RESEARCH ARTICLE
www.afm-journal.deUnraveling the Role of Spacer Cations: Toward Constructing
Ideal Dion–Jacobson Halide Perovskites
Zhengxun Lai, Yi Shen, Bei Jiang, Yuxuan Zhang, You Meng, Di Yin, Boxiang Gao,
Weijun Wang, Pengshan Xie, Yan Yan, SenPo Yip, Lei Liao,* and Johnny C. Ho*

Dion–Jacobson-type 2D halide perovskites (DJPs) present an ideal alternative
to their 3D counterparts due to their superior stability and exceptional
optoelectronic properties. Despite the numerous DJPs proposed in recent
years, the impact of diﬀerent spacer cations on DJPs remains unclear. This
understanding is crucial for researchers to select suitable materials and is anurgent requirement for the development of higher-performance DJPs-based
devices. In this study, the inﬂuence of the chain-like spacer cations withvarying branch chains and chain lengths is thoroughly examined using boththeoretical and experimental methods. The ﬁndings reveal that spacer cations
with high polarity components along the main chain direction enhance thestability and photoelectric properties of DJPs. Additionally, it is found that the
chain length of the spacer cation plays a critical role. Chain lengths that are
too long or too short can detrimentally aﬀect the photoelectric performanceand stability of DJPs. These insights will guide researchers in selecting
suitable spacer cations and in innovating new types of DJPs.

Z. Lai, B. Jiang, L. Liao
Changsha Semiconductor Technology and Application Innovation
Research InstituteCollege of Semiconductors (College of Integrated Circuits)
Hunan University
Changsha 410082, China
E-mail: liaolei@whu.edu.cn
Y. Shen, Y. Zhang, Y. Meng, D. Yin, B. Gao, W. Wang, P. Xie, Y. Yan, J. C. Ho
Department of Materials Science and Engineering
City University of Hong Kong
Kowloon, Hong Kong SAR 999077, China
E-mail: johnnyho@cityu.edu.hk
Y. Meng, J. C. Ho
State Key Laboratory of Terahertz and Millimeter Waves
City University of Hong Kong
Kowloon, Hong Kong SAR 999077, China
S. Yip, J. C. Ho
Institute for Materials Chemistry and Engineering
Kyushu University
Fukuoka 816–8580, JapanThe ORCID identiﬁcation number(s) for the author(s) of this article
can be found under https://doi.org/10.1002/adfm.202409987© 2024 The Author(s). Advanced Functional Materials published by
Wiley-VCH GmbH. This is an open access article under the terms of theCreative Commons Attribution-NonCommercial-NoDerivs License,
which permits use and distribution in any medium, provided the original
work is properly cited, the use is non-commercial and no modiﬁcations
or adaptations are made.

DOI: 10.1002/adfm.202409987

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1. Introduction

The past decade has seen a surge of interestin halide perovskites due to their excellent
optoelectronic performance.[1–6] This captivating material has now moved beyond theconﬁnes of the laboratory and is progressively being utilized in industrial production. However, as halide perovskites venture toward industrial output and commercialization, they are encountering a host ofchallenges.[7–10] To address future issues,a fundamental understanding of the inherent structures and properties of diﬀerent types of halide perovskites is essential. The conventional 3D halide perovskites
follow the chemical formula ABX3 , where“A” represents a large alkali metal like Rb
and Cs or short-chain ammonium, suchas methylamine (MA) or formamidine, “B”
denotes a transition metal element likeSn or Pb, and “X” symbolizes a halogen.
Despite their potential, the stability of the 3D halide perovskites is poor, rendering them unsuitable for use in ambientenvironments.[11,12] To enhance the stability of 3D halide perovskites, long-chain organic molecules are incorporated to form
2D halide perovskites. Currently, there are three types of 2D
halide perovskites: the Dion–Jacobson, Ruddlesden–Popper, andalternating cation in the interlayer space types.[13–15] Amongthese, the Dion–Jacobson type 2D halide perovskites (DJPs)
show the most promise, exhibiting greater stability and superior
optoelectronic performance due to their advanced lattice structure, devoid of weak van der Waals bonds.[16,17] The chemical
formula of DJPs is deﬁned as LAn−1 Bn X3n+1 , where A, B, and
X are consistent with the 3D counterparts, and L represents along-chain organic molecule, known as a spacer cation, which
separates n layers of PbI6 octahedra. In recent years, signiﬁcantadvancements have been made in the ﬁeld, with various typesof DJPs and their corresponding devices being reported.[18,19]
For instance, Niu et al. fabricate solar cells with the DJPs based
on 1,4-butanediamine cations, achieving an enhanced power
conversion eﬃciency compared to the RPP-based ones.[20]
Zheng et al. also achieved a high-sensitivity X-ray imager with
Dion–Jacobson and Ruddlesden–Popper halide perovskites,
which is promising for next-generation low-dimensional X-ray
detectors.[21,22] Yang et al. fabricated 2D/3D halide perovskites
with amidino-based spacer cations. The 2D DJP component
decreased the concentrations of voids and bulk defects of the© 2024 The Author(s). Advanced Functional Materials published by Wiley-VCH GmbH

