One-dimensional (1D) [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) nanorods as an efficient additive for improving the efficiency and stability of perovskite solar cells

Article abstract of DOI:10.1039/c6ta03605h

We report a novel one dimensional (1D) [6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM) nanorod material as an efficient additive to form a wrinkle-like bicontinuous perovskite layer, where 1D PCBM nanorods can distribute homogenously throughout the film with an enlarged grain size. The resultant interconnected 1D PCBM nanorod within the perovskite material can also efficiently facilitate the photo-generated charge separation and carrier transportation process. An improved solar cell performance, from 9.5% up to 15.3%, was achieved using the optimized 1D PCBM nanorod content, along with enhanced device working stability. This work gives a new insight towards designing high performance organic-inorganic perovskite solar cells.

Full text of DOI:10.1039/c6ta03605h

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One-dimensional (1D) [6, 6]-phenyl-C61-butyric acid methyl ester
(PCBM) nanorods as efficient additive for improving the efficiency
and stability of perovskite solar cells
Received 00th January 20xx,
Accepted 00th January 20xx
Chenxin Ran,a Yonghua Chen,b† Weiyin Gao,a Minqiang Wang*a and Liming Dai*b
DOI: 10.1039/x0xx00000x
www.rsc.org/
We report a novel one dimensional (1D) [6, 6]-phenyl-C61-butyric for perovskite solar cells with a large aperture area > 1 cm2,
acid methyl ester (PCBM) nanorod material as an efficient additive showing potentials for large-scale commercialization.13
to form a wrinkle-like bicontinuous perovskite layer, where 1D However, it is highly desirable to further enhance the PCE and
PCBM nanorods can distribute homogenously throughout the film stability of perovskite solar cells for specific practical
with an enlarged grain size. The resultant interconnected 1D applications.
PCBM nanorod within the perovskite material can also efficiently
To improve the performance of perovskite solar cells, most
facilitated the photo-generated charge separation and carrier state-of-the-art studies have paid much attention to interface
transportation process. An improved solar cell performance, from engineering,4,5 compositional optimization,12,14-19 solvent
9.5% up to 15.3%, was obtained using the optimized 1D PCBM selection,20-22 and device fabrication23-25. Of particular interest,
nanorod content, along with an enhanced device working certain small molecules, such as fullerene derivative (e.g.
stability. This work gives a new insight towards designing high PCBM
，
A10C60),26-28 Lewis bases thiophene and pyridine,29,30
performance organic-inorganic perovskite solar cells.
and halogen-bonded supermolecular complex,31 have been
used to successfully passivate the interface trap states at the
grain boundaries by forming continuous pathways for charge
carriers within the perovskite film. Among these materials,
Introduction
Since 2012, organic–inorganic halide perovskite solar cells, PCBM has already been widely used as electron acceptor and
mainly based on CH3NH3PbX3 materials (X = Cl, Br, I), have electron transport layer in organic solar cells. Recently, one-
become the most significant development in the field of solar dimensional (1D) PCBM nanorods have been successfully
cells.1 This is because the CH3NH3PbI3-based materials possess synthesized by a solution-based method.32 Compared to C60
34
multiple advantages, including (i) strong and broad range of nanorods,33,
these newly-developed 1D PCBM nanorods
visible light absorption; (ii) ambipolar transport of electrons have even a higher surface area and transport channels with a
and holes; (iii) high charge mobility; (iv) small exciton binding higher electron mobility. They are predicted to be ideal
energy, and (v) long exciton diffusion length.2 Consequently, acceptor materials in organic solar cells.32
the power conversion efficiency (PCE) of the CH3NH3PbX3
Herein, we report a significantly improved performance for
perovskite solar cells has rapidly exceeded 15% within two CH3NH3PbI3 perovskite solar cells by blending a small amount
4-
years,3 followed by further improvement of PCE up to ~18%,
of 1D PCBM nanorods (typically, 0 - 960 µg/mL) within the
and the highest efficiency has exceeded over 20% in 2015.7, PbI2/ CH3NH3I (MAI) precursor solution in DMF (1.2 M, 0.208
Very recently, an accredited PCE of 15% has been reported mL) during the device fabrication (vide infra). Our results
showed that the 1D PCBM nanorods can help forming a
12
12
bicontinuous morphology and bulk-heterojunction structure
through the thickness of the perovskite film, which can (i)
enlarge the perovskite grain size, (ii) induce a large interfacial
contact area between the PCBM nanorod and perovskite
matrix, and (iii) improve the charge carrier separation and
transportation within the perovskite film. As a result, the
newly-developed perovskite solar cell with the PCBM nanorod
additive exhibited an enhanced PCE of 15.3%, compared with
9.5% for its counterpart without any additive, along with an
increased open-circuit voltage (VOC, from 0.74 to 0.90 V) and
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fill factor (FF, from 0.57 to 0.74), but with no photocurrent
hysteresis.
