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Voroshilov, Pavel; Ovchinnikov, Victor; Papadimitratos, Alexios; Zakhidov, Anvar; Simovski,ConstantinLight Trapping Enhancement by Silver Nanoantennas in Organic Solar Cells
Published in:ACS Photonics
DOI:10.1021/acsphotonics.7b01459
Published: 10/04/2018
Document VersionPeer reviewed version
Please cite the original version:Voroshilov, P., Ovchinnikov, V., Papadimitratos, A., Zakhidov, A., & Simovski, C. (2018). Light TrappingEnhancement by Silver Nanoantennas in Organic Solar Cells. ACS Photonics, 5(5), 1767-1772.https://doi.org/10.1021/acsphotonics.7b01459
Light Trapping Enhan ement by Silver
Nanoantennas in Organi Solar Cells
Pavel M. Voroshilov,
∗,†,‡Vi tor Ov hinnikov,
‡Alexios Papadimitratos,
¶Anvar A.
Zakhidov,
†,§,¶and Constantin R. Simovski
†,‡
†ITMO University, Kronverkskiy pr. 49, 197101 St. Petersburg, Russia
‡Aalto University, P.O. Box 15500, 00076 Aalto, Finland
¶The University of Texas at Dallas, Ri hardson 75080, USA
§National University of S ien e and Te hnology MISiS, Mos ow 119049, Russia
E-mail: p.voroshilov�metalab.ifmo.ru
Abstra t
In this paper, we study experimentally the en-
han ement of parameters of a typi al organi
thin �lm solar ell (OSC) a hieved by an orig-
inally proposed light trapping stru ture (LTS).
Our LTS is an array of silver nanoantennas
supporting olle tive modes, what provide sub-
wavelength light enhan ement in the substrate.
Low losses in the metal result from the advanta-
geous opti al �eld distribution that is a hieved
in the broad range of wavelengths. We study
the enhan ement of all the main photovoltai
(PV) parameters observed in OSC with our
LTS, su h as power onversion e� ien y (PCE),
whi h is e�e tively in reased by 18%, �ll fa -
tor (FF), that an also be e�e ted positively
and short- ir uit urrent Isc improvement. Al-though the Ag nanoantenna o upies up to 40%
of the photoa tive OSC area, it provides the
nearly twi e enhan ed opti al absorption, re-
sulting in overall higher Isc and FF. Improve-
ments of the opti al and ele tri al design of
OSC ar hite tures are dis ussed for further en-
han ement of performan e.
Keywords
Metasurfa e, Nanoantennas, Small mole ules,
Organi solar ells, Light trapping, Domino-
modes
The photovoltai industry regularly requests
new s ienti� and te hni al solutions for the
su essful global ommer ialization of renew-
able energy.
1
Considerable e�orts have been
made to enhan e the e� ien y of thin �lm so-
lar ells whi h an �nd numerous appli ations
(see e.g. in
1�4
). There is a variety of strate-
gies to in rease the e� ien y of thin �lm solar
ells, whi h in lude material development,
5,6
in-
terfa e engineering,
7
improvements in fabri a-
tion and pro essing,
8
devi e ar hite ture mod-
ernization
9
and optimization of harge trans-
fer.
10
Photon management by light-trapping nanos-
tru tures represents one of the promising ways
to improve the performan e of su h solar ells,
whose e� ien y is limited due to insu� ient ab-
sorption of light in the a tive (PV) layer. Ab-
sorption is limited by PV layer, whi h thi kness
lays in subwavelength range at frequen ies of
operation. Organi solar ells of ex iton type
(we do not refer dye-sensitized and perovskite
solar ells as OSCs) have a fundamental limi-
tation on the thi kness of the PV layer � be-
low 100-150 nm � due to small di�usion length
of Frenkel's ex itons.
11
Opti al absorption of
organi PV materials is also low. Therefore,
the larger part of the in ident sunlight passes
through the PV layer. An anti-re�e ting oat-
ing does not resolve the issue of low absorption.
1
Even if the thi kness of the PV layer e�e tively
doubled by a polished rear ele trode it is insuf-
� ient. Moreover, many OSCs imply a trans-
parent ( ondu ting oxide) or absorbing (e.g. Al
foil) rear ele trode that does not ompletely re-
�e t light ba k to the a tive layer. In this situa-
tion, one has to apply LTSs, usually performed
as plasmoni nanostru tures.
