Molecular design with silicon core: Toward commercially available hole transport materials for high-performance planar p-i-n perovskite solar cells

Article abstract of DOI:10.1039/c7ta09716f

Organic hole transport layers (HTL) play a very important role for realizing high performance and low-cost planar p-i-n perovskite solar cells (pero-SCs). In this work, we synthesized two X-shaped organic HTL materials, Si-OMeTPA and SiTP-OMeTPA, with silicon cores and triphenylamine (TPA) derivative branches. This molecular design strategy can substantially simplify synthetic procedures making them available for reducing device costs. This is particularly applicable for Si-OMeTPA since it can be synthesized by two steps from commercial raw materials showing a total yield of over 60%. This molecularly designed Si-OMeTPA possesses advantages of high thermal stability, high crystallinity with a long range ordered lamellar structure, and excellent hole mobility. The resulting HTL can also facilitate the sequential growth of high-quality perovskite films, giving a significantly enhanced photovoltaic performance with a best power conversion efficiency of 19.06%, which is one of the highest PCE among the planar p-i-n pero-SCs to date. In addition, the devices exhibit negligible hysteresis, good reproducibility, and stability. To the best of our knowledge, this is the first example of an easily synthesized HTL material in planar p-i-n pero-SCs that shows superior performance relative to the well-known poly(bis(4-phenyl)(2,4,6-trimethylphenyl)amine) (PTAA) HTL.

Full text of DOI:10.1039/c7ta09716f

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10.1039/C7TA09716F.
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ISSN 2050-7488
PAPER
Kun Chang, Zhaorong Chang et al.
Bubble-template-assisted synthesis of hollow fullerene-like
MoS2 nanocages as a lithium ion battery anode material
rsc.li/materials-a

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Molecular Design with Silicon Core: Toward Commercially
Available Hole Transport Materials for HighꢀPerformance
Planar pꢀiꢀn Perovskite Solar Cells
Rongming Xue, a Moyao Zhang, a Guiying Xu, a Jingwen Zhang, a Weijie Chen, aHaiyang Chen,
a Ming Yang,c Chaohua Cui,a Yaowen Li,* a Yongfang Li*a, b
a
Laboratory of Advanced Optoelectronic Materials, College of Chemistry, Chemical
Engineering and Materials Science, Soochow University, Suzhou 215123, China
*Email: ywli@suda.edu.cn (Li. Y. W.)
b CAS Research/Education Center for Excellence in Molecular Sciences, CAS Key Laboratory
of Organic Solids, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190,
China
*Email: liyf@iccas.ac.cn (Li. Y. F.)
c College of Chemistry, Chemical Engineering and Materials Science, Soochow University,
Suzhou 215123, China
1

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Abstract
Organic hole transport layer (HTL) play very important role for realizing high performance
and lowꢀcost planar p-i-n perovskite solar cells (peroꢀSCs). In this work, we synthesized two
Xꢀshaped organic HTL materials SiꢀOMeTPA and SiTPꢀOMeTPA with silicon cores and
triphenylamine (TPA) derivative branches. This molecular design strategy can substantially
simplify synthetic procedure making them available for reducing device cost. Particularly for
SiꢀOMeTPA, it can be synthesized by two steps from commercial raw materials showing a
total yield of over 60%. This molecular design SiꢀOMeTPA possesses advantages of high
thermal stability, high crystallinity with a long range ordered lamellar structure, and excellent
hole mobility. The resulting HTL can also facilitate the sequent growth of highꢀquality
perovskite films, giving a significantly enhanced photovoltaic performance with a best power
conversion efficiency of 19.06%, which is one of the highest PCE among the planar p-i-n
peroꢀSCs to date. In addition, the devices exhibit negligible hysteresis, good reproducibility
and stability. To the best of our knowledge, this is the first example of an easily synthesized
HTL materials in planar p-i-n peroꢀSCs that shows even superior performance relative to the
wellꢀknown poly(bis(4ꢀphenyl)(2,4,6ꢀtrimethylphenyl)amine) (PTAA) HTL.
Keywords: hole transport materials; Xꢀshaped molecules; long range ordered lamellar
structure; silicon core; planar p-i-n perovskite solar cell.
2

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1. Introduction
The planar p-i-n organicꢀmetal halide perovskite solar cells (peroꢀSCs) with lowꢀ
temperatureꢀprocessed organic/polymer hole transport layer (HTL) on ITO as bottom
electrode have attracted great attention in recent years, because of its high efficiency and less
hysteresis of photovoltaic performance, low cost and easy fabrication by low temperature
solutionꢀprocessing.1ꢀ5 In the p-i-n peroꢀSCs, poly(3,4ꢀethylenedioxythiophene)/
poly(styrenesulfonic acid) (PEDOT:PSS) is the most commonly used organic HTL material.
But the PEDOT:PSSꢀbased p-i-n peroꢀSCs show poor longꢀterm stability due to its acidic and
hygroscopicity nature,6 and lower openꢀcircuit voltage (Voc) (0.90ꢀ0.95 V)7 due to the energy
mismatch between the PEDOT:PSS HTL and the perovskite photoactive layer. Therefore,
exploiting neutral and hydrophobic organic/polymer HTL that possessing matched energy
levels are particularly important to fully unveil the potential of p-i-n peroꢀSCs. Although
many HTL materials such as poly(3ꢀhexylthiophene) (P3HT),8 poly[N,N’ꢀbis(4ꢀbutylphenyl)ꢀ
N,N’ꢀbis(phenyl)benzidine] (polyꢀTPD),9 polytriarylamine (PTAA),10ꢀ12 1,4’ꢀbis(4ꢀ(diꢀ
ptoyl)aminostyryl)benzene (TPASB),13 TAPC,14 TruxꢀOMeTAD15 and PhNaꢀ1T16 have been
exploited to enhance the Voc and power conversion efficiency (PCE) of the p-i-n peroꢀSCs. It
is still needed to develop new HTL materials with easy synthesis and high reproducible
photovoltaic performance.
