Green-Solvent-Processable, Dopant-Free Hole-Transporting Materials for Robust and Efficient Perovskite Solar Cells

Article abstract of DOI:10.1021/jacs.7b04949

In addition to having proper energy levels and high hole mobility (μ_h) without the use of dopants, hole-transporting materials (HTMs) used in n-i-p-type perovskite solar cells (PSCs) should be processed using green solvents to enable environmentally friendly device fabrication. Although many HTMs have been assessed, due to the limited solubility of HTMs in green solvents, no green-solvent-processable HTM has been reported to date. Here, we report on a green-solvent-processable HTM, an asymmetric D-A polymer (asy-PBTBDT) that exhibits superior solubility even in the green solvent, 2-methylanisole, which is a known food additive. The new HTM is well matched with perovskites in terms of energy levels and attains a high μ_h (1.13 × 10^-3 cm²/(V s)) even without the use of dopants. Using the HTM, we produced robust PSCs with 18.3% efficiency (91% retention after 30 days without encapsulation under 50%-75% relative humidity) without dopants; with dopants (bis(trifluoromethanesulfonyl) imide and tert-butylpyridine, a 20.0% efficiency was achieved. Therefore, it is a first report for a green-solvent-processable hole-transporting polymer, exhibiting the highest efficiencies reported so far for n-i-p devices with and without the dopants.

Full text of DOI:10.1021/jacs.7b04949

Subscriber access provided by UNIVERSITY OF ADELAIDE LIBRARIES
Article
Green-Solvent-Processable, Dopant-Free Hole-Transporting
Materials for Robust and Efficient Perovskite Solar Cells
Junwoo Lee, Mahdi Malekshahi Byranvand, Gyeongho Kang,
Sung Y. Son, Seulki Song, Guan-Woo Kim, and Taiho Park
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b04949 • Publication Date (Web): 16 Aug 2017
Downloaded from http://pubs.acs.org on August 16, 2017
Just Accepted
“Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted
online prior to technical editing, formatting for publication and author proofing. The American Chemical
Society provides “Just Accepted” as a free service to the research community to expedite the
dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts
appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been
fully peer reviewed, but should not be considered the official version of record. They are accessible to all
readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered
to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published
in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just
Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor
changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers
and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors
or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
Journal of the American Chemical Society is published by the American Chemical
Society. 1155 Sixteenth Street N.W., Washington, DC 20036
Published by American Chemical Society. Copyright © American Chemical Society.
However, no copyright claim is made to original U.S. Government works, or works
produced by employees of any Commonwealth realm Crown government in the course
of their duties.

Page 1 of 8
Journal of the American Chemical Society
Green-Solvent-Processable, Dopant-Free Hole-Transporting Materi-
als for Robust and Efficient Perovskite Solar Cells
Junwoo Lee,‡ Mahdi Malekshahi Byranvand,‡ Gyeongho Kang, Sung Y. Son, Seulki Song, GuanꢀWoo
Kim, and Taiho Park*
1
2
3
4
5
6
7
8
9
Department of Chemical Engineering, Pohang University of Science and Technology, San 31, Namꢀgu, Pohang, Gyeongbuk
790ꢀ780, Korea
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
ABSTRACT: In addition to having proper energy levels and high hole mobility (ꢀh) without the use of dopants, holeꢀtransporting
materials (HTMs) used in nꢀiꢀpꢀtype perovskite solar cells (PSCs) should be processed using green solvents to enable environmenꢀ
tally friendly device fabrication. Although many HTMs have been assessed, owing to the limited solubility of HTMs in green solꢀ
vents, no greenꢀsolventꢀprocessable HTM has been reported to date. Here we report on a greenꢀsolventꢀprocessable HTM, an
asymmetric D–A polymer (asyꢀPBTBDT) that exhibits superior solubility even in the green solvent, 2ꢀmethylanisole, which is a
known food additive. The new HTM is well matched with perovskites in terms of energy levels and attains a high ꢀh (1.13 × 10−3
cm2/Vs) even without the use of dopants. Using the HTM, we produced robust PSCs with 18.3% efficiency (91% retention after 30
days without encapsulation under 50%–75% relative humidity) without dopants; with dopants (bis(trifluoromethanesulfonyl) imide
and tꢀbutylpyridine, a 20.0% efficiency was achieved. Therefore, it is a first report for a greenꢀsolventꢀprocessable holeꢀtransporting
polymer, exhibiting the highest efficiencies reported so far for nꢀiꢀp devices with and without the dopants.