12
In
13,14
we suggested a on ept of light trap-
ping employing a metasurfa e of metal nanoan-
tennas, whose resonan es over a very broad
band of wavelengths. These resonan es orre-
spond to olle tive os illations that keep even
for a perfe tly ondu ting metal. The �eld is
lo ally enhan ed in the gaps between the metal
elements and the losses in the metal are almost
absent. Su h advantageous distribution origins
from leaky modes
15
supported by the metasur-
fa e suggested in.
13
We all them leaky domino-
modes by analogy with guided domino-modes
whi h initially were revealed in far-infrared
16
and visible
17
ranges for arrays of metal nano-
bars. Leaky domino-modes were studied theo-
reti ally in
15
and experimentally in.
18
The aim of the present paper is to experimen-
tally demonstrate the enhan ement of a typi al
OSC by LTS with leaky domino-modes. Sin e
the pioneering work by Ching W. Tang
19
on
two-layer organi OSC based on a mole ular
donor-a eptor stru ture with a PCE of about
1%, the signi� ant progress has been a hieved
in the improvement of OSC systems, leading
to a PCE of over 11%
20
in single-jun tion and
over 13%
21
in tandem small-mole ule based de-
vi es with a polished rear ele trode. However,
the main drawba k inherent to typi al OSCs by
C.W. Tang keeps � the absorption of light is in-
su� ient even in these advan ed OSCs. There-
fore, high opti al losses still determine the om-
paratively low e� ien y of organi solar photo-
voltai s ompared to sili on photovoltai s.
Usually, in papers on LTSs, the in rease of op-
ti al absorption in the a tive layer is laimed as
a su� ient eviden e of the e� ien y enhan e-
ment be ause it results in the orresponding in-
rease of the short- ir uit urrent Isc.22
How-
ever, the total PCE of a solar ell is a�e ted
not only by Isc but also by other parameters
su h as FF and open ir uit voltage Voc:
PCE =Voc · Isc · FF
Pin
, (1)
where Pin is the power of the in ident light (re-
ferred to the surfa e of our thin �lm solar ell
and integrated over the spe trum). In a or-
dan e with Eq. 1 an LTS in order to grant the
enhan ement must not harm both FF and Voc,
otherwise, the overall e� ien y may de rease.
In this paper, we study the e�e t of proposed
LTS on all mentioned parameters. Our target
is the enhan ement of PCE.
An expli it example of an OSC that an be
drasti ally improved by our LTS is a small-
mole ule OSC
14
operating in the near-infrared
and transparent for the visible light. In that
work, we have hosen a bulk-heterojun tion
small-mole ule OSC with phthalo yanine as a
donor and fullerene as an a eptor. We pre-
ferred this design as the typi al one that is suf-
� iently simple in fabri ation.
In our theoreti al work,
14
we have shown
that proposed LTS in reased the absorption of
infrared solar light in the tin phthalo yanine
(SnP , donor) � fullerene (C60, a eptor) lay-
ers of the OSC more than triple. That OSC had
two indium tin oxide (ITO) ele trodes (both an-
ode and athode), that made it transparent for
visible light, though by a pri e of very low e�-
ien y.
3
Our LTS with its triple enhan ement of
the useful absorption promises a nearly double
in rease of the short- ir uit urrent and, per-
haps, a twi e higher PCE (though this ques-
tion was not studied). Meanwhile, the trans-
mittan e of visible light is kept su� ient for the
e�e tive transparen y of a window into whi h
su h the enhan ed OSC ould be in orporated.
For estimation of the e�e tive in rease of the
short- ir uit urrent, both proper opti al and
ele tri al simulations should be performed, as
in has been done e.g. in.
23
It is not easy to pre-
di t how both opti al absorption and photo ur-
rent an be optimally improved simultaneously.
