Naphtho[1,2-: B:4,3- b ′]dithiophene-based hole transporting materials for high-performance perovskite solar cells: Molecular engineering and opto-electronic properties

Article abstract of DOI:10.1039/c8ta02071j

As a pump for extracting carriers from the absorber to the cathode, hole transporting materials (HTMs) play indispensable roles in promoting the power conversion efficiencies (PCE) of perovskite solar cells (PSCs). In this context, we present two simple HTMs using 2,2′-(1,2-phenylene) bisthiophene or naphtho[1,2-b:4,3-b′]dithiophene as a π-bridge to link two electron-donor units (4′,4′′-dimethoxytriphenylamine): PBT and NDT. Their frontier molecular orbital energies match well with those of perovskites as is consistent with the DFT calculation results. The PCE measured under mimetic solar illumination (AM 1.5, 100 mW cm^-2) increases from 13.6% (PBT) to 18.8% (NDT), which surpass that of the spiro-OMeTAD (18.1%) reference. The modified annulated core in NDT makes it perform better in photovoltaic conversion than PBT. The reason for this is discussed from aspects of hole mobility, charge collection probability (P_c) and recombination kinetics.

Full text of DOI:10.1039/c8ta02071j

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ISSN 2050-7488
PAPER
Kun Chang, Zhaorong Chang et al.
Bubble-template-assisted synthesis of hollow fullerene-like
MoS nanocages as a lithium ion battery anode material
2
rsc.li/materials-a

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DOI: 10.1039/C8TA02071J
Journal Name
PAPER
Naphtho[1,2-b:4,3-b’]dithiophene-Based Hole Transporting
Materials for High-performance Perovskite Solar Cells: Molecular
Engineering and Opto-electronic Properties†
Received 00th January 20xx,
Accepted 00th January 20xx
a
ac
c
c
de
ac
Bin-Bin Cui,* Ning Yang, Congbo Shi, Shuangshuang Yang, Jiang-Yang Shao, Ying Han, Liuzhu
b
b
de
b
DOI: 10.1039/x0xx00000x
Zhang, Qingshan Zhang, Yu-Wu Zhong and Qi Chen*
www.rsc.org/
As a pump for extracting carriers from the absorber to the cathode, hole transporting material (HTM) has indispensable
meanings to promote the power conversion efficiencies (PCE) of perovskite solar cells (PSCs). In this context, we present
two simple HTMs using a 2,2’-(1,2-phenylene) bisthiophene or a naphtha[1,2-b:4,3-b’]dithiophene as π-bridges to link two
electron-donor (4’,4’’-dimethoxytriphenylamine): PBT and NDT. Their frontier molecular orbital energies match with that of
perovskite well as is consistent to the DFT calculation results. The PCE measured under mimetic solar illumination (AM 1.5,
-
2
1
00 mW·cm ) increases from 13.6% (PBT) to 18.8% (NDT), which surpass that of spiro-OMeTAD (18.1%) reference. The
modified annulated core in NDT make it perform better in photovoltaic conversion ability than PBT. The reason for this is
discussed from aspects of hole mobility, charge collection probability (Pc) and recombination kinetics.
