Materials Today Electronics 8 (2024) 100100

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Materials Today Electronics
journal homepage: www.elsevier.com/locate/mtelec

Aromatic spacer engineering for 2D halide perovskites and their application
in solar cells
Yi Shen a, Siliang Hu a, You Meng a, SenPo Yip b, Johnny C. Ho a, b, c, *a

Department of Materials Science and Engineering, City University of Hong Kong, Kowloon 999077, Hong Kong SAR, PR China
Institute for Materials Chemistry and Engineering, Kyushu University, Fukuoka 816-8580, Japanc
State Key Laboratory of Terahertz and Millimeter Waves, City University of Hong Kong, Kowloon 999077, Hong Kong SAR, PR Chinab

A R T I C L E I N F O

A B S T R A C T

Keywords:
Aromatic
Spacer engineering
2D halide perovskite
solar cell

Perovskites have emerged as a promising new generation of photovoltaic conversion materials, gradually sur­passing traditional silicon-based materials in solar cell research. This development is primarily due to theirsuperior power-conversion efficiency (PCE), simple fabrication process, and cost-effective production. However,the low stability of perovskite ionic crystals poses a significant challenge to their stability, hindering the progressof perovskite materials and devices. Although two-dimensional (2D) perovskites offer improved stability, addingorganic amine ions results in a quantum confinement effect that reduces the optoelectronic performance ofdevices. To counter this issue, the strategic design of suitable spacer cations offers a potential solution. Aromatic
amine ions possess greater polarity and structural adjustability compared to aliphatic amine ions, making themadvantageous in mitigating the quantum confinement effect. This review focuses on phenylethylammonium(PEA) as a representative aromatic spacer cation. It categorizes the evolution of these cations into four trajec­
tories: alkyl chain modification, substitution of hydrogen atoms on the aromatic ring with specific substituents,
replacement of benzene rings with aromatic heterocycles, and utilization of multiple aromatic rings instead of a
monoaromatic ring. The structure, properties, and corresponding device performance of aromatic spacer cations
utilized in reported 2D perovskites are discussed, followed by the presentation of a series of factors for selecting
and designing aromatic amine ions for future development.

1. Introduction

improve their photoelectric conversion efficiency further. As a result,developing a new generation of solar cells has become an urgent priority[2].
Three-dimensional (3D) organic-inorganic halide perovskite solar
cells (PSCs) are widely regarded as the most promising photovoltaic
devices currently being developed [3-6]. Over the past decade, PSCs
have evolved significantly in terms of PCE [7-12]. Several key features of
3D perovskites contribute to their impressive power conversion effi­
ciency. These include efficient carrier transport [13-15], high absorption
coefficient [16-19], high defect tolerance [20], low trap density [8],ambipolar nature [21], and ease of solution processing [22-24]. Perov­
skites offer potentially higher efficiency than traditional silicon-basedmaterials, simpler manufacturing processes, and reduced costs, mak­ing them promising for large-scale commercial applications. However,
due to the low decomposition energy of the perovskite material itself,
the ions are easy to diffuse and decompose into various sub-phases [25,26]. When moisture infiltrates the grain boundaries of the perovskite

Global climate change is one of the key issues threatening life onearth, which has increasingly attracted the attention of scientists andpoliticians [1]. Fossil fuels, namely coal, oil, and natural gas, have been
identified as the primary contributors to global climate change. Suchfuels account for more than 75 % of the total global emissions ofgreenhouse gases and nearly 90 % of all carbon dioxide emissions. The
greenhouse gases emitted by these fuels envelop the earth, trapping the
sun’s heat and resulting in global warming and climate change. As theenergy crisis and environmental damage endanger our planet, devel­
oping and utilizing renewable energy sources have become increasingly
important. Solar energy, recognized as a highly sustainable source ofclean energy, has witnessed a rapid acceleration in its development andutilization. At present, the solar cells available on the market are pre­dominantly silicon-based. However, those silicon cells are characterizedby significant defects and impurities, and it has proven difficult to* Corresponding author.
E-mail address: johnnyho@cityu.edu.hk (J.C. Ho).

https://doi.org/10.1016/j.mtelec.2024.100100
Received 29 December 2023; Received in revised form 31 March 2024; Accepted 17 April 2024
Available online 30 April 2024
2772-9494/© 2024 The Author(s). Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/bync-nd/4.0/).

Y. Shen et al.

Materials Today Electronics 8 (2024) 100100

thin film, it can lead to the degradation of the perovskite structure [27,28]. This low stability seriously hinders the future commercialization of
perovskites. To overcome this issue, various strategies such as interface
engineering [29-32], doping [33-35], and encapsulation [36-39] havebeen attempted. While these methods have successfully modified theproperties of perovskite materials, they cannot fundamentally alter its
inherent structure. As a result, the potential for enhancing stability
remained limited. Fortunately, recent studies have shown that intro­
ducing large-volume hydrophobic organic amine ions can significantly
enhance the stability of 3D perovskites by cleaving their structure along
crystallographic planes forming 2D sheet structures as shown in Fig. 1a[40-43]. Notably, when the introduction of organic cation improves the
moisture stability of perovskite, the heat stability, light stability andphase stability can also get a significant enhancement. Besides, unlike
traditional 3D perovskites, 2D perovskites possess structural diversity.
This diversity results because various organic synthesis methods can beutilized to obtain and study a large variety of organic ammonium ions[44-48]. Consequently, the choice of organic ammonium ions lends ahigh degree of flexibility in tuning the photoelectric characteristics of 2D
perovskites. Additionally, manipulating the number of inorganic layers
can result in different optoelectronic properties.
The most common structure of 3D perovskites can be represented by
2+
2+
the general formula AMX3[A = Cs+, CH3NH+
3 (MA); M = Pb , Ge ,2+−−−
Sn ; X = Cl , Br , I ]. In terms of 2D perovskites, we mainly have the
Ruddlesden-Popper (RP) phase, Dion-Jacobson (DJ) phase andAlternating-Cation-Interlayer (ACI) phase (Fig. 1b). The ACI phase is lessstudied and can only be constructed using guanidinium. Therefore, wewill focus on the RP and DJ phases here. The RP phase has the formulaR2An-1MnX3n+1, while the DJ phase is represented by R’An-1MnX3n+1 (ndenotes the number of layers of continuous inorganic octahedra be­
tween adjacent organic layers). Here, R refers to a bulky organic mon­oammonium spacer cation, while R’ represents a bulky organic
diammonium spacer cation. The ideal structure of 2D perovskites ischaracterized by a systematic and regular alternating of layers con­taining organic ammonium ions and octahedral layers with a specificnumber of continuous octahedral layers. In general, obtaining the phasewith n = 1, which signifies pure 2D perovskites, is relatively straight­
forward. Nevertheless, preparing a pure phase of quasi-2D perovskites
with high n values (ranging from 2 to ∞) is challenging since different nphases have similar crystallization energy. As a result, quasi-2D perov­
skites often contain multiple n phases, and the nominal n values
generally represent the stoichiometry of different compositions in the
precursor solutions. In 2D RP perovskites, the organic and inorganic
layers are interconnected through hydrogen bonding, while van der
Waals forces bind the two ammonium ion layers within the organic
layer. Due to the tilt of the Pb - I octahedra, there is a fixed displacement
between the inorganic layers in 2D RP perovskites. On the other hand, in
2D DJ perovskites, each diammonium ion can form hydrogen bonds
with adjacent octahedral layers on both sides, and no van der Waals
effect exists. The linearity of diamine cations in 2D DJ perovskites re­sults in no lateral displacement between neighboring inorganic layers.
Although the stability of 2D perovskites has a great improvement,
the PCE still cannot be comparable to 3D perovskite. This is due to theappearance of Quantum Wells (QWs) which is caused by the quantum
and dielectric confinement effects, leading to the poor charge transportability and diffusion length [49,50]. As Fig. 1c shown, the organic layers

Fig. 1. (a) Structure schematic of 3D and 2D perovskites with n values of 1, 2, and 3, respectively. (b) Schematic of Ruddlesden-Popper (RP), Dion-Jacobson (DJ) andAlternating-Cation-Interlayer (ACI) perovskite structures. (c) Quantum Wall schematic of the 2D perovskite where organic layers act as barriers and inorganic layers
act as walls. (εw represents the dielectric constant of inorganic layers; εb represents the dielectric constant of organic layers; LUMO: lowest unoccupied molecularorbital; HOMO: highest occupied molecular orbital; CB: conduction band; VB: valence band).
2

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Materials Today Electronics 8 (2024) 100100

act as the potential “barriers” and inorganic layers serve as the potential“wells”. To be specific, the quantum confinement effect can result in thesignificant promotion of exciton binding energy (Eb) and the mismatch
between dielectric constants of organic layers and inorganic layers also
leads to the increase of Eb. This high Eb causes that photo-generated
carriers primarily exist as excitons rather than free electrons and holes
and to be confined in the QWs, which finally inducing the energy losses.
Also, the transport of carriers is hindered by these multiple QWs andmainly occurs through the inorganic sheets. Therefore, improving car­rier transport capability between the inorganic and organic layers is
crucial for enhancing the electrical performance of 2D perovskite ma­
terials. This challenge can potentially be addressed by the reasonable
design of new organic ammonium ions. Hence, a comprehensive review
and summary of all previously studied ammonium ions is paramount.
This knowledge from such a review could pave the way for innovativeideas in future designs.
In both RP and DJ types of 2D perovskites, the spacer ammonium
ions can be categorized as aliphatic or aromatic ammonium ions. Some
examples of aliphatic ammonium ions for RP perovskites include pro­pylammonium (PA) [51], butylammonium (BA) [52,53], iso­butylammonium (IBA) [54-56], while examples of aromatic ammonium
ions for 2D RP perovskites include phenylethylammonium (PEA) [57]and 2-thio-phenemethylammonium (TMA) [58]. For 2D DJ perovskites,
examples of spacer ammonium ions are propyldiammonium (PDA) [59],butyldiammonium (BDA) [60], and 1,4-phenylenedimethanammonium(PDMA) [61], among others. In recent studies on the development of
spacer cations, aromatic spacers have garnered significant attention. In
Fig. 2, we show the representative aromatic spacers of RP and DJ pe­
rovskites. The complete PCE and stability summaries of common RP and
DJ perovskite solar cells are shown in Table 1. It is important to note thatcompared to aliphatic spacers, aromatic spacers primarily possess thefollowing advantages: (a) Aromatic spacers generally exhibit a larger
dielectric constant due to their high dipole moment. This high dielectric
constant is favorable for reducing the quantum confinement effect be­
tween organic and inorganic layers, enhancing interlayer carrier trans­
fer capability. (b) Aromatic spacers have demonstrated the ability to
induce the perpendicular growth of the perovskite lattice, facilitating
efficient electron and hole transfer between the substrate and film sur­face. (c) The inherent structural rigidity and bulk volume of aromatic
amine ions, in contrast to aliphatic chain amine ions, effectively limit
slippage between the organic layers of 2D perovskites, thus enhancing
overall stability. (d) Aromatic molecules offer diverse structural possi­
bilities and can be finely tuned through various chemical synthesis
methods. This includes heteroatom substitution of alkyl chains orhydrogen on the aromatic ring and the utilization of aromatic hetero­cycles. These structural adjustments can lead to tailored properties of 2D
perovskites, enabling researchers to obtain perovskite materials withspecific characteristics for various applications. Considering theimmense potential for further exploration, it becomes paramount toretrospectively evaluate the reported species and characteristics of ar­omatic amine ions and their impact on the performance of 2D perov­
skites. This retrospective analysis will provide invaluable guidance forthe future development of aromatic spacer cations.
This comprehensive review provides a meticulous overview of
research conducted on aromatic spacer cations in 2D perovskites. Our
primary focus emphasizes the intrinsic properties of these aromatic
spacers, including volume, hydrophobicity, dipole moment, and so on,
as well as their influence on the performance of 2D perovskites. These
properties influence various aspects, such as the distribution patterns ofdifferent n-phases, grain growth direction, distortion degree of inorganic
PbI6 octahedra, and stacking mode of organic amine ions. Additionally,
we explore the implications of these factors on device efficiency andstability. Eventually, we underscore a set of factors for selecting anddesigning aromatic amine ions while speculating on potential avenues
for future research.

