development, fabrication and analysis annual report …

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JPL N0. 9950-597 TDRL NO. 74/DRD NO. SE DOE/JPL-955089 - 81112

Line Item No. 7 Distribution Category UC-63

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SILICON SOLAR CELL PROCESS

DEVELOPMENT, FABRICATION AND ANALYSIS

ANNUAL REPORT (PHASE 111)

For Period Covering1 July 1980 to 31 June 1981

By:

H.I. Yoo, P.A. Iles, and D.C. Leung

JPL Contract No. 955089

OPTICAL COATING LABORATORY, INC.Photoelectronics Division

15251 E. Don Julian RoadCity of Industry, California 91746

"The JPL Low-Cost Silicon Solar Array Project is sponsored by the U.S. Department of Energyand forms part of the Solar Photovoltaic Conversion Program to initiate a major effect towardthe development of low-cost solar arrays. This work was performed for the Jet PropulsionLaboratory, California Institute of Technology by agreement between NASA and DOE."

I

^.+A:iA-LLt-1v3787) .illt_li, Ai, c:tLl knC' r.i:i ^bL-11,^1J

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DRL NO. 74/DRD NO. SE DOE/JPL-953089 - 81/12Line Item No. 7 Distribution Category UC-63

SILICON SOLAR CELL PROCESS

DEVELOPMENT, FABRICATION AND ANALYSIS

ANNUAL REPORT (PHASE III)

For Period Covering1 July 1980 to 31 June 1981

By:

H.I. Yoo, P.A. Iles, and D.C. Leung

JPL Contract No. 955089

OPTICAL COATING LABORATORY, INC.Photoelectronics Division15251 E. Don Julian Road

City of Industry, California 91746

"The JPL Low-Cost Silicon Solar Array Project is sponsored by the U.S. Department of Energyand forms part of the Solar Photovoltaic Conversion Program to initiate a major effect towardthe development of low-cost solar arrays. This work was performed for the Jet PropulsionLaboratory, California Institute of Technology by agreement between NASA and DOE."

't

"This report was prepared as an account of work sponsored by the UnitedStates Government. Neither the United States nor the United StatesDepartment of Energy, nor anyof their Employees, nor any of theircontractors, subcontractors, or their employees, makt- q any warranty,express or implied, or assumes any legal liability or responsibility for theaccuracy, completeness or usefulness of any information, apparatus,produce or process disclosed, or represents that its use would not infringeprivately owned rights."

f

iABSTRACT

S

ISolar cells were fabricated from EFG ribbons (,Mobil-Tyco), Dendritic webs

(Westinghouse), Cast Ingots by Heat Exchanger Method (HEM, Crystal Systems), and Cast

fIngots by Ubiquitous Crystallization Process (UCP, Semix). Baseline and other process

variations were applied to fabricate solar cells.

EFG ribbons grown in a carbon-containing gas atmosphere showed significant

Improvement In silicon quality. This new EFG demonstrated an average baseline

efficiency of 10.3% as opposed to 8.0% AM 1 for the earlier EFG. The best efficiency

achieved by the advanced process was 13.8% under AM 1 conditions.

Baseline solar cells from dendritic webs of various runs indicated that the quality

of the webs under investigation was not as good as the conventional CZ silicon, showing an

average minority carrier diffusion length of about 60um versus 120um of CZ wafers.

Detail evaluation of large cast ingots by HEM showed ingot reproducibility

problems from run to run and uniformity problems of she°t quality within an ingot.

Junction shunting was a major problem area, probably caused by the particulate inclusions

observed in the bulk.

Initial evaluation of the wafers prepared from the cast polycrystalline ingots by

UCP suggested that the quality of the wafers from this process is considerably lower than

the conventional CZ wafers. Overall performance was relatively uniform, except for aI

few cells which showed shunting problems caused by inclusions.

I.

li.

III.

IV.

V.

ABSTRACT i

TABLE OF CONTENTS

LIST OF FIGURES

LIST OF TABLES

INTRODUCTION 1

TECHNICAL DISCUSSION 2

A. EFG Solar Cells 2

1.0 Solar Cell Fabrication 22.0 Solar Cell Performance and Characterization 3

B. Dendritic Web Solar Cells 24


iC. Solar Cells From Cast Ingots by HEM 33

1.0 Solar Cells From A Cut Section (#41-07) 332.0 Solar Cells From A Small Cast Ingot (#41-24) 443.0 Solar Cells From A Large Cast Ingot (#41-41C) 534.0 Solar Cells From A Large Cast Ingot 041-48) 64

D. Solar Cells From Cast Ingots by UCP 74


CONCLUSIONS AND RECOMMENDATIONS 83

WORK PLAN STATUS 85

REFERENCES 86

APPENDICES

1. Time Schedule

11. Abbreviations

-ii-

LIST OF FIGURES

FIGURE NO. PAGE

1 Spectral Response of Baseline Solar Cells From EFG 8Material

2 Spectral Response Of The Baseline Solar Cells From 9EFG With CO-On

3 Spectral Response of High Efficiency Solar Cells 11From EFG Ribbons (CO 2 Ambient)

4 Small Light Spot Scanning of Typical Earlier EFG 12Material

S Small Light Spot Scanning of Typical New EFG Ribbons 13with CO2

6 Microscopic Pictures of. The Surface of Earlier EFG 14Chemical Etching (200X Magnification)

7 Microscopic Pictures of The Surface of New EFG With 15CO Ambient After Chemical Etching (200X Magnifi-cation)

8 Spectral Response of The Baseline Solar Cells From 27Dendritic Webs

9 Average Efficiency vs. Position of The Horizontally 36Cut HEM Cube (#-,1-07)(Baseline Process)

10 Spectral Response of Solar Cells From a HEM Cube 37(041-07XRaseline Process)

11 Spectral Response of Solar Cells From a HEM Cube 39041-07)(Gettering + Baseline Process)

12 Spectral Response of Solar Cell From HEM Cube(1/41-07) 41(GET, SJ, Narrow Gridlines, BSF, MLAR Process)

13 L 's vs. Number of Suns For Horizontally Cut HEM(;kMN41-07)Cube Cells (Cells No. in Parenthesis)

42

14 Cross Section of A Vertically Cut HEM Ingot(#41-24) 46

13 Short Circuit Current Density (AM 1, No AR) Mapping 47of a Vertically Cut HEM Wafer From A Cast Ingot (#41 -24)

-iii-

FIGURE NO. PAGEt j

16 Open Circuit Voltage (AM 1, No AR) Mapping of a 48Vertically Cut HEM Wafer From A Cast Ingot 041-24)

17 Efficiency (AM 1, No AR) Mapping of a Vertically Cut 49HEM Wafer From a Cast Ingot 041-24)

18 Efficiency (Normalized W R T The Control Cells) SOMapping of A Vertically Cut HEM Wafer From A CastIngot 041-24)

19 Efficiency (Normalized WRT The CZ Control Cells) 51Mapping of Vertically Cut HEM 041-24) Baseline CellsWith The Gettering Steps Added

