Towards device stability of perovskite solar cells through low-cost alkyl-terminated SFX-based hole transporting materials and carbon electrodes

Abstract

Developing cost-effective, high-efficiency, and stable hole transporting materials (HTMs) is crucial for replacing traditional spiro-OMeTAD in perovskite solar cells (PSCs) and achieving sustainable solar energy solutions. This work presents two novel air-stable HTMs based on a spiro[fluorene-9,9′-xanthene] (SFX) core functionalized with N-methylcarbazole (XC2-M) and N-hexylcarbazole (XC2-H) rings. These HTMs were synthesized via a straightforward, three-step process with good overall yields (∼40%) and low production costs. To further reduce device cost, carbon back electrodes were employed. The resulting PSCs, with a structure of FTO/SnO2/Cs0.05FA0.73MA0.22Pb(I0.77Br0.23)3/HTM/C achieved power conversion efficiencies (PCEs) of 13.5% (XC2-M) and 10.2% (XC2-H), comparable to the reference spiro-OMeTAD device (12.2%). The choice of alkyl chain on the HTM significantly impacts film morphology and device stability. The XC2-H device exhibited exceptional long-term stability, retaining approximately 90% of its initial PCE after 720 h of storage in 30–40% humidity air without encapsulation. This surpasses the performance of both the spiro-OMeTAD (55% retention) and XC2-M (68% retention) devices. The superior stability of XC2-H is attributed to its highly hydrophobic nature and the formation of a compact, smooth film due to interdigitation of the hexyl chains. The straightforward synthesis of XC2-H from commercially available materials offers a promising approach for large-scale PSC production.

Full text

Towards device stability of
perovskite solar cells through low-
cost alkyl-terminated SFX-based
hole transporting materials and
carbon electrodes
Jeeranun Manit1, Pongsakorn Kanjanaboos2, Phiphob Naweephattana1, Atittaya Naikaew2,
Ladda Srathongsian2, Chaowaphat Seriwattanachai2, Ratchadaporn Supruangnet5,
Hideki Nakajima5, Utt Eiamprasert3 & Supavadee Kiatisevi1,4
Developing cost-eective, high-eciency, and stable hole transporting materials (HTMs) is crucial
for replacing traditional spiro-OMeTAD in perovskite solar cells (PSCs) and achieving sustainable
solar energy solutions. This work presents two novel air-stable HTMs based on a spiro[uorene-9,9′-
xanthene] (SFX) core functionalized with N-methylcarbazole (XC2-M) and N-hexylcarbazole (XC2-H)
rings. These HTMs were synthesized via a straightforward, three-step process with good overall
yields (∼40%) and low production costs. To further reduce device cost, carbon back electrodes were
employed. The resulting PSCs, with a structure of FTO/SnO2/Cs0.05FA0.73MA0.22Pb(I0.77Br0.23)3/HTM/C
achieved power conversion eciencies (PCEs) of 13.5% (XC2-M) and 10.2% (XC2-H), comparable
to the reference spiro-OMeTAD device (12.2%). The choice of alkyl chain on the HTM signicantly
impacts lm morphology and device stability. The XC2-H device exhibited exceptional long-term
stability, retaining approximately 90% of its initial PCE after 720 h of storage in 30–40% humidity air
without encapsulation. This surpasses the performance of both the spiro-OMeTAD (55% retention) and
XC2-M (68% retention) devices. The superior stability of XC2-H is attributed to its highly hydrophobic
nature and the formation of a compact, smooth lm due to interdigitation of the hexyl chains. The
straightforward synthesis of XC2-H from commercially available materials oers a promising approach
for large-scale PSC production.
Keywords Perovskite solar cell, Hole transporting material, Spiro[uorene-9,9’-xanthene], Carbon electrode,
Carbazole
Over the past decade, perovskite solar cells (PSCs) have made tremendous progresses in photovoltaic eciency
comparable to crystalline silicon solar cells aer the discovery of the ecient TiO2 sensitization by organolead
halide perovskite for visible-light conversion into electricity in 2009 by Miyasaka and co-workers1. e power
conversion eciency (PCE) record for PSCs currently exceeds 26.2%2. e eciency and stability of the devices
are mainly aected by the PSC structure as well as the ability of charge transporting materials, particularly
HTMs. e functions of HTMs are as follows: (i) extracting photogenerated holes as a result of the HOMO of
the HTM which is located above the valence band of the perovskite; (ii) transporting the holes to electrodes
and blocking a direct contact between the perovskite layer and the electrode; and (iii) suppressing nonradiative
recombination3.
Ecient HTMs should have following properties such as (i) being cost-eective and simply synthesized
which are signicant for the practical development of PSCs, (ii) possessing appropriate HOMO and LUMO
1Department of Chemistry, Faculty of Science, Mahidol University, Rama VI Rd, Ratchathewi, Bangkok 10400,
Thailand. 2School of Materials Science and Innovation, Faculty of Science, Mahidol University, Nakhon Pathom
73170, Thailand. 3Department of Chemistry, Faculty of Science and Technology, Rajamangala University of
Technology Thanyaburi, Pathum Thani 12110, Thailand. 4Center of Sustainable Energy and Green Materials, Faculty
of Science, Mahidol University, Putthamonthon, Nakhon Pathom 73170, Thailand. 5Synchrotron Light Research
Institute, Nakhon Ratchasima 30000, Thailand. email: supavadee.mon@mahidol.edu
OPEN
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energy levels relative to the valence band energy level of the perovskite, (iii) showing a good thermal stability and
hydrophobicity to prevent degradation and enhance long-term device stability, as well as good solubility in lm
formation, and (iv) having an intrinsically high hole mobility and a high hole conductivity.
