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NBSIR 78-1513

Measurement Techniques for

Solar Cells, Quarterly Report:

January 1 to March 31, 1978

D. E. aawyer, H. K. Kessler,

and H A. Schafft

Electron Devices Division

Center for Electronics and Electrical Engineering

National Bureau of Standards

Washington, D.C. 20234

September 1978

Issued 1978

Prepared for the Department of Energy

Division of Solar Technology

\dvanced Materials R&D Branch

Jnder Task Order A054-SE of Interagency

Agreement EX-77-A-01 -6010

100 t

;U56

78-1513

} Q~l O

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NBSIR 78-1513

• ' 4 MEASUREMENT TECHNIQUES FOR

SOLAR CELLS, QUARTERLY REPORT:

JANUARY 1 to MARCH 31, 1978

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

and H. A. Schafft

Electron Devices Division

Center for Electronics and Electrical Engineering

National Bureau of Standards

Washington, D C. 20234

September 1978

Issued 1978

Prepared for the Department of Energy

Division of Solar Technology

Advanced Materials R&D Branch

Under Task Order A054-SE of Interagency

Agreement EX-77-A-01-6010

mb U. u

'

U.S. DEPARTMENT OF COMMERCE. Juanita M. Kreps, Sacrattiy

Dr. Sidney Harman, Under Secretary

Jordan J. Baruch, Assistant Secretary for Scienca and Technology

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director

TABLE OF CONTENTS

Page

Preface iv

Executive Summary 1

1. Introduction 2

2. Work Performed during Reporting Period 3 2.1 Development of Solar Cell Device and Material

Measurement Techniques 3 2.1.1 Analysis 3

2.1.2 Equipment for Light-Biasing Cells 10

2.1.3 Laser Flying-Spot Scanner Modifications 10

2.2 Workshops and Symposia 11

3. References 11

Appendix A 12

LIST OF FIGURES

Page

Figure 1. An end-on view, not to scale, of an idealized

solar cell showing the portion between a con- tiguous pair of parallel metallization stripes 5

Figure 2. The one-dimensional electrical representation of

figure 1 for the case of laser scanning with a line of light 5 Figure 3. A sketch of eqs (5) and (6) for a representative

value for (^Z/2) of 1.30 7 Figure 4. The I-V characteristics for the dark and illuminated

idealized solar cell is shown in (a) , and (b) shows

the equivalent circuit 7

Figure 5. Air-mass-one insolation levels plotted versus desired

scanning-sensitivity ratios for various combinations

of cell stripe spacing and emitter sheet resistance . . 9

iii
PREFACE

This work was conducted as a part of the Semiconductor Technology Program of the National Bureau of Standards (NBS) . This program serves to

focus NBS research to enhance the performance, interchangeability, and

reliability of integrated circuits and other semiconductor devices including solar cells through improvements in measurement technology for use in specifying materials and devices in national and international

commerce and for use by industry in controlling device fabrication processes. This research leads to carefully evaluated and well-documented

test procedures and associated technology. Special emphasis is placed

on the dissemination of the results of the research to the appropriate

technical community. Application of these results by industry will

contribute to higher yields, lower cost, and higher reliability of semiconductor devices. Improved measurement technology also leads to greater

economy in government procurement by providing a common basis for the

purchase specifications of government agencies and, in addition, provides

a basis for controlled improvements in fabrication processes and in essential device characteristics.

The segment of the Semiconductor Technology Program described in this

quarterly report is supported by the Division of Solar Technology of the

Department of Energy (DOE) under DOE Task Order A054-SE of Interagency

k

Agreement eX-77-A-01-6010. The contract is monitored by Dr. Donald L.

Feucht, Chief of DOE's Advanced Materials R&D Branch. The NBS point

of contact for information on the various task elements of this project

is Dr. David E. Sawyer of the Electron Devices Division at the National

Bureau of Standards

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.

