Polyx multicrystalline silicon solar cells processed by PF+ 5 unanalysed ion implantation and rapid thermal annealing

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Polyx multicrystalline silicon solar cells processed by

PF+ 5 unanalysed ion implantation and rapid thermal

annealing

W.O. Adekoya, Li Jin Chai, M. Ajaka, J.C. Muller, P. Siffert

To cite this version:

Polyx multicrystalline

silicon solar cells

_{processed by}

_PF+5

_unanalysed

ion

implantation

and

_rapid

thermal

_annealing

W. O.

_Adekoya,

Li Jin Chai

_(*),

M.

_Ajaka,

J. C. Muller and P. Siffert

CRN

_(IN2P3),

Laboratoire PHASE

_(UA

du CNRS n°

_{292), 23,}

rue du _Loess, 67037

_Strasbourg

Cedex, France

(Reçu

le 26

_{septembre 1986, accepté}

le 8 décembre

₁₉₈₆₎

Résumé. 2014

Le recuit transitoire des défauts induits dans le silicium par

l’implantation

d’ions _peut être une

technique compétitive

par rapport au recuit

_classique

_{pour la fabrication des}

_{photopiles à}

_usage terrestre. Il

reste

_{cependant quelques inconvénients,}

tels _qu’une faible durée de vie des _porteurs

_{photogénérés-}

et une

tension en circuit ouvert _{peu élevée.}

_Nous

avons tenté de surmonter ces _{difficultés pour le silicium}

multicristallin

_(POLYX)

_{produit par la CGE}

_(France)

en choisissant une

_température

de recuit de la _jonction

dopée au

_phosphore,

d’une _part, assez _élevée, 800 °C

_(afin

d’obtenir une bonne recristallisation et une

activation suffisante du

_dopant)

et d’autre _part, suffisamment _faible, 900 °C

_(afin

de réduire les effets de

dégradation

des

_{propriétés électriques}

de la

_base).

Le but de ce travail est d’étudier les

_{caractéristiques}

I-V des

photopiles

et de les _{comparer à} celles obtenues

_par

recuit

_classique

ou _par diffusion du

_dopant.

Abstract. 2014

Rapid

thermal

_annealing

of

_damage

induced _{by implantation} in silicon can be a cost effective

technology

for the

_processing

of terrestrial solar cells as

_compared

to classical furnace or _pulsed laser

annealing. Unfortunately,

drawbacks as _{poor bulk lifetime} or low

_{open-circuit-voltage}

occur as well. We have

attempted to overcome these limitations for POLYX

_{multicrystalline}

cast _{silicon grown}

_by

CGE

_(France)

_by

keeping

the

_annealing

_temperature of the

_{phosphorus doped layer}

as

_high

as 800 °C

_(to

ensure a _good

crystalline

quality

and a

_{high dopant activation)}

while

_being

less than 900 °C

_(to

minimize the effect of

degradation

of the base

_properties).

_{The purpose of the present} work is to

_investigate

the I-V characteristics of the cells and to _compare to those obtained with classical furnace

_annealing

or with classical diffusion process.

Classification

Physics

Abstracts

60.30 - 72.40 - 61.70T - 81.60

1. Introduction.

Multicrystalline

silicon can be considered as an attractive alternative to

_single

_crystal

silicon as base material for the fabrication of solar cells. It offers the

_possibility

of

_achieving

_strong

reduction of cell cost in

_spite

of a

_relatively

small decrease in

ef-ficiency.

Parallel to the work on new

_crystal

_growth

methods

_including

ribbon

_sheet,

_pulling

and cast

ingot growing,

the

_increasing

use of «

off

grade »

low cost silicon necessitates the

_optimization

of the cell

manufacturing

process and

especially

the

_junction

formation.