In general, HTMs can be categorized into three types; inorganic, polymeric, and small-organic molecule
HTMs. Although the inorganic materials have high stability, high internal hole mobility, and low material cost4,
solvents commonly used for their deposition are likely to dissolve the perovskite layer resulting in reduced
device stability5. Polymeric HTMs such as poly(triarylamine) (PTAA) and polyaniline (PANI) seem to present
a high hole mobility for PSCs and good lm-forming properties6. However, their limitations such as dicult
purication, low solubility, and broad molecular weight distribution lead to the development of organic
materials to meet industry requirements. Small-organic molecule HTMs oer potential advantages such as
simple purication, exible molecular structure, controllable molecular weight, and manageable production
costs. Additional signicant properties such as good solubility and high thermal stability can be achieved
by using the small-organic molecule HTMs with a non-planar 3D geometry, as featured by the widely used
spiro-OMeTAD which contains a spirobiuorene (SBF) core unit. However, low mobility (≈ 10− 5 cm2V− 1s− 1),
limited conductivity (≈10− 7 Scm− 1),7,8 high cost (∼90 to ∼460 USD per gram depending on its purity)9, and
synthetic complexity of spiro-OMeTAD make large-scale production impractical. erefore, the need to develop
alternative HTMs with cost-eectiveness, high eciency, and good stability for sustainable PSCs remains a
challenge to be addressed.
Recently, various conjugated moieties and structures similar to spiro-OMeTAD have been used to
improve the hole mobility of organic HTMs including the eciency and stability of PSCs3,10,11. A spiro-based
HTM that proves 20% PCE is found to be characterized by spiro[uorene-9,9’-xanthene] (SFX) as the core
skeleton12–17. SFX is known for its concise and economic synthesis through a one-pot reaction between phenol
and uorenone18–20. Furthermore, SFX-OMeTAD was found to exhibit much faster dissolution than spiro-
OMeTAD. e performances of PSCs based on SFX derivatives reportedly range from 12.4 to 20.8% comparable
to those based on SBF12–16.
Carbazole-based HTMs have demonstrated remarkable potential in PSCs, achieving PCEs of up to
17−21%.21–23 Notably, an SBF-based HTM incorporating an extended π-conjugated N-ethylcarbazole unit
exhibited an impressive PCE of 21.76%.23 e presence of the ethyl side chain eectively alleviated unfavorable
phase-separation in the lm, reduced π−π stacking interactions, and enhanced solubility.
With the goal of developing alternative HTMs with cost-eectiveness, high eciency, and good stability
for PSCs, we designed two new SFX-based HTMs by introducing carbazole motifs with two dierent alkyl
groups as peripheral substituents. ese HTMs, XC2-M (N2,N7-bis(4-methoxyphenyl)-N2,N7-bis(9-methyl-9H-
carbazol-3-yl)spiro[uorene-9,9’-xanthene]-2,7-diamine) and XC2-H (N2,N7-bis(9-hexyl-9H-carbazol-3-yl)-
N2,N7-bis(4-methoxyphenyl)spiro[uorene-9,9’-xanthene]-2,7-diamine), exhibit a distinctive buttery-shaped
structure as depicted in Fig.1. In order to further reduce the cost of PSCs, we used a carbon (C) electrode instead
of Au or Ag due to its abundant source, low cost, and ecient charge collection with a proper work function
(WF) of −5eV similar to that of Au24,25. Although the performance of carbon-based PSCs is notably less ecient
compared to devices employing a metal-based electrode, the highly hydrophobic nature and chemical inertness
of the carbon electrode are reportedly advantageous for the long-term stability of PSCs without the need for
encapsulation23.
With the introduction of methyl and hexyl groups, the XC2-M and XC2-H based devices using the carbon
electrode and Cs0.05FA0.73MA0.22Pb(I0.77Br0.23)3 (triple-cation perovskite) showed short-circuit photocurrent
densities (JSC) of 22.2mA cm−2 and 21.1mA cm−2, and ll factors (FF) of 0.64 and 0.48, yielding the PCEs of
13.5% and 10.2%, respectively. e XC2-H-based device without encapsulation exhibits remarkable stability,
retaining 88% of its original power conversion eciency (PCE) aer 720 h of storage in 30−40% humidity-
controlled air at ambient temperature. In contrast, devices based on the reference spiro-OMeTAD and XC2-M
retain only 55% and 68% of their initial PCEs under the same conditions, respectively. Additionally, aer
annealing at 65°C for 720h, the XC2-H-based device retains approximately 40% of its initial PCE. is stability
is signicantly superior to XC2-M- and spiro-OMeTAD-based devices, which retain only 20% and 5% of their
initial PCEs, respectively, and underscores the advantage of incorporating long alkyl chains into HTM design,
providing a viable approach to address both lm formation and stability issues.
Experimental
Materials
Commercially available reagents were purchased from Sigma-Aldrich, TCI and used without further
purication. Toluene was freshly distilled from sodium prior to use. Other solvents were used directly. Pd2(dba)3
and dicyclohexyl[2′,4′,6′-tris(propan-2-yl)[1,1′-biphenyl]-2-yl]phosphane (Xphos) were purchased from
Sigma Aldrich. Chromatography was performed using Merck silica gel (60 Å, 70−230 mesh). Nuclear Magnetic
Resonance (NMR) spectra were recorded on a Bruker Ascend™ 400MHz spectrometer using CDCl3 and DMSO-
d6 as solvents. Tetramethylsilane (TMS) and non-deuterated residue solvents were used as internal standards.
High-resolution mass spectra (HRMS) were recorded with an HR-TOF-MS Micromass model VQTOF2 mass
spectrometer and reported in m/z unit. UV−vis absorption spectra were recorded in CH2Cl2 solution at room
temperature on Shimadzu UV-2600 spectrophotometer. e UV− vis absorption spectra of the HTMs in
thin lm state were recorded using an Ocean Optics DH-2000-BAL light source. e emission spectra were
recorded in CH2Cl2 solution on a HORIBA FluoroMax® Plus. Steady-state photoluminescence (PL) spectra were
obtained using either a SHIMADZU RF-5301PC or Edinburgh Instruments FLS980 spectrometer. PL spectra
were measured using an integration time of 0.1s, an excitation wavelength of 625nm, excitation and emission
slit widths of 10nm and 5nm, respectively, and an emission range of 650–850nm. Cyclic voltammetry (CV)
measurements were performed on an electrochemical workstation (CH Instrument model 620E voltammetric
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Fig. 2. Synthesis of XC2-M and XC2-H.
