A linear D-π-A based hole transport material for high performance rigid and flexible planar organic-inorganic hybrid perovskite solar cells

Article abstract of DOI:10.1039/c9tc03941d

A facile and less expensive hole transport material is essential to enhance the power conversion efficiency (PCE) of perovskite solar cells (PSC) without compromising the ambient stability. Here, we designed and synthesized a new class of HTM by introduci

Full text of DOI:10.1039/c9tc03941d

Journal of
Materials Chemistry C
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PAPER
A linear D–p–A based hole transport material
for high performance rigid and flexible planar
organic–inorganic hybrid perovskite solar cells†
Cite this: J. Mater. Chem. C, 2019,
, 13440
7
a
a
a
Haeun Kwon,‡ Saripally Sudhaker Reddy, ‡ Veera Murugan Arivunithi,
a
a
a
b
Hyunjung Jin, Ho-Yeol Park, Woosum Cho, Myungkwan Song * and
a
Sung-Ho Jin
*
A facile and less expensive hole transport material is essential to enhance the power conversion
efficiency (PCE) of perovskite solar cells (PSC) without compromising the ambient stability. Here, we
designed and synthesized a new class of HTM by introducing donor–p–acceptor (D–p–A). The HTM
was synthesized by combining the moieties of triphenylamine, biphenyl and oxadiazole derivatives as
0
0 0
electron donating, p-spacer and electron withdrawing moieties, respectively, named
4
-(5-(4-
0
0
00 00 0 0 0
(
hexyloxy)phenyl)-1,3,4-oxadiazol-2-yl)-N,N-bis(4-methoxyphenyl)-[1,1 :4 ,1 :4 ,1 -quaterphenyl]-4-amine
TPA-BP-OXD). The p–p conjugation is increased by introducing the biphenyl p-spacer. The HTM was
(
terminated with an OXD-based moiety and framed as a D–p–A-based HTM that trigged improvement in
the charge transportation properties due to its p–p interactions. We rationally investigated the HTM by
characterizing its photophysical, thermal, electrochemical, and charge transport properties. The great
features of the HTM stimulated us to explore it on rigid and flexible substrates as a dopant-free HTM in
planar inverted-perovskite solar cells (i-PSCs). The device performance in solution processed dopant-free
HTM based i-PSC devices on both rigid and flexible substrates showed PCEs of 15.46% and 12.90%,
respectively. The hysteresis is negligible, which is one of the most effective results based on a TPA-BP-OXD
HTM in planar i-PSCs. The device performance and stability based on the TPA-BP-OXD HTM are better due
to higher extraction and transportation of holes from the perovskite material, reduced charge recombination
at the interface, and enhanced hydrophobicity of the HTM to compete for a role in enhancing the stability.
Overall, our findings demonstrate the potentiality of the TPA-BP-OXD based HTM in planar i-PSCs.
Received 20th July 2019,
Accepted 23rd September 2019
DOI: 10.1039/c9tc03941d
rsc.li/materials-c
9
device architectures. In spite of the high PCE in mesoporous
PSCs, the processing temperature for mesoporous based PSCs
Introduction
In the photovoltaics field, organic–inorganic hybrid perovskite (4500 1C) is high for structural modification of mesoporous
1
0–12
2
solar cells (PSCs) are one of the most effective solar light TiO , which restricts their processing with flexible substrates.
harvesting technologies, due to their broad absorption spectra, high PSCs of planar-type devices are classified into two types: ETL/
extinction coefficient, high charge carrier mobility, low exciton perovskite/HTL as regular and HTL/perovskite/ETL as inverted-
1–7
13–24
binding energy, and excellent semiconducting properties. The type PSCs (i-PSCs).
Among them, i-PSCs have emerged as a
excellent features of perovskites have improved the power promising tactic to solve the complications accompanying the
conversion efficiency (PCE) of PSCs, and it has skyrocketed to device structures mentioned above. The simplistic fabrication
8
2
4.2% by improving the quality of the perovskite layer and process of these i-PSCs at low temperature and with low hysteresis
reducing recombination by placing suitable interfacial layers via evidenced that they are extremely suitable for large scale and
10,25–31
flexible PSCs as well.
