A star-shaped cyclopentadithiophene-based dopant-free hole-transport material for high-performance perovskite solar cells

Article abstract of DOI:10.1039/d1cc02396a

A new cyclopentadithiophene (CPDT)-based organic small molecule serves as an efficient dopant-free hole transport material (HTM) for perovskite solar cells (PSCs). Upon incorporation of two carbazole groups, the resulting CPDT-based HTM (C-CPDT) shows an impressive power conversion efficiency (PCE) of 19.68% with better stability compared with those of spiro-OMeTAD.

Full text of DOI:10.1039/d1cc02396a

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A star-shaped cyclopentadithiophene-based
dopant-free hole-transport material
for high-performance perovskite solar cells†
Cite this: Chem. Commun., 2021,
57, 6444
Received 6th May 2021,
Accepted 1st June 2021
Kun-Mu Lee,abc Jui-Yu Yang,d Ping-Sheng Lai,e Ke-Jyun Luo,d Ting-Yu Yang,d
g
d
Kang-Ling Liau,f Seid Yimer Abate
and Yan-Duo Lin
*
DOI: 10.1039/d1cc02396a
rsc.li/chemcomm
A new cyclopentadithiophene (CPDT)-based organic small mole-
A thiophene-fused polyaromatic, cyclopenta[2,1-b:3,4-b0]dithi-
cule serves as an efficient dopant-free hole transport material ophene (CPDT), has been shown to have excellent charge-
(HTM) for perovskite solar cells (PSCs). Upon incorporation of two transport ability due to its rigid structure with an extended
carbazole groups, the resulting CPDT-based HTM (C-CPDT) shows p-conjugation system. CPDT derivatives have been employed in
an impressive power conversion efficiency (PCE) of 19.68% with organic light-emitting diodes (OLEDs), organic field-effect transis-
better stability compared with those of spiro-OMeTAD.
tors (OFETs), and organic photovoltaic devices (OPVs).11–13
Recently, we have demonstrated that CPDT derivatives incorporat-
Perovskite solar cells (PSCs) are rapidly advancing as a potential ing a dithiolane ring in the central core with two triphenylamine
alternative to conventional silicon solar cells, with their power groups can be used as HTMs in PSC fabrication, affording an
conversion efficiency (PCE) boosted to over 25% in the last decade impressive PCE of 19.07%, presumably owing to the intermolecu-
because of the significant progress in the optimization of the lar SꢀꢀꢀS interactions.14 However, the hygroscopic dopants Li-TFSI
composition of the perovskite materials and development of new and t-BP were still needed in these CPDT-based HTMs.
hole-transport materials (HTMs).1–5 To date, the most widely used
In this work, we report two new star-shaped CPDT-based HTMs,
organic small molecular HTM in PSCs has been 2,20,7,70-tetrakis- denoted as T-CPDT15 and C-CPDT, based on a central CPDT core
(N,N-di-p-methoxyphenylamine)9,90-spirobifluorene (spiro-OMeTAD), endowed with four aryl amine moieties (Fig. 1) and their applica-
which exhibited a promising PCE of 25.17%.6 Nonetheless, it suffers tion in PSCs. The difference between T-CPDT and C-CPDT is the
from a high production cost and tedious synthetic procedures. In replacement of two p-methoxytriphenylamine units in the central
addition, dopants like 4-tert-butylpyridine (t-BP) and lithium bis(tri- core of T-CPDT with two p-methoxydiphenylamine-substituted
fluoromethanesulfonyl)imide (Li-TFSI) are needed for spiro-OMeTAD carbazole segments. The introduction of the carbazole groups on
in order to obtain high device performance because of its intrinsic C-CPDT was found to increase the planarity of molecules, leading
low hole mobility. However, hygroscopic additives will accelerate the to the formation of a smooth and uniform thin film, and thus
deterioration of the perovskite, thus resulting in a decrease in the hole-transporting ability. More importantly, devices fabricated
long-term device stability of the cell.7–9 Consequently, further devel- using the C-CPDT HTM without using any dopant achieved an
opment of efficient and dopant-free HTMs is highly desirable.10
impressive PCE of 19.68%, which is significantly higher than that
of the device based on T-CPDT (11.45%).
a Department of Chemical and Materials Engineering, Chang Gung University,
Taoyuan 33302, Taiwan
b Division of Neonatology, Department of Pediatrics, Chang Gung Memorial
Hospital, Linkou Taoyuan 33305, Taiwan
c Center for Green Technology, Chang Gung University, Taoyuan 33302, Taiwan
d Department of Chemistry, Soochow University, Taipei 11102, Taiwan.