good solubility, 2,2’,7,7’-Tetrakis(N,N-di-p-methoxy-phenylamine)-
9,9’-spirobiuorene which was abbreviated as spiro-OMeTAD stands
out from the crowd, becoming one kind of commercial and the most
Introduction
Intense light-absorption, narrow exciton binding energy, high charge-
carrier mobility, long diffusion length, low-cost fabrication and the
skyrocketing power conversion efficiencies (PCE) based on these
advantages have attracted abroad attention to organic-inorganic halide
6
popular HTM for efficient PSCs. For example, Snaith and co-
7
workers showed a maximum PCE of 15.4% in 2013, based on
3 3 x
CH NH PbI3-xCl deposited as thin layers with spiro-OMeTAD as the
hole conductor, and a PCE as high as 19.3% was reported by Yang
1
perovskite solar cells (PSCs). Historic breakthroughs were made
8
and his co-workers in 2014. However, complicated synthesis,
since the first demonstration on a solid-state PSC in 2012 by Park’s
difficult purification, and environmental instability seriously limit the
2
group with long-term stability and a PCE of 9.7%, and a prominent
9
commercial applications of spiro-OMeTAD to PSCs. In such
3
PCE of 15.0% was demonstrated by Grätzel’s group in 2013. The
situations, to develop low-cost and effective HTMs for PSCs is a
challenging and investigation with far-reaching significates.10
Recently, lots of simple triphenylamine-based HTMs in typical donor-
π-donor structure employing π-linkers such as thiophene,11
carbazole,12 triazatruxene,13 3,4-ethylenedioxythiophene14 and so on
have been developed for PSCs. It has been predicted by theoretical
best-performing devices configuration for PSCs comprises an active
perovskite layer sandwiched between an electron transporting
material (ETM) and a hole transporting material (HTM), and this is
4
also commonly used in PSCs’ fabrication procedure. In addition to
intrinsic opto-electronic properties of perovskite layer, as a pump for
extracting carriers from the absorber to the cathode, HTM play an
essential role in promoting PCEs of PSCs quickly to increase from 3.8
to a certified 22.1%.1
a,5
With well-matched frontier molecular orbital energies, sufficient
hole mobility, simplicity and convenience in film-fabrication, and
a.Advanced Research Institute of Multidisciplinary Science, Beijing Institute of
Technology, Beijing 100081, P. R. China
*
E-mail: cui-chem@bit.edu.cn.
b.
Department of Materials Science and Engineering, Beijing Institute of
Technology, Beijing 100081, P. R. China
*
E-mail: qic@bit.edu.cn
c.
School of Chemistry and Chemical Engineering, Beijing Institute of Technology,
Beijing 102488, P. R. China
Beijing National Laboratory for Molecular Science, CAS Key Laboratory of
Photochemistry, Institute of Chemistry, Chinese Academy of Sciences, Beijing
single-crystal X-ray
structure of NDT
PBT
NDT
d.
Fig. 1 Chemical structures of PBT, NDT and thermal ellipsoid plot of the X-
ray single-crystal structure of NDT (probability: 50%). Atom colour code: C,
grey; N, blue; S, yellow; O, red.
1
00190, P. R. China
e.
University of Chinese Academy of Sciences, Beijing 100049, P. R. China
Electronic Supplementary Information (ESI) available: [details of any supplementary
information available should be included here]. See DOI: 10.1039/x0xx00000x
This journal is © The Royal Society of Chemistry 20xx
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Journal Name
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DOI: 10.1039/C8TA02071J
a 100
c
e
o
NDT
NDT
435 C
90
80
70
60
50
40
o
3
32 C
0
.92 eV
LUMO
HOMO
-1.39 eV
-1.42 eV
-4.72 eV
16.8 eV
PBT
PBT
PBT
NDT
0
.93 eV
16.9 eV
18.5 18.0 17.5 17.0 16.5 16.0 15.5
-4.69 eV
200
400
600
800
3.0 2.5 2.0 1.5 1.0 0.5 0.0 -0.5

Binding Energy (eV)
Binding Energy (eV)
PBT
N D T
Temprature ( C)
d
f
b
1
.0
.8
spiro-OMeTAD
PBT
NDT
0
o
1
75 C
0.6
.4
0.2
.0
0
PBT
NDT
o
2
13 C
0
75
100 125 150 175 200 225 250
400
500
600
700
800
900

Temprature ( C)
Wavelength (nm)
Fig. 2 (a) Thermogravimetric analysis of PBT and NDT at a scan rate of 10 ᵒC/min. (b) DSC of PBT and NDT with a scan rate of 20 ᵒC/min. (c) UPS
spectrum of PBT, and NDT films on Si substrates. (d) Normalized UV-visible spectra of PBT, NDT, and spiro-OMeTAD films, at ambient temperature. (e)
HOMO and LUMO energies of PBT and NDT calculated by DFT at the B3LYP/6-31G*/CPCM level and their electron density distribution of HOMO and
2 2
LUMO in CH Cl . (f) Relative energy levels diagram of each component in PSCs.
