Fluoranthene-based dopant-free hole transporting materials for efficient perovskite solar cells

Article abstract of DOI:10.1039/c7sc05484j

Significant efforts have been devoted to developing new dopant-free hole transporting materials (HTMs) for perovskite solar cells (PVSCs). Fluoranthene is one typical cyclopentene-fused polycyclic aromatic hydrocarbon with a rigid planarized structure, and thus could be an ideal building block to construct dopant-free HTMs, which have not been reported yet. Here, we report a new and simple synthetic method to prepare unreported 2,3-dicyano-fluoranthene through a Diels-Alder reaction between dibenzofulvene and tetracyanoethylene, and demonstrate that it can serve as an efficient electron-withdrawing unit for constructing donor-acceptor (D-A) type HTMs. This novel building block not only endows the resulting molecules with suitable energy levels, but also enables highly ordered and strong molecular packing in solid states, both of which could facilitate hole extraction and transport. Thus with dopant-free HTMs, impressive efficiencies of 18.03% and 17.01% which are associated with enhanced stability can be achieved based on conventional n-i-p and inverted p-i-n PVSCs respectively, outperforming most organic dopant-free HTMs reported so far.

Full text of DOI:10.1039/c7sc05484j

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Fluoranthene-based Dopant-free Hole Transporting Materials for
Efficient Perovskite Solar Cells
Xianglang Sun,a† Qifan Xue,c† Zonglong Zhu,*b Qi Xiao,a Kui Jiang,b Hin-Lap Yip,c He Yan*b and
Zhong’an Li*a
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
Significant efforts have been denoted on developing new dopant-free hole transporting materials (HTMs) for perovskite
solar cells (PVSCs). Fluoranthene is one typical cyclopentene-fused polycyclic aromatic hydrocarbon with rigid planarized
structure, and thus could be an ideal building block to construct dopant-free HTMs, which, however, has not been
reported yet. Here, we report a new and simple synthetic method to prepare unreported 2,3-dicyano-fluoranthene though
a Diels-Alder reaction between dibenzofulvene and tetracyanoethylene, and demonstrate it can serve as efficient electron-
withdrawing unit for constructing donor-acceptor (D-A) type HTMs. This novel building block not only endows the resulting
molecules with suitable energy levels, but also enables highly ordered and strong molecular packing in solid states, both of
which could facilitate the hole extraction and transport. Thus as dopant-free HTMs, impressive efficiencies of 18.03% and
17.01% associated with enhanced stability can be achieved based on conventional n-i-p and inverted p-i-n PVSCs,
respectively, outperforming most organic dopant-free HTMs reported so far.
www.rsc.org/
mobility, and thus needs to be improved through a chemical
doping process by ionic dopants such as Li-
Introduction
bis(trifluoromethanesulfonyl)imide (Li-TFSI).12-16
However,
Organic-inorganic hybrid perovskite solar cells (PVSCs) have
triggered a worldwide attention due to their impressive
research progress within a short time and very promising
market prospects.1-5 Recently, the record-high power
conversion efficiency (PCE) of PVSCs has reached the certified
22%, almost rivaling that of the crystalline silicon based
photovoltaics.6 When fabricating PVSCs, the introduction of
suitable interfacial materials, i.e. electron transporting
materials (ETMs) and hole transporting materials (HTMs), is
very critical to achieve high device performance, because they
not only improve the charge carrier transport/collection
efficiency, but also act as the protective layer for perovskites
to enhance device stability.7-11
doping process always induces a negative effect on device
stability due to the sophisticated oxidation process associated
with undesired ion migration/interactions.17, 18 For example,
the device PCEs derived from well-known doped HTM,
2,2′,7,7′-tetrakis(N,N-bis(p-methoxyphenyl)amino)-9,9′-
spirobifluorene (spiro-OMeTAD), often vanished after 30-day
ambient storage, even the maximum of its initial PCEs has
been close to 19%.19 Thus, development of efficient dopant-
free HTMs is urgently needed, however less dopant-free HTMs
can show comparable PCEs to the doped spiro-OMeTAD.18, 20-26
Normally, HTMs would not need an additional doping
process if they exhibit hole mobility up to 10-4~10-3 cm2V-1s-1.27
To this end, two molecular design strategies, donor-acceptor
(D-A) type and star-shaped structure, have been mainly used
For HTMs applied on PVSCs, organic semiconductors are
more popular over inorganic counterparts attributed to their
milder processing conditions compatible with perovskites.