_During

the last ten _{years, ion}

implanta-tion has

_gained

_acceptance

as a

_highly

effective

junction

formation _process. The results of several studies indicates that excellent solar cells can be fabricated

_{[1, 2],}

and the ion

_implantation

_process

(*)

On leave from the lab. of Ion-beam

_Physics,

Wuhan Univ. Wuhan

_(China)

has been

_applied

to the fabrication _{of space}

_[3],

flat

plate

[4],

concentrator

_[5]

and

_{high efficiency [6]}

solar

_cells,

with notable

_performances

achieved for each _type of cell. Nevertheless the

_high

cost of conventional ion

_implanters

has

_prevented

wide-spread

use of

_implantation

for solar cell manufac-ture, so

_that,

alternative

_procedures

_using

unana-lysed

ion

_{implantation [7, 8],}

or

_{glow discharge [9]}

have been

_proposed.

In the case of

_large

_grains

_{multicrystalline}

silicon solar

_cells,

realised

_by

ion

_{implantation,}

_only

few studies are

_reported

in literature :

_phosphorus

and arsenic

_implantation

_followed

_by

_incoherent

_light

annealing

[10, 11] ;

PFS

glow

discharge

plus

UV laser

annealing

[12], PFS

unanalysed

implantation

+ YAG

and

_CO2

laser

_{annealing [13, 15].}

In this

_work,

we _propose for POLYX material the realization of shallow N+ P

_junction

_by

_coupling

an

unanalysed

ion

_implantation

_procedure

with a

_rapid

thermal

_{annealing processing.}

Fast

_dopant

activation

696

as well as

_negligible

redistribution are the two

essential

_advantages

of this

_{annealing procedure.}

But the two

_{major problems,}

which are the reduction of the

_minority

carrier lifetime in the bulk and the presence of residual defects in the space

charge

as well as in the front

_{region, imply}

serious limitations to _{the process} as the

_open-circuit

_voltage

and the

quantum

efficiency

are affected. We have therefore

attempted

to overcome these limitations in

multicrys-talline silicon solar cells

_{through keeping}

the

anneal-ing

temperature

in the _{range of 800} to 900 °C. The results of

_experiments

carried out on devices from POLYX

_(CGE)

material with

_implanted

N+ P

junc-tions are

_presented.

The electrical

_properties

were

analysed by

using

sheet resistance measurements, dark and AMI illumination 1 V

_{characteristics,}

as well as

_spectral

_response.

Our

choice of

_annealing

parameters was based on results from studies carried -out on

_single

_crystal

silicon of a similar

_resistivity

as the wafer used in this work. Various

_analysis

like

Rutherford

_{Backscattering}

_(RBS),

_Transmission

Electron

_{Microscopy (TEM),}

Sheet

_resistance,

car-rier concentration and

_{mobility profiling,}

and

_Deep

Level Transient

_Spectroscopy

_(DLTS)

measure-ments, all show that an

_annealing

_{temperature range} of 800-850 °C for 4-5 s, appears to be

_optimal

for our

experimental

conditions. 2.

_Experimental

conditions.

2.1 DOPING. - As industrial

_necessity

of

_high

production

rates over

_large

surface wafers results in

using

sophisticated

and

_expensive

ion

_implantation

equipment,

we have

_proposed

the realization of solar cells

_by

_using

a low cost,

high

current, low energy,

unanalysed

ion

_implantation

_procedure

_[7],

which needs no

_{sophisticated}

_magnetic

network for

isotope

selection,

nor

_long

beam lines. This

_doping

processing

has been called AMI

_(Atomic

and

TIME(sec)

Fig. 1. - Schema of the

Rapid

Thermal

_Annealing

_(RTA)

set _{up together} with the

_{microprocessor}

controlled thermal

cycle.

Molecular ion

_{Implantation). Phosphorus}

ions were

implanted

at 30 keV _energy

_{by using}

this

_technique

at an

_optimized

dose

_of

2-3 x

1015 ion/cm2.

The

uniformity

is about 10 % accross the wafer and

10

% wafer to wafer as determined

_by

means of the four

points

probe

technique performed

after

_annealing.