IV
Measurement Techniques for Solar Cells

Quarterly Report

January 1 to March 31, 1978

By

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

and H. A. Schafft

EXECUTIVE SUMMARY

This report covers research performed in the period January 1

to March 31, 1978 on the Program on Solar Cell Measurement Technique Development and Other Services by the Electron Devices

Division of the National Bureau of Standards. The objectives

of the program are to assist the DOE thin-film photovoltaic effort by developing solar cell device and material measurement

techniques using the NBS-developed laser flying-spot scanner,

and by assisting the Department of Energy (DOE) in organizing

and hosting appropriate workshops and symposia and providing

general consultation and liaison services.

A inajor portion of the program is the development of techniques

using the scanner to reveal solar cell quantities of interest,

such as emitter sheet resistance and portions of the metallization making poor ohmic contact to the underlying emitter. A

technique simpler than one using light modulated at high frequencies (described in the previous Quarterly Report [1] ) to

reveal these quantities was instrumented during the present

quarter. It employs forward biasing of the cell during scanning.

The case of scanning between parallel emitter metallization

stripes using a line of unmodulated light was analyzed. The

analysis takes a form quite similar to that employed last quarter for the case of microwave-modulated light. It is shown for

the present case that the minimum-to-maximum signal ratio depends only on the emitter sheet resistance, the stripe separation, and the cell conductance per unit area; the last can be

1
varied by adjusting the cell current. The analysis predicts

that the emitter sheet resistance can be obtained by fitting

the analytical value for the minimum-to-maximum ratio to the

experimentally observed ratio.

Apparatus development work underway this quarter included the

design and initial construction of an intense insolation source

for light-biasing cells while scanning and the construction of

a preamplifier to couple the low- impedance illuminated cell to

the laser scanner display electronics. Work begun last quarter

to increase the area scanned is continuing.

The announcement and program for the May 1-3, 1978, Stability

of (Thin Film) Solar Cells and Materials Workshop were mailed

to prospective attendees and to news sources likely to be read

by the photovoltaic community.

1 . INTRODUCTION

This report covers work performed in the period January 1 to March 31,

1978 on the Program on Solar Cell Measurement Technique Development and

Other Services by the Electron Devices Division of the National Bureau

of Standards under Task Order A054-SE of Interagency Agreement EX-77-A01-6010.

The objectives of the program are to assist the Department of Energy

(DOE) Advanced Materials R&D Branch photovoltaic effort in the following ways:

1. by developing solar cell device and material measurement techniques using the laser flying-spot scanner originally developed

at NBS for use on integrated circuits and discrete transistors,

and

2. by assisting DOE in organizing and hosting appropriate workshops and symposia, and by providing general consultation and

liaison services.

Activities and accomplishments in the various project areas during this

reporting period are described in section 2. The announcement and pro2
gr?m for the first workshop, the one on Stability of (Thin Film) Solar

Cells and Materials, is included as Appendix A.

2. WORK PERFORMED DURING REPORTING PERIOD

2.1 Development of Solar Cell Device and Material Measurement Techniques

2.1.1 Analysis

Description of New Technique

One of the major objectives of the program is the development of techniques which can be used to determine solar cell faults, such as regions

of poor metallization, and to determine important cell parameters such

as "emitter" sheet resistance. During this reporting period a way to

achieve this objective was conceived which is simpler than using

microwave-modulated light in the manner described in the previous Quarterly Report [1]

The new method makes use of the distributed nature of the cell under

forward-bias conditions. The low-frequency small-signal equivalent circuit of the biased and scanned cell is a three-dimensional resistive array, and the scanning light spot is represented by a current generator

moving within the array as the spot scans the cell. The array components are normally made up of the cell emitter sheet resistance and the

distributed resistance for the p-n junction. This distributed resistance is the local slope of the voltage-current curve for each increment

in cell area. A simplified, one-dimensional analysis of this array is

presented in this section to show that the new technique, with the cell

adequately forward-biased, can be expected to reveal all the cell qualities revealed by the technique using microwave-modulated light.

The analysis proceeds in a manner similar to the one for microwavemodulated light in the previous report; in the present case the local

voltage-current slope associated with the p-n junction plays a role

similar to the distributed transition-region junction capacitance for

the microwave-modulated light case. Implementation of the newer technique should be easier because high-frequency equipment and techniques

are not needed. Cell forward-biasing can be achieved in either of the

following ways, or by a combination of both: (1) uniform, unmodulated

3
illumination of the cell with no steady-state current in the cell load, or (2) connecting the cell to a current source. The explanation of the

technicjue is facilitated by the use of figures 1 and 2.

