Modified Fullerenes for Efficient Electron Transport Layer-Free Perovskite/Fullerene Blend-Based Solar Cells

Article abstract of DOI:10.1002/cssc.201700180

A variety of novel chemically modified fullerenes, showing different electron-accepting capabilities, has been synthesized and used to prepare electron transport layer (ETL)-free solar cells based on perovskite/fullerene blends. In particular, isoxazolino[60] fullerenes are proven to be a good candidate for processing blend films with CH₃NH₃PbI₃ and obtaining enhanced power conversion efficiency (PCE) ETL-free perovskite solar cells (PSCs), improving the state-of-the-art PCE (i.e., 14.3 %) for this simplified device architecture. A beneficial effect for pyrazolino and methano[60]fullerene derivatives versus pristine [60]/fullerene is also shown. Furthermore, a clear correlation between the LUMO energy level of the fullerene component and the open circuit voltage of the solar cells is found. Apart from the new knowledge on innovative fullerene derivatives for PSCs, the universality and versatility of perovskite/fullerene blend films to obtain efficient ETL-free PSCs is demonstrated.

Full text of DOI:10.1002/cssc.201700180

Accepted Article
Title: Modified fullerenes for Efficient Electron Transport Layer-Free
Perovskite:Fullerene Blend-Based Solar Cells
Authors: Juan L. Delgado, Rafael Sandoval-Torrientes, Nazario
Martín, Ramón Tena-Zaera, Silvia Collavini, ivet kosta, Jorge
Pascual, and Inés García-Benito
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ChemSusChem
10.1002/cssc.201700180
FULL PAPER
Modified fullerenes for Efficient Electron Transport Layer-Free
Perovskite:Fullerene Blend-Based Solar Cells
[
a]
[c, d]
[a]
[d]
[c]
Rafael Sandoval-Torrientes, Jorge Pascual,
Inés García-Benito, Silvia Collavini, Ivet Kosta,
[
c]
[a, b]
[d, e]
Ramón Tena-Zaera,* Nazario Martín,*
and Juan Luis Delgado*
Abstract: A variety of novel chemically modified fullerenes, showing
different electron accepting capabilities, has been synthesized and
used to prepare electron transport layer(ETL)-free solar cells based
on perovskite:fullerene blends. In particular, isoxazolino[60]
fullerenes are proven to be a good candidate for processing blend
Perovskite-based solar cells (PSCs) have revolutionized the
photovoltaic field, showing tremendous leap in power
a
[4]
conversion efficiency in the last years. Within the field of
regular PSCs, the big majority of the research work has been
devoted to the preparation of perovskite (i.e. including those
based on advanced composition engineering with multiple
3 3 3
films with CH NH PbI and obtaining enhanced power conversion
[
5]
efficiency (PCE) ETL-free perovskite solar cells, improving state-of-
the-art PCE (i.e. 14.3%) for this simplified device architecture.
Beneficial impact for pyrazolino and methano[60]fullerene
derivatives versus pristine [60]fullerene is also shown. Furthermore,
a clear correlation between the LUMO energy level of the fullerene
component and the open circuit voltage of the solar cells is found.
Apart from the new knowledge on innovative fullerene derivatives for
perovskite solar cells, the universality and versatility of
perovskite:fullerene blend films to obtain efficient ETL-free
perovskite solar cells is demonstrated.
anions and/or cations) films with improved microstructural
[
6]
properties and innovative organic hole transporting materials
[
7]
(HTMs), to afford highly efficient PSCs (Figure 1A).
Titanium dioxide is often the material of choice as electron
transporting material (ETM) in regular PSCs. The titanium
dioxide used in these solar cells needs generally a sintering
process, which requires treatment at high temperatures. This
thermal treatment implies a drawback in energy cost for a future
commercialization of these devices. Moreover, the use of
titanium dioxide influences the rigid morphology of the cell,
[8]
limiting its use on curved surfaces. Thus, the replacement of
TiO as ETM by organic materials is a very important subject for
2
the suitable development and future commercialization of PSCs.
