Ci a ion: Ros, E.; Fe nández, S.;
O ega, P.; Taboada, E.; A nedo, I.;
Gandía, J.J.; Voz, C. Impac o
G aphene Monolaye on he
Pe o mance o Non-Con en ional
Silicon He e ojunc ion Sola Cells
wi h MoOxHole-Selec i e Con ac .
Ma e ials 2023,16, 1223. h ps://
doi.o g/10.3390/ma16031223
Academic Edi o s: Johann Bouclé
and Ma in Mølle G e e
Recei ed: 28 No embe 2022
Re ised: 17 Janua y 2023
Accep ed: 28 Janua y 2023
Published: 31 Janua y 2023
Copy igh : © 2023 by he au ho s.
Licensee MDPI, Basel, Swi ze land.
This a icle is an open access a icle
dis ibu ed unde he e ms and
condi ions o he C ea i e Commons
A ibu ion (CC BY) license (h ps://
c ea i ecommons.o g/licenses/by/
4.0/).
ma e ials
A icle
Impac o G aphene Monolaye on he Pe o mance o
Non-Con en ional Silicon He e ojunc ion Sola Cells
wi h MoOxHole-Selec i e Con ac
Eloi Ros 1, Susana Fe nández 2,* , Pablo O ega 1, Elena Taboada 3, Is ael A nedo 3,4 , JoséJa ie Gandía2
and C is óbal Voz 1,*
1
Depa amen o de Ingenie ía Elec ónica, Uni e si a Poli ècnica de Ca alunya (UPC), 08034 Ba celona, Spain
2Di isión de Ene gías Reno ables, CIEMAT, A da. Complu ense 40, 28040 Mad id, Spain
3das-Nano, Polígono Indus ial Tallun xe II, Calle M-10, Tajona , 31192 Na a a, Spain
4
Depa amen o Ingenie ía Eléc ica, Elec ónica y de Comunicación, Uni e sidad Pública de Na a a, Campus
A osadía, 31006 Pamplona, Spain
*Co espondence: [email p o ec ed] (S.F.); [email p o ec ed] (C.V.);
Tel.: +34-913466039 (S.F.); +34-934017488 (C.V.)
Abs ac :
In his wo k, a new design o anspa en conduc i e elec ode based on a g aphene
monolaye is e alua ed. This hyb id elec ode is inco po a ed in o non-s anda d, high-e iciency
c ys alline silicon sola cells, whe e he con en ional emi e is eplaced by a MoO
x
selec i e con ac .
The de ice cha ac e iza ion e eals a clea elec ical imp o emen when he g aphene monolaye
is placed as pa o he elec ode. The cu en – ol age cha ac e is ic o he sola cell wi h g aphene
shows an imp o ed FF and V
oc
p o ided by he on elec ode modi ica ion. Imp o ed conduc ance
alues up o 5.5 mS a e achie ed o he g aphene-based elec ode, in compa ison wi h 3 mS o
ba e ITO. In addi ion, he de ice e iciency imp o es by a ound 1.6% when g aphene is inco po a ed
on op. These esul s so a open he possibili y o no iceably imp o ing he con ac echnology o
non-con en ional pho o ol aic echnologies and u he enhancing hei pe o mance.
Keywo ds:
g aphene; hole- anspo -laye ; ansi ion me al oxides; non-con en ional silicon
he e ojunc ion sola cells
1. In oduc ion
G aphene is an a ac i e candida e o a new gene a ion o anspa en conduc i e
elec odes (TCEs) wi h huge impac in di e en esea ch ield domains, such as displays,
ouch sc eens, o sola cells, and i is ecen ly se ing oo in he comme cial
ma ke [1–4]
.
G aphene, de ined as a single laye (monolaye ) o ca bon a oms, shows unique cha ac-
e is ics ha make i a e y e sa ile ma e ial. Among hem, i s mechanical, elec ical,
and op ical p ope ies a e conside ed e y a ac i e o ene gy-gene a ing de ices, which
makes g aphene a e y p omising ma e ial o nea - u u e ene gy echnology [
5
,
6
]. Since
i s disco e y in 2004 by And éGeim and Kos ya No oselo a he Uni e si y o Manch-
es e , se e al ab ica ion echniques ha e been de eloped and well-es ablished: mechanical
ex olia ion o highly o ganized g aphi e shee s [
7
], supe sonic sp ay p epa a ion [
8
], lase -
assis ed p ocesses [
9
], o chemical apo deposi ion (CVD) [
10
]. The main limi a ions
and obs acles o in eg a ing g aphene in de ice echnologies emain he ollowing (i) he
achie emen o cos -e ec i e high-quali y c ys alline g aphene; (ii) compa ibili y o he
pa ame e s used du ing g aphene ans e ence; and (iii) scale-up o mass p oduc ion o
co e ing la ge a eas. These challenges emain open e en a labo a o y scale [
11
–
13
]. In his
sense, CVD is conside ed one o he mos p omising app oaches ha allows he syn hesis
o high-quali y g aphene ma e ial in a con ollable and ep oducible way [
14
]. Due o he
po en ial bene i o inco po a ing g aphene in o de ices o a ious applica ions, he e ha e
been signi ican e o s ocused on de eloping e icien and eliable ans e me hods. This
Ma e ials 2023,16, 1223. h ps://doi.o g/10.3390/ma16031223 h ps://www.mdpi.com/jou nal/ma e ials
Ma e ials 2023,16, 1223 2 o 9
ac i i y has al eady enabled he success ul inco po a ion o g aphene playing in di e se
oles as a anspa en elec ode [
15
,
16
], in e acial laye , o an elec on accep o [
17
,
18
] in
di e en pho o ol aic echnologies (i.e., o ganic, dye-sensi ized, and e en silicon). How-
e e , esea ch on echnologies con aining g aphene in he s uc u e is s ill a a labo a o y
scale, and mo e e o s a e needed o implemen i in o he chain p oduc ion.
