Revised April 2004 • NREL/SR-520-35884 

R.A. Sinton 

Sinton Consulting, Inc. 

Boulder, Colorado

Development of an In-Line 

Minority-Carrier Lifetime 

Monitoring Tool for Process 

Control during Fabrication of 

Crystalline Silicon Solar Cells 

Annual Subcontract Report 

June 2003 

National Renewable Energy Laboratory 

1617 Cole Boulevard 

Golden, Colorado 80401-3393 

NREL is a U.S. Department of Energy Laboratory 

Operated by Midwest Research Institute • Battelle 

Contract No. DE-AC36-99-GO10337 
Revised April 2004 • NREL/SR-520-35884 

Development of an In-Line 

Minority-Carrier Lifetime 

Monitoring Tool for Process 

Control during Fabrication of 

Crystalline Silicon Solar Cells 

Annual Subcontract Report 

June 2003 

R.A. Sinton 

Sinton Consulting, Inc. 

Boulder, Colorado

NREL Technical Monitor: D. Mooney 

Prepared under Subcontract No. ZDO-2-30628-08 

National Renewable Energy Laboratory 

1617 Cole Boulevard 

Golden, Colorado 80401-3393 

NREL is a U.S. Department of Energy Laboratory 

Operated by Midwest Research Institute • Battelle 

Contract No. DE-AC36-99-GO10337 
This publication was reproduced from the best available copy 

Submitted by the subcontractor and received no editorial review at NREL

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iii

Table of Contents 

Abstract ....................................................................................................................................................... 1 

Introduction ................................................................................................................................................. 2 

Task 1. Prototype Industrial Applications Using Existing or Modified Instruments............................... 3 

Task 2. Design, Engineer and Test a Small Sample Head Suitable for Use Inline.................................. 5 

 1. An Instrument for Measuring the Lifetime on Solar CZ Silicon Ingots ................................ 6 

 2. A Multi-Crystalline Block Scanner........................................................................................ 9

 3. An Instrument for Measuring Lifetime on High-Lifetime FZ Boules ................................. 13 

 4. An Instrument for the rapid In-Line Measurement of Wafers ............................................. 15 

Task 3. Develop Application Notes for Industrial Use of Lifetime Testing in the 

Production Line......................................................................................................................... 21 

References ................................................................................................................................................. 23 

List of Figures 

Fig. 1. The starting point for this subcontract. Over 100 Sinton Consulting lifetime testers are 

used worldwide. .......................................................................................................................... 2 

Fig. 2 The Pseudo-IV curve from a high-quality 100 cm2

 solar cell..................................................... 4 

Fig. 3. A demonstration of the repeatability and speed of the measurement.......................................... 5 

Fig. 4. A small sample head used in a configuration allowing the measurement of the lifetime 

in CZ silicon ingots ..................................................................................................................... 7 

Fig. 5. The shape of the sensor is optimized for flexible measurements of blocks ................................ 7 

Fig. 6. QSSPC lifetime data from a CZ solar silicon instrument for 3 wavelengths of light ................. 8 

Fig. 7. An example of a simplified interface .......................................................................................... 9 

Fig. 8. This instrument performs line scans down the length of a Multi-crystalline silicon block ...... 11 

Fig. 9. A detail of the block scanner, the sense head........................................................................... 11 

Fig. 10. A different view of the block scanner indicating the sample head as it scans the block........... 12 

Fig. 11. A line scan. This data averages over 30 mm, about 1⁄4 of the block width, yet has a 

2-3 mm resolution in the growth direction................................................................................ 12 

Fig. 12. A transient measurement on a 1.6 Ohm-cm p-type FZ boule of silicon ................................... 13 

Fig. 13. Analyzed data from a transient measurement of a 1.6 Ohm-cm FZ boule of silicon................ 14 

Fig. 14. An instrument that was optimized for measurements on as-grown boules of highlifetime float-zone silicon ......................................................................................................... 15 

Fig. 15. This sample head is optimized for in-line measurements of wafers or flat surfaces of 

uniform blocks .......................................................................................................................... 17 

Fig. 16. The wavelength dependence of photoconductance has proven to be a powerful tool in 

R&D.......................................................................................................................................... 18 

Fig. 17. The photon distribution that results from 3 filters and a flashlamp .......................................... 19 

Fig. 18. The lifetime measured on industrial multi-crystalline wafers with a nominal resistivity 

of 1-ohm-cm.............................................................................................................................. 19 

Fig. 19. The data from the bare multi-crystalline wafers from Fig. 18, also showing the 

correction that can be applied if the lifetime measured with visible light is much less 

than the lifetime measured with infra-red light......................................................................... 20 
1

Abstract: 

As the production volumes of crystalline silicon manufacturing lines have grown in 

recent years, the demand for improved process control and process monitoring in 

manufacturing has increased. Since 1995, Sinton Consulting Inc. has been providing 

several tools to both Universities and industry for their R&D laboratories. For example, 

over 100 Sinton minority-carrier lifetime instruments are in use worldwide. The purpose 

of the work reported here is to adapt these successful R&D instruments and techniques to 

be directly applicable to process control and monitoring in production environments. 