Additionally, for commercial application
a
b
of flexible and rigid i-PSCs, interfacial engineering is required for
Department of Chemistry Education, Graduate Department of Chemical Materials,
Institute for Plastic Information and Energy Materials, Pusan National University,
Busandaehakro 63-2, Busan 46241, Republic of Korea. E-mail: shjin@pusan.ac.kr
Materials Center for Energy Convergence, Surface Technology Division, Korea
Institute of Materials Science (KIMS), 797 Changwondaero, Sungsan-Gu,
Changwon, Gyeongnam 642-831, Republic of Korea. E-mail: smk1017@kims.re.kr
Electronic supplementary information (ESI) available. See DOI: 10.1039/c9tc03941d
These authors contributed equally to this work.
3
2
improvement of the PCE and long term stability. Particularly,
a HTM is essential for PSCs, which should decrease the recom-
bination rate and realize high efficiency by facilitating a high
hole transportation ability. Till now, PSCs based on rigid and
flexible substrates widely utilized commercially available state-
of-the-art HTMs such as PEDOT:PSS or spiro-OMeTAD. Apart
†
‡
1
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from their advantages, the high hygroscopic nature of PEDOT:
PSS, and lower mobility and conductivity, acidic nature, and
synthesis complexity of spiro-OMeTAD restrict the further devel-
opment of PSCs and stability issues have been noticed as key
3
3–35
limitations.
spiro-OMeTAD and PEDOT:PSS, researchers attempt with inor-
ganic HTMs such as NiO , CuSCN, Cu O, and V to replace
To overcome these limitations accompanying
x
2
2 5
O
1
0
Fig. 1 Synthetic route of HTM TPA-BP-OXD.
spiro-OMeTAD and PEDOT:PSS. On the other hand, various
conjugated polymers have been incorporated in PSCs as HTMs
and have shown higher PCEs than PEDOT:PSS and spiro-
3
6–38
OMeTAD.
Despite the high PCE of inorganic HTMs, they 1 was further mixed with B pin , CH COOK and Pd(dppf)Cl ,
2 2 3 2
require a high temperature for post-annealing, and thus hinder which resulted in a yield of 63% of compound 2. The reaction
4
4
the fabrication of flexible i-PSCs.
conditions were followed according to our previous reports.
Therefore, organic HTMs are also recommended for high Subsequently compound 3 was synthesized as shown in Fig. S1
19,39–41
performance rigid and flexible i-PSCs.
Specifically, organic (ESI†). Last, by coupling of compound 2 and compound 3 under
small molecule (SM) HTMs are used as an efficient candidate for Suzuki conditions, TPA-BP-OXD was synthesized. To confirm
1
13
rigid and flexible PSCs. Recently, through rational design and TPA-BP-OXD, the material was characterized by H NMR,
C
synthesis, we have successfully demonstrated the potentiality of NMR, and HR-MS, and the corresponding detailed synthetic
3
2,39
SM HTMs and their multi-applications in PSCs and OSCs.
From procedures and spectra are provided in the ESI.†
The structure of TPA-BP-OXD is shown in Fig. 2(a). The
the above observations, organic SM HTMs are the most promising
candidates and the importance of their design is highlighted to compound consists of TPA as an excellent electron donating
maximize high performance and stability for both OSCs and PSCs. moiety, and due to its excellent features, it has been extensively
Since triarylamines have been paid enormous attention in utilized in organic optoelectronic applications. By introducing
optoelectronic applications due to their hole transporting biphenyl as a p-spacer, it extends the molecular structure,
3
2,35,39,41
ability, and chemical and morphological stability,
we which enhances the p–p conjugation. Finally, the HTM was
have chosen one as one of the key fragments, and further terminated with an OXD-based moiety and framed as a D–p–A-
extended with a biphenyl core as a spacer, which increases the based HTM that helps to improve the charge transportation
4
2
p–p interactions and stability of the material, and eventually properties due to its p–p interactions.
