SERI/STR-211-2451 

UC Category: 63 

DE85000507 

Research on the Basic 

Understanding of High 

Efficiency in Silicon 

Solar Cells 

Annual Report 

1 December 1982 - 30 November 1983 

A Subcontract Report 

A. Rohatgi 

P. Rai-Choudhury 

Westinghouse R&D Center 

Pittsburgh, Pennsylvania 

September 1984 

Prepared under Subcontract No. X8-3-02090-4 

SERI Technical Monitor: J. Milstein 

Solar Energy Research Institute 

A Division of Midwest Research Institute 

1617 Cole Boulevard 

Golden, Colorado 80401 

Prepared for the 

U.S. Department of Energy 

Contract No. DE-AC02-83CH10093 
Printed in the United States of America 

Available from: 

National Technical Information Service 

U.S. Department of Commerce 

5285 Port Royal Road 

Springfield, VA 22161 

Price: 

Microfiche A01 

Printed Copy AO? 

NOTICE 

This report was prepared as an account of work sponsored by the United States 

Government. Neither the United States nor the United States Department of Energy, 

nor any of their employees, nor any of their contractors, subcontractors, or their 

employees, makes any warranty, express or implied, or assumes any legal liability 

or responsibility for the accuracy, completeness or usefulness of any information, 

apparatus, product or process disclosed, or represents that its use would not 

infringe privately owned rights. 
1 • 

2. 

3. 

CONTENTS 

SUMMARY •••••••••••••••••••••••••••••••••••••••••••••••••••••••• 1 

INTRODUCTION ••••••••••••••••••••••••••••••••••••••••••••••••••• 5 

2.1 

2.2 

2.3 

Objecti ve .................................................. . 

Technical Approach ••.••.•••.•.•••.••••••••..•...•.•••••..• 

2.2.1 

2.2.2 

2.2.3 

2.2.4 

Theory and DesigT'l •••••••••••••••••••••••••••••••••• 

Heavy Doping ann Recombination ••..•••.••..•.•.•.••. 

Resistive Losses and Grid Desig~ ••••••••••••••••••• 

Optical Losses •.••.••.•.•••••.••...•.•••..•.•.••••• 

Experimental Approac~ ••••••••••••••••••••••••••••••••••••• 

2.3.1 

2.3.2 

2.3.3 

Material and Dopant Selection ••••••...••..•........ 

Oxide Passivation •.•.••.•••..••••••••••.•••..•••.•. 

Oxide-Passivated Solar Cells with High-Low 

5 

5 

5 

11 

13 

15 

15 

16 

Emitter.......... ..••••............................ 16 

2.3.4 Oxide-Passivated Cells with High-Low Emitter 

and MIS-Type Contacts.............................. 16 

2.3.5 Oxide-Passivated Cells with Multilayer AR 

Coating and Reflective Back Metal ....•••.••....•.•. 17 

TECHNICAL PROGRESS ••••••••••••••••••••••••••••••••••••••••••••• 18 

3.1 

3.2 

3.3 

3.4 

3.5 

Model Calculations and Discussion •..••...•••.•.••••.••.... 

Baseline 

B::lseline 

Material Selection and Qualification ••••••• 

+ + 

n -p-p Solar Cells on 4 ohm-ern 

Float-Zone Silicon •••••••••••••••••••••••• 

Cell Testing With and Without a Metal Mas~ •••••••••••••••• 

Oxide-Passivated 17.2% Efficient Solar Cells on 

4 0.-cm Float-Zone Silicon ........................... . 

3.5.1 Ellipsometric Measurements on 17.2% Efficient 

18 

24 

27 

29 

30 

Cel1s •••••••.. 1 •••••••••••••••••••••••••••••••••••• 32 

3.').2 

3.5.3 

I-V Analysis of the 17.2% Efficient Cells .•.•... 36 

Reflectivity and Spectral Response Measurements 

on 17.2% Efficient Cells........................... 36 

i 
4. 

5. 

6. 

7. 

8. 

3.5.4 Diffusion Length Determination from Spectral 

Response Data.oo.ooo ••••••••• oo •••••••••••••• oo.ooo 42 

3.6 

3.5.5 

3.5.6 

Spreading Resistance Measurements on 17.2%' 

Efficient Cells •••••••••••••••••••••••••••••••••••• 

Effectiveness of Front- and Back-Surface 

Passivation in the Oxide-Passivated Cells •••••••••• 

3.5.7 Analytical Summary of 17.2% Efficient Cells •••••••• 

Oxide-Passivated Solar Cells Fabricated on 0.1-0.2 

Q-cm Boron-Doped Float-Zone Silicon ••••••••••••••••••• 

3.7 Oxide-Passivated Solar Cells Fabricated on 0.75 Q-cm 

44 

4h 

48 

50 

Boron-Doped Float-Zone Silicon •••••• 0· ......... 0........... 53 

3.8 Oxide-Passivated Solar Cells on 0.3-0.7 Q-cm 

Gallium-Doped Czochralski Silicon......................... 57 

3.9 

3.10 

Gallium-Diffused Back-Surface Field ••••••••••••••••••••••• 

3.9.1 

3.9.2 

3.9.3 

3.9.4 

Chemistry of the OTG Process ••••••••••••••••••••••• 

The OTG Diffusion System ••••••••••••••••••••••••••• 

Process Sequence Used for Open-Tube Gallium 

Diffusion for Back-Surface Field Formation ••••••••• 

+ + n -p-p Solar Cells with Ga Back-Surface Field ••••• 

Plasma-Depos~ted Thin Films for Multilayer 

60 

60 

63 

64 

~4 

Antireflective Coating................................... ~8 

3.11 High-Low Emitter, MIS Contacts, and Reflective 

Back Contact ••••••••••••••••••••••••.••••••••••••.••.••. o 70 

CONCLUSIONS I) I) I) •• 0 I) 0 • I) (I " I) " .01) 0 I) 0 I) 0 0 •• 0 I) I) • I) I) 0 0 I) • I) I) • 0 • I) I) •••• I) 13 • I) I) • 

PROGRAM STATUS 0 ••••• I) •••• I) ••••• I) ••••• I) •• I) ••••••• 0 0 •• 

5.1 Present Status •••••••••••••••••••••••••••••••••••••••••••• 

5.2 Future Activity .•....... I) I) •••••••• I) ••••••• I) ••••••••••••••• 

RE F E REN CE S •• 0 0 I) ••• 0 • • • • • • I) 0 • • • • • I) •• 0 0 • • • • • • • •••• 0 •••• CI ••••••• I) • 

ACKNOWLEOCMENTS •••••••• I) •••• I) I) •••••••• I) • I) •••• I) I) •• I) •• I) • I) •••••••• 

APP E ND I CE S I) ••• I) ••••••••• I) I) ••••••••• 0 • &I ••• 0 ••• 0 •••••••• 0 •••••••• 

Importance and Considerations of High-Efficiency 

2 

75 

75 

75 

78 

79 

Solar Cells &I •••••••••••••••••••••••• 0 ••• 00 •••••••••• a o. •• • 79 

2 Design Fabrication and Analysis of 17-18% Efficient 

Surface-Passivated Silicon Solar Cells •••••••••••••••••••• 

11 

95 
Figure 1. 

Figure 2. 

LIST OF FIGURES 

Internal recombination velocity plots for 

back-surface field solar cells ••••••••••••••••••••••••• 

Calculated efficiency as a function of molybdenum 

concentration for a narrow-base, back-surface 

8 

field, high-efficiency cell............................ 12 

Figure 3. 

Figure 4. 

Figure 5. 

Reflectance data for two- and three-layer antireflective coating designs for oxide-passivated 

surfaces. (The data following the curve numher in 

the legend represent the refractive indices and 

thicknesses of the antireflective coatings.) ••••••••••• 

Schematic diagram of the OLTS set-up ••••••••••••••••••• 

OLTS spectra for the baseline 4 ohm-cm 

14 

25 

float-zone silicon..................................... 26 

Figure 6. Capacitance versus time curve for determining the 

minority-carrier generation lifetime in baseline 

float-zone silicon..................................... 28 

Figure 7. 

Figure 8. 

Figure 9. 

Cell configuration ••••••••••••••••••••••••••••••••••••• 

Schematic diagram of oxide-passivated cells •••••••••••• 

Dark I-V data taken at SERI on a 17.2% oxidepassivated cellon 4 0.-cm float-zone silicon ••••••••••• 

Figure 10. Lighted I-V data taken at SERI on a 17.2% 

oxide-passivated cell fabricated on 4 Q-cm 

31 

33 

38 

float-zone silicon..................................... 39 

Figure 11. Reflectivity and relative spectral response of 

a 17.2% efficient oxide-passivated cellon 

4 r,-cm silicon......................................... 40 

Figure 12. Quantum efficiency versus wavelength plot for an 

unpassivated cell and 17.2% efficient oxidepassivated cellon 4 0.-cm float-zone silicon........... 41 

iii 
Figure 13. Diffusion length plot for an unpassivated cell 

and 17.2% efficient cellon 4 D.-cm float-zone 

silicon ................................................ 43 

Figure 14. 

Figure 15. 

Figure 16. 

Figure 17. 

Figure 18. 

Figure 19. 

Figure 20. 

Figure 21. 

Dopant profile in the emitter of a 17.2% efficient 

oxide-passivated cellon 4 ~-cm float-zone silicon ••••• 

Quantum efficiency versus wavelength plot for an 

unpassivated cell and a 16.9% efficient oxide-passivated cellon 0.1-0.2 D.-cm float-zone silicon •••••••••• 

Diffusion length plot for an unpassivated cell 

and a 16.9% efficient oxide-passivated cellon 

0.1-0.2 0.-cm float-zone silicon •••••••••••••••••••••••• 

Quantum efficiency versus wavelength plot for an 

unpassivated cell and a 16.7% efficient oxidepassivated cellon 0.75 0,-cm float-zone silicon •••••••• 

Diffusion length plot for an unpassivated cell 

and a 16.J% efficient oxide-passivated cellon 

0.75 0.-cm float-zone silicon •••••••• 4.~ •••••••••••••••• 

E!fect of abrupt doping concentration in the 

p region on the minimum effective recombination 

velocity at the p-p+ interface. W + was optimized 

at eac h concentrat1onP •••••••••••••••••••••••••••••••••• 

Effect of surface recombination velocity, S , and 

width of p+ region (W +) on the effective 0 

b o ° 1 ° P + ° f recom.1nat10n ve OC1ty at p-p 1nter ace ••••••••••••••• 

Arrangement for open-tuhe gallium diffusion ••••••••••• 

45 

52 

54 

55 

56 

hI 

62 

63 

Figure 22. Gallium back-surface field profile after 12300C 

open-tuhe gallium diffusion............................ 65 

Figure 23. Research deposition reactor............................ 67 

iv 
Table 1. 

Table 2. 

Table 3. 

Table 4. 

Table 5. 

Table 6. 

Table 7. 

Table ~. 

Table 9. 

Table 10. 

Table 11. 

Table 12. 

Table 13. 

LIST OF TABLES 

Calculated Performance for Narrow-Base Rack-Surface 

Field Solar Cells (4 ohm-cm, p-base, width 250 ~m) ••••• 

Example Output of Contact Design Program ••••••••.•••••• 

+ + Process Sequence for Fabricating n -p-p Solar Cells ••• 

Solar Cells (n+-p-p+) Fabricated on Baseline 

9 

13 

29 

4 ohm-cm Float-Zone Silicon............................ 30 

Cell Testing With and Without the Metal Mask With 

1 em x 1 em Window..................................... 32 

Process Sequence for Fabricating Oxide-Passivated 

Solar Cells............................................ 34 

Oxide-Passivated Solar Cells on Boron-Doped 4 

n-cm Float-Zone Silicon .•••.•••.•...•.••.•.••..•••..... 

A Comp~rison of the Cell Parameters of a Baseline 

n+-p-p Cell and 17.2% Oxide-Passivated Cell 

Fabricated on 4 0.-cm Float-Zone Silico~ •••••••••••••••• 

Solar Cell Data With: a) Both Surfaces Passivated, 

b) Only Front-Surface Passivation, and c) With No 

35 

37 

Surface Passivation.................................... 47 

Solar Cell Data on 0.1-0.2 0.-cm Float-Zone Silicon, 

With and Without Oxide Passivation ••••••••••••••••••••• 

A Comp~rison of the Cell Parameters of a Baseline 

n+-p-p Cell and 16.9% Oxide-Passivated Cell 

Fabricated on 0.1-0.2 0.-cm Float-7.one Silicon •••••••••• 

Solar Cell Data on 0.75 0.-cm Float-Zone Silicon, 

51 

57 

With and Without Oxide Passivation..................... 58 

A Comparison of the Cell Parameters of a Baseline 

n+-p-p+ Cell and 16.7% Oxide-Passivated Cell 

Fabricated on 0.75 0.-cm Float-Zone Silicon............. 59 

v 
Table 14. 

Table 15. 

Table 16. 

Baseline n+-p-p+ Cells and Oxide-Passivated 

+ + O-n -p-p -0 Solar Cells on 0.3 to 0.7 0,-cm 

Gallium-Doped Czochralski Crystal Grown From 

a Synthetic Quartz Crucible............................ 59 

A Comparison of n+-p-p+ Solar Cells with Diffused 

Boro~OBac~3Surface Field at 9500C (~surface ~ 10 cm ,W + ~ 0.5 ]..lm) and Galllum 'RacK-Surface 

Field Formed b~ OpeY8Tub~3Ga Diffusion at 12300C 

(N f ) ~ 5 x 10 cm , W + ~ 2 \.1m ••••••••••••••••• 

sur ace p 

Results of Single-Layer Plasm~-Dep~sited Silicon 

Nitride Coating on Baseline n -p-p Cells 

(AR Coating Thickness = 890 A and Refractive 

66 

Index = 1.95).......................................... 70 

vi 
1. SUMMARY 

The overall objective of this program is to develop a basic 

understanding of high efficiency silicon solar cells and achieve cell 

efficiencies greater than 17% by employing innovative concepts of 

material preparation, cell design, and fabrication technology. The 

program consists of a theoretical effort to develop models for very 

high-efficiency cell designs, experimental verification of the designs, 

and an improved understanding of efficiency-limiting mechanisms such as 

heavy doping effects and bulk and surface recombination. 

In this program we have used 4 ohm-em, high lifetime, (Ill) 

float-zone silicon obtained .from Monsanto as our baseline material, in 

addition to low-resistivity gallium-doped Czochralski silicon and some 

boron-.doped float-zone silicon in the resistivity range of 0.1-0.8 )I-cm. 

DLTS and generation lifetime measurements were performed on the 

4 )I-em baseline float-zone material to confirm its high quality, and no 

deep levels were detected in this material. The minority-carrier generation lifetime, determined by pulse MOS technique, was about 350 usecs. 

+ + 0 Baseline n -p-p solar cells were fabricated by 850 C/50 min 

POC13 diffusion and 9500 C/20 min RBr3 diffusion to form the emitter and 

the back-surface field, respectively. Cells were tested under 100 mH 

AMI illumination, and an average efficiency of ~ 14.5% was obtained for 

4 )I-cm base cells with the maximum exceeding 15%. Short-circuit current 

density and open-circuit voltage were 33 mA/cm2 and 580 mV, 

respectively. 

~-le have used a theoretical model which involves an internal 

recombination velocity to assess the minority-carrier loss mechanisms in 

various regions of the solar cell. Using the guidelines of our model, 

we passivated the cell surfaces by thermal oxidation of silicon at 

1 
8000 e. We have successfully fabricated the oxide-passivated cells on 

4 0.-cm base material with efficiencies in the range of 17 to 17.5%, 

open-circuit voltage # nOD mV, and short circuit current ~ 36 rnA/cm2 • 

As predicted by our model, oxide passivation provided an increase of 

about 20 mV in open-circuit voltage, ~ 3 rnA/cm2 increase in J sc ' and 

about 2% improvement in absolute cell efficiency compared to the 

counterpart 15% efficient unpassivated cells. These cells have heen 

tested and verified at SERlo 

Detailed dark I-V and spectral respon~e measurements were 

performed on the cells, with and withotlt passivation, to unnerstann the 

oxide-passivation-inducen improvement. Transformed dark I-V data, in 

which bulk and junction recombination components were properly separated, 

showed a decrease in reverse saturation current (Jo ) from 3.7 x 10- 12 

A/cm2 to 2.0 x 10-12 A/cm2 as a result of surface passivation. This 

decrease in J • coupled with an increase of 2-3 ~\/cm2 in J ,accounts o sc 

for the observed 20 mV increase in Voc. Spectral response and 

reflectivity measurements showed that a 17.2% efficient cellon 4 0.-cm 

material has significantly higher quantum efficiencies in the wavelength 

range of 0.4-0.7 urn and somewhat higher quantum efficiency in the range 

of 0.9-1.1 urn, compared to the counterpart unpassivated cell. This 

indicates that oxide passivation is quite effective on both surfaces and 

reduces the loss of carriers via surface recombination. 

We found that removal of back oxide from the passivated cells 

2 reduces J sc by ~ 0.5 rnA/cm and Voc by ~ 5 mV. The majority of increase 

in Voc comes from the front-surface passivation, while hoth front and 

back-surface passivation contribute to the increase in J

sc

' Thus, both 

front- and back-surface passivation are important ann effective in our 

devices. It appears that without any passivation, emitter surface 

recombination velocity controls the reverse saturation current (Jo 

J ); therefore, front passivation alone increases V and J • After 

oe oc sc 

the front-surface passivation, Joe is reduced and the J

o 

or Voc tends to 

hecome base depennent, and therefore an additional ~ 5 mV increase in V 

oc 

is observed when the back surface is passivated. 
+ + We have fabricated both baseline n -p-p and the oxine-passivated 

+ + O-n -p-p -0 cells on a gallium-doped 0.3 to 0.7 ~-cm Czochralski crystal 

which was pulled from a synthetic quartz crucible to minimize residual 

+ + impurity concentration. Efficiencies of the n -p-p solar cell were ~ 

14.75% and the oxide-passivated cell efficiencies were 

maximum exceeding 16.3%. 

the passivated cells. 

V was ~ 615 mV and J was 

oc sc 

16% with the 

33 mA/cm2 in 

High-efficiency solar cells on boron-doped, lower-resistivity, 

float-zone silicon have also been fabricated. Without the oxide 

passivation, solar cell efficiencies on 0.1-0.2 ~-cm material were 

~15.5%, and after passivation the efficiency went up to 16.7% with Voc 

of ~620 mV and J sc of ~33 mA/cm2 • On 0.75 ~-cm float-zone silicon, the 

cell efficiency was also ~15.5% without the oxide passivation, and went 

up to 16.9% as a result of oxide passivation. The open-circuit voltage 

was ~624 mV and the short-circuit current density was 34 mA/cm2

• In 

both cases we saw ~1-2 mA/cm2 improvement in J sc and about 10-15 mV 

improvement in V as a result of oxide passivation, which is somewhat 

oc 

less than what we found for 4 Q-cm material. This particular run had a 

patchy antireflective coating due to a slight problem in the photoresist 

step. We therefore anticipate better results on these crystals. 

In the case of 0.1-0.2 Q-cm material, quantum efficiency 

improvement was observed only in the shorter wavelength range probably 

because these wafers were ~ 350 ~m thick and cell width to diffusion 

length ratio was somewhat larger. This is consistent with the 

relatively smaller improvement observed in these cells due to the oxide 

passivation, compared to 4 ~-cm cells. 

Consistent with the model design, we fabricated a gallium backsurface field with a surface concentration of ~ 5 x 1018 cm- 3 , a depth 

of ~ 2 ~m by open-tube gallium (OTG) diffusion at 12300 C. In this first 

attempt, despite the desired surface concentration, we observed a 

. + + degradation in the n -p-p solar cell efficiencies from 15% to 13%. 

This loss in cell performance is attributed to a decrease in carrier 

3 
lifetime, suggesting that the gallium diffusion conditions need to be 

optimized. 

Few attempts have been made to deposit silicon nitride coating 

by a plasma-enhanced technique using silane and ammonia gases. Initial 

results on baseline n+-p-p+ cells look quite promising, indicating -45% 

enhancement in the short-circuit current and >50% improvement in cell 

efficiency. Open-circuit voltage is higher than for the cells with 

spin-on coating, suggesting that these films can also passivate the 

silicon surface. Multilayer AR coating with varying refractive index is 

being investigated. 

Attempts were made to apply reflective back contacts by 

replacing Ti-Pd-Ag with AI, Ag, and Au. Ag and Au back contacts peeled 

off hecause of poor adhesion with the back oxide but the results with Al 

back contact were encouraging. More work needs to be done in this area. 

One attempt was made to make a hi~h-low emitter using epitaxial 

growth. Poor lifetime in the epi-layer resulted in low cell 

efficiency. Further attempts are being made to improve this lifetime. 

4 
2. INTRODUCTION 

This report is a summary of activities conducted under Phase I 

of a SERI-sponsored program to develop high-efficiency solar cells. 

2 • 1 0 b j e c t i ve 

The objective of this program is to develop a basic understanding 

of high-efficiency silicon solar cells and to achieve cell efficiencies 

greater than 17%. Important considerations for improving silicon solar 

cell efficiency include high-purity silicon, proper cell design, and 

careful cell processing. Specifically, the current phase of the program 

is concerned with: 1) theoretical study or models leading to highefficiency cell designs, 2) material selection and experimental verification of the designs and analysis, and 3) identification and improved 

understanding of mechanisms which degrade or limit cell performance. 