Results and discussions
We used a simple and efficient liquid-liquid interfacial
precipitation (LLIP) method reported recently32 to synthesize
the 1D PCBM nanorods. Briefly, PCBM nanosheets were first
formed at the interface between the chloroform and methanol
phases in a PCBM solution. Then, the interface was strongly
disturbed by ultrasonication, and brown aggregated
precipitates appeared at the bottom after 24 h standing still
(Fig. S1). After another 30 min sonication, 1D PCBM nanorods
formed by folding the preformed PCBM nanosheets.32 Fig. 1a
shows TEM images of the pristine PCBM powder with random
sphere morphology. After interface assembling by the LLIP
method, PCBM molecules arranged into sheets as shown in
Fig. 1b. Further ultrasonication of the PCBM sheets resulted in
scroll-like PCBM nanorods structures. The HRTEM image given
in Fig. 1d shows an individual scrolled PCBM nanorods with a
diameter of ~ 10 nm, and length of 100~400 nm (Fig. 1c),
Fig. 2 (a) Digital images of PbI2/MAI precursor solution (left) and
PbI2/MAI precursor solution with 240 μg/mL 1D PCBM nanorods
(right). (b) UV-Vis, (c) XRD and (d) PL spectrum excitation at 470 nm of
the corresponding perovskite film.
which depends on the size of the PCBM sheets. The S3). Moreover, the produced 1D PCBM nanorods showed
corresponding low magnification SEM images of 1D PCBM similar optical property with respect to the pristine PCBM
nanorods compared with PCBM sheets (Fig. S2) demonstrated powder (Fig. S4), indicating that the PCBM molecular structure
that most PCBM sheets scrolled up into nanorods structure. was not affected during the self-assembling LLIP method and
From the XRD pattern in Fig. S2, we can see that the the sonication process.
assembling of PCBM powder into sheets led to a new distinct
The effect of 1D PCBM nanorods on the properties of
peak at ~6.0°, whose intensity is further enhanced after the CH3NH3PbI3 perovskite film was then characterized. It was
scroll of PCBM sheets. From the theoretical calculation,32 this found that the PbI2/MAI precursor with 1D PCBM nanorods
new peak is originated from the stacking of PCBM bilayer can form a homogenous dispersion in light brown colour (Fig.
sheets, which have a intermolecular distance of
∼15 Å, 2a), which can be cost into uniform perovskite films. UV-Vis
corresponding to XRD peak at ~6.0°. After being scrolled into spectroscopic measurements (Fig. 2b) showed that the
1D PCBM nanorods, the overlapping of PCBM bilayer sheets perovskite film with 1D PCBM exhibited higher absorption
resulted in an enhanced XRD peak intensity (red curve in Fig.
