Well-Defined Thiolated Nanographene as Hole-Transporting Material for Efficient and Stable Perovskite Solar Cells

Article abstract of DOI:10.1021/jacs.5b06493

Perovskite solar cells (PSCs) have been demonstrated as one of the most promising candidates for solar energy harvesting. Here, for the first time, a functionalized nanographene (perthiolated trisulfur-annulated hexa-peri-hexabenzocoronene, TSHBC) is employed as the hole transporting material (HTM) in PSCs to achieve efficient charge extraction from perovskite, yielding the best efficiency of 12.8% in pristine form. The efficiency is readily improved up to 14.0% by doping with graphene sheets into TSHBC to enhance the charge transfer. By the HOMO-LUMO level engineering of TSHBC homologues, we demonstrate that the HOMO levels are critical for the performance of PSCs. Moreover, beneficial from the hydrophobic nature of TSHBC, the devices show the improved stability under AM 1.5 illumination in the humidity about 45% without encapsulation. These findings open the opportunities for efficient HTMs based on the functionalized nanographenes utilizing the strong interactions of their functional groups with perovskite.

Full text of DOI:10.1021/jacs.5b06493

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Communication
Well-Defined Thiolated Nanographene as Hole-Transporting
Material for Efficient and Stable Perovskite Solar Cells
Jing Cao, Yu-Min Liu, Xiaojing Jing, Jun Yin, Jing Li, Bin Xu, Yuan-Zhi Tan, and Nanfeng Zheng
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.5b06493 • Publication Date (Web): 18 Aug 2015
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Well-Defined Thiolated Nanographene as Hole-Transporting
Material for Efficient and Stable Perovskite Solar Cells
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Jing Cao,1,†Yu-Min Liu, 1,†Xiaojing Jing, 1 Jun Yin, 1,2 Jing Li,2 Bin Xu,3 Yuan-Zhi Tan,1,* Nanfeng Zheng1,*
1 Collaborative Innovation Center of Chemistry for Energy Materials, State Key Laboratory for Physical Chemistry of Solid Surfaces, Engineering
Research Center for Nano-Preparation Technology of Fujian Province, and Department of Chemistry, College of Chemistry and Chemical Engi-
neering, Xiamen University, Xiamen 361005, China
2 Pen-Tung Sah Institute of Micro-Nano Science and Technology, Xiamen University, Xiamen 361005, China.
3 State Key Laboratory of Supramolecular Structure and Materials, Jilin University, Changchun 130012, China
Supporting Information
LUMO energy level, electron-donating ability, as well as their processabil-
ABSTRACT: Perovskite solar cells (PSCs) have been demonstrated as
ity, are ready to be modulated by rational functionalization,28,29 which
one of the most promising candidates for solar energy harvesting. Here,
render nanographenes promising candidates for the HTMs of PSCs.
for the first time, a functionalized nanographene (perthiolated tri-sulfur-
However, to the best of our knowledge, none of nanographenes has been
annulated hexa-peri-hexabenzocoronene, TSHBC) is employed as the
employed in PSCs as HTMs up to date. Meanwhile, the function groups
hole transporting material (HTM) in PSCs to achieve efficient charge
at the periphery of nanographenes provide the possibility to tune the
extraction from perovskite, yielding the best efficiency of 12.8% in pristine
interactions (such as coordination interactions) with perovskite layer in
form. The efficiency is readily improved up to 14.0% by doping with
PSCs, which would facilitate the charge extraction and transport from
graphene sheets into TSHBC to enhance the charge transfer. By the
perovskite to nanographenes; therewith improve the efficiencies of PSCs
HOMO-LUMO level engineering of TSHBC homologues, we demon-
but was rarely studied at current stage.
strate that the HOMO levels are critical for the performance of PSCs.
Here we use a thiolated nanographene perthiolated tri-sulfur-
annulated hexa-peri-hexabenzocoronene (TSHBC, the structure shown
in Fig. 1a) as the HTM in the pristine form in PSCs. The thiol groups at
the periphery form Pb-S coordination-bonds at the interface of perov-
skite and HTM. The tight binding of TSHBC helps to rapidly extract
charge from perovskite, resulting in a low energy loss at the interface. The
performance is readily improved by doping with graphene sheets into
TSHBC to enhance the hole transporting property within HTM. By
tuning the HOMO level of TSHBCs, the HOMO level of HTMs is re-
vealed as a key issue for the performance of HTMs in PSCs. Moreover,
the long-term stability is the biggest remaining challenging issue for the
commercialization of PSCs. In order to resolve this issue, various strate-
gies have been proposed and investigated.9,22,30-33 Hydrophobic thiolated
nanographenes reported in this work would provide as an effective mo-
lecular sealing approach to improve the stability of PSCs.