In this regard, some of us have recently described the “Fullerene
Saturated Approach” as an innovative strategy to prepare
efficient PSCs, while avoiding energy cost processing issues,
Introduction
In order to face important issues such as global warming,
pollution and environmental degradation, the search for clean
and sustainable energy sources is nowadays a priority for
humankind. In this regard, photovoltaics are amongst the most
[
9]
associated to the use of TiO
2
as ETM. Nevertheless, the
development of novel electron transporting materials (ETMs)
[1]
able to replace the TiO
2
10]
is just restricted to a few examples,
promising alternatives to address these environmental issues.
In particular, perovskite-based solar cells have recently
[
[11]
such as azaacenes,
pristine fullerenes (Figure 1B),
[
12]
[
2]
mesostructures fullerene architectures
and cross-linkable
surpassed the all-organic photovoltaics and dye sensitized
[
13]
[3]
fullerenes.
solar cells in terms of power conversion efficiency.
Inspired by the outstanding effects of the “Fullerene
Saturated Approach”, we have recently reported a strategy to
prepare electron transport layer-free solar cells based on
[
14]
[
[
a]
b]
R. Sandoval-Torrientes, I. García-Benito, Prof. N. Martín
IMDEA Nanociencia
Ciudad Universitaria de Cantoblanco
C/ Faraday 9, 28049 Madrid, Spain
Prof. N. Martín
perovskite–[70]fullerene blends (Figure 1C).
This robust
solution-processing protocol for the deposition of pinhole-free
methylammonium lead triiodide-70fullerene blend films on
fluorine-doped tin oxide (FTO) simplifies the solar cell fabrication
process (i.e. no ETL deposition step) and provide efficient
devices with enhanced stability. Interestingly, significant
photostability improvement has also been recently reported for
inverted architecture perovskite solar cells based on
Departamento de Química Orgánica
Facultad C. C. Químicas, Universidad Complutense de Madrid
28040 Madrid, Spain
E-mail: nazmar@ucm.es
[c]
J. Pascual, Dr. I. Kosta, Dr. R. Tena-Zaera
IK4-CIDETEC
[
15]
Parque Tecnológico de San Sebastián
Paseo Miramón 196, 20009 Donostia–San Sebastián, Spain
E-mail: rtena@cidetec.es
perovskite:PCBM films. Indeed, the latter were also proven to
[
16]
improve the fill factor and suppress the hysteresis. However,
there is rather limited knowledge on the interactions between the
[
[
d]
e]
J. Pascual, S. Collavini, Prof. J. L. Delgado
POLYMAT, University of the Basque Country UPV/EHU
Avenida de Tolosa 72, 20018 Donostia–San Sebastián, Spain
E-mail: juanluis.delgado@polymat.eu
Prof. J. L. Delgado
[
17]
fullerene (or derivatives) and the perovskite. Therefore, there
is need for further research evaluating the use of different
chemically modified fullerene derivatives in perovskite-based
blend films and effects on the solar cell performance. Apart from
its simplicity, ETL-free device architecture has been chosen due
to the absence of interfaces between the perovskite crystals and
other electron acceptor materials that might cause some
Ikerbasque, Basque Foundation for Science
4
8013 Bilbao, Spain

These authors contributed equally to this work
Supporting information for this article is given via a link at the end of the
document.
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ChemSusChem
10.1002/cssc.201700180
FULL PAPER
interference in the analysis of the fullerene (or derivatives)
influences on the solar cell performance. However, in order to
prove the versatility and scope of this processing methodology
to prepare efficient electron transport layer-free solar cells, it is
necessary to fabricate solar devices based on blends of
perovskite with different chemically modified fullerene
derivatives.
isoxazolino60fullerene molecules, as well as known
methano60fullerene derivatives, namely DPM-6 and PC61BM.
Figure 1. Different types of regular architecture perovskite solar cells
discussed in the text.
