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CMMUNICATION

This journal is © The Royal Society of Chemistry [year] [journal], [year], [vol], 00–00 | 1

Dual Functional Additive for HTM Layer in Perovskite Solar Cells

Hong Zhang,a Yantao Shi,*

a Feng Yan,

b Liang Wang,

a Kai Wang,

a Yujin Xing,

a Qingshun Dong,

a and

Tingli Ma*a,c

Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX

DOI: 10.1039/b000000x 5

The ionic liquid N-butyl-N’-(4-pyridylheptyl)imidazolium

bis(trifluoromethane)sulfonimide (BuPyIm-TFSI) was used as

a dual-functional additive to improve the electrical properties

of the hole-transporting material (HTM) for perovskite solar

cells. BuPyIm-TFSI improved the conductivity of HTM and 10

reduced dark current of the solar cell simultaneously, thereby

greatly increasing the power conversion efficiency.

As a typical 3rd generation photovoltaic cell, mesoscopic solar

cells have been studied intensively since 1991.1 However, no

breakthroughs are available for conventional dye-sensitized solar 15

cells (DSCs), regardless of the many so-called strategies that have

been tested experimentally or theoretically. In 2012, this stagnant

situation was successfully overcome by using organolead halide

perovskites as sensitizer in all-solid-state mesoscopic solar cells.2

Up to now, power conversion efficiencies (PCEs) of higher than 20

15% have been reported by the groups of H. J. Snaith3 and M.

Grätzel,4 either based on a 3D porous framework or a 2D planar

structure. Undoubtedly, perovskite solar cells open up a new and

promising avenue for future thin-film solar cell development.5

Perovskite solar cells generally comprise a conductive 25

substrate, a compact TiO2 layer, an organolead halide perovskite

light-absorption layer (single or combined with a porous scaffold),

a hole-transporting layer (HTM), and a metal cathode (Ag or Au).

As of this writing, studies have mainly focused on designing

scaffold layers,6 optimizing the perovskite layer,3, 4, 7 exploring 30

alternative structures,8 and probing into inherent mechanisms.9

As one of the key components in perovskite solar cells, HTM

separates photo-excited electron-hole pairs and transports the

holes to the external circuit. Thus photovoltaic performance of

perovskite solar cells is highly dependent on the properties of 35

HTM in which 2,2′,7,7′-tetrakis(N,N-di-p--methoxyphenyl-

amine)9,9′-spirobifluorene (spiro-OMeTAD) is currently widely

used as the matrix for hole transportation.2-7 However, spiro-

MeOTAD alone in HTM is insufficient for obtaining high PCEs

because of low conductivity and other problems. Adding different 40

kinds of functional additives to spiro-MeOTAD can further

improve the properties of HTM and ensure the high efficiency

perovskite solar cells. For example, lithium10 or cobalt salts4 are

frequently used for p-doping to increase the hole concentration.

Despite the high volatility and unpleasant smell, TBP (4-tert-45

butylpyridine) is usually added into HTM to suppress charge-

recombination, as in traditional DSCs.11 For the first time, we use

a stable ionic liquid N-butyl-N’-(4-pyridylheptyl)imidazolium

bis(trifluoromethane) sulfonimide (BuPyIm-TFSI) as a dual-

functional additive to simultaneously improve the electrical 50

property of HTM and suppress charge combination in perovskite

solar cells. BuPyIm-TFSI improved the conductivity of HTM and

reduced the dark current of the solar cell.The PCE greatly

increased from 3.83% (with no additive in HTM) to 7.91%, and

these rates are comparable to those in conventional HTM 55

containing lithium salt and TBP. Moreover, using such dual-

functional additive can effectively simplify the components of

HTM and help reduce the production cost.

Fig. 1. Molecular structures of the BuPyIm-TFSI and spiro-OMeTAD 60

Molecular structures of BuPyIm-TFSI and spiro-MeOTAD are

shown in Fig. 1, the molecular structure of BuPyIm-TFSI was

characterized by NMR spectra in Supporting Information. As

illustrated in Fig. 1, BuPyIm-TFSI is an ionic liquid and its cation

comprises imidazolyl, pyridyl and alkyl chains that are used to 65

either link or modify the two heteroatom rings. Unlike ordinary

organic additives, ionic liquids are nearly non-volatile and

usually have excellent stability. As shown in Fig. 2a, subsequent

thermal gravimetric analysis indicated that the decomposition

temperature of BuPyIm-TFSI was higher than 300°C. 70

Undoubtedly, such satisfactory thermal stability will be

meaningful to the long-term durability of perovskite solar cells.