ability in the range from 550 nm to 750 nm, and different
absorption behaviour in the infrared range compared with the
control film without any additive, due to the
thermodynamically favoured bonding of PCBM to defective
halides, and this passivation effect is also beneficial for
perovskite solar cell application.26 Besides, it is observed that
both film with or without 1D PCBM showed typical perovskite
XRD peaks (Fig. 2c). Compared to the XRD pattern for the
perovskite with no additive (Fig. 2c), the perovskite film with
1D PCBM nanorods exhibited relative sharp and narrow peaks
with a relative small full width at the half maximum (FWHM)
peak, suggesting the presence of highly crystallized perovskite
particles with an enlarged grain size. Fig. 2d shows the PL
spectra of the two perovskite films. It is well-known that
fullerenes and their derivatives are strong electron acceptors,
and the C60 carbon core is able to accommodate even multiple
electrons at the same time.36 Here, it is clearly observed a
more striking PL quenching effect in the perovskite/1D PCBM
composite film than that of the control perovskite film, which
indicates that efficient photo-generated charge separation and
extraction took place at the interface of perovskite material
and 1D PCBM nanorods. This PL quenching effect also
indicates the large contact area between the perovskite and
1D PCBM nanorods.
Fig. 1 TEM image of (a) pristine PCBM powder, (b) PCBM sheets
produced by LLIP method and (c) scroll-like PCBM nanorods after
sonication. (d) HRTEM of 1D PCBM nanorods. (e) Schematic procedure
for 1D PCBM nanorod formation.
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Fig. 3 SEM images of perovskite films with increasing content of 1D PCBM nanorods as additives under different magnifications: (a) 0
96 g/mL, (c) 240 g/mL, (d) 960 g/mL, and (e) 2400 g/mL. Scale bar: 1 m. (f-j) The corresponding histograms of the grain size distribution of the
five perovskite films, inset shows the mean grain size of the film.
µg/mL, (b)
µ
µ
µ
µ
µ
Morphologies of the films made from the PbI2/MAI nanorods with respect to the control perovskite film with no
precursor with different 1D PCBM nanorod concentrations additive. The previous works that using PCBM powder as
were further characterized. As shown in Fig. 3a-d, a uniform additive didn’t show such unique morphology and the
pinhole-free morphology with enlarged grain size was enlarged grain size, no matter one-step fabrication of
observed for all perovskite films as the 1D PCBM concentration CH3MH3PbI3/PCBM film26 or two-step fabrication of PbI2-PCBM
of the precursor solution increased from 0 to 960
However, some pinholes and cracks appeared when the 1D morphology in this work (Figure 3e), when using excess PCBM
PCBM content became too high (2400 g/mL, Fig. 3e), though powder in the film in both previous works, the sphere particle
µ
g/mL. film27. Besides, different from the obvious strip-like
µ
the grain size of the film remained increasing. Perovskite film shape aggregation phase were observed,26,27 demonstrating
without any additive showed a mean grain size of 180 nm with the important role of 1D PCBM nanorods as additive on
the smallest 40 nm and largest 510 nm (Fig. 3f, Table S1). CH3NH3PbI3 film formation.
When a small amount of 1D PCBM (96
µ
g/mL) was added, the
It is well known that a larger perovskite grain size is
mean grain size increased up to 240 nm (Fig. 3f, Table S1). beneficial to the improvement of the solar cell performance
Further increasing the content of 1D PCBM nanorods resulted because a larger grain size can lead to (i) a reduced interfacial
in an enlarged grain size of the perovskite film, which is in area between the perovskite grains and suppressed charge
agreement with the XRD result above (Fig. 2c). The direct trapping to eliminate hysteresis, and (ii) a lower bulk defect
proportional relationship between the 1D PCBM content and density and higher carrier mobility, allowing for the
the grain size of the perovskite flim shown in Fig. 3f-j and Fig. photogenerated charge carriers to transport through the
S5 implies that the addition of 1D PCBM nanorods during the perovskite layer without frequent encounters with defects and
perovskite film formation can significantly affect the crystalline impurities.4 Furthermore, the homogeneous appearance of the
process to increase the perovskite grain size and reduce the wrinkle-like morphology on the surface of the perovskite
grain boundaries. The enlarged grain size may origin from the particles (Fig. 3b-e) as well as within the perovskite film (Fig.
