A hydrophobic hole transporting oligothiophene for planar perovskite solar cells with improved stability

Article abstract of DOI:10.1039/c4cc04680c

An oligothiophene derivative named DR3TBDTT with high hydrophobicity was synthesized and functioned as the hole transporting material without an ion additive. 8.8% of power conversion efficiency was obtained for CH ₃NH₃PbI_3-xCl_x based planar solar cells with improved stability, compared to devices using Li-TFSI doped spiro-MeOTAD.

Full text of DOI:10.1039/c4cc04680c

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A hydrophobic hole transporting oligothiophene
for planar perovskite solar cells with improved
stability†
Cite this:DOI: 10.1039/c4cc04680c
Received 19th June 2014,
Accepted 28th July 2014
Lingling Zheng,a Yao-Hsien Chung,a Yingzhuang Ma,a Lipei Zhang,a Lixin Xiao,*ab
Zhijian Chen,ab Shufeng Wang,ab Bo Quab and Qihuang Gonga
DOI: 10.1039/c4cc04680c
www.rsc.org/chemcomm
An oligothiophene derivative named DR3TBDTT with high hydro- be avoided, because it will not only increase the costs, but also
phobicity was synthesized and functioned as the hole transporting seriously deteriorate the stability of the perovskite due to its high
material without an ion additive. 8.8% of power conversion effi- hydroscopicity leading to the decomposition of perovskite.29,30
ciency was obtained for CH3NH3PbI3ꢀxClx based planar solar cells
Oligothiophenes containing a backbone structure of a benzo-
with improved stability, compared to devices using Li-TFSI doped dithiophene (BDT) unit as the central block and ethylrhodanine
spiro-MeOTAD.
as the end group (DR3TBDTT) are highly efficient donors with a
high hole mobility of 10ꢀ4 cm2 Vꢀ1 sꢀ1 in the bulk heterojunction
Recently, organic–inorganic hybrid perovskite materials have rapidly solar cells.31–33 In this article, we modified the alkyl groups of
developed and attracted significant research interest due to their DR3TBDTT to obtain a low-lying energy level of the highest
excellent absorption and transporting properties. CH3NH3PbI3 and occupied molecular orbital (HOMO) (5.39 eV), matching with the
CH3NH3PbBr3 were firstly used as the light absorbers in liquid HOMO of perovskite (5.43 eV) (Fig. 1). As a result, the highest PCE
dye-sensitized solar cells by Miyasaka et al.1 Benefiting from the of 8.8% was achieved using DR3TBDTT adding a small amount of
solid-state hole transporting material (HTM) of 2,20,7,70- polydimethylsiloxane (PDMS) as the HTM without the ion additive,
tetrakis(N,N-di-p-methoxyphenylamine)-9,90-spirobifluorene (spiro- comparable to 8.9% for spiro-MeOTAD doped with Li-TFSI and
MeOTAD), much higher power conversion efficiencies (PCEs) have tBP. Moreover, the resultant oligothiophene possessed a high
been achieved in solid-state cells than those of liquid ones,2–7 and hydrophobicity with a water contact angle of 107.41, which can
exceeded 15% in a short term.8–13 N. J. Jeon et al. changed p-OMe be expected to prevent water penetration. So, the perovskite in
substituents of the conventional spiro-MeOTAD, 16.7% of PCE was devices without the hydroscopic Li-TFSI showed improved stability
obtained using the derivative with o-OMe substituents for meso- resulting from the moisture resistance of oligothiophene. This
structured perovskite solar cells.13 Owing to the difficulty in the work provides an alternative candidate for HTM dispensed with
purification of spiro-MeOTAD, many other triphenylamine deriva- the ion additive for stable perovskite solar cells.