In this report, perovskite-based blend films with a variety of
new fullerene derivatives endowed with different organic
addends and with tailored electronic properties are studied. The
fullerene-based
materials
include
two
novel
two new
and two
[
18]
pyrazolino[60]fullerene derivatives (PI-1 and PI-2),
[
19]
isoxazolino[60]fullerene derivatives (IS-1 and IS-2)
[
20]
Figure 2. Fullerene derivatives synthesized in the present study.
methano[60]fullerene derivatives (DPM-6 and PC61BM), all of
them shown in Figure 2.
It is important to underline here that pyrazolino[60]fullerenes
and isoxazolino[60]fullerenes have never been previously used
in PSCs. Therefore, to the best of our knowledge, this study
represents the first attemp of their use in PSCs. In the case of
methano[60]fullerenes, the most employed fullerene derivative in
PSCs is by far PC61BM ([6,6]-phenyl-C61-butyric acid methyl
Owing to the presence of a heteroatom directly connected to
the
fullerene
cage,
pyrazolino60fullerenes
and
isoxazolino60fullerenes display better electron accepting
[
22]
properties than methano60fullerenes.
presence of highly electron withdrawing fluorine atoms on the
phenyl rings of pyrazolino60fullerenes and
isoxazolino60fullerenes also contribute to the enhancement of
Moreover, the
[
21]
ester).
However, the related DPM-6 endowed with two aryl
groups in the cyclopropane ring remains unexplored in regular or
inverted PSCs. Therefore, additionally to the unambiguous proof
of universality and versatility of perovskite:fullerene blend films
to obtain efficient ETL-free perovskite solar cells, new
knowledge about innovative fullerenes in the PSC field is
provided.
[23]
these electron accepting properties.
materials 1-4, were prepared in excellent yields by reaction of
,5‒bis(trifluoromethyl)benzaldehyde or
The novel precursor
3
pentafluorobenzaldehyde with hydroxylamine to afford oximes 1
and 2, and with pentafluorophenylhydrazine to afford
hydrazones 4 and 3. The innovative fullerene materials were
obtained by in situ cycloaddition of the corresponding precursors
(
1-4) with NBS in the presence of trimethylamine and
60fullerene, to afford isoxazolino60fullerenes IS-1 and IS-2
Results and Discussion

and pyrazolino60fullerenes PI-1 and PI-2 in good yields
Synthesis
[
22b]
(
Scheme 1).
Methano60fullerene DPM-6 was prepared
In this study, we have designed and synthesized new fullerene
materials displaying tailored electron accepting properties, to
investigate its effect on the performance of electron transport
layer-free solar cells based in perovskite–fullerene blends. For
[24]
according to a protocol described by our group. PC61BM was
[25]
synthesized following an already reported protocol.
All the
novel precursors and fullerene derivatives were fully
1
13
19
characterized by means of H NMR, C NMR, F NMR, UV-Vis
spectroscopy, MALDI-MS and Cyclic Voltammetry (See
Supporting Information).
this
purpose,
we
have
prepared
and
two
two
novel
novel
pyrazolino60fullerene
molecules
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ChemSusChem
10.1002/cssc.201700180
FULL PAPER
The electrochemical properties of IS-1, IS-2, PI-1, PI-2,
DPM-6 and PC61BM were determined by means of cyclic
voltammetry in o‒DCB/MeCN (4:1) at room temperature and
using a three-electrode cell equipped with a glassy carbon
working electrode, a Pt wire counter electrode and a Ag/AgNO
3
reference electrode. DPM-6 and PC61BM showed a lower
electron accepting ability than pyrazolino60fullerenes and
isoxazolino60fullerenes, with a first reduction wave centered at
-
0.430 V and -0.450 V, respectively. Isoxazolino60fullerenes
IS-1 and IS-2 showed first reduction waves centered at -0.347 V
and -0.352 V, respectively.
