Inverted polymer fullerene solar cells exceeding 10% efficiency with poly(2-ethyl-2-oxazoline) nanodots on electron-collecting buffer layers

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Polymer solar cells have been spotlighted due to their potential for low-cost manufacturing but their efficiency is still less than required for commercial application as lightweight/flexible modules. Forming a dipole layer at the electron-collecting interface has been suggested as one of the more attractive approaches for efficiency enhancement. However, only a few dipole layer material types have been reported so far, including only one non-ionic (charge neutral) polymer. Here we show that a further neutral polymer, namely poly(2-ethyl-2-oxazoline) (PEOz) can be successfully used as a dipole layer. Inclusion of a PEOz layer, in particular with a nanodot morphology, increases the effective work function at the electron-collecting interface within inverted solar cells and thermal annealing of PEOz layer leads to a state-of-the-art 10.74% efficiency for single-stack bulk heterojunction blend structures comprising poly[4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-b']dithiophene-alt-3-fluorothieno[3,4-b]thiophene-2-carboxylate] as donor and [6,6]-phenyl-C71-butyric acid methyl ester as acceptor.

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### ARTICLE

Received 27 Apr 2015 | Accepted 19 Oct 2015 | Published 14 Dec 2015 DOI: 10.1038/ncomms9929 OPEN
## Inverted polymer fullerene solar cells exceeding 10% efficiency with poly(2-ethyl-2-oxazoline) nanodots on electron-collecting buffer layers

Sungho Nam 1,2, Jooyeok Seo 1, Sungho Woo 3, Wook Hyun Kim 3, Hwajeong Kim 1,4, Donal D.C. Bradley 2,5 &
Youngkyoo Kim 1

Polymer solar cells have been spotlighted due to their potential for low-cost manufacturing but

their efficiency is still less than required for commercial application as lightweight/flexible

modules. Forming a dipole layer at the electron-collecting interface has been suggested as one

of the more attractive approaches for efficiency enhancement. However, only a few dipole layer

material types have been reported so far, including only one non-ionic (charge neutral) polymer.

Here we show that a further neutral polymer, namely poly(2-ethyl-2-oxazoline) (PEOz) can be

successfully used as a dipole layer. Inclusion of a PEOz layer, in particular with a nanodot

morphology, increases the effective work function at the electron-collecting interface within

inverted solar cells and thermal annealing of PEOz layer leads to a state-of-the-art 10.74%

efficiency for single-stack bulk heterojunction blend structures comprising poly[4,8-bis(5-(2ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-b 0 ]dithiophene-alt-3-fluorothieno[3,4-b]thiophene
2-carboxylate] as donor and [6,6]-phenyl-C71-butyric acid methyl ester as acceptor.

1 Organic Nanoelectronics Laboratory, School of Applied Chemical Engineering, Kyungpook National University, Sankyuk-dong University Road 80,
Daegu 702-701, Republic of Korea. 2 Blackett Laboratory, Department of Physics and Center for Plastic Electronics, Imperial College London, London SW7
2AZ, UK. 3 Green Energy Research Division, Daegu Gyeongbuk Institute of Science and Technology, Daegu 711-873, Republic of Korea. 4 Research Institute of
Advanced Energy Technology, Kyungpook National University, Daegu 702-701, Republic of Korea. 5 Departments of Engineering Science and Physics, Division
of Mathematical, Physical and Life Sciences, University of Oxford, Oxford OX1 3PD, UK. Correspondence and requests for materials should be addressed to
Y.K. (email: [ykimm@knu.ac.kr)](mailto:ykimm@knu.ac.kr) or H.K. (email: [khj217@knu.ac.kr)](mailto:khj217@knu.ac.kr) or D.D.C.B. (email: [donal.bradley@mpls.ox.ac.uk).](mailto:donal.bradley@mpls.ox.ac.uk)

olymer solar cells have been extensively studied because
of their potential to enable high-throughput,
low-temperature roll-to-roll fabrication of flexible solar
# P
modules 1–7 . Since the landmark early works on the concept
of polymer:fullerene bulk heterojunction (BHJ) solar cells 8–14, the
power conversion efficiency (PCE) has been remarkably
improved up to B10% for polymer:fullerene solar cells by
introducing new light-absorbing conjugated polymers and
fullerene derivatives 15–26 . Most of the high-efficiency
polymer:fullerene solar cells adopt an inverted-type device
structure, because a stable charge-collecting buffer layer
can be made using metal oxides such as zinc oxide (ZnO) and
titanium oxide instead of poly(3,4-ethylenedioxythiophene):
poly(styrenesulfonate) (PEDOT:PSS). PEDOT:PSS is strongly
acidic, leading to deterioration of the active layers and/or
bottom electrodes in the case of normal-type device
structure 27,28 . Moves have also been made to replace
PEDOT:PSS with alternative hole-collecting buffer layers, such
as CuSCN 29,30 .

In the case of inverted polymer:fullerene solar cells, further
efficiency enhancement can be achieved by applying an organic
interfacial layer (interlayer) between the electron-collecting
buffer layer (ECBL)—typically a metal oxide—and the BHJ
photogeneration layer. The dipolar nature of the interlayer results
in a larger built-in potential and enhanced electron transfer from
the fullerene lowest unoccupied molecular orbital 31,32 . Another
role that such interlayers can perform is to improve adhesion
between the metal oxide and BHJ layers 33,34 . Polymer interlayers
that lead to high efficiencies can be divided into two categories.
First, charged polymers with counter-ions such as PFNBr,
PFN-OH, WPF-oxy-F, PFNSO, PF6NO, P3ImHT, PTMAHT
and so on 35–43 . Concerns exist, however, over cost, tendency to
aggregate and the exciton-quenching and/or charge-carrier
trapping effects of the ionic species. Second, neutral
polymers, for which the key example reported to date is
poly(ethylene imine) (PEI) (also known as poly(ethylene
imine)-ethoxylated) 44–47 . PEI is expected to be relatively
free from exciton-quenching and/or charge-trapping effects.
Interestingly, PEI is typically synthesized from poly(2-ethyl-2oxazoline) (PEOz, Fig. 1a) by hydrolysis and/or other
reactions 48–50, offering the intriguing prospect of using PEOz in
its place. Considering the presence of nitrogen atoms in the welldefined backbone of PEOz, it is expected that PEOz chains can
act as an electron donor in a similar manner to PEI when they
contact the surface of electron-accepting materials. However,
PEOz has never been tried for solar cell applications, even though
it has been used for biomedical applications such as drug delivery
and biosensors because of its excellent biocompatibility and
stealth behaviour 51–55 .