Fig. 1. (a) Chemical structures of the HTMs, XC2-M and XC2-H. (b) Optimized molecular geometries of
XC2-M and XC2-H obtained by DFT calculations at B3LYP/6-31G(d) in dichloromethane as the solvent
parameter.
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HTMs λmax, absa)
[nm] λmax, ema)
[nm] λintb)
[nm] Egc)
[eV] Eoxonset
[eV] EHOMO
[eV] ELUMO
[eV] μ
[cm2V−1s−1]
XC2-M 374 433 416 2.98 0.28 −5.08e)/−4.95f) −2.10e)/−1.97f) 2.67 × 10−3
XC2-H 374 433 417 2.97 0.27 −5.07e)/−4.90f) −2.10e)/−1.93f) 1.27 × 10−3
Spiro-OMeTAD 377 430 404 3.07 0.33 −5.13e)/−5.07f) −2.06e)/−2.00f) 2.55 × 10−3
Tab le 1. Summary of spectroscopic and photophysical data, oxidation potential, and hole mobility of XC2-M,
XC2-H, and spiro-OMeTAD. a)λmax, abs and λmax, em were measured in acetonitrile solution; b)λint is the
wavelength at the intersection of the absorption and emission spectra; c)Eg was calculated by the equation of
Eg = 1240/λint; d)Redox potentials were calibrated versus ferrocene (EFc/Fc+) as a reference; e)EHOMO = −4.8 −
Eoxonset and ELUMO = EHOMO + Eg; f)EHOMO and ELUMO were estimated from UPS spectra.
Fig. 3. (a) Absorption (solid line) and emission (dash line) spectra of XC2-M(blue), XC2-H (red), and
spiro-OMeTAD (black) in acetonitrile. (b) CV results of XC2-M, XC2-H, and spiro-OMeTAD (10−4 M in
dichloromethane). (c) Energy level diagram of XC2-M and XC2-H in comparison with perovskite and spiro-
OMeTAD measured by CV and UPS techniques.
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HTMs Jsc
[mA cm− 2]Voc
[V] FF Rs
[Ω cm2] PCE [%] (Champion) Retaining PCE [%]
(aer 30 days)
XC2-M 17.4 ± 3.0 0.97 ± 0.03 0.68 ± 0.07 11.3 ± 2.0 11.4 ± 1.3 (13.5) 68
XC2-H 20.1 ± 1.1 1.00 ± 0.01 0.46 ± 0.03 19.5 ± 2.4 9.18 ± 0.9 (10.2) 88
Spiro-OMeTAD 16.9 ± 3.3 1.02 ± 0.01 0.63 ± 0.12 13.6 ± 3.9 10.5 ± 1.0 (12.2) 55
Tab le 2. Photovoltaic parameters determined from J–V measurements of PSCs based on XC2-M, XC2-H, and
spiro-OMeTAD as HTMs.
Fig. 4. (a) J–V curves from reverse scans of the best cells using XC2-M, XC2-H, and spiro-OMeTAD with
an active area of 0.04cm2. (b) J–V characteristics of XC2-M-, XC2-H-, and spiro-OMeTAD-based hole-
only devices. (c) Normalized power conversion eciency decay for PSCs (ISOS-D-1) under dark ambient
conditions with RH of 30−40%. (d) ermal stability test (ISOS-D-2) at 65°C in the dark under ambient
conditions.
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analyzer) with a platinum counter electrode, a Ag/Ag+ reference electrode, and a glassy carbon working
electrode in 0.1M tetrabutylammonium hexauorophosphate (TBAPF6) in CH2Cl2 solution at a scan rate of 100
mV s−1. Ferrocene/ferrocenium (Fc/Fc+) was used as the internal standard. Band diagram data were obtained
by performing ultraviolet photoelectron spectroscopy (UPS) measurement using synchrotron radiation at the
Fig. 5. Top-view SEM, cross-section SEM, and AFM morphology images (Z-scale = 60nm) of FTO/SnO2/
perovskite/spiro-OMeTAD (a, d, and g, respectively), FTO/SnO2/perovskite/XC2-M (b, e, and h, respectively),
and FTO/SnO2/perovskite/XC2-H (c, f, and i, respectively). e AFM scan size is 10μm × 10μm for all
conditions.
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Beamline 3.2Ua. e measurements were conducted under the electrical bias of − 9.51V, an incident light
energy of 60eV, and a scan step resolution of 0.02eV at Synchrotron Light Research Institute (ailand). A light
source, specically a high-quality AAA-class 7520-LED with an LSS-7120 LED controller (VeraSol), was used to
deliver 1 sun irradiation at an intensity of 100 mW cm−2. e light intensity was calibrated using a silicon diode
(Hamamatsu S1133). Solar cell performance and stability test were carried out using a Keithley 2400 source
meter under 1 sun irradiation conditions with an active cell area of 0.04cm2, as summarized in Table S2. ese
solar cells featured carbon electrodes in conjunction with FTO electrodes. Photocurrent density−voltage (J−V)
curves were recorded within a voltage range of 1.10V to −0.10V with a scan step of 0.05V and a delay time
of 0s. e measurements were taken at room temperature under ambient air, and no protective encapsulation
was used. Hole mobility was investigated via the space-charge-limited current (SCLC) method using a diode
conguration of FTO/poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT: PSS)/triple-cation
perovskite/HTM/C. e detailed analysis method can be found in the supplementary data. Surface and cross-
sectional morphology was observed by eld emission scanning electron microscopy (FESEM; JSM7610FPlus
JEOL, tungsten lament electron source, 20kV, and secondary electron mode). Topography images were done
via atomic force microscopy (AFM, Park NX-10) installed using a non-contact probe (ACTA) with a spring
constant k = 37N m−1 and a resonance frequency of 300kHz. All samples were scanned at a speed of 0.5μm s−1
with a 4nm set point.