www.afm-journal.deperovskite ﬁlms, leading to high-performance solar cells witha high power conversion eﬃciency of up to 24.9%.[23]Kanatzidis et al. designed mixed Pb/Sn DJPs (3AMPY)(3AMPY=3-(aminomethyl)(MA)n−1 Pbx Snn−x Br3n+1
pyridinium), which exhibit desirable low bandgaps beneﬁcial to optoelectronic applications.[24] They also found that theposition of amino groups of aromatic diammonium cations canlead to the distortion of the Pb-I octahedra and contribute tobroader white-light emission of the DJPs.[25] Despite these advancements, there is still considerable work to be done to realize
high-performance, long-term stable DJPs and fully industrialize
this promising material. Importantly, a comprehensive understanding of how spacer cations impact the stability and device
performance of diﬀerent DJPs is still lacking. This knowledge
is crucial for the study and development of next-generation
DJPs. Therefore, it is both essential and urgent to investigate theinﬂuence of diﬀerent organic spacers on DJPs.
In this study, we aim to address the development challenges of
halide perovskites demonstrated above, particularly the DJPs. We
systematically investigate the impact of various spacer cations on
DJPs using both theoretical and experimental methods. Our ﬁndings reveal that the polarity of spacer cations signiﬁcantly inﬂuences the stability and photoelectric properties of DJPs. A spacer
cation with a larger polarity component along the main chain direction can eﬀectively enhance the stability and photoelectric device performance of DJPs. Additionally, the chain length of thespacer cation also plays a pivotal role. As the chain length increases, the stability of the DJP structure initially improves, thenweakens, suggesting that an optimal chain length is crucial forachieving satisfactory device performance. These results providea systematic understanding of the inﬂuence of spacer cations on
DJPs, oﬀering valuable insights to researchers for the design andselection of suitable spacer cations and the development of new
types of DJPs.

2. Results and Discussion
For the long-chain diamine-based spacer cation of DJPs, itsfeature can be deﬁned by its chain length and branch chains.
It needs to be pointed out that to simplify the study, we only
focus on the chain-like spacer cations without aromatic ringsin this work. Some aromatic rings can also be incorporated
into some spacer cations, but they can also be seen as branchchains, which is not included in this work.[26,27] For the chain-likespacer cations of the DJPs, their chain length can be determinedby the number of C atoms in the main chain between these
two amines. As shown in Figure 1a, the number of C atoms
in methanediamine (MDA) is 1, in ehylendiamine (EDA) is
2, and in propanediamine (PDA) is 3. Herein, we will ﬁrst
investigate the role of branch chains in DJPs and their inﬂuenceon the stability and optoelectronic properties by ﬁrst-principle
calculations. As for the branch chain, we use PDA as the basemolecule and gradually add branch chain alkyls to it (Figure 1b).
Then, diﬀerent spacer cations can be obtained: N-Methyl1,3-propanediamine (MPDA), 3-Dimethylaminopropylamine(DMPA),
N,N′-Dimethyl-1,3-propanediamine(DMPDA),
N,N,N′-Trimethyl-1,3-propanediamine (TMPDA). From thesederived spacer cations from PDA in Figure 1b, one can ﬁnd that
PDA and DMPDA are perfectly symmetric; for DMPA, there