So in reased opti al absorption (and number
of photogenerated arriers) an be a hieved by
in reasing photoa tive layer thi kness, however
this may lead to overall de rease of the expe ted
2
PV layersPV layers
ITO cathode Al cathode
0 1E field, arb. unit-
λ = 840 nmλ = 480 nm
0 1E-field, arb. unit
glass
ITO anode
CuPc
SnPc
C60
BCP
Al cathode
Ag
glass
ITO anode
CuPc
SnPc
C60
BCP
ITO cathode
Ag
a b c
x
z
yx
z
y
x
z z
y x y
z z
Transparent device
Opaque device
P1P 1P 2
P2
Figure 1: Normalized ele tri �eld intensity distributions in the horizontal planes P1 and P2 (top
panel) and in the slightly tilted ross-se tions (bottom panel) for an OSC ontaining ITO anode
and athode, λ =840 nm (a) and for a similar OSC with ITO anode and Al athode, λ =480 nm
( ). A tive layer boundaries are shown as white dotted lines (bottom panel). Ag nanoantennas are
lo ated on top of ITO anode, embedded into the PV layer. In panel (b) we show the OSC s hemati s
for these two ases. Horizontal planes P1 and P2 are shown as bla k dashed lines. Orange arrows
show the wave in iden e.
PCE enhan ement due to poor harge olle tion
from thi ker layer (due to e.g. short di�usion
length of arriers, and higher series resistan e
of thi ker layer) and thus overall de reased �ll
fa tor. Therefore in future, we will qualitatively
validate the numeri al model of nanoantennas
in orporated in a solar ell with the experimen-
tal results for further development of the e�-
ient light-trapping on ept for high PCE solar
ells.
Unfortunately, in the present work, we did not
manage to fabri ate a transparent ITO ath-
ode. Therefore, the experimental realization of
the exa tly same stru ture as in
14
turned out to
be impossible. We have repla ed ITO for the
athode by Al and dropping the fun tionality
of the transparen y on entrated on the e�e t
of light trapping and its impli ations. Sin e in
this ase the transparen y of the PV layer is
not required, we have deviated from the design
of
14
adding a nanolayer of opper phthalo ya-
nine (CuP ) as an additional donor and hole
transport layer (HTL). It allowed us to reate
a double heterojun tion and to in rease the ini-
tial e� ien y of the prototype. Our new design
is typi al and orresponds to works.
9,11
In or-
der to demonstrate the advantageous operation
of our LTS, we do not modify the design pa-
rameters of the prototype. In this meaning, our
study fairly demonstrates the real enhan ement
granted by the LTS to a typi al OSC.
The HTL of CuP has maximal absorption
in the visible region. Therefore, onversion of
solar radiation o urs in the extended spe tral
range, whi h in ludes both the near infrared
(750-900 nm) part where the absorbing mate-
rial is SnP and the visible part (550-750 nm),
where CuP absorbs. One more di�eren e of
the present design from that of
14
is the slight
shift of the metasurfa e with respe t to the ITO
anode. Instead of embedding the nanoantennas
into ITO, we lo ate them on the ITO surfa e
that allowed us to strongly simplify the fabri-
ation.
3
Glass
ITO
LTS
25 mm
2 mm
hITO
Glass
Dx
Dy
c
ab
e
z
xy
300 400 500 600 700 800 900 1000
Wavelength, nm
0
20
40
60
80
100
Tran
smit
tan
ce o
r re
flec
tan
ce, %
T (substrate)
T (LTS)
R (substrate)
R (LTS)
a b c
Figure 2: (a) Photo of the sample with our LTS. (b) S heme of the metasurfa e geometry. The
geometri al parameters are as follows: Dx = Dy = 1000 nm, a = 300 nm, b = 150 nm, c = 300
nm, e = 250 nm, h = 35 or 50 nm (two di�erent samples). ( ) Transmission and re�e tion spe tra
were measured for 50-nm sample (LTS) and for bare ITO area (substrate).
400 nm
Figure 3: S anning ele tron mi ros ope image
of our LTS.
Although the design modi� ations listed
above may seem insigni� ant, they an dras-
ti ally a�e t the performan e of the LTS. It
be omes ne essary to study the in�uen e of the
Al athode on the lo al ele tri �eld distribu-
tion inside the PV ell. We have performed
full-wave (CST) ele tromagneti simulations of
two PV devi es: one with a transparent ITO
athode and another with an opaque Al ath-
ode. Complex refra tive indexes were taken
from.
