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2D Perovskites Based on Halogen-Substituted Spacer Cations in Solar Cells

Chaoyang Wu , Chao Wang , Feifan Chen , Xinhe Dong , Haiying Zheng

Progress in Chemistry ›› 2025, Vol. 37 ›› Issue (4) : 575 -592.

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Progress in Chemistry ›› 2025, Vol. 37 ›› Issue (4) : 575-592 DOI: 10.7536/PC240618
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2D Perovskites Based on Halogen-Substituted Spacer Cations in Solar Cells
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Abstract

Two-dimensional （2D） perovskite materials have been receiving considerable attention owing to their high stability. Despite this，there is still significant potential for improving their power conversion efficiency. Designing effective spacer cations is one of the crucial methods to improve the photoelectric performance of 2D perovskite solar cells. Among the various strategies，halogen substitution has emerged as a particularly effective approach，which can fine-tune the stability and optical properties of the perovskite crystal structure，leading to notable improvements in photoelectric conversion efficiency as well as long-term stability. In recent years，there has been significant and notable progress of two-dimensional （2D） perovskites based on various halogen-substituted spacer cations in the preparation of high-performance perovskite solar cells. This paper initially provides a comprehensive overview of the development status of 2D perovskite materials and devices that employ different spacer cations. Following this，the focus shifts to an in-depth review of the advancements made in the fabrication of 2D perovskite solar cells （PSCs） and the surface modification of three-dimensional （3D） perovskites，specifically emphasizing the role of spacer cations that have been singly or multiply substituted with halogens such as fluorine，chlorine，and bromine. Finally，we present a concise discussion on the current challenges faced in this field and offer insights into the potential future directions for further research and development.

Contents

1 Introduction

2 2D perovskite materials and devices with different spacer cations

3 Characteristics of halogen-substituted spacer cation-based 2D perovskites and their applications in photovoltaic devices

3.1 Research on halogen-substituted spacer cation-based 2D perovskites and photovoltaic devices

3.2 Research on halogen-substituted 2D perovskite surface modification of 3D perovskites

4 Conclusion and future perspectives on halogen-substituted 2D perovskites

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Keywords

perovskite solar cells / 2D perovskite materials / spacer cations / halogen substitution / stability

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Chaoyang Wu, Chao Wang, Feifan Chen, Xinhe Dong, Haiying Zheng. 2D Perovskites Based on Halogen-Substituted Spacer Cations in Solar Cells. Progress in Chemistry, 2025, 37(4): 575-592 DOI:10.7536/PC240618

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1 Introduction

In recent years, perovskite solar cells (PSCs), as a new generation of photovoltaic technology, have attracted significant attention in the field of sustainable energy due to their long carrier diffusion length and lifetime[1-3], excellent charge carrier mobility[4], tunable optical bandgap[5-7], high and stable photoluminescence quantum efficiency[8-10], and low-cost solution-based fabrication processes.
Based on these excellent properties, an increasing number of researchers have devoted themselves to improving the performance of PSCs[11-13], and their power conversion efficiency (PCE) has rapidly increased from the initial 3.8% to over 26%[14-15], making them comparable with traditional silicon-based solar cells. However, the instability of perovskite materials under humidity, temperature, and light prevents their long-term use, severely restricting the commercialization progress of PSCs[16]. Therefore, to enhance the stability of PSCs, researchers have proposed many effective strategies, such as: defect passivation engineering[17-19], interface modification engineering[20-22], and device encapsulation engineering[23-25]. In addition, 2D perovskite materials with unique structures have begun to attract attention. Generally, in the layered structure of low-dimensional or 2D perovskites, organic spacer cations in the form of alkylammonium cations are inserted between inorganic octahedra. Due to the hydrophobicity of alkylamines, 2D perovskites can exhibit superior thermal and humidity stability compared to 3D perovskites[26-28]. Among them, Songyuan Dai from North China Electric Power University and Xu Pan from the Institute of Plasma Physics, Chinese Academy of Sciences, were among the first to propose applying 2D perovskite materials into perovskite solar cells. They successfully fabricated a mixed-dimensional perovskite solar cell with an efficiency of 15.17%, which showed only less than 0.5% efficiency degradation after being stored for over 20 days in an environment with humidity greater than 90%, demonstrating extremely high stability[29-30]. This strategy of introducing 2D perovskites has pointed out a new path for researchers to potentially solve the stability issues of perovskite solar cells. Of course, simply transitioning from a 3D to a 2D phase may also produce adverse effects; for example, as the n value decreases, the optical bandgap and exciton binding energy increase, leading to a decline in PCE[31-33]. At the same time, different organic spacer cations may also affect the properties of 2D perovskites. Therefore, to improve the performance of 2D perovskites, it becomes crucial to rationally control the n value and design appropriate organic spacer cations.
In this review, we first introduced 2D perovskite materials containing different spacer cations, and then focusing on several typical halogen-substituted spacer cations, such as fluorine-, chlorine-, and bromine-substituted spacer cations, we critically discussed the roles and impacts of these halogen-substituted 2D perovskite materials in the preparation of efficient and stable PSCs. In addition, we summarized the current challenges faced by halogen-substituted spacer cation-based 2D perovskite materials and provided an outlook on their future research directions.