Nevertheless, most organic HTMs exhibit relatively low hole
for designing dopant-free HTMs with better intermolecular
interactions to ensure sufficient hole mobility.17,
28-34
By
integrating these two strategies, Nazeeruddin et al. prepared a
new class of star-shaped D-A molecules serving as dopant-free
HTMs towards ~19% PCE with enhanced device stability.19, 35
Nonetheless, one fact is that most of reported D-A type HTMs
come from relatively complicated scaffolds with multistep
synthesis and purification. We recently reported a new dipolar
chromophore based dopant-free HTM via a facile synthesis,
which afforded a high PCE of 16.9%.36 Following these
successes, it is very crucial to exploit new D-A combinations
towards high performance dopant-free HTMs with low
synthetic complexity.
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diphenylamine units
(
3
-4). So far, facile synthesis of
functionalized fluoranthene derivatives remains a challenge.
One of the most widely methods of affording fluoranthenes is
through typical Diels-Alder (D-A) reaction based on the
acenaphthenequinone derivatives
(Scheme 2a), but the
functionalized positions of this method are always limited,
43
Another typical method
mainly on C7-C10 positions.42,
proposed by Stubbs and Tucker et al. employs fluorene
derivatives as the starting materials (Scheme 2b), but the total
synthetic efficiency is too poor due to the tedious multistep
reactions.44
Through a careful searching, we encouragingly found in
1949, dibenzofulvene and maleic anhydride were reported as
the starting materials by Wang et al. to yield 2,3-substituted
fluoranthene based on the D-A reaction following with
dehydrogenation (Scheme 2c).45 Although the yield of this
reaction is low, only ~10%, the simple and facile procedure is
very attractive for us. Thus, with a purpose of synthesizing
more electron-deficient fluoranthene core, tetracyanoethylene
(TCNE) was selected as the dienophile to react with
Scheme 1. Synthetic route of fluoranthene-cored HTMs (BTF1-4).
One of key factors for designing high performance HTMs is
designing a suitable core structure. Fluoranthene is one typical
cyclopentene-fused polycyclic aromatic hydrocarbon, and
exhibits a rigid planarized structure that can favor to form
enhanced π-π stacking. More importantly, fluoranthene, as a
substructure of fullerene, shows a typical electron-deficient
character due to the cyclopenta-fused nonalternant
structure.37-40 Thus, fluoranthene may be an ideal electron-
withdrawing building block to construct D-A type HTMs for
PVSCs, which however has not been reported possibly due to a
lack of efficient molecular design strategy. In this study, we
describe a new and facile synthetic method to prepare
unreported 2,3-dicyano-fluoranthene moiety as efficient core
towards high performance dopant-free HTMs.
dibenzofulvene
(Scheme 2d) through D-A reaction but
following with dehydrocyanic acid. Excitingly, 2,3-dicyano-
fluoranthene was successfully obtained as we expected, in
spite of with a low isolated yield of 3.5%. With an indirect
method, we prepared diphenylamine moieties substituted
dibenzofulvene (
intermediates to react with TCNE as shown in Scheme 1, which
successfully obtained our desired products, BTF3 , with
8-9) first, and then used them as the critical
-4
enhanced isolated yields (29.1% for BTF3 and 43.0% for BTF4).
The electron-rich substitutions were found to activate the
reactivity of fulvene. This can be further evidenced by the
higher yield of BTF4, as methoxyl groups in
9 make diene more
electron-rich. Moreover, due to the facile synthesis, all
synthetic costs, 7.1 $/g, 11.4 $/g, 53.1 $/g and 62.8 $/g for
Results and discussion
The synthetic route of HTMs is shown in Scheme 1, while the
BTF1-4 (Table S1-S4), respectively, are much cheaper than that
details are provided in Experimental Section in Supporting
of spiro-OMeTAD, making them good material candidates
suitable for large scale production.