24�26
Ele tri �eld intensity distributions
for these two ases are presented in Fig. 1.
Indeed, the opaque metal �lm fully hanges
the hot spots arrangement in the bulk of the
stru ture at all wavelengths. Obviously, in
presen e of a transparent athode, an e�e tive
Fabry-Perot avity is formed in the stru ture
that works together with domino-modes ex-
ited in our nanoantennas and leads to the
more advantageous lo ation of hot spots in the
near-infrared range (700-850 nm). Meanwhile,
in this on�guration, these spots do not appear
at other wavelengths. The situation hanges
when Al repla es ITO as a athode material:
hot spots now also appear in the visible range
exa tly in favorable lo ations. However, in the
near-infrared band hot spots shift away from
the PV layer. Anyway, this does not result
in noti eable parasiti losses be ause these hot
spots do not interse t with the metal. In gen-
eral, simulations on�rmed our expe tations �
the enhan ement of the useful absorption keeps
after the repla ement of ITO by an aluminum.
More details on this modeling an be found in
the Supporting information.
We have prepared two samples with 50 nm
and 35 nm thi k Ag nanoantennas on top of
the ITO-glass substrate. These samples on-
tain 1.5 · 106 and 3 · 106 unit ells of our
LTS, respe tively. Fabri ation was done using
ele tron-beam lithography (EBL) and ele tron-
beam evaporation (see Methods and Support-
ing information). SEM analysis showed a very
high quality of our nanoantenna arrays, as an
be guessed from Figs. 2 and 3. We measured
transmittan e T and re�e tan e R for a 50-nm
thi k LTS (in iden e from the glass side) and
ompared them with those obtained for the bare
4
BCP [10 nm]C70 [55 nm]SnPc:C70 [30 nm]CuPc [15 nm]
Glass substrate
ITO anode
Al cathode50 nm
PV layers
Solar light
ITO(an
od
e)
Cu
Pc
SnPc
C70
BC
P
Al(cath
od
e)
-4.8- 34.
-5.2
-3.5-4.0
-5.1- 24.
-6.1
-3.0
-6.4
Cu
Pc
SnPc
C70
Active layers
Ener
gy
rela
tive
to
vac
uu
m le
vel,
eV
e-
h+
Device Isc [mA/cm2] Voc [V] FF [%] PCE [%]
W/o NAs 4.6 41 0.366 41.6 0.702
W/ NAs 4.955 0.276 42.9 0.587
Cu
rren
t (m
A/c
m)²
Without NAsWith NAs
Voltage (V)
a b c
Silver
0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4
0
-1
-2
-3
-4
-5
Figure 4: (a) Cross-se tion s hemati s of the OSC with C70 as an a eptor layer and 50-nm thi k
silver nanoantennas. (b) Energy-level band diagram for the urrent on�guration. ( ) IV urves
measured under illumination for two ases: with (red line) and without (blue line) nanoantennas.
substrate (glass and ITO). The result is shown
in Fig. 2 ( ). The averaged re�e tan e growth
is about 10%. Noti e, that this negative e�e t
keeps for the OSC, where it is over ompensated
by the light-trapping e�e t.
The target of these simulations is twofold.
Firstly, we may estimate the in rease of the
parasiti absorption A introdu ed by our LTS.
Sin e the losses in Ag elements are negligibly
small, parasiti absorption an be attributed
to opti al losses in ITO. From energy balan e,
we have A = 1 − R − T . For integral absorp-
tion in the operational range λ =400-900 nm
we have nearly 5% in absen e of our LTS and
nearly 8% in its presen e. This is an a ept-
able harm. Se ondly, we may see that the re-
�e tan e grows mu h less that it an be esti-
mated using the geometri al opti s. Really, Ag
nanoantennas over almost 40% of the area, but
the re�e tan e is λ-dependent and mu h lower
than 0.4. This is so be ause the eigenmodes ofour LTS are ex ited by the normally in ident
plane wave, what makes geometri al opti s in-
adequate.