Nowadays, he PV ma ke con inues o be domina ed by c ys alline-silicon ech-
nology, which equi es new non-con en ional solu ions o educe cos s. In his sense,
silicon-he e ojunc ion (SHJ) echnology is conside ed a eliable low- empe a u e and high-
e iciency solu ion. In his scena io, ma e ials used in o he eme ging echnologies (i.e.,
ansi ion- me al-oxides (TMOs), o alkaline sal s) a e being in ensi ely s udied as al e -
na i e cha ge-ca ie collec o s o adi ional doped amo phous silicon laye s. The main
ad an ages o using hese ma e ials include he ollowing: (i) simple deposi ion ech-
niques a low- empe a u e; (ii) no haza dous gas p ecu so s a e employed; and (iii) less
pa asi ic abso p ion compa ed wi h adi ional he e ojunc ions. In his espec , success ul
high-e iciency sola cells implemen ing such ma e ials ha e al eady been epo ed [
19
–
21
].
Rega ding he use o g aphene in SHJ echnology, hyb id concep s ha e al eady been
s udied, demons a ing he possibili y o enhance he pho o ol aic
pe o mance [22,23]
. Pa ic-
ula ly, g aphene laye s ha e been in oduced on o con en ional anspa en -conduc i e-oxide
(TCO) elec odes such as indium- in-oxide (ITO). This combined s uc u e p o ides a signi i-
can educ ion in he de ice se ies esis ance, which esul s in a highe ill ac o . Fu he mo e,
he TCO con inues o play i s ole as an an i e lec ion coa ing due o he e y high op ical
ansmi ance o he g aphene laye [
24
]. I echnical issues can be add essed, his solu ion can
de ini ely imp o e SHJ sola cells.
In his wo k, a g aphene monolaye is inco po a ed on he on elec ode o a non-
con en ional c ys alline silicon sola cell. Speci ically, a 50 nm- hick molybdenum oxide
(MoO
x
) hole-selec i e laye eplaced he p-doped amo phous silicon laye o con en ional
he e ojunc ion sola cells. The g aphene monolaye s o m pa o he anspa en elec ode
and a e ans e ed a he end o he ab ica ion ou e, jus be o e he las me aliza ion s ep
(Figu e 1). The compa ibili y o he condi ions used o g aphene ans e ence wi h he
in eg i y o he sola cell is ea ed as a key issue. Finally, mechanisms ha could explain
he imp o emen in de ice pe o mance due o g aphene inco po a ion a e p esen ed
and discussed.
Ma e ials 2023, 16, x FOR PEER REVIEW 2 o 10
his sense, CVD is conside ed one o he mos p omising app oaches ha allows he syn-
hesis o high-quali y g aphene ma e ial in a con ollable and ep oducible way [14]. Due
o he po en ial bene i o inco po a ing g aphene in o de ices o a ious applica ions,
he e ha e been signi ican e o s ocused on de eloping e icien and eliable ans e
me hods. This ac i i y has al eady enabled he success ul inco po a ion o g aphene play-
ing in di e se oles as a anspa en elec ode [15,16], in e acial laye , o an elec on ac-
cep o [17,18] in di e en pho o ol aic echnologies (i.e., o ganic, dye-sensi ized, and e en
silicon). Howe e , esea ch on echnologies con aining g aphene in he s uc u e is s ill a
a labo a o y scale, and mo e e o s a e needed o implemen i in o he chain p oduc ion.
