A structure-property study of fluoranthene-cored hole-transporting materials enables 19.3% efficiency in dopant-free perovskite solar cells

Article abstract of DOI:10.1039/c9sc01697j

To date, most of the prevailing organic hole-transporting materials (HTMs) used in perovskite solar cells (PVSCs), such as spiro-OMeTAD and PTAA, generally require a sophisticated doping process to ensure their reasonable hole-transporting properties. Unfortunately, the employed dopants/additives and the associated oxidation reactions have been shown to deteriorate the long-term device stability seriously. The exploitation of efficient and stable dopant-free HTMs is thus strongly desired for PVSCs. However, effective molecular design strategies for dopant-free HTMs are still lacking. Thus far, only a few of them yielded comparable performance to their doped counterparts, while their synthetic costs are still high. In this work, a new class of cost-effective small molecule dopant-free HTMs have been developed using readily available fluoranthene as the structural framework. The structure-property correlation of the fluoranthene-based HTMs was carefully investigated by tuning their structural geometry (linear vs. branched), connection between electron-donating and electron-withdrawing moieties (single bond vs. ethylene), and the substitution position of the methoxy side-groups (para-vs. meta-). As a result, the optimized molecule, FBA3, was demonstrated to serve as an efficient dopant-free HTM in a conventional PVSC to deliver an impressive power conversion efficiency of 19.27%, representing one of the best cost-effective dopant-free organic HTMs reported thus far.

Full text of DOI:10.1039/c9sc01697j

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DOI: 10.1039/C9SC01697J
ARTICLE
A Structure-Property Study of Fluoranthene-cored Hole-
Transporting Materials Enables 19.3% Efficiency Dopant-free
Perovskite Solar Cells
Received 00th January 20xx,
Accepted 00th January 20xx
Xianglang Sun,†a Fei Wu, †b Cheng Zhong,c Linna Zhu*b and Zhong’an Li*a
DOI: 10.1039/x0xx00000x
To date, most of the prevailing organic hole-transporting materials (HTMs) used in perovskite solar cells (PVSCs), such as
Spiro-OMeTAD and PTAA, generally require a sophisticated doping process to ensure their reasonable hole-transporting
property. Unfortunately, the employed dopants/additives and the associated oxidation reactions have been shown to
deteriorate the long-term device stability seriously. The exploitation of efficient and stable dopant-free HTMs is thus
strongly desired for PVSCs. However, effective molecular design strategies for dopant-free HTMs are still lacking. Thus far,
only few of them yielded comparable performance to the doped counterparts, while their synthetic costs are still high. In
this work, a new class of cost-effective small molecule dopant-free HTMs have been developed using readily available
fluoranthene as the structural framework. The structure-property correlation of the fluoranthene-based HTMs was
carefully conducted by tuning their structural geometry (linear Vs branch), connection between electron-donating and
electron-withdrawing moieties (single bond Vs ethylene), and the substitution position of the methoxy side-groups (para-
Vs meta-). As a result, the optimized molecule, FBA3, was demonstrated to serve as an efficient dopant-free HTM in a
conventional PVSC to deliver an impressive power conversion efficiency of 19.27%, representing one of the best cost-
effective dopant-free organic HTMs reported thus far.