2.2 Technical Approach 

2.2.1 Theory and Design 

Recent efforts to increase the efficiency of silicon solar cells 

have been correctly focussed on the problem of raising the open-circuit 

voltage. Current collection efficiency has reached near the theoretical 

limits.(1-3) This emphasis has led directly to the problem of maximizing 

the effective diffusion length in all parts of the device while using 

substrate material with the highest possible donor or acceptor concentrations. These provisions act in opposition and are modified by the specific 

details of the device structure. It is thus evident that an optimization 

is possible. The problem is greatly complicated by imperfectly understood 

mechanisms or imprecise knowledge of the controlling parameters, and by 

experimental difficulties in separating the effects of these mechanisms. 

5 
The major problems of efficiency improvement fall in the above 

categories; however, there are additional design requirements for 

efficient contacts and antireflective coatings. Although these areas 

are better understood, they are not trivial and must he satisfied in a 

manner compatible with the requirements of the rest of the device. 

The mechanisms to be dealt with can be more or less separated 

into heavy-doping effects and recombination processes, resistive losses, 

and optical losses. These three mechanisms are discussed in the 

following subsections. 

2.2.2 Heavy Doping and Recombination 

Basic diode theory provides a simple relationship to examine the 

doping requirements for a high-efficiency cell. We assume an idealized 

device consisting solely of a high lifetime base region with acceptor 

concentration, Na , and width, W, bounded on the top side by a collecting 

junction and on the bottom side by a zero recomhination back contact. 

In the steady state, under low-injection and open-circuit conditions, 

the electron concentration will be nearly constant throughout the base; 

the value of the electron concentration is governed by the generation 

(G) and recombination (R) rates, which must be equal: 

G = R 

The generation rate is given by: 

G = NphlW 

R n IT 

p n 

where Nph is the photon flux, n 

p 

is the electron concentration, and 

the lifetime. If this device is to produce an open-circuit voltage 

v ,then the eJect ron concentration at the edge of the space-charge oc 

region, n , is given by: p 

6 

T 

n 

of 
n 

p 

n exp(V IVT) po oc (n~/N ) exp (V IVT ) 1. a oc 

V

T = kT/q and the other symbols have their usual meaning. Combining 

these gives the desired relationship: 

N 

a 

Wn.2 = _=-1._ exp(V IV

T ) 

T N h oc n p 

We see that for a given open-circuit voltage, the base doping 

level must increase as the base lifetime becomes smaller or the base 

width is increased. If we assume a photon flux, Nph = 2.5 x 10 17 /cm2_ 

sec, a flux equivalent to a maximum short-circuit current of 40 rnA, then 

for a V of 0.7 volts, width of 150 ~m, and a base lifetime of 200 ~s, oc 

h .. b d' i -. 7 x 1 0 1 6 I cm3 _ W d t e m1.n1.mum ase op1.ng s -- . e cannot expect to 0 

better than this idealized analysis implies. Our proposed approach 

attempts to follow its guidelines; that is, minimize the basewidth 

consistent with absorption, maximize lifetimes, and use the highest 

doping levels which do not lead to the heavy doping penalties of bandnarrowing and Auger recombination. 

To contend with these design questions we have developed several 

simple models which, although neither complete nor fully verified, 

provide good agreement with experiment in most cases. For the present 

problem, the "effective recomhination velocity model" or Se model is 

particularly useful.(4) This model includes the effect of band 

narrowing, Fermi statistics, trap and Auger recombination, and the 

effects of recombination at the device surfaces. While there are 

uncertainties as to the proper constants for Auger recombination and 

band narrowing, model results are in good general agreement with 

experimental data. This model is described in more detail in Section 

3.1. Figure 1 shows results of voltage calculations for a back-surfacefield (BSF) device using three different BSF structures with two 

different base diffusion lengths (see Table 1). The significance of the 

velocity is that the saturation current can be expressed in terms of the 

value of Se at the space-charge region boundary: 

7 
o 

I 

p+ 

Diffused 

Back-Field 

1 Distance, 11m 

- - - L=4671lm 

L = 209 Ilm 

Curve 725471-8 

149.75 l~ 

_____ -i 

/ 

------------

N+ 

Diffused 

Emitter 

Back 

Surface 

Hi-Lo 

Junction 

Base Region Junction 

Depletion 

Region 

Front 

Surface 

Figure 1. Internal recombination velocity plots for 

back-surface field solar cells. 

8 
Tahle 1. Calculated Performance for Narrow-Base Back-Field 

Solar Cells (4 ohm-cm, p-base, width 150 ~) 

Intrinsic 

Base Diffusion 

Length, 

\lm 

209 

209 

209 

467 

467 

467 

Effective 

Base Diffusion 

Length, 

\lm 

324 

330 

336 

630 

1090 

1470 

J 

on 

V oc' J sc' n, 

mV rnA % 

567 34 15.4 

571 34 15.5 

573 35 15.6 

587 37 16.9 

599 37 17.4 

605 37 17.8 

and the open-circuit voltage is: 

V = V ~n[J /(J +J )] oc T sc on op 

Remarks 

Ohmic back surface 

Passive back surface 

Passive back surface; 

reduced BSF doping 

Ohmic back surface 

Passive back surface 

Passive back surface; 

reduced BSF doping 

The highest voltage results with the lowest values of Se at the 

space-charge edge. Our model calculations show that a narrow back-field 

(~1 \lm) is superior to the more typical wide BSF, provided that the back 

surface has a low recombination velocity, where "low" means smaller than 

the diffusion velocity, Dn/Ln' in the p+ region. 

Figure 1 illustrates the rationale behind our proposed approach. 

Referring to the BSF region in the figure, the upper curve is for a high 

surface recombination ohmic back while the lower two curves are for 

oxide-passivated backs. The lowest curve was obtained by lowering the 

BSF doping to lOI9/ cm3, thus reducing the heavy doping effects. In the 

9 
base region the solid curves are for the lower lifetimes. It is clear 

that better performance would be obtained if the basewidth were reduced. 

The emitter in this example is a conventional high-surf ace-concentration 

phosphorus diffusion emitter, but the emitter surface is oxide-passivated. 

Calculations indicate that high efficiency should be obtained 

with deep high-low emitter structures, provided the impurity profile at 

the n-p junction is very abrupt and the donor concentration is low 

enough to minimize heavy doping effects (i.e., ~ 5 x 101S/cm3 ). While 

currently available experimental data do not agree well with the 

prediction, this is believed to be a result of excessively graded 

junction profiles. Although reduced emitter concentration is indicated, 

low emitter doping results in high sheet resistance and a deep emitter 

structure would provide substantial assistance in the design of an 

efficient contact grid and should be further investigated. 

The contradictory requirements for passivating the surfaces while 

providing them with electrical contacts is approached in two ways. The 

first is to use a very small total area of actual ohmic contact, made by 

grid delineation through the passivating back oxide. This is less than 

ideal because of the difference between surface recombination velocity at 

the metal and that under the oxide (Figure 1). This translates into a 

difference of two orders of magnitude in the value of Se at the base 

boundary of the BSF region. Although the contact area is small, its 

effect is coupled to a very large volume of the base because of the large 

diffusion length. This situation can be substantially improved by 

employing tunneling MIS contacts.(S) While we do not at present have an 

effective explanation of the properties of such an interface, the 

passivation benefit seems real. MOS theory indicates that the contact 

properties depend on the metal-silicon barrier heights and require that 

the metal work functions be such as to result in a surface accumulation 

layer. This implies that the metal work function should be less than 

that of silicon for contact to an n-type layer (e.g., Al or Mg) and the 

converse for a p-type contact (e.g., Au, Pd, or Pt). 

10 
A final consideration involves the question of impurities. In 

the course of an intensive investigation of the effects of impurities on 

. (6 7' silicon and silicon solar cells conducted for JPL, ' J it was found that 

many impurities result in considerable lifetime degradation at concentrations in the parts per trillion range, well below the detection limits 

of practically every analytical method. An example of this behavior is 

shown in Figure 2 where, for molybdenum at a concentration of lOlO/cm3 , 

cell efficiency is reduced from 15.35% to 14.9%. The effect is more 

pronounced for higher efficiency cells. This observation indicates that 

extreme care must be exercised to prevent contamination of the silicon 

both in processing and during purification and growth. 

A benefit in impurity control may be obtained by using gallium as 

a primary p-type dopant. Gallium has a small effective segregation 

coefficient and will therefore remain in the melt at high concentrations 

during crystal growth. This may provide a degree of gettering for trap 

impurities. However, a more important reason for using gallium is the 

experimental fact that higher lifetimes are observed than in the case of 

comparable boron-doped material. It may also be conjectured that gallium 

acceptors will have a slightly higher threshold for bandgap narrowing 

because of their higher activation energies and a better fit in the 

silicon lattice. 

2.2.3 Resistive Losses and Grid Design 

We have developed a design model which minimizes the total energy 

losses of the contact system. The model has been verified experimentally 

for numerous designs. Included in the analysis are the losses due to 

metal coverage, ohmic losses in the emitter and the BSF layer, ohmic 

losses in the metal grid and back contact, ohmic losses in the metalsilicon interface (both front and back), and ~hmic losses due to bulk base 

resistance. An example of grid design data is illustrated in Table 2. 

11 
...... 

N 

>-

u 

C 

Q) 

.~ 

--

L!...I 

"C 

Q) 

N 

ro .5 

E 

La 

Z 

o Efficiency Projected by Impurity Model 

6 Efficiency Calculated by Finite Element Model 

lOll 

Mo Concentration, cm-3 

Figure 2. CAlculAted efficiency as a function of molybdenum 

concentrAtion for a narrow-hase, hack-surface field, 

high-efficiency cell. 

Curve 726545-A 
Table 2. Example Output of Contact Design Program 

Cell Module: 1.0000 x 1.0000 cm Grid Module: 0.9000 x 0.9000 cm 

Pad: 0.060 x 0.060 cm Number of Fingers: 11 Width: 5 urn 

Thickness: 6 urn 

Metal Resistivity: 

Diffused Sheet Resistance: 40 ohms/square 

-6 1.68 x 10 ohm-cm Base Resistivity: 4.00 ohm-cm 

Base ~.Jidth: 175 l1m BSF Sheet Resistance: 60 ohms/ square 

Back Contact Area Coverage Factor: 0.0020 Contact Dot Radius: 10 um 

Number: 637 Specific Interface Resistance: 2.0 x 10-5 ohm-cm2 

Intrinsic Efficiency: 17.50% Light Current Density: 27.00 mA/cm2 

Corner Angle, A2 = 26.57 

Energy Losses (LaPlace Solution) 

Sheet: 0.029 mW Finger: 0.110 mW Shadow: 0.207 mW 

Front Interface: 0.002 mW Base Loss: 0.096 mW 

Back Con·tact Interface Loss: 0.000 mW Back Sheet Loss: 0.0484 mW 

Total Joule Losses: 0.285 mW Total Losses: 0.493 mW 

Contact Coverage Area: 0.012 cm

2 (1.18%) 

R = 0.213 ohms 

s 

R = 0.213 ohm-cm2 

sp 

Efficiency 17.01% 

2.2.4 Optical Losses 

Use of a multilayer antireflective coating is capable of 

increasing cell efficiency by about 5%. In addition, incorporation of 

an optically reflecting back can minimize absorption losses resulting 

from the narrow hase. The reflector -- for example, a gold film deposit 

on an oxide -- will be a highly efficient mirror for unabsorbed low~ 

energy photons. The design of an AR coating is complicated by the 

presence of the passivating oxide. An example of the reflectance 

calculations for this case is shown in Figure 3. 

13 
40 

30 

~ 

Q.> 

U ...... C ~ co 20 -

U 

Q.> 

-

Q.> 

0::: 

10 

o 

Curve 732176-A 

1: L 54-100; 2.00-750 

2: L46~ 100; 2.37.-650; 1..46:-1300 

3: 1.46-100; 2.37.-500; 1046-1100 

0.6 0.7 0.8 0.9 LO 

Wavelength. 11 m 

Figllre 1. Reflect!wce clata for two- ancl three-layer antireflective 

cOrlt;'n~ clesigns for oxide-passivaterl surfilce •. (The rlrltil 

fo 110wing the curve nUlTlher in t}w legencl represent the 

refractive inclfcps ilncl thicknesses ()f the antireflective 

coatings.) 

1.1 
2.3 Experimental Approach 

The following subsections describe systematically our 

experimental approach for making high-efficiency solar cells. After 

selecting the right material, we have added features in our baseline 

cell design process to achieve AMI cell efficiencies in excess of 17%. 

2.3.1 Material and Dopant Selection 

In arldition to clever cell design, it is extremely important to 

select the proper starting material, dopants, and doping concentrations 

in all regions of the solar cell. The most important criterion for the 

starting material is high carrier lifetime (> 500 ~s). For the highest 

efficiencies, such lifetimes must be obtained in low-resistivity 

material (~ 0.1 ohm-cm). We have used a high-quality float-zone silicon 

as our starting substrate. We have also made a few cells on 0.1 ~-cm 

and 0.75 ~-cm boron-doped float-zone silicon. Prior to advan~ed cell 

design or processing, we fabricated baseline n+-p-p+ type solar cells 

followed by carrier lifetime and DLTS measurements. 

We have investigated the use of Ga as a dopant (in addition to 

the conventionally used boron) in the substrate material as well as in 

+ the p region. Ga fits the silicon lattice better than boron and has an 

activation energy greater than that of boron; therefore, it may have 

less severe heavy doping effects. We used the Westinghouse open-tube 

gallium diffusion technique to form the p+ region.(8) It is possible 

that Ga-doped crystals may even have higher carrier lifetimes because of 

the gallium-rich melt-assisted gettering (the segregation coefficient of 

Ga is several orders of magnitude smaller than that of boron). 

With + + regard to the doping concentrations in the nand p 

19 -3 regions, surface concentrations should be limited below 5 x 10 cm , 

and then a compatible contact grid should be designed to minimize the 

sheet resistance losses. Some attempts have been made to do this, 

although the majority of runs in this phase were done by straight POC13 

and BBr3 diffusions with high surface concentration. 

15 
2.3.2 Oxide Passivation 

Our mod~l dictates that it is necessary to passivate cell 

surfaces for higher open-circuit voltages; therefore, we have passivated 

the cell surfaces by growing a very thin layer of thermal oxide on top 

f + d + . o n an p reglons. We have opened an aperture pattern through the 

back oxide layer to establish communication between the back metal and 

the p+ region and we have made some attempt to optimize the oxidation 

conditions and oxide thickness for the best results. 

2.3.3 Oxide-Passivated Solar Cells with High-Low Emitter 

Fabrication of a six-layer cell structure consisting of oxiden+-n-p-p+-oxide could result in a further improvement over the oxidepassivated cell design. Our preliminary model calculations indicate 

that to gain the maximum benefit from this high-low emitter design, the 

+ 

n region should be very shallow « 0.1 ~m) and the n region should be 

about 1 ~m wide, with a doping density of less than 10 18 -3 

cm and 

diffusion length of several microns. The transition from n to p should 

be highly abrupt. It is possible to produce such a region by either 

atmospheric-pressure silane epitaxy or a reduced-pressure CVD 

technique. An attempt was made by epitaxial-growth to form an n-region 

and further work is in progress. 

2.3.4 Oxide-Passivated Cells with High-Low Emitter and MIS-Type Contacts 

In this cell design use of all the above-mentioned features 

along with a very thin layer of oxide « 30 A) underneath the front and 

back contact metals is proposed. Thus, the contact regions have an MIS 

structure with very thin oxide. This avoids direct metal-to-silicon 

contact to keep the surface recombination velocity of silicon low. The 

important requirements are that the oxide layer should be thin enough to 

keep the tunneling impedance low and that its oxide quality should be 

good enough to offer surface passivation. r,reen(5) has shown that it is 

possible to grow such an oxide layer, although it requires clever 

16 
contact grid design to minimize losses. Attempts are in progress to 

make such a structure. 

2.3.5 Oxide-Passivated Cells wi.th Multilayer AR Coating 

and Reflective Back Metal 

Use of multilayer antireflective coatings ann reflective back 

metal contact can enhance the oxide-passivated cell performance. 

Initial model calculations (described in Section 2.2.1) indicate that 

the reflective losses can be minimized by using a multilayer AR coating 

in place of the conventionally used single-layer coating with a 

refractive index of ~ 2. Similarly, if we make very thin-base solar 

cells, we can gain from a reflective back mirror to contain the light. 

Plasma-deposited Si N coatings have been formed with varying refractive 

x y 

index in the range of 1.5-3.8. Solar cells with single layer coatings 

has been fabricated, and multilayer coatings are heing investigated. 

17 
3. TECHNICAL PROGRESS 

3.1 Model Calculations and Discussion 

We have developed a simplified analytical model which provides 

useful insight and guidelines for fabricating high-efficiency solar cells. 

The model includes the effect of bandgap narrowing, Auger recombination, 

and recombination at the device surfaces, but it neglects the electric 

f · Id fl' f h d' f d . .. (9,10) le e ects resu tLng rom t e gra lent 0 oplng concentratl0ns. 

The model is based on the use of an internal recombination velocity 

as a measure of the minority-carrier losses in the various regions of the 

device. This directly provides the junction saturation current and thus the 

dark voltage-current characteristic which exerts primary control over cell 

performance. It is apPA.rent that the maj or henefit of reducing 

recombination is the increase in open-circuit voltage, V ,which follows 

oc 

from reductions of the saturation current, J • 

o 

Solar cell efficiency is directly proportional to the open-circuit 

voltage and the short-circuit current. Roth, of course, are subject to 

recombination losses, but here we will only concentrate on the variation of 

V oc' Open-circuit voltage is ino,rersely related to the saturation current 

(J ). 0 J 

V V

T Q,n [ 1 oc 

+ 2..£.1 J . (1) 

0 

W D W 

2 

[~: cosh (~) + n sinh 

qn. D 'L L (~: II 

J J ob+ J __ 1 • n n n 

0 oe NA L Wb n cosh (~) + S sinh (L b lJ n 

n n n 

W D W (2) 

2 

[~ cosh (~) + ~ sinh(~) ] qn. D L L L 

1 -2. 12 12 P + -N- • L W W 

D P cosh (~) + S sinh(Le ) 'L p 

p p p 

1R 
(D ,L ) and (D ,L ) are the diffusivity and diffusion length of the 

n n p p 

minority carriers in the p-base and n+ emitter region, Wb and We are the 

base and emitter widths beyond the junction edges, Sn and Sp are the 

surface recombination velocities at the front of the n+ emitter region 

and back of the p-base region, respectively, and Job and Joe are the 

base and emitter contribution to the reverse saturation current.(IO) 

The recombination at the silicon surface can 

i d " 1 h" h" "(11-14) b " ntro uc~ng a ow- ~g ]Unctlon or Y grow~ng 

detailed analysis of low-high structures by Gunn(lS) 

be reduced by 

an oxide laye r. 

provides a 

A 

beginning place for our model. The starting expression for the present 

discussion is an equation, derived from the carrier transport equations 

which transform the surface recombination velocity of the device (So) to 

an effective recombination velocity seen by minority carriers at the low 

side of the low-high interface as a function of the properties of the 

low and high regions. This equation for a p-p+ structure is: 

+ + 

S L W 

+ + 

.....2.....!!. + 

+ tanh ( -t) 

n D D L -E. --.!!. n n S (3) 

e + + + 

n L S L W 

P n 1 + o n tanh (-E.) 

D+ L+ 

n n 

concentration, diffusion length, and diffusivity of 

the minority carrier in the heavily doped p+ 

region. 

W; width of the heavily doped region. 

S surface recombination velocity at the back of the 

o high region. 

S effective recombination velocity at the low side 

e of the low-high junction. 

n minority-carrier concentration in the low region. p 

19 
We introduce the necessary modifications in the above expression 

to include the effects of degeneracy and handgap narrowing. We have 

devised an empirical expression for the effective bandgap narrowing, 

(AVG) based on the data of Lanyon(16) and Lindholm(17) corrected for 

degeneracy effects. The values thus adjusted for Fermi statistics can 

be used in the usual Holtzman expression for the nn product, i.e., 

np 

2 

n. 1.e exp 

2 

n. 1. 

where 6V~ and 6VC are the effective bandgap narrowing in the high and 

low regions, respectively. 

[ 3/4 10 20 

t:...vG 0.231 (--) 

, N 

and E 2 N N (_.J2.) n. exp 1. c v V

T 

Equation 3 can now be written as: 

+ 

NA Dn 

+ + N L A n 

exp 

+ - /:,V - tJ.V

G (G ') 

V 

T 

1-2/3 

I 

+ 1 i 

J 

+ + S L W 

. ....Q.....!l + tanh (--..12.) + + D L 

n n 

1 + 

+ S L ~ 

+ D 

n 

tanh 

Empirical expressions were derived to relate the diffusion 

length and diffusivity to the impurity concentrations (N). 

(4) 

(5) 

The eXpression for D was obtained from the data of Conwell as 

. (18) given 1.n Grove. 