tives were reported as the HTMs.14–20 However, almost all these
The solution and thin-film UV-vis absorption spectra of
HTMs reported for solution-process perovskite solar cells needed DR3TBDTT are presented in Fig. S1a.† DR3TBDTT in diluted
the addition of ion additives, e.g., lithium bis(trifluoromethyl-
sulfonyl)imide (Li-TFSI) with 4-tert-butylpyridine (tBP) to achieve
improved hole mobility and device performance.13–27 Few HTMs in
their pristine form can compete with additive doped spiro-
MeOTAD for perovskite solar cells. P. Qin et al. reported a branched
conjugated HTM working alone for meso-structured perovskite
solar cells with a high PCE of 12.8%.28 The use of Li-TFSI should
a State Key Laboratory for Mesoscopic Physics and Department of Physics,
Peking University, Beijing 100871, China. E-mail: lxxiao@pku.edu.cn
b New Display Device and System Integration Collaborative Innovation Center of the
West Coast of the Taiwan Strait, Fuzhou 350002, China
† Electronic supplementary information (ESI) available: Experimental details;
physical properties of DR3TBDTT; EDS; absorption spectra; stability of devices;
repeatability of devices; device performance and EIS fitting results. See DOI:
10.1039/c4cc04680c
Fig. 1 (a) The chemical structure of DR3TBDTT. (b) The relative energy
level diagram of a perovskite solar cell. (c) Cross-sectional SEM image of
the planar perovskite solar cell using DR3TBDTT as the HTM.
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chloroform solution exhibited an absorption peak at 528 nm. solution than the surface energy of the perovskite,36 which made it
The film spectrum showed two absorption peaks at 588 nm and extremely difficult to form a continuous and uniform film on the
614 nm, one of which is a vibronic shoulder, corresponding to perovskite layer with a roughness of tens of nanometers.4,35 With
the p-orbital overlap between the molecule backbones.31–33 The the aid of 0.1 mg mlꢀ1 PDMS, full coverage on the highly textured
optical band gap was estimated to be 1.84 eV by the absorption surface of perovskite was obtained, since the difference of the
onset of the film. The HOMO energy level was measured by surface tension and the defects of the film could be eliminated by
photoelectron spectroscopy to be 5.39 eV (Fig. S2†), only 0.04 eV its low surface tension and high viscosity.36 The introduction of
higher than the HOMO of CH3NH3PbI3ꢀxClx, indicating an Li-TFSI and tBP into DR3TBDTT solution led to a decreased
extremely low energy barrier for hole transporting. Density coverage, appearing as the smaller domains of HTM on the
functional theory (DFT) calculations showed that the HOMO perovskite. The HTM-coated perovskite films showed additional
of DR3TBDTT was mainly located at the BDT unit, and its absorption at 500–670 nm consistent with the absorption of
LUMO was mainly located at ethylrhodanine of both ends, DR3TBDTT, and slightly higher absorption for the film containing
revealing an acceptor–donor–acceptor (A–D–A) backbone structure PDMS in this region also indicated more adhesion of HTM as
(Fig. S1b†). The calculated HOMO energy level was 5.25 eV, close to shown in the SEM images (Fig. S5†).
the experimental result. Thermogravimetric analysis (TGA) and
80 nm thick Au was thermally evaporated under vacuum on
differential scanning calorimetry (DSC) analysis showed excellent the HTM as the cathode. As a comparison, devices containing
stability of DR3TBDTT, with a high melting point of 224 1C and widely used spiro-MeOTAD + Li + tBP as the HTM were also
decomposition temperature of 398 1C (Fig. S3†).
fabricated. The performance of each device under 100 mW cmꢀ2
To test the performance of DR3TBDTT, planar perovskite solar simulated AM1.5G irradiation are shown in Fig. 3a and listed in
cells based on the structure of FTO/compact TiO2/CH3NH3PbI3ꢀxClx/ Table S1.† The PCE of the device with DR3TBDTT as HTM was
HTM/Au were fabricated (Fig. 1). A 350 nm thick CH3NH3PbI3ꢀxClx 4.9%, with a low short-circuit current ( Jsc) of 12.6 mA cmꢀ2 and a
layer was fabricated by sequential deposition, using a mixture low fill factor (FF) of 0.42. By using DR3TBDTT + PDMS as the
of PbI2–PbCl2 as the precursor according to our previous HTM, the performance improved significantly with a PCE of
report.34,35 The HTM layers were deposited by spin-coating. 8.8%, a Jsc of 15.3 mA cmꢀ2, a FF of 0.60 and an open circuit
Besides using DR3TBDTT alone, we blended it with a widely- voltage (Voc) of 0.95 V, comparable to 8.9% PCE for spiro-
used flow agent of PDMS to improve the morphology.31,32,36 MeOTAD + Li + tBP based devices with a Voc of 0.93 V. A higher
The influences of the conventional additive Li-TFSI and tBP on Voc of DR3TBDTT + PDMS devices attributed to the deeper
the performance and stability were also studied.