Introduction
It is worth noting that the employed charge-transporting
layers (CTLs) also play a critical factor in affecting the overall
Since the debut in 2009,1 organic-inorganic hybrid perovskite
solar cells (PVSCs) have made an impressive achievement and
the record power conversion efficiency (PCE) has exceeded
23% recently.2 With such great performance, more recent
research focus pertaining to PVSCs has been shifted to their
long-term operational stability in order to fulfill further
stability of the derived PVSCs.3-9 For example, the commonly
used electron-transporting material (ETM), titanium dioxide
(TiO2), has been shown to engender photodegradation of
perovskite materials under ultraviolet light, thereby
deteriorating the device performance.10 Meanwhile, the
prevailing organic/polymeric hole-transporting materials
commercial applications,3, because the perovskites have
4
(HTMs),
such
as
2,2′,7,7′-tetrakis(N,N-di-p-
(Spiro-OMeTAD,
shown to possess a poor stability under external stimuli such
as light, moisture, oxygen, and heat. Besides using device
encapsulation to address such disadvantage, more
fundamental approaches including composition engineering
and interface engineering have been vigorously developed in
recent years aiming to intrinsically enhance the materials’
robustness.5-8
methoxyphenylamine)-9,9′-spirobifluorene
Figure 1) and poly(triarylamine) (PTAA) have also been
demonstrated to be unfavorable for long-term operational
device stability owing to the required ionic doping process.3, 4, 9
Although the doping indeed improves the charge transport
property of the HTMs, the used dopants/additives, like
bis(trifluoromethane)sulfonimide lithium salt (LiTFSI) and 4-
tert-butylpyridine (tBP), are very hygroscopic and deliquescent
and the oxidation process is generally associated with
undesired ion migration/interactions. All these factors
contribute to an inferior long-term operational stability of the
derived PVSCs.11, 12
On one hand, using inorganic HTMs to replace the organic
counterparts seem to be a feasible solution to address this
shortage since they generally possess higher hole mobility and
chemical stability than the latter.13 Nevertheless, most of the
inorganic HTMs require harsh processing conditions that are
a.Key Laboratory for Material Chemistry of Energy Conversion and Storage,
Ministry of Education, School of Chemistry and Chemical Engineering, Huazhong
University of Science and Technology, Wuhan, 430074, P. R. China
E-mail: lizha@hust.edu.cn
b.Chongqing Key Laboratory for Advanced Materials and Technologies of Clean
Energy, Faculty of Materials & Energy, Southwest University, Chongqing, 400715,
P. R. China E-mail: lnzhu@swu.edu.cn
c. Department of Chemistry, Wuhan University, Wuhan, 430072, P. R. China
†Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/x0xx00000x
† These authors contributed to this work equally.
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incompatible with the perovskites, which impedes the
fabrication of efficient PVSCs especially for the n-i-p device
configuration. Compared to the inorganic materials, the
organic semiconductors generally possess milder processing
conditions and have a much better flexibility in the molecular
design.14-16 Therefore, their optoelectronic properties could be
effectively tailored through synthetic manipulation. It is worth
noting that some recent studies have developed HTM-free
PVSCs based on the carbon electrode; however, all of them
seem to compromise the resultant efficiency.17-19 As a result,
the development of dopant-free HTMs is urgently desired to
solve the stability issue caused by the doped HTMs. To date,
View Article Online
DOI: 10.1039/C9SC01697J
only few of the dopant-free HTMs have been reported to show
21
comparable PCEs to the doped Spiro-OMeTAD,20,
which
reveals effective molecular design strategies for dopant-free
HTMs are still lacking so far.
Figure 1. Chemical structures of spiro-OMeTAD and the studied fluoranthene-based
The dopant-free HTMs exploited thus far, including both
small molecules22-28 and polymers29-34, are mainly based on the
HTMs.
donor-acceptor (D-A) type design. For example, Sun et al.25, 26 fluoranthene core (single bond Vs ethylene), as well as the
and Yang et al.27, 28 have independently reported a series of A- substitution positions of methoxy groups in the TPA units
D-A type small molecule dopant-free HTMs using two (para- Vs meta-) are also tuned to elucidate how these
dimensional benzo[1,2-b:4,5:b′]dithiophene derivatives (2D- structural variations affect HTM properties and the resultant
BDT) as the donor moiety to realize efficient PVSCs (PCE > performance in PVSCs.
17%). Meanwhile, Park et al. prepared a new class of 2D-BDT-
based D-A type conjugated polymers to serve as efficient
Results and discussion
dopant-free HTMs.29-31 On the other hand, the branched
structure has also been successfully adopted in designing
Notably, we have previously used the fluoranthene as the
efficient dopant-free HTMs.35-39 By integrating these two core structure to synthesize BTF2
(Figure 1), which exhibits a
molecular strategies, Nazeeruddin et al. developed a new class very low lab synthetic cost of 11.41 $/g.46 Unfortunately,
of star-shaped D-A type dopant-free HTMs using quinolizino owing to its high-lying highest occupied molecular orbital
acridine (FA) and triazatruxene as the core moiety to yield a (HOMO) level (-4.8 eV) and low hole mobility of 2.13
×
10-5
41
high PCE of 19.03%.40,
Note that, despite these cm2V-1s-1, BTF2 cannot serve as an efficient dopant-free HTM
achievements, all of those top-performing dopant-free HTMs to realize high-performance PVSCs. On this basis, we further
developed so far are derived from complicated π-conjugated designed a new synthetic chemistry based on the typical Diels-
scaffolds requiring tedious synthesis and purification, such as Alder reaction (Scheme S1), by which electron-withdrawing
BDT25-31, quinolizino acridine (FA)40, triazatruxene41 and so on, cyano groups were introduced into the fluoranthene core that
making their synthetic costs too expensive to be used for not only down-shifts the HOMO level (~5.02 eV) but also
widespread applications. Therefore, it motivates us to develop improve the hole mobility (> 10-4 cm2V-1s-1). Consequently,
high-performance dopant-free HTMs with low synthetic while serving as the dopant-free HTM in the n-i-p planar PVSC,
complexity for scale-up to afford a well balance between the dicyanofluoranthene-cored BTF4 (Figure 1) could deliver a
efficiency, stability, and materials cost.