D 

D 0 

+ A (6) 

(~) 0.6 0 

1 + 

10 

20 
where for p-type 

1) 

o 

D 

no 

1.8 

and for n-type 

n 

o 

A 

o 

D 

po 

35 

12.5 

The diffusion length is obtained using a Kendall's lifetime 

function(19) for bulk silicon (TK) combined with Beck and Conradt's(20) 

data for Auger recombination TAo 

where 

T 

n 

-1 

(_1 + _1 ) 

TA TK 

T 

a 

1 + N 

7 x 1015 

(7) 

(8) 

Note the value chosen for TO is related to the quality of 

silicon. We have found that a value of 20n to 400 us for TO is more 

appropriate for modeling high-performance devices. 

The Auger lifetime is given by: 

1.2 for p-type 

2.8 for n-type 

1 

Then the diffusion length is obtained from: 

L 

21 

(9) 

(10) 
It should be noted that the derivation of Equation 5 is 

completely general in that it may be applied to the calculation of an 

effective recombination velocity across an arbitrarily chosen region 

anywhere in the device as, for example, within the base region where the 

doping is constant. In this case we set NA = N~ = NA(base). This 

velocity then characterizes the total recombination beyond this plane. 

Assuming appropriate recombination velocity on the cell surface 

as the boundary conditions and from known doping profiles, we can now 

iteratively apply this calculation using Equation 5 from the back 

surface, across the base to the edge of the depletion region, and thus 

account for all the recombination processes in the base and back by 

calculating SejbO A similar calculation for the n region above the 

junction can give Sejb" Relating Equation 2 in terms of Se as given in 

Equation 5 permits expressing the base and emitter component of the 

saturation current density as: 

J 

o 

(11 ) 

where (Sejb,NA) and (Seje,ND) are the recombination velocities and 

doping densities at the edges of the depletion region. Then, neglecting 

current contributions from the depletion region, 

v 

oc 

J 

sc 

VT ~n (-J--'+'-"-J-) 

on on 

(12 ) 

Figure 1 shows the results of model calculations for a backsurface field solar cell using three different back-surface field (BSF) 

structures with two different base diffusion lengths. In Figure 1 

recombination velocity is plotted as a function of distance into the 

+ + 

n -p-p solar cell. Referring to the BSF region in the figure, the 

upper curve is for a high recombination ohmic back (So = 10° em/sec), 

while the lower two curves are for the oxide-passivated backs with 

reduced So. The lowest curve was obtained by lowering the BSF surface22 
d i . 1019 

op ng concentratlon to cm, -3 t h us re d' uClng t e h h eavy d' oplng 

effects. In the 4 Q-cm base region, the solid curves are for a lower 

base diffusion length of 209 urn while the dotted curves are for longer 

base diffusion length of 467 urn. In this example we have assumed a 

diffused emitter with high-surface concentration with the emitter 

surface passivated (So ~ 500 cm/sec). 

If the emitter surface is not passivated in this 4 Q-cm cell, 

the S. »S 'b' J »Job ' and V is limited by Joe. Model eJe eJ oe oc 

calculations in Figure 1 show that with the emitter surface passivated, 

S . > S 'b' but due to much higher doping density in the emitter (~ en ~ _3J 

10 cm ) at the edge of the depletion region(4) compared to the base 

15 -3 doping (3.5 x 10 cm ), Job »Joe • Therefore, with emitter surface 

passivation, Job limits J

o

• Now both Sejb and the reverse saturation 

current can be reduced further by back-surface passivation or lower BSF 

doping (Figure 1) to gain additional increase in Voc. Figure 1 shows 

that a long diffusion length (L > 3W) in the finished device is 

necessary to realize this benefit of back-surface passivation and lower 

BSF doping, otherwise S 'b is limited by the diffusion velocity (D/L) of eJ 

the carriers in the base. Calculations also indicate that for a 

diffusion length of 467 Urn, with both surfaces passivated, a Voc of 599 

(21) 2 mV, J sc of 37 rnA/cm , and a cell efficiency of 17.4% can be 

obtained. If the surface concentration of the BSF region is reduced to 

19 -3 10 cm , then a Voc of 605 mV and a cell efficiency of 17.8% can be 

achieved on this 4 Q-cm substrate. 

Use of a lower resistivity substrate(22) with similar diffusion 

length can further reduce the base component of the reverse saturation 

current and thus give greater improvements in Voc. The design of the 

emitter could be more important for the low-resistivity substrate 

because Joe may dominate J

o

' even after front-surface passivation. 

Reduced doping in the emitter can lower Joe further and provide 

additional gain in V A similar effect of doping concentration is oc 

shown in Figure 1 for the heavily doped p+ region when Job dominates J

o

• 

23 
3.2 Baseline Material Selection and Qualification 

In this program we selected a 4 ohm-cm, p-type, boron-doped, 

(111) float-zone silicon grown with zero dislocation density as a 

baseline material. These wafers are 2 inches in diameter and ~10 mils 

thick. The front side is chern-mechanically polished and the back side 

is chemically etched. This crystal was grown at Monsanto and is 

supposed to have high minority-carrier lifetime. 

In order to evaluate the material quality ourselves we 

fabricated 30 mil diameter Schottky barrier diodes on the wafers by 

evaporating Ti-Au contacts to perform DLTS measurements. Deep-level 

transient spectroscopy (DLTS) is a capacitance transient technique which 

is used to detect the deep levels that may degrade carrier lifetime in 

semiconductors. The capacitance transients are generated from the deep 

levels by applying voltage pulses repetitively to a reverse-biased 

depletion region of the device. This technique is well-documented in 

the literature.(23-24) Figure 4 shows a schematic diagram of our DLTS 

set-up which uses a lock-in amplifier and Boonton capacitance meter. 

Figure 5 shows DLTS spectra for the baseline float-zone silicon. 

There are no detectable deep levels in this material because no peaks 

were observed in the spectra. It is important to recognize that the 

DLTS detection limit for 4 ohrn-cm material is 3.5 x lOll cm3; 

therefore, lifetime-killing centers with lower than 3.5 x lOll cm3 

1 concentration will not show up in this spectra. Since 1r ~ ONtV th 

(where 1r is the recombination lifetime, 0 is the capture cross section 

of the deep level, Nt is deep-level concentration, and Vth is the 

thermal velocity of carriers (~ 107 cm/sec), the DLTS data suggest that 

if there are deep levels with Nt < 3.5 x lOll cm- 3• then the lifetime of 

this material would be greater than 300 ~secs provided the minoritycarrier capture cross section of those deep levels is smaller than 

10-15 cm2 • 

In another experiment we determined the minority-carrier 

generation lifetime in this float-zone silicon by fabricating MOS 

24 
Pulse 

Generator 

HP - 214A 

PE 5160 

Pulse 

Transformer 

I 

I 

'f 

T.C. Sample ~~ 

capacitance 

Meter ../~ ~i;; Boonton-12B 

Out 

T(mV) 

Owg. 1711896 

TIme Base Generator 

X = .02N msec. 

Ref 

Track and LDck- In 

Ho I d HIi"-!--::.;I n",,! Amp Ii fi e r 

SHC - 85 ORTEC-9503-SC 

Out 

x - y Recorder 

l-----+---4--~-----___I H;V;045A 

Keith ley-l60 

Voltmeter 9 

TEK-545 A 

Ext. Trig. 

Figure 4. Schematic diagram of the nLTS set-up. 

25 
1 

I 

-5.5 ~4. 5 

T 

I 

-3. 5 

I 

I 

-2. 5 

Temperature ( mV) 

Curve 743363-A 

I I 

I I 

-1. 5 -0. 5 

Figure 5. nLTS spectra for the baseline 4 9hm-cm float-zone silicon. 

26 

o 
capacitors. The wafers were first oxidized at 11000 c for 1 hour in pure 

O2, then in-situ annealed for 20 min in N2 at 11000C, and finally slowcooled at a rate of 10C/min. Aluminum metal dots were evaporated 

followed by a 450oC/30 min sintering in H2 • The generation lifetime 

(Tg) was determined by the pulse MOS technique in which MOS capacitors 

are pulsed from inversion into deep depletion and the recovery to 

inversion is recorded as a function of time (Figure 6). This technique 

is well-documented in the literature.(25) Figure 5 shows the typical 

c-t curve for these samples which gave us a generation lifetime of 

350 ~secs. It should be pointed out that the generation lifetime (Tg) 

and the recombination lifetime (T r ) are equal only when the lifetimecontrolling deep center lies in the middle of the bandgap: 

where ~E is the energy difference between the deep level and the midgap. 

3.3 + + Baseline n -p-p Solar Cells on 4 ohm-cm Float-zone Silicon 

To verify the material quality, a large number of 1 cm x 1 cm 

+ + 

n -p-p solar cells were fabricated on 4 ohm-cm float-zone silicon by 

8500 C POCl3 diffusion for a n+ emitter and 9500 C BBr3 diffusion for a p+ 

back-surface field. + + The process sequence for fabricating n -p-p solar 

cells is shown in Table 3. Solar cell data consisting of short-circuit 

current density, open-circuit voltage, fill factor, and cell efficiency 

are shown in Tahle 4. All cells were t'ested under 100 mW / cm

2 AMI 

illumination using a quartz-iodine lamp. 

The data in Table 4 show that the short-circuit current density 

(lsc) is about 33 mA/cm2 and the open-circuit voltage is in the range of 

575-585 mV. Average solar ce1l efficiency is ~ 14.75% with the maximum 

exceeding 15%. 

27 
-"-c.. 

Q) 

U 1-..) C :Xl «:II 

:t:= 

u 

«:II 

c.. 

«:II 

U 

Curve 743202-A 

CF = 70 pf ( Inversion) 

Figure n. Capacitance versus time cllrve for determining the 

minority-carrier generation lifetime in hRseline floatzone silicon. 

------; 
Table 3. + + Sequence for Fabricating n -p-p Solar Cells 

Wafer clean 

Silox deposition to cap emitter side 

Clean 

BBr3 diffusion to form p+ back surface field 

Oxide removal 

+ Silox deposition on p region 

Clean 

POCl3 diffusion to form n+ emitter 

Oxide removal 

Pre-metal deposition clean 

Front and back metal evaporation 

Apiezon wax to protect back metal 

Photoresist front side and expose grid patterns 

Metal etch 

Photoresist front and expose mesa around 

grid patterns 

Silicon etch 

Contact sintering 

Test 

Anti-reflective coating on front 

Test 

3.4 Cell Testing With and Without a Metal Mask 

Figure 7 shows that our cell configuration consists of twelve 

1 cm x 1 cm solar cells on two-inch diameter wafers. An 8 to 10 urn deep 

mesa is etched around each cell to isolate it. In order to verify that 

we are not collecting any appreciable current from the etched baresilicon region between the cells, we tested a few cells by placing a 

metal mask on top of the wafer. The mask was painted black and had a 

precise 1 cm x 1 cm window so that by proper placement or alignment, only 

29 
Cell ID 

1 

2 

3 

4 

5 

6 

7 

8 

9 

10 

11 

12 

Run: 1+-7, 

100 mW/cm 2 

Table 4. Solar Cells (n+-p-p+) Fabricated on Baseline 

4 ohm-cm Float-Zone Silicon 

Short-Ci rcuit Open-Circuit Cell 

Current J Voltage V Efficiency / 2 sc oc 

rnA cm Volts Fill Factor % 

33.5 0.582 0.767 14.9 

33.3 0.582 0.767 14.8 

33.1 0.584 0.756 14.6 

33.2 0.582 0.781 15.1 

33.4 0.582 0.777 15.1 

33.0 0.579 0.780 14.9 

32.9 0.582 0.786 15.0 

33.2 0.579 0.776 15.0 

33.6 0.577 0.763 15.0 

33.0 0.579 0.777 14.9 

32.9 0.581 0.772 14.7 

32.9 0.581 0.752 14.4 

Wafer B-1 

AMl illumination 

.one cell is exposed to the light during testing. Table 5 shows that 

with and ~ithout the metal mask, the difference in J sc and cell efficiency is less than 2%; this is within the experimental error of 

measurement and the manual alignment of the metal mask, although without 

the metal mask absolute cell efficiencies appear to be 0.2 to 0.3% 

higher. 

3.5 Oxide-Passivated 17.2% Efficient Solar Cells on 4 n-cm 

Float-Zone Silicon 

Following the guidelines of our model, the solar cell surfaces 

were passivated by growing a thermal oxide at 800°C. The oxide thick-

+ + 

ness on top of the n region was about 110 A and on top of the p region 

30 
Figure 7. Cell configuration. 

31 

RM-213 
Table 5. Cell Tes ting Wi th- and Without the Metal 

Mask with 1 cm x 1 cm Window 

Cell 

No. Without Metal Mask With Metal Mask 

J V n J V n sC2 oc sc 2 oc 

(rnA/cm ) (volts) % (rnA/cm ) (volts) % 

1 34.9 .588 16.0 33.7 .590 15.7 

2 34.9 .589 16.0 34.4 .591 15.9 

3 35.4 .596 16.5 34.9 .596 16.2 

4 35.0 .594 16.4 34.0 .596 16.1 

5 35.1 .594 16.3 34.5 .596· 16.1 

it was 45 A. A grid pattern was opened through the back oxide. 

Figure 8 shows a schematic diagram of the oxide-passivated cell, while 

the process sequence for the oxide-passivated cells is shown in 

Table 6. In this sequence we slow-cooled all the high-temperature steps 

at a rate of lOC/min to preserve the bulk lifetime. In addition, 

phosphorus was diffused through a very thin oxide to preserve the 

surface quality. Front and back contacts were Ti-Pd-Ag and the 

Ti02/5i02 antireflective coating was spun on these cells prior to 

photolithography and metallization. Our cell design (O-n+-p-p+-O), 

coupled with careful cell processing and controlled AR coating 

deposition, has resulted in cell efficiencies in excess of 17% with a 

maximum of 17.2%. These cells were tested and verified at SERI. As 

shown in Table 7, our measurements gave V of 600 mV, J of 

oc sc 

2 36.2 rnA/cm , fill factor of 0.793, and cell efficiency of 17.2%. SERI 

measurements on the same cell gave V of 604 mV, J of 35.6 rnA/cm2 , oc sc 

fill factor of 0.795, and cell efficiency of 17.11%. 

3.5.1 Ellipsometric Measurements on 17.2% Efficient Cells 

A Rudolph Auto-EL ellipsometer was used to determine the initial 

oxide thickness and the thickness of spin-on AR coating on the 17% . 

32 
AR 

p 

Back Metal 

AR 

p 

Oxide 

Dwg. 9344A42 

Baseline Cell 

Oxide Passivated 

Cell 

Figure 8. Schematic diagram of oxide-passivated cells. 

33 
Table h. Process Sequence for Fabricating 

Oxide-Passivated Solar Cells 

Clean 

Silox deposition to cap emitter side 

Clean 

BBr3 diffusion to form p+ back-surface field 

Oxide etch 

+ Silox deposition p region 

+ POCl 3 diffusion to form n emitter 

Oxide etch 

Clean 

Oxidation - passivation 

+ Anti-reflective coating on n side 

Photoresist both sides 

Expose grid pattern on both sides 

Antireflective coating etch 

Oxide etch 

Evaporate metal on front side 

Reiect metal/remove PR from the back 

Evaporate back metal 

Build front metal thickness by electroplating Ag 

Photoresist front side and expose mesa mask around 

grid patterns 

Protect hack by apiezon wax 

Etch AR coating 

Etch silicon for mesa 

Remove wax from back 

Remove photoresist from front 

Test 

Sinter contact 

Test 

34 
Cell ID 

HIEFY 4-4 

-5 

-6 

-7 

-8 

HIEFY 4-4 

-5 

-6 

-7 

-8 

*AM1, 100 

Table 7. Oxide-Passivated Solar Cells on 

Boron-Doped 4 n-c.m Float-Zone Silicon 

Cells Tested at Westinghouse 

Area J V FF 

2 sc 2 oc 

(cm ) (rnA/cm ) (mV) (%) 

1.0 36.1 599 79.4 

1.0 36.2 600 79.3 

1.0 36.4 598 78.5 

1.0 36.2 599 79.1 

1.0 36.3 597 79.5 

Above Cells Tested at SERI 

1.01 35.5 604 79.6 

1.01 35.6 604 79.5 

1.01 35.9 605 78.6 

1.01 35.4 604' 79.7 

1.01 35.3 603 79.8 

2 mW/cm Illumination 

*Run #HIEFY 4, Base 2 

n 

(%) 

17.1 

17.2 

17.1 

17.2 

17.2 

17.1 

17.1 

17.1 

17.0 

17.0 

efficient cells. Just after the oxidation step the measured front oxide 

thickness was 110 A and the back oxide was ~ 50 A thick. After complete 

cell fabrication, ellipsometric measurements were made with the help of 

a computer program for two-layer coatings since the AR coating consists 

of a thin Si02 layer underneath the spin-on coating. The measurements 

show that the AR coating of a 17.2% efficient cell consists of an oxide 

thickness of 112 A with a refractive index of 1.458, and a spin-on antireflective coating on top of this oxide with a thickness of 555 A and a 

refractive index of 2.03. 

35 
3.5.2 I-V Analysis of the 17.2% Efficient Cells 

Dark I-V measurements(26) were performed in conjunction with the 

lighted I-V data to obtain resistances, bulk recombination, and junction 

recombination components. An effective base 

2 

lifetime is calculated from 

J using an approximation o 

J qni ~ 

o = ~ {~-T--· Notice that in this calculaA eff 

tion, surface effects are neglected; also, it is assumed that J o is dominated by the base, which may not necessarily be the case. As discussed 

in Section 3.3.6, for these 4 Q-cm material cells, J o ~ Joe without any 

passivation; however, after front-surface passivation, J becomes 

o 

dependent upon Job (J o1 = Joe + Job)' The I-V analysis and its 

approximations are described in detail in the second quarterly r~port.(27) 

Table 8 shows the comparison of cell parameters of a 15.2% 

b 1 · + + 11 .J h . d . d 17 2a, ase 1ne n -p-p ce anu 1°1 t e counterpart OX1 e-pass1vate • k ce • 

We observe a 2.8 rnA increase in J , 17 mV increase in V , and ahout a 

sc oc 

factor of 1.85 decrease in J o1 • Dark and lighted I-V curves taken at 

SERI on a 17.2% cell are shown in Figures 9 and 10, respectively. There 

is reasonable agreement between the lighted and dark I-V data taken at 

Westinghouse and SERI. We generally measure slightly lower open-circuit 

voltage because of the lack of temperature control during cell testing. 

3.5.3 Reflectivity and Spectral Response Measurements 

on 17.2% Efficient Cells 

Our equipm~nt for reflectivity and spectral response measurements includes a tungsten halogen light source, a manochrometer. and a 

silicon photodetector.(27) Photocurrent from the cell is compared 

automatically to that from a standard silicon detector of calibrated 

spectral response. Reflectivity and relative spectral response of the 

17.2% cell is shown in Figure 11. Absolute spectral response (amps/watt) 

is obtained by: 

Relative Spectral Response x Standard (A/W) 

l-R(A) 

36 

(13 ) 
Table 8. A Comparison of the Cell Parameters of a Baseline 

n+-p-p+ Cell and 17.2% Oxide-Passiv8ted Cell 

Fabricated on 4 n-cm Float-Zone Silicon 

._------ -----------

Parameter 

Baseline 

+ + n -p-p Cell 17.2% Oxide-Passivated Cell 

FF 

n 

Leff 

2 33.4 rnA/cm 

583 

.78 

15.2% 

2 0.5 n-cm 

-12 2 3.7 x 10 A/cm 

213 llsecs 

2 36.1 mA/cm 

600 

.794 

17.2% 

2 0.21 it-cm 

709 \.Isecs 

7.7 X 10-7 

From these data; internal quantum efficiency, QE(A)' as a function of 

wavelength is determined according to: 

he QE(A) = CiT· Absolute Spectral Response (iL; ) 

Internal quantum efficiency is defined as the ratio of the number of 

collected electrons to the numb~r of photons entering the material at a 

given wa~elength per unit time.(10) 

37 
Curve 74485+-A 

0 

Cell ID: Hiefy-4-5 

-1 Sample: SD2183 

Date: Aug 5,1983 15:38 

-2 Temp. = 28. OOC 

Area = L 010 cm2 

Vl 

c.. -3 Rshunt = 803 Kohms E 

ro Rseries = O. 390 oh ms 

...-

- Equ iv. Rs = O. 2Z1 oh ms c: 

Q) -4 j ~ 

~ 

:J 

U 

- -5 1 C"l 

0 

....J 

R 

-6 

--7 

-8 ~~~~~~~~~~~~~~~~~~~~~ 

0.00 . 10 . 20 . 30 .40 .50 .60 . 70 

Voltage (volts) 

Figure 9. Dark I-V data taken at SERl on a 17.2% oxide-passivated cell 

on 4 0,-cm float-zone silicon. 