E-mail: ydlin@scu.edu.tw
e Department of Applied Chemistry, National Chiayi University, Chiayi 60004,
Taiwan
f Department of Chemistry, National Central University, Taoyuan 32001, Taiwan
g Institute of Chemistry, Academia Sinica, Taipei 115024, Taiwan
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
d1cc02396a
Fig. 1 Chemical structures of T-CPDT and C-CPDT.
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The detailed synthetic route and the NMR and mass spectra good overlap between the HOMO and LUMO is expected to be
characterization of C-CPDT are illustrated in the ESI.† Fig. 2a advantageous to hole-transport ability. According to DFT
shows the normalized UV-vis absorption and photoluminescence calculations, the HOMO and LUMO energy levels of T-CPDT
spectra of T-CPDT and C-CPDT in chlorobenzene and the corres- and C-CPDT were determined to be ꢁ4.16/ꢁ1.66 eV and
ponding data are summarized in Table 1. These CPDT-based ꢁ4.27/ꢁ2.01 eV, respectively. Thus, the Eg values of T-CPDT
HTMs exhibited intense absorption in the range of 300–500 nm. and C-CPDT were estimated to be 2.50 eV and 2.26 eV,
The maximum absorption peaks of T-CPDT and C-CPDT are respectively (Fig. 2b).
centered at 413 nm and 410 nm respectively, and are attributed
The HOMO levels of T-CPDT and C-CPDT were measured
to the p–p* transitions. However, both HTMs showed a weak using differential pulse voltammetry (DPV) in THF solution
absorption band at around 540 nm, which may have resulted (1.0 ꢂ 10ꢁ3 M) and the results are shown in Fig. 2c. The energy
from the intramolecular charge transfer (ICT) transitions from the level diagrams of T-CPDT and C-CPDT are plotted in Fig. 2d.
side arm triphenylamine moieties to the central CPDT core. This is The HOMO energy levels of T-CPDT and C-CPDT were esti-
supported by time-dependent density functional theory (TDDFT) mated to be ꢁ5.12 eV and ꢁ5.29 eV, respectively, which are
calculations shown in Fig. S1 and Table S1 (ESI†). The photolumi- more positive than the valence band of perovskite (ꢁ5.43 eV),
nescence spectra of T-CPDT and C-CPDT exhibited a maximum indicating that the new CPDT-based HTMs are suitable for
peak at 508 nm and 512 nm, respectively. An optical bandgap (Eg) of efficient hole extraction from the perovskite layer to HTMs.
2.70 and 2.74 eV for T-CPDT and C-CPDT, respectively, was calcu- It is noted that the deeper HOMO energy level of C-CPDT may lead
lated according to the normalized Uv-Vis and PL spectra. It was to a higher open-circuit voltage (Voc) of the corresponding PSC
found that the UV and PL profiles are similar in the solution, device. On the other hand, the LUMO levels of T-CPDT and
presumably ascribable to the very small structural difference C-CPDT were determined from the HOMO and Eg values to be
between T-CPDT and C-CPDT.
ꢁ2.42 and ꢁ2.55 eV, respectively. They are higher than the
In order to gain an insight into the correlation between the conduction band of the perovskite, disfavoring the undesired
molecular geometry and electronic properties, the optimized electron transfer from the perovskite layer to the metal electrode.
structure and frontier HOMO and LUMO of the two derivatives However, the HOMO values obtained by DPV were consistent with
were simulated by density functional theory (DFT) (Fig. 2b and the calculated values from DFT results. We also measured the
Fig. S2, ESI†). It was found that the two carbazole units in solid-state ionization potential (Ip) values of T-CPDT and C-CPDT
C-CPDT adopt a highly planar geometry compared with its on thin films using photoelectron spectroscopy in air (PESA),
triphenylamine counterpart (T-CPDT), suggesting that a close whose results are shown in Fig. S3 (ESI†). The Ip values of
molecular packing may benefit the hole transportation between T-CPDT and C-CPDT were estimated to be ꢁ4.95 and ꢁ5.03 eV,
the neighboring molecules. The electron density of the HOMO respectively, which exhibit the same trend as that of oxidation
mainly delocalizes over the half backbone of the molecule, potential data obtained from DPV and DFT calculations.
