A dimeric fullerene derivative for efficient inverted planar perovskite solar cells with improved stability

Article abstract of DOI:10.1039/c7ta00362e

Fullerene derivatives can efficiently passivate the interfacial defects of perovskite layers to improve the performance of perovskite solar cells. In this work, a new dimeric fullerene derivative (D-C₆₀) with two [6,6]-phenyl-C₆₁-butyric acid methyl ester (PC₆₁BM) units was designed, synthesized and applied as the electron transporting material (ETM) in perovskite solar cells (PSCs) taking advantage of its appropriate energy levels, relatively fast electron mobility, and easy solution processability compared to the widely used PC₆₁BM, D-C₆₀ can efficiently passivate the trap states between the perovskite and fullerene layers, leading to improved electron extraction and overall photovoltaic performance. Devices based on D-C₆₀ as the ETM achieved power conversion efficiencies (PCEs) of 16.6%, which is significantly higher than that observed with PC₆₁BM (14.7%). In addition, the more hydrophobic and compact D-C₆₀ layer resulted in higher device stability than that with PC₆₁BM. These results show that covalently linked dimeric fullerene derivative can act as efficient electron transporting materials (ETMs) for high performance PSCs.

Full text of DOI:10.1039/c7ta00362e

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Kochiss, E. Castro, G. Betancourt-Solis, H. Han and L. Echegoyen, J. Mater. Chem. A, 2017, DOI:
1
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Volume
4
Number
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January 2016 Pages 1–330
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ISSN 2050-7488
PAPER
Kun Chang, Zhaorong Chang et al.
Bubble-template-assisted synthesis of hollow fullerene-like
MoS nanocages as a lithium ion battery anode material
2
rsc.li/materials-a

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A dimeric fullerene derivative for efficient inverted planar perovskite solar cells with
improved stability
†
‡
†
†
§
Chengbo Tian, Kevin Kochiss, Edison Castro, German Betancourt-Solis, Hongwei Han, and
Luis Echegoyen.*†
†
Department of Chemistry, University of Texas at El Paso, 500 West University Avenue, El Paso,
TX 79968, United States
§
Michael Grätzel Center for Mesoscopic Solar Cells, Wuhan National Laboratory for
Optoelectronics, School of Optical and Electronic Information, Huazhong University of Science
and Technology, Wuhan 430074, Hubei, People’s Republic of China
‡
Department of Chemistry and Biochemistry, Center for Polymers and Organic Solids, University
of California, Santa Barbara, CA 93106, United States
*
E-mail: echegoyen@utep.edu
Abstract
Fullerene derivatives can efficiently passivate the interfacial defects of perovskite layers to
improve the performance of perovskite solar cells. In this work, a new dimeric fullerene derivative
(D-C60) with two [6,6]-Phenyl-C61-butyric acid methyl ester (PC61BM) units was designed,
synthesized and applied as the electron transporting material (ETM) in perovskite solar cells (PSCs)
taking advantage of its appropriate energy levels, relatively fast electron mobility, easy solution
processability. Compared to the widely used PC61BM, D-C60 can efficiently passivate the trap
1

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states between the perovskite and fullerene layers, leading to improved electron extraction and
overall photovoltaic performance. Devices based on D-C60 as the ETM achieved power conversion
efficiencies (PCEs) of 16.6%, which is significantly higher than that observed with PC61BM
(14.7%). In addition, the more hydrophobic and compact D-C60 layer resulted in higher device
stability than that with PC61BM. These results show that covalently linked dimeric fullerene
derivative can act as efficient electron transporting materials (ETMs) for high performance PSCs.
1
. Introduction
The unique properties of organic-inorganic hybrid perovskite materials (typically CH
such as high light absorption coefficients,1,2 long carrier diffusion lengths,3-5 high electron/hole
3
NH
3
PbI ),
3
6
7
mobilities, simple device architectures, low-temperature fabrication processes, compatibility
8
,9
10
with flexible substrates, negligible hysteric behavior, and efficiencies comparable to those of
1
1
commercial standards, make them promising candidates for the development of the next-
generation photovoltaic solar cells for commercial applications. Currently, PSCs are the most
rapidly developing class, with certified efficiencies rising from 3.8% in 2009 to 22.1% in 2016.1,12
In inverted planar PSCs, the perovskite layer is surrounded by an electron transporting layer (ETL)
and a hole transporting layer (HTL). The HTL, PEDOT:PSS or NiOx,8,13 is connected to the anode,
ITO or FTO, while the ETL is typically a fullerene derivative that is direct contact with the metal
cathode, Al.
PC61BM is the most widely used fullerene derivative in photovoltaic applications, and has become
the standard in the field, due to its efficient electron transporting and solution processable
properties.14,15
2