terminated the HTM with an oxadiazole (OXD) fragment. OXD UV-vis absorption spectra of TPA-BP-OXD in CHCl
is not only known for its good electron accepting property with and the film state are shown in Fig. 2(b). From the onset
3
solution
opt
high PL but also for high thermal and hydrolytic stability and absorption spectrum, the optical energy gap (E
g
) was measured
4
3
resistance to oxidative degradation. The main aim of this to be 2.67 eV. From Fig. 2(b), the higher energy absorption peak
linear design is to enhance p–p interactions with enhanced appearing at 317 nm was attributed to the n–p* transition of
stability. With all aspects herein, we design and report a novel triphenylamine, while the low energy band around 358 nm was
0
0 0
donor–p–acceptor (D–p–A) HTM, 4 -(5-(4-(hexyloxy)phenyl)- attributed to intra-molecular charge transfer (ICT). The improved
0
0
00 00 0 0 0
1
,3,4-oxadiazol-2-yl)-N,N-bis(4-methoxyphenyl)-[1,1 :4 ,1 :4 ,1 -
ICT in the film state could be ascribed to the efficient p–p
quaterphenyl]-4-amine (TPA-BP-OXD), by introducing biphenyl stacking of the HTM in the film state. Furthermore, the
as a p-spacer, and it extended the molecular structure, which temperature dependent and concentration dependent absorption
enhances the p–p conjugation. Finally, the framed D–p–A-based spectra in chlorobenzene solution reveal the existence of ICT as
HTM helps to improve the charge transportation properties due illustrated in Fig. S2 and S3 (ESI†), respectively. Even though a
to its p–p interactions. It possesses suitable highest occupied similar tendency was observed in the film and solution absorp-
molecular orbital (HOMO) energy, high mobility, good solubility, tion spectra, there was a shift towards longer wavelength in the
and thermal stability. The dopant-free TPA-BP-OXD-based device absorption spectrum. The absorption peak at higher energy is
exhibited significantly improved device performance in solution- almost the same as that of the solution, whereas the lower energy
processed i-PSCs with PCEs of 15.46% on rigid and 12.90% on peak was shifted toward longer wavelength in the film state,
flexible substrates. Negligible hysteresis is observed for the rigid which appeared around 380 nm. This is ascribed to intra-
and flexible PSC devices. Additionally, the devices exhibited long molecular charge transfer between the TPA and the OXD moiety.
term stability over 400 h with minimal loss of the initial perfor- The maximum emission of TPA-BP-OXD in solution was at
mance for both rigid and flexible PSCs.
511 nm (Fig. S4, ESI†). Thermogravimetric analysis (TGA) and
differential scanning calorimetry (DSC) were performed to
explore the thermal stability of TPA-BP-OXD. From Fig. 2(c) and
Results and discussion
(
d), TPA-BP-OXD displayed high T (corresponding to 5% weight
d
g
Fig. 1 and Fig. S1 (ESI†) show the systematic synthesis of the loss) and T of 411 1C and 72 1C, respectively. The high thermal
TPA-BP-OXD, D–p–A based HTM. The detailed synthetic proce- stability of HTM is attributed to its linearity.
dures are given in the experimental section. Initially, the Suzuki
The electrochemical properties of TPA-BP-OXD were examined
coupling reaction is used to synthesize compound 1. Compound using cyclic voltammetry (CV) analysis. Fig. 2(e) shows an
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Fig. 2 (a) Molecular structure of the HTM TPA-BP-OXD, (b) UV-vis absorption spectra in solution and the film state, (c) TGA thermogram, (d) DSC trace
ꢀ1
2 2
recorded at a heating rate of 10 1C min , (e) cyclic voltammogram of TPA-BP-OXD in CH Cl solution, and (f) calculated frontier molecular orbitals:
calculated spatial distributions of the HOMO and LUMO of TPA-BP-OXD.
irreversible oxidation process of TPA-BP-OXD around 1.2 V in
The device configuration of the rigid and flexible substrates
methylene chloride (CH Cl ) solution, which was due to the is ITO/TPA-BP-OXD/perovskite/PC61BM/ZnO NP/Ag, and PET/
2
2
oxidation of TPA. From CV, the HOMO energy of TPA-BP-OXD ITO/TPA-BP-OXD/perovskite/PC61BM/ZnO NP/Ag, respectively,
was found to be ꢀ5.10 eV. The lowest unoccupied molecular to explore the potential of the HTM in i-PSCs. The device
orbital (LUMO) energy was calculated from the HOMO energy architecture and energy level diagrams are presented in Fig. 3(a)
opt
and Eg . From this result, the obtained LUMO value was and 4(a) and (b) respectively. The short-circuit current density–
ꢀ
2.43 eV. TPA-BP-OXD’s obtained HOMO is well matched to voltage ( J–V) characterization of the i-PSCs was carried out under
the perovskite material valence band, which is beneficial for AM 1.5G with simulated solar irradiation at an intensity of
ꢀ
2
efficient charge transfer at the interface.