2.2 ANNEALING. - The

_system

of

_Rapid

Thermal

Annealing

(RTA)

used here is a

_commercially

available fumace

₍₃₄

_{kW power} HEATPULSE machine

_[16])

with a

_compressed

air and water

cooled reflector to ensure a

_good

_temperature

homogeneity

of the

_sample

holder. A continuous

high purity

argon gas flow is maintained

through

the chamber

_during

the whole

_annealing

_{process. The}

temperature

is controlled

_by

an

_optical

_pyrometer

connected to a

_{microprocessor.}

The

_heating

rate is fixed at about 50 °C/s and the

_cooling

rate is about 100 °C/s

_{(see figure 1).}

In order to _compare the RTA

annealing

to the classical

_{one, 10}

cm2 POLYX

slices from the same

_{ingot (n°}

_633,

_resistivity

1.96

_n.cm)

were cut in 2

cm2

_samples,

so that there were _many

samples

for each slice number. For the electrical

analysis,

we have restricted our

_study

to two slices

(number

33 and

₄₃₎

_coming

from the middle of the

ingot.

The measurements were

_performed

on 3-4

samples

for each

_experimental

_condition,

so that the maximum

_scattering

of the

_results,

were taken in consideration.

2.3 CELL PROCESSING. - The contacts of the cells were obtained

_{by simple}

vacuum

_evaporation

of a Au

_layer

on the

back,

while a

_Ag

_grid

covers about 12.5 % of the front surface. The cells were tested on a simulator

_adjusted

to deliver 100

mW/cm 2

_(AMI

conditions).

The measurements were taken at 28 °C. 3.

_{Characteristics}

of the

_{doped layer.}

3.1 REGROWTH BEHAVIOUR. - We

_report

in

_figure

2 the RBS _spectra obtained

_{for ~100~}

monocrystal-line silicon

_samples

annealed at

_615,

_{725, 770,}

820 and 980 °C for 4 s. The

_comparison

between the

aligned

and the random curves shows that within the

temperature range 615-770

°C,

the

damaged

layer

is still

_amorphous

while an

_annealing

at 820 °C is sufficient to have a

_{crystal quality}

close to that of a

virgin sample

as confirmed

_by

_measuring

the minimum

_yield

_{X min which reaches} a value of 5 %.

Complementary

Transmission Electron Mi-croscopy

(TEM) [17]

measurements confirm the relative

_{good crystal}

_quality

for 820 °C/4 s

_annealing

with

_only

_{the presence of small} 100-200

À

diameter

defects,

probably

precipitates ;

whereas at

tempera-tures above 950 °C dislocation

_loops

and

_rod-shaped

defects appears.

3.2 DLTS ANALYSIS. - Since

Fig. 2. - Rutherford

Backscattering

(RBS)

measurements of the residual disorder after RTA process.

Fig. 3. - DLTS _signal for a

PFS

_implanted

_sample

annealed at 615 °C/4 s, 820 °C/4 s and 1100 °C/4 s.

temperature/time

compromise

in order to achieve

good crystal quality,

we have

_{investigated, using}

the DLTS

_technique,

a series of

samples

annealed under these conditions as well as above and below it in order to see the evolution of

electrically

active defects within the range 600-1100 °C. Our main results are summarized in

figure

3.

These results indicate that one defect level at 542 meV with a concentration and effective cross section of 2.7 x

1014 cm- 3

and 2.1 x

10-14 cm-2, respectively

had formed at 615 °C/4 s. This same level was

observed in all the other

_samples

annealed _up to

1100 °C/4 s, we remark that its concentration

_greatly

diminished at the

_highest

temperature.

At 820 °C/4 s, two other levels are seen to have formed one of which is not _yet characterizable. The other level shows up at 180 meV with a concentration of 4.8 x

1013 cm- 3

and an effective cross-section 9.3 x

10-16 cm2.