Xꢀshaped organic molecule HTL materials as shown in Fig. 1a, consisting of a core and
four branches, have drawn particular interest in the n-i-p peroꢀSCs due to their wellꢀdefined
structure, easier purification, and fineꢀtuned energy levels. For example, 2,2’,7,7’ꢀ
tetrakis(N,Nꢀdiꢀpꢀmethoxyphenylꢀamine)9,9’ꢀspiroꢀbifluorene (SpiroꢀOMeTAD) and its
analogues17, 18 as top HTLs in n-i-p peroꢀSCs demonstrated high photovoltaic performance
with the highest PCE over 20%.19, 20 However, they usually require additive to improve their
conductivities21 which makes the device processing complicated and results in hysteresis and
poorer device stability. Besides, the spiro or fusedꢀring core derivatives are expensive in
3

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considering future application. More recently, Li et al. reported a series of Xꢀshaped HTL
materials (such as TPPꢀSMeTAD or TPPꢀOMeTAD) using 3ꢀD triphenylamine (TPA) moiety
as branches and pꢀxylene as core, and used them in p-i-n peroꢀSCs. The resulting HTLs can
efficiently enhance charge extraction and passivate surface traps of the perovskite film,
realizing a PCE of over 16% for the p-i-n peroꢀSCs.22 However, the PCE of the p-i-n peroꢀ
SCs needs further improved and the structureꢀproperty relationships between the HTL
materials and the perovskite films remain less understood.
Siliconꢀcontaining organic semiconductor materials have been known to exhibit altered
properties with respect to their carbon analogues,23, 24 which have been reported to show
promising characteristics as photoactive layer materials for thinꢀfilm transistors25 and polymer
solar cells.26 It was demonstrated that incorporation of silicon atom in organic semiconductor
materials can be used to optimize molecular orbital shapes and energies, as well as molecule
packing property by rational molecule design.27 However, this promising strategy for
designing HTL materials of planar pꢀiꢀn peroꢀSCs has not been reported.
In this work, two Xꢀshaped small molecule HTMs, named SiTPꢀOMeTPA and Siꢀ
OMeTPA (see Fig. 1), were developed and used in planar p-i-n peroꢀSCs. In the molecules,
two siliconꢀbased derivatives (tetraphenylsilane or silicon atom) are employed as the cores,
and TPA with methoxyl end group are used as the four branches. This molecular design
strategy can substantially reduce the expensive synthetic procedure and improve the overall
yields. In particular, chosing silicon atom as core for SiꢀOMeTPA plays an important role in
simplifying synthetic procedure with a straightforward twoꢀsteps synthesis from commercial
raw materials. In addition, silicon atom core can stabilize electronic properties of the
molecules due to the high bond interaction between lowꢀlying σ* orbital of silicon atom and
the π* orbital of neighboring conjugated moiety.28 As a result, SiꢀOMeTPA exhibits high
crystallinity, excellent thermal stability, as well as appropriate energy levels to transfer holes.
The p-i-n planar peroꢀSCs with the SiꢀOMeTPA HTL, showed a high PCE of 19.06% with
4

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negligible hysteresis and excellent device stability. To our best knowledge, 19.06% is one of
the highest PCE among the planar p-i-n peroꢀSCs to date.
2. Results and discussion
2.1 Synthesis and Physicochemical Properties
Fig. 1 (a) A schematic illustration of Xꢀshaped molecule and their advantages. (b)
Molecular structures and corresponding DFTꢀcalculated electronic structures of (a) SiTPꢀ
OMeTPA and (c) SiꢀOMeTPA.
The synthetic routes of SiTPꢀOMeTPA and SiꢀOMeTPA are shown in Scheme S1 and
S2 respectively, and the detailed synthetic processes are described in the Experimental section.
Molecular structures of the two molecules were characterized by 1H and 13C NMR spectra and
MALDIꢀTOF mass spectrometry, as shown in Fig. S1ꢀS6 in Electronic supplementary
information (ESI). Briefly, tetrakis(4ꢀbromophenyl)silane was synthesized by the reaction of
SiCl4 and 1,4ꢀdibromobenzene with a 83.2% yield. Then it was coupled with two different
arylamines (OMeTPAꢀBPIN and bisꢀOMeAm) through Suzuki coupling or Buchwaldꢀ
Hartwig reaction, giving the target SiTPꢀOMeTPA and SiꢀOMeTPA with yields of 70.2% and
72.6%, respectively. It is worth noting that SiꢀOMeTPA can be synthesized by only two steps
with a total yield over 60%. Meanwhile, the two materials are readily soluble in common
organic solvent such as chlorobenzene (CB) and chloroform but insoluble in water and N,Nꢀ
dimethylformamide (DMF). From this point of view, SiTPꢀOMeTPA and SiꢀOMeTPA are
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suitable to be used as the HTL of p-i-n peroꢀSCs. It is worth mentioning that we also roughly
estimate the synthesis cost of 1ꢀg SiꢀOMeTPA and SiTPꢀOMeTPA according to the cost
model that is described by Pablo et al.29 and Osedach et al.30. As shown in Table S2 and Table
S3 in ESI, the estimated synthesis cost of SiꢀOMeTPA and SiTPꢀOMeTPA are ca. 29.57 $/g
and 35.1 $/g, respectively, which are much cheaper than that of PTAA (423.3 $/g).
Density functional theory (DFT) calculations were performed to further investigate the
electronic structures of the two silicon based HTL materials. As shown in Fig. 1b and 1c, the
electron density of SiꢀOMeTPA is more fully delocalized within the whole molecule than that
of SiTPꢀOMeTPA owing to the higher bond interaction between lowꢀlying σ* orbital of the
silicon atom and π* orbital of the conjugated TPA moeity. Moreover, the incorporation of
silicon atom in the center of SiTPꢀOMeTPA and SiꢀOMeTPA can also stabilize their
electronic properties, as evidenced from the strong reversibility of redox peaks in the
electrochemical cyclic voltammetry test (Fig. S7 in ESI).