Information (SI). First, we prepared fluoranthene-cored BTF1
-2
via a simple Buchwald-Hartwig coupling reaction between the
41
readily accessible 3,8-dibromo-fluoranthene
The ground-state geometric structures of BTF3 (Triclinic, P-1
space group) and BTF4 (Monoclinic, P21/c space group) were
refined by X-ray crystallography, and the detail data are shown
(
2)
and
in Table S5-S6 in SI. Figure 1 shows the packing arrangements
of BTF3-4. According to the relative arrangement between
molecules and contact positions, four types of molecular
packing modes are totally found (Figure 1a and S3-S4),
including H-type dimeric stack through dipole-dipole
interactions due to dicyano-substitutions (mode 1), H-type
dimeric stack through π-π interactions between two adjacent
molecules (mode 2), J-type dimeric stack through π-π
interactions between two adjacent diphenylamine units (mode
3), and herringbone stack through π-π interactions between
two adjacent molecules (mode 4).
As shown in Figure 1b, BTF3 molecules pack into highly
ordered and extended H-aggregates along the (0-11) plane,
wherein two types of molecular stacks (mode 1 and 2) appear
in an alternating fashion. In stack mode 1, the stack distance
between two dicyano-fluoranthene units is 3.515 Å, while that
in stack mode 2 is 3.435 Å. Except short stack distance, the
Scheme 2. Synthetic methods for functionalized fluoranthenes.
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Figure 1. a) Schematic representations of the π-stack modes of BTF3-4 based on single-crystal X-ray analyses, while block arrows mean the
direction of dipole moments, and double arrow lines represent the positions of π-stack; b) π-stacks of BTF3 viewed along a axis; c) π-stacks of BTF4
viewed along b axis; d) π-stacks of BTF4 viewed along a axis. For clarity, the hydrogen atoms and solvent molecules in BTF3 are omitted. The π-
stack distances are noted in green color.
contact area in mode 2 is much larger than that in mode 1, as crystal of BTF1 for structure analysis, which however indicates
the diphenylamine units also show a close contact with each an absence of strong π-π interactions (Figure S5). Based on
other with a distance of 3.593 Å. Moreover, it is encouragingly these results, we can conclude cyano-substitution plays a
found BTF3 molecules in two adjacent H-aggregates can pack critical role in enabling strong intermolecular interactions in
into highly ordered J-like aggregates along the (100) plane solid states, which thus are expected to be beneficial to
based on the stack mode 3 with a π-π-distance of 3.451Å. achieve efficient charge transport.
Thus, the solid state structure of BTF3 can be classified as a
two-dimensional “brick-wall”-like assembly.
BTF4 molecules interestingly show
a highly ordered
herringbone assembly with partial edge-to-face packing along
the (101) plane (stack mode 4, Figure 2c). And due to the
asymmetrical nature of cyano-fluoranthene, they contact in
three different positions (Figure S4) with a π-π-distance of
3.464 Å, 3.471 Å, and 3.617 Å, respectively, making the
molecular packing very dense. Different from those observed
in BTF3, the neighboring herringbone assemblies interact with
each other along both (110) and (101) planes, based on stack
mode 3 and 1, respectively. Thus, the packing structure of
BTF4 can be classified as
herringbone assembly. The edge-to-face π-π-distance within
a
quasi-three-dimensional
dimer in stack mode 3 for BTF4 is slightly longer (3.669 Å) than
that in BTF3
. However, a much shorter dipole-dipole
Figure 2. a) The film absorption spectra. b) The CV curves versus Fe/Fe+
(0.66 V) measured in DCM solution. c). The hole injection characteristics
measured by SCLC method based on the device structure of
ITO/V2O5/HTMs/V2O5/Al. d) Steady state PL spectra of the bilayered
perovskite films with or without capping different HTLs.
interaction distance of 3.37 Å is obtained for BTF4 in stack
mode 4, attributed to that a close edge-to-face π-contact with
a distance of 3.564 Å between diphenylamine units also
occurs. On the contrary, we only obtained a low-quality single
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with data summarized in Table
reversible oxidative and reductive processes, while only
oxidative processes observed for BTF1 . The HOMO levels of
BTF1 are obtained as -4.94, -4.80, -5.19, and -5.02 eV,
respectively, versus Fc/Fc+. As shown, the cyano-substitution
results in a decrease in HOMO levels for BTF3 , making them
more compatible with perovskites to facilitate hole extraction
Figure S10 and S17). Moreover, we noted the introduction of
electron-rich methoxy groups can induce an increase in HOMO
levels. The LUMO levels of BTF3 are calculated from the
onset of reduction potential, -3.46 and -3.38 eV, respectively,
while those of BTF1 are determined as -2.68 and -2.59 eV,
respectively, by subtracting Eopt from HOMO levels.
