The role of organic spacers in 2D/3D hybrid perovskite solar cells

Abstract

The long-term stability of three-dimensional (3D) perovskite solar cells (PSCs) has become the major challenge for their commercial applications. To solve this problem, the development of 2D/3D hybrid perovskites by incorporating bulky organic spacers into 3D perovskite films has been proposed as an efficient strategy to combine the high efficiency of 3D PSCs and high stability of 2D PSCs. In this review, we present the structure of 3D and 2D perovskite materials, discuss the transformation process from 3D to 2D perovskite, and summarize the key organic spacers used in 2D/3D PSCs. Most importantly, based on the appropriate selection of functional groups in organic spacers, we highlight the role of organic spacers in the fabrication process and formation mechanism of different types of 2D/3D perovskite heterojunctions including the bulk incorporation model, surface treatment model, and bulk incorporation and surface treatment coexisting model as well as other feasible models.

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1. Introduction

The power conversion efficiency (PCE) of three-dimensional (3D) organic–inorganic metal hybrid perovskite solar cells (PSCs) has increased sharply from the initial 3.8%¹ to a certified value of 26.1%² in the past decade. Moreover, many studies have demonstrated that the excellent PCE of 3D PSCs primarily stems from the unique properties of perovskite materials, such as high carrier mobility,³ high absorption coefficient,⁴ long carrier lifetime^5,6 and low exciton binding energy.⁷ These optoelectronic characteristics make perovskite materials outstanding light absorbers for the next generation of photovoltaic devices. However, despite the ultrahigh PCE of state-of-the-art PSCs, which is comparable to that of silicon-based solar cells, long-term stability remains a major challenge for commercialization due to issues such as moisture⁸ and oxygen⁹ attack, heat stress,¹⁰ light soaking¹¹ and ion migration.¹²

In comparison, low dimensional perovskites, especially 2D perovskites consisting of 2D phases and quasi-2D phases, have been proven to be more durable than 3D perovskites due to the incorporation of bulky and hydrophobic organic spacers.¹³ In 2014, Smith et al. demonstrated Ruddlesden–Popper (RP) type 2D PSCs by introducing a phenylethylammonium (PEA) organic spacer.¹⁴ Although the corresponding photovoltaic device exhibited a low PCE of 4.73%, the layered structure of 2D perovskite contributed to the formation of high-quality films that exhibited greater moisture resistance compared to the 3D MAPbI₃ (MA refers to methylammonium) film. Consequently, researchers began to devote significant efforts to the study of 2D PSCs and subsequently developed various organic spacers.^15–18 However, even though 2D perovskites demonstrate enhanced resilience to heat and humidity, the PCE of 2D PSCs still lags behind that of state-of-the-art 3D PSCs.¹⁹

To combine the high efficiency of 3D PSCs with superior stability of 2D PSCs, a dimensional engineering strategy has been proposed and developed. Through dimensional engineering, 2D/3D hybrid heterojunctions can be formed at different positions, wherein 3D perovskites ensure a wider absorption range for achieving high efficiency, while 2D perovskites guarantee that the device will not degrade rapidly under severe conditions. In addition to improving device stability, the construction of 2D/3D heterojunctions is also helpful in energy level regulation, ion migration suppression, defect passivation and residual stress release.²⁰ Generally, 2D/3D heterojunctions with the aforementioned high performance primarily fall into three types: (1) arranged in the bulk of 3D perovskite (referred to as the bulk incorporation model), (2) located on the top of 3D perovskite (referred to as the surface treatment model), and (3) situated both in the bulk and on the top of 3D perovskite (referred to as the bulk incorporation and surface treatment coexisting model).

However, despite the development of many organic spacers for constructing 2D/3D heterojunctions, a clear summary of the role of different types of organic spacers in the preparation process of these 2D/3D perovskites is still lacking. In this review, we first introduce the structures of 3D and 2D perovskite materials and explain the transformation process from 3D to 2D perovskite. Subsequently, we summarize the primary organic spacers that have been used in reported 2D/3D PSCs so far and further analyze their effects on the photoelectric performance of devices. Finally, based on the appropriate selection of functional groups in organic spacers, we mainly focus on the role of organic spacers in the fabrication process and formation mechanism of various 2D/3D perovskite heterojunctions, including the bulk incorporation model, surface treatment model, bulk incorporation and surface treatment coexisting model, and other feasible models.