Fig. 2 (a) DSC thermograms of SiTPꢀOMeTPA and SiꢀOMeTPA at a heating rate of 10
oC/min in N2. (b) Normalized absorption spectra of SiTPꢀOMeTPA and SiꢀOMeTPA films
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with or without thermal annealing. (c) XRD patterns of SiꢀOMeTPA films and (d) J-V
characteristics of the holeꢀonly devices with the structure of ITO/PEDOT:PSS/SiTPꢀ
a
o
b
OMeTPA or SiꢀOMeTPA/Au. Thermal annealed HTL at 150 C for 30 min. HTL doped
with 3 wt% F4ꢀTCNQ then thermal annealed at 150 oC for 30 min.
Both silicon based materials exhibit good thermal stability, with the decomposition
temperatures of 435 °C for SiTPꢀOMeTPA and 426 °C for SiꢀOMeTPA, at 5% weight loss
(Fig. S8 in ESI). Meanwhile, differential scanning calorimetry (DSC) thermograms (see Fig.
2a) reveal that SiTPꢀOMeTPA is amorphous in nature (glass transition temperature (Tg)
detected at 147 °C). While SiꢀOMeTPA shows a melting peak at around 345 °C and
crystallization peak at around 323 °C during the heating and cooling process, respectively,
which indicates SiꢀOMeTPA is crystalline after thermal annealing. These results can be
further confirmed by the redꢀshifted maximum absorption peak of the annealed SiꢀOMeTPA
film (at 150 oC for 30 min), while the amorphous SiTPꢀOMeTPA shows an almost unchanged
absorption spectra (see Fig. 2b). The Xꢀray diffraction (XRD) pattern of the SiꢀOMeTPA film
on silicon wafer (see Fig. 2c) shows (100), (200) and (300) three diffraction peaks, revealing a
lamellar molecule packing structure with a long range order in the crystalline state.31, 32
Meanwhile, this ordered structure could be well maintained even adding 3 wt% 2,3,5,6ꢀ
Tetrafluoroꢀ7,7,8,8ꢀtetracyanoquinodimethane (F4ꢀTCNQ) additive. In comparison, the SiTPꢀ
OMeTPA films show no diffraction peak even after thermal annealing due to its amorphous
nature (Fig. S9 in ESI).
Spaceꢀchargeꢀlimited current (SCLC) method was then used to investigate the effect of
molecular packing structure on hole mobilities (ꢀh) of SiTPꢀOMeTPA and SiꢀOMeTPA films.
As shown in Fig. 2d and Table 1, the ꢀh of the thermal annealed SiꢀOMeTPA film shows
more than one order of magnitude higher than that of the asꢀcast film, due to the formation of
long range ordered lamellar structure in the crystalized SiꢀOMeTPA film after thermal
annealing. The ꢀh was further enhanced by adding 3 wt% F4ꢀTCNQ additive, resulting in a ꢀh
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of 2.96×10ꢀ4 cm2/V
s. The result agrees well with the resistance reduction of PTAA HTL by
•
doping F4ꢀTCNQ.33 Although the amorphous SiTPꢀOMeTPA films also showed an increased
ꢀh after thermal annealing and doping F4ꢀTCNQ, the relatively lower values indicate the
molecule arrangment can dramatically influence hole transport ability. Therefore, it can be
concluded that the long range ordered lamellar structure of the crystalized SiꢀOMeTPA film is
one of the ideal structures for πꢀconjugated molecules to facilitate vertical hole transport.
Fig. 3 (a) Transmission spectra of glass/ITO, glass/ITO/PTAA, glass/ITO/SiTPꢀOMeTPA and
glass/ITO/SiꢀOMeTPA electrodes. UPS spectra in the cutoff (b) and the onset (c) energy
regions of SiTPꢀOMeTPA and SiꢀOMeTPA films. (d) The images of the water and DMF
droplets contact angles on different surfaces of the HTLs.
Transmittance and matched energy levels of the HTLs are also important factors that
significantly affects the current density and charge extraction ability of the peroꢀSCs. As
shown in Fig. 3a, ITO coated with 30ꢀnmꢀthick SiꢀOMeTPA film showed almost the same
transmittance, while the SiTPꢀOMeTPA and control PTAA films led to an abviously
decreased transmittance in the wavelength range of 340ꢀ430 nm. This result reveals that the
SiꢀOMeTPA film is a superior candidate as the HTL of p-i-n peroꢀSC without loss of incident
light. Ultraviolet photoelectron spectroscopy (UPS) was then used to accurately evaluate the
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highest occupied molecular orbital (HOMO) energy levels34 of the two silicon based HTLs on
ITO. The cutoff (Ecutoff) and onset (Ei) energy regions in the UPS spectra are shown in Fig. 3b
and 3c, respectively. According to the equation ϕ=21.2ꢀ(Ecutoff ꢀ Ei)35, the HOMO energy
levels of SiTPꢀOMeTPA and SiꢀOMeTPA were calculated to be ꢀ5.34 and ꢀ5.39 eV,
respectively. The lowest unoccupied molecular orbital (LUMO) levels of ꢀ2.30 eV for SiTPꢀ
OMeTPA and ꢀ1.96 eV for SiꢀOMeTPA were obtained from their corresponding optical
bandgaps (3.04 eV and 3.43 eV) and HOMO energy levels. Compared with the work function
or HOMO energy level of conventional HTL, such as ꢀ5.12 eV of PEDOT:PSS and ꢀ5.13 eV
of PTAA,36, 37 the deeper HOMO energy level of SiTPꢀOMeTPA and SiꢀOMeTPA closing to
the valence band (VB) of MAPbI3 (5.4 eV) could enhance charge transfer from MAPbI3 to the
HTL via an energetically favorable pathway without a significant loss in energy. Meanwhile,
the higher LUMO (ꢀ2.3 eV and ꢀ1.96 eV) of the two silicon based HTLs can also effectively
block electrons to prevent recombination at anode.