In this study, we report the use of PEOz as a cathode interlayer
in inverted BHJ solar cells with ZnO as ECBL. The polymer:
fullerene BHJ layer that we used comprises poly[4,8-bis(5(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-b 0 ]dithiophene-alt3-fluorothieno[3,4-b]thiophene-2-carboxylate] (PTB7-Th) and

[6,6]-phenyl-C71-butyric acid methyl ester (PC71BM). PEOz
layers were spin coated from methanol and the influence of
solution concentration (thickness) on film morphology and
work function was investigated by atomic force microscopy
(AFM), Auger electron microscopy (AEM), scanning electron
microscopy (SEM), ultraviolet photoelectron spectroscopy (UPS)
and electric force microscopy (EFM). A distinctive nanodot (ND)
morphology was obtained for B4 mg ml � 1 PEOz solution
concentrations, resulting in maximum PCE ¼ 10.74%. Korea
Institute of Energy Research-certified performance data for a
slightly less optimized (PCE ¼ 9.6%) PEOz interlayer solar cell are
presented.

20 PEOz –4.5
(mg ml –1 )

_n_
ZnO 4 mg ml –1

ITO 8 mg ml –1

PEOz in MeOH

0 mg ml –1

1 mg ml –1

PEOz

2 mg ml –1

4 mg ml –1

ITO
Glass

–4.6

–4.7

–4.8

–4.9

–5.0

0
1
2
4
8

19 18 17 0

18 17 0 4 8

Binding energy (eV)

d Vacuum level e

100

PEOz (mg ml –1 )

ITO
ITO/ZnO
ITO/ZnO/PEOz(4)
ITO/ZnO/PEOz(8)

(2)

(1) –4.3 - +

- +

ITO - +

- +

–7.7    - +

–4.3

300 450 600 750 900

Wavelength (nm)

ITO

–4.9

–7.7

+
+
+
+
+
+
+

Figure 1 | PEOz-modified ZnO layer. (a) Chemical structure of PEOz and
schematic of PEOz coating on the ZnO ECBL. (b) Energy cutoff region UPS
spectra as a function of PEOz concentration. (c) Work function energies for
PEOz-coated ZnO, measured using the EFM system. (d) Simplified energy
band diagram for the work function shift in the ZnO layer resulting from
PEOz addition: (1) 0.15 eV for 4 mg ml � 1 and (2) 0.28 eV for 8 mg ml � 1

PEOz methanol solutions. (e) Optical transmittance spectra for ITO,
ITO/ZnO and ITO/ZnO/PEOz, where PEOz(4) and PEOz(8) denote films
deposited from 4 and 8 mg ml � 1 solutions.

Results and Discussion
PEOz-induced changes in ZnO work function. As a first step,
we examined the effect of PEOz interlayers on the ECBL work
function. PEOz films were spin coated (1,500 r.p.m.) from different methanol solution concentrations (Fig. 1a) and after drying
the PEOz-coated ZnO samples were subjected to UPS and EFM
measurements (c.f. details in Methods section). As observed in
Fig. 1b, the cutoff part of the UPS spectrum shifted towards
higher binding energy as the PEOz solution concentration
increased, indicating a reduction in the magnitude of the ZnO
work function with increasing coverage (film thickness) 31,56,57 .
This trend was also confirmed by the EFM measurements (see
Fig. 1c), which are performed in an inert (nitrogen) environment
at atmospheric pressure so that actual work function change can
be measured under similar conditions to those appertaining
following coating of the active layer. The EFM data reveal that the
ECBL work function shifts byB0.15 eV (from - 4.83 to

- 4.68 eV) and 0.28 eV (from - 4.83 to - 4.55 eV),
respectively, following the spin coating of 4 and 8 mg ml � 1
PEOz solutions. The reduction in work function associated with
the PEOz dipole layer is illustrated in the simplified energy band

diagram of Fig. 1d, in which the valence band (7.7 eV) and
conduction band (4.3 eV) energy of the ZnO layer was obtained
from the UPS spectrum in Fig. 1b and its optical band gap (see
Methods section) 56–59 . We note that the discrepancy in work
function between the UPS and EFM data can be attributed
primarily to the different measurement environments (in vacuo
for the UPS measurement and in ambient for the EFM
measurement) as previously well documented 58–60 . Figure 1e
shows 300–900 nm optical transmittance spectra for ITO, ITO/
ZnO and ITO/ZnO/PEOz samples, where 4 and 8 mg ml � 1 PEOz
data are labelled PEOz(4) and PEOz(8), respectively. The average
transmittance between 400 and 900 nm exceeds 80% for all
samples, with the optical quality evident from the Fabry–Perot
interference fringes. This result confirms that the addition of a
PEOz interlayer on top of the ZnO ECBL does not harm its
optical transmittance, ensuring that there should be a negligible
effect on the light intensity reaching the PTB7-Th:PC71BM
BHJ layer.

PEOz-induced changes in solar cell performance. Based on the
observed changes in ZnO work function (Fig. 1), inverted PTB7Th:PC71BM BHJ solar cells were fabricated as shown in Fig. 2a.
The PEOz solution concentration was again varied from 0 to
8 mg ml � 1, to investigate the interlayer coverage (thickness)
effect on solar cell performance. Taking into account the work
function shift presented in Fig. 1d, the overall flat energy band
diagram for our PTB7-Th:PC71BM solar cells can be represented

as per Fig. 2b. It is useful to note that relatively small changes in
the conduction band energy level (work function) of the n-type
ZnO layer will significantly affect the device built-in potential
(electric field). A close look at the optical absorption spectra in
Fig. 2c shows that the PTB7-Th component has a vibronically
structured absorption band spanning the range from B550
to 800 nm, coincident with the high transmittance range of the
ITO/ZnO/PEOz stack.