Nowadays, he PV ma ke con inues o be domina ed by c ys alline-silicon echnol-
ogy, which equi es new non-con en ional solu ions o educe cos s. In his sense, silicon-
he e ojunc ion (SHJ) echnology is conside ed a eliable low- empe a u e and high-e i-
ciency solu ion. In his scena io, ma e ials used in o he eme ging echnologies (i.e., an-
si ion- me al-oxides (TMOs), o alkaline sal s) a e being in ensi ely s udied as al e na i e
cha ge-ca ie collec o s o adi ional doped amo phous silicon laye s. The main ad-
an ages o using hese ma e ials include he ollowing: (i) simple deposi ion echniques
a low- empe a u e; (ii) no haza dous gas p ecu so s a e employed; and (iii) less pa asi ic
abso p ion compa ed wi h adi ional he e ojunc ions. In his espec , success ul high-e -
iciency sola cells implemen ing such ma e ials ha e al eady been epo ed [19–21].
Rega ding he use o g aphene in SHJ echnology, hyb id concep s ha e al eady been
s udied, demons a ing he possibili y o enhance he pho o ol aic pe o mance [22,23].
Pa icula ly, g aphene laye s ha e been in oduced on o con en ional anspa en -con-
duc i e-oxide (TCO) elec odes such as indium- in-oxide (ITO). This combined s uc u e
p o ides a signi ican educ ion in he de ice se ies esis ance, which esul s in a highe
ill ac o . Fu he mo e, he TCO con inues o play i s ole as an an i e lec ion coa ing due
o he e y high op ical ansmi ance o he g aphene laye [24]. I echnical issues can be
add essed, his solu ion can de ini ely imp o e SHJ sola cells.
In his wo k, a g aphene monolaye is inco po a ed on he on elec ode o a non-
con en ional c ys alline silicon sola cell. Speci ically, a 50 nm- hick molybdenum oxide
(MoOx) hole-selec i e laye eplaced he p-doped amo phous silicon laye o con en ional
he e ojunc ion sola cells. The g aphene monolaye s o m pa o he anspa en elec ode
and a e ans e ed a he end o he ab ica ion ou e, jus be o e he las me aliza ion s ep
(Figu e 1). The compa ibili y o he condi ions used o g aphene ans e ence wi h he
in eg i y o he sola cell is ea ed as a key issue. Finally, mechanisms ha could explain
he imp o emen in de ice pe o mance due o g aphene inco po a ion a e p esen ed and
discussed.
Figu e 1.
Schema ic ep esen a ion o he ab ica ed silicon he e ojunc ion sola cell using molyb-
denum oxide as he hole-selec i e con ac and implemen ing a g aphene monolaye on he on
anspa en elec ode.
Ma e ials 2023,16, 1223 3 o 9
2. Ma e ials and Me hods
The g aphene monolaye s we e ob ained by chemical- apo -deposi ion (CVD) by
he Spanish company G aphenea S.L. [
25
]. The CVD ab ica ion echnique was p e e ed
because i can po en ially be scaled up main aining high-pu i y and ela i ely good quali y
ma e ial. The ab ica ion was ca ied ou on coppe oil, using CH
4
as a p ecu so , hen
p epa ed o ans e wi h a polyme hyl me hac yla e (PMMA) coa ing, and inally ans-
e ed o he desi ed subs a e. In he ans e p ocess, en i onmen s such as O
2
plasma,
UV-O
3
ac i a ion, and high- empe a u e annealing p ocesses we e a oided. The main
eason is ha hey can nega i ely a ec he de ice, as non-con en ional he e os uc u es
a e o en less s able [
26
]. Hence, he empe a u e used in he ans e p ocess did no exceed
120 ◦C [27]
. Raman spec a we e ob ained using a Jobin-Y on LabRam HR 800 sys em wi h
an A exci a ion lase sou ce emi ing a 514 nm. The Raman spec a we e used o con i m
ha g aphene monolaye s we e posi i ely ans e ed and o alida e hei quali y.
The sola cells we e ab ica ed on n- ype (2
Ω·
cm) 280
µ
m- hick la c-Si wa e s wi h a
(100) o ien a ion. The wa e s we e i s dipped in dilu ed HF (1%) o e ch he na i e silicon
oxide. Then, in insic and n-doped amo phous silicon laye s (i/n s ack) we e deposi ed
on he ea side by plasma-enhanced CVD o ob ain a good e e ence elec on-selec i e
con ac [
28
]. Following, a 50 nm- hick hole-selec i e MoO
x
laye was he mally e apo a ed
on he on side. The MoO
x
ilm was coa ed by an ITO anspa en elec ode ha was
much hinne han no mal (<20 nm). This s ack s ill wo ked p e y well as an an i- e lec ion
coa ing, since he e ac i e index o MoO
x
is simila o ha o ITO a he wa eleng h o
in e es [
29
]. The ITO coa ing p o ec s he MoO
x
laye and also se es as a anspa en
elec ode. In e es ingly, i can be kep much hinne because he inal shee esis ance o he
de ice will be u he educed by he use o g aphene. Consequen ly, much less indium
is needed in his s uc u e compa ed wi h s anda d on anspa en elec odes [
30
]. The
ea side was inished by e apo a ing a 1
µ
m- hick aluminium con ac ha was p o ec ed
by a pho o esis be o e con inuing he ab ica ion ou e. Fu he mo e, he nex s ep was
al eady he ans e ence o a g aphene monolaye on he on side ollowing a p ocedu e
de eloped in a p e ious wo k [
27
]. Nex , sola cells o 1 cm
2
and 4 cm
2
we e de ined by
pho oli hog aphy. Finally, he de ices we e comple ed by e apo a ing a 2
µ
m- hick Ag
g id con ac ing 4.5% o he de ice a ea. A se o sola cells was ab ica ed, skipping he
g aphene ans e s ep o se e as a e e ence.