much higher PCE of 18.03% than that of BTF2 (10.45%).46
To find out the inherent regularity of designing low-cost but
Note that replacing diphenylamine by TPA units could be an
high-performance dopant-free HTMs, we herein develop a alternative approach to down-shift the HOMO level by
new class of fluoranthene-cored HTMs Figure
and weakening the material’s donating ability.48 This molecular
(
1)
systematically investigate their structure-property correlation. design has also been proven as an effective approach to
The fluoranthene unit is selected as the main structural increase the hole mobility as a result of the enhanced π-π
framework due to its several promising advantages including interactions.49, 50 In this manner, we prepared FBA1 and FTA1
cheap price, highly planar structure, easy functionality, and with linear and branched structure, respectively, wherein the
good thermal/electrochemical stability.42-44 Moreover, the para-methoxy substituted TPA units are connected with
unique central cyclopenta-fused ring endows the fluoranthene fluoranthene through the single bond. Meanwhile, the
with an electron-deficient character,45 which thereby can be ethylene connection between TPA units and fluoranthene core
used as an electron-withdrawing core to construct various D-A was also employed to afford linear FBA2 and branched FTA2
,
47
type HTMs.46,
Herein, we functionalize the fluoranthene with a purpose of increasing the molecular planarity to
moiety with methoxy-substituted triphenylamine (TPA) units improve the intermolecular π-π interactions. Besides the
on the 3-, 8- positions or 3-, 8-, 9- positions to form a linear linking bridge, the meta-methoxy substituted TPA units was
and star-shaped structure, respectively. In addition, the also utilized to prepare FBA3 in parallel with FBA2 consisting of
connection between the TPA units and the central the para-methoxy substituents. As compared to the para-
2 | J. Name., 2012, 00, 1-3
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Table1. Relevant synthetic costs, photophysical properties and charge transfer properties of the studied HTMs.
View Article Online
DOI: 10.1039/C9SC01697J
b
b
c
e
f
h
h
Costa
($/g)
λsol
λfil
Eg
HOMOd
(eV)
Tg
Td
Mobilityg
(cm2V−1S−1)
τ1
τ2
HTMs
BTF2
FBA1
FBA2
FBA3
FTA1
FTA2
(nm)
(nm)
(eV)
(℃)
(℃)
[ns]
[ns]
11.41
12.55
40.50
14.75
19.39
86.10
482
493
2.21
-4.80
-5.00
-4.98
-5.07
-5.00
-4.99
/
411
2.89×10-5
8.91×10-5
1.36×10-4
2.12×10-4
4.83×10-5
1.07×10-4
2.45
16.37
430
473
455
417
463
436
474
456
426
466
2.48
2.24
2.29
2.49
2.21
85
95
371
429
429
435
376
1.56
1.67
1.10
1.71
1.51
11.39
10.44
8.90
78
110
100
12.35
9.80
a Synthetic costs. b Absorption maxima of low-energy band in dichloromethane solutions and as thin films. c Optical bandgaps calculated from solution absorption
edges. d Measured from electrochemistry experiments, EHOMOs are calculated according to an equation of EHOMO = – (4.8 + Eox) eV. e Glass transition temperature
detected by DSC analyses under nitrogen with a heating rate of 10 oC/min. f The 5% weight loss temperature detected by the TGA analyses under nitrogen at a
h
heating rate of 10 oC/min. g Hole mobilities measured by SCLC method. τ1 and τ2 correspond to the fitted fast and slow decay lifetime, respectively, based on
methoxy substituted TPA, the meta-methoxy substituted TPA of Spiro-OMeTAD (170~475 $/g),54 attributed to the facile
could further lower the HOMO level of the derived HTMs synthesis with simple purification processes. The highest
according to an inductive effect.51, 52 Through these different synthetic cost of FTA2 is mainly due to its low isolated yield
structural modifications, we thus are able to conduct a (15.7%) caused by a large loss during purification.
systematic structure-property investigation to provide
valuable insights for designing efficient dopant-free HTMs.