The best electron accepting properties were shown by PI-1
and PI‒2, exhibiting first reduction waves centered at -0.322 V
and -0.316 V, respectively (Figure 3 inset). PI-1 and PI-2
showed first reduction waves shifted 30 mV compared with its
isoxazolino60fullerenes analogous (IS-2 and IS-1), due to the
presence of the electron withdrawing pentafluorophenyl ring
3
directly connected to the sp
N atom of the pyrazoline
heterocycle (Table 1). The origin of the different electron
capabilities of these materials stem from the presence of a
nitrogen or oxygen atom directly connected to the fullerene cage,
in pyrazolino60fullerenes and isoxazolino60fullerenes,
Scheme 1. Reagents and conditions used for the preparation of precursors
1‒4 and fullerene materials IS-1, IS-2, PI-1 and PI-2.
[
22a]
respectively.
Optical and Electrochemical Properties
The optical properties of the fullerene derivative materials
prepared in this study were investigated by means of UV-Vis
spectroscopy. As it can be seen in Figure 3, the fullerene
materials display an intense absorption in the UV region, while
the visible region remains almost transparent. All the fullerene
derivatives display the characteristic absorption peak at ~ 430
nm, which is a typical feature of 1,2‒dihydrofullerenes, due to
Table 1. Reduction potentials (CV) and energy levels of the LUMO for
fullerene materials IS-1, IS-2, PI-1, PI-2, DPM-6, PC61BM and C60
.
1/2 [a]
red
ε
[b]
Fullerene Derivative
E
LUMO (eV)
IS-1
IS-2
-0.347
-0.352
-0.322
-0.316
-0.430
-0.450
-0.349
-4.055
-4.048
[
26]
the saturation of a double bound on the 60fullerene cage.
PI-1
-4.078
PI-2
-4.084
DPM-6
PC61BM
-3.970
-3.950
C
60
-4.050
[
a] V vs. Ag=AgNO
PI‒1, PI‒2, DPM-6 and PC61BM were recorded at a scan rate of 100 mV s
GCE as working electrode. Reduction potentials were measured in
o‒DCB/MeCN (4:1) solution (Bu NPF 0.1 M as supporting electrolyte) at
3
. Cyclic voltammograms for compounds IS‒1, IS‒2,
–1
.
4
6
room temperature. All potentials are reported with reference to an internal
standard of the ferrocene/ferrocenium couple (Fc/Fc+ = 0.00 V). [b] The
energetic level of the LUMO for each fullerene material has been estimated
1/2
using the following equation ELUMO= -[E red + 4.4] eV .
Figure 3. UV-Vis absorption spectra of the fullerene derivatives synthesized in
-
5
the present study, measured in dichloromethane (1x10 M). The inset shows
the cyclic voltammograms of selected fullerene materials (DPM-6, IS-2 and
PI‒2), illustrating its different electron accepting capabilities by showing their
first reduction wave. UV-Vis spectra of the different fullerenes in DMF are
shown in Figure S35.
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FULL PAPER
Electron Transport Layer-Free Solar Cells
(PCE) of the solar cells prepared with blend films including
fullerene or derivatives is significantly higher than the reference
device (i.e. with perovskite film with no fullerene). Furthermore,
the performance of solar cells based on blend films with
fullerene derivatives is better than the analogue with pristine
fullerene. This finding suggests that all here studied chemical
modifications have a beneficial impact on the fullerene for their
use in perovskite:fullerene blend films for photovoltaic
applications.
To evaluate the synthesized fullerene derivatives in ETL-free
PSCs, perovskite:fullerene derivative films were prepared by the
[
14]
recently described protocol.
was prepared by dissolving 7.71 mmol of CH
mmol of PbCl (molar ratio 3:1) in 3 mL of DMF and stirring
Briefly, the perovskite solution
3
NH and 2.57
2
2
overnight. Prior to deposition, each fullerene derivative (IS‒1,
IS‒2, PI‒2, DPM‒6 and PC61BM) was added to the perovskite
solution to prepare the solar cells. It is noted that PI-1 was not
used in this photovoltaic study due to its lack of solubility in DMF.