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Figure 1. Schematics of spacer cations with a) diﬀerent chain lengths andb) diﬀerent branch chains, where the hydrogen atoms are hidden. c) Optimized structures of the DJPs with the spacer cations in b).

are two groups of alkyls on one side of PDA, the asymmetry ofwhich is larger than that of MPDA (one group of alkyl on the
one side of PDA) and TMPDA (one group of alkyl on the one
side and two group of alkyl on the other side of PDA). Therefore,
the degree of symmetry of these spacer cations can be acquired:
PDA, MPDA > DMPDA, TMPDA > DMPA.
After the construction of the spacer cations, 2D DJPs with then value of 1 are constructed using these spacer cations, and thelattice and physical properties are detailly studied and compared.
It needs to be pointed out that to exclude the inﬂuence of then value, 2D DJPs (n = 1, with the molecular formula of LPbI4 ,L represents the spacer cations) are chosen in this work. Another important reason to choose the DJPs with the n value of
1 is that when n>1, the ﬁlm fabricated experimentally is a mixed
phase, which is not conducive to this study.[28] Besides, to construct the DJ phase structure with the spacer cations mentioned
above, we ﬁrst designed and built the original lattice structures.
After that, we performed lattice optimization on the structureswith low energy and atomic convergent forces (Experimental Section). The systems were convergent and maintained the DJ-type
structure after the optimization, ensuring the formation of DJ
phases. Figure 1c depicts the lattice structure of the DJPs after
the lattice optimization, where these DJPs are referred to as theircorresponding spacer cations below, i.e., PDA, MPDA, DMPA,DMPDA, and TMPDA. One can see a slight distortion of the DMPDA lattice and a blatant distortion of the PDA lattice, where one
can also notice that both PDA and DMPDA spacer cations are
symmetric. The distorted lattice can lead to their relatively poor
stability. Accordingly, the stability of these DJPs is studied ﬁrst.

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www.afm-journal.deFigure 2. a) The number of branch C, formation energies and the length of the hydrogen bonds in the DJPs. b) The COHP and the ICOHP values of the
DJPs. c) I 3d XPS spectra for the DMPA and PDA perovskite ﬁlms. d) Charge density diﬀerence maps of the DJPs with diﬀerent branch chains viewing
along the b axis. e) The length of the spacer cations, interlayer spacings, and bandgaps of the DJPs. f) The schematics of the DJPs’ lattice structures withsymmetric and asymmetric spacer cations, respectively. g) Absorption and h) reﬂectivity spectra of DMPA and PDA acquired by DFT calculations.

We use formation energy to characterize their stability. Since the
DJPs are fabricated experimentally using these precursors: thespacer cations, PbI2 and HI, the formation energy can be deﬁned
as E0 = EDJP −ESpacer −EPbI −EHI , where EDJP , ESpacer , EPbI , and EHI
are the internal energies of the DJPs, spacer cations, PbI2 and HI,
respectively. The calculated formation energies of these DJPs areshown in Figure 2a, where the left axis shows the number of the
branch alkyls of the spacer cations. Notably, the formation energy
has no direct relation with the number of branch alkyls. The DJPs
PDA and DMPDA with symmetric spacer cations show larger formation energies, which conﬁrms their poor stability comparedwith the other asymmetric ones. To visually illustrate the symmetry of these spacer cations, the dipole moments of them werealso calculated and presented in Figure S1 (Supporting Information). For the symmetric spacer cations PDA and DMPDA, thedirection of the dipole moments is perpendicular to the main
chain of the molecules. However, for the asymmetric MPDA,
DMPA, and TMPDA, the dipole moments lean to the side withmore branch alkyls. Especially for DMPA, the asymmetry is thelargest as demonstrated above. This observation can also be conﬁrmed by its largest dipole moment of 2.95 D with the direction even parallel to the main chain. Therefore, we can suppose