such as lithium bis(trifl uoromethanesulfonyl) imide (Liꢀ
TFSI), and ꢀbutylpyridine ( ꢀBP), primarily to control
hole mobility ( h). However, it is well known (and we
INTRODUCTION
t
t
Following the first use of perovskites as light absorption
materials by Miyasaka et al.,1 perovskite solar cells
(PSCs) have been widely considered for use in nextꢀ
generation solar cells.2ꢀ11 In general, the nꢀiꢀpꢀtype PSCs
comprise a layering of fluorineꢀdoped tin oxide (FTO), an
electron transporting layer (ETL), perovskites, a holeꢀ
transporting layer (HTL), and a counter electrode (thus,
the nꢀiꢀp typology). Tremendous efforts have been exꢀ
pended in the development of new materials (ETLs, perꢀ
ovskites, and HTLs) to improve power conversion effiꢀ
ciency (PCE). Among the various components, the develꢀ
opment of new HTLs has proven attractive not only to
materials scientists but also to academic and industrial
engineers, as the current HTL fabrication involves comꢀ
plicated fabrication steps and processes that affect the
work conditions and production cost. 2,2′,7,7′ꢀ
Tetrakis(N,Nꢀdiꢀpꢀmethoxyphenylamine)ꢀ9,9′ꢀ
spirobifluorene (Spiro-OMeTAD) is a common holeꢀ
transporting material (HTM) that has been widely emꢀ
ployed in the development of PSCs with PCEs up to
22.1%.12 However, despite its low molecular weight relaꢀ
tive to polymeric HTMs, it can only be processed in chloꢀ
rinated or harmful solvents such as chlorobenzene (CB),
dichlorobenzene (DCB), or toluene. To enable environꢀ
mentally friendly fabrication of PSCs, it will therefore be
necessary to find a greenꢀsolventꢀprocessable HTM. Nuꢀ
merous other HTMs have been developed,13ꢀ21 including
those developed by our group, 22ꢀ24 that exhibit high PCE
values. For instance, Jen et al., Nazeeruddin et al., Sun et
al., and Soek et al. independently reported a crosslinked
vinylbenzyl etherꢀfunctionalized small molecular HTM
(TCTAꢀBVP) (PCE = 18.3%),25 dissymmetric fluoreneꢀ
dithiophene (PCE = 20.2%),26 and several small molecuꢀ
lar and polymeric triarylamineꢀbased HTMs (PCEs =
16.7%–20.8%).27,28,29 However, these high PCE values
were achieved only through processing with halogenated
or harmful solvents with severe acute toxicity in human
body (see Figure S1 for the respective MSDS hazard
symbols: GHS06, GHS07, GHS08, and GHS09). In addiꢀ
tion, it was necessary to use polar additives (dopants),
ꢀ
have demonstrated) that these dopants exhibit deliquesꢀ
cence, leading to the destruction of device functions.24
Given this background, the key specifications of an ideal
HTM would include (1) a proper energy level, (2) a high
ꢀh value, (3) no use of dopants, and (4) high solubility in
nonꢀharmful solvents in order to produce a greenꢀsolventꢀ
processable PSC with a high PCE and longꢀterm stability.
As mentioned earlier, many studies have reported high
PCEs using various HTMs, including Spiro-OMeTAD
with sophisticatedly designed energy levels (criterion 1),
high h values (criterion 2), and dopantꢀfree HTMs (criteꢀ
,
ꢀ
rion 3);30ꢀ38 however, only few of these attained reasonaꢀ
ble PCEs (with efficiencies of 16.2%,39 16.3%,40 16.9%,41
and 14.5%42 attained by Yang et al., Grätzel et al., Jen et
al., and Marks et al., respectively). It is noticed that, in a
pꢀiꢀn inverted PSC, Chen et al. reported a truxeneꢀbased
HTM (TruxꢀOMeTAD), annealed at 150°C (PCE =
18.6%).43
Our group also reported a dopantꢀfree, polymeric HTM
(random copolymer (RCP)), achieving a high PCE of
17.3% and longꢀterm stability under a 75% relative huꢀ
midity (RH) condition.24 However, all of these HTMs
were processed with chlorinated or harmful solvents (and
therefore did not meet criterion (4)). The need for green
HTMs for PSCs with high efficiency and longꢀterm staꢀ
bility that meet all four of the above mentioned criteria is
clear.
Here we report a greenꢀsolventꢀprocessable, polymeric
HTM comprising benzothiadiazole (BT) and benzo[1,2ꢀ
b:4,5:b′]dithiophene (BDT) (asy-PBTBDT in Figure 1). We
design a novel D (eꢀdonor)–A (eꢀacceptor)ꢀtype conducting
polymer to easily obtain appropriate highest occupied molecuꢀ
lar orbital (HOMO) and achieve high ꢀh,22 allowing effective
hole extraction from perovskites to the HTM and hole transꢀ
portation through the HTM without the use of LiꢀTFSI and tꢀ
BP, respectively.24 To improve solubility, the length or numꢀ
ber of alkyl substituents can, in theory, be increased generally,
although as we have demonstrated,44,45 this can sometimes
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 2 of 8
Synthesis and characteristics. Detailed synthesis and
result in decreased ꢀh. Instead, we adopt asymmetric alkyl
substituents on BT units in place of the conventional symmetꢀ
ric alkyl substituent (PBTBDT) to produce irregularity within
a repeating unit of the polymeric (asy-PBTBDT) HTM.23 The
resulting novel polymeric HTM is not only soluble in the variꢀ
ous halogenated or harmful solvents mentioned above but also
highly soluble in 2ꢀMA (ca. 20 mg/ml), which is used as a
food additive (Figure 1 and see Figure S2 for that 2ꢀMA did
not destroy a perovskite layer).46 Based on this, we firstly reꢀ
port a greenꢀsolventꢀprocessable HTM, asy-PBTBDT. Thereꢀ
fore, it is a first step of replacing harmful solvents in fabricaꢀ
tion of perovskites solar cells. This makes it possible to proꢀ
duce highly stable devices with 18.3% and 20.0% without and
with the dopants, which are the highest efficiencies reported so
far for the nꢀiꢀp PSCs using polymer HTMs, employing green
solvent.
