NAT-LINST. OF STAND & TECH

I I II I III I III II III I AlllOfc, 3A7372

NBSIR 80-2181

Measurement Techniques for

Solar Cells, Annual Report

December 15,1 978 to

December 14, 1979

D. E. Sawyer, H. K. Kessler, T. J. Russell,

W. F. Lankford, and H. A. Schafft

"\

Electron Devices Division

Center for Electronics and Electrical Engineering

National Engineering Laboratory

U.S. Department of Commerce

National Bureau of Standards

Washington, DC 20234

January 1981

-yc—

100

. U56

80-2181

1981

c. 2

Prepared for Solar Energy Research Institute

Golden, CO 80401

NBSIR 80-2181

MEASUREMENT TECHNIQUES FOR

SOLAR CELLS, ANNUAL REPORT

DECEMBER 15, 1978 TO

DECEMBER 14, 1979

National Bureau of Standards

Library, E-01 Admin. Bldg.

MAR 2 1981

nu cue - - 7W

Qqoo

/> A,

D. E. Sawyer, H. K. Kessler, T. J. Russell,

W. F. Lankford, and H. A. Schafft

Electron Devices Division

Center for Electronics and Electrical Engineering

National Engineering Laboratory

U.S. Department of Commerce

National Bureau of Standards

Washington, DC 20234

January 1981

Prepared for Solar Energy Research Institute

Golden, CO 80401

U.S. DEPARTMENT OF COMMERCE, Malcolm Baldrige, Secretary

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director

Table of Contents

Page

1. Executive Summary 1

2. Application of the Laser Flying-Spot Scanner to Solar Cells and

Materials 3

2. 1 Overview 3

2.2 Experimental Zinc Phosphide Schottky Diodes 4

2.3 Silicon Tandem Junction Cells 4

2.4 Ungridded C^S/CdS Cells 6

2.5 EFG Polycrystalline Silicon Cells 8

2.5.1 Defect Location 8

2.5.2 Silicon Carbide Particles 8

2.5.3 Crack Parallel to a Metallization Finger 11 2.5.4 Back Contact Faults 11 2.5.5 Deliberate Scratches 18

2.6 New Applications . . 18

3. Mathematical Modeling 19

4. NBS-22, A Solar Cell Test Pattern 19

4. 1 Description of the Pattern 20

4.2 Sample Preparation and Data Collection 24

5. Laser Scanner Ancillary Technique and Apparatus Development ... 24

5.1 Improved Ref lected-Light Detection System 24

5.1.1 Alignment of Ref lected-Light Optical Path ...... 28

5.1.2 Photomultiplier Detector 30

5.2 Specimen Birefringence Microscope Attachment 30

5.3 A Light Source for Biasing Large-Diameter Cells 34

5.4 Optical Parameters of Various Lens Combinations 35

5.5 Variable Specimen Temperature 40

5.5.1 Heating Stage 40

5.5.2 Cooling Stage, Thermoelectric Type 40

5.5.3 Cooling Stage, Compressor Type 41

6. Workshops 43

6. 1 Stability of (Thin Film) Solar Cells and Materials 43

6.2 Photovoltaic Material and Device Measurements ... 43

7. Consultation and Liaison Activities 43

7.1 Consultations to SERI 43

7.2 Visits from Industry and Universities to the National Bureau

of Standards * 44

References 46

Appendix A 48

Appendix B 49

iii
List of Figures

Page

1. Sketch of semicircular magnesium metallization contact patterns

deposited to form Schottky barriers on p-type Zn3

P

2 5

2. Sketch of a tandem junction solar cell 5

3. Ungridded Cu2 S/CdS cell response to 0.633-um scanning light ... 7

4. Ungridded Cu2 S/CdS cell response after scratching the surface . . 7

5. Photoresponse map to 0.633-ym light of an early low-cost EFG

cell with 500-mA forward bias 9

6. Photoresponse of an EFG cell near a silicon carbide particle

producing a point shunt through the cell indicated by the

reduced signal as the laser spot approaches the particle 10

7. Predicted photoresponse of a cell with a point shunt located at

0.5 (dotted lines) compared to a perfect cell (solid lines) ... 10

8. Line scan through the region of a cell where a silicon carbide

particle is producing a point shunt 12

9. Line scan in the region of a cell when a silicon carbide

particle has not produced a shunt through the cell 12

10. Raster scan of an EFG cell under a few suns light bias to

identify a hairline crack between the second and third

metallization finger from the left 13

11. Line scan of the cracked region of the cell in figure 10 ... . 13

12. Expanded line scan of the previous cell before the crack has

spread to the bus bar metallization 14

13. Same cell as in figure 12 at a later time when the crack has

interrupted the flow of current along the bus bar 15

14. Line scan adjacent to the bus bar showing the reduction in

photoresponse due to the crack 15

15. Photoresponse of a high quality 1-cm by 4-cm EFG cell 16

16. Cell of figure 15 under about 3 suns light bias 16

17. Cell of figure 15 under no light bias but with 1.15-ym laser

light indicating the photoresponse at the back contact more

clearly 17

xv
*

Page

18. NBS-22 test pattern which includes four solar cells (A, B,

C, and D) surrounded by arrays of microelectronic test

structures ...... ..... 21

19. Expanded view of the D solar cell which contains the doubly

diffused and the singly diffused regions of the cell and

also the regions where the metallization does not contact

the emitter surface, shown as the white breaks 22

20. The microelectronic test structure pattern which surrounds

the solar cell 23

21. Sheet resistance measurement locations around cell A for

values given in table 2 27

22. New ref lected-light asssembly using a germanium photodiode

visible on the right-hand component 29

23. Reflected light configuration with a photomultiplier

detector 31

24. Raster scan of most of an NBS-22 solar cell using the

configuration of figure 23 in the large area scan mode of

figures 25 and 26 32

25. Optical diagram for the large area scan configuration 38

26 Photograph of the large area scan configuration 39

27. Photograph of the compressor type cooling stage (top) and

cover (bottom) 42

List of Tables

Page

1. Test Structure List 25

2. Sheet Resistance + Standard Deviation 26

3. NBS Laser Scanner Specimen to Objective-Lens Working Distance

and Scan Excursion for the 0.633- pm Laser Wavelength Tabulated

Using a 10X Ocular Lens, a Total Optical Scanning Angle of 20

Deg at the Ocular Lens, and with Various Objective Lenses .... 36

4. NBS Laser Scanner Scanning Resolution and Ref lected-Light Bias,

for 0.633- and 1.15-ym Laser Wavelengths Tabulated Using a 10X

Ocular Lens and with Various Objective Lenses 37

v
Disclaimer

*

Certain commercial equipment, instruments, or materials are identified in

this report in order to adequately specify the experimental procedure. In no case does such identification imply recommendation or endorsement by the

National Bureau of Standards, nor does it imply that the material or equipment identified is necessarily the best available for the purpose.

vi
Measurement Techniques for Solar Cells

Annual Report

December 15, 1978 to December 14, 1979

D. E. Sawyer, H. K. Kessler,

T. J. Russell, W. F. Lankford, and H. A. Schafft

1 . EXECUTIVE SUMMARY

This report covers research performed in the period December 15,

1978 to December 14, 1979 on Solar Cell Measurement Technique

Development and Other Services by the Electron Devices Division of

the National Bureau of Standards (NBS ) * The objectives of the

project are to provide support to the Department of Energy (DoE)

thin-film photovoltaic effort in the following ways: (1) by

developing solar cell device and measurement techniques using the

NBS-developed laser flying-spot scanner and (2) by assisting the

DoE in organizing appropriate workshops and symposia and providing

general consultation and liaison services.

In this annual report, further applications of the NBS scanning

technique are described. The technique has been applied to

experimental solar cells and cell prototypes of interest in the

Solar Energy Research Institute (SERI) Photovoltaic Program Office

( PVPO ) . Cells and prototypes scanned include Cu2 S/CdS cells,

silicon tandem junction cells, Zn2 P

2 Schottky diode specimens,

and edge-fed growth (EFG) polycrystalline silicon cells. Scanning

of these cells was performed with the cell suppliers present to aid

in the rapid transfer of scanning results and dissemination of

scanning techniques. For the Cu2 S/CdS cells, the results show

that it is possible to screen cells for quality before the most

expensive fabrication step, gridding, is performed. For this cell

type, the importance of proper handling techniques was underscored

by laser scanning an ungridded cell before and after the cell was intentionally damaged by scratching the cell surface. Scanning the

tandem junction cells pinpointed a source of cell-efficiency loss

and motivated the redesign of the cell. Scanning of the Zn2

P

2 specimens yielded values of the effective minority carrier diffusion length in the Zn 2 P

2 in satisfactory agreement with the

results obtained using other techniques. The results of scanning

the EFG cells were interpreted by the supplier to yield the locations of missing antiref lective (AR) coating, metallization finger

breaks and loss of ohmic contact of metallization, back contact

problems, hairline cracks, and regions having reduced minority carrier lifetime. The electrical effects of silicon carbide inclusions were also observed. Another result of the EFG cell scanning

was that the supplier was able to pinpoint a fabrication-related

source of excess cell resistance.

* presently employed by Chevron Research Co., Solar Research Division,

P. 0. Box 1627, Richmond, CA 94802.

t Retired.

1
The results of the EFG scanning work were interpreted by the

supplier as consistent with the first-order mathematical modeling

performed by NBS and with the more detailed modeling performed by

K. Lehovec and A. Fedotowsky at the University of Southern

California as part of this effort. These USC workers have

completed a mathematical analysis which shows that the

response-equivalency of line and point-light scanning is a general

result and holds for cells having defects as well as for perfect

cells. Also treated is the scanning response of a cell containing

a point short-circuit and a cracked cell having various values of

crack shunt conductance with the cell crack both parallel to the

metallization fingers and at various angles to the fingers.

Although the modeling was based on single-crystal cells, the

results are expected to be applicable to other, and more exploratory, cell materials, e.g., polycrystalline silicon: shorts

are commonly observed in polycrystalline silicon cells, and some of

the effects which can occur at grain boundaries can be modeled in

terms of localized junction shunting.