As shown in Fig. 3a, the current density–voltage (J–V) curves
under simulated solar light (air mass 1.5 G, 100 mW cm � 2 )
illumination noticeably improved on addition of a PEOz layer on
top of the ZnO ECBL. However, the improvement was not
monotonic in PEOz concentration: the best performance was
measured for the PEOz(4) device (ITO/ZnO/PEOz(4)/PTB7Th:PC71BM/MoO3/Ag) and higher concentrations gave worse
performance. Considering the work function changes for the ZnO
layer in the presence of PEOz (see Fig. 1), PEOz(8) devices might
have been expected to perform better than PEOz(4) devices. This
result may reflect that the thicker PEOz layer could relatively
restrict electron transport to the ZnO ECBL from the PTB7Th:PC71BM BHJ layer, owing to a slightly higher electrical
resistance. This would be consistent with the observation that the
change in short circuit current density (JSC) was more
pronounced than that for open circuit voltage (VOC). An

ITO/ZnO/PEOz(8)

ITO/ZnO

ITO/ZnO/PEOz(4)

PEOz
(mg ml –1 )

C4H9 C2H5
C4H9

_n_

PTB7-Th

_n_

_n_ Ag

PEOz

MoO3

C2H5

0.0

PTB7-Th:PC71BM

ZnO

ITO

Glass

b –2.3 c

–3.6

PEOz

300

PTB7-Th:PC71BM

PTB7-Th

C4H9

0.2 0.4 0.6 0.8

Voltage (V)

ITO/ZnO/PEOz(4)

ITO/ZnO

PC71BM

–3

–6

–9

–12

–15

–18

100

C2H5

PC71BM

450 600 750

Wavelength (nm)

600

ITO

–4.9

- –4.2
–4.3

- +

- + –6.1

- +

–4.7
–5.2 –5.3

–6.1

750 900

–7.7

+
+
+
+
+
+
+
+

Figure 2 | Device/energy band structure and absorption coefficient data.
(a) Solar cell device stack and chemical structures for PTB7-Th, PC71BM
and PEOz. (b) Flat energy band diagram for the PTB7-Th:PC71BM BHJ solar
cells with ZnO/PEOz ECBL/interlayer. The conduction band and valence
band energy values of the ZnO layer were measured in this work (see the
Methods section). The blue arrow denotes the shift in ZnO work function
caused by the PEOz. (c) Absorption coefficient (a) spectra for pristine films
of PTB7-Th and PC71BM, and for a BHJ PTB7-Th:PC71BM blend film.

400 500 600 700 800 900
Wavelength (nm)

Figure 3 | Solar cell performances. (a) Light (air mass 1.5G,
100 mWcm � 2 ) J–V curves for PTB7-Th:PC71BM solar cells fabricated using
PEOz solution concentrations in the range 1–8 mg ml � 1 . Data are also
shown for a control device with no PEOz interlayer. (b) EQE spectra for
PTB7-Th:PC71BM solar cells with a bare ZnO ECBL and with a PEOz(4)coated ZnO ECBL.

enhanced JSC for the PEOz(4) device was also confirmed by
external quantum efficiency (EQE) spectra measurements (see
Fig. 3b). The EQE value is higher for the PEOz(4) device than the
control device (ITO/ZnO/PTB7-Th:PC71BM/MoO3/Ag) across
the entire wavelength range, suggesting that the effect is not
predominantly interference related. This result supports a
conclusion that the improved solar cell performance is mainly
attributed to enhanced charge (electron) collection in the
presence of a PEOz interlayer due to the increased built-in
potential represented in Fig. 1.

The detailed variation in solar cell parameter values as a
function of PEOz concentration (extracted from the data shown
in Fig. 3a) is shown in Fig. 4. In all cases, there is a nonmonotonic variation, with 4 mg ml � 1 providing the extreme

18.6 0.81

18.3

18.0

17.7

17.4

17.1

16.8

55
160

140

120

100

0.80

0.79

0.78
10.0

9.5

9.0

8.5

8.0

0 2 4 6 8 0 2 4 6 8
PEOz (mg ml –1 ) PEOz (mg ml –1 )

Figure 4 | PEOz coating dependence of solar cell parameters. JSC, VOC, FF,
PCE, RS and RSH for the PTB7-Th:PC71BM solar cells as a function of the
PEOz concentration (N.B. these data were taken from the light J–V curves in
Fig. 3 with error bars for more than 15 devices).

value for each parameter. The short circuit current (Jsc) peak is
mirrored by a fill factor (FF) maximum and a series resistance
(RS) minimum that are consistent with otherwise electron
transport-limited behaviour. The solar cell shunt resistance
(RSH) peak at 4 mg ml � 1 PEOz concentration also signals an
advantageous reduction in leakage current for this device
structure.

In contrast, the VOC value only very slightly increases up to
4 mg ml � 1 and is then almost constant for higher concentrations;
the maximum is only marginally optimal in that regard. The
rising VOC value is consistent with the work-function shifts seen
in UPS measurements but the magnitude of the effect is not. VOC
increases at best by 0.005 eV compared with a work-function shift
of 0.15 eV for PEOz(4) relative to PEOz(0) devices (see Fig. 1). In
addition, the work-function shift continues, reaching 0.28 eV for
PEOz(8), whereas VOC is saturated beyond PEOz(4). This
strongly suggests that other energy levels limit the open circuit
voltage, because the offset energy between the PTB7-Th highest
occupied molecular orbital and PC71BM lowest unoccupied
molecular orbital isB1 eV 61,62, and the thicker PEOz(8) layer
inevitably has a relatively higher electrical resistance.

Finally, the solar cell PCE significantly improved from 8.81 to
9.53% by depositing a PEOz(4) film (see Table 1). This 9.53%
PEOz(4) device was subsequently tested and certified at 9.57% by
the National Solar Cell Accreditation Center of the Korea
Institute for Energy Research (see Supplementary Fig. 1).
Additional experiments were undertaken to assess the ubiquity
of PEOz and specifically the optimal 4 mg ml � 1 concentration.
In particular, poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b 0 ]
dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl) carbonyl]thieno

[3,4-b]thiophenediyl]] (PTB7) was substituted for the PTB7-Th
donor used here and BHJ blend solar cells were fabricated, again
with PC71BM as the acceptor. As shown in Supplementary Fig. 2,
the best performance for the PTB7:PC71BM blend was also
obtained for PEOz(4) solar cells.

Surface morphology of the PEOz-modified ZnO ECBL. The
non-monotonic dependence of solar cell performance parameters
on PEOz solution concentration suggests that more needs to be
known about the microstructure and morphology of the films
deposited. To investigate how such additional factors may contribute to the improved device performance, alongside a lower
ZnO ECBL work function, the surface morphology of PEOzcoated ZnO was examined using AFM measurements. As shown
in Fig. 5a, a fine-grain morphology was measured for bare ZnO
(without the PEOz layer), with a root-mean-square roughness of
0.88 nm. A striking morphology change was found for the
ZnO/PEOz(4) samples, as shown in Fig. 5b, where randomly

Table 1 | Solar cell performances as a function of PEOz concentration in methanol.

