Similar or different: the same Spiro-core but different alkyl chains with apparently improved device performance of perovskite solar cells

Article abstract of DOI:10.1007/s11426-018-9432-x

By intelligently utilizing the odd-even effect existing in the melting points of alkanes as presented in the basic textbook of Organic Chemistry, different alkoxy groups were introduced to modify the structure of commercial Spiro-OMeTAD to give new Spiro derivatives of Spiro-OEtTAD, Spiro-OPrTAD, Spiro-OiPrTAD and Spiro-OBuTAD, with the aim to adjust the molecular packing status in perovskite solar cells as hole transporting compounds. Excitedly, with the introduction of ethoxy groups instead of the methoxy ones in Spiro-OMeTAD, Spiro-OEtTAD-based perovskite solar cells demonstrated the highest device performance of 20.16%, higher than that of Spiro-OMeTAD (18.64%).

Full text of DOI:10.1007/s11426-018-9432-x

SCIENCE CHINA
Chemistry
•ARTICLES•
https://doi.org/10.1007/s11426-018-9432-x
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Similar or different: the same Spiro-core but different alkyl chains
with apparently improved device performance of perovskite solar
cells
Fan Liu1†, Shiqing Bi2†, Xiaorui Wang1, Xuanye Leng2, Mengmeng Han1, Baoda Xue2,
Qianqian Li1*, Huiqiong Zhou2* & Zhen Li1,3*
1Department of Chemistry, Hubei Key Lab on Organic and Polymeric Opto-Electronic Materials, Wuhan University, Wuhan 430072, China;
2CAS Key Laboratory of Nanosystem and Hierachical Fabrication, CAS Center for Excellence in Nanoscience National Center for
Nanoscience and Technology, Beijing 100190, China;
3Institute of Molecular Aggregation Science, Tianjin University, Tianjin 300072, China
Received November 7, 2018; accepted January 28, 2019; published online February 25, 2019
By intelligently utilizing the odd-even effect existing in the melting points of alkanes as presented in the basic textbook of
Organic Chemistry, different alkoxy groups were introduced to modify the structure of commercial Spiro-OMeTAD to give new
Spiro derivatives of Spiro-OEtTAD, Spiro-OPrTAD, Spiro-OiPrTAD and Spiro-OBuTAD, with the aim to adjust the molecular
packing status in perovskite solar cells as hole transporting compounds. Excitedly, with the introduction of ethoxy groups instead
of the methoxy ones in Spiro-OMeTAD, Spiro-OEtTAD-based perovskite solar cells demonstrated the highest device perfor-
mance of 20.16%, higher than that of Spiro-OMeTAD (18.64%).
perovskite solar cells, alkyl chains, molecular packing
Citation:
Liu F, Bi S, Wang X, Leng X, Han M, Xue B, Li Q, Zhou H, Li Z. Similar or different: the same Spiro-core but different alkyl chains with apparently
improved device performance of perovskite solar cells. Sci China Chem, 2019, 62, https://doi.org/10.1007/s11426-018-9432-x
1 Introduction
HTMs exhibit a critical role in extracting photogenerated
holes from the perovskite layer, transporting these charges to
the back contact metal electrode, and minimizing re-
combination losses at the TiO2/perovskite/HTM interface,
directly affecting the device performance [3]. Accordingly,
the investigation of HTMs has always been one of the hottest
topics in PSCs [4]. So far, most of HTMs can be divided into
inorganic compounds, such as CuSCN, CuI and CuOx [5],
and organic ones including p-type polymers and small mo-
lecules [6,7]. Among them, unique merits of organic small
molecule-based HTMs have some advantages of well-de-
fined chemical structures, precise molecular weight and
batch-to-batch reproducibility, contributing to the exploring
of the relationship between molecular design and device
performance.
Perovskite solar cells (PSCs), as a renewable energy source,
have become one of the most promising approaches to solve
the current energy crisis and environmental pollution. From
2009 to now, PSCs have quickly gotten their photoelectric
conversion efficiency (PCE) over 23% [1]. Most of the
highly efficient PSCs are based on the device configuration
of n-i-p [2], with the perovskite absorber being sandwiched
between a titanium dioxide semiconductor as electron-
transporting materials (ETMs) and a p-type semiconductor
as hole-transporting materials (HTMs). In PSC devices,
†These authors contributed equally to this work.
