Hole transporting materials based on benzodithiophene and dithienopyrrole cores for efficient perovskite solar cells

Article abstract of DOI:10.1039/c7ta11314e

The development of highly efficient hole transporting materials (HTMs) for perovskite solar cells (PSCs) is still one of the most thrilling research subjects in the development of this emerging photovoltaic technology. Inner ring engineering of the aromatic core of new HTMs-consisting of three fused rings endowed with four triarylamine units-reveals major performance effects over the fabricated devices. In particular, substitution of the central pyrrole ring in dithienopyrrole (DTP) by a benzene ring-benzodithiophene (BDT)-allows enhancing the power conversion efficiency from 15.6% to 18.1%, in devices employing mixed-perovskite (FAPbI₃)_0.85(MAPbBr₃)_0.15 (MA: CH₃NH₃⁺, FA: NHCHNH₃⁺) under 1 sun illumination. In comparison, 2,2′,7,7′-tetrakis(N,N-di-p-methoxyphenylamine)-9,9′-spirobifluorene (spiro-OMeTAD) yielded a power conversion efficiency of 17.7%. The novel HTM molecules show an efficient quenching of the perovskite photoluminescence, indicating an efficient charge transfer from the active layer to the HTM, along with a good conductivity (comparable to that of the spiro-OMeTAD reference). Density functional theory (DFT) calculations allowed rationalizing the electrochemical and optical properties and predicting a reorganization energy (λ) for the best performing BDT-based HTM (0.101 eV) significantly lower than that computed for the benchmark spiro-OMeTAD (0.139 eV).

Full text of DOI:10.1039/c7ta11314e

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torrientes, I. Zimmermann, J. Calbo, J. Aragó, J. Santos, E. Orti, N. Martín and M. K. Nazeeruddin, J. Mater.
Chem. A, 2018, DOI: 10.1039/C7TA11314E.
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Kun Chang, Zhaorong Chang et al.
Bubble-template-assisted synthesis of hollow fullerene-like
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2
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DOI: 10.1039/C7TA11314E
Journal of Materials Chemistry A
ARTICLE
Hole Transporting Materials Based on Benzodithiophene and
Dithienopyrrole Cores for Efficient Perovskite Solar Cells
a
b
c
c
a
c
a,d
R. Sandoval-Torrientes, I. Zimmermann, J. Calbo, J. Aragó, J. Santos, E. Ortí,* N. Martín* and
M. K. Nazeeruddin*b
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
The development of highly efficient hole transporting materials (HTMs) for perovskite solar cells (PSCs) is still one of the
most thrilling research subjects in the development of this emerging photovoltaic technology. Inner ring engineering of the
aromatic core of new HTMs –consisting in three fused rings endowed with four triarylamine units– reveals major
performance effects over the fabricated devices. In particular, substitution of the central pyrrole ring in dithienopyrrole
www.rsc.org/
(
DTP) by a benzene ring –benzodithiophene (BDT)– allows enhancing the power conversion efficiency from 15.6% to 18.1%,
in devices employing mixed-perovskite (FAPbI NH
3 3
)0.85(MAPbBr )0.15 (MA: CH
3
3
+, FA: NH=CHNH +) under 1 sun illumination. In
3
comparison, 2,2′,7,7′-tetrakis(N,N-di-p-methoxyphenylamine)-9,9′-spirobifluorene (spiro-OMeTAD) yielded 17.7%. The
novel HTM molecules show an efficient quenching of the perovskite photoluminescence, indicating an efficient charge
transfer from the active layer to the HTM, along with a good conductivity (comparable to that of spiro-OMeTAD reference).
Density functional theory (DFT) calculations allowed rationalizing the electrochemical and optical properties, and predict a
reorganization energy (λ) for the best performing BDT-based HTM (0.101 eV) significantly lower than that computed for the
benchmark spiro-OMeTAD (0.139 eV).