PEOz (mg ml � 1 ) VOC (V) JSC (mA cm � 2 ) FF (%) PCE (%) RS (X) RSH (kX)
0 0.793 (0.004) 17.23 (0.14) 63.42 (0.64) 8.81 (0.11) 135.0 (11.5) 14.47 (1.29)
1 0.794 (0.005) 17.44 (0.23) 63.92 (0.83) 8.94 (0.23) 114.5 (6.1) 15.43 (2.20)
2 0.796 (0.006) 18.01 (0.22) 66.36 (1.11) 9.35 (0.20) 92.3 (6.8) 15.98 (2.25)
4 0.798 (0.005) 18.10 (0.23) 67.95 (0.82) 9.53 (0.12) 88.0 (4.5) 18.89 (1.78)
4* 0.806 17.21 68.97 9.57 85.3 19.2
6 0.797 (0.005) 17.97 (0.26) 66.27 (1.97) 9.29 (0.26) 107.2 (15.3) 16.68 (3.53)
8 0.797 (0.004) 17.48 (0.17) 65.56 (2.05) 9.11 (0.33) 122.8 (17.2) 15.74 (3.25)

FF, fill factor; JSC, short circuit current density; PCE, power conversion efficiency; PC71BM, [6,6]-phenyl-C71-butyric acid methyl ester; PEOz, poly(2-ethyl-2-oxazoline); PTB7-Th, poly[4,8-bis(5-(2ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-b 0 ]dithiophene-alt-3-fluorothieno[3,4-b]thiophene-2-carboxylate] (PTB7-Th); RS, series resistance; RSH, cell shunt resistance; VOC, open circuit voltage; ZnO,
zinc oxide.
‘4*’ denotes the certified results for the inverted PTB7-Th:PC71BM solar cells with the ZnO/PEOz(4) layer, which were measured in the National Solar Cell Accreditation Center of Korea Institute of
Energy Research.

a
Glass/ITO/ZnO

schematically illustrated in Fig. 6b but we note that it is difficult to
probe such a situation using standard measurements. As shown
in Supplementary Fig. 5 the surface nanomorphology of
PTB7-Th:PC71BM BHJ films is little altered (other than in terms
of its surface roughness) by being deposited on top of a bare
ZnO ECBL or one coated with a PEOz(4) or PEOz(8) film. In
addition, high-resolution transmission electron microscope
images of focused ion beam prepared PEOz(4) device crosssections only vaguely reveal the presence of the PEOz film (see
Supplementary Fig. 6). Further detailed studies will be required to
gain a clearer insight but our initial considerations suggest that
the role of PEOz ND structures may be different from the
conventional island-interface effect bestowed by metal (Ag)
clusters, conical island titanium oxide (TiO2), and silicon (Si)
nanodots, in which surface plasmon and effects have been
proposed to play a significant role 63–65 .

Further device optimization and stability (lifetime) studies. To
increase solar cell performance even more, the ZnO/PEOz(4)
layer was thermally annealed before deposition of the BHJ blend
film; this can lead to better adhesion between the ZnO ECBL and
the PEOz interlayer. As shown in Supplementary Fig. 7, 120 �C
was found to be the optimal annealing temperature and fabrication of a set of devices using this protocol yielded B10.7% PCE
(see Fig. 7a, where VOC ¼ 0.794 V, JSC ¼ 19.0 mA cm � 2 and
FF ¼ 71.2%, and hence PCE ¼ 10.74%). PCE ¼ 10.74% represents
state-of-the-art performance for a single-stack polymer:fullerene
solar cell, as summarized in Supplementary Table 1. Our 10.74%
solar cells were next subjected to a brief stability test under
continuous illumination with simulated solar light (air mass
1.5 G, 100 mW cm � 2 ). As shown in Fig. 7b, both the optimized
PEOz(4) device and the control device (bare ZnO ECBL) exhibited a rapid decrease in JSC with illumination time, whereas FF
was marginally more stable for the optimized device. The VOC
trend was different in that the optimized PEOz(4) device showed
a significantly slower reduction, probably reflecting the PEOz ND
layer’s role in retarding interfacial degradation. As a result, the
overall PCE stability of the optimized PEOz(4) device
(t1/2B98 min) was marginally better than for the control device
(t1/2B81 min). Nevertheless, in reality both devices exhibited
poor stability; this can be ascribed directly to device degradation,
as it was evident that continuous illumination drastically alters
the BHJ film colour (see inset photographs in Fig. 7b). In this
regard, the poor stability is likely to be associated with PTB7-Th,
mirroring the known instability of the closely related PTB7
polymer 66–68 .

Conclusion
In summary, coating a ZnO ECBL with PEOz leads to formation
of an interfacial dipole layer that induces a significant change in
work function; reductions of 0.15 and 0.28 eV are achieved for
PEOz(4) and (8) films, respectively. The corresponding performance of inverted ZnO-ECBL-containing PTB7-Th:PC71BM BHJ
solar cells is noticeably improved (certified PCE B9.57%) by
insertion of the PEOz interlayer but the improvement peaks for
films prepared from 4 mg ml � 1 solutions. Beyond this concentration there is a decline or (for Voc) saturation in cell parameters,
flagging the importance of factors other than the change in work
function. In addition, even for PEOz(4) films, the observed
increase in VOC is rather smaller than would have been expected
from the work function shift, probably owing to the practical
difference between film surface (no influence of electron transport
and contact resistance) and bulk device property (strong
influence of electron transport and contact resistance). The
morphology/microstructure of the deposited PEOz films was, in

500 nm

Glass/ITO/ZnO/PEOz(4) LND SND

Glass/ITO/ZnO/PEOz(8)

500 nm

Figure 5 | AFM measurements of the surface morphology. Threedimensional height-mode (left) and two-dimensional phase-mode (right)
AFM images. (a) Glass/ITO/ZnO, (b) glass/ITO/ZnO/PEOz(4) and
(c) glass/ITO/ZnO/PEOz(8). ‘SND’ and ‘LND’ in b represent ‘small NDs’
and ‘large NDs’, respectively.

distributed NDs cover the whole surface (see also the SEM images
in Supplementary Fig. 3). In closeup, the AFM (Supplementary
Fig. 4) and SEM images (Supplementary Fig. 3 right) reveal the
coexistence of both small NDs (SNDs) and large NDs (LNDs)
(the diameter of LNDs and SNDs is 80B120 and 10B20 nm,
respectively). As shown in Fig. 6a, the AEM measurements
confirmed the co-existence of SNDs and LNDs on the ZnO/
PEOz(4) sample surface (note that ‘N’ and ‘C’ atoms are present
only in the PEOz polymer). In addition, the AEM images suggest
that the spaces between NDs may also be covered by a thin
PEOz(4) nanolayer as indicated by the (colour) difference
between the bare ZnO and PEOz(4)-coated ZnO surfaces. In
contrast, the ZnO/PEOz(8) samples show a more uniform surface
morphology that is less rough than the ZnO layer and
pronouncedly different from the ZnO/PEOz(4) case. However,
the presence of a pit-like defect on the PEOz(8) surface might
be responsible for the relatively lower shunt resistance of devices
(see Fig. 4).