The cu en densi y s. ol age (J–V) elec ical cha ac e is ics o comple e de ices we e
measu ed in a ou -p obe con igu a ion using a 2601B Sou ce Me e (Kei hley Ins umen s,
Solon, OH, USA). The J-V cu es unde s anda d es condi ions (100 mW/cm
2
,
AM1.5 g
sola spec um, 25
◦
C) we e measu ed using an ORIEL 94021A (Newpo Co po a ion,
I ine, CA, USA) sola simula o . The ex e nal-quan um e iciency cu es (EQE) o he sola
cells we e measu ed using a comme cial ins umen , QEX10 (PV Measu emen s, Boulde ,
CO, USA). Quasi-s eady-s a e open-ci cui (QSSV
oc
) measu emen s we e acqui ed wi h a
sys em made in-house [
31
]. This echnique p o ides pseudo-J-V cu es, elimina ing he
e ec o pa asi ic se ies esis ance. This in o ma ion will be aluable o assess any e ec
a e g aphene ans e ence o he han educing he shee esis ance. Addi ionally, he
conduc ance o he anspa en elec odes was e alua ed di ec ly using a con ac less, non-
des uc i e Onyx sys em om he das-Nano Company [
32
]. This pa en ed measu emen is
based on e lec ion-mode e ahe z ime-domain spec oscopy (THz-TDS) in a equency
ange om 0.1 THz o 5 THz [
33
]. This sys em p o ides a ull-a ea map wi h in o ma ion
abou he elec ical p ope ies, he homogenei y, and he quali y o he 2D ma e ials and
hin ilms [34].
3. Resul s and Discussion
A e g aphene was ans e ed on o he on elec ode, Raman spec oscopy was
used as a eliable me hod o ensu e bo h he p esence and quali y o he ilm (Figu e 2).
The Raman spec um shows he cha ac e is ic peaks expec ed om high-quali y g aphene
monolaye s [
35
], mainly he G, G * and G’ (also named 2D) bands appea ing a ~1590 cm
−1
,
Ma e ials 2023,16, 1223 4 o 9
~2450 cm
−1
and ~2690 cm
−1
. Small signals in he D and D’ bands imply he exis ence o
sp
3
-C de ec s. Ne e heless, he con ibu ion o bo h peaks is e y small, and de ec s a e
possibly localized o due o bounda y e ec s. Fu he mo e, he p esence o sp
3
hyb idized
ca bon could also be explained by la e adso p ion due o ai exposu e o consequence o
he we ans e p ocess.
Ma e ials 2023, 16, x FOR PEER REVIEW 4 o 10
3. Resul s and Discussion
A e g aphene was ans e ed on o he on elec ode, Raman spec oscopy was
used as a eliable me hod o ensu e bo h he p esence and quali y o he ilm (Figu e 2).
The Raman spec um shows he cha ac e is ic peaks expec ed om high-quali y g aphene
monolaye s [35], mainly he G, G * and G’ (also named 2D) bands appea ing a ~1590 cm−1,
~2450 cm−1 and ~2690 cm−1. Small signals in he D and D’ bands imply he exis ence o sp3-
C de ec s. Ne e heless, he con ibu ion o bo h peaks is e y small, and de ec s a e pos-
sibly localized o due o bounda y e ec s. Fu he mo e, he p esence o sp3 hyb idized
ca bon could also be explained by la e adso p ion due o ai exposu e o consequence o
he we ans e p ocess.
Figu e 2. Raman ull spec a om 150 cm−1 o 3000 cm−1 and an inse co esponding o he a ea o
in e es wi h espec o g aphene.
Figu e 3 shows conduc ance maps measu ed by e ahe z e lec ion spec oscopy o
a e e ence sola cell (a) compa ed wi h he same de ice coa ed wi h a g aphene mono-
laye (b). The Onyx sys em om das-Nano is able o esol e he busba and inge s o he
on me allic g id in he e ence de ice [32]. A backg ound shee conduc ance o 3 mS is
measu ed on he ITO egion, which inc eases up o 4.50 mS on he busba (Figu e 3a).