Current research on PSCs focuses on two well-differentiated
approaches, the first one searching the modification of the
photo-active perovskite layer. Manipulation of perovskite’s
1
. Introduction
Among the different strategies utilized to harness solar energy,
the photovoltaic (PV) approach is the only one able to directly
convert Sun’s light into electrical energy. The over-all PV
capacity installed in 2015 was calculated to be 227.1 GW, which
represents 0.6% of the global power generation.[1] Virtually, all
the installed PV capacity relies on silicon-based solar cells (SCs).
Despite the dramatic reduction of the production costs, quality
PV silicon production still represents an enormous energetic
burden. The so-called emerging PV technologies (organic cells,
quantum dot cells, perovskite cells, etc.) are promising future
alternatives that aim to fulfill niche markets where silicon cells
cannot compete, when not directly competing with
conventional silicon based SCs. Among these emerging
technologies, perovskite solar cells are seriously positioning
themselves as competitor candidates to silicon solar cells. No
more than seven years after they were first reported by Kojima
et al.,[2] perovskite solar cells (PSCs) already reach efficiencies
exceeding 22%,[3] competitive with the well-established silicon
SCs, which is an unprecedented development in the field of
photovoltaics.
3
ABX structure by anionic and cationic substitution allows
controlling its absorption properties. Modifying the size of the
organic A cation has been observed to have a measurable effect
on the band gap. Substitution of methylammonium (MA) by
formamidinium (FA) leads to a nearly 0.1 eV gap narrowing.[4]
2+
2+
Replacement of Pb by Sn (cation B) has also demonstrated
to substantially affect the light absorption capabilities, allowing
3
band gaps as narrow as 1.20 eV for MASnI .[5] However, the
most significant band gap manipulations have been achieved by
substitution of the halide anion X. The lattice changes induced
by increasing size of the halide anion (Cl < Br < I) reduce the
band gap and result in a spectral shift towards longer
wavelengths.[2,6] The combination of the aforementioned
structural and electronic modifications in mixed compositional
perovskites
Cs
such
(100−x)Pb(I0.83Br0.17
as
MAPb(I1-xBr
3 0.15
0.85[MAPbBr ]
x 3
) ,
x
(MA0.17FA0.83
)
)
3
or [FAPbI
3
]
has redounded in perovskites with enhanced stability rendering
efficiencies up to 22%.[3,7,8,9]
The second major approach in PSCs concerns the synthesis and
development of new and more efficient, both in terms of cost
and performance, charge transporting materials. Examples of
electron
azaacenes[10,11] or fullerene derivatives[12–14] have been
reported in order to replace the TiO as the archetype ETM.
More importantly, hole transporting materials (HTMs) research
may be considered as one of the main topics in PSCs as the field
is still dominated by the expensive compound 2,2′,7,7′-
transporting
materials
(ETMs)
employing
a.
IMDEA-Nanociencia, Ciudad Universitaria de Cantoblanco, Madrid, 28049, Spain.
EPFL VALAIS, Sion, 1951, Switzerland. *Email - nazmar@ucm.es
Instituto de Ciencia Molecular, Universidad de Valencia, Paterna, 46980, Spain.
Facultad de Ciencias Químicas, Universidad Complutense de Madrid, Madrid,
b.
c.
2
d.
2
8040, Spain.
Electronic Supplementary Information (ESI) available: Synthetic and computational
details, device fabrication and characterisation, and supplementary figures. See
DOI: 10.1039/x0xx00000x
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tetrakis(N,N-di-p-methoxyphenylamine)-9,9′-spirobifluorene
(
system, along with providing a stronger electrVoienw AdrtoiclneaOtnilninge
spiro-OMeTAD).[15,16] So far, a high number of new HTMs has character. The PSCs fabricated makingDOuIs:e10.1o0f39t/hCe7TsAe113tw14oE
been tested making use of many types of chemical structures materials as HTLs show efficient charge transport to the
and constituents. Nevertheless, it may be unquestionably electrodes that allows obtaining photocurrent conversion
asserted that structures exhibiting triarylamine functionalities efficiencies (PCEs) over 18%.