Under the PV Manufacturing R&D subcontract “Development of an In-Line, MinorityCarrier Lifetime Monitoring Tool for Process Control during Fabrication of Crystalline 

Silicon Solar Cells”, Sinton Consulting developed prototypes for several new instruments 

for use in the manufacture of silicon solar cells. These instruments are based on two 

families of R&D instruments that were previously available, an illumination vs. opencircuit-voltage technique and the quasi-steady state RF photoconductance technique for 

measuring minority-carrier lifetime. Compared to the previous instruments, the new 

prototypes are about 20 times faster per measurement, and have automated data analysis 

that does not require user intervention even when confronted by challenging cases. For 

example, un-passivated multi-crystalline wafers with large variations in lifetime and 

trapping behavior can be measured sequentially without error. Five instruments have 

been prototyped in this project to date, including a block tester for evaluating cast or 

HEM silicon blocks, a CZ ingot tester, an FZ boule tester for use with long-lifetime 

silicon, and an in-line sample head for measuring wafers. The CZ ingot tester and the FZ 

boule tester are already being used within industry and there is interest in the other 

prototypes. For each instrument, substantial R&D work was required in developing the 

device physics and analysis as well as for the hardware. This work has been documented 

in a series of application notes and conference publications, and will result in significant 

improvements for both the R&D and the industrial types of instruments. 

Acknowledgements: Special thanks to Victoria Nosal, Ed Witt, Christie Johnson, 

Carolyn Lopez and David Mooney for guiding us through this first proposal and 

subsequent DOE subcontract. Much of the technical work in this report was made 

possible by the contributions of Tanaya Mankad and Stuart Bowden. 
2

Introduction. 

The objective of this subcontract over its two-phase, two year duration is to design and 

develop improvements to the existing Sinton Consulting R&D minority-carrier lifetime 

testers. The envisioned improvements would enable the possibilities for performing 

various in-line diagnostics on crystalline silicon wafers and cells for solar cell 

manufacturing lines. This would facilitate manufacturing optimization and improved 

process control. 

The scope of work for Phase I was to prototype industrial applications for the improved 

instruments. A small-sample-head version of the instrument was to be designed and 

developed in this effort. This new instrument was to be complemented by detailed 

application notes detailing the productive use of minority-carrier lifetime measurements 

for process optimization and routine process control. 

Fig. 1. The starting point for this subcontract. Over 100 Sinton Consulting lifetime 

testers are used worldwide. This instrument is primarily applied to taking detailed 

measurements on single wafers in an R&D setting. Industrial applications and industrial 

versions of this measurement technique will be discussed in this annual subcontract 

report. 
3

Task 1. Prototype Industrial Applications Using Existing or Modified Instruments. 

The R&D minority-carrier lifetime tester(1) from Sinton Consulting requires 

considerable skill and understanding for successful measurements. An important goal of 

Task 1 in this subcontract was to refine the instrument, the measurement procedures, and 

the software in order to eliminate the necessity of operator discretion and intervention for 

measuring individual wafers. 

This goal was largely achieved by working with the original R&D instrument, the WCT100, and optimizing the procedures, software analysis and the hardware. In the new 

procedure, the measurement is referenced to an undoped wafer instead of the dark 

conductance of the wafer under test. In this way, the instrument can be tuned once for an 

undoped wafer and then left unattended. Each measurement will then be taken 

referenced to this undoped wafer. By doing this, the full non-linear calibration curves for 

the instrument output response vs. conductance could be implemented. These calibration 

curves are also referenced to an undoped wafer. This improved the accuracy of the 

instrument as well as the usability. Some hardware improvements were incorporated in 

order to minimize the long-term instrument drift from this initial calibration. 

In order not to lose sensitivity by referencing a conductance far from the conductance of 

the wafer under test, auto-scaling of the gain and voltage offset was implemented into the 

hardware and software. This improvement removed the necessity for the user to be an 

oscilloscope expert. Now, results from a novice user are rather indistinguishable from 

the results that an experienced scientist would obtain. 

Data published from the R&D instrument generally uses the full injection-level lifetime 

curve in order to draw conclusions about the data. At the start of this project, it was 

thought that analysis simplifications would be required in order to achieve the high-speed 

measurements that would enable in-line wafer testing or block scanning. One of the first 

accomplishments of this work was to implement a fast data acquisition card to replace the 

digital oscilloscope previously used. This, in combination with better database 

implementation, allowed the complete R&D analysis to be so quick that these functions 

did not limit the rate at which data could be obtained, analyzed and logged. It was found 

that the inclusion of the full analysis allows the direct porting of methods used in the 

R&D labs to the industrial applications, without compromising the analysis flexibility or 

data quality. A new interface sits on top of the full data analysis in order to present the 

user with a limited spectrum of options completely specialized for the task at hand. In 

the background, the complete analysis is implemented and this reports back a simplified 

set of results to the user. 

This results in a façade of a simple program and interface, while the results can have the 

same credibility as those that might be published from the R&D lab. An example of a 

simplified interface is presented under Task 2. 