However, the solubility of the other fullerene derivatives was
around 0.3 mg/mL (i.e. significantly higher than that of pristine
Table 2. Photovoltaic parameters for the best PCE solar cell (average
values of a series of devices, see statistical analysis in Figures S36-39).
Fullerene
Derivative
-2
J
sc (mA cm )
Voc (mV)
FF (%)
PCE (%)
[
60]fullerene: 0.1 mg/mL). The blend films resulting from the use
of the five different fullerene derivatives were used to fabricate
solar cells in the following configuration
glass/FTO/CH NH PbI :fullerene derivative/spiro-OMeTAD/Au
detailed information can be found in the experimental section).
IS-1
IS-2
16.7 (17.2)
16.1 (15.9)
16.5 (16.0)
16.1 (16.1)
14.5 (14.6)
1030 (1013)
1062 (1046)
1022 (1002)
1043 (1012)
1064 (1050)
68.7 (67.2)
73.8 (72.3)
69.4 (68.7)
69.2 (66.1)
72.2 (71.2)
11.8 (11.7)
12.7 (12.0)
11.7 (11.0)
11.6 (10.8)
11.2 (10.9)
3
3
3
(
PI-2
The top-view SEM micrographs of perovskite:fullerene blends
did not show major differences other than the elimination of a big
DPM-6
PC61BM
[
14]
amount of pinholes (as occurred in perovskite:C70 blend films ).
Figure S43). However, some effect of the fullerene derivative on
(
the blend film roughness cannot be fully ruled out Solar cells
based on perovskite films with pristine fullerene ((i.e.
Fullerene-
free
17.0 (15.2)
1040 (950)
69.6 (61.2)
10.5 (9.0)
glass/FTO/CH
3
NH
3
PbI
3
:C60/spiro-OMeTAD/Au) and without
fullerene (i.e. glass/FTO/CH
3
NH PbI /spiro-OMeTAD/Au) were
3
3
It is worth to note that the main improvement versus devices
with pristine fullerene was detected for the Jsc. This may be due
to the better solubility in DMF exhibited by the fullerene
derivatives, making possible to prepare processing solutions
also prepared for comparison purposes. Figure 5 shows the J–V
curves, under AM1.5G simulated sunlight, of the best devices.
2
with fullerene/PbCl ratios of 1:7200 (i.e. close to those claimed
[
14]
as the best from recent studies with [70]fullerene). However,
further comparative analysis of the photovoltaic parameters
indicates that in general the open circuit voltage (Voc) increases
in
the
following
order:
pyrazolino60fullerenes
<
isoxazolino60fullerenes < methano60fullerenes, following a
ε
similar trend to that observed for the LUMO energy estimated
by electrochemistry (Table 1). This experimental observation
suggests, additionally to the increase of fullerene solubility, the
chemical functionalization may play a significant role in the solar
cell performance. In particular, it seems that the use of less
electron accepting fullerenes within these blends results in an
increase in the Voc. Although further device characterization is
required in order to determine the involved mechanisms, this
finding may provide some useful practical guidelines for
selecting fullerene derivatives for their use in PSCs. Another
relevant aspect to bear in mind is the functionalization of the
fullerenes. More in particular, the isoxazolino moiety might
represent a binding site for a better interaction with perovskite.
As the best PCE was obtained for devices based on
Figure 5. J–V characteristics, under AM1.5G simulated sunlight, of the best
PCE ETL‒free solar cells based on CH NH PbI :fullerene blends.
