New Challenges and Possibilities for Silicon Based Solar Cells

New challenges and possibilities for silicon based solar cells

Marisa Di Sabatino Dept of Materials Science and Engineering NTNU 2

Outline

-What is a solar cell? -How does it work? -Silicon based solar cells manufacturing -Challenges and Possibilities -Concluding remarks 3 What is a solar cell?

• It is a device that converts the energy of the sunlight directly into electricity.

CB PHOTON

Eph=hc/λ

Electron-hole pair 4

How does a solar cell work? 5 How does a solar cell work?

Current

Semiconductor

Eg + - P-n junction

Metallic contacts

Turid Worren Reenaas 6

How does a solar cell work?

- Charge generation (electron-hole pairs) - Charge separation (electric field) - Charge transport 7 - Charge generation (electron-hole pairs) - Charge separation (electric field) - Charge transport

Eph>>Eg Eph<Eg

Eg CB Eph

VB 8 - Charge generation (electron-hole pairs) - Charge separation (electric field) - Charge transport

p-type C - - - - Fermi level - + + + + + n-type n-p junction

+ - Electric field 9 - Charge generation (electron-hole pairs) - Charge separation (electric field) - Charge transport

Back and front side of a silicon solar cell 10 Semiconductors

Materials for solar cells 11 Why Silicon?

Silicon: abundant, cheap, well-known technology 12

Silicon solar cell value chain 13 Silicon solar cells value chain

From sand to solar cells…

Raw Materials Efficiency Crystallization Isc, Voc, FF

Feedstock Solar cell process

Photo: Melinda Gaal 14 Multicrystalline silicon solar cells Directional solidification 15 Monocrystalline silicon solar cells Czochralski process 16

Crystallization methods for PV silicon

• Multicrystalline silicon ingots: – Lower cost than monocrystalline – More defects (dislocations and impurities) • Monocrystalline silicon ingots: – Higher cost and lower yield – Oxygen related defects – Structure loss 17 Czochralski PV single crystal growth

• Dominating process for single crystals • Both p (B-doped) and n (P-doped) type crystals • Growth rate 60 mm/h

Challenges: •Productivity low due to slow growth, long cycle time... •Defects point defects: vacancies, interstitials, oxygen defects •Segregation

kB=0.8 (for p-type); kP=0.35 (for n-type) •Oxygen

Contamination from SiO2 crucible 18 Continuous feeding in Czochralski growth

•Increase of productivity -no need to cool down & recharge •Adjust composition during growth -compensate for segregation -crystal of uniform composition 19 Recharging / Continuous feeding Accumulated impurities 1.20E-04

RC - Ingot 1 RC - Ingot 2 RC - Ingot 3 1.00E-04 RC - Ingot 4 CCz - Ingot 1 CCz - Ingot 2 CCz - Ingot 3 8.00E-05 CCz - Ingot 4

6.00E-05

Fe[ppB]

4.00E-05

2.00E-05

0.00E+00 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 Length [m]

Recharging CCZ Norsun 20 Point defects during Czochralski growth

PV monocrystals are grown at high rates in vacancy mode but starts in interstitial mode. Defect zone durig V/G low: transition. interstitials

V/G high: vacancies

Sketch of grown-in microdefects distribution in an axial section of a Cz ingot

* Y. Hu, PhD Thesis at NTNU, 2012 21 Directional solidification Bridgman / Vertical Gradient Freeze (VGF)

• Dominating process for PV • Multicrystalline ingots • Bach size, trend towards larger ingots >800 kg • Growth rate > 60 mm/h • Defects deteriorate properties – Structural (grain boundaries, dislocations…) – Impurities 22Challenges:

Reduce cost •Directional solidification, VGF •Bridge gap between multi- and mono •New Si materials •Less pure •Doping elements, B, P •Light elements, O, C, N •Metals, Fe, Cr, Cu, Ti PV installation growth

Improve efficiency •Control structure •Grain size, orientation •Defect structure, •Dislocations, grain boundaries •Control impurities 23 Metal contamination