2. Aromatic monoamine spacer cations of 2D RP perovskites
2D RP perovskites have attracted considerable research interest in
recent years, with a significant growth in the diversity of aromaticmonoamine cations applied in 2D RP perovskites. Structurally, in 2D RP
perovskites, the amino end of the monoamine cation is tethered to the
inorganic octahedral layer via hydrogen bonds. In contrast, the opposing
end is linked to amine ions through van der Waals interactions. Notably,various aromatic monoamine ions can be viewed as derivatives of PEA
which was firstly reported in 2014 [41], following four distinct path­ways: modification of alkyl chains, replacement of hydrogen atoms on
the aromatic ring with specific substituents, substitution of benzene
rings with aromatic heterocycles, and replacement of a monoaromaticring with polyaromatic rings. This summary will summarize thedifferent types of aromatic monoamine ions and their applications in 2D
RP perovskites, considering these four aspects.
2.1. Modification of alkyl chains
The modification of alkyl chains in aromatic spacer amine ions has asignificant impact on the performance of 2D RP perovskites. However,
the related research is limited. The primary modifications of alkyl chains
include adjusting the length of the alkyl chain, incorporating conjugateddouble bonds, and introducing heteroatoms.
Kamminga et al. studied 2D RP perovskite single crystals using
phenylalkylammonium cations [57]. These cations had varying
numbers of carbon atoms between the phenyl ring and amino group,
namely PMA (1), PEA (2), PPA (3), and PBA (4) (Fig. 3a-c). The researchfindings revealed an interesting trend in the photoluminescence (PL)spectrum as the carbon chain length increased from 1 to 4 (Fig. 3c).
Specifically, there was a noticeable blue-shift in the PL peak. Upon
analyzing the single crystal structures, it was observed that compounds
corresponding to PMA and PEA, which had shorter ligand lengths,
exhibited an inorganic structure composed of corner-sharing PbI6
octahedra (Fig. 3a, b). As the alkyl chain length increased, the Pb-I-Pb
bond angle also increased, indicating a larger distortion in the PbI6
octahedra sheet. On the other hand, the two compounds with longer
alkyl chains, PPA and PBA, showed a different connection mode withboth corner- and face-sharing PbI6 octahedra sheets (Fig. 3a, b). The
electronic structures were calculated using density functional theory(DFT), and the results indicated that the increase in spacer cation length
led to systematic changes in the energy band structure. This observation
was consistent with the structural changes from 3D to 2D and 1D with
face-sharing PbI6-octahedra.
In 2020, Wu et al. achieved a significant breakthrough by incorpo­
rating a unique aromatic amine molecule with a double bond in the alkyl
chain (3-phenyl-2-propen ammonium or PPeA (6)) as a spacer in 2D RP
perovskites [92]. This development led to the successful synthesis of asingle crystal structure denoted as (PPeA)2(FA0.5MA0.5)n− 1PbnI3n+1(where n = 2, 3, 4) (Fig. 3d). Through analysis of the single crystal
structure, it was observed that the presence of sp2 hybridization in the-CH––CH- group allowed for the formation of multiple hydrogen bonds
between the halide ions of the PbI6 octahedra and the spacer cations,facilitating deeper penetration of organic spacers. Therefore, theperovskite structure with alternating organic layers and inorganic layers
was effectively stabilized. Moreover, the network of hydrogen bonds
between PPeA and the PbI6 octahedra played a crucial role in linking
adjacent inorganic layers, thus inhibiting in-plane crystal growth of the
2D RP perovskite films. As a result of incorporating the PPeA spacer into
the 2D RP perovskite structure, an outstanding PCE of 14.76 % wasachieved for the composition with n = 4. Furthermore, these perovskite
devices exhibited excellent stability under high temperature and hu­
midity conditions, highlighting their potential for practical applications.
Derived from primary amines, the secondary, tertiary and quater­nary amines were also studied in perovskites [65,93-95]. Jesus et al.
figured out that with the increase of the number of substitution located3

Y. Shen et al.

Materials Today Electronics 8 (2024) 100100(caption on next page)4

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Materials Today Electronics 8 (2024) 100100

Fig. 2. Aromatic monoamine and diamine cations. Aromatic monoamine cations: 1, Benzylammonium (PMA) 2, Phenylethylammonium (PEA) 3, 3-phenyl-1propylammonium (PPA) 4, 4-Phenyl-1-butylammonium (PBA) 5, 2-Fluoro-2-phenylethan-1-aminium (β-FPEA) 6, 3-Phenyl-2-propenammonium (PPeA) 7, Phenyl­
trimethylammonium (PTA) 8, Benzamidine (PhFA) 9, 2-Thiophenemethylammonium (TMA) 10, 2-Thiopheneformamidinium (ThFA) 11, 2-Thiophene ethyl­ammonium (TEA) 12, 2-Furfurylammonium (FuMA) 13, 2-Selenophenemethylammonium (SeMA) 14, 4-(Aminoethyl)pyridine (4-AEP) 15, 4Pyridinylmethylammonium (PyA) 16, 4-Fluoro-phenethylammonium (4F-PEA/pF1PEA) 17, 3-Fluoro-phenethylammonium (mF1PEA) 18, 2-Fluoro-phenethylammo­nium (oF1PEA) 19, 4-Methoxyphenethylammonium (MeO-PEA) 20, 4-Chloro-phenylethylammonium(4Cl-PEA) 21, 4-Bromo-phenylethylammonium (4Br-PEA) 22,
4-Fluoro-benzylammonium (pFBA) 23, 3-Fluoro-benzylammonium (mFBA) 24, 2-Fluoro-benzylammonium (oFBA) 25, 3-bromobenzylammonium (3BBA) 26, 4-Flu­
oro-benzamidine (p-FPhFA) 27, 2,3,4,5,6-pentaFluoro-phenethylammonium (F5-PEA) 28, 1-Naphthalenemethylammonium (NpMA) 29, 9-Anthracenemethylammo­nium (AnMA) 30, 1-Naphthylamine (1-NA) 31, amino(thieno[3,2-b]thiophen-2-yl)methaniminium (TTFA) 32, [2,2′-bithiophen]− 5-yl(amino)methaniminium(BTFA) 33, Bithiophenemethylammonium (BThMA) 34, Biphenemethylammonium (BPhMA) 35, (9H-carbazol9-yl)butyl‑1-ammonium (CA) Aromatic diamine
cations: 36, 1,4-Phenylenedimethanam-monium (PDMA) 37, 2,3,5,6-Tetrafluoro-1,4-benzenedimethanammonium (TFBDA) 38, 1,5-diaminonaphthalene (NDA) 39,
2,5-thiophenedimethylammonium (ThDMA) 40, Thieno[3,2-b]thiophene-2,5-diyldimethanaminium (TTDMA) 41, Bithiophene dimethylammonium (BThDMA) 42,
3-(aminomethyl)pyridinium (3AMPY) 43, 4-(aminomethyl)pyridinium (4AMPY).

on the amino group, the effective interaction between organic spacers
and octahedral sheets may be weakened owing to the reduction ofhydrogen bonds and increase of steric hindrance. In Chen’s work, theysuccessfully used phenyl-based primary (PA), secondary (NPA), tertiary(DPA) and quaternary ammonium (PDA)(7) to prepare 2D perovskites
with n = 4, which show PCEs of 3.9 %, 7.8 %, 4.3 % and 11.53 % in solar
cell devices, respectively. The higher PCE of PDA-based PSC wasascribed to its high-quality film, low defects and suitable energy levels.
Fluorination is a common strategy to augment the performance of 2D
perovskite devices. Still, most related research has primarily focused on
substituting fluorine into the benzene ring of aromatic amines. Due tothe complexity and difficulty of the synthetic route, only limited
research has been conducted on fluorine substitution on the alkyl chainof aromatic amines. Recently, Zhang et al. synthesized a new aromatic
spacer cation called β-FPEA (5), which introduced a fluorine substituent
on the first carbon atom of the alkyl chain in PEA [78]. DFT calculations
revealed that β-FPEA possessed a dipole moment of 1.71 Debye, greaterthan that of PEA (1.26 Debye) (Fig. 3e). The higher polarity was bene­
ficial to facilitate exciton dissociation and charge transport. Moreover,the presence of fluorine atoms of β-FPEA resulted in positive charge
accumulation on NH+
3 due to its strong electron-pulling capability. This
fact leads to enhanced interaction between β-FPEA spacers and inor­
ganic octahedral layers. By combining β-FPEA cations with FA cations,theresearchers
prepared2D
perovskite
filmsof(β-FPEA)2(FA)n− 1PbnI3n+1 and (PEA)2(FA)n− 1PbnI3n+1 (n = 5) via a
simple spin-coating process. X-ray photoelectron spectroscopy (XPS)
measurements indicated a shift of approximately 0.1 eV towards lower
binding energy in the Pb and I peaks of β-FPEA-based 2D perovskite
films compared to PEA-based films, reflecting a change in the chemical
environment of Pb and I atoms (Fig. 3f). Scanning electron microscopy(SEM) revealed that the crystals in β-FPEA-based 2D perovskite films
exhibited a sheet-like shape with a vertical growth direction relative tothe substrate. In contrast, PEA-based 2D perovskite films displayedneedle-shaped crystals parallel to the substrate, accompanied by larger
grain boundaries. X-ray diffraction (XRD) and grazing-incidence
wide-angle X-ray scattering (GIWAXS) measurements demonstrated
that PEA-based 2D perovskite films possessed (0k0) diffraction peaks
representative of a lower n phase, whereas the (111) diffraction peaks of
β-FPEA-based 2D perovskite films were notably stronger (Fig. 3g). These
differences indicated that introducing a fluorine atom on the alkyl chain
improves the crystallinity of 2D perovskites and promotes crystal growth
perpendicular to the substrate. Time-resolved PL measurements showedthat after 10 min, the PL peak of PEA-based 2D perovskite films shifted
towards shorter wavelengths, accompanied by the appearance of newemission peaks associated with the low n phase. However, β-FPEA-based
2D perovskite films exhibited nearly unchanged spectra in PL mapping.
Time-resolved XRD tests yielded consistent results, further indicating
the improved stability of β-FPEA-based 2D perovskite films. Finally,
performance studies of PSCs based on PEA and β-FPEA revealed that
introducing fluorine on the alkyl chain of PEA led to an enhanced PCEfrom 12.81 % to 16.77 %. Additionally, when devices were prepared on
the PTAA substrate, a higher PCE of 19.11 % was achieved. This