This level was

_again

observed after 1 100 °C/4 s

_annealing,

however,

with a

_higher

con-centration as shown in

figure

3.

The kinetics of formation of these levels is the

subject

of another

_study,

but their

_importance

at the

moment is seen in the fact that under

quasi-ideal

annealing

conditions,

they

remain in the

_junction.

3.3 ELECTRICAL PROPERTIES. - In

figure

4,

the sheet resistance of the N+

_doped

_layer

of POLYX

multicrystalline

silicon is

_plotted

_a)

versus the

an-nealing

temperature in the range 700-1 000 °C for a 4 s

_duration,

_b)

versus

_annealing

time for a tempera-ture of 850 °C. ’ It is

_noteworthly

that the sheet resistance reaches a minimum value of 90-110 03A9/~ when the

_annealing

_temperature

is more than 800 °C. This value is as low as those obtained for the classical thermal _process

_{(three-step annealing}

550 °C, 1

h ;

850 °C,

1/4

h ;

550

°C,

1

_h)

_{(Fig. 4c).}

Moreover,

it is known that to avoid

_degradation

effects on the lifetime of

_minority

carriers in the

bulk,

it is necessary to utilize the lowest

_annealing

temperature

which

_gives

a

_{good recrystallization}

of the

_amorphized

_layer.

For this _reason, we decided to

carefully analyse

the 800-900 °C _{range for}

_annealing

times between 2 and 8 s.

Fig. 4. - Evolution of the sheet resistance

as

_a)

a function

of RTA _temperature for 4 s

_heating

_time,

_b)

a function of

698

In

_figure

_5,

we have

_reported

the carrier

concen-tration

_profiles

as deduced from anodic

_stripping

technique

for

_~100~

_{monocrystalline}

silicon

_samples

annealed at different

_{temperature-time couples,}

which

_give

a maximum activation of the

_dopant.

The

phosphorus

peak

activity

reaches a level close to the

equilibrium solubility

limit

_(about

4 x

1020

cm- 3)

for

temperature-time couples

such as

₍₇₅

°C,

15

_s)

or

(900 °C, 2 s).

The

_phosphorus

active

_fraction,

as deduced from the total incident dose of the

E

PFn

ions is about 24 % to 30 %. It is also to be noted that no diffusion occurs

_{by increasing}

the

temperature

due to shorter

_processing

durations.

Fig. 5. - Carrier concentration

profiles

of ~100~

mono-crystalline silicon _samples annealed at different RTA

temperature-time couples.

4. Photovoltaic

_{performances.}

In table

_I,

we _present a

_comparison

_{of the results of} the AMI

_performance

of the

_single

_crystal

and

polycrystalline

cells. Our results for the former are

compared

with those obtained from cells

_prepared

by

implantation

of P+ and

_PFS

ions followed

_by

a similar incoherent

_{light annealing}

_[18]

or

_{by liquid}

phase

laser

_annealing

_[12-14]

and with those

_reported

for SILSO material

_[10-12].

In the case of

_single

_crystal

_cells,

we observed that the short circuit current

_(Isc)

is almost

_equivalent

in all the

_processed

_cells,

whereas the _open circuit

voltage

(V oc)

of the

_PF+

_implanted

_samples

is low

(below

500

_mV)

when

_compared

with results ob-tained for P+

_{implantation [18]}

and for

_PFS

after laser

_annealing

_[12-14].

This is also the case with the

polycrystalline

cells.

These

low

_Voc

values can be attributed to the

_persistence

of a defective

_region

near the

_junction

as confirmed

_by

DLTS and also to

the _{presence of} fluorine which inhibits the

_regrowth

during annealing

[17, 19].

The results from the cells

Fig. 6. -

Evolution of the

_{V,., I.}

and ~ as a function of

RTA :

a)

temperature and

_b)

_{duration ;}

_c)

_classical

processed

on the POLYX

_ingot

633

_using

the RTA

annealing

set-up

are

_compared

_{with those from cells}

prepared using

the classical

_{three-step annealing}

procedure

(i.e.

