regularly bound atoms

< 1μm No wafer

5.1 Why still silicon?


> 90% silicon and multisilicon Silicon has the potential for high efficiency Silicon is available unlimited

second most element of the earth‘s crust The involved materials and processes

are non-toxic and do not harm the environment The silicon technology already exists

and is reliable Already exists a broad knowledge

of the materials and the devices

Global PV-market



1. Thermal oxidation:

Reduction of the density of states on the interface or surface

Oxygen streams over the hot wafer surface and reacts with silicon to SiO2

This results in an amorphous layer Temperature of the process ~ 1000°C Thickness of the layer > 35nm efficiency decreases Time goes on and the velocity of the growth of the oxidic

layer decreases



2. Passivation with SiNx

Reduction of the density of states on the interface Gases silane SiH4 and methane NH3 form a layer of Si3N4

Temperature of the process ~ 350°C Passivation quality rises with silane amount S ~ 20 cm/s – 240 cm/s depending on the refraction index

advantages: lower production temperature Nitride seems also to work better as an anti reflection layer for

solar cells better passivation



3. Passivation with only silane

The quality of the passivation is enormous Passivation layer on the emitter should be

very thin (10nm) high absorption prefer SiNx-Process on the emitter

The process temperature is ~225°C The passivation seems independet of

contaminations of the silicon surface brought in during the manufacturing process

An example is the HIT-Solar Cell from Sanyo

Layer of monocristalline silicon between amorphous silicon layers

Efficiency of ~ 18,5%

Passivierqualität als Funktion der a-Si:H-Schichtdicke

HIT solar cell



4. Back Surface Field (BSF)A thin layer of p-doped material to prevent the minorities from

moving to the back contact where they recombinate

e.g. use aluminium for a back contact, which melts (T ~ 500°C) into the silicon and creates a positive doped BSF. Besides it serves as a reflection layer.



Intrinsic gettering:Contaminations will be collected at one area in the crystall and

afterwards will be removed

Extrinsic gettering:Contaminations will be transported to the crystall surface and

afterwards be removed

e.g. aluminium Foreign atom will be freed out of their bonds diffuse into

the Al-Si alloy 30 minutes at T = 800°C to eliminate most of the

contaminations, depends on the diffusion length of the atom

5.3 Reflection


1. Anti reflection layer One or more layers reduction from 30-35% to 5%-10% Mainly 600nm transmission Silicon nitride or transparent layers, e.g. SiO2; TiO2; Ta2O5 ITO can be used as anti reflection layer and at the same time as a

transparent contact Double anti reflection layers ZnS or MgF2

2. Texturing (light trapping) Use NaOH, KOH in etching baths The etching works anisotropic 2μm - 10μm

big pyramids on (100) oriented crystall planes

5.3 Reflection


advantages: At least second reflection The effective absorption length of the silicon layer will be

reduced the light way through the layer increases The area of the surface becomes bigger Total reflection on the inside of the front layer possible Reflection can be reduced about 9/10 of the former reflection

Examples of light trapping

5.3 Reflection


disadvantage: More difficult to form it on multi-/polycrystalline silicon layers

no sufficient reflection reduction The surface area is increased higher surface carrier

recombination rates

New: A focused laser scans the

wafer surface to form a dotted matrix The damage on the surface of the

crystall will be etched away afterwards Advantage: it is better for the

environment and can be used on different materials

Reflection can be reduced from ~35% to 20%Laser texturized poly chrystall silicon

5.3 Reflection


3. Back side reflection Two different layers at the backside:

Patterns of microscopic spheres of glass within

a precisely designed photonic crystall Capture and recycle the photons Large-scale manufacturing techniques are

being developed

advantage: Reflects more light than the aluminium layer Light reenters the silicon at low angle light

bounces around inside Efficiency can be increased up to 37%

a) represents the aluminium layer b) represents the new version



Why using laser? All for Si-PV-technology used materials absorb light A small optical/thermical penetration depth is given for λL < 1µm

Laser can focuse very good (size of structure 10µm – 100µm) Minimal mechanical demands on the fragile Si-wafer Screen printing process can be prevented

Laser`s high quality output beams and unique pulse characteristics coupled with low cost –of-ownership



• p-doped layer is coated with an outer layer of n-doped silicon to form a large pn-junction• n-doped layer coats the entire wafer recombination pathways between front and rear surfaces

Edge isolation:groove is continuously scribed completely through the n-type layer right next to the edge of the cell

Requirements:• Rp should be kept high; FF > 76%• Little waste of solar cell area• 1000 wafer/h• Flexibility (thin wafers)

Groove to isolate the front and rear side of the cells



Front surface contacts:Burried contacts to minimize the area

obscured by the front contacts

electrodes with a high volume

and collection surface

Depth and width 20μm – 30μm every 2mm-3mm

Laser Fired ContactsElectrically and thermo-mechanically

advantageous to include passivation

layer, which is non-conducting

laser creates localized Al/Si- alloys

Efficiency of ~ 21%

Over 1000 rear side local metal point-contacts created per solar cell

Laser generated groves on the cell surface


Saturn-solar-cell Laser Grooved Buried Contact (LGBC) Laser will burn a trench in the front side of the solar cell Trench is 35µm deep and 20µm wide and has form of a „U“ or a „V“ Trench will chemically be filled up with the front contact material, usually

silver a large metal hight-to-width aspect ratio

allows closely spaced metal findgers

low parasitic resistance losses

advantages: Shading losses will only be 2% to 3% Reduction of metall grid and contact

resistance Reduction of emitter resistance

because of very close fingers Possible efficiency >17%

LGBC-cell



Prevent obscuration of the solar cell or high reflection and absorption of the silver grids.

