Unified Lifetime Measurement for Silicon PV - PVMC

Unified Lifetime Measurement for Silicon PV
Authors
Lubek Jastrzebski 1 , Marshall Wilson 1 , Jacek Lagowski 1 , Piotr Edelman 1 ,Alexandre Savtchouk 1
Andrew Findlay 1 , Sara Olibet 2 and Valentin Mihailetchi 2
1
Semilab SDI LLC, Tampa, FL 3361, USA
2
International Solar Energy Research Center-ISC Konstanz, Germany
Outline of presentation
1. Introduction to the Approach: Method and Goals
2. QSS-µPCD small perturbation lifetime, τ eff.d
3. Quality of Decay Control Method
4. Results and Discussion
• Parameter free extraction of steady-state lifetime, τ eff.ss
• Correlation with QSSPC
• Advantages of τ eff.d and J 0 based on Basore-Hansen
5. Example of applications to wafer mapping
6. Summary and Conclusions
7. Acknowledgements
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Introduction
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Microwave detected photoconductance decay, µPCD is a powerful
metrology of recombination properties that is widely used in silicon photovoltaics.
The advantages are:
parameter free non-contact fast suitable for mapping
Further advantages in precision and control of the injection level were demonstrated
with QSS-µPCD (Quasi-Steady-State) that operates under a small perturbation carrier
decay condition originally described by Basore and Hansen (1990).
Further improvements to QSS-µPCD reliability, precision, and application range, where
demonstrated using a novel “Quality of Decay Control” method that recognizes and
eliminates common problems of non-exponential decays.
QSS-µPCD with Quality of Decay Control enables enhanced precision measurements
of not only the carrier decay lifetime, and also carries advantages of a parameter-free
determination of the effective steady-state lifetime.
The goal of the present work is to present a unified lifetime measurement approach that
provides self-consistent, parameter free determination of carrier decay lifetime and
steady-state lifetime. Excellent correlation with QSSPC effective lifetime is
demonstrated.
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Apparatus for QSS-µPCD with QD Control
Key elements:
• adjustable large spot pulsed laser excitation
• adjustable steady-illumination up to 25 suns
(980nm laser)
• Semilab proprietary high sensitivity
resonance microwave detection
Discussion of large spot benefits can be found
in several publications; for example: Basore
and Hansen (1990) or Aberle et.al (1999).
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QSS-µPCD is Based on Small Perturbation
Measurement; Basore-Hansen (1990)
Small perturbation decay lifetime, τ eff.d , is also referred to as “differential lifetime”
Brendel (1995); Aberle, Schmidt and Brendel (1996).
• The steady-state illumination, I, is varied in steps I 1 , I 2 …I k
• Small perturbation decay lifetime τ eff.d is measured for each step
• During decay, the injection level is locked at the corresponding steady-state values, n k .
Reducing decay distortions due to injection dependent recombination.
• For mono-exponential decay, measurement related factors should also be eliminated.
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Common Factors Causing Non-exponential Decay
A. Wafer Related
• injection dependent bulk recombination
• injection dependent surface recombination
• trapping of excess carriers
• modulation of depletion region by excess carriers
B. Measurement Related
• nonlinear microwave reflection response to excess carriers
• too small carrier excitation spot compared to microwave detection area
• lateral spreading of injected carriers during decay measurement interval
• too small or non-uniform light bias spot
Factor A can be eliminated using strictly controlled small perturbation conditions.
Factor B can be controlled by tuning microwave detection system and by careful
adjustment of illumination system.
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Quality of Decay, QD [arb. units]
Two Elements of Quality of Decay Control
Quality of Decay Parameter, QD
Quality of Decay Control Chart
1.30
1.20
small light spots
optimized condition
1.10
1.00
0.90
UCL = 1.02
LCL = 0.98
0.80
0.70
0.01 0.1 1 10 100
Steady-state Light Intensity [suns]
QD parameter is obtained as a ratio
of half-life values in progressing carrier decay.
Ideally QD=1 ; QD1 identifies
measurements that erroneously underestimate
and overestimate the lifetime, respectively.
QD Control chart which specifies QD
limits for mono-exponential decay.
Optimized condition with QD close to 1
eliminates measurement errors in broad
intensity range from 0.02 to 25suns.
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Typical Photoconductance Decay Distortions
Progressively faster decay typical for passivated
emitter measured without bias light.