Predicted Behavior

Figure 1 is an end-on view, not to scale, of an idealized solar cell

showing the portion between a contiguous pair of (parallel) metallization stripes, and figure 2 is its one-dimensional electrical representation for the case of scanning with a line of light. This type of scan- ning will probably not be performed in the program, but it is useful to

analyze this case because the solution can be obtained as a closed-form

expression which may be examined to anticipate the important results for

the more realistic two-dimensional case. In figure 2 the cell emitter

is represented by the continuous resistance element, and the small-signal

voltage-current ratios for the elemental diodes making up the cell between the stripes are represented by the discrete resistors connected

with dotted lines. In the absence of significant shunting by the latter,

the photocurrent will find its way to the load through one path or another. Thus, the load voltage (the scanner display screen signal)

,

while it may vary from one light spot location to another due to variations in such quantities as local lifetime, will not be influenced by

the proximity to a defect such as a region of poor metallization contact.

In short, without significant internal shunting, the scanner does not

reliably detect cell defects such as cracks, regions of poor contacting,

regions of excessive emitter sheet resistance, etc., which may seriously

influence cell reliability and conversion efficiency.

The values of the shunt resistances can be varied by varying the diode

forward bias. For the simple case in which each elemental diode is

equivalent and all are biased to the same voltage, the shunt conductance

per unit area is the same for all portions of the cell. With Op representing this conductance, and flg the emitter sheet resistance, the signal current and voltage for each increment of distance dx satisfy the

pair of differential equations:

4
Figure 1. An end-on view, not to scale, of an idealized solar cell showing the portion between a contiguous pair of parallel metallization stripes.

LOAD

Figure 2. The one-dimensional electrical representation of figure 1 for the case of laser scanning with

a line of light. The cell emitter is represented by

the continuous resistance element, and the small-signal

voltage-current ratios for the elemental diodes making

up the cell between stripes are represented by the discrete resistors shown connected with dotted lines.

5
( 1 ) di (x)

dx

v(x) aP

dv(x)

dx

i(x)fl

s

(2)

The solution of eqs (1) and (2) is carried out in a manner identical to

that for the solution of the resistance-capacitance case analysis in the

previous report [1], that is, eq (2) is differentiated and substituted

into eq (1) .

where

d^v(x)

dx2

3^v(x)

3 = a ) . s p

0 (3)

(4)

The voltage across the load Z^, i.e., the output voltage the

scanning light line at the arbitrary position 0 £ p £ J, is

V = i z {sinh[3QZ- - p) ] + sinh(3p)}

'^out Z 23Z

{sinh(3S') + — — [cosh(3^-) -11)

s

(5)

As the laser light line is swept from one grid stripe to another,

undergoes a maximum-minimum-maximum excursion having the ratio

V ^ (min) - out 1

V ^(max) cosh(3V2)

out

( 6 )

Equation (5) is plotted in figure 3 for a representative value of (3^-/2) .

The desired forward biasing of the cell may be achieved by shining light

on the cell. It is appropriate now to relate the shunt conductance

to the cell current and associated-air-mass one (AMI) insolation level.

Figure 4 shows the I-V characteristics for the dark and illuminated

idealized solar cell and its equivalent circuit representation. The

well-known effect of shining light on the cell is to displace the I-V

characteristic along the I-axis an amount equal to the external current

which would exist if the diode were connected to a short circuit. This

current will be designated I . An alternate description states that

charge flows through the cell in the forward-bias direction. For the

6
Figure 3. A sketch of eqs (5) and (6) for a representative value for (35-/2) of 1.30.

Figure 4. The I-V characteristics for the dark and illuminated idealized solar cell is shown in (a) , and (b) shows the equivalent circuit.

In the latter, the strength of the current generator is proportioned

to the light intensity.