In addition, surface wettability of HTLs has a significant impact on the quality of the
subsequent growth of perovskite film on it in the fabrication of the p-i-n peroꢀSCs. Generally,
polar solvents have good wettability on the surface of conventional HTLs like PEDOT:PSS,
as confirmed by the small contact angles of water droplet (19.7o, Fig. 3d). Recent reports
reveal that the nonꢀwetting HTL (such as PTAA) surface can facilitate the growth of highꢀ
aspectꢀratio crystalline grain perovskite films and thus lead to higher photovoltaic
performance of the peroꢀSCs.38 Fortunately, as shown in Fig. 3d, the two silaneꢀcontaining
HTLs exhibit a comparable surface wettability with that of PTAA (contact angle of water
droplet (θ)=87.2o), particularly SiꢀOMeTPA shows a same contact angle (θ=87.2o) with that
of PTAA. We then further used DMF droplet instead of water to measure contact angles for
accurately evaluating wettability of PbI2 precursor. It can be seen that the contact angles of
DMF droplet on different HTL surfaces are obviously smaller than those of water droplet due
to the lower polarity of DMF, but exhibit a similar trend except that the contact angles on the
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surfaces of SiꢀOMeTPA and SiTPꢀOMeTPA are slightly lower than that on PTAA surface.5
Therefore, it can be expected that the improved hydrophily of two silaneꢀcontaining HTL
materials will facilitate the PbI2 deposition from DMF solution leading to a full coverage of
perovskite materials.39 Such HTLs with balanced wettability should be beneficial for
achieving highꢀquality perovskite films.
2.2 Photovoltaic Properties
Based on the above results, we then fabricated planar p-i-n peroꢀSCs with a structure of
glass/ITO/HTL/MAPbI3/PCBM/Al. As shown in Fig. 4a, the MAPbI3 film can well grow on
the hydrophobic surfaces of SiTPꢀOMeTPA and SiꢀOMeTPA films showing compact and
homogeneous perovskite morphologies, in particular for the MAPbI3 film grown on Siꢀ
OMeTPA showing a larger average grain size over ~600 nm. Accordingly, in the crossꢀ
section SEM image of SiꢀOMeTPA based device, a thick perovskite layer (~420 nm) with
grain boundaries perpendicular to the substrate is obtained, which can effectively minimize
the grain boundary energy.5 It can be seen from Fig. 4a that all functional layers can be clearly
distinguished, confirming that the planar heterojunction configuration was well formed in the
peroꢀSCs. XRD patterns are commonly used to evaluate the quality of perovskite film in the
peroꢀSCs. Fig. S10 in ESI shows the XRD patterns of MAPbI3 grown on the SiꢀOMeTPA and
SiTPꢀOMeTPA HTLs. It can be seen that diffraction peak at 12.4° (assigned to the (001)
plane of PbI2) is weaker for the MAPbI3 film grown on SiꢀOMeTPA than that on SiTPꢀ
OMeTPA, indicating a lower PbI2 content and more complete perovskite phase of the
photoactive layer prepared on the SiꢀOMeTPA HTL. Low PbI2 content has been found to be
beneficial for perovskite photovoltaics due to its passivation effect at grain boundaries that
reduces charge recombination.40 Fig. 4b provides the energyꢀlevel diagram of the related
materials in the peroꢀSCs. As the CB of perovskite is ꢀ5.40 eV,7 both SiTPꢀOMeTPA and Siꢀ
OMeTPA HTLs show a perfect energy band alignment with the perovskite photoactive layer
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with small HOMO energy level offset, for efficient hole transfer and collection from the
photoactive layer to the HTL.
Table 1. Summary of Photovoltaic Parameters of the PeroꢀSCs.
c
HTL
Treatment
ETL/Cathode
Voc
FF
Jsc
PCE
(%)
ꢀh
(V)
(%)
(mA/cm2)
(cm2/V•s)
SiTPꢀOMeTPAa
SiTPꢀOMeTPAa
SiTPꢀOMeTPAa
SiꢀOMeTPAa
SiꢀOMeTPAa
SiꢀOMeTPAa
SiꢀOMeTPAa
SiꢀOMeTPAb
asꢀcast
PCBM/Al
PCBM/Al
PCBM/Al
PCBM/Al
PCBM/Al
PCBM/Al
0.81 58.42
0.98 63.31
0.99 71.69
0.98 57.48
1.01 64.86
1.07 74.23
14.54
18.10
19.85
15.29
19.77
22.96
23.08
22.98
6.85
1.39×10ꢀ6
annealed
11.28 1.27×10ꢀ5
doped/annealed
asꢀcast
14.15
8.58
6.3×10ꢀ5
4.24×10ꢀ6
8.8×10ꢀ5
annealed
12.89
18.23 2.96×10ꢀ4
doped/annealed
doped/annealed
doped/annealed
PCBM/C60/BCP/Ag 1.07 77.17
PCBM/C60/BCP/Ag 1.07 75.97
19.06
18.68
a Reverse scan: 1.2 V
ꢀ
0 V, scanꢀrate 200 mV/s, delay time 100 ms. bForward scan: 0
ꢀ
1.2 V,
scanꢀrate 200 mV/s, delay time 100 ms. c ꢀh calculated from J-V curves of holeꢀonly devices.
Fig. 4 (a) Crossꢀsection SEM images of the planar p-i-n peroꢀSCs based on SiꢀOMeTPA HTL
(left). Topꢀview SEM images of the perovskite films grown on SiꢀOMeTPA HTL (right top)
and SiTPꢀOMeTPA HTL (right bottom). (b) The corresponding energyꢀlevel diagram of peroꢀ
SCs. (c) JꢀV curves of the planar p-i-n peroꢀSCs based on different HTLs. (d) The steadyꢀstate
PL spectra and (e) TRPL decay transient spectra of the perovskite films prepared on ITO or
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a
o
b
different HTLs coated substrate. Thermal annealed HTL at 150 C for 30 min. HTL doped
with 3 wt% F4ꢀTCNQ then thermal annealed at 150 oC for 30 min.