are the most successful and repeated ones. The reason for this In order to gain a better understanding of the structural and
success lies in the highest-occupied molecular orbital (HOMO) electronic properties of the new BDT and DPT derivatives,
energy levels this functionality provides, that allows efficient density functional theory (DFT) calculations were performed at
overlapping with the perovskite’s valence band. Triarylamines the B3LYP/6-31G** level. Interestingly, the predicted
have been conjugated to spiranic central cores resembling that reorganization energy for the best performing BDT-based HTM
of spiro-OMeTAD, delivering efficiencies comparable to spiro- (0.101 eV) resulted to be significantly lower than that computed
OMeTAD or even superior.[17–20] Being aware that energy for the benchmark spiro-OMeTAD (0.139 eV).
matching is not the only concurring criteria, much attention has
been devoted to developing novel central cores able to
2
. Results and Discussion
promote efficient crystalline ordering. Well-ordered hole
transporting layers (HTLs) allow efficient charge transport and
extraction to the electrodes, and motifs that promote
aggregation (aromatic surfaces for π−π stacking, sulphur
heteroatoms for S···S interactions, etc.) have been
demonstrated to be very effective.[21–25]
2
.1. Materials Synthesis and Properties
The synthesis of the new HTMs was carried out as shown in
Scheme 1 with good yields. Molecule TTPA-DTP was obtained
in a straightforward manner comprising four synthetic steps;
the DTP central core was attained following a modification of
the procedure reported by Wong and collaborators.[29]
Starting from commercially available 3,3’-dibromo-2,2’-
bithiophene, the DTP core is obtained in moderate yield by a
Buchwald-Hartwig cross-coupling reaction with p-anisidine.
Polybromination of the DTP core is achieved in the form of a
white insoluble solid after one-pot sequential treatment with
NBS at room temperature, followed by reflux after addition of
bromine. The final product TTPA-DTP is accomplished by a four-
fold Suzuki-Miyaura reaction with four equivalents of p-
methoxytriphenylaminepinacol boronate (4). On the other
Fig. 1 Molecular structures of the HTMs based on the use of benzodithiophene (BDT)
and dithienopyrrole (DTP) as cores bearing four triphenylamine (TPA) moieties.
hand, TTPA-BDT was prepared in two steps from commercially
available dibenzothiophene, consisting in tetrabromination by
treatment with excess of bromine, followed by a four-fold
Electron rich molecules like benzodithiophene (BDT) and
dithienopyrrole (DTP) have been widely employed in organic
photovoltaics with varying degree of success.[26–28] These two
building-blocks offer structural elements, like having large π-
conjugated surfaces and the presence of heteroatoms, that help
promoting intermolecular interactions. Furthermore, the two
thiophene rings that both molecules possess bear the potential
of being substituted with up to four triarylamine moieties.
Combined all together, these characteristics prompted us with
the idea of designing two new molecules for their use as HTMs
in PSCs, where BDT and DTP central cores are substituted with
four N,N-bis(4-methoxyphenyl)aniline units through the 2- and
Suzuki-Miyaura reaction with boron pinacolate 4. The structural
characterization of the synthesized products was consistent
with the molecular formulae of TTPA-DTP and TTPA-BDT. Full
characterization and synthetic details of the products is
provided in the Electronic Supplementary Information (ESI).
To gain more insight into the structural and electronic
properties of the BDT and DPT derivatives, density functional
theory (DFT) calculations were performed at the B3LYP/6-
3
1G** level for the TTPA-BDT and TTPA-DTP compounds in the
presence of the solvent (CH Cl ). Their respective cores (BDT
2
2
and DTP), the pendant triarylamine (TPA) moiety, and the
reference spiro-OMeTAD compound were also computed for
comparison purposes (see the Supporting Information for full
computational details).
3
-positions of the thiophene rings giving rise to the
tetrakistriphenylamine (TTPA) derivatives sketched in Fig. 1
.
The substitution of the inner core benzene ring of BDT by a
pyrrole ring in DTP allows adding an extra heteroatom to the
2
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DOI: 10.1039/C7TA11314E
Scheme 1. Synthetic route for the preparation of the heterocyclic TTPA-BDT and TTPA-DTP HTMs.