improvement in device performance can be attributed to the increased
carrier diffusion length and lifetime and decreased trap density in
β-FPEA-based 2D perovskites. This work pioneered a new way of pro­moting the performance and stability of 2D RP perovskite devices byintroducing substituents to modify the alkyl chain of aromatic amines.
2.2. Aromatic heterocyclic ringAromatic heterocyclic ring spacers have great application potential
in 2D RP perovskites. Specially, the thiophene ring has garnered sig­
nificant attention due to its promising properties in various fields, suchas organic chemistry, organic semiconductor materials, electrontransporting materials, and hole-transporting materials. Incorporating
thiophene rings and their derivatives in 2D RP perovskites is believed toregulate the energy band arrangement [58,64,70,96-101]. Furthermore,
the presence of S atoms strengthens the interaction between inorganicand spacer cations layers, thereby contributing to stabilizing the 2D
perovskite lattices [64]. Additionally, researchers have extensively
explored other heteroaromatic rings such as furan [74,102], seleno­
phene [72], and pyridine [66,68,83,103-106] for their potential to be
introduced into the spacer cations layers of 2D RP perovskites.
In 2018, Chen et al. successfully incorporated the thiophene ring into
2D RP perovskites [58]. They introduced a thiophene-based spacer
cation known as 2-thiophenemethylammonium (ThMA)(9) and syn­thesized (ThMA)2(MA)n− 1PbnI3n+1 (n = 3) perovskite films. Adding
MACl additives aided in achieving a nanorod-like structure for theperovskite films (Fig. 4a). By adjusting the weight ratio of MACl/MAI
between 0 and 1, the researchers discovered that a ratio of 0.5 yieldedperovskite films with superior crystallinity and an ordered structure, as
depicted in Fig. 4a. Furthermore, at this optimal weight ratio, theperovskite films exhibited a preferential growth direction perpendicular
to the substrate, which was advantageous for charge transport. The
fabricated PSCs device utilizing (ThMA)2(MA)n− 1PbnI3n+1 (n = 3)
perovskite films demonstrated an impressive PCE of 15.42 %.
Formamidinium (FA) is a crucial cation that can substitute the MA
cation in the 3D perovskite lattice. Liu and colleagues incorporated theThMA spacer cation into the FAPbI3 perovskite system, resulting in theformation of n = 3 perovskite films, namely (ThMA)2(FA)2Pb3I10 [96].
Compared to MA-based (ThMA)2(MA)2Pb3I10 PSCs, the introduction of
FA cation in the devices increased PCE from 15.42 % to 16.18 %. This
improvement can be attributed to the FA cation’s ability to narrow theband gaps. A crystal growth technique assisted by the use of 4-(tri­fluoromethyl) benzyl ammonium iodide was employed in this study toenhance perovskite crystallinity, minimize trap density, and reduce
nonradiative recombination losses. The high-resolution transmission
electron microscopy (HRTEM) and GIWAXS characterization revealed
the presence of low n values, high n values, and 3D phase in the 2D
perovskite films. As a result, the final PCE of the optimized devices
displayed a significant enhancement, reaching up to 19.06 %.
2-Thiophene ethylamine (TEA) (11) is an organic spacer cation witha longer alkyl chain than TMA. A study by Liang et al. utilized TEA as aspacer cation in synthesizing 2D RP perovskite films, with PEA as a5

Materials Today Electronics 8 (2024) 100100

Y. Shen et al.

Table 1The summaries of representative PCE and stability for common RP and DJ perovskite solar cells.
Device structure

2D Perovskite

PCE
(%)

Stability

Ref

FTO/c-TiO2/perovskite/spiroOMeTAD/Au
ITO/PTAA/perovskite/PCBM/PEI/Ag(PEA)2(MA)2Pb3I10(PEA)2MA4Pb5I16

4.73
11.10

[41]
[62]

ITO/PEDOT:PSS//perovskite/PCBM/BCP/Ag
ITO/PEDOT:PSS/perovskite/PCBM/Bphen/Al
ITO/SnO2/perovskite/spiroOMeTAD/Ag
FTO/C60/perovskite/spiroOMeTAD/Au
ITO/PEDOT:PSS/perovskite/PCBM/BCP/Ag(PEA)2(MA)4Pb5I16(TEA)2(MA)4Pb5I16(PTA)2(MA)3Pb4I13(4-AEP)2MA4Pb5I16(PhFA)2MA4Pb5I16− xClx

14.10
7.20
11.53
11.68
12.92

ITO/PEDOT:PSS/perovskite/spiroOMeTAD/Au
ITO/PTAA/perovskite/C60/BCP/Cu
ITO/PEDOT:PSS/perovskite/PCBM/BCP/Ag
ITO/PEDOT:PSS/perovskite/PCBM/BCP/Ag
ITO/PTAA/perovskite/PCBM/BCP/Ag(BA0.9PyA0.1)2MA3Pb4I13(PPeA)2(FA0.5MA0.5)3Pb4I13(ThMA)2(MA)2Pb3I10(ThFA)(Cs)4Pb5I16(pFBA)2(MA)4Pb5I16

13.01
14.76
15.42
16.00
17.12

ITO/PEDOT:PSS/perovskite/PDTL/PCBM/BCP/Ag(SeMA)2(MA)4Pb5I16

17.25

ITO/PEDOT:PSS/perovskite/PCBM/BCP/Ag(NpMA)2(MA)3Pb4I13

17.25

ITO/PEDOT:PSS/perovskite/PCBM/BCP/Ag(p-FPhFA)2(MA)4Pb5I16

17.37

ITO/PEDOT:PSS/perovskite/PCBM/BCP/Ag(FuMA)2MA3Pb4I19Cl4

18.00

ITO/PTAA/perovskite/PCBM/Cr/Au
ITO/TiO2/perovskite/Spiro-OMeTAD/Au
ITO/PEDOT:PSS/Perovskite/PCBM/BCP/Ag
ITO/PTAA/perovskite/PCBM/BCP/Ag

3BBA-based perovskite(CA)2(MA)4Pb5I16(BThMA)2(MA)4Pb5I16(β-PEA)2(MA)4Pb5I16

18.2
18.23
18.85
19.11

ITO/PEDOT:PSS/perovskite/PCBM/BCP/ Ag(BTFA)2(MA)4Pb5I16

15.43

ITO/PEDOT:PSS/perovskite/PCBM/BCP/ Ag(TTFA)2(MA)4Pb5I16

19.41

FTO/TiO2/perovskite/spiroOMeTAD/Au
ITO/PTAA/perovskite/PCBM/PEI/Ag(4F-PEA)2(MA)4Pb5I16(4F-PEA)2MA4Pb5I16

13.64
17.34

ITO/PTAA/perovskite/PCBM/PEI/Ag(4F-PEA)2(MA)3Pb4I13

18.10

ITO/PTAA/perovskite/PCBM/BCP/Ag(4F-PEA)2(FA)4Pb5I16

21.07

FTO/cp-TiO2/mp-TiO2/perovskite/spiroOMeTAD/Au
FTO/PEDOT:PSS/perovskite/C60/BCP/Ag
ITO/PEDOT:PSS/perovskite/PCBM/Bphen/Ag
FTO/c-TiO2/perovskite/Spiro-OMeTAD/Au
FTO/TiO2/SnO2/perovskite/Spiro-OMeTAD/Au(PDMA)(FA)2Pb3I10

7.00

NA
70 % retained after 500 h (Air, 60 % RH, without encapsulation)
65 % retained after 500 h (N2, 55 ◦ C, without encapsulation)
90 % retained after 1080 h (Air, 30 % RH, with encapsulation)
80 % retained after 270 h (Air, 60 % RH, 25 ◦ C, without encapsulation)
90 % retained after 1600 h (N2, RT, without encapsulation)
95 % retained after 1000 h (Air, 30 % RH, RT, without encapsulation)
68 % retained after 3000 h (N2, without encapsulation)
77 % retained after 450 h (N2, continuous illumination, without
encapsulation)
98 % retained after 1200 h (N2, without encapsulation)
94 % retained after 600 h (Air, 85 % RH, 85 ◦ C, with encapsulation)
90 % retained after 1000 h (N2, without encapsulation)
92 % retained after 960 h (N2, without encapsulation)
100 % retained after 2000 h (Ar, without encapsulation)
83 % retained after 600 h (Air, 60 % RH, 25 ◦ C, without encapsulation)
100 % retained after 1008 h (Air, 30 % RH, without encapsulation)
86 % retained after 1008 h (N2, 60 ◦ C, without encapsulation)
97 % retained after 1008 h (N2, continuous illumination, without
encapsulation)
91 % retained after 4100 h (N2, RT, without encapsulation)
89 % retained after 1000 h (Air, 30 ± 5 % RH, without encapsulation)
80 % retained after 600 h (N2, 80 ◦ C, without encapsulation)
79 % retained after 600 h (N2, continuous illumination, without
encapsulation)
99 % retained after 3000 h (N2, without encapsulation)
82 % retained after 450 h (N2, continuous illumination, without
encapsulation)
94 % retained after 1850 h (N2, without encapsulation)
83 % retained after 768 h (N2, 85 ◦ C, without encapsulation)
80 % retained after 768 h (N2, continuous illumination, without
encapsulation)
82 % retained after 2400 h (Air, 40 % RH, RT, without encapsulation)
85 % retained after 2000 h (Air, 65 % RH, 25 ◦ C, without encapsulation)
89 % retained after 670 h (Air, 45 % RH, without encapsulation)
90 % retained after 720 h (70 ◦ C, without encapsulation)
86 % retained after 780 h (Air, 35 % RH, without encapsulation)
78 % retained after 777 h (N2, continuous illumination, without
encapsulation)
90 % retained after 777 h (N2, continuous illumination, without
encapsulation)
65 % retained after 576 h (Air, 75 ◦ C, without encapsulation)
93 % retained after 500 h (Air, 60 % RH, without encapsulation)
94 % retained after 500 h (N2, 55 ◦ C, without encapsulation)
90 % retained after 720 h (Air, 45 % RH, without encapsulation)
80 % retained after 720 h (N2, 80 ◦ C, without encapsulation)
97 % retained after 1500 h (N2, 80 ◦ C, without encapsulation)
90 % retained after 1200 h (Air, 40 % RH, 25 ◦ C, without encapsulation)
85 % retained after 60 days (Air, 30− 50 % RH, without encapsulation)(3AMPY)(MA)3Pb4I13(PDMA)MA5Pb6I19(PDMA)(MA)3Pb4I13(NDA)(MA)3Pb4I13

9.20
11.00
15.81
15.08

ITO/SnO2/perovskite/Spiro-OMeTAD/Au(TFBDA)(MA)9Pb10I31

15.24

ITO/PEDOT:PSS/perovskite/PCBM/BCP/Ag
ITO/PEDOT:PSS/perovskite/PCBM/BCP/Ag
ITO/PEDOT:PSS/Perovskite/PCBM/BCP/Ag(TFBDA)(MA)3Pb4I13(ThDMA)(MA)4Pb5I16(BThDMA)(MA)4Pb5I16

16.62
15.75
18.10

ITO/PEDOT:PSS/perovskite/PCBM/BCP/Ag(TTDMA)(MA)3Pb4I13

18.82

comparative reference [64]. XPS measurements revealed a noticeable
trend of decreasing binding energy from MAPbI3 to PEA and TEA, as
shown in Fig. 4b. This trend, represented by the change in the charac­
teristic peaks of Pb, confirmed the distinct interaction between S atoms
and Pb atoms. This interaction induced electronic coupling between the

NA
NA
NA
75 % retained after 1000 h (Air, 60 % RH, without encapsulation)
70 % retained after 200 h (85 ◦ C, without encapsulation)
93 % retained after 1300 h (Air, 40–70 % RH, without encapsulation)
80 % retained after 100 h (80 ◦ C, without encapsulation)
93 % retained after 1839 h (N2, RT, without encapsulation)
95 % retained after 1655 h (N2, RT, without encapsulation)
100 % retained after 3100 h (N2, without encapsulation)
100 % retained after 500 h (Air, 30–50 % RH, without encapsulation)
82 % retained after 300 h (N2, 85 ◦ C, without encapsulation)
99 % retained after 4400 h (N2, without encapsulation)
94 % retained after 740 h (N2, continuous illumination, without
encapsulation)
94 % retained after 740 h (N2, 80 ◦ C, without encapsulation)
98 % retained after 960 h (Air, 30 % RH, RT, without encapsulation)