Fig. 4c shows the current densityꢀvoltage (J-V) characteristics of the peroꢀSCs with
SiTPꢀOMeTPA and SiꢀOMeTPA HTLs treated by different conditions, under the illumination
of AM1.5G 100 mW/cm2, and Table 1 listed the photovoltaic parameters data of the
corresponding peroꢀSCs. The peroꢀSCs with the asꢀcast HTL exhibited quite poor PCE of
6.85% for the device with SiTPꢀOMeTPA HTL and 8.58% for SiꢀOMeTPA HTL with the
low Voc, short circuit current density (Jsc), and fill factor (FF), which could be attributed to the
low hole mobilities of the asꢀcast HTLs (1.39×10ꢀ6 for SiTPꢀOMeTPA HTL and 4.24×10ꢀ6
o
cm2/V•s for SiꢀOMeTPA HTL).41 After thermal annealing of the HTLs at 150 C for 30 min
before depositing perovskite films, the PCEs of the SiTPꢀOMeTPA and SiꢀOMeTPA based
devices were dramatically increased to 11.2% and 12.89%, respectively, which could be
ascribed to the significantly enhanced hole mobilities of the HTLs that improves Ohmic
contact and charge extraction ability of the device. For the devices based on the HTLs with 3
wt% F4ꢀTCNQ and thermal annealing (doped/annealed), the PCEs of the peroꢀSCs with
SiTPꢀOMeTPA and SiꢀOMeTPA HTLs were further improved to 14.15% and 18.23%,
respectively. Notably, the SiꢀOMeTPA HTL based peroꢀSCs exhibits ~30% higher PCEs than
that of the SiTPꢀOMeTPA based devices. The superior photovoltaic performance of the Siꢀ
OMeTPA HTL could be attributed to its higher ꢀh and higher transparency originating from
long range ordered lamellar phase structure and more fully delocalized electron in Siꢀ
OMeTPA.
The charge transfer properties of the peroꢀSCs were further investigated by steadyꢀstate
PL spectra and timeꢀresolved PL (TRPL) measurements, as showed in Fig. 4d and 4e, and
Table S1 in SI. The perovskite film shows the strongest PL quenching and reduced life time
(22.35 ns) when depositing it on the doped/annealed SiꢀOMeTPA HTL in comparison to
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those on the asꢀcast SiꢀOMeTPA, annealed SiꢀOMeTPA, and even the doped/annealed SiTPꢀ
OMeTPA HTLs. This means that the excitons produced in the perovskite film can be easily
dissociated and the holes can be more efficiently extracted by the doped/annealed Siꢀ
OMeTPA HTL. The results agree with the improved photovoltaic performance of the peroꢀ
SCs with the doped/annealed SiꢀOMeTPA HTL. In addition, from the obviously improved Jsc
and FF for the doped/annealed SiꢀOMeTPA based device, we can also conclude that the
crystalized SiꢀOMeTPA with dopant has less traps in it or at the surface of Siꢀ
OMeTPA/perovskite, which facilitates charge transportation and suppresses charge
recombination.
Fig. 5 (a) Charge collection probability as a function of internal voltage; (b) light intensityꢀ
dependence of Voc for the peroꢀSCs with the doped/annealed SiTPꢀOMeTPA or SiꢀOMeTPA
HTLs.
In order to understand the device operation mechanism of the p-i-n peroꢀSCs. The plots
of photocurrent (Jph) versus the effective applied voltage (Veff) for the devices were measured
to investigate the influence of different HTLs (doped/annealed SiTPꢀOMeTPA and Siꢀ
OMeTPA) on the exciton dissociation and charge collection. As can be seen from Fig. 5a,
when Veff arrives at ~2 V, Jph values for two devices reach saturation (Jsat), suggesting that
charge recombination is minimized at higher voltage due to the high internal electric field in
the devices. The charge extraction probability (CEP) can be calculated from the ratio of
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Jph/Jsat.42 We can find that the CEP increases from 0.923 to 0.944 by changing doped/annealed
SiTPꢀOMeTPA to doped/annealed SiꢀOMeTPA HTL. This result indicates better charge
extraction ability of peroꢀSC can be achieved by using the crystalized SiꢀOMeTPA HTL with
long range ordered lamellar structure.
Fig. 5b shows the charge recombination kinetics of devices by measuring Voc at varied
logarithmically (ln(I)) with light intensity. The slopes for the devices with the doped/annealed
SiTPꢀOMeTPA and doped/annealed SiꢀOMeTPA HTL are 1.43 kT/e and 1.19 kT/e,
respectively. The significant decreased slope of the device with the doped/annealed Siꢀ
OMeTPA HTL, that are closed to the trapꢀfree device with a slope of 1.0 kT/e43, 44, indicates
the reduced trap state in the perovskite film. This result could be ascribed to that the
doped/annealed SiꢀOMeTPA HTL is more beneficial for the growth of MAPbI3 with less
traps, which can effectively depress the charge carriers recommbination thus achieve higher
device performance.
Fig. 6 (a) J-V curves of the peroꢀSCs with SiꢀOMeTPA as HTL and C60/BCP as double
cathode buffer layers in reverse and forward scan measured under the illumination of
AM1.5G, 100 mW/cm2. Forward scan: ꢀ0.2 Vꢀ1.2 V, scan rate 200 mV/s, delay time 100
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ms; Reverse scan: 1.2 Vꢀꢀ0.2 V, scan rate 200 mV/s, delay time 100 ms. (b) EQE spectra of
the corresponding peroꢀSCs. (c) Histograms of device PCE measured for 50 corresponding
peroꢀSCs. (d) The plot of PCE values versus synthetic steps of organic HTLs used for the
MAPbI3 based p-i-n peroꢀSCs reported in literatures and this work.
In order to further improve photovoltaic performance of the p-i-n peroꢀSCs with the
doped/annealed SiꢀOMeTPA as HTL, we used the thermally evaporated C60/bathocuproine
(BCP) double layers as cathode buffer layer in the device, in considering that the BCP film as
an excitonꢀtransporting and holeꢀblocking layer could realize ehancement of device
performance.45 The device structure is glass/ITO/SiꢀOMeTPA/MAPbI3/PCBM/C60 (20
nm)/BCP (5 nm)/Ag. As shown in Fig. 6a and Table 1, the devices showed a further improved
PCE of 19.06 % measured in the reverse scan direction, with a Jsc of 23.08 mA/cm2, a Voc of
1.07 V, a FF of 77.17%. which is quite consistent with the PCE measured in the forward scan
direction (18.68%), with a Jsc of 22.98 mA/cm2, a Voc of 1.07 V, a FF of 75.97%, indicating
negligible J-V hysteresis. The performance enhancement and negligible hysteresis behavior
could be attributed to the wellꢀgrown perovskite film on the SiꢀOMeTPA HTL and the
improved electronꢀcollection and holeꢀblocking property of the C60/BCP double cathode
buffer layers.