38 
Curve 744850-A 

40.0 

35.0 

Cell 10 : H iefy-4-5 

30.0 Sample :SD 2183 

25.0 Date: Aug Sf 1983 15: 38 

20.0 

Temp. = 28. OOC 

A rea = 1. 010 cm 2 

-

« 15.0 V = O. 6042 volts 

E oc 2 - 10.0 J sc = 35.60 mA/cm -c:: 

Q,) 

"- Fill Factor = 7 9. 54 0/0 "- 5.0 ::J Efficiency = 17. 11 0/0 u 

0.0 V = 0 5064 V max • 

-5.0 I = 34.12 rnA max 

-10.0 I = 35.96 rnA 

sc 

-15.0 

-20.0 

-.20 -.10 0.0 . 10 .20 .30 .40 .50 .60 

Voltage (volts) 

Figure 10. Lighted I-V data taken at SERI on a 17.1% oxide-passivated 

cell fabricated on 4 ~-cm float-zone silicon. 

39 

.70 
Curve 744852-A 

50 1.3 

40 1.1 

-'f!!.. 

-

>. 

:::: 30 O. 9 

.:::: 

..... 

~ 

-w 20 O~ 7 ~ 

10 0.5 

0~ __ ~~~ __ ~ ____ ~ _____ ~ _____ -L __ ~O.3 

o. 4 O. 5 O. 6 O. 7 O. 8 O. 9 1. a 1. 1 

Wavelength (microns) 

Figure 11. Reflectivity ann relative spectral response of a 17.2% 

efficient oxide-passiv~ted cellon 4 0.-cm silicon. 

40 

Q) 

V') 

c: 

0 

0-

V') 

Q) 

~ 

ro 

'-

..... 

u 

Q) 

c.. 

V') 

Q) 

.::: 

..... 

ro 

Q) 

0::: 
-..... 

c: 

Cl.) 

u 

'-

Cl.) 

c.. 

>-

u 

c: 

Cl.) 

.-

.~ 

--

LU 

E 

:::s 

..... 

c: 

ro 

:::s 

0 

Curve 744845-A 

100 

90 

80 

70 

60 

50 

40 • Without ('xide Passivation 

30 o With Oxide Pass ivation 

20 

10 

a 

425 525 625 725 825 925 1025 

Wavelength (nonometers) 

Figure 12. Quantum efficiency versus wavelength plot for an 

unpassivated cell and 17.2% efficient oxide-passivated cell 

on 4 Po-cm float-zone silicon. 

41 

1120 
Figure 12 compares the quantum efficiency of the oxide-passivated 

+ + 17.2% efficient cell and the counterpart 15.2% efficient n -p-p cell 

without any oxide passivation. The data clearly verify the benefits of 

oxide passivation. i~e observe a very significant increase in the quantum 

efficiency at the shorter wavelengths (0.4 - 0.55 ~m) which are primarily 

absorbed in the emitter. (At a wavelength of 0.4 ~m, the absorption 

length is ~0.1 ~m.) This supports the fact that the front oxide reduces 

the loss of carriers to the front surface. The data show that even at 

0.4 ~m the quantum efficiency can be raised from 0.4 to 0.72 by proper 

surface passivation, which means that a majority of photogene rated 

carriers near the emitter surface are not lost due to its bulk 

recombination properties (Auger recombination and bandgap narrowing), but 

a very significant number are lost into the front surface. Thus, the 

emitter region should not be regarded as a dead layer because even for 

0.3 ~m deep junctions, the quantum efficiency of the light absorbed in 

the emitter can he made greater than 75% by proper oxide passivation. 

Figure 12 indicates that the quantum efficiency in the wavelength 

range of 0.75 to 0.95 ~m is nearly the same. In this range, most light 

is absorbed within the bulk of silicon, away from surfaces, because 

absorption length is in the range of 15 to 60 ~m. This suggests that 

oxide passivation has apparently not changed the true diffusion length in 

the base material. 

At higher wavelengths, > 0.95 Wm, we again see a small 

enhancement in the· quantum efficiency of the passivated cells. This 

indicates that back-surface passivation is also helping to raise cell 

performance. These data are consistent with our expectation that both 

front- and back-oxide passivation contributes to the increase in J sc' 

while the majority of increase in V comes from front-surface 

oc 

passivation. 

3.5.4 Diffusion Length Determination from Spectral Response Data 

Figure 13 shows a plot of photon attenuation length (l/a, where a 

is the photon absorption coefficient) and a variable X-I, where X is 

42 
Vl 

LQ.) 

..... Q.) 

E 

0 

'-

u .-E 

-ro 

..c: 

c.. 

« 

-... 

...... 

ISO 

135 

120 

105 

90 

75 

60 

45 

30 

15 

0 

-1 -. 8 -. 6 -. 4 -. 2 0 

Curve 744844-A 

e Without Oxide Passivation 

o With Oxide Pass ivation 

Slope = L = 263 ~ m 

.2 .4 .6 .8 1 

X-I 

1.2 1.4 1.6 1.8 2 

Figure 13. Diffusion length plot for an unpassivated cell and 17.2% 

efficient cellon 4 0.-cm float-zone silicon. 

43 
the ratio of the number of photons absorbed per unit of time in the base 

to the number of electrons per unit of time in the external circuit. 

-u(d + w)/Q X = e E, where d is the junction depth, w is the width of the 

depletion region at zero bias, and QE is the quantum efficiency. The slope 

of l/u versus X-I gives an effective diffusion length(28) which would be 

equal to -the base diffusion length if the base width is much greater than 

the diffusion length; otherwise the effective diffusion length is a 

combination of bulk diffusion length and surface recombination velocity 

Figure 13 shows such plots for an oxide-passivated 17.2% efficient cell and a 15.2% efficient unpassivated cell. It is interesting 

to note that the two curves overlap in the l/u range of 4.4-6.0 wm 

(wavelength range 0.7-0.94 \lm). The slope of the two curves in this 

range gives a bulk diffusion length of 263 \lm, but as we go to longer 

wavelengths the slope of the unpassivated cell becomes less steep, 

indicRting that the effective bulk diffusion length becomes smaller due 

to back-surface recombination. Relative to the unpassivated cell, the 

slope of the oxide-passivated cell is only very slightly affected at the 

larger wavelengths, supporting the reduction in the hack-surface 

recombination velocity 

Assuming 263 \lm to be the true bulk diffusion length in the 

finished cell, we obtain a minority-carrier diffusion velocity (D/L) of 

1254 cm/sec for the bulk. Only a very slight drop in the effective 

diffusion length at longer wavelengths (Figure 13) in the oxidepassivated cell confirms that back-surface recombination velocity has 

been reduced considerably. However, a downward bending suggests that S 

at the p-p+ interface is still greater than 1254 em/sec (D/L). If the 

back surface has no influence at all (infinite bulk), then there will be 

no curvature in Figure 13 at longer wavelengths (higher values of 1/u). 

3.5 5 Spreading Resistance Measurements on 17.2% Efficient Cells 

Figure 14 shows the carrier concentration profile obtained by 

the spreading resistance measurements. The data indicate a surface 

44 
Curve 744853-A 

- 1018 ~ 

I 

E 

u 

c:: 

.~ 

1017 -ra 

"- -c:: 

Q) 

u 

c:: 

0 

10 u 16 b-

.~ 

"-

bra 

U 

1015 

1013 ~ ______ ~ _________ ~ ____ ~ _________ ~ ______ ~ ____ -J 

o O. 1 0.2 0.3 0.4 0.5 0.6 

Depth (. microns) 

Figure 14. Dopant profile in the emitter of a 17.2% efficient oxidepassivated cellon 4 n-cm float-zone silicon. 

45 
concentration of 4 x 10 19 

cm3 and a junction depth of - 0.3 ~m. The 

sheet resistance measured by four-point probe after diffusion was - 80 

0./square, but the sheet resistance calculated from the spreading 

resistance data was - 240 0./square. More measurements are required to 

resolve this discrepancy. The surface concentration in Figure 14 indeed 

appears lower than expected despite the fact that oxidation consu~es 

50 A of the silicon surface. On the other hand, n-type dopants tend to 

segregate in the silicon at the Si/Si02 interface. A lower doping 

concentration does reduce heavy doping effects; however, the sheet 

resistance of the cell cannot be very high because the measured series 

resistance is only - 0.25 ohms. 

3.5.6 Effectiveness of Front- and Rack-Surface ~assi.vation in the OxidePassivated Cells 

In Section 3.5.2 we showed that oxide passivation reduces the 

reverse saturation current J where J = J (emitter) + J ob (base). 0' 0 oe 

In order to find out which component of J o 

has heen reduced most or 

which surface passivation is more effective in improving V , we removed 

oc 

the back metal and the back oxide, protecting the front-surface, and 

then remetallized the back. These cells now have only front oxide 

passivation. Table 9 shows the cell data with a) both surfaces 

passivated, b) the same cells with only front oxide passivation, and 

+ + c) baseline n -p-p cells with no surface passivation. 

The data in Table 9 indicate that the hack oxide removal results 

2 in about 0.5 rnA/cm loss in Jsc ' - 5 mV loss in Voc ' and about 0.5 to 

0.75% decrease in absolute cell efficiency. Compared to the unpassi-

+ + vated n -p-p cells, Voc of the front-surf ace-passivated cell is 10 to 

15 mV higher and J sc is also somewhat higher, resulting in about 1% 

higher absolute cell efficiency. This implies that emitter surface 

reco~bination velocity controls the reverse saturation current or V 

oc 

(J - J ) in the cells without any surface passivation and the fronto oe 

surface passivation lowers Joe and improves Voc. It appears that with 

the front-surface passivAtion, .T

n 

tends to hecome base controlled and 

46 
Table 9. Solar Cell Data With a) Both Surfaces Passivated, 

b) Only Front-Surface Passivation, and c) with No 

Surface Passivation 

Cell 10 

2 

3 

4 

5 

6 

7. 

8 

v J 

oc 

volts 

sc 2 

rnA/cm 

(a) Both Surfaces Passivated 

.599 

.598 

.595 

.593 

.592 

.594 

.593 

.589 

34.6 

34.6 

34.6 

34.8 

34.5 

34.8 

34.3 

34.6 

FF 

.792 

.789 

.793 

.795 

.792 

.793 

.792 

.796 

(b) Above Cells with Only Front-Surface Passivation 

2 

3 

4 

5 

6 

7 

8 

1 

2 

3 

4 

5 

(c) 

.595 

.594 

.592 

.584 

.583 

.590 

.589 

.571 

33.9 

34.0 

33.8 

34.0 

33.8 

34.2 

33.7 

33.9 

.7Q5 

.793 

.795 

.788 

.787 

.793 

.793 

.779 

+ + 

n -p-p Cells Without Surface Passivation 

.582 

.582 

.582 

.579 

.577 

33.5 

33.3 

33.2 

33.0 

33.6 

47 

.767 

.767 

.781 

.780 

.763 

n 

% 

16.5 

16.4 

16.3 

16.4 

16.2 

16.4 

16.1 

16.2 

16.0 

16.0 

15.9 

15.7 

15.5 

15.9 

15.7 

15.1 

14.9 

14.8 

15.1. 

14.9 

15.0 
therefore back-surface passivation gives an additional 5 mV improvement 

in V (back oxide removal showed a 0.5 mV drop in V ). Passivating oc oc 

both surfaces reduces the loss of minority carriers to surfaces; 

therefore, we observe some increase in.1 just by front-surface 

sc 

passivation and a further increase when both surfaces are passivated. 

3.5.7 Analytical Summary of 17.2% Efficient Cells 

Table 7 shows that oxide passivation coupled with careful cell 

processing can give solar cell efficiencies greater than 17% (AMI) on 

good quality 4 ~-cm float-zone silicon, with Voc ~ 600 mV, 

J ~ 36 mA/cm2, and fill factor ~ 0.795. These parameters are in good sc 

agreement with the model calculations in Section 3.1. These cells are 

among the best cells reported to date, especially on 4 ~-cm material. 

The data in Table 8 show ~ 20 mV improvement in V , 3 mA/cm2 increase 

oc 

in J sc ' and about 2% (absolute) increase in cell efficiency in the 

oxide-passivated cell compared to the counterpart unpassivated cell. 

Dark I-V measurements (Table 8) indicate a decrease in reverse 

-12 -2 -12 -2 saturation current, J o ' from 3.7 x 10 cm to 2.0 x 10 cm· 

Using the expression V ~ KT/q 2n(J /.1), this decrease in J

o coupled oc sc 0 

with the measured increase in J

sc essentially accounts for the observed 

18 mV increase in Voc due to passivation in this pair of cells. Since 

J J (emitter) + J b (base), to find out which component of J has 

o oe 0 0 

been reduced, back-oxide passivation from some lower efficiency cells 

(16.5%) was removed and then they were retested (Table 9b) after 

+ remeta11izing the p back surface. The data in Table 9 indicate that, 

out of the ~18 mV increase in V due to oxide passivation, a 13 mV 

oc 

improvement comes from the front-surface passivation and the remaining 

comes from the back-surface passivation. This also implies that without 

any surface passivation in these cells, J ~ J and, therefore, fronto oe 

surface passivation alone is able to increase V • If J b was limiting oc 0 

J

o

' then the front-surface passivation would not have much influence on 

J or V It appears that after front-surface passivation in these 

o oc 

4 ~-cm cells, J becomes base limited (J ~ J b) and, therefore, ba·cko 0 0 

48 
surface passivation gives an additional 5 mV improvement in Voc 

(Tables 9a-9b). These observations are entirely consistent with our 

model predictions in Section 3.1, Figure 1. 

Greater improvements in V can be achieved from front-surface 

oc 

passivation in lower resistivity base material(22,29) with similar 

diffusion velocity (niL). In these cells Job will no longer limit J o ; 

instead, the value of S achievable from front-surface passivation will 

define the lower limit of J or J. However, if J remains much 

oe 0 oe 

greater than Job' even after front-surface passivation, then the backsurface passivation will not increase V much further. Some attempts oc 

are being made to verify this by fabricating cells on 0.2-0.3 rG-cm 

material specially grown for high lifetime. 

Figure 12 explains the reason for the observed increase in J

sc 

due to oxide passivation. Shorter wavelengths « 0.55 ~m) are primarily 

absorbed in the emitter, while longer wavelengths (> 0.95 ~m) are mostly 

absorbed near the back surface. The figure shows that front-surface 

passivation results in a significant improvement in the quantum 

efficiency at shorter wavelengths, while the back-surface passivation 

increases the quantum efficiency at longer wavelengths. This is 

consistent with the data in Table 9, which show that both front- and 

back-surface passivation contribute to the observed increase in J sc ' 

since passivation reduces the number of photogene rated carriers that are 

lost to the surfaces. The quantum efficiency at ~ 0.4 ~m wavelength 

could be raised from 40 to 75% by emitter surface passivation, which 

implies that in the unpassivated cell, a majority of the photogene rated 

carriers near the emitter surface are not collected because of high 

surface recombination, and not because of high bulk recombination in the 

emitter as a result of Auger and bandgap narrowing effects. Thus, the 

emitter region should not be regarded as a "dead layer" because simply 

by proper oxide passivation, emitter quantum efficiency can be made 

greater than 75%, and further improvements may be possible by reducing 

Auger recombination and bandgap narrowing effects in the emitter. 

Surface recombination can be minimized further by growing a thin tunnel 

49 
oxide underneath the front and back metal to completely avoid direct 

metal/silicon contact. 

Figure 12 indicates that quantum efficiency in the wavelength 

range of 0.85-0.95 ~m is nearly the same for both unpassivated and 

passivated cells. Most of the light in this wavelength range is 

absorbed within the bulk silicon, away from the surfaces, since the 

absorption length is in the range of 20-60 ~m. This suggests that oxide 

passivation has apparently not changed the actual diffusion length of 

the base material, which is calculated to be 263 ~m in this wavelength 

range (Figure 13). At longer wavelengths (higher values of l/a), the 

slope of the unpassivated cell becomes less steep relative "to a 

passivated cell, indicating that its effective diffusion length 

decreases near the back surface due to high surface recombination 

velocity. The slope of the curve for the oxide-passivated cell in 

Figure 13 is only slightly affected at longer wavelengths, supporting 

the notion that a good back-surface passivation has been achieved. 

3.6 Oxide Passivated Solar Cells Fabricated on 0.1-0.2 ~-cm 

Boron-Doped Float-Zone Silicon 

A few solar cells have been fabricated on 0.1-0.2 ~-cm Wacker 

Waso-S (100), boron-doped float-zone silicon. The thickness of these 

wafers was ~ 350 ~m. In this particular run the antireflective coating 

ended up a little thin and patchy on the oxide-passivated cells because 

of some problem in the photoresist step; however, solar cell efficiencies 

still look quite pr6mising. Table 10 shows the solar cell data on 

0.1-0.2 ~-cm material, with and without passivation. Without passivation, 0.1-0.2 ~-cm material gave J sc ~ 31.8, Voc ~ 0.613, fill factor 

~ .802, and cell efficiency = 15.6%. After oxide passivation, the best 

cell in this run had J sc ~ 33.0, Voc ~ 0.627, fill factor ~ .815, and 

cell efficiency ~ 16.9%. Partly because of poor AR coating, we only saw 

about 1.5 mA/cm2 improvement in the J sc and about 14 mV improvement in 

Voc due to oxide passivation. 

50 
Table 10. Solar Cell Data on 0.1-0.2 Q-cm Float-Zone Silicon, 

With and Without Oxide Passivation 

J V FF 

sc 2 oc 

Cell In rnA I cm volts % 

(a) Without Oxide Passivation 

C-1 31.2 .612 .785 

-2 31.8 .6 t3 .802 

-4 31.3 .612 .798 

-5 31.7 .612 .797 

-9 30.9 .607 .799 

(b) With Oxide Passivation 

C-4 33.0 .627 .815-

-8 33.2 .624 .793 

C3-4 32.3 .620 .809 

-9 32.4 .617 .811 

C4-8 32.6 .617 .800 

*Cell Area 1 2 AMI, 100 mW/cm 2 cm , Illumination 

n 

% 

15.0 

15.6 

15.3 

15.5 

15.0 

111.9 

16.4 

16.3 

16.2 

16.1 

Quantum efficiency data in Figure 15 show that somewhat smaller 

enhancement in J sc is also because the back-surface passivation is not 

effective in increasing the long wavelength response in these cells. In 

Section 3.5.6 we showed that the back-sur'face passivation on 4 Q-cm 

material gave ~ 0.5 mA/cm2 improvement in J and about 5 mV increase in sc 

Voc. One of the reasons why back-surface passivation is not as effective 

{oj 375).lm . is because the cell thickness to diffusion length ratio, L (= 300 ).lm)' 1.S 

W 250 ).lm more than unity in these cells as opposed to a L (= 263 ).lm) of less than 

unity for 4 Q-cm cells. In low-resistivity silicon with good lifetime, 

51 
-..... 

c: 

Q) 

u 

'-

Q) 

0-

>-. 

u 

c: 

Q) 

.-

.5:2 

..... 

-

UJ 

E 

::J 

..... 

c: 

ro 

::J 

0 

Curve 744846-A 

100 

90 

80 

10 

60 

50 

40 

30 • Without Oxide Passivation 

o With Oxide Pass ivation 

20 

10 

0 

425 525 625 125 825 925 1025 

Wavelength (nonometers) 

Figure 15. Quantum efficiency versus wavelength plot for an 

unpassivated cell and a 16.9% efficient oxide-passivated 

cellon 0.1-0.2 0.-cm float-zone silicon. 

52 

1125 
Job is quite small and, even after front-surface passivation, if Joe 

dominates J

o

' then back-surface passivation will not affect J o or improve 

Voc A diffusion length of 300 ~m in these cells was obtained from the 

slope of I/o versus X-I in the wavelength range of 0.7-0.95 ~ in 

Figure 16. Such a high diffusion length in the finished cells on 0.1-0.2 

0,-cm material is quite respectable, and we think we could have done 

better on this material if the AR coating was proper. The overlap of two 

curves in Figure 16, especially at longer wavelengths, again supports the 

premise that oxide passivation on the back of these cells is not helping 

the current response at all. 

Dark I-V data in Table 11 show a decrease in J o1 by a factor of 

~1.4 due to oxide passivation. This is consistent with the observed 

increase in Voc· 

3.7 Oxide-Passivated Solar Cells Fabricated on 0.75 0,-cm Boron-Doped 

Float-Zone Silicon 

Solar cells were also fabricated on 0.75 Q-cm Wacker Waso-S 

boron-doped float-zone silicon using the oxide passivation technique. 

Solar cell data on this material. with and without passivation, are 

sho\¥n in Table 12. Without the oxide passivation, this material gives 

2 J sc 32.3 rnA/cm • Voc = .607 mV, fill factor = .802, and cell 

efficiency of 15.7%. After oxide passivation, the best cell gave 

J sc = 33.7, Voc = 0.624, fill factor = .792, and cell efficiency of 

16.7%. The scatter in the cell data was due to the nonuniform and 

patchy AR coating since these cells were also processed with the 

0.1-0.2 0-cm cells described in Section 3.6. Compared to 4 Q-cm 

material, we see a smaller oxide passivation-induced improvement. 

J went up bv 1.5 mA/cm2 and V improved by 17 mV. Quantum efficiency sc J oc 

data in Figure 17 show improvement in the entire spectrum range; 

however, quantum efficiency improvement in the short wavelengths is not 

as good as seen for 4 0.-cm material. This may be another reason, in 

addition to a poor AR coating, why we did not see as much improvement in 

Jsc ' Slopes of the l/a versus X-I plot in Figure 18 give a bulk 

53 
VI 

L.. 