Fig. S1 collects the most representative bond length values HTMs and the spiro-OMeTAD reference compound. The
calculated at the B3LYP/6-31G** level for the BDT and DPT topology of the HOMO of TTPA-BDT and TTPA-DTP clearly
cores and the TTPA-BDT and TTPA-DTP HTMs. The central resembles that obtained for the corresponding cores with
benzene ring of the C2h-symmetry BDT core mostly preserves its important contributions from the TPA moieties attached to the
delocalized structure with carbon–carbon (CC) bond distances 2-position of the thiophene rings, especially in the case of TTPA-
between 1.390 and 1.427 Å. The terminal thiophene rings lose DTP. The HOMOs of the pristine BDT and DTP cores, calculated
part of their typical structural pattern with alternating single at −5.55 and −5.30 eV, respectively, undergo a noticeable
and double CC bonds showing a significant difference between destabilization in passing to TTPA-BDT (−4.62 eV) and TTPA-DTP
the terminal CC double bond (1.356 Å) and the bond fused to (−4.51 eV) upon the insertion of the strong electron-donating
the benzene ring (1.427 Å). In contrast, the DTP core exhibits a TPA moieties. This enhancement of the electron-donor
clear single–double CC alternating backbone similar to that character brings the HOMOs of the TTPA-BDT and TTPA-DTP
found for other related thiophene oligomers.[30,31] The HTMs close to that calculated for the reference spiro-OMeTAD
insertion of the TPA units to obtain TTPA-BDT and TTPA-DTP (−4.44 eV). It is important to notice that a significant charge
hardly causes any substantial change in the bond distances of transfer occurs from the peripheral TPA groups to the central
the cores, with the exception of the terminal CC and carbon– cores, which hold a total net charge of −0.30e and −0.22e for
sulfur bonds (Fig. S1).
TTPA-BDT and TTPA-DTP, respectively. The electronic structure
Fig. 2 shows the frontier molecular orbitals computed for the of the TTPA-BDT and TTPA-DTP HTMs is therefore found to be
TPA unit, the BDT and DPT cores, the TTPA-BDT and TTPA-DTP remarkably polarized.
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Fig. 2 Energy diagram displaying the frontier molecular orbitals computed at the B3LYP/6-31G** level for the TPA unit, the BDT and DTP cores, the TTPA-BDT and TTPA-DTP HTMs,
and the reference spiro-OMeTAD.
The electrochemical properties of the sulfur-rich TTPA-BDT and which enables a more efficient hole extraction, with
TTPA-DTP HTMs were studied by cyclic voltammetry (CV) in consequences over the overall performance of the PSC devices
CH Cl solution (Fig. 3a). CV experiments evidence the reductive built.
2 2
character expected from these electron-rich materials.
Compound TTPA-BDT presents a quasireversible oxidation
ox
process (E1/2 = 0.92 V vs. NHE), that shows a subtle shoulder
which is rationalized as the sequential oxidation of the two TPA
moieties fully conjugated with the core (linked to the BDT
through the thiophene 2- and 2’-positions), followed by the
oxidation of the TPAs attached to the thiophene 3- and 3’-
positions. This apparent subtlety makes the 2,2’ TPAs being
stronger donors due to their effective conjugation with the BDT
core, in line with the HOMO computed for TTPA-BDT localized
at the core and the 2,2’ TPAs. In contrast, derivative TTPA-DTP
has a much richer redox activity, showing up to four
ox
quasireversible oxidation waves (E1/2 = 0.54, 0.71, 0.91, and
1
.20 V vs. NHE). In this case, the lower oxidation potentials are
ascribed to the better electron-donor properties of the DTP
core, which is expected to participate in the oxidation process.