HOMO energy level (5.39 eV) of DR3TBDTT than that of
The scanning electron microscope (SEM) images were employed spiro-MeOTAD (5.22 eV).14 But the very close HOMO between
to observe the morphology of the HTM on the perovskite layer
(Fig. 2). All the relative elements can be observed in the respective
energy dispersive spectra (EDS) (Fig. S4†). From a solution of
DR3TBDTT, the perovskite layer was only partially covered by
HTM, probably due to the much larger surface tension of the
Fig. 3 (a) J–V curves, (b) IPCE, and (c) the dark current of the planar
perovskite solar cells with different HTMs. (d) Nyquist plots and the fitting
line of perovskite solar cells in the perovskite/Au interface under the dark
Fig. 2 Top-view SEM images of perovskite layers covered by DR3TBDTT
based HTMs (scale bar = 5 mm and 1 mm).
at respective Voc. (e) Rs and Rseries obtained from EIS analysis compared to
R#series and FF deduced from the J–V curves.
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DR3TBDTT and perovskite is unfavorably for charge transfer, Moreover, DR3TBDTT with a higher content of PDMS exhibited
resulting in the lower Jsc. Devices with DR3TBDTT + Li + tBP an abnormal J–V curve demonstrating a huge resistance in the
showed the poorest performance, especially of Voc and FF. As device derived from the excess addition of insulated PDMS.
shown in Fig. 3c, at higher forward bias, the dark current Besides, the XRD results revealed no obvious difference of
increased from DR3TBDTT + PDMS to DR3TBDTT and to molecule packing between the DR3TBDTT film and the
DR3TBDTT + Li + tBP based devices, illustrating increased DR3TBDTT + PDMS film (Fig. S8†). To sum up, the role of
recombination leading to decreased Voc.37,38 As a result, the insulated PDMS was limited as a flow agent, which assisted the
0.17 eV HOMO level shift was not reflected in the extra Voc for formation of a continuous, uniform and thin HTL, resulting in
the devices based on DR3TBDTT and DR3TBDTT + Li + tBP. The a significant enhancement of device performance.
incident photon to current conversion efficiency (IPCE) (Fig. 3b)
All the devices exhibited good stabilities when stored under
showed the same shapes for all devices, suggesting that the a relatively low humidity of B20% (Fig. S9†). After 13 days of
absorption of the very thin DR3TBDTT has a negligible effect on aging, the PCEs of spiro-MeOTAD + Li + tBP and DR3TBDTT +
the solar cells.14,15 The integrated Jsc calculated from the IPCE Li + tBP based devices decreased from 8.3% to 7.7% and 3.3% to
are well matched with the Jsc from J–V curves.
3.1%, respectively, with a B6% reduction relative to the initial
Electrochemical impedance spectra (EIS) were employed to PCEs. While the performance decreased slightly from 8.3% to
explain the significant difference of FF and the relations with 8.1% for DR3TBDTT + PDMS based devices (B2% reduction) and
the morphology of the HTM. The high-intermediate frequency did not decline for DR3TBDTT based devices. However, when the
arcs are shown in Fig. 3d, representing two individual processes, devices of different HTMs were stored in a high relative humidity
corresponding to the charge-transport resistance of the Au/HTM of above 50%, the differences of device stability can be observed
interface and overlapping partially with the resistance in the obviously (Fig. 4a). Perovskite in devices containing Li-TFSI totally
HTM.37–41 We fitted the high frequency feature with a parallel decomposed from dark brown to yellow after 3 days, with a sharp
R–C circuit. The series resistance (Rs) can be obtained from the decline of PCE from 3.7% to 0.9% and 8.9% to 3.7% for
intersections of these arcs, corresponding to the resistance of the DR3TBDTT + Li + tBP and spiro-MeOTAD + Li + tBP based devices,
conducting glass, contacts and wires. The fitted resistance (Rpc) respectively (Fig. S10†). On the other hand, the perovskite in
from the arcs mostly attributed to the resistance at the Au/HTM devices using DR3TBDTT and DR3TBDTT + PDMS still appeared
interface, because the HTM formed very thin overlayers (Fig. S6†) dark brown, with the PCE changing from 4.9% to 4.4% and 8.8%