2 2D Perovskite Materials and Devices with Different Spacer Cations

According to the structural differences caused by different interlayer cations, 2D perovskites can be divided into the following three types: Ruddlesden-Popper type (RP type), Dion-Jacobson type (DJ type), and alternating cations in the interlayer space type (ACI type) (Figure 1 shows the crystal structures of 3D perovskite and RP, DJ, ACI 2D perovskites). These types of perovskites have different structural characteristics. The RP-type 2D perovskite has two staggered layers of cations, while the DJ-type 2D perovskite contains only one layer of cations. Their general formulas are A2Bn-1MnX3n+1 and A’Bn-1MnX3n+1, respectively, where A and A’ are organic monoamines and diamine cations (such as butylamine and butanediamine), and B is a small-sized organic cation (methylammonium (MA+) or formamidinium (FA+))[34-37]. The ACI-type 2D perovskite differs from the above two types; its general formula is A''BnMnX3n+1, where A'' and B are larger cations (e.g., guanidinium salts (GA+)) and smaller cations (MA+, FA+), respectively, arranged alternately. M represents a divalent metal cation (e.g., Pb2+, Sn2+), X is a halogen anion, and n denotes the number of inorganic framework layers between octahedra[38-39].
In 2D perovskites, different spacer cations exhibit varying regulatory capabilities; therefore, they have been widely studied to enhance the performance of 2D PSCs. Long-chain organic spacer cations such as butylammonium (BA) are among the representatives. As early as 2015, Cao et al. reported a 2D perovskite film (BA)n-1PbnI3n+1 (where n=1, 2, 3, and 4) prepared using BA and conducted detailed characterization of its optical and electronic properties. The results indicated that this material has potential application value in light-emitting diodes and ultimately achieved a PCE of 4.02%[40]. Since then, an increasing number of long-chain organic spacer cations, such as pentylamine[41], hexyl[42], and dodecyl[43], have been used to prepare 2D perovskite materials to improve the performance of PSCs. In addition to long-chain organic spacer cations, cyclic spacer cations have also attracted widespread attention due to their unique properties. First are aromatic spacer cations; for example, Zhang et al. and Fu et al. employed phenethylammonium as the spacer cation and reported the use of NH4SCN and NH4SCN+NH4Cl as additives to enhance the efficiency of 2D PSCs, respectively. The former successfully fabricated vertically oriented, highly crystalline 2D (PEA)2(MA)4Pb5I16 films via a one-step spin-coating method and improved the PSC efficiency from 0.56% to 11.01%[44]. The latter achieved a PCE of 14.1% for devices based on (PEA)2(MA)4Pb5I16 through the synergistic effect of two additives[45]. Additionally, larger-sized phenyl-based organic spacer cations such as phenylbutylammonium (PBA+), α-methylbenzylammonium (MBZA+)[46], benzylammonium (AL+)[47]+)[48] have gradually received increasing attention. Second, heterocyclic compounds containing N and S have entered people's vision due to their more diverse electronic structures and properties. For instance, Lu et al. developed a large-sized spacer cation based on thiophene, 2,5-thiophenedimethylammonium (ThDMA+), and fabricated a 2D (ThDMA)MA4Pb5I16 perovskite film using a one-step method, achieving a PCE of 15.75% along with better stability[49]. Moreover, other N- and S-containing organic spacer cations have been utilized in fabricating 2D PSCs, including 2-thiopheneethylamine (TEA+)[50], 2-thiophenemethylformamidinium (ThFA+)[51], and 4-(aminoethyl)pyridine (4-AEP+)[52]. Finally, to improve the performance of PSCs, some halogens with strong electronegativity and hydrophobicity have also been attempted to be incorporated into organic spacer cations. For example, Xu et al. employed para-fluorophenethylammonium (F-PEA+) and guanidinium salts (GA+) together as organic spacers to enhance the stability of RP-type perovskites, achieving a PCE of 17.50% for devices based on (F-PEA0.8GA0.2)2MA3Pb4I13[53]. Besides fluorine, other halogen elements (such as chlorine and bromine) have also been introduced into organic cations for preparing 2D perovskites (e.g., X-PEA+, X: Cl, Br)[54].
In summary, it can be seen that different spacer cations have distinct mechanisms for enhancing the performance of 2D perovskite materials. Among them, halogen-substituted spacer cation-based 2D perovskites combine the unique properties brought by halogens with the characteristics of different spacer cations, enabling more diverse combinations and gradually demonstrating great potential in improving the performance of PSCs.

3 Characteristics and Photovoltaic Device Applications of Halogen-Substituted Spacer Cation-Based 2D Perovskites

The structure and properties of organic spacer cations have a significant impact on the performance of 2D perovskite materials. Introducing new organic spacer cations is the most fundamental and effective method to further enhance the efficiency of 2D PSCs. Therefore, researchers in the field of 2D PSCs are increasingly focusing on the design and optimization of organic spacer cations. Consequently, numerous novel organic spacer cations have emerged within just a few years, among which halogen-substituted spacer cations have attracted growing attention due to their unique properties. First, halogen atoms possess high electronegativity; they can not only form strong halogen-hydrogen or halogen-carbon bonds, enhancing the hydrophobic properties of the material surface, but also induce electron transfer during chemical bond formation due to differences in electronegativity, thereby altering the local electronic distribution of the material. Second, halogen atoms exhibit high chemical inertness; introducing halogens can reduce reactions between the material and other chemicals, thus improving its chemical stability. Lastly, electron-withdrawing halogens in organic cations help reduce quantum and dielectric confinement effects, making them effective at lowering exciton binding energy[55]. In terms of application, halogen-substituted spacer cations such as para-fluorophenethylamine (4F-PEA)[56], 4-trifluoromethylphenylethylammonium iodide (CF3PEAI)[57], and 2-chloroethyl ethylamine (CEA+)[58] have already been widely applied. These organic cations, benefiting from the unique properties of halogens, demonstrate excellent performance in preparing highly efficient and stable 2D perovskite materials and enhancing the stability of 3D PSCs. They are gradually becoming one of the ideal materials for improving solar cell efficiency and extending service life. Additionally, halogen-substituted spacer cations have shown great potential in applications such as light-emitting diodes[59] and photodetectors[60].
In conclusion, halogen-substituted organic spacer cations play a significant role in the design and optimization of 2D perovskite materials through their unique properties, providing new possibilities for improving the performance of optoelectronic conversion devices and expanding application fields.