Progressively slower decay observed for higher
injection levels due to lateral carrier spreading
Note: “Linear” decay is a consequence of injection dependent effective lifetime
τ − 1
eff
const ∆n →
∂∆n
∂t
= − ∆n
τ eff
= −const
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Effective Lifetime [s]
Lifetime Overestimation in Measurements
Outside QD Control Limits
p + /n/p + emitter test structure
1.0E-02
outside QDC limits
too small illumination
spots
1.0E-03
within QDC limits
optimized measurement
1.0E-04
0.01 0.1 1 10 100
Steady-state Light Intensity [suns]
Note: illustrated errors are caused by injection dependent recombination and
lateral carrier spreading during decay measurement interval.
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Effective Lifetime [s]
Parameter-Free Determination of τ eff.ss
• Steady-state lifetime τ eff.ss is obtained from decay lifetime τ eff.d by integration
over light intensity (procedure of Schuurmans et. al 1997)
• No wafer parameters are required
• Results are compared with QSSPC
1.0E-03
1.0E-03
Effective Lifetime [s]
1.0E-04
t eff
Integrated Steady State teff
1.0E-04
Integrated Steady State teff
Sinton QSSPC
t eff
t eff
teff.d from QSS-uPCD with QDC
t eff.d
Sinton QSSPC
t eff
p + /n/p + test structure
1.0E-05
0.01 0.1 1 10 100
Steady-state Light Intensity [suns]
1.0E-05
0.01 0.1 1 10 100
Steady-state Light Intensity [suns]
• Integrated and QSSPC values practically coincide
• Consistent with the theory, in the emitter dominated range τ eff.d ≈ 1 2 τ eff.ss
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τ eff.d and τ eff.ss Correlation with QSSPC
• A series of measurements of the integrated τ eff.ss were done with stringent quality of
decay control values within 0.98 and 1.02 control limits
• The same wafers were measured with QSSPC for comparison.
Small Perturbation Decay Lifetime teff,d [s]
1.0E-03
1.0E-04
1.0E-05
0.02 sun
1 sun
5 sun
10 sun
1.0E-05 1.0E-04 1.0E-03
Unified Steady-state teff [s]
1.0E-03
0.02 sun
1 sun
R 2 = 0.9947
5 sun
10 sun
1.0E-04
1.0E-05
1.0E-05 1.0E-04 1.0E-03
Sinton QSSPC t eff [s]
QSSPC t eff [s]
• The results demonstrate 1:1 correlation with R 2 = 0.9947
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Measurement of J 0 and Self-consistency Test
• Small perturbation decay lifetime vs. light intensity enables a direct determination of the
−2
emitter saturation current, J 0 from the slope of τ eff.d vs. G ss (Basore-Hansen)
• τ eff.d data after integration gives the steady-state lifetime τ eff.ss . The injection level is
calculated as ∆n = G ss ∙ τ eff.ss , and J 0 is obtained from τ eff.ss
−1
vs. ∆n (Kane-Swanson)
• Quantitative 1:1 correlation between two J 0 methods proves self-consistency of the approach.
10000
10000
J0 from Steady-State Lifetime [fA/cm 2 ]
1000
100
(Kane-Swanson Method)
R 2 = 0.9952
Sinton QSSPC J0 [fA/cm 2 ]
1000
100
(Kane-Swanson Method)
R 2 = 0.9934
10
(Basore-Hansen Method)
10 100 1000 10000
J 0 from Small Perturbation Lifetime [fA/cm 2 ]
(Basore-Hansen Method)
10
10 100 1000 10000
J 0 from Small Perturbation Lifetime [fA/cm 2 ]
• QSS-µPCD J 0 values correlate very well with results of Sinton QSSPC
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J 0e Wafer Mapping
Wafer Mapping Example with QSS-µPCD on p + /n/p + Emitter Test Structure
• Advantage of direct J 0 mapping using small perturbation decay lifetime at
two light intensities
J 0e [fA/cm 2 ]
Avg = 92.27
Max = 160.2
Min = 27.8
• Mapping exposes weak PV spots with high J 0 that can degrade solar cell performance
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Conclusions
• Quality of Decay Control offers an effective solution to problems with nonexponential
photoconductance decay.
• The high precision small perturbation QSS-µPCD technique unifies carrier decay
lifetime and steady-state lifetime measurements and enables parameter-free
determination of both lifetimes.
• One to one correlation is demonstrated with QSSPC results
• Unique usefulness of τ eff.d is demonstrated for silicon PV wafers by the example of
direct determination of J 0 and J 0 mapping
Acknowledgements
• We would like to thank Prof. Isidro Martin of Universitat Politecnica de Catalunya for
his stimulating ideas regarding the relationship between small perturbation and
steady state lifetimes. We would also like to thank Dr. Paul Basore for bringing to
our attention his excellent work on mPCD.
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