in this study and its resistance is too small to resolve.37 Therefore, to 8.0%, respectively, representing a superior stability. The
the total series resistance of the devices can be calculated as obvious difference of device stability resulted from greatly differ-
Rseries = Rs + Rpc, which has a serious effect on FF (Table S1†). As ent hydrophobicity of the HTL (Fig. 4b). DR3TBDTT films showed
a consequence, DR3TBDTT + Li + tBP based devices with the a very big water contact angle of 107.41, so that the hydrophobic
largest Rseries have the lowest FF, and DR3TBDTT + PDMS based HTLs can efficiently prevent the water penetration into the
devices achieved the highest FF of 0.60 with the smallest Rseries perovskite layer.21 With the addition of PDMS, the angle declined
of only 4.1 O cm2. The Rseries calculated from EIS matched the slightly to 99.51, still much bigger than that of spiro-MeOTAD +
series resistance calculated from the J–V curve (R#series) well and Li + tBP films. Both films containing Li-TFSI showed the smaller
had a contrary tendency of FF (Fig. 3e). A large interface resistance water contact angle of around 801, revealing an increased affinity
was derived from the partial bareness of the perovskite, leading to of water caused by Li-TFSI. Such hydroscopic ion additives should
a direct contact of perovskite and Au.38 A full coverage of HTM on be avoided in practical applications for perovskite solar cells due
the perovskite layer is advantageous to fabricate an efficient device to their negative influence on device stability.
with a high FF. Therefore, an optimized hole transporting layer
In summary, the hole transporting material DR3TBDTT was
(HTL) should be a continuous, uniform film with the minimum successfully synthesized with a well-matched HOMO energy level
thickness on top of the perovskite, consistent with the conclu- of 5.39 eV and excellent hydrophobicity for planar perovskite solar
sions made by A. Dualeh et al.37
cells. By the addition of PDMS, a continuous thin capping layer on
Li-TFSI in our study failed to enhance device performances,
likely stemmed from two effects. Firstly, the introduction of
Li-TFSI accompanying with its polar solvent into the solution of
DR3TBDTT led to a reduced coverage on the perovskite result-
ing in low FF and PCE. Secondly, the thickness of HTL using
conventional spiro-MeOTAD in devices usually needs to reach
more than 140 nm, so the ion salt is essential to assist the hole
transporting in such a thick film due to the poor pristine
mobility of spiro-MeOTAD.23,26,37 In our case, the thin capping
layer of DR3TBDTT with higher hole mobility as its analog as
reported31,32 does not need the ion additive.
To define the role of PDMS, a device exclusively using PDMS
to replace the HTM was fabricated and the performance of such
Fig. 4 (a) Devices exposed to a relative humidity 450% in air for 3 days at
room temperature without encapsulation under illumination. (b) Water
a device was as poor as the device without any HTM (Fig. S7†). contact angles of each HTM films on a glass substrate.
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¨
17 N. J. Jeon, J. Lee, J. H. Noh, M. K. Nazeeruddin, M. Graztel and
perovskite was formed as the HTL. The highest PCE for devices
using DR3TBDTT + PDMS as the HTM reached 8.8% without the
addition of Li-TFSI, comparable to 8.9% obtained from spiro-
MeOTAD + Li + tBP. However, the introduction of Li-TFSI into the
HTMs led to decreased device stability due to the hydroscopicity
of the ion additives. HTL composed of hydrophobic DR3TBDTT
can efficiently protect the perovskite layer from moisture, so that
the devices exhibited excellent stability. This work provides an
efficient candidate of ion additive-free HTMs for highly stable
perovskite solar cells.
We acknowledge the financial support from the National
Natural Science Foundation of China (61177020 and 11121091) and
the National Basic Research Program of China (2013CB328700).
The authors are thankful to Mr Xiao Yu and Prof. Dechun Zou of
the College of Chemistry and Molecular Engineering of Peking
University for their kind help in the assistance in the electro-
chemical impedance measurements.