3.1 Study on Halogen-Substituted Spacer Cation-Based 2D Perovskites and Photovoltaic Devices

Low-dimensional perovskite materials, such as two-dimensional (2D) and quasi-two-dimensional (Q-2D) perovskites, have attracted significant attention due to their excellent stability. Although they still lag behind 3D perovskites in terms of PCE, they are considered one of the promising alternatives to 3D perovskites owing to their high stability and unique physical properties. To address the issue of low efficiency in 2D perovskite devices, various novel spacer cations have been designed and developed. Among them, halogen-substituted spacer cations have gradually emerged due to their outstanding performance. For example, fluorophenethylammonium (F-PEA)[61], para-fluorophenylformamidinium (p-FPhFA)[62], and 3-bromobenzylammonium iodide (3BBAI)[63] can all enable 2D perovskites to exhibit superior optoelectronic properties. By introducing these halogen-substituted spacer cations, the performance of 2D perovskite materials has been significantly enhanced, providing new directions and opportunities for further optimization and application of 2D perovskite materials.

3.1.1 2D Perovskites with Monofluoro-Substituted Phenethylammonium Groups

Among various halogen-substituted spacer cations, fluorine substitution has been most extensively studied, and the development of fluorine-substituted spacer cations has gradually started from fluorinated phenethylamine (Table 1 summarizes the performance and stability of 2D PSCs based on monofluorinated phenethylamine). In 2014, Smith et al. first reported the layered perovskite material (PEA)2(MA)2Pb3I10, but the device efficiency at that time was only 4.73%[64]. To improve the efficiency of 2D perovskite devices, various approaches have been adopted, such as additive engineering and large cation engineering. However, the effects of these methods remain unsatisfactory. Therefore, to fundamentally address the issue of low efficiency in 2D perovskite devices, it is essential to start with the properties of the inserted spacer cations.
Due to its high dipole moment and strong hydrophobic properties, fluorine has been most extensively studied among halogen substitutions for various spacer cations. PEA, as the most commonly investigated large-size cation, has been thoroughly studied in terms of fluorination. Hu et al. investigated the influence of chemical properties of organic cations on the performance of 2D perovskites and devices. It was found that selective mono-fluorination at different positions of the benzene ring in PEA (oF1PEA, mF1PEA, and pF1PEA) significantly affects the performance of 2D PSCs (n=4). Among them, the 2D perovskite based on pF1PEA exhibited the highest efficiency of over 10%, followed by mF1PEA (over 10%), PEA (over 7%), and oF1PEA (less than 1%). This study provided valuable insights into designing more complex fluorinated phenethylammonium spacer cations and interactions between organic components. On this basis, Shi et al. conducted further in-depth research on para-fluorinated phenethylamine (F-PEA). They utilized F-PEA as a spacer cation to fabricate quasi-2D (4FPEA)(sub)2(/sub)(MA)(sub)4(/sub)Pb(sub)5(/sub)I(sub)16(/sub) films (Figure 2b), achieving an efficiency of 17.3% along with high stability. The study also demonstrated that fluorination plays a crucial role in morphology, defect density, mixed-phase distribution, and charge dissociation dynamics of 2D perovskite thin films, thereby enhancing device performance. Wang et al. conducted extensive studies on the effects of halogen substitution on crystal orientation and multiphase distribution in PEA-based perovskite films. By substituting different halogens at the para-position of the benzene ring in PEA (X-PEA(sup)+(/sup), X=F, Cl, Br), they revealed the regulatory mechanism of halogen-substituted spacers and fabrication processes on crystal orientation and n-phase distribution of quasi-2D perovskites (Figure 2c). Subsequently, they fabricated quasi-2D (X-PEA)(sub)2(/sub)MA(sub)3(/sub)Pb(sub)4(/sub)I(sub)12(/sub) films, where the 2D perovskite based on F-PEA(sup)+(/sup) achieved a PCE of 18.10%, significantly higher than PEA(sup)+(/sup) (12.23%), Cl-PEA(sup)+(/sup) (7.9%), and Br-PEA(sup)+(/sup) (6.08%). This study offered a simple approach to enhance PSC performance by adjusting the molecular structure of organic spacers. Meanwhile, Wei et al. and Shao et al. proposed another method to further improve the performance of F-PEA-based 2D perovskites, which involved using FA(sup)+(/sup) instead of MA(sup)+(/sup). The former demonstrated that FA(sup)+(/sup) plays an important role in promoting vertical crystal growth and improving film microstructure, fabricating quasi-2D perovskite film (FPEA)(sub)2(/sub)(FA)(sub)8(/sub)Pb(sub)9(/sub)I(sub)28(/sub) with excellent moisture and thermal stability, achieving a PCE of 16.15%, bringing hope for commercial application of 2D perovskite materials. The latter experimentally proved that FA(sup)+(/sup)-based 2D RP perovskites possess unique phase distribution characteristics, where a large amount of 3D-like phases are embedded within low-n phases, forming a conductive network throughout the entire film, thus greatly improving charge transport (Figure 2d). With assistance from two additives, MACl and PbCl(sub)2(/sub), they prepared high-quality (4F-PEA)(sub)2(/sub)(FA)(sub)4(/sub)Pb(sub)5(/sub)I(sub)16(/sub) films, achieving an ultra-high efficiency of 21.07% while exhibiting better thermal and environmental stability. This study provides broad prospects for fabricating highly efficient and stable RP PSCs. In addition, Zhang et al. proposed another method to improve the relatively poor photovoltaic efficiency of PEA-based 2D PSCs. By fluorinating hydrogen atoms on the β-carbon atom of PEA, they synthesized a new type of spacer cation, β-fluorophenethylamine (β-FPEA). This novel spacer cation improved crystallinity, phase distribution, and preferred orientation of 2D RP perovskite films, leading to enhanced stability (Figure 2e). Similar to previous studies, they also used FA(sup)+(/sup) instead of MA(sup)+(/sup), preparing (β-FPEA)(sub)2(/sub)(FA)(sub)4(/sub)Pb(sub)5(/sub)I(sub)16(/sub) films, achieving a PCE of 19.11% in a PTAA-based device architecture. This work presents a feasible method for fabricating highly efficient and stable 2D RP PSCs. Apart from the above studies, Liang et al. addressed the issue of strong n=1, 2 signals observed when exciting PEA- and F-PEA-based 2D perovskites from the bottom of the perovskite film by preparing layered nickel oxide substrates (lamellar-NiO(sub)x(/sub)) as hole transport layers via hydrothermal methods. By creating vertical charge transport pathways at the bottom of the perovskite film, they bypassed charge trapping and recombination centers present in low-n regions of PEA- and F-PEA-based 2D perovskites, effectively overcoming limitations related to carrier transport near the bottom of the films. They demonstrated that lamellar NiOx is an ideal substrate material, achieving a PCE of 15.2% based on (FPEA)(sub)2(/sub)(MA)(sub)3(/sub)Pb(sub)4(/sub)I(sub)13(/sub) films.