[63]
[64]
[65]
[66]
[67]
[68]
[69]
[58]
[70]
[71]
[72]

[73]

[67]
[74]

[75]
[76]
[77]
[78]
[79]
[79]
[80]
[62]
[81]
[82]
[61]
[83]
[84]
[85]
[86]
[87]
[88]
[89]
[90]
[91]

organic and inorganic layers, stabilizing the lattice structure of inor­
ganic octahedral layers and enhancing exciton dissociation and inter­
phase charge transfer. Moreover, this interaction facilitated thenucleation of the 3D phase at the 2D phase grain boundaries, promoting
the formation of 2D/3D heterojunctions (Fig. 4c). The presence of these6

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Materials Today Electronics 8 (2024) 100100

Fig. 3. (a) Single crystal structure of (PMA)2PbI4, (PPA)3Pb2I7, and (PBA)3Pb2I7 perovskites at 100 K, projected along the [100] direction. (b) Single crystal structureof (PMA)2PbI4, (PEA)2PbI4, (PPA)3Pb2I7, and (PBA)3Pb2I7, projected along the [001] direction. The organic molecules are not shown for clarity. (c) Selected PL
spectra for Compounds 1 − 4, which are referred to as (PMA)2PbI4, (PEA)2PbI4, (PPA)3Pb2I7, and (PBA)3Pb2I7, respectively. (d) Chemical structure of PPeA andstability measurement of PPeA-based PSCs. (e) Calculated ESP maps for β-FPEA and PEA spacers. (f) XPS spectra of the Pb 4f orbitals of PEA- and β-FPEA based 2D RP
perovskite films. (g) GIWAXS results of PEA-based and β-FPEA-based 2D RP perovskite films. (h) AFM images of PEA-based and β-FPEA-based 2D RP perovskite films.
(a-c) Reproduced with permission from Ref. [57]. Copyright 2016, American Chemical Society. (d) Reproduced with permission from Ref. [92]. Copyright 2020,
American Chemical Society. (e-h) Reproduced with permission from Ref. [78]. Copyright 2023, Wiley-VCH.

7

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Materials Today Electronics 8 (2024) 100100

Fig. 4. (a) SEM images of ThMA-based perovskite films with different MACl/MAI weight ratios from 0 to 1 before thermal annealing (top) and after thermal
annealing (bottom). (b) XPS spectra of Pb 4f for MAPbI3 perovskite, PEA-based, and TEA-based 2D perovskite thin films, respectively. (c) Schematic illustration of
2D/3D mixed phases in TEA-based 2D RP perovskites. (d) STEM images and corresponding EDS elemental mapping of TEA-based 2D RP perovskites. (e) TEM andmagnified HR-TEM image for TEA-based 2D RP perovskites. (a) Reproduced with permission from Ref. [58]. Copyright 2018, American Chemical Society. (b-e)
Reproduced with permission from Ref. [64]. Copyright 2019, Wiley-VCH.

heterojunctions was validated by techniques such as TEM and
Energy-Dispersive X-ray Spectroscopy (EDS) (Fig. 4d). The formation of
2D/3D heterojunctions extended exciton diffusion length and carrier
lifetime, leading to a higher PCE of 11 % in TEA-based 2D PSCs
compared to the PEA-based counterpart. Additionally, the TEA-based
2D PSCs demonstrated remarkable phase and device performance sta­bility. In Yao’s research, perovskite single crystals ((TEA)2(MA)2Pb3I10)
based on TEA with n = 3 were dissolved in a designated solvent (mixed
DMAc: TOL (HI) solvent), yielding a special solution that acted as pre­cursor solution before spin-coating. Importantly, the 2D perovskite
films, derived from this precursor solution via the thermal spin-coating
method, maintained a consistent primary phase (n = 3) analogous to thedissolved single crystal. Compared to PSCs prepared using conventional
stoichiometric precursors, the PSCs fabricated through this novel
approach achieved a peak PCE of 14.68 % after optimization. This co­ordination engineering provided an important avenue for obtaining
high-quality 2D RP perovskite films with a narrow n-phase distribution.
2-Thiopheneformamidinium (ThFA)(10), a fusion of thiophene andFA cation was initially synthesized and employed in 2D RP perovskite(ThFA)2MA2Pb3I10 by Liu et al. in 2020 [96]. They pioneered a Pre­
cursor Organic Salts-Assisted Crystal Growth (PACG) methodology toenhance the quality of 2D RP perovskite films. By employing PACG
technology, the solar cell devices demonstrated an impressive PCE of
16.72 %. This significant achievement was attributed to the extended
carrier lifetime, increased carrier mobility, and reduced nonradiative
recombination observed in the ThFA-based 2D RP perovskite films.
Building upon their previous work, Liu and colleagues further expanded
the application of ThMA and ThFA spacer cations by incorporating theminto CsPbI3-based perovskite [70]. This incorporation led to the devel­
opment of 2D RP perovskite films and PSC devices with an n-value of 5,
specifically (ThMA)2Cs4Pb5I16 (ThMA-Cs) and (ThFA)2Cs4Pb5I16(ThFA-Cs). SEM images displayed in Fig. 5a revealed that ThFA-Cs
perovskite films exhibited superior morphological quality, character­ized by enlarged grain sizes. In contrast, ThMA-Cs perovskite films dis­played smaller grain sizes and noticeable pinholes. The presence oflarge, substrate-perpendicular grains in ThFA-Cs films contributed toenhanced carrier transport ability and reduced electron-hole pairrecombination. Graphical interpretation of GIWAXS results in Fig. 5b8

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Materials Today Electronics 8 (2024) 100100

Fig. 5. (a) SEM image of ThMA-Cs and ThFA-Cs perovskite films. (b) GIWAXS results of ThMA-Cs and ThFA-Cs perovskite films. (c) XRD results of THFMA-based andFuMA-based 2D perovskite films. (d) XRD patterns of (THFMA)2PbI4 and (FuMA)2PbI4 crystal powders. (e) GIWAXS results of THFMA-based and FuMA-based 2D
perovskite films. (f) XRD patterns of (4-AEP)2MAn− 1PbnI3n+1 (n = 1, 3, 4, 5) and (PEA)2MA4Pb5I16 perovskite films before thermal annealing and (g) after thermal
annealing. (h) Laser particle size measurement results of the BA perovskite and 0.1PyA perovskite precursor solution. (a, b) Reproduced with permission from
Ref. [70]. Copyright 2022, American Chemical Society. (c-e) Reproduced with permission from Ref. [74]. Copyright 2022, American Chemical Society. (f-h)
Reproduced with permission from Ref. [66]. Copyright 2019, American Chemical Society.

indicated that ThFA-Cs and ThMA-Cs perovskite films exhibited highcrystallinity with a preferred vertical orientation. Stability assessments
performed over 720 h revealed that perovskite films based on ThMA andThFA exhibited an unchanged phase, highlighting enhanced stability.
This improved stability can be attributed to the incorporation of organic
spacer cations, effectively alleviating inherent stress within the 3D
CsPbI3 perovskite structure. Ultimately, the PSC device utilizing
ThFA-Cs-based perovskite films demonstrated an outstanding PCE of
16.00 %.
The furan ring, analogous to the thiophene ring, represents a fivemembered heterocyclic compound with an oxygen atom, whereas
thiophene contains a sulfur atom. Despite their structural similarities,
the heteroatom oxygen in furan has a greater electronegativity and asmaller radius than the sulfur atom in thiophene. These characteristics
make furan a promising candidate for spacer cations in 2D RP perov­
skites. In a groundbreaking development, Zheng and colleagues intro­
duced a furan-based spacer cation, 2-furfurylammonium (FuMA)(12),for the first time in 2022 [102]. Under the facilitation of MACl additives,the crystal grains within the 2D RP perovskite (FuMA)2(MA)4Pb5I16
films exhibited an enhanced orientation of growth perpendicular to thesubstrate and larger size, which resulted in an improvement of PCE from
4.9 % to 15.66 % for PSCs. Notably, even when fabricating a 2D RP

perovskite device under ambient air conditions, the PCE can still achieve
an impressive value of up to 15.24 %. Tetrahydrofuran (THF) is struc­turally similar to furan, with both compounds possessing five-memberedrings containing an oxygen atom. However, the absence of aromaticity
in THF distinguishes it from furan. In the same year, Liu and his
co-workers developed a novel spacer cation, (tetrahydrofuran-2-yl)
methanaminium chloride (THFMACl), and investigated its differencesfrom FuMA in the context of perovskite films and device application[74]. The results showed that the observed shift of peaks in XRD tests of
FuMA-based perovskite films towards higher 2θ angles, as shown in
Fig. 5c, d compared with THFMA-based perovskite film, suggestinglattice compression. This compression can be attributed to the slight
reduction in the size of the five-membered ring in FuMA, coupled with
its enhanced intermolecular forces of π − π. The analysis through
GIWAXS displayed sharp Bragg spots in FuMA-based perovskite films,
suggesting their high crystallinity and vertical growth direction related
to the substrate (Fig. 5e). The HRTEM and fast Fourier transform (FFT)
measurements showed different phases (n = 1, 2, 3, 4 …) coexisting in
FuMA- and THFMA-based 2D perovskite films. Finally, the PCE of
FuMA-based perovskite devices, utilizing an ITO/PEDOT: PSS/(Fu­MA)2MA3Pb4I9Cl4/PCBM/BCP/Ag architecture, achieved a remarkablevalue of 18.00 %, which surpassed that of THF-based perovskite devices
9

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Materials Today Electronics 8 (2024) 100100

and also exceeded the PCE reported by Zheng et al. for FuMA-based PSCs
with an ITO/SnO₂/(FuMA)2(MA)4Pb5I16/Spiro-OMeTAD/Au structure.
Selenium, an element of the chalcogen family, along with sulfur andoxygen, possesses a larger atomic radius than its counterparts. This large
size leads to a greater dipole moment in selenium-containing fivemembered rings, specifically in selenophene. Notably, selenophene ex­hibits pronounced Se-Se interactions in various organic electronic ma­
terials [107,108]. Due to these properties, selenophene shows potential
as a spacer cation in 2D RP perovskites. In 2023, Liu’s group synthesized
selenophene-based ammonium cation (SeMA)(13) through a complex
route [72]. Computational analysis revealed that SeMA had a dipole
moment of 9.21 D, surpassing that of thiophene-based spacer (ThMA) at
8.51 D and furan-based spacer (FuMA) at 7.73 D using the same calcu­
lation method. This higher dipole moment was believed to reduce theinfluence of quantum confinement between organic and inorganic
layers. Using the normal spin-coating method, 2D RP perovskite films ofn = 5 (SeMA)2MA4Pb5I15− xClx were obtained. Comparing these perov­
skite films with MAPbI3 perovskite films, XPS testing indicated a
directional shift towards lower energy in the characteristic peak repre­
senting the binding energy of Pb. This shift suggested alterations in thechemical environment of Pb. PL and UV–vis absorption spectra revealed
the presence of multiple phases ranging from low n = 1 phase to high nphases in SeMA-based perovskite films. Surface morphology analysis
through SEM exhibited compact and smooth surfaces, while XRD and
GIWAX analyses confirmed the vertical growth direction of crystal
grains. This study’s highlight was using predeposition transport layer(PDTL) techniques based on newly designed cations to mitigate surface
defects in 2D RP SeMA-based perovskite films. This approach signifi­
cantly enhanced the charge carrier mobility and reduced non-radiative
recombination processes in the perovskite films. Consequently, thePCE of SeMA-based perovskite solar cells reached high values of 17.25 %and 19.03 %, using methylammonium (MA) and formamidinium (FA) as
the A site cation in 2D RP perovskites, respectively.
Recent studies have explored using thiophene-, furan-, andselenophene-based spacer cations (ThMA, FuMA, and SeMA) as theorganic layer in 2D RP perovskites. These spacer cations have shown asimilar effect on the perovskite films’ morphology and grain growthorientation. Specifically, they can promote the formation of large crys­talline grains and induce a preferential vertical growth direction,
thereby enhancing the performance of devices. However, despite thesesimilarities, a detailed investigation into the subtle differences among
these three spacer cations in the context of 2D RP perovskites iscurrently lacking, according to our knowledge. Moreover, we think
exploring the interactions between different five-membered heterocy­cles based on S, O, and Se atoms in 2D RP perovskite may be a direction
worthy of exploration.
In addition to aromatic rings incorporating heteroatoms derived
from chalcogen elements, the potential of other heterocycles as spacer
cations in 2D RP perovskites, such as the pyridine ring, remains largely
unexplored. Compared with amino group, the nitrogen atom on thepyridine ring is more difficult to be protonated, which requires higher
temperature for successful protonation. This limitation means that only
the amino group can be protonated under conventional conditions for
preparing spacer ammonium ions. A breakthrough was made by Wanget al. in 2019, who discovered a pyridine-based spacer cation known as
4-(aminoethyl)-pyridine (4-AEP)(15) [66]. They found that 4-AEP can
modulate the crystallization rate of 2D RP perovskite films. To investi­
gate the impact of 4-AEP on perovskite films, the authors prepared aseries of films with the formula (4-AEP)2MAn− 1PbnI3n+1 (n = 1, 3, 4, 5)
and (PEA)2MAn− 1PbnI3n+1 (n = 5). XRD analysis revealed that, before
annealing, 2D RP perovskite films based on 4-AEP exhibited weak or
negligible peaks, except for the substrate diffraction peak (Fig. 5f).
However, after annealing, two distinct characteristic peaks, (111) and(202), emerged (Fig. 5g). In contrast, 2D perovskite films based on PEA
displayed evident perovskite characteristic peaks of (111) and (202)even before annealing. These results suggested that compared to PEA