CI.) 

'tU 

E 

0 

L.. 

.i:2 

E 

ro 

..c::: 

0-

« 

--....-4 

ISO 

135 

120 

105 

90 

15 

60 

45 

30 

15 

0 

-1 -. 8 -. 6 ~. 4 -. 2 0 

Curve 744847-A 

• Without Oxide Passivation 

o With Oxide Pass ivation 

Slope = L = 300 \.1 m 

.2 .4 .6 .8 1 1.21.41.61.8 2.0 

X-I 

Figure 16. Diffusion length plot for an unpassivated cell and a 16.9% 

efficient oxide-passivated cellon 0.1-0.2 ~-cm float-zone 

silicon. 

54 
-...... 

c: 

0) 

·U 

sO) 

c.. 

-

>. 

U 

c: 

0) 

.-

.5:2 

--I..LJ 

E 

::::J 

...... 

c: 

ro 

::::J 

0 

Curve 744849-A 

100 

90 

80 

70 

60 

50 

40 

• Without Oxide Passivation 

30 o With Oxide Pass ivation 

20 

10 

0 

425 525 625 725 825 925 1025 

Wavelength (nanometers) 

Figure 17. Quantum efficiency versus wavelength plot for an 

unpassivated cell and a 16.7% efficient oxide-passivated 

cellon 0.75 ~-cm float-zone silicon. 

55 

1125 
--V') 

1-

Q,) 

1i5 

E 

0 

l-

.~ 

E 

ro 

.C 

0.. 

<:( 

-~ l"""'l 

150 

135 

120 

105 

90 

75 

60 

45 

30 

15 

0 

-1 -.8 -6-4-2 . . . 0 

Curve 744848-A 

• Without Oxide Passivation 

L = 300 ~m 

o With Oxide Passivation 

L =400 ~m 

.2 .4 .6 .8 1 1.2 1.4 1.6 1.8 2 

X-I 

Figure lR. Diffusion length plot for an unpassivated cell and a 16.7% 

efficient oxide-passivated cellon 0.75 Q-cm float zone 

silicon. 

56 
Table 11. A Comparison of the Cell Parameters of a Baseline 

n+-p-p+ Cell and 16.9% Oxide-Passivated Cell 

Fabricated on 0.1-0.2 Q-cm Float-Zone Silicon 

Parameter 

J 

sc 

Voc 

FF 

n 

Rs 

Rsh 

J o1 

Teff 

J 02 

Baseline 

+ + + 

n -p p Cell 

2 31.8 rnA/cm 

.613 volts 

.802 

15.6% 

0.76 Q-cm2 

8.3 kQ-cm2 

7.1 x 10- 13 A/cm2 

0.5 )Jsecs 

2 x 10-:- 6 A/cm2 

16.9% Oxide-Passivated Cell 

2 33.0 rnA/cm 

~627 volts 

.815 

16.9% 

0.26 Q-cm2 

35 kQ-cm2 

5.0 x 10- 13 A/cm2 

1.06 )Jsecs 

5 x 10- 6 A/cm2 

diffusion length of ~300 )Jm in the unpassivated cell and ~400 )Jm in the 

passivated cell. Curves in Figure 18 tend to diverge at longer 

wavelengths, indicating that the back-surface passivation is working. 

This is also consistent with the ~ = ~ < 1 for the passivated cell. 

Dark and lighted I-V data in Table 13 show that J o decreases by 

about a factor of 2 in the oxide-passivated cells. This decrease 

coupled with the observed 1.5 rnA/cm2 increase in J sc accounts for the 

17 mV increase in V due to oxide passivation. oc 

3.8 Oxide-Passivated Solar Cells on 0.3-0.7 Q-cm Gallium-Doped 

Czochralski Silicon 

In order to see the effect of dopant on the base material, we 

obtained 0.25 to 0.67 Q-cm Ga-doped Czochralski crystal with (100) 

orientation. This crystal was pulled from a synthetic quartz crucible 

to minimize the residual impurity concentration. Table 14 shows that 

+ + 

n -p-p solar cells fabricated on this material gave cell efficiencies 

exceeding 15%. Voc was greater than 600 mV and J

sc 

was ~ 31 rnA/cm2 •. 

57 
Table 12. Solar Cell Data on 0.75 \(-cm Float-Zone Silicon, - With and Without Oxide Passivation 

J V FF n sc 2 oc 

Cell ID rnA/cm Volts % % 

( a) Without Oxide Passivation 

R-1 31.7 .605 .794 15.2 

R-2 32.3 .607 .802 15.7 

-3 32.0 .606 .800 15.5 

-4 32.9 .006 .7% lS.R 

-5 31.9 .602 .801 15.4 

-6 31.6 .605 .793 lS.2 

-8 31.5 .600 .801 15.1 

-10 31.6 .606 .796 15.2 

(b) With Oxide Passivation 

Rl-4 33.2 .612 .804 16.4 

-5 33.7 .624 .792 1n.7 

-8 33.2 .613 .805 16.4 

-9 33.4 .608 .801 16.3 

RI-8 33.5 .608 .791 16.1 

5 33.2 .609 .791 16.1 

*Cell Area 1 2 AMI, 100 mW/cm 2 cm , Illumination 

Table 14 also shows that the oxide-passivated cells on the Gadoped wafers give efficiencies in excess of 16% with J ~ 33 mA/cm2 and 

sc 

+ V as high as 613 mV. The oxide thickness on top of the n region was 

oc 

~ 100 ~ and on top of the p+ region it was ~ 40 ~. Consistent with our 

model calculations, 650 ~ thick AR coating was spun to reduce reflection 

losses. 

58 
Tahle 13. A Comparison of the Cell Parameters of a Baseline 

n+-p-p+ Cell and 16.7% Oxide-Passivated Cell 

Fabricated on 0.75 0.-cm Float-Zone Silicon 

Baseline 

Parameter + + 

n -p-p Cell 16.7% Oxide-Passivated Cell 

J 32.3 mAl.em 2 33.7 rrtA/em2 

se 

Voc .607 volts .624 volts 

FF .802 .792 

n 15.7% 16.7% 

R 0.9 0.-em 2 .46 2 Si-cm' s 2 

Rsh 1.6 M0.-em2 53 k0.-cm 

J 01 1.8 x 10-12 9.4 x 10-13 

Teff 19 l-Isees 67 l-Isees 

J 02 1.3 x 10-6 1.1 x 10-6 

Table 14. Baseline n+-p-p+ Cells and Oxide-Passivated O-n+-p-p+-O 

Solar Cells on 0.3 to 0.7 Si-cm Gallium-Doped Czochralski 

Crystal Grown From a Synthetic Ouartz Crucible 

J V n 

sc 2 oe 

Cell ID (rnA/cm ) (volts) FF % 

a) Baseline + + 

n -p-p Cells (HIEFY-7) 

1 31.2 .603 .798 15.1 

2 31.3 .603 .798 15.1 

3 31.2 .602 .797 15.0 

4 31.2 .603 .784 14.7 

b) Oxide-Passivated + + O-n -p-p -0 Cells (HIEFY-5) 

1 33.6 .608 .794 16.3 

2 33.5 .608 .795 16.2 

3 32.8 .613 .802 16.2 

4 32.9 .612 .800 16.2 

*Cell Area 1 2 AMI 100 mW/em 2 

em , illumination 

59 
3.9 Gallium-Diffused Back-Surface Field 

Our model calculations (Figure 19) show that surface recombina-

+ tion velocity (8 ) can be minimized at the p-p interface by e 

back-surface field with a doping concentration of ~ 5 x 10 18 

an abrupt 

-3 em 

Model calculations in Figure 20 indicate that the width of the backs1lrface field region should be greater than 10 wm for a cell design 

+ ( 6 which consists of metal on top of the p region So ~ 10 em/sec). 

+ However, if the p surface is passivated (S ~ 500 em/sec), then the 

+ width of the p region should be reduced below 1 to 2 wm. 

~allium fits the silicon lattice better than boron and may 

therefore show lower bandgap-narrowing effects. Therefore, we decided 

to minimize heavy doping effects in the HSF region without losing its 

effectiveness by forming a Ga back-surface field with a surface 

concentration of ~ 5 x 1 n18 em -3 and \.Jp + < 2 wm. He used an open-tube 

gallium (OTG) diffusion process for this step. 

The open-tube gallium (OTC) diffusion process uses carbon 

monoxide to reduce gallium oxide, Ga 20'3' at controlled temperature to 

generate gaseous gallium, which diffuses into the silicon wafer. The 

process equipment consists of three main parts: the diffusion furnace, 

the source furnace, and the gas-control system. This process can 

achieve three functions: the predeposition oxidation, the gallium 

deposition, and the drive-in diffusion with oxidation. 

3.9.1 Chemistry of the OT~ Process 

Chemical reactions involved in the OTG deposition process can be 

simplified as follows: 

Ga203 (s) + 2 CO (g) ""'" Ga 20 (g) + 2 CO 2 (g) (15 ) 

Ga 20 (s) + CO (g) + 2 Ga (g) + CO 2 (g) (16 ) 

Ga203 (s) + CO (g) + 2 GaO (g) + CO 2 (g) (17) 

Ga 20 3 (s) + 3 CO (g) + 2 Ga (g) + 3 CO 2 ( g) (IR) 

60 
~ V\ 

---E 

10 u 3 

Q.) 

V') 

-0 

Q.) 

~ 

> "' 

E 

102 ~ 

E 

c: 

:E 

Figure 19. 

Curve n S6P'?-A 

NA = 4 X 1015 em-3 

6 So = 10 eml sec 

Effect of abrupt doping concentration in the p+ region on 

the minimum effective recombination velocity at the p-p+ 

interface. W + was optimized at each concentration. 

p 

61 
Figure 20. 

10 

. 1 

15 -3 N

A =4xlO em 

Curve 715683-A 

18 -3 N + = 3 x 10 em Abrupt p 

4 So = 125 x 10 

1 

w + (!-lm) p 

10 100 

Effect of surface recombination velocity, S , and width of 

p+ region (W +) on the effective recomhinat~on velocity at 

+ . P 

p-p Interface. 

62 
where (s) and (g) represent solid phase and gas phase, respectively. It 

is known that the concentration of GaO in Equation 17 is negligihle. 

Hence, the chemical reactions in the system may be further simplifed as 

Ga 2

0 3 (s) + 3 CO (g) + 2 Ga (g) + 3 CO 2 (g) (19) 

Therefore, at a controlled carbon monoxide gas flow rate and 

controlled gallium oxide source temperature, e.g., 87SoC ± 2 OCt a 

finite amount of gaseous gallium can be generated. Using an inert gas 

such as argon can hring the gallium into the diffusion furnace and 

diffuse the gallium impurity uniformly into the silicon wafers. 

The chemistry of pyrogenic oxidation is the same as conventional 

wet oxidation, the details of which are not included here. 

1.9.2 The OTG Diffusion System 

The following simplified diagram (Figure 21) shows the OTG 

diffusion system. Gallium trioxide, Ga 203 , as shown is the diffusion 

source reagent. The source furnace and diffusion furnace temperatures 

are set at Ts 875°C ± 2°C and TD = l2300C + 1°C, respectively. 

Dwg" 7772A36 

Ar 

= Quartz Diffusion Furnace Tube call 

.t l~ rH2" HCI 

Silicon Wafers ~::§~"~~~~iJ~==~;;~~?z::::=T C 

__ C\-L-~I=II::d=I~I===-_~G~as!.!Fl~0~~...J/203--c:.""'",., ;;3 T: C: 

""\ T =8750 ±~C L 

s 0 

2 Quartz Boat 

Figure 21. Arrangement for open-tube gallium diffusion. 

&3 
Silicon wafers sit in the diffusion flat zone, which has a temperature 

controlled to within an accuracv of ± lOCo The timer controls various 

gases and turn-on and turn-off at a pre-set time sequence. 

3.9.3 Process Sequence Used for Open-Tube Gallium Diffusion for 

Back-Surface Field Formation 

a. Mask front surface by 1200 ~ Silox + 1200 ft silicon nitride 

deposited by CVD technique to prevent Ga diffusion into the 

front surface. 

b. Load wafers at AOOoe with 2 liter/min Argon flow in the 

tube. 

c. 

d. 

e. 

f. 

Heat wafers from 600°C to l2300 C in 45 minutes. 

Turn on 900 cc/min CO for '3 min\ltes. 

Slow cool wafers from 12'300 C to 9000 e at a rate of 

1 • SoC / mi n • 

o At 900 C stop the flow of CO. 

g. Slow pull the wafers from the furnace in 30 minutes. 

Fig\lre 22 shows that the ahove process sequence gave a hack-surface 

concentration of ~ 5 x 10 18 cm-'3 and a p+ depth of ~ 2 microns. 

However, on the front surface (Figure 8) we observed an out-diffusion of 

horon probably into the oxide because p-type dopants tend to segregate 

into the oxide. Since the boron concentration decreased suhstantially 

in the first 5 to 10 micron region, we chern-mechanically polished a 

~ 25 ~m thick region from the top surface prior to POCI'3 emitter 

diffusion. 

3.9.4 + + n -p-p Solar Cells with Ga Back-Surface Field 

We have fabricated baseline n+-p-p+ cells with a Ga back-surface 

field, and the oxide-passivated cells with Ga BSF are in process. Cell 

data in Table 15 inrlicate that, rlespite the desired surface concentra-

') 

tion, we observed 2 to 3 mA/cm- reduction in J sc ' a decrease of 

64 
-

I' 

E 

u 

c: 

0 

-ro 

s.... 

-c: 

Q) 

u 

c: 

0 

u 

E 

::J 

ro 

<.!) 

(1) 

> 

:;:; 

u 

« 

1019 ,..---_-r-__ -.,... ___ ,..--__ ....,..-__ ---;-__ ---, 

101? 

1016 

1015 

1014

0

'----. ..J..1---2"-----..J..3---41.....---...J..S-----'6 

Distance (1..1 m) 

Figure 22. o Gallium back-surface field profile after 1230 Copen-tube 

gallium diffusion. 

65 
Table 15. A Comparison of n+-p-p+ Solar Cells with Diffused Boron BackSurface Field at 9500 C (N f ~ 10 20 

cm -3, W + ~ (). 5 flm) sur ace p 

and Gallium Back-Surface Field Formed by Open-Tube -Ga ., 0 ( 18 -3 W + ) DlffUSlon at 1230 C Nsurface ~ 5 x 10 cm , p ~ 2 flm 

J V T! sC_ 2 oc 

Cell ID (rnA/cm ) (volts) FF ~{ 

a) Roron Back-Surface Field 0'1-7 ) 

'33.5 .582 .767 14.9 

2 33.3 .5R2 .767 14Js 

3 33.1 .584 .756 14.6 

4 33.2 .582 .781 15. 1 

b) Gallium Back-Surface Field CHIEFY) 

29.5 .552 .773 12.6 

2 30.0 .555 .777 12.Q 

3 30.5 .557 .783 13.3 

4 30.4 .556 .778 13.2 

* 2 ? 

Cell area 1 cm , AMI 100 mW/cm~ Illumination 

~ 0.25 mV in V , and about 2% loss in cell efficiency compared to the oc 

boron-diffused back-surface field which has a surface concentration of 

~ 1020 cm- 3• Dark I-V analysis showed a decrease in the effective bulk 

lifetime from 276 flsecs (J o1 ~ 3.3 x 10- 12 A/cm2 ) to 38 flsecs 

(J o1 ~ 8.9 x 10-12 A/cm2 ) by switching from boron BSF to gallium BSF. 

The high-temperature (12300 C) diffusion during this OTG process appears 

to degrade the bulk lifetime. It is possible that a slower cooling rate 

may be required since the cleanliness of the OTC process is sacrificed 

at such high temperatures. Possibly, this disadvantage can he overcome 

by incorporating HCl gettering and very slow cooling (Figure 21). 

66 
Figure 23. Research deposition reactor. 

67 

RH-99783 
It should be recognized that a BSF width of 2 ~m with NA ~ 5 x 

10 18 is not appropriate for a n+-p-p+ cell design but is desirable for 

the oxide-passivated cells (Figure 20). A few oxide-passivated cells 

with Ga BSF were also processed; however, due to loss in carrier 

lifetime we did not see high efficiencies. 

3.10 Plasma-Deposited Thin Films for Multilayer Antireflective Coating 

Our model calculations (Figure 3) indicate that reflection losses 

can be reduced significantly by a properly tailored multilayer AR coating 

instead of the conventionally used single- or double-layer coatings. 

Plasma techniques can provide a way of depositing AR-coated layers with 

controlled thickness and refractive index; therefore, we are developing 

and investigating this approach. Generally, hard and chemically stahle 

thin films of refractory materials such as Si0 2 , Si3N4 , SiC, and Ti0 2 can 

be deposited with high optical perfection by plasma deposition 

techniques. The deposition reaction is carried out in a glow discharge at 

pressures of about 1 torr. The "hot" electrons of plasma-producing 

radicals combine on the substrate under ion bombardment to form the films. 

An illustration of our research deposition reactor is shown in Figure 21. 

The films can be deposited with substrate temperatures of 200-

300 0 C. Typically, the reaction consists of silane with an oxidant. At 

present we are using NH3 gas to deposit nitride hase coating and in the 

future we could go to Ti0 2 base coatings, if necessary. The following 

reaction is used to produce Si3N4 films: 

Because of the nonequilibrium nature of the deposition process, wide 

departures from stoichiometry are possible; the NIsi ratio can be varied 

from 0 to about 1.5, with corresponding changes in refractive index from 

about 3.5 to 1.B. Also, the films include bonded hydrogen in varying 

amounts. These films are useful for AR coating because the refractive 

index can be selected. 

08 
The optimum single-layer index has been calculated at 2. We 

have obtained this index value by adjusting the SiN stoichiometry to be 

x 

slightly Si rich. We have actually deposited single-layer plasma AR 

coating on solar cells and are getting ready to deposit multilayer 

coatings. In these single-layer films, no visible absorption is 

observed but is expected if the N!Si ratio becomes smaller than that 

required for the high index coatings needed for multilayer AR 

coatings.(30) Mixtures such as Si02!Ti02 will be needed for these. 

Solar cell data with a single-layer plasma AR coating are shown 

in Table 16. This first experiment was performed on unpassivated cells, 

and the thickness of the AR coating (890 A) turned out to be higher than 

the desired value of 750 A. The measured refractive index was 1.95. 

The AR coating was deposited after cell fabrication (without AR coating) 

to determine the relative improvement without the interference of any 

process effects. Table 16 shows that despite the higher thickness the 

relative improvement was nearly 45% in J • 

sc 

+ + The n -p-p cell efficiency 

with the spin-on Ti0 2!Si02 AR coating is generally about 15%, but the 

best cell efficiency with this plasma coating was 15.9%. Efficiency 

improvement is even greater than 50% because we also see an appreciable 

enhancement in Voc. With the conventional spin-on coating, Voc is about 

580 mV, but with plasma coating V was ~590 mV, suggesting that this 

oc 

coating may be helping in passivation of the emitter surface. This 

could happen because atomic hydrogen in the discharge can tie up some 

dangling silicon bonds. In a second experiment with single layer 

coating, we deposited 750 A thick coating which resulted in 45-47% 

improvement in J

sc Thus, initial results on single-layer plasma Si3Nx 

coating look quite promising, and experiments are in progress to apply 

this on oxide-passivated cells. This will be followed by the multilayer 

AR coating structure. 

69 
Table 16. Resul ts of Single-Lay~r Plasma-Deposited Silicon Nitride 

Coatin~ on Baseline n -p-p+ Cells (AR Coating Thickness 

= 890 and Refractive Index = 1.95) 

J V FF n 

sc 2 oc 

Cell ID rnA/cm Volts % % 

(a) Before AR Coating Deposition 

4 23.3 .566 .765 10.1 

6 22.7 .557 .760 9.6 

7 23.6 .567 .7f>1 10.2 

8 23.5 .567 .765 10.2 

9 23.4 .566 .771 10 .2 

10 22.8 .553 .7f>S 9.7 

(b) Af ter AR Coating Deposition 

4 33.5 .588 .773 15.2 

6 33.9 .592 .792 15.9 

7 33.6 .588 .779 1').4 

8 33.3 .589 .760 14.9 

9 33.6 .590 .773 15.4 

10 33.4 .586 .785 15.4 

3.11 High-Low Emitter, MIS Contacts, and Reflective Back Contact 

One attempt has been made to fabricate a high-low n+-n emitter 

by epitaxially depositing a 2 ].lm thick n-layer with a doping of 10 17 

-3 

cm We were able to get the proper thickness and doping hut our 

diffusion length appeared to be short; therefore, J

sc 

and the cell 

efficiencies were lower than the baseline cells. Another run with 

special precautions for high-lifetime epitaxial growth is in progress. 

\.Je have also made one run to study reflective back contact using 

Al, Au, and Ag metals in addition to standard Ti-Pd-Ag contact. These 

metals were deposited over the gridded back oxide. Au and Ag metals 

70 
showed poor adhesion to the back oxide and consequently peeled off. The 

Al metal looked somewhat promising with current densities in some cases 

about 0.5 mA/cm2 better than Ti-Pd-Ag contact. This area needs to be 

investigated further. 