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2409987 (3 of 8)that the dipole moment component along the main chain direction can strengthen the hydrogen bond between the spacer
cation and the inorganic layers, enhancing the stability of the
DJPs with asymmetric spacer cations, just as Figure 2a shows. Accordingly, the crystal orbital Hamilton population (COHP) analysis between the I and H atoms was carried out, which can provide the strength of the hydrogen bonds directly (Figure 2b).[29]Evidently, all the states of the I–H atoms of these DJPs below
the Fermi level are the binding states, contributing to their stable structures. The integrated COHP (ICOHP) of these samples
is also labeled in the ﬁgure. The binding states of the DJPs PDA(ICOHP = −0.64 eV) and DMPDA (ICOHP = −0.65 eV) withsymmetric spacer cations are obviously less than those of theasymmetric ones (ICOHP = −0.83, −0.90, and −0.94 eV). The
COHP analysis results conﬁrm the stronger hydrogen bonds inthese “asymmetric” DJPs. Additionally, a shorter hydrogen bondgenerally means it is stronger. The average length of the hydrogen bonds in the DJPs is also shown in Figure 2a, corresponding to their formation energy, conﬁrming the hydrogenbonds’ crucial role. To experimentally verify the enhanced hydrogen bonds in the “asymmetric” DJPs, the PDA and DMPA DJP
ﬁlms with respectively symmetric and asymmetric spacer cations© 2024 The Author(s). Advanced Functional Materials published by Wiley-VCH GmbH

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Table 1. Lattice parameter a and Pb–I–Pb angles of the DJPs.
DJP

Lattice
Pb-I-Pb angle 1 Pb-I-Pb angle 2
parameter a [Å]
[deg]
[deg]

Averagevalue [deg]

PDA

11.77

133.51

157.43145.47

MPDA

11.45

155.56174.33164.95

DMPA

11.48

152.97171.87162.42

DMPDA

11.83

134.71

155.17144.94

TMPDA

11.66

147.72169.05158.39were fabricated by spin-coating method, and X-ray photoelectron
spectroscopy (XPS) measurement was conducted to them. As illustrated in Figure 2c, the binding energy of I 3d in DMPA shifts
to a lower value than PDA, which is attributed to the stronger I–H
hydrogen bonds.[30,31] Besides, one can see the line width of the
PDA XPS peaks is much larger than that of DMPA, indicating
the diﬀerent chemical bonding of I atoms in PDA caused by the
structural distortion. To demonstrate how the hydrogen bonds
are strengthened in the DJPs’ lattice, the charge density diﬀerence maps of these DJPs were also simulated and presented in
Figure 2d and Figure S2 (Supporting Information), where the yellow and red regions represent the accumulation and depletion ofelectrons, respectively. The red areas in the “asymmetric” DJPs’
lattices are larger, indicating more electron transfer caused by thepolarity of the asymmetric spacer cations. All these results suggest that a spacer cation with a large polarity component along
the main chain direction can eﬀectively enhance the stability of
the DJPs by both strengthening the hydrogen bonds and reducing the distortion of the lattice.
Figure 2e shows the bandgap, interlayer spacing, and spacer
length (the molecule length of the spacer cation) of the DJPs.
The interlayer spacing is supposed to correlate positively withthe chain length of the spacer cation. However, one can see
in Figure 2e that it does not increase as the spacer length increases: although the chain length of the symmetric PDA andDMPDA spacers are not the largest, their interlayer spacings areeven larger than the asymmetric ones with longer chain length.
This discovery indicates that a spacer cation with high polarity
along the main chain direction can decrease the interlayer spacing, making the DJP crystals more compact. To visually demonstrate this conclusion, the schematics of DJPs’ lattice structureswith symmetric and asymmetric spacer cations are illustrated
in Figure 2f. For the asymmetric ones, the much higher polarity along the main chain direction will strengthen the hydrogen bonds between the spacer cations and the inorganic layers,
and at the same time, draw the inorganic layers close. For thestrictly symmetric ones, because the polarization is perpendicular to the molecule, the polarization will interact more with eachother but not the inorganic layers, leading to larger interlayer
spacings and distorted inorganic layers, which further aﬀects thestability of the DJPs, as demonstrated in Figure 1c. The distortionof the inorganic layer can also be compared directly by the two
Pb–I–Pb angles, as is shown in Figure S3 (Supporting Information), where the ideal Pb–I–Pb angle is 180°.[32,33] As is shown in
Table 1, the lattice parameter a (the lattice parameter perpendicular to the stacked layers) and the two Pb–I–Pb angles of these
DJPs are listed. One can see that the change of the lattice param-

