1
DOI: 10.1002/(( ))
Article type: Communication
Isomer-pure bis-PCBM assisted crystal engineering of perovskite solar cells showing
excellent efficiency and stability
Fei Zhang, Wenda Shi, Jingshan Luo, Norman Pellet , Chenyi Yi, Xiong Li, Xiaoming Zhao, T.
John. S. Dennis*, Xianggao Li, Shirong Wang*, Shaik Mohammed Zakeeruddin, Dongqin Bi*,
Michael Grätzel*
F. Zhang, X. Zhao, Prof. X. Li, Prof. S. Wang*
School of Chemical Engineering and Technology, Tianjin University
300072 Tianjin, China
Email: wangshirong@tju.edu.cn
F. Zhang, Dr. J. Luo, N.Pellet, Dr. C. Yi,Dr. X. Li, Dr. S. M. Zakeeruddin, Dr. D. Bi *, Prof.
M. Grätzel*
Laboratory of Photonics and Interfaces, Institute of Chemical Sciences and Engineering,
École Polytechnique Fédérale de Lausanne (EPFL)
Station 6 CH-1015, Lausanne, Switzerland
Email: dongqin.bi@epfl.ch ; michael.graetzel@epfl.ch
F. Zhang, X. Zhao, Prof. X. G. Li, Prof. S. Wang*
Collaborative Innovation Center of Chemical Science and Engineering(Tianjin)
300072 Tianjin, China
W. Shi, X. Zhao, Dr. T. J. S. Dennis*
School of Physics and Astronomy, Queen Mary University of London,
327 Mile End RoadLondon, E1 4NS, UK
Email:j.dennis@qmul.ac.uk
Keywords: stability; PCBM; perovskite solar cell;bis-PCBM
Abstract: We prepared the novel fullerene derivative (a-bis-PCBM) by separating it from the
as-produced bis- phenyl-C
61
-butyric acid methyl (bis-[60] PCBM) ester isomer mixture using
preparative peak-recycling high performance liquid chromatography (HPLC). We employed
the compound as a templating agent for the solution processing of metal halide perovskite
films by the antisolvent method. Perovskite solar cells (PSCs) containing a-bis-PCBM
perovskite achieve better stability, efficiency, and reproducibility compared with those
employing traditional PCBM. The a-bis-PCBM can fill the vacancies and grain boundaries of
the perovskite film, enhancing the crystallization of perovskites and addressing the issue of
2
slow electron extraction. In addition, it can also resist the ingression of moisture, protect the
interfaces from chemical erosion, and passivate the voids or pinholes generated in the hole-
transporting layer. As a result, we obtain an outstanding power conversion efficiency (PCE)
of 20.8 % compared with 19.9 % by PCBM, accompanied by excellent stability under heat
and simulated sunlight,. The PCE of unsealed devcies dropped by less than 10% in ambient
air (40% RH) after 44 days at 65 ℃ and by 4% after 600 h under continuous full sun
illumination and maximum power point tracking respectively.
Hybrid organic-inorganic lead halide perovskite solar cells (PSCs) have emerged as a
promising candidate for the next generation photovoltaic technology due to their low
manufacturing cost and high performance.
1−6
Through judicious manipulation of perovskite
morphology and improvement of interfacial properties,
2−6
PSCs have reached a certified
power conversion efficiency (PCE) up to 22.1%.
7
Generally, the PSCs with the best
performance employ a sandwich configuration, composed of a layer of TiO
2
electron selective
contact, which is infiltrated by the intrinsic perovskite light harvester, followed by a layer of
hole transport material (HTM) as p-type contact and a metal back contact.
8
Despite of these stunning advances, several challenges still remain before PSCs become
a competitive commercial technology, one crucial issue being the device stability.
9−11
Uncontrolled film morphology associated with poor crystallity of the perovskites results in
low efficiency and poor reproducibility of the device performance.
12
Previous studies have
indicated that the degradation of PSCs is primarily governed by the ingress of atmospheric
oxygen and water vapor into the film upon exposure to air, which in turn causes undesired
reactions with the active materials.
13,14
Various methods have been tried to modify the
morphology of perovskite films aiming to improve the stability, for example, poly(methyl
methacrylate) (PMMA) was used as a template to control nucleation and crystal growth,
resulting in considerable increase in both the device efficiency and stability when kept under
3
dry condition in the dark.
15
Other studies use additives like 1-methyl-3-(1H,1H,2H,2H-
nonafluorohexyl)-imidazolium iodide,
16
or phenyl-C
61
-butyric acid methyl ester (PCBM)
17-19
or alkali metal ions
20
in the pervokite precursor solution film to improve film formation and
increase the environmental stability. However, very few investigations achieve stability under
both prolonged heat and light soaking stress with PSCs exhibiting a high PCE and good
moisture resistance.
21
A bis-analogue of PCBM, bis-PCBM, was successfully utilized in polymer solar cells to
improve the open-circuit voltage (V
oc
) over simple PCBM.
22,23
However, the increase in PCE
is not as high as expected from the increase in V
OC,
which is because that bis-PCBM exists as
a mixture of 19 structural isomers, which leads to morphological and energetic disorder in the
active layer of photovoltaic devices, resulting in a degrading effect on the J
sc
24
. The disorders
induced by the isomer mixture may be removed through fabricating devices from isomer-pure
samples. Hence, using isomer-pure samples may potentially lead to both higher voltage and
higher current in the polymer solar cells, compared to the isomer mixture.