More importantly, the elaborate structural design enables dual

function of the BuPyIm-TFSI. On one hand, based on the

mechanism proposed by H. J. Snaith, free proton can be released 75

from imidazolyl for p-doping and thus increase the conductivity

of HTM by partial oxidation of spiro-MeOTAD.12 On the other

hand, pyridyl can function in the same way as TBP to suppress

the charge recombination that occurs at the TiO2/perovskite

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2 | Journal Name, [year], [vol], 00–00 This journal is © The Royal Society of Chemistry [year]

Table 1 Photovoltaic parameters of the peroviskite solar cells under 100 mWcm-2 light illumination.

HTM Compositions VOC (V) JSC (mA cm-2) FF PCE (%)

HTM1 Pristine spiro 0.63±0.03 12.26±0.20 0.50±0.08 3.83 ±0.20

HTM2 Spiro+BuPyIm-TFSI 0.87±0.04 16.26±0.30 0.56±0.03 7.91 ±0.30

HTM3 Spiro+Li+TBP 0.91±0.05 15.56±0.50 0.57±0.05 8.16±0.25

Fig. 2. (a) Thermal gravimetric analysis curves of BuPyIm-TFSI; (b) J-V 5

curves of CE/HTM/CE devices for different HTM.

interface.

In total, three HTM samples we prepared and designated as

HTM1, HTM2 and HTM3, respectively. HTM1 contained only

spiro-MeOTAD, whereas HTM2 contained BuPyIm-TFSI and 10

spiro-MeOTAD. For comparison, we prepared HTM3 using

spiro-MeOTAD, lithium salt (LiTFSI) and TBP based on

published works.4 Conductivities of these three HTMs were

characterized using linear sweep voltammetry by sandwiching

HTM between two platinum electrodes, as reported previously.13 15

As shown in Fig. 2b, BuPyIm-TFSI improved the conductive

property of HTM2 compared with HTM1 and HTM3. Based on a

previously reported theory, an ionic liquid such as BuPyIm-TFSI

is considered as Brϕnsted acid and can release free protons to

facilitate the oxidization of spiro-MeOTAD, which increase the 20

hole concentration and the subsequent conductivity of HTM.12

We fabricated our perovskite solar cells using a previously

described method, in which one sequential deposition route was

used to well control the morphology of CH3NH3PbI3.4 The cross

section and the top view of the perovskite solar cells are shown in 25

Fig. 3. In addition to the necessary layer of dense TiO2, we also

added one mesoporous layer of anatase TiO2 (approximately 300

nm thick) to support the CH3NH3PbI3, which was prepared by

two deposition steps using PbI2 and CH3NH3I solutions

consecutively. The formation of CH3NH3PbI3 was confirmed by 30

XRD spectroscopy (Fig. S1). We also verified that the perovskite

was uniformly distributed throughout the mesoporous TiO2 films

by performing cross-sectional scanning electron microscopy with

elemental mapping via energy dispersive X-ray analysis (Fig. S2)

35

Fig. 3. (a) Cross-sectional SEM image of a complete photovoltaic device

based on HTM2. (b) SEM image of the top view of perovskite layer.

Photovoltaic performance of the perovskite solar cells based on

HTM1, HTM2, and HTM3 was measured under AM 1.5, 100

mW/cm2 light illumination. J-V curves of these three perovskite 40

solar cells are illustrated in Fig. 4 and the detailed parameters

have been summarized in Table 1. All of the above mentioned

measurements were repeated more than three times. HTM2-based

perovskite solar cells showed a much better performance than

HTM1-based ones that contained only spiro-MeOTAD. The open-45

circuit voltage (Voc) and short-circuit current density (Jsc) of

HTM1-based perovskite solar cells were ca. 0.63 V and 12.26 mA

cm-2, respectively. The Voc and Jsc values were greatly enhanced

to ca. 0.87 V and 16.26 mA cm-2, respectively, in HTM2-based

perovskite solar cells with BuPyIm-TFSI. The fill factor, which 50

reflects the inherent resistance and the degree of chare

recombination, also increased from ca. 0.50 to 0.56. HTM2-based

perovskite solar cells achieved a PCE of 7.91% because of the

improved parameters, and this PCE value was much higher than

that of HTM1-based cells (3.83%).This result was comparable to 55

that obtained for HTM3-based cells containing LiTFSI and TBP,

as shown in Table 1.

