Highly thermal-stable perylene-bisimide small molecules as efficient electron-transport materials for perovskite solar cells

Article abstract of DOI:10.1039/d0tc04241b

Perylene-bisimide (PDI)-based small molecules (PDI-Ph, PDI-PhCN, PDI-PhCN-2Br and PDI-PhCN-4Br) were synthesized via imidization of perylene bisanhydride and core-bromided perylene bisanhydride. The physical, optical and electronic properties of these molecules were characterized by thermogravimetric analysis (TGA), UV-Vis, X-ray diffraction (XRD), cyclic voltammetry and space charge-limited current (SCLC). PDI-Ph, PDI-PhCN and PDI-PhCN-2Br show excellent thermal stability with decomposition temperatures above 400 °C (610 °C for PDI-PhCN) and high crystallinity with strong π-π stacking. The three molecules also exhibit high electron mobility with average values of 0.169 cm2 V-1 s-1 for PDI-Ph, 0.212 cm2 V-1 s-1 for PDI-PhCN and 0.119 cm2 V-1 s-1 for PDI-PhCN-2Br. Utilizing these molecules as the single electron-transporting layer (ETL), inverted perovskite solar cells with a configuration of ITO/NiOx/MAPbClxI3-x/ETL/Ag were fabricated. A power conversion efficiency of 14.6% was achieved from the device using PDI-PhCN as ETL. Furthermore, when BCP was used as the hole-blocking layer, the identical structured perovskite device achieved a high efficiency of 18.8% for the PDI-PhCN/BCP combination, which was better than the standard cell (17.4%) using C60/BCP as ETL. The superior performance of PDI-PhCN compared to PDI-Ph, PDI-PhCN-2Br and PDI-PhCN-4Br comes from its higher electron mobility and better matched energy levels with that of the absorber MAPbClxI3-x. Our results demonstrate that PDI-based small molecules are very promising electron-transporting materials for highly efficient, low-cost perovskite solar cells. This journal is

Full text of DOI:10.1039/d0tc04241b

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Materials for optical, magnetic and electronic devices
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DOI: 10.1039/D0TC04241B
Highly Thermal-Stable Perylene-Bisimide Small Molecules as Efficient
Electron-Transporting Materials for Perovskite Solar Cells
Weibo Yan, Zhaoxia He, Jingjing Jiang, Di Lu, Yuancai Gong, Wensheng Yang, Ruidong Xia, Wei Huang, Hao Xin*
Key Laboratory for Organic Electronics and Information Displays & Jiangsu Key Laboratory for Biosensors, Jiangsu National
Synergetic Innovation Center for Advanced Materials (SICAM), Nanjing University of Posts & Telecommunications, 9 Wenyuan
Road, Nanjing 210023, China. E-mail: iamhxin@njupt.edu.cn
Keywords: perylene-bisimide, electron-transporting material, thermal stability, mobility, perovskite solar cells
Abstract: Perylene-bisimide (PDI) based small molecules (PDI-Ph, PDI-PhCN, PDI-PhCN-2Br and PDI-PhCN-4Br) were
synthesized via imidization of perylene bisanhydride and core-bromided perylene bisanhydride. The physical, optical and
electronic properties of these molecules were characterized by thermogravimetric analysis (TGA), UV-Vis, X-ray diffraction
(XRD), cyclic voltammetry and space charge limited current (SCLC). PDI-Ph, PDI-PhCN and PDI-PhCN-2Br show excellent
o
o
thermal stability with decomposition temperature above 400 C (610 C for PDI-PhCN) and high crystallinity with strong π-π
stacking. The three molecules also exhibit high electron mobility with average value of 0.169 cm2V-1s-1 for PDI-Ph, 0.212 cm2V-
1s-1 for PDI-PhCN and 0.119 cm2V-1s-1 for PDI-PhCN-2Br. Utilizing these molecules as single electron-transporting layer (ETL),
inverted perovskite solar cells with a configuration of ITO/NiOx/MAPbClxI3-x/ETL/Ag were fabricated and power conversion
efficiency of 14.6% was achieved from device using PDI-PhCN as ETL. Further, when BCP was used as hole-blocking layer,
identical structured perovskite device achieved a high efficiency of 18.8% for PDI-PhCN/BCP combination, better than standard
cell (17.4%) using C60/BCP as ETL. The superior performance of PDI-PhCN than PDI-Ph, PDI-PhCN-2Br and PDI-PhCN-4Br
comes from its higher electron mobility and better matched energy levels with that of absorber MAPbClxI3-x. Our results
demonstrate PDI-based small molecules are very promising electron-transporting materials for highly efficient and low-cost
perovskite solar cells.
