A novel wrap-around metal contact optimized for radial p-n ... · p+nn+ s corporates ors (B155 f or...
Transcript of A novel wrap-around metal contact optimized for radial p-n ... · p+nn+ s corporates ors (B155 f or...
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1
Supporting Information
A novel wrap-around metal contact optimized for
radial p-n junction wire solar cells
Sun-Mi Shin, Jin-Young Jung, Kwang-Tae Park, Han-Don Um, Sang-Won Jee,
Yoon-Ho Nam and Jung-Ho Lee*
Department of Chemical Engineering, Hanyang University, 55 Hanyangdaehak-ro, Sangnok-
gu, Ansan, Kyeonggi-do 426-791, Republic of Korea
Email: [email protected]
Department of Chemical Engineering
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Fig. S1
wire co
By em
realizat
cell siz
contact
samples
because
affected
with fin
factor b
the sma
(sample
increasi
(finger/
IEEE P
note tha
irrespec
1 J-V curve
ntacts. Diff
mploying t
ion of robu
zes and/or p
with conve
s which we
e the fill f
d by top-co
nger/bus ba
behaviors du
all one was
e size: 11
ing the sam
/bus bar) ne
Photovoltaic
at the I-V cu
ctive of the
es for solar
ferent cell si
the wrap-ar
st, reliable
pattern desi
entional me
ere difficult
factors for
ontact struct
ar electrode
ue to differ
cut (for m
cm2) had
mple sizes.
eed to be o
c Specialists
urves for th
cell sizes (s
cells with
izes are com
round cont
collection o
ign of met
etal grids, w
to fabricate
these smal
ture. For ex
es (Fig. S1a
ent cell are
easurement
been measu
In case of
optimized a
s Conferenc
he wrap-aro
see Fig. S1b
2
(a) conven
mpared for e
tact design
of charge ca
al grids. To
we actually
e metal grid
ll cells can
xample, foll
a) demonstr
eas although
t with differ
ured. In ge
convention
according to
ce, Washing
ound contact
b).
ntional plan
each scheme
n, we have
arriers which
o specifical
y thought th
ds on their
nnot suffici
lowing I-V
rate the com
h they are id
rent grid de
eneral, the
nal top elec
o the cell s
ton, D.C., U
t cells are o
ar grids, an
e.
e intended
h is irrespec
lly compare
hat too sma
tops were r
iently refle
curves of p
mpletely dif
dentically f
esign) right
fill factors
ctrodes, the
sizes (Ref.
USA, 1978)
observed to
nd (b) wrap
to emphas
ctive of var
re our wrap
all area (
-
Fig. S2
embedd
contacts
S2(a)] d
removin
was neg
was obs
Fig. S3
wavelen
wavelen
2 (a) A dig
ded in PDM
s (right), an
Our light
detached fro
ng from the
gligible [Fi
served from
Absorption
ngths from
ngths were
gital photog
MS, Si micro
nd (b) transm
absorptance
om the sub
e substrate b
g. S2(b)]. A
m the wire ar
n spectra of
400 nm and
notable to r
graph comp
owires only
mittance spe
e (Fig. 1c)
strate. How
because we
After remov
rrays contai
f PDMS. Lo
d 1100 nm,
reveal high
3
paring tran
(left), Si m
ectra of the
data were
wever, the el
have confir
ving from t
ining Ag ele
ow average
, but two pe
absorption
nsparency o
microwires s
sample (rig
measured b
lectrical pro
rmed that li
the substrat
ectrodes.
absorption
eaks at shor
(>10%) of P
of two Si m
urrounded b
ght) in (a).
by using th
operties wer
ight absorpt
e, low tran
of ~0.8% w
rt (1100)
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Fig. S4
microw
~80% w
our wor
of a mic
wire sur
which t
4 External
wire sample
The reason
was originat
rk, despite
crowire-arra
The wire a
rfaces in wh
then resulted
Quantum
which is me
ning why th
ted mostly
the remarka
ayed-absorb
arrays were
hich the RIE
d in degrada
Efficiency
easured in F
he short circ
from the fa
ably increa
ber removed
fabricated b
E proceeded
ation (~0.7
4
(EQE) re
Fig. 2b.
cuit current
act that there
sed surface
d from the s
by RIE proc
d for long t
μs) in mino
esult of the
t was poore
e was no su
e-to-volume
substrate.
cess causing
ime in orde
ority carrier
e wrap-arou
r than the li
urface passi
ratio becau
g severe pla
er to pattern
life time.
ound Ag co
light absorp
ivation proc
use of the p
asma damag
n 60-μm-lon
ontacted
ptance of
cesses in
presence
ge to the
ng wires,
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Fig. S5
solar ce
Step A:
Step B:
5 Schematic
ell with wrap
: A periodic
An 1.5-μm
wafer usin
The substr
mixture of
Periodic S
etching pr
adopted to
The wire p
determined
Park et al.