anisotropy of rod like shape 1D PCBM during spin coating 4b) indicates that the 1D PCBM nanorods not only cover the
process. In contrast, addition of the pristine PCBM powder did perovskite grains to enlarge the interface between the
not cause an increase in the grain size of the perovskite film perovskite material and 1D PCBM nanorods, but also
(Fig. S6), which is in consistent with the work using PCBM as distributed uniformly though the thickness to form
additive.26,27 Besides, the films with 1D PCBM nanorods bicontinuous bulk heterojunction structure, which greatly
showed wrinkle-like surface morphology (Fig. 3b-e). benefits the charge separation and transportation process.
Moreover, the cross-section SEM images in Fig. 4a-c depicted
a
a
A recent theoretical study has shown that PCBM could bind
that a bicontinuous composite structure can be formed within to iodide-rich defect sites,26 which explains both the observed
the whole thickness of the perovskite film with 1D PCBM
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µ
L, 1 mg/mL in DMF) into 1.2 M PbI2/MAI (115 mg/40 mg)
precursor solution to form a homogenous mixture solution
with the final concentration of 1D PCBM from 0 g/mL to 960
g/mL. As shown in Fig. S7, all the solar cells containing the 1D
µ
µ
PCBM nanorods showed an enhancement of the PCE
compared to the control device without any additive. Table 1
lists the numerical results from Fig. S7. It is worth to notice
that the performance of the perovskite solar cells first
increased with increasing content of the 1D PCBM nanorods
up to about 240
µ
nanorods content caused
g/mL. Further increase in the 1D PCBM
decrease in the solar cell
a
performance. More specifically, the PCE initially increased
from 9.54% to 14.34% as the content of 1D PCBM nanorods
increased from 0 to 240 µ
PCBM nanorods up to 960
g/mL, and further addition of the 1D
g/mL reduced the PCE to 11.48%.
µ
The control device prepared under the same conditions, but
without any additive, showed a PCE of 9.5% only. Here, the
concentration of PbI2/MAI is 1.2 M and the thickness of the
film is ~500 nm, which is too thick to achieve a high
performance for the pure CH3NH3PbI3 device, as demonstrated
by others.3,25,35,39,40 The curves of VOC, JSC, FF and PCE as a
function of the content of the 1D PCBM nanorods given in Fig.
S8 all show a similar optimum content of the 1D PCBM
nanorods around 240 µg/mL for the best performing device.
The value of VOC an FF highly depends on the quality of the
CH3NH3PbI3 film. High film quality requires dense and
continuous film structure as well as high crystallinity of the film,
and any factor that lower the film quality will lead to
decreased VOC as well as FF. Before optimized content of 1D
PCBM nanorods, the increased Voc was due to an improved
perovskite film quality (high crystalline, low defect and large
grain size) while the increased FF originated from the positive
effect of 1D PCBM nanorods on the charge separation and
transportation within the perovskite film, but further
increasing content of 1D PCBM nanorods over the optimized
dosage scarified the quality of the film, leading to both
decreased Voc and FF. However, Jsc showed little change with
increasing content of the 1D PCBM nanorods resulted from a
counterbalance between the following two opposite effects: (i)
photoinduced charge transfer was increasingly more impeded
when Voc increased;41 (ii) 1D PCBM nanorods facilitated the
charge transportation process to increase Jsc. Based on the
Fig. 4 Cross-section SEM image of the (a) CH3NH3PbI3 and (b)
CH3NH3PbI3/1D PCBM nanorod film. (c) Scheme of planar perovskite
solar cell using 1D PCBM nanoribbons as the additives. Scale bar: 0.5
µ
m.