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eter a is consistent with the interlayer spacings: symmetric PDA
and DMPDA are larger than asymmetric MPDA, DMPA, and
TMPDA. Expectedly, the Pb–I–Pb angles of PDA and DMPDA
distorted more extensively than those of MPDA, DMPA, and TMPDA, contributing to the poor stability of PDA and DMPDAbased DJPs. The stability of the symmetric PDA and asymmetric DMPA-based DJPs was also studied using molecular dynamics simulation through ab initio molecular dynamic (AIMD) calculation with VASP (Figure S4a, Supporting Information). The
temperature was set at 100 °C (373 K) in the simulation. For
asymmetric DMPA, the atoms vibrate near their original location.
However, for symmetric PDA, the lattice started to decompose after 10 ps. The energies of these two systems are also delineatedin Figure S4b (Supporting Information). It is expected that the
total energy of DMPA remains stable, but the energy of PDA has
a sharp increase at 10 ps. The AIMD results conﬁrm the superiorstability of the DJPs with asymmetric cations.
Apart from the stability of the halide perovskites, their optoelectronic properties also need our attention, which is signiﬁcantto the performance of the optoelectronic devices. The interlayer
spacing of these DJPs is supposed to be related to the quantum
conﬁnement eﬀect. This way, one can see in Figure 2e that thechanges in the bandgaps of these DJPs are strictly consistent with
the interlayer spacing caused by the quantum conﬁnement effect, indicating the bandgaps are mainly determined by the interlayer spacings.[34,35] The band structures of these DJPs are also
shown in Figure S5 (Supporting Information). The fundamental bandgaps of them all appear at a Г high symmetry point. It isevident that diﬀerent from the DJPs PDA and DMPDA with symmetric spacer cations, there is substantial overlap between Pb andI atomic orbitals in the other three (marked by red circles), which
can reduce the bandgap. The result suggests that the distortion of
the lattice structures of the symmetric spacer cations-based DJPs
also plays a crucial role in enlarging their bandgaps. In addition,
the narrowed interlayer spacing of the DJPs with asymmetricspacer cations will also be supposed to alleviate the obstruction ofelectronic transmission along the layer-stacking direction in 2D
halide perovskite and further enhance the optoelectronic performance. Therefore, the optical properties of these DJPs with symmetric spacers PDA and asymmetric spacers DMPA were studied
by ﬁrst-principle calculations. Figure 2g displays their absorption
spectra. The absorption edge of DMPA has a red shift compared
to PDA due to the smaller bandgap. The reﬂectivity of DMPA
is visibly smaller than that of PDA (Figure 2h), which is favorable in optoelectronic devices to increase eﬃciency. The dielectric
constant and conductivity of the DJPs are also shown in Figure
S6 (Supporting Information). The DJPs with symmetric spacer
cations show larger dielectric constants and conductivities, beneﬁting from the spacer cations’ larger polarities, which can contribute to the transport of the carriers.
To further prove the results acquired by the ﬁrst-principle calculations above, the DJP ﬁlms with symmetric spacers PDA andasymmetric spacers DMPA were also used to study their stability and optoelectronic performance experimentally. As we havementioned, the DJP ﬁlms with n = 1 phase are chosen in thiswork because of their pure-phase nature. The ﬁlms were ﬁrst
characterized by X-ray diﬀraction (XRD) (Figures 3a,d). TheirXRD patterns are consistent with the simulation ones, further
conﬁrming the pure-phase DJP ﬁlms. The scanning electron© 2024 The Author(s). Advanced Functional Materials published by Wiley-VCH GmbH