3.1.2 Other Spacer Cation-Based 2D Perovskites with Monofluoro Substitution

In addition to the mono-fluorinated PEA-based spacer cations, other spacer cation skeletons are also being actively explored as mono-fluorinated alternatives (Table 2 summarizes the performance and stability of 2D PSCs based on other mono-fluorinated spacer cations). Among them, introducing fluorinated aromatic alkyl ammonium spacer cations is considered an effective strategy for enhancing the performance of quasi-2D PSCs [71]. Therefore, many researchers have started focusing on spacer cations that meet these criteria and contain similar functional groups, such as benzylamine, phenethylamine, and phenylformamidinium. To reduce lead's environmental toxicity, Park et al. adopted tin-based perovskites instead of lead-based ones and utilized large-sized spacer cations, namely distyryl derivative ethylammonium iodide (FSAI, 2-(4-(3-fluoro)distyryl)ethylammonium iodide), to fabricate tin-based 2D perovskite films based on [(FSA)₂(MA)(ₙ₋₁)SnₙI(₃ₙ₊₁)] (n=1, 2). They demonstrated that fluorination could extend the stability of tin-based perovskites [60]. Li et al. developed a class of aromatic formamidinium (ArFA) spacers, specifically phenylformamidinium (PhFA) and p-fluorophenylformamidinium (p-FPhFA) (Figure 3a), systematically investigating the influence of fluorine in the organic spacer layer on the performance of 2D RP perovskites. They emphasized the potential of fluorination in fabricating high-performance PSCs and pointed out that the strong multiple hydrogen bonding interactions between I⁻ in the [PbX₆]⁴⁻ octahedral layers and hydrogen atoms in the organic spacers played a major role in enhancing stability. The 2D perovskite device based on p-FPhFA, (p-FPhFA)₂MA(ₙ₋₁)PbₙI(3ₙ₊₁₋ₓ)Clₓ (n=5), achieved a PCE of 17.37%. This study provided a feasible approach for designing novel FA derivative-based spacers for fabricating 2D RP PSCs [62].
Lai et al. employed meta-fluorinated benzylammonium iodide (3FBAI) as the spacer cation to prepare quasi-2D PSCs. Unlike conventional preparation of other low-dimensional perovskites usually requiring inert gas-filled gloveboxes, this film fabrication exhibited more advantages and could be conducted in a humidity-controlled air environment. They prepared the perovskite films with a molar ratio of 3FBAI:methylammonium chloride (MACl):PbI2=1.6:2.5:3, achieving a vertically oriented, nearly pinhole-free high-quality film through volume effects, which led to a PCE of 20.12%. This work laid the foundation for fabricating high-quality perovskite films using simple processes and enhanced the attractiveness of low-cost industrialization of PSCs in the future[72]. Wang and Shi et al. both used para-fluorobenzylamine (F-BZA) as the spacer cation to fabricate 2D PSCs. The former conducted an in-depth investigation into how organic cation doping influenced the crystallization and intermolecular interactions of perovskites (Figure 3b). Through comprehensive studies on crystallization, carrier dynamics, and device performance, they elucidated the mechanism by which fluorinated benzylamine improves charge transport in Q-2D RP perovskites, achieving a PCE of 16.82% and demonstrating negligible hysteresis and excellent stability under ambient conditions[73]. The latter proved theoretically and experimentally that the large dipole moment and electron-inductive effect of 4-fluoroaniline (FPA+) improved the hydrogen bonding between H and I atoms (Figure 3c). Devices based on (FPA)2Cs4Pb5I16 achieved a PCE of 15.18%, providing deeper insights into fluorine-substituted organic cations and enriching our understanding of this field[74]. Additionally, Lehner et al. investigated meta-fluorinated benzylamine as an organic spacer cation and employed MACl as an additive to assist in forming (3FBA)2(MA)6Pb7I22 films. By combining macroscopic, microscopic, and spectroscopic studies, they successfully clarified how the MACl additive influences nucleation and crystal growth processes (Figure 3d), showing that MACl additives induce vertical alignment of perovskite crystals. They achieved a PCE of 18.5% along with significantly enhanced moisture stability. Their study provides a basis for designing rational additives to achieve highly oriented perovskite films in future research[75].
From the above research, it can be seen that fluorinated benzylamine as a spacer cation exhibits superior performance compared to its non-fluorinated counterpart. However, the specific effects of fluorination at different positions of benzylamine on 2D perovskites are not yet fully understood. Therefore, researchers have conducted detailed comparative studies on the impact of fluorination at various positions of benzylamine in 2D perovskites. Yan et al. utilized X-fluorobenzylammonium (X-FBA, where X denotes different substitution positions: o-ortho, m-meta, and p-para) as a spacer cation. After systematically evaluating the crystal orientation, morphology, and device performance of the resulting films, they found that pFBA exhibited a larger dipole moment and lower formation energy compared with BA, oFBA, and mFBA (Figure 3e). In the film, the low-n phase of pFBA was distributed at the surface, while the large-n phase was located at the bottom, effectively improving the interfacial contact between the hole transport layer and the perovskite film, as well as enhancing the hydrophobicity of the top film. The 2D perovskite device based on (pFBA)2MA4Pb5I16 achieved a PCE of 17.12%, significantly higher than those based on BA (14.07%), oFBA (12.89%), and mFBA (14.67%). This study further highlights the importance of structural design of spacer cations in constructing highly efficient and stable 2D PSCs[71].