Adding a g aphene monolaye clouded he con as o e he de ice a ea, making i ha de
o dis inguish he inge s om he ITO laye unde nea h (Figu e 3b). The shee conduc -
ance o he backg ound is in his case 4.5 mS, which inc eases o 6.5 mS in he busba
egion. The esul s om his expe imen a e summa ized in Table 1, whe e no malized
alues o he se ies esis ance a e also shown. Summa izing, he g aphene laye posi i ely
con ibu es o educing he se ies esis ance o he on elec ode. The e ec is simila o
ha obse ed in con en ional silicon he e ojunc ion sola cells, whe e he de ice pe o -
mance imp o ed by adding a g aphene monolaye s [23].
Figu e 2.
Raman ull spec a om 150 cm
−1
o 3000 cm
−1
and an inse co esponding o he a ea o
in e es wi h espec o g aphene.
Figu e 3shows conduc ance maps measu ed by e ahe z e lec ion spec oscopy
o a e e ence sola cell (a) compa ed wi h he same de ice coa ed wi h a g aphene
monolaye (b)
. The Onyx sys em om das-Nano is able o esol e he busba and inge s o
he on me allic g id in he e ence de ice [
32
]. A backg ound shee conduc ance o
3 mS
is measu ed on he ITO egion, which inc eases up o 4.50 mS on he busba (Figu e 3a).
Adding a g aphene monolaye clouded he con as o e he de ice a ea, making i ha de
o dis inguish he inge s om he ITO laye unde nea h (Figu e 3b). The shee conduc ance
o he backg ound is in his case 4.5 mS, which inc eases o 6.5 mS in he busba egion. The
esul s om his expe imen a e summa ized in Table 1, whe e no malized alues o he
se ies esis ance a e also shown. Summa izing, he g aphene laye posi i ely con ibu es o
educing he se ies esis ance o he on elec ode. The e ec is simila o ha obse ed in
con en ional silicon he e ojunc ion sola cells, whe e he de ice pe o mance imp o ed by
adding a g aphene monolaye s [23].
Fu he in o ma ion can be ex ac ed om he cu en - ol age cha ac e is ics (Figu e 4)
and he Ex e nal Quan um E iciency (EQE) cu es o he sola cells (Figu e 5). Table 2
compa es he main pho o ol aic pa ame e s o he e e ence and g aphene-coa ed sola
cells, e idencing ha his modi ica ion clea ly imp o ed he inal pe o mance. Howe e ,
he e is also a sligh educ ion in he sho -ci cui cu en densi y (J
sc
) o he g aphene-
coa ed cell (31 mA/cm
2
) compa ed wi h he e e ence de ice (32 mA/cm
2
). This di e ence
could a p io i be ela ed o op ical abso bance by he g aphene laye . Howe e , EQE
measu emen s indica e ha he main di e ence in pho ocu en collec ion is obse ed a
wa eleng hs be ween 800 nm and 1100 nm. The beha io in his nea -in a ed egion o he
EQE cu e is gene ally associa ed wi h ea su ace ecombina ion and he quali y o he
back e lec o . A possible explana ion is ha he ea con ac su e ed some deg ada ion
du ing he we ans e o he g aphene shee . The ea side was p o ec ed by a hick
pho o esis o a oid any damage om he eac i e ma e ial used in his p ocess. Thus,
Ma e ials 2023,16, 1223 5 o 9
deg ada ion could be ela ed o he he mal s ep (150
◦
C) ha is also in ol ed in he
g aphene ans e ence. In o de o con i m his, we submi ed e e ence de ices o a simila
he mal s ep, and we obse ed a qui e simila deg ada ion (Supplemen a y Figu e S4).
This could be unde s ood bo h as some deg ada ion o he ea su ace passi a ion as well
as a dec ease in back e lec ance. In u u e wo k, his deg ada ion can be minimized by
in e cala ing a anspa en -conduc i e-oxide laye be ween he hin amo phous silicon
ilms on he ea side and he me allic con ac .
Ma e ials 2023, 16, x FOR PEER REVIEW 5 o 10
(a)
(b)
Figu e 3. Maps o shee conduc ance measu ed by e ahe z e lec ion spec oscopy o (a) e e ence
de ice (a) and wi h he inco po a ion o a g aphene monolaye (b).
Table 1. Summa y o he shee conduc ance measu emen s by e ahe z e lec ion spec oscopy.