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www.afm-journal.deFigure 3. Experimental results of the DMPA and PDA ﬁlms and photodetectors. XRD curves and the corresponding simulation results of a) DMPA andd) PDA ﬁlms. The SEM images of b) DMPA and e) PDA ﬁlms (the scale bars are both 50 μm), where the insets are their optical pictures (the scale bars
are both 10 mm). Absorption spectra of c) DMPA and f) PDA ﬁlms after a long time storage. g) TGA curves of the DMPA and PDA ﬁlms. h) I–t curvesof the DMPA and PDA-based photodetectors under the illumination of the chopped laser. Long-time operation of the i) PDA and j) DMPA ﬁlms-based
photodetectors with continuous on–oﬀ switching.

microscope (SEM) image of the DMPA ﬁlm (Figure 3b) suggeststhat its grains are larger than 10 μm, indicating its good crystallinity. The PDA ﬁlm also shows a smooth surface (Figure 3e).
From their optical images in the insets, one can see that the
DMPA ﬁlm is orange and the PDA ﬁlm is yellow, which results from the smaller bandgap of DMPA than PDA. Their thicknesses can also be acquired by the cross-sectional SEM images
in Figure S7 (Supporting Information), where the thickness of
DMPA and PDA ﬁlms is ≈909 and 788 nm, respectively. Next,
the stability of these two DJP ﬁlms was studied. Figures 3c,fshow the absorption spectra of the DJP ﬁlms before and after
storage in a humid environment (the humidity is ≈70%). After 20 days of storage, the absorption spectrum of DMPA ﬁlms
remained unchanged, suggesting its excellent long-term stability. However, for the PDA ﬁlm, the absorption edge blue shiftedfrom 469 to 455 nm, indicating the decomposition of the PDA
ﬁlm remaining the inorganic PbI2 residue. The thermal stability of the ﬁlms was also studied by thermogravimetric analysis(TGA) (Figure 3g). The PDA ﬁlm started to decompose at thetemperature of 90 °C. However, for DMPA ﬁlm, the decomposition temperature was as high as 270 °C, conﬁrming its much

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better thermal stability. In addition, the DJP ﬁlms were also conﬁgured into photodetectors to further investigate their performance and operation stability in devices, which is signiﬁcant for
their future application. Figure 3h shows the on–oﬀ switching
curves of the photodetectors under a 450-nm laser. It is evident
that both the photocurrent and on/oﬀ ratio of the DMPA-based
photodetector is much larger than that of PDA, conﬁrming thebetter optoelectronic properties of DMPA. The long-term on/oﬀ
switching stability of the photodetectors was then studied: thesetwo photodetectors were put into continuous operation with abias voltage of 1.5 V and the illumination of a chopped laser(Figures 3i,j). The wavelength of the laser was 450 nm with aﬁxed light intensity of 100 mW cm−2 . One can see the photocurrent of the PDA-based photodetector gradually decreased during
the operation. On the other hand, for DMPA, the photodetector could keep stable without any decay for more than 40 000 s.
The experimental results conﬁrm the superior stability and optoelectronic performance of the asymmetric spacer cations-based
DJPs.
As demonstrated above, apart from the branch chain, the main
chain length is also a main factor of the spacer cation, as shown in© 2024 The Author(s). Advanced Functional Materials published by Wiley-VCH GmbH

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www.afm-journal.deFigure 4. a) Schematics of spacer cations with diﬀerent chain lengths and branch chains. b) Formation energies of all the DJPs in a). Interlayer spacingsand formation energies of the DJPs constructed by the spacer cations with the nC value of c) two and d) one.