20 L vs. No. of Suns For Vertically Cut HEM 041-24) 52ZRis (Cells No. in Parenthesis)

21 Wafer Identification Within the Large HEM Ingot SS

22 A Mapping of Normalized n (% To Control) For A Center 56Layer of Vertically Cut HEM (#41-41C)

23 A Mapping of Normalized n (% to Control) For a Quarter 57Layer of Vertically Cut HEM (141-41C)

24 A Mapping of Normalized n (% to Control) For an Edge 58Layer of Vertically Cut HEM 041-41C)

23 A Mapping of Normalized n (% to Control) For the Top 59Horizontal Layer of HEM (41-41C)

26 A Mapping of Normalized n (% to Control) For The 60Middle Horizontal Layer of HEM (#41-41C)

27 A Mapping of Normalized n (% to Control) For The 61Bottom Horizontal Layer of HEM (#41-41C)

28 Spectral Responses of Selected Samples From a Center 62Layer of Vertically Cut HEM 041-41C)

29 A Mapping of Normalized n (% to Control) From a Center 66Layer of Vertically Cut HEM (#41-48)

30 A Mapping of Normalized n (% to Control) For A 67Quarter Layer of Vertically Cut HEM 041-48)

31 A Mapping of Normalized n (% to Control) For an Edge 68Layer of Vertically cut HEM (141-48)

(32 A Mapping of Normalized n (% to Control) of A Top 69

Layer of Horizontally Cut HEM 441-48)

-iv-

FIGURE NO. PAGE

33 A Mapping of Normalized ri (% to Control) of A 70Middle Layer of Horizontally Cut HEM 041-48)

34 A Mapping of Normalized Tl (% to Control) Of A 71Bottom Layer of Horizontally Cut HEM 041-48)

33 Microscopic Picture of Inclusions Observed 72In A HEM Ingot (1141-48), 200X Magnification

36 Spectral Responses Of Selected Samples Of A Center 73Layer of Vertically Cut HEM (1141-48)

37 Mapping of The 2x2 Cells in SEMIX Wafer 76

38 Microscopic Photographs of Inclusions (or Precipi- 77tates) Observed in UCP Wafers (20OX Magnification)

39 Spectral Response of Baseline Solar Cells (No AR) 78From UCP

40 Small Light Spot Scanning of A Baseline Cell From UCP 79

-v-

1

LIST OF TABLES

TABLE NO. PAGE

Summary of Baseline Cells From EFG Ribbons With 16CO2 Ambient

2 Summary of High Efficiency Cells From New EFG 17Ribbons With CO2 Ambient

3 Average Cell Parameters of EFG Ribbons With And 18Without CO In Ambient

4 Average Short Circuit Current Density (,Jsc mA/cm 2) 19For The Two Step Diffusion Process (730 C, 9 Hoursin POCL3)

S EFG Material With Low Temperature Annealing (6000C, 2030 Hours)

6 "A" Factors From Dark I-V Measurements 21

7 Effective Minority Carrier Diffusion Lengths of EFG 22Ribbons With CO2

8 Effective Minority Diffusion Length of Selected Solar 23Cells From EFG Ribbons With and Without CO in Ambient

9 Analysis of Dendritic Web Samples 28}

10 Dendritic Web Solar Cell From Baseline Process 29

11 Dendritic Web Solar Cells From Advanced Process 30

12 Summary of The Pre-Characterized Web Wafers 31

13 Minority Carrier Diffusion Lengths of The Pre- 32Characterized Web Cells

14 Summary of High Efficiency Cells From a Selected 43Portion of A HEM Ingot (#41-07)

13 Summary of Results of The Solar Cells From Cast 81Ingot By UCP

16 Effective Minority Carrier Diffusion Length of Solar Cells 82Made From UCP Wafers

-vi-

I. INTRODUCTION

The objective of this program Is to investigate, develop, and utilize

technologies appropriate and necessary for improving the efficiency of solar

cells made from various unconventional silicon sheets. During the third phase of

the program, work has progressed in fabrication and characterization of solar

cells from EFG ribbons (Mobil-Tyco), cast ingots by Heat Exchanger Method

(HEM, Crystal Systems), and cast ingots by Ubiguitotis Crystallization Process

(UCP, Semix). Solar cells were fabricated using a baseline process typical of

those used currently in the silicon solar cell industry. Other process

modifications were included to give indications of the possible improved

performance obtainable fram various sheets. Process variations used included

gettering by diffusion glasses, low temperature annealing, shallow junction

formation, application of fine front grid lines, formation of back surface field

(BSF) and back surface reflector (BSR), and application of better AR coating

(i.e., mu!ti-layer anti-reflective coating).

The solar cell parameters measured included open circuit voltage (Voc),

short circuit current density (Jsc), curve fill factor (CFF) and conversion

efficiency n (all taken under AM 1 illumination). Also, measurements for typical

cells included spectral response, dark I-V characteristics, minority carrier

diffusion length, and photoresponse by fine light spot scanning.

The results were compared to the properties of cells made from

conventional single crystalline Czochralski silicon with an emphasis on statistical

evaluation. Limited efforts were also made to identify impurity levels in certain

portions of HEM ingots using Deep Level Transient Spectroscopy (DLTS).

-1-

E;

11. TECHNICAL DISCUSSION*

A. EFG Solar Cells

1.0 Solar Cell Fabrication

EFG Ribbons •ith C Ambient

The new EFG ribbons were grown with CO 2 mixed with argon in the

ambient as opposed to pure argon for the earlier EFG ribbons. The resistivity

was slightly lower, i.e., 0.8 ohm-cm as opposed to 1-2ohm-cm (estimated by zero

bias capacitance measurement) of the previous ribbons. The new material was

covered by ,a thin layer of oxidized carbon compound which made an extra

surface etching step necessary. Otherwise, the processes used were the same.

Baseline cells were fabricated along with earlier EFG material and CZ control

silicon for comparison. In addition, some high efficiency cells were fabricated

by applying shallow junction diffusion technique, incorporating back surface

reflector (BSR) or back surface field (BSF), narrow grid lines, and multi-layer

antireflection dating. Appendices in References (1) and (2) provide details of

the processes used and reference (3) gives technical details of the EFG process.

EFG Ribbons With CO Ambient

ribbons evaluated had been grown in a furnace with CO-off (Run # 18-

191-0) and CO-on (Run #18-196-1) atmosphere. The ribbons were sliced into

2x2cm blanks using a dicing saw, and baseline solar cells were fabricated along

with CZ silicon (control cell) for comparison.