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In 2013, Jeng and co-workers reported a PCE of 3.9% for inverted planar PSCs using PC61BM as
the ETM. The reasonable efficiency of this type of devices demonstrated that fullerene derivatives
were promising ETMs in PSCs.16 Recently, Nie and Im’s group reported a device with an
efficiency of 18% by developing a new perovskite deposition technique.17,18 Huang’s group
recently obtained a remarkable efficiency, as high as 19.4%, which is comparable to those of
mesoporous perovskite solar cells, by means of a simple solvent annealing process to improve the
surface of the perovskite, resulting in a better electron extraction into the fullerene layer.19
To explore the effect of functional groups on the performance of fullerene-based PSCs, double
fullerene layers or fullerene derivatives with different functional groups have been studied.20-27
However, up to now, all of the fullerene derivatives reported in PSCs are monomeric derivatives
and to the best of our knowledge, dimeric fullerene derivatives as ETMs in PSCs have never been
explored. There is a limited number of dimeric fullerene derivatives that were investigated in
polymer solar cells.28,29 Due to the excellent performance of PC61BM-based PSCs, a dimeric
fullerene with two PC61BM units was expected to show improved performance in PSCs.
30
Herein, we report the synthesis, characterization and photovoltaic properties of a fullerene dimer
derivative (D-C60), the structure of which is shown in Scheme 1. Due to suitable energy levels and
adequate electron mobility of D-C60, the best efficiency obtained from a device based on this
compound was 16.6%. This value was higher than that for the analogous PC61BM based PSCs,
1
4.7%. Meanwhile, the higher hydrophobicity of D-C60 compared to that of PC61BM resulted in
better device stability.
3

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2
. Experimental and methods
Materials Synthesis
6,6]-phenyl-C61-butyric acid (PC61BA) was synthesized by following previously reported
[
3
1
procedures. The synthetic procedure followed to prepare (D-C60) is presented in Scheme 1. EDC
(
25.6 mg, 0.13 mmol) was added slowly to a pre-cooled solution of 2,2-diethyl-1,3-propanediol
6.6 mg, 0.05 mmol), PC61BA (100.0 mg, 0.11 mmol), and a small amount of DMAP in 40 mL of
(
dry dichloromethane. The mixture was stirred for 36 h at room temperature. After drying under
vacuum, the residual mixture was purified by silica gel column chromatography (toluene as eluent).
1
D-C60 was precipitated from methanol as a dark-brown solid. H NMR (CDCl
3
, 400 MHz) (Figure
S1): δ (ppm) 0.88 (6H, t, J=5.16 Hz), 1.42 (4H, m), 2.19 (4H, m), 2.53 (4H, t, J=7.52 Hz), 2.91
(4H, m),3.68 (4H, s), 7.47 (2H, t, J=7.32 Hz), 7.55 (4H, t, J=7.46 Hz), 7.93 (4H, d, J=7.52 Hz).
13C NMR (CDCl
, 100 MHz) (Figure S2): δ (ppm) 173.48, 148.80, 147.81, 145.85, 145.19, 145.15,
3
1
1
7
45.08, 145.03, 144.79, 144.66, 144.50, 144.42, 144.01, 143.76, 143.12, 143.03, 142.99, 142.93,
42.23, 142.18, 142.13, 142.11, 140.99, 140.75, 138.03, 137.57, 136.73, 132.09, 128.44, 128.24,
9.86, 51.85, 51.68, 33.88, 33.67, 29.70, 22.37. MALDI-TOF-MS m/z: calcd 1888.3, found 1888.6.
Device Fabrication
3
2
Methylammonium iodide (CH
3
NH I) was prepared using a previously reported procedure. PSCs
3
with a configuration of FTO/PEDOT:PSS/CH
3
NH
3
PbI
3
/PC61BM (or D-C60)/Al were fabricated on
2
FTO-coated glass substrates with a resistivity of 10 Ω/cm . The patterned FTO glass substrates
were cleaned sequentially with detergent, deionized water, and ethanol, each step for 15 min dried
with nitrogen gas and finally treated in a UV-ozone oven for 20 min. After passing through a 0.45
μm PVDF filter, the PEDOT:PSS solution (Baytron P VP AI 4083) was spin-coated onto the
4