Moreover, beneficial from the hydrophobic nature of TSHBC, the devic-
es show the improved stability under AM 1.5 illumination in the humidity
about 45% without encapsulation. These findings open the opportunities
for efficient HTMs based on the functionalized nanographenes utilizing
the strong interactions of their functional groups with perovskite.
The solution-processed hybrid perovskite solar cells (PSCs) have
drawn dramatically increased attentions as a real competitor to silicon
solar cell,1-10 due to their high power conversion efficiencies, easy fabrica-
tion and low-cost. Clearly, a large amount of the converted photon energy
is wasted by the surface recombination resulting from the imperfect crys-
tal passivation and undesirable interfacial behavior. Therefore, further
efficiency enhancement mainly relies on minimizing the interface losses
by interface modification between the active layers.11-13 More recent stud-
ies have revealed that the interfacial modification between perovskite and
electron collecting layers can enhance the efficiencies of PSCs.12-15 While,
the hole transporting materials (HTM) at the other side of perovskite
layer, also play an important role in the most efficient PSCs. An ideal
HTM generally requires compatible energy level and sufficient charge
extraction and transferability.3-8,16-18 Carbon materials with unique con-
ductivity properties, such as porous carbon,9,12 graphene,19 graphdiyne,20
grapheneoxide21 and single-walled carbonnanotube,22 have been success-
fully utilized as hole extraction and transfer materials in PSCs.
TSHBC was synthesized following a recently reported method (See
Supporting Information for details),34 and used as HTM for the fabrica-
tion of PSCs. The fabricated PSCs had a typical p-i-n configuration of
FTO/TiO2/CH3NH3PbI3/TSHBC. In a typical fabrication procedure,
the compact TiO2 layer and mesoporous TiO2 layer were deposited on a
FTO (fluorine doped tin oxide) conducting glass substrate, to achieve
electron extraction and transport. Contrary to the conventional PSCs
using Spiro-OMeTAD as HTM, we employed here TSHBC (20 nm
thickness) on top of perovskite layer as HTM. The entire fabrication
procedure was completed in the open air at a common relative humidity
of 45%. The fabricated perovskite devices were characterized by SEM
(Fig. 1b), UV-Vis (Fig. S1), and XRD (Fig. S2).
Nanographenes are nano-sized graphene molecules with defined
chemical structures.23 Their physical and chemical nature is essentially
determined by their size and peripheral structure.24,25 In past decades,
nanographenes have been intensively investigated as organic semicon-
ductors due to its excellent self-assembly properties and large conjugated
To evaluate whether TSHBC could serve as an effective material to
extract photogenerated holes from the perovskite layer, the J-V character-
istics of FTO/TiO2/CH3NH3PbI3/TSHBC device were measured un-
der the illumination of AM 1.5, 100 mW cm-2. As shown in Fig. 1c, when
~20 nm thick TSHBC was deposited on the top of the perovskite layer as

system.26 In general, nanographenes are typical p-type
semiconductors26,27 and their electronic properties such as HOMO-
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HTM, the best efficiency of 12.8% was successfully achieved. The open-
circuit voltage (Voc), short-cut current (Jsc) and fill-factor (FF) of the cell
were measured to be 0.95 V, 20.56 mAcm-2, 65.79%, respectively. The
measurements over 30 fabricated devices gave an average efficiency of
10.6 ± 2.2% (Fig. 1d).The incident photon-to-current conversion effi-
ciency spectra (IPCE) of the device spanned from the UV region to 800
nm (Fig. S3), matching well with its UV-Vis absorption spectra (Fig. S1).
PSCs devices with 20 nm TSHBC-tBu and TSHBC-CF3 layers were
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fabricated, and their PCEs were measured. As illustrated in Fig. 2b and S6
and S7, the devices based on TSHBC-tBu offered an average efficiency of
9.9 ± 2.3% with the best efficiency of 12.2%. In comparison with devices
based on TSHBC, TSHBC-tBu based devices had a similar Jsc, but a
slightly lower Voc. The lower Voc is attributed to the higher HOMO level
of TSHBC-tBu than that of TSHBC. In remarkable contrast, the
TSHBC-CF3-based devices showed a much poorer photovoltaic perfor-
mance with the best efficiency of 6.1% (the average efficiency of 4.5 ±
2.7%). This value was even lower than that of the devices with no HTM
(Fig. S7 and Table S2), indicating that TSHBC-CF3 hindered the hole
transfer from perovskite due its lower HOMO level than the top of val-
ance band of perovskite. The longer PL decay time of the film with
TSHBC-CF3 (Fig. S8) also suggested the similar result. These results
from three different TSHBC homologues clearly demonstrate that, in
order to maximize the overall performance of PSCs, the HOMO level of
HTMs should be higher than but close enough to the top of valance band
of perovskite.