Synthesis of the HTMs
Synthesis of XC2-M
A mixture of N2,N7-bis(4-methoxyphenyl)spiro[uorene-9,9’-xanthene]-2,7-diamine (3) (360.2 mg, 0.63
mmol), 5 (342.5mg, 1.32 mmol), Pd2(dba)3 (5.8mg, 0.006 mmol), Xphos (12.0mg, 0.025 mmol), and t-BuONa
(168.7mg, 1.76 mmol) was added in an oven-dried round-bottom ask. e mixture was evacuated and backlled
with N2 gas for three cycles. Freshly distilled toluene (2.6 mL) was added to the mixture, which was then heated
to reux at 120°C for 4h under a nitrogen atmosphere. Aer cooling down to room temperature, water was
added and the reaction mixture was extracted with ethyl acetate. e combined organic phase was washed
with brine, and dried over anhydrous Na2SO4. Aer solvent removal in vacuo, the residue was further puried
by silica gel column chromatography (20% ethyl acetate in hexane) to aord the analytically pure XC2-M as a
yellow solid (405.3mg, 69% yield). 1H NMR (400MHz, d6-DMSO): δ (ppm) = 7.89 (d, J = 7.8Hz, 2H, aromatic),
7.78 (d, J = 2.1Hz, 2H, aromatic), 7.55 (dd, J = 14.6 Hz, J = 8.3Hz, 4H, aromatic), 7.45–7.40 (m, 4H, aromatic),
7.15 (dt, J = 22.7Hz, J = 7.5Hz, 4H, aromatic), 7.05 (dd, J = 8.7Hz, J = 2.1Hz, 2H, aromatic), 7.01–6.91 (m, 8H,
aromatic), 6.80–6.76 (m, 4H, aromatic), 6.71 (dd, J = 8.4 Hz, J = 2.2Hz, 2H, aromatic), 6.62 (d, J = 2.2 Hz, 2H,
aromatic), 6.58 (d, J = 7.8Hz, 2H, aromatic), 3.81 (s, 6H, CH3), 3.69 (s, 6H, CH3). 13C NMR (100 MHz, d6-
DMSO): δ (ppm) = 155.39, 154.61, 150.49, 148.08, 141.09, 140.56, 138.86, 137.60, 131.47, 128.35, 127.25, 126.21,
125.93, 125.23, 124.50, 123.68, 122.70, 121.53, 120.49, 120.02, 119.15, 118.57, 117.06, 116.39, 115.51, 114.76,
110.17, 109.18, 55.19, 53.61, 29.03. HRMS (ESI) m/z: calcd for [M]+, C65H48N4O3 932.3726; found, 932.3727.
Synthesis of XC2-H
A mixture of N2,N7-bis(4-methoxyphenyl)spiro[uorene-9,9’-xanthene]-2,7-diamine (3) (200.97 mg, 0.35
mmol), 6 (240.0mg, 0.73 mmol), Pd2(dba)3 (3.2mg, 0.0035 mmol), Xphos (6.67mg, 0.014 mmol), and t-BuONa
Fig. 6. (a) Water contact angles of XC2-M-, XC2-H-, and spiro-OMeTAD-based perovskite lms on FTO
substrates. (b) Steady-state PL spectra of the perovskite lms with and without HTMs.
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(94.2mg, 0.98 mmol) was added in an oven-dried round-bottom ask. e mixture was evacuated and backlled
with N2 gas for three cycles. Freshly distilled toluene (2.6 mL) was added to the mixture which was then heated
to reux at 120°C for 4h under a nitrogen atmosphere. Aer cooling down to room temperature, water was
added and the reaction mixture was extracted with ethyl acetate. e combined organic phase was washed with
brine, and dried with anhydrous Na2SO4. Aer complete solvent removal, the residue was puried by silica gel
column chromatography (20% ethyl acetate in hexane) to aord the analytically pure XC2-H as a dark-yellow
solid (262.7mg, 70% yield). 1H NMR (400 MHz, d6-DMSO): δ (ppm) = 7.83 (d, J = 7.8Hz, 2H, aromatic), 7.75
(d, J = 2.1Hz, 2H, aromatic), 7.50 (d, J = 8.4Hz, 4H, aromatic), 7.39–7.36 (m, 4H, aromatic), 7.16–7.05 (m,
4H, aromatic), 7.01–6.89 (m, 10H, aromatic), 6.76–6.68 (m, 6H, aromatic), 6.63 (br d, 2H, aromatic), 6.57 (dd,
J = 8.1 Hz, J = 1.6Hz, 2H, aromatic), 4.29 (t, J = 7.1Hz, 4H, NCH2), 3.69 (s, 6H, CH3), 1.71 (t, J = 7.1Hz, 4H,
CH2), 1.22 (ddt, J = 20.1Hz, J = 14.3Hz, J = 8.4Hz, 12H, CH2), 0.89–0.70 (m, 6H, CH3). 13C NMR (100 MHz, d6-
DMSO): δ (ppm) = 155.80, 155.01, 151.00, 148.53, 140.98, 140.95, 139.27, 137.35, 131.93, 128.70, 127.65, 126.63,
126.31, 125.72, 124.86, 123.21, 122.04, 120.94, 120.41, 119.66, 118.92, 117.45, 116.79, 116.11, 115.16, 110.60,
109.69, 55.59, 54.09 ,42.79, 31.41, 28.95, 26.59, 22.45, 14.28. HRMS (ESI) m/z: calcd for [M–H]+, C75H68N4O3
1073.5364; found, 1073.5311.
Device fabrication
Preparation of perovskite solutions. Stock solutions of 1.5M CsI in dimethyl sulfoxide (DMSO), 1.5M PbBr₂ in
a 1:4 (v/v) DMSO: N,N-dimethylformamide (DMF), 1.5M PbI₂ in a 1:4 (v/v) DMSO: DMF were prepared one
day prior to use. A separate formamidinium lead iodide (FAPbI₃) solution was prepared by mixing 198.6mg
formamidinium iodide (FAI) with the PbI₂ stock solution containing 9% PbI₂ powder excess. Similarly, a
methylammonium lead tribromide (MAPbBr3) solution was created by mixing 38.6mg CH3NH3Br (MABr)
with 9mol% PbBr2 excess. It has been reported that incorporating excess PbI₂ and PbBr₂ into the perovskite
precursor solution passivates defect, facilitates charge extraction, and consequently improves overall device
performance26,27. ese FAPbI3 and MAPbBr3 solutions were then combined in a 23:77 volume ratio to form a
double-cation solution. e triple-cation perovskite solution with the formula Cs0.05FA0.73MA0.22Pb(I0.77Br0.23)3
was prepared by combining the double-cation solution with 5 vol% of the CsI stock solution to prevent
perovskite phase separation. e resulting triple-cation perovskite solution was kept under constant stirring
at room temperature for 1h and then ltered with 0.22μm PTFE CNW syringe lter prior to lm fabrication.