The third oxidation wave (0.95 V) seems to be more intense and
probably arises from the TPA moieties attached to the DTP
through thiophene 3,3’-positions, which, as for TTPA-BDT
,
cannot experience an effective conjugation with the core. This
difference in donating strengths is reflected in the HOMO
energies estimated from the oxidation potentials, obtaining ‒
5
.36 eV for TTPA-BDT and ‒4.98 eV for TTPA-DTP. In
comparison, spiro-OMeTAD presents a HOMO level of ‒5.16
eV.[32] The HOMO level observed for TTPA-BDT is closer in
energy to the valence band of the perovskite (ca. ‒5.65 eV),
4
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Fig. 3 a) Cyclic voltammograms of TTPA-BDT and TTPA-DTP recorded in CH
at a scan rate of 100 mV s‒1. b) Normalized absorption and emission spectra of TTPA-BDT
and TTPA-DTP recorded in CH Cl solution.
2 2
Cl solution
values estimated for TTPA-BDT and TTPA-DTP are 0.101 and
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0
.285 eV, respectively. Note that these λ values are significantly
2
2
smaller than those obtained for their corresponding fragments
0.166, 0.467 and 0.274 eV for BDT, DTP and TPA, respectively)
(
Table 1 Electrochemical and optical properties of TTPA-BDT and TTPA-DTP.
due to the fact that charge extraction involves both the core and
the peripheral TPA moieties (especially those linked through the
ox
1/2
abs
max
ems
max
HTM
E
[
E
[eV]
‒5.36
‒4.98
HOMO
[b]
λ
λ
E
[eV]
2.73
2.76
0-0
[c]
E
[eV]
‒2.63
‒2.22
LUMO
[d]
V][a]
0.92
0.54
[nm]
408
407
[nm]
505
485
2
,2’-positions of the thiophene rings). TTPA-BDT exhibits a low
TTPA-BDT
TTPA-DTP
reorganization energy similar to those found for other excellent
p-type semiconducting organic materials[33,34] and smaller
than that obtained for the reference spiro-OMeTAD (0.139 eV)
[
=
a] Measured vs. normal hydrogen electrode (NHE). [b] EHOMO estimated as EHOMO
ox
‒4.44 eV ‒ E1/2 . [c] Optical band gap estimated from the intersection of the at the B3LYP/6-31G** level of theory.[24] TTPA-DTP shows a
absorption and emission spectra. [d] ELUMO estimated as ELUMO = EHOMO + E0-0
.
higher reorganization energy because the formation of the
cation implies important changes in the single-double CC bond-
length alternation pattern of the DTP core. Therefore, TTPA-
BDT presents a great potential as hole transporting material for
PSC devices owing to its small λ value and the appropriate
energy level alignment with the valence band edge of the
perovskite.
To gain a better understanding of the electrochemical
properties of the HTMs studied, the oxidized species (up to the
tetracation) of TTPA-BDT and TTPA-DTP were also calculated at
2 2
the B3LYP/6-31G** level in CH Cl . Table S1 collects the charges
accumulated by the constituting fragments (central cores and
TPA units) as oxidation occurs. The evolution of the charges
upon oxidation indicates that for the cation species the charge
is initially extracted from the TPA units (especially from those
linked to the 2.2’-positions of the thiophene rings) with
significant contributions from the central BDT and DTP cores.
This effect is more pronounced for TTPA-DTP, for which the
conjugation through the central core is more effective. Upon
further oxidation (dication, trication and tetracations), the
charge is gradually extracted from all the TPA units. The
evolution of the charges extracted (Table S1) evidences the
most favorable electronic communication between the TPA
moieties introduced in 2,2’-positions and the conjugated BDT
and DTP cores. It should be noted that, for TTPA-BDT, the
ionization energies (IEs) required for passing from the neutral
molecule to the cation (IE1), from the cation to the dication
Despite the electrochemical differences found between TTPA-
BDT and TTPA-DTP, both HTMs show very similar absorption
profiles with strong absorption features below 470 nm (Fig. 3b).