: Formation
A cost-effi
bottom sid
This appr
direction o
a front side
First, boro
cs showing
p around Ag
c array of sil
m-thick SiO
ng high-dens
rate was pat
f CF4 and C
SiO2 dot arr
rocess, inc
o define the
pitch (a cent
d by the pa10 described
ns of a radia
ficient, conf
des of a waf
roach produ
of a wafer, w
e and a BSF
on and phos
the overal
g contacts (
licon micro
O2 layer wa
sity plasma
tterned usin
HF3 plasma
rays (2 μm
luding etch
high aspect
ter-to-cente
atterned siz
d more detai
al p–n juncti
formal dopi
fer using dif
uces a high
which conco
F on backsid
phorus silic
5
ll process s
(without det
owires
as deposited
chemical v
ng low-level
a.
in diamete
hing (SF6)
t ratio Si mi
er distance in
ze of a phot
ils about thi
ion and bac
ing techniqu
fferent dopa
h-efficiency
omitantly in
de.
cate precurs
sequences f
taching wire
d on a 8-in
vapor depos
l lithograph
er) were use
and polym
icrowire arr
n between m
to mask. Pr
is procedure
k surface fi
ue was emp
ants in one t
y p+nn+ s
ncorporates
sors (B155 f
for fabricati
es from the
ch n-doped
ition
hy and react
ed as etch m
merization
rays.
microwires)
revious exp
e.
eld (BSF)
ployed to co
thermal cyc
tructure ac
a radial p–n
for B and P
ing a Si m
substrate).
d (5 Ωcm),
tive ion etch
masks. The
(C4F8) ste
) and diame
perimental w
o-dope the
cle.
cross the th
–n junction M
P509 for P, s
icrowire
Si(100)
hing in a
e plasma
ps, was
eter were
work by
top and
hickness
MWs on
supplied
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6
by Filmtronics) were spin-coated on each dummy wafer.
Then, the SOD dummy wafers were baked at 150 °C for 20 min. To form the BSF, a
phosphorus-coated dummy wafer was directly contacted to the bottom side of a
target wafer so as to degenerately dope boron into the backside.
The boron-coated dummy wafer, however, was located in proximity mode on the
front side of the target wafer to tailor the doping profile of boron diffusion.
Simultaneous diffusion doping was carried out in a tube furnace under a mixed
ambient of N2 and O2 at 900°C for 5 min. Previous work by Jung et al.14 reported
more details about this procedure.
Step C: Spin-coating of PDMS.
A mixed solution (10:1 wt %) of PDMS base (sylgard 184) and curing agent was
spin-coated on the wire arrays at 4000 rpm for 480 sec.
To maintain the height of PDMS uniform, a solution should be stirred enough to
cover whole sample surface prior to dropping for spin-coating.
Then, the coated sample was cured at 80°C for 8 hrs. The vacuum oven was
necessary to remove air bubbles originated from the mixing process of PDMS base
with curing agent.
A drastic change in temperature should be avoided because the PDMS film can be
cracked due to the difference in thermal expansion coefficients. Previous work
performed by Putnam et al.4 reported more details about this procedure.
Step D: PDMS etching
Spin-coated PDMS was etched using a solution of tetrabutylammonium fluoride
(TBAF) mixed with dimethyl fluoride (DMF) in a 1:1 volume ratio. The etch rate
was 10 μm/min, but was normally sensitive to temperature.
A height of PDMS was variable by adjusting dipping time. After dipping in the
solution, the sample was rinsed in DMF solvent. This process was to remove the
PDMS residues on wire arrays.
Then, cleaning using deionized (DI) water was conducted. This cleaning process was
important because the PDMS residue was likely remained in between wire arrays.