25
Herein, we report on the separation isolation of the pure a-bis-PCBM isomer from the
reaction mixture of isomers by preparative peak-recycling high performance liquid
chromatography (HPLC). We employ it as a templating agent in the chlorobenzene
antisolvent to enhance the electronic quality and PV performance of solution processed
perovskite films. Our results show that the a-bis-PCBM leads to (i) enlargement of the
perovskite grain size, (ii) passivation of the interface trap states at the grain boundaries, and
(iii) improvement of the charge carrier separation and transportation within the perovskite
film.
26-28
As a result, the newly developed PSCs using a-bis-PCBM as an additive in the
antisolvent to template the nucleation and growth of the PSC attain a PCE of 20.8 %,
compared with 18.8 % for the PCBM-free reference and 19.9 % for PCBM itself- Importantly
this PCE increase comes along with enhanced stability under heat and illumination.
4
Figure 1 shows a schematic illustration of the a-bis-PCBM or PCBM-assisted growth
process for the perovskite- a-bis-PCBM or PCBM layer. We produce the mixed cation
perovskite [(FAI)
0.81
(PbI
2
)
0.85
(MABr)
0.15
(PbBr
2
)
0.15
] films in a single step from a solution of
formamidinium iodide (FAI), PbI
2
, methylammonium bromide (MABr) and PbBr
2
in a mixed
solvent of dimethyl formamide (DMF) and dimethyl sulfoxide (DMSO). The perovskite spin-
coating procedure employed 2000 rpm for 10 s followed by 6000 rpm for 30 s. During the last
15 s of second spin coating step 100 ml of a-bis-PCBM containing chlorobenzene (CB) was
dropped onto the above film to template the nucleation and growth of the perovskite crystals.
Figure S2 shows the scanning electron microscopy (SEM) images of corresponding
perovskite films deposited on m-TiO
2
/c-TiO
2
/FTO substrate. As illustrated from the top-view
SEM, the grain size of the perovskite film increases with the use of PCBM / a-bis-PCBM
and directs most of the grain boundaries to assume a perpendicular orientation to the substrate.
Hence, the PCBM or a-bis-PCBM is expected to uniformly trigger heterogeneous nucleation
over the perovskite precursor film, improving the grain size and facilitating the perovskite to
grow in preferred direction. Reflections of facets with (111) indices become dominant
because of the low symmetry of the trigonal perovskite (p3m1) phase as indicated in the XRD
pattern in Figure 2a.
15
Whereas the cross-sectional SEM of the reference film reveals
numerous grain boundaries, very few appear in the PCBM or a-bis-PCBM-containing film.
The high-angle annular dark-field scanning transmission electron microscopy (HAADF-
STEM) indicates that the a-bis-PCBM can fill in the vacancies and grain boundaries of the
perovskite film(Figure S3).We also measured the contact angle between the corresponding
films and CB and present the results in Figure S4. The angle of a CB droplet on the a-bis-
PCBM containing film was 61.70°, while for PCBM and the controls, the values were
69.40°and 73.46°, respectively, indicating a better wetting of the a-bis-PCBM containing
perovskite by the CB compared to the other two samples.
11
5
To further examine the crystal structure of the perovskite films, we conducted thin film
X-ray diffraction (XRD) measurements for deposited on m-TiO
2
/c-TiO
2
/FTO substrates
(Figure 2a). All the samples show the same trigonal perovskite phase with the dominant
(111) lattice reflection. The peak at 12.5°arises from the (001) lattice planes of PbI
2
. The
excess PbI
2
is believed to passivate surface defects, increasing the solar cell performance.
29,30
By taking the full width at half maximum (FWHM) of the (111) reflection, we calculate the
crystallite size using Scherrer's equation. Their size increases from 38 nm to 49 nm and 67 nm
for the control, PCBM and a-bis-PCBM respectively. We attribute the larger crystal sizes to
the templating effect of PCBM or a-bis-PCBM on the crystal growth. These observations
indicate that a-bis-PCBM -containing perovskite film has a higher crystal quality. We ascribe
the high-quality crystallization as one of the main factors for improved device performance.
31-
33
We exposed unsealed films of pristine, PCBM and a-bis-PCBM - containing perovskite
film to ambient environments, and periodically recorded their film X-ray diffraction patterns.
The decomposition of perovskite in moist air is known to lead to the formation of PbI
2
phase.
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In Figure 2a, the ratio of PbI
2
(12.5°)/ perovskite (13.8°) of pristine control perovskite
increases faster than that of PCBM-containing and a-bis-PCBM -containing perovskite film
after 40 days. The a-bis-PCBM-containing perovskite film turned out to be the most stable
one in this test environment, which can also be seen from the Figure 2b. The color of a-bis-
PCBM -containing perovskite film was persistent whereas that of the PCBM-containing and
pristine control perovskite film faded from black to yellow. Figure 2c shows measurements
of the contact angle formed by a deionized water droplet with the pristine control, PCBM and
a-bis-PCBM -containing perovskite film. The derived contact angles are 71.56°, 63.30° and
47.10° for the a-bis-PCBM, PCBM-containing film and the pristine control respectively. This
trend reflects a strong increase in hydrophobicity of the perovskite upon incorporation of the