*Corresponding authors (email: qianqian-alinda@163.com; zhouhq@nanoctr.cn;
lizhen@whu.edu.cn)
© Science China Press and Springer-Verlag GmbH Germany, part of Springer Nature 2019
chem.scichina.com link.springer.com

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Liu et al. Sci China Chem
The modification of the molecular packing has always
been the focus in the research field of organic opto-electronic
materials [8]. As to HTMs, the molecular packing is also
vital to achieve highly efficient PSCs. For example, Na-
zeeruddin and co-workers [9] designed KR216 (4,4′-di-
methoxydiphenylamine-substituted 9,9′-bifluorenylidene) as
HTM, in which, the double bond resulted in three different
types of packing orientation for the optimization of the de-
vice performance. Asiri and co-workers [10] have designed
three D-π-D type organic semiconductors incorporating TPA
units with thiophene or thienothiophene moiety through an
ethylene unit as the π-bridge, making the molecules stack
with the edge-to-face packing mode, to facilitate the efficient
charge extraction from perovskite layer. More recently, Zhu
and co-workers [11] used an electron deficient quinoxaline
unit as core to construct D-A-D type HTMs of TQ1 and TQ2
with phenyl and methylthiophene as substituent groups re-
spectively, to achieve tight molecular packing for the im-
provement of the device performance. Actually, carefully
analyzing the molecular structures of small organic HTMs
with device performances over 20% (Chart S1, Supporting
Information online) [12–19], most of the excellent HTMs
possess the spiro structure, partially adjusting the molecular
packing status in solid state to enhance the device perfor-
mance.
On the other hand, from the basic textbook of Organic
Chemistry, it is well known that the carbon number of al-
kanes affects their melting points badly, especially for those
with the carbon number less than 15. For alkanes with the
even carbon number, the increasing trend of the melting
point is much larger than that of the odd ones, due to the
much tighter molecular packing of alkanes with the even
carbon numbers (Figure 1), which is named as the odd-even
effect. Thus, it is reasonable that the same aromatic core with
different alkyl substituents would exhibit different molecular
packing status in solid state, as evidenced by the previous
molecular design of ours and other scientists [20]. However,
regrettably, the odd-even effect of alkyl chains has not re-
ceived great interests in the design of high performance
HTMs for PSCs.
Figure 1 The molecular design idea and molecular structures of target
HTMs (color online).
Herein, we would like to present their syntheses, character-
ization, and device performance in detail.
2 Experimental
1
Experimental Subheading: H and 13C NMR spectra were
measured on a Bruker Avance-400 spectrometer (Germany).
Mass spectra were measured on a ZAB 3F-HF spectro-
photometer. Elemental analyses of carbon, hydrogen, and
nitrogen were performed on a CARLOERBA-1106 micro-
analyzer. UV-vis absorption spectra were recorded on a
Shimadzu UV-2500 recording spectrophotometer (Japan).
Cyclic voltammograms were carried out on a CHI660E
electrochemical workstation at room temperature in nitro-
gen-purged anhydrous dichloromethane with tetrabutylam-
monium hexafluorophosphate (TBAPF6) as the supporting
electrolyte at a scanning rate of 100 mV s−1. A platinum disk
and a Ag/AgCl electrode were used as the working electrode
and quasi-reference electrode, respectively. The ferrocene
was used as the internal reference for calibration.
Synthesis: the materials, detailed synthetic routes and
synthesis of intermediates are shown in Supporting In-
formation online.
General procedure for the preparation of HTMs: to a
Prompted by the above points, and also considering that
the commercialized Spiro-OMeTAD is the most popular
HTM, in this article, different alkyl chains were introduced
to the Spiro core, to yield the corresponding new Spiro-
derivatives with the different length of the alkoxy chain as
Spiro-OMeTAD, Spiro-OEtTAD, Spiro-OPrTAD, Spiro-
OiPrTAD and Spiro-OBuTAD (Figure 1). Really, as ex-
pected, these derivatives demonstrated different melting
points, conductivities, packing modes and device perfor-
mance. The best PSC performance employing Spiro-OEt-
TAD as the hole transporting layer, achieved a maximum
PCE value of 20.16%, outperforming that of Spiro-OMe-
TAD (18.64%) under the same experimental conditions.