Sample Name
Shee Conduc ance
(mS)
Shee Resis ance
(kΩ)
Se ies Resis ance
(Ω·cm2)
e e ence
3.0–3.5
0.3–0.35
3.35
g aphene-coa ed
4.5–5.5
0.2–0.22
2.95
Fu he in o ma ion can be ex ac ed om he cu en - ol age cha ac e is ics (Figu e
4) and he Ex e nal Quan um E iciency (EQE) cu es o he sola cells (Figu e 5). Table 2
compa es he main pho o ol aic pa ame e s o he e e ence and g aphene-coa ed sola
cells, e idencing ha his modi ica ion clea ly imp o ed he inal pe o mance. Howe e ,
he e is also a sligh educ ion in he sho -ci cui cu en densi y (Jsc) o he g aphene-
coa ed cell (31 mA/cm2) compa ed wi h he e e ence de ice (32 mA/cm2). This di e ence
could a p io i be ela ed o op ical abso bance by he g aphene laye . Howe e , EQE meas-
u emen s indica e ha he main di e ence in pho ocu en collec ion is obse ed a wa e-
leng hs be ween 800 nm and 1100 nm. The beha io in his nea -in a ed egion o he
EQE cu e is gene ally associa ed wi h ea su ace ecombina ion and he quali y o he
back e lec o . A possible explana ion is ha he ea con ac su e ed some deg ada ion
du ing he we ans e o he g aphene shee . The ea side was p o ec ed by a hick pho-
o esis o a oid any damage om he eac i e ma e ial used in his p ocess. Thus, deg a-
da ion could be ela ed o he he mal s ep (150 °C) ha is also in ol ed in he g aphene
ans e ence. In o de o con i m his, we submi ed e e ence de ices o a simila he mal
s ep, and we obse ed a qui e simila deg ada ion (Supplemen a y Figu e S4). This could
be unde s ood bo h as some deg ada ion o he ea su ace passi a ion as well as a de-
c ease in back e lec ance. In u u e wo k, his deg ada ion can be minimized by in e ca-
la ing a anspa en -conduc i e-oxide laye be ween he hin amo phous silicon ilms on
he ea side and he me allic con ac .
Figu e 3.
Maps o shee conduc ance measu ed by e ahe z e lec ion spec oscopy o (
a
) e e ence
de ice (a) and wi h he inco po a ion o a g aphene monolaye (b).
Table 1. Summa y o he shee conduc ance measu emen s by e ahe z e lec ion spec oscopy.
Sample Name Shee Conduc ance
(mS)
Shee Resis ance
(kΩ)
Se ies Resis ance
(Ω·cm2)
e e ence 3.0–3.5 0.3–0.35 3.35
g aphene-coa ed 4.5–5.5 0.2–0.22 2.95
Ma e ials 2023, 16, x FOR PEER REVIEW 6 o 10
Figu e 4. JV cu es o he e e ence and g aphene-coa ed sola cells measu ed unde AM1.5 g illu-
mina ion (100 mW/cm2) a oom empe a u e (25 °C).
Figu e 5. Ex e nal Quan um E iciency ex ac ed om he ab ica ed sola cells.
Table 2. Main pho o ol aic pa ame e s o he e e ence and g aphene-coa ed sola cells. An absolu e
1.6% inc ease in e iciency was achie ed by his modi ica ion o he on elec ode.
De ice
Voc (mV)
Jsc (mA/cm2)
FF (%)
PCE (%)
e e ence
498
32
65.7
10.4
g aphene-coa ed
580
31
67.2
12
The slope o he JV cu e a ound sho -ci cui is also a bi highe o he g aphene-
coa ed sola cell, which poin s o a lowe shun esis ance. This e ec could be explained
by some addi ional cu en leakage be ween he pho oac i e a ea o he de ice and he
subs a e. Some esidues be ween he de ices emained on he g aphene-coa ed subs a e
a e he pho oli hog aphic and e ching s eps we e comple ed o isola e he de ices. These
Figu e 4.
JV cu es o he e e ence and g aphene-coa ed sola cells measu ed unde AM1.5 g
illumina ion (100 mW/cm2) a oom empe a u e (25 ◦C).

Ma e ials 2023,16, 1223 6 o 9
Ma e ials 2023, 16, x FOR PEER REVIEW 6 o 10
Figu e 4. JV cu es o he e e ence and g aphene-coa ed sola cells measu ed unde AM1.5 g illu-
mina ion (100 mW/cm2) a oom empe a u e (25 °C).
Figu e 5. Ex e nal Quan um E iciency ex ac ed om he ab ica ed sola cells.
Table 2. Main pho o ol aic pa ame e s o he e e ence and g aphene-coa ed sola cells. An absolu e
1.6% inc ease in e iciency was achie ed by his modi ica ion o he on elec ode.
De ice
Voc (mV)
Jsc (mA/cm2)
FF (%)
PCE (%)
e e ence
498
32
65.7
10.4
g aphene-coa ed
580
31
67.2
12
The slope o he JV cu e a ound sho -ci cui is also a bi highe o he g aphene-
coa ed sola cell, which poin s o a lowe shun esis ance. This e ec could be explained
by some addi ional cu en leakage be ween he pho oac i e a ea o he de ice and he
subs a e. Some esidues be ween he de ices emained on he g aphene-coa ed subs a e
a e he pho oli hog aphic and e ching s eps we e comple ed o isola e he de ices. These
Figu e 5. Ex e nal Quan um E iciency ex ac ed om he ab ica ed sola cells.