External quantum efficiency (EQE) spectra of the corresponding peroꢀSCs are shown in
Fig. 6b. The Jsc value calculated from the integration of the EQE spectrum was 21.9 mA/cm2,
which agrees well with the Jsc value obtained from the J-V curve within 5% deviation. Fig. 6c
shows a histogram of the PCE values of 50 devices. Half of the devices based on the
doped/annealed SiꢀOMeTPA as HTL and with the C60/BCP double cathode buffer layers
demonstrated PCEs over 18.0%. Fig. 6d plots PCE values versus synthetic steps of the HTLs
used for the p-i-n peroꢀSCs reported in literatures and this work. It can be seen that 19.06% is
the highest PCE in the planar p-i-n peroꢀSCs with the HTL synthesized by only two steps,
indicating that SiꢀOMeTPA is a promising cheap HTL material for future application of the p-
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i-n peroꢀSCs. It is worth noting that this PCE is only lower than the highest reported value of
the device with PTAA HTL for the MAPbI3 system, where additive and antisolvent dripping
treatments were simultaneously employed to further improve the quality of MAPbI3 film.12
However, these treatments need stringent processing and toxic solvent, in particular for the
antisolvent technology, which are unfavorable for largeꢀarea solar cell fabrication and device
reproducibility. Encouragingly, when fabricating devices under the same condition (without
additional treatments to MAPbI3), the SiꢀOMeTPA HTL based device showed an
approximately 10% enhanced PCE and slightly higher Voc compared to that of the device
using PTAA as HTL (PCE of 17.47% consistent with previous reports. 10, 33, see in Fig. S11,
ESI), which probably attributes to the well grown perovskite film on SiꢀOMeTPA as well as
better matched device energy levels.
Fig. 7 (a) Stability of the p-i-n peroꢀSCs in (a) nitrogen atmosphere and (b) ambient
atmosphere with 70ꢀ80% RH.
Stability of the peroꢀSCs is an another critical issue for their application. First of all, we
investigated the the effect of intrinsic thermal annealing stability of the SiꢀOMeTPA HTL on
the performance of the p-i-n peroꢀSCs. Considering the decomposition of perovskite film and
F4ꢀTCNQ at high thermal annealing temperature, SiꢀOMeTPA without dopant was thermal
o
annealed at different temperatures (100ꢀ200 C) with different time (5ꢀ30 min), then used as
HTL to fabricate the device directly. As shown in Fig. S12 in ESI, the PCEs of the devices
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with the SiꢀOMeTPA HTL exhibit only slight variation with increasing the thermal annealed
temperature and with extending the annealing time on the HTL, indicating a promising
thermal stability of the SiꢀOMeTPA HTL without influencing its long range ordered lamellar
structure as well as electronic coupling between SiꢀOMeTPA and MAPbI3. For the planar p-i-
n peroꢀSCs, the chemical stability of HTL is also particularly important due to its influence on
the interfaces between the HTL and perovskite film and between the HTL and ITO or FTO
bottom transparent electrode. The storage stability of the p-i-n peroꢀSCs based on Siꢀ
OMeTPA HTL and control PEDOT:PSS HTL without encapsulation were studied in the
nitrogen and ambient atmosphere, respectively, as shown in Fig. 7. In the nitrogen atmosphere,
the PCE of the control peroꢀSC based on PEDOT:PSS HTL degraded to 74% of its initial
value when keeping in the dark condition for 500 h (see Fig. 7a), while the SiꢀOMeTPA HTLꢀ
based device retained over 92% of its initial PCE. One possible reason responsible for the
improved device stability could be ascribed to the neutral charateristic of the SiꢀOMeTPA
HTL, which can effectively prevent the corrosion agaist ITO and perovskite film. For
investigation of the stability of the devices in ambient atmosphere, we fabricated the peroꢀSCs
using a structure of ITO/PEDOT:PSS or SiꢀOMeTPA/perovskite/PCBM/C60/BCP/Ag, and the
unencapsulated devices were stored under ambient atmosphere with a ~60ꢀ80% relative
humidity (RH). As shown in Fig. 7b, after 130 h exposure, the SiꢀOMeTPA based device
retained almost 90% of its initial PCE, while the PCE of the PEDOT:PSS based devices
dramatically degraded to 49%. We could attribute the obviously enhanced stability of the Siꢀ
OMeTPA based devices to the hydrophobicity of the SiꢀOMeTPA HTL, which will protect
the MAPbI3 layer from water ingress.
3. Conclusion
In conclusion, we synthesized two lowꢀcost Xꢀshaped organic molecules SiꢀOMeTPA
and SiTPꢀOMeTPA with silicon core and TPA branches, and used them as HTL in the planar
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p-i-n peroꢀSCs. Among the two HTLs, SiꢀOMeTPA can be simply synthesized by two steps
from commercial raw materials with a total yield of over 60%, and possesses excellent
thermal stability, high crystallinity with a long range ordered lamellar structure and higher ꢀh.
Therefore, the SiꢀOMeTPA HTL allows us simultaneously address several major challenges
of the p-i-n peroꢀSCs, including facilitaing the growth of highꢀquality and less trap states
polycrystalline perovskite films as well as improving hole extracting ability and band
alignment at interface. As a result, the p-i-n peroꢀSCs with the doped/annealed SiꢀOMeTPA
as HTL and C60/BCP as double cathode buffer layers showed significantly enhanced PCE of
19.06% with negligible hysteresis behavior and high stability. The results indicate that Siꢀ
OMeTPA is a promising lowꢀcost HTL materials for future application of the p-i-n peroꢀSCs.