S. I. Seok, J. Am. Chem. Soc., 2013, 135, 19087.
18 J. Wang, S. Wang, X. Li, L. Zhu, Q. Meng, Y. Xiao and D. Li, Chem.
Commun., 2014, 50, 5829.
19 S. Lv, L. Han, J. Xiao, L. Zhu, J. Shi, H. Wei, Y. Xu, J. Dong, X. Xu,
D. Li, S. Wang, Y. Luo, Q. Meng and X. Li, Chem. Commun., 2014,
50, 6931.
20 J. H. Heo, S. H. Im, J. H. Noh, T. N. Mandal, C.-S. Lim, J. A. Chang,
¨
Y. H. Lee, H.-j. Kim, A. Sarkar, M. K. Nazeeruddin, M. Graztel and
S. I. Seok, Nat. Photonics, 2013, 7, 486.
21 Y. S. Kwon, J. Lim, H.-J. Yun, Y.-H. Kim and T. Park, Energy Environ.
Sci., 2014, 7, 1454.
22 H. Chen, X. Pan, W. Liu, M. Cai, D. Kou, Z. Huo, X. Fang and S. Dai,
Chem. Commun., 2013, 49, 7277.
¨
23 H. J. Snaith and M. Graztel, Adv. Mater., 2007, 19, 3643.
24 U. B. Cappel, T. Daeneke and U. Bach, Nano Lett., 2012, 12, 4925.
25 R. Scholin, M. H. Karlsson, S. K. Eriksson, H. Siegbahn, E. M. J.
¨
Johansson and H. Rensmo, J. Phys. Chem. C, 2012, 116, 26300.
26 A. Abate, T. Leijtens, S. Pathak, J. Teuscher, R. Avolio, M. E. Errico,
J. Kirkpatrik, J. M. Ball, P. Docampo, I. McPherson and H. J. Snaith,
Phys. Chem. Chem. Phys., 2013, 15, 2572.
27 R. Katoh, M. Kasuya, S. Kodate, A. Furube, N. Fuke and N. Koide,
J. Phys. Chem. C, 2009, 113, 20738.
¨
28 P. Qin, S. Paek, M. I. Dar, N. Pellet, J. Ko, M. Graztel and
M. K. Nazeeruddin, J. Am. Chem. Soc., 2014, 136, 8516.
29 J. H. Noh, S. H. Im, J. H. Heo, T. N. Mandal and S. I. Seok, Nano Lett.,
2013, 13, 1764.
30 G. Niu, W. Li, F. Meng, L. Wang, H. Dong and Y. Qiu, J. Mater. Chem.
A, 2014, 2, 705.
Notes and references
1 A. Kojima, K. Teshima, Y. Shirai and T. Miyasaka, J. Am. Chem. Soc.,
2009, 131, 6050.
2 H.-S. Kim, C.-R. Lee, J.-H. Im, K.-B. Lee, T. Moehl, A. Marchioro,
¨
S.-J. Moon, R. Humphry-Baker, J.-H. Yum, J. E. Moser, M. Graztel 31 J. Zhou, Y. Zuo, X. Wan, G. Long, Q. Zhang, W. Ni, Y. Liu, Z. Li,
and N.-G. Park, Sci. Rep., 2012, 2, 591.
3 M. M. Lee, J. Teuscher, T. Miyasaka, T. N. Murakami and H. J. Snaith,
Science, 2012, 338, 643.
4 Q. Chen, H. Zhou, Z. Hong, S. Luo, H.-S. Duan, H.-H. Wang, Y. Liu,
G. Li and Y. Yang, J. Am. Chem. Soc., 2014, 136, 622.
5 N. Pellet, P. Gao, G. Gregori, T.-Y. Yang, M. K. Nazeeruddin, J. Maier
G. He, C. Li, B. Kan, M. Li and Y. Chen, J. Am. Chem. Soc., 2013,
135, 8484.
32 J. Zhou, X. Wan, Y. Liu, Y. Zuo, Z. Li, G. He, G. Long, W. Ni, C. Li,
X. Su and Y. Chen, J. Am. Chem. Soc., 2012, 134, 16345.
33 Y. Liu, Y. Yang, C.-C. Chen, Q. Chen, L. Dou, Z. Hong, G. Li and
Y. Yang, Adv. Mater., 2013, 25, 4657.
¨
and M. Graztel, Angew. Chem., Int. Ed., 2014, 53, 3151–3157.