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1. Introduction
Perovskite solar cell (PSC) has undergone tremendous development since its first report with an efficiency of 3.8% in 20091 and
power conversion efficiency (PCE) greater than 25% has recently been achieved through optimization of device configuration2,3
and engineering of absorber crystallinity4-6 and interface property.7-10 Interface engineering has proved to be an effective strategy
to improve the performance of PSCs, which significantly improves carrier extraction and transportation from perovskite layer to
electrodes.7-11 One of the most effective interface engineering is design and selection of hole-transporting material (HTM) and
electron-transporting material (ETM).11,12 Current widely used and efficient ETMs of fullerene (C60)13 and [6,6]-phenyl-C61-
butyric acid methyl ester (PCBM)14 are very expensive, accounting for a large proportion of the cost of PSCs, and also sensitive
to oxygen, affecting the stability of PSCs. To solve the above deficiencies of fullerene ETMs, many non-fullerene substitutes
have been explored, however, satisfied ones are still very limited.15-18 The currently reported non-fullerene ETMs mainly include
18-23
a few polymers consisting of naphthalene-diimide (NDI) or perylene-diimide (PDI) units
and small molecules.24,25 While
polymers exhibit good solution processability, few of them have realized high efficiency without the presence of fullerene
derivatives.19,26 The best performing perovskite solar cell using polymer ETM was reported by Kim et al,27 in which a NDI-based
polymer with strong electron withdrawing dicyanothiophene was developed and used as ETM in device with a configuration of
ITO/PEDOT:PSS/MAPbI3−xClx/ETM/BCP/Ag cell and achieved a 17.0% efficiency. Compared to polymer ETMs, small-
molecule ETMs23 including naphthalene-diimide,24 perylene-diimide,25,28,29 pyrazine-azaacene derivatives,30,31 and fused-ring
electron acceptors,32,33 not only possess some intrinsic advantages, such as definite molecular structure, high purity, good batch to
batch repeatability and high mobility, but also demonstrate better performance than polymers.23 For example, the three
naphthalene-diimide
molecules
(N,N’-bis(phenylmethyl)naphthalene-1,4,5,8-tetracarboxylic
diimide,
N,N’-bis(1-
indanyl)naphthalene-1,4,5,8-tetracarboxylic diimide and N,N’-bis(1-phenylethyl) naphthalene-1,4,5,8-tetracarboxylic diimide)
reported by Kwon group24 were used as single ETM in device with configuration of ITO/PEDOT:PSS/FAPbI3−xBrx/ETM/Al and
achieved high efficiencies of 19.6%, 20.2% and 20.5%, respectively. An n-type sulfur-containing small molecule
hexaazatrinaphtho[2,3-c][1,2,5] thiadiazole reported by Zhang group31 resulted in 18.1% efficient inverted perovskite solar cell
when used as single ETM in device of ITO/ PEDOT:PSS/MAPbI3/ETM/LiF/Ag.
Among the small-molecule ETMs mentioned above, pyrazine-azaacene derivatives and fused-ring electron acceptors require
painstaking synthesis procedures and thus are not economic for large-scale application. Instead, naphthalene-diimide and
perylene-diimide based small molecules that use commercially available raw materials and take fewer synthetic procedures are
better choice. Although perylene-diimide small-molecules currently show inferior performance than naphthalene-diimide ones as
ETMs in PSCs,23 they have great potential because they have higher electron mobility and thermal stability due to their large
planarity and conjugation degree.34 However, few perylene-diimide small-molecules have demonstrated satisfactory performance
as ETMs of perovskite solar cells, which was attributed to dramatic decrease of mobility caused by irregular molecular
aggregation when fabricated via solution-process.35,36 In this work, we have synthesized a series of perylene-diimide small
molecules that have simple structures and high thermal stability as ETM of perovskite solar cell. The high thermal stability
enables vacuum thermal deposition of these molecules which ensures regular molecular packing to attain excellent electron
mobility. Perovskite solar cells with a structure of ITO/NiOx/perovskite/ETM/Ag have been fabricated and power conversion
efficiencies of 10.9%, 14.6%, 12.7%, and 7.71% have been obtained from devices respectively using PDI-Ph, PDI-PhCN, PDI-
PhCN-2Br, and PDI-PhCN-4Br as single ETL. When combined with BCP as ETL, an efficiency of 18.8% was achieved from
PDI-PhCN/BCP based device, which is superior to standard device (17.4%) using C60/BCP as ETL. Our results demonstrate PDI-
based molecules are very promising electron-transporting materials for highly efficient, low-cost and stable perovskite solar cells.
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2. Experimental Section
2.1. Materials
3,4,9,10-Perylenetetracarboxylic dianhydride, aniline, 4-aminobenzonitrile and anhydrous zinc acetate were purchased from
Zhongshenghuateng Technology Co., Ltd. 1,3-Dibromo-5,5-dimethylhydantoin was purchased from Nanjingwanqing Chemical
Glass Instrument Co., Ltd. PbI2, PbCl2, C60 and Ag were purchased from Alfa Aesar Chemical Co., Ltd. BCP was purchased
from Xi’ an Polymer Light Technology Co., Ltd. Indium tin oxide (ITO) coated glass substrates with a sheet resistance of 10 Ω/□
were purchased from South China Science & Technology Co., Ltd. CH3NH3I was synthesized according to the literature37 and
recrystallized prior to use.