We are in the process of making solar cells with front and back 

M1S contacts by growing thin « 50 A) oxides, oxynitrides, and lowpressure CVD nitride. The front metal grid and the back metal will he 

deposited directly on these thin dielectrics to completely avoid direct 

metal/silicon contact. 

71 
4. CONCLUSIONS 

Consistent with our model calculations we have successfully 

fabricated greater than 17% efficient solar cells by passivating the 

cell surfaces. Compared to the unpassivated cells, oxide passivation 

gave ~ 3 mA/cm2 improvement in J

sc

' ~ 20 mV increase in V

oc

' and greater 

than 2% increase in the absolute cell efficiency. Cell efficiencies of 

17.2% on 4 0-cm float-zone silicon have been achieved by 110 ~ thick 

front oxide and ~ 50 A thick hack oxide grown at ROOoC. In these cells 

the majority of increase in Voc comes from the front-surface 

passivation, although back-surface passivation also provides additional 

~ 5 mV increase in Voc. Even in 4 0-cm base cells, emitter surface 

recombination seems to limit J or V prior to any passivation. o oC' 

In these 4 0-cm base cells, both front- and back-surface 

passivation contribute to the increase in J

sc by reducing the loss of 

photogenerated carriers to the surfaces. Spectral response measurements 

showed a very significant improvement in the quantum efficiency at 

shorter wavelengths, associated with appreciable increase in the quantum 

efficiency at longer wavelengths. This is consistent with the observed 

increase in J

sc 

and supports the fact that both front- and back-surface 

passivation are effective and working. No significant increase in the 

spectral response was observed in the wavelength range of 0.75 to 0.95 

urn when the light is absorbed well within the bulk away from the 

surfaces. This indicates that oxide passivation does not appreciably 

change the intrinsic diffusion length of the hulk silicon in the 

finished cell. The effective diffllsion lerigth, at longer wavelengths, 

is much lower for the unpassivated cells but is hardly affected by the 

surface in the case of passivated cells. 

72 
Analysis of the I-V data shows that oxide passivation in 17.2% 

efficient cells lowers J o1 by about a factor of 2. This decrease 

coupled with - 3 mA/cm2 increase in J sc accounts for the observed 20 mV 

increase in the V in our 4 n-cm float-zone silicon cells. oc 

Solar cells fabricated on 0.75 n-cm silicon show an increase in 

2 J sc of - 1.5 rnA/cm , Voc improvement of 17 mV, and absolute cell 

efficiency increase of about 1%. The best cell efficiency was 16.7%. 

The bulk diffusion length (- 400 ~). calculated from the spectral 

response measurements, was less than the cell width; therefore, we saw 

some increase in the quantum efficiency at longer wavelengths but not as 

much as in the case of 17.2% efficient 4 n-cm cells. Due to the 

effectiveness of oxide passivation, J o was reduced by a factor of two 

and quantum efficiency at shorter wavelengths was significantly 

improved. 

Contrary to 4 n-cm and 0.75 ~-cm cells, the oxide-passivated 

0.1-0.2 D.-cm cells did not show an improvement in the spectral response 

at longer wavelengths. This could be partly because the hulk diffusion 

length (300 ~, calculated from spectral response measurements) was less 

than the cell thickness (375 urn). The best cell efficiency on this 

material was 16.9%, in spite of the poor" antireflective coating. 

The bulk diffusion length in the 0.75 D.-cm finished cell 

(- 400 ~) and in the 0.1-0.2 0.-cm cell (- 300 urn) is actually greater 

than the bulk diffu~ion length in the 17.2% efficient 4 D.-cm cell 

(263 ~). These low-resistivity crystals are indeed of very high 

quality_ Their efficiency at present is slightly lower than the 17.2% 

cells made on 4 n-cm material, partly because they are - 375 ~ thick 

and the back-surface passivation is therefore not as effective, and 

partly because we had poor antireflective coating on low-resistivity 

cells due to a problem in the photoresist step. More cells from lowresistivity material are being fabricated. 

+ + Solar cells (n -p-p ) have also been fabricated on 0.3 to 

0.7 D.-cm gallium-doped Czochralski crystal which gave - 15% efficient 

73 
cells. After oxide passivation, the hest cell efficiencies were 

16.5%. 

A gallium back-surface field formed by open-tube gallium 

diffusion at 12300C for 3 min resulted in a surfAce concentration of 

~ 5 x 10 18 cm- 3 and a depth of ~ 2 ~m. Despite the desired surface 

concentration, we saw a 2% drop in cell efficiency due to a decrease in 

carrier lifetime during the high-temperature diffusion. 

Initial results on plasma-deposited antireflective coatings look 

very promising. The single-layer coating alone gives ) 45% enhancement 

in J sc and >50% enhancement in efficiency. An appreciahle increase in 

V suggests that the plasma Si3N coating may be passivating the oc . x 

silicon surface due to the presence of ato~ic hydrogen in the discharge. 

An initial attempt has been made to fabricate high-low emitter, 

MIS contacts and reflective back contact. Further experiments are in 

progress to overcome some of the problems observed in the initial 

experiments. 

We have ordered 0.3 n-cm float-zone, high-lifetime silicon from 

Wacker for future work. He believe this material can give much higher 

efficiencies than the 17% already achieved on 4 n-cm substrates. 

74 
5. PROGRAM STATUS 

5.1 Present Status 

During this phase of the program we attempted all the tasks and 

successfully fabricated great~r than 17% efficient cells. Specifically we: 

• Fabricated 17.2% efficient cells on 4 ~-cm float-zone silicon 

which were tested and verified at SERI. 

• Analyzed the 17.2% efficient cells by spectral response 

measurements, I-V measurements, ellipsometric measurements, 

and spreading resistance measurements. 

• Fabricated and analyzed 16.7% efficient oxide-passivated 

cells on 0.75 ~-cm float-zone silicon. 

• Fabricated and analyzed 16.9% efficient oxide-passivated 

cells on 0.1-0.2 ~-cm float-zone silicon. 

• Set up and calibrated the equipment for plasma-deposi~ed 

antireflective coating, and fabricated and analyzed n -p-p+ 

cells with single-layer Si3Nx coatings. 

• Initiated experiments for high-low emitter and reflective and 

MIS back contacts. 

• Ordered 0.3 ~-cm high lifetime float-zone wafers from Wacker 

for future work. 

5.2 Future Activity 

We expect to get low-resistivity, high-lifetime wafers from Wacker 

in January 1984. We plan to fabricate OXide-passivated, high-efficiency 

cells on this new material. We plan to design and make new masks for 2 cm 

x 2 cm cells with two or more different grid patterns. We will continue 

to investigate the MIS contacts and multilayer AR coating. We will also 

initiate experiments on dendritic web silicon, especially the use of H+ 

implantation for improving web solar cell performance. 

75 
6. REFERENCES 

1. F. A. Lindholm and J. G. Fossum, 14th IEEE Photovoltaic Specialists 

Conference Proceedings, p. 680 (1980). 

2. A. Neugroschel, F. A. Lindholm, ann S. C. Pao, Appl. Phys. Lett., 

33:168 (1978). 

3. R. B. Godfrey and ~. A. Green, Appl. Phys. Lett., 34:790 (197g). 

4. J. R. Davis and A. Rohatgi, 15th IEEE Photovoltaic-Specialists 

Conference Proceedings, p. 680 (1980). 

5. A. W. Blakers and M. A. Green, Appl. Phvs. Lett. 39:483 (981). 

6. J. R. Davis et al., IEEE Trans. Electron Devices, ED-27, p. 67 

(1980). 

7. R. H. Hopkins et al., Final Report JPL/DOE-954331-81/14 (1982). 

8. R. N. Ghoshtagore, Solid-State Electronics, 22(10):877 (1980). 

9. M. Wolf, "Designing Practical Si.licon Solar Cells Approaching the 

'Limit Conversion Efficiency' ," Proc. 14th IEEE Photovoltil.ic 

Specialists Conf., p. 563 (1980). 

10. H. J. Hovel, Semiconductor and Semimetals, Vol. II, Solar Cells, 

Academic Press, p. 20 (1975). 

11. M. P. Godlewski, C. R. Barona, and H. W. Brandhorst, Jr., "Low-High 

Junction Theory Applied to Solar Cells," Proc. 10th Photovoltaic 

Specialists Conf., p. 40 (1973). 

12. J. G. Fossum, "Physical Operation of Back Surface Field Solar 

Cells," IEEE Trans. Electron. Devices, Vol. ED-24, p. 322 (1977). 

13. A. Sinha and S. K. Chattopadhyaya, "Effect of Back Surface Fieln on 

Photocurrent in a Semiconductor Junction," J. Solid State Elect., 

Vol. 21, p. 943 (1978). 

14. J. G. Fossum and E. to Burgess, "High-Efficiency p + -n-n + BatkSurface-Field Silicon Solar Cells," Appl. Phys. Lett., Vol. 33, p. 

238 (1978). 

76 
15. J. B. Gunn, "On Carrier Accumulation and the Properties of Certain 

Semiconductor Junctions," J. Electron. Control, Vol. 4, p. 17 

(1958) • 

16. M. P. D. Lanyon and R. A. Tuft, "Bandgap Narrowing in Heavily Doped 

Silicon," Proc. IEDM, Washington, D.C. p. 316 (1978). 

17. F. A. Lindholm, private communication. 

18. A. S. Grove, Physics and Technology of Semiconductor Devices, John 

Wiley, New York, 1967. 

19. D. Kendall, presented at the Conf. Physics and Application of Lithium 

Diffused Silicon, NASA, Goddard Space Flight Center, Dec. 1969. 

20. J. D. Beck and Conradt, Solid State Communications, Vol. 13, p. 93 

(1973). 

21. J. A. Amick and A. K. Ghosh, "Practical Limiting Efficiencies for 

Crystalline Silicon Solar Cells," J. Electrochem. Soc., Vol. 130 (1), 

p. 160 (1983). 

22. J. G. Fossum, F. A. Lindholm, and M. S. Shibib, "The Importance of 

Surface Recombination and Energy-Bandgap Narrowing in p-n-Junction. 

Silicon Solar Cells," IEEE Trans. Electron Devices, Vol. ED-26. ~. 

1294 (1979). 

23. D. V. Lang, J. Appl. Phys., 45: 3023 (1974). 

24. G. L. Miller, D. V. Lang, and L. C. Kimerling, Ann. Rev. Material 

Science, 377 (1977). 

25. D. K. Schroder, and J. Guldberg, Solid State Electron.,~, 1285 

(1971). 

26. A. Rohatgi et a1., "Effect of Titanium, Copper and Iron on Silicon 

Solar Cells," J. Solid State Electronics, 23, 415 (1980). 

27. A. Rohatgi and P. Rai-Choudhury, Second Quarterly Report on "Research 

on the Basic Understanding of High Efficiency in Silicon Solar Cells," 

SERI Contract No. XB-3-02090-4, June 1983. 

28. N. D. Arora, S. G. Chamberlain and D. J. Roulston, "Diffusion Length 

Determination in p-n Junction Diodes and Solar Cells," Apple Phys. 

Lett. 37, 3, p. 325 (1980). 

29. Mark Spitzer, Spire Corporation, private communication. 

30. C. C. Johnson and T. Wydeven, "Plasma-Enhanced cvn Silicon Nitri-de 

Antireflection Coatings for Solar Cells," J. Solar Energy l.!...(4) , 355 

(1983). 

77 
7. ACKNOWLEDGMENTS 

The authors would like to thank T. W. O'Keeffe, n. L. ~eier> 

and n. N. Schmidt for their help in obtaining spectral response data; 

J. B. ~cNally and W. Cifone for cell processing; R. R. Adams for 

oxidation and diffusion; F. S. Youngk for plating; S. Karako for cell 

testing; W. D. Partlow and R. ~adia for plasma-deposited antireflective 

coatings; L. Chen for open-tube gallium diffusion; H. M. Hohgood for 

supplying Ga-doped silicon; J. B. Milstein and C. Osterwald for cell 

testing at SERI; M. Spitzer of Spire Corp. for supplying 0.1-0.2 0-cm 

wafers; J. Minahan and M. Wolf for supplying 0.75 0-cm wafers; and 

G. S. Law for reading and preparing the manuscript. 

78 
APPENDIX 1 

IMPORTANCE AND CONSIDERATIONS OF HIGH-EFFICIENCY SOLAR CELLS 

A. Rohatgi and E. F. Federmann 

Westinghouse R&D Center 

Pittsburgh, PA 15235 

SUMMARY 

This paper discusses tpe importance of high efficiency solar 

cells in reducing the cost of photovoltaic modules and systems. In 

order for photovoltaics to compete with other forms of energy on a 

large-scale basis, the cost of cell modules should come down to about 

70¢/watt (1980 dollars) and, more importantly, module efficiencies should 

be about 16% because of high area-related costs. Based on recent trend 

in cell efficiency improvement, it is projected that 16% efficient 

modules could be realized within the next five years. The silicon 

material, processing, and design considerations for achieving 18-20% 

efficient cells are also discussed. It is shown that early price 

reduction in photovoltaic systems will occur because of the increase 

in cumulative production and steep learning curve, but when modules 

reach a price that assures substantial implementation then higher 

efficiency becomes the key element in reducing the cost and increasing 

the market. 

79 
IMPORTANCE AND CONSIDERATIONS OF HIGH-EFFICIENCY SOLAR CELLS 

A. Rohatgi and E. F. Federmann 

Westinghouse R&D Center 

Pittsburgh, PA 15235 

The importance of energy has increased tremendously in the past 

decade. Among the various alternatives available, solar energv is 

particularly attractive because it is essentially unlimited and has 

minimal pollution or physical danger associated with it. The photovoltaic cell 

is a very desirable solar energy conversion device because it converts 

sunlight directly into electricity rather than some intermediate form of 

energy. The solar cell was invented in the lQ50s, but at that time its 

cost was too high and efficiency too low to compete with other forms of 

energy. Fortunately, with the advent of the space program in the 1960s 

and the thrust for terrestrial applications in the mid-1970s, the cost 

of solar cells has corne down by about an order of magnitude to 'V$10;'v3tt, 

and in large quantities, prices as low as $4.95/Watt have been quoted. 

In order for photovoltaics to compete with other forms of 

energy on a large-scale basis, the cost of solar cell modules should 

corne down to about 70¢/watt (1980 dollars) and, more importantly, 

module efficiencies should be about 16% because area-related costs become 

extremely important for large systems. It is now recognized increasingly 

in the photovoltaic community that high efficiency is a major attribute 

80 
that will determine the large-scale applicability of solar photovoltaic 

systems. Present-dai solar cell efficiencies fall considerably short of 

the module efficiency requirements for very large-scale applications. 

Assuming 5% reflection absorption losses. 1% mismatch losses, and 94% packing 

factor for rectangular cells, 18% efficient cells will be required 

for 16% efficient modules. Current module efficiencies are about 12% 

in production. 

In the last two to three years, solar cell efficiencies have 

been in the range of 14-17% even at the research level, and there has 

been some concern that single-crystal silicon cell efficiency has 

reached its practical limit, (1) although the theoretical limit(2) 

is ~25% (AM1). However, recent breakthroughs have occurred in silicon 

cell efficiency at the research level. Cell efficiencies in the range 

of 17-18.5% have been reported (Table 1), which is existing proof that 

photovoltaic know-how and technology today can provide 18% efficient cells, 

although it could take another five years before such cells are 

made reproducibly in mass production. The high efficiencies in Table 1 are the 

result of high-quality silicon, innovative cell design, very careful cell 

processing, reduced reflection losses, and improved understanding of 

efficiency-limiting mechanisms such as bandgap narrowing, interband 

Auger recombination, and surface recombination. 

Based on recent progress toward high-efficiency cells (Table 1), 

it is not unreasonable to expect 20% efficient cells at the research 

level by 1986-1988 and at the production level by 1995. However, it 

will take a considerable amount of basic research involving high-quality 

81 
silicon material, process development, cell design, and basic 

understanding of efficiency-limiting factors. Some of the considerations 

needed for 18-20% efficient cells are discussed below. 

Material and Carrier Lifetime Considerations 

There is a need to understand the crystal defects and 

imperfections even in the very best silicon crystals grown today because 

the measured lifetimes are well below the ultimate value. Based on the 

best measured lifetime values of the order 6f- 1 msec, Fossum et al. (7) 

have hypothesized a vacancy-related fundamental defect in silicon 

crystals which limits the lifetime in nondegenerate silicon. However, 

in good--quality crystals, it is difficult to detect any deep level defect 

even with the help of the most sensitive techniques, such as deep-level 

transient spectroscopy that are available today. There is a need to 

identify and minimize such crystal defects by improved methods of 

crystal growth, which may include high-purity melt, improved crucible materials 

and instrumentation, optimized growth conditions, and multiple 

pass float zone refining. 

There is some discrepancy in the lifetime data in the low-

, " (8) 'h' b d A ff' , d reslstlvlty range, suggestlng t at lnter an uger coe lClents nee 

to be reevaluated. Any change in these coefficients would have 

significant impact on the design of heavily doped regions. A lowresistivity (0.1-0.3 u-cm) and high lifetime (~500 wsecs) ribbon silicon 

would be il very desirilble substrate for high-efficiency, low-cost solar cell 

modules. 

82 
Process Considerations 

Substantial research is also required in the area of device 

processing for high-efficiency cells. It is extremely important to 

select process conditions that will not introduce new defects or 

unwanted impurities; otherwise, the high lifetime of the starting 

material will become academic. High-temperature oxidations should be 

avoided to minimize stacking faults and dislocation-type defects. 

Special care must be taken during substrate cleaning, and favorable 

gettering ambients consisting of POC~3 and HC~ gas should be utilized 

whenever possible. Slow cooling and gradual wafer withdrawal from the 

furnace could also be important in preserving or even increasing the 

carrier lifetime of the starting material. 

Design Considerations 

If a high carrier lifetime cannot be obtained or retained in the 

cell, then clever cell design can still give high-efficiency cells. As 

suggested by the recent model calculations of Sah, (9) 20% efficient 

p+-n-n+ cells can b~ realized with a base lifetime of only 20 ~secs 

provided the cell thickness is reduced to 50 ~m and the back-surface field 

penetration is 20 ~m with an ND 18 -3 "-'5 x 10 cm . Use of a p+ emitter 

reduces the interband Auger recombination in the emitter because of 

sma 11 er A uger b · t' S' '1 1 d 1 1 1 . (10) recom lna lon rates. lml ar y, our mo e_ ca cu atlons 

suggest that for a n+-p-p+ cell with metal over p+ region, the BSF 

region should be deep (> 10 ~m), with a surface concentration of 

3 x 1018 cm3 . However, if the p+ surface is passivated by growing a 

83 
thin layer of oxide, then the BSF region should be very thin in order 

to take advantage of the low surface recombination velocity. 

In addition to the bulk material and BSF region, the emitter 

plays a very important role in determining cell efficiency. Most current 

cells employ a surface dopant concentration of ~1020 cm3 In this range 

Auger recombination limits the minority-carrier lifetime values to nanosecond range, while bandgap narrowing increases the saturation current 

and consequently reduces the open-circuit voltage. Another very 

important factor which lowers the current as well as the voltage is the 

emitter surface recombination velocity. We have found that just by 

passivating the emitter surface, the quantum efficiency of the cells at 

short wavelengths (0.4-0.5 ~m) can be raised from 40% to greater than 

"75% (Figure 1). Therefore, the emitter region should not be regarded as a 

"dead layer" because by proper passivation coupled '''ith reduced emitter 

doping, it may be possible to raise the quantum efficiency at shorter 

wavelengths to greater than 80-90%. However, special care must be taken 

in designing the contact for reduced emitter doping; otherwise, series 

resistance can degrade the fill factor. The back surface of the cells 

in Figure 1 was also passivated resulting in an increase in the quantum 

efficiency at longer wavelengths. Passivation of these cells also 

resulted in about 20 mV increase in V 

oc 

Another factor that will help achieve 18-20% efficient cells 

is the design and fabrication of multilayer antireflective coatings with 

varying refractive index to minimize reflection losses. Most good 

84 
single-layer and double-layer AR coatings are able to increase J by sc 

about 45%, but a multilayer coating could give an additional ~5% 

improvement. For use in modules, the AR coating should be designed 

for the proper interface. 

There is further potential for efficiency improvement by using 

tandem cells with silicon as the base. 

Impact of Cell Efficiency on Photovoltaic System Costs and Markets 

Just why do we need high efficiency cells? There are some who 

might say that the important factor is the cost per watt; and in instances 

where the system area related placement costs are small, this indeed 

seems to be true at this time. However, as photovoltaic module prices 

comedown and market penetration becomes greater, efficiency of the cell 

becomes the vital factor in further module cost reduction. 

Figure 2 shows our projection of module efficiency and price 

to the year 2000. The bands indicate a 90% probability the value will 

be within the range shown in the band. For the module price, the dashed 

curve shows the likely price. 

Early price reduction will occur primarily because of 

increasing cumulative production and a rather steep learning curve. 

However, as the cell cost becomes a less dominant part of the module 

cost and conventional components that have been in high volume production 

for years become more important, the module learning curve becomes less 

steep and efficiency improvement is the key element in further cost 

reduction. Our evaluations indicate that this will occur at a module 

price of about $1.50/watt, and that below $l/watt, further price reduction 

will depend almost entirely on efficiency. 