Further, inspe ting the opaque design de-
pi ted in Fig. 1 we de ided to additionally fab-
ri ate a variant of the OSC with C70 fullerene
derivative instead of C60. If the impa t of
our LTS will be more signi� ant for the stru -
ture with C70, it will on�rm the shift of light-
trapping regime (predi ted by our simulations)
from the IR range to the visible range for Al
athode. This should be so be ause C70 has
higher absorption than C60 below λ =700 nm,
whereas C60 has higher absorption at longer
waves.
Our �rst sample with 50 nm-thi k nanoan-
tennas has the following stru ture above the
glass substrate: ITO anode (150 nm) / CuP
(15 nm) / SnP :C70 (30 nm) / C70 (55 nm)
/ BCP (10 nm) / Al (70 nm). The or-
responding s hemati is shown in Fig. 4
(a). This on�guration with o-evaporated ph-
thalo yanine and fullerene leads to the for-
mation of two donor-a eptor heterojun tions:
CuP /C70 and SnP /C70. We illuminated our
sample with an AM 1.5G solar simulator from
the side of glass through a 1 mm aperture.
We measured the IV dependen ies of our sam-
ple illuminating either the 1 mm areas om-
prising our nanoantennas or 1 mm areas free
of them. In this way, we have olle ted su�-
ient statisti s for a reliable plotting two IV-
urves � one for an OSC enhan ed by our LTS
and another for a bare OSC (with only an-
tire�e tive oating). These urves are shown
in Fig. 4 and demonstrate a noti eable in-
rease of Isc aused by our LTS. Photo urrent
generated in the nanoantenna area rea hes the
value 5 mA/cm2, whereas the stru ture with-
out nanoantennas gives 4.6 mA/cm2. This 8%
in rease of the photo urrent orresponds to a
higher in rease of the PV absorban e due to a
nonlinear urrent response of su h OSC.
27
Due to 40% of the area overed by Ag nanoan-
tennas, the photoa tive area that photogener-
ates arriers and provides Isc is only 60% of the
similar area in a referen e OSC. Therefore the
5
a tual enhan ement of Isc, provided by the LTSis not 5 mA/cm2
, but 5/0.6 = 8.3 mA/cm2.
The Ag nanoantenna array o upied area is
not reating arriers but is reating on entra-
tion and enhan ement of the photoni ele tri-
al �eld whi h results in the nearly twi e en-
han ement of the opti al absorption of light,
resulting in 8.3 mA / 4.6 mA = 1.8 times en-
han ed on entration of arriers. We assume
here that the edges of sharp Ag stru tures do
not provide better harges olle tion (sin e the
nanoantennas an olle t only ele trons by low
work fun tion), but on the ontrary, are reat-
ing leakage of arriers and quen hing of ex i-
tons. Despite all those negative e�e ts of Ag
nanoantennas: shading the useful area, wrong
work fun tion, re ombination of ele trons and
holes on Ag nanostru tures, we have still ob-
served the overall sizable enhan ement of Isc =5 mA (8% more than in the referen e ell) at a
nearly twi e small photoa tive area.
Fill fa tor al ulated from IV hara teristi s
is improved by 2.6% for our LTS (see inset of
Fig. 4 ( )). Unfortunately, the open- ir uit
voltage Voc of the area with nanoantennas de-
reased by 0.09 V, i.e. 25% ompared to the
bare OSC. This harm overshadows the posi-
tive gain in photo urrent and negatively a�e ts
the overall PCE. The possible reason for the
fall in Voc ould be quite substantial dimen-
sions of metal nanoantennas ompared with ul-
trathin a tive layers. Really, Voc in OSCs is
dire tly proportional to the di�eren e between
the Fermi levels, and the ITO ele trode having
the bandgap in the ultraviolet has an impa t on
Voc. Silver elements having the ohmi onta t
with ITO may modify the e�e tive bandgap of
the ele trode. As a result, the di�eren e of the
Fermi levels squeezes and Voc be omes smaller.
Parameters of the OSC with nanoantennas
and without nanoantennas are shown in the in-
set of Fig. 4 ( ). These data are averaged over
several sample areas, as explained above.