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o
treated FTO substrates at 5000 rpm for 30 s, and heated at 150 C for 15 min in air. Then a solution
of 1 M PbI
2
in DMF was spin-coated at 3000 rpm for 30 s on top of the PEDOT:PSS coated
o
substrates, and dried on a hot plate at 70 C for 10 min. The FTO/PEDOT:PSS/PbI
2
substrates
were then transferred to a vacuum oven to be converted to CH
3
NH
3
PbI
3
by means of a vapor-
assisted gas–solid crystallization process following previously reported methods.27,33 After the
CH
dissolved in chlorobenzene was spin-coated onto the CH
complete the devices, Aluminum electrodes (100 nm) were deposited by thermal evaporation
3
NH
3
PbI
3
films were formed and then cooled to room temperature, 2 wt% PC61BM (or D-C60
)
3
NH PbI layer at 1200 rpm for 30 s. To
3
3
-6
under a pressure of 2×10 Torr through a shadow mask. The active area of the devices was
determined by a mask with an aperture in the middle (7 mm2).
Device characterization
J–V characteristics of photovoltaic cells were tested using a Keithley 2420 source measure unit
under a Photo Emission Tech SS100 Solar Simulator, and the light intensity was calibrated by a
standard Si solar cell. The external quantum efficiencies (EQEs) were measured using a Bentham
(from Bentham Instruments Ltd) measurement system. The light intensity was calibrated using a
single-crystal Si photovoltaic cell as the reference. The J-V and EQE measurements were carried
out in air. The SEM images were collected using a ZEISS Sigma FE-SEM, where the electron
beam was accelerated in the range of 500 V to 30 kV. Film thicknesses were measured using a
KLA Tencor profilometer. The contact angles of water were determined using a Rame-Hart model
́
2
50 goniometer using pure deionized water at room temperature at a constant volume of 5 μL. A
total of ten static measurements were analyzed and averaged for each ETM layer. The steady
photoluminescence (PL) spectra were obtained using an Edinburgh instruments FLS980
fluorescence spectrometer.
5