Preparation of HTM solutions. To prepare each HTM solution (spiro-OMeTAD, XC2-M, and XC2-H), 1 mL
of 65.3 mM HTM in chlorobenzene was mixed with 17.5 µL LiTFSI solution (520mg in 1 mL acetonitrile) and
28.5 µL of 4-tert-butylpyridine (t-BP). HTM solutions were stirred overnight at room temperature in a glove box.
Device fabrication. e PSC devices were fabricated using a previously reported method15,28–32. FTO glass
substrates were cleaned sequentially using ultrasonication in Alconox solution, deionized (DI) water rinse, and
isopropanol rinse. e cleaned substrates were treated with UV-ozone for 20min to further enhance surface
cleanliness. To obtain a compact layer of SnO2 on FTO, a solution of 0.2M SnCl2·2H2O in ethanol (EtOH) was
prepared and aged at room temperature for 2 days before use. is SnO2 precursor solution was spin-coated
onto the FTO substrates at 3000rpm for 30s with an acceleration of 1500rpm/s under ambient conditions. e
spin-coated lm was then annealed at 180°C for 1h and allowed to cool to room temperature. Deposition of the
perovskite layer onto the FTO/SnO2 substrates was achieved by spin-coating 50 µL of the triple-cation perovskite
solution at 3500rpm for 35s with an acceleration rate of 700rpm s−1. Anisole (100 µL) was added onto the
rotating substrate as an antisolvent at 30th s aer the program started. e lm was subsequently thermally
treated at 100°C for 30min. All spin coating and annealing processes were performed in a glove box lled with
N2. 50 uL of the HTM solution was spread and rested on the perovskite layer for 30s. e deposition of the hole
transporting layer was achieved using spin coating for 30s at a spin speed of 2000rpm with an acceleration of
1000rpm s–1. To prepare a carbon electrode, the ethanol solvent interlacing technique was used following the
previously reported procedure33,34. A commercial carbon ink was rst coated on a glass slide using a doctor
blade to form a carbon electrode with a 80-µm thickness and soaked in ethanol for two hours. e carbon lm
was peeled o the glass substrate, dried at room temperature, and cut into 0.04cm2-sheets for further use. To
complete the fabrication, the device with all stacking layers was pressed and annealed at 60 ºC for 5min33,34.
Results and discussion
Synthesis of HTMs
e new spiro-based HTMs exhibit a simple structure consisting of the central SFX core attached to two
diarylamine arms with N-methyl- or N-hexylcarbazole, as presented in Fig.1. e synthetic routes of XC2-M
and XC2-H are shown in Fig.2 and detailed synthetic procedures and material characterization are available
in the Supporting Information (Figs. S1−S2). N-Alkylcarbazole (5 and 6) and 2,7-dibromospiro[uorene-9,9’-
xanthene] (2) were prepared according to the literature12,35. p-Anisole fragment was then introduced to yield
the intermediate 3 through palladium-catalyzed Buchwald–Hartwig amination. e nal products were also
synthesized by Buchwald–Hartwig amination, giving XC2-M and XC2-H with the isolated yields of 69% and
70%, respectively. e overall yields of these three-step syntheses are 38% and 42%, respectively. According to
green-chemistry protocols, XC2-M and XC2-H were prepared without inert atmosphere and toxic reagents,
lowering unfavorable environmental impact. Notably, these HTMs exhibit exceptional stability under ambient
conditions, eliminating the need for glovebox storage in contrast to spiro-OMeTAD. Structures of the synthesized
HTMs were characterized using standard spectroscopic techniques such as 1H NMR, 13C NMR, and UV–vis.
High-resolution mass spectrometry (HRMS) conrmed the presence of XC2-M and XC2-H.
In terms of solubility, both new HTMs showed good solubility in various organic solvents, such as
dichloromethane, ethyl acetate, acetonitrile, toluene, anisole, as well as in chlorobenzene as shown in Fig. S3c.
is is likely caused by the perpendicular spiro-core structure of SFX15. Furthermore, calculated costs for the
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synthesis of XC2-M and XC2-H are estimated to be about 120 USD/grams and 140 USD/grams, respectively,
whereas spiro-OMeTAD costs are approximately 310 USD/grams (Table S4 and S5). is cost-competitiveness
suggests the potential for mass production of XC2-M and XC2-H, making them more economically viable
alternatives to spiro-OMeTAD.
Optical properties and electron density distribution analysis
Figure3a shows the UV-vis absorption and uorescence spectra of XC2-M, XC2-H, and spiro-OMeTAD in
acetonitrile solution at room temperature, and Table1 summarizes the corresponding spectral data. e optical
bandgap (Eg) was estimated from the intersection of the absorption and emission bands36. e maximum
absorption bands (λmax, abs) of XC2-M and XC2-H, located at 374nm, correspond to the π−π* transition of the
entire π−conjugated backbones. ese absorption bands exhibit a slight blue shi compared to that of spiro-
OMeTAD (377nm), which is likely attributed to the highly twisted geometries of the arylamine side groups.
e results indicate that the substitution of a methyl group with the hexyl group on carbazole moieties causes
no signicant change in the absorption properties. e absorption spectra of the HTM lms are presented in
Fig. S3a (Supporting Information). In their lm state, both XC2-M and XC2-H exhibit a slight red shi in their
maximum absorption wavelength, suggesting the formation of J-aggregates, a common phenomenon observed
in π-conjugated molecules. Minimal light absorption by these HTMs in the visible region (> 430nm) indicates
a minimal impact on the overall light harvesting eciency of the resulting devices. In addition, no signicant
changes in the absorption edges of the perovskite lms modied with spiro-OMeTAD and the new HTMs were
observed (Fig. S3b).