2. Structures of 3D and 2D perovskites

2.1. Structure of 3D perovskite

To gain a deeper understanding of the properties of perovskite materials, it is crucial to delve into their structures. Conventional perovskite materials are generally defined as a series of compounds with the formula of ABX₃. In this formula, the A site typically represents a monovalent cation, such as CH₃NH₃⁺ (MA⁺), NH=CHNH₃⁺ (FA⁺) and Cs⁺. The B site usually denotes a divalent metal cation (Pb²⁺, Sn²⁺), while X site refers to halogen ions including Cl⁻, Br⁻, and I⁻.²¹ Within the perovskite framework, B site ions coordinate with X site ions to form a [BX₆]⁴⁻ octahedron, in which the B site ions reside in the center of the octahedron and the X site ions occupy the apex positions. These [BX₆]⁴⁻ octahedrons are further interconnected by vertices, extending into 3D space. Meanwhile, A site cations occupy the vacant space within the network formed by the [BX₆]⁴⁻ octahedrons (Fig. 1).²²

Fig. 1 Representative structures for 3D perovskite and 2D perovskite of the RP phase (a), DJ phase (b) and ACI phase (c).

2.2. Structure of 2D perovskite

The tolerance factor (t) is often used to account for octahedral distortions in the crystal structure of perovskite to predict the 3D to 2D transition, in which t is defined by the following Goldschmidt's formula:²³

The archetypal 3D perovskites have t values in the range of 0.8 < t < 1.1. When the A site cations become too large to fit well within the [BX₆]⁴⁻ octahedral cavity, the initial cubic structure of the 3D perovskite could transform into a layered 2D structure to maintain energy stability, thus leading to the deviation from the above range. Therefore, based on the arrangement of the 3D perovskite unit cells, 2D perovskites can be classified into (100), (110) and (111) orientations, where large organic spacers separate the inorganic layered [BX₆]⁴⁻ octahedron and tightly connect the different layers through hydrogen bonding and van der Waals forces.²⁴ Depending on the types of organic spacers, 2D perovskite can be categorized into three main classifications: RP phase, Dion–Jacobson (DJ) phase, and alternating cations in the interlayer-space (ACI) phase (Fig. 1). These are expressed as A′₂A_n−1B_nX_3n+1, A′′A_n−1B_nX_3n+1 and A′′′AB_nX_3n+1, respectively. Here, A′ represents monovalent organic cations like PEA and butylammonium (BA), A′′ denotes a divalent organic cation, and A′′′ typically refers to guanidinium (GA). In the case of ACI perovskite, the large GA spacer and small A cations are arranged in the spacer layer, exhibiting the dual characteristics of RP and DJ structures. The value of n signifies the number of inorganic layers or the metal-halide frameworks located between the organic spacer layers.²⁵

2.3. Organic spacer cations

2D perovskite materials offer significant advantages in terms of environmental stability. However, the presence of large organic spacers in the structure hinders the migration of photogenerated carriers and increases the exciton binding energy, ultimately leading to inferior photovoltaic performance of devices.²⁶ Therefore, the selection of effective organic spacers, while ensuring the high environmental stability of 2D perovskite and excellent photovoltaic performance of 3D perovskites, has emerged as a research focus for the development of efficient 2D/3D PSCs. In pursuit of this goal, scholars have conducted a series of investigations encompassing the regulation of organic spacers, optimization of surface morphology and orientation, and the development of a diverse range of organic spacers (Fig. 2 and 3).²⁰

Fig. 2 Representative aromatic spacers for 2D/3D hybrid PSCs.

Fig. 3 Representative aliphatic and alicyclic spacers for 2D/3D hybrid PSCs.

3. 2D/3D heterojunctions in mixed dimensional perovskite solar cells

Mixed dimensional 2D/3D perovskite films can be fabricated through two common processes: bulk incorporation^27–31 and surface treatment.^32–35 In the bulk incorporation method, small amounts of large organic ammonium salts or 2D perovskite seed crystals are added to the 3D perovskite precursor. This allows for the growth of highly crystalline and the large grained 2D/3D perovskite with favorable out-of-plane layer orientation.^27,31 On the other hand, in surface treatment, 2D perovskite is formed in situ on the deposited 3D perovskite film by spin-coating a solution containing bulky organic ammonium salts³⁶ or 2D perovskite seed crystals.³⁴ Some researchers have also explored alternative methods such as evaporating bulky organic ammonium salts on the top of 3D perovskite films^37,38 or hot-pressing 2D perovskite films onto 3D perovskite films to create 2D/3D heterojunctions.³⁴ The performance enhancement achieved through the formation of a thin mixed 2D/3D heterojunction at the interface between the perovskite layer and the hole transport layer can be attributed to surface defect passivation, accelerated charge extraction, and the introduction of hydrophobic spacers to enhance humidity stability.