Figure 2 illustrates the density functional theory (DFT)-
optimized structures of these new fluoranthene-based HTMs.
The synthetic route of FBA1-3 and FTA1-2 is shown in After introducing ethylene linkages, the torsional angles
Scheme S2, while the experimental details are described in between fluoranthene core and the attached phenyl groups
Supporting Information (SI). The synthesis is really can be significantly decreased. This is particularly pronounced
straightforward, only involving two- or three- reaction steps for the angle setting on the 8 position of fluoranthene core,
under mild reaction conditions. As shown, by controlling the from 32.9o (FBA1) to 2.4o (FBA2) and 3.0o (FBA3), respectively.
bromination of fluoranthene, 3, 8-dibromofluoranhe (
8, 9-tribromofluoranthene ( ) can be readily obtained, which TPA units and fluoranthene core could make the structure
then underwent a typical Pd-catalyzed Suzuki or Heck - more coplanar, which would be beneficial to hole transport.
coupling reaction with different functionalized TPA units ( On the other hand, larger torsional angles on the 8 and 9
and ) to yield the desired linear HTMs (FBA1 ) and branched positions of fluoranthene are observed after forming branched
HTMs (FTA1 ), respectively. All these new molecules were structure. As seen, from FBA1 to FTA1, such torsional angle
1) and 3, These results thus suggest the ethylene connection between
2
3, 4
7
-3
-
2
well-characterized by spectroscopic methods, and gave increases from 32.9 o to 51.2o, while from FBA2 to FTA2, it
satisfactory data (See SI for the details). The material costs are increases from 2.4o to 23.1o. We speculate it possibly arises
estimated following the referenced method53 (see details in from the steric interactions between the neighboring TPA units
Table S1
-
S5). As shown in Table 1, most of them exhibit on 8, 9-positions of fluoranthene.
The thermal properties of these new fluoranthene-cored
relatively low lab synthetic costs from 12.55 $/g (FBA1) to 86.1
$/g (FTA2), which are much lower than the commercial price HTMs were measured using differential scanning calorimetry
(DSC, Figure 3a) and thermogravimetric analysis (TGA, Figure
S1a), and the related data are summarized in Table 1. All the
HTMs show high thermal stabilities, with the 5% weight loss
o
temperatures (Tds) over 370 C. For an ideal HTM, a high glass
transition temperature (Tg) is often required to maintain a
robust film morphology during device fabrication and
operation.15 As seen, most of our prepared HTMs show high
Tgs with the highest one of 110 oC, being comparable to the
value of Spiro-OMeTAD. It is interesting to note that there is a
clear correlation between the molecular structure and the
resulting Tgs. The Tgs were effectively increased from 85 oC
o
o
o
(
FBA1) and 95 C (FBA2) to 110 C (FTA1) and 100 C (FTA2),
respectively. This result indicates the branched structure
possesses better thermal stability. The linking bridge between
TPA units and fluoranthene core play a less important role in
Figure 2. The DFT-optimized geometrical structure of studied HTMs.
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solutions are given in Figure S3, wherein most ofVtihewemArticdleisOpnllainye
DOI: 10.1039/C9SC01697J
reversible oxidative processes, except for FBA3. Their HOMO
levels versus Fc/Fc+ were determined based on the average
oxidation potentials or onset ones and are summarized in
Table 1. Figure 3c illustrates their energy-level alignment
relative to Spiro-OMeTAD and perovskite. As expected, by
replacing the diphenylamine unit in BTF2 with the TPA unit,
the resultant HOMO level of FBA1 can be efficiently decreased
by ~0.20 eV, which is comparable to that of BTF4 with cyano-
substitutions on the fluoranthene core.46 While changing the
linear structure to the branched structure or changing single
bond linking bridge to ethylene linking bridge, the resultant
HOMO levels show a negligible variation. It is interesting to
note that FBA3 possesses a deeper-lying HOMO level than
FBA2, which is more compatible with the valence band (VB) of
Figure 3. (a) DSC curves under nitrogen with a heating rate of 10 oC/min. (b)
The absorption spectra in DCM solutions. (c) Corresponding energy levels
relative to Spiro-OMeTAD and perovskite. (d) Color variation of pristine
fluoranthene-cored HTMs films on a glass substrate before (up) and after
(down) exposure to I2 vapor for 5 min.
perovskite. This result is similar to those reported in the
52
literatures,51,
and could be attributed to the decreased
electron-donating ability resulted by meta-methoxy
substituents.
affecting the resultant Tg. FBA2 exhibits a higher Tg than FBA1
The HOMO and lowest unoccupied molecular orbital
possibly due to its more planar structure. However, the meta- (LUMO) levels of these molecules were also simulated by DFT
methoxy substitutions were found to decrease Tg of FBA3 by calculations, with results shown in Figure S4. The LUMOs of all
o
~17 C by comparison with FBA2, possibly due to the more HTMs are located mainly on the central fluoranthene core,
asymmetric structure.