Both systems exhibit a broad band covering the 250–470 nm
range with two well-resolved peaks. The first peak is registered
4
−1
−1
at ca. 407 nm (6.5 and 8.9 × 10 M cm for TTPA-BDT and
TTPA-DTP, respectively) along with a shoulder in the 360–370
nm range. The second intense absorption peak is observed at
around 310 nm. Theoretical simulations of the absorption
spectra for TTPA-BDT and TTPA-DTP
from B3LYP/6-31G** time-dependent DFT (TDDFT) calculations
of the lowest-energy singlet excited electronic states (S ) in the
presence of CH Cl Table S2). The theoretical spectra correctly
(Fig. S5) were performed
n
2
2
(
reproduce the experimental absorption spectra with two
intense bands calculated at 448 and 305 nm (TTPA-BDT) and
(
IE2), from the dication to the trication (IE3) and from the
4
63 and 307 nm (TTPA-DTP). For TTPA-BDT, the band at 448 nm
results from the S → S and S → S electronic transitions
calculated at 450 and 414 nm, respectively, and gives rise to the
experimental peak registered at 407 nm. These S → S and S
transitions involve one-electron promotions from the
trication to the tetracation (IE4) are predicted to have relatively
close values (4.56, 4.79, 5.09 and 5.17 eV, respectively). The
difference in IE values is smaller between IE1 and IE2 (0.22 eV)
and especially between IE3 and IE4 (0.08 eV). These differences
explain the electrochemical behavior found for TTPA-BDT
showing a unique four-electron oxidation wave with a shoulder
at low potentials corresponding to the first two oxidation
processes (Fig. 3a). In contrast, TTPA-DTP, which presents a
stronger donor character with a smaller IE1 value (4.37 eV) in
line with the electronic properties discussed above, exhibits a
larger separation of 0.46 eV between the IE1 and IE2 (4.83 eV)
energies, whereas IE3 (5.10 eV) and IE4 (5.21 eV) remain quite
close at 0.11 eV. These ionization energies justify the well-
separated oxidation waves, the third one implying a two-
electron process, registered for TTPA-BDT (Fig. 3a). The fourth
oxidation wave of TTPA-DTP recorded at 1.22 V should mainly
correspond to the oxidation of the DTP core. Theoretical
calculations therefore clarify the different electrochemical
0
1
0
3
0
1
0
→
S
3
HOMO−2 and HOMO to the LUMO, and imply some charge
transfer from the peripheral TPA units to the central BDT moiety
(Fig. S3). The band predicted at 305 nm, which is related to the
experimental band peaking at 310 nm, shows a complex nature
with significant participation of many electronic transitions. For
TTPA-DTP, a similar absorption spectrum is predicted with slight
bathochromic shifts of the absorption bands.
The emission spectra show broad bands centered at 505 and
4
85 nm for TTPA-BDT and TTPA-DTP, respectively. Optical band
gaps of 2.73 and 2.76 eV (for TTPA-BDT and TTPA-DTP
,
respectively) are estimated from the intersection of the
absorption and emission spectra.
behavior found for TTPA-BDT and TTPA-DTP
.
Hole reorganization energies (λ) were calculated at the
B3LYP/6-31G** level in gas phase to evaluate the capability of
the TTPA-BDT and TTPA-DTP compounds as HTMs (see the
Supporting Information for full computational details). The λ
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experiences a glass transition at 71.1 °C, followed by two
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crystallization processes at 105.3 and 156.1 °C, to finally melt at
3
11.5 °C, during the first heating cycle. In the second cycle, the
material undergoes a glass transition at 112.8 °C, to further
crystallize at 211.1 °C and melt at 306.9 °C.
2
.2. Photovoltaic Characterization
The new hole transporting materials TTPA-BDT and TTPA-DTP
were employed in perovskite-based solar cells to compare their
performance with the most commonly used compound spiro-
OMeTAD as a reference. PSCs were prepared following the
standard n-i-p configuration having a stack of fluorine-doped tin
oxide (FTO) conductive glass│compact titania (30
nm)│mesoporous titania (200 nm)│perovskite (600 nm)│HTM
(
50‒300 nm│gold electrode (70 nm). Owing to the exceptional
advantages it presents, the compositional modification
MAPbBr 0.85 previously introduced by Seok et al.