85 
Perhaps a hypothetical example would be appropriate here. 

Assume a module priced at $l/watt using a 16% cell that results in a 

12% efficiency module due to high mismatch losses and low packing factor. 

-The module price per square meter is $120 in th is case. I f an 18~~ 

2 improved cell at the same module cost/meter can be developed with a 

narrow statistical efficiency variation bet\oJeen cells and a form suited 

to a high packing factor, module efficiency would be 16%. This would 

result in a module price of $l/watt x .12 ~ .16 or SO.75/watt. If 

') 

the system placement cost for a particular application is $50/m~, the 

cost of installing the array for the 16% cell will be .05 ~ .12 = $0.42f,oJatt, 

making the installed array cost $1.42/watt of array output under standard 

conditions and neglecting array wiring losses. For the 18% cell, the 

placement cost will be .05 + .16 = $0.31/watt, and the total installed 

array cost, $1.06/watt. In this case, the reduction is $0.36/watt -

$0.25/watt for the module and $O.ll/watt for area related system costs. 

How important are modest module price reduce ion to the photovoltaic potential? Figure 3 shows - with the usual 90% band - the 

dependence of annual Megawatt peak sales on module price. For the 

likely case, a reduction of module price from $l/watt to S.70/watt is 

projected to result in a more than threefold increase in market size from 

2500 1:fHp to 8000 MHp. 

These speculations on the impact of cell efficiency improvement 

have assumed that all other attributes of the cell and module remain the 

same. Attributes of importance are: for the cell - life, yield, narrow 

statistical variation in efficiency, adaptability to minimum steps and 

86 
automated techniques for cell and module manufacture, and adaptability 

to high packing factor; for the module - life maintenance and 

reliability, degradation, and automated processes of connection and 

assembly. 

Trade-off of the above attributes with efficiency can have an 

important impact on system costs. In particular, life can have a profound 

effect. System life is usually taken to be 20 or 30 years. If the 

module life is less, not only does the system cost increase due to purcahse 

of new modules - the cost of physical replacement must also be added. 

To summarize, when photovoltaic modules reach a price that 

assures substantial implementation, the major factor in reducing module 

cost will be increased efficiency. The increase in utilization of 

photovoltaic systems is dramatically affected in this implementation range 

by modest reductions in module price. As a bonus, reduction of placement 

costs for higher efficiency modules will further enhance system 

implementation. Thus, just at the time when photovoltaic systems become 

viable for many applications, further increase in efficiency is the key 

element in future market buildup. 

87 
TABLE , 

1-

SaNE RECENT HIGH EFFICIENCY SILICON SOLAR CELLS 

TESTED UNDER ONE seN AMI ILLU11I:\!ATION 

J V Substrate 

sc oc 1l Resistivity 

mA/cm2 mV FF % ~-cm Source 

36.0 625 .805 18.1 0.15 Applied Solar Energy 

36.5 610 .775 17.2 10 Corporation (3) 

36.2 600 .793 17.2 4 W ., (4) estlngnouse 

35.9 627 .800 18.0 0.3 Spire Corp. (5) 

34.9 643 .813 18.1 U ~) University of NeTw 

South \'}a1es 

Australia(6) 

33.0 653 .810 17.5 0.3 Sandia National 

Concentrator Laboratories(ll) 

Cell 

34.0 624 .820 17.6 0.3 Applied Solar Energy 

Concentrator Corporation(ll) 

Cell 

35.1 623 .780 17 .1 0.3 Catholic University of 

Leuven, Belgium (12) 

88 
...-

c 

w 

~ w 

CL 

>-. 

u 

c 00 W 

\0 .-

U .-'+- '+-

L..W 

E 

::J 

...-

c 

co 

::J 

0 

100 

90 

80 

70 

60 

50 

40 

30 

20 

10 

0 

315 475 

Curve 744777-A 

o Without Oxide Passivation 

• With Oxide Passivation 

575 675 775 875 975 

Wavelength (Nanometers) 

1025 

Fig. I - Quantum efficiency vs. wavelength plot for an unpassivated cell 

and 17.2% efficient oxide passivated cellon 4 Q-cm float zone silicon 
Curve 745908-A 

Price Reduction 

Primarily Due to Volume Primarily Due to Efficienc,y 

20 ... ... 10 

18 9 

16 8 * - N 

~ 14 7 00 

0' 

- 12 6 

...-4 

. -

:t::: 5 10 5 coo 

8 4 ~ 

--* 6 3 

4 2 

0 1 

0 

1983 1985 1990 1995 2000 

Year 

Fig. 2 - Proj ected modul e prices and effic ienc ies 

90 
3 

* N 

00 

0--

...-4 

- 2 ..... 

..... 

co 

~ 

""-

* 1 

30 

Low 

100 

High 

1000 

MWp!Year 

Cu rye 745007-A 

Fig. 3. -Module MWp/year as function of price 

91 

10,000 
REFERENCES 

1. J. A. Amick and A. K. Ghosh, J. Electrochem. Society 1300),160 

(1983). 

2. H. Wolf, Proc. of 14th IEEE Photovoltaic Specialists Conf., p. 674 

(1980) . 

3. P. A. lIes, Applied Solar Energy Corporation, private communication. 

4. A. Rohatgi and P. Rai-Choudhury, 3rd Quarterly Report on "Research 

on Basic Understanding of High Efficiency in Silicon Solar CeJls," 

SERI Contract No. XB-3-02090-4, July 1983, also submitted for 

17th Photovol taic Specialis ts Conf. (1984). 

5. M. Spitzer, Spire Corporation, private communication; 3rd Quarterly 

Report on Research on Basic Understanding of High Efficiency in 

Silicon Solar Cells," SERI Contract [\0. ZB3-02090-3 also submitted 

for 17th Photovoltaic Specialists Cunf. (1984). 

6. A. W. Blakers, M. A. Green, and Shi Jigun, ISES Perth Meeting, 

Australia, 1983. 

7. J. G. Fossum and D. S. Lee, Solid State Electronics 25 (8), 741 (1982). 

8. H. Wolf, Analysis and Evaluation in the Production Process and 

Equipment Area of the Low Cost Solar Array Project, DOE/JPL Final 

Report of Contract No. 956034, April 1982. 

9. C. T. Sah, "Study of Relationships of Material Properties and High 

Efficiency Solar Cell Performance on Material Composition, First 

Technical Report, DOE/JPL Contract No. 956289-83/1, July 1983. 

10. J. R. Davis and A. Rohatgi, Proc. of 14th IEEE Photovo1taic 

Specialists Conf., p. 569 (1980). 

92 
11. H. T. Weaver and R. D. Nasby, Proc. of 16th IEEE Photovo1taic 

Specialists Conf., p. 361 (1982). 

12. J. Nijs et a1., Solar Cell, 7(3), p. 331 (1982). 

93 

APPENDIX 2 

83-9F4-HIEFY-P 2 

December 1983 

Unrestricted 

DESIGN, FABRICATION, AND ANALYSIS OF 17-18% EFFICIENT 

SURFACE-PASSIVATED SILICON SOLAR CELLS 

A. Rohatgi and P. Rai-Choudhury 

ABSTRACT 

A simple analytical model has been developed which provides 

useful guidelines for fabricating high-efficiency silicon solar cells. 

, + + Consistent with the model calculations, both surfaces of n -p-p solar 

cells were passivated by a thin layer of thermally grown Si0 20 Oxide 

passivation resulted in 17.2% efficient solar cells on 4 Q-cm base 

material. Passivated cells show about 3 ~~/cm2 increase in J sc ' -20 

improvement in V oc' and -2% increase in absolute cell ef fitiency 

compared to the counterpart 15.2% efficient unpassivated cells. The 

mV 

majority of improvement in Voc came from the emitter surface 

passivation, while both front- and back-surface passivation contributed 

to the increase in J sc • The emitter region should not be regarded as a 

"dead layer" because emitter surface passivation can increase the 

quantum efficiency at short wavelengths from 40% to greater than 75%. 

95 
1. INTRODUCTION 

The theoretical maximum efficiency of a silicon solar cell is 

about 25%, although present-day cells fall considerably short of this 

limiting value. This is largely a consequence of heavy doping effects, 

bandgap narrowing, and high recombination at and near the cell surfaces. ~ecent efforts to increase the efficiency of silicon solar cells 

have been focused appropriately on the problem of raising the opencircuit voltage since current collection efficiencies are approaching 

theoreticallimits.(1-3) This emphasis has led directly to the problem 

of maximizing the effective diffusion length in all parts of the device, 

while using substrate material with the highest possible donor or 

acceptor concentrations. These provisions act in opposition and are 

modified by the specific details of the device structure. It is thus 

evident that an optimization is possible. The problem is complicated by 

imperfectly understood mechanisms or imprecise knowledge of the 

controlling parameters, and by experimental difficulties in separating 

the effects of these mechanisms. The major problems of efficiency 

improvement fall into the above categories; however, there are 

additional design requirements for efficient contacts and antireflective 

coatings. Although these areas are better understood, they are not 

trivial and must be satisfied in a manner compatible with the requirements of the rest of the device. In this paper we describe our effort 

of achieving high-efficiency silicon solar cells by a combination of 

modelling, inriovative cell design, compatible cell processing, and a 

detailed analysis of high-efficiency cells. 

2. MODEL CALCULATIONS 

We have developed a simplified analytical model which provides 

useful insight and guidelines for fabricating high-efficiency solar 

cells. The model includes the effect of bandgap narrowing, Auger 

recombination, and recombination at the device surfaces, but it neglects 

96 
the electric field effects resulting from the gradient of doping 

concentrations.(4,5) 

The model is based on the use of an internal recombination 

velocity as a measure of the minority-carrier losses in the various 

regions of the device. This directly provides the junction saturation 

current and thus the dark voltage-current characteristic which exerts 

primary control over cell performance. It is apparent that the major 

benefit of reducing recombination is the increase in open-circuit voltage, Voc' which follows from reductions of the saturation current, J o ' 

Solar cell efficiency is directly proportional to the opencircuit voltage and the short-circuit current. Both, of course, are 

subject to recombination losses, but here we will only concentrate on 

the variation of Voc. Open-circuit voltage is inversely related to the 

saturation current (Jo). 

2 

qni = J ob+ J oe- --- • - NA 

WB 

D [SnCOSh (1) n n 

L D W 

n _n (_B) L cosh L 

n n 

D 

• .....E. 

L 

p 

D 

+ ....!l sinh L 

n 

+ S sinh 

n 

(1) 

(2 ) 

(Dn,Ln ) and (Dp,Lp) are the diffusivity and diffusion length of the 

+ minority carriers in the p-base and n emitter region, WB and WE are the 

base and emitter widths beyond the junction edges, Sn and Sp are the 

+ surface recombination velocities at the front of the n emitter region 

and back of the p-base region, respectively, and Job and Joe are the 

base and emitter contribution to the reverse saturation current.(S) 

97 
The recombination at the back surface can be reduced by 

introducing a back-surface field or low-high junction,(6-9) while the 

recombination at the front surface can be reduced by growing an oxide 

layer. A detailed analysis of low-high structures by Gunn(lO) provides 

a beginning place for our model. The starting expression for the present 

discussion is an equation, derived from the carrier transport equations 

which transform the surface recombination velocity of the device (So) t) 

an effective recomhination velocity seen by minority carriers at the low 

side of the low-high interface as a function of the properties of the 

low and high regions. + This equation for a p-p structure is: 

where + L+ n+ np ' n' n 

s 

o 

s 

e 

n + n+ 

-E. ~ 

+ 

n L 

p n 1 + 

+ S L 

o n 

n+ 

n 

(3) 

tanh 

concentration, diffusion length, and diffusivity of 

the minority carrier in the heavily doped p+ 

region. 

width of the heavily doped region. 

surface recombination velocity at the back of the 

high region. 

effective recombination velocity at the low side 

of the low-high junction. 

minority-carrier concentration in the low region. 

We introduce the necessary modifications in the above expression 

to include the effects of degeneracy and bandgap narrowing. We have 

devised an empirical expression for the effective bandgap narrowing, 

(~VG) based on the data of Lanyon(ll) and Lindholm(12) corrected for 

degeneracy effects. The values thus ad.iusted for Fermi statistics can 

be used in the usual Boltzman expression for the np product, i.e., 

98 
2 

np = n ie = exp 

where ~v~ and ~VG are the effective bandgap narrowing in the high and 

low regions, respectively. 

~VG _ 0.231 [(1~20 /14 + 1 ]-2/3 

and 

N N exp c v 

Equation 3 can now be written as: 

N D+ ~V+ - A n ( G S = -- exp e N+ L+ V A n T 

~V~ ) 

S L+ w+ 

-2....!!. + tanh (....E.) 

D+ L+ 

n n 

S L+ w+ 

1 + -2...E. tanh 

D+ 

(-E.) 

L+ 

n n 

Empirical expressions were derived to relate the diffusion 

length and diffusivity to the impurity concentrations (N). 

(4) 

(5) 

The expression for D was obtained from the data of Conwell as 

given in Grove.(13) 

D 

D 0 = + A (6 ) 0.6 

(~) 0 

1 + 

where for p-type 10 

Do = Dno = 35 

Ao = 1.8 

and for n-type 

Do Dpo = 12.5 

A = 1 0 

99 
The diffusion ~ength is obtained using a Kendall's lifetime 

function(14) for bulk silicon ('K) combined with Reck and Conradt's(15) 

data for Auger recombination lAo 

where 

-1 

l = (_1 + _1 ) 

n 'A TK 

l 

a 

N 1 + ----:-:::-

7 x 10 15 

(7) 

(8 ) 

Note the value chosen for TO is related to the quality of 

silicon. We have found that a value of 200 to 400 us for TO is more 

appropriate for modeling high-performance devices. 

The Auger lifetime is given by: 

1.2 for p-type 

2.8 for n-type 

1 

TA = 31 2 (K • 10- )N A 

Then the diffusion length is obtained from: 

L 

(9) 

(10 ) 

It should be noted that the derivation of Equation 5 is 

completely general in that it may be applied to the calculation of an 

effective recombination velocity across an arbitrarily chosen region 

anywhere in the device as, for example, within the base region where the 

+ doping is constant. In this case we set NA = NA = NA(base). This 

velocity then characterizes the total recombination beyond this plane. 

Assuming appropriate recombination velocity on the cell surface 

as the boundary conditions and from known doping profiles, we can now 

iteratively apply this calculation using Equation 5 from the 'back 

100 
surface, across the base to the edge of the depletion region, and thus 

account for all the recombination processes in the base and back by 

calculating SejbO A similar calculation for the n region ahove the 

junction can give SejbO Relating Equation 2 in terms of Se as given in 

Equation 5 permits expressing the base and emitter component of the 

saturation current density as: 

(11 ) 

where Sejb,NA and Seje,ND are the recombination velocities and doping 

densities at the edges of the depletion region. Then, neglecting 

current contributions from the depletion region, 

v 

oc 

J 

sc V T R.n (-J-"'::+"':::"'J-) 

on on 

(12 ) 

Figure 1 shows the results of model calculations for a backsurface field solar cell using three different back-surface field (BSF) 

structures with two different base diffusion lengths. In Figure 1 

recombination velocity is plotted as a function of distance into the 

+ + 

n -p-p solar cell. Referring to the BSF region in the figure, the 

upper curve is for a high recombination ohmic back (So = 106 em/sec), 

while the lower two curves are for the oxide-passivated backs with 

reduced So •. The lowest curve was obtained by lowering the BSF surfacedoping concentration to 10 19

cm- 3 , thus reducing the heavy doping 

effects. In the 4 n-cm base region, the solid curves are for a lower 

base diffusion length of 209 ~m while the dotted curves are for longer 

base diffusion length of 467 ~m. In this example we have assumed a 

diffused emitter with high-surface concentration with the emitter 

surface passivated (So ~500 cm/sec). 

If the emitter surface is not passivated in this 4 n-cm cell, 

the S. »S 'b' J »Job ' and V is limited by Joe. Model eJe eJ oe oc 

calculations in Figure 1 show that with the emitter surface passivat~d, 

101 
Seje ~ Sejb' but due to much higher doping density in the emitter 

(~1017 cm- 3 ) at the edge of the depletion region(7) compared to the base 

15 -3 doping (3.5 x 10 . cm ), Job »Joe " Therefore, with emitter surface 

passivation, Job limits J o • Now both Sejb and the reverse saturation 

current can be reduced further by back-surface passivation or lower BSF 

doping (Figure 1) to gain additional increase in Voc. Figure 1 shows 

that a long diffusion length (L > 3W) in the finished device is 

necessary to realize this benefit of back-surface passivation and lower 

BSF doping, otherwise Sejb is limited by the diffusion velocity (D/L) of 

the carriers in the base. Calculations also indicate that for a 

diffusion length of 467 ~m. with both surfaces passivated, a V of 599 oc 

mV, J

sc (22) of 37 mA/cm2 , and a cell efficiency of 17.4% can be 

obtainpd. If the surface concentration of the BSF region is reduced to 

10 19 cm3 , then a Voc of 605 mV and a cell efficiency of 17.8% can be 

achieved on this 4 Q-cm substrate. 

Use of a lower resistivity suhstrate with similar diffusion 

length can further reduce the base component of the reverse saturation 

current and thus give greater improvements in Voc. The design of the 

emitter could be more important for the low-resistivity substrate because 

J mav dominate J , even after front-surface passivation. Reduced oe < 0 

doping in the emitter can lower J further and provide additional gain oe 

in Voc. A similar effect of doping concentration is shown tn Figure 1 

+ for the heavily doped p region when Job dominates J o ' 

3. EXPERIMENTAL WORK 

Consistent with our model calculations, we passivated the solar 

cell surfaces to obtain higher open-circuit voltage. Solar cells with 

an n+-p-p+ structure were fabricated on 4 ~-cm, p-type, boron-doped, 10 

+ mils thick, <111> float-zone silicon. The n emitter was formed by a 

850°C POCl3 diffusion which resulted in a junction depth of 0.3 ~m and a 

+ sheet resistance of 60-80 ~isquare. The p back-surface fteld (BSF) was 

fabricated by a 9500C boron diffusion Thin thermal oxide was grown at 

11)2 
BOOoC, which resulted in an oxide thickness of -100 A on top of the n+ 

region and -50 A on the p+ surface. All the high-temperature steps were 

followed by a slow cool at -loC/min in order to preserve the high 

lifetime of the starting material. About 600 A thick Ti0 2/Si0 2 

antireflective coating was applied by a spin-on process on top of the 

front oxide. Ti-Pd-Ag contacts were made on front and back. The cell 

area was 1 cm2 and the front contact grid design had an a1ea coverage of 

2%. A similar grid pattern was opened through the back oxide to estab-

+ lish communication between the p region and the back metal contact. 

Solar cells were tested under 100 mW/cm 2 AMI spectrum with the 

help of a quartz-iodine simulator. Current-voltage measurements were also 

taken in the dark to separate cell resistances, bulk recombination, and 

iunction recombination components numerically and/or graphically.(16,17) 

The hulk component of the transformed I-V data(16) had a slope equal to 

one and the reverse saturation current (Jo ) was ohtained from the 

intercept at V = O. 

Both reflectivity and spectral response measu~ements were 

performed over a wavelength range of 0.4 ~ to 1.1~. In the spectral 

response measurements, photocurrent under short-circuit conditions from 

the cell is compared automatically to that from a standard silicon 

detector of calibrated spectral response. Absolute spectral response 

(amps/watt) was obtained by: 

,~elative Spectral Response (X) x Standard Detector (A/W) 

l-R (X) 

From these data, internal quantum efficiency, QE(X), as a function of 

wavelength was determined according to: 

) =Absolute Spectral Response (X) x Energv of Photons 

°E(A q 

Internal quantum efficiency is defined as the number of 

electron-hole pairs collected relative to the number of photons entering 

the material.(5) 

103 
The effective diffusion length_in the base was obtained from the 

plot of photon attenuation length (l/a, where a is the photon absorption 

coefficient) and a variable X-I, where X is the ratio of photons 

absorbed per unit time in the base to the number of electrons per unit 

of time in the external circuit.(18) X = e-a(d+w)/QE, where d is the 

junction depth and w is the width of the depletion region at zero 

bias. The effective diffusion length would be equal to t~1e time base 

diffusion length if the base width is much greater than the diffusion 

length; otherwise, it is a combination of hulk diffusion length and 

surface recombination velocity. 

Ellipsometric measurements were performed to determine the 

thickness of the oxide layer and the antireflective coating separately 

and in combination. 

4. RESULTS 

All solar cells were fahricated on high-lifetime 4~-cm floatzone silicon. Consistent with the high lifetime of the starting 

material, deep-level transient spectroscopy (DLTS) performed on Schottky 

barrier diodes(19) showed no deep levels. Table 1 shows the cell data 

for the baseline n+-p-p+ solar cells, withotlt any oxi<ie passivation. 

Short-circuit current density (J ) in these cells is ahout 33 rnA/cm 2 , sc 

open-circuit voltage (Voc ) is -580'mV, and the average cell efficiency 

is about 14.75% with the maximum exceeding 15%. 