The reported AFM and AEM measurements raise two
questions: whether and how the ND morphology might
contribute to the enhanced performance of PEOz(4) PTB7Th:PC71BM solar cells. At the present time we are not able to
directly answer these questions but we note that charge (electron)
transport from the BHJ layer to the ZnO ECBL might, for
instance, be affected by the influence of the ND structure on local
electric fields. In addition (or alternatively), the physical contact
(adhesion) to the BHJ blend and/or its effective active area might
be enhanced by an increase in PEOz surface area. This is

Nanodots Nanodots

LND SND MoO3

PTB7-Th:PC71BM

ZnO

Glass/ITO/ZnO

5 μm

Glass/ITO/ZnO/PEOz(4)

5 μm

PEOz
nanodot

layer

ZnO

ITO

Glass

Figure 6 | AEM measurements of the surface atom distribution and formation of NDs. (a) AEM images for individual atoms (zinc (Zn), oxygen (O),
carbon (C) and nitrogen (N)). (b) Illustration for the proposed formation of the PEOz(4) nanolayer comprising SNDs and LNDs, and a contiguous coating
between them.

particular, found to have a key influence on device function. A
characteristic ND-like surface morphology was observed for the
PEOz(4)-coated ZnO ECBL, leading to an increased contact
surface area with, and an enhanced electron collection from, the
BHJ blend. This ND morphology is absent for PEOz(8)-coated
ZnO ECBL samples, with the corresponding absence of any
increase in VOC and JSC then attributable to relatively higher
electrical resistance. Our results have clearly demonstrated the
correlation between solar cell performance and the morphology
change, which can be controlled by varying the concentration of
the PEOz solution. Additional optimization by thermal annealing
of the PEOz(4)-coated ZnO ECBL at 120 �C resulted in
PCE ¼ 10.74%, one of the best efficiencies reported for a singlestack polymer:fullerene solar cell. Annealing at higher temperatures still (150 �C, data not shown) was, however, then
detrimental to device performance, resulting in a loss of the
desirable ND microstructure. These results demonstrate a clear
correction between our solar cell performance and the PEOz film
morphology, controlled both by varying the PEOz solution
concentration and by annealing. Future work will need to
focus on enhancing device stability but this may require
replacement of the PTB7-Th donor polymer in the BHJ blend,
which, similar to the closely related PTB7 donor polymer, appears
to be highly susceptible to photo-oxidation. The use of PEOz
with other active solar cell materials will be the subject of ongoing
studies.

Methods
Materials and solutions. PTB7-Th (weight-average molecular weight ¼ 126 kDa,
polydispersity index ¼ 2.5) and PC71BM (purity499%) were purchased from
1-Material (Canada) and Nano-C (USA), respectively. PEOz (weight-average

molecular weight ¼ 50 kDa, polydispersity index ¼ 3–4) was supplied by
Sigma-Aldrich (USA) and used without further purification. The PEOz solutions
were prepared by varying the concentration up to 8 mg ml � 1 in methanol. Binary
polymer:fullerene solutions were prepared using chlorobenzene as a solvent in the
presence of 1,8-diiodooctane (chlorobenzene:1,8-diiodooctane ¼ 97:3 by volume)
at a solid concentration of 20 mg ml � 1 (PTB7-Th:PC71BM ¼ 1:1.5 by weight) and
were vigorously stirred at room temperature for 12 h before spin coating. The ZnO
precursor solutions were prepared by dissolving zinc acetate dihydrate (SigmaAldrich, 1 g) and ethanolamine (Sigma-Aldrich, 0.28 g) in 2-methoxyethanol
(Sigma-Aldrich, 10 ml) and stirred at 60 �C for 3 h and then at room temperature
for 12 h before spin coating.

Thin film and device fabrication. ITO-coated glass substrates (sheet
resistance ¼ 10 O cm � 2 ) were patterned by photolithography/etching processes.
The pre-patterned ITO-glass substrates were cleaned using acetone and isopropyl
alcohol in an ultrasonic cleaner, followed by drying under flowing nitrogen. The
dried ITO-glass substrates were treated inside an ultraviolet–ozone chamber for
20 min, to remove any remnant organic residues on the surface of the substrates
and to make the ITO surface hydrophilic. ZnO precursor solutions were spin
coated onto the cleaned ITO-glass substrates and the resulting ITO/ZnO samples
were annealed at 200 �C for 1 h in air. After cooling down to room temperature, the
PEOz solutions were spin coated onto the surface of the ZnO layer, followed by
soft-baking for 15 min at 60 �C (in the case of unoptimized structures) or annealing
for 15 min at 120 �C (in the case of fully optimized structures) or (detrimentally) at
150 �C. The PEOz-coated samples were transferred to a nitrogen-filled glove box
for the active layer coating. The PTB7-Th:PC71BM BHJ layers were spin-coated on
top of the PEOz-coated ZnO layers and dried for 20 min inside the glove box. Next,
these samples were transferred to a vacuum chamber in the argon-filled glove box,
where MoO3 (10 nm) and Ag (80 nm) were sequentially deposited on top of the
BHJ layers through a shadow mask in a vacuum of 2 � 10 � 6 Torr. The active area
of devices was 0.05 or 0.09 cm 2 . For the measurement of surface morphology and
nanostructure, all samples were prepared using the ITO-glass substrates in the
same way as for the device fabrication, whereas the samples were prepared by spin
coating on quartz substrates for optical measurements. Attempts to measure the
exact thickness of the PEOz layers gave unreliable results, because the PEOz layers
were too thin.