3 3 3
The photovoltaic parameters extracted from the J-V curves
are summarized in Table 2. It is noted that, additionally to the
best values, the mean ones (i.e. extracted from the full series,
see statistical analysis in Figures S36-39) are also included in
the table. As it can be seen, the power conversion efficiency
3 3 3
CH NH PbI :IS-2 blend films, a further optimization protocol was
applied on the deposition of these films by using o-xylene as a
[
14]
co-solvent. As a result, solar cells with improved photovoltaic
performance were obtained (Figure 6a shows a micrography of
the cross section of the device). In particular, champion devices
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ChemSusChem
10.1002/cssc.201700180
FULL PAPER
–
2
showing a JSC = 17.3 mA cm , VOCꢀ=ꢀ1077 mV, FF = 78% and
an overall PCE of 14.3% (Figure 6b). Good agreement between
the measured Jsc and value estimated from the eQE spectrum
1.0
(
Figure S44), is noted. It is worth to mention that 14.3 % PCE
0.8
0.6
0.4
results in an improvement versus the recently reported on
perovskite:[70]fullerene blend-based solar cells, meaning, to the
best of our knowledge, the highest PCE value achieved for ETL-
[
27]
free perovskite solar cells up to date.
In addition, this
Fullerene-free
IS-2
PI-2
DPM-6
C60
formulation leads to highly reproducible devices, with rather
narrow PCE dispersion as it can be seen in Figure 6c.
0.2
.0
0
0
40
80
120 160 200 240
t (min)
Figure 7. Evolution of PCE (normalized values) versus time under AM1.5
simulated sunlight for non-encapsulated solar cells containing the best working
fullerene for each type.
Therefore,
the
present
results
point
out
isoxazolino[60]derivatives as very good fullerene candidates to
be used in the perovskite solar cells, placing them in an
advantageous position versus the very well-known PC61BM. This
finding anticipates significant potential and may open wide
avenues for isoxazolino[60]derivatives, which up to our best
knowledge have not been previously proposed and/or evaluated
for photovoltaic applications. Although the relatively lower
photovoltaic performance detected for devices based on
perovskite:PI-2 blend film, the pyrazolino60fullerenes may also
have some opportunities in perovskite solar cells. The scenario
seems indeed to be quite different to their application in organic
solar cells, for which the highest reported PCE using this kind of
Figure 6. a) Cross-section micrograph obtained by FE-SEM means and b)
J‒V curve and PV parameters for champion perovskite:IS-2 device; c)
distribution of champion device PCE values, pointing out its reproducibility.
As discussed in the introduction, the beneficial impact of
using perovskite:fullerene blend films on the solar cell stability
[
28]
fullerene
derivative
is
~
1%.
Concerning
the
[
14,15]
methano[60]fullerene bisadducts, it is concluded that DPM-6
seems to be a competitive alternative to the well-known PC61BM
also to be used in PSCs, as previously demonstrated for organic
has been recently claimed.
Therefore, the effect of the
different fullerene derivatives on the photostability of the
unencapsulated perovskite solar cells was also investigated.
Figure 7 shows the evolution of PCE versus time under
continuous AM1.5G simulated sunlight for solar cells based on
blend films including the best-performing fullerene from each
derivative family (i.e. IS-2, PI-2 and DPM-6). As can be seen in
Figure 6, there are no clear differences, concluding that all
fullerenes have similar beneficial impact on the photostability of
the device. This finding suggests fullerene core (i.e.
irrespectively of the chemical modification) as the main cause of
the stability enhancement. Indeed, all perovskite:fullerene
derivative blend films exhibited similar water contact angle
[
29]
solar cells.
Conclusions
A variety of novel (IS-1, IS-2, PI-1, PI-2) and well-known
fullerene materials (DPM-6, PC61BM) showing different electron
accepting capabilities have been designed and synthesized to
study the effect over the photovoltaic performance of electron
transport layer-free solar cells based on perovskite:fullerene
blends. In particular, 14.3% PCE ETL-free perovskite solar cells,
(
Table S1), pointing out similar behaviour versus moisture which
based on CH
were obtained. The photovoltaic performance improvement, in
comparison to devices based on CH NH PbI :[60]fullerene blend
points out the beneficial
3 3 3
NH PbI :isoxazolino[60] derivative blend films,
may be one of the main degrading agents for PSCs. In addition,
it is noted that FF is the most relevant parameter concerning
photostability as Voc and Jsc did almost not vary with exposure
time (Figure S40-42).