A demonstration of the implemented improvements is shown in Figure 1. One subtask of 

Task 1 was to demonstrate the Suns-Voc technique(2) at high speed, suitable for 
4

automation. This technique allows the construction of a current-voltage curve as soon as 

the diffusion is formed on the wafer by simply probing the two polarity regions of the 

wafer. The open circuit voltage as a function of the illumination intensity is then 

measured and converted to a presentation analogous to the final solar cell I-V curve. 

Pseudo Light IV curve without the effect of Rs

0

0.005

0.01

0.015

0.02

0.025

0.03

0.035

0.04

0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70

Voltage (V)

Current (A/cm 2)

0

0.002

0.004

0.006

0.008

0.01

0.012

0.014

0.016

0.018

Power Density (W/cm 2)

11/14/2002 Suns

Fig. 2. The Pseudo-IV curve from a high-quality 100 cm2 solar cell. This is the result 

that would be expected from a solar cell that had no series resistance. 

It is most easily used after Al back-surface field formation, when the substrate is 

electrically accessible simply by placing the wafer on a conductive chuck. The resulting 

curve gives the upper bound possible for efficiency, since the data is taken under opencircuit conditions and involves no current flow. This illumination vs. open-circuit 

voltage technique was implemented using this new hardware and software interface. 

Fig. 2 and 3 show examples of data taken at a high rate, one wafer per two seconds.

This data in Figure 2 required that accurate voltage and illumination data be taken over 

two orders of magnitude (in illumination) within a fraction of a second. The fast data 

acquisition and analysis allows a large number of points to be taken at high resolution. 

The result has quite low noise. The longer term stability and standard deviation for a 

large number of measurements taken at high speed is shown in Fig. 3. For example, the 

measured “Pseudo fill factor” of this curve is 0.7906 +/- 0.0003, giving a standard 

deviation of 0.04% over these 3000 measurements taken in 100 minutes. 
5

0

2

4

6

8

10

12

14

16

18

0 500 1000 1500 2000 2500 3000

Measurement (30 per minute)

Pseudo Efficiency (%)

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

FF, Vmp (V), Jmp (A/W)

"Pseudo" Efficiency = 16.46 +/- 0.01 %

FF = 0.7906 +/- 0.0003

Vmp = 0.5094 +/- 0.0014 V

Jmp = 0.0323 +/- 0.00009 A/cm2

Voc = 0.5949 +/- 0.0003 V

Fig. 3. A demonstration of the repeatability and speed of the measurement. 3000 

measurements were taken on a single wafer at a rate of 30 per minute. With automation 

of the wafer handling, this could have been 3000 wafers. 

Demonstration of the spectral response of the photoconductance was another subtask 

called out in Task 1. This was implemented on several instruments and used to optimize 

measurements on bare wafers and silicon blocks. This is reported under Task 2. 

Task 2. Design, Engineer and Test a Small Sample Head Suitable for Use Inline. 

The intention of this task was to develop a small sample-head version of the lifetime 

tester with the flexibility to address applications for inline wafer testing and 

measurements on industrial multi-crystalline and CZ blocks or boules of silicon. 

This task was expected to result in a prototype design that could be used as the basis for 

an instrument for industrial early-adopters of lifetime testing. 

In fact, it quickly became obvious that this task should be accelerated, in order to obtain 

proof of concepts for the instruments. The ideal proof of concept is one that gives a 

portfolio of data analysis results that can be sent around in order to focus a discussion 

with industry on near-term as well as long-term applications of the instrument to 

particular problems. Once this is done, industrial interest drives easy cooperation with 

significant industry involvement including sample exchanges, discussions, and on-site 

experiments. This helps to define the most relevant and useful designs and requirements 

for the instruments. 

This strategy resulted in prototypes of four different lifetime-test instruments in this first 

year. Two of these have already been placed in industry, and are generating published as 

well as unpublished data that is contributing to further improvements in the instruments 

and measurement techniques. The following will briefly introduce these four 
6

instruments, as well as a key result or two from each. These instruments are an ingot 

lifetime tester for use on CZ material prior to sawing(3), a block-test line-scanner for use 

on multicrystalline blocks prior to sawing, an FZ boule tester capable of characterizing 

the new generations of long-lifetime, low-cost FZ material to be used in low-cost 20% 

production silicon solar cells(4,5,6), and a small sample head suitable for integration into 

a production line as an in-line lifetime tester for each wafer at the start of the process(7) 

as well as at latter stages such as directly after the phosphorus diffusion(16). 

A key enabling technology for these instruments was the design of a small, maneuverable 

instrument that could be placed on irregularly shaped samples (cylindrical, flat, etc.). 

Also, the light source was integrated into the instrument so that the illumination and the 

photoconductance sensing is from the same side of the sample. As shown in figure 1, the 

previous R&D instrument had open-air illumination giving a uniform, large area 

illumination on a wafer and a reference cell. The photoconductance sensor was 

underneath the wafer on the opposite side from the illumination. In order to measure 

blocks, this had to be changed so that the portion of the block that would be illuminated 

on one side was also the portion where the sensor would monitor photoconductance. The 

reference cell was made to be internal to the instrument as well. This compact design 

forms the basis for all four instruments in this section. Each instrument and analysis is 

different for the four applications. However, many of the design parts are in common, 

simplifying the manufacture of instruments to serve these different applications. 