100 mW cm . To explore the potentiality of TPA-BP-OXD as a
Density functional theory (DFT) has been used to explore HTM, both rigid and flexible substrate based i-PSCs were fabri-
and gain further understanding of the geometrical and electro- cated and optimized. The accomplished photovoltaic parameters
nic distribution of TPA-BP-OXD. The electron density of the are summarized in Tables 1 and 2. As shown in Fig. 3(b), the
HOMO was localized on TPA, whereas the LUMO was distrib- TPA-BP-OXD HTM based i-PSC on a rigid substrate attained
uted on the OXD moiety as shown in Fig. 2(f). The calculated an open-circuit voltage (V ) of 1.03 V, current density ( J ) of
OC
SC
ꢀ
2
HOMO and LUMO of PA-BP-OXD were ꢀ4.64 and ꢀ1.64 eV, 21.15 mA cm and fill factor (FF) of 69.59%, and a resulting PCE
opt
g
respectively. E
from DFT was found to be 3.0 eV. The dipole of 15.19% in the forward scan; in the reverse scan the given PCE
ꢀ
2
moment was found to be 5.43 D. Furthermore, the dihedral was 15.46% with a VOC of 1.03 V, JSC of 21.23 mA cm and FF of
angles are shown in Fig. S5 (ESI†). The obtained experimental 70.56%. In addition, the TPA-BP-OXD HTM based device exhibits
values and the calculated theoretical values are in good accordance almost similar J–V curves in the forward and reverse scans.
with each other.
PCEReverse ꢀ PCEForward
The HTM’s hole transporting behavior is a major influence
on the performance of PSCs. We performed the space-
charge-limited current (SCLC) method to elucidate the charge
HI ¼
PCEReverse
From this result, the calculated hysteresis index (HI) was
transport potentiality of TPA-BP-OXD with a configuration of almost negligible for TPA-BP-OXD (0.013) compared with PEDOT:
ITO/PEDOT:PSS/TPA-BP-OXD/MoO /Ag as displayed in Fig. S6 PSS (0.019). A similar trend was observed in the flexible substrate
3
(
1
ESI†). The hole mobility obtained of TPA-BP-OXD was 3.12 ꢁ based device as shown in Fig. 4(c). The TPA-BP-OXD HTM based
ꢀ5
2
ꢀ1 ꢀ1
ꢀ2
0
cm V
s
.
flexible i-PSC shows a VOC of 1.02 V, JSC of 18.81 mA cm and FF
1
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Fig. 3 (a) Rigid device structure of MAPbI
3
based i-PSC, (b) J–V characteristics, (c) IPCE spectra of rigid i-PSCs with the TPA-BP-OXD HTM, (d) histogram
of the rigid i-PSC efficiencies, (e) steady state PCE as a function of time and (f) real PCEs of the i-PSCs as a function of time in days (inset: contact angle of
TPA-BP-OXD).
3
Fig. 4 (a) Flexible device structure, (b) energy level diagram of the MAPbI based i-PSC, (c) J–V characteristics, (d) IPCE spectra of flexible i-PSCs with
TPA-BP-OXD as a HTM, (e) histogram of the flexible i-PSC efficiencies, (f) steady state PCE as a function of time and (g) real PCEs of i-PSCs as a function
of time in days.
of 67.14% with a resulting PCE of 12.90% in the forward scan. In and shunt resistance (Rsh) from the dark J–V are summarized in
the reverse scan direction, the attained PCE was 12.70% with a Tables 1 and 2. The standard material using PEDOT:PSS as a
ꢀ2
VOC of 1.02 V, JSC of 18.68 mA cm and FF of 66.27%. The HTM was compared with the i-PSCs on the flexible substrate
calculated HI was almost negligible for TPA-BP-OXD (0.015) with attributed results of TPA-BP-OXD.