mixture
of
2,2′,7,7′-tetrabromo-9,9′-spirobi[fluorene]
(1 mmol), diarylamines (4.8 mmol), PtBu3 (0.12 mmol), Pd
(OAc)2(0.04 mmol) and potassium tert-butoxide (1.4 mmol)
in a nitrogen flushed round bottom flask, toluene (30 mL)
was added and the mixture was bubbled with nitrogen for
15 min. Then the reaction was refluxed for 48 h before
quenching by water. The resultant mixture was extracted for
three times by CH2Cl2, the organic layer was collected,
combined, and dried over Na2SO4 for 2 h. After removal of
solvent, the crude product was purified by column chroma-
tography on silica gel.
Spiro-OMeTAD: a light yellow powder with the yield of
79%. 1H NMR (400 MHz, THF-d8, δ): 7.40–7.38 (d, 4H, J=

3
Liu et al. Sci China Chem
8.3 Hz, –ArH), 6.90–6.88 (m, 16H, –ArH), 6.77–6.75 (m,
3 Results and discussion
20H, –ArH), 6.51 (s, 4H, –ArH), 3.72 (s, 24H, –CH2). 13C
NMR (100 MHz, THF-d8, δ): 156.72, 151.19, 148.61,
142.47, 136.37, 126.26, 123.23, 120.71 118.49, 115.34,
55.69; MS (ESI, m/z): [M+H]+ calcd for: C81H68N4O8,
1225.4; Found, 1224.7. Anal. Cald for C81H68N4O8: C, 79.39;
H, 5.59; N, 4.57. Found: C, 79.42; H, 5.64; N, 4.77.
Spiro-OEtTAD: a light yellow powder with the yield of
79%. 1H NMR (400 MHz, THF-d8, δ): 7.39–7.37 (d, 4H, J=
8.3 Hz, –ArH), 6.87–6.86 (d, 16H, J=8.9 Hz, –ArH), 6.76–
6.73 (m, 20H, –ArH), 6.51–6.50 (d, 4H, J=2.0 Hz, –ArH),
3.97–3.92 (m, 16H, –CH2), 1.36–1.32 (t, 24H, J=7.0 Hz,
–CH3). 13C NMR (100 MHz, THF-d8, δ): 156.07, 151.19,
148.62, 142.33, 136.39, 126.24, 123.27, 120.66, 118.54,
115.89, 64.31, 15.41; MS (ESI, m/z): [M+H]+ calcd for:
C89H84N4O8, 1337.6; Found, 1336.7. Anal. Cald for C89H84
N4O8: C, 79.91; H, 6.33; N, 4.19. Found: C, 80.39; H, 6.29;
N, 4.46.
Spiro-OPrTAD: a light yellow powder with the yield of
79%. 1H NMR (400 MHz, Acetone-d6, δ): 7.48–7.46 (d, 4H,
J=8.3 Hz, –ArH), 6.92–6.89 (m, 16H, –ArH), 6.84–6.82 (m,
16H, –ArH), 6.79–6.77 (dd, 4H, J=8.3, 2.2 Hz, –ArH), 6.44
(s, 4H, –ArH), 3.91–3.88 (t, 16H, J=6.5 Hz, –CH2), 1.79–
1.74 (dd, 16H, J=14.0, 6.6 Hz, –CH2), 1.03–0.99 (t, 24H, J=
7.4 Hz, –CH3). 13C NMR (100 MHz, THF-d8, δ): 156.22,
151.19, 148.61, 142.36, 136.37, 126.25, 123.23, 120.65,
118.47, 115.90, 70.40, 23.76, 11.09; MS (ESI, m/z): [M+H]+
calcd for: C97H100N4O8, 1449.9; Found, 1449.8. Anal. Cald
for C97H100N4O8: C, 80.36; H, 6.95; N, 3.86. Found: C,
80.74; H, 6.93; N, 4.05.
The straightforward synthetic routes of Spiro-derivatives are
illustrated in Scheme S1 (Supporting Information online).