2.2. Instrumentations
1H NMR spectra were measured on a Bruker AC-400 using tetramethylsilane (TMS) as internal standard. Mass spectrometry
(MS) was recorded on a Fourier transform ion cyclotron resonance mass spectrometer (Bruker, autoflex™ speed MALDI-TOF).
Thermo-gravimetric analyses (TGA) were carried out on STA-2500-Regulus instrument at an elevation temperature rate of 10
°Cmin-1 under nitrogen flow. Cyclic voltammetry (CV) was performed with a computer controlled CHI660E electrochemical
work station using a conventional three-electrode configuration consisting of a compartment electrolysis cell with a platinum
button as a working electrode, platinum wire as a counter electrode, and Ag/AgCl as a reference electrode. The samples were
thermally deposited onto the working electrode surface in high vacuum to form a thin film for CV-test. UV-Vis spectra were
obtained with a LAMBDA 35 spectrophotometer. Fluorescence spectra were recorded with a shimadzu (RF-6000 Plus) spectra
photometer with an exciting wavelength of 420 nm. X-ray powder diffraction (XRD) patterns were performed on a Bruker D8
Advanced/X-ray diffractometer with Cu-Kα radiation at a generator voltage of 40 kV and a current of 100 mA. Scanning electron
microscope (SEM) images were carried out on SEM Hitachi S-4800 microscope. Film thicknesses were measured using a
DektakXT Probe Profilometer, Bruker. The current density-voltage (J-V) curves of photovoltaic devices were measured on a
Keithley 2400 source-measure unit under 100 mWcm-2 AM 1.5G irradiation in ambient environment using a xenon lamp solar
simulator [Oriel 300 W solar simulator]. The external quantum efficiency (EQE) of the solar cells were acquired on an Enlitech
QE-R3018 using calibrated Si diodes as reference.
2.3. Synthesis of PDI-based Molecules
N,N’-diphenylperylene-3,4:9:10-tetracarboxydiimide
(PDI-Ph)
and
N,N’-di(4-cyanophenyl)per-ylene-3,4:9:10-
tetracarboxydiimide (PDI-PhCN) were synthesized according to the reference,38 and the synthesis and characterization of N,N’-
di(4-cyanophenyl)-1,7- dibromoperylene-3,4:9:10 -tetracarboxydiimide (PDI-PhCN-2Br) and N,N’-di(4-cyanophenyl)-1,6,7,12-
tetrabromoperylene -3,4:9:10-tetracarboxydiimide (PDI-PhCN-4Br) are described in Supplementary Section.38
2.4. Solar Cell Device Fabrication
Perovskite solar cells with an inverted configuration (ITO/NiOx/perovskite/PDI-molecule (or PDI-molecule/BCP)/Ag) were
fabricated. Here perovskite MAPbClxI3-x was employed as the representative light-harvesting material to explore the performance
of the PDI-molecules as electron-transporting material (ETM) because it possesses long carrier lifetime, high/balanced hole and
electron mobility,39 which enables decent device performance and valid comparison. A thin NiOx layer (~15 nm) was first
deposited on ITO-coated glass substrate according to the reference.37 Then MAPbClxI3-x (~300 nm) was fabricated on top of NiOx
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by one-step fast deposition-crystallization procedure, specifically, a DMF solution of MAPbClxI3-x (43%, wt%, PbI2:PbCl2:MAI
(molar ratio) = 0.95:0.05:1) was spin-coated at 6000 rpm for 6.5 s, followed by immediate exposure to chlorobenzene to induce
crystallization, then span at 6000 rpm for 50 s. MAPbClxI3-x film was annealed at 100 oC for 10 min. Then, a single ETM (PDI-
Ph, PDI-PhCN, PDI-PhCN-2Br or PDI-PhCN-4Br) or bi-layer ETM (PDI-Ph/BCP, PDI-PhCN/BCP, PDI-PhCN-2Br/BCP, PDI-
PhCN-4Br/BCP or C60/BCP) was deposited on top of the perovskite layer by thermal evaporation under high vacuum. Finally,
Ag (100 nm) was thermally deposited as the top contact through a shadow mask.
3. Results and Discussion
3.1. Synthesis
The synthetic methods of the PDI-based molecules are outlined in Fig. 1. Compound PDI-Ph and PDI-PhCN were synthesized
via aryl-amination catalyzed by anhydrous Zn(OAc)2 in quinoline solvent.40 The synthesis of compound PDI-PhCN-2Br and
PDI-PhCN-4Br proceeded on two steps: 1) bromination of starting material PDO with a strong brominating agent of 1,3-
dibromo-5,5-dimethylhydantoin in H2SO4 (100%); 2) aryl-amination with HOAc as both catalyst and solvent in order to avoid the
reaction of aryl-amine and bromium on the bay position of PDO.38
Figure 1. Synthetic route to the PDI-based molecules.