compared with PEDOT:PSS (0.027). The TPA-BP-OXD based i-PSCs
For the confirmation of JSC, the incident photon-to-electron
showed superior device performance compared to the PEDOT:PSS conversion efficiency (IPCE) was measured for the fabricated
based i-PSCs without any additional dopants on the rigid and i-PSCs of TPA-BP-OXD, as shown in Fig. 3(c) and 4(d). The
flexible substrates, due to the slightly enhanced JSC and integrated JSC (the forward and reverse scans are 21.28 and
ꢀ
2
improved FF. The extracted values of the series resistance (R
s
)
21.39 mA cm , respectively) of the TPA-BP-OXD based devices
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Table 1 Photovoltaic performance of rigid i-PSCs with TPA-BP-OXD as dopant-free HTMs
ꢀ2
ꢀ2
2
2
HTM
Scan direction
VOC (V)
JSC (mA cm
)
FF (%)
PCE (%)
Integrated JSC (mA cm
)
R
s
(O cm )
R
sh (O cm )
7
TPA-BP-OXD
TPA-BP-OXD
PEDOT:PSS
PEDOT:PSS
Forward
Reverse
Forward
Reverse
1.03
1.03
1.03
1.04
21.15
21.23
20.03
19.69
69.59
70.56
66.56
68.18
15.19
15.46
13.76
14.04
21.28
21.39
20.86
20.32
2.21
2.03
3.29
3.75
3.06 ꢁ 10
6.30 ꢁ 10
6.77 ꢁ 10
8.90 ꢁ 10
7
6
6
Table 2 Photovoltaic performance of flexible i-PSCs with TPA-BP-OXD as dopant-free HTMs
ꢀ2
ꢀ2
2
2
HTM
Scan direction
VOC (V)
JSC (mA cm
)
FF (%)
PCE (%)
Integrated JSC (mA cm
)
R
s
(O cm )
R
sh (O cm )
6
6
6
6
TPA-BP-OXD
TPA-BP-OXD
PEDOT:PSS
PEDOT:PSS
Forward
Reverse
Forward
Reverse
1.02
1.02
1.03
1.03
18.81
18.68
18.66
18.58
67.14
66.2
52.09
53.81
12.90
12.70
10.07
10.35
19.02
18.94
18.88
18.72
4.21
4.89
7.31
7.10
6.23 ꢁ 10
5.10 ꢁ 10
4.20 ꢁ 10
4.62 ꢁ 10
calculated from the IPCE spectra values are well matched with reduced the decay time, which is beneficial for better extraction
the JSC values (the forward and reverse scans are 21.15 and of holes from the photogenerated excitons. From the above
ꢀ
2
2
1.23 mA cm , respectively, for TPA-BP-OXD) resulting from investigations, we have demonstrated that TPA-BP-OXD was
the J–V curves. The same trend is detected in the flexible able to extract and transport holes to the electrode efficiently.
substrate based i-PSCs. The integrated JSC of TPA-BP-OXD in A stability test was performed for the fabricated i-PSCs based
ꢀ
2
the flexible i-PSCs was 19.02 and 18.94 mA cm in the forward on TPA-BP-OXD (unencapsulated, temperature: 25 ꢂ 2.5 1C,
and reverse scans, respectively, which are well matched with humidity: 35 ꢂ 5%), as displayed in Fig. 3(f) and 4(g) for rigid
ꢀ
2
the JSC values (18.81 and 18.68 mA cm in the forward and and flexible i-PSCs, respectively. The TPA-BP-OXD based i-PSCs
reverse scans, respectively) from the J–V curves. Moreover, the with a rigid substrate exhibited high long term stability over
values revealed by the J–V (obtained JSC) and IPCE spectra 720 h. To know the reason behind the long term stability, a
(integrated JSC) obtained values are in good agreement with a water contact angle (WCA) study was carried out for TPA-BP-OXD.
narrow error range. Fig. 3(d) and 4(e) show the histogram of the The WCA result for TPA-BP-OXD was 63.51 as shown in Fig. 3f
rigid and flexible substrate PCEs based on both PEDOT:PSS and (inset). From the stability study, the TPA-BP-OXD based devices were
TPA-BP-OXD. To further confirm the consistency of the J–V much more stable than the PEDOT:PSS based i-PCEs. For the rigid
measurement, the PCEs of the device were biased at VMP values i-PSCs, a similar trend has been followed, but the retained PCE
(voltage at the maximum power point) as a function of time, as is slightly higher than for the flexible devices. The higher hydro-
shown in Fig. 3(e). The stabilized PCE of the rigid i-PSCs based phobicity of TPA-BP-OXD is ascribed to the alkoxy tail group found
on PEDOT:PSS and TPA-BP-OXD was linear in time (s) and in the HTM design. This phenomenon would restrict the moisture
was found to be 14.04 and 15.46%, respectively, after 60 s. penetration into the perovskite layer, which in turns helps in
39
Following a similar technique on flexible substrates for PEDOT: enhancing the long term stability.