1. An instrument for measuring the lifetime on solar CZ silicon ingots. 

This instrument was optimized to measure lifetime in the range of 1-100 microseconds on 

ingots that have no sample preparation. Ingots would simply be pulled from the existing 

processes and measured. This instrument is shown in Fig. 4, 5. Lifetime data from this 

instrument is shown in Fig. 6. The lifetime as a function of the wavelength of light is 

shown in comparison data from a transient measurement and simulations from PC1D(7). 

With the critical lifetime to be measured normally in the range of 10 μs, the Quasisteady-state photoconductance (QSSPC) method gives much more robust results than the 

transient PCD method with this instrument(1). Infra-red light is used as the excitation 

source. This light is absorbed deep within the silicon. Much of the photogeneration is 

deeper than the diffusion length in this material. In this case, the measured lifetime 

becomes relatively indicative of the bulk lifetime, even for as-sawn or as-ground surfaces 

with very high surface recombination velocities. 

The theory for QSSPC on blocks of silicon within the range of solar silicon has be 

investigated, with the results developed into an application note(8). Some of these results 

were presented at the 2003 PVSEC conference in Osaka as well(7). 
7

Figure 4. A small sample head used in a configuration allowing the measurement of the 

lifetime in CZ silicon ingots. A section of an ingot is shown here. 

Fig. 5. The shape of the sensor is optimized for flexible measurements of blocks. The 

relatively large area gives a large photoconductance signal, and insures that the device 

physics of the analysis is primarily one-dimensional. The long slender shape of the 

sensor permits measurements on cylindrical shapes, since the sensor and illumination is 

nearly planar to the sensed portion of a cylindrical ingot when used in the geometry 

shown in Fig. 4. 
8

 Fig. 6. QSSPC lifetime data from a CZ solar silicon instrument for 3 wavelengths of 

light (see Fig. 16). The top graph shows the simulation in the range of solar CZ silicon. 

This data was taken on a 1.6 Ohm-cm FZ sample as a calibration of the QSSPC method. 

The data is compared to a transient PCD measurement and simulation curves from 

PC1D. 

PC1D Simulation

0

10

20

30

40

50

0 10 20 30 40 50

Bulk lifetime (μs)

Measured lifetime (

μs)

700 nm

950 nm

1000 nm

1050 nm

Simulation with Sparse Data from 1.6 Ohm-cm p-type block

0

200

400

600

800

1000

1200

0 200 400 600 800 1000 1200

Bulk lifetime (μs)

Measured lifetime ( μs)

Transient measurement

1000 nm IR-pass

850 nm IR-pass

VIS band-pass

1050 nm

1000 nm

950 nm

700 nm
9

Solar Ingot Lifetime and Resistivity Control

Top lifetime 4.2 μs

Ohm-cm 0.9 Ohm-cm

Tail lifetime 4.1 μs

Ohm-cm 0.9 Ohm-cm

LOGGED

New Boule

Top

Tail

Record Data Check 

Calibration

Fig. 7. An example of a simplified interface. Although the device physics analysis is 

done with the same completeness as in the R&D instrument, a small subset of user input 

and analysis output is displayed on the user interface. This standardizes the 

measurement for a wide range of user types. To the user, the system is simple and the 

data is repeatable. The analysis results are color coded. Red results “fail”. 

green results “pass”. 

Figure 7 shows an example of a simplified user interface suitable for industrial 

application. In this case, the user pushes a button, and a window pops up to accept the 

sample name. To measure the silicon, the operator places the instrument on the sample 

and pushes a button (“top” or “tail”). When finished, the data is logged to the database. 

Unseen by the operator, the QSS analysis is applied with a fixed interpretation previously 

set up by the engineer. This data analysis can specify the injection level to report the 

result, a bias light analysis to correct for trapping, and any of the other analysis 

techniques common to the QSSPC method. This system results in very uniform data 

results that are simple to obtain and are rather operator independent since there is little 

discretion in the measurement or analysis at the operator level.

2. A Multi-Crystalline Block Scanner. 

Figure 8 shows an instrument for measuring the lifetime of a multicrystalline block. This 

instrument performs a line scan down the length of the block, with high resolution (2 

mm). This data can be used to determine the good region of the block from which to saw 

wafers. The data will also be used to optimize the growth process. Knowing the quality 

and spatial uniformity of the silicon directly after growth (and cutting into blocks) will 

allow short-loop optimization of the growth process. This information will be especially 

valuable since it can be done directly after growth, and it is independent of variations that 

might be introduced during wafer fabrication. Also, fabrication processes usually 
10

scrambles the wafer order, so that spatial information about the block is lost by the time 

the solar cell efficiency is determined. 

A detailed view of the sense head is shown in Fig. 9. This arrangement, with a thin line 

of illumination, was optimized for the block-scan application. The long line averages 

over about 1⁄4 of the width of the block. The thin dimension of the line is in the direction 

of the scan. Some data from a line scan is shown in Fig. 11. The transition between 

material that is suitable for fabricating solar cells, and material destined for re-cycling is 

fairly abrupt. The 2 mm resolution, coupled with a 1-2 mm step size will yield 

information sufficient to specify a cut to an accuracy of less than a millimeter. 