Based on the aforementioned statements, the main types of 2D/3D heterojunctions are illustrated in Fig. 4 as follows: (1) 2D/3D heterojunctions generated in the bulk of 3D perovskite films, causing the bulk incorporation model; (2) 2D/3D heterojunctions formed on the surface of 3D perovskites, leading to the surface treatment model; and (3) 2D/3D heterojunctions arranged on both the surface and bulk of 3D perovskite, resulting in the bulk incorporation and surface treatment coexisting model. We will analyze these structures and the role of large organic spacers in each structure in detail in the following discussion.

Fig. 4 Schematic diagram of the bulk incorporation model, surface treatment model, and bulk incorporation and surface treatment coexisting model.

3.1. Bulk incorporation model

3.1.1. Aromatic spacers. In 2018, Ho-Baillie et al. reported the benefits of incorporating phenethylamine iodide salt (PEAI) into the (FAPbI₃)_0.85(MAPbBr₃)_0.15 perovskite and demonstrated the presence of a higher-band-gap PEA-based quasi-2D perovskite at grain boundaries using Kelvin probe force microscopy (KPFM).³⁹ After performing measurements in dark mode, the contact potential difference (CPD) at grain boundaries (CPD_GB) and in the grain interior (CBD_GI) were determined and the averaged differences of CPD_GB and CBD_GI were calculated. Notably, the value of ‘CPD_GB – CBD_GI′ in the film with 4.5% PEAI (−44 mV) was lower than that without PEAI (−13 mV), indicating a change in the electronic properties of the grain boundaries due to the formation of a quasi-2D phase at the grain boundaries. Additionally, in the same year, Yang et al. developed a fabrication technique for the pure α-phase FAPbI₃ perovskite by incorporating PEA₂PbI₄ 2D perovskite seed crystals into the precursor solution.²⁸ In this process, 2D PEA₂PbI₄ passivated the defective grain boundaries of the 3D perovskite through aromatic rings and longer alkyl chains, resulting in a significantly improved hydrophobic property of the film. Furthermore, the 3D perovskite and PEA₂PbI₄ exhibited type I band alignment, which reduced charge recombination and facilitated charge separation and collection. Meanwhile, 2D perovskite regions at grain boundaries experienced a downward band bending under illumination, creating a high-potential barrier for holes and resulting in superior PL lifetime and photovoltaic performance.

In 2021, Liu et al. reported for the first time a class of aromatic formamidinium (ArFA) salts, which included benzamidine hydrochloride (PhFACl), 4-hydroxybenzamidinium hydrochloride (p-HOPhFACl), and 4-fluorobenzamidinium hydrochloride (p-FPhFACl), for 2D/3D hybrid PSCs (Fig. 5a).⁴⁰ As shown in Fig. 5b, the ¹H NMR spectra revealed strong NH⋯I hydrogen-bonding interactions between the iodide anions in the corner-sharing [PbI₆]⁴⁻ octahedron layers and the ammonium fragment of organic spacers. The crystallographic quality and orientation of the perovskite crystals were confirmed using grazing-incidence wide-angle X-ray scattering (GIWAX). In contrast to the diffraction rings observed in the pristine film, the sharp and distinct Bragg spots in the ArFA films (Fig. 5c) suggested that the incorporation of ArFA spacers led to a preferred vertical orientation of the crystal with respect to the substrate. As a result, the optimized PhFA-based 2D/3D PSC exhibited a champion PCE of 23.36% with a high short-circuit current density (J_SC) of 24.67 mA cm⁻², an increased open-circuit voltage (V_OC) of 1.16 V and a notable fill factor (FF) of 81.54%.

Fig. 5 (a) Chemical structures of spacer cations FA, PhFA, p-HOPhFA, and p-FPhFA. (b) ¹H NMR spectra of pristine PhFACl and mixed PhFACl/PbI₂. (c) GIWAX patterns of the control, PhFACl-, p-HOPhFACl-, and p-FPhFACl-based perovskite films. Reproduced with permission.⁴⁰ Copyright 2021, American Chemical Society. (d) Top-views and cross-sectional SEM images of pristine, NpMAI-modified and (NpMA)₂PbI₄-modified PbI₂ films and SEM images of the corresponding perovskite films. (e) J–V curves of the pristine, NpMAI-modified and (NpMA)₂PbI₄-modified devices. Reproduced with permission.⁴¹ Copyright 2022, John Wiley & Sons, Inc.