spacer cations, 4-AEP spacer cations could slow down the crystallization
rate of perovskite films before annealing, which can facilitate the for­mation of high-quality perovskite films with dense and smooth surfaces,
as confirmed by SEM measurements. Further analyses using FTIR and
XPS techniques indicated that the nitrogen atoms within the pyridine
ring can effectively coordinate with Pb ions in PbI2, forming an inter­mediate species that decelerated the crystallization rate. The resulting
PSCs based on (4-AEP)2MAn-1PbnI3n+1 (n = 5) exhibited a PCE of 11.68% and improved moisture stability. Another pyridine-based spacer
cation, 4-pyridinylmethylammonium (PyA)(14), with a shorter alkyl
chain compared to 4-AEP, was introduced by Xu and his co-workers in
2022 [68]. They combined PyA spacer with the aliphatic spacer BA toprepare a mixed spacer 2D RP perovskite, namely (BA1− xPyAx)2­
MA3Pb4I13 (x = 0, 0.1, 0.2, 0.4). The pure BA-based 2D RP perovskite
served as a reference sample. Morphological characterization revealed
obvious cracks in the BA-based 2D perovskite film. However, with theaddition of PyA at a 0.1 ratio, uniformly smooth 2D perovskite films
were obtained, and the cracks disappeared. As the PyA ratio increased
from 0.1 to 0.4, the perovskite film quality deteriorated. Therefore, theoptimal PyA ratio was determined to be 0.1. Laser particle size analysis
demonstrated that the presence of the PyA spacer had an inhibitory
effect on the formation of large colloidal particles in the precursor so­lution. This was evidenced by the larger colloidal size (> 1000 nm)
observed in the BA-based 2D perovskite precursor solution compared to
the 0.1PyA-based 2D perovskite precursor solution (5 nm) (Fig. 5h).
Suppressed precursor aggregation contributed to the formation of large
grains, reducing defects and nonradiative recombination. Consequently,
the 0.1PyA-based solar cells achieved an impressive PCE of 13.01 % andexhibited high stability.
2.3. Substituents on aromatic rings
The substitution of specific functional groups (-F, -Br, -Cl, -OCH3,-CF3, etc.) for hydrogen on the benzene ring is an essential strategy fordeveloping novel spacer cations, enhancing the performance, andimproving the stability of 2D RP perovskites. Different substituents
exhibit varying electron-withdrawing abilities, hydrophobicity, andsize, leading to distinct effects on the properties of the substituted
benzene ring. For instance, fluorine atoms can increase the dipole
moment of benzyl spacer cations, thereby reducing the mismatch indielectric constants between the organic and inorganic layers in 2D RP
perovskites. Methoxy groups (-OCH3) exhibit strong hydrophobicity,
contributing to the stability of the 2D RP perovskite lattice. Further­more, these substituents also impact the size of 2D RP perovskite crystal
grains, growth orientation, exciton separation, charge carrier transport,
etc.
Within the halogen group, fluorine (F) stands out due to its minimal
atomic volume and strong electron-withdrawing ability. Consequently,
extensive research has been conducted on fluorine-substituted spacer
cations for use in 2D RP perovskites. The simplest fluorinated benzyl
spacer cation is 4-fluorophenethylamine (4F-PEA)(16), which isbelieved to enhance the charge transfer ability between inorganicoctahedral layers [62,67,80]. For example, Zhu et al. conducted anin-depth investigation into the structural and performance differences of
2D RP perovskite based on 4F-PEA and PEA to study the impact offluorine atoms [80]. Initially, the author prepared 2D RP perovskite
solar cells of n = 1 (4F-PEA)2PbI4 and (PEA)2PbI4 with identical struc­tures based on PEA and 4F-PEA. The results showed that devices of(4F-PEA)2PbI4 exhibited a higher PCE of 1.90 %, compared with PCE of
1.34 % of (PEA)2PbI4. The measurement of time-resolved microwave
conductivity (TRMC) also displayed about 7 times out-of-plane micro­
wave mobility of (4F-PEA)2PbI4 than that of (PEA)2PbI4, which meantenhanced out-of-plane charge transfer capability for (4F-PEA)2PbI4(Fig. 6a). Furthermore, from the perspective of single crystal structure
analysis, as shown in Fig. 6b, in organic layers of (4F-PEA)2PbI4, the4-FPEA cations adopted a face-to-face stacking arrangement. In contrast,10

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Materials Today Electronics 8 (2024) 100100

Fig. 6. (a) TRMC comparison of out-of-plane and in-plane transport for (PEA)2PbI4 and (4F-PEA)2PbI4 films. (b) Single crystal structures of (PEA)2PbI4 and (4FPEA)2PbI4. (c) UV–vis absorption and (d) PL peak from the back side (top) and the front side (bottom) of PEA-, oF1PEA-, mF1PEA-, and pF1PEA-based 2D perovskites(n = 4). (e) Single crystal structures of (PEA)2PbI4, (oF1PEA)2PbI4, (mF1PEA)2PbI4 and (pF1PEA)2PbI4. (f) Calculated formation energy for (PEA)2PbI4,(oF1PEA)2PbI4, (mF1PEA)2PbI4 and (pF1PEA)2PbI4. (g) PL decay profiles of PEA-, 4F-PEA-, 4Cl-PEA-, and 4Br-PEA-based 2D RP perovskites on the glass substrate.
(h) TA spectra at different delay times with top and bottom excitation for PEA-, 4F-PEA-, 4Cl-PEA-, and 4Br-PEA-based 2D RP perovskites films. (a, b) Reproducedwith permission from Ref. [80]. Copyright 2019, American Chemical Society. (c-f) Reproduced with permission from Ref. [109]. Copyright 2022, Springer Nature. (g)Reproduced with permission from Ref. [46]. Copyright 2019, American Chemical Society. (h) Reproduced with permission from Ref. [81]. Copyright2020, Wiley-VCH.

the stacking arrangement was edge-to-face for (PEA)2PbI4. By compar­
ison with edge-to-face packing, face-to-face packing could lead to
reducing the ring centroid-to-centroid distances for 4F-PEA spacers andincreased pi-orbital overlap, which was conducive to the charge trans­
port in the out-of-plane direction. Additionally, a comparative analysis
was also undertaken for high n-value (n = 5) perovskites based on
4F-PEA and PEA, (4F-PEA)2MA4Pb5I16 and (PEA)2MA4Pb5I16. Surface
morphology tests indicated that (4F-PEA)2MA4Pb5I16 films had a morehomogeneous surface with reduced grain boundaries compared to(PEA)2MA4Pb5I16 films. Results from TPRL and TRMC tests demon­
strated that (4F-PEA)2MA4Pb5I16 possessed extended carrier lifetimes
and reduced defect density. The improvements in stacking, morphology,
and electronic performance were attributed to the introduction offluorine atoms, which ultimately resulted in an improved PCE of 13.64% for solar cells based on (4F-PEA)2MA4Pb5I16. But the (PEA)2­
MA4Pb5I16-based PSCs only had a PCE of 9.6 %. More importantly, due
to the hydrophobicity induced by fluorination, the (4F-PEA)2­
MA4Pb5I16-based devices showed higher stability than (PEA)2­
MA4Pb5I16-based devices under high temperature and ambientconditions. In another study by Shao et al., with the assistance of
NH4SCN additives, the PCE of solar cells based on (4F-PEA)2MA4Pb5I16
further achieved a breakthrough [62], reaching 17.3 %. It is worth
mentioning that Shao’s work additionally pointed out that fluorinatedspacer cations 4F-PEA can induce systematically distributed patterns fordifferent n-phases in 2D RP perovskites films. Specifically, the low
n-phases were observed at the bottom, while the high n-phases were11

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Materials Today Electronics 8 (2024) 100100