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Figure 1.(a) The structures of TSHBC. (b) Cross-sectional SEM image of the
device of FTO/TiO2/perovskite/TSHBC film. The best I-Vcharacteristics
(c), comparison of the performance distributions of 30 individual devices (d)
of the cells.
The excellent performance of PSCs involving the use of TSHBC as
HTM suggested a unique hole-extracting interface between perovskite
and TSHBC. Infrared (IR) spectra of the films during the fabrication of
devices were thus measured to evaluate the interactions between perov-
skite and TSHBC. In the IR spectrum of the TiO2/perovskite/TSHBC
film (Fig. S4), there was an appearance of absorption peak at 432 cm-1
that was assigned to Pb-S vibration.15 And the C-S absorption peak at 700
cm-1 became weaker and slightly blue-shifts than that of pure TSHBC.
These results clearly suggested the formation of Pb-S coordination-bond
between TSHBC and perovskite. We propose that the formation of Pb-S
coordination–bond between TSHBC and perovskite contributes the
efficient electron extraction at the interface.
Figure2.(a) The energy level diagram of the materials used in PSCs. (b) The
best I-Vcharacteristics of the cells. (c) The best I-V characteristics of the cells
based on TSHBC with different thickness.
With twelve phenylthio groups decorated at their periphery, TSHBC
and its homologues are not strictly planar and thus not expected to serve
as an excellent hole transporting material. Indeed, space-charge-limited
currents measurement revealed negligible hole transport mobility (μ=
4.0×10-15 cm2V-1s-1) for TSHBC (Fig. S9).35 Similarly, both TSHBC-tBu
and TSHBC-CF3 also exhibited poor hole transporting properties (Table
S1). Considering the poor hole transporting property of TSHBC and its
homologues is deleterious to the overall performance of PSCs, one can
suppose that a thicker film of TSHBC in the device would lead to a poor-
er performance. As expected, a much poorer performance was observed
on the devices with thicker (60 nm) TSHBC layers (Fig. 2, Fig. S10 and
Table S3). The best performance of the cells was only 3.2%. In contrary,
the PCEs should be improved by decreasing the thickness of TSHBC
layer. Unfortunately, due to the high roughness of pervoskite layer, when
the thickness of TSHBC in the device was further decreased below 20 nm,
a lot of pinholes were present in the TSHBC layer (Fig. S10b). Conse-
quently, the fabricated PSCs showed an overall performance similar to
that of devices with no HTM layer (Fig. S11 and Table S3).
In order to have photogenerated holes effectively extracted into
HTM, the matching of HOMO level of HTM with the valence-band level
of perovskite should be a critical factor. The easy chemical modification
of TSHBCs allows us to verify this hypothesis experimentally. By decorat-
ing electron-donating or with drawing groups at the periphery of TSHBC,
the HOMO-LUMO levels of TSHBC homologues can be reasonably
modulated without changing other properties, making it possible to in-
vestigate the impact of the HTM’s HOMO level on the efficiencies of
PSCs. Two TSHBC homologues (TSHBC-tBu and TSHBC-CF3) bear-
ing twelve 4-t-butylphenyl groups and 4-trifluoromethylphenyl groups at
the periphery were synthesized. The HOMO-LUMO energy levels of
TSHBC homologues were obtained by the cyclic voltammetric meas-
urements under the same condition with ferrocene as an internal stand-
ard (Fig. S5 and Table S1). As shown in Fig. 2a, the HOMO levels of
TSHBC-tBu and TSHBC-CF3 were measured to be -5.27 and -5.70eV,
0.13 eV higher and 0.3 eV lower than that of TSHBC, respectively.
The variation in the energy levels is expected to impact the perfor-
mances of PSCs using TSHBC-tBu and TSHBC-CF3 as HTMs. The
To further understand the impact of thickness of TSHBC on the
cells′ performances, electrochemical impedance spectra (EIS) of PSCs
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with TSHBC of various thicknesses were measured in the dark condition. ity of ~45% (Fig. S17), the devices with TSHBC retained over 85% of
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At the 0.8 mV forward bias voltage, the greatly increased charge-transport
resistance, obtained from the semi-circles at high-intermediate frequen-
cy,36,37 was observed as the thickness of HTM increased from 20 nm to 60
nm (Fig. S12). The increased resistance could be ascribed to the disad-
vantageous charge transfer within the thicker HTM layer,3 in good
agreement with the J-V characteristics above mentioned.
their original efficiency after 10-day, while the cell based on
TSHBC/graphene kept more than 90% of its original efficiency. In con-
trast, under the same storage condition, the cells with spiro-OMeTAD
lost ~80% of their efficiency after 10 days. As shown in Fig. 4b, the effi-
ciency of devices with spiro-OMeTAD was decreased to 25% within 60
min under illumination at AM 1.5 G. However, under the same illumina-
tion condition for 400 min, the device with TSHBC retained over 60% of
its original efficiency, and the cell with TSHBC/graphene still kept more
than 70% of its original efficiency. The rapid decrease of the efficiency can
be described to accelerating decomposition effects from intrinsic thermal
instability during illumination, consistent with the reported result.38
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Figure 3.The best I-V characteristics of the cells (a), the transient PL
spectra of films (b), based on 20 nm HTM layers.