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3
. Results and Discussion
The synthetic approach to prepare D-C60 is presented in Scheme 1. In this approach the two
fullerenes are linked by an alkyl bridge between the ester functional groups on the PC61BMs. D-
C
60 exhibits a slightly higher lowest unoccupied molecular orbital (LUMO) energy level than the
3
0
mono-adduct fullerene derivatives, making it a good candidate to act as the ETM in PSCs. 2,2-
diethyl-1,3-propanediol was used to connect the two PC61BM units in order to increase the
solubility of the dimer and to result in a homogeneous and compact film, with high hydrophobicity
to improve water inclusion and enhance the device stability.
PC61BM (99%) was bought from Nano-C. PC61BA was synthesized heating PC61BM and
hydrochloric acid under reflux in toluene for 12 h. D-C60 was obtained from the reaction of
PC61BA with 2,2-diethyl-1,3-propanediol in anhydrous dichloromethane. The final product was
1
13
characterized by MALDI-TOF mass spectrometry, H and C NMR (see supporting information).
Due to the flexible connecting group, D-C60 exhibits excellent solubility in normal organic solvents
such as dichloromethane, chloroform, and chlorobenzene, which makes it suitable for solution-
processed PSCs.
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Scheme 1. The synthetic route for the synthesis of D-C60
.
The energy levels of fullerene derivatives are important to assess their electron extraction and hole
blocking properties in PSCs. The electrochemical properties of PC61BM and D-C60 were studied
+
via cyclic voltammetry (CV) in the range of 0 to -2.0 V vs. Fc/Fc . Both exhibit three independent
well-defined reversible reduction waves (see Figure 1a and Figure S3 ). The half-wave potentials
for the reduction processes are listed in Table S1. The LUMO energy levels of the compounds
표푛
were calculated from their onset reduction potentials (퐸푟푒푑) using the equation; LUMO energy
level = -e (퐸 + 4.80) (eV). 34 These values are listed in Table 1. The LUMO energy levels were
표푛
푟푒푑
estimated at -3.88 and -3.90 eV for D-C60 and PC61BM respectively, which are similar to previous
2
9
reports. These values indicate that both compounds can be used as efficient ETMs in PSCs.
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Based on their maximum absorption onsets, as shown in Figure 1b and Table 1, and the values of
the LUMO energy levels, D-C60 and PC61BM were estimated to have HOMO values of -5.61 and
-5.63 eV, respectively, deep enough to block holes.
Figure 1. a) Cyclic voltammetric scans for D-C60 and PC61BM at a scan rate of 100 mV/s. b) UV-
Vis absorption spectra of D-C60 and PC61BM in toluene solution.
Table 1. Electrochemical and photo-physical data of D-C60 and PC61BM
풐풏
Compound
D-C60
λ
abs (nm)
E
g
(eV)
푬
(V) LUMO (eV) HOMO (eV)
풓풆풅
717
1.73
1.73
0.92
0.90
-3.88
-3.90
-5.61
-5.63
PC61BM
718
For PSC devices, photovoltaic performance is heavily influenced by the carrier mobilities of the
charge transporting layers. High electron mobility of the fullerene derivatives improves the short-
circuit current density (Jsc) and thus the PCEs. The electron mobility of D-C60 and PC61BM were
8