Other Processes Used

Since significant improvement in solar cell performance, especially short

*Pertinent figures and tables for each section (A,B, etc.) are placed at the end

of these sections.and reproduce the tests. circuit current, was reported (4) at

-2-

the 1980 European Photovoltaic Solar Energy Conference by utilizing two step

jdiffusion, an experiment was carried out to try and reproduce the tests. Silicon

sheets under test were SILSO (Wacker), EFG, and poly silicon from CZ growth.

i The first step diffusion included 9 hours of POC1 3 diffusion at 750oC-, it was

hoped that preferential diffusion at grain boundaries would occur at this stage.

Normal diffusion at 8750C diffusion followed thereafter and baseline solar cells

were fabricated. Half of the samples were inserted during the second diffusion

(normal 8750C diffusion), serving as control cells, to compare the results of the

cells with two step diffusion. No AR coating was applied to these cells.

In an effort to reduce potential residual thermal stress before the cell

processing, low temperature annealing was tried on earlier EFG ribbons (Mobil-

Tyco Run 4187-3C series). The EFG blanks were thoroughly cleaned and

annealed at 60000 for 48 hours in nitrogen atmosphere. Baseline solar cells were

made from both annealed and un-annealed EFG ribbons and the cell performance

was compared to see the effect of :he annealing.

2.0 Solar Cell Performance and Characterization

Characteristics Under Wumination

All solar cell,: were of 2cm x 2cm size. The products from the ba--•-line

process had SiO AR coatings and 90% ac*ive area with Ti-Pd-Ag metallization.

Solar cell parameters such as Isc, Voc, C'FF, and n were measured under AM I

conditions at 280C. Descriptions of the simulator and measurement are given in

References (1) and (2).

Parameters of the baseline solar cells from EFG ribbons grown in CO2

ambient a-e shown in Table 1, where results of the cells from the earlier EFG

ribbons grown without: carbon containing gas atmosphere are also shown for

comparison. Note; the two types of the ribbons were processed together.

The new EFG with C'02 ambient was found to be superior in all parameters,

with an average n of 10.3% AM 1 as opposed to 8.0% AM 1 for the earlier EFG.

The slightly lower resistivity of the new EFG would account only partly for the

higher Voc values. Other factors such as increased minority carrier diffusion

length are needed to account for the rest of the per for manse difference; this

will be discussed later. The results of the high efficiency processes are

summarized in Table 2. The best efficiency achieved from the ribbons was

13.8% under AM I conditions.

The bas,!line EFG grown in CO ambient showed improvement in all cell

parameters, resulting in an average efficiency of 10.7% AM I compared with

8.6% of the EFG cells without CO in ambient. Table 3 summarizes the results.

Table 4 indicates results of an effort to passivate grain boundaries by two

step diffusion (refer to section 1.0 for the details), showing a comparison of short

circuit current density Jsc of the solar cells with and without the twe step

diffusion. Note; Poly CZ and SILSO wafers were processed with EFG. Jsc of

poly CZ and SILSO cells stayed about the same, while EFG cells showed

reduction in Jsc by the process.

Summary of performance of EFG cells with low temperature annealing are

given in Table S, suggesting the annealing step did not result in improvement of

the sheet quality.t

_4_

,

` Dark Current Characteristics

Dark forward currents were measured. The "A" factor, which is defined byv

ff

Isc = 1 (exp ( AKT )-1), of the baseline samples are listed in Table 6 with their

1 CFF values. The table indicates "A" values equal or more than 2 showing

problems of shunting and leading to lower CFF's. All these samples were edge-

trimmed so that these values represent leakage in the junction rather than the

j edge. From the limited number of samples here, it seems that the earlier EFG

{ material is more likely to form leaky junctions. If the high "A" values (>2) are

not counted, the rest of the "A" factors with one exception, are within the 1.6-

1.8 range both for the new and earlier EFG.

i

Spectral Response

Spectral response was measured by a filter wheel method (see Reference 1

for the details). Results for selected baseline and high efficiency cells are

presented in Figures 1 to 3. It is clear that the new material grown in carbon

containing gas atmosphere is superior. Also, a significant enhancement of long

i wavelength response was noticed in the high efficiency cells. This indicates that

the new EFG material responds more positively to BSF or BSR treatments than

the earlier EFG. This can be due to longer diffusion lengths (2) as will be seen in

the following section.

Minority Carrier Diffusion LTVh

Effective minority carrier diffusion length (L D) was measured by the filter

wheel method using the short circuit current method (see Reference I for the

details). Selected samples were measured both in the form of whole samples and

at selected areas as shown in the figure attached to Table 7 which gives the

results of EFG with CO2. Lp values of the new EFG range from 30 to 40um

r

-5-

i

while the earlier EFG ranged from 20 to 30um. This significant increase in L is a

the most decisive factor that accounts for better Isc, because of better carrier

collectio i. Also, the saturation current (Io) decreases with longer L and since

Voc =AK In Isc, the increase of Lp also accounts for part of the increase of

Voc. The light bias effect on L for the new EFG has not been studied in this

report period. Work in this area will be useful to evaluate the true significance

of L under one sun illumination.

Table 8 shows results of EFG ribbons with CO ambient, indicating a range

of the diffusion length between 35 and 50um for the EFG witi, CO-on, and 25 and

35um for the EFG with CO-off. L of the CZ control cell was about 140um.

Photorespoi by Small Light Spot Scanning

Localized photoresponse of selected solar cells was made using a small

light spot scanning technique (see Reference 1 for the detailed description and

procedures of the measurement). Figure 4 and Figure 5 show the relative

responses of the typical earlier and new EFG, respectively, and also CZ control

cells. It is obvious from the presented data that the earlier EFG material has

more dips in the response and also has some areas of low response. Chemical

etching study in the following section indicates that structural defects are partly

responsible for the low response.

Structural Defects

Efforts were made to show structural defects by chemical etching

techniques. The etching solution used was diluted Sirtl etch (10 grams of C 20in 60 ml of deionized distilled water, and an equal volume of concentrated HF).

Materials Research Inc. in Utah, reported that 50 seconds in this etching

-6-

k

solution distinctly revealed grain boundaries, twin boundaries, and dislocations

for EFG (S). Figure 6 and Figure 7 show microscopic pictures of the etched

surface of the earlier and new EFG, respectively; (a) for a typical region and (b)

for a area of high dislocation. Dislocation density of the earlier EFG is

considerably higher than the new EFG grown in carbon containing gas

atmosphere. The earlier EFG also showed areas of dislocation clusters, (b) in

Figure 6, which is expected to reduce photoresponse significantly. The low

localized response in Figure 4 could possibly be due to this dislocation clusters.