In addition to the spacers with a single aromatic ring, multiple-ring aromatic spacers such as naphthyl-⁴¹ and pyrenyl-based spacers⁴² have also been applied in bulk 2D/3D PSCs. In 2022, Liu et al. reported the crystal growth regulation of 2D/3D perovskite films through a two-step spin-coating method by using 1-naphthalenemethylammonium iodide (NpMAI) or 2D perovskite (NpMA)₂PbI₄ as seed crystals in PbI₂ precursor solution.⁴¹ They found that the pristine PbI₂ film showed a dense film morphology, while the PbI₂ film incorporated small quantity of (NpMA)₂PbI₄ or NpMAI, showing a mesoporous morphology, especially for the (NpMA)₂PbI₄ incorporated film. This mesoporous morphology benefitted the penetration and reaction of organic salts with PbI₂ in the second deposition step, resulting in a higher quality perovskite film with large grain size (Fig. 5d). Moreover, the application of (NpMA)₂PbI₄ seed doping can not only improve the crystallinity of the perovskite film but also prolong the lifetime of charge carriers, thereby leading to a better photovoltaic performance of devices. Finally, the 2D/3D hybrid films with (NpMA)₂PbI₄ 2D perovskite exhibited a superior PCE of 24.37%, higher than that of the NpMAI-modified film (23.28%) and the control one (21.83%) (Fig. 5e). For pyrenyl-based spacers, 1-(ammonium acetyl)pyrene (PEY) was proposed to fabricate MAPbI₃-based 2D/3D perovskite by Hayase's group in 2018.⁴² They demonstrated the enhancement of ultraviolet irradiation and oxygen stability due to the large functional organic pyrene group with high humidity resistance and strong absorption in the ultraviolet region.

Furthermore, the spacers based on heterocyclic rings such as thiophene and pyridine can also regulate the crystal arrangement and orientation through π–π interactions as well as hydrogen bonding interactions. In 2019, Liu et al. demonstrated highly efficient and stable 2D/3D hybrid PSCs using 2-thiophenemethylammonium (ThMA) as a spacer.⁴³ ThMA could partially insert into the 3D perovskite crystal lattice and form 2D perovskite phases, which aligned perpendicular to the substrate as confirmed by XRD and GIWAX measurements. Additionally, the introduction of a large ThMA spacer inhibited ion migration and improved the overall hydrophobic performance of 2D/3D films, leading to enhanced humidity stability in atmospheric environments. Remarkably, the ThMA spacer-based 2D/3D PSCs exhibited significantly improved performance, including a large V_OC of 1.16 V, a J_SC of 22.8 mA cm⁻², and a very notable FF of 81.0%, resulting in an impressive PCE of 21.49%. In contrast to benzene-based spacers, the pyridine ring possesses a lone electron pair on the N atom, allowing pyridine to partially replace DMSO by coordinating with Pb²⁺.⁴⁴ Moreover, the weak alkalinity of N atoms in the pyridine unit enables the interaction between the pyridine ring with an amino terminal group and the [PbI₆]⁴⁻ framework, ultimately forming DJ-type 2D perovskite.³⁰ Recently, Zhao et al. conducted a detailed investigation of the crystallization dynamics of 2D/3D hybrid perovskite by designing a 2D/3D heterojunction based on the FAPbI₃ 3D perovskite and DJ-type (4AP)PbI₄ 2D perovskite (4AP = 4-amidinopyridine).³⁰ As shown in Fig. 6a and b, the FA-rich α phase perovskite rapidly crystalized on the film's surface after dropping the anti-solvent, resulting in a sharp increase in PL intensity. Subsequently, the entrapped DMSO solvent volatilized from the bulk film during the annealing process, causing the surface perovskite to dissolve and recrystallize leading to a gradual decrease in PL intensity. Eventually, the evaporation of DMSO caused volume collapse of the internal perovskite, resulting in void formation at grain boundaries. In contrast, the addition of (4AP)PbI₄ 2D perovskite could slow down the nucleation and growth process of FA-rich α phase 3D perovskite, and the presence of (4AP)PbI₄ provided a skeleton that aided in the hetero-nucleation of 3D perovskite and filled the voids at grain boundaries caused by DMSO evaporation, resulting in more uniform and larger grains. Through this method, the 2D/3D films achieved a PCE of 24.9% in a rigid device and a PCE of 22.3% in a flexible device, with the PCE achieved by rigid device setting a record for 2D/3D hybrid PSCs to date (Fig. 6c–e).

Fig. 6 (a) Schematic illustration of the crystallization mechanism of 4-AP modified perovskite films. (b) In situ PL measurements for the formation kinetics of perovskite films. J–V curves of rigid solar cells(c) and flexible solar cell (d). (e) Normalized PCE with curvature from flat to R = 2 mm. Reproduced with permission.³⁰ Copyright 2023, Elsevier.