3
3
3
[
14]
films and [70]fullerene analogues,
impact of the fullerene chemical modification. Furthermore, an
interesting experimental correlation between the open circuit
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ChemSusChem
10.1002/cssc.201700180
FULL PAPER
+
voltage value of the devices and the LUMO energy level of the
fullerene material, which may be very helpful in future materials
design for perovskite solar cells, has been found out as well. All
in all, the current photovoltaic study, which includes ETL-free
ferrocene/ferrocenium couple (Fc/Fc = 0.00 V). UV–Visible spectra were
recorded in a Varian Cary 50 spectrophotometer in dichloromethane
-
5
(1x10 M)
perovskite solar cells based on
a
large variety of
Device fabrication
perovskite:fullerene derivative blends, allows to confirm the
universality of using solution processed perovskite:fullerene
blend films as light absorber material for ETL-free solar cells.
It is important to remark that the synthetic costs associated to
Glass/FTO samples were cleaned by the following procedure: the
samples were sonicated in distilled water with soap for 5 min, rinsed
thoroughly with distilled water, dried and sonicated in acetone and
2‒propanol for 15 min in each solvent. The perovskite solution was
the
fabrication
of
pyrazolino60fullerenes
and
prepared by dissolving 7.71 mmol of MAI and 2.57 mmol of PbCl (molar
2
isoxazolino60fullerenes, are similar to those required for
PCBM, evidencing the high potential of these fullerene
derivatives in this emerging field. This gained knowledge paves
the way to the use of new modified fullerenes as well as non-
fullerene electron acceptors with a controlled LUMO level.
ratio 3:1) in 3 mL of DMF and stirring overnight. Prior to deposition, the
perovskite solution was saturated with the corresponding fullerene
material (i.e. materials IS-1, IS-2, PI-2, DPM-6 and PC61BM.). It is noted
-1
that fullerene solubility in the perovskite solution was around 0.3 mg ml
for all of them. The resulting solution was spin coated on the substrates
following a two-step protocol, which consisted of a first step of 500 rpm
for 5 s followed by a second step of 2000 rpm for 45 s. Subsequently, the
samples were annealed at 100°C for 2 h to ensure complete perovskite
formation. On top of the perovskite layer, the spiro-OMeTAD hole-
Experimental Section
Materials
selective contact was deposited from
a solution that contained
Chemicals and reagents for synthesis were purchased from commercial
suppliers and used as received. All solvents were dried according to
standard procedures. Air-sensitive reactions were carried out under
argon atmosphere. Flash chromatography was performed using silica gel
spiro‒OMeTAD (108.4 mg) in chlorobenzene (953.43 mL), LiTFSI
-
1
solution in MeCN (17.17 mL, 520 mg mL ) and tert-butylpyridine (29.4
mL). The HTL was deposited by spin coating the solution at 3000 rpm for
30 s. The samples were left in a desiccator overnight. Finally, an array of
2
(
Merck, Kieselgel 60, 230-240 Analytical thin layer chromatography
TLC) was performed using aluminum coated Merck Kieselgel 60 F254
round Au back contacts (~ 0.07 cm ) was deposited by thermal
–6
(
evaporation at more than 5x10 torr with a NANO38 (Kurt J. Lesker)
apparatus with a shadow mask.