-7-

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-14-

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-15—

TABLE 1

SLWARY OF BASELINE CELLS FROM EFG RIBBONS WITH

CO2 AMBIENT

NEW EFG EARLIER EFG CZ CONTROL

Voc

(mV)

AV. 568 524 588

S.D. 7 5 -

R 560-580 '.116-534 588

Jsc

(mA/cm2)

AV. 24.3 21.4 28.0

S.D. .7 .8 -

R 23.0-25.0 20.5°23.0 27.8-28.3

CFF

M

AV. 75 71 76

S.D. 3 3 -

R 69~77 65-75 75-77

M

AV. 10.3 8,0 12.6

S.D. 0.6 0.4 -

R 9.6-11.1 1.4-8.7 12.3.12.8

NOTE: 1) 2x2 cm Cells with SiO AR coating measured under AM1

at 280C test block temperature.

2) New EFG: Improved grains, grown in CO2

gas atmosphere.

-16-

f TABLE. 2

l

SUMMARY OF HIGH EFFICIENCY CELLS FROM NEW EFG RIBBONS WITH

CO2 NIBIENT

Voc (mV) Jsc (mA/cm2 ) CFF (%) (X)

AV. 565 28.4 76 12.1

S.D. 9 1.1 2 .7

R 550-574 27.0-30.3 72-78 11.5-13.6

(a)

Voc (mV) Jsc (mAicm ? ) CFF (^;} _^ (%)

578 30.3 79 13.8

(b)

NOTE: 1) 2x2 cm cells with AR coating at 280C test

block temperature under AM1 conditions.

2) Process Used:

(a) SJ,BSR, and MLAR,

(b) SJ, BSF, and MLAR.

-17-

TABLE 3

AVERAGE SELL PARAMETERS OF EFG RIBBONS WITH AND

WITHOUT CO IN AMBIENT_

Voc, mV mA scm2 CFF, % , %

WITHOUT CO 540 22.9 70 8.6

WITH CO 567 25.1 76 10.7

CZ CONTROL 582 28.2 78 12.7

NOTE: Baseli,.e Solar Cells (2x2cm) With SiO AR Measured

at 28cs. Under AM1.

-18-

EFGPOLY

SILSOHAMCO

17.9 22.1 22.4

15.3 22.1 22.3

Jsc of Control: 23.4

ss on 2x2 cells without AR, measured at

FG material without CO in growth)

—19—

TABLE 4

AVERAGE SHORT CIRCUIT CURRENT DENSITY (Jsc mA/cm2)

FOR TWO STEP DIFFUSION PROCESS (750 0C, 9Hr. in POC13)

_ TABLE 5

EFG MATERIAL WITH LOW TEMPERATURE ANNEALING (600'C,3O Hr)

Voc,mVmA/cm2

CFF

NOT ANNEALED 493 13.2 74 4.8

ANNEALED 493 13.3 73 4.7

CZ CONTROL 568 20.1 74 8.5

BASELINE PROCESS ON 2x2 CELLS WITHOUT AR MEASURED AT AM1 AT

28°C. (EFG MATERIALS GROWN WITHOUT CARBON CONTAINING GAS

ATMOSPHERE)

-20-

TABLE 6_

"A" FACTORS FROM DART: I-V MEASUREMENTS

CELL NOS. A CFF

1 1.71 .77

1.69 .75

NEW EFG

WITH CO21 1.69 .76

AMBIENT 9 1.62 .75

11 2.05 .70

12 1.75 .74

14 1.79 .73

16 2.14 .69

17 1.63 .72

EARLIER EFG19 2.0 .70

21 1.7 .75

22 1.45 .74

23 1.73 .74

C-3 1.83 .75CZ CONTROL

C-4 1.27 .78

-21-

F TABLE 7P'

rEFFECTIVE MINORITY CARRIER DIFFUSION LENGTHS OF

EFG RIBBONS WITH CO2

0 000 0

Diameter of the Circle = 3mm

DIFFUSION LENGTH m

WHOLE 1 2 3 4 5

Cz Control119 119 120 126 126 120

C-3

New EFG (CO2)40 50 48 40 40 34

#7

New EFG (CO2)

#1328 26 25 18 23 34

EARLIER EFG

#161R 14 24 18 20 17

EARLIER EFG I

#2428 34 30 32 3 t . 19

-22-

TABLE 8

EFFECTIVE MINORITY CARRIER DIFFUSION LENGTH OF

SELECTED SOLAR CELLS FROM EFG RIBBONS WITH AND WITHOUT

CO IN AMBIENT

EFG WITH CO EFG WITHOUT CO CZ

CONTROL

CELL I.D. #8 #10 #7 #3

DIFFUSIONLENGTH 50 35 35 25 140

NOTE: Diffusion Length (effective) Measured on Whole

Area (2x2cm) Using Short Circuit Current Method.

-23-

r^

B. Deneritk Web Solar Cells


Blank shaping ( 2x2cm ) and removal of the surface deposit (SiO) were

carried out using the same method described in Section E of Reference (1).

Baseline solar cells were fabricated from dendritic webs for various runs; Run

17-1373 0176-213, 8.5 ohm-cm), 17-1377 (J220-2.6, 3.4 ohm-cm), and 17-1390

0183- 1.5, 9.4 ohm-cm). Efforts were made to improve the performance by

forming a shallow junction, using fine grid lines, BSF and MLAR coating.

Baseline solar cells were also fabricated from the same webs and the results

were compared. Dendritic webs tested were #17-1387 (J180-2 .7, 8 ohm-cm), 17-

1388 0181-3.7, 11 ohm-cm), 17-1389 0181-3.8, ilohm-cm), 17-1402 (AA009-4.4,

3 ohm-cm).

Baseline solar cells were also fabricated on 2x2 wafers pre-characterized

in terms of dislocation density by MRI (Utah), in an effort to correlate solar cell

parameters with defects. Table 9 shows dislocation densities provided by MRI.

The table indicates there is no significant difference in dislocation count

between the samples. See Reference (6) for the details of the Web process.


Characteristics Under Wumination

Finished solar cells were tested under AM 1 condition at 280C test block

temperature. Table 10 summarizes cell parameters of the baseline process,

indica!ing an average efficiency slightly higher than 11% for dendritic web cells.

NOTE: The average efficiency of the CZ control cells (starting substrate

resistivity of 1-3 ohm-cm) was about 13%. Lower efficiency of the web cells

was mainly due to lower open circuit voltage, caused by the higher starting

-24-

substrate resistivity of the webs. Short circuit current density of the web cell

was lower than the control cells, (by about 1mA/cm 2) suggesting that the quality

of the webs under investigation is not quite so good as that of conventional CZ!

silicon. Cell parameters from the advanced process are summarized in Table 11,

showing an average efficiency of 12.5% which is an efficiency improvement of

about 1% over the baseline solar cells. Open circuit voltage enhancemnent by

the BSF process does not seem to be as effective as in earlier tests. The reason

1for the small improvement in Voc is not known at present.