while the LUMO spreads mainly on the central CPDT core. The
To explore the hole-transport abilities of T-CPDT and
C-CPDT with respect to spiro-OMeTAD, hole-only devices with
the structure of ITO/PEDOT:PSS/HTM/Au were fabricated to
determine the hole mobility by the space-charge-limited
current (SCLC) method (Fig. S4, ESI†). Under the identical
conditions, the hole mobility of pristine T-CPDT and C-CPDT
was estimated to be 2.37 ꢂ 10ꢁ4 and 4.39 ꢂ 10ꢁ4 cm2 Vꢁ1 sꢁ1
,
respectively, indicating that the incorporation of a carbazole unit
in C-CPDT indeed improves the hole mobility, due to the strong
p–p stacking and thus enhanced intermolecular interactions
caused by a coplanar carbazole moiety. The hole-mobility of doped
spiro-OMeTAD is also recorded (2.30 ꢂ 10ꢁ4 cm2 Vꢁ1
s
ꢁ1) using
the same measurement method.
The steady-state photoluminescence (PL) and time-resolved
photoluminescence (TRPL) spectra were also measured to evaluate
the hole-extraction capabilities of the CPDT-based HTMs. As
illustrated in Fig S5 (ESI†), it is clear that significant quenching
of PL intensity was observed when HTMs were deposited on top of
the perovskite layer. MAPbI3/C-CPDT showed more quenching in
PL intensity than MAPbI3/T-CPDT, suggesting that the former
HTM exhibited better hole extraction capability at the interface
of the bilayer. In addition, the hole transport properties were
further validated by TRPL experiments. From the TRPL decay
profile, the perovskite/HTM bilayer exhibited a much fast time
decay, much shorter than those of the pristine perovskite layer.
Fig. 2 (a) Normalized absorption and photoluminescence spectra of
T-CPDT and C-CPDT in chlorobenzene solution. (b) The electron densities
of HOMO and LUMO levels of T-CPDT and C-CPDT calculated by DFT at
the B3LYP/6-31(d,p) level. (c) Differential pulse voltammetry (DPV) of
T-CPDT and C-CPDT in THF solution. (d) Energy level diagrams of
T-CPDT and C-CPDT used in perovskite solar cells.
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Table 1 Photophysical and electrochemical data of T-CPDT, C-CPDT and spiro-OMeTAD
labs (nm) (e ꢂ 10ꢁ4/Mꢁ1 cmꢁ1
)
lfa (nm)
EHOMO (eV)
E0–0 (eV)
ELUMO (eV)
Ip (eV)
EHOMO (eV)
ELUMO (eV)
a
b
c
d
e
f
f
HTM
T-CPDT
C-CPDT
spiro-OMeTAD
413 (8.26)
410 (7.09)
389 (18.37)
508
512
428
ꢁ5.12
ꢁ5.29
ꢁ5.15
2.70
2.74
3.02
ꢁ2.42
ꢁ2.55
ꢁ2.13
ꢁ4.95
ꢁ5.03
ꢁ4.98
ꢁ4.16
ꢁ4.27
ꢁ4.21
ꢁ1.66
ꢁ2.01
ꢁ0.61
a
b
c
Maximum of the absorption and fluorescence bands in chlorobenzene solution. Determined by differential pulse voltammetry. The value of
d
E0–0 was obtained from the intersection of normalized absorption and photoluminescence spectra. Energy of the LUMO of the compounds
estimated by EHOMO + E0–0
e
f
. Ionization potential was measured by the photoemission in the air method from films. TDDFT/B3LYP/6-31G(d,p)
level calculated values.