Fig. 8 shows the apparatus after a line scan. The data is on screen, and shows the lifetime 

as a function of position, as well as the resistivity as a function of position. Another 

view of the instrument is shown in Fig. 10. 

If desired, the instrument can do raster scans to accomplish a 2-D mapping. The present 

prototype is not automated for scanning in the second dimension, although a manual 

stage movement is incorporated in this 2nd axis. 

As with the Solar CZ measurement, the preferred methodology of measurement and 

analysis with this equipment is the QSSPC method. Infra-red light creates 

photogeneration that extends into the silicon beyond a diffusion length from the surface. 

PC1D simulations give a good approximate calibration for the difference between 

“measured” and actual lifetime as shown in Fig. 6. A final correlation will probably be 

done by the industrial customer, by comparing the measured lifetimes to the resulting 

final solar cell efficiencies in order to map the complex relationship between the lifetimes 

as measured on as-grown materials with this method and the final solar cell 

efficiencies(3). Since the properties of the silicon wafer will change during hightemperature processing, the relationship between as-grown lifetime and final lifetime will 

be complex. 
11

Fig. 8. This instrument performs line scans down the length of a Multi-crystalline silicon 

block. The sample head, (behind the block) is run along a computer-controlled stage. 

Fig. 9. A detail of the block scanner, the sense head. This spot size is about 3 mm wide, 

and 30 mm high giving high resolution when stepped in the long (growth) axis of the 

block. The result is averaged over 30 mm in the axis perpendicular to the growth 

direction. The total illuminated area is large enough to give very good signal to noise. 
12

Fig. 10. A different view of the block scanner indicating the sample head as it scans the 

block. 

0

5

10

15

20

25

0 5 10 15 20

Cm

Lifetime (

μs)

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

ohm-cm

Fig. 11. A line scan. This data averages over 30 mm, about 1⁄4 of the block width, yet has 

a 2-3 mm resolution in the growth direction. This data was taken at a 1 mm step size in 

the growth direction. Lifetime is in blue (left axis), resistivity in pink (right axis). 
13

3. An Instrument for Measuring Lifetime on High-Lifetime FZ Boules. 

This application is unique for the series of instruments, in that it utilized the transient 

PCD technique. In contrast to the QSSPC technique, for a transient measurement the 

light is abruptly terminated, and then the decay of the photoconductance in the dark is 

monitored. Data taken in this way is shown in Fig. 12. 

This is the optimal situation for FZ silicon. This silicon typically has very high lifetime. 

Recombination at the surface would have a major effect on the measured lifetime of this 

material as determined by the QSSPC technique. By terminating the light abruptly, the 

surface recombination quickly eliminates electrons from near the surface. After this 

initial rapid decay, the measured lifetime asymptotically approaches the actual bulk 

lifetime. PC-1D simulations have been applied to verify this behavior, and study it in 

detail(9). 

An instrument measuring an FZ boule is shown in Fig. 13. The instrument is nominally 

the same as the CZ instrument. However, the analysis is different, with an optimization 

for this high-lifetime material. In addition, more light of shorter wavelengths is used in 

the excitation to obtain the highest possible initial photoconductance, since the surface 

modes are allowed to decay away before the bulk lifetime is determined(8). 

Illumination at Reference Cell and PC Signal

0.01

0.10

1.00

10.00

100.00

0.0E+00 2.0E-03 4.0E-03 6.0E-03 8.0E-03 1.0E-02

Time (s)

Illumination Intensity (Suns)

0.000

0.001

0.010

0.100

1.000

10.000

Photo-Conductance Signal (V)

Intensity (Suns)

PC signal

First point analyzed

Fig. 12. A transient measurement on a 1.6 Ohm-cm p-type FZ boule of silicon. 

(sample courtesy of Jan Vedde, Topsil A/S). 

Data from an 1.6 Ohm-cm p-type boule is shown in Fig. 13. In contrast to measurements 

on wafers, it is difficult to calibrate the injection-level scale precisely, since the carriers 
14

are not confined. They can diffuse deep into the bulk away from the RF sense head. This 

effect is a function of the diffusion length. As a result, the minority-carrier injection 

levels shown on the x axis are approximate. 

At high injection levels, (which occur just after the light is extinguished), the measured 

lifetime is short as the surface recombination quickly annihilates the electrons near the 

surface. Boron-doped p-type material generally has lower lifetime at low injection levels 

than at higher injection, due to the bulk recombination. The competition between these 

trends leads to a maximum lifetime during this measurement at moderate carrier density. 

This lifetime, 1.4 ms for this boule, indicates that very long lifetime can be characterized 

despite the lack of surface passivation on the boule. 

This instrument and measurement promises instant feedback to the grower for 

optimization of the material growth. In addition, it allows for specification of the boule 

for lifetime as a characteristic that can be given to the solar cell fabrication line or wafer 

customer. The layout of this instrument, with a boule to be measured, is shown in 

Fig. 14. 

Fig. 13. Analyzed data from a transient measurement of a 1.6 Ohm-cm FZ boule of 

silicon. Despite a lack of an surface preparation, lifetimes exceeding 1 ms can be 

measured. The accuracy of the measurement results has been modeled in PC1D and 

detailed in an application note(8). 