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Step E:
Step F:
Step G
: Ag deposi
Ag electro
wires were
metal thick
The reason
physically
between th
PDMS rem
PDMS coa
: Selective A
The Ag co
solution (m
etching so
etching tim
infiltrated
ition
odes were th
e embedded
knesses wer
n why we
y separate
he substrate
mained) was
ating and etc
Ag removal
oated on pro
methanol :
olution likel
me was requ
into the inte
hermally ev
d in PDMS r
re varied fro
coated Ag
Ag electro
e and Ag. In
s fixed at 10
chback
and D abo
protect a p
protruded
which the
by wet etc
l
otruded part
ammonia :
ly penetrate
uired to prec
erfacial reg
7
vaporated on
remained. T
om 100 to 1
on top of P
ode from th
n our work
0 m.
PDMS c
ove, were re
planar Ag la
Then, the
out of PDM
Ag in prot
ching.
ts of wires
hydrogen
ed along th
cisely limit
ion of PDM
nto Si micro
The depositi
1300 nm by
PDMS rem
he substrat
k, the gap (c
coating and
epeated onc
ayer, not sid
e Ag-coated
MS [see left
truded parts
was remov
peroxide=4
he interface
in order to
MS/wire (see
owires in wh
ion rate was
adjusting d
mained at wi
te while co
correspondi
etchback s
ce again bec
dewall-coate
d wire top a
t figures, Fi
s could be s
ed for 1 mi
4:1:1, volum
between A
prevent the
e figures be
hich the bo
s ~10Å/sec,
deposition ti
wire bottoms
ontrolling
ing to the h
steps, i.e.,
cause we ne
ed Ag.
and sidewa
igs. S5(a) &
selectively r
in using Ag
me ratio). S
Ag and PDM
e overetchin
elow).
ttoms of
, and the
ime.
s was to
the gap
height of
steps C
eeded to
alls were
& (d)], in
removed
g etching
ince the
MS, the
ng of Ag
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Step H
Fig. S6
coating
of PDM
formed
: Finally, th
6 SEM imag
and etchba
MS after etc
after remov
he bottom co
ges (a-c) de
ack processe
chback proc
val of PDM
ontact was t
escribing ho
es. (d) Ag-c
cess; (e) se
MS. (d) and (
8
thermally de
ow the cont
coated-top r
elective rem
(f) correspo
eposited on
tact height
regions of a
moval of Ag
nd to (a) an
to a back si
was adjuste
microwire
g; (f) wrap-
nd (c), respe
ide of the so
ed by using
were protru
-around Ag
ectively.
olar cell.
g PDMS
uded out
g contact
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Contac
area com
where ρ
A circu
Ag/Si c
where r
Ag/Si w
For this
by ITO/
surroun
ct resistanc
We extrac
mpared to A
ρc is contact
ular area of
contact wher
The wire t
r is a wire ra
when the L i
s reason, a b
/Si, which c
We can al
nd top-regio
ITA
e of ITO/Si
cted the con
Ag/Si contac
t resistivity,
ITO/Si con
re L denote
tops are calc
adius of 1 μ
is 500 nm.
black dot co
corresponds
lso calculat
ons of micro
TO/Si A
i [Fig. 3(c)]
ntact resista
cts. Contact
, and A is c
ntact is dire
s a height o
culated by
topA
μm. Note th
500L nA
onnected w
s to the sam
e the Rc of
owires. The
ITO/SiA
500L nm
9
]
ance of ITO
t resistance
ccR A
ontact area
ectly compa
of a cylinder
p region
hat the area
2nm rL
with red dott
me area form
f ITO/Si wh
contact are
2A r
top-regA
2A
O/Si while
(Rc) is dete
. The ρc of
ared with a
r.
2r
of wire-top
500L nmL
ted lines [Fi
med by Ag/S
hen ITO at
a can be de
L
gion A
500LrL
maintaining
ermined by:
ITO/Si is 7
sidewall ar
p is equal to
ig. 3(c)] sta
i contact at
ttempts to t
scribed usin
0 2nm r
g the same
1.43×10-3 m
rea of a cy
o the contact
ands for a p
L of 500 nm
three-dimen
ng L as follo
r L
contact
(eq. 1)
mΩ cm2.
lindrical
(eq. 2)
t area of
(eq. 3)
planar Rc
m.
nsionally
ows:
(eq. 4)
(eq. 5)
(eq. 6)
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where Δ
using th
To extr
necessa
ΔA and ΔL
he equations
ract the co
ary; then, th
L stand for
s above, we
R
ontact resist
e unit was c
_ ITO/ScR
the extende
e could sum
_ ITcR
_ ITO/SicR
tance per u
converted fr
i52 Lr
10
ed contact a
m up the cont
_TO/Si
I
c
A
_
5002c
Lr
unit cell ar
rom [Ω] into
_ ITO/Si
500 2c
nm r
area and co
tact resistan
_ ITO/Si
ITO/Si
_ ITO/Si
0 2nm r
rea, the inf
o [Ω cm2].