3.1.2. Aliphatic spacers. Among aliphatic molecules, BA is a classical organic spacer commonly used to fabricate RP type 2D perovskites,^45–47 suggesting its potential for outstanding performance in 2D/3D perovskite systems. In 2017, Snaith et al. introduced BA spacer into a mixed-cation and mixed-halide FA_0.83Cs_0.17Pb(I_yBr_1−y)₃ 3D perovskite.²⁷ Notably, plate-like 2D perovskites were observed to intersperse between highly orientated 3D perovskite grains, significantly enhancing crystallinity and suppressing non-radiative charge recombination through the formation of a classical type-I heterojunction (Fig. 7a–c). The device based on BA_0.09(FA_0.83Cs_0.17)_0.91Pb(I_0.6Br_0.4)₃ with a wide band gap of 1.72 eV achieved an optimized PCE of 17.2%, while the device utilizing the BA_0.05(FA_0.83Cs_0.17)_0.95Pb(I_0.8Br_0.2)₃ perovskite with a narrow band gap of 1.62 eV as the photoactive layer demonstrated a champion PCE of 20.6%. In 2021, Seok et al. found that the chemical reaction between the isopropyl alcohol (IPA) solvent and methylammonium chloride (MACl) in the precursor would generate isopropylammonium cations (iPAmH⁺) in a two-step spin-coating process. Furthermore, iPAmH⁺ could also be generated by adding IPA directly to the precursor in a one-step spin-coating process, where iPAmH⁺ played a role in stabilizing the α-phase FAPbI₃.⁴⁸ Based on these findings, they directly added 15 mol% isopropylammonium chloride (iPAmHCl) to the precursor for the fabrication of the FAPbI₃ film, ensuring the specific effects of the iPAmH⁺ cation. Similar to MACl, a substantial amount of the introduced iPAmHCl volatilized during the annealing process, with little iPAmHCl remaining at the FAPbI₃ grain boundaries, contributing to the stabilization of the α-phase and enabling the fabrication of PSCs with a certified PCE of 23.9%.

Fig. 7 (a) SEM images and 2D X-ray diffraction measurements of post-annealed perovskite films with different BA concentrations. (b) Enlarged SEM images of x = 0.03, 0.09 and 0.16 films. (c) Proposed electronic band offsets of the 2D/3D heterojunction. Reproduced with permission.²⁷ Copyright 2017, Springer Nature. (d) Schematic diagram of with/without BDA-seed-induced growth and the images of the PbI₂ film with/without 2D seed under a confocal laser scanning microscope at different reaction times after depositing organic salts. (e) Dynamic light scattering spectra of the solutions. XRD patterns (f) and different peak intensity ratios (g) of perovskite films with various seed concentrations. (h) J–V curves of control and BDA-modified PSCs. Reproduced with permission.²⁹ Copyright 2022, Elsevier.

The diammonium spacers with two ammonium groups can bond with a [PbI₆]⁴⁻ octahedron on two sides, forming a DJ-type 2D perovskite. This structure provides favorable nucleation sites for the highly oriented growth of 3D perovskites.^49,50 In 2022, Zhao et al. introduced (BDA)PbI₄ (BDA = butane-1,4-diaminium) seed crystals into the PbI₂ precursor as nucleation centers for 2D/3D perovskite formation. The results revealed that the perovskite film bypassed the nucleation stage and directly entered the growth stage after being covered with organic salts.²⁹ The process of seed crystal-induced crystallization rapidly consumed the solute and significantly decreased the solution concentration. Consequently, it inhibited the random nucleation of the remaining 3D perovskite region without seeds, leading to the dominance of seed crystal-induced crystallization in the film formation process. It is noteworthy that the (BDA)PbI₄ itself inherently exhibited a high orientation of (00L). Therefore, the template-induced 3D perovskite demonstrated dominant and well-defined (001) and (002) peaks instead of the unfavorable (111) peaks. Eventually, the 2D phase transformed into grain boundaries after completing the orientation-induced growth, forming a type-I band alignment with 3D perovskite to passivate GB defects. This transformation resulted in a boost in PCE from 21.03% to 23.95% in mixed-dimensional PSCs (Fig. 7d-h). More reported organic spacer cations applied in bulk-incorporation type PSCs can be found in Table 1.

Table 1 An exhaustive list of recently reported organic spacer cations applied in bulk-incorporation type PSCs