1
2
3
4
5
6
7
8
characterization of PBTBDT and asy-PBTBDT are deꢀ
scribed in Figure S3–5 and Note S1 in the Supporting
Information. Numberꢀaverage molecular weights (Mn)
and molecular weight distributions (polydispersity indices
(PDI)) were determined by GPC using CB as the eluent
and a calibration curve of polystyrene standards (Figure 1;
also see Table S1). PBTBDT is highly soluble in CB,
DCB, chloroform, toluene, oꢀxylene, and mesitylene (see
Figure S6ꢀ7 for the Hansen solubility diagram). Similarly,
asy-PBTBDT is highly soluble even in 2ꢀMA, although
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
the Mn (ca. 188
slightly higher than that of PBTBDT (180
k
Da and 1.55 PDI) of asy-PBTBDT is
Da, 1.66 PDI).
k
The solubility of asy-PBTBDT measured in this study
was ca. 20 mg/mL, which was sufficient to prepare a thick
polymer layer on perovskites. The high solubility of asy-
PBTBDT can be accounted for by the asymmetric alkyl
substituents on BT units, which lead to irregularities in
the repeating unit of the polymer.
The two polymers exhibited nearly identical intrinsic
properties in terms of absorption, oxidation, and thermal
degradation as measured using UV–Vis absorption specꢀ
troscopy (Figure S8), cyclic voltammetry (Figure S9),
thermal gravimetric analysis (Figure S10), and differential
scanning calorimetry (DSC) (see insert in Figure S10).
Although the DSC experiments produced no distinctive
thermal transitions such as glass transition or heat of meltꢀ
ing, the GIWAXS measurements on the polymer films
produced weak crystal peaks, as shown in Figure 2a (see
also insets in the GIWAXS images). Judging from a comꢀ
parison of the respective inꢀplane (100) and outꢀofꢀplane
(010) scattering peak intensities, the crystallinity of asy-
PBTBDT film processed with CB (denoted by asy-
PBTBDTCB) was lower than that of PBTBDTCB. The
lower crystallinity in asy-PBTBDTCB can be attributed to
the irregularity of its alkyl chains, enabling superior soluꢀ
bility even in 2ꢀMA. The GIWAXS images of the asy-
PBTBDT film processed with 2ꢀMA (denoted by asy-
PBTBDT2-MA) are similar to those of the asy-PBTBDTCB
film (Figure S11). The crystallinity of the asy-PBTBDT
film was not significantly influenced by the processed
solvents.
Figure 1. Chemical structures obtained by gel permeation
chromatography (GPC) showing their solubilities in 2ꢀ
methylanisole (2ꢀMA).
RESULTS AND DISCUSSION
Figure 2. Crystallinity characterization, electronic properties, and energy diagrams. (a) Grazingꢀincidence wideꢀangle Xꢀray
scattering (GIWAXS) of PBTBDT and asy-PBTBDT asy-PBTBDT shows lower crystallinity than that of the control. (b) Doꢀ
pantꢀfree asy-PBTBDT2-MA exhibited hole mobility compatible to doped common Spiro-OMeTADCB. (c) Energy diagram obꢀ
.
tained using UV–Vis spectrometry and cyclic voltammetry. The energy level of asy-PBTBDT is matched to the use of HTMs in
PSCs.
ACS Paragon Plus Environment

Page 3 of 8
Journal of the American Chemical Society
1
2
3
4
Table 1 Summary of the photovoltaic parameters obtained from the best devices. The table lists the average PCE (PCEavg),
MPP efficiency (PCEMPP), and current density (JMPP) results for two HTMs prepared using Spiro-OMeTADCB with difꢀ
ferent dopants and for an HTM prepared using asy-PBTBDT2-MA. Measurements were performed under AM 1.5 solar
illumination. The cell size is 0.09 cm2.
5
6
7
8
9
Device conditions
J-V curves
MPP efficiency
VOC
(V)
FF
PCE (PCEavg
)
PCEavg.hys
(%)
PCEMPP
(%)
JSC
JMPP
HTM
solvent
dopant
X
(mA/cm2)
(mA/cm2)
(%)
(%)
18.3 (17.4)
17.5
18.5 (17.4)
17.7
asy-PBTBDT2-MA (R) 2ꢀMA
22.4
21.8
22.1
22.0
19.8
20.1
1.11
1.10
1.11
1.09
0.99
0.98
73.2
72.5
74.9
73.5
50.4
42.0
17.9
18.1
9.1
20.5
20.6
13.4
17.4
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
(F)
Spiro-OMeTADCB (R)
CB
CB
O
X
17.5
9.1
(F)
Spiro-OMeTADCB (R)
(F)
9.9 (7.7)
8.2
Figure 3. PSC characterization. (a) Dopantꢀfree asy-PBTBDT2-MA (red solid points) and doped Spiro-OMeTADCB (blue solid
points) device statistics taken from measurements of 20 devices with an average PCE of 17.4%. (b) J–V curves of the respective
PSCs. The dopantꢀfree asy-PBTBDT2-MA exhibited performance compatible to that of doped Spiro-OMeTADCB. (c) IPCE of
devices using dopantꢀfree asy-PBTBDT2-MA and doped Spiro-OMeTADCB. (d) MPP curves of the champion cells. These values
are compatible to the corresponding J–V curve results.
Note that in our results, we denote “a polymer film processed
with a solvent” as Polymersolvent, as a processing solvent might
influence the properties of the polymer film.
PBTBDT2-MA
was in the range of 1.13–1.34 × 10−3
or CB
cm2/Vs (Figure 2b and Figure S12), which is similar to that of
the doped Spiro-OMeTADCB containing 2 mM LiꢀTFSI and
0.07 mM tꢀBP (1.80 × 10−3 cm2/Vs). Note that the ꢀh value
(ca. 1.40 × 10−4 cm2/Vs) of the dopantꢀfree Spiro-OMeTADCB
was one order of magnitude lower than that of the doped
HTM. Addition of two types of dopants to asy-PBTBDT2-MA
Hole mobility and energy levels. The hole mobility (ꢀh) of
asy-PBTBDT was measured using the space charge limited
current method. The ꢀh value of the dopantꢀfree asy-
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 4 of 8
potentially highly reliable method to estimate device perforꢀ
resulted in significant increases in its ꢀh values to 4.71 ×
or CB
10−3 and 4.82 × 10−3 cm2/Vs, respectively, along with effective
charge carrier density (Figure S13 and Table S2).