4. Experimental Section
4.1 Materials synthesis
4.1.1 Synthesis of SiTP-OMeTPA
4-bromo-N,N-bis(4-methoxyphenyl)aniline (OMeTPA-Br) 4ꢀbromoaniline (5.00 g, 29.2
mmol), 4ꢀiodoanisole (14.4 g, 61.3 mmol), CuI (0.28 g, 1.46 mmol), KOH (12.8 g, 228 mmol),
and 1,10ꢀphenanthroline (0.26 g, 1.46 mmol) were added in degassed anhydrous toluene (50
mL) and the reaction mixture was heated to 110 ºC. Reaction was continued for 36 h and then
the reaction mixture was cooled to room temperature, diluted with CH2Cl2 (100 mL), and
washed three times with 150 mL water. The combined organic phases were dried over
anhydrous MgSO4, and the solvent was evaporated subsequently. The crude product was
purified by silica gel chromatography using petroleum ether/dichloromethane (3/1, v/v) as
1
eluent to afford the final product as yellow solid (6.02 g, 53.8%). H NMR (300 MHz, d6ꢀ
DMSO) δ 7.31 (d, J = 8.8 Hz, 2H), 7.04 (d, J = 8.9 Hz, 4H), 6.92 (d, J = 8.9 Hz, 4H), 6.67 (d,
J = 8.9 Hz, 2H), 3.74 (s, 6H). 13C NMR (75 MHz, CDCl3) δ 156.06, 147.93, 140.56, 131.77,
127.74, 126.58, 114.79, 55.50.
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4-methoxy-N-(4-methoxyphenyl)-N-(4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-
yl)phenyl)aniline (OMeTPA-BPIN) OMeTPAꢀBr (2.0g, 5.2 mmol) was added into a dry 250
ml Schlenk flask, and 50 mL anhydrous THF was transferred into the flask, immediately. 2.72
mL (6.53 mmol) 2.4 M nꢀBuLi solution was then injected dropwise by syringe into the THF
solution, and kept stirring at ꢀ78 ºC for 2 hours. After that, isopropoxyboronic acid pinacol
ester (1.21 g, 6.53 mmol) was added dropwise into the above solution by syringe. The
reaction mixture was firstly kept stirring at ꢀ78 ºC for 1 hour and then warmed up to room
temperature for overnight. Deionized water (20 mL) is added to quench the reaction. The
phases are separated and the aqueous layer is extracted with dichloromethane (3×100 mL).
The combined organic phases are dried over anhydrous MgSO4, the solvent was removed by
rotorꢀvapor. The crude product was purified by silica gel chromatography using petroleum
ether/dichloromethane (3/1ꢀ1/1, v/v) as eluent to afford the final product as white solid (1.63 g,
1
72.6%). H NMR (300 MHz, CDCl3) δ 7.60 (d, J = 8.6 Hz, 2H), 7.10 ꢀ 7.02 (m, 4H), 6.89 ꢀ
6.78 (m, 6H), 3.79 (s, 6H), 1.32 (s, 12H). 13C NMR (75 MHz, CDCl3) δ 156.20, 151.39,
140.42, 135.77, 127.14, 118.64, 114.73, 83.40, 55.47, 24.86.
4',4''',4''''',4'''''''-silanetetrayltetrakis(N,N-bis(4-methoxyphenyl)-[1,1'-biphenyl]-4-amine)
(SiTP-OMeTPA) Tetra(4ꢀbromophenyl)silane (0.5 g, 0.77 mmol), OMeTPAꢀBPIN (1.6 g,
3.70 mmol), Pd(PPh3)4 (0.027 g, 0.023 mmol) were dissolved in a mixture of Toluene (30
mL) and 2M K2CO3 aqueous solution (10 mL). This solution was proted by N2 and then
stirred at 100 ºC for 48 hours. After cooling to room temperature, the reaction solution was
diluted with 20 mL CH2Cl2, and washed three times with 50 mL water. The combined organic
phases were dried over anhydrous MgSO4, and the solvent was evaporated subsequently. The
crude product was purified by silica gel chromatography using petroleum
ether/dichloromethane (4/1ꢀ1/1, v/v) as eluent to afford the final product as light green solid
(0.837 g, 70.2%). 1H NMR (300 MHz, CDCl3) δ 7.67 (d, J = 8.1 Hz, 8H), 7.58 (d, J = 8.1 Hz,
8H), 7.44 (d, J = 8.6 Hz, 8H), 7.07 (t, J = 8.6 Hz, 16H), 6.98 (d, J = 8.4 Hz, 8H), 6.89ꢀ6.78 (m,
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16H), 3.80 (s, 24H). MS (MALDIꢀTOF): calcd for C104H88N4O8Si [M]+, 1548.64; found,
1548.73.
4.1.2 Synthesis of Si-OMeTPA
Tetrakis(4-bromophenyl)silane (Si-TBP) 1,4ꢀdibromobenzene (4.00 g, 16.95 mmol) and 50
mL anhydrous THF were added into a dry 250 ml Schlenk flask. Then 7.41 mL (17.8 mmol)
2.4 M nꢀBuLi solution was injected dropwise by syringe at ꢀ78 ºC, and the mixture was
stirring at ꢀ78 ºC for 2 hours. After that, a 5 mL THF solution of SiCl4 (0.65 g, 3.85 mmol)
was added into the mixture and kept stirring at ꢀ78 ºC for 1 hour, then warmed up to room
temperature for overnight. The reaction solution was quenched by 50 mL deionized water,
and obtained aqueous solution was extracted by dichloromethane (100 mL) for three times.
The combined organic phase were dried over anhydrous MgSO4, the solvent was removed by
rotorꢀvapor. The crude product was then purified by silica gel chromatography with petroleum
ether to yield white powder product (2.06 g, 82.3%). 1H NMR (400 MHz, CDCl3) δ 7.54 (d, J
= 7.9 Hz, 8H), 7.34 (d, J = 7.9 Hz, 8H). 13C NMR (150 MHz, CDCl3) δ 137.58, 131.43 (d, J =