PSS and TPA-BP-OXD, the PCE was found to be 10.35 and
2.70%, respectively, after 60 s (Fig. 4f).
The surface morphology of the pristine PEDOT:PSS and TPA-
BP-OXD and PEDOT:PSS with perovskite and TPA-BP-OXD with
1
We measured the photoluminescence spectra (PL) and time- perovskite materials was studied using atomic force microscopy
resolved PL decay (Fig. S7, ESI†) to confirm the extent of the (AFM) analyses, as exhibited in Fig. S8 (ESI†). The bare HTM
hole extraction behavior of TPA-BP-OXD. The samples were film possesses a smooth surface morphology, clearly showing
prepared with the configurations ITO/perovskite (MAPbI
ITO/PEDOT:PSS/MAPbI and ITO/TPA-BP-OXD/MAPbI . Fig. S7(a) interface. The bare PEDOT:PSS (root mean square (RMS) = 1.21 nm)
ESI†) shows the MAPbI film’s high intensity PL spectrum without and PEDOT:PSS with perovskite (RMS = 8.56 nm) had slightly
3
), the possibility of efficient charge separation at the HTM/perovskite
3
3
(
3
a HTM, whereas with the introduction of TPA-BP-OXD and reduced RMS values when compared to those of the bare TPA-
PEDOT:PSS (TPA-BP-OXD/MAPbI and PEDOT:PSS/MAPbI ) the BP-OXD (RMS = 1.42 nm) and TPA-BP-OXD with perovskite
MAPbI absorber intensity was dramatically reduced. In particular, (RMS = 11.44 nm) films.
as evidenced by its reduced intensity, TPA-BP-OXD showed efficient Furthermore, electrochemical impedance spectroscopy (EIS)
3
3
3
PL quenching compared to PEDOT:PSS. Furthermore, for these was performed to gain insight into the interface interactions
samples, the transient PL decays were measured as shown in between PEDOT:PSS/perovskite and TPA-BP-OXD/perovskite
1 2
Fig. S7(b) (ESI†). t and t are denoted as fast and slow decays, based PSCs to depict their hole/charge transport behavior
respectively. t1 and t2 of TPA-BP-OXD are 0.83 and 4.68 ns, (Fig. S9, ESI†). The equivalent circuit model previously reported
3
9
respectively. Whereas t1 and t2 of PEDOT:PSS are 1.89 and was used to fit the attained EIS curves. Rrec decreased in the
.12 ns, respectively. The average transient decay values achieved order TPA-BP-OXD 4 PEDOT:PSS at low and high voltages,
were 8.68 ns for ITO/MAPbI , 1.89 ns for ITO/PEDOT:PSS/ which exhibits a long carrier lifetime for excellent transportation
MAPbI , and 0.83 ns for ITO/TPA-BP-OXD/MAPbI . Compared to with the i-PSCs of TPA-BP-OXD, and thereby improves the
PEDOT:PSS and MAPbI , the TPA-BP-OXD based film effectively performance of the PSCs. Additionally, the hole conductivity
5
3
3
3
3
1
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HTM) was increased with increased voltage due to the higher
gradient of the hole density in the HTM. As evidenced by the
above investigations, the HTM performance of TPA-BP-OXD was
better than that of PEDOT:PSS under the optimized conditions.
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In summary, we designed, and well optimized conditions with a
simple synthetic approach were used to synthesize, a novel
D–p–A based HTM, TPA-BP-OXD. It exhibited a hole mobility of
ꢀ
5
2
ꢀ1 ꢀ1
10 J. Seo, J. H. Noh and S. I. Seok, Acc. Chem. Res., 2016, 49, 562.
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.12 ꢁ 10 cm V
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: 411 1C),
1
1
1
1
1
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charge transportation of TPA-BP-OXD is due to the p–p interactions.
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PCE. The hydrophobic nature of TPA-BP-OXD enhanced the stability
of the device. Remarkably, forward and reverse scans of dopant-free
TPA-BP-OXD based devices displayed identical performance with
negligible hysteresis through excellent compatibility of the linear
shaped D–p–A type HTM. Overall, this study demonstrates an
efficient linear shaped D–p–A based HTM which can enhance the
PCE with less hysteresis, and good stability in i-PSCs on rigid and
flexible substrates as well.
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Conflicts of interest
There are no conflicts to declare.
Acknowledgements
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