1
2
3
4
5
6
7
8
mance under real operational conditions. Similarly, the PCE
for the dopantꢀfree Spiro-OMeTADCB was ca. 9.9% (Figure.
3b), which is close to its stabilized PCEMPP value (Figure S22)
The HOMO and lowest unoccupied molecular orbital
of asy-PBTBDT were placed at ca. −5.36 and −3.55 eV,
respectively (Table S1), based on estimates of the oxidaꢀ
tion potential in cyclic voltammograms and the bandgap
value (1.81 eV) in the UV–Vis absorption spectra. This
energy value was in good agreement with a value (ꢀ5.3
eV) obtained from UPS measurement (Figure S14). Based
on this range, it would be expected that only perovskiteꢀ
generated holes could be easily transferred to asy-
PBTBDT HTL showing higher HOMO level than that of
the perovskite (ca. –5.52 eV) (Figure 2c and Figure S15).
but much lower than that for the doped Spiro-OMeTADCB
,
primarily owing to the decreased FF value in both the scan
directions. This result indicates the importance of a high HTL
ꢀh value to achieve robust photovoltaic performance in this
type of solar cell.24
The high performance of asy-PBTBDT2-MA can be primariꢀ
ly ascribed to its favorable film formability as a result of its
high solubility in 2ꢀMA and secondarily to its high ꢀh value,
which is attained using a green solvent without dopants. It was
noted that the solubility of Spiro-OMeTAD in 2ꢀMA was not
as high as that in CB and that the photovoltaic performances
of the dopantꢀfree and doped Spiro-OMeTAD2-MA (which met
criteria (1, 3, and 4) and (1, 2, and 4), respectively) were
slightly poorer than those of the corresponding Spiro-
OMeTADCB (Figure S23). A ca. 0.14 VOC value difference
would be expected between two devices using doped Spiro-
OMeTADCB and dopantꢀfree asy-PBTBDTCB based on the
energy levels shown in Figure 2c, as the HOMO level of asy-
PBTBDT (−5.36 eV) is deeper than that of Spiro-OMeTAD
(−5.22 eV).27 However, the HOMO energy level, which can
only be measured by cyclic voltammetry without dopants,
represents the neutral state of Spiro-OMeTAD in this study.
Indeed, the VOC value for the dopantꢀfree Spiro-OMeTADCB
was 0.99 V, which was ca. 0.12 V smaller than that for the
doped Spiro-OMeTADCB. It is possible that the increased VOC
value for doped Spiro-OMeTADCB is caused by the soꢀcalled
pꢀdoping (direct partial oxidization of the central amine in
Spiro-OMeTAD with LiꢀTFSI, see Figure S24a),48 which
might lower the HOMO energy level in an actual doped sysꢀ
tem.
We further investigated the photovoltaic performance of
asy-PBTBDTCB prepared with a conventional processing solꢀ
vent (CB) in the absence of any dopants (Table 2, Figure S25ꢀ
26). This type of dopantꢀfree asy-PBTBDTCB would possess
criteria 1, 2, and 3. The PCE, PCEavg.hys, and PCEMPP of the
device were 17.1%, 16.7%, and 16.6%, respectively (Figure
S27).
We noted that the PCE was similar to the value (PCE =
17.3%) obtained from the D–A type of an RCP possessing
criteria 1, 2, and 3, as previously reported by our group.24
However, the PCE from the dopantꢀfree asy-PBTBDTCB was
slightly lower than that of the dopantꢀfree asy-PBTBDT2-MA
produced in an identical manner (PCE = 18.3%).
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Photovoltaic performance. Twenty devices were fabricated
using dopantꢀfree asy-PBTBDT2-MA (Table 1). The overall
PCE values ranged from 16.0% to 18.3% (Figure S16ꢀ17),
which is comparable to that of conventional control devices
employing doped Spiro-OMeTADCB (Figure 3a and Figure
S18ꢀ19). In reverse scanning (scan rate = 0.05 V/s), the best
device employing dopantꢀfree asy-PBTBDT2-MA reached a
PCE of 18.3%, which is equivalent to the results obtained by
the best control device employing doped Spiro-OMeTADCB
(PCE of 18.5%) (Figure 3b). Furthermore, The PCE of 18.3%
is the highest value among the nꢀiꢀp PSC devices employing
dopantꢀfree polymeric HTMs up to date. To confirm the JSC
values from the J–V curves, the incident photonꢀtoꢀcurrent
efficiencies (IPCE) were measured and then used to calculate
the JSC.cal values of 21.6 and 21.3 mA/cm2 for the dopantꢀfree
asy-PBTBDT2-MA and the doped Spiro-OMeTADCB, respecꢀ
tively, which were both in close agreement with the JSC values
from the J–V curves (Figure 3c).
The maximum power point (MPP) efficiency (PCEMPP) at
each MPP was measured to estimate the device performance
under real operational conditions (Figure 3d).47 The stabilized
PCEMPP and photocurrent density (JMPP) of the dopantꢀfree
asy-PBTBDT2-MA devices were found to be 17.4% and 20.5
mA/cm2, respectively, which is equivalent to the correspondꢀ
ing values in the doped Spiro-OMeTADCB device (Table 1).