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measured by the space-charge limited current (SCLC) method (Figure S4) and the Mott–Gurney
3
5
law, based on electron-only devices with the structure ITO/Cs
2
CO /fullerene derivative/Ca/Al,
3
-
4
-4
2
-1 -1
and found to be 9.83×10 and 6.85×10 cm V s , respectively. The higher electron mobility of
D-C60 suggests that the fullerene cages are better packed for D-C60 than for PC61BM. The higher
electron mobility of D-C60 leads to a higher short-circuit current density and PCE than that of
PC61BM-based PSC devices.
To test the performance of D-C60 as the ETM in PSCs, planar structure devices were prepared, as
shown in Figure 2a, with a configuration: FTO/PEDOT:PSS/CH
3
NH
3
PbI /PC61BM (or D-C60)/Al.
3
The energy-level alignment of all materials involved in the device is shown in Figure 2b. The
perovskite layer of the device was prepared by a vapor-assisted gas–solid crystallization process,
3
3
previous reported. After this process, the perovskite layer on PEDOT:PSS, as shown in Figure
c, exhibits homogeneous and well packed grains with no pinholes. As shown in the cross-section
2
of the device (Figure 2d), excellent contact results between the perovskite and the fullerene layers.
The thickness of each layer was measured using a profilometer, showing that the PEDOT:PSS
layer, perovskite layer and fullerene derivative layer are around 40 nm, 350 nm and 60 nm,
respectively.
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Figure 2. (a) Configuration scheme of the inverted structure planar perovskite solar cells, (b)
Energy level diagram of the materials used in PSCs, (c) Top-view SEM image of the perovskite
film, and (d) Cross-sectional SEM image of the device FTO/PEDOT:PSS/perovskite/fullerene
derivative film.
The current-voltage (J–V) curves of PSCs with PC61BM and D-C60 as the ETMs were obtained
-2
under AM 1.5G irradiation (100 mW cm ) in air. The J-V curves of the devices are shown in
Figure 3a and the key photovoltaic performance parameters Voc, Jsc, FF, and PCE are listed in
Table 2. The most efficient device using D-C60 as ETM yielded a PCE of 16.6%. The efficiencies
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are significantly higher than those of devices based on PC61BM as ETM, which resulted in an
optimal PCE of 14.7%.
In order to better compare the device performances, a statistical histogram of the efficiencies of 20
individual devices using PC61BM and D-C60 as the ETMs are shown in Figure 3c. The average
efficiencies for D-C60 and PC61BM based devices are 16.1±0.5% and 13.9±0.7%, respectively.
The narrower distribution of the key photovoltaic parameters illustrate that D-C60 is an excellent
ETM for inverted planar structure PSCs.
The higher Jsc values of the D-C60 based devices were verified by the incident photon-to-electron
conversion efficiency (IPCE) spectrum, as shown in Figure 3b. The integrated current densities
from the IPCE curve for devices involving D-C60 and PC61BM as the ETMs are 21.5 and 20.4 mA
-2
cm , respectively, which are consistent with the values from the J-V curves. Furthermore, the
higher Jsc and FF of D-C60 based devices can be related to the higher electron mobility and electron
extraction ability.
1
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Figure 3. Device performance and reproducibility of perovskite solar cells fabricated by using D-
60 and PC61BM as ETMs. a) J–V curves of devices using D-C60 and PC61BM. (b) EQE spectra
C
for perovskite solar cells fabricated by using D-C60 and PC61BM as ETMs. (c) Histograms of
device PCE measured for 20 individual devices based on PC61BM and D-C60 ETMs. d) Maximal
steady-state photocurrent output at the maximum power point for D-C60 based device at 0.82 V
and its corresponding power output.
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2