3.1.3 Multifluoro-Substituted Spacer Cation-Based 2D Perovskites

Compared to mono-fluorine substitution in the spacer cations, multi-fluorine substitution, by increasing the number of fluorine substituents, can not only modulate the electronic structure and optical properties of 2D perovskites to a greater extent, but also significantly enhance their stability. Therefore, multi-fluorine substitution may bring unexpected improvements to 2D perovskite solar cells compared to mono-fluorine substitution (see Table 3 for the performance and stability of 2D PSCs based on multi-fluorine substituted spacer cations).
Wang and Lv et al. both employed 2,3,5,6-tetrafluoro-1,4-phenylenedimethylammonium (TFBDA or 4F-PhDMA) as the spacer cation. The former demonstrated that the hydrophobicity of fluorine atoms and stronger interactions between the fluorinated organic spacers and inorganic framework can improve device stability. A film based on (TFBDA)MA9Pb10I31 achieved a PCE of 15.24%, significantly higher than that of the non-fluorinated BDA (9.16%)[76]. The latter primarily investigated the multiple covalent interactions of cation incorporation into 2D perovskites, theoretically proving that besides NH···I hydrogen bonding interactions, CH···F and F···I interactions also exist in the (4F-PhDMA)(MA)3Pb4I13 film (Figure 4a), with these interactions exhibiting high dissociation energies. Based on this finding, their fabricated device showed a PCE of 16.62%. This study further confirmed the importance of various non-covalent interactions for achieving high-performance DJ PSCs[77]. In addition to the aforementioned polyfluorinated spacer cations, researchers have developed other fluorinated spacer cations. To enhance carrier separation and transport in low-dimensional PSCs, Tan et al. combined 2,2,2-trifluoroethylamine (F3EA+) with butylammonium (BA+) as hybrid spacer cations. Experimental results indicated that the high-dipole-moment fluorinated cations can improve carrier dissociation through polarization of the organic spacers and reduce carrier recombination (Figure 4b), enabling devices based on [(BA)0.94(F3EA)0.06]2(MA)3Pb4I13 to achieve improved fill factor and open-circuit voltage, resulting in a PCE of 12.51%[78]. Chen et al. developed new spacers, phenylthienylmethylammonium iodide (PTMAI) and pentafluorophenylthienylmethylammonium iodide (5FPTMAI), demonstrating that the assembly of spacers can effectively regulate perovskite nucleation and crystallization processes (Figure 4c). Devices based on ((5FPTMA)0.1(PTMA)0.9)2MA4Pb5I16 achieved a PCE of 18.56%. This study proposed an effective strategy for controlling the growth of 2D perovskite films[79]. Zhang et al. developed two heterocyclic azetidinium secondary ammonium spacers with different electron-withdrawing groups: 3-hydroxyazetidine (3OHAz) and 3,3-difluoroazetidine (3,3-DFAz). Due to stronger dipole-dipole and hydrogen bonding interactions between the 3,3-DFAz spacer and [PbX6]4- octahedra, compared to devices based on 3OHAz, (3,3-DFAz)2MAn-1PbnI3n+1-xClx (n=4) 2D perovskite devices exhibited higher PCE (19.28%). When MA0.95FA0.05 was used as the mixed A-site cation, the PCE further increased to 19.85%. This work provided a feasible method for tuning energy level alignment in 2D RP PSCs (Figure 4d)[80].

3.1.4 2D Perovskites with Other Halogen-Substituted Spacer Cations

In addition to fluorine-substituted spacer cations, researchers are also designing and developing halogen-substituted spacer cations such as chlorine and bromine to enhance the performance of 2D perovskite materials. The introduction of halogens like chlorine and bromine can bring more chemical tunability and changes in physical properties to 2D perovskite materials. Through careful design, researchers have successfully prepared a series of 2D perovskite materials based on chlorine- and bromine-substituted spacer cations with excellent optoelectronic properties, providing new possibilities for the practical application of 2D perovskite solar cells. For example, Yang et al. used 3-bromobenzylammonium iodide (3BBAI) as an organic spacer cation to fabricate a 2D perovskite device with an optimal power conversion efficiency (PCE) of 18.20% based on a specific molar ratio (3BBAI:MACl:PbI2=2:2:3). The device exhibited almost no change in photovoltaic parameters after immersion in water for 60 seconds, demonstrating its advantages in stability. Additionally, due to suppressed non-radiative recombination, this material also shows potential applications in light-emitting diodes (Fig. 4e)[63]. Li et al. first employed a novel organic spacer cation, 4-chlorophenylmethylamidinium (CPFA+), to prepare MA-based 2D PSCs based on CPFA2MA8Pb9(I0.857Cl0.143)28. By incorporating MACl, they facilitated the growth and orientation of the perovskite film, assisting in the formation of high-quality films. This novel structure improved crystal orientation, reduced trap state density, extended carrier lifetime, and achieved better energy level alignment. Ultimately, this 2D PSC device achieved a PCE of 14.78%, and exhibited higher environmental stability compared to 3D MAPbI3[81]. Furthermore, Liu et al. adopted a strategy involving the incorporation of halogenated secondary spacer cations by introducing 4-fluorobenzylammonium (FBA) and 4-chlorobenzylammonium (CBA) as secondary spacer cations into an ethanolamine (HEA)-based 2D perovskite. Results showed that halogenated aromatic cations possess excellent passivation effects, significantly enhancing film performance (Fig. 4f). Devices based on (HEA0.9CBA0.1)2(Cs0.1FA0.9)8Pb9(I0.95Br0.05)28 achieved a PCE of 18.75%[82]. Moreover, Cheng et al. chemically modulated the spacer cations by selecting para-halogen substituted benzylammonium iodides (3X-BAI, X=F, Cl, Br, I) as spacer cations, proving that different halogens can significantly affect the orientation of low-dimensional perovskites, charge transport from the perovskite to the hole extraction layer, and device performance. The 2D perovskite phases based on 3F-BAI and 3I-BAI exhibited random orientations, whereas the other two displayed vertical growth perpendicular to the substrate. Finally, quasi-2D PSCs based on the 3Br-BAI film (n=3) exhibited a PCE of 13.21%, significantly higher than those based on 3F-BAI (9.49%), 3Cl-BAI (12.09%), and 3I-BAI (9.21%)[83].
With continuous emphasis on the design and optimization of spacer cations, halogen-substituted spacer cation-based 2D PSCs have made significant progress. These new materials exhibit significantly superior stability compared to traditional 3D PSCs; however, how to achieve efficiency comparable to that of 3D perovskites while maintaining high stability remains a key research focus for these 2D PSCs. Therefore, researchers are actively working to explore new material design strategies and process optimization methods. Additionally, mechanistic studies on the influence of halogens on the performance of 2D perovskites are also essential.