A solar cell test pattern (NBS-22) was conceived and an initial

design completed during the previous reporting year (September 15,

1977 to December 14, 1978) with the objective of providing

precisely known artifacts to link the solar cell modeling work with

cell laser scanning under controlled conditions in order to put

laser scanning on a firm theoretical, and reproducible, basis. The

final design, fabrication, and initial testing of NBS-22 were completed during this present reporting period. NBS-22 was fabricated from single-crystal silicon, because only this material

permits the required degree of fabrication precision. The test

pattern consists of four solar cells, each surrounded by arrays of

microelectronic test structures from which various cell and

material properties and parameters can be determined. In each

array, test structures to measure emitter and metallization sheet

resistance, metal-to-emitter contact resistance, substrate

resistivity, and carrier density in the base as a function of

distance below the metallurgical junction are included. One of the

solar cells contains no intentional defects; one contains

separations in the metallization contact to the cell of various

controlled sizes; one has no separations, but does have an emitter

sheet resistance which varies periodically in a stripe pattern; and

one contains both separations and the emitter sheet resistance

variation described above.

Development of and improvements in ancillary techniques and

apparatus have continued. A new ref lected-light optical system was

constructed, and a simple method for aligning the ref lected-light

path was developed. An attachment for revealing specimen

birefringence by analyzing the 1.15-ym scanning reflected light was successfully designed and tested. A substrate heater stage for

heating specimens from room temperature to 160 °C while they are being scanned was constructed and found to hold the desired

temperature within this range to +0.5°C. Cooling stages for cooling

2
cells from room temperature to -20 °C (thermoelectric type) and to

-6Q°C (compressor type) were made and tested. These three items,

the birefringence attachment, the heating, and the cooling stages, were constructed in anticipation of future investigations to

separate various physical mechanisms which may be operative

simultaneously at grain boundaries to reduce the efficiency of

polycrystalline silicon solar cells.

The resolution of the scanner, both for the 0.633-ym and the

1.15-ym scanning wavelengths, was tabulated, along with the light

spot deflection and the specimen working distance, for the various

optical components which are on hand. This information was compiled to facilitate selection of the most appropriate optical

arrangement for a given scanning assignment and to serve as a time-saving guide for others who may construct scanners based on

the one developed at NBS.

The proceedings of the Workshop on Stability of (Thin Film) Solar

Cells and Materials, held May 1-3 1978 at NBS, were published [1] as was a companion document "A Perspective of a Workshop on Stability of (Thin Film) Solar Cells and Materials" [2]. The

Photovoltaic Material and Device Measurements Workshop (with focus

on polycrystalline thin film cells) was held June 11-13, 1979 in

Arlington, Virginia with 210 registrants. NBS assisted the SERI

PVPO in conducting this workshop. The general purpose of the

workshop was to accelerate the development of thin film cells by

improving the versatility and reliability of material and device

measurement techniques.

A patent application, "Nondamaging Technique for Locating

Semiconductor Device Defects and Measuring Device Resistivities,"

D. E. Sawyer, was filed in the U.S. Patent and Trademark Office by

the DoE on May 25, 1979.

Eighteen visits to NBS were made by PVPO, industry, or academic

staff during the year to discuss various aspects of the laser

scanner to measure experimental solar cells whose development is

being supported by PVPO.

The publications and patent applications associated with the work

conducted in this reporting period are listed in Appendix A.

2. APPLICATION OF THE LASER FLYING-SPOT SCANNER

TO SOLAR CELLS AND MATERIALS

2. 1 Overview

During this reporting period, the scanner was used to investigate samples of

both conventional single-crystal silicon p-n junction solar cells and cells

or prototypes constructed using either less well-developed semiconductor

materials or exploratory device concepts. From the latter group, four

examples of cells or cell concepts have been selected for discussion. This

is because the results obtained should be of interest to the main readership

3
of this report: those members of the photovoltaic community working with newer materials or exploratory device concepts. Scanning of the cells and

prototypes discussed in this section was performed with the suppliers present

to assist in the rapid transfer of the scanning results and effective

dissemination of the scanning techniques used.

2.2 Experimental Zinc Phosphide Schottky Diodes

Measurements were made on a Zn3

P

2 Schottky diode specimen provided by the

Institute of Energy Conversion (IEC) at the University oi; Delaware. The

purpose of the measurements was to assess the spatial uniformity of the

minority carrier diffusion length. The Zn3 P

2 specimen was p-type with a

carrier concentration of about 10 ^ cm“3.

An opaque layer of magnesium was deposited by the IEC to form Schottky

barriers in the form of two semicircular patterns as illustrated in figure

1; their 3-mm diameters were separated by about 0.2 mm. Specimens prepared

in this form have been used by the IEC in their work using two experimental

procedures for determining the effective minority carrier diffusion length in

Zn 3 p

2 through spectral response measurements [3] . A third and simpler

method used by the IEC to determine the diffusion length employs a focused

light spot. By moving the specimen with a micrometer stage and measuring the

short-circuit photocurrent as a function of the distance of the light spot

from the edges of the diameters of the opaque Schottky barrier patterns, an effective minority carrier diffusion length can be obtained. This method

gives results which are in good agreement with the other two [4]. However,

the focused light-spot technique is tedious and time-consuming since the

specimen is moved manually under the light spot, particularly so when one wishes to compare diffusion lengths for several specimen locations.

The work performed at NBS with IEC personnel on one of their Schottky barrier

specimens showed that an assessment of diffusion length uniformity can be

made in a straightforward and rapid manner using the laser scanner. Scanning

was performed using 0.633-ym laser light. The photoresponse maps obtained by

photographing the display screen were subsequently interpreted by the IEC as showing that a good degree of spatial uniformity was exhibited for the

minority carrier diffusion length. They also interpreted these results to

obtain a mean value which was in satisfactory agreement with that previously

obtained using the more tedious fixed- light-spot traveling-specimen method.

2.3 Silicon Tandem Junction Cells

Tandem Junction Cells (TJC) provided by Texas Instruments Inc. were scanned.

The design of these devices is novel in that the cell has junctions on both

the illuminated and nonilluminated faces, but the junction on the illuminated

face is left to "float" electrically; contacts are made only to the

nonilluminated (back) face [5]. This permits all of the top cell surface to

be used for photovoltaic conversion, and it also allows a great deal of

flexibility in connecting cells into modules. A sketch of a TJC is shown in

figure 2. The cell is modeled as a phototransistor, and ultimate AMI

efficiencies of 20 percent or better are anticipated [5].

4
- <—0.2 mm

Figure 1. Sketch of semicircular magnesium metallization contact patterns

deposited to form Schottky barriers on p-type Zn-jP2

*

METAL CONTACTS

Figure 2. Sketch of a tandem junction solar cell. The light path is shown

striking the back surface at greater than the critical angle of 15 deg to

reflect along the path of the dashed line.

5
Scanning was performed on typical TJCs with the cells unbiased using both

0.633- and 1.15-ym laser light. Decreases in the photoresponse to 0.633-ym

light over the p

+ base and to 1.15-ym light over both the p

+ base and the

n

+ collector metallization were detected. These observations led to

several speculative interpretations that suggested design modifications that

could increase the overall efficiency of these cells [5].

An expression of the value of the scanner is contained in the following

quotation [5]

:

"This very preliminary work indicates the potential power of laser

scanning as a diagnostic tool. Further development of the technique and

of the data interpretation could be very beneficial in the development

of high efficiency solar cells."

2.4 Ungridded Cu2 S/CdS Cells

Scanning of a Cu2 S/CdS cell prior to gridding showed that it is possible to screen cells for quality before the most expensive fabrication step,

gridding, is undertaken. Additionally, the need for careful handling of

cells during fabrication was underscored by making measurements on the

ungridded cell before and after scratching a cell as might occur during

careless handling or by the use of improper tools. The cell scanned was made

by the IEC using their current standard process. Cells of this type are fabricated on 2 by 2 cm thin copper sheets which serve as the back contact to

the cell. It was anticipated by the IEC that a temporary, yet satisfactory,

contact to the top side of the junction could be made with copper-coated

brass clips contacting edges of the cell, provided that the cell was properly

biased for the assessment.

Figure 3 shows the response for the ungridded cell using 0.633-ym scanning

light and before the cell was scratched. The top side was contacted as described above. The cell was reverse biased 0.6 V. Reverse-biasing greatly

reduces the cell distributed shunt conductance, a conductance inherent in the

voltage-current nature of the cell. With the conductance reduced, collection

of photocurrent becomes relatively insensitive to the resistance of the

emitter between the point of photoexcitation and the top collecting

electrode. This means that an assessment of the photocurrent produced

anywhere over the cell area can be made without taking steps (by, e.g.

,

gridding the cell) to ensure a low-resistance emitter current path. In

figure 3, the vertical deflection of a scan line at a given point is

proportional to the photocurrent produced by the cell. The overall

photoresponse is quite similar to that observed when scanning gridded

Cu2 S/CdS cells except, of course, there are no grids in the present case to

shadow the cell from the scanning light spot. The depressions in the picture

at the bottom, and at the upper left, are due to shadowing by the contact

clips.

Figure 4 shows the response for the same cell and operating conditions after

dragging one tip of a pair of steel tweezers across the cell surface. The

effect of this localized damage can be seen by the lowered photoresponse in

the scratched region about one-third the way down from the top of the figure.

Plastic tweezers are recommended by the IEC for cell handling, but sometimes

6
Figure 3. Ungridded Cu2 S/CdS cell response to 0.633-ym scanning light.

Figure 4. Ungridded Cu2 S /CdS cell response after scratching the surface.

7
steel tweezers are inadvertently used by the production line workers.

Comparison of figures 3 and 4 underscores the need to maintain proper

handling techniques.

2.5 EFG Polycrystalline Silicon Cells*

2.5.1 Defect Location

A study of EFG (Edge Fed Growth) thin film silicon solar cells was made on cells furnished by the Mobil-Tyco Solar Corporation. This work is reported

in somewhat greater detail than other visits as an example of the productive

collaboration that can occur with industry in the use of the scanner and in

recognition of the excellent work of Dr. Yates.

Raster scans were first made to identify regions in solar cells where various

types of defects were seen to occur. Line scans were then made through these

regions of interest to make a more quantitative analysis of the defects. For

example, figure 5 is the photoresponse map of an early low-cost n-on-p EFG

cell scanned using 0.633-pm light while forward-biased at 500 mA using an external current source. The display mode used was intensity-modulation of

the cathode ray beam with the "grey scale" reversed from that conventionally

used; i.e., in figure 5 the darker regions correspond to regions of greater

signal collection. Some cell features are readily identifiable; the white

patch above finger 1 coincides with missing antireflection coating. This was established by visual inspection, and confirmed by the fact that the decrease

in signal response occurs for both "bias off" and "bias on" scanning

conditions to rule out conduction defects. A break is shown in finger 2;

lighter areas such as the region to the left of this break indicate reduced

current collection. Breaks also occur in fingers 3, 4, 5, 7, 8, etc. The

white dots above finger 11 are due to the presence of silicon carbide

particles; their "signatures" are apparently much larger than the actual

particle sizes which may indicate low minority carrier lifetime near the

particles due to surrounding impurities or defects. Alternatively, the

particles may cause localized junction shunting. The calculations of Lehovec

and Fedotowsky [6] also show that the photoresponse in the vicinity of

localized shunts should decrease as the scanning light spot approaches the

shunts.