3.2. Thermostability and Molecular Packing
The performance of organic electric devices highly depends on the quality of the functional materials which require good stability
and crystallinity. The thermogravimetric analyses (TGA) of the four PDI-based molecules are shown in Fig. 2a. All the
o
molecules exhibit good thermal stability with the decomposition temperatures starting at 553, 610, 416 and 398 C for PDI-Ph,
o
PDI-PhCN, PDI-PhCN-2Br and PDI-PhCN-4Br, respectively. The decomposition temperature increases approximately 60 C
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when using electron-withdrawing benzonitrile on “N” to replace electron-donating phenyl group. But the decomposition
temperature of bromine-PDI decreases with the increase of bromine number on PDI bay. The decomposition temperature of these
molecules exceeds the naphthalene-diimide molecules, pyrazine-azaacene derivatives and fused-ring electron acceptors (300-400
oC),24,41,42 which not only enables the possibility of thermal deposition, but also ensures the thermal stability of PSC using them
as ETM.
Powder X-ray diffraction (XRD) measurements were carried out to investigate the packing of the molecules in the solid state
and the results are shown in Fig. 2b. Sharp diffraction peaks are observed for all four molecules in diffraction angle between 20o
and 30o. PDI-Ph, PDI-PhCN and PDI-PhCN-2Br exhibit two diffraction peaks, respectively at 27.79o and 25.00o, 26.15o and
23.83o, and 25.26o and 24.53o, corresponding to d-spacing of 3.21 and 3.56 Å, 3.41 and 3.73 Å, and 3.47 and 3.62 Å,
respectively. The larger angle diffraction peak of this pair is attributed to H-type aggregation of the cofacial π-π stacking of PDI
plane with shorter intermolecular distance, while the smaller angle one is attributed to J-type aggregation of the head-to-tail π-π
stacking of PDI molecules.43,44 For PDI-PhCN-4Br, only one sharp diffraction peak at 19.32o is observed, which corresponds to a
d-spacing of 4.58 Å and can be attributed to J-type aggregation. The increased bulkiness and twist by the four bromine
substituents hamper the face-to-face packing of PDI-PhCN-4Br molecules which required for H-type aggregation.43,44 The sharp
π-π stacking diffraction of the four compounds reveals the high crystallinity of the deposited films, which is beneficial for
electron transport.
Figure 2. (a) TGA curves of PDI-Ph, PDI-PhCN, PDI-PhCN-2Br and PDI-PhCN-4Br measured from room temperature to 700
oC with a heating rate of 10 °Cmin-1 under N2 atmosphere. (b) XRD patterns of PDI-Ph, PDI-PhCN, PDI-PhCN-2Br and PDI-
PhCN-4Br films vacuum thermally deposited on glass substrates.
3.3. Photophysical and Electrochemical Property
To determine the energy levels of the four PDI molecules, the absorption spectra and cyclic voltammetry (CV) were investigated
and the results are shown in Fig. 3. Benefited from the PDI chromophores of the molecules, the vacuum deposited films exhibit
strong and broad absorption in wavelength range from 300 to 600 nm (Fig. 3a). Compared to PDI-Ph, the absorption edge of
PDI-PhCN is slightly red-shifted for 6 nm due to the electron-withdrawing benzonitrile on “N”, increasing the molecule
conjugation. With two bromine on the PDI bay, the absorption edge of PDI-PhCN-2Br red-shifts for 20 nm due to p-π
conjugation between “Br” and PDI ring. Further increase the number of bromine, the absorption of PDI-PhCN-4Br greatly blue-
shifted because of the reduced molecular delocalization due to tetrabromide substituents inducing PDI plane distortion and weak
π-π stacking as revealed by XRD results (Fig. 2b). The optical bandgaps (Eg,opt) of PDI-Ph, PDI-PhCN, PDI-PhCN-2Br and PDI-
PhCN-4Br extracted from the absorption edges are 2.06, 2.04, 1.97 and 2.05 eV, respectively.
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Within the cyclic voltammetry scanning range of -3.0 to +3.0 V measured (Fig. 3b), no oxidation waves were observed for all
the four molecules, indicating they are insensitive to oxygen. Two reduction peaks are observed for PDI-Ph, PDI-PhCN and PDI-
PhCN-2Br whereas only one for PDI-PhCN-4Br. The onsets of the first reduction potential are -0.88, -0.77, -0.96 and -1.40 V
(compared to that of ferrocene (Fc)) for PDI-Ph, PDI-PhCN, PDI-PhCN-2Br and PDI-PhCN-4Br, respectively. The LUMO
levels are thus calculated to be -3.92 eV for PDI-Ph, -4.03 eV for PDI-PhCN, -3.84 eV for PDI-PhCN-2Br and -3.40 eV for PDI-
PhCN-4Br based on equation ELUMO = -(Ered + 4.8).18 According to the formula of Eg,opt = ELUMO EHOMO 18 the HOMO levels of
,
PDI-Ph, PDI-PhCN, PDI-PhCN-2Br and PDI-PhCN-4Br, are calculated to be -5.98, -6.07, -5.81 and -5.45 eV, respectively.