It was noted that owing to the high FF value in the dopantꢀfree
asy-PBTBDT2-MA device, its stabilized JMPP (20.5 mA/cm2)
approached the JSC value (22.4 mA/cm2) in its J–V curve.
The average efficiency (PCEavg.hys) of the reverse and forꢀ
ward scans for the dopantꢀfree asyꢀPBTBDT2ꢀMA was 17.9%,
which was similar to that for the doped Spiro-OMeTADCB
(Figure S20ꢀ21). This value was comparable to the correꢀ
sponding stabilized PCEMPP value (17.4%). The result shows
that measurement of the stabilized PCEMPP at an MPP is a
Table 2 Summary of the photovoltaic parameters obtained from best devices, showing average PCE (PCEavg), MPP effiꢀ
ciency (PCEMPP), and current density (JMPP). The devices were fabricated using different types of asy-PBTBDT using
different solvents and dopants. Measurements were performed under AM 1.5 solar illumination. The cell size is 0.09 cm2.
Device conditions
J-V curves
MPP efficiency
VOC
(V)
FF
(%)
PCE (PCEavg
)
PCEavg.hys
(%)
PCEMPP
(%)
JSC
JMPP
HTM
solvent
CB
dopant
X
(mA/cm2)
(mA/cm2)
(%)
17.1 (15.9)
16.2
20.0 (19.2)
18.8
asy-PBTBDTCB (R)
21.2
21.1
22.8
22.7
22.2
22.0
1.12
1.10
1.12
1.06
1.14
1.12
72.3
70.0
79.4
78.6
79.6
77.0
16.7
19.4
19.6
19.5
21.1
21.1
16.6
(F)
asy-PBTBDT2-MA (R)
(F)
asy-PBTBDTCB (R)
(F)
2ꢀMA
CB
O
O
19.0
18.9
20.0 (19.5)
19.1
ACS Paragon Plus Environment

Page 5 of 8
Journal of the American Chemical Society
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
Figure 4. Doped device testing of asy-PBTBDT, hole extraction ability, and long-term stability. (a) Device with doped asy-
PBTBDT2-MA (red line) shows enhanced PCE owing to increasing h with a low hysteresis phenomenon. (b) The IPCE of doped
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
ꢀ
asy-PBTBDT2-MA (red line) confirms the current density predicted by the J–V and MPP curves. (c) Pulse width of the excitation
source (=70 ps) obtained from the timeꢀresolved photoluminescence measurement of glass/perovskite/HTMs samples. (d) Longꢀ
term stability test: devices with dopantꢀfree asy-PBTBDT2-MA (red filled circle line) cells, with doped cells (open red circle
line), and the device with doped Spiro-OMeTADCB (open blue diamond line) were stored at 50%–75% RH and room temperaꢀ
ture. Comparison of J–V curves between new devices and devices aged 30 days revealed high stability in the dopantꢀfree asy-
PBTBDT2-MA cell. 2 mM LiꢀTFSI + 0.07 mM
tꢀBP were used as dopants. Noticed that the best efficiency at the first day is obꢀ
tained with doped asyꢀPBTBDT, whereas the best stability is obtained with unꢀdoped polymer.
The reason for this discrepancy is not clear, but is likely
ascribable to the polarity ( p) and boiling temperature
among all of the reported polymeric HTMs with dopants.
Using the IPCE spectrum, the JSC values were also conꢀ
firmed (see inset in Figure 4b); given the high PCEavg.hys
(19.4%) and FF values (Figure 4b), the measured high
stabilized PCEMPP (19.0%) and JMPP (21.1 mA/cm2) were
not surprising. Using CB as a processing solvent in the
presence of the dopants (thus meeting criteria 1 and 2)
(Figure S28), nearly identical efficiencies (PCE = 20.0%,
PCEavg.hys = 19.6%, and PCEMPP = 18.9%) and current
densities (JSC = 22.2 mA/cm2, JSC.cal = 22.4 mA/cm2, JMPP
= 21.1 mA/cm2) were achieved (Table 2); this was also
the result of a high ꢀh value (4.82 × 10−3 cm2/Vs)
achieved without pꢀdoping (Figure S24b).
The excellent photovoltaic performances of the devices
employing asy-PBTBDT2-MA with and without additives
were further investigated using steadyꢀstate photolumiꢀ
nescence (SSPL) (see Experimental section and Figure
S29 for details). The photoluminescence peak at 760 nm,
which originates from excitation of perovskite at 464 nm,
δ
(BP) differences between 2ꢀMA (δp = 4.7 MPa1/2, and BP
= 182°C) and CB (δp = 3.6 MPa1/2, and BP = 131°C) (the
cohesive energy density of polarity and boiling temperaꢀ
ture were calculated using the YamamotoꢀMolecular
Break (YꢀMB) method in HSPiP).49 These results suggest
that 2ꢀMA can enable better contact between a relatively
hydrophobic polymer and a hydrophilic rough perovskite
surface.50,51 As mentioned earlier, the addition of the doꢀ
pants LiꢀTFSI and
significant increase in its
t
ꢀBP to asy-PBTBDT2-MA resulted in a
ꢀ
h values to 4.71 × 10−3 cm2/Vs.
The effects of these dopants on photovoltaic performance
were therefore further studied. Earlier in this study, we
reported HTM satisfying criteria 1, 2, and 4 (Table 2).