2.5.2 Silicon Carbide Particles

The presence of silicon carbide particles is a common problem when graphite

crucibles are used to contain the molten silicon in EFG techniques. Some

insight into the way these particles affect cell performance may be obtained

by comparing the photoresponse shown in figure 6 to that predicted by Lehovec

for a point shunt, as seen in figure 7, where the effect of the point shunt

is shown by the dotted lines. The reduced photoresponse as the laser spot

approaches the particle is thus the signature of a point shunt. The

difference between a shunting and a nonshunting silicon carbide particle.

* The work reported in this section was performed at NBS by Dr. D. A. Yates

of the Mobil-Tyco Solar Energy Corporation with the cooperation of NBS. It was abstracted from Mobil-Tyco trip reports and edited, and appears with

the permission of the Corporation.

8
Figure 5. Photoresponse map to 0.633-ym light of an early low-cost EFG cell

with 500-mA forward bias. Breaks in metallization mentioned in text are indicated with arrows.

9
Figure 6. Photoresponse of an EFG cell near a silicon carbide particle

producing a point shunt through the cell indicated by the reduced signal as the laser spot approaches the particle.

*- 0.5

Figure 7. Predicted photoresponse of a cell with a point shunt located at

0.5 (dotted lines) compared to a perfect cell (solid lines).

10
which shadows the junction, is shown in the line scan in figures 8 and 9,

respectively.

2.5.3 Crack Parallel to a Metallization Finger

The detection of cracks in forward-biased single crystal silicon cells with a laser scanner was previously reported by NBS [7]. Figures 10 and 11 show

that cracks may also be found in EFG cells and detected using the same scanning technique. The cell scanned is 2 cm by 2 cm and was fabricated

under DoE sponsorship. Figure 10 is the 0.633-ym photoresponse for a steady-state irradiance of a few suns and with the cell dc open-circuited.

The grey scale is reversed from that used for figure 5; i.e., regions of

greater photoresponse now appear lighter. There appears to be a linear

defect (as indicated by a dark line) between the second and third fingers

from the left. Figure 1 1 is a line scan through the defect region. The

photoresponse is discontinuous across the defect; also, the photoresponse at

the two edges of the defect is such that the edge closer to a metallization

finger provides the greater photoresponse. This behavior is predicted for a cracked cell by Lehovec and Fedotowsky ' s analysis [6]. Use of an optical

microscope confirmed that the cell indeed was cracked in the region indicated

by the scanner display. The nonzero photoresponse slope at the crack edges

indicates either ohmic shunting across, or minority carrier recombination at,

the cracking boundary, or a combination of these two. It is possible that

the contributions from these two mechanisms may be separated through the use

of ancillary measurements. The shunting contribution might be computed from

a measurement of temperature rise at the crack for a given value of cell

reverse bias. Localized temperatures can be measured using commercially

available microradiometers, or by using a thermal phosphor sheet pressed

against the cell surface [8]

Further studies of the cell with a crack parallel to a metallization finger

were made which showed the difference between the signature of a crack in the

active cell region and one in which the crack also extends to the bus bar

metallization. The same cell as used for figures 10 and 1 1 is shown in

figures 12 and 13 in a scan over only the cracked region. Figure 12 shows

the characteristic photoresponse signature as predicted by theory [6] for

high electrical resistance across the crack. At a later time when the crack

had spread to partially interrupt the bus bar, the effect is seen as a lower

photoresponse even near the right-hand finger as compared to the left-hand

side, figure 13. The extent of the reduction of current collected past the

partial break is shown in the line scan in figure 14 directly adjacent to the

bus bar. Only about three-fifths of the normal current is collected from the

right side of the crack.

2.5.4 Back Contact Faults

Figures 15, 16, and 17 are scanner display photographs near the end of a 1 -cm

by 4-cm high-quality (11 percent efficient) EFG cell. Figures 15 and 16 are

for 0.633-pm laser light scanning. The cell is unbiased in figure 15 and

under a light bias of about 3 suns for figure 16. Figure 17 is for 1.15-ym

scanning with the cell unbiased. The causes of some of the "features" which

appear in both figures 15 and 17 and are enhanced with irradiance (fig. 16) are presently not explained. Microscope observations did not reveal cracks.

11
Figure 8. Line scan through the region of a cell where a silicon carbide

particle is producing a point shunt.

Figure 9. Line scan in the region of a cell when a silicon carbide particle

has not produced a shunt through the cell.

12
Figure 10. Raster scan of an EFG cell under a few suns light bias to

identify a hairline crack between the second and third metallization finger

from the left.

Figure 11. Line scan of the cracked region of the cell in figure 10.

13
Figure 12. Expanded line scan of the previous cell before the crack has spread to

the bus bar metallization. Direction of scan on cell is opposite to that for the one used in figure 11.

14
Figure 13. Same cell as in figure 12 at a later time when the crack has

interrupted the flow of current along the bus bar. Note the lowered response

all the way to the right-hand grid.

Figure 14. Line scan adjacent to the bus bar showing the reduction in

photoresponse due to the crack.

15
Figure 15. Photoresponse of a high quality 1-cm by 4-cm EFG cell,

light is 0.63 ym with no light bias.

Figure 16. Cell of figure 15 under about 3 suns light bias.

Laser

16
Figure 17. Cell of figure 15 under no light bias but with 1.15-ym laser

light indicating the photoresponse at the back contact more clearly.

17
but cracks cannot be ruled out. These features include the lowered

photoresponse of the far-right cell portion between fingers 1 and 6 (counting

from the top), and the diagonal line between fingers 1 and 4. The use of

1.15-ym scanning light (fig. 17) enhanced these features and added a few more

such as the diffuse patterns near the center. The very regular shape of the

rectangular patch overlapping fingers 6 and 7 in figure 17 argues against its

being due to a crystal defect. The same region exhibited a reduced 0.633-pm

photoresponse with 3 suns background (fig. 16), indicating a conduction

defect. Taken together, these two observations suggest that the patch in

question originated during cell fabrication. Cells are fabricated from

p-type EFG sheets by first diffusing a n

+ skin to form the n

+p photo junction.

This diffusion creates an n

+p junction in the back side as well. Tabs are used to support the ends of the diffused wafers during evaporation of

aluminum onto the back side. The wafer is then heated in vacuum to alloy the

aluminum through the back side n

+ skin and to make ohmic content with the

P~type base. The specimen is then placed in another fixture (which supports

the specimen by its edges), and a trimetal mixture of chromium, titanium and

silver is deposited over the aluminum for protection against corrosion. The

rectangular patch visible in figure 17 matches the geometry of the end tab

used to hold the wafer during aluminum evaporation. Because this tab masks a portion of the specimen against the aluminum evaporation, the n

+p barrier

beneath it remains intact. This creates a high-resistance base contact

region and explains the lowered photoresponse of the forward-biased cell

between fingers 6 and 7 in figure 16. The interpretation of the enhanced

1.15-ym photoresponse which allows a "shadow" of the tab to be visible in

figure 17 is that the reflection of 1.15-ym light is greater from the p-base,

aluminum interface than it is from the multilayer consisting of the p-base, n

+ skin, and trimetal composition. A greater reflection allows more of the

1.15-ym light (which is but weakly absorbed within the silicon) to make a second pass through the photosensitive junction region and add to the cell

photoresponse. It is noteworthy that, to the eye, all portions of the back

contact appeared to be the same. The missing aluminum was masked from

observation by the trimetal film.

2.5.5 Deliberate Scratches

To study the effect of surface scratches on the photoresponse, deliberate

scratches were made with a diamond scribe before the plating step. If the

scratch caused an ohmic shunt, reduced photoresponse should be observed as the laser spot approaches the scratch. This was not observed and subsequent

Liquid Crystal Thermography using reverse bias on the cell confirmed that

there was no evidence of high leakage current at the scratch site. This

suggests that the laser scanner can differentiate between serious deep cell

scratches and relatively harmless shallow ones.

2.6 New Applications

Two new areas in which the laser scanner may be able to provide important

measurements of solar cell operating characteristics were identified during

the contract period. One is a test of bandgap narrowing as a function of

dopant density. This is an area of considerable current theoretical activity

[9-11] and one in which careful experimental work is needed. The

contribution of the electron-hole pair separation energy to the bandgap was

18
calculated by H. P. D. Lanyon as a function of dopant density and

temperature [9]. To check the temperature dependence, cooling stages were

designed and built as described in sections 5.5.2 and 5.5.3. Preliminary

measurements of the change in photoresponse over a range of 23°C to -60 °C

gave the expected reduction in photoresponse with lower temperature for

dopant densities measured. However, additional work is needed to establish a quantitative dependence of this change on dopant density.

The second new area is an attempt to separate the resistive effects that

limit the current collected in a solar cell from those due to recombination

in the cell. A suggestion was made by the principal investigator (D. E.

Sawyer) to attack this task by measuring the photoresponse of a forwardbiased test cell with the laser scanner and then using an infrared detection

system to map the recombination radiation in the same cell area and for the same value of forward-bias current. This idea was stimulated by G. W.

Turner ' s paper at the Photovoltaic Material and Device Measurements Workshop

[ 12 ].

3. MATHEMATICAL MODELING

A report on the initial modeling work performed by Drs. Kurt Lehovec and

Andre Fedotowsky of the University of Southern California under subcontract

has been published [6] . It extends the work previously performed by them on the analysis of laser scanning techniques applied to solar cells. Work

previously reported [7] developed the basic equations and applied them to

fault-free cells scanned either by a line-shaped light beam or a focused spot

and showed that these two light geometries give the same analytical results.

The report concluded with a general solution for cells having noncontacting

electrode sections. During the present reporting period the equivalency of

line and spot scanning, for all cells including those with defects, was proven, and results for the following were obtained: cells containing point

shorts and cells cracked both parallel to the metallization fingers and at

various angles to these fingers, and with various values of crack shunt

conductance. The complete report is published separately [13].

4. NBS-22, A SOLAR CELL TEST PATTERN

One of the needs of the laser scanning program is a means of determining the

degree of agreement between cell scanning results and the results predicted

analytically [6] , both for "perfect" cells and for cells with various

defects. To answer this need, NBS-22, a solar cell test pattern, was conceived in the first year of this program, and its final design was completed in the first half of 1979.