calculation that HTMs with planar cores performed better in the PCEs synthesized through straightforward Suzuki-Miyaura cross-
of PSCs than HTMs with no planar cores.15 More than one couplings17 with high yields (for PBT, 68%; NDT, 79%) by binding
thiophthene derivative16 have been reported to own higher hole the suitable donor groups (4’,4’’-dimethoxytriphenylamine) to a 2,2’-
mobility than spiro-OMeTAD, making them to be promising (1,2- phenylene)bisthiophene or a naphtha[1,2-b:4,3-b’]dithiophene
competitors and alternatives to spiro-OMeTAD.
core (Scheme 1), which can be obtained in a simple synthetic route
In present work, we introduce two simple HTMs using a 2, 2’- starting from commercial products, and the synthesis details are in the
1,2-phenylene)bisthiophene or a naphtha[1,2-b:4,3-b’]dithiophene as
(
π-bridges
to
link
two
electron-donor
units
(4’,4’’-
dimethoxytriphenylamine) which are denoted as PBT and NDT as
shown in Fig. 1. Ultroviolet photoelectron spectroscopy (UPS), hole
mobility and opto-electronic properties including PCE, incident
photon-to-current efficiency (IPCE), photoluminescence (PL) in
PSCs, charge collection probability (Pc) and recombination kinetics
were found to be intensely related to the different π-bridge between
the donor groups. With the annulated core of naphtha[1,2-b:4,3-
b’]dithiophene, NDT tends to stack through strong π-π and O-π
interactions and leads to excellent hole mobility and performance in
PSCs.
Scheme 1. Synthetic Route for Obtaining PBT and NDT: (a) 2-
(
tributylstannyl)thiophene, Pd(PPh
3
)Cl
2
,
DMF, 130 °C; (b) NBS,
o
o
CHCl
3
/AcOH, 0 C to RT; (c) DDQ, BF
3
OEt
2
2
, dry CH Cl
2
, 0 C to RT; (d) 4-
Results and discussion
Synthesis
methoxy-N-(4-methoxyphenyl)-N-(4-(4,4,5,5-tetramethyl-1,3,2-
3 4 2 3 2
dioxaborolan-2-yl)phenyl) aniline, Pd(PPh ) , 2M K CO , THF/H O (10 : 1),
reflux.
Thiophene-triphenylamine based derivatives, PBT and NDT were
2
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Table 1 Electrochemical and photophysical data of spiro-OMeTAD, PBT and NDT.
ARTICLE
View Article Online
DOI: 10.1039/C8TA02071J
a
b
c
d
d
Compound
E
HOMO, exp (eV)
Abspeak (nm)
Absedge (nm)
E
g
(eV)
E
LUMO, exp (eV)
E
HOMO, cal (eV)
E
LUMO, cal (eV)
PBT
-5.23
-5.32
367
386
450
445
417
2.76
2.79
2.97
-2.47
-2.53
-2.15
-4.69
-1.39
NDT
-4.72
-1.42
-5.1218
305, 384
-4.1618
-0.5418
Spiro-OMeTAD
a
b
. cELUMO, exp = EHOMO, exp + E
. dDFT-calculated energies of HOMO
Determined from UPS measurements. The absorption edges of films determined the E
g
g
2 2
and LUMO at the B3LYP/6-31G*/CPCM level of theory including solvent effects (CH Cl ).
supplementary information (SI). The PBT central core (3, a VG Scienta R4000 analyzer. The valence band energies (EVB
)
dibrominated derivative) was prepared by Stille cross coupling18 corresponding to HOMO energy levels of PBT and NDT are estimated
between commercially available 1,2-dibromo-4,5-dimethoxybenzene to be -5.23 and -5.32 eV below vacuum level, respectively (Table 1),
and 2-(tributylstannyl)thiophene, followed by
employing N-bromosuccinimide (NBS). The
dichlorodicyano-p-benzoquinone (DDQ)/acid-mediated oxidative electrode (-5.10 eV), indicating that they have appropriate energy
cyclization19 of compound 3 gives rise to the formation of the π- levels to extract the hole from perovskite (CH
NH PbI ) to the Au
a
bromination which are coincident located between the HOMO of perovskite
subsequent (CH NH PbI , -5.43 eV) and the valence-band maximum of Au
3
3
3
3
3
3
extended NDT central core under a mild synthetic condition. All the electrode like pumps. The π-conjugation effect from annulated cores
above steps have ideal yields, and the intermediate products can be in NDT leads to its 0.09 eV lower in EHOMO than PBT.21
obtained by sample recrystallization without further purification. The
According to the normalized UV-vis absorption spectra in Fig. 2d
overall yields from compound 1 to PBT and NDT are 55% and 59%, (Table 1), films of PBT, NDT and spiro-OMeTAD on ITO substrates
respectively, and the estimated cost are 98.1 and 78.8 $/g for PBT and show their maximum absorption wavelength (λmax) at 367 nm, 386 nm
NDT, respectively, which are much lower than the purchased spiro- and 384 nm, respectively. We can attribute the bathochromic
OMeTAD (≈271.4 $/g). Molecular structure of the two new HTMs responding in spectrum to the enhancement of the conjugation in NDT
were confirmed by H and 13C (400 MHz) nuclear magnetic resonance by the annulated core.22 The emission maximum of PBT, NDT and
1
(
NMR), and high resolution mass spectra (HRMS).