Except for PDI-PhCN-4Br, the energy levels of PDI-Ph, PDI-PhCN and PDI-PhCN-2Br match well with commonly used
perovskite materials like MAPbClxI3-x or MAPbI3.39 The noticeably higher LUMO level of PhCN-4Br compared to others is
attributed that “Br” forms the p-π conjugation with PDI plane, thus enhances the electron density of the PDI bay, where the
electronic clouds of LUMO focus on (Fig.S3).
Figure 3. (a) UV−vis absorption spectra of PDI-Ph, PDI-PhCN, PDI-PhCN-2Br and PDI-PhCN-4Br in film obtained by vacuum
thermal deposition. (b) Cyclic voltammetry curves of PDI-Ph, PDI-PhCN, PDI-PhCN-2Br and PDI-PhCN-4Br.
Table 1. Summary of the optical and electrochemical properties of the PDI-based molecules.
PDI small molecule
PDI-Ph
λonset (nm)
603
Eg,opt (eV) a
2.06
HOMO (eV) b
-5.98
LUMO (eV)
-3.92
-4.03
-3.84
-3.40
.
PDI-PhCN
609
2.04
-6.07
PDI-PhCN-2Br
PDI-PhCN-4Br
630
1.97
-5.81
605
2.05
-5.45
a: Calculated from the absorption band edge. b: Calculated according to equation EHOMO = ELUMO - Eg,opt
3.4. Electric Property
The electron mobility of the PDI-based molecules was investigated by space charge limited current (SCLC) model.18 The SCLC
is described by J = 9ε0εrµV2/8L3, where ε0 is the permittivity of free space, εr is the dielectric constant of the PDI-molecules
(3.8~5.3), µ is the hole mobility, V is the voltage drop across the device, L is the PDI-molecule thickness, and V = Vappl – Vr - Vbi,
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where Vappl is the applied voltage to the device, Vr is the voltage drop due to contact resistance and series resistance across the
electrodes, and Vbi is the built-in voltage due to the difference in work function of the two electrodes. The J-V curves of the
electron-only devices with a structure of ITO/ZnO/PDI molecule/LiF/Al measured in dark are plotted in Fig. 4a and the J-V
curves of PDI-Ph device under different illumination conditions are plotted in Fig. 4b. The mobility statistic data of the four
molecules are presented in Fig. S3 and Table 2. The electron mobility extracted from the J-V curve fitting are 0.190 cm2V−1s−1
for PDI-Ph, 0.330 cm2V−1s−1 for PDI-PhCN, 0.121 cm2V−1s−1 for PDI-PhCN-2Br, and 0.00411 cm2V−1s−1 for PDI-PhCN-4Br.
The mobility for PDI-Ph, PDI-PhCN and PDI-PhCN-2Br (Table 2) are comparable to the reported values of PDI based
molecules in high crystallinity state.45,46 This means vacuum deposition method can solve the weakness of low mobility derived
from disordered aggregation of PDI molecules from solution-process. The electron mobility of PDI-Ph extracted from the dark J-
V curve is only 0.129 cm2V−1s−1 and increases significantly to 1.405 cm2V−1s−1 under 100 mWcm-2 illumination (Fig. 4b). Under
work condition, the ETM of PDI-Ph in cell ITO/NiOx/perovskite(PVK)/PDI-Ph/Ag will expose to a small fraction of the 100
mWcm-2 illumination passing through ITO/NiOx/PVK (see Fig. S4). Under the same illumination condition, the electron mobility
of PDI-Ph is measured to be 0.428 cm2V−1s−1, much higher than that under darkness. More electron-only devices for PDI-Ph
were investigated and the mobility variation shown the same phenomena: the mobility is enhanced with the increase of light
intensity (in Fig. S3). The mobility of PDI-PhCN, PDI-PhCN-2Br and PDI-PhCN-4Br (in Fig. S3) demonstrates the same trend
as that of PDI-Ph. These results indicate these molecules possess the characteristic of photo-induced mobility enhancement,
which will improve the electron transportation of illuminated devices using them as ETMs.
Figure 4. (a) J-V curves of the PDI-molecule films measured from their electron-only device with a structure of ITO/ZnO/PDI-
molecule/LiF/Al under darkness. The solid lines are the curve fittings using SCLC model. The thickness of the films are given in
the legends. (b) J-V curves of the PDI-Ph films measured from the identical devices with that in (a) under different illumination
conditions.
Table 2. Summary of the mobility of PDI-based electron-transporting materials under darkness.
