by means of characterisation and simulation Pietro P. Altermatt - - PowerPoint PPT Presentation

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Understanding the fill factor by means of characterisation and simulation

Pietro P. Altermatt Leinbiz University of Hannover, Germany SPREE Seminar @ UNSW, 19th March 2015

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Which parameters influence the fill factor?

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…the lumped series resistance Rs

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… the Voc

M.A. Green, Solar Cells, 1992, (ISBN 0 85823 580 3), p. 96 M.A. Green, Solar Cells 7, 337 (1982)

n = 1.0 Rs = 0

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… der ideality factor n

M.A. Green, Solar Cells, 1992, (ISBN 0 85823 580 3), p. 96 M.A. Green, Solar Cells 7, 337 (1982)

Rs = 0

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Analytical approximation using Voc, n and Rs

Jsc = 39 mA/cm2 Detailed overview of analytical equations for FF:

• E. Sanchez and G.L. Araujo, Solar Cells 20, 1 (1987)

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Do we here have a „FF problem“ ?

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No

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FF in relation to Voc

It is advantageous to consider FF in relation to Voc

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FF depends on n

The idality factor may influence FF as strongly as Rs.

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Contents

(Loss)-analysis using simulations Characterization using I-V measurements

1 2

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Contents

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Characterization using I-V measurements

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Measurement of I-V curves

Measurement

Dark 1-sun 1.1-sun  0.9 sun (Jsc-Voc)

1-sun Jsc-Voc dark Voc Vmpp Bias [V] Current-density [mA/cm2]

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I-V curves – logarithmic

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I-V curves – logarithmic

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I-V curves – logarithmic

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I-V curves – logarithmic

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I-V curves – logarithmic

Jsc-Voc Voc Vmpp 1-sun dark Bias [V] Current-density [mA/cm2]

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Extraction of Rs

Measurement Extraction

• f Rs

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Rs extraction from I-V curves

Overview: P.P. Altermatt et al, Prog. PV 4, 399 (1996)

1-sun Jsc-Voc dark Voc Vmpp Jsc-Voc DLL Bias [V] Current density [mA/cm2] Series resistance [cm2] Bias [V]

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Tripple light-level (TLL) method

• K. F. Fong, K. R. McIntosh, A. W. Blakers, Prog. PV 21, 490 (2013)

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Tripple light-level (TLL) method

• K. F. Fong, K. R. McIntosh, A. W. Blakers, Prog. PV 21, 490 (2013)

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Large J0 → I-V curve is higher

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Large J0 → I-V curve is higher

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Rs extraction from I-V curves

• K. F. Fong, K. R. McIntosh, A. W. Blakers, Prog. PV 21, 490 (2013)

1-sun Jsc-Voc dark Voc Vmpp Jsc-Voc DLL

If possible, use the tripple light-level method to measure Rs

Bias [V] Current-density [mA/cm2] Series resistance [cm2] Bias [V]

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Simulation of the metallised parts

• Y. Yang et al, Prog. PV 20, 490 (2012)

Device simulation Circuit simulation

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Simulation of the metallized parts

Device simulation Internal Rs Jsc-Voc DLL. Bias [V] Series resistance [cm2] Circuit simulation

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Parametrization of Rs

Measurement Extraction

• f Rs

Parametrization

• f Rs(V)

If simulation: internal Rs Proper distinction between Rs- and recombination losses

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Rs(V) as polynome 2nd degree

Jsc-Voc

where V0 is offen Voc

Internal Rs DLL. Bias [V] Series resistance [cm2]

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Rs-corrected I-V curves

Measurement Extraction

• f Rs

Parametrization

• f Rs(V)

Rs(V)-free I-V curves If simulation: internal Rs Proper distinction between Rs- and recombination losses Rs(V) polynome

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Rs-corrected I-V curves

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• 1

10 10

1

Current density [mA/cm

2]

0.65 0.60 0.55 0.50 0.45 0.40

External bias [V]

Exp Rs(V) Rs(Vmpp)

FF Exp = 78.52 FF Rs(V) = 83.18

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Rs-corrected I-V curves show recombination losses

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• 1

10 10

1

Current density [mA/cm

2]

0.65 0.60 0.55 0.50 0.45 0.40

External bias [V]

Exp Rs(V) Rs(Vmpp)