Using 2ꢀMA solvent, The PCE values were of this HTM
were enhanced from 18.3% to ca. 20.0%, primarily owing
to the resulting increase in the FF value (73.2% →
79.4%) (Figure 4a). The efficiency is the highest value
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 6 of 8
was significantly quenched in perovskite layers covered
with doped Spiro-OMeTADCB, doped asy-PBTBDT2-MA
Corresponding Author
1
2
3
4
5
6
7
8
,
* taihopark@postech.ac.kr
or dopantꢀfree asy-PBTBDT2-MA HTLs, indicating effecꢀ
tive hole extraction from the perovskites to the HTLs in
all three cases. The HTL quenching rates were measured
using timeꢀresolved photoluminescence, producing simiꢀ
lar results for the dopantꢀfree asy-PBTBDT2-MA and
doped Spiro-OMeTADCB but a faster quenching rate in
the doped asy-PBTBDT2-MA (Figure 4c). Identical trends
were observed in HTLs processed with CB (See Figure
S30ꢀ31 for a comparison of the dopant effects on the reꢀ
spective quenching rates).
Author Contributions
‡These authors contributed equally.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENT
This work was supported by grants from the Center for Advanced
Soft Electronics under the Global Frontier Research Program
(Code No. NRFꢀ2012M3A6A5055225) and the National Research
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Finally, nonꢀencapsulated devices were stored under 50%–
75% RH conditions to measure longꢀterm stability. The 30ꢀ
day efficiency of a device processed with green solvent (2ꢀ
MA) without dopants retained ca. 91% of its initial PCE (Figꢀ
ure 4d) owing to the intrinsic hydrophobicity of the dopantꢀ
free asy-PBTBDT2-MA (Figure S32). This value was much
higher than that for the doped asy-PBTPBDT2-MA, which lost
approximately 25% of its efficiency over the test period. Furꢀ
thermore, the doped Spiro-OMeTADCB showed that the effiꢀ
ciency remarkably decreases by around 60%. The result is
consistent with the results previously reported by our group.24
Overall, the tests described above confirm that asy-PBTBDT
is the first reported HTM to possess all four of the following
criteria: (1) a proper energy level, (2) a high ꢀh value, (3) no
use of dopants, and (4) high solubility in nonꢀharmful solꢀ
vents.
Foundation
of
Korea
(NRF)
grant
(Code
No.
2015R1A2A1A10054230) funded by the Korea government
(MSIP).
REFERENCES
(1) Kojima, A.; Teshima, K.; Shirai, Y.; Miyasaka, T. J. Am. Chem.
Soc. 2009, 131, 6050ꢀ6051.
(2) Park, N.ꢀG. Mater. Today 2015, 18, 65ꢀ72.
(3) CorreaꢀBaena, J.ꢀP.; Abate, A.; Saliba, M.; Tress, W.; Jacobsson,
T. J.; Grätzel, M.; Hagfeldt, A. Energy. Environ. Sci. 2017, 10, 710ꢀ
727.
(4) Byranvand, M. M.; Song, S.; Pyeon, L.; Kang, G.; Lee, G.ꢀY.;
Park, T. Nano Energy 2017, 10, 181ꢀ187.
(5) Choi, J.; Song, S.; Hörantner, M. T.; Snaith, H. J.; Park, T. ACS
Nano 2016, 10, 6029ꢀ6036.
(6) Song, S.; Hörantner, M. T.; Choi, K.; Snaith, H. J.; Park, T. J.
Mater. Chem. A 2017, 5, 3812ꢀ3818.
(7) Song, S.; Moon, B. J.; Hörantner, M. T.; Lim, J.; Kang, G.; Park,
M.; Kim, J. Y.; Snaith, H. J.; Park, T. Nano Energy 2016, 28, 269ꢀ
276.
(8) Manser, J. S.; Christians, J. A.; Kamat, P. V. Chem. Rev. 2016,
116, 12956ꢀ13008.
(9) Ito, S.; Mizuta, G.; Kanaya, S.; Kanda, H.; Nishina, T.;
Nakashima, S.; Fujisawa, H.; Shimizu, M.; Haruyama, Y.; Nishino,
H. Phys. Chem. Chem. Phys. 2016, 18, 27102ꢀ27108.
(10) Niemann, R. G.; Gouda, L.; Hu, J.; Tirosh, S.; Gottesman, R.;
Cameron, P. J.; Zaban, A. J. Mater. Chem. A 2016, 4, 17819ꢀ17827.
(11) Guerrero, A.; You, J.; Aranda, C.; Kang, Y. S.; GarciaꢀBelmonte,
G.; Zhou, H.; Bisquert, J.; Yang, Y. ACS Nano 2015, 10, 218ꢀ224.
(12) Yang, W. S.; Park, B. –W.; Jung, E. H.; Jeon, N. J.; Kim, Y. C.;
Lee, D. U.; Shin, S. S.; Seo, J.; Kim, E. K.; Noh, J. H.; Seok, S. I.
Science 2017, 356, 1376ꢀ1379.
(13) Reddy, S. S.; Gunasekar, K.; Heo, J. H.; Im, S. H.; Kim, C. S.;
Kim, D. H.; Moon, J. H.; Lee, J. Y.; Song, M.; Jin, S. H. Adv. Mater.
2015, 28, 686ꢀ693.
(14) Zhang, J.; Xu, B.; Johansson, M. B.; Hadadian, M.; Correa
Baena, J. P.; Liu, P.; Hua, Y.; Vlachopoulos, N.; Johansson, E. M.;
Boschloo, G. Adv. Energy Mater. 2016, 6, 1502536.