7
idealized case discussed, and with uniform illumination, I is equal to

Xj the product of the cell junction area and the cell current density J

1j due to illumination. In turn, J is related to the cell voltage V and

Li J^, the junction saturation-current density, by

J = J - 1) . (7)

L o

In the scanning method described, the cell is dc open-circuited and illuminated by a constant light source while it is scanned with a much

weaker (laser) source. The constant source, by forward-biasing the cell,

produces a small-signal junction conductance per unit area readily calculated by differentiating eq (7)

a = dJ /dV = (q/kT) (J + J ) . (8)

p L L o

The current J will usually be several orders of magnitude larger than

Li J^, and so the quantity multiplying (q/kT) for all practical purposes is

simply J . At room temperatxire , and with J expressed in aitperes per

L L

unit area,

a = 38.4 J siemens per unit area . (9)

p L

^

The insolation level for a representative cell and measurement condition

can be readily calculated. The assumed conditions are stripe separation,

1 = 2 mm, and emitter sheet resistance, =50 9./Q i 0.9 is the value of

s Vout (n^n) /Vq^^

(

max) judged to just yield adequate scanning sensitivity.

From eqs (4) and (6), the a value required is 0.442 S/cm^, and from

eq (9), the required value of light-generated current is 11.5 mA/cm'^.

The short-circuit photocurrent from a good quality silicon cell exposed

to an insolation of one sun with an air-mass-one spectral distribution

is about 38 mA/cm^ [2]. Using this value, the required cell insolation

level in the present example is found to be approximately 0.30 suns, a value which can be reached quite easily in the laboratory. In figure 5,

required insolation levels are plotted versus desired scanning-sensitivity

ratios for various combinations of stripe spacing and emitter sheet resistance. The insolation values required to give unambiguous v(min)/

v(max) values of 0.5 for line-scanning typical nonconcentrator cell de8
INSOLATION.

AMI

SUNS

Figure 5. Air-mass-one insolation levels plotted versus desired

scanning-sensitivity ratios for various combinations of cell stripe

spacing and emitter sheet resistance.

9
signs having ^-values from 1 to 3 mm and values from 50 to 200 ^/u

s can be achieved with an insolation source of 10 suns or less, according

to this figure. Such an insolation source is reaching completion in the

laboratory.

The second way suggested to achieve the desired internal resistive shunt

ing, by using a current source, is conceptually simpler than flooding

the cell with light. Preliminary experiments performed near the end of

the present reporting period indicate that this method may offer a con- venient way to detect a variety of cell defects.

2.1.2 Equipment for Light-Biasing Cells

One of the project needs mentioned in the previous section is an illuminator to bias cells optically while they are being scanned. An interim

specification on the illuminator is that it should produce, for flat

plate (nonconcentrator) cells, a short-circuit current response equivalent to exposing the scanned area of the cell to an insolation of 20 suns. It is obvious that noise and modulation of the light by, e.g., an inadequately filtered lamp power supply should be a minimum. One

type of light source which should satisfy the requirements is the quartz

halogen incandescent lamp. An illuminator using such a lamp has been

designed this quarter and its construction is reaching completion. Its

optical system uses four groups of heat filters and condensing and focus

ing lenses arranged symmetrically around the long axis of the lamp.

This arrangement captures almost all of the light availcible from the

lamp and it is designed so that it focuses the light onto the receiving

ends of four flexible fiber-optic light pipes which direct the light to

the cell area being scanned. Preliminary measurements indicate that

more than 30 suns insolation can be achieved at the cell surface and

that extraneous noise sources are negligible.

2.1.3 Laser Flying-Spot Scanner Modifications

The capacitance associated with the injection of minority carriers in

forward-biased solar cells may severely limit the spot deflection rates

allowable, when one employs the method to pin-point cell defects described in section 2.1.1. One way of solving this problem is to reduce

the RC time-constant of the cell and its load. This reduction can be

10
accomplished by coupling the cell to the scanner display electronics

with a transformer input. Initial and promising results have been

achieved using a transformer having an input impedance of 1.75 0.. The transformer secondary was connected to the 3.3-kfi display screen input, and a 9,000-yF electrolytic capacitor was put in series with the

primary to block the cell dc voltage.