Spacers	Device architecture	V OC (V)	FF (%)	J SC (mA cm^−2)	PCE (%)	Ref.
PEA	ITO/SnO2/FA0.98Cs0.02PbI3/spiro-OMeTAD/Au	1.130	76.11	24.00	20.64	28
PABA	FTO/c-TiO2/MAPbI3/spiro-OMeTAD/Au	1.080	77.62	20.70	17.35	51
PhFA	ITO/SnO2/FA0.84MA0.16PbI3/spiro-OMeTAD/MoO3/Ag	1.160	81.54	24.67	23.36	40
p-OHPhFA	ITO/SnO2/FA0.84MA0.16PbI3/spiro-OMeTAD/MoO3/Ag	1.160	81.89	23.21	22.05	40
p-FPhFA	ITO/SnO2/FA0.84MA0.16PbI3/spiro-OMeTAD/MoO3/Ag	1.160	81.73	23.24	22.03	40
MTIm	FTO/c-TiO2/m-TiO2/[(MTIm)2(Cs0.05FA0.95)29Pb30I91]0.85(MAPbBr3)0.15/spiro-OMeTAD/Au	1.140	79.12	23.56	21.25	52
4-AEP	ITO/SnO2/MAPbI3/spiro-MeOTAD/Au	1.080	81.00	23.58	20.70	44
4AP	FTO/TiO2/FAPbI3/spiro-OMeTAD/Au	1.180	81.81	25.70	24.90	30
ThMA	ITO/SnO2/FA0.84MA0.16PbI3/spiro-OMeTAD/MoO3/Ag	1.160	81.00	22.80	21.49	43
NpMA	ITO/SnO2/FA0.84MA0.16PbI3/spiro-OMeTAD/MoO3/Ag	1.180	81.36	25.30	24.37	41
PEY	FTO/SnO2/(PEY2PbI4)0.02MAPbI3/spiro-OMeTAD/Au	1.050	66.10	21.15	14.70	42
iPAm	—	1.159	79.80	25.79	23.85	48
BA	FTO/SnO2/PCBM/BA0.05(FA0.83Cs0.17)0.95Pb(I0.8Br0.2)3/spiro-OMeTAD/Au	1.140	80.00	22.70	20.60	27
TBA	ITO/PEDOT:PSS/TBA2MA19Pb20I61/PCBM/BCP/Ag	1.032	80.53	23.70	19.70	53
HEA	FTO/c-TiO2/m-TiO2/(HEA)2(Cs0.1FA0.9)29Pb30I91/spiro-MeOTAD/Au	1.100	79.63	22.81	19.84	54
Gly	FTO/TiO2/CsPbI2Br(Gly-X)/spiro-OMeTAD/Au	1.330	80.92	16.04	17.26	55
GBA	PEN/ITO/PTAA/FA0.7MA0.25Cs0.05Pb(I0.93Br0.07)3/PCBM/BCP/Ag	1.140	80.02	23.51	21.45	56
EDA	ITO/PEDOT:PSS/FASnI3/PCBM/BCP/Ag	0.634	69.43	19.32	8.47	49
BDA	ITO/SnO2/FA0.75MA0.25PbI3/spiro-OMeTAD/Au	1.140	84.70	24.20	23.95	29
ODA	ITO/SnO2/Cs0.05FA0.85MA0.10Pb(I0.97Br0.03)3/spiro-OMeTAD/Ag	1.120	75.61	24.80	20.94	50
TETA	TO/SnO2/Cs0.05FA0.85MA0.10Pb(I0.97Br0.03)3/spiro-OMeTAD/Ag	1.100	74.03	25.22	20.57	50
EDBE	TO/SnO2/Cs0.05FA0.85MA0.10Pb(I0.97Br0.03)3/spiro-OMeTAD/Ag	1.150	76.66	25.88	22.68	50
TBA	ITO/PEDOT:PSS/TBA2MA19Pb20I61/PCBM/BCP/Ag	1.032	80.53	23.70	19.70	53
BiPi	FTO/TiO2/MAPbI3/spiro-OMeTAD/Au	1.100	77.00	23.51	20.03	57

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3.2. Surface treatment model

The surface passivation through the deposition of an organic spacer or a 2D perovskite layer is another crucial method to build 2D/3D heterojunctions in order to achieve both high efficiency and ultrastability in PSCs.⁵⁸ In general, defects on the surface of 3D perovskite, such as halide vacancies and undercoordinated Pb²⁺, can be significantly passivated by introducing large spacer cations and their corresponding 2D perovskites. Simultaneously, the formation of 2D/3D heterojunctions through the surface passivation method rearranges the energy level structure at the interface between the 3D perovskite and the charge carrier transport layer, creating a staircase-like energy level structure that effectively enhances the carrier transport capacity. Additionally, the presence of large spacer cations at the interface can prevent water invasion and ion migration, thereby promoting excellent long-term stability of 2D/3D devices in various complex environments.