Solid state difusion of iron from crucible into ingot Low lifetime ”red zone”

125 C/C_ mm cr

1E+16 10 1E+15 m0 Ref crucible m

1E+14

3 -

1E+13 total iron cm 1E+12 interstitial iron

1E+11

1E+10 0,0 50,0 100,0 Ref crucible with Distance from edge [mm] silica coating

HP crucible

* T. Nærland, PiP, 2009 * Y. Boulfrad, PhD Thesis, 2012 24

Reusable Si3N4 crucibles for oxygen control

20 18 P1R R5 R6 16 • Nitride crucibles allow 14 low oxygen levels 12 10

(ppma) • Can be reused several i 8 SiO2 crucible

O times (>5) 6 4 2 Si3N4 crucible 0 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 solid fraction

*Modanese et al, J Cryst. Gr. 354 (2012) 25 Dislocation structures in mc Si •Form during solidification, cooling and deformation •Often associated with grain boundaries •Interact with impurities •Prevents gettering •Depend on crystal orientation •Increase with fraction solid

Si3N4 (B) and SiC (A) melt precipitates 26 Seed-assisted growth Mono-like silicon ingots

Larger grains but lower cost and higher yield than CZ process

- A seed-structure consisting of six equally oriented <110>-seeds - Grown in a pilot-scale Crystalox furnace, 12 kg

*K. E. Ekstrøm et al, CSSC7, 2013 27 Seed-assisted growth

Low level of defects

• Lifetime maps before and after gettering • Dislocations develop at seed-interfaces and also depend on seed misorientation

*K. E. Ekstrøm et al, CSSC7, 2013 28

DEVELOPMENT OF NEW MODELING SOFTWARE SiSim

Task Title Activities

1 SiSim software Software framework, releases, manuals, pre and post, user courses.

2 SiSim numerical methods Implementation of numerical methods

3 Global furnace modelling Furnace modelling (multi, mono) validation

4 Impurity transport and defect Modelling impurity transport phenomena, formation thermodynamics and particle formation, defect formation

5 Mechanical modelling Stresses and deformation in furnace components, elastic and plastic deformation, crystal plasticity

6 Electromagnetism Electromagnetic field, coupling with fluid flow and consequences 29 Cyberstar furnace: fluid and gas flows

Gas flow

Melt flow 30 Cyberstar furnace: Stresses and strains

Thermo-elastic stresses

Max 23

Max 29

Grain structure 0 18 Von Mises stress [MPa] 31 Crystallox furnace: impurities Gas/melt flow Heat transfer

EM forces

Boron

[C/C0]

With EM Without EM 32

Cost for production of solar modules 33

Silicon solar cells production

What happens to the wafer?

Courtesy Radovan Kopecek, ISC and Erik Stensrud Marstein, IFE 34 Silicon solar cells production

Saw-damages removal

Texturing

Emitter-diffusion

Anti-reflection coating (ARC)

Metal contacts/Screen printing

Metal contacts/Co-firing

Edge isolation 35

Ag n+ p p+ 36 Efficiency limits

= 33% 1 sun (Eg=1.4eV) Theoretical limit by Shockley and Queisser, 1961

Si (Eg = 1.1 eV): η= 31%

Typical values for commercial solar cells:

Sola cells Efficiency: Solar cells Efficiency: 19-22% 16-19% 37

G3 G2

G1 38 39

http://pvinsights.com/ R. Kopecek, ISC, CSSC9, 2016 40 41

120MW total installed capacity in 2016

Global radiation on inclined surface: 42 Concluding Remarks

-Silicon based solar cells still dominating the marked

-Since 2016, prices are falling -> good for low cost PV but bad for PV industry

-Room for innovations (e.g. direct wafer, tandem etc) is extremely tight

-Material quality and solar cell architectures still key parameters

-Bifacility is coming! 120MW total installed capacity in 2016 and in 2017 at least doubled