small and high grids, which will become smaller towards the edge of the cell

COSIMA (Contacts to a-Si:H passivated wafers

by means of annealing): Amorphous silicon (silane process) on mono-

crystalline silicon Aluminium on theses layers results in

contacting the monocrystalline silicon Process temperature ~ 200°C No photolithography

Solar cell with a-Si:H-rear passivation and COSIMA contacts



Advantages: Simplifies thin film manufacturing process Efficiency values about 20%

Combination with doted contacts: Screen printed interface layer (little holes) good passivation Aluminium on the interface layer COSIMA

Advantages: Can be used on thinner wafers no bending The passivation abbility of the amorphous layer will be kept after the

annealing process The contact resistivity is 15mΩcm2 Increase of the quantum yield in the infrared wavelength range Reduces Seff to 100 cm/s (4% metallization)

EWT/MWT


Front (left) and rear (right) of a EWT-solar cell. The front contacts are brought to the rear of the solar cell by many dots.

Emitter Wrap through (EWT)

• Emitter on the front surface is wraped with the rear surface by little holes• Edges of the holes are also emitter areas, which transport emitter current• Power-conveying busbars and the grid are moved to the rear surface• Use double sided carrier collection (n+pn+) increases the efficiency• 100µm holes are made by laser

EWT- cell with n+pn+ - structure



Disadvantage:Manufacturing process is very complex

Metal wrap throug (MWT)• Absence of the bus bars (on the rear side) connection by holes• Less serial resistance losses because of interconnection of the modules on the back• FF ~77%; efficiency ~ 16%

Advantages:• Eliminate grid obscuration no high doping high Isc high efficiency• n+pn+- structure use lower quality solar grade silicon• Uniform optical appereance improves asthetics• Silicon solar cell < 200μm• Efficiency around 18%• gain in active cell area•Diffusion length can be reduced to the half

MWT-cell



MWT EWT

Voc [mV] 617 596

Jsc [mA/cm2] 36,1 37,7

FF [%] 75,1 72,8

η [%] 16,7 16,3

Area [cm2] 189,5 61,5



Cross section of a partially plated laser groove.



A 300 solar cell: Negative conducting silcon wafer Emitter and all contacts on the back side No obscuration on the front side Efficiency value > 21%

5.6 OECO-cell (Obliquely Evaporated COntacts)


Standard OECO cell:

• front contacts are evaporated on the flanks of the ditch by self obscurance • flat homogeneous emitter because of one step phosphor diffusion• very thin contacts of metall are possible• development of a ultra thin tunnel oxid between metal and semiconductor, which forms high sufficient MIS contacts• passivation layer on the front and rear side (SiNX or SiO2)• efficiency ~ 20%

5.6 OECO-cell


Advantages: reduces the oscuration easy manufacturing processes and

environmentally friendly efficiency value > 20%

Standard OECO solar cell

Mass production

5.6 OECO-cell

19.04.23

Both contacts are on the rear side The back of this cell accords to the

standard OECO cell The front has a texturized surface Deep phosphorous emitter on

almost

the whole back side

Advantages: Reduction of impurity shunt

resistance and serial resistance Reduction of obscurance at the

front Double sided light-sensitivity

bifaciale solar cell efficiency for both sides ~ 22%

possible

Back – OECO - cell



There is also high potential in improvents for the manufacturing process development of a

„solar silicon“

1. Sawing process has to be improved

2. Automation processes have to be developed

3. New contact processes

4. Fast processes with low cycle time



Annual consumption of electricity per person:1000kWh/a

Annual solar cell power 1000W/m2a800 – 1200 hours of sun in Germany with 80% ca. 800kWh/m2a out of a photovoltaic system

Efficiency of 15% 120kWh/m2a

To cover the annual consumption of electricityper person you need ~ 8,3m2

Multicrystalline solar cell (15x15x0,03cm3) has a peak power of 3,5W and is made out of 24gsilicon (+ loss during production) 6,8kg silicon

2030 silicon needed per year = 160,000t !



Russia – Saint Petersburg

Germany - Munich

Nominal power (crystalline silicon)

1kW 1kW

Incline of the modules

42° 37°

Losses because of temp.

6,4% 6,5%

Losses because of reflection 2,9% 2,9%

Losses in general 15,0% 15%

Complete losses 24,3% 24,4%

Power production out of a PV constructed for 1kW per year

865kWh 1009kWhBy http://re.jrc.ec.europa.eu/pvgis/apps/pvest.php?lang=de

6. Bibliography


http://www.isfh.de

http://www.fv-sonnenenergie.de

http://www.solarserver.de

http://www.laser-zentrum-hannover.de

http://www.hmi.de

http://www.coherent.com

http://www.bine.de

http://www.lexsolar.de

http://www.diss.fu-berlin.de

http://www.energieinfo.de

http://www.wikipedia.de

Rudden M.N., Wilson J., „Elementare Festkörperphysik und Halbleiterelektronik“, Spektrum Akademischer Verlag, ©1995, 3. Auflage

Würfel P., „Physik der Solarzellen, Spektrum Akademischer Verlag, ©2000, 2.Auflage

Kaltschmitt M., Streicher W., Wiese W. (Hrsg.), „Erneuerbare Energien Systemtechnik, Wirtschaftlichkeit, Umweltaspekte“, Springer Verlag, ©1993, 3. Auflage

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