spiro-OMeTAD are located at 482 nm, 502 nm and 421 nm,
respectively. The correspondent transition energy which also called
energy gaps (E ) are estimated to be 2.76, 2.79 and 2.97 eV for PBT,
g
X-ray Crystallography
More importantly, the single crystal X-ray structure of NDT NDT and spiro-OMeTAD respectively, with their absorption edges
showing an obvious annulated core has been determined (Fig. 1). are located at 450, 445 and 387 and 417 nm, respectively. Therefore,
Slow volatilization of a CH
crystal of NDT. Crystallographic data for NDT (CCDC 1814908): molecular orbitals (LUMOs) energy levels of PBT and NDT are
, space group P-1(2), triclinic, a = 11.237(2), b = 12.606(3), calculated to be -2.47 and -2.53 eV (Fig. 2f), meeting requirements
c = 19.298(4) Å,  = 90.32º,  = 95.79º,  = 109.07º, U = 2568.5 (10) that to be higher than the conduction band edge of CH NH PbI (-3.93
eV). We listed all the experimental and calculated values in Table 1.
2 2 g
Cl solution help us obtained the single based on the experimental EHOMO and E , the lowest unoccupied
56 2 6 2
C N O S
3
3
3
Å.
Thermomechanical analysis
As the thermogravimetric analysis (TGA) data (Fig. 2a) showing,
NDT exhibits an excellent thermal stability and start decomposing at
temperatures (T
PBT presents a melting temperature (T
differential scanning calorimetry (DSC) of NDT reveals a crystalline
behavior with a T of 213 °C and there is no an obvious cooling
d
) at 435 °C, while the T
d
for PBT is 332 °C. When
m
) of 175 °C (Fig. 2b), the
m
crystallization temperature (Tcr) in the cooling progress (Fig. S1a). No
melting and cooling crystallized phenomenon in the second heat/cool
cycle both for PBT and NDT, too (Fig. S1).
Experimental and calculated energy levels of HTMs matching
with perovskite
Determination of the highest-occupied molecular orbital (HOMO)
energy levels of organic molecules by ultroviolet photoelectron
spectroscopy (UPS) is a common and more reliable method.20 As
shown in Fig. 2c, the valance band (VB) spectra of PBT and NDT
were measured with a monochromatic He I light source (21.2 eV) and
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1
00
a 25
b
c40
spiro-OMeTAD
PBT
NDT
2
0
5
0
8
6
4
2
0
0
0
0
0
20
15
10
5
0
1
1
5
0
30
20
10
PVSK
PBT
NDT
spiro-OMeTAD
PCEs: 18.1%
PBT
13.6%
NDT
18.8%
0
0
.0 0.2 0.4
0.6
0.8
1.0
1.2
300
400
500
600
700
800
900
8
10
12
14
PCE (%)
16
18
Volt (V)
Wavelength (nm)
e
f
d
PVSK
PVSK
spiro-OMeTAD
PBT
NDT
spiro-OMeTAD
PBT
NDT
0
0
0
0
0
.8
.6
.4
.2
.0
7
00 725 750 775 800 825 850
0
50
100
150
Time (ns)
200
250
300
Wavelength (nm)
-2
Fig. 3 (a) Current (J)–voltage (V) curves of the solar cells with PBT, NDT, and spiro-OMeTAD under 1 sun (AM 1.5, 100 mW·cm ) with a scan rate of 20
-1
mV/s . (b) Incident photon-to-electron conversion efficiency (IPCE) for the PSCs in (a). (c) The statistics of PCE distribution for devices with HTMs of PBT,
NDT, and spiro-OMeTAD under ambient conditions. 30 devices were fabricated for each tests. (d) A cross-sectional SEM image of a full cell of NDT (the
perovskite absorber is around 600 nm). (e) Continuous wave (CW) photoluminescence (PL) spectra, excitation at 600 nm. (f) Time-resolved PL spectra for
the pristine perovskite (PVSK) film and perovskite/HTM bilayer, excitation at 445 nm and monitored at 765 nm.