Table 2.
Main pho o ol aic pa ame e s o he e e ence and g aphene-coa ed sola cells. An absolu e
1.6% inc ease in e iciency was achie ed by his modi ica ion o he on elec ode.
De ice Voc (mV) Jsc (mA/cm2)FF (%) PCE (%)
e e ence 498 32 65.7 10.4
g aphene-coa ed 580 31 67.2 12
The slope o he JV cu e a ound sho -ci cui is also a bi highe o he g aphene-
coa ed sola cell, which poin s o a lowe shun esis ance. This e ec could be explained
by some addi ional cu en leakage be ween he pho oac i e a ea o he de ice and he sub-
s a e. Some esidues be ween he de ices emained on he g aphene-coa ed subs a e a e
he pho oli hog aphic and e ching s eps we e comple ed o isola e he de ices. These can
be obse ed by compa ing he de ice pic u es shown in Figu es S1 and S2 (Supplemen a y
In o ma ion). Howe e , his seems no o be he cause o he lowe shun esis ance because
cu en leakage did no educe a e sc ibing he de ices. Some pinholes may be seen on he
on elec ode o he g aphene-coa ed sola cell (inse o Figu e S1). This indica es ha we
ac ually ha e in e nal connec ion pa hs wi hin he ac i e a ea o he de ice. Then, his s ep
o he ab ica ion ou e would s ill need some u he op imiza ion o minimize his p ob-
lem. The lowe shun esis ance can be limi ing he ill ac o (FF) o he g aphene-coa ed
de ice, which would pa ially hide he e ec o he educed on con ac esis ance. To
analyze his u he , QSSVoc (Suns-Voc) measu emen s we e comple ed o ob ain pseudo-JV
cu es o he e e ence and g aphene-coa ed sola cells (Supplemen a y Figu e S3). The
pseudo ill ac o (pFF) o he e e ence sola cell eached a a he good alue o 84%, while
he pFF o he g aphene de ice only eached 80%. These alues o he pseudo ill ac o
con i m ha he e e ence sola cell was indeed be e isola ed. Ne e heless, he eal FF
inally measu ed in he sola cells was be e o he g aphene-coa ed de ice (Table 2).
De ini ely, his can be a ibu ed o a educ ion in he esis ance o he on con ac due o
he e ec o he g aphene laye . The o al se ies esis ance o he de ice can be calcula ed
using his equa ion [36]:
Rs=
Voc
Jsc
·1−FF
pFF(1)
Acco ding o his, he R
s
alue o he e e ence sola cell is 3.35
Ω·cm2
and i dec eases
o 2.99
Ω·cm2
o he de ice inco po a ing g aphene. These alues, calcula ed om di ec
elec ical cha ac e iza ion (compa ing JV and QSSVoc measu emen s), a e he o al se ies
Ma e ials 2023,16, 1223 7 o 9
esis ance o each de ice. I is obse ed a e y good coincidence wi h he alues deduced
om e ahe z measu emen s, which a e 3.35
Ω·cm2
o he e e ence and 2.95
Ω·cm2
o
he g aphene-coa ed on con ac (Table 1). The con ac less e ahe z e lec ion spec oscopy
only senses he con ibu ion o he on elec ode o he se ies esis ance. Then, his esul
clea ly indica es ha he ea elec ode is con ibu ing much less o he measu ed se ies
esis ance. In he e e ence de ice, wi hin expe imen al accu acy, he se ies esis ance is
de e mined by he on con ac . In e es ingly, he con ibu ion o he ea con ac could be
es ima ed a a ound 40
mΩ·cm2
o he sample wi h g aphene. I could be a gued ha he
ea con ac esis ance inc eased du ing he we -p ocess used o g aphene ans e . This
e ec could also be ela ed o he EQE deg ada ion de ec ed in he nea -in a ed egion o
he spec um.