550 °C,

1 ;

850

_°C,

1/4

_{h ;}

_{550 °C,}

1 h).

For the purpose of

comparison,

we show in

_figure

6 the evolution of the _open circuit

_voltage

_(Voc),

the short circuit current

_(Isc)

and the

_{efficiency (~)}

for the fast thermal

_{processing a)}

in the

_temperature

range 700-1 000 °C for a fixed

_annealing

time 4 s,

b)

at a fixed

_temperature

850 °C for different

_times,

and

_c)

for the

_classically

_{annealed reference}

samples.

For these series of measurements

per-formed on POLYX

_material,

we have a

_large

scattering

in the

_experimental

results more

_especially

for the

_Isc

current and for

_efficiency

values

_(about

± 10

_%)

than for the sheet resistance and the

Voc

values. This

_scattering

effect is in _agreement with other results obtained on SILSO material for a similar

_{annealing technique}

_{[10, 11].}

One can see on this

_{figure :}

- that the

_Voc

values in the _{range 800-900 °C} are

as

_good

as the values

_reported

for classical

_annealing

and that the

_annealing

duration

_plays

a

_negligible

role in this case ;

- that

_higher

current values than those obtained

for the

_three-step

_process can be obtained for the lower

_annealing

_temperature

of 800 °C which from RBS

_results,

assures a sufficient

_regrowth

of the

damaged

layer.

However,

the

_annealing

duration is

important

here as the

_Isc

values decrease as a function of

_increasing

the _{duration in the range}

2-8 s ;

- finally,

the

_efficiency

values confirm these tendencies so that the

_rapid

thermal

_processed

cells have about one _per cent more conversion

_efficiency

than those obtained from the

_{three-step annealing.}

However,

the results obtained to date

_by

the association of the

_Rapid

Thermal

_{Annealing (RTA)}

with the Atomic and Molecular ion

_Implantation

(AMI)

have not reached the level of those

_processed

using

for

_doping,

classical ion

_implantation

or dif-fusion of the

_dopant,

and

_pulsed

laser for the

annealing (see

Table

_1).

5. Conclusion.

The aim of this

_study

was the

_{investigation}

of the

possibilities

offered

_by

the

_Rapid

Thermal

_Annealing

(RTA)

to _{regrow the POLYX} silicon

_damaged

_layer

due to the

_doping

_process

_using

the Atomic and

Table I. -

Photovoltaic properties

of

mono and

_{polycristalline}

cells

_produced

_by

AMI

_{implantation followed}

_by

classical and RTA

_annealing.

Some

_results,

obtained

_by

classical ion

_implantation

or

by

classical

_diffusion

_of

the

phosphorus

dopant

are

_given

_for

_comparison.

700

Molecular ion

_Implantation

_(AMI)

_process with

PF5

ions.

Our main results show that cells have not reached in the

_present

_processing

conditions,

the level of cells

_processed

with the conventional

_technique,

even

_though

the

regrowth

and electrical activation

seem to be sufficient. The

V oc

and

_{efficiency limiting}

factor which result from this

_processing

is due to

residual

defects

and

to _{the presence} of fluorine. A better controled and

_possibly

slower

_cooling

rate can be _{the way} to eliminate this serious drawbacks.

Acknowledgments.

We would like to thank Miss A. Grob and S. Barthe for

_performing

RTA

_annealing

and Miss C.

_Weymann

and M. J.-P. Schunck for the contacts of the cells.

We are also

_grateful

to the French

_{Energy Agency}

AFME

_(Agence

_Française

_pour la Maîtrise de

l’Energie)

and to the PIRSEM from the French CNRS

_(Centre

National de la Recherche

Scientifi-que)

for financial _support.

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[20]

Photovoltaic results obtained on POLYX

_ingot

n° 633 and

_given

_by CGE

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Marcoussis