close binding between 1D PCBM nanorods and CH3NH3PbI3
structure as well as homogenous distribution of 1D PCBM
nanorods in the perovskite matrix to form
a bulk-
heterojunction structure. Similar to other inorganic
polycrystalline solar cells, such as silicon, cadmium telluride
(CdTe), and copper indium gallium selenide (CIGS) solar cells,37
passivating the dangling bonds in the polycrystalline solar cells,
which cause a large density of ‘surface states’ in the band gap,
is one of the essential methods to minimize the charge
recombination at the perovskite material surface.38 Therefore,
the unique morphology changes induced by the addition of the
1D PCBM nanorods is expected to have a positive impact on
the performance of perovskite solar cells. We then used a
simple one-step method to fabricate the CH3NH3PbI3
perovskite solar cells (see Experimental Section for details of
the device fabrication). Fig. 4c schematically shows the device
structure for a typical perovskite solar cell developed in this
study.
above experimental observations, we envision that
continuous network of the 1D PCBM nanorods forms at the
optimum 1D PCBM content (i.e., 240 g/mL) via a percolation
a
µ
threshold process, and that further increase in the 1D PCBM
content above the percolation threshold does not lead to any
obvious further increase in the pathways for charge
transportation, but a significant reduction in the active
perovskite material and its associated light absorbance, along
with a concomitant morphology change of the perovskite
phase (cf. Fig. 3e).
By introducing an increasing amount of the 1D PCBM
nanorods into the light absorb layer within the perovskite solar
cells (cf. Fig. 4c), we observed an enhancement in the
photovoltaic performance with respect to its counterpart
without any additive. The mixed light absorb layers with
different amounts of the 1D PCBM nanorods were prepared by
dispersing different amounts of the 1D PCBM solution (0-200
Fig. 5a shows J−V curve of the best performing cell obtained
by further optimizing the CH3NH3PbI3 perovskite solar cell with
240 µg/mL of the 1D PCBM nanorods, which exhibited a high
JSC of 22.88 mA/cm2, a VOC of 0.90 V, and a FF of 0.743,
resulting in a PCE of 15.30%. Furthermore, this device showed
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Table 1. Performance parameters of perovskite solar cells with different 1D PCBM additions.a
1D PCBM
concentration
(μg/mL)
VOC (V)
JSC (mA/cm2)
FF
PCE (%)
0
48
0.74±0.01
0.76±0.01
0.85±0.01
0.89±0.01
0.81±0.02
0.78±0.02
0.87±0.02
22.49±0.18
22.98±0.20
21.80±0.22
22.43±0.24
22.02±0.28
21.51±0.27
18.93±0.20
0.57±0.01
0.61±0.02
0.66±0.02
0.74±0.01
0.68±0.02
0.69±0.02
0.74±0.01
9.54±0.29
10.55±0.34
12.28±0.29
14.34±0.38
12.18±0.27
11.48±0.31
12.59±0.22
96
240
480
960
240(PCBM)
a Average and standard deviation values were obtained based on 8 cells for each of the data.
Fig. 5 (a) J-V curves for the best performing device. (b) J-V curves
obtained by different bias scan direction: from forward to reverse
(blue one) and from reverse to forward (green one). (c) IPCE spectrum
of the best performing device. (d) J-V curves of the device using 1D
PCBM (blue one) and PCBM (red one) as the additives, respectively.
Fig. 6 (a) JSC, (b) VOC, (c) FF and (d) PCE variations of the perovskite
solar cells fabricated with and without 1D PCBM nanorod additives
under continuous simulated sunlight irradiation.