a
0

Certified (PCE = 9.6%)

(from table 1)

Optimized (PCE = 10.7%)

0.4
_V_ APP (V)

0.6 0.8

–5

–10

–15

–20

1.0

0.9

0.8

0.7

0.6

0.5

1.0

0.9

0.8

0.7

1.0

0.9

0.8

0.7

0.6

1.0

0.9

0.8

0.7

0.0 0.2

ITO/ZnO

ITO/ZnO/PEOz(4)

0.6
0.5
0.4
0.3
0 30 60 90

120 150 180

Time (min)

Figure 7 | Fully optimized device performance and stability data. (a) Light
J–V characteristics of a fully optimized PTB7-Th:PC71BM solar cell
(JSC ¼ 19.0 mA cm � 2, VOC ¼ 0.794 V, FF ¼ 71.2%, PCE ¼ 10.74%) together
with certified data for a 9.6% PCE cell fabricated with a simple soft-baking
step (without the optimized PEOz annealing step). (b) Change of
normalized solar cell parameters for PEOz(4) and PEOz-free solar cells as a
function of exposure time under continuous AM1.5G, 100 mWcm � 2

illumination. The inset photographs in the VOC graph show the striking
colour change of the PTB7-Th:PC71BM BHJ that accompanies the decrease
in performance.

Measurements. The thickness of films was measured using a surface profiler
(Alpha Step 200, Tencor Instruments), whereas the optical absorption and transmittance of film samples were measured using an ultraviolet-visible spectrometer
(Optizen 2120, MECASYS). The UPS spectra were measured using an ultra-high
vacuum UPS system (ESCALAB 250Xi, Thermo Scientific) at B1 � 10 � 9 mbar
with a He I (21.2 eV) ultraviolet source: all samples were biased at - 5 V, whereas
the energy scale of the UPS spectra was calibrated to the Fermi level of a thermally
evaporated, cleaned Ag substrate. The valence band energy of the ZnO layer was
obtained as 7.7 eV from the low binding energy part of corresponding UPS
spectrum after calibration with the clean Ag reference, whereas the conduction

band energy of the ZnO layer was calculated by subtracting its optical band gap
(3.4 eV) (refer to Fig. 1e) from the valence band energy. The valence band
maximum of the PEOz-coated ZnO layers was calculated using the equation,
EVBM ¼ PIN � (ECF � EON), where EVBM, PIN, ECF and EON are the valence band
maximum, the incident photon energy (21.2 eV), the binding energy at cutoff
region and the onset binding energy, respectively 69 . The work functions of the ZnO
and ZnO/PEOz nanolayers were also measured using an EFM (XE-150, Park
Systems) and calibrated against highly ordered pyrolytic graphite. The surface
morphology was measured using an AEM (scanning Auger nanoprobe, PHI700,
Physical Electronics), AFM (Nanoscope IIIa, Digital Instruments) and SEM
(S-4800, Hitachi). Device cross-sections were measured using a high-resolution
transmission electron microscope (Titan G2 ChemiSTEM Cs Probe, FEI Company)
equipped with a focused ion beam system (Versa3D LoVac, FEI Company).
The J–V curves of devices were measured using a solar cell measurement system
equipped with a solar simulator (92250A-1000, Newport-Oriel) and an
electrometer (Model 2400, Keithley). The EQE measurements were carried out
using a specialized EQE measurement system equipped with a light source
(Tungsten-Halogen lamp, 150W, ASBN-W, Spectral Products) and a
monochromator (CM110, Spectral Products).

References
1. Van Franeker, J. J., Turbiez, M., Li, W., Wienk, M. M. & Janssen, R. A. J.
A real-time study of the benefits of co-solvents in polymer solar cell processing.
Nat. Commun. 6, 6229 (2015).
2. Wu, J.-S., Cheng, S.-W., Cheng, Y.-J. & Hsu, C.-S. Donor-acceptor conjugated
polymers based on multifused ladder type arenes for organic solar cells. Chem.
Soc. Rev. 44, 1113–1152 (2015).
3. Kim, H. et al. Organic solar cells based on conjugated polymers: history and
recent advances. Korean J. Chem. Eng. 31, 1095–1104 (2014).
4. Dang, M. T., Hirsch, L., Wantz, G. & Wuest, J. D. Controlling the morphology
and performance of bulk heterojunctions in solar cells. Lessons learned from
the benchmark poly(3-hexylthiophene):[6,6]-phenyl-C61-butyric acid methyl
ester system. Chem. Rev. 113, 3734–3765 (2013).
5. Krebs, F. C. All solution roll-to-roll processed polymer solar cells free
from indium-tin-oxide and vacuum coating steps. Org. Electron. 10, 761–768
(2009).
6. Krebs, F. C. Roll-to-roll fabrication of monolithic large-area polymer solar
cells free from indium-tin-oxide. Sol. Energy Mater. Sol. Cells 93, 1636–1641
(2009).
7. Voigt, M. M. et al. Gravure printing for three subsequent solar cell layers
of inverted structures on flexible substrates. Sol. Energy Mater. Sol. Cells 95,
731–734 (2011).
8. Yu, G., Gao, J., Hummelen, J. C., Wudl, F. & Heeger, A. J. Polymer photovoltaic
cells-enhanced efficiencies via a network of internal donor-acceptor
heterojunctions. Science 270, 1789–1791 (1995).
9. Shaheen, S. E. et al. 2.5% efficient organic plastic solar cells. Appl. Phys. Lett. 78,
841–843 (2001).
10. Padinger, F., Rittberger, R. S. & Sariciftci, N. S. Effects of postproduction
treatment on plastic solar cells. Adv. Funct. Mater. 13, 85–88 (2003).
11. Kim, Y. et al. Device annealing effect in organic solar cells with blends of
regioregular poly(3-hexylthiophene) and soluble fullerene. Appl. Phys. Lett. 86,
063502 (2005).
12. Reyes-Reyes, M., Kim, K. & Carroll, D. L. High-efficiency photovoltaic devices
based on annealed poly(3-hexylthiophene) and 1-(3-methoxycarbonyl)-propyl1-phenyl-(6,6)C61 blends. Appl. Phys. Lett. 87, 083506 (2005).
13. Li, G. et al. High-efficiency solution processable polymer photovoltaic
cells by self-organization of polymer blends. Nat. Mater. 4, 864–868
(2005).
14. Kim, Y. et al. A strong regioregularity effect in self-organizing conjugated
polymer films and high-efficiency polythiophene:fullerene solar cells. Nat.
Mater. 5, 197–203 (2006).
15. Chen, H. Y. et al. Polymer solar cells with enhanced open-circuit voltage and
efficiency. Nat. Photon. 3, 649–653 (2009).
16. Deng, Y. et al. Dithienocarbazole and isoindigo based amorphous low bandgap
conjugated polymers for efficient polymer solar cells. Adv. Mater. 26, 471–476
(2014).
17. Ho, C.-C., Chen, C.-A., Chang, C.-Y., Darling, S. B. & Su, W.-F. Isoindigobased copolymers for polymer solar cells with efficiency over 7%. J. Mater.
Chem. A 2, 8026–8032 (2014).
18. Guo, X. et al. Polymer solar cells with enhanced fill factors. Nat. Photon. 7,
825–833 (2013).
19. He, Z. et al. Single-junction polymer solar cells with high efficiency and
photovoltage. Nat. Photon. 9, 174–179 (2015).
20. Tao, Q. et al. D-A1-D-A2 copolymers with extended donor segments for
efficient polymer solar cells. Macromolecules 48, 1009–1016 (2015).
21. Zhang, S. et al. Side chain selection for designing highly efficient
photovoltaic polymers with 2D-conjugated structure. Macromolecules 47,
4653–4659 (2014).