Table 2 Photovoltaic parameters of the PSCs based on three HTMs and HTMs’ hole mobilities
a
a
FFavga
(%)
PCEavga
(%)
b
V
OC
V
OCavg
J
SC
J
SCavg
FF
(%)
PCE
(%)
Hole mobility
(cm /V/s)
Compound
-
2
(mA·cm-2)
2
(
V)
(V)
(mA·cm )
1.06
1.11
1.09
0.97
1.05
1.04
16.53
23.47
22.21
14.83
21.52
20.35
63
74
73
57
70
68
13.56
18.78
11.03
16.25
16.13
9.736 × 10-4
5.101 × 10-3
4.243 × 10-3
PBT
NDT
Spiro-OMeTAD
18.05
a
b
Average VOC，JSC，FF and PCE obtained from 30 identical devices of each HTM under the same condition. Hole mobility was calculated by the SCLCs
method using the Mott-Gurney law.
Density functional theory (DFT) calculations were employed to as the reference. Fig. 3a displays the current density-voltage (J-V)
evaluate the HOMOs and LUMOs of the two new HTMs in theory curves (forward: from -0.2 to 1.2 V) of the PSCs prepared with PBT,
aspect. As shown in Fig. 2e, the orbital density in the HOMOs of PBT NDT and spiro-OMeTAD as HTMs. The corresponding photovoltaic
(
-4.69 eV) and NDT (-4.72 eV) are delocalized in the whole molecule, parameters of devices are listed in Table 2. It can be observed that
whereas the orbital density of the LUMOs of PBT (-1.39 eV) and NDT NDT based device has the best performance with a power conversion
-1.42 eV) are mainly localized on the bridge groups. Similar donor- efficiency (PCE) above 18.7%, a short-circuit currents (JSC) of 23.47
(
-
2
π-donor structure of PBT and NDT leads to their similar orbital mA·cm , an open-circuit voltage (Voc) of 1.11 V, and a fill factor (FF)
density, which is agrees with the similarity of photophysical and of 74%, which are all slightly higher than that of reference (spiro-
electrochemical properties.
OMeTAD with a PCE of 18.1%), indicating that NDT can act as a
promising HTM for PSCs. PBT based PSCs show a PCE of 13.6%
under the same conditions. The statistical data of the PSCs based on
2 3 3 3
Devices in the configuration of ITO/SnO /CH NH PbI /HTM/Au 30 uniform devices with each HTM are shown in Fig. 4c, and the
Photovoltaic conversion
as shown in Fig. 3d were fabricated to investigate the performance of average PCE values of the PSCs employing PBT, NDT and spiro-
PSCs employing PBT and NDT as HTM which was measured under OMeTAD as HTMs are 11.03%, 16.25%, and 16.13%, respectively.
1
sun irradiation, when devices with spiro-OMeTAD were fabricated Average values of Jsc, FF, and Voc in Figure 3c are also listed in
4
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Table 2. As shown in Fig. S2, J-V curves of the devices were Photoluminescence (PL) and time-resolved photoVliuewmAirntieclsecOennlicnee
DOI: 10.1039/C8TA02071J
measured by different voltage scan directions (forward and reverse) (TRPL)
with a 20 mV/s-1 scan rate. Although they all have relatively large
To further explore the advantage of the modified annulated core
hysteresis, hysteresis of the devices based on NDT is the smallest
in NDT on carriers’ transfer at the perovskite/NDT interface, we
among these three kinds of PSCs, indicating the least trap states in
measured the continuous wave (CW) photoluminescence (PL, Fig.