Fig. 4. J-V curves of the peroviskite solar cells employing different HTM.

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This journal is © The Royal Society of Chemistry [year] Journal Name, [year], [vol], 00–00 | 3

To better understand the inherent mechanism underlying the

improvements in HTM2-based perovskite solar cells, we first

provide insight into the process of charge recombination by

characterizing the dark current shown in Fig. 4. An HTM2-based

solar cell exhibited a much lower dark current density than an 5

HTM1-based solar cell, indicating that BuPyIm-TFSI can

suppress charge recombination. Similar to the pronounced effect

of TBP on the interfaces of either perovsikte/TiO2 in perovskite

solar cells or dye/TiO2 in traditional solid state DSCs, BuPyIm-

TFSI also form a barrier layer that retards charge recombination 10

between electrons in TiO2 and holes in the HTM.

Fig. 5. IPCEs of the peroviskite solar cells employing different HTM

Incident photon-to-current conversion efficiencies (IPCEs) of

our perovskite solar cells were characterized and shown in Fig. 5. 15

For this type of solar cells, the spectrum rang, within which

sunlight can be effectively converted into electricity, is very wide.

The spectrum ranged from 300 nm to higher than 750 nm, with

the maximum at ca. 520 nm. Compared with HTM1-based solar

cells, HTM2- and HTM3-based perovskite solar cells 20

demonstrated higher IPCE within this broad range, especially

from 350 nm to 750 nm, where more than 50% of the photons can

be successfully converted into electricity. This result was in

agreement with J-V measurements.

In summary, the validity of ionic liquid BuPyIm-TFSI is 25

reported for the first time as a stable and dual-functional additive

to enhance HTM conductivity and suppress charge combination

in perovskite solar cells simultaneously. Various

characterizations were carried out and some relevant mechanisms

were determined to support our conclusions. BuPyIm-TFSI in 30

HTM improved photovoltaic parameters, such as photocurrent,

photovoltage, and fill factor. The PCE was enhanced remarkably.

Using such dual functional additive can effectively simplify the

components of HTM and help reduce production cost.

This work is supported by the National Natural Science 35

Foundation of China (Grant No. 51273032), Doctoral Found of

Ministry of Education of China (Grant No. 2110041110003),

International Science & Technology Cooperation Program of Chi

na (Grant No. 2013DFA51000),Fundamental Research Funds for

the Central Universities (Grant No. DUT12RC(3)57) and Open 40

Project Program of the State Key Laboratory of Physical

Chemical of Solid Surfaces, Xiamen University (Grant No.

201210). This research was also supported by the State Key

Laboratory of Fine Chemicals of China. We also thank Prof. Yi

Xiao and Mr. Youdi Zhang for helping evaporate sliver electrode. 45

Notes and references

a State Key Laboratory of Fine Chemicals, School of Chemistry, Dalian

University of Technology (DUT), 2 Linggong Rd., 116024, Dalian, P. R.

China. Fax: +86-411-84986230; Tel: +86-411-84986230;

E-mail: tinglima@dlut.edu.cn; shiyantao@dlut.edu.cn 50 b Jiangsu Key Laboratory of Advanced Functional Polymer Design and

Application, Department of Polymer Science and Engineering, College of Chemistry, Chemical Engineering and Materials Science, Soochow

University, Suzhou, 215123, P. R. China c Graduate School of Life Science and Systems Engineering,Kyushu 55

Institute of Technolog, 2-4 Hibikino, Wakamatsu, Kitakyushu, Fukuoka,

808-0196, Japan

† Electronic Supplementary Information (ESI) available: [Experimental

details, fabrication and characterization of devices].See

DOI: 10.1039/b000000x/ 60

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View Article OnlineDOI: 10.1039/C3CC49458F

The ionic liquid N-butyl-N’-(4-pyridylheptyl) imidazolium bis(trifluoromethane) sulfonimide

(BuPyIm-TFSI) was used as an additive to improve HTM conductivity and reduce dark current of

the solar cell simultaneously, thereby greatly increasing the power conversion efficiency.

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