located at the top surface, which proved advantageous for charge sep­aration and transport. In Liu’s research, a fluorine atom was introduced
at the para-position of FA-based spacer (Benzamidine (PhFA)), resulting
in a new aromatic spacer, 4-Fluoro-benzamidine (p-FPhFA)(24) [67].
Similarly, introducing fluorine at the para-position significantly
enhanced the quality of the 2D perovskite thin film and promoted grain
growth perpendicular to the substrate. Non-radiative recombination was
suppressed, whereas the mobility and lifetime of the carriers were
improved. Ultimately, PSCs with p-FPhFA spacers pushed the PCE to
17.37 %.
The position of fluorine substitution on the benzene ring is critical to
the 2D perovskite structure and morphology, as well as the performanceof the corresponding prepared solar cells [71,109,110]. You andco-workers had conducted in-depth research on 2D RP perovskites basedon monofluorinated PEA with different substitution positions of benzene
moiety (2-fluorophenethylammonium (oF1PEA)(18), 3-fluorophenethy­
lammonium(mF1PEA)(17)and
4-fluorophenethylammonium(pF1PEA/4F-PEA)) [109]. UV testing revealed distinct distributions ofdifferent n phases within oF1PEA, mF1PEA, and pF1PEA 2D perovskite
films (Fig. 6c). Also, PL characterization, excited from both front andback sides, indicated that high-n phases perovskites were concentrated
near the surface in contact with air, while low-n phases perovskites werelocated at the bottom for 2D perovskite films with mF1PEA and pF1PEA
spacers, as shown in (Fig. 6d). However, in oF1PEA-based 2D perovskite
films, the distribution of n phases was random. Compared to the uniform
surface of PEA-based 2D perovskite films with few pinholes,
oF1PEA-based 2D perovskite films had a rough and uneven surface.
Nonetheless, mF1PEA and pF1PEA-based 2D perovskite films were
uniform and smooth. Furthermore, 2D perovskite single crystals(Fig. 6e) with n = 1 for PEA, oF1PEA, mF1PEA, and pF1PEA weresynthesized to study their differences in molecular stacking and theinterlayer interactions between inorganic octahedral layers and organic
layers. It is noted that in pF1PEA2PbI4, these spacer cations between
adjacent organic layers exhibited a completely ordered face-to-facestacking arrangement. However, in the single layer of mF1PEA cations
in mF1PEA2PbI4, there was a rotation in the angle between adjacentaromatic cation groups, which led to a form of herringbone arrange­
ment. The oF1PEA2PbI4 and PEA2PbI4 had a similar structure, where thespacer cations between adjacent organic layers were arranged in anedge-to-face stacking manner. The calculated formation energy((Fig. 6f)) based on these single crystals showed a trend related to crystal
stacking and disorder: pF1PEA2PbI4 > mF1PEA2PbI4 > PEA2PbI4 >oF1PEA2PbI4. These differences were possibly caused by different
dipole-dipole interactions, resulting in the variations of PCE of solar cells
based on these spacer cations. As a result, pF1PEA- and oF1PEA-based
PSCs showed the highest and lowest PCEs, at over 10 % and below 1%, respectively. Another parallel research (22, 23, 24) focusing on the
disparate effects of fluorine substitution at various positions of benyl
moiety in 2D RP perovskites was predicated on PMA cations and was
conducted by Zhang et al. [71]. In this research endeavor, the influenceof fluorine substitution at distinct positions on the morphology, phase
distribution, crystallinity, and device performance of 2D RP perovskite
films closely aligned with the findings in You’s work. Similarly, solar
cells fabricated based on fluorine substitution at the para position of thebenzene ring in PMA demonstrated the highest PCE of 17.12 %.
Aside from fluorine, which belongs to the halogen family, chlorine
and bromine have different atomic radius and electronegativities. In the
study conducted by Huang et al., they used 3-bromobenzylammonium
iodide (3BBAI)(25) to prepare 2D RP perovskites and achieved high
PCE of 18.2 % for corresponding solar cells [75]. They indicated that
3BBA can result in the distribution of low n phases at the bottom of film
and high n phases at the top of film, which is benefited to the chargetransportation. Subsequently, Wang et al. investigated the impact ofchloro (Cl), and bromo (Br) substitutions on the para position in PEA
cations in 2D RP perovskites [46]. They designed a series of spacer
cations, namely PEA, 4F-PEA, 4Cl-PEA (20), and 4Br-PEA (21), andcalculated dipole moments for them were approximately 11.83 D, 13.16
D, 13.49 D, and 13.34 D, respectively. It could be seen that the intro­
duction of halogen atoms with electron-withdrawing properties
increased the dipole moment, which can benefit charge dissociation andintramolecular charge transport. Subsequently, they prepared corre­
sponding 2D RP perovskite films with a fixed n value of 3 using thesespacer cations, PEA2MA2Pb3I10, 4F-PEA2MA2Pb3I10, 4Cl-PEA2­
MA2Pb3I10, and 4Br-PEA2MA2Pb3I10. XRD and GIWAX measurements
revealed that H-PEA2MA2Pb3I10 and 4F-PEA2MA2Pb3I10 films possesseda preferential growth orientation perpendicular to the substratecompared to 4Cl-PEA2MA2Pb3I10 and 4Br-PEA2MA2Pb3I10 films. Thisdifference may be because the radius of the Cl and Br atoms is larger
than that of the H atom and F atom. The structural analysis for PEA2PbI4,
4F-PEA2PbI4, 4Cl-PEA2PbI4, and 4Br-PEA2PbI4 single crystals was con­ducted to study the distortion in the perovskite inorganic octahedral
structure. The results indicated that the angle of Pb− I− Pb changed from
151.3 ◦ for PEA2PbI4 to 152.91 ◦ for 4F-PEA2PbI4, then to 153.6 ◦ for
4Cl-PEA2PbI4 and finally to 152.6 ◦ for 4Br-PEA2PbI4. The smallerPb− I− Pb angle indicated a stronger influence of spacer cations on the
PbI6 octahedra. And the exciton-binding energy, characterized by PL
lifetime test, showed a clear difference in the order of PEA2PbI4 >4Br-PEA2PbI4 > 4Cl-PEA2PbI4 > 4F-PEA2PbI4, as shown in Fig. 6g.
Among these perovskites, the solar cells based on 4F-PEA2MA2Pb3I10
films obtained the highest PCE of 5.83 %. This study pointed out that theappropriate selection for electron-withdrawing atomic substituents canbe used to regulate the optoelectronic performance of the devices. Zheng
et al. conducted further research on utilizing these four spacer cations in
2D RP perovskites with fixed n = 4 [81]. Their research underscored theordered gradient arrangement of different n-values within PEA- and
4F-PEA-based perovskite thin films due to nucleation competition be­
tween high n phases and low n phases. From the bottom to the top of theperovskite film, there was a gradual increase in the n value of theperovskite phases, leading to the formation of graded band arrange­
ment, which ultimately facilitated more efficient charge transportation(Fig. 6h). However, for 4Cl-PEA- and 4Br-PEA-based perovskite thinfilms, the distribution of high n phases and low n phases was random,
generating a disorderly band arrangement. Similar to Wang’s work, in
Zheng’s findings, 4F-PEA-based perovskite solar cells achieved thehighest PCE at 18.10 %, followed by a PCE of 12.23 % for PEA-basedsolar cells. Solar cells based on 4Cl-PEA and 4Br-PEA showed theworst performance, with PCEs recorded at 7.93 % and 6.08 %, respec­tively. In addition, the results of stability measurements showed thatdevices employing these four spacer cations exhibited commendable
moisture stability versus 3D perovskites. However, in terms of thermal
stability, solar cells based on PEA and 4F-PEA notably outperformed
those based on 4Cl-PEA and 4Br-PEA, possibly due to the disorderly
distribution of different n-phases.
In addition to investigating substituents from the halogen group,
which possess high electron-withdrawing characteristics, substituentswith electron-donating abilities, such as methoxy groups, have also been
explored. For example, Fu et al. chose F and MeO (19) as substituents forthe para-position hydrogen atoms on benzene ring of PEA and con­structed three different 2D RP perovskites with n = 5, (4F-PEA)2(­
MA)4Pb5I16, (PEA)2(MA)4Pb5I16 and (MeO-PEA)2(MA)4Pb5I16 [111].
Regarding stability, 2D perovskite films utilizing 4F-PEA and MeO-PEA
as spacer cations demonstrated superior performance compared to
PEA-based perovskite films (Fig. 7a-b). The authors conducted compu­
tational calculations to determine the specific reasons for this observa­
tion. The results suggested that the enhanced stability of the
MeO-PEA-based 2D perovskite could be attributed to the significant
volume of the MeO group, which imparts a certain repulsive effect onwater molecules. On the other hand, despite the small size of the F atom
as the substituent in 4F-PEA cations, its electronegativity can still inducea repulsive effect on water molecules, thereby contributing to theimproved stability of the 4F-PEA-based perovskite. However, while MeO
increased the resistance of the perovskite lattice to water, it
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Materials Today Electronics 8 (2024) 100100

Fig. 7. (a) The binding energy curves of water and perovskites with PEA, 4F-PEA, and MeO-PEA spacer ligands. (b) Absorption spectra of PEA-, 4F-PEA-, and MeOPEA-based 2D RP perovskites under humidity conditions of 40 – 50 %. (c) Single-crystal structures of (PEA)2PbI4, ((PEA)0.5(F5-PEA)0.5)2PbI4 and (F5-PEA)2PbI4. (a,b) Reproduced with permission from Ref. [111]. Copyright 2019, Wiley-VCH (c) Reproduced with permission from Ref. [112]. Copyright 2019, American Chem­
ical Society.

simultaneously negatively impacted the performance of the perovskite
devices due to its large volume. This effect resulted in a low PCE of 9.4 %for MeO-PEA-based solar cells. Among the devices based on these threespacer cations, solar cells utilizing 4F-PEA as spacers achieved thehighest PCE at 13.5 %.
Another spacer cation developed for 2D RP perovskites is 2,3,4,5,6pentafluorophenethylammonium (F5-PEA)(27), which is a derivative of
PEA where all hydrogen atoms on the phenyl ring are replaced byfluorine [112,113]. In a study by You et al., mixed cation 2D RP pe­
rovskites with n = 4 were prepared using PEA and F5-PEA cations [112].
After adjusting the ratio of the two spacer cations, the authors found thatthe optimal performance of the 2D perovskite thin films was optimal
when the ratio of PEA to F5-PEA was 1:1. Deviation from this ratio,
either above (1: 0.75) or below (0.75: 1), resulted in a decrease in device
performance. Under the 1:1 ratio of PEA to F5-PEA, the surface of the 2D
perovskite thin films was made uniform and smooth without any holes,
and it exhibited an ordered vertical distribution of different phases. At
the optimal ratio, 2D perovskite solar cells displayed a high PCE
exceeding 10 % with enhanced stability. Further single-crystal testing
results for (PEA)2PbI4, ((PEA)0.5(F5-PEA)0.5)2PbI4, and (F5-PEA)2PbI4
revealed that only a face-to-face stacking structure was observed in((PEA)0.5(F5-PEA)0.5)2PbI4 as shown in Fig. 7c. This result was caused byquadrupole− quadrupole interactions between PEA and F5-PEA, leading
to the stabilization of the perovskite lattice and enhancement of device
performance.

2.4. Multiple aromatic rings
The successful application of single-ring aromatic spacer cations in
2D RP perovskites has sparked interest in investigating multiple-ringaromatic spacer cations [73,77,79,114,115]. By leveraging the highdipole moment resulting from extended π-electron delocalization,
incorporating multiple aromatic ring amines offers the potential tomitigate the dielectric mismatch between organic and inorganic layers,
thereby minimizing the effects of quantum confinement. However, theresearch on multiple-ring aromatic organic amines is relatively scarce,
primarily focusing on naphthyl-based and bithiophene-based ammo­nium ions, which have already demonstrated significant practical ap­
plications. As such, exploring the applicability of other multi-ringaromatic amines in this context is still necessary.
Xu and his colleagues used 1-naphthalenemethylammonium (NpMA)(28) and 9-anthracenemethylammonium (AnMA)(29) as spacer cations
to fabricate 2D RP perovskite thin films and corresponding devices witha normal n value of 4 [73]. They confirmed the existence of N − H⋅⋅⋅I
interactions between the spacer ammonium ion and the inorganicoctahedra framework through 1H NMR measurements (Fig. 8a). The PL
and UV− vis absorption spectrum indicated that different n phases
coexisted in NpMA-based 2D perovskite films, further verified by
HRTEM tests as shown in Fig. 8b. But in the AnMA-based 2D perovskite
films, 3D-like phases were dominated, possibly owing to bulk steric ef­
fects of AnMA spacer. Besides, the perovskite films based on NpMA
exhibited superior crystallinity and vertical growth orientation to thesubstrate. Utilizing TA spectroscopy, the authors clarified the chargetransport pathways within 2D perovskites. They proposed that13

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Materials Today Electronics 8 (2024) 100100

Fig. 8. (a) Chemical structure and corresponding 1H NMR spectra of MAI, AnMAI, and (AnMA)2MA3Pb4I13. (b) HRTEM image of the NpMA-based (left) and AnMAbased 2D perovskite films (right). (c) Calculated HOMO and LUMO results for (BPhMA)2PbI4 and (BThMA)2PbI4 and proposed energy levels of organic and inorganic
components, and conductivity and statistics of mobilities diagrams (d) of corresponding 2D RP perovskite films. (e) SEM images of the BTFA-based and TTFA-based
2D RP perovskite films. (f) The structural evolution monitored by in situ GIWAXS during the heating from 40 ◦ C to 54 ◦ C at a rate of 1 ◦ C/30 s of the as-deposited
BTFA-based and TTFA-based 2D perovskite films. (a, b) Reproduced with permission from Ref. [73]. Copyright 2020, American Chemical Society. (c, d) Reproducedwith permission from Ref. [77]. Copyright 2023, Wiley-VCH. (e, f) Reproduced with permission from Ref. [79]. Copyright 2023, Wiley-VCH.