Minority-Carrier Lifetime vs. Carrier Density 

0.000

0.001

0.002

0.003

1.0E+13 1.0E+14 1.0E+15 1.0E+16

Estimated Minority- Carrier Density (cm-3)

Measured Lifetime (sec)

Minority-Carrier Density

Maximum Lifetime

Apparent Carrier Density

Tau = 1403.2 μs at 4.8E+14 cm-3
15

Figure 14. An instrument that was optimized for measurements on as-grown boules of 

high-lifetime float-zone silicon. (Photograph courtesy of Jan Vedde, Topsil A/S). 

4. An Instrument for the rapid In-Line Measurement of Wafers. 

The last instrument to be discussed here is a small sample head that was optimized for 

measuring wafers. This instrument is shown in Fig. 15. Again, for ease of automation, 

the internal light source configuration was used in contrast to the open-air method for the 

R&D instrument. However, this small sample head has a large illuminated sense area for 

very high sensitivity. This will allow for the measurement of unpassivated thin wafers. 

This case, unpassivated wafers with no surface recombination, is quite difficult to 

measure due to low effective lifetimes in the wafers, coupled with highly-variable levels 

of trapping, which tends to bury the signal that would be due to minority-carrier 

photoconductance(10). 

This instrument applies a correction for trapping that is based on obtaining the full 

injection-level dependence of the measured lifetime(10). In addition, the wavelength 

dependence of the lifetime has been applied to better separate out the surface from the 

bulk effects in order to more accurately track the bulk lifetime(7,13). This was reported 

at the PVSEC in Osaka(7), and detailed in an application note as well(11). For the case 

of typical industrial solar cell wafers, (300-μm-thick, 1 ohm-cm p-type wafers) guidelines 

for QSSPC measurement have recently been detailed in a handbook(12). Fig. 16 shows a 

key result from this work. For wafers with good surface passivation, such as a 

phosphorus emitter diffusion with a grown oxide or nitride, the measured lifetime will 
16

correspond closely to the bulk lifetime, for lifetimes less than 50 μs. For unpassivated 

wafers 1-ohm-cm multicrystalline wafers, the lifetime has been determined to be greater 

than 1 x 105

 cm/s by using the wavelength dependence of the measured lifetime to 

separate bulk and surface effects(7,13). Filters were integrated into the light path for the 

instrument shown in Fig. 15. This photon distribution from these filters is shown in Fig. 

16. This instrument configuration enables straightforward application of the technique of 

Bail(13) to measure the surface recombination velocity. By noting the dramatically 

different lifetimes measured with visible and IR light (Fig. 17), the results from Fig. 15 

could be used determine the surface recombination velocity. 

This predictable high surface recombination allows a correction to be made to the 

lifetime data that will allow “good” from “bad” unpassivated wafers to be discriminated 

with better precision. The effect of this correction is shown in Fig. 18. 

For this measurement to be valid, it has been found that the trapping correction is 

essential, therefore this correction is incorporated into the instruments intended for this 

application. 
17

Fig. 15. This sample head is optimized for in-line measurements of wafers or flat 

surfaces of uniform blocks. The illuminated area is large, to enable high sensitivity for 

measuring unpassivated wafers just entering the process line, and giving very onedimensional results on phosphorus-diffused wafers. 
18

1

10

100

1000

10000

100000

1000000

0.01 0.10 1.00 10.00 100.00

Measured effective lifetime (μs)

Surface Recombination Velocity (cm/s)

1.0E-15

1.0E-14

1.0E-13

1.0E-12

1.0E-11

1.0E-10

1.0E-09

Emitter Joe (A/cm 2 )

Fig. 16. The wavelength dependence of photoconductance has proven to be a powerful 

tool in R&D. These curves indicate the different lifetimes that would be measured on 

industrial 1 ohm-cm, p-type substrates with bulk lifetime of 0.1, 1, 10, and 100 μs, as a 

function of the surface recombination. For well passivated wafers, the measured lifetime 

is the bulk lifetime. For unpassivated wafers, the difference between the lifetime 

measured under blue and infra-red light can be used to accurately determine bulk 

lifetime and surface recombination velocity(Bail, Sinton). 

Two application notes have been developed for the instrument in Fig. 15. The first is 

coupled with a special version of the software analysis, and is optimized for bare 

wafers(11). 

The second application note is focused on measuring wafers after the phosphorus 

diffusion(16). There is already ample data from industry using the WCT-100 R&D 

instrument that the lifetime measured after the phosphorus diffusion is very predictive of 

the final solar cell efficiency. This measurement has been used to qualify both the 

process and the wafer at this stage, where the cost of screen print and final test can still be 

avoided if the wafer or emitter diffusion is bad. Also, any process problems can be 

quickly identified almost as soon as they occur giving unambiguous feedback very close 

to the source of the problem. 
19

0

1

2

3

4

5

6

7

200 400 600 800 1000 1200

Wavelength (nm)

Photons - nm -1 (A.U.)

α =1 μm

α = 42 μm

α = 160 μm

α =14μm

Fig. 17. The photon distribution that results from 3 filters and a flashlamp. The 

flashlamp spectrum was measured[14] and the filter characteristics were taken from 

specification sheets. The three filters are visible-pass, IR-pass with a 850 nm threshold, 

and 1000 nm IR-pass. 