Material
PDI-Ph
Thickness (nm)
108
µ (cm2V-1s-1)a
0.169±0.083
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PDI-PhCN
98
77
75
0.212±0.127
0.119±0.069
PDI-PhCN-2Br
PDI-PhCN-4Br
0.00388±0.00390
a: Measured in the electron-only devices in the space charge limited current (SCLC) regime. The average value was calculated
based on ~10 devices.
3.5. Perovskite Solar Cells Using PDI-Molecules as Electron-Transporting Material
Using the PDI-molecule as electron-transporting layer, planar p-i-n perovskite solar cells with a configuration of
ITO/NiOx/MAPbClxI3-x/PDI-molecule/Ag or ITO/NiOx/MAPbClxI3-x/PDI-molecule/BCP/Ag (in Fig. 5a) were fabricated and
compared to the reference device using C60/BCP as ETL. Fig. 5b shows the energy diagram of the PDI-molecules and other
materials used in the device. The LUMO energy levels (3.8-4.0 eV) of PDI-Ph, PDI-PhCN and PDI-PhCN-2Br match well that of
MAPbClxI3-x (3.8 eV), which is expected to efficiently transfer electron to cathode, while excessively higher LUMO energy level
of PDI-PhCN-4Br (3.4 eV) than MAPbClxI3-x (3.8 eV) can form a barrier to electron transfer. The HOMO energy levels of PDI-
Ph (5.98 eV), PDI-PhCN (6.07 eV) and PDI-PhCN-2Br (5.81 eV) are lower than that of MAPbClxI3-x (5.4 eV), which indicates
that PDI-Ph, PDI-PhCN and PDI-PhCN-2Br can be employed individually as ETM to block hole from penetrating to the cathode,
while the close HOMO energy level of PDI-PhCN-4Br (5.45 eV) with MAPbClxI3-x (5.4 eV) cannot play the role of blocking
holes. Further combining these PDI-molecules with BCP, which has deep HOMO energy level of 7.0 eV, can more efficiently
block hole than PDI-only ETL. The top view scanning electron microscopy (SEM) image of MAPbClxI3-x film and cross section
SEM image of the finished device with PDI-PhCN as ETM are shown in Fig. 5c and 5d. A uniform and dense perovskite
MAPbClxI3-x film with large grains in micrometer sizes can be clearly seen in Fig. 5c. From Fig. 5d, under the perovskite layer, a
continuous and compact NiOx (~15 nm) hole-transporting layer is formed; on top of the perovskite layer, the PDI-PhCN
molecules form a compact and flat ETL with excellent adherence to both perovskite and top BCP. These features ensure decent
device performance and thus valid comparison for PSCs based on PDI-molecule/BCP and C60/BCP.
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Figure 5. (a) Schematic illustration of the device configuration with PDI-molecule ETM structure. (b) Energy levels of each
layer and charge transfer in a representative inverted PSC. (c) Top view SEM image of MAPbClxI3-x film on ITO/NiOx substrate.
(d) Cross-section SEM image of the PSC device with PDI-PhCN as ETM.
The current density-voltage (J−V) and external quantum efficiency (EQE) curves of the best performing devices using the
PDI-molecule or PDI-molecule/BCP as ETM and the reference device using C60/BCP are shown in Fig. 6. The average device
parameters with standard deviations are summarized in Table 3. The effect of the PDI-molecule or BCP thickness on the device
performance has been investigated and the results are given in Tables S1-S6. The thickness of the sole PDI-molecule as ETM is
optimized to be approximately 25 nm. The J-V curve of device without ETM is also shown for comparison, which has very poor
performance with PCE < 1.0%.
Figure 6. J−V curves of the best performing MAPbClxI3-x solar cells (a) with PDI-molecule as single ETM. (b) with PDI-
molecule(or C60)/BCP as bilayer ETM. (c) EQE spectra of the devices in (a). (d) EQE spectra of the devices in (b).
Table 3. Summary of perovskite solar cell device parameters using different ETMs.
a
a
a
ETL
(nm)
None
VOC
JSC
FFa
(%)
ηavg
ηmax
(%)
0.48
(V)
(mAcm-2)
0.91±0.44
(%)
0.49±0.14
44.0±1.9
0.29±0.19
PDI-Ph(25)
0.84±0.04
0.93±0.04
18.8±1.2
21.9±1.4
63.9±0.6
65.8±5.9
10.2±0.67
13.1±1.50
10.9
14.6
PDI-PhCN(25)
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PDI-PhCN-2Br(25)
PDI-PhCN-4Br(25)
0.83±0.05
0.87±0.04
1.05±0.03
0.91±0.04
1.02±0.04
0.88±0.03
0.92±0.04
21.8±2.9
19.4±1.8
21.2±1.8
21.7±1.9
22.4±1.3
21.9±1.3
20.6±2.3
67.2±4.0
40.2±4.4
72.2±3.7
67.7±6.5
73.4±5.3
70.2±6.8
49.6±10.6
11.6±1.10
6.98±0.73
16.3±1.10
13.3±1.65
17.4±1.40
14.4±1.55
9.07±1.70
12.7
7.71
17.4
15.0
18.8
16.0
10.8
C60/BCP(40/10)
PDI-Ph/BCP(25/7)
PDI-PhCN/BCP(25/7)
PDI-PhCN-2Br/BCP(25/7)
PDI-PhCN-4Br/BCP(25/7)
a: The average value was calculated based on ~50 devices.