34 Y. Ma, L. Zheng, Y.-H. Chung, S. Chu, L. Xiao, Z. Chen, S. Wang,
B. Qu, Q. Gong, Z. Wu and X. Hou, Chem. Commun, 2014, DOI:
10.1039/C4CC01962H.
6 H.-S. Kim, S. H. Im and N.-G. Park, J. Phys. Chem. C, 2014, 118, 5615.
7 H. J. Snaith, J. Phys. Chem. Lett., 2013, 4, 3623.
8 J. Burschka, N. Pellet, S.-J. Moon, R. Humphry-Baker, P. Gao, M. K. 35 L. Zheng, Y. Ma, S. Chu, S. Wang, B. Qu, L. Xiao, Z. Chen, Q. Gong,
¨
Nazeeruddin and M. Graztel, Nature, 2013, 499, 316.
Z. Wu and X. Hou, Nanoscale, 2014, 6, 8171.
9 D. Liu and T. L. Kelly, Nat. Photonics, 2014, 8, 133.
10 J. T. Wang, J. M. Ball, E. M. Barea, A. Abate, J. A. Alexander-Webber,
36 J. W. Du, in Coatings Technology Handbook, ed. A. A. Tracton,
CRC Press, Boca Raton, FL, USA, 3rd edn, 2006, ch. 75.
´
J. Huang, M. Saliba, I. Mora-Sero, J. Bisquert, H. J. Snaith and R. J. 37 A. Dualeh, T. Moehl, N. Tetreault, J. Teuscher, P. Gao, M. K.
¨
Nicholas, Nano Lett., 2014, 14, 724. Nazeeruddin and M. Graztel, ACS Nano, 2014, 8, 362.
11 K. Wojciechowski, M. Saliba, T. Leijtens, A. Abate and H. J. Snaith, 38 E. J. Juarez-Perez, M. Wußler, F. Fabregat-Santiago, K. Lakus-Wollny,
Energy Environ. Sci., 2014, 7, 1142.
12 M. Liu, M. B. Johnston and H. J. Snaith, Nature, 2013, 501, 395.
E. Mankel, T. Mayer, W. Jaegermann and I. Mora-Sero, J. Phys. Chem.
Lett., 2014, 5, 680.
13 N. J. Jeon, H. G. Lee, Y. C. Kim, J. Seo, J. H. Noh, J. Lee and S. I. Seok, 39 V. Gonzalez-Pedro, E. J. Juarez-Perez, W.-S. Arsyad, E. M. Barea,
J. Am. Chem. Soc., 2014, 136, 7837.
14 A. Krishna, D. Sabba, H. Li, J. Yin, P. P. Boix, C. Soci, S. G. Mhaisalkar
and A. C. Grimsdale, Chem. Sci., 2014, 5, 2702.
F. Fabregat-Santiago, I. Mora-Sero and J. Bisquert, Nano Lett., 2014,
14, 888.
40 H.-S. Kim, I. Mora-Sero, V. Gonzalez-Pedro, F. Fabregat-Santiago,
E. J. Juarez-Perez, N.-G. Park and J. Bisquert, Nat. Commun., 2013,
4, 2242.
15 T. Krishnamoothy, F. Kunwu, P. P. Boix, H. Li, T. M. Koh,
¨
W. L. Leong, S. Powar, A. Grimsdale, M. Graztel, N. Mathews and
S. G. Mhaisalkar, J. Mater. Chem. A, 2014, 2, 6305.
41 Y. Xu, J. Shi, S. Lv, L. Zhu, J. Dong, H. Wu, Y. Xiao, Y. Luo,
S. Wang, D. Li, X. Li and Q. Meng, ACS Appl. Mater. Interfaces,
2014, 6, 5651.
¨
16 H. Li, K. Fu, A. Hagfeldt, M. Graztel, S. G. Mhaisalkar and A. C.
Grimsdale, Angew. Chem., Int. Ed., 2014, 53, 4085.
Chem. Commun.
This journal is ©The Royal Society of Chemistry 2014