22. Nguyen, T. L. et al. Semi-crystalline photovoltaic polymers with efficiency
exceeding 9% in a B300 nm thick conventional single-cell device. Energy
Environ. Sci. 7, 3040–3051 (2014).
23. Liu, Y. et al. Aggregation and morphology control enables multiple cases of
high-efficiency polymer solar cells. Nat. Commun. 5, 5293 (2014).
24. Ballantyne, A. M. et al. Studies of highly regioregular poly(3-hexylselenophene)
for photovoltaic applications. Adv. Mater. 19, 4544–4547 (2007).
25. Faist, M. A. et al. Competition between the charge transfer state and the singlet
states of donor or acceptor limiting the efficiency in polymer:fullerene solar
cells. J. Am. Chem. Soc. 134, 685–692 (2012).
26. Faist, M. A. et al. Understanding the reduced efficiencies of organic solar cells
employing fullerene multiadducts as acceptors. Adv. Energy Mater. 3, 744–752
(2013).
27. Waldauf, C. et al. Highly efficient inverted organic photovoltaics using solution
based titanium oxide as electron selective contact. Appl. Phys. Lett. 89, 233517
(2006).
28. Hau, S. K., Yip, H.-L. & Jen, A. K.-Y. A review on the development
of the inverted polymer solar cell architecture. Polym. Rev. 50, 474–510
(2010).
29. Yaacobi-Gross, N. et al. High efficiency organic photovoltaic cells based
on the solution-processable hole transporting interlayer copper thiocyanate
(CuSCN) as a replacement for PEDOT:PSS. Adv. Energy Mater. 5,

1401529 (2015).
30. Treat, N. D. et al. Copper thiocynate (CuSCN): An attractive hole transport/
extraction layer for use in organic photovoltaic cells. Appl. Phys. Lett. 107,
013301 (2015).
31. Lee, B. R. et al. Amine-based polar solvent treatment for highly efficient
inverted polymer solar cells. Adv. Mater. 26, 494–500 (2014).
32. Wang, G. et al. Interface modification of ZnO-based inverted PTB7:PC71BM
organic solar cells by cesium stearate and simultaneous enhancement of device
parameters. ACS Sustain. Chem. Eng. 2, 1331–1337 (2014).
33. Cheng, Y.-J., Hsieh, C.-H., He, Y., Hsu, C.-S. & Li, Y. Combination of
indene-C60 bis-adduct and cross-linked fullerene interlayer leading to highly
efficient inverted polymer solar cells. J. Am. Chem. Soc. 132, 17381–17383
(2010).
34. Hau, S. K. et al. Interfacial modification to improve inverted polymer solar cells.
J. Mater. Chem. 18, 5113–5119 (2008).
35. Duan, C., Zhang, K., Zhong, C., Huang, F. & Cao, Y. Recent advances in water/
alcohol-soluble p-conjugated materials: new materials and growing applications
in solar cells. Chem. Soc. Rev. 42, 9071–9104 (2013).
36. Yang, T. et al. Inverted polymer solar cells with 8.4% efficiency by conjugated
polyeletrolyte. Energy Environ. Sci. 5, 8208–8214 (2012).
37. Na, S.-I., Oh, S.-H., Kim, S.-S. & Kim, D.-Y. Efficient organic solar cells
with polyfluorene derivatives as a cathode interfacial layer. Org. Electron. 10,
496–500 (2009).
38. Duan, C. et al. Conjugated zwitterionic polyelectrolyte-based interface
modification materials for high performance polymer optoelectronic devices.
Chem. Sci. 4, 1298–1307 (2013).
39. Guan, X., Zhang, K., Huang, F., Bazan, G. C. & Cao, Y. Amino N-oxide
functionalized conjugated polymers and their amino-functionalized precursors:
new cathode interlayers for high-performance optoelectronic devices. Adv.
Funct. Mater. 22, 2846–2854 (2012).
40. Kesters, J. et al. Imidazolium-substituted polythiophenes as efficient electron
transport materials improving photovoltaic performance. Adv. Energy Mater. 3,
1180–1185 (2013).
41. Chang, Y.-M. & Leu, C.-Y. Conjugated polyelectrolyte and zinc oxide stacked
structure as an interlayer in highly efficient and stable organic photovoltaic
cells. J. Mater. Chem. A 1, 6446–6451 (2013).
42. Xia, R. et al. Investigation of a conjugated polyelectrolyte interlayer for high
performance inverted polymer solar cells. Adv. Energy Mater. 3, 718–723
(2013).
43. Xu, W. et al. Fluorene-based cathode interlayer polymers for high performance
solution processed organic optoelectronic devices. Org. Electron. 15, 1244–1253
(2014).
44. Zhou, Y. et al. A universal method to produce low-work function electrodes for
organic electronics. Science 336, 327–332 (2012).
45. Kang, H., Hong, S., Lee, J. & Lee, K. Electrostatically self-assembled
nonconjugated polyelectrolytes as an ideal interfacial layer for inverted polymer
solar cells. Adv. Mater. 24, 3005–3009 (2012).
46. Woo, S. et al. 8.9% single-stack inverted polymer solar cells with electron-rich
polymer nanolayer-modified inorganic electron-collecting buffer layers. Adv.
Energy Mater. 4, 1301692 (2014).
47. Zhou, Y., Fuentes-Hernandez, C., Shim, J. W., Khan, T. M. & Kippelen, B. High
performance polymeric charge recombination layer for organic tandem solar
cells. Energy Environ. Sci. 5, 9827–9832 (2012).
48. Jeong, J. H., Song, S. H., Lim, D. W., Lee, H. & Park, T. G. DNA transfection
using linear poly(ethylenimine) prepared by controlled acid hydrolysis of
poly(2-ethyl-2-oxazoline). J. Controlled Release 73, 391–399 (2001).