FF Exp = 78.52 FF Rs(V) = 83.18 FF Rs(Vmpp) = 83.09

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Comparison of pseudo-FF with 1FF

n = 1.0 Rs = 0

M.A. Green, Solar Cells, 1992, (ISBN 0 85823 580 3), p. 96 M.A. Green, Solar Cells 7, 337 (1982)

pFF is often smaller than 1FF

FF Exp = 78.52 FF Rs(V) = 83.18 FF Rs(Vmpp) = 83.09 FF n=1 = 83.28

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Loss analysis

If simulation: internal Rs Proper distinction between Rs- and recombination losses Rs(V) polynome Recombination losses pFF Loss analysis Predictions Measurement Extraction

• f Rs

Parametrization

• f Rs(V)

Rs(V)-free I-V curves 1FF

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Content

(Loss)-Analysis using simulations

2

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Domain & discretization

Domain

2D Entire cell Finger

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Reproduction of the ideality factor

Ultimate test

• S. Steingrube et al. Energy Procedia 8, 263 (2011)

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Which is the most likely current-path?

V

dark

applied forward bias

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Exponentially increasing recombination rates

39

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Dark I-V curve = recombination rate

40 30 20 10

Stromdichte [mA/cm

2]

700 600 500 400 300 200 100

• 100

Spannung [mV]

many few Number of defects

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Illuminated I-V curve is shifted to 4th quadrant

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Think of G – R

G

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J(V) = G – R(V)

R(V)

G

J(V) = G – R(V)

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Losses in the various cell regions

R(V)

Recombination current [mA/cm2] Voltage [V]

Total Emitter Base Al-BSF

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Predictions

Loss analysis Predictions Measurement Extraction

• f Rs

Parametrization

• f Rs(V)

Rs(V)-free I-V curves

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After improvement of the emitter in a PERC cell

Recombination current [mA/cm2] Bias [V]

Total Emitter Base Al-BSF Voc = 614 mV Voc = 633 mV FF = 75.4 FF = 76.3

Bias [V] Standard cell Improved emitter

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Losses in the p-type Cz base

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Ladungsträgerkonzentration n [cm-3]

1012 1013 1014 1015 1016 1017

Lebensdauer eff[µs]

10-5 10-4

B-dotiertes Cz-Si Ndop=5.110

15cm

• 3

nach Beleuctung (1Sonne 60 Stunden) nach Tempern (200°C 10 min)

Injection dependent lifetime in the p-type base

p/n=10

n



p n

  

              

1 1 SRH

( ) ( ) ( )

p n

n n n p p n n n p n

Injection density n [cm-3] B-O complex

Ndop=5.1x1015 cm-3

Effective lifetime

• J. Schmidt, A. Cuevas, J. Appl. Phys. 86 (1999) 3175
• S. Rein, S.W. Glunz, Appl. Phys. Lett. 82 (2003) 1054
• K. Bothe R. Sinton, J. Schmidt, Prog. PV 13 (2005) 287

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Deactivated B-doped Cz wafers

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• D. Waler et al, Appl. Phys. Lett. 104, 042111 (2014)

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Improved emitter → smaller FF because of base!

Recombination current [mA/cm2] Bias [V]

Total Emitter Base Al-BSF Voc = 614 mV Voc = 633 mV FF = 75.4 FF = 76.3

Bias [V] Standard cell Improved emitter

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FF, pFF und 1FF

Two cells with low FF 1) Mainly due to Rs  pFF is close to 1FF 2) Mainly due to n  pFF is far from1FF  Determine FF and pFF, if possible using Rs(V), and 1FF

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More recent progress of PERC cells (1)

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Inital Improved emitter

79.69 80.38

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More recent progress of PERC cells (2)

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Improved base and rear Improved emitter

80.80 80.38

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More recent progress of PERC cells (3)

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Improved base Improved base and rear

81.46 80.80

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Emitter losses increase…

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Inital Improved emitter… …base …base and rear

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…because Vmpp increases

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Inital Improved emitter… …base …base and rear

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Main points

• Extraction of Rs(V) from three I-V curves (TLL method)
• Clear distinction between Rs(V) and recombination losses
• Rs(V)-corrected I-V curve

→ pFF < 1FF ?

• Further analysis and prediction with simulations

FF is not only determined by Rs, but also by the ideality factor, i.e.by recombination, especially in good cells (where the base or the rear surface dominates)

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Thank you!

altermatt@solar.uni-hannover.de