Table 2 shows the data for the solar cells with both surfaces 

2 passivated. In oxide-passivated cells, J

sc 

-36 rnA/cm , Voc -600 mV, and 

cell efficiencies are greater than 17%. A lighted I-V curve is shown in 

Figure 2. Table 3 shows a comparison of cell parameters of a 17.2% 

efficient oxide-passivated cell and a 15.2% efficient unpassivated 

cell. Oxide passivation results in an increase in J ,V • and fill 

sc oc 

factor associated with a decrease in the reverse saturation current 

104 
Table 1 

Baseline Unpassivated Solar Cells + + (n -p-p ) Fabricated 

on 4 ohm-cm Float-Zone Silicon 

Short-Circuit Open-Circuit Cell 

Curre~t J sc Voltage Voc Efficiency 

Cell ID rnA/cm Volts Fill Factor % 

1 33.3 0.582 0.767 14.8 

2 33.1 0.584 0.756 14.6 

3 33.0 0.579 0.777 14.9 

4 32.9 0.581 0.772 14.7 

5 32.9 0.581 0.752 14.4 

6 33.4 0.583 0.780 15.2 

Table 2 

Oxide-Passivated Solar Cells on Boron-Doped 4 Q-cm 

Float-Zone Silicon 

Are~ J V FF n 

Cell ID (cm ) (~/cm2) (~~) (% ) (% ) 

HIEFY 4-4 1.0 36.1 599 79.4 17. 1 

-5 1.0 36.2 600 79.3 17.2 

-6 1.0 36.4 598 78.5 17. 1 

-7 1.0 36.2 599 79.1 17.2 

-8 1.0 36.3 597 79.5 17.2 

HIEFY 3-1 1.0 34.6 599 79.2 16.5 

-2 1 .• 0 34.6 598 78.9 16.4 

Figure 3 compares the internal quantum efficiency of the oxidepassivated 17.2% efficient cell and the counterpart 15.2% efficient 

unpassivated cell. The data show a significant increase in the quantum 

efficiency at shorter wavelengths (0.4-0.55 ~m) along with an 

appreciable increase ~n the quantum efficiency at longer wavelengths 

(0.9-1.1 ~) as a result of passivating both cell surfaces. 

105 
Table 3 

A Comparison of Cell Parameters of a 15.2% Efficient Cell 

and 17.2% Oxide-Passivated Cell 

Parameter 

J sc 

V oc 

FF 

n 

~ . . 19ure 

Unpassivated 

+ + n -p-p Cell 

33.4 mA/cm 2 

583 

.78 

15.2% 

0.5 n 

104 k0 

3.7 x 10-12 A/cm2 

Oxide-Passivated 

+ + O-n -p-p -0 Cell 

~ 36.1 mA/cm~ 

600 

.794 

17.2% 

0.21 n 

1~ k0 

2 x 10- 12 A/cm2 

4 shows the l/a versus X-I plot for the oxide-passivated 

as well as the unpassivated cell. The two curves overlap in l/a range 

of 15-60 wm and the slope of the two curves in this range gives a hulk 

diffusion length of 263 ~, but as we go to longer wavelengths the slope 

of the unpassivated cell curve becomes less steep, indicating a relative 

decrease in tbe effective diffusion length. 

In order to gain more insight into the effect of surface 

passivation, the hack metal and the back oxide were removed from a few 

16.5% efficient oxide-passivated cells, protecting the front surface, 

~ 

and then the p' back surface was remetallized. These cells now only 

have front-surface passivation. Table 4 shows the cell data with 

a) both surfaces passivated, b) the same cells with only the front 

surfaces passivated, and c) unpassivated cells. The data (Table 4b-4c) 

show that front-surface passivation in 'these 4 Q-cm cells provides a 

significant increase in Voc (~13 mV) and an appreciable increase in J sc 

(0.5-1 mA/cm2). Rack-surface passivation (Table 4a-4b) gives an 

additional increase in Voc of ~5 mV and J sc of 0.5 mA/cm2• 

Ellipsometric measurements on 17.2% efficient oxide-passivated 

cells showed that the antireflective coating consists of passivating 

oxide of thickness 112 A with a refractive index of 1.458 and a spin-on 

106 
Table 4 

Solar Cell Data With a) Both Surfaces Passivated, 

b) Only Front-Surface Passivated, and 

c) With No Surface Passivation 

V J S7 2 n 

Cell ID vgrts rnA cm FF % 

( a) Both Surfaces 'Passivated: + + o-n -p-p -0 

1 .599 34.6 .792 16.5 

2 .598 34.6 .789 16.4 

(b) Above Cells with Only Front-Surface Passivated: + + o-n -p-p 

1 .595 33.9 .795 16.0 

2 .594 3-4.0 .793 16.0 

(d Cells Without Surface Passivation: + + n -p-p 

1 .582 33.5 .767 14.9 

2 .582 33.3 .767 14.8 

layer of 555 A on top of the passivating oxide with a refractive index 

of 2.03. 

5. DISCUSSION 

Tahle 2 shows that oxide passivation coupled with careful cell 

processing can give solar cell efficiencies greater than 17% (AMI) on 

good quality 4 Q-cm float-zone silicon, with Voc ) 600 mV, J sc 

~36 mA/cm2

• and fill factor -0.795. These parameters are in good 

agreement with the model calculations in Section 2. These cells are 

among the best cells reported to date, especially on 4 Q-cm material. 

The data in Table 3 show -20 mV improvement in Voc ' -3 mA/cm2 increase 

in J sc ' and about 2% (ab~olute) increase in cell efficiency in the 

oxide-passivated cell compared to the counterpart unpassivated cell. 

Dark I-V measurements (Table 3) indicate a decrease in reverse 

saturation current, J o • from 3.7 x 10- 12 cm- 2 to 2.0 x 10- 12 cm- 2• 

107 
Using the expression Voc ~ KT/q £n(Jsc/Jo )' this decrease in J

o coupled 

with the measured increase in J

sc essentially accounts for the observed 

18 mV increase in Voc due to passivation in this pair of cells. Since 

J

o 

Joe (emitter) + Job (base), to find out which component of J

o 

has 

been reduced, back-oxide passivation from some lower efficiency cells 

(16.5%) was removed and then they were retested (Table 4b) after 

+ remetallizing the p back surface. The data in Table 4 indicate that, 

out of the ~18 mV increase in V due to oxide passivation, a 13 mV 

oc 

improvement comes from the front-surface passivation and the remaining 

comes from the back-surface passivation •. This also implies that without 

any surface passivation in these cells, 10 ~ Joe and, therefore, fr6ntsurface passivation alone is able to increase Voc. If J oh was limiting 

J , then the front-surface passivation would not have much influence on 

o 

J or V • It appears that after front-surface passivation in these 

o oc 

4 Q-cm cells, J

o 

becomes base limited (Jo ~ Job) and, therefore, backsurface passivation gives an additional 5 mV improvement in V 

oc 

(Tables 4a-4b). These observations are entirely consistent with our 

model predictions in Section 2, Figure 1. 

Greater improvements in Voc can be achieved from front-surface 

. . iIi i' b . 1 ( 20 , 21 ) . th . . 1 pasSlvatlon n ower res st vlty ase materla Wl Slml ar 

diffusion velocity (D/L) In these cells Job will no longer limit J o ; 

instead, the value of S achievahle from front-surface passivation will 

define the lower limit of Joe or J o • However, if J remains much 

oe 

greater than Job' even after front-surface passivation, then the backsurface passivation will not increase Voc much further. Some attempts 

are being made to verify this by fabricating cells on 0.2-0.3 Q-cm 

material specially grown for high lifetime. 

Figure 3 explains the reason for the observed increase in J

sc 

due 

to oxide passivation. Shorter wavelengths «0.55 urn) are primarily 

absorbed in the emitter, while longer wavelengths ()0.95 ).1m) are mostly 

absorbed near the back surface. The fig~re shows that front-surface 

passivation results in a significant improvement in the quantum efficiency 

at shorter wavelengths, while the back-surface passivation increases the 

IUS 
quantum efficiency at longer wavelengths. This is consistent with the 

data in Table 4, which show that both front- and back-surface passivation 

contribute to the observed increase in J sc ' since passivation reduces the 

number of photogenerated carriers that are lost to the surfaces. The 

quantum efficiency at ~.4 ~m wavelength could be raised from 40 to 75% by 

emitter surface passivation, which implies that in the unpassivated 

cell, a ma.;ority of the photogenerated carriers near the emitter surface 

are not collected because of high surface recombination, and not because 

of high bulk recombination in the emitter as a result of Auger and bandgap 

narrowing effects. Thus, the emitter region should not be regarded as a 

"dead layer" because simply by proper oxide passivation, emitter quantum 

efficiency can be made greater than 75%, and further improvements may be 

possible by reducing Auger recombination and bandgap narrowing effects in 

the emitter. Surface recombination can be minimized further by growing a 

thin tunnel oxide underneath the front and back metal to completely avoid 

direct metal/silicon contact. 

Figure 3 in~icates that quantum efficiency in the wavelength 

range of 0.85-0.95 ~ is nearly the same for both unpassivated and 

passivated cells. Most of the light in this wavelength range is 

absorbed within the bulk silicon, away from the surfaces, since the 

absorption length is in the range of 20-60 ~m. This suggests that oxide 

passivation has apparently not changed the actual diff~sion length of 

the base material, which is calculated to be 263 ~m in this wavelength 

range 9'igure 4) •. At longer wavelengths (higher values of 1/ <l), the 

slope of the unpassivated cell becomes less steep relative to a 

passivated cell, indicating that its effective diffusion length 

decreases near the back surface due to high surface recombination 

velocity. The slope of the curve for the oxide-passivated cell in 

Figure 4 is only slightly affected at longer wavelengths, supporting the 

notion that a good back-surface passivation has been achieved. 

le9 
6. CONCLUSIONS 

Consistent with our model calculations, we fabricated greater 

than 17% efficient solar cells by passivating the cell surfaces. In the 

4 D-cm base material used in this study, both front- and back-surface 

passivation by a thin layer of Si0 2 resulted in about 3 mA/cm2 increase 

in J sc ' -20 mV improvement in Voc' and -2% increase in the absolute cell 

efficiency. In these cells, the malority of increase in Voc comes from 

front-surface passivation, although back-surface passivation also 

provides -5 mV increase in Voc. Both front- and back-surface 

passivation contribute to the increase in J sc by reducing the loss of 

photogenerated carriers to the surfaces. Even in a 4 ~-cm base cell, 

the emitter surface recomhination velocity seems to limit J o or Voc when 

tell surfaces are not passivated. 

Greater improvements in Voc can be achieved by front-surface 

passivation in lower resistivity base cells provided the same rliffusion 

velocity (D/L) and diffusion length to base width (L/W) ratio can be 

maintained. In such cells, Job will not limit J o ; instead, the lower 

limit of J

o 

will be defined hy the value of S achievable from the frontsurface passivation. The emitter region should not be treated as a 

"dead layer" because proper surface passivation can result in greater 

than 75% quantum efficiencies at short wavelengths «0.55 urn), and . further improvements are possible by reducing heavy doping effects or by 

growin~Jtunnel oxide underneath the metal grid. 

7. ACKNOWLEDGMENTS 

The authors would like to acknowledge the invaluable 

contribution of the late J. R. Davis for device modelling and 

guidance. They would like to thank T. W. O'Keeffe, D. L. Meier, and D. 

No Schmidt for help in spectral response measurements; J. R. McNally, F. 

S. Youngk, and S. Karako for cell fabrication and testing; and G. S. Law 

for reading and preparing the manuscript. They would also like to thank 

110 
J. B. Milstein for his encouragement and support and C. Osterwald for 

verifying the cell efficiencies at SERI. This work was partially 

supported hy SERI Contract No. XB-3-o2090-4. 

III 
8. REFERENCES 

1. F. A. Lindholm and J. G. Fossum, "Review of Physics Underlying 

Recent Improvements in Silicon Solar Cell Performance," Proc. 14th 

IEEE Photovoltaic Specialists Conf., p. 680 (1980). 

2. A. Neugroschel, F. A. Lindholm, S. C. Pao, and J. G. Fossum, 

"Emitter Current Suppression in a High-Low-Junction Emitter Solar 

Cell Jsing an Oxide-Charge-Induced Electron Accumulation Layer," 

Appl. Phys. Lett. ~, 168 (1978). 

3. R. B. Godfrey and M. A. Green, "655 mV Open Circuit Voltage 17.6% 

Efficient Silicon MIS Solar Cells," Appl. Phys. Lett., 34, 790 

(1979). 

4. M. Wolf, "Designing Practical Silicon Solar Cells Approaching the 

'Limit Conversion Efficiency'," Proc. 14th IEEE Photovoltaic 

Specialists Conf., p. 563 (1980). 

5. H. J. Hovel, Semiconductor and Semimetals, Vol. II, Solar Cells, 

Academic Press, p. 20 (1975). 

6. M. P. Godlewski, C. R. Barona, and H. W. Brandhorst, Jr., "Low-High 

Junction Theory Applied to Solar Cells," Proc. 10th Photovoltaic 

Specialists Conf., p. 40 (1973). 

7. J. G. Fossum, "Physical Operation of Rack Surface Field Solar 

Cells," IEEE Trans. Electron. Devices, Vol. ED-24, p. 322 (1977). 

8. A. Sinha and S. K. Chattopadhyaya, "Effect of Rack Surface Field on 

Photocurrent in a Semiconductor Junction," J. Solid State Elect., 

Vol. 21, p. 943 (1978). 

9. J. G. Fossum and E. L. Burgess, "High-Efficiency p+-n-n+ BackSurface-Field Silicon Solar Cells," Appl. Phys. Lett., Vol. 33, p. 

238 (1978). 

10. J. B. Gunn, "On Carrier Accumulation and the Properties of Certain 

Semiconductor Junctions." J. Electron. Control, Vol. 4, p. 17 

(1958) • 

11. M. P. D. Lanyon and R. A. Tuft, "Bandgap Narrowing in Heavily Doped 

Silicon," Proc. IEDM, Washington, D.C. p. 316 (1978). 

12. F. A. Lindholm, private communication. 

13. A. S. Grove, Physics and Technology of Semiconductor Devices, John 

Wiley, New York, 1967. 

112 
14. D. Kendall, presented at the Conf. Physics and Application of 

Lithium Diffused Silicon, NASA, Goddard Space Flight Center, Dec. 

1969. 

IS. J. D. Beck and Conradt, Solid State Communications, Vol. 13, p. 93 

(1973). 

16. A. Rohatgi, J. R. Davis, R. H. Hopkins, P. Rai-Choudhury, P. G. 

McMullin, and J. R. McCormick, "Effect of Titanium, Copper, and Iron 

on Silicon Solar Cells," J. Solid State Elect., Vol. 22, p. 415 

(1980). 

17. A. Neugroschel, F. Lindholm, and C. T. Sah, "A Method for 

Determining the Emitter and Rase Lifetimes in p-n Junction Diodes," 

IEEE Trans. Electron. Devices, Vol. ED-24, p. 662 (1974). 

18. N. D. Arora, S. G. Chamberlain, and D. J. Roulston, "Diffusion 

Length Determination in p-n Junction Diodes Solar Cells," Appl. 

Phys. Lett., Vol. 37, p. 325 (1980). 

19. A. Rohatgi, "Application of DLTS Technique to Study Junctions and 

Interfaces," Vacuum-Surfaces-Thin Films, K. L. Chopra and T. C. 

Goel, Ed., Vanity Books, Delhi, p. 115 (1979). 

20. J. G. Fossum, F. A. Lindholm, and M. S. Shihib, "The Importance of 

Surface Recombination and Energy-Bandgap Narrowing in p-n-Junction 

Silicon Solar Cells," IEEE Trans. Electron Devices, Vol. ED-26, p. 

1294 (1979). 

21. Mark Spitzer, Spire Corporation, private communication. 

22. J. A. Amick and A. K. Ghosh, "Practical Limiting Efficiencies for 

Crystalline Silicon Solar Cells," J. Electrochem. Soc., Vol. 130 

(I), p. 160 (1983). 

113 
LIST OF FIGURES 

Figure 1. Internal recomhination velocity plots for back-surface field 

solar cells 

Figure 2. Lighted I-V data for a 17.2% oxide-passivated eell fabricated 

on 4 0~cm float-zone silicon 

Figure 3. Internal quantum efficiency versus wavelength plot for an 

unpassivated 15.2% efficient cell and 17.2% efficient oxidepassivated cellon 4 P-cm float-zone silicon 

Figure 4. Diffusion length plot for an unpassivated 15.2% efficient 

cell and 17.2% efficient oxide-passivated cellon 4 P-cm 

float-zone silicon. 

114 
p+ 

I Jifbsed 

ISack-Field 

Back Hi-Lo 

S wriace J u"lction 

Distance. \.1m 149.75 1:1) __________________ ---L ____ ~ 

- L=467 j.Jm 

L = 2091Jm 

------

-----

Base R egicn 

N+ 

C ifft.:sed I r:;r·it1er ,-

Junction Front 

~'e~ietion 

Region 

5 u r~ac e 

Figure I. I~ternal reco~~inat1on velocity plots for ~ack-surface 

field solar cells. 

115 
<t 

E 

-c 

Q.) 

lI-

:::::l 

U 

.10,0 

35.0 

30. 0 

25.0 

20. 0 

15. 0 

10.0 

5.0 

O. 0 

-5.0 

-10.0 

-15.0 

- I .~ 

v = O. &JO volts 

OC 2 

J = 36.2 rnA/em 

se 

Fill Foctor = 79.3 % 

Efficiency = 17. 2 % 

-~.O~ __ ~ __ ~ __ ~ ____ ~ __ ~ __ ~ ____ ~ __ ~ __ ~ 

-. 20 -. 10 0.0 . 10 . 20 . 30 . 40 .50 .60 .70 

Voltage (volts) 

Figure 2. lighted I-V data for a 17.2~ oxide-passivated cell fabricated 

on 4 ~-cm fleat-zone silicon. 

116 
C~r\le 7~~e45"A 

100 r----r-, 

90 ..- -

c 80 Q) 

U 

!... 

Q) 

0.. 70 >, 

u 

c:: 

Q) 60 .~ 

"---

L....I 50 

E 

:::l 40 ..... 

c • Without Oxide Pass lvation ro 

:::l 

0 30 o With Oxide Passivation 

20 

10 

0 

425 525 625 725 825 1025 

Wavelength (nonometers) 

Figure 3. Internal quantum efficiency versus ~avelength plot for an 

u~passivated 15.2% efficient cell and 17.2% efficient oxidepassivated cellon 4 ~-crn float-zone silicon. ' 

117 

1120 
V'l 

\... 

(!.) 

..-

(!.) 

E 

0 

\... 

.~ 

E 

('tl 

..c 

0.. 

<: 

-....... 

150 

l35 

120 

1J5 

7Q 

75 

&0 

45 

30 

15 

0 

-1 -,8 -,6 -,4 -,2 0 

• \'nthout Oxide Passivation 

o V,' ith Oxide PElSS ivation 

Siope=L=263~m 

.2.4',6.81 

X-l 

1.21.4 1.6 1.8 2 

Fig~re 4. Diffusion length plot for an unpassivated 15.2% efficient 

cell and 17.2% efficient oxide-passivated cellon 4 ~-cm 

flcat-zone silicon. 

118 
Document Control 11. SERI Report No. 12. NTIS Accession No. 

Page SERI/STR-211-245l 

3. Recipient's Accession No. 

4. Title and Subtitle 5. Publication Date 

Research on the Basic Understanding of High Efficiency 

in Silicon Solar Cells 6. 

September 1984 

7. Author{s) 

A. Rohatgi and P. Rai-Choudhury 

9. Performing Organization Name and Address 

Westinghouse R&D Center 

1310 Beulah Road 

Pittsburgh, Pennsylvania 15235 

12. Sponsoring Organization Name and Address 

Solar Energy Research Institute 

1617 Cole Boulevard 

Golden, Colorado 80401 

15. Supplementary Notes 

Tprhnirrll tv;nnitnr· Joseoh Mil stein 

16. Abstract (Limit: 200 words) 

8. Performing Organization Rept. No. 

10. Project/Task/Work Unit No. 

3434.10 11. Contract (C) or Grant (G) No. 

(C) XB-3-02090-4 

(G) 

13. Type of Report & Period Covered 

Technical Report 

14. 

This report presents results of research designed to develop a basic understanding of high-efficiency silicon solar cells and achieve cell efficiencies 

greater than 17% by employing innovative concepts of material preparation, cell 

design, and fabrication technology_ The research program consisted of a theoretical effort to develop models for very high-efficiency cell designs, expermental verification of the designs, and improved understanding of efficiencylimiting mechanisms such as heavy doping effects and bulk and surface recombination. Research was performed on high-lifetime float-zone silicon, the baseline 

material, low-resistivity gallium-doped czochralski silicon, and boron-doped 

float-zone silicon. 

17. Document Analysis 

a.Descriptors Silicon; Solar Cells; Theoretical Data; Experimental Data 

b.ldentifiers/Open-EndedTerms High Efficiency Silicon Solar Cells 

c. UC Categories 

63 

18. Availability Statement 

National Technical Information Service 

u.S. Department of Commerce 

5285 Port Royal Road 

Springfield, Virginia 22161 

Form No. 0069 (3-25-82) 

19. No. of Pages 

129 

20. Price 

A07