The hole mobilities (μ) of BTF1 without adding any
1. Both BTF3-4 display
Table 1. Optical, electrochemical, and hole mobility properties of BTF1-4.
a)
λsol
b)
c)
Eopt
d)
λfil
EHOMO
ELUMO
μf)
-2
HTMs
(nm)
(nm)
(eV)
(eV)
(eV)
(cm2V-1s-1)
-4
BTF1 304,475 310,481 2.24
BTF2 305,482 308,493 2.16
BTF3 344,609 347,629 1.70
BTF4 344,632 344,654 1.59
-4.94
-4.80
-5.19
-5.02
-2.68e) 1.46ꢀ10-5
-2.59e) 2.13ꢀ10-5
-3.46d) 6.36ꢀ10-5
-3.38d) 1.17ꢀ10-4
-
4
(
[a] Absorption maxima in DCM solutions. [b] Absorption maxima of thin films. [c]
Optical band gap calculated from film absorption edges. [d] Measured from
-4
electrochemistry experiments. [e] Calculated by an equation of ELUMO
=
E
HOMO+Eopt. [f] Measured by SCLC method.
-2
The absorption spectra of BTF1
-4
in diluted solutions and as
thin films are shown in Figure S6 and Figure 2a, respectively,
with related data listed in Table 1. All spectra show two
distinct absorption bands, ascribed to localized π-π*
transitions and intramolecular charge transfer (ICT). By
comparing the film spectra with their corresponding solution
-
4
dopants were measured by space-charge-limited-current
(SCLC) method (Figure 2c). As shown in Table 1, the μ values of
BTF3
much higher than those of BTF1
(2.36
10-5 cm2V-1s-1). The high mobility of BTF4 would fulfill
-4 are 6.36ꢀ ꢀ
10-5 and 1.17 10-4 cm2V-1s-1, respectively,
-2 and spiro-OMeTAD
spectra, it is found the low-energy absorption bands of BTF3
-4
ꢀ
are more red-shifted than those of BTF1-2, suggesting
the needs of dopant-free HTMs. This can be attributed to its
quais-three-dimensional supramolecular assembly associated
with much closer molecular stacks. We also characterized the
steady-state photoluminescence (PL) spectra (Figure 2d) of bi-
layered perovskite/non-doped HTL films to check their ability
of extracting holes. As seen, the PL of all bilayered films can be
quenched due to the introduction of HTLs, and particularly
aggregation of former are more pronounced, consistent with
the results from crystal structure analysis. Both methoxy- and
cyano- substitutions result in a red-shift in absorption bands,
while the red-shift degree for latter is much larger, up to 150
nm of absorption maximum for BTF4, ascribed to more
effective ICT. The absorption edges of BTF1-4 in thin films are
determined to be 554, 575, 731 and 781 nm, respectively,
most effectively for BTF4, followed by BTF3
BTF1. It is worth noting that the quenching effectiveness of
dopant-free BTF3 is even comparable to that of doped spiro-
OMeTAD (Figure 2d), suggesting holes are extracted efficiently
from perovskites for BTF3 even without adding any dopants.
, BTF2 and then
corresponding to the optical bandgap (Eopt, Table 1) of 2.24,
2.16, 1.70, and 1.59 eV, respectively.
-4
Their HOMO and lowest unoccupied molecular orbital
(LUMO) levels were determined by cyclic voltammetry (CV),
-
4
and the CV curves in DCM solutions are given in Figure 2b
,
Figure 3. J-V curves of the best-performing conventional (a) and inverted (b) PVSCs with different dopant-free HTMs. c) Stable output current of
dopant-free BTF4 based conventional PVSC under a constant bias of 0.865 V d) The stability test of the conventional PVSCs in ambient air with a
humidity of 40~50%.