A number of ways are being pursued to increase the scanning area. When

the scanner was used for discrete transistor-like devices and ICs, a large scanning area was not needed, and the ability to achieve a scan- ning area 3 mm on a side was quite adequate. For solar cells, one would

like to increase this value by at least an order of magnitude. Various

ways are being investigated to achieve this by modifying the scanner optics through replacing or supplementing the original optical elements.

2.2 Workshops and Symposia

The plans for the workshop "Stability of (Thin Film) Solar Cells and

Materials" scheduled to be held at NBS, Gaithersburg, Maryland, on May 1-3, 1978 were completed this quarter. The announcement and program

were mailed to a number of piablications read by the photovoltaic community; copies are included as Appendix A. This material with a general information sheet, hotel reservation card, etc., were mailed to about 800

people believed to be engaged in various aspects of solar cell work.

3 . REFERENCES

1. Sawyer, D. E. , Kessler, H. K. , and Schafft, H. A., Measurement Techniques for Solar Cells, Quarterly Report, September 15 to December

31, 1977, NBSIR 78-1488 (July 1978).

2. Hovel, H. J. , Solar Cells, Semiconductors and Semimetals, R. K. Willardson and A. C. Beers, Eds, Vol. 11, p, 47 (Academic Press,

New York, 1975)

.

11
APPENDIX A

WORKSHOP

Stability of (Thin Film) Solar Cells and Materials

May 1-3, 1978

National Bureau of Standards

Gaithersburg, Maryland

The National Bureau of Standards is conducting a workshop on the Stability of (Thin Film) Solar Cells and Materials for the Department of Energy's Advanced Materials R&D Branch. The workshop is part of the Department of Energy's National Photovoltaic Program and is directed to those

engaged in research on various types of cells. One of its main purposes

is to identify and discuss problems and obstacles to achieving 20-year

life for terrestrial solar cells and to plan how these may be overcome

by the use of test and measurement procedures designed to enhance the

prediction of material and device stability.

Three different groups of exploratory solar cell materials and concepts

will be addressed. These are: (1) [CdZn] S/CU2S , CdS/Cu-ternaries

,

CdS/InP, and amorphous Si; (2) polycrystalline, MIS, and conducting oxide Si; and (3) polycrystalline and AMOS GaAs. Modes and mechanisms for

failure and degradation of these systems and the status of present reliability testing will be reviewed by current workers. A nuittoer of speakers have been invited to discuss measurements and test approaches that

have been used for achieving high reliability in related device technologies. For example, representatives from DoD, NASA, and the power

transistor commvmity will speak to provide enhanced perspective for the

solar cell community and to promote cross-fertilization. Time will be

available for the presentation of a limited number of unsolicited papers

that provide data pertinent to the topics of the workshop. Discussion

groups will be formed to identify tests and measurement procedures that

can be used to enhance the prediction of material and device stability

for each of the three solar cell groups.

Advanced registration is requested. For more information, including

registration and hotel reservation forms, contact Mrs. Elaine Cohen,

Division 425.00, National Bureau of Standards, Building 225, Room A-327,

Washington, D.C. 20234. Telephone: (301) 921-3625.

12
Workshop on Stability of (Thin Film) Solar Cells and Materials

National Bureau of Standards

Washington, D.C.

May 1-3, 1978

MONDAY, May 1 8:30-9:15 a.m. 9:15-10:00 a.m.

10:00-1:00 p.m.

1 :00-2 :00 p.m.

2:00-2:30 p.m.

2:30-5:00 p.m.

6:30 p.m.

7 : 30 p.m.