3.2 Study on Surface Modification of 3D Perovskites by Halogen-Substituted 2D Perovskites

2D perovskites exhibit high stability, while 3D perovskites demonstrate significant advantages in photovoltaic performance. To fully utilize the benefits of both materials, researchers have recently begun exploring the integration of 2D and 3D perovskites to form 2D/3D heterojunctions. This type of heterojunction can combine the high stability of 2D perovskites with the high efficiency of 3D perovskites. In this research direction, surface modification techniques play a crucial role in constructing efficient 2D/3D heterojunctions. Particularly, when halogen-substituted 2D perovskites are used as surface modification layers, their unique chemical properties can effectively regulate charge transport at the interface. For instance, monofluorinated phenethylammonium-based 2D perovskites and polyfluorinated alternative spacer cation-based 2D perovskites can both enhance carrier separation efficiency and suppress undesirable charge recombination by controlling their surface modification process on 3D perovskites, thereby significantly improving the overall power conversion efficiency (PCE) of the device. This strategy of 2D/3D heterojunctions not only combines the respective advantages of 2D and 3D perovskites but also provides new possibilities for the practical application of perovskite materials in optoelectronic devices.

3.2.1 Monofluoro-Substituted Spaced Cation-Based 2D Perovskites for Surface Modification

Compared to other substitution strategies, mono-fluorinated spacer cation-based 2D perovskites exhibit significant advantages as surface modification materials. Firstly, due to the strong electronegativity of fluorine compared to other elements, surface modification can significantly enhance environmental stability and prolong the service life of 3D perovskites. Secondly, mono-fluorinated spacer cations can effectively modify the surface of perovskite materials and passivate acceptor defects, thereby improving device efficiency[84]. Finally, compared to other complex substitution strategies, the synthesis process of mono-fluorinated spacer cations is relatively simple, which helps reduce costs, simplify production processes, and facilitate practical application. In summary, mono-fluorinated spacer cation-based 2D perovskite surface modification offers many advantages and has therefore attracted considerable attention in practical research (Table 4 summarizes the performance and stability of PSCs based on mono-fluorinated spacer cation-based 2D perovskites used for surface modification).
Lee et al. tailored the dimensionality and surface morphology of the perovskite film by employing 4-fluorophenethylammonium iodide (FPEAI), forming a 2D (FPEA)(sub)2(/sub)PbI(sub)4(/sub) perovskite thin film on the 3D surface, thereby inducing a novel grain boundary passivation effect (Fig. 5a), suppressing non-radiative charge recombination, reducing surface defects and lowering trap density. They achieved power conversion efficiencies (PCEs) of 20.53% and 16.82% on devices with areas of 0.09 cm(sup)2(/sup) and 2.00 cm(sup)2(/sup), respectively, suggesting that FPEAI could be a simple and efficient method to enhance the performance of PSCs [85]. Li et al. introduced a small amount of the organic cation FPEA(sup)+(/sup) onto the surface of (MAPbI(sub)3(/sub))(sub)0.75(/sub)(FASnI(sub)3(/sub))(sub)0.25(/sub), forming low-italic n (/italic)-value (1 ≤ italic n (/italic) ≤ 4) layered 2D perovskites that induced Pb-Sn perovskite crystals to grow vertically aligned with respect to the substrate. At the same time, they observed for the first time that phase separation in the original 3D Pb-Sn perovskite was suppressed due to the presence of the 2D perovskite. Ultimately, they achieved a PCE of 17.51% along with good stability [86]. Similarly, Jiang and Xiong et al. both utilized para-fluorophenethylammonium as a 2D capping layer to optimize the performance of PSCs. Jiang et al. introduced 4-fluorophenethylammonium iodide (FPEAI) onto the surface of the 3D perovskite (FAPbI(sub)3(/sub))(sub)0.85(/sub)(MAPbBr(sub)3(/sub))(sub)0.15(/sub) to form a 2D perovskite layer, which provided additional supramolecular interactions, facilitated interfacial hole transport, improved defect density, and resulted in a PCE of 20.5% alongside excellent stability [87]. Xiong et al. annealed para-fluorophenethylammonium acetate (FPEAAc) on the surface of the 3D perovskite (FAPbI(sub)3(/sub))(sub)0.95(/sub)(MAPbBr(sub)3(/sub))(sub)0.05(/sub), allowing it to melt and uniformly cover the 3D perovskite; the organic solvent anchored onto the 3D perovskite ensured the formation of a 2D perovskite. This approach successfully passivated the 3D perovskite surface (Fig. 5b), ultimately achieving a PCE of 22.53% and enhanced stability [88]. Additionally, Mohammed et al. used formamidinium bromide (FABr) as an additive introduced into the 2D perovskite precursor, forming a 2D perovskite layer containing 4FPEAI spacing cations, thus obtaining a 3D/2D heterostructure layer. This structure significantly passivated grain boundaries and suppressed charge recombination between the perovskite and the hole transport layer, enhancing the charge transport process in PSCs. When the molar ratio of FABr was 5%, a PCE of 21.79% and improved stability were achieved, highlighting the importance of 2D perovskite interface modification for high-performance PSCs [89]. Kareem et al. proposed a two-step engineering approach. Firstly, propionic acid (PA) was introduced as an acidic additive into the 3D solution as a crystallization promoter; subsequently, the 3D perovskite device was post-treated using an FPEAI solution doped with nitrosonium tetrafluoroborate (NOBF(sub)4(/sub)), forming a 2D/3D heterostructure. NOBF(sub)4(/sub) accelerated the charge transport process between the perovskite and the hole transport layer, ultimately achieving a PCE of 23.18% and outstanding stability [90].