The absorption spectra of these HTMs in dichloromethane entire π-conjugated system, particularly for linear molecules.
solutions and as thin films are presented in Figure 3b and S1b These results reveal a substantial overlap between LUMO and
while the relevant HOMOs are almost distributed over the
,
respectively, with relevant data listed in Table 1. The lowest- HOMO that will be beneficial to the generation of excitons and
energy absorption band around 400~600 nm is ascribed to the hole transport.55, 56 In addition, the trend of calculated HOMO
intramolecular charge transfer (ICT) interactions between TPA levels is also in good accordance with those from CV
units (D) and fluoranthene core (A), while the localized π−π* measurements. By introducing ethylene linkage or forming
transition of TPA units correspond to other higher energy branched structure, the calculated HOMOs of corresponding
absorption bands. As introduced earlier, the diphenylamine HTMs can be increased accordingly, while meta-methoxy
unit has a stronger electron-donating ability than the TPA unit. substituents are found to effectively reduce the calculated
Consequently, BTF2 shows the most red-shifted absorption HOMO of FBA3 with a value of -4.86 eV.
band compared to others. Due to the extended conjugation of
The HTM’s antioxidant capacity is also an important
the ethylene linking bridge, the absorption maxima (λabs) of parameter for the long-term stability of its derived device,
both FBA2 and FTA2 exhibit a red-shift of ~40 nm when which is closely related to its HOMO level.57 To clarify this, we
comparing with FBA1 and FTA1. Interestingly, when the linear have tested the chemical stability of these fluoranthene-cored
structures (FBA1/FBA2) convert to the branched structures HTMs using I2 vapor, given that the HOMO levels of ~5.0 eV for
-
(
FTA1/
FTA2), there will be a blue-shift in λabs observed, these compounds are close to the oxidation potential of I-/I3
possibly due to the more twisted structure of the latter that and the reaction with I2 from perovskites might occur at the
will make ICT interaction less efficient. On the other hand, associated interface. As displayed in Figure 3d, after exposure
FBA3 exhibits a blue-shifted λabs compared with FBA2, which to I2 vapor for 5 min, a notable color change from light orange
confirms the weaker electron donating ability of meta- to brown was observed for the film
methoxy substituted TPA unit relative to the para-methoxy owing to their higher-lying HOMO levels. In contrast, FBA1 and
substituted one as discussed earlier. The optical bandgap (Eopt FTA1 films only showed a slight color change. As considering
s of BTF2, FBA2, and FTA2
,
Table 1) were calculated from the absorption edges in DCM that FBA2 and FTA2 only possess slightly higher HOMO levels
solutions, with the values from 2.21 eV to 2.49 eV. Moreover, (< 0.02 eV) than FBA1 and FTA1, the readily oxidation with I2 of
all the materials’ film absorption spectra exhibit similar former could be partially due to the existence of unstable
features to their solution spectra albeit with slightly red- ethylene group. However, despite containing the ethylene
shifted, broadened absorption band, indicating the absence of groups, the film of FBA3 remained its original color, showing
aggregation.
the best chemical stability among the studied HTMs. This
The energy levels of these fluoranthene-cored HTMs were result strongly underlines the importance of the deep-lying
characterized using electrochemical cyclic voltammetry (CV). HOMO level for an ideal HTM.
Their corresponding CV curves measured from the DCM
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Figure 4. (a) The hole injection characteristics measured by the SCLC method based on a device structure of ITO/PEDOT:PSS/HTM/MoOx(10 nm)/Ag. (b) Steady PL spectra
and (c) time-resolved PL spectra of bare perovskite (PV) films and bi-layered perovskite films capped with different fluoranthene-cored HTMs.