Briefly, the derivatives were synthesized by classical Buch-
wald-Hartwig C–N coupling reaction using the Spiro-core
tetrabromide and 4,4′-dialkoxydiphenylamine. The detailed
synthetic procedures and the related nuclear magnetic re-
sonance (NMR) spectroscopy, mass spectroscopy, and ele-
mental analysis are presented in Supporting Information
online (Figures S1–S15).
The absorption spectra of HTMs are shown in Figure 2(a),
with the corresponding photophysical data summarized in
Table 1. In dichloromethane, all Spiro-derivatives exhibit an
absorption peak at 387 nm with the same optical band gap
(Eg) of about 2.95 eV, regardless of their different alkoxy-
substitution. As measured by cyclic voltammetry in di-
chloromethane (Figure 2(b) and Table 1), the Spiro-deriva-
tives show very similar highest-occupied molecular orbital
(HOMO) levels, and the values (from −5.20 to −5.06 eV) are
lower than that of the perovskite (CH3NH3PbI3) valence band
(−5.43 eV) [21], assuring the successful extracting and
transferring of holes from the perovskite layer to the metal
contact.
In order to gain more insight into the electronic properties
of Spiro-derivatives, the optimized structures (Figure S16)
and frontier molecular orbitals (Figure 3) were obtained
through density functional theory (DFT) calculations
(B3LYP/6-31g*). All HTMs exhibit a largely twist 3D
configuration as the result of the Spiro-space geometry
structure, which plays an important role in forming excellent
amorphous layers with full coverage on the perovskite layer
and facilitates the isotropic hole transport. The calculated
Spiro-OiPrTAD: a light yellow crystal with the yield of
1
63%. H NMR (400 MHz, DMSO-d6, δ): 7.48–7.45 (d, 4H,
J=8.3 Hz, –ArH), 6.83–6.76 (m, 32H, –ArH), 6.71–6.68 (dd,
J=8.3, 2.1 Hz, 4H), 6.17–6.18 (s, 4H, –ArH), 4.50–4.47 (m,
8H, –CH–), 1.24–1.22 (d, 48H, J=6.0 Hz, –CH3). 13C NMR
(100 MHz, THF-d8, δ): 154.86, 151.18, 148.59, 142.32,
136.38, 126.22, 123.24, 120.63, 118.54, 117.36, 70.70,
22.61; MS (ESI, m/z): [M+H]+ calcd for: C97H100N4O8,
1449.9; Found, 1448.8. Anal. Cald for C97H100N4O8: C,
80.36; H, 6.95; N, 3.86. Found: C, 80.72; H, 7.06; N, 3.98.
Spiro-OBuTAD: a light yellow powder with the yield of
39%. 1H NMR (400 MHz, Acetone-d6, δ): 7.48–7.46 (d, 4H,
J=8.3 Hz, –ArH), 6.91–6.89 (m, 16H, –ArH), 6.84–6.81 (m,
16H, –ArH), 6.79–6.76 (dd, 4H, J=8.3, 2.1 Hz, –ArH), 6.44
(s, 4H, –ArH), 3.96–3.93 (t, 16H, J=6.4 Hz, –CH2), 1.75–
1.72 (dd, 16H, J=8.6, 6.4 Hz, –CH2), 1.52–1.46 (m, 16H, –
CH2), 0.98–0.94 (t, 24H, J=7.4 Hz, –CH3); 13C NMR
(100 MHz, THF-d8, δ): 156.21, 151.17, 148.57, 142.37,
136.31, 126.24, 123.15, 120.66, 118.41, 115.83, 68.51,
32.65, 20.35, 14.42; MS (ESI, m/z): [M+H]+ calcd for:
C105H116N4O8, 1562.1; Found, 1561.9. Anal. Cald for
C105H116N4O8: C, 80.73; H, 7.49; N, 3.59. Found: C, 80.99;
H, 7.62; N, 3.70.
Figure 2 (a) UV-vis absorption spectra of the Spiro-HTMs in CH2Cl2
solutions. (b) Cyclic voltammograms of Spiro-HTMs in CH2Cl2 solutions.
(c) Thermogravimetric analysis curves of Spiro-HTMs under nitrogen at
10 °C min−1 of heating rate. (d) Differential scanning calorimetry of Spiro-
HTMs under nitrogen at heating rate of 20 °C min−1 (color online).