NDT based devices.1d, 23 It can be seen that the open-circuit voltages
4
e) and time-resolved photoluminescence (TRPL, Fig. 4f) with the
of devices based on different HTMs are quite similar while the
performance of the devices employing NDT as HTM improved
remarkably due to the increased JSC and FF.
device structure ITO/perovskite/HTM/Au. It is noted that the bilayer
of perovskite/NDT show an estimate dramatic quenching value of
9
estimate 80% and 65% for spiro-OMeTAD and PBT. The comparison
of the TRPL decay at 770 nm between the bilayers and the bare
perovskite deposited on glass is showed in Fig. 4f. The estimated PL
0% when the quenching value of perovskite/HTM bilayer film are
The solar cells’ incident photon-to-current efficiency (IPCE)
spectra which are fabricated with PBT, NDT and spiro-OMeTAD are
also compared with each other in Fig. 4b. The integrated photocurrent
densities of PBT, NDT and spiro-OMeTAD were shown to be 22.66,
e
decay time τ values are 15.3 ns for pure perovskite, 6.1 ns for
-
2
2
1.34 and 16.77 mA·cm , respectively, which are well consistent
perovskite/spiro-OMeTAD, 12.2 ns for perovskite/PBT and 4.5 ns for
perovskite/NDT, respectively. The PL and TRPL results demonstrate
that the modified annulated core of NDT is beneficial to hole transfer
which may promote the PCE of PSCs directly.
with the measured photocurrent density (Table 2). In a broad spectral
scope from 400 to 750 nm, IPCE of NDT and spiro-OMeTAD based
devices exceed 75%, and reach to a maximum 85% (600 nm).
Conversely, the device employing PBT as HTM showed a rather poor
IPCE (65%). Thus, the enhancement in JSC and FF of NDT can be Hole mobility analysis
attribute to the better optical absorption.24
The hole-transport ability of PBT, NDT and spiro-OMeTAD was
a
b
π·····π
O····π
a
100
10
1
b
0.1
spiro-OMeTAD
PBT
0.01
NDT
c
1
2
3
4
5
Applied Bias (V)
1
.10
.05
.00
.95
.90
d
e 1
c
1
PBT
NDT
PBT
NDT
10
1
0
0
1
0
.1
0.85
PBT
NDT


 1
 1
0
.80
.75
0
0
.01
0.1
1
1
10
100
1
10
100
Vint (V)
Light Intensity (mW/cm2)
Light Intensity (mW/cm2)
Fig. 4 (a) J-V curves obtained from the hole-only ITO/PEDOT:PSS/HTM (PBT, NDT, spiro-OMeTAD)/Au device. (b) Crystal packing diagrams of the single
crystals of NDT: π···π distance of 3.34 Å, O···π distance of 3.56 Å. (c) Charge collection probability VS. internal voltage on a log-log scale for PSCs (with
PBT and NDT). (d) JSC of PSCs (with PBT and NDT) VS. light intensity on a log-log scale. (e) VOC of PSCs (with PBT and NDT) VS. light intensity on a log-
log scale.
measured by space-charge-limited current (SCLC) technique (Fig. different HTMs are summarized in Table 2, and the NDT film exhibits
2
-1 -1
3
a) with the device structure ITO/PEDOT:PSS/HTM/Au, and the hole a hole mobility of 4.9 × 10-3 cm ·V ·s which is much higher than
mobility was calculated using the Mott-Gurney law.1d, 25 Detailed PBT’s 9.8 × 10-4 cm ·V ·s when spiro-OMeTAD exhibits a hole
2
-1 -1
introduction to Mott-Gurney law and the hole-only devices’ mobility of 4.2 × 10-3 cm ·V ·s . The higher hole mobility of NDT
fabrication are in the supporting information. For PBT, NDT and can be attributed to its intrinsic crystal structures and layer-by-layer
spiro-OMeTAD, the thickness are 190, 210 and 230 nm respectively, stacking geometry.26 From two adjacent crystal cells of NDT, two π-
which are measured by the step profiler. The hole mobility values of π stacking with distances of 3.34 Å between two NDT molecules in
2
-1 -1
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 5
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J Po lue r an sa el od foMna ot et rai ad l js u Cs ht emm ai rs gt ri yn sA
Page 6 of 8
ARTICLE
one cell and two O-π interactions with distances of 3.56 Å between
Journal Name
We demonstrated two new hole transporting materiaVileswinArtdicolenOonrl-inπe-
DOI: 10.1039/C8TA02071J
two close cells are measured (Fig. 3b). This indicates that the donor constitution, PBT and NDT, using
a
2,2’-(1,2-
annulated core is beneficial to NDT’s intermolecular interactions phenylene)bisthiophene or a naphtha[1,2-b:4,3-b’]dithiophene as π-
leading to a higher hole mobility, and predicts NDT could be a kind bridges to link two electron-rich triphenylamine units (4’,4’’-
of effective HTM for high efficient PSCs with the matching frontier dimethoxytriphenylamine).They have similar matching frontier
molecular orbital energy.