3.2.1. Aromatic spacers. In fact, aromatic compounds have long been used to construct 2D/3D heterojunctions using a surface deposition method.^59–61 In 2016, Docampo et al. reported 2D/3D perovskite by spin-coating a mixture solution of MAI:PEAI/IPA on the surface of 3D perovskite, and successfully observed (00l) diffraction peaks of (PEA)₂(MA)₄Pb₅I₁₆ by XRD measurement.⁵⁹ Two years later, Wang et al. developed a method by spin-coating PEAI solution on 3D perovskite and in situ formed a layered 2D PEA₂PbI₄.⁶² Such a PEA₂PbI₄ 2D perovskite capping layer induced Fermi-level splitting of the 2D/3D perovskite film under light illumination, leading to an enhanced V_OC and thus a PCE of 18.51% in 2D/3D PSCs. In 2022, Sun et al. reported in situ formed 2D perovskite by the deposition of a mixed solution of a hole transport material (HTM) (spiro-OMeTAD) and PEA spacers.⁶³ In this process, the high polar solvent like IPA was not used anymore, which reduced the damage to the 3D perovskite film, thus leading to a PCE of 21.09%.

In light of the unique properties of PEA spacers,^64,65 researchers have explored the molecular structure and extended the use of phenyl spacers such as benzylammonium (BnA),⁶⁶ phenylbutylammonium (PBA),⁶⁷ 4-fluorophen-ethylammonium (FPEA),^68,69 and pentafluorophenylethylammonium (FEA),³² among others.^{33,35,70–74} In 2019, Grätzel et al. employed FEA in highly efficient bilayer 2D/3D PSCs and successfully demonstrated the dominant generation of 2D perovskite on the surface of the 2D/3D bilayer through X-ray reflectivity (XRR) and grazing incidence X-ray diffraction (GIXRD) characterization. The (FEA)₂PbI₄ 2D perovskites exhibited approximately four layers in 2D/3D bilayers (about 8 nm) and displayed a larger angular distribution in the out-of-plane direction. Additionally, the perfluorinated 2D perovskite layer exhibited super hydrophobicity and inhibited ion motion, leading to comprehensive enhancement in moisture resistance and intrinsic stability of the obtained 2D/3D PSCs. In 2019, Sargent et al. creatively utilized 4-vinylbenzylammonium (VBA) to create well-ordered perovskite quantum wells (PQWs) (also known as low dimensional perovskites) on top of 3D perovskite layers.³³ As shown in Fig. 8a–d, VBA featured an additional terminal vinyl group on the para position of the ammonium group in the benzene ring. The adjacent vinyl groups formed an extended conjugate system undergoing photochemical cross-linking under UV photoexcitation (∼250 nm) to produce new covalent bonds when forming 2D perovskites.^75,76 These cross-linked VBA-based 2D/3D perovskite films exhibited improved crystallinity, resulting in enhanced V_OC, negligible hysteresis, and increased hydrophobic properties compared to uncross-linked counterparts such as PEA-based and allylammonium (ALA)-based heterojunctions. To gain further insight into the formation mechanism of VBA-based 2D/3D heterojunction films, in situ GIWAX measurements were conducted to gather information regarding the orientation and d-spacing of diffractive species before, during, and after the exposure of 3D perovskites to VBA spacers.³⁵ For the (MAPbBr₃)_0.05(FAPbI₃)_0.95 perovskite, upon exposure to VBA spacers, 2D perovskite with n = 3 initially formed and its dimension gradually decreased in subsequent stages, undergoing a progressive dimensional reduction from 3D fragments to n = 3 → 2 → 1 (Fig. 8e and f).

Fig. 8 (a) Schematic illustrating the process by which formation of cross-linked 2D perovskites atop a 3D layer is achieved using VBABr. (b) Fourier transform infrared spectra of films treated with VBABr, PEABr, and ALABr and the 2D/3D VBABr-treated films with different durations of UV light (254 nm) exposure. (c) XRD patterns for the 3D film and the 2D/3D film with/without UV exposure. (d) J–V curves of VBA-based 2D/3D perovskites. Reproduced with permission.³³ Copyright 2019, American Chemical Society. (e) Proposed 3D-to-2D conversion mechanism by VBA spacers. (f) In situ GIWAXS patterns for VBA-treated films. Reproduced with permission.³⁵ Copyright 2021, Springer Nature.