When PDI-Ph was used as the single ETM, the best performing device shows an efficiency of 10.9% with JSC, VOC and FF of
20.2 mAcm-2, 0.85 V and 0.633, respectively (Fig. 6a, Table 3, S1). For PDI-PhCN as the single ETM, the solar cell
performance is significantly improved with JSC, VOC, FF and PCE of 22.3 mAcm-2, 0.96 V, 0.683 and 14.6% (Fig. 6a, Table 3,
S2), respectively. Whereas, for PDI-PhCN-2Br as the single ETM, the device performance decreases compared to PDI-PhCN,
with JSC, VOC, FF and PCE of 22.2 mAcm-2, 0.80 V, 0.711 and 12.7%, respectively (Fig. 6a, Table 3, S3). For PDI-PhCN-4Br as
the single ETM, the solar cell shows the worst performance with 7.71% efficiency. The better device performance for PDI-PhCN
is probably attributed to the higher electron mobility (Table 2) and deeper HOMO energy level of PDI-PhCN (Table 1), which
reduces device resistance and charge carrier recombination, thus decreases voltage loss and enhances charge collection.47 The
poor performance of PDI-PhCN-4Br based devices with low JSC and FF is due to its two order of magnitude lower electron
mobility (Fig. 6a, Table 2, S4) than other three PDI-molecules and unmatched energy levels with perovskite, which cannot
efficiently transfer electrons and block holes.
Although solely used PDI-Ph, PDI-PhCN and PDI-PhCN-2Br have played important ETM role, the solar cell efficiencies are
unsatisfied due to relatively lower VOC and FF than reported devices with the identical structure. In view of the excellent electron
mobility of these molecules, energy-level alignment and interfacial contact of them are the main factors that restrict their device
performance. From the aspect of energy-level alignment, the HOMO energy level of either molecule is not deep enough to
efficiently block holes transferring to cathode, which can lead to the adverse carrier recombination. From the other aspect of
interfacial contact, intense photoluminescence (PL) quenching of pristine MAPbClxI3-x by PDI-Ph, PDI-PhCN, PDI-PhCN-2Br
and PDI-PhCN-4Br (Fig.S6) indicates excellent electron extraction capability of these PDI-molecules, as well as good interfacial
contacts between MAPbClxI3-x and PDI-molecules. Subsequently, the surfaces morphology of PDI-molecules on perovskite were
investigated by AFM and SEM (Fig.7). SEM images showed that except the perovskite grain boundaries become unclear, the
surfaces of PDI-Ph/MAPbClxI3-x, PDI-PhCN/MAPbClxI3-x, PDI-PhCN-2Br/MAPbClxI3-x have no evident change compared to
MAPbClxI3-x, while PDI-PhCN-4Br showed visible aggregation on MAPbClxI3-x surface. AFM images exhibited the roughness
(Ra) increases from 5.91 to 8.78, 9.30, 9.34 and 13.6 nm after deposition of PDI-molecules on the MAPbClxI3-x, probably due to
aggregation of PDI-molecules, which will impact interface contacts with Ag electrode. All these factors combined could lead to
interface charge accumulation and recombination, thus the lower VOC and FF of the devices.
Therefore we further added an excellent hole-blocking layer of BCP between PDI-molecule and Ag. First, we fabricated a
reference device using a conventional combination of C60 (40 nm) and BCP (10 nm) bilayer as ETM, the solar cell performance
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with JSC, VOC, FF and PCE of 21.5 mAcm-2, 1.07 V, 0.755 and 17.4% was achieved, which is comparable to similar structured
PSC in the literature.39 Then on top of the optimal 25 nm thick PDI-molecule layer, BCP layer was deposited and its optimized
thickness is approximately 7 nm (Table S5, S6). For PDI-Ph/BCP as the ETM, a champion device with JSC, VOC, FF and PCE of
22.1 mAcm-2, 0.92 V, 0.740 and 15.0% was achieved (Fig. 6b, Table 2, S5). Except for JSC, all the other parameters are lower
than that of the reference device. When PDI-PhCN/BCP was used as the ETM, the device exhibited superior performance to
reference device with JSC, VOC, FF and PCE of 22.7 mAcm-2, 1.05 V, 0.787 and 18.8% (Fig. 6b, Table 2, S6), respectively. For
PDI-PhCN-2Br/BCP as the ETM, the best device efficiency of 16.0% is slightly inferior to the reference device (17.4%) due to
decreased VOC of 0.90 V (Fig. 6b, Table 2, S6). For PDI-PhCN-4Br/BCP as the ETM, the device performed far worse than the
reference device with an efficiency of 10.8%. Overall, the device performance using PDI-molecule/BCP as the ETM is
significantly improved for about 30-40% than that solely using the PDI-molecule. Among these PDI-molecule/BCP
combinations, PDI-PhCN/BCP realizes the best device efficiency, while PDI-Ph/BCP and PDI-PhCN-2Br/BCP both exhibit
medium performance with PDI-PhCN-2Br/BCP showing the worst performance, which is mainly attributed to difference in the
electron mobility of PDI-molecules (Table 2).