REGISTRATION

SESSION I. Welcoming, Perspective, and Workshop Overview

SESSION II. Status of Present Reliability Testing,

Failure Modes, Failure Mechanisms, and

Data for Advanced-Cell Materials

(a) [CdZn] S/CU2 S , CdS/InP, (c) polycrystalline

CdS/Cu-Ternaries , and AMOS GaAs

amorphous Si

(b) polycrystalline, MIS and

conducting oxide Si

LUNCH

SESSION II. (continued)

SESSION III. Types of Measurements and Tests Used to

Define Stability

gies

in Related Technolo-

(a) DoD Experience (e) Concentrator Cells

(b) Semiconductor Power Devices (f) Corrosion and

(c) Methods and Tests at BTL

(Solar Cell and Other)

Measurement Tools

for Corrosion

(d) "JPL/NASA" Experience (g) (Inter) Diffusion

Phenomena

CASH

(h) Encapsulants

BAR (Ramada Inn)

WORKSHOP DINNER (Ramada Inn)

TUESDAY, May 2 9:15-11:00 a.m. SESSION III. (continued)

11:00-1:00 p.m. SESSION IV. Working Groups Sessions

1:00-2:00 p.m.

2:00-5:00 p.m.

LUNCH

SESSION IV. Working Groups Sessions (continued)

WEDNESDAY, May 3 9:15-11:20 a.m. SESSION IV. Working Groups Sessions (concluded)

11:20-1:00 p.m. SESSION V. Preparation of 'Working Groups Summaries

1:00-2:00 p.m.

2:00-4:00 p.m.

LUNCH

SESSION VI

.

Working Groups Present Summaries and

Recommendations

4:00 p.m. END OF WORKSHOP

13
N BS-1 1 4 A I R E V ; 31

U DEPT. OF COMM. 1, |>1'H1 1( A 1 ION OK K 1 POR I' NO. 2 . (.ov’t Acu ssion BIBLIOGRAPHIC DATA No. SHEET

3, Ri i‘ ipit ni ’ s Acct ssiA>n No.

4. 1 1 IT.l AND sUlvn 1 1 1 Measurement Techniques for Solar Cells: Quarterly Report

for the Period January 1 to March 31, 1978

5. Publication Date

6. Performing Organization ( ode

7. AITHOR(S)

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

8. Performing Organ. Report No.

9. PEREORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Project /Task /Work Unit No.

11. C ontract /Grant No.

Task Order A054-SE

12. 'Sponsoring Organization Name and Gompletc Address (Street, City, State, ZIP)

Advanced Materials R&D Branch

Division of Solar Technology

Department of Energy

20 Massachusetts Avenue, N.W. Washington, D. C. 20545

13. Type of Report & Period Covered January T - March 31, 1978

14. Sponsoring Agency Code

15. SUPPI.KMENTARY notes

16. ABSTRA( r (A 200-word or less factual summary of most significant information, ff document includes a significant

bibliography or literature survey, mention it here.) This is the quarterly report of the work performed in the three month period

January 1 - March 31, 1978. The objectives of the program are to assist the DOE

thin-film photovoltaic effort by developing solar cell device and material measurement

techniques using the NBS-developed laser flying-spot scanner, and by assisting DOE in

organizing and hosting appropriate workshops and symposia and providing general consultation and liaison services,

A technique simpler than the one using light modulated at high frequencies, described

in the previous quarterly report, is set forth which employs forward-biasing solar

cells during scanning to pin-point certain cell defects and to obtain values of cell

quantities such as emitter sheet resistance. An analysis appropriate for laser scan- ning forward-biased cells with a line source is presented. Results from initial

experiments suggest that the new technique should work quite well on real-world solar

cells. Apparatus development work included the design and initial construction of a high-sun insolation source for forward-biasing cells by light while scanning, and the

construction of a matching network to couple the low-impedance illuminated cell to the scanner display electronics.

The announcement and program for the May 1-3, 1978 Workshop on the Stability of (Thin

Film) Solar Cells and Materials at NBS is presented.

17. KE.Y WORDS (six to twelve entries; alphabetical order; capitalize only the first letter of the firs' key word unless a proper name; separated by semicolons

)

Device measurements; laser scanning; light-biasing; metallization; ohmic contacts;

reliability; semiconductor measurements; sheet resistance; solar cells; solar cell

stabi 1 ity.

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(THIS REPORT)

l ot Official Distribution. Do Not Release to NTIS

UNC.L ASSIEIED

I rom Sap. of Ooc., U.S. Government Printing Office 20. SECURITY ( LASS 22. Price

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