molecular orbital energies to perovskite layer which conforms to DFT
calculation results well. The annulated core of NDT make it perform
better in thermal properties, hole mobility and photovoltaic
Charge collection probability (Pc) and recombination kinetics
Only the higher hole mobility of NDT actually can’t give an conversion in PSCs than PBT. The PCE which was measured under
-
2
absolute explanation for the better photovoltaic conversion ability simulated solar illumination (AM 1.5, 100 mW·cm ) increases from
-
2
than PBT. Thus, under illumination at 100 mW/cm , we examined 13.6% (PBT) to a marvelous 18.8% (NDT), a value which is superior
charge collection probability (Pc) versus internal voltage (Vint)24 of to the 18.1% of spiro-OMeTAD. Charge collection probability (P
) of
PBT and NDT, and their recombination kinetics by measuring JSC and
OC at various light intensities were also compared to demonstrate
C
devices (Fig. 4c) based on NDT and PBT where
V
Pc = Jph/Jph,sat
(1)
that the optimization of π-bridge in HTMs is beneficial to the
performance of PSCs by improving charge extraction and hole-
transport within the device.
V
int = Vbi − Vapplied
(2)
in which the photocurrent density Jph = J
L
− J
D
(J
L
and J
D
are the
current density under illumination and in the dark), the built-in
potential (Vbi) is the voltage when J = J
applied voltage.27 The saturated photocurrent (Jph,sat) is the Jph value
when Jph saturates at high Vint (2 V).5a
bi for NDT and PBT based
L
D
, and the Vapplied is the
Conflicts of interest
There are no conflicts to declare.
V
devices are 1.18 and 1.20 V, respectively. The values with no obvious
difference in Vbi can be attribute to the similarity in HOMO levels of
two HTMs. As shown in Fig. 4c, better charge extraction ability of
NDT can be judged because of the NDT’s Pc increases from PBT’s
Acknowledgements
This work was supported by funding from the National Natural
Science Foundation (21703008) and the Young Talent Thousand
Program of China. The J-V measurement was performed via the
solar simulator (SS-F5-3A, Enlitech) along with AM 1.5G
spectra whose intensity was calibrated by the certified standard
silicon solar cell (SRC-2020, Enlitech) at 100 mV/cm . The
external quantum efficiency (EQE) data were obtained by using
the solar-cell spectral-response measurement system (QE-R
Enlitech).
9
3.2% to 97.5% at short-circuit condition.
Study the recombination kinetics in HTMs of devices based on
different HTMs by measuring JSC and VOC with respect to gradually
-
2
varied light intensities (from 100 to 1 mW/cm ) on a log-log scale
2
can give us another viewpoint to compare charge extraction or transfer
ability of NDT and PBT.28 The dependence of JSC upon illumination
intensity (I) follows the power law which is generally expressed as
，
J
SC ∝ Iα
(3)
where α = 1 for both PBT and NDT based PSCs, as shown in Fig. 4d,
indicating that, at short-circuit condition, almost all holes are swept
out before recombination.5a, 27a
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Journal of Materials Chemistry A
Page 8 of 8
View Article Online
DOI: 10.1039/C8TA02071J
TOC graphic and abstract:
25
20
15
10
5
0
spiro-OMeTAD
PCEs: 18.1%
PBT
13.6%
NDT
18.8%
0.0 0.2 0.4
0.6
0.8
1.0
1.2
Volt (V)