To further study the eect of the alkyl-terminated carbazole units on the structural and electronic properties
of the new HTMs, density functional theory (DFT) calculations and time-dependent density functional theory
(TDDFT) were performed with B3LYP and CAM-B3LYP functionals, respectively, including 6-31G(d) basis
set. Table S1 and Fig. S8 illustrate the highest occupied molecular orbitals (HOMOs) and the lowest unoccupied
molecular orbitals (LUMOs) involved in the electron transitions of XC2-M and XC2-H. e calculation results
reveal that the HOMOs of both HTMs are predominantly localized on the central uorene core with some
extensions to the adjacent aromatic rings of the peripheral units, while their LUMOs spread mainly over the
uorene core with some extensions to the carbazole. is implies that charge transfer in these new HTMs
occurs from the peripheral OMe-phenyl groups towards the central SFX core and peripheral carbazole units.
Furthermore, XC2-M and XC2-H form similar dihedral angles of ca. −33° and −144° between the uorene
cores attached to phenyl rings and carbazole units, respectively (Fig.1b), leading to their similar absorption
properties (Fig. S7). e result is in good agreement with their UV−vis data in which the new HTMs exhibit
the absorption bands at the same wavelength. Signicantly, both XC2-M and XC2-H show twisting motions of
N-alkylated arylamine side arms and highly distorted, “buttery-like”, spatial structures due to the SFX core unit.
is unique shape further reduces intermolecular interactions between HTM molecules, ultimately enhancing
their overall solution processability.
Electrochemical properties
To understand how eciently the HTMs extract charge carriers, cyclic voltammetry (CV) measurements were
performed in dichloromethane solution (Fig.3b). e experiments were conducted in TBAPF6/CH2Cl2 in the
presence of ferrocene/ferrocenium (Fc/Fc+) redox couple as an internal standard at a scan rate of 100 mV s−1.
Redox potentials of XC2-M, XC2-H, and spiro-OMeTAD are presented in Table1. XC2-M and XC2-H exhibit
a similar oxidation wave at an onset potential (Eoxonset) of 0.27−0.28eV, whereas the Eoxonset of spiro-OMeTAD
is 0.33eV. e cathodic shi of the rst oxidation potential of the new HTMs by 0.05eV compared to spiro-
OMeTAD indicates their high electron donating ability, consistent with the TD-DFT results (Fig. S9), leading
to the higher HOMO energy. is could be ascribed to the electron donation from the diarylamino side arms.
e EHOMO and ELUMO of the HTMs were deduced from the CV curves and absorption spectra as illustrated in
Fig.3c.
e CV measurements reveal that the HOMO energy levels of XC2-M and XC2-H are very similar, at
−5.08eV and −5.07eV versus vacuum, respectively. Both HTMs exhibit slightly higher HOMO levels compared
to spiro-OMeTAD (by 0.05eV and 0.06eV, respectively). Given the carbon electrode’s work function of −5.0eV,
ecient hole transfer from the HTMs to the electrode is facilitated (Fig. S3d). e LUMO levels of XC2-M,
XC2-H, and spiro-OMeTAD are calculated to be −2.09, −2.11, and − 2.03eV, respectively.
To conrm the energy levels of the HTMs, we employed ultraviolet photoelectron spectroscopy (UPS).
Figure S4 shows the UPS spectra with the cut-o (Ecut-o) and onset (Eonset) energy. According to the equation:
EF = Ecut-o − 60 (synchrotron energy)17, the HOMO levels of spiro-OMeTAD, XC2-M, and XC2-H are found to
be − 5.07, − 4.95, and − 4.90eV, respectively. By using the relationship: EHOMO = EF − Eonset and ELUMO = EHOMO
+ Eg37, the LUMO levels are found to be − 2.00, − 1.97, and − 1.93eV, respectively. Figure3c illustrates the UPS
results which show the same trend as those obtained from the CV experiments.
In terms of the device performance, it is crucial to consider the alignment of energy levels between the
perovskite and the HTM materials. e UPS measurement and Tauc plot exhibit the HOMO and LUMO levels
of the triple-cation perovskite of − 5.60 and − 3.94eV, respectively (Figs. S4−S5). Figure S6a shows the energy
levels of all HTMs and the triple-cation perovskite obtained by UPS. e HOMO levels of all three HTMs are
higher than HOMO level of the perovskite (−5.60eV). is result indicates a good match between the HOMO
levels of the new HTMs and the perovskite layer, suggesting that XC2-M, XC2-H, and spiro-OMeTAD can
eciently extract holes from the perovskite layer at the interface. is ecient charge extraction is a critical
factor for achieving optimal performance in PSCs. Meanwhile, the deeper LUMO level of perovskite than that of
all HTMs is benecial for blocking electron leakage from the perovskite layer to the HTM layer, resulting in the
reduction of non-radiative recombination38.
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Photovoltaic properties
To evaluate the performance of the new small-molecule HTMs, the device conguration of FTO/SnO2/
perovskite/HTM/C was used following the previously reported procedure39,40. Spiro-OMeTAD was used as a
reference HTM in this study. e current−voltage (J−V) measurements of the devices based on these HTMs
were performed under simulated AM 1.5G conditions (Fig.4a), and the corresponding photovoltaic parameters
are presented in Table2.
Although spiro-OMeTAD exhibits the higher VOC (1.02V) than the new HTMs (0.97–1.00V) due to the
deeper HOMO level of spiro-OMeTAD, XC2-M-based device yields an impressive PCE of 13.5% with a short-
circuit photocurrent density (JSC) of 17.4mA cm− 2, and a ll factor (FF) of 0.68, compared to 12.2% obtained
from the spiro-OMeTAD-based device (JSC = 16.9mA cm− 2, FF = 0.63). In contrast, the XC2-H-based device
shows the lowest performance with a PCE of 10.2% (JSC = 20.1mA cm− 2, FF = 0.46). e low PCE of XC2-H
is clearly a result of the low FF (0.46) and high series resistance (Rs) of the device. e observed dierence in
performance might be due to variations in the interactions between the HTM and perovskite layers. A detailed
analysis of these interactions is discussed in the following sections. It should be noted that the PCEs achieved by
our new HTM-based devices are comparable to those reported in the literature (Table S6 and S7).