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Table 2. Device parameters of (FAPbI3)0.85(MAPbBr3)0.15-based dopant-free PVSCs using BTF1-4, spiro-OMeTAD and PEDOT:PSS as HTMs.
HTMsa
BTF1
Device
n-i-p
n-i-p
n-i-p
n-i-p
n-i-p
p-i-n
p-i-n
p-i-n
p-i-n
p-i-n
Voc (V)
Jsc (mA/cm2)
FF (%)
PCE (%)
0.91(0.90±0.01)
0.85(0.84±0.02)
1.08(1.07±0.02)
1.06(1.05±0.01)
17.5(16.2±1.3) 62.6 (60.7±1.8)
19.5(18.0±1.4) 63.1(60.7±2.1)
20.4(19.2±1.1) 74.3(72.4±1.8) 16.34(15.28±1.13)
22.5(21.5±1.1) 75.6(73.7±1.9) 18.03(16.97±1.05)
9.97(8.84±1.12)
10.45(9.16±1.02)
BTF2
BTF3
BTF4
Spiro-OMeTAD
BTF1
1.02(1.01 ±0.01) 14.3(13.0±1.3) 64.0(62.4±1.5)
9.33(8.21±1.09)
0.96(0.95±0.01)
0.91(0.90±0.02)
16.1(14.9±1.2) 72.4(70.7±1.5) 11.19(10.02±0.98)
17.7(16.5±1.1) 74.3(72.7±1.6) 11.96(10.51±1.02)
BTF2
BTF3
1.03(1.02 ±0.01) 19.3(18.3±1.1) 75.9(74.1±1.7) 15.09(14.14±0.96)
1.03(1.01 ±0.02) 21.5(20.5±1.0) 76.8(75.3±1.5) 17.01(16.14±0.85)
0.96(0.94±0.01)
BTF4
PEDOT:PSS
22.4(21.5±1.0) 76.4(74.7±1.6) 16.42(15.48±0.94)
We fabricated conventional n-i-p planar devices with a sipro-OMeTAD (18.8%). A slight enhancement on PCEs was
configuration of FTO/SnO2/PCBM/mixed perovskite/HTL/MoO3 observed for the doped BTF3 based devices, while the
/Au to evaluate the efficacy of the BTF1 as dopant-free highest PCE of 18.94% was yielded for BTF4. These results, on
-4
-4
HTMs. Mixed perovskite (FAPbI3)0.85(MAPbBr3)0.15 (FA: the other hand, suggest our dicyano-fluoranthene-cored
NH=CHNH3+; MA: CH3NH3+) was used as the active light- molecules can serve as highly efficient HTMs without needing
harvesting layer. The device fabrication details are described in dopants.
SI. The morphology of BT1
-4
thin films onto perovskites has
Today, most HTMs used in inverted p-i-n devices are those
been studied through scanning electron microscope (SEM), polymer HTMs such as poly(3,4-ethylenedioxythiophene)
and the images are shown in Figure S11. As can be clearly polystyrenesulfonate (PEDOT:PSS) and polytriarylamine
seen, all HTLs possess smooth surface without pin-holes.