(unit r L
ontact heigh
nce of ITO/S
L
formation o
cell area)
ht, respectiv
Si.
of a cell a
vely. By
(eq. 7)
(eq. 8)
area was
(eq. 9)
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Fig. S7
adoptin
emitter
shallow
depth sh
provide
emitter
area, w
radial ju
top side
of wire
contacts
wire cel
7 Schematic
ng wrap-arou
Radial p-n
area, yieldi
wer the junc
hallower, p
e a conform
layer, with
we are curre
unction wir
e of wires, i
e arrays. Th
s will furth
lls.
cs of (a) con
und contact
n junction w
ing high Au
ction depth
lasma dopin
mally doped
a very abru
ntly develo
re solar cell
it is possibl
his selectiv
her improve
nventional r
ts
wire solar
uger recomb
while redu
ng techniqu
d, ultrashall
upt junction
oping the ad
ls. Since ou
le to remov
ve emitter c
cell conve
11
radial p-n ju
cells conve
bination; thu
ucing the e
ues23 would
low junctio
n doping pro
dvanced sel
ur contact is
ve the unnec
concept ad
ersion effici
unction, and
entionally r
us, we need
emitter surf
be highly r
on undernea
ofile. To fur
lective emit
s located cl
cessary emi
dopting plas
iencies repo
d (b) selecti
require a la
d to find out
face area. T
recommend
ath a degen
rther reduce
tter structur
ose to the w
tters positio
sma doping
orted to date
tive emitter
arge amoun
t how to eff
To make a j
dable becaus
nerately dop
e the emitter
re (see Fig.
wire bottom
oned at the
g and wrap
e in radial j
concept
nt of the
fectively
junction
se it can
ped thin
r surface
S7) for
ms, not a
top side
p-around
junction
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Fig. S8
Experim
thickne
*At eac
8 Plots of
mentally ob
ss with thei
Met
thickn
(nm
100
200
300
400
700
100
130
ch condition
f FF value
btained, cha
ir standard d
tal
ess*
m)
FF
0
0
0
0
0
00
00
n, five samp
es and a
ampion and
deviations.
F@champio
29.69
29.56
37.85
51.86
61
65.4
64.42
ples were ide
12
table show
averaged F
on, %FF
entically fab
wing the d
FF data are
F@ average
%
27.8
28.4
36.7
37.3
57.9
62.5
63.3
bricated to
detailed sa
shown as a
e Sta
dev
1
1
1
3
2
1
1
evaluate the
ample infor
a function o
andard
viation
1.65
1.65
1.28
3.81
2.85
1.89
1.58
e fill factors
rmation.
of metal
s.
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Fig. S9
1.
2.
A detailed
At Line 57
s oR =R
The influe
unchanged
determine
to only kn
The Rc val
(1) used fo
3. We ca
procedure d
7, Page 2, th
c m+R +R =
ence of Ro
d while var
ed by wafer
now the Rc a
lues can be
or plotting f
Rc
an also know
describing h
he following
o e=R +R w(base and
rying the m
r resistivity
and Rm at th
obtained as
figure 3c.
cothRWLT
□
w the Re val
13
how to extra
g equation i
where Re=(Rc
emitter res
metal thickn
and emitter
he same tim
s a function
)h(L/LT
Figure 3c
lues from a
act Rs from
is denoted,
+Rm), Ro=co
sistances) c
ness adopte
r sheet resis
me.
of contact h
cotWρR c □
right-hande
the variable
onstant
an be precl
ed for a fro
stance. To e
height (L) f
)ρRh(L
c
□
ed, y-axis of
es of Rc and
luded becau
ont electrod
extract Rs, w
from the equ
(1)
f Fig. 2a be
d Rm.
use it is
de. Ro is
we need
uation
low.
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4.
Since the
below). A
of Rc and R
Re=Rc+Rm,
s a result, w
Rm.
Rm can be
we can final
14
Figure 2a
e obtained f
lly extract a
Figure 3d
from the val
a total serie
lues of Re a
es resistance
and Rc (see
e (Rs) as a
e Fig. 3d
function
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