49. Brissault, B. et al. Synthesis of linear polyethylenimine derivatives for DNA
transfection. Bioconjugate Chem. 14, 581–587 (2003).
50. De la Rosa, V. R., Bauwens, E., Monnery, B. D., De Geest, B. R. &
Hoogenbom, R. Fast and accurate partial hydrolysis of poly(2-ethyl-2oxazoline) into tailored linear polyethylenimine copolymers. Polym. Chem.
5, 4957–4964 (2014).
51. Hoogenboom, R. Poly(2-oxazoline)s: a polymer class with numerous potential
applications. Angew. Chem. Int. Ed. 48, 7978–7994 (2009).
52. Viegas, T. X. et al. Polyoxazoline: chemistry, properties, and applications in
drug delivery. Bioconjugate Chem. 22, 976–986 (2011).
53. Konradi, R., Acikgoz, C. & Textor, M. Polyoxazolines for nonfouling surface
coatings—A direct comparison to the gold standard PEG. Macromol. Rapid
Commun. 33, 1663–1676 (2012).
54. Bogomolova, A. et al. Study of complex thermosensitive amphiphilic
polyoxazolines and their interaction with ionic surfactants. Are hydrophobic,
thermosensitive, and hydrophilic moieties equally important? J. Phys. Chem. B
118, 4940–4950 (2014).
55. Ramiasa, M. N. et al. Plasma polymerized polyoxazoline thin films for
biomedical applications. Chem. Commun. 51, 4279–4282 (2015).
56. Ishii, H., Sugiyama, K., Ito, E. & Seki, K. Energy level alignment and interfacial
electronic structures at organic/metal and organic/organic interfaces. Adv.
Mater. 11, 605–625 (1999).
57. Sun, Y., Seo, J. H., Takacs, C. J., Seifter, J. & Heeger, A. J. Inverted polymer solar
cells integrated with a low-temperature-annealed sol-gel-derived ZnO film as
an electron transport layer. Adv. Mater. 23, 1679–1683 (2011).
58. Hwang, J., Wang, A. & Kahn, A. Energetics of metal-organic interfaces:
new experiments and assessment of the field. Mater. Sci. Eng. Rep. 64, 1–31
(2009).
59. Kim, J. S. et al. Kelvin probe and ultraviolet photoemission measurements
of indium tin oxide work function: a comparison. Syn. Metal 111, 311–314
(2000).
60. Schlaf, R., Murata, H. & Kafafi, Z. H. Work functioni measurements on indium
tin oxide films. J. Electron Spectrosc. Relat. Phenom. 120, 149–154 (2001).
61. Boudreault, P.-L. T., Najari, A. & Leclerc, M. Processable low-bandgap
polymers for photovoltaic applications. Chem. Mater. 23, 456–469 (2011).
62. Zhou, H., Yang, L. & You, W. Rational design of high performance conjugated
polymers for organic solar cells. Macromolecules 45, 607–632 (2012).
63. Westphalen, M., Kreibig, U., Rostalski, J., Lu¨th, H. & Meissner, D. Metal cluster
enhanced organic solar cells. Sol. Energy Mater. Sol. Cells 61, 97–105 (2000).
64. Kim, W.-R., Park, H. & Choi, W.-Y. Conical islands of TiO2 nanotube arrays in
the photoelectrode of dye-sensitized solar cells. Nanoscale Res. Lett. 10, 63
(2015).
65. Roczen, M. et al. Structural investigations of silicon nanostructures grown by
self-organized island formation for photovoltaic applications. Appl. Phys. A
108, 719–726 (2012).
66. Alem, S. et al. Degradation mechanism of benzodithiophene-based conjugated
polymers when exposed to light in air. ACS Appl. Mater. Interfaces 4,
2993–2998 (2012).
67. Soon, Y. W. et al. Correlating triplet yield, singlet oxygen generation and
photochemical stability in polymer/fullerene blend films. Chem. Commun. 49,
1291–1293 (2013).
68. Razzell-Hollis, J. et al. Photochemical stability of high efficiency PTB7:PC70BM
solar cell blends. J. Mater. Chem. A 2, 20189–20195 (2014).
69. Liao, S.-H. et al. Single junction inverted polymer solar cell reaching power
conversion efficiency 10.31% by employing dual-doped zinc oxide nano-film as
cathode interlayer. Sci. Rep. 4, 6813 (2014).

Acknowledgements
This work was financially supported by grants from the Korean Government
(NRF_2015R1A2A2A01003743, NRF_2014R1A1A3051165, NRF-2014R1A6A3A03055861,
Basic Science Research Program_2009-0093819, Human Resource Training Project for
Regional Innovation_MOE/NRF_2014H1C1A1066748 and Basic Research Laboratory
Program_2011-0020264) and partly by the DGIST R&D Program of the Ministry of
Science, ICT and Future Planning of Korea (15-EN-03).

Author contributions
S.N. fabricated devices, measured devices and carried out SEM and TEM measurements.
W.H.K. performed sample preparation for UPS and EFM measurements. S.W.
contributed to various experimental systems and protocols. H.K carried out AES
and AFM measurements and analysis. Y.K designed and directed the research and
experiments. D.D.C.B contributed valuable advice on interpreting the results. S.N.,
D.D.C.B. and Y.K. wrote the manuscript.

Additional information
Supplementary Information accompanies this paper at [http://www.nature.com/](http://www.nature.com/naturecommunications)
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Competing financial interests: The authors declare no competing financial interests.

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How to cite this article: Nam, S. et al. Inverted polymer fullerene solar cells exceeding
10% efficiency with poly(2-ethyl-2-oxazoline) nanodots on electron-collecting buffer
layers. Nat. Commun. 6:8929 doi: 10.1038/ncomms9929 (2015).

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