Our next example is totally identi al to the
previous one ex lude repla ement of C70 by
C60 and thinner Ag elements (35 nm instead
of 50 nm). The stru ture is shown in Fig. 5
(a). It onsists of the following layers on the
glass plate: ITO (150 nm) / CuP (15 nm)
/ SnP :C60 (30 nm) / C60 (25 nm) / BCP
(10 nm) / Al (70 nm). Sin e C60 is less ab-
sorptive material in the visible range and its
thi kness de reased, the impa t of our LTS in
the photo urrent also de reased ompared to
the previous ase � 4.6% instead of 8%, as
an be seen from Fig. 5 ( ). However, thin-
ner metal elements improved FF by 19% versus
2.6%, whereas Voc almost kept un hanged. To-
tal averaged value of the PCE enhan ement is
18% that is a quite ompetitive result and is
higher than the gain provided by known plas-
moni ounterparts of our LTS for similar OSCs
(see e.g.
28
).
It should also be noted, that our samples were
not en apsulated. Therefore, in order to pre-
vent the degradation of organi materials and
oxidation of Ag, our measurements were per-
formed at room temperature and in an inert
atmosphere. In order to prevent Ag nanoanten-
nas tarnishing during transportation from the
lean room to the laboratory, where the solar
ell was prepared, we used a polymer prote -
tive oating. This oating was removed in the
laboratory with a etone. However, all samples
were treated by gentle UV-ozone for additional
leaning before the organi layer deposition.
29
So, a ertain oxidation of the thin silver ele-
ments might take pla e during this treatment.
This is another reason, why the in rease of the
PV absorption granted by our LTS in reality
turned out to be lower than that predi ted by
numeri al simulations. Hopefully, the experi-
mental enhan ement o�ered by our nanoanten-
nas will in rease on e more, if the fabri ation
fa ilities will allow lithography pro ess and so-
lar ell preparation in the same lean room.
To summarize, in this paper, we have the-
oreti ally and experimentally proved that our
original LTS from silver nanoantennas an suf-
� iently in rease the e� ien y of OSCs. First
of all, our numeri al simulations have shown ad-
vantageous �eld distributions that predi ted en-
han ement of the useful absorption. Se ondly,
we have fabri ated several OSCs with di�erent
materials and measured gain in PV parameters
aused by nanostru tures. Sin e total PCE of
a solar ell is a�e ted not only by Isc, but alsoby �ll fa tor FF and open ir uit voltage (Voc),
6
Solar light
ITO(an
od
e)
Cu
Pc
SnPc
C06
BC
P
Al(cath
od
e)
-4.8- 34.
-5.2
-3.5-4.0
-5.1 - 54.
- 26.
-3.0
-6.4
Cu
Pc
SnPc
C06
Active layers
Ener
gy
rela
tive
to
vac
uu
m le
vel,
eV
e-
h+
Cu
rren
t (m
A/c
m)²
Voltage (V)
a b c
Silver
BCP [10 nm]C60 [25 nm]SnPc:C60 [30 nm]CuPc [15 nm]
Glass substrate
ITO anode
Al cathode35 nm
PV layers
Without NAsWith NAs
Device Isc [mA/cm2] Voc [V] FF [%] PCE [%]
W/o NAs 2 869. 0.396 36 5. 0.415
W/ NAs 2 997. 0. 763 4 .3 4 0.489
0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45
0
-0.5
-1
-1.5
-2
-2.5
-3
-3.5
Figure 5: (a) Cross-se tion s hemati s of the OSC with C60 as an a eptor layer and 35-nm thi k
silver nanoantennas. (b) Energy-level band diagram for the urrent on�guration. ( ) IV urves
measured under illumination for two ases: with (red line) and without (blue line) nanoantennas.
we have onsidered several design solutions and
dis ussed the in�uen e of the devi e ar hite -
ture on LTS performan e. Experimental data
for our OSCs based on C60 fullerene and en-
han ed by 35 nm thi k nanoantennas show the
enhan ement 18% that qualitatively �ts the re-
sults of simulations. Other experimental data
are also in line with theoreti al expe tations.
We believe that the present results are helpful
for further development of LTSs for thin �lm
solar ells. In our future designs, we will hoose
nanoantennas with proper work fun tion on-
tributing into olle tion of holes at ITO side
and with better lo ation allowing us to avoid
the 40% shading of the PV layer.