For the pre-characterized cells, list in Table 9, baseline solar cell

9

parameters are summarized in Table 12. As expected, no significant difference

in various parameters is indicated. The average efficiency was about 10.8% AM I

which is a slightly lower value than that of the CZ control cells of similar

substrate resistivity, mainly due to lower Jsc.

a Spectra! Response

Absolute spectral response (A/W) was obtained using a filter wheel set-up.

The results are plotted in Figure 8 for the baseline solar cells. The web cells

show response close to those of the CZ control cells, yet showing slightly lower

response in the long wavelength region.

Minority Carrier Diffusion Length

Minority carrier diffusion length (L D) was measured by the filter wheel

method using th- -snort circuit current method. Selected samples were measured

by illuminating the whole area. The results showed values of about 130um for #6

cell (from web I.D.// 17 .. 1 3), and 1 10um for #14 cell (from web I.D. #17-1377),

while L n of the CZ control cell indicated about 150um.

-25-

^.

Y

The results of the samples from the pre-characterized webs are listed in a.

Table 13 and they are quite uniform. The whole range of variation was between

58 and 68um which agrees well with the MR[ data in terms of defect counts.

-26-

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TABLE 9

ANALYSIS OF DENDRITIC WEB SAMPLES

JPLSAMPLE #

NO. OF DISLOCATIONSPITS/FIELD

NO. OF DISLOCATIONSPITS/um2

J250-4.7-A 17.808 2.737 x 10-4

J250-4.7-B 14.946 2.298 x 10-4

J250-4.7-C 12.146 1.867 x 10-4

J250-4.7-D 16.614 2.554 x 10-4

J250-4.7-E 15.526 2.387 x 10-4

J250-4.7-F 15.800 2.429 x 10-4

J250-4.7-K1 15.828 2.433 x 10-4

J250-4.7-K2 16.615 2.554 x 10-4

J250-4.7-L 1 37.424 5.753 x 10-4

J250-4.7-L 2 27.082 3.702 x 10-4

-28-

TABLE 10

DENDRITIC WEB SOLAR CELL FROM BASELINE PROCESS

WEB I.D. NO.

CZCONTROL11-173

Pr-8. 0-CM11-1 773

p-3.4sa-cm

17-1390

p-9.49-cm

Voc

(mV)

AV. 532 534 515 588

S.D. 530-534 532536 512518 584--590

R 2 2 3 3

Jsc

(mA/cm2 )

AV. 28.8 28.1 28.6 29.8

S.D. 0.5 0.3 0.5 0.5

R 28.s-29.4 27.8-28.4 27.4-29.0 2q.3-30.0

CFF

(X)

AV. 76 76 75 74

S.O. 1 1 1 3

R 75-76 75-76 7477 70-76

M

AV. 11.6 11. 4 11.0 13.0

SEP0.1 0.1 0.2 0.6

411.7 11.311.5 10.611.3 12.2,-13.5

NOTE: 1) 2x2 cm cells under AM1 measured at 28 0C test block

temperature.

2) CZ Control Cells: 1-3 ohm-cm

-29-

TABLE 11

DENDRITIC WEB SOLAR CELLS FROM ADVANCED PROCESS

CONTROL CELLS (NO BSF)

WEB WEB CZ

Voc

(mV)

AV. 545 531 581

S.D. 14 11 -

R 582-558 514-546 578-582

Jsc

(mA/cm2 )

AV. 29.2 28.1 29.9

S.D. 0.6 0.5 -

R 28.5-29.8 27.4-28.8 29.3 -30.4

CFF

M

AV. 79 78 78

S.D. 1 1 1

R 78-80 75-•79 77-79

rQM

AV. 12.5 11.7 13.5

S.D. 0.6 0.5 -

R 11.8-13.0 10.9-12.2 13.2-13.7

NOTE: 1) Measured under AM1 at 280C test block temperature.

2) Advanced process: SJ+BSF+MLAR

3) CZ Ccntrol Cells: 1-3 ohm-cm

-30-

• TABLE 12

SUMMARY OF THE PRE-CHARACTERIZED WEB WAFERS

i

ffi Voc (m V) Jsc (mA/cm 2)

CFFM

1 rl M

AVERAGE 1 334 1 26.3 1 77 1 10.8

iSTANDARD

1 .1 1 .2DEVIATION

RANGE 1 532-534 1 26.2-26.5 1 76-78 1 10.6-10.8 11

-31-

TABLE 13

MINORITY DIFFUSION LENGTHS OF THE

PRE-CHARACTERIZED WES CELLS

SAMPLE I.D. L (um)

C 65

T2 62

F 58

K-1 62

K-2 63

L 62

Control #11 121

11

i

-32-

. C. Solar Cells From Cast Silicon ingots by Heat Exchanger Method (HEM)

Four HEM cast silicon ingots were evaluated during the contract period; a

r.st cube section ( 411x4 1I x 411 , Crystal System ID #41-07), a small cast ingot (N611x

11 11 11 116 x 4 ,Crystal System !D #41-24), and two large cast ingots (-12"x 12 x 6 ,

Crystal System ID # Is 41-41C and 41-48).1

lThe crystals used semiconductor grade starting materials, and the purpose

of the study was to test material uniformity by measuring solar cell performance

as a function of the positio in the crystals. Reference (7) provides technical

details of the HEM process.

1.0 Solar Cells From A Cube Section (#41- 07)

i —

Solar Cell Fabrication

The cube was cut horizontally with respect to its casting position (refer to

Figure 9 for wafer preparation) and wafers were marked relative to the top of

the ingot. The ingot was mostly single crystal and measurement of the

resistivity indicated about 2 ohm-cm with P-type conductivity. 2x2 blanks were

cut from the ingot and they were all marked such that their positions were

traceable. Baseline process was applied to fabricate cells and for some of the

cells, an extra step of gettering by diffusion glass was applied before the

baseline process. ,also, an attempt to anneal the wafers was made on some

samples before the baseline process. Because this crystal showed a clear

correlation between performance and position, high efficiency processing

(including gettering, shallow junction with narrow grid lines, BSF, MLAR) was

applied to the best portion of the crystal.

*In Section C, figures and tables are placed at the ends of sub-sections 10, 20,

etc., since these subsections are fairly long.

C.harae-Wistics Under Wumination

Solar cells made by the baseline process had 90% active area and SiO AR

coating while high efficiency cells had 94% active area and MAR coating.