7.0 Hz), 125.42.
4,4',4'',4'''-silanetetrayltetrakis(N,N-bis(4-methoxyphenyl)aniline) (Si-OMeTPA) SiꢀTBP (0.5
g, 0.77 mmol), bis(4ꢀmethoxyphenyl)amine (bisꢀOMeAm) (1.26 g, 3.85 mmol), Pd2(dba)3
(0.022 g, 0.023 mmol) and sodium tertꢀbutoxide (0.44 g, 4.62 mmol) were dissolved in 10 ml
of refinedꢀanhydrous toluene in a 50 mL twoꢀnecked flask, and then the solution was
protected with N2. Then a solution of triꢀtertꢀbutylphosphine in toluene (1 M, 46 ꢁl, 0.046
mol) was injected. The mixture was heated to 110 ºC and stirred for 48 h. After cooling to
room temperature, the mixture was diluted with dichloromethane and washed by brine for
three times. The organic phase was combined and dried over MgSO4. After the solvent was
removed, the crude product was purified by silica gel chromatography using petroleum
ether/dichloromethane (4/1ꢀ1/1, v/v) as eluent to afford the final product as white power
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1
(0.696 g, 72.6%). H NMR (300 MHz, CDCl3) δ 7.33 (d, J = 8.5 Hz, 8H), 7.09 ꢀ 7.02 (m,
16H), 6.88 ꢀ 6.78 (m, 24H), 3.78 (s, 24H). 13C NMR (75 MHz, CDCl3) δ 155.99, 149.47,
140.65, 137.04, 127.02, 125.58, 118.83, 114.65, 55.48. MS (MALDIꢀTOF): calcd for
C80H72N4O8Si [M]+, 1244.51; found, 1244.39.
4.2 Measurements and Instruments
1H NMR and 13C NMR spectra were measured using a Varian Mercuryꢀ400 NMR.
Timeꢀofꢀflight mass spectra were recorded with a Broker MALDIꢀTOF mass system. Thermal
gravimetric analysis (TGA) was carried out on DiscoveryꢀTG, with a heating rate of 10
oC/min under nitrogen flow. Differential scanning calorimetry (DSC) was performed under
o
nitrogen flushing at a heating rate of 10 C/min with DiscoveryꢀDSC Q20. Electrochemical
measurements of the two HTL materials were performed with a Bioanalytical Systems
BAS100 B/W electrochemical workstation. The cyclic voltammetry (CV) diagrams of the
HTMs were obtained by using nꢀBu4NPF6 as supporting electrolyte in acetonitrile solution
with a glass carbon working electrode, a platinum wire counter electrode and a Saturated
calomel electrode (SCE) reference electrode under N2 atmosphere. UPS was tested by Kratos
Axis Ultra DLD. PL was tested by FLS980 (Edinburgh Instrument, UK), and the transient
photoluminescence also used FLS980 (Edinburgh Instrument, UK) at the wavelength of
768ꢂnm. XRD patterns were collected using X’Pert Pro MPD (PANalytical B.V.). The SEM
images were collected on SU8010 produced by Hitachi, where the electron beam accelerated
at 5kV. The J-V characteristics of the devices was measured with a computerꢀcontrolled
Keithley 2400 Source Measure Unit under the illumination of AM1.5G white light source, the
optical power at the sample was 100 mW/cm2. And the light intensity was corrected by a
standard silicon solar cell. The EQE was obtain using a solar cell spectral response
measurement system (Enli Technology Co., Ltd., QEꢀR3011). The light intensity at each
wavelength was also calibrated with a standard Si solar cell. The transmittance and reflection
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spectra were measure with Ultraviolet Spectrometer (Agilent Technologies Cary 5000 UVꢀ
VisꢀNIR).
4.3 Device Fabrication
ITO glass substrates were sequentially washed with isopropanol, acetone, distilled water,
and ethanol for 20 min, then dried with N2. The sythesized HTL materials (SiꢀOMeTPA and
SiTPꢀOMeTPA) and control PTAA were dissolved in CB with a concentration of 5 mg/mL
and 10 mg/mL, respectively. Then, they were spinꢀcoated on the ITO/glass substrates at the
speed of 6000 rpm for 30 s. For the doped HTLs, the 3 wt% and 1 wt% F4ꢀTCNQ were added
into sythesized HTL materials and PTAA solution, respectively, before depositing. The
resulting HTLs were thermally annealed at 150 °C for 30 min (SiꢀOMeTPA and SiTPꢀ
o
OMeTPA) and 100 C for 10 min (PTAA). The perovskite layer was fabricated by twoꢀstep
method. PbI2:MAI precursor solution (35 ꢁL, molar ratio = 1.3:0.3, dissolved in DMF/DMSO,
o
volume ratio = 9:1) was stirred at 70 C overnight on the hot plate. On the top of HTL, the
PbI2:MAI precursor solution was firstly spinꢀcoated at 6000 rpm for 15s. Then MAI solution
(40 ꢁL, 35 mg/mL, dissolved in 2ꢀpropanol) was spinꢀcoated at 3700 rpm for 45 s on
PbI2:MAI film. After that, the deposited films were thermal annealed at 100 oC for 30 min in a
nitrogenꢀfilled gloveꢀbox. The resulting perovskite films were coated with PCBM film by
spinꢀcoating 20 mg/mL PCBM solution (in CB) at 2000 rpm for 30s. Finally, Al (100 nm)
electrode or C60 (20 nm)/BCP (5 nm)/Ag (100 nm) were deposited on PCBM by vacuum
evaporation under 1*10ꢀ6 mbar.
Electronic supplementary information Materials synthesis and device fabrication.
1
13
Characterization figures showing H NMR, C NMR, TGA, DSC, XRD of SiTPꢀOMeTPA
and SiꢀOMeTPA. Characterization of perovskite film including XRD and TRPL data. PCE
evolution of peroꢀSCs with thickness and annealing time, cost analysis of HTL materials.
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Acknowledgment
This work was supported by the National Natural Science Foundation of China (Grant Nos.
51673138 and 91633301), the Priority Academic Program Development of Jiangsu Higher
Education Institutions (PAPD), the Jiangsu Provincial Natural Science Foundation (Grant No.
BK20160059), and Natural Science Foundation of the Jiangsu Higher Education Institutions
of China (16KJB430027). Project Funded by China Postdoctoral Science Foundation (Grant
No. 2017M610347).
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Journal of Materials Chemistry A
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DOI: 10.1039/C7TA09716F
Table of Contents
We synthesized a low-cost silicon containing HTL materials, achieving an excellent PCE of
19.06% for planar p-i-n perovskite solar cells.