CONCLUSION
In this study, we demonstrated that the asymmetric substituted
polymer asy-PBTBDT has superior solubility in the edible
food additive 2ꢀMA relative to the symmetric polymer
(PBTBDT) despite the identical carbon numbers in their reꢀ
spective alkyl groups. The ꢀh value of dopantꢀfree asy-
PBTBDT2-MA was found to be ca. 1.13 × 10−3 cm2/Vs, which
is nearly identical to that of doped Spiro-OMeTADCB (1.80 ×
10−3 cm2/Vs). Dopantꢀfree asy-PBTBDT2-MA processed using
a green solvent can be used to fabricate PSCs with high longꢀ
term stability and photovoltaic performance (i.e., a PCE of
18.3%). Addition of dopants to asy-PBTBDT2-MA significantꢀ
ly improves photovoltaic performance to up to PCE = 20%
owing to dopantꢀinduced enhancement of the ꢀh. These effiꢀ
ciencies represent the highest values in nꢀiꢀp PSCs using roꢀ
bust polymeric HTMs in case of dopantꢀfree and use of doꢀ
pant, respectively. The approach described in this paper has
the potential to commercialize the PSCs by enhancing solubiliꢀ
ty in green solvent and to further improve photovoltaic perꢀ
formance by increasing HTL ꢀh values, which in turn could
significantly step up the commercialization pathway of PSCs.
This development strategy could also be applied to a variety of
polymerꢀbased optoelectronics, including bulk heterojunction
solar cells and organic lightꢀemitting or organic fieldꢀeffect
transistors.
(15) Rakstys, K.; Abate, A.; Dar, M. I.; Gao, P.; Jankauskas, V.;
Jacopin, G. n.; Kamarauskas, E.; Kazim, S.; Ahmad, S.; Grätzel, M. J.
Am. Chem. Soc. 2015, 137, 16172ꢀ16178.
(16) Malinauskas, T.; Saliba, M.; Matsui, T.; Daskeviciene, M.;
Urnikaite, S.; Gratia, P.; Send, R.; Wonneberger, H.; Bruder, I.;
Graetzel, M. Energy. Environ. Sci. 2016, 9, 1681ꢀ1686.
(17) Xu, B.; Bi, D.; Hua, Y.; Liu, P.; Cheng, M.; Grätzel, M.; Kloo,
L.; Hagfeldt, A.; Sun, L. Energy. Environ. Sci. 2016, 9, 873ꢀ877.
(18) Li, M.ꢀH.; Hsu, C.ꢀW.; Shen, P.ꢀS.; Cheng, H.ꢀM.; Chi, Y.;
Chen, P.; Guo, T.ꢀF. Chem. Commun. 2015, 51, 15518ꢀ15521.
(19) Cheng, M.; Aitola, K.; Chen, C.; Zhang, F.; Liu, P.;
Sveinbjörnsson, K.; Hua, Y.; Kloo, L.; Boschloo, G.; Sun, L. Nano
Energy 2016, 30, 387ꢀ397.
ASSOCIATED CONTENT
Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: xx.xxxx/jacs.xxxxxxx.
Experimental details, synthesis of polymers, device fabrication,
and detailed experimental procedures
(20) Cho, I.; Jeon, N. J.; Kwon, O. K.; Kim, D. W.; Jung, E. H.; Noh,
J. H.; Seo, J.; Seok, S. I.; Park, S. Y. Chem. Sci. 2017, 8, 734ꢀ741.
(21) Park, S.; Heo, J. H.; Cheon, C. H.; Kim, H.; Im, S. H.; Son, H. J.
J. Mater. Chem. A 2015, 3, 24215ꢀ24220.
AUTHOR INFORMATION
ACS Paragon Plus Environment

Page 7 of 8
Journal of the American Chemical Society
(22) Kwon, Y. S.; Lim, J.; Yun, H.ꢀJ.; Kim, Y.ꢀH.; Park, T. Energy.
Environ. Sci. 2014, 7, 1454ꢀ1460.
(23) Kim, G. W.; Kim, J.; Lee, G. Y.; Kang, G.; Lee, J.; Park, T. Adv.
Energy Mater. 2015, 5, 1500471.
(51) Zhang, F.; Jespersen, K. G.; Björström, C.; Svensson, M.;
Andersson, M. R.; Sundström, V.; Magnusson, K.; Moons, E.;
Yartsev, A.; Inganäs, O. Adv. Funct. Mater. 2006, 16, 667ꢀ674.
1
2
3
4
5
6
7
8
9
(24) Kim, G.ꢀW.; Kang, G.; Kim, J.; Lee, G.ꢀY.; Kim, H. I.; Pyeon,
L.; Lee, J.; Park, T. Energy. Environ. Sci. 2016, 9, 2326ꢀ2333.
(25) Li, Z.; Zhu, Z.; Chueh, C. C.; Luo, J.; Jen, A. K. Y. Adv. Energy
Mater. 2016, 6, 1601165.
(26) Saliba, M.; Orlandi, S.; Matsui, T.; Aghazada, S.; Cavazzini, M.;
CorreaꢀBaena, J.ꢀP.; Gao, P.; Scopelliti, R.; Mosconi, E.; Dahmen,
K.ꢀH. Nat. Energy 2016, 1, 15017.
(27) Jeon, N. J.; Lee, H. G.; Kim, Y. C.; Seo, J.; Noh, J. H.; Lee, J.;
Seok, S. I. J. Am. Chem. Soc. 2014, 136, 7837ꢀ7840.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
(28) Yang, W. S.; Noh, J. H.; Jeon, N. J.; Kim, Y. C.; Ryu, S.; Seo, J.;
Seok, S. I. Science 2015, 348, 1234ꢀ1237.
(29) Xu, B.; Zhang, J.; Hua, Y.; Liu, P.; Wang, L.; Ruan, C.; Li, Y.;
Boschloo, G.; Johansson, E. M. J.; Kloo, L.; Hagfeldt, A. Jen, A. K.