3.2.2 Multifluoro-Substituted Spacer Cation-Based 2D Perovskites for Surface Modification

Compared to monofluorinated spacer cations, polyfluorinated spacer cations also offer certain advantages in the field of surface modification. Firstly, introducing more fluorine atoms can enhance the chemical bond strength between the perovskite and other atoms, effectively preventing erosion from moisture and other harmful factors in the external environment. Secondly, by adjusting the substitution position and number of fluorine atoms, precise control over critical properties such as light absorption, charge separation, and transport of the perovskite material can be achieved. Furthermore, polyfluorination helps improve the interfacial energy of the perovskite surface, offering greater potential for interface engineering. Therefore, polyfluorinated spacer cation-based 2D perovskites demonstrate excellent application potential in the field of surface modification (Performance and stability of PSCs using polyfluorinated spacer cation-based 2D perovskites for surface modification are summarized in Table·5).
Xu et al. used 2,2,2-trifluoroethylamine (FEA) to passivate the perovskite surface, forming (FEA)2PbI4. Due to the electronegativity of fluorine, this approach achieved effective passivation, reducing interfacial defects and suppressing non-radiative recombination, leading to a power conversion efficiency (PCE) of 19.24% for films based on (FAPbI3)0.85(MAPbBr3)0.15[91]. Liu et al. instead deposited pentafluorophenethylammonium (FEA) onto the surface of a 3D film, forming a (FEA)2PbI4 structure. The formation of this 2D phase consumed the FAPbI3 phase at the film surface, demonstrating the novelty of this method. The fully fluorinated molecule provided excellent stability to the perovskite film, suppressed interlayer ion migration, and facilitated hole extraction, ultimately achieving a PCE of 22.1%[92]. Similarly, in their study on printable hole-conductor-free PSCs, Chen et al. also treated the perovskite with pentafluorophenethylammonium (F5PEAI). They found that F5PEAI could spontaneously penetrate into the MAPbI3 surface, forming a low-dimensional phase (Figure 5c). This enabled hole-conductor-free triple-cation PSCs based on MAPbI3 to achieve a PCE of 17.47%[93]. Compared to the direct use of pentafluorophenethylammonium for passivation mentioned above, Ye et al. proposed a new strategy by partially replacing phenylethylammonium (PEAI) with the larger-sized pentafluorophenethylammonium (F5PEAI) as a 2D passivation layer. This created strong non-covalent interactions between F5PEAI and PEAI, resulting in longer carrier lifetimes, lower exciton binding energy, reduced trap density, and higher conductivity in the perovskite film (Figure 5d). Ultimately, PSCs based on a wide bandgap (1.68 eV) achieved a PCE of 21.10%[94]. In addition to studies on phenethylamine fluorination, compounds with similar properties such as benzylamine and methylphenethylamine have also attracted attention. For example, Paek et al. designed pentafluorobenzylammonium iodide (5FBzAI), a highly hydrophobic cation capable of forming a (5FBzAI)2PbI4 overlayer on the surface of the perovskite film. While providing interfacial passivation, it reduced charge recombination and enhanced device stability, ultimately achieving a PCE of 21.65% (Figure 5e)[95]. Xia et al. introduced 3-(trifluoromethyl)phenethylammonium iodide (CF3PEAI) to the perovskite surface and found that this passivating agent could deeply penetrate into the film (>30 nm) while forming a 2D perovskite, further passivating internal defects, improving long-term stability, and ultimately achieving a PCE of 21.05%[96]. Additionally, Byeon et al. suggested that besides the type of functional group connected to the spacer molecule, conformational changes in the fluorine isomers on layered perovskites may also affect optoelectronic performance. In their study, using meta-CF3- and para-CF3-phenethylammonium iodide (PEAI) spacers formed a 2D phase (n=1) passivation layer, and it was found that the meta-CF3 passivation layer showed better performance, effectively suppressing non-radiative recombination and enhancing carrier lifetime, achieving a PCE of 22.4%, proving that the position of fluorinated groups can further enhance device stability[97]. Regarding whether more fluorination is always better, Wang et al. provided some answers by investigating how the degree of fluorination in large aromatic cations affects the performance of resulting PSCs. In this study, (FxPEA)2PbI4 (x=1, 2, 3, 5) 2D perovskites were in situ grown on the surface of 3D perovskites to investigate the effects of different degrees of fluorination on PSC performance. Based on single-crystallography results and density functional theory (DFT) calculations, it was found that increased fluorination could influence dipole moments and formation energies. Appropriate dipole moments and formation energies can promote efficient separation of photogenerated carriers, thereby improving device performance. Specifically, experimental results showed that the surface modification based on F3PEAI spacer cations achieved a PCE of 22.74%, higher than those based on F1PEAI (21.09%), F2PEAI (22.06%), and F5PEAI (21.86%), indicating that appropriate fluorination can better enhance PSC performance[98].