The NBS-22 test pattern contains four solar cell test structures which are surrounded by arrays of microelectronic test structures which can be used to

measure electrically various solar cell and material parameters. One cell is

perfect and the other three have combinations of process variations including

regions where the metal does not contact the silicon and regions with

different sheet resistance. The arrays of microelectronic test structures

include van der Pauw cross-sheet resistors [14], a contact resistor [15],

a four-probe resistor [16], an MOS capacitor [17], a dc MOSFET dopant

profiler [18], and a junction diode [19]. These structures can be used to

19
determine sheet resistance, contact resistance, substrate resistivity, dopant

profiles, and defect recombination centers. It has been demonstrated [20]

that these measurements and others can aid in the control of a semiconductor

fabrication process. With the microelectronic test structures deployed

around each of the four solar cells, not only may "spot" values of the

quantities of interest be obtained, but their variation over the wafer may

also be inferred. The sheet resistance determined by electrical measurements

may be compared with independent measurements made with the laser scanner. A measure of the contact resistance, also obtained by electrical

measurements, served to check the quality of the metallization contacts to

both the solar cells and the test structures. The remaining structures were included for possible use in connection with future studies of solar cell

measurement techniques.

Initially, efforts to fabricate the photomasks needed to produce the test

pattern encountered problems due to the requirement that the

computer-controlled pattern generator work both with the very small

microelectronic test structures and with the much larger solar cells. These

problems were surmounted, and satisfactory photomasks and wafers containing

the test pattern were fabricated during this reporting period.

4. 1 Description of the Pattern

The solar cell test pattern, NBS-22, is illustrated in figure 18, and the

solar cell test structures, which are approximately 1 cm on a side, are labeled A, B, C, and D. Surrounding these cells are the arrays of

microelectronic test structures which are 2.44 by 1.60 mm on a side. The

arrays have a step-and-repeat distance of 2.540 mm in both the x- and the

y-directions. In the actual test pattern, no array occurs in the rectangles

shown in the figure which contain either a diagram of the array or an "x."

To fabricate the test pattern, four photomasks are required: level 1 delineates the doped regions; level 2, the oxide contact openings; and level

3, the metal probe pads and metal interconnects. To alter the emitter sheet

resistance systematically, there is a fourth mask, level 4, used to pattern a second doped region within the area of the first. In figures 18 to 21, the

contact openings are shown as cross-hatched areas, while the doped and metal

regions are enclosed by solid lines. Cell A is an unaltered or "perfect"

solar cell. In cell B, there are regions in the vertical fingers where

contact openings are not made in the oxide resulting in no contact to the

cell. As illustrated in figure 19, the "opens" in contact fingers 1 and 3

range in length by doubling from 0.25 to 16 mm, starting from the top; the

pattern is reversed on fingers 5 and 7. Cell C has regular contacts as in

cell A, but has alternate columns of double- and single-diffused regions to

produce alternate columns of low and high sheet resistance in the emitter

region. Both noncontacted regions and areas of low and high sheet resistance

are included in cell D.

The pattern of microelectronic test structures which surround each solar cell

is shown in figure 20. The test structures are designed with probe pads as an integral part of the structure; they are arranged in eight 2 by 10

columns, and the eight columns are separated by 300 ym. The probe pads in

these columns are 80 ym on a side and they are separated by 80 ym. This

20
Figure 18. NBS-22 test pattern which includes four solar cells (A, B, C, and

D) surrounded by arrays of microelectronic test structures. Arrays are present in every one of the larger rectangles not containing a diagram of the

array or an "x".

21
Figure 19. Expanded view of the D solar cell which contains the doubly

diffused and the singly diffused regions of the cell and also the regions

where the metallization does not contact the emitter surface, shown as

the white breaks. The dimensions are given in micrometers.

22
21

Figure

20.

The

microelectronic

test

structure

pattern

which

surrounds

the

solar

cell.

The

structures

are

numbered

and

described

in

table

1.
approach [21] allows automated measurements using one probe card on a wafer

prober. Alternate pairs of double- and single-diffused cross test structures

head the columns. The single-diffused cross structure is used to construct

the last column. In the figure each structure is numbered; table 1 lists

each structure together with additional information on its design and use.

4.2 Sample Preparation and Data Collection

Test pattern NBS-22 was delineated using a contact printer and conventional

photolithography onto two 2-in. (51-mm) diameter p-type silicon wafers, one pattern to a wafer. In an inert atmosphere from a solid source, two

successive phosphorus diffusions at 950°C for 15 min were used to produce the

variation of emitter sheet resistance. The metal interconnects and contact

pads were formed by a layer of aluminum produced by electron-beam

evaporation.

The dc electrical measurements of the test structures were made on the two

wafers using a computer-controlled acquisition system consisting of a wafer

prober, electrical scanners, a current source, and a digital voltmeter.

The results of the measurements of sheet resistances on the two wafers for

the singly diffused region are given in table 2. The location of each

measurement is shown in figure 21. Each table entry is the mean and standard

deviation of the set of four measurements made along the top row in each test

array surrounding cell A. The mean sheet resistance and standard deviation

for the whole cell indicates greater variation over the cell dimensions of

approximately 1 cm than over the test arrays with an approximate dimension of

2 mm. Nominal sheet resistance for the singly diffused region is 51.5 ft/n

while that for the doubly diffused region is 31.5 ft/.

Measurements of the contact-to-emitter resistance were also made. These

measurements served to determine that there exists uniform contact resistance

across the wafer.

5. LASER SCANNER ANCILLARY TECHNIQUE AND APPARATUS DEVELOPMENT

5.1 Improved Ref lected-Light Detection System

By collecting the light reflected from a cell during scanning onto a photodetector, one may present a picture of the cell topography on the scanner display screen. This is useful by itself when one wishes to observe

specimen birefringence (cf sec. 5.2). One may also mix this ref lected-light

signal with the specimen response signal. Mixing in this manner permits cell

response features to be located accurately with respect to topographical

features such as metallization fingers.

The reflected light system previously employed used a mirror inserted into

the microscope illuminator port to deflect the reflected light to an "outboard" focusing lens and a germanium photodiode mounted on a bracket

fastened to the laser scanner frame. One disadvantage of this arrangement

was that it was difficult to shield the ref lected-light system against room

light. This light can be picked up by the lens or by the photodiode, and it

produces a spurious display signal modulated at 120 Hz (twice the power line

24
Table 1 . Test Structure List

Structure

Letter/Number

Test Structure

Name

Specif ication/notes

Description

Parameter Measured Reference

A solar cell no process modification -

B solar cell noncontacted regions -

C solar cell alternate regions of high

and low sheet resistance *-•

D solar cell noncontacted regions and

alternate regions of high

and low sheet resistance

1 van der Pauw

cross resistor

double-doped region sheet resistance of

double-doped region [14]

2 contact resistor metal-to-doped region contact resistance [15]

3 van der Pauw

cross resistor

metal region metal sheet

resistance [14]

4 contact resistor metal-to-substrate contact resistance [15]

5 four-probe

resistor

substrate

i base resistivity

[16]

6 van der Pauw

cross resistor

single-doped region sheet resistance of

the single-doped

region [14]

7 junction diode recombination centers [19]

8 MOS capacitor field oxide over single-doped region

recombination centers,

Si02/Si interface

states [17]

9 substrate contact [15]

10 dc MOSFET dopant

profiler

four-terminal MOSFET substrate dopant densi

ty profile [18]

1 1 mask, alignment

marker p

12 NBS-22 Logo -

25
Table 2. Sheet Resistance + Standard Devia

tion [ohms per square]

Location,

Figure

Wafer #1 Wafer #2

1 51.75 + 0.10 49.52 + 0.36

2 51.47 + 0.02 48.79 + 0.30

3 51.42 + 0.03 48.25 + 0.26

4 51.47 + 0.07 47.93 + 0. 18

5 51.28 + 0.03 47.00 + 0. 17

6 50.89 + 0.08 46.46 + 0. 15

7 50.65 + 0.09 46. 14 + 0.15

8 50.42 + 0.08 45.91 + 0. 15

9 50.36 + 0.09 46. 14 + 0. 17

10 50.49 + 0.07 46.62 + 0.30

11 50.93 + 0.13 47.43 + 0.38

12 51.55 + 0.20 48.48 + 0.47

13 52.43 + 0.21 50.07 + 0.57

14 52.36 + 0.20 50.22 + 0.61

15 52.35 + 0.20 50.24 + 0.63

16 52.32 + 0.20 50.30 + 0.59

Mean = 51.38 + 0.72 48.08 + 1.61

26
Figure 21. Sheet resistance measurement locations around cell A for values

given in table 2.

27
frequency). A second disadvantage is that the microscope illimination port,

because it was occupied by the mirror, was no longer available for other

purposes. The port is actually a cylindrical tube fastened to the microscope

body. It contains slots designed to hold filters for changing the spectrum

of the light used to view specimens when the microscope is used in the

service for which it was designed. One of these slots is needed for the

rotatable analyzer used for specimen birefringence work, but it is not

accessible with a mirror inserted into the illumination port.

A new ref lected-light assembly was constructed to overcome the above

objections. A photograph of the assembly is shown in figure 22. It screws

into the illuminator port and leaves the filter slots unobstructed. The

assembly incorporates a standard 10X microscope ocular, a germanium

photodiode having a 3-mm diameter photosensitive surface, and a photodiode

mounting arrangement which permits the diode to be readily aligned and locked

in position to receive the light reflected from the scanned specimen. The

alignment method is described in section 5.1.1. A few features of the

assembly should be pointed out. The cutout in the barrel is used for

insertion of the ocular lens and the germanium photodiode and it is normally

covered with black tape. The germanium photodiode is used in the

photovoltaic mode with a 10 ki2 terminating resistor, and it is wired directly

to the bnc output connector. The detector socket is mounted on a circuit

board which in turn is fastened to the metal rod which protrudes from the

rear of the case. The rod slides in a sleeve, and the sleeve may be moved

eccentrically with respect to the case axis. During alignment, the rod is

locked to the sleeve by tightening a 6/32 locking screw, and the eccentric

location is fixed by tightening a knurled ring.