Finally, he open-ci cui ol age (V
oc
) o he g aphene-coa ed sola cell is signi ican ly
highe compa ed wi h he e e ence de ice. This migh no be expec ed, as he se ies
esis ance should ha e no di ec in luence on he measu ed open-ci cui ol age. The e is
no cu en lowing h ough he sola cell in open-ci cui condi ions. On he o he hand, he
V
oc
alue may indeed be ela ed o he quali y o he selec i e con ac . Gene ally, hicke
ITO elec odes coa he MoO
x
laye o p o ide high la e al conduc ance and good con ac
wi h he me allic g id. Howe e , in his wo k, he ITO hickness was educed below
20 nm
o in es iga e g aphene as an al e na i e indium- ee anspa en elec ode. In silicon
he e ojunc ion echnology, i is known ha he elec ode wo k unc ion can signi ican ly
impac he band alignmen a he co esponding con ac [
37
–
39
]. Some hing simila may be
expec ed o he non-con en ional sola cells s udied he e. Namely, band alignmen a he
MoO
x
/silicon in e ace could be modi ied by he addi ion o a g aphene laye , gi en ha he
ITO elec ode is e y hin. Ac ually, i has been epo ed ha g aphene imp o ed he quali y
o MoO
x
hole-selec i e con ac s o pe o ski e sola cells [
40
]. Ne e heless, he e is ano he
e ec ha could also explain he inc ease in V
oc
o he g aphene-coa ed sola cell. Exci a ion
o su ace-plasmon-pola i ons (SPP) on g aphene monolaye s has been demons a ed [
41
],
and a ious applica ions in op oelec onic de ices ha e been epo ed [
42
]. Pa icula ly,
g aphene su ace plasmons can signi ican ly inc ease abso p ion in he subs a e sola
cells [
43
,
44
]. This plasmonic e ec would con ibu e o he highe open-ci cui ol age
and could also assis cha ge-ca ie ex ac ion ia op ical exci a ion [
45
]. This seems o be
he case he e, wi h g aphene posi i ely inc easing he V
oc
by 80 mV compa ed wi h he
e e ence sola cell. This, oge he wi h he highe FF alue, ansla es in o an o e all 1.6%
inc ease in he powe con e sion e iciency (PCE) due o he g aphene inco po a ion.
4. Conclusions
The goal o his esea ch was o e alua e he e ec o inco po a ing a g aphene laye
on he on anspa en elec ode o n- ype silicon sola cells wi h non-con en ional MoO
x
hole-selec i e con ac . Fo ha pu pose, g aphene monolaye s ab ica ed by CVD we e
ans e ed unde condi ions compa ible wi h he in eg i y o hese de ices. The Raman
cha ac e iza ion showed a high quali y o he ans e ed g aphene monolaye s. A no el
con ac less elec ical cha ac e iza ion by e ahe z e lec ion spec oscopy e idenced a
50% inc ease in shee conduc ance ( om 3.0–3.5 mS o 4.5–5.5 mS) by he inco po a ion
o g aphene. Consequen ly, he JV cu e o he g aphene-coa ed sola cell shows be e
V
oc
and FF alues o a e y ema kable absolu e inc ease in PCE o 1.6%. Hence, his
in es iga ion has iden i ied possible applica ions o g aphene-based elec odes in non-
con en ional sola cells. Fu he mo e, his use could also be in e es ing o applica ions
demanding lexible o anspa en elec onics be ing, on a educed en i onmen al impac .
Supplemen a y Ma e ials:
The ollowing a e a ailable online a h ps://www.mdpi.com/a icle/
10.3390/ma16031223/s1, Figu e S1: Pho og aphy o he g aphene-based sola de ices ab ica ed in
his wo k; Figu e S2: Pho og aphy o he e e ence sola de ices ab ica ed in his wo k; Figu e S3:
Pseudo-JV cu es calcula ed om QSSVoc cha ac e iza ion o he e e ence and g aphene-coa ed
sola cells s udied in his wo k; Figu e S4: EQE cu es o e e ence sola cells be o e and a e
being submi ed o a he mal s ep (150
◦
C) simila o he one in ol ed in he p ocess o g aphene
Ma e ials 2023,16, 1223 8 o 9
ans e ence. The e is a dec ease in EQE he in a ed egion simila o he deg ada ion obse ed in
he g aphene-coa ed sola cell.
Au ho Con ibu ions:
Concep ualiza ion, C.V., J.J.G. and S.F.; me hodology, C.V., P.O. and E.R.;
alida ion, E.R.; o mal analysis, E.R., P.O., C.V., I.A. and E.T.; in es iga ion, C.V., J.J.G. and S.F.;
esou ces, E.R. and S.F.; da a cu a ion, E.R., I.A. and E.T.; w i ing—o iginal d a p epa a ion, E.R.
and S.F.; w i ing— e iew and edi ing, C.V., P.O., I.A. and J.J.G.; supe ision, C.V. and J.J.G.; p ojec
adminis a ion, C.V., J.J.G. and S.F.; unding acquisi ion, C.V., J.J.G. and S.F. All au ho s ha e ead
and ag eed o he published e sion o he manusc ip .
Funding:
This esea ch was unded by MCIN/AEI/10.13039/501100011033, g an numbe s PID2019-
109215RB-C41 and PID2019-109215RB-C42.
Ins i u ional Re iew Boa d S a emen : No applicable.
In o med Consen S a emen : No applicable.
Da a A ailabili y S a emen :
The da a p esen ed in his s udy a e a ailable on eques om he co e-
sponding au ho s. The da a a e no publicly a ailable due o indus ial pa icipa ion in
his esea ch
.
Con lic s o In e es : The au ho s decla e no con lic o in e es .
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