Fig. 18. The lifetime measured on industrial multi-crystalline wafers with a nominal 

resistivity of 1-ohm-cm. A much lower lifetime is measured using blue light, indicative of 

a surface recombination velocity greater than 105

 cm/s. Once this high surface 

recombination velocity is verified for a sample, an analysis can be used to better estimate 

the true bulk lifetime of these samples. The passivated samples, shown on the right, have 

wavelength independent measured lifetimes, as predicted by the modeling in Fig. 16. The 

colors correspond to the three photon distributions shown in Fig. 16. 

0.01

0.1

1

10

100

0123456789

Sample

Measured lifetime (

μs)

Bare multicrystalline wafers

high-lifetime n-type, diffused+ ox

Multi, after P diff
20

Fig. 19. The data from the bare multi-crystalline wafers from Fig. 18, also showing the 

correction that can be applied if the lifetime measured with visible light is much less than 

the lifetime measured with infra-red light. This correction, based on the sample 

thickness, removes the effect of the wafer thickness from the measured lifetime, and gives 

a much better approximation of the actual bulk lifetime than the measured lifetime does. 

This allows a more informed selection criteria for the acceptance or rejection of bare 

un-passivated wafers. 

0.01

0.1

1

10

100

01234567

Sample

Measured lifetime (

μs)

Pass

Fail

Corrected Lifetime
21

Task 3. Develop Application Notes for Industrial Use of Lifetime Testing in the 

Production Line. 

The intent of this task was to develop application notes for industrial applications of the 

instrument. These notes should operate on several levels. First, they should allow a 

simple procedure to obtain immediate and valid results. Second, they should provide an 

explanation of the device physics connecting the measurement procedure and results 

solidly to the scientific basis for the measurement. 

These notes have been completed for 4 applications: 

1) Measurements on bare wafers(11). A special version of the software was constructed 

that is optimized to be very robust for this measurement. The application note details the 

basis for the measurement. The technique chosen for this optimization involves the 

spectral response of the measured lifetime to correct for the high-surface recombination 

velocity. The study and qualification of this technique was reported in a conference 

paper(7), as well as under the discussion of Task 2 in this report. 

2) Measurements on relatively uniform blocks or boules of material. An application note 

details both the use of an optimized instrument and QSSPC to measure CZ solar silicon, 

and the use of transient PCD for measurements on FZ silicon. Some details of the 

QSSPC measurement were presented in a conference paper(11). The FZ analysis is as 

yet unpublished other than the application note itself(8). 

3) Measurements on wafers after phosphorus diffusion, or after other good surface 

passivations. This application note(15) focuses primarily on the optimum measurement 

strategies for industrial wafers after the phosphorus diffusion, but has general discussion 

of other wafer types as well. The use of high-resistivity wafers to qualify a diffusion 

process is also covered. Some details that are particular to multi-crystalline wafers, and 

the interpretation of data from wafers with widely varying grain lifetimes was presented 

in a conference paper(15). This paper discusses the type of lifetime average that results 

from the circuit connection through the junction that is present during measurements on 

wafers after the phosphorus diffusion. 

In addition to these application notes, several other activities are in progress. These 

include the complete specification of typical instruments for characteristics vs. ambient 

temperature range, the determination of the lifetime measurement accuracy that can be 

guaranteed, and traceability of lifetime measurements to a standard. This is ongoing, 

largely because rather than a “typical” instrument, we have developed 4 instruments 

during the Task 2 activities. As a result, the characterization of these instruments has 

been broad, across the varied instruments, rather than deep. More work remains to be 

done on MTBF and other longer-term characteristics of each individual type of 

instrument. 

The function of measuring sheet resistance or resistivity with the conductance sensor in 

these instruments has been formalized in two applications. In the block scanner, this 
22

resistivity is measured and plotted as a function of position. For the wafer measurement 

tools, this function has been implemented so that the measurement is taken automatically 

whenever a lifetime is measured. 
23

References: 

(work developed with support from this subcontract is marked with an *) 

(1) Sinton, R.A. and Cuevas, A., Contactless determination of current-voltage characteristics and minority-carrier 

lifetimes in semiconductors from quasi-steady-state photoconductance data. Applied Physics Letters, 1996, 69(17), pp. 

2510-2512. 

(2) R. A. Sinton and A. Cuevas, in A Quasi-Steady-State Open-Circuit Voltage Method for Solar Cell Characterization, 

Proc. 16th European Photovoltaic Solar Energy Conference, Glasgow, Scotland, 2000. 

(3) *John L. Coleman, Jeff Nickerson, and Ronald A. Sinton, “Empirical Modeling of Ingot and Post Oxidation Level 

QSS Lifetime Measures in a Production Environment” to be presented at the NREL Workshop on Crystalline Si 

Materials and Processes, August, 2003. 

(4) Jan Vedde, Leif Jensen, and Thomas Clausen, “Float zone silicon for High Volume Production of Solar Cells”, To 

by published in Proc. PVSEC, Osaka, Japan, May 2003. 