5.1.1 Alignment of Ref lected-Light Optical Path

The first step is to align the scanner mechanically so that the scanning

raster for both the visible, 0.633 ym, and near-infrared, 1.15-ym, laser sources coincide. This is done by varying the mechanical adjustments for

both light paths and locking the adjustments after coincidence has been

achieved. A semiconductor specimen responsive to both wavelengths is used

for this purpose. Coincidence of the 0.633- and 1.15-ym scanning rasters

permits the near-infrared reflected light portion of the scanning system to

be aligned automatically when the system is aligned visually with the

0.633-ym laser.

The ref lected-light assembly, i.e., the photodetector and associated focusing

lens, is removed and a reflecting pattern is fastened to the specimen rocking

stage. A satisfactory reflecting pattern is provided by a fine metal-grid

mesh fastened to a small, flat mirror. The particular mesh used in developing this technique was a rectangular pattern with a series of 25-ym wide

bars with a 375-ym separation. The 0.633-ym (visible) laser is selected, and

the scanning mirrors are energized. A white card or a piece of frosted glass

is positioned in the (unfocused) reflected light beam, and the rocking stage

micrometer heads are adjusted so that the amount of light scattered from the

card, observed visually, is maximized. The card is then removed and the

reflected light assembly put back in place. The detector portion of the

assembly is designed so that the detector may be moved axially with respect

to the lens, and eccentrically with respect to the optical axis of

28
<D -P U C 3 <D Cn 3

•H

O I

22.

New

reflectedlight

assembly

using a

germanium

photodiode

visible

on

the

right-hand

com-
the lens. With the detector serving as the input to the scanner display

electronics, the detector is moved eccentrically to place the ref lected-light

image of the grid pattern in the center of the display screen, and axially to

maximize the signal. Both adjustments are then locked and the ref lectedlight system alignment is conplete.

The desired semiconductor specimen can now be fastened to the rocking stage.

Because the incident and ref lected-light paths are both now properly aligned,

the only mechanical adjustments that have to be made to obtain reflectedlight information are the rocking stage micrometer heads. These are adjusted

for a maximum specimen ref lected-light signal using the desired laser source.

5.1.2 Photomultiplier Detector

The improved ref lected-light detection system described in the previous

section still suffers from the rather limited area of the specimen that can

be displayed with the reflected light. The reason for this is that the

system depends on the light reflected from the specimen being intercepted by

the objective of the microscope. When areas larger than about 1 mm on a side are scanned, the angle of incidence of the laser beam on the specimen is such

that no practical lens can intercept the reflected light.

To overcome this limitation, another approach was explored. This approach

uses a partially reflecting mirror under the objective lens to deflect the

reflected light to an appropriate detector. The flexibility in the location

and area of the mirror allows the interception of reflected light from a larger area than before. The exploratory effort described here used a thin

glass microscope slide for the mirror and a photomultiplier, with an S-1

response, to monitor the light reflected from the glass slide as shown in

figure 23. A neutral density filter with a transmission of approximately 1 percent, placed in front of the photomultiplier, was used to reduce light

reaching the cell from the sample. While no effort was made to optimize the

system and to determine scanning area limits for the approach, areas as large

as one centimeter on a side could be displayed in the reflection mode of the scanner. The uniformity of the response of the photomultiplier of its face

was sufficiently uniform that no optical focusing of the reflected light was necessary in this exploratory effort. An example of the response obtained

using this reflective mode to scan the surface of a solar cell A on Test

Pattern NBS-22 is shown in figure 24.

5.2 Specimen Birefringence Microscope Attachment

The general usefulness of the laser scanner for investigating semiconductor

interface phenomena in polycrystalline silicon cells was enhanced by adding a birefringence detection capability to the ref lected-light optical circuit for

the plane-polarized 1.15-ym scanning laser.

The 1.15-nm laser light is weakly absorbed in single crystal silicon at room

temperature. Much of the scanning light can be reflected from the cell backcontact metallization and collected by the reflected light detection and

imaged on the display screen. Of course, light directly reflected from

surface features, such as metallization fingers is also imaged. Additionally,

30
Figure 23. Reflected light configuration with a photomultiplier detector.

The laser spot can be seen on the solar cell, the microscope slide reflector,

and the filter in front of the detector. The location of the spots are indicated with arrows.

31
Figure 24. Raster scan of most of an NBS-22 solar cell using the

configuration of figure 23 in the large area scan mode of figures 25 and 26.

The metallization bus bar is above the area scanned. The lower portion of

the scan includes about three and a half arrays of microelectronic test

structures. The seven peaks in this portion are due to the pairs of MOS

capacitors in each array.

32
light may also be reflected from the semiconductor surface itself, or from

surface laminates, due to optical mismatches. Thus, the ref lected-light

display signal is a composite of these several images.

Birefringence can be used to detect crystal strain [22] which may be present

near regions of material mismatch, e.g., near twin boundaries and near grain

boundaries. Strain may cause bandgap dilation or narrowing, and this in turn

can influence the minority carrier recombination properties at the interface.

Comparison of cell photoresponse scanning measurements with birefringence

measurements on a spatial point -by-point basis may help elucidate the interface processes acting to reduce cell conversion efficiencies. This expectation was the motivation for adding the birefringence capability for 1.15-ym

scanning.

The light emitted by the 1.15-ym scanning laser is plane-polarized. Detection of bulk birefringence is performed by first extinguishing, through the

use of a 1.15-ym analyzer, the light reflected to the photodetector from a scanned flat metal mirror target. This places the axis of the analyzer

perpendicular to the plane of polarization of the laser light. The target is

then replaced by the cell to be investigated. The cell is (in principle)

rotated in the scanning plane, and any resultant intensity variations in the

display screen indicate the location of strain-induced birefringence in the

cell. It is impractical to rotate solar cell specimens as they are connected

for photoresponse measurements made in conjunction with birefringence ones,

and so the plane of polarization of the 1.15-ym light is rotated instead.

This is accomplished by rotating a half-wave plate in the laser beam; the

well-known result of rotating a half-wave plate through an angle of 0 in one direction is to cause the plane of polarization of the emerging light to

rotate an angle 20 in the opposite direction.

The scanner microscope has an opening for inserting filters, polarizers, etc.

in its body between the ocular and specimen (objective) lenses. A twoaperture, two-position holder was made to slide within this opening. In one position, the optical path is unobstructed. The other position contains a 1.15-ym half-wave plate which can be rotated 360 deg by an external control

to rotate the plane of polarization of 1.15-ym scanning light passing through

the microscope and incident on the specimen. The analyzer for the reflected

light path is in the form of a circular disk fitted with a lever finger, and

this fits into a filter-holder slot in the illumination port between the

microscope body and the reflected light assembly (see sec. 5.1).

The birefringence apparatus was completed, and a preliminary test was performed during this reporting period. The scanned target was a mirror

partially covered with a thin, transparent, plastic sheet. By rotating the

half-wave plate in its holder and finding a corresponding orientation of the

ref lected-light analyzer, it was possible to extinguish the signal from the

mirror-only portion while retaining that from the plastic-covered portion,

and vice versa . Intermediate rotation angles permitted hybrid combinations

of the extinguishing-enhancing behavior to be seen, confirming that the

apparatus was working as designed.

33
5.3 A Light Source for Biasing Large-Diameter Cells

A new optical system was developed to replace the fiber optic system when

light-biasing large-diameter cells during laser scanning. The fiber optic

system uses four fiber-optic bundles to transmit the light from a single

quartz-halogen lamp to specimens having dimensions of 2 cm by 2 cm or less. This system is convenient when high power, short working distance

lenses are used. This is because the fiber optic ends can be inserted within

the limited space between the edge of the objective lens and its scanned

specimen, thus placing the light source close to the scanned cell. With

cells having diameters larger than about 2 cm, it is not possible to

illuminate the cell uniformly using this system. For these larger cells, the

entire microscope optics with the exception of the eyepiece, is replaced by a 55-mm focal length single-lens reflex-camera lens ( cf sec. 5.4) which yields

a 200-mm lens-to-specimen working distance. With the larger working

distance, shadowing of the bias light by the scanner optics is not a problem,

and the fiber optic light-biasing arrangement can be replaced by a simpler

one which illuminates the cell from a longer distance.

A satisfactory illumination system was developed using a pair of 12-V

oblong-shaped quartz lamps. These are sold as automobile "passing lights"

and are available from most automobile supply houses. Several minor

modifications were made to make them suitable for their new service. For

their intended application, the lamps are designed to produce parallel light.

To achieve this objective, they are assembled with the quartz-halogen bulb in

front of, and at the focal point of, a curved metal reflector. A metal

shield is clipped to the front of the bulb to ensure that all the light

emerging from the lamp is first incident on the reflector. The scanning of

large-diameter cells calls for a different configuration. The beam of light

from a lamp needs to be about 50 mm in diameter when the lamp is about 300 mm

from the scanned cell. This requirement was satisfied by increasing the

distance between the bulb and the reflector. This was done by removing the

lamp sockets which previously protruded behind the reflector and remounting

them so that they also served as spacers between the bulbs and the

reflectors. When operated at their new focal length, it was observed that

the bulb shields were no longer needed. In fact, their presence caused a decrease in light intensity at the center of the light spot on the specimen.

The shields were removed to enhance light uniformity. Additional uniformity

of the light with the 50-mm diameter spot was achieved by rotating the bulb

sockets 90 deg to place the axis of the bulb filaments parallel to the minor

axes of the reflectors.

In addition, the lamps were clamped to the scanner frame to eliminate motion

of the lamp relative to the cell specimen due to, e.g. , building vibrations

which can cause undesired modulation of the bias light. This in turn can mask the response display of the specimen to the (weaker) laser scanning

light. By fastening the lamps to the scanner frame, the lamps and specimen

tend to move in unison minimizing the undesired light-bias modulation.

Measurements made using a pair of lamps converted in the manner described

yielded short-circuit currents in 5-cm diameter single-crystal silicon cells

equivalent to about 3 suns insolation.

34
5.4 Optical Parameters of Various Lens Combinations

Two sets of optical specifications for the NBS laser scanner using different

lens combinations are given in tables 3 and 4. The parameters of interest

shown in table 3 indicate distances between the objective lens of the system

and the sample, and the corresponding total scan excursions possible at this

distance. The Zeiss standard research microscope is used with an intermediate body attachment installed between the top of the microscope limb

and the body tube holding the eyepiece, which would normally be attached to

the top of the limb. The intermediate body holds the polarized light

analyzer used in the reflected light mode. The extra distance introduced

requires corrector lenses in both the top and bottom of the attachment to

maintain the same optics as with the body tube alone. However, if the bottom

lens is removed, the spatial resolution of the system is increased, although

at the expense of greater working distance. This effect is shown in table 4

for the normal configuration (both lenses in place), and for the modified

configuration (bottom lens removed). In all cases, the eyepiece is a 10X

ocular lens.