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Liu et al. Sci China Chem
Table 1 The electrochemical, photophysical data of the HTMs and photovoltaic parameters for the PSCs devices based on Spiro-derivatives
c)
d)
HTM
Eg (eV) a)
2.95
EHOMO b)/ELOMO (eV) λabs,max (nm)
Td (°C) e)
388
Tm (°C) f)
248
Tg (°C) g)
167
Tc (°C) h)
Spiro-OMeTAD
Spiro-OEtTAD
Spiro-OPrTAD
Spiro-OiPrTAD
Spiro-OBuTAD
−5.20/−2.25
−5.09/−2.13
−5.06/−2.11
−5.13/−2.19
−5.17/−2.12
387
387
387
387
388
177
232
–
2.96
306
239
229
2.95
362
268
226
2.94
354
322
–
–
2.95
292
233
188
–
a) Band gaps estimated from the optical absorption band edge of the solution. b) Calculated from the onset oxidation potentials of the compounds. c)
Estimated using the empirical equation ELUMO=EHOMO+Eg. d) Values derived from absorption spectra in dilute CH2Cl2 solution. e) 5% weight loss temperature
measured by TGA under N2. f) Melting temperature measured by DSC under N2. g) Glass transition temperature measured by DSC under N2. h)
Crystallization temperature measured by DSC under N2.
Spiro-derivatives in the in-plane direction, indicating that
after prolonging alkyl chains, Spiro-OEtTAD, Spiro-OPr-
TAD, Spiro-OiPrTAD and Spiro-OBuTAD have a new la-
mellar stacking with a more promoted preferring face-on
orientation. In addition, all HTMs show the similar π-π
packing peaks in the in-plane direction and present the si-
milar orientation in the out-of-plane direction. As a result,
Spiro-derivatives exhibit larger extent of packing and en-
hanced charge transporting properties in comparison with
Spiro-OMeTAD.
To evaluate the photovoltaic properties of these Spiro-
Figure 3 The HOMO/LUMO distribution of the Spiro-HTMs (color
online).
derivatives in PSC devices, we fabricated the devices with
the standard planar n-i-p device configuration of FTO/TiO2/
Perovskite (CH3NH3PbI3)/HTM/Au. As depicted in Figure 5
(a), Spiro-derivatives acted as new HTMs and Figure 5(b)
shows the energy level diagrams of the related HTMs. All
devices were fabricated by solution spin-coating processes
and the detailed device fabrication procedures were pre-
sented in Supporting Information online. Figure 6(a) shows
the current density-voltage (J-V) curves of PSCs, with the
corresponding parameters summarized in Table 2. The Spiro-
OEtTAD-based device shows the highest PCE value of
20.16%, with the open-circuit voltage (Voc) of 1.11 V, the
short-circuit current density (Jsc) of 23.93 mA cm−2, and the
fill factor (FF) of 75.65%. The device based on Spiro-
OBuTAD also yields the outstanding performance with the
PCE of 19.04% (Voc of 1.10 V, Jsc of 23.01 mA cm−2 and FF
of 75.77%). In comparison, upon changing the even alkoxy
to odd one of the peripheral group, the PCE decreases to
18.64% for Spiro-OMeTAD, 16.86% for Spiro-OPrTAD,
and 16.59% for Spiro-OiPrTAD. The Jsc values matched with
the intergrated Jsc values from the IPCE spectra shown in
Figure 6(b). Moreover, J-V curves of Spiro-derivatives-
based PSCs under different scan conditions are shown in
Figure 6(c) and Figure S18, and both of them exhibit small
hysteresis (hysteresis indexs are 0.014, 0.004, 0.001, 0.001
and 0.002, respectively).
trend of HOMO and LUMO matches the electrochemical
data, and no obvious difference was found in frontier mo-
lecular orbital distribution among the Spiro-derivatives.