3.2.3 2D Perovskites with Other Halogen-Substituted Spacer Cations for Surface Modification

In addition to fluorine-substituted spacer cation-based 2D perovskate surface modifications, spacer cations substituted with other halogens such as chlorine and bromine have also shown significant potential in the fabrication of 2D/3D perovskite devices (Performance and stability of PSCs using 2D perovskites based on spacer cations substituted with other halogens for surface modification are summarized in Table 6). Therefore, surface modification using 2D perovskites based on spacer cations substituted with other halogens has begun to attract increasing attention. Through a series of design and optimization efforts, this approach offers the possibility of achieving more efficient and stable 2D/3D PSCs.
Liu et al. designed four different spacer cations (phenethylamine (BA), para-fluorophenethylamine (FBA), para-chlorophenethylamine (CBA), and para-bromophenethylamine (BBA)) to form a low-dimensional passivation layer (LDP) on the surface of 3D perovskite Cs0.05FA0.95PbI2.7Br0.3 (Figure 6a). With the buffering capacity and hydrophobicity provided by LDP, a PCE of 21.13% and significantly enhanced stability were ultimately achieved[99]. Karabag et al. deposited a series of synthesized halogenated PEA+ iodide salts (x-XPEAI, where x: ortho (o), meta (m), para (p), and X: F, Cl, Br) onto the surface of 3D perovskite (Figure 6b). The formation of 2D perovskite was confirmed through X-ray diffraction (XRD) and grazing-incidence wide-angle X-ray scattering (GIWAXS). Density functional theory (DFT) analysis revealed that meta-substitution exhibited lower formation energy and higher interfacial dipole moments compared to ortho- and para-substitutions, thereby enhancing device performance. Ultimately, devices treated with m-BrPEAI demonstrated a maximum PCE of 23.42%. However, considering stability, average efficiency, and reproducibility comprehensively, devices treated with m-ClPEAI showed the best overall performance[100]. Ma et al. employed molecular engineering to design four organic cations with π-conjugated structures (4Tm+, F4Tm+, Cl4Tm+, Br4Tm+) and utilized these cations to construct stable 2D perovskite films on the surface of 3D perovskite (Figure 6c). They demonstrated that fine-tuned conjugated ligands could form more stable 2D structures and suppress ion migration, thus improving film performance. As a result, PSCs treated with Cl4Tml exhibited a PCE of 24.63% along with higher humidity and thermal stability[101]. Beyond rigid thin-film cells, Dai et al. proposed a method to enhance the performance of flexible PSCs. They grew self-assembled monolayer molecules on the electron transport layer (ETL) and used 3-chlorobenzylammonium (3-CBA) to grow low-dimensional perovskite films on the hole transport layer (HTL). Through the synergy of these two strategies, they achieved a PCE of 21.03% and improved operational stability (Figure 6d)[102].
From the above literature reports, it can be seen that halogen-substituted spacer cations have shown significant advantages in the surface modification of 3D perovskites, including excellent environmental stability and tunability, making them highly promising for the fabrication of efficient and stable 2D/3D PSCs. However, further in-depth investigation into the mechanism of how halogen substitution affects the properties of 2D perovskite materials, and the synergistic enhancement of photovoltaic performance and stability of 2D/3D PSCs based on halogen-substituted spacer cations through various modulation strategies, are still highly necessary.

4 Conclusion and Prospect

In recent years, with an in-depth understanding of the stability issues of 3D perovskites, 2D perovskites have become an important part of PSC research due to their unique layered structure and excellent stability. Their applications in PSCs mainly fall into the following three categories: (1) Direct fabrication of 2D PSCs, where the long-chain ammonium salts in the 2D perovskite structure provide better stability to the device and slow down the degradation rate, but the organic layers in the layered structure also limit the transport of electrons and holes within the material, resulting in lower PCE. (2) Coating a 2D perovskite layer onto the surface of a 3D perovskite, which can enhance the device stability and interfacial passivation effect compared to 3D perovskite devices, thereby reducing interface recombination. However, precise control over the thickness of the 2D perovskite layer is required; if the 2D modification layer is too thick, it may hinder charge transport. Additionally, since the introduced 2D perovskite component exists only on the surface, its improvement in stability is limited. (3) Adding n=1 2D perovskite crystals into the precursor solution of the 3D perovskite, during which the 2D layers typically form at the bottom of the 3D structure, creating a 3D/2D perovskite heterostructure. This method allows for better bandgap alignment, facilitating charge transport; however, the recrystallization growth of the dissolved 2D perovskite crystals becomes uncontrollable, thus affecting the reproducibility of device performance. With continuous exploration of their application methods and processes, more approaches are being developed to fabricate and utilize different 2D perovskite materials to improve the performance of PSCs. Among these various 2D perovskite materials, halogen-substituted spacer cation-based 2D perovskites have successfully fabricated devices with higher efficiency and improved stability due to the tunability of halogens on the material's electronic structure and optical properties, as well as their inherent hydrophobicity, making them worthy of further investigation. Nevertheless, compared to 3D PSCs, halogen-substituted spacer cation-based 2D PSCs still exhibit significant gaps in efficiency, indicating that many challenges and unresolved issues remain in transitioning this technology from laboratory research to practical applications.
First, although halogen substitution has been proven to be an effective way to modulate the performance of 2D perovskites, its mechanism and influence still require further investigation. For example, the impact mechanisms of single and multiple substitutions with different halogen atoms on the optical properties, stability, and charge transport of materials are not yet fully understood and need to be revealed through experimental and theoretical calculations. At the same time, whether introducing varying numbers of halogen atoms would lead to defects that affect the material structure also needs further study and understanding in order to achieve precise control over material performance. Second, the current preparation methods and processes are not yet mature and controllable enough. Although the most commonly used solution method is simple and feasible, it is difficult to ensure consistency in material quality due to excessive control conditions. While many processes have been developed nowadays, none of them fully meet industrial requirements; therefore, process parameters need continuous optimization and costs reduced to rapidly adapt to commercial demands. Finally, although 2D perovskite materials based on halogen-substituted spacer cations exhibit good long-term stability in PSCs owing to the unique properties of halogens and the 2D structure, there remains a significant gap compared to other commercial solar cells. Therefore, continuous innovation and attempts are needed—for instance, selecting spacer cations with different numbers and types of halogen substitutions—to prepare 2D perovskite materials with stronger stability that can meet practical application requirements for reliability and durability.
In conclusion, 2D perovskites with halogen-substituted spacer cations have significant potential and room for development, but further in-depth studies are still required to achieve technological breakthroughs. It is believed that in the future, through comprehensive research involving theoretical simulations, material design, and optimization of fabrication processes, precise control and optimization of material properties can be realized, thereby promoting their widespread application in optoelectronic fields and making greater contributions to the development of PSCs.

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Funding

National Natural Science Foundation of China(52102196)

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