Y.; Sun, L. Chem, 2017, 2, 676ꢀ687.
(30) Liu, J.; Wu, Y.; Qin, C.; Yang, X.; Yasuda, T.; Islam, A.; Zhang,
K.; Peng, W.; Chen, W.; Han, L. Energy. Environ. Sci. 2014, 7, 2963ꢀ
2967.
(31) Kazim, S.; Ramos, F. J.; Gao, P.; Nazeeruddin, M. K.; Grätzel,
M.; Ahmad, S. Energy. Environ. Sci. 2015, 8, 1816ꢀ1823.
(32) Song, Y.; Lv, S.; Liu, X.; Li, X.; Wang, S.; Wei, H.; Li, D.; Xiao,
Y.; Meng, Q. Chem. Commun. 2014, 50, 15239ꢀ15242.
(33) Franckevičius, M.; Mishra, A.; Kreuzer, F.; Luo, J.; Zakeeruddin,
S. M.; Grätzel, M. Mater. Horiz. 2015, 2, 613ꢀ618.
(34) Zhao, X.; Zhang, F.; Yi, C.; Bi, D.; Bi, X.; Wei, P.; Luo, J.; Liu,
X.; Wang, S.; Li, X. J. Mater. Chem. A 2016, 4, 16330ꢀ16334.
(35) Zhang, J.; Xu, B.; Yang, L.; Mingorance, A.; Ruan, C.; Hua, Y.;
Wang, L.; Vlachopoulos, N.; Lira‐Cantú, M.; Boschloo, G.; Hagfeldt,
A.; Sun, L.; Johansson, E. M. J. Adv. Energy Mater. 2017, 7, 1602736
(36) Lu, H.; Ma, Y.; Gu, B.; Tian, W.; Li, L. J. Mater. Chem. A 2015,
3, 16445ꢀ16452.
(37) Jiang, X.; Yu, Z.; Zhang, Y.; Lai, J.; Li, J.; Gurzadyan, G. G.;
Yang, X.; Sun, L. Sci. Rep. 2017, 7, 42564.
(38) Zhang, F.; Liu, X.; Yi, C.; Bi, D.; Luo, J.; Wang, S.; Li, X.; Xiao,
Y.; Zakeeruddin, S. M.; Grätzel, M. ChemSusChem 2016, 9, 2578ꢀ
2585.
(39) Liu, Y.; Hong, Z.; Chen, Q.; Chen, H.; Chang, W. H.; Yang, Y.
M.; Song, T. B.; Yang, Y. Adv. Mater. 2016, 28, 440ꢀ446.
(40) Zhang, F.; Yi, C.; Wei, P.; Bi, X.; Luo, J.; Jacopin, G.; Wang, S.;
Li, X.; Xiao, Y.; Zakeeruddin, S. M. Adv. Energy Mater. 2016, 6,
1600401.
(41) Li, Z. a.; Zhu, Z.; Chueh, C.ꢀC.; Jo, S. B.; Luo, J.; Jang, S.ꢀH.;
Jen, A. K.ꢀY. J. Am. Chem. Soc. 2016, 138, 11833ꢀ11839.
(42) Liao, H. C.; Tam, T. L. D.; Guo, P.; Wu, Y.; Manley, E. F.;
Huang, W.; Zhou, N.; Soe, C. M. M.; Wang, B.; Wasielewski, M. R.;
Chen, L. X.; Kanatzidis, M. G.; Facchetti, A.; Chang, R. P. H.; Marks,
T. J. Adv. Energy Mater. 2016, 6, 1600502.
(43) Huang, C.; Fu, W.; Li, C.ꢀZ.; Zhang, Z.; Qiu, W.; Shi, M.;
Heremans, P.; Jen, A. K.ꢀY.; Chen, H. J. Am. Chem. Soc. 2016, 138,
2528ꢀ2531.
(44) Lee, G.ꢀY.; Han, A.ꢀR.; Kim, T.; Lee, H. R.; Oh, J. H.; Park, T.
ACS Appl. Mater. Interfaces 2016, 8, 12307ꢀ12315.
(45) Son, S. Y.; Kim, Y.; Lee, J.; Lee, G.ꢀY.; Park, W.ꢀT.; Noh, Y.ꢀ
Y.; Park, C. E.; Park, T. J. Am. Chem. Soc. 2016, 138, 8096ꢀ8103.
(46) Zhang, H.; Yao, H.; Zhao, W.; Ye, L.; Hou, J. Adv. Energy
Mater. 2016, 6, 1502177.
(47) Tao, C.; Neutzner, S.; Colella, L.; Marras, S.; Kandada, A. R. S.;
Gandini, M.; De Bastiani, M.; Pace, G.; Manna, L.; Caironi, M.
Energy. Environ. Sci. 2015, 8, 2365ꢀ2370.
(48) Abate, A.; Leijtens, T.; Pathak, S.; Teuscher, J.; Avolio, R.;
Errico, M. E.; Kirkpatrik, J.; Ball, J. M.; Docampo, P.; McPherson, I.;
Snaith, H. J. Phys. Chem. Chem. Phys. 2013, 15, 2572ꢀ2579.
(49) Sherwood, J.; Constantinou, A.; Moity, L.; McElroy, C. R.;
Farmer, T. J.; Duncan, T.; Raverty, W.; Hunt, A. J.; Clark, J. H.
Chem. Commun. 2014, 50, 9650ꢀ9652.
(50) Giovambattista, N.; Debenedetti, P. G.; Rossky, P. J. J. Phys.
Chem. B 2007, 111, 9581ꢀ9587.
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 8 of 8
1
2
For Table of Contents Only
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
ACS Paragon Plus Environment