Solar cell parameters such as Voc, lsc, CFF, and n were measured at 29 0C under

AM 1 conditions (see Reference 1 for description of simulator). In order to

illustrate the relations between solar cell performance and position within the

crystals, Figure 9 gives average solar cell efficiency (using the baseline process)

as a function of position. It is obvious that a definite pattern can be 'ablished;

best solar cell performance is found in the upper middle portion of the crystal,

the top portion is slightly inferior, and performance deteriorates in the lower

halt with the worst cells close to the seed. After gettering treatme pt, most

improvement was found in the lower middle portion of the crystal. Annealing at

6000C made no significant improvement in cell performance. For the best

portion of the crystal, cell performance, whether baseline process or high

efficiency process is comparable to the single crystal CZ controls. The result of

high efficiency processing is given in Table 14. Also, no significant deviation

was detected between the edge and center portions of any wafer, nor between

polycrystalline and single crystalline portions. For this horizontally cut (lt41-07)

crystal, the only positional variable was the distance from the bottom surface.

Spectral esponse

Absolute spectral response (A/W) was obtained using a filter wheel set-up.

Figures 10 to 12 show the response of selected samples of baseline, gettered +

baseline, and high efficiency processes for samples from the horizontally cut

crystal W41-07).

-34-

Light Bias Minority Carrier Diffusion Length Measurement

The varying performance of solar cells from different portions of HEM

crystals indicate the possibility of existence of recombination centers in the low

Eperformance positions. Light bias minority carrier diffusion length

measurements should indicate their existence. Selected samples from both

crystals were measured by the SSI 100 diffusion length meter which ha, :s built in

light bias source.

The difference between the SSI system and a filter wheel is that the SSI

can only measure a relatively small localized area of the solar cell. The bias

source was a tungsten-halogen lamp. The bias level varied from approximately

1/3 sun to 6.8 suns. The results are shown in Figure 13. The cells with lower L

showed higher relative increase in L with illumination. Also observed were

photondegradation effect in some of the cells as indicated by cell 2-3.

DLTS Measurements

In order to look at the defect structure from a different direction, some

preliminary deep level transient spectroscopy (DLTS) measurements for the

horizontally cut crystals were made. The DLTS system used (reference 118 for

the DLTS system) heloiigs to JPL. Not enough measurements were made to

achieve consistency of the results. However, preliminary indications were that

extra distributions of defect levels existed in the lower portion of the crystal (8

and 12 series) while these levels could not be found in the upper portion (5 and 2

series). This is consistent with the light bias results. There is also some

indication that a common defect level exists in all cells.

-35-

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FIGURE 13 »-1

LO's vs. No. of Suns for Horizontally Cut HEM Cube Cells (#41-07)

(Cells No. in Parenthessis)

o a (5-22)0

160

120

0

7 ^ Q ^] ^'1 f rJNI

-a2-

TABLE 14

SUMMARY OF HIGH EFFICIENCY CELLS FROM A SELECTED PORTION

OF A HEM INGOT (#41-07)

Voc (mV) Jsc (mA/cm2 CFF (%) (%)

AV 597 32.4 78 15.0

S.D. 3 .6 2 .5

R 588-602 30.6-33.3 75-80 13.5-15.7

NOTE: 1) 2x2 cm cells with AR coating measured under AM1

at 280C test block temperature.

2) Solar cells were fabricated from a portion of the

ingot (#41-07), where good baseline performance

was obtained.

3) Processes used were Gettering, SJ, BSF, and MLAR.

-43-

2.0 Solar Cells From A Small Cast kwt (041-24)

Solar Cell Fabrk:atktn

A whole cast ingot was cut vertically with respect to casting position and

only central parts of the crystal were used for evaluation (Figure 14 for the

details). 2x2cm silicon blanks were prepared from the wafers and baseline solar

cells were fabricated.

Gettering by diffusion glass was also carried out. Two gettering

experiments were made; one on the cleaned saw-cut wafers and the other on the

chemically polished wafers.

Characteristics Under Ulumination

Finished baseline solar cells had no AR coating and about 90% active area

with Ti-Pd-Ag metallization. Solar cell parameters, soich as Isc, Voc, CFF, and

n, were measured under an AM 1 conditions at 280C test block temperature.

Figure 15 to l8 show mappings of the solar cell parameters; a definite

pattern cannot easily be established. Judging from Isc (Figure 15) it seems that

the edges except on the bottom section .ire lowest and across the ingot the upper

middle portion is the worst section. (Only Might deterioration occured close to

the seed in contra-,t to the horizontally cut crystal.) All the cells were inferior

to the CZ control cells.

Normalized efficiencies of the baseline (with respect to the CZ controls)

HEM cells with the gettering process are shown in Figure 19. The result

suggested; 1) dependence of cell performance on the location is similar with the

-44-

- - - - --y

i

baseline process, and 2) no significant improvement in cell performance was t

noticed after the gettering.

Minorittr Carrier Diffusion !,e!Wh

Light bias minority carrier diffusion length was carried out using the same

method described in section 1.0. The results of the selected samples are plotted

in Figure 20, indicating similar results with the cells fromthe cube (041-01).

I-45-

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Lp vs. No. of Suns for Vertically Cut HEM ( #41-24) Cells

(Cells' No. in Parenthesis)um120

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-52-

tI

3.0 Solar Cells From A Large Cast Ingot (041-41C)


Baseline solar cells were fabricated on wafers cut from a large cast ingot

(ingot #41-41C was 12"x 12"x 6" in dimension, v40kg). The positions where the

Iwafers were cut is illustrated in Figure 21. Cross section layers of half ingots

were cut at three vertical positions and three horizontal positions for each ingot.

About 48 - 2x2cm cells were fabricated on each layer chosen to represent each

ingot position. Cell fabrication was completed in all six layers for the ingot.

Resistivity measured by four point probe was about 2 ohm-cm with, p-type

conductivity.

Characteristics Under Illumination

Finished baseline solar cells had SiO AR coating and about 90% active area

with Ti-Pd-Ag metallization. Solar cell parameters, such as lsc, Voc, CFF, and n

were measured under AM 1 conditions at 280C test block temperature. Figures

22 to Figure 27 show the mappings of cells normalized n (to CZ control) on all six

layers for the ingot. This ingot demonstrated the best performance for all the

HEM ingots measured to date. Except for the region close to the seed at center

bottom, with some areas showing low Jsc and shunting problems (top layer), the

cell performances were quite uniform throughout the ingot including the

polycrystalline area. (See the Figures in text.) The overall efficiency of the

usable area of the ingot is about 87% of that of the baseline CZ control cells.

Spectral Response

Absolute spectral response (A/W) measurements were made using a filter

wheel set-up. Response versus wavelength of selected cells from a center lay: r

-53-

is given in Figure 28. The figure shows a variation of the response (mostly in

long wavelength region) from cell to cell depending on the position.