The hole-transporting properties of these fluoranthene- FTA2
> FBA1 > FTA1 > BTF2, which is closely consistent with
cored HTMs were evaluated by the space-charge-limited- mobility. Time-resolved PL measurements were also
current (SCLC) method without adding any dopants in a device conducted to study their hole-extraction efficiency. As shown
configuration of ITO/PEDOT:PSS/HTMs/MoOx/Ag. The in Figure 4c and Table 1, after introducing HTM layers, the
characterized curves are shown in Figure 4a, with the decay time of corresponding bi-layered films was significantly
estimated hole mobilities given in Table 1. As seen, the hole shortened by comparison with that of bare perovskite film
mobility (μh) of the studied compounds can be significantly (~49 ns). Besides, the decline trend of decay time is quite
improved by rational structural modifications. In addition to similar to that of PL quenching efficiency, in which FBA3 shows
BTF2 and FTA1, all the compounds show respectable μh close the shortest decay time of 8.9 ns. All these results confirm an
to or beyond 10-4 cm2 V-1 s-1, exceeding the typical value of efficient hole transfer from perovskite to HTMs.
non-doped Spiro-OMeTAD (~2
×
10-5 cm2 V-1 s-1)46. This result
To realize an efficient n-i-p PVSCs, besides the respectable
indicates their promising potential to serve as dopant-free hole-transporting capability, the HTMs need to form
HTMs in the n-i-p PVSCs. Note that, from BTF2 to FBA1 and homogenous films atop the perovskite layer since the high
FBA2, the μhs has gradually increased from 2.91
×
10-5 cm2 V-1 s- quality HTM layer can effectively reduce the interfacial charge
to 8.91× ×
10-5 cm2 V-1 s-1 and 1.36 10-4 cm2 V-1 s-1. Meanwhile, recombination loss and prevent the contact between
1
FTA2 also exhibits a much higher μh than FTA1. These perovskite layer and metal electrode.9, 14-16 To confirm their
observations therefore strongly manifest that both the thin-film formation capabilities, atomic force microscopy
replacement of diphenylamine by TPA unit and the (AFM) images of the prepared fluoranthene-cored HTM films
introduction of ethylene linkage can effectively improve the atop the perovskite layer were investigated as shown in Figure
hole-transporting capability of the derived HTMs, largely due
5. In principle, the morphology is highly related to the
to the enhanced π-π interactions. Further, the meta-methoxy molecular structure of a molecule, in addition to solubility and
substituted TPA units endow the FBA3 with a highest μh processability.58 As seen, from BTF2 to FBA1 and FBA2, the
among the studied HTMs (2.12
comparable to that of doped Spiro-OMeTAD (4.87
×
10-4 cm2 V-1 s-1), which is even film quality was improved accordingly, accompanied with a
10-4 cm2 V-1 reduction of pin-holes. On the other hand, the films of FBA3
×
,
s-1, Figure S6). Unexpectedly, we noticed that both FTA1 and FTA1, and FTA2 showed more uniform morphology. For FTA1
FTA2 exhibit decreased μhs compared to the FBA1 and FBA2; it and FTA2, it can be easily rationalized by their more
might be due to their weak intermolecular interactions in the amorphous state confirmed by their high Tgs and weak
solid-state caused by three-dimensional branched structure. intermolecular interactions. Besides, their branched structure
These data thus are consistent with those theoretical
calculation results. As demonstrated in Figure 2, the ethylene
linking bridge makes the structure more coplanar to enhance
intermolecular interactions, while the steric effect derived
from the substituents on the neighboring 8, 9 positions of
fluoranthene core potentially reduces the intermolecular
interactions of branched materials (FTA1 and FTA2).
To assess the hole transfer efficiency between the studied
compounds
photoluminescence (PL) spectra of the bi-layered MAPbIxCl3-
x/non-doped HTM films were examined as shown in Figure 4b
and
perovskite,
the
steady-state
.
As shown, the PL of perovskites at ~780 nm was effectively
quenched when capping with different HTM layers. This result
suggests the designed HTMs are capable of extracting holes
from the perovskite even in the absence of dopants. Notably,
Figure 5. AFM images of fluoranthene-cored HTM films atop perovskite layer.
the quenching efficiency follows a trend of FBA3
≈ FBA2 >
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might also contribute to the improved film-forming ability. As achieve the best performance (Figure S8). Based on this new
View Article Online
DOI: 10.1039/C9SC01697J
Figure 6. (a) Cross-sectional SEM image of the complete PVSC device. (b) J–V curves of the champion PVSCs with different dopant-free HTMs and doped Spiro-OMeTAD. (c)
Histograms of photovoltaic parameters. Environmental stability of PVSCs based on dopant-free FBA2, FBA3 and FTA2 and doped Spiro-OMeTAD performed under simulated
sunlight in a nitrogen ambience (d) or in ambient air with a humidity of 20–30% (e).