Methods
E-beam exposurePolymer spin-coating Al sputteringSubstrate cleaning
Al etchingDevelopmentAg evaporationLift-off
Glass
ITO
MMA
PMMA
Al
Ag
Figure 6: Pro ess �ow diagram illustrating fab-
ri ation steps of LTS.
Several samples with silver nanoantennas
were fabri ated on top of 150 nm-thi k ITO
oated glass substrates pur hased from Lumi-
nes en e Te hnology Corp by EBL and lift-o�
pro ess in the Mi ronova Nanofabri ation Cen-
tre of Aalto University. Pro ess �ow diagram is
shown in Fig. 6. Substrates were preliminary
leaned with a etone and isopropanol (IPA) in
an ultrasoni bath at 40
◦C and rinsed in DI-
water. A opolymer resist (MMA) with on-
entration of solids 11% in Ethyl La tate and
polymer resist (950PMMA A) with on entra-
tions of solids 2.25% in Anisole (both from Mi-
ro hem) were sequentially deposited by spin-
oating. Samples were dried on a hot plate
for 3 minutes at a temperature of 160 degrees
after ea h spin- oating pro edure resulting in
formation of 400 and 100 nm layers of MMA
and PMMA, respe tively. A thin (20 nm) sa -
ri� ial layer of aluminum was sputtered. The
purpose of this layer is to solve the problem
with e-beam fo using on a diele tri layer dur-
ing EBL. E-beam exposure was done using Vis-
te EBPG 5000Plus ES tool at 100 keV with
4 nA urrent beam and 1000 µC/cm2e-beam
dose. Then, samples were pro essed in et hing
mixture PWS 80-16-4 (65) from Honeywell on-
taining phosphori a id and nitri a id to om-
pletely remove the sa ri� ial aluminum layer.
Next, PMMA and MMA were developed in so-
lution of 1:3 MIBK:IPA during 30 se and then
rinsed in IPA during 30 se to terminate de-
velopment and prevent s umming. Silver was
deposited by using ele tron beam evaporation
at a rate of 2 Å/s and operating pressure of
10−7mbar. Finally, non-exposed areas of poly-
7
mer layers were washed out together with sil-
ver on their surfa e by a etone in the ultra-
soni bath. Samples were hara terized by us-
ing AFM (DI3100), SEM (Zeiss Supra 40), and
multifun tional tool for re�e tion (R), trans-
mission (T) and quantum e� ien y (QE) mea-
surements (QEX10).
Small mole ule OSCs were prepared in the
multi-sour e high-va uum system (Angstrom
Engineering In ., Canada) by sequential ther-
mal evaporation onto ITO-glass substrates with
our nanostru tures of the following �lms: a
15 nm thi k opper(II) phthalo yanine (from
H.W. Sanders Corp) HTL, a 30 nm thi k o-
evaporated in equal quantity SnP :fullerene
mixed D:A layer, an additional ontinuous
fullerene layer (25 or 55 nm), BCP (10 nm)
and Al (70 nm) layers. We used C60 and
C70 fullerenes (both >98% from Nano-C). Base
pressure of the hamber was kept around 10−8
mbar during evaporation. Evaporation rate
for separate layers was 0.5 Å/s, while o-
evaporation rate was equal to 0.25 Å/s for ea h
parti ular material. OSCs were hara terized
with an AM 1.5G solar simulator alibrated to
one sun (100 mW/cm2) through aperture holes
of 1 mm in diameter, whi h provided sele tive
illumination of nanostru tured regions only.
A knowledgement This work was par-
tially supported by the Ministry of Edu ation
and S ien e of the Russian Federation (Grant
14.Y26.31.0010) and Nokia Foundation. The
authors thank Mikhail Omelyanovi h, Kristina
Ian henkova, and Matthias Mes hke for useful
dis ussions. We also a knowledge the provision
of fa ilities and te hni al support by Aalto Uni-
versity at Mi ronova Nanofabri ation Centre.
Supporting information
The Supporting Information (SI) is available
free of harge on the ACS Publi ation website.
The SI in ludes details of 3D ele tromagneti
modeling and additional simulated ele tri �eld
distributions at di�erent wavelengths. More-
over, the SI ontains omplementary SEM and
AFM analysis of samples.
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