The stability of the perovskite solar cells under device
working conditions is one of the most important issues needs
to be tested for practical applications.42 In this regard, we have
also characterized the photostability of un-encapsulated
perovskite solar cells fabricated with and without the addition
of 1D PCBM nanorods under continuous simulated sunlight
irradiation in air at room temperature (room humidity:
30±1%RH). Since Ca/Al cathode is not stable in air, we used Ag
as the cathode for testing the photostability of the perovskite
solar cells in air, though the performance using Ag cathode
decreased a little compare with Ca/Al cathode (Figure S9). As
can be seen in Fig. 6, the photostability of the perovskite solar
cell with 1D PCBM nanorods showed an enhanced stability
negligible hysteresis (Fig. 5b) as the fullerene based materials
could effectively passivate charge traps in perovskite materials
to eliminate the notorious photocurrent hysteresis.38 The
photon-to-current efficiency (IPCE) was high over the whole
visible-light range with a maximum value close to 85% around
490 nm (Fig. 5c), and the JSC value integrated from the IPCE
spectrum was found to be in good agreement with that
measured from the J–V curve (i.e., 22.88 mA/cm2).
We also compared the performance of the best device to its
counterpart with the pristine PCBM powder as an additive at
the same concentration (240 μg/mL) in Table 1. As shown in
Fig. 5d, the best performing device using the pristine PCBM
powder as the additive showed a JSC of 19.04 mA/cm2, a VOC of
0.88 V, a FF of 0.75, and a PCE of 12.57%. Most of these values
are better than those of the control device without any PCBM
(cf. Fig. S7), but worse than the corresponding values for the
perovskite solar cells using optimized content of 1D PCBM
nanorods as the additive, indicting, once again, that the
addition of the 1D PCBM nanorods with an increased grain size
and unique continuous structure could significantly improve
the solar cell performance.
over 3600
s continuous simulated sunlight irradiation
compared with its counterpart without any additives. Fig. 6a, c
indicates that the improved stability is mainly owing to the
reduced decay of JSC and FF during the test while the VOC didn’t
show any decay (Fig. 6b) for perovskite solar cells either with
or without the 1D PCBM additive.
Previous studies have indicated that water permeates the
perovskite crystal along the grain boundaries to cause gradual
degradation of the film.43, 44 Thus, it is reasonable for us to
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study is caused by the reduced grain boundaries upon the
introduction of the 1D PCBM nanorods. It has also been
reported that the photostability of certain organic
fluorescence semiconductors can be improved by carbon-
based additives, such as fullerenes,45 graphene or graphene
composites,46,47 and carbon nanotubes,48 through the efficient
photo-charge transfer effect, as also indicated by the PL
quenching associated with the addition of the 1D PCBM
nanorods into the CH3NH3PbI3 perovskite film in this study (Fig.
2d). Besides, the above-mentioned strong binding capability of
PCBM to iodide-rich defect sites, which passivates the
interface of the perovskite grains and hence the associated
moisture adsorption, may have also contributed to the
improved photostability of our perovskite solar cells.
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improved CH3NH3PbI3 perovskite solar cell performance by
using 1D PCBM nanorods as additive. It was found that the
presence of scroll-like 1D PCBM nanorods can effectively
enlarge the grain size of the perovskite film and form an
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bulk-heterojunction structure within
a
bicontinuous
morphology, which provides an efficient charge transportation
pathway for charge carriers. As a result, a PCE as high as 15.3%
and much improved device working stability have been
obtained for the CH3NH3PbI3 perovskite solar cell with an
optimum amount of the 1D PCBM nanorods (240 μg/mL, Fig.
6). This work provides a new and efficient approach for
enhancing performance of perovskite solar cells. The
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Acknowledgements
This work has been financially supported by 111 Program (No.
B14040), Natural Science Foundation of China (Grant No.
51572216 and 61176056) and the NSFC Major Research Plan
on Nanomanufacturing (Grant No. 91323303). The authors
gratefully acknowledge financial support from the industrial
science and technology research project in shaanxi province
(2015GY005) and the open projects from Institute of Photonics
and Photo-Technology, Provincial Key Laboratory of
Photoelectronic Technology, Northwest University, China. The
authors also sincerely appreciate the support from the China
Scholarship Council (CSC).
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