(5) Keith R. MacIntosh, Michael J. Cudzinovic, David D. Smith, and William P. Mulligan and Richard Swanson, “ 

The Choice of Silicon Wafer for the Production of Rear-Contact Solar Cells”, To by published in Proc. PVSEC, Osaka, 

Japan, May 2003. 

(6) Thomas Clausen, Jan Vedde, Theis Leth Larsen, and Leif Jensen, and “Thin Monocrystalline Solar Cells 

Fabrication on Float Zone Silicon”, To by published in Proc. PVSEC, Osaka, Japan, May 2003. 

(7) *Ronald A. Sinton, “Practical measurement of bulk lifetime and surface recombination by using wavelength 

dependence”, To by published in Proc. PVSEC, Osaka, Japan, May 2003. 

(8) *Ronald A. Sinton , “Boule Test Application Note”, Application note developed under DOE subcontract ZDO-2-

30628-08. Sintonconsulting.com, Unpublished. 

(9) P. A. Basore and D. A. Clugston, 26th PVSC Anaheim, 1977 pg. 207. 

(10) Daniel Macdonald, Ronald A. Sinton, and Andres Cuevas, “On the use of a bias-light correction for trapping 

effects in photoconductance-based lifetime measurements of silicon”, J.A.P. Vol 89, (5) 1 March 2001. 

(11) *Ronald A. Sinton, “Testing Bare Wafers using the Sinton Consulting WCT-100 Lifetime Tester”, Application 

note developed under DOE subcontract ZDO-2-30628-08. Sintonconsulting.com, Unpublished. 

(12) A. Cuevas and R. A. Sinton, “Characterization and diagnosis of silicon wafers and devices”, Practical Handbook 

of Photovoltaics, T. Markvart and L. Castener Eds., to be published by Elsevier, Oxford. 

(13) Bail, M. and Brendel, R., 16th European Photovoltaic Solar Energy Conference, Glasgow, 2000, pp. 98-101. 

(14) Measurement by Daryl Myers, NREL, 2001. 

(15) *Ronald A. Sinton, “Predicting Multi-Crystalline Solar Cell Efficiency From Lifetime Measured During Cell 

Fabrication”, To by published in Proc. PVSEC, Osaka, Japan, May 2003. 

(16)*Ronald A. Sinton, “Testing Solar Cell Wafers After Phosphorus Diffusion”, Application note developed under 

DOE subcontract ZDO-2-30628-08. Sintonconsulting.com, Unpublished. 

(17) R. A. Sinton, “Measuring and Monitoring Electronic Properties of Si During Industrial Material Preparation and 

Cell Fabrication, To be published in the Proc. NREL PVAR&D meeting”, March, 2003. 
REPORT DOCUMENTATION PAGE Form Approved 

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1. AGENCY USE ONLY (Leave blank) 2. REPORT DATE

April 2004 

3. REPORT TYPE AND DATES COVERED 

Annual Subcontract Report 

June 2003 

4. TITLE AND SUBTITLE

 Development of an In-Line Minority-Carrier Lifetime Monitoring Tool for Process Control 

during Fabrication of Crystalline Silicon Solar Cells: Annual Subcontract Report, 

June 2003 

6. AUTHOR: 

R.A. Sinton 

5. FUNDING NUMBERS

PVP46101 

ZDO-2-30628-08

7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES) 

Sinton Consulting, Inc. 

1132 Green Circle 

Boulder, Colorado 80305

8. PERFORMING ORGANIZATION 

REPORT NUMBER 

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National Renewable Energy Laboratory 

1617 Cole Blvd. 

Golden, CO 80401-3393

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AGENCY REPORT NUMBER

NREL/SR-520-35884 

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13. ABSTRACT (Maximum 200 words): Under the PV Manufacturing R&D subcontract “Development of an In-Line, Minority-Carrier 

Lifetime Monitoring Tool for Process Control during Fabrication of Crystalline Silicon Solar Cells”, Sinton Consulting developed 

prototypes for several new instruments for use in the manufacture of silicon solar cells. These instruments are based on two 

families of R&D instruments that were previously available, an illumination vs. open-circuit-voltage technique and the quasisteady state RF photoconductance technique for measuring minority-carrier lifetime. Compared to the previous instruments, 

the new prototypes are about 20 times faster per measurement, and have automated data analysis that does not require user 

intervention even when confronted by challenging cases. For example, un-passivated multi-crystalline wafers with large 

variations in lifetime and trapping behavior can be measured sequentially without error. Five instruments have been 

prototyped in this project to date, including a block tester for evaluating cast or HEM silicon blocks, a CZ ingot tester, an FZ 

boule tester for use with long-lifetime silicon, and an in-line sample head for measuring wafers. The CZ ingot tester and the 

FZ boule tester are already being used within industry and there is interest in the other prototypes. For each instrument, 

substantial R&D work was required in developing the device physics and analysis as well as for the hardware. This work has 

been documented in a series of application notes and conference publications, and will result in significant improvements for 

both the R&D and the industrial types of instruments. 

15. NUMBER OF PAGES 14. SUBJECT TERMS: PV; thin film; solar cell; manufacturing; process control; crystalline 

silicon; minority carrier; open-circuit voltage; photoconductance; quasi steady state; floatzone silicon; phosphorus diffusion; 16. PRICE CODE

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