Thermogravimetric analysis (TGA) exhibits the good ther-
mal stability of the Spiro-derivatives, with the decomposi-
tion temperature (Td) higher than 290 °C (Figure 2(c) and
Table 1). The differential scanning calorimetry (DSC) mea-
surements show their glass transition temperature (Tg) of
167 °C for Spiro-OMeTAD [21], 229 °C for Spiro-OEtTAD,
226 °C for Spiro-OPrTAD, and 188 °C for Spiro-OBuTAD
(Figure 2(d) and Table 1). Undoubtedly, their melting points
are different (248 °C for Spiro-OMeTAD, 239 °C for Spiro-
OEtTAD, 268 °C for Spiro-OPrTAD, 322 °C for Spiro-
OiPrTAD and 233 °C for Spiro-OBuTAD) as the result of the
different linked alkyl chains, confirming the influence of the
alkyl chains on the molecular packing. Furthermore, grazing-
incidence wide-angle X-ray scattering (GIWAXS) mea-
surements could confirm the different influence of alkyl
chains once again. As shown in Figure 4, Figure S17 and
Table S1 (Supporting Information online), with the change
from the methoxy chain to other alkoxy ones, the values of
the crystal correlation length (CCL) are similar but with a
little decreasing trend. In detail, the CCL values were cal-
culated to be about 1.587 to 1.154 nm in the in-plane di-
rection as well as 1.478 to 1.111 nm in the out-of-plane
direction for the Spiro-derivatives. In comparison with
Spiro-OMeTAD, a new peak (q≈0.4 Å−1) appeared for other
In order to further investigate the reasons for different
photovoltaic performance of the PSC devices based on these
Spiro-derivatives, we investigated the hole mobility in CH3
NH3PbI3 with HTMs using single carrier devices, which

5
Liu et al. Sci China Chem
Figure 4 GIWAXS patterns and the corresponding in-plane and out-of-plane profiles (inset) for pure films of (a) Spiro-OMeTAD, (b) Spiro-OEtTAD, (c)
Spiro-OPrTAD, (d) Spiro-OiPrTAD, (e) Spiro-OBuTAD and (f) in-plane line cuts of the Spiro-derivatives (color online).
respectively. Also, as shown in Figure S20, hole re-
organization energies (λhole) were evaluated at the B3LYP/6-
31G* level, and the λhole value (Table S3) of Spiro-OMeTAD
(166 meV) is larger than those of other Spiro-derivatives (the
values of Spiro-OEtTAD to Spiro-OBuTAD are 151, 153,
162, and 151 meV, respectively). Obviously, perovskite
layers combined with even alkoxy Spiro-derivatives exhibit
the higher hole mobility in comparison with odd ones, and
the λhole values of even alkoxy Spiro-derivatives are smaller
Figure 5 Device structure of the planar PSCs (a) and energy level dia-
than those of odd alkyl Spiro-derivatives. Furthermore, we
conducted the conductivity measurements to test the elec-
trical conductivity (Figure S21) of Spiro-derivatives. The
corresponding conductivity values were calculated using
ohm’s law (Table S2), and the highest conductivity value was
found for Spiro-OEtTAD (1.76×10−5 S cm−1) followed by
gram of the Spiro-derivatives (b) used in this study (color online).
Spiro-OBuTAD
(1.72×10−5 S cm−1),
Spiro-OPrTAD
(1.68×10−5 S cm−1), Spiro-OMeTAD (1.67×10−5 S cm−1) and
Spiro-OiPrTAD (1.63×10−5 S cm−1). Thus, accompanying
with changing the length of alkyl, the conductivity values
show a similar trend with the hole moblity of perovskite and
Spiro-derivatives, and λhole values. These results demon-
strated that the intermolecular charge transfer of Spiro-de-
rivatives was significantly affected by the alkyl groups. The
different hole mobility and conductivity of Spiro-derivatives
can led to their different Jsc values and devices performance.
With the highest hole mobility and conductivity, the Spiro-
OEtTAD-based device shows the best performance.
The morphology of these Spiro-derivatives on the per-
ovskite film was studied by atomic force microscope (AFM).
Apparently, the root mean square (RMS) of even-alkoxy
Spiro-derivatives was smaller than that of Spiro-OMeTAD,
which was beneficial to the overall performance (Figure
S22). Moreover, steady state PL spectra of the perovskite
films on different HTM substrates were measured (Figure 7
(b)), to investigate the influence of Spiro-derivatives on the
hole extraction in CH3NH3PbI3. Upon the addition of Spiro-
derivatives, an enhanced PL quenching effect was observed
Figure 6 (a) J-V characteristics of the PSC devices employing Spiro-
derivatives as HTMs. (b) IPCE spectra of the PSC devices based on dif-
ferent HTMs. (c) J-V characteristics of Spiro-OEtTAD-based PSCs under
different scan conditions (color online).