for FBA3, the improved film morphology may be attributed to device structure, BTF2 yielded a higher PCE of 13.44% than the
that in comparison with FBA2, the molecular symmetry can be previously reported value of 10.45%. Encouragingly, the PVSCs
further destroyed due to meta-methoxy substitutions on the using the newly-prepared HTMs all show improved
TPA units.59
photovoltaic performance than the control BTF2 device. It thus
To test the effectiveness of these prepared fluoranthene reveals the success of rational molecular modifications in our
derivatives as the dopant-free HTMs, a typical n-i-p planar design of dopant-free HTMs. Impressively, the best performing
PVSC was fabricated in
a
device configuration of PVSC with a promising PCE of 19.27% was derived from FBA3,
ITO/C60/perovskite/HTL/MoO3/Ag. The device fabrication outperforming the control devices (17.57%) using doped Spiro-
details are described in SI. Shown in Figure 6a is the cross- OMeTAD HTM. This result is also among the best for dopant-
sectional scanning electron microscopy (SEM) image of the free HTMs reported thus far.20-41 In addition, FBA2 and FTA2
FBA3-based device, in which the stratified device configuration also delivered respectable PCEs of 18.70% and 17.73%,
+
can be seen obviously. MAPbIxCl3-x (MA: CH3NH3 ) was selected respectively. The stabilized PCE and photocurrent of the
as the photoactive layer instead of (FAPbI3)0.85(MAPbBr3)0.15 by champion PVSCs using these dopant-free HTMs near the
considering that its VB of 5.4 eV matches with the HOMO maximum power point were also tested as shown in Figure S9
.
levels of our designed HTMs much better. For comparison, a These results clearly manifest the high reliability of our J-V
control device using the regularly doped Spiro-OMeTAD as the curves and the absence of current hysteresis, combined with
HTM was also fabricated with the same fabrication results from those J-V curves with forward and reverse
procedures.
scanning direction (Figure S7). Figure S10 displays their
The current density–voltage (J–V) curves of the champion corresponding incident photon-to-electron conversion
PVSCs measured under AM 1.5 G irradiation at 100 mWcm-2 efficiency (IPCE) spectra, in which the integrated short-circuit
are shown in Figure 6b and S7, and the related photovoltaic current density (Jsc) values show well consistency with the
parameters are summarized in Table 2. The processing values obtained in the J-V measurements, affirming the well
solution concentration of each HTM was carefully optimized to reliability of these device results.
6 | J. Name., 2012, 00, 1-3
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DOI: 10.1039/C9SC01697J
Table 2. Device parameters of MAPbIxCl3-x-based PVSCs using dopant-free fluoranthene-cored HTMs and LiTFSI/tBP doped spiro-OMeTAD.
HTMs
Voc (V)
Jsc(mAcm-2)
FF
PCE (%)
BTF2
0.96(0.96±0.02)
1.05(1.04±0.01)
1.06(1.06±0.01)
1.09(1.08±0.01)
1.01(1.01±0.01)
1.03(1.02±0.01)
1.07(1.06±0.01)
20.19(20.01±0.32)
21.57(21.36±0.29)
22.32(22.03±0.33)
22.12(21.88±0.32)
20.76(20.52±0.30)
22.04(21.83±0.33)
21.24(20.95±0.37)
0.693(0.667±0.021)
0.742(0.729±0.014)
0.790(0.773±0.019)
0.799(0.781±0.019)
0.723(0.703±0.017)
0.781(0.765±0.015)
0.773(0.758±0.019)
13.44(12.76±0.52)
16.80(16.24±0.46)
18.70(17.97±0.55)
19.27(18.46±0.59)
15.15(14.52±0.47)
17.73(17.12±0.57)
17.57(16.83±0.65)
FBA1
FBA2
FBA3
FTA1
FTA2
Doped Spiro-OMeTAD
The histograms of device parameters of PVSCs are shown in Therefore, the effect of dopants on device stability can be the
Figure 6c. In general, the resulting Vocs are closely related to study focus. As shown in Figure 6d, after exposure to the
the HOMO levels of HTMs.14-16 As discussed earlier, by simulated sunlight for 500 h, over 90% of its initial PCEs can be
replacing diphenylamine with the TPA units, the resulting retained for all the dopant-free devices, while the PCE of
HOMO levels can be downshifted to be better compatible with doped spiro-OMeTAD was decayed to 75% of its initial value.