plates. The materials used in the photovoltaic study were obtained from
commercial suppliers in high purity and used without further purification:
glass/FTO (TEC15, Hartford Glass), methylammonium iodide (MAI,
Device characterization
DYESOL), PbCl
Chemicals Limited), lithium bis(trifluoromethane) sulfonimidate (LiTFSI,
9.9%, Solvionic), tert‒butylpyridine (96%, Sigma–Aldrich), DMF (extra
pure, Scharlab), 2-propanol (synthetic grade, Scharlab), acetone
technical grade, Scharlab), chlorobenzene (99.8%, Sigma–Aldrich) and
2
(98%, Sigma–Aldrich), spiro-OMeTAD (99%, Feiming
The J–V characteristics of the solar cells were measured under a xenon
arc lamp simulator equipped with an AM1.5G spectral filter (Sun 2000,
ABET Technologies). The intensity was adjusted to provide 1 sun
9
-2
illumination (100 mW cm ) by using a calibrated silicon solar cell. The
(
J‒V characteristics were recorded by scanning the potential from higher
acetonitrile (UV HPLC grade, Scharlab).
-1
than the VOC to zero (i.e., “reverse mode”) at approximately 300 mV s .
Before the measurement, a voltage of approximately 1.2 V was applied
to the devices for 1 min. It is noted that unmasked devices were
generally used for the J-V characterization. However, only relatively
minor differences were detected when mask was used (Fig.
S45).Stability measurements were carried out under continuous AM1.5G
simulated sunlight, recording the J–V characteristics (in “reverse mode”
and without applying any previous external voltage) every 10 min during
the first 60 min and from there every 30 min.
Characterization
1
NMR spectra were recorded on a Bruker Advance 300 ( H: 400 MHz;
13
19
C: 101 or 75 MHz) and Bruker Advance DRX-500 ( F: 471 MHz)
spectrometers at 298 K using partially deuterated solvents as internal
standards. Coupling constants (J) are denoted in Hz and chemical shifts
(
δ) in ppm. Multiplicities are denoted as follows: s = singlet, d = doublet, t
triplet, m = multiplet, q = quartet. UV-Vis spectra were recorded in a
Varian Cary 50 spectrophotometer. Matrix assisted Laser desorption
ionization (coupled to Time-of-Flight analyzer) experiments
MALDI‒TOF) were recorded on a MAT 95 thermo spectrometer and a
Bruker REFLEX spectrometer respectively. Cyclic voltammograms for
=
a
(
Acknowledgements
–
1
compounds were recorded at a scan rate of 100 mV s using an air-tight
single-compartment three-electrode cell equipped with a glassy carbon
We thank Sandra Rodriguez for the excellent technical support.
This work was partially supported by the European Research
Council (ERC-320441-Chirallcarbon), Basque Government
(PC2015-1-03 (16-79)), Comunidad de Madrid (FOTOCARBON
Project S2013/MIT-2841), MINECO of Spain (Grant Numbers
CTQ-2014-52045-R, MAT2013-47192-C3-2-R and CTQ2015-
working electrode, Pt wire counter electrode, and Ag/AgNO
electrode. Reduction potentials were measured in o-DCB/MeCN (4:1)
solution (Bu NPF 0.1 M as supporting electrolyte) at room temperature.
All potentials are reported with reference to an internal standard of the
3
reference
4
6
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ChemSusChem
10.1002/cssc.201700180
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[
11] K. Wojciechowski, T. Leijtens, S. Siprova, C. Schlueter, M. T. Hörantner,
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Basque Foundation for Science, for an “Ikerbasque Research
Fellow” contract, Polymat Foundation and MINECO of Spain for
IEDI-2015-00666 grant. J.P. and J.L.D. acknowledge Iberdrola
Foundation for financial support.
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FULL PAPER
Novel CH
based on
derivatives (i.e. isoxazolino, pyrazolino
and methano[60]fullerenes) are
3
NH
a
3
PbI
3
:fullerene blends
R. Sandoval-Torrientes, J. Pascual, I.
García-Benito, S. Collavini, I. Kosta, R.
Tena-Zaera,* N. Martín,* J. L. Delgado*
variety of fullerene
demonstrated to fabricate electron
transport layer-free solar cells that
efficiently (i.e. up to 14.3%) convert
sunlight into electricity.
Page No. – Page No.
Modified fullerenes for Efficient
Electron Transport Layer-Free
Perovskite:Fullerene Blend-Based
Solar Cells
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