were attained by sandwiching the perovskite layer and Spiro-
derivatives between ITO/PEDOT:PSS (bottom) and Au (top)
contacts. Figure 7(a) and Table S2 show the curves and data,
and the hole mobility values for Spiro-OMeTAD to Spiro-
OBuTAD with perovskite were calculated to be 4.52×10−9,
5.83×10−9, 4.76×10−9, 4.76×10−9 and 5.97×10−9 cm2 V−1 s−1,

6
Liu et al. Sci China Chem
Table 2 The electrochemical, photophysical data of the HTMs and photovoltaic parameters for the PSCs devices based on Spiro-derivatives
a)
HTM
Jsc (mA cm−2)
22.53
Voc (V)
1.10
FF (%)
75.04
75.65
69.27
70.14
75.77
PCE (%)
Calculated Jsc (mA cm−2)
Spiro-OMeTAD
Spiro-OEtTAD
Spiro-OPrTAD
Spiro-OiPrTAD
Spiro-OBuTAD
18.64 b) (18.08±0.52) c)
20.16 (19.75±0.42)
16.86 (16.73±0.53)
16.59 (16.50±0.66)
19.04 (19.47±0.71)
21.58
22.35
21.67
21.29
21.56
23.93
1.11
22.98
1.06
22.57
1.05
23.01
1.09
a) The integrated current density from the IPCE spectra. b) The best PCE values of the devices. c) The statistical data of the device performance with 20
individual cells.
4 Conclusions
In conclusion, we have developed and synthesized new
Spiro-bifluorenes molecules (Spiro-derivatives) as hole
transporting materials, in which, different alkoxy chains
(OMe, OEt, OPr, OiPr and OBu) were introduced to the para
position of diphenylamine. As expected, the different alkoxy
groups lead to many different properties, mainly as the result
of the different molecular packing in solid state. Excitedly,
PSCs based on Spiro-OEtTAD achieved the highest PCE
value of 20.16%, much higher than that of Spiro-OMeTAD
(18.64%). This confirms the importance of the molecular
packing, providing a new approach to further optimize the
present excellent HTMs and the next rational molecular
design.
Figure 7 (a) The J-V curves measured by SCLC characteristic based on
single carrier devices. (b) Steady state PL spectra of the perovskite films
with or without the capping HTMs. (c) The unencapsulated devices sta-
bility test of the devices in ambient (room temperature, 30% RH) (color
online).
Acknowledgements This work was supported by the National Natural
Science Foundation of China (21734007, 51573140, 51673151, 21773045),
the Natural Science Foundation of Hubei Province (2017CFA002), and the
Fundamental Research Funds for the Central Universities (2042017kf0247,
2042018kf0014).
with the pristine perovskite film, and the larger degree of PL
quenching appeared for the perovskite with even-alkoxy
Spiro-derivatives bilayer rather than the odd-alkoxy ones.
Reasonably, the perovskite/Spiro-OEtTAD bilayer exhibits
the largest PL quenching degree, meaning its highest hole
extraction ability for the corresponding best device perfor-
mance. Thus, all the obtained experimental results of steady
PL decay, λhole, mobility, and conductivity, could give a fully
explanation for the different device performance.
Conflict of interest The authors declare that they have no conflict of
interest.
Supporting information The supporting information is available online at
http://chem.scichina.com and http://link.springer.com/journal/11426. The
supporting materials are published as submitted, without typesetting or
editing. The responsibility for scientific accuracy and content remains en-
tirely with the authors.
To check if changing the alkyl chain can improve the
stability of devices, we measured the stability of none-
ncapsulation devices with Spiro-derivatives and the related
PCE decay curves are presented in Figure 7(c). After being
stored in ambient air under a relative humidity of 30% RH,
the PCE of device with Spiro-OMeTAD decreased quickly
and reduced to 55% of the original efficiency after being
stored for 22 d. However, the other four HTMs’ devices
could retain more than 75% of the original PCE, especially
the devices based on Spiro-OEtTAD and Spiro-OBuTAD
still obtained over 85% of the original PCE. That is to say,
the longer alkyl chain of Spiro-core HTMs, the better devices
stability.
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