For the movie projector zoom lenses, values are given for the two extremes of

the adjustable focal lengths. In the last case listed, the microscope body

is omitted, and the 35-mm single-lens reflex camera lens is installed to

provide a wider area of scan. More detailed illustrations of this system are given in figures 25 and 26.

Table 4 extends the optical data for some of the combinations of table 3 and

adds five others. The first four columns give the scanner resolution

measured with a standard calibrated grid. The resolution, a measure of

effective laser spot size, is obtained from the relative intensity of

transmitted light through the mask between opaque grid lines for various grid

spacings. The resolution stated in micrometers is the width of the openings

between grid lines of the calibrated grid which provides a value of the

modulation percentage which falls within the range of about 20 to 40 percent.

The modulation percentage is defined as 100 times the ratio x/y where x is

the peak-to-peak variation in the transmitted light intensity as the laser

spot is scanned across the grid, and y is the unobstructed intensity of the

laser spot. At an extreme, 100-percent modulation corresponds to all the

laser spot being transmitted between the opaque grid lines. A more detailed

description of the measurement is given in elsewhere [23].

Columns 5 and 6 of table 4 give, in arbitrary units, the relative amount of

light reflected from the objective lenses themselves. The measurements were

made with a black piece of paper at the sample position and the reflected

light recorded by the system shown in figure 25. This illustrates an important consideration: in the ref lected-light mode, light reflected from

the surfaces of the optical elements themselves introduces an interference

into the solar cell scanning measurements. A value of 50 or less in this

table gives satisfactory performance; however, this generic problem suggests

that mirrors may be preferred as optical elements wherever possible. It is seen that the problem is worse for the infrared light than for the visible

beam.

35
Table

3.

NBS

Laser

Scanner

Specimen

to

Objective-Lens

Working

Distance

and

Scan

Excursion

for

the

0.633-jjm

Laser

Wavelength

Tabulated

Using a

10X

Ocular

Lens, a

Total

Optical

Scanning

Angle

of

20

Deg

at

the

Ocular

co

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39
5.5 Variable Specimen Temperature

One of the project objectives is to develop techniques which, when used in

conjunction with the laser scanner and other measurement methods, can be used

to separate the several phenomena which may be operating in concert at grain

boundaries to reduce the conversion efficiency of polycrystalline silicon

solar cells. A classic method for analyzing the physical behavior of

experimental specimens is to perform appropriate measurements as a function

of specimen temperature. Toward this objective, solar cell heating and

cooling stages were constructed to fit between the scanned specimen and the

specimen translation stage of the microscope

5.5.1 Heating Stage

The heater stage has a temperature range from room temperature to about

160°C, that is, to slightly below the eutectic temperature (183°C) of the

lead-tin solder commonly used for cell contacts. The stage was built around

a commercially available heater element sold as a substrate heater for a thermocompression bonder, which is designed to be energized from a dc source and maintained at the desired temperature by an on-off controller. The

heater element is rectangular and measures 3.75 cm by 6.25 cm and 2.5 cm

thick. To fit it for scanner service, it was only necessary to thermally

insulate the heater from the specimen translation stage. This was done by

bonding, with silicone rubber, a 2.5-cm thick block of plastic foam to the

base of the heater. Temperatures are readily set and maintained to +0.5°C

over the desired range. No problems were encountered using the 0.633-ym

source. Some controller noise, due to on-off heater current transients, was observed on the scanner display screen when the weaker 1.15-ym cell

photosignals were displayed. This annoyance may perhaps be eliminated by

changing the controller to a zero-crossing type.

5.5.2 Cooling Stage, Thermoelectric Type

A stage for cooling specimens over the range from room temperature to -20 °C was constructed using a flat, single-stage themoelectric element having the

dimensions 3.75 cm by 3.75 cm and 1 cm thick. The thermoelectric element is

energized by an adjustable, low-voltage, direct-current source and cools by

pumping heat from the top surface on which the specimen is mounted to the

bottom surface. The specifications for the element place an upper limit on the temperature of the bottom surface. In order to achieve the desired

cooling range without exceeding the rated bottom surface temperature, one must place this surface in thermal contact with a heat sink. A water-cooled

copper block was constructed to serve this purpose. Adequate thermal contact

between the thermoelectric element and the copper block is achieved by

lightly coating the lower surface of the element with silicone "heat sink"

grease and bolting the two units together. This assembly is set into the

cut-out portion of a plastic-foam disk 9.50 cm in diameter and 3.1 cm thick.

The disk and a specimen cover prevent moisture in the room air from

condensing on the scanned specimen; the small-scale effect of condensation

(which occurs at temperatures at and below the dew point) is to coat the

specimen with a multitude of tiny water "lenses" which degrades the optical

scanning resolution. The specimen cover is a clear plastic dish through

which dry nitrogen is flowed to eliminate this problem.

40
The cooling stage worked satisfactorily and was used to cool specimens

between room temperature and about -2°C without dry nitrogen flow while they

were laser scanned using both 0.633- and 1.15-ym wavelengths.

5.5.3 Cooling Stage, Compressor Type

At the suggestion of Dr. R. A. Forman of the Electron Devices Division, a means for cooling specimens over the range of from room temperature to about

-60°C was constructed using a commercially available refrigerator compressor.

The compressor is hermetically sealed to a stainless steel flexible tube with

an outer diameter (OD) of about 1.0 cm. This tube is coaxial to allow the

liquid refrigerant to be pumped through the center capillary to the tip of

the tube where its refrigerant expands to a gas. The expanding gas absorbs

best from the tube, cooling it, as the gas returns to the compressor through

the outer portion of the coaxial line. The flexible tube is coiled into a 3.7-cm deep circular channel cut into the bottom of a solid cylindrical

aluminum block which is 4.7 cm high and 10.2 cm in diameter. The OD of the

channel is 8. 8 cm and the width of the channel is just large enough to

accommodate the flexible tube. A groove, cut into the bottom edge of the

wall formed by the channel, allows the extension of the flexible tube to exit

from the aluminum block. A circular aluminum plate, 0.35 cm thick, is

screwed to the bottom of the block to contain the coiled tube. The volume

remaining in the channel that is not occupied by the tube is filled with

silicone mixed with copper powder to enhance the thermal conductance between

the tube and the block.

The top of the block is provided with screw holes for use in bolting a specimen holder to the block. Also on the top of the block is a slightly

flattened 0.5-cm OD copper tube. This tube is sealed at one end and is bent

and fastened in such a way that it circles the area on the block for the

specimen holder and has the open end extend out from the side of the block.

Distributed on the top of the circular portion of the tube are small diameter

holes. The purpose of this tube is to conduct dry nitrogen into the specimen

chamber which is formed by placing a plastic cover over the top of the

aluminum block. To thermally insulate the chamber, the plastic cover is

fitted with plastic foam. The cover contact to the block is by way of the

plastic foam which is such that it allows the nitrogen introduced into the

chamber to escape. The cover is also fitted with a clear plastic window

which is situated centrally to allow for the entrance of the laser beam from

the scanner. The purpose of the dry nitrogen is to prevent frosting inside

the specimen chamber. A similar approach is used to prevent frosting on the

top side of the window. A thin-walled tube, 0.3 cm OD, was bent and fastened

so that it circles the top side of the specimen window. Holes in the tube,

0.15 cm in diameter, face to the center so as to direct the dry nitrogen over

the face of the window. The tube is, in turn, covered by an inverted plastic

cup 0.7 cm high and 6.6 cm in diameter which has a circular hole cut to allow

for the escape of the nitrogen. The hole is of such a size that its diameter

is somewhat larger than the window but smaller than the diameter of the

circle that the tube forms. This is to help guide the dry nitrogen over the

window.

With the aluminum block encased in plastic foam material and with a low-pressure dry nitrogen source for each of the above described tubes, the

41
Figure 27. Photograph of the compressor type cooling stage (top) and cover

(bottom)

42
cooling stage was able to cool specimens to as low as -60 °C with no frosting

or condensation on the optical parts or the specimen. A photograph of the

cooling stage without the insulation around the aluminum block is shown in

the upper part of figure 27 and the top cover is shown in the lower part.

6. WORKSHOPS

6.1 Stability of (Thin Film) Solar Cells and Materials

The proceedings of the Workshop on Stability of (Thin Film) Solar Cells and

Materials, held May 1-3 1978 at NBS, were published (1], as was a companion

document which summarized the conclusions of this workshop and their

implications [2]. The orientation of the workshop and the topics addressed

were discussed in the previous annual report [24].

6.2 Photovoltaic Material and Device Measurements

The Photovoltaic Material and Device Measurements Workshop (with focus on polycrystalline thin film cells) was held June 11-13, 1979 in Arlington,

Virginia with 210 registrants. The workshop was conducted for the Department

of Energy by the Photovoltaics Program Office of SERI in association with the

Electron Devices Division of NBS. The general purpose of the workshop was to

accelerate the development of thin film cells by improving the versatility

and reliability of material and device measurement techniques. No workshop

proceedings were issued as such, but contributions from presenters at the

workshop have been the subject of two special issues of the journal. Solar

Cells [25]. The preface to these issues, by D. E. Sawyer as Guest Editor,

and their tables of contents, are provided in Appendix B.

7. CONSULTATION AND LIAISON ACTIVITIES

7.1 Consultations to SERI

At the suggestion of the SERI PVPO, D. E. Sawyer attended the December 18,

1978 Photovoltaics Session of the Workshop on Reliability of Materials for

Solar Energy at Denver, Colorado and participated in the session discussions.

The workshop was organized by the Materials Branch of SERI's Research

Division.

D. E. Sawyer evaluated for the PVPO four measurements -related proposals which

they received under their Innovative Concepts Solicitation, and one unsolicited proposal.

A number of activities were engaged in by D. E. Sawyer and H. K. Kessler to

aid the PVPO in duplicating the NBS laser scanner for routine evaluation of

cell specimens provided the PVPO by PVPO contractors. These activities

include sending the PVPO detailed information on the NBS scanner parts and

appropriate NBS publications and copies of the scanning resolution tables

(table 4 and 5). Reviews and discussions of the theory, construction, and

use of the scanner were held during visits of D. E. Sawyer to the PVPO and

during visits of SERI PVPO personnel to NBS. The latter visits are listed

below

43
April 1979 -- Mr. Richard Matson of the Solar Energy Research Institute

(SERI) (Golden, CO) visited D. E. Sawyer and H. K. Kessler to

discuss the Photovoltaic Program Offices (PVPO) project to set

up a laser scanner at SERI similar to the one at NBS.