(
3 3
)0.15(FAPbI )
was used for the photoactive perovskite layer.[9] The
perovskite layer was fabricated following a one-step spin-
coating procedure involving anti-solvent treatment. The HTMs
were applied by spin-coating their corresponding solutions in
chlorobenzene onto the annealed perovskite film. Tert-
butylpyrydine, lithium tris[bis(trifluoromethylsulfonyl)imide]
(Li-TFSI)
and
tris[2-(1H-pyrazol-1-yl)-4-tert-
butylpyridine]cobalt(III) (FK209) were added in equimolar
amounts as additives to all HTMs solutions. Detailed description
Fig. 4 a) Thermogravimetric analysis of the HTMs under nitrogen at a heating rate of 10 of the solar cell fabrication process can be found in the
C min‒1. b) Differential scanning calorimetry of TTPA-DTP (red) and TTPA-BDT (black) Supporting Information. Fig. 5a shows a cross-section scanning

under nitrogen at a heating rate of 20 C min‒1 (1st and 2nd cycle).
electron microscopy (SEM) image of a typical solar cell prepared
using the aforementioned solar cell configuration. For both
The thermal properties of the HTMs were determined by molecules, TTPA-BDT and TTPA-DTP, the thickness of the HTM
thermogravimetric analysis (TGA) and differential scanning layer for the best performing devices was shown to be between
calorimetry (DSC) (Fig. 4). Both TTPA-BDT and TTPA-DTP show 50 and 100 nm, which is slightly thinner than the 300 nm that is
good thermal stability displaying a plateau until 411.7 and 424.7 usually observed in devices with spiro-OMeTAD, but consistent
°C, respectively, when they start decomposing. Upon heating, with other reports on alternative small molecule HTMs.[24] Fig.
the BDT-based compound shows a glass transition at 164.3 °C 5b shows the energy level diagram of the different solar cell
prior to experiencing a crystallization process at 218.3 °C, components. Both HTMs, TTPA-BDT and TTPA-DTP, have their
eventually melting at 282.5 °C during the first heating cycle. The HOMO levels close to the valence band of the perovskite and
second heating cycle just shows a lower temperature glass should therefore be able to extract holes from the perovskite
transition at around 138.5 °C. Similarly, the DTP-based material absorber.
6
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Fig. 5 a) Cross-sectional SEM image of a PSC fabricated with TTPA-DTP as HTM layer. b) Energy level diagram of the different solar cell components. c) Forward and reverse current
density-voltage (J−V) plots of PSC devices measured under reverse bias employing different HTMs, i.e. TTPA-BDT, TTPA-DTP and spiro-OMeTAD as a reference. d) Current-voltage
curves of TTPA-BDT, TTPA-DTP and spiro-OMeTAD as a reference measured on substrates having interdigitating gold electrodes with a channel length of 2.5 µm. Conductivity values
were calculated using Ohms law.
Reverse and Forward current density-voltage (J−V) plots of the DTP. In fact, the HOMO energies calculated theoretically for
devices employing the different HTMs, along with the TTPA-BDT and TTPA-DTP differ in only 107 meV. One reason for
corresponding photovoltaic parameters of the champion cells, the underperformance of the device using TTPA-DTP could be
are respectively displayed in Fig. 5c and Table 2. The reference the poor solubility of this HTM in chlorobenzene, which leads to
spiro-OMeTAD cells did not show any significant hysteresis, in a less uniform film formation (see Fig. S7).
contrast with the new HTMs; therefore, it may be concluded
that the hysteresis observed for the cells employing TTPA-BDT
Table 2 Photovoltaic parameters of the champion PSCs.