Devices using PDI-molecule as single ETM show similar external quantum efficiency (EQE) curves in the wavelength range
of 300-790 nm (Fig. 6c). The JSC integrated from the EQE spectra are 19.4, 21.0, 20.9, and 18.7 mAcm-2 for PDI-Ph, PDI-PhCN,
PDI-PhCN-2Br and PDI-PhCN-4Br based solar cells, respectively, in excellent agreement with that measured from the J−V
characterization in Fig. 6a. The PDI-PhCN and PDI-PhCN-2Br devices show higher EQE than PDI-Ph and PDI-PhCN-4Br
devices in all wavelength ranges. Devices using PDI molecule/BCP as ETM show similar EQE curves to that of C60/BCP
reference device in the wavelength range of 300-790 nm (Fig. 6d). The JSC integrated from the EQE spectra are 20.6, 21.0, 21.4,
21.1 and 19.2 mAcm-2 for C60/BCP, PDI-Ph/BCP, PDI-PhCN/BCP, PDI-PhCN-2Br/BCP and PDI-PhCN-4Br/BCP based solar
cells, respectively, in excellent agreement with that measured from the J−V characterization in Fig. 6b. Except PDI-PhCN-
4Br/BCP, the PDI-Ph/BCP, PDI-PhCN/BCP and PDI-PhCN-2Br/BCP based devices show higher EQE than C60/BCP reference
device in wavelength ranges of 370–500 nm (Fig. 6d), probably due to the higher mobility and thinner thicknesses of PDI-
molecules (~0.212 cm2V-1s-1, 25 nm) than C60 (~0.03 cm2V-1s-1, 40 nm),48 reducing the device resistance and carrier loss during
the transmission. PDI-PhCN/BCP device also exhibits higher EQE than all others between 650 and 740 nm because of higher
mobility of PDI-PhCN. The higher EQE and PCE of PDI-PhCN/BCP device than the reference device demonstrates the
advantage of using PDI-PhCN molecule to substitute C60 as ETM for PSCs.
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Figure 7. Top view SEM and AFM images of MAPbClxI3-x (a, a’), PDI-Ph/MAPbClxI3-x (b, b’), PDI-PhCN/MAPbClxI3-x (c, c’),
PDI-PhCN-2Br/MAPbClxI3-x (d, d’) and PDI-PhCN-4Br/MAPbClxI3-x (e, e’) films on NiOx/ITO substrates.
4. Conclusions
In summary, four perylene-bisimide (PDI) based small molecules (PDI-Ph, PDI-PhCN, PDI-PhCN-2Br, PDI-PhCN-4Br) have
been synthesized via aryl-amination and bromination of perylene anhydride. TGA, UV-Vis, XRD, and CV characterizations
o
reveal the PDI-molecules have outstanding thermostability with decomposition temperature higher than 400 C (over 600 oC for
PDI-PhCN, most-thermostable known small-molecule semiconductor), high crystallinity with strong π-π stacking, and well
matched energy levels (except PDI-PhCN-4Br) to that of perovskite absorbers. These molecules exhibit high electron mobility
(as high as ~0.2 cm2V-1s-1) and photo-induced mobility enhancement. Utilizing these molecules as single ETM, perovskite solar
cells with an inverted configuration of ITO/NiOx/MAPbClxI3-x/ETM/Ag have been successfully fabricated and power conversion
efficiency of 10.9%, 14.6% 12.7% and 7.71% have been achieved from PDI-Ph, PDI-PhCN, PDI-PhCN-2Br and PDI-PhCN-4Br,
respectively. When combining these molecules with BCP as ETL, the device efficiencies were respectively improved to 15.0%,
18.8%, 16.0% and 10.8%. The solar cell performance of PDI-PhCN/BCP device is superior to that of reference cell using
C60/BCP as ETM (PCE of 17.4%) due to more excellent electron mobility of PDI-PhCN (~0.212 cm2V-1s-1) than C60 (~0.03
cm2V-1s-1). Further modification of PDI-molecule structure to lower its HOMO level enough to more efficiently block hole can
avoid using BCP to realize higher efficiency. Our results demonstrate that PDI-based small molecules are very promising
electron-transporting materials for more efficient and economical perovskite solar cells.
Supporting Information
The supporting information contains complementary data of synthetic compounds and device performance.
Conflicts of interest
The authors declare no competing financial interest.
Acknowledgements
This work was financially supported by the National Natural Science Foundation of China (Grant nos. 61704087).
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