photoexcited excitons originating from the low-n phases initiallymigrated to the neighboring high-n phases, subsequently dissociating
into free charges. The 2D PSCs with NpMA and AnMA achieved a PCE of
17.25 % and 14.47 % with long-term stability, respectively. Similar to
NpMA, 1-naphthylamine (1-NA)(30) was also developed into spacer
cation to prepare Cs-based solar cells with n = 4 by Jin et al., achieving
an impressive PCE of 16.62 %. The authors pointed out that comparedwith the common PEA cations, 1-NA can further reduce the binding
energy of 2D perovskites, thereby promoting the dissociation of exci­
tons. In addition, they also suggested that the strong π-π interactions not
only enhanced the stability of perovskite lattices but also facilitated theformation of high-quality films with vertical orientation, which
improved device performance.
Yuan et al. designed a novel spacer derived from carbazole, (9Hcarbazol-9-yl)butyl‑1-ammonium (CA)(35), and used it into 2D RP pe­
rovskites with n = 5 [75]. Employing the anti-solvent method, they
induced a distribution of low n phases at the top of perovskite film,
which led to outstanding stability against water. Besides, under theassistance of NH4SCN additives, the perovskite crystals can grow in thevertical orientation, resulting in the enhanced carrier transport ability
and reduced non-radiative recombination. Finally, the PSCs based on CA
had an excellent PCE of 18.23 % and long-term stability.
Recently, Liu et al. utilized bithiophenemethylammonium(BThMA)(33) and biphenemethylammonium (BPhMA)(34) in 2D RP perovskites
as spacer cations, and they arrived at a concept of “quantum confine­ment breaking” [77]. To be specific, through the DFT calculation results,
as shown in Fig. 8c-d based on perovskite structure of n = 1, they
discovered that there was a strong orbital coupling for (BThMA)PbI4
between the octahedral framework of inorganic layers and organic
conjugated molecular layers, derived from the fact that its HOMO was
located in inorganic layers. In contrast, LUMO was mainly located inmolecule layers. However, no such coupling was seen in (BPhMA)2PbI4
perovskites because both its HOMO and LUMO were located in the
inorganic octahedra layers. Owing to this orbital coupling, (BThMA)PbI4
showed enhanced conductivity and electron and hole mobility
compared with (BPhMA)2PbI4 in the direction vertical to the substrate.
In the study for perovskites with higher n values, BThMA-based 2D
perovskite films displayed superior crystallinity and uniform surfaces
with large grains. As a result, the PCE of PSCs based on the BThMA
spacer had an impressive value of 18.05 %, higher than that of
BPhMA-based devices (12.69 %). This work provided a novel thought toaddress or impair the quantum confinement effect in 2D RP perovskites.
In another research conducted by Liu et al., they combined
fused-bithiophene and unfused-bithiophene with the FA group to
develop FA-based multiple aromatic ring spacer cations, TTFA(31) andBTFA(32) [79]. This research focused on studying the nucleation and
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Materials Today Electronics 8 (2024) 100100

crystallization process for 2D perovskite films through situ GIWAXS and
optical microscopy measurement (Fig. 8e-f). The authors discovered thatthe binding energy Eb of TTFA between contiguous molecules was
greater than that in BTFA, which led to larger clusters in 2D perovskite
precursor solution based on TTFA. The larger clusters subsequently
accelerated the formation of large crystal grains, generating high-quality
films for TTFA-based 2D perovskite. In contrast, BTFA-based 2D
perovskite films showed rough surface morphology accompanied bysmall grains. Differences in morphology ultimately resulted in variations
in performance, with TTFA-based solar cells exhibiting a superior PCE of
19.41 %.

DJ-type diamine ions, which are primarily based on benzene and thio­
phene compounds. Therefore, although the development of DJ-type
spacers can also be divided into four directions (similar to RP-type
spacers), this section will discuss these spacers and corresponding pe­
rovskites from three aspects, benzene, thiophene, and pyridine
derivatives.
3.1. Benzene derivatives
The divalent organic cation 1,4-phenylenedimethanam-monium(PDMA)(36) has been widely used to create 2D DJ-phase perovskites.
The initial use of PDMA was realized by Gratzel et al. in 2019. They
conducted a detailed study on the morphology and optoelectronic
properties of 2D DJ (PDMA)FAn-1PbnI3n+1 perovskite films withdifferent n values (n = 1, 2, 3, and 4) [61]. As shown in Fig. 9a-b, withthe value of n increases, the morphology of the 2D DJ perovskite films
transitioned from large-sized grains (n = 1) to needle-like structures (n =2) and finally to a similar island-like pattern covering the film surface (n= 3), indicating a weakening in the crystallinity of films. Decreased
crystallinity was also corroborated by XRD testing. Besides, UV and PL
exhibited a redshift phenomenon similar to RP-type perovskites as thevalue of n increases (Fig. 9c-d). A PCE exceeding 7 % was achieved in 2D
DJ perovskite devices with n phase of 3, which also had extremely highair stability. Followingly, Etgar et al. Used PDMA spacers to prepare 2D

3. Aromatic diamine spacer cations of 2D DJ perovskites
Recently, there has been a marked increase in the exploration of 2D
DJ perovskites. The DJ-phase structure incorporates a divalent organic
cation connecting each perovskite sheet layer, which results in a wellordered lamellar structure. This phase contrasts with the RP phaseperovskite structure, which displaces adjacent 2D perovskite sheet
layers with half an octahedron. The DJ-phase structure enhances the
structural stability of the 2D layered perovskite by completely occu­
pying the van der Waals gaps in the RP lamellar structure, positioning it
as a hopeful candidate for high-efficiency, stable perovskite devices.
However, there have been relatively few research studies on aromaticFig. 9. (a) Schematic structure of (PDMA)FAn− 1PbnI3n+1 perovskites with different n compositions (n = 1 − 4). (b) SEM images of (PDMA)FAn− 1PbnI3n+1 perovskite
films with different n compositions (n = 1 − 4). (c) Optical images of (PDMA)FAn− 1PbnI3n+1 perovskite films with different n compositions (n = 1 − 4) and (d) the
corresponding UV− vis absorption and PL spectra. (e) HR-TEM images and schematic crystal structures of the confinement 2D and non-confinement 2D perovskites.
(f) Chemical structures of PDMA and TFBDA and single crystal structure of (TFBDA)PbI4. (a-d) Reproduced with permission from Ref. [61]. Copyright 2019,
American Chemical Society. (e) Reproduced with permission from Ref. [84]. Copyright 2019, Wiley-VCH. (f) Reproduced with permission from Ref. [87]. Copyright2021, Wiley-VCH.
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DJ perovskite devices with n = 10 and achieved an impressive PCE of
15.6 % without other molecules or additives [116]. They suggested that
PDMA spacers played a crucial role in determining the orientation of theperovskite structure. In 2019, Liang’s group investigated the inner lat­tice structures of PDMA-based 2D DJ perovskite [84]. They systemati­
cally explored the impact of varying the molar proportion of PDMA on
the charge transport and crystal structure of perovskite films. The re­searchers found that when n <= 2, the structure of the 2D perovskite
was either a confinement or nonconfinement mixture. However, when n> 2, the structure became entirely nonconfinement. This transformation
was confirmed by HR-TEM measurements, as shown in Fig. 9e, and thereason can be attributed to the octahedral layers’ extrusion of the PDMA
spacer layer. Besides, the authors then compared the UV–vis absorption
spectra for samples with n value ranging from 1 to 10. For samples of n =1, 2, and 3, there were obvious exciton absorption peaks, but with nvalue increasing to higher, these peaks disappeared and a distinct ab­sorption band edge emerged at higher wavelength, which was further
evidence of the reduced quantum confinement effect. As illustrated inFig. 9e, the spacing of inorganic layers in the nonconfinement structure
was significantly shorter than that in the confinement structure. That
was consistent with the calculated exciton binding energy of the non­confinement structure (70 meV), less than that of the confinement
structure (140 meV). Finally, the solar cells with an n value of 6 achieved
the best PCE of 11 %. Moreover, Liu and co-workers also prepared 2D DJ
perovskites using PDMA as spacers with an n value of 4. Their researchhighlighted the hot spin-coating method, which resulted in more uni­
form, high-quality thin films with a more orderly distribution ofdifferent n phases [85]. Carrier transport ability was increased, and ahigh PCE of 15.81 % was obtained for the corresponding devices.
As a fluorinated derivative of PDMA, 2,3,5,6-tetrafluoro-1,4-benze­
nedimethanammonium (TFBDA)(37) was designed, synthesized, andused to construct 2D DJ perovskites by Zheng et al. in 2021 [87]. As
shown in Fig. 9f, the researchers found that the prepared (TFBDA)PbI4
single crystal showed a layered structure, and the stack arrangement of
the adjacent TFBDA2+ ligands was in an edge-to-face stacking way.
Additionally, the TFBDA-based 2D DJ perovskite films preferentiallygrew in a vertical orientation, which caused a higher PCE of 15.24 % for
PSCs with n = 10, larger than the PCE of BDA-based devices. Impor­tantly, owing to the hydrophobic nature of the fluorinated cations,perovskite devices with spacers of TFBDA displayed increased long-term
stability. Similarly, Liu’s group also used TFBDA as a spacer cation toprepare 2D DJ perovskites devices with n = 4, achieving a higher PCE of
16.62 % [88]. Importantly, Liu and colleagues specifically pointed out
from another point of view that the improved stability of perovskite
devices based on TFBDA was due to the existence of multiple
non-covalent interactions between TFBDA cations and octahedral
layers, such as F…I, CH…F and NH…F interactions. This interaction
greatly stabilized the lattice structure of 2D DJ perovskites.
In addition, 1,5-diaminonaphthalene (NDA)(38) with extended
π-conjugation length was developed as spacer for 2D DJ perovskites bySatapathi et al. [86]. Under the assistance of NH4SCN additives,
NDA-based perovskite films had a better surface morphology and highcrystallinity, which contributed to its decreased trap density andimproved carrier transport ability. Notably, the solar cells based on NDA
spacer reached an impressive value of 15.08 % and showed long-term
stability against humidity and heat.