the VB of perovskites. This thereby enables significantly Furthermore, we also checked the effect of dopant-free HTMs
enhanced Vocs from 5.2% improvement (FTA1) to 13.5% with different Tgs on the thermal stability of fabricated PVSCs
improvement (FBA3). As seen, despite producing a high PCE of in a nitrogen ambience. As shown in Figure S11, the dopant-
18.7% for FBA2-based PVSC, it shows a limited Voc value of free device based on FTA2 with a Tg of 100 oC clearly shows an
1.06 V. To further down-shift the HOMO level and thus to enhanced thermal stability than that from FBA3 with a Tg of 78
o
increase the resultant Voc, a slightly structural regulation on oC; when successively heating the devices at 80 C, 90 oC, 100
o
FBA2 was then carried out by changing the para- into meta- of oC and 110 C for 20 min, the final PCE of FTA2-based device
methoxy-substitutions. As shown in Table 1, the derived FBA3 can retain 94.9% of the original PCE, while that for FBA3-based
shows a deeper-lying HOMO level than FBA2, thereby enabling device is only 91.9%. Moreover, we found that both dopant-
an increased Voc value up to 1.09 V without sacrificing other free devices show much better thermal stability than the
parameters to deliver the highest PCE of 19.27%. Our results doped Spiro-OMeTAD control device during heating
therefore highlight the importance of relative energy level treatments, indicating the removal of dopants/additives can
between perovskite layer and HTMs for achieving high Voc.
also effectively enhance the thermal stability of resulting
The improvement in Jsc and fill factor (FF) has been also PVSCs given the fact that the t-BP additive is a low boiling
demonstrated by fine-tuning the molecular structure. From point solvent.
BTF2 to FBA1 and FBA2, the Jsc value was enhanced from 20.19
We further evaluated the stability of these un-encapsulated
mA cm-2 to 21.57 mA cm-2 (6.8% improvement) and 22.32 mA devices in ambient air with a relative humidity of 20~30%. As
cm-2 (10.5% improvement), respectively, while the FF value shown in Figure 6e, all the tested devices showed a faster
was enhanced from 0.693 to 0.742 (7.1% improvement) and degradation than the case kept in nitrogen ambience. After
0.79 (11.4% improvement), respectively. It certainly originates stored for 180 h, the PCEs of the devices using dopant-free
from their gradually improved hole mobility/hole extraction HTMs and doped Spiro-OMeTAD were degraded by 20% and
ability and better film morphology as discussed earlier. After 43%, respectively. Furthermore we have tested the device
forming branched structure, the corresponding hole stability in ambient air with a high humidity of 80% as shown in
mobility/hole extraction ability are reduced, and thus the Figure S12. After stored for 144 h, the PCE of the doped Spiro-
obtained Jscs and FFs are lower as compared to the linear OMeTAD device was decayed to only 7% of its original PCE,
HTMs. Nonetheless, HTMs consisting of ethylene linking bridge while for the dopant-free devices, the PCEs can still retain
show better performance than other HTMs, indicating its 33%~49% of their original PCEs after stored for 168 h. These
effectiveness in structural design towards dopant-free HTMs.
We finally compared the stability of PVSCs derived from indeed improves the operational stability of PVSCs.
dopant-free HTMs (FBA2 FBA3, and FTA2) and doped spiro- Nonetheless, we have to note that our device stability against
observations therefore indicate that removal of dopants
,
OMeTAD under the same aging conditions to check whether moisture is still imperfect. This is because the device stability is
removal of ionic dopants can improve device stability. First, all not only related to the dopants/additives in the HTMs, but also
the un-encapsulated devices were tested in nitrogen ambience is highly dependent on the HTM film quality atop perovskite
to exclude the influence of external moisture and oxygen. layer, film morphology stability, and the interface between
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perovskite and HTM.60 Actually, the dopant-free devices based
on FBA3 and FTA2 showed an enhanced stability in the high
humidity conditions than that from FBA2, which, we think,
could be due to their better film quality atop perovskite layer
as evidenced by AFM measurements (Figure 5).
Notes and references
View Article Online
DOI: 10.1039/C9SC01697J
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DOI: 10.1039/C9SC01697J
For table of contents
R
MeO
OMe
R
R
N
R
R:
R
FBA1
R
FTA1
PCE = 16.80%
PCE = 15.15%
1
10
9
2
4
3
8
R
R
7
R
R
FTA2
PCE = 17.73%
6
5
BTF2
OMe
N
PCE = 13.44%
MeO
N
R
R
FBA3
PCE = 19.27%
FBA2
MeO
OMe
PCE = 18.70%
A systematic structure-property correlation was conducted to preliminarily elucidate an inherent
regularity governing the structure of dopant-free HTMs.