Figure 1a. Hence, the inﬂuence of the main chain length on DJPs
is then investigated in detail. We use nC to represent the numberof C atoms between the two amino groups in the spacer cations.
The nC values of the spacer cations we studied above in Figure 1b
are all three. In these cases, there is a letter “P” in their abbreviations, PDA, MPDA, DMPA, DMPDA, and TMPDA, representing “propyl”, which comes from the nC value three. Accordingly,
when we reduce the nC value to two and one, the letters “E” and“M” can be used to represent ethyl and methyl. As depicted in
Figure 4a, these derived DJPs can be referred to as XDA, MXDA,
DMXA, DMXDA, and TMXDA, where X represents M, E, or P.
Figure 4b shows the formation energies of the DJPs. One can seethat for all these DJPs with diﬀerent branch chains, the formation energy decreases as the chain length increases, suggestingenhanced stability. However, it needs to be pointed out that when
the nC value comes to four, DMXDA perovskite cannot maintaina stable lattice, indicating that a symmetric spacer cation with amuch longer chain is harmful to the stability of DJPs. To thoroughly study the eﬀect of chain length on the spacer cations, theinterlayer spacings and formation energies of the DJPs with nC
values of two and one are also presented (Figures 4c,d). Similar
to the nC value of three discussed above, for the DJPs with thenC value of two, the DJPs with symmetric spacer cations EDA
and DMEDA show the highest formation energies and largest
interlayer spacings, which is attributed to their zero componentof polarity along the main chain direction as we demonstrated
above. However, the DJPs with nC value of one in Figure 4d differ from those with nC values of two and three, where the symmetric MDA and DMMDA no longer show the highest formation energies. It is because these spacer cations with nC = 1 are
too small, and are even able to act as the A site cations in 3D
halide perovskites. These small spacer cations can excessively reduce the interlayer spacings, which will acutely twist the latticeof the DJPs. The length of the hydrogen bonds of nC = 1 and 2
are shown in Figure S8 (Supporting Information). One can see
the length of the hydrogen bonds in the DJPs with nC = 2 havethe same trend with the interlayer spacings and formation ener-

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gies, conﬁrming the crucial role of the polarity of spacer cations.
But for the DJPs with nC = 1, there is an apparent deviation between the length of the hydrogen bonds and the formation energies, indicating the strong hydrogen bonds can no longer help
stabilize the structure of the DJPs. Besides, from the lengths ofthe hydrogen bonds in Figure 2a and Figure S8 (Supporting Information), we can conclude that a spacer cation with a shorter
chain length tends to bring about even stronger hydrogen bonds
between the spacer cations and inorganic layers, which further
proves the strong hydrogen bonds cannot guarantee high stability of the DJPs with nC = 1. The distortion of the inorganic
layers can be visually examined by the length of the six Pb─I
bonds, marked from 1 to 6 in Figure S9 (Supporting Information). The lengths of the Pb─I bonds of all the DJPs with diﬀerent chain lengths are listed in Tables S1–S5 (Supporting Information), where the largest and smallest values are marked red
and pale blue, respectively. It is obvious that the octahedral distortion is reduced as the nC value increases, which conﬁrms thata spacer cation with a chain length that is too short can makethe inorganic layers distorted. These results reveal that a spacer
cation that is too short will distort the inorganic layers in DJPs.
Still, a too-long one will weaken the hydrogen bonds between theorganic spacer and inorganic layers. As a result, to obtain robust
DJP materials, a spacer cation with an optimal chain length is also
important.
Apart from the chain-like spacer cations studied in this work,
there are also aromatic compound-based spacer cations. Similar to the chain-like spacer cations, the polarity and chain lengthof the aromatic compounds-based spacer cations are also supposed to inﬂuence the DJPs following the same roles as thechain-like ones. That aromatic compound can also be seen as
one kind of branch chain of the molecule. Besides, the DJPs withthe aromatic compounds-based spacer cations are supposed tohave higher humidity stability because of their larger molecules.
However, they also suﬀer from larger steric hindrance, which candistort the inorganic layers, leading to poor crystallinity and structure instability.