solution can effectively regulate the growth and orientation of 2D DJ
perovskite crystals and improve the film morphology. The effect reduced
the density of defective states and enhanced the carrier transport effi­
ciency. The optimized devices achieved a notable PCE of 15.75 %.
Subsequently, Liu et al. further developed a bithiophene-based spacer,
namely thieno[3,2-b] thiophene-2,5-diyldimethanaminium (TTDMA)(40), and it was successfully used as a spacer cation for 2D DJ perov­
skites (Fig. 10a-b) [91]. The longer π-conjugation length of TTDMA
compared to ThDMA resulted in enhanced intermolecular interactions
between organic spacers in the layered 2D chalcogenide phases. Pre­
cursor aggregates of ~1 µm were detected in the TTDMA-Pb chalco­
genide precursor solution, which led to the formation of large grain sizes
in the perovskite thin films with enhanced crystallinity and verticalorientation as shown in Fig. 10c-d. In addition, the extended conjugate πlength also led to a decrease in exciton binding energy, which wasbeneficial for efficient charge transfer. As shown in Fig. 10e, the ThDMA
film displayed dominant n = 2 and n = 3 phases, while TTDMA film had
dominant phases of n = 3 and n = 4. The ultrafast decay times of TTDMA
films were τ1, n = 3 = 0.26 ps and τ1,n = 4 = 13.18 ps. Whereas, ThDMA
film showed longer decay times of τ1,n = 2 = 0.32 ps, τ1,n = 3 = 0.46 ps,and τ1,n = 4 = 18.57 ps. Ultimately, the PCE of TTDMA-based solar cells
reached a high value of 18.82 %. In 2022, another new spacer cation
with two covalently attached thiophene rings, namely bithiophene
dimethylammonium (BThDMA)(41) was also successfully developed by
Liu et al.. They investigated the interactions between the intermediate
spacer and the inorganic layer [90]. The results showed that there werestrong orbital interaction effects between the BThDMA spacer layers andthe adjacent inorganic layers. This interaction could also induce thegrowth of crystals with large grains and preferential vertical orientation,
which reduced the trap density and improved the charge carrier
mobility. As a result, the perovskite devices based on BThDMA had an
excellent PCE of 18.1 %.
Seferos et al. established structure-property relationships by tuning
the structure of the rings (benzene and thiophene rings) and substituentsof 2D DJ perovskite spacer molecules [117]. They firstly designed asymmetric conjugated linker structure [1,1′-biphenyl]− 4,4′-diyl-dime­
thylamine (PP), and then the symmetry was disrupted by substituting ina sequence of fluorine atoms to achieve a structure of (2-fluoro-[1,1′-biphenyl]− 4,4′-diyl)-dimethylamine (FPP). In the end, heterocyclic
substitution was employed to obtain a structure of (5-(4-(amino­
methyl)− 2-fluorophenyl)thiophene-2-yl)methanamine (FPT) (Fig. 10f).
The large in-plane deformation of the octahedron was determined by thestrength of the electrostatic interactions between the positively charged
ends of the organic ligand and the negatively charged inorganic layer.
This deformation resulted in a larger band gap which was represented byblue-shifted absorption and emission peaks. The authors verified thatenhanced π-stacking interactions between conjugated organic ligands
positively improved the stability of the structure.
As previously mentioned, the hydrogen on the pyridine ring can beprotonated under high-temperature reaction conditions. In 2018,
Kanatzidis et al. reported two novel diamine cations, 4-(aminomethyl)
pyridinium (4AMPY)(42) and 3-(aminomethyl)pyridinium (4AMPY)(43), and applied them in 2D DJ perovskites [83]. The authors discov­ered that the varying positions of nitrogen in pyridine relative to thealkyl chain significantly impacted the crystal structure, as shown in
Fig. 10g-j. Compared to 3AMPY, 4AMPY had higher symmetry, which
can help maintain the octahedra’s orderly arrangement in the inorganic
layers (eclipsed conformation). Also, the adjacent 4APMY cations
adopted a face-to-edge stacking in organic layers. As for 3AMPY, its
asymmetry led to a slight misalignment of the inorganic layer. In
contrast to 3AMPY, 4AMPY-based 2D DJ perovskite structure showed asmaller Pb-I-Pb angle and a larger distortion for the octahedral layers.
Besides, the penetration depth of the NH+
3 of 3AMPY into the inorganic
layer was deeper than that of 4AMPY, which reduced the spacing be­
tween inorganic layers and led to a small band gap. It is worth notingthat as the value of n increased, the Pb-I-Pb angle increased for both3.2. Aromatic heterocyclic derivatives
As previously discussed, thiophene is one of the most commonly usedfunctional heterocycles, and its significant application value in 2D RPtype perovskites has already been demonstrated. Similarly, thiophene
can also be applied in 2D DJ-type perovskites. In 2020, Liu et al. syn­thesized 2,5-thiophenedimethylammonium (ThDMA)(39) and con­structed the 2D DJ perovskites with n = 5 [89]. They found that
introducing a strong coordination molecule DMSO into the precursor
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Fig. 10. (a) Chemical structures of MA, ThDMA, and TTDMA spacers. (b) Schematic crystal structures of (ThDMA)(MA)n− 1PbnI3n+1 (n = 4, ThDMA-Pb) and(TTDMA)(MA)n− 1PbnI3n+1 (n = 4, TTDMA-Pb). (c) Top-view SEM images of ThDMA-Pb and TTDMA-Pb perovskite films. (d) Dynamic light scattering (DLS) spectra of
ThDMA-Pb and TTDMA-Pb perovskite precursor solution. (e) TA spectra at selected probe times of ThDMA-Pb and TTDMA-Pb perovskite films. (f) Chemical
structures and the optimized molecular conformations for PP, FPP, and FPT. (g) Single crystal structure from the side and top views of (3AMPY)(MA)Pb2I7 and(4AMPY)(MA)Pb2I7 perovskite. (h) Average Pb-I-Pb angles for (xAMPY)(MA)n− 1PbnI3n+1 structures (x = 3 or 4, n = 1 − 4). (i) Comparison for closest NH⋅⋅⋅I distanceof (3AMPY)(MA)Pb2I7 and (4AMPY)(MA)Pb2I7 perovskites. (j) Comparison for closest interlayer distance (left) and closest I⋅⋅⋅I distance (right) of (3AMPY)(MA)Pb2I7
and (4AMPY)(MA)Pb2I7 perovskites. (a-e) Reproduced with permission from Ref. [89]. Copyright 2021, Wiley-VCH. (f) Reproduced with permission from Ref. [117].
Copyright 2021, American Chemical Society. (g-j) Reproduced with permission from Ref. [83]. Copyright 2019, American Chemical Society.

3AMPY and 4AMPY-based perovskite structures, which meant thestructural disorder gradually weakened. Also, differences in the struc­tures affected the optical properties of DJ perovskites. Finally, theperovskite devices based on 3AMPY spacers with n = 3 obtained an
outstanding PCE of 9.2 %.

greatly enhanced stability against humidity, heat and light, owing to theintroduction of organic hydrophobic ammonium ions. Despite the sig­
nificant progress in the PCE of 2D perovskites, it generally ranges only
from 16 % to 20 %, which is considerably lower than the PCE of over 25% achieved by 3D perovskites. The reason is that when large organicammonium ion groups reduce the dimensionality of perovskites fromthree dimensions to two, it also acts as a “Quantum Well”, resulting in aloss in the optoelectronic performance of devices. There are primarily
two strategies to address this issue: (1) Enhancing the carrier transport
capability in the organic spacer cation layers of 2D perovskites. (2)Inducing the growth of 2D perovskites perpendicular to the substrate

4. Conclusion
In recent years, the importance of 2D perovskites in photovoltaic
devices has become increasingly prominent. Compared with traditional
3D perovskites, the advantages of 2D perovskites primarily lie in their17

Materials Today Electronics 8 (2024) 100100

Y. Shen et al.

reduces carrier transport in the organic layers and encourages carriertransport in the inorganic octahedral layers as much as possible. How­ever, both approaches require us to identify suitable organic spacer
cations for implementation. Recent developments in novel spacer cat­ions have mainly focused on aromatic ammonium ions. Compared toaliphatic spacer cations, aromatic spacer cations have a greater advan­
tage in weakening the influence of quantum confinement effect andimproving cross-layer carrier transport ability due to their inherent highdipole moment and aromaticity. The discussion above summarizes thetypes, characteristics, and impacts on 2D perovskite films and devices ofthe major aromatic spacer cations reported so far. The difference amongaromatic spacer cations is mainly reflected in volume, dipole moment,
types and positions of substituents, heterocycles, and conjugate length.
Importantly, we think their impact on the performance of perovskite
devices is principally manifested through influencing the following
characteristics: (a) Quantum barriers: Due to the mismatch in the
dielectric constants between the organic and inorganic layers in 2D
perovskites, the organic layers can be regarded as a quantum barrier,which restricts the transmission of charges between the inorganic layers.
Aromatic spacer cations with high dipole moments have correspond­
ingly larger dielectric constants, which can reduce the impact of thequantum barrier and enhance the interlayer charge transfer capability.
(b) Film quality: Appropriate aromatic spacer cations can induce 2D
perovskite films to grow in the preferred perpendicular orientation to
the substrate, thereby reducing charge transfer between the organic andinorganic layers and enhancing out-of-plane charge transfer capability.
High-quality films with fewer grain boundaries and larger grains canalso enhance the performance of the corresponding devices. (c) Phase
distribution: Phases with different n values typically coexist in 2D
perovskite films. Aromatic spacer cations such as F-PEA and PEA, which
can induce the formation of an orderly arranged n-phase distribution
from low to high, are more beneficial for separating and transporting
excitons. (d) Humidity stability: How well the aromatic spacer ions canrepel water molecules is strongly related to the stability of the 2D pe­
rovskites. (e) Interactions between the organic and inorganic layers:
Different aromatic spacer cations have different levels of interaction
with the inorganic octahedra. Stronger interaction can better stabilize
the perovskite structure. (f) Interactions between the organic layers: For
2D RP-type perovskites, the organic layer contains two layers ofammonium ions. Different ammonium ions can lead to different inter­layer stacking modes of ammonium ions. Generally speaking, face-toface stacking is more beneficial to the performance of perovskites.
Considering the impact of all these aspects is crucial for selecting ordesigning aromatic spacer cations. Overall, suitable aromatic spacer
cations always exhibit the following characteristics: high dielectric
constant, high hydrophobicity, appropriate volume, long conjugatedlength, and ability to strongly interact with the inorganic octahedral
layers and organic spacer layers (only for RP-type) in perovskite lattices.
However, incorporating all these characteristics into a single aromatic
spacer cation is often difficult. Therefore, making appropriate trade-offs
or employing a variety of spacer cations is often necessary.
As we discussed above, the development of aromatic spacer cations
can be mainly divided into four directions: (1) modification of alkyl
chains, (2) replacement of hydrogen atoms on the aromatic ring withspecific substituents, (3) substitution of benzene rings with aromaticheterocycles, and (4) replacement of a monoaromatic ring with multiple
aromatic rings. If we think about it carefully, we can notice that
although changes in spacers structure always lead to multiple effects on
2D perovskites, every direction of modifying or designing spacers
possess its unique advantage and impact. For example, the effect ofmodified alkyl chains of spacers is mainly manifested in changing theinteraction between the spacers and the inorganic octahedral sheets.
That always makes a difference in stability of perovskites. Introducing
specific substituent group on the aromatic ring gives us the freedom toadjust the dipole moment of spacers and different substituent group canlead to different stacking mode of inorganic layers and phases

distribution in 2D perovskites. And multiple aromatic ring spacers havegreater advantages in weakening or even breaking the quantum
confinement effect. It may be possible that the future design of spacer
cations can combine these design directions.
Indeed, designing new aromatic spacers to achieve higher PCE for 2D
perovskite is promising, but there are also huge challenges in design andsynthesis of spacers. However, based on the above discussion, we believethat in the future, the device performance of 2D perovskites could rivalor even surpass that of 3D perovskites.
CRediT authorship contribution statement
Yi Shen: Writing – original draft, Project administration, Formal
analysis, Conceptualization. Siliang Hu: Writing – original draft,
Investigation. You Meng: Formal analysis. SenPo Yip: Writing – review& editing. Johnny C. Ho: Writing – review & editing, Writing – originaldraft, Funding acquisition.
Declaration of competing interest
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
the work reported in this paper.
Data availability
No data was used for the research described in the article.
Acknowledgment
This work was financially supported by the General Research Fund(CityU 11306520) of the Research Grants Council of Hong Kong SAR,
China, and the City University of Hong Kong (project no. 7020088 and
9229138).
Supplementary materials
Supplementary material associated with this article can be found, inthe online version, at doi:10.1016/j.mtelec.2024.100100.
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