Furthermore, the hole mobility of the new HTMs was studied using the space-charge limited current (SCLC)
method described in the Supporting Information. e hole-only devices with the structure of FTO/PEDOT:
PSS/Perovskite/HTM/C were prepared and used for the measurement. e hole mobility of XC2-M, XC2-H,
and spiro-OMeTAD were found to be 2.67 × 10−3, 1.27 × 10−3, and 2.55 × 10−3 cm2 V− 1 s− 1, respectively (Fig.4b;
Table1). is is likely caused by the intimate contact between the HTM and perovskite in XC2-M and spiro-
OMeTAD devices. e inferior hole transfer at the interface between the perovskite and XC2-H layers, which
is consistent with the initial device performance result, is likely caused by a non-conductive barrier of long
alkyl chains in the XC2-H molecules that hinders the charge transfer17. Despite exhibiting a slightly lower hole
mobility compared to XC2-M, the exceptional long-term stability of XC2-H highlights the importance of factors
beyond just ecient hole transport in HTM design for PSCs.
Stability test
To study the eect of humidity on PSC performances, we examined the devices collected in the dark under
ambient air with relative humidity (RH) of 30−40% for 720h without encapsulation (ISOS-D-1). Aer 720h
of storage under the ambient air, the PCE value of the spiro-OMeTAD-based device decreases dramatically
and retains only approximately 55% of its original value, as shown in Fig.4c and Table S3 (Supplementary
Information). In contrast, the XC2-M-based device retained approximately 68% of its initial eciency,
demonstrating superior stability. Remarkably, the XC2-H-based device, equipped with peripheral hexyl chains,
exhibited the highest stability, retaining an impressive 88% of its initial PCE aer 720h of storage. e high
uniformity and homogeneity observed in the XC2-H lm coating the perovskite layer might explain the good
stability results achieved by the device. Furthermore, we further investigate the stability of the non-encapsulated
devices at 65°C following the ISOS-D-2 protocol (Fig.4d). All devices exhibited a signicant decrease in PCE
aer 120h, with XC2-H-based devices losing approximately 40% and others experiencing a 60% reduction.
Previous studies have shown that unsealed PSCs degrade rapidly at high temperatures possibly due to thermal
breakdown of the perovskite layer. However, in contrast, our XC2-H-based device exhibited remarkable stability.
Aer 720h of exposure at 65°C, it retained approximately 40% of its initial PCE. is is probably owing to
better hydrophobicity of the XC2-H-based hole transport layer compared to those based on XC2-M and spiro-
OMeTAD, which showed substantial degradation under identical conditions. Notably, the spiro-OMeTAD-
based device exhibited the most severe degradation at high temperature, retaining only 5% of its initial PCE
aer 720h. ese results suggest that the long alkyl chains incorporated into the HTM molecule play a critical
role in enhancing the long-term stability of the device.
To further evaluate the properties of XC2-M and XC2-H, their thermal stability was examined using the
dierential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). As shown in Fig. S10a in the
Supporting Information, XC2-H exhibits a glass transition temperature (Tg) of 147°C, surpassing that of XC2-M
(138°C). Notably, both new HTMs demonstrated higher thermal stability compared to the benchmark material,
spiro-OMeTAD (121°C)41. Figure S10b displays the TGA spectra of the new HTMs. e TGA results also show
the same trend in which a decomposition temperature (5% weight loss, Td95) of XC2-H (410°C) is signicantly
higher than Td95 of XC2-M (354°C). e results conrm that the incorporation of long alkyl chains enhances the
thermal stability of the HTMs, in good agreement with the stability test discussed above.
Film-forming abilities
To further probe morphological variations in the HTM layers on the perovskite, we employed scanning electron
microscopy (SEM) and atomic force microscopy (AFM). We hypothesize that the exceptional long-term stability
of XC2-H is attributable to its unique lm morphology. is morphology is likely inuenced by the interaction
between the HTM and perovskite layers which results in better contact and charge extraction leading to reducing
charge recombination losses at the interface. Figure S6b shows a top-view SEM image of perovskite grains with
the average size of ca. 300–400nm. e cross-section SEM images highlight the smooth surface of multilayer
structure consisting of a layer of HTM (ca. 200–250nm) covering the top of the perovskite layer (ca. 600nm) for all
HTMs (Fig.5d−f). e combination of sucient HTM layer thickness to fully encapsulate perovskite grains and
the good solubility of the HTMs likely contributes to the formation of smooth HTM lms (Fig.5a−c). While the
XC2-H-based device displayed the smoothest surface (Fig.5c), it formed a separate layer on top of the perovskite
(Fig.5f). In contrast, the XC2-M- and spiro-OMeTAD-based devices exhibited a more uniform interfacial layer
that seemed to blend more seamlessly with the perovskite material (Fig.5d−e). is intimate contact between
the HTM and perovskite could potentially enhance charge extraction in XC2-M and spiro-OMeTAD devices,
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contributing to their higher initial PCEs. However, their lms displayed some imperfections. e XC2-M layer
contained grain boundaries, cracks, and small surface defects, while tiny pinholes were observed on the lm of
spiro-OMeTAD. Conversely, XC2-H formed a more compact and uniform layer without pinholes. is likely
arises from the interdigitation of the hexyl chains within the XC2-H lm. Furthermore, AFM images (scan area:
10μm x 10μm) reveal that both XC2-H and spiro-OMeTAD exhibited a smooth surface when deposited on
the perovskite crystals, with root-mean-square roughness (Rq) values of 9.5 and 9.1nm, respectively (Fig.5g−i).