and TTPA-DTP originates from the HTM layer. This behaviour
may arise from an unbalanced charge extraction at the
perovskite/HTM interface, as well as small pinholes that are
present due to the non-uniform film.[35] With a power
conversion efficiency (PCE) of 18.1%, measured under
HTM
V
OC
J
SC
FF
[%]
PCE
[%]
−
2
[
V]
[mA cm ]
23.0
23.0
22.2
22.2
TTPA-BDT (reverse)
TTPA-BDT (forward)
TTPA-DTP (reverse)
TTPA-DTP (forward)
spiro-OMeTAD (reverse)
spiro-OMeTAD (forward)
1.07
1.06
1.07
1.07
1.10
1.10
73.6
70.0
65.8
53.7
72.5
72.4
18.1
17.2
15.6
12.8
17.7
17.6
‒1
simulated sunlight (AM 1.5 G; 100 mW cm ), TTPA-BDT shows
an excellent performance, similar to the reference compound
spiro-OMeTAD (17.7%) (PCE values of the 15 devices fabricated
per HTM are shown in Fig. S9). A high short circuit current of
22.3
22.2
‒
2
2
2
3.0 mA cm could be observed for TTPA-BDT compared to
To get further insight into the charge transport properties of the
new HTMs, conductivity measurements were carried out. Spiro-
OMeTAD, TTPA-BDT and TTPA-DTP were applied on
interdigitated gold substrates by spin-coating their
corresponding solutions in chlorobenzene. The molecules were
doped using 6 mol% FK209, representing the same amount used
in the devices. The results are shown in semi logarithmic I−V
plots in Fig. 5d and the corresponding conductivity values were
calculated using Ohms law. The highest conductivity value was
‒2
2.3 mA cm for spiro-OMeTAD. On the other hand, the open
circuit voltage is 30 mV lower than the reference cell. TTPA-DTP
with a PCE of 15.6% due to the relatively low fill factor caused
by low conductivity. Although the HOMO level of the two
molecules differs by around 400 meV, no significant difference
in the Voc of the devices could be observed. This can be related
to the different electrochemical behavior observed for both
HTMs: a unique four-electron oxidation wave for TTPA-BDT and
a well-separated one-electron first oxidation wave for TTPA-
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4
‒1
found for spiro-OMeTAD (3.74 × 10 S cm ) followed by TTPA- This work was supported by the H2020-ICT-2014-1, grant
View Article Online
DOI: 10.1039/C7TA11314E
BDT (1.26 × 10− S cm ) and TTPA-DTP (4.91 × 10 S cm‒1). agreement n°643791. We also thank the European Research
4
‒1
−5
Compared to spiro-OMeTAD, the lower conductivity showed by Council (ERC-320441-Chirallcarbon), the CAM (FOTOCARBON
TTPA-BDT is likely compensated by a thinner HTM layer, that in project S2013/MIT-2841) and the Spanish Ministry of Economy
‒
term yields a higher short circuit current (23.0 vs. 22.2 mA cm
)
and Competitiveness MINECO (projects CTQ2014-52045-R,
2
. In contrast, in the case of TTPA-DTP the conductivity might CTQ2015-71154-P, CTQ2016-81911-REDT, and Unidad de
be too low for an efficient charge extraction, thus adding further Excelencia María de Maeztu MDM-2015-0538), the Generalitat
explanation for the lower device efficiency observed for this Valenciana (PROMETEO/2016/135) and European Feder funds
molecule. The smaller conductivity might be due to the higher (CTQ2015-71154-P). J.A. is grateful to MINECO for a “JdC-
reorganization energy calculated for TTPA-DTP (see above). incorporación” post-doctoral fellowship. J.C. acknowledges the
From steady-state photoluminescence (PL) as well as transient Generalitat Valenciana for a Vali+d post-doctoral fellowship.
PL we found that the new molecules behave in fact very similar. IMDEA Nanociencia acknowledges support from the 'Severo
For both TTPA-BDT and TTPA-DTP, the steady state PL show an Ochoa' Programme for Centres of Excellence in R&D (MINECO,
efficient quenching of the perovskite emission, evinced by a Grant SEV-2016-0686).
sharp decline of the curves within the 5‒10 ns timescale (see
Figure S8). Altogether, the PL behavior of the HTMs/perovskite
blends is indicative of an efficient charge transfer from the
active layer to the hole transporting layer which, in turn, is
Notes and references
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High efficiency (18.1%) perovskite solar cells are demonstrated by using a hole transporting
material with very low reorganization energy (λ)