MIWIty Carrier Diffusion Length

Minority carrier diffusion length (LD) was measured on the same solar cells

used to measure spectral response, using the short circuit current method. The

results of spectral response and minority carrier diffusion length measurements

indicate that there is substantial variation of L from top to bottom in the

ingots (from 15um to 120um, mostly above 30um). This variation is not reflected

as strongly from the Jsc data because Jsc does not drop substantially until L

40um. However, one would expect that more pronounced differences will show

up when a back surface field is added.

-54-

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4.0 Solar Cells From A Large Cast Ingot 041-48)


Methods like those described in section 3.0 were used for the evaluation of

another large cast ingot (12"x 12"x 6", 40kg). Measured resistivity was about 6

ohm-cm with P-type conductivity.i

Characteristics Under Mumination

Baseline solar cells had about 90% active area with SiO AR coating. Solar.,.

cell parameters such as Voc, Jsc, CFF, and n were obtained under AM 1

simulation conditions at 280C test block temperature.

Figure 29 to Figure 31 show mappings of normalized n (to CZ control) of

cells for each of the three vertically cut layers and Figures 32 to Figure 34 show

results of the three horizontally cut layers. The solar cell performance of this

ingot is much worse than that of #41-41C. Voc and CFF indicated severe

shunting problems in relatively large portions of the ingot ; especially close to the

edge. (The shunted areas are separated out in the figures and excluded from the

averaging of efficiency). The cause of the shunting is not immediately known.

Portions of the shun 1ting area had high density of micro-precipitates as shown in

the microscope pict..:-es of Figure 35. In ;ome cases these micro-precipitates

were observed at grain boundaries of very fine grain areas. However, these fine

grain areas only account for a small portion of the total shunting area.

Inclusions were also noted in some other areas in the bulk.

The shunted area is estimated to be about 30 to 35% of the whole ingot,

mostly occurring along the edge and on the top of the upper half of the ingot (see

figures). The top polycrystalline area where the final solidification occurred

ri

-64-

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seems to be shunted m st as judged by the cell performance. Other areas

showing shunting problems are polycrystalline areas along the edge downward.

SThe single crystalline area and the lower polycrystalline areas away from the

edge are relatively free of the shunting problems. It could be speculated that

the shunting is caused by inclusions, most likely precipitates at the final

{ solidification stage. These results merit further material study. The total

[ effectiveness of the ingot was estimated to be about 60% compared to a CZ

control ingot.

Spectral Response

Absolute spectral response (A/W) measurements were made using a filter

wheel set-up. Response of the selected cells from a vertically cut center layer

is shown in Figure 36, indicating poor performance of some cells in the long

wavelength region.

Minority Carrier Diffusion Length

Minority carrier diffusion length (Lp) was measured on the finished solar

cells using the short circuit current method. Measurements gave similar results

with the ingot (#41-41C) in previous section, showing a wide variation of L

from 10 to l 10um depending on the location.

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Microscopic Picture of Inclusions G'served

in a HEM Ingot (4.1-48), 20OX Magnification

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-71

D. Sorer Cells From Gut kwt Bt ► Ubiguitekss Ca"aUlzation Rocco (UCP) -


Six (6) polycrystalline UCP wafers ( 10xlOcm) representatinr, six (6)

dif_event groups of material were delivered. Each wafer was polished &,, cut to

2x2cm blanks with their position marked (see Figure 37 for the positio n ;!). With

`. the orientation of each wafer and its position in each group of material known,

one can Use the results of the cell performance to correlate with the properties

of each material group.

All wafers were polycrystalline with mm size grains. Measured resistivity

was about 3 ohm-cm. Baseline process was applied to fabricate solar cells.

Refer to reference (9) for the details of UCP process.


Characteristics Under Illumination

Solar cell parameters, such as Jsc, Voc, CFF, and n were measured under

AM 1 conditions at 280C test block temperature. The results of the c::ils are

summarized in Table 15. One can see from the Table that the cell performance

was relatively Lnifor . n. The few cells which had shunting problems showed

inclusions under T crosc ,--pe observation. Figure 38 shows microscopic pictures

of the inclusions Ayierved in a cell which showed severe shunting problems.

Analysis of data and visual observation indicated that the variation of Jsc

was related to the grain size of the material and the average Jsc was lower than

the CZ control by more than 11%. The lower Voc could be partly accounted for

by slightly higher resistivity of the material (- 3ohm -corn) than the CZ control (-1

ohm-cm). The average efficiency was consido rably lower than the ^ ' , . control;

10.6% versus 13.1%.

-74-

y

S _at_ Response

Absolute spectral response (A/W) was measured using the filter wheel set-(( up. Plots of the response of representative cells without AR coating are given in

is Figure 39. The UCP cells gave lower response than the CZ control cell,i

especially at long wavelength ( > 0.6um), suggesting reduced minority carrier

diffusion length.

1Minority Carrier Diffusion Length

Effective minority carrier diffusion length (Lp) was obtained using the

short circuit current method of the finished solar cells. Results from selected

samples are summarized in Table 16, in which short circuit current density

information is given in right hand column for reference. The table indicates a

range of L from 20 to 80um, with a typical L of around 40-50um.

Photoresporw ! By Small Light Spot Scanning

Localized photoresponse of the UCP solar cells was obtained by light

scanning. Refer to Appendix of Reference (1) for the details of the

measurement. Typical scanning results are given in Figure 40, in which (A)

represents scanning of a cell which is re!iL Lively free from grain boundaries, and

(B) response of a cell with small grain structure. The cell with smaller grains

showed reduced response, and wide spectral variations. Photoresponse of the CZ

control cells are shown.

The figures indicate that response in the bule of the UCP cell is lower than

the CZ controls.

-75-

FIGURE 37

MAPPING OF THE 2 X 2 CELLS IN SEMIX WAFER

1 2 i 3 i 4I I I i 'I ( I I I

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I I ^

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i I I I ^1 13 1 14 I 15 1 16 II I I I I

— ---1 -----L -----I-- ---^

1-4 10 cm

Shown here are the cells' number and their positions with the orientation

of the wafer pre-determined.

10cm

-76-

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FIGURE 38. Microscopic Phonographs of Inclusions

,(or Precipitates) Observed in UCP Wafers

200X Magnification, (a) From a Cell

(#E-13), (b) From a Cell (#E5).