May 1979 - Mr. Phil Pierce of SERI (Golden, CO) visited D. E. Sawyer to

discuss the photovoltaic program conducted at NBS for SERI.

June 1979 - Dr. Robert Lewis of PVPO, SERI (Golden CO) visited D. E. Sawyer

to discuss applications of laser scanner to SERI's needs.

June 1979 - Mr. Alan Postlethwaite, program manager in the DOE Photovoltaic

Office (Washington, DC), visited D. E. Sawyer and H. K. Kessler

to see the laser scanner and discuss its use in photovoltaic

work.

July 1979 — Mr. Ted Cannon and Mr. Clyde Carter of SERI (Golden, CO) visited

D. E. Sawyer, H. K. Kessler, and H. A. Schafft to see the laser scanner in operation and to familiarize themselves with the

apparatus.

7.2 Visits from Industry and Universities

to the National Bureau of Standards

The following visits and consultations were conducted with the authors:

Dec. 1978 - Drs. George Storti and John Wohlgemuth of Solarex (Rockville,

Maryland) visited D. E. Sawyer and H. K. Kessler to conduct

laser scanning on experimental polycrystalline and vertical

junction solar cells being developed under DoE sponsorship.

Jan. 1979 - Professor Wayne Anderson, SUNY (Buffalo, New York), and Alan E.

Delahoy, a graduate student at Rutgers University (New

Brunswick, New Jersey), visited to scan experimental solar

cells.

Jan. 1979 — Walter Manson of Texas Instruments (Dallas, Texas) visited D. E. Sawyer and H. K. Kessler and performed measurements on experimental tandem junction solar cells.

Feb. 1979 - George Storti and Scott Johnson of Solarex visited for further

discussions on laser scanning of solar cells being developed

under DoE/SERI sponsorship.

Feb. 1979 - Brian Plunkett and Patrick Lasswell of the Institute for Energy

Conversion, University of Delaware (Wilmington, Delaware),

worked with Sawyer and Kessler on techniques for laser scanning

exploratory CdS/C^S and CdfZnJS/C^S solar cells being

developed under SERI sponsorship.

Feb. 1979 - Masashi Nagasi, VLSI Technology Research Association Cooperative

Labs, visited to discuss the laser scanner.

44
Mar. 1979 — Brian Plunkett and Patrick Lasswell, Institute for Energy

Conversion, visited NBS to study equipment and techniques for

laser scanning exploratory CdS/Cu2 S and Zn2 P

2 solar cells

and test specimens being developed under SERI sponsorship.

Mar. 1979 — Mr. William Morris, staff engineer at the Electronics

Laboratory, General Electric Co. (Syracuse, New York), visited

D. E. Sawyer, D. W. Berning, and H. K. Kessler. He is compiling

a failure-analysis handbook for the Rome Air Development

Command, New York and expressed considerable interest in the

scanning techniques developed for this application and also for

device design for determining processing-related defects.

Apr. 1979 — Dr. Douglas Yates of Mobil Tyco Solar Energy Corporation

(Waltham, Massachusetts) visited NBS to make laser scanner measurements on EFG samples.

Apr. 1979 — David Lischnow of Bell Laboratories (Allentown, Pennsylvania)

visited D. W. Berning. Mr. Berning assisted Mr. Lischnow in

finding inverted regions in transistors they had made by using

the laser scanner.

June 1979 — Dr. Gary Turner of ARCO-Solar (Chatsworth, California) visited

Dr. Sawyer to study the laser scanner prior to constructing his own and to exchange cell measurement techniques.

Aug. and Dr. Douglas Yates of Mobil Tyco made two additional visits to

Nov. 1979 — conduct further laser scanner studies on silicon EFG solar cells

manufactured by his company.
REFERENCES

1. Sawyer, D. E., and Schafft, H. A., Eds., Semiconductor Measurement

Technology: NBS/DOE Workshop, Stability of (Thin Film) Solar Cells and

Materials, NBS Spec. Publ. 400-58 (August 1979).

2. Schafft, H. A., and Sawyer, D. E. , A Perspective of a Workshop on Stability of (Thin Film) Solar Cells and Materials, NBSIR 79-1778 (July

1979).

3. Convers Wyeth, N. , and Catalano, A., Spectral Response Measurements of

Minority-Carrier Diffusion Length in Zn3P2

» J. Appl. Phys. 50,

1403-1407 (1979).

4. Catalano, A., private communication.

5. Carbajal, B. G. , High Efficiency Cell Development, DOE/JPL/954881-5,

Final Technical Progress Report (1978).

6. Lehovec, K. , and Fedotowsky, A., Scanning Light Spot Analysis of Faulty

Solar Cells, Solid-State Electronics Ti_, 565-576 (June 1980).

7. Sawyer, D. E., Kessler, H., and Schafft, H. A., Measurement Techniques

for Solar Cells, Quarterly Report: April 1 to June 30, 1978, NBSIR

79-1909 (November 1979), pp. 12-18.

8. Yates, D. A., private communication.

9. Lanyon, H. P. D. , Bandgap Narrowing in Moderately to Heavily Doped

Silicon, IEEE Trans. Electron Devices ED-26 , 1014-1018 (1979).

10. Mahan, G. D. , Energy Gap in Si and Ge: Impurity Dependence, J. Appl . Phys. 51^, 2634-2646 (1980).

11. Lowney, J. R. , Kahn, A. H. , Blue, J. L. , and Wilson, C. L. , Disappearance of Impurity Levels in Si and Ge Due to Screening, to be published

in J. Appl. Phys

12. Turner, G. W. , Fan, J. C. C. , and Salerno, J. P. , Infrared Electroluminescence as a Diagnostic Tool for Polycrystalline GaAs Solar Cells,

Solar Cells 2 (3), 261-262 (May 1980)

13. Lehovec, K. , and Fedotowsky, A., Solar Cell Mathematical Analysis and

Computer Simulation, NBS-GCR-285 (February 1981).

14. Buehler, M. G. , Grant, S. D. , and Thurber, W. R. , Bridge and van der

Pauw Sheet Resistors for Characterizing the Line Width of Conducting

Layers, J. Electrochem. Soc. 125 , 650-654 (1978).

15. Buehler, M. G. , Semiconductor Measurement Technology: Microelectronic

Test Pattern NBS-3 for Evaluating the Resistivity-Dopant Density

Relationship of Silicon, NBS Spec. Publ. 400-22 (May 1976), p. 13.

46
16 . Buehler, M. G. , and Thurber, W. R. , A Planar Four-Probe Test Structure

for Measuring Bulk Resistivity, IEEE Trans, Electron Devices ED-23 , 968-974 (1976).

17. Sze, S. M., Physics of Semiconductor Devices, pp. 444-467 (John Wiley & Sons, New York, 1969).

18. Buehler, M. G. , The D-C MOSFET Dopant Profile Method, J, Electrochem.

Soc, 127 , 701-704 (1980).

19. Sah, C. T., Detection of Recombination Centers in Solar Cells from

Junction Capacitance Transients, IEEE Trans, Electron Devices ED-24 , 410-419 (1977).

20. Russell, T. J., Maxwell, D. B. , Reimann, C. T. , and Buehler, M. G. , A

Microelectronic Test Pattern for Measuring Uniformity of an Integrated

Circuit Fabrication Technology, Solid State Technology 22_ (2), 71-74

(1979).

21. Buehler, M. G. , Comprehensive Test Patterns with Modular Test

Structures: The 2 by N Probe-Pad Array Approach, Solid State Technology

22 (10), 89 (1979).

22. Jessop, H. T., and Harris, F. C. , Photoelasticity, Principles and

Methods (Dover Publications, New York, 1950).

23. Sawyer, D. E. , and Berning, D. w. , Semiconductor Measurement Technology:

A Laser Scanner for Semiconductor Devices, NBS Spec. Publ. 400-24

(February 1977).

24. Sawyer, D. E. , Kessler, H. K. , and Schafft, H. A., Measurement

Techniques for Solar Cells: Annual Report for the Period September 15,

1977 to December 14, 1978, NBSIR 80-2027 (July 1980).

25. Solar Cells 1 (2) (February 1980) and 2_ (3) (May 1980).

47
APPENDIX A

Associated Publications

A . 1 Publications

Schafft, H. A., and Sawyer, D. E. , A Perspective of a Workshop on Stability of (Thin Film) Solar Cells and Materials, NBSIR 79-1778 (July

1979).

Sawyer, D. E. , and Schafft, H. A., Eds., Semiconductor Measurement

Technology: NBS/DoE Workshop, Stability of (Thin Film) Solar Cells and

Materials, NBS Spec. Publ. 400-58 (August 1979).

Sawyer, D. E. , Kessler, H. K. , and Schafft, H. A., Measurement

Techniques for Solar Cells, Quarterly Report, April 1 to June 30, 1978,

NBSIR 79-1909 (October 1979).

Sawyer, D. E. , Kessler, H. K. , and Schafft, H. A., Measurement

Technologies for Solar Cells, Annual Report, September 15, 1977 to

December 14, 1978, NBSIR 80-2027 (July 1980).

Lehovec, K. , and Fedotowsky, A., Scanning Light Spot Analysis of Faulty

Solar Cells, Solid-State Electronics 23, 565-576 (June 1980).

A. 2 Patent Applications

Sawyer, D. E. , Nondamaging Technique for Locating Semiconductor Device

Defects and Measuring Device Resistances, United States Patent and

Trademark Office (filed May 25, 1979).

48
APPENDIX B

Preface to

So lav Cells

Volume 1, February 1980

The papers in this special issue were selected by peer review from those

submitted following their presentation at the first meeting of the

Photovoltaic Material and Device Measurements Workshop held in Arlington,

Virginia, U.S.A., June 11-13, 1979. The general purpose of these workshops,

conducted for the U.S. Department of Energy by the Photovoltaics Program

Office of the Solar Energy Research Institute, Golden, Colorado, is to

accelerate the development of thin film solar cells by improving the

versatility and reliability of terrestrial solar cell material and device

measurement techniques. This first workshop, conducted in association with

the U.S. National Bureau of Standards, Washington, DC, focused on polycrystalline materials and the associated thin film cells. Workshops to

examine other photovoltaic materials and/or technologies will be held in the

future.