(PTAA).46-49 Doping process is also required for PTAA to
The current density-voltage (J-V) curves of the best- improve device performance.50 PEDOT:PSS belongs to a class
performing conventional PVSCs using dopant-free HTMs are of self-doped polymers, however, it always suffers a large
shown in Figure 3a, with related photovoltaic parameters potential loss and inherent acidity-induced stability problem.51,
listed in Table 2. The champion devices based on dopant-free 52.Therefore, it is also important to develop dopant-free HTMs
BTF1 and BTF2 showed limited PCEs of 9.97% and 10.45%, suitable for inverted PVSCs.53, 54 But less molecular HTMs have
respectively, which are only slightly enhanced compared to been reported to be applied on both conventional and
that of spiro-OMeTAD-based dopant-free control devices inverted planar PVSCs. In this end, inverted PVSCs with a
(9.33%) under same fabrication conditions. By contrast, the configuration of ITO/HTL/mixed perovskite/PCBM/LiF/Ag were
device performance of dopant-free BTF3-4 can be significantly also tested using BTF1-4 as the dopant-free HTMs, while
improved, due to their more suitable HOMO levels and PEDOT:PSS-based control device was fabricated for
enhanced hole mobilities. The champion PVSC based on comparison. The J-V curves of best-performing devices are
dopant-free BTF4 delivered a very impressive PCE of 18.03% shown in Figure 3b. From BTF1 to BTF4, the resulting PCEs of
with an open-circuit voltage (Voc) of 1.06, a short-circuit inverted devices increase gradually (Table 2), similar trend as
photocurrent (Jsc) of 22.5 mA cm-2, and a FF of 75.6%, which is that observed in conventional devices. The champion PCE from
among best for dopant-free devices reported so far.25 While BTF4-based inverted PVSCs is 17.01%, higher than that of
for BTF3, its dopant-free devices exhibited a slightly decreased PEDOT:PSS-based control device (16.42%).
device PCE of 16.34% mainly due to a reduction in Jsc value
caused by its inferior hole mobility compared to that of BTF4
The stabilized PCE and photocurrent of champion devices of
BTF4 near the maximum power point (Figure 3c and S18b
.
)
To understand the charge transfer behaviors of these dopant- were tested to evaluate the efficacy of J-V curves. The results
free HTLs, the electrochemical impedance spectroscopy (EIS) clearly manifest the high reliability of our J-V curve with an
of fabricated cells was further measured at open circuit with a absence of current hysteresis, when combing with those J-V
frequency ranging from 1 Hz to 1 MHz, and the Nyquist plots curves with forward and reverse scanning direction at different
are shown in Figure S14. Similar to hole mobility, the obtained scan rates (Figure S12b and S18c). We also measured the
recombination reisistance (Rrec) also manifests an order of incident photon-to-electron conversion efficiency (IPCE)
Rrec
(
(
BTF4) > Rrec
(
BTF3) > Rrec
(
BTF2)~Rrec
(
spiro-OMeTAD) > spectra of all champion conventional and inverted devices
Rrec
BTF1). This therefore suggests BTF3 and BTF4 can based on dopant-free BTF1-4
(
Figure S15 and S20), wherein
efficiently reduce the recombination process with higher the integrated JSC values is consistent with those obtained
recombination resistance to produce higher FFs.
from the experimental J-V measurements (Table 2).
Doped conventional devices were also fabricated through
To check if removing doping process can improve device
using Li-TFSI as the dopant with additive of 4-tert-butyl stability of PVSCs over doped spiro-OMeTAD, we compared
pyridine (TBP). As shown in Figure S16, the device PCEs of the environmental stability of PVSCs derived from dopant-free
doped BTF1-2 can be enhanced by 40% due to an effective BTF4 and doped spiro-OMeTAD without encapsulation. The
improvement on both Voc and FF. Nonetheless, such PCE decay curves are presented in Figure 3d, wherein it is
enhancement is still poor by comparison with that of doped easily seen the performance of both devices decreased quickly
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at the beginning when being stored in ambient air under a 4.
relative humidity of 40~50%. However, dopant-free BTF4 5.
based device can still retain over 50% of its original PCE after 6.
stored for 30 days, while the PCE of the doped spiro-OMeTAD 7.
based device almost vanished. Moreover, BTF4-based inverted 8.
devices also showed an enhanced stability and can retain 55%
of its original PCE after being stored in air for 7 days, while 90% 9.
of PCE of PEDOT:PSS-based control device was lost (Figure
S18e). Thus, our designed fluoranthene-cored dopant-free
HTMs not only deliver a high PCE comparable to doped spiro- 10.
OMeTAD, but also show an enhanced device stability, 11.
suggesting they are very promising material candidates 12.
towards efficient PVSCs.
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This work is funded by National Science Foundation of China
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Chemical Science
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DOI: 10.1039/C7SC05484J
Unreported 2,3-dicyano-fluoranthene was first prepared as an efficient electron-
withdrawing building block for constructing D-A type dopant-free hole transporting
materials.