HTM in PSCs usually play both roles to extract holes from perov-
skite and transport holes to the current collecting electrode. The results
discussed above have demonstrated that TSHBC molecules readily
achieve effective hole extraction from perovskite. But their poor hole
transporting behavior would be a limiting factor to further improve the
performance of PSCs. To resolve this issue, highly conductive graphene
sheets were incorporated into the TSHBC layer (Fig. S13). When the
thickness was 20 nm for the HTM layer, the ratio of graphene and
TSHBC was optimized (Fig. S14 and Table S4). For the devices base on
the optimized TSHBC/graphene (5:1, weight ratio), the significantly
enhanced performance with the best efficiency of 14.0% (Fig. 3a) was
observed. Both Voc and Jsc were improved. The measurements over 30
fabricated devices gave an average efficiency of 12.0 ± 2.1% (Fig. S7). This
result demonstrated the introduction of highly conductive materials
helped to improve the hole transporting property of TSHBC and thus
enhanced overall photovoltaic performance of PSCs. However, it should
be noted that the presence of photocurrent hysteresis was revealed in
both cells with TSHBC and TSHBC/graphene as HTMs (Fig. S15 and
Table S5), which could be due to the nature of the perovskite films.
Figure 4.(a) The water contact angles. (b) The efficiency variation of the
devices stored under illumination at AM 1.5 G with the humidity of 45%.
In summary, thiolated nanographene molecules have been success-
fully demonstrated as a new type of hole transporting material for fabri-
cating high-performance PSCs. The best efficiency of 12.8% was achieved
by using TSHBC as HTM. The energy level matching between the
HOMO of TSHBC and valence band of perovskite was critical to the
overall performance of PSCs. Detailed studies revealed that the TSHBC
layer had a rather poor charge transfer characteristics. The further im-
proved overall efficiency of PSCs up to 14% was readily achieved by dop-
ing the HTM layer of TSHBC with graphene to enhance the efficient
hole transport. More importantly, due to the hydrophobic nature of
TSHBC, the devices involving TSHBC exhibited significantly improved
device stability. All these findings validate nanographenes as a promising
HTM for PSCs. However, the poor hole mobility of these nanographene
molecules is one important factor to limit the fill-factor and thus overall
efficiency of perovskite solar cells based on them. The performance of
nanographenes as HTMs in PSCs can be further improved in terms of
edge functionalization and core-structure modulation. This work also
suggests the possibility to use functional graphene materials, for example
the thiolated graphene nanoribbons and thiolated graphene sheets, as the
HTMs in PSCs.
In order to further verify the improved hole transfer within the
TSHBC/graphene layer, the transient photoluminescence (PL) spectra
were measured. As illuminated in Fig. 3b, compared to the perovskite-
only film with a decay time (τ) of 73.6 ns, the introduction of TSHBC on
the top of perovskite layer shortened the PL decay time to 6.1 ns. Fur-
thermore, the film with perovskite-TSHBC/graphene displayed a shorter
PL decay time of 3.5 ns. The quenched photoinduced excitons at the
pervoskite/HTMs interface clearly confirmed that introducing graphene
enhanced the hole transporting property of TSHBC as HTM.
ASSOCIATED CONTENT
Supporting Information. Experimental details, details of the XRD, UV-vis
spectra, cross-sectional SEM images and the transient PL spectra of the cells
based on different TSHBC homologues.
Another important advantage of thiolated nanographenes as HTM
lies in their hydrophobicity which readily provides as an effective molecu-
lar sealing approach to improve the stability of PSCs. In our previous
work,15 we found that the hydrophobic thiols on perovskite surface readi-
ly inhibited the entrance of water molecules into perovskite film to signif-
icantly enhance the stability of PSCs. As shown in Fig. 4a and S16, the
contact angles of water droplets on both TSHBC and TSHBC/graphene
were ~107°, while the angle was only 70° on spiro-OMeTAD. The excel-
lent hydrophobic properties of thiolated TSHBC and TSHBC/graphene
should improve the long-term durability of the devices. In this work, the
stabilities of the devices with 20 nm TSHBC and TSHBC/graphene
layers were measured under AM 1.5 illumination in the humidity about
45% without any encapsulation. When stored in air with a relative humid-
AUTHOR INFORMATION
Corresponding Author
nfzheng@xmu.edu.cn; yuanzhi_tan@xmu.edu.cn
Notes
† J. Cao and Y.-M. Liu contributed equally to this work.
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