The subjects included in the first "measurements" workshop, and thus the

topics treated in this special issue, were selected based on questionnaires

distributed at the Department of Energy's Photovoltaic R&D Review Meeting

held on October 24-26, 1978, Vail, Colorado. The attendees, who represented

the forefront workers in the U.S. thin film photovoltaic community, were asked for their measurement needs and concerns, and the workshop was constructed accordingly. Because of this genesis, photovoltaic researchers

throughout the world should find the papers which follow to be timely and of

great interest.

David E. Sawyer, Guest Editor

Electron Devices Division

National Bureau of Standards

Washington, DC 20234

49
SOLAR CELLS, Vol. 1, No. 2 February 1980

CONTENTS

Preface 113

Perspective on photovoltaic material and device measurements .... 115

A. Rothwarf (Newark, Del., U.S.A.)

Measuring trace elements in semiconductors: methods and pitfalls . . 117

R. M. Lindstrom (Washington, D.C., U.S.A.)

Electron-beam-induced current characterization of polycrystalline

silicon solar cells 123

J. I. Hanoka (Waltham, Mass., U.S.A.)

Backscattering and transmission electron microscopy studies of

layered structures 141

J. W. Mayer (Pasadena, Calif., U.S.A.)

New techniques for the study and control of grain boundary

effects 145

R. F. Wood, R. T. Young, R. D. Westbrook, J. Narayan,

W. H. Cristie, and J. W. Cleland (Oak Ridge, Tenn. , U.S.A.)

Grain size and its influence on efficiency in polycrystalline GaAs

solar cells 149

A. E. Blakeslee and S. M. Vernon (Yorktown Heights,

N.Y., U.S.A.)

Characterization of EFG silicon ribbons by ion beam techniques . . . 153

M. Hage-Ali, R. Stuck, M. Toulemonde, and P. Siffert (Strasbourg,

France

)

Structural and chemical characterization of photovoltaic materials

and devices 158

J. D. Meakin (Newark, Del., U.S.A.)

The Znx

Cd-|_ xS/Cu2 S hetero junction: review and recent

measurements 159

L. C. Burton (Blacksburg, Va., U.S.A.)

Scanning light spot analysis of faulty solar cells 175

K. Lehovec and A. Fedotowsky (Los Angeles, Calif., U.S.A)

Correlation of grain boundary electrical properties with grain

boundary impurities in multigrained silicon using surface analytical

techniques 178

L. L. Kazmerski and P. J. Ireland (Golden, Colo., U.S.A.)

* Extended abstract.

50
Etching of CdS films 183

F. A. Shirland (Pittsburgh, Pa., U.S.A.)

Effect of substrate orientation on the properties of the Si/PbS

hetero junction 199

H. Elabd, A. J. Steckl, and W. Vidinski (Troy, N.Y., U.S.A.)

*Photoconductivity as a probe of polycrystalline films ........ 209

R. H. Bube (Stanford, Calif., U.S.A.)

*Optical properties of polycrystalline semiconductor films 213

A. H. Clark (Orono, Me., U.S.A.)

^Electronic transport and optical properties of dislocations in

deformed compound semiconductors 216

C. Elbaum (Providence, R.I., U.S.A.)

*Electron diffusion lengths in the CdS/Cux

S cell from spectral

response measurements 218

C. Moses (Canton, N.Y., U.S.A.) and D. Wasserman (Ithaca,

N.Y., U.S.A.)

^Effective diffusion length in polycrystalline semiconductor thin

films 222

T. L. Chu, E. D. Stokes, and S. S. Chu (Dallas, Tex., U.S.A.)

The design and utilization of microprocessor-controlled absolute

spectral response system 225

L. M. Kilgren, N. C. Wyeth, and W. E. Devaney (Newark,

Del., U.S.A.)

*Eiectron-beam-induced current and scanning light spot techniques

for investigating the response of polycrystalline solar cells .... 233

N. Inoue, S. M. Goodnick, and C. W. Wilmsen (Fort Collins,

Colo. , U.S.A.

51
SOLAR CELLS, Vol. 1, No. 3 May 1980

i CONTENTS

Some investigations on the influence of defects/grain boundaries on photovoltaic mechanisms in polycrystalline silicon films 237

B. L. Sopori and A. Baghdadi (Phoenix, Ariz., U.S.A.)

Scanned laser response studies of metal-insulator-silicon solar cells

in polycrystalline Czochralski silicon 251

J. R. Szedon, T. A. Temofonte, and T. W. O'Keeffe (Pittsburgh,

Pa., U.S.A.)

Infrared electroluminescence as a diagnostic tool for

polycrystalline GaAs solar cells 261

G. W. Turner, J. C. C. Fan, and J. P. Salerno (Lexington, Mass.,

U.S.A.

)

Study of grain boundaries in GaAs by scanning light microscopy . . . 263

R. M. Fletcher, D. K. Wagner, and J. M. Ballantyne (Ithaca,

N.Y., U.S.A.)

Diffusion length measurements in C^S 271

W. J. Biter and T. W. O'Keeffe (Pittsburgh, Pa., U.S.A.)

Ellipsometry of thin silicon dioxide films on rough polycrystalline

silicon surfaces 272

T. D. Burleigh, S. Wagner, and T. F. Ciszek (Golden, Colo.,

U. S.A.

)

Evaluation of silicon-on-ceramic using light-beam-induced

currents 273

J. D. Zook (Bloomington, Minn., U.S.A.)

Experimental determination of the photon economy in polycrystalline

thin film photovoltaic materials and devices 275

J. A. Bragagnolo and E. A. Fagen (Newark, Del., U.S.A.)

Grain boundary defects in thin semicrystalline material 285

Z. C. Putney and W. F. Regnault (Gaithersburg, Md. , U.S.A.)

Barrier heights and passivation of grain boundaries in

polycrystalline silicon 293

C. H. Seager and D. S. Ginley (Albuquerque, N.M., U.S.A.)

Electrical transport properties in inhomogeneous media 297

R. Landauer (Yorktown Heights, N.Y. , U.S.A.)

Grain boundary resistance measurements in polycrystalline GaAs . . . 298

M. J. Cohen, J. S. Harris, Jr., and J. R. Waldrop (Thousand Oaks

Calif., U.S.A.)

* Extended abstract.

52
Physical models for recombination currents in polycrystalline

silicon p-n junction solar cells 302

J. G. Fossum, A. Neugroschel, F. A. Lindholm, and J. A. Mazer

Gainesville, Fla., U.S.A.)

Effect of interface recombination at p-n junction perimeters on photoluminescence and current 304

C. H. Henry (Murray Hill, N.J., U.S.A.)

Grain boundary effects and conduction mechanism studies in chromium

metal-insulator-silicon solar cells on polycrystalline silicon . . . 305

W. A. Anderson, K. Rajkanan (Buffalo, N.Y., U.S.A.), A. E.

Delahoy, and S. L. Hyland (Piscataway, N.J., U.S.A.)

Study of spatial inhomogeneities in photosensitive materials

using direct current and microwave techniques 311

P. Herczfeld, L. Hanlon, and J. Wargin (Philadelphia, Pa.,

U.S.A.

Noise spectral density as a device reliability estimator 315

J. DuBow and C. Osterwald (Fort Collins, Colo., U.S.A.)

Capacitance as a tool for investigating thin film CdS/Cud2 S

hetero junctions 321

W. J. Mant’ney and N. C. Wyeth (Newark, Del., U.S.A.)

Measurement of the resistivity of thin CdS films on brass

substrates 323

S. Hogan, S. Wagner (Golden, Colo., U.S.A.), and F. Barnes

(Boulder, Colo., U.S.A.)

Measurement techniques in thin film polycrystalline materials and

devices (solar cells) 327

H. K. Charles, Jr., R. J. King, and A. P. Ariotedjo (McLean,

Va., U.S.A.)

53
NBS-I14A (rev. 2 -ac) U.S. DEPT. OF COMM. 1 BIBLIOGRAPHIC DATA

SHEET (See instructions)

4. TITLE AND SUBTITLE

. PUBLICATION OR 2 REPORT NO.

NBSIR 80-2181

. Performing Organ. Report NoJ 3, Publication Date January 1981

Measurement Techniques for Solar Cells, Annual Report

December 15, 1978 to December 14, 1979

5. AUTHOR(S)

D. E. Sawyer, H. K. Kessler, T. J. Russell, W. F. Lankford, and H. A. Schafft

6. PERFORMING ORGANIZATION (If joint or other than NBS, see instructions) 7. Con traci/Grant No. NATIONAL BUREAU OF STANDARDS Task Order A054SE

DEPARTMENT OF COMMERCE 8. Type of Report & Period Covered

WASHINGTON, D.C. 20234 Final, Dec. 15, 1978 to

Dec. 14, 1979

9. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street. City. State, ZIP)

Solar Energy Research Institute

1617 Cole Boulevard

Golden, CO 80401

10. SUPPLEMENTARY NOTES

Document describes a computer program; SF-185, FIPS Software Summary, is attached.

11. ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant

bibliography or literature survey, mention it here)

The NBS-developed laser scanner was used to examine a variety of devices: Cu2S/CdS

cells, silicon tandem junction cells, Zn3P2 Schottky diode specimens, and edge-fed

growth polycrystalline silicon cells. The results show that it is possible to detect

cell design- and processing-induced losses in conversion efficiency, areas of missing

antireflection coating, lack of ohmic contact of the metallization to the cell, breaks

in cell metallization fingers, fine cracks, scratches, and silicon carbide inclusions.

Observation of photoresponse caused by point shunts in the junction and cell cracks is

consistent with modeling work. A solar cell test pattern is described and the results

of cell sheet resistance measurements are discussed. The test pattern includes four

solar cells (three with intentional defects). It also includes microelectronic test

structures to determine certain material and design characteristics of these cells.

The test pattern serves as a vehicle for linking theoretical and experimental results

and placing the scanner technique on a firm theoretical base. Modifications to the

laser scanner system are described including an improved ref lected-light detection

system, a birefringence capability, temperature-controlled stages for heating (to

160°C) and cooling (to -60°), and a new light source for biasing large-diameter cells.

Support to the Photovoltaic Program Office of the Solar Energy Research Institute

through the conduct of workshops, consultations, and liaison activities is described.

12. KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names; and separate key words by semicolons)

Defect identification; laser scanner; measurement techniques; photovoltaics ; solar

cells; test pattern.

13. AVAILABILITY 14. NO. OF

PRINTED PAGES

[2^5 Unl imited

[ J For Official Distribution. Do Not Release to NTIS

J Order From Superintendent of Documents, U.S. Government Printing Office, Washington, D.C.

20402.

58

15. Price

] Order From National Technical Information Service (NTIS), Springfield, VA. 22161

USCOMM-DC 8043-P80

i