0110 ENNT, PAGE I5

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TABLE 15

SUMMARY OF RESULTS OF THE SOLAR CELLS FROM CAST INGOT

BY UCP

Voc Jsc I CFF 'n NO.OFWAFER # (Inv) (mA/cin (9b) (96) CELLS

Ave. 559 25.1 78 10.9A-5 S.D. 6 0.9 1 0.5 14

c<ange 546-570 23.0-26.4 74-79 9.9-11.8

Ave. 554 25.1 76 10.6

B-3 S.D. 9 1.2 2 0.7 15Range 540-568 23.1-26.9 70-79 9.6-12.0

IAve. 550 25.5 76 10.7

C-1 S.D. 5 0.5 1 0.4 12Range 542-558 24.4-26.4 73-77 9.7-11.1

Ave. 557 26.0 76 11.0

D-3 S.D. 8 0.7 2 .6 12Range 542-568 25.0-26.8 70-78 9.5-11.7

Ave. 543 25.4 72 9.9E-7 S.D. 14 0.6 10 1.5 12

Range 504-558 24.0-26.1 44-78 5.5-11.2

Ave. 555 ( 24.9 75 10.4F-3 S.D. 7 I 0.8 2 0.5 13

Range 540-570 I 23.1-26.1 72-78 i 9.4-11.5 i

Combining .Ave. 553 25.3 ^ 76 -^ 10,.6 ' 7°All Wafers Range 504-570 23.0-26.0 I 44-79 5.5-12.0

Ave. 586 28.7 78 13.1CZ. Control S.D. 0.2 1 I 0.1 3

Range - 28.5-28.9 77-79 ; 13.0-13.2

-81-

TABLE 16

EFFECTIVE MINORITY CARRIER DIFFUSION LENGTH OF rOLAR CELLS

MADE FROM UCP WAFERS

Jsc(mA/cm 2)CELL NO. LD(um) (No AR)

A-5-10 48 18.6

GOOD CELLS B-3-5 72 19.2

D-3-1 63 18.7

A-5-15 44 17.6

AVE.CELLS C-1-14 48 17.6

E-7-1 46 17.6

A-5-12 28 16.3

BAD CELLS B-3-2 23 16.2

F-3-6 37 16.3

CONTROL #2 177 20.5

-82-

III. CONCLUSIONS AND RECOMMENDATIONS

The conclusions and recommendations reached after processing and

evaluation of the sheets are as follows:

EFG Ribbons

o EFG ribbons grown in carbon containing gas atmosphere showed significant

improvement in sheet quality. Chemical etching study indicated that new

i EFG showed better structural perfection; i.e.. larger grain size and lessi

dislocation density, and the diffusion length was significantly higher.

a The new EFG demonstrated an average baseline col efficiency of 10.3%

AM 1 as opposed to 8.0% AM 1 for the earlier EFG. The best efficiency

achieved by the advan =ed process was 13.8% under AM 1 conditions.

o Baseline solar cells frorn dendritic webs of various runs suggested that

quality of the webs studied in this period was not as good as the

conventional CZ silicon, about 60um versus 120um, and was slightly lower

than that of the better web samples tested earlier.

o Performances -)f the baseline solar cells agree well with the dislocation

counts analyzed by MRI, especially in terms of uniformity.

-83-

i^^

HEM

o Detail evaluation of four ingots suggested that HEM has problems of ingot

reproducibility from run to run and also in uniformity of sheet quality

within an ingot. Junction shunting was major cause of the problem, which

could be due to the particulate inclusions observed in the bulk.

o Gettering and low temperature annealing experiment did not improve cell

efficiency significantly. Detailed analysis of defects and impurities is

necessary to identify the potential causes of degraded silicon sheet quality.

UCP

o Initial evaluation of the wafers prepared from the cast ingot by the process

suggested that the quality of the wafers by this process is considerably

lower than the CZ control; average baseline efficiency 10.6% versus 13.1%.

o Overall cell performance was relatively uniform with an exception of few

cells which showed shunting problems caused by particulate inclusions.

-84-

IV. WORK PLAN STATUS

Next phase of the program is expected to continue efforts of the previous

pnases, with an emphasis on improvement of solar cell efficiency by process

optimization and development of new techniques tailored to suit the specific

sheet form. Efforts will be extended to fabricate larger area solar cells. Increased

efforts will be made to correlate the results of defect studies at other laboratories,

with the cell resul*c!.

14,f

-85-

V. REFERENCES

1. H.I. Yoo, et.al . "Silicon Solar Cell Process Development, Fabrication and

Analysis". JPL Contract No. 955089, Annual Report (Phase I), June 1979.

2 H.I. Yoo, et.al . "Silicon Sola r Cell Process Development, Fabrication and

Analysis". JPL Contract No. 955089, Annual Report (Phase II), June 1980.

3. F.V. Wald, et.al . "Large Area Silicon Sheet by EFG", JPL Contract No.

954355, Technical Reports for the LSA Project, Mobil-Tyco.

4. F. Ferraris and F.C. Matacotta, "Optimized Diffusion Parameters for Cell

Made byColumnar Silicor;". Proc. of European Photovoltaic Solar Energy

Conf., pp.625, October 1980.

5. R. Natesh, et.al. "Quantitative Analysis of Defects in Silicon". Final

Report for the LSA Project, JPL, April ;980.

6. C.S. Duncan, !t.al., "Silicon Web Process". JPL Contract No. 954654,

Technical Report, Westinghouse.

7. F.Schmid, et.al ., "Silicon Ingot Casting Heat Exchanger Method, Multi-Wire

Slicing-Fixed Abrasive Slicing Technique". JPL Contract No. 954373,

Technical Reports for the LSA Project, Crystal System.

lV

-86-

8. D.V. Lange 11 0eep Level Transient Spectroscopy: A New Method to

Characterize Traps in Semiconductors", Journal of Applied Physics, Vol.45,

P.3023-3032, 1974.

9. "Semicrystalline Casting Process Development and Verification", SEMIX

Inc., Technical Reports, n6E Contract No. DE-FC01-80ET-23199.

i

-87-

APPENDIX

TIME SCHEDULE

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APPENDIX II

ABBREVIATIONS

VCC: Open Circuit Voltage

ISC: Short Circuit Current

iSC' Short Circuit Current Density

ISCR' Short Circuit Current (Red Response) at Wavelength Above .6um

ISCB: Short Circuit Current (Blue Response) at Wavelength Below .6um

CFF: Curve Fill Factor

'9 : Solar Cell Conversion Efficiency

L: Minority Carrier Diffusion Length (D.L.)

IMAX' Current at Maximum Power Point

VMAX' Voltage at Maximum Power Point

BSF: Back Surface Field

BSR: Back Surface Reflector

V B Bias Voltage

Io: Diode Saturation Current

HEM: Heat Exchanger Method

EFG: Edge Defined Film-Fed Growth

SOC: Silicon on Ceramic

RTR: Ribbon-to- Ribbon

UCP: Ubiquitous Crystallization Process

SPV: Surface Photovoltage

MLAR: Multi-Layer Anti-Reflective

Rs : Series Resistance

development, fabrication and analysis annual report …

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