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Table 2. Summary of device performance analysis in Figure 3. The calculated Jsc values are
obtained from the EQE curves.
J
sc
Calculated Jsc
Type
Voc (V)
FF (%)
PCE (%)
2
2
(
mA/cm )
(mA/cm )
PC61BM
D-C60
0.94
0.96
20.96
20.4
21.5
74.4
78.8
14.7
16.6
21.89
The maximal steady-state power and photocurrent outputs were used to further verify device
-2
efficiencies. As shown in Figure 3d, D-C60 based devices resulting in 20.1 mA cm at 0.82 V had
-2
PCEs of 16.5%. PC61BM based devices resulting in 18.2 mA cm at 0.80 V, had PCEs of 14.5%
Figure S5). Both efficiencies are very similar to the values determined from the J-V data.
(
It is worth noting that the D-C60 based devices exhibited a slightly higher Voc than those of PC61BM
based devices, which can be reasonably attributed to the slightly higher LUMO energy level of D-
C
60.36-38 The hysteresis effects of the devices were also studied. As shown in Figure S6, no obvious
J-V curve differences were observed, regardless of the scan direction for the D-C60 based devices.
The negligible hysteresis effect in the PSCs with D-C60 as the ETMs further indicate that the
fullerene derivatives effectively passivate the charge traps at the fullerene/perovskite interface.10,39
To investigate the electron extraction ability of D-C60 as the ETM in PSCs, steady state and time-
resolved photoluminescence (PL, TRPL) spectra were recorded. Figure 4a shows the steady-state
PL spectra of perovskite, perovskite/PC61BM and perovskite/D-C60 thin film substrates. When the
perovskite layer forms a contact with PC61BM or D-C60, a significant PL quenching effect was
observed. Notably, devices based on D-C60 show more efficient PL quenching than those based on
PC61BM, which indicates that D-C60 more efficiently inhibits electron-hole recombination.9
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Additionally, TRPL, was measured by monitoring the emission peak of the perovskite layer as a
function of time as shown in Figure 4b. Perovskite/D-C60 devices exhibit a shorter PL decay time
(11.1 ns) than that observed for the perovskite/PC61BM (14.0 ns) devices, which implies that
electrons are more efficiently transferred to D-C60 than to PC61BM. The faster decay is the result
of PL quenching due to electron transfer at the interface, which suppresses electron-hole
recombination and increases the Jsc and FF of the device.40
To probe charge transfer processes at the perovskite/D-C60 interface, electrical impedance
spectroscopy (EIS) measurements were conducted for both perovskite/D-C60 and
perovskite/PC61BM devices. The data were fit to a relatively simple equivalent circuit (Figure S7
and Table S2) consisting of a contact resistance between the fullerene/perovskite or
4
1
perovskite/PEDOT:PSS interface and the recombination resistance (Rrec). The Nyquist plots for
the perovskite/D-C60 devices show lower contact resistances and larger recombination resistances
than those for perovskite/PC61BM devices, thus D-C60 can decrease charge recombination more
efficient than PC61BM at the fullerene/perovskite interface and electron extraction into the ETL is
facilitated with D-C60. This must be associated with the differences of charge recombination
efficiencies at the perovskite/D-C60 or perovskite/PC61BM interface since the
4
2
PEDOT:PSS/perovskite interface is identical for both cases. This result is totally consistent with
the higher PL quenching observed for perovskite/D-C60 devices.
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Figure 4. (a) steady-state photoluminescence spectra, and (b) TRPL decay transient spectra of
FTO/perovskite/PC61BM and FTO/perovskite/D-C60
.
Assessment of the stability of devices with D-C60 and PC61BM as the ETMs was conducted under
a 50% humidity in air at room temperature without encapsulation. As shown in Figure 5b, the
devices with D-C60 retained more than 75% of its original performance after 10 days, while devices
with PC61BM as the ETM retained only 25% after 10 days. The higher stability of the D-C60-based
devices may result from the higher hydrophobicity of D-C60 as well as the more compact self-
assembled layer. As shown in Figure 5a, the hydrophobic properties of the different fullerene
derivatives were investigated. Water droplets on perovskite/PC61BM and perovskite/D-C60 layers
result in contact angles of 82° and 96°, respectively. These values indicate that the two C60 subunits
in D-C60 make the fullerene derivative more hydrophobic than PC61BM.
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Figure 5. (a) The images of the water droplet contact angles on different surfaces of fullerene
derivative film. (b) Normalized PCE of perovskite solar cells employing D-C60 and PC61BM ETMs
as a function of storage time in air.
4
. Conclusions
In summary, we synthesized a dimeric fullerene derivative D-C60 and applied it as an ETM for
PSCs taking advantage of its appropriate energy levels, relatively fast electron mobility, easy
solution processability and device stability. As evidenced by CV, SCLC, EIS, PL and TRPL
studies, D-C60 can effectively passivate the surface defects of the perovskite film, as well as
efficiently extract and transport electrons. As a result, using D-C60 as ETM, PSC devices reached
up to 16.6%, PCEs, higher than those of PC61BM based devices. Due to its higher hydrophobicity
D-C60, also enhanced the stability of the devices compared to PC61BM. We believe that dimeric
fullerene derivatives have the potential to become high performance alternatives for ETMs in PSCs.
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Acknowledgements
L.E. thanks the US National Science Foundation (NSF) for generous support of this work under
the NSF-PREM program (DMR 1205302) and the CHE-1408865. The Robert A. Welch
Foundation is also gratefully acknowledged for an endowed chair to L.E. (Grant AH-0033).
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Table of contents
Efficiency and stability are improved in inverted planar
perovskite solar cells by using D-C as electron transport
6
0
material