However, we can observe some defects and pinholes for the lm with spiro-OMeTAD. In comparison, the XC2-
M-based device exhibited a rougher surface with the Rq value of 14nm, which could be a result of the defects
in morphology (Fig.5h). erefore, XC2-H obviously forms a more uniform morphology on the perovskite
layer than XC2-M. ese results emphasize the superior lm morphology of the long-alkyl chain HTM. is
compact morphology signicantly reduces water penetration and subsequent perovskite degradation, ultimately
contributing to the desirable long-term stability observed in XC2-H-based devices.
Furthermore, water contact angle measurements of XC2-M-, XC2-H-, and spiro-OMeTAD-based perovskite
lms deposited on FTO substrate were conducted to investigate surface wettability of the new HTMs since a
hydrophobic surface is desirable for high-performance PSCs to reduce the penetration of moisture into the
perovskite layer42. Figure6 presents the results of the water contact angle measurements of XC2-M, XC2-H and
spiro-OMeTAD, deposited on the perovskite lm. e largest water contact angle of 79° was obtained on XC2-H,
when compared with XC2-M (64°) and spiro-OMeTAD (70°). e surface of the device based on XC2-H shows
the highest degree of hydrophobicity, much better than that of XC2-M and spiro-OMeTAD, which can be used to
justify its high stability in the long term. Our nding is in accordance with previous reports demonstrating that
alkyl chains enhance the hydrophobicity of hole transporting layers, thereby improving PSC device stability43,44.
e high hydrophobicity of XC2-H, arising from its hexyl terminal groups, likely contributes to the exceptional
stability of XC2-H -based devices. By acting as a protective barrier against moisture, XC2-H prevents moisture
diusion into the perovskite layer.
Charge transfer and transport properties
To investigate the hole-transporting ability and nonradiative recombination losses of the HTMs, the steady-
state photoluminescence (PL) spectra of three perovskite/HTM lms coated on a bare glass substate were
measured. In Fig.6b, the perovskite lm shows an intense emission peak around 750nm whereas the perovskite/
HTM lms obviously exhibit the PL quenching. e PL intensity was quenched in the decreasing order by the
perovskite/XC2-H, perovskite/XC2-M, and perovskite/spiro-OMeTAD lms, in accordance with the trend of Jsc
values (Table2). Remarkably, the perovskite/XC2-H lm demonstrates a strongest PL quenching eect which is
in good agreement with its remarkable Jsc of 20.1mA cm− 2. is suggests that XC2-H is the most ecient HTM
in terms of hole extraction at the perovskite/HTM interface when compared with XC2-M and spiro-OMeTAD.
e results together with the compact lm morphology, thermal stability and surface hydrophobicity explain the
superior performance of the PSC device based on XC2-H in a long term.
Conclusions
Two new SFX-based HTMs, XC2-M and XC2-H, incorporating N-methylcarbazole and N-hexylcarbazole rings,
respectively, were synthesized using cost-eective and environmentally-friendly protocols. Both HTMs exhibit
similar molecular geometries, comparable electrochemical and photophysical properties but remarkably distinct
wettability and lm morphology on the perovskite layer, leading to the signicant dierence in long-term device
stability. Compared to the champion PCE of a spiro-OMeTAD-based device using the carbon electrode (12.2%),
the initial eciencies of 13.5% and 10.2% were achieved under the same device structure based on XC2-M and
XC2-H, respectively, in line with the trend of hole mobility as follows: XC2-M > spiro-OMeTAD > XC2-H. is
study reveals that the newly developed HTM with peripheral hexyl chains, XC2-H, exhibited the signicantly
higher water contact angle (78°) compared to XC2-M (64°) and spiro-OMeTAD (67°). In addition, XC2-H shows
eective hole transfer at the perovskite/HTM interface which is in agreement with its remarkable Jsc of 20.1mA
cm− 2. Crucially, stability analysis in the dark at ambient humidity following ISOS-D-1 (room temperature) and
ISOS-D-2 (65 ºC) shows that the XC2-H-based device exhibits the excellent stability, surpassing XC2-M and
spiro-OMeTAD. e exceptional long-term stability of XC2-H highlights the importance of factors beyond just
hole transport eciency in designing HTMs for PSCs.
e success of XC2-H can be attributed to its unique combination of properties. Its highly-twisted 3D
structure enables good solubility in organic solvents, facilitating spin coating. e resulting lms exhibit
excellent non-wetting behavior, high thermal stability, and a smooth, compact morphology. ese attributes
are derived from the presence of peripheral long hexyl chains, which repel moisture and minimize pinholes,
ultimately enhancing charge extraction. ese characteristics, along with its appropriate energy level alignment,
enable XC2-H to maintain a high-quality interface with the perovskite layer over time, leading to superior device
stability. is nding emphasizes that careful consideration of the HTM’s molecular structure and its ability to
form a dense, smooth lm, in addition to hole mobility and energy levels, is crucial for designing ecient HTMs
for high-performance PSCs.
Data availability
e datasets used and/or analyzed during the current study are available from the corresponding author on
reasonable request.
Received: 20 May 2024; Accepted: 30 September 2024
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Acknowledgements
JM acknowledges the Development and Promotion of Science and Technology Talents Project (DPST) for her
scholarship. We acknowledge the Center of Excellence for Innovation in Chemistry (PERCH-CIC), Ministry of
Higher Education, Science, Research, and Innovation.
Author contributions
J.M. carried out the experiments. P.K., P.N., and U.E. coordinated on data analysis. A.N. and L.S. coordinated on
solar cell fabrication. C.S. coordinated on AFM analysis. R.S. and H.N. coordinated on UPS measurement. J.M.
and S.K. analyzed the data and wrote the manuscript. S.K. initiated the ideas and supervised the project.
Funding
is work was nancially supported by the Mahidol University (Basic Research Fund: scal year 2024). JM
acknowledges the Development and Promotion of Science and Technology Talents Project (DPST) for her schol-
arship. We acknowledge the CIF-CNI grant, Faculty of Science, Mahidol University.
Declarations
Competing interests
e authors declare no competing interests.
Additional information
Supplementary Information e online version contains supplementary material available at https://doi.
org/10.1038/s41598-024-74735-4.
Correspondence and requests for materials should be addressed to S.K.
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