The lifetimes of T-CPDT and C-CPDT deposited on the perovskite reference using the same fabrication procedure. A cross-section
film were estimated to be 21.39 and 16.57 ns, respectively, which SEM image of the complete device with the C-CPDT HTM is shown
are shorter than that of the pristine perovskite film (31.78 ns), in Fig. S8 (ESI†). The typical photocurrent density–voltage (J–V)
indicating that C-CPDT has better hole extraction ability than characteristics based on dopant-free T-CPDT, C-CPDT, and doped
T-CPDT (Table S2, ESI†). These results are consistent with that spiro-OMeTAD are shown in Fig. 3a. The corresponding photo-
obtained from SCLC experiments.
voltaic parameters are summarized in Table 2. The devices based
In addition to the hole transport properties, the morphological on T-CPDT gave a poor PCE of 11.45%, an open-circuit voltage (Voc)
properties of the HTM films on the surface of the perovskite layer of 0.88, a short-circuit density (Jsc) of 21.37 mA cmꢁ2 and a fill
also played a key role in the device performance. The morphology factor (FF) of 60.9%. In contrast, the optimized PSCs based on
of perovskite films coated with T-CPDT and C-CPDT was examined C-CPDT as the HTM exhibited a promising PCE of 19.68%,
using scanning electron microscopy (SEM). As shown in Fig. S6 together with a Voc of 1.14 V, a Jsc of 23.27 mA cmꢁ2, and an FF
(ESI†), the top-view SEM images of T-CPDT and C-CPDT present a of 74.2%, which are superior to those of the device using doped
homogeneous and fully covered layer on top of the perovskite film. spiro-OMeTAD (Voc = 1.09 V, Jsc = 23.42 mA cmꢁ2, FF = 75.5%, and
The surface roughness of the perovskite with HTMs was studied by PCE = 19.27%). The device with C-CPDT showed a significantly
atomic force microscopy (AFM). The root-mean-square (RMS) improved PCE compared to that with T-CPDT, which could be
roughness values of T-CPDT and C-CPDT are 11.53 and 8.29 nm, ascribed to the higher hole mobility, a better film-forming property
respectively (Fig. S7, ESI†). Apparently, C-CPDT showed smoother and strong hole-extraction capability. A near coplanar structure by
and more uniform film coverage on the perovskite surface. fusing the carbazole moiety on C-CPDT is suggested to have a
The better uniformity of C-CPDT may have likely resulted from contribution. Fig S9 and Table S3 (ESI†) show a small hysteresis
the better planarity of carbazole units on the structure.
between the forward and reverse scans for the C-CPDT-based
To exploit the potential of T-CPDT and C-CPDT as dopant-free device. The corresponding incident photon-to-electron conversion
HTMs in PSCs, a device structure of FTO/c-TiO2/mp-TiO2/ efficiency (IPCE) spectra are depicted in Fig. 3b. The integrated
CH3NH3PbI3/HTM/Ag was fabricated. The details of the fabrication photocurrents of devices with undoped T-CPDT, C-CPDT, and
process can be found in the ESI.† For parallel comparison, the doped spiro-OMeTAD are 21.16, 22.97 and 23.08 mA cmꢁ2
,
device with spiro-OMeTAD as a HTM was also prepared as a respectively, which are in good agreement with the Jsc values
measured in J–V scanning. The stable power output was measured
at the maximum power point (MPP) to further verify the measured
PCEs from the J–V curves for the devices based on T-CPDT and C-
CPDT up to 300s (Fig. 3c). The T-CPDT- and C-CPDT-based devices
exhibited reliable PCEs of 19.52% and 11.03%, respectively. Fig. 3d
shows an average PCE value of 18.82% with a narrow distribution
for devices based on C-CPDT, compared to that of T-CPDT-based
devices (9.88%). These results indicate good reproducibility of the
data for the devices made from C-CPDT.
The long-term stability of PSC devices based on undoped
T-CPDT and C-CPDT, and doped spiro-OMeTAD was further
studied in ambient air without encapsulation, with a relative
humidity of around B40% at a temperature of 20–25 1C in the dark.
Table 2 Photovoltaic parameters of the best-performing PSCs with
dopant-free T-CPDT and C-CPDT and doped spiro-OMeTAD as HTMs
HTM
Voc [V]
Jsc [mA cmꢁ2
]
FF (%)
PCEmax [%]
Fig. 3 (a) J–V curves, (b) IPCE spectra of the best-performing PSCs, (c)
stable output PCE measured as a function of illumination time and (d)
statistical efficiency histograms of PCEs with different HTMs.
T-CPDT
C-CPDT
Spiro-OMeTAD
0.88
1.14
1.09
21.37
23.27
23.42
60.9
74.2
75.5
11.45
19.68
19.27
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As shown in Fig. S10 (ESI†), the PSCs with spiro-OMeTAD Notes and references
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Y.-D. Lin is thankful for the support from the Ministry of Science
and Technology (Grant Number MOST 109-2113-M-031-007). K.-M.
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