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www.afm-journal.de3. Conclusion

Supporting Information

In conclusion, to ﬁgure out the inﬂuence of spacer cations’molecular structures on the structural and optoelectronic properties of DJPs, we detailly investigated a series of DJPs with different spacer cations through theoretical and experimental methods. We found that the polarity of the spacer cations plays a crucial role in determining the structure stability and optoelectronic
properties of the DJPs. For the spacer cations with a large polarity component along the main chain direction, the high polarity
can not only strengthen the hydrogen bonds between the organic
spacer layers and inorganic octahedral layers but also create lessdistortion to the octahedral layers and shorten the interlayer spacings. All these promotions contribute to the high stability and superior optoelectronic properties of the DJPs. In addition, an optimal chain length of the spacer cations was found to be crucial toconstructing ideal DJPs. A spacer cation that is too short or too
long will weaken the lattice structures of the DJPs. These valuable ﬁndings will help people further cognize the role of spacer
cations and discover more diverse and remarkable DJPs that assist their development and commercialization.

Supporting Information is available from the Wiley Online Library or fromthe author.

4. Experimental Section
Synthesis of Materials: The DMPAPbI4 and PDAPbI4 DJP precursors
were prepared by dissolving DMPAI2 (PDAI2 ) and PbI2 in dimethylformamide at a molar ratio of 1:1 to create a 1 m solution. The precursors
were stirred in the glovebox for 4 h before use.
Film and Device Fabrication: The glass slides were ultrasonicallycleaned sequentially with acetone, ethanol, and deionized water. They werethen treated with an oxygen plasma to enhance hydrophilicity. Next, 40 μL
of precursor solution was spin-coated onto the slides at 3000 rpm for 30s, then thermal annealing at 100 °C for 10 min. Using shadow masks, 100nm-thick Au electrodes were thermally evaporated onto the DJP ﬁlms tofabricate the photodetectors. The photodetectors have a channel length of
10 μm and a width of 70 μm.
Film and Device Characterization: A Thermo Scientiﬁc ESCALAB 250Xi
system was employed to perform the XPS measurement. The XRD patterns were obtained using a Bruker D2 Phaser with Cu K𝛼 radiation. The
ﬁlm morphologies were characterized with a FEI Quanta 450 FEG SEM.
Absorption spectra were recorded using a Hitachi UH 4150 UV–vis absorption spectrophotometer. The thermal stability of the ﬁlms was assessed by TGA (PerkinElmer Simultaneous Thermal Analyzer 6000). The
photodetection performance of the fabricated photodetectors was evaluated using a standard electrical probe station equipped with an Agilent
4155C semiconductor analyzer. A 405 nm laser served as the light sourcefor photodetector measurements, with incident irradiation power measured by a PM400 power meter from Thorlabs.
Computational Details: The ﬁrst-principles calculations were performed by the Vienna Ab initio Simulation Package (VASP) and the Cambridge Sequential Total Energy Package module in Material Studio software based on density functional theory (DFT).[36,37] The generalizedgradient approximation for exchange–correlation energy proposed by
Perdew, Burke, and Ernzerhof was used.[38] The convergence criteria forthe energy and atomic forces were set to 5 × 10−4 eV and 0.01 eV Å−1 , respectively. The Brillouin zone was sampled with 3 × 3 × 3 k point meshes
for structural optimization and denser 5 × 5× 5 k point meshes for theelectronic and optical properties. The COHP and the ICOHP were calculated by the Local Orbital Basis Suite Towards Electronic-Structure Reconstruction (LOBSTER) program.[39] The DFT calculations for the polarity of the spacer molecules were performed using the Gaussian09 program package.[40] The geometry optimizations of the spacer cations werecarried out at Becke’s three-parameter functional and Lee-Yang-Parr functional (B3LYP) level of theory with the 6–31G (d, p) basis.[41,42]

Adv. Funct. Mater. 2024, 2409987

2409987 (7 of 8)

Acknowledgements
This work was supported by a fellowship award from the Research
Grants Council of the Hong Kong Special Administrative Region, China(CityURFS2021-1S04) and the City University of Hong Kong (project no.
9229138).

Conﬂict of Interest
The authors declare no conﬂict of interest.

Data Availability Statement
The data that support the ﬁndings of this study are available from the corresponding author upon reasonable request.

Keywords
Dion–Jacobson, halide perovskite, optoelectronic, photodetector, spacer
cation, stability
Received: June 8, 2024
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