Thiophenyl aromatic compounds are also important organic spacers for the construction of 2D/3D heterojunctions on the surface of 3D perovskite.^36,77 As mentioned above, 2D/3D heterojunctions can regulate the arrangement of energy levels to enhance charge extraction. To investigate this mechanism more thoroughly, Grancini et al. developed an innovative technique called ultraviolet photoemission spectroscopy (UPS) depth profiling. This technique allowed them to probe the energetic landscape within the bulk of materials and across buried interfaces, accurately measuring the progression of the vacuum level and valence band positions as a function of film depth. They further utilized this technique to provide an in situ measurement of the interfacial energy level alignment in a 2D/3D system constructed using ThMAX (X = Cl, Br, or I) (Fig. 9a).⁷⁸ The valence bands of 2D perovskites composed of ThMAI and ThMABr bent downward, resulting in an energy level alignment that promoted hole extraction. On the other hand, the valence bands of 2D perovskites composed of ThMACl showed an upward slope, creating a certain energy barrier for hole transport. Importantly, the 2D perovskites based on ThMAI and ThMABr exhibited a clear transition from n-type to p-type materials compared to the 3D perovskite. This transition led to the formation of a p–n junction at the interface between the 3D and 2D perovskites, significantly enhancing charge extraction and reducing interface recombination (Fig. 9b and c). Additionally, Grancini and colleagues also investigated the inhibitory effects of thiophenyl-based 2D perovskites on ion migration.⁷⁹ Pure 2D perovskites based on ThMAI and 3-thiophenemethylammonium iodide (3-TMAI) can embed small MA cations that migrate from the 3D bulk underneath and immobilize them to form a new quasi-2D structure. In this transition process, the MA cations were consumed. On the other hand, 2D perovskites based on 2-thiopheneethylammonium iodide (2-TEAI) physically blocked the MA cations at the 2D/3D interface, inhibiting ion migration and preserving the purity of the 2D phase. Therefore, although different thiophenyl-based 2D perovskites employed different mechanisms to block ion migration, 2D/3D hybrid devices fabricated by using ThMAI, 3-TMAI and 2-TEAI all exhibited improved long-term stability compared to the pristine device.

Fig. 9 (a) Schematic representation of the UPS depth profiling technique. (b) Measured UPS spectra and the corresponding energetic level diagrams of ThMAI, ThMABr, and ThMACl with the 3D perovskite layer. (c) J–V curves of the champion cells of 3D and 2D/3D PSCs employing ThMAI, ThMABr, and ThMACl. Reproduced with permission.⁷⁸ Copyright 2021, Elsevier. (d) Chemical structures of FBImBr and FBImI. (e) SEM images and water contact angle images of FBImBr, FBImI and control perovskite films. (f) Environmental stability for the FBImBr, FBImI and control devices. Reproduced with permission.⁸⁰ Copyright 2023, Springer Nature. (g) Schematic illustration of the structure of Dion–Jacobson phase 2D/3D (C₆₀-BPAM). (h) 2D-GIXRD profiles of 3D and 2D/3D perovskite films. Reproduced with permission.⁸¹ Copyright 2022, John Wiley & Sons, Inc.

Imidazole (IZ) is a unique diammonium molecule with an aromatic structure that can form additional hydrogen bonding with a [PbI₆]⁴⁻ framework. Due to its delocalized π bonding, it may exhibit low reactivity with its surroundings, leading to additional electrical conductivity.^82,83 However, IZ has an unfavorable ionic radius of 258 pm, which is close to the upper limit of the tolerance factor. Therefore, it is not considered a suitable spacer for 2D perovskite fabrication.⁸² In 2022, Tu et al. successfully demonstrated the formation of 2D/3D heterojunctions through post-treatment with imidazole hydrobromide (IZBr) and achieved an outstanding PCE of 23.25% for MA-free (FA_0.9Cs_0.1PbI₃) PSCs.⁸⁴ Other imidazolium-based cations such as benzimidazolium (BIZ) and benzodiimidazolium (Bdi) spacers have larger ionic radii and stronger π conjugation effects, making them more suitable for 2D/3D heterojunction fabrication.⁸⁵ In 2021, Feng et al. reported the facile in situ formation of a layered 2D (BIZ)₂PbI₃Br perovskite by incorporating benzimidazolium iodine (BIZI) on a 3D CsPbI₂Br absorber. The resulting 2D (BIZ)₂PbI₃Br exhibited remarkable defect passivation and moisture resistance for the 3D perovskite, leading to improved photovoltaic performance in an inverted (p–i–n) PSC.⁸⁶ More recently, based on benzimidazole, Chen et al. designed highly conjugated fluorinated benzimidazolium halides (FBImX, X = I or Br) as passivators for 2D/3D perovskite. By introducing double fluoride atoms in the meta-position of the imidazole group, fluoride effectively enhanced the hydrophobicity of the film and passivated both anion and cation vacancies present in 3D perovskite.⁸⁰ As a result, the best PCE of 23.00% was achieved for FBImBr-incorporated PSCs, along with a high V_OC of 1.17 V and an FF of 81.80%. The FBImBr-modified device exhibited excellent operational stability, maintaining 86% of the initial PCE after 1300 h under 25% relative humidity (Fig. 9d–f).

Generally, fullerene (C₆₀)⁸⁷ and its derivatives, such as phenyl-C₆₁-butyric acid methyl ester (PCBM),⁸⁸ C₆₀-substituted benzoic acid self-assembled monolayer (C₆₀-SAM)⁸⁹ and [6,6]-phenyl-C₆₁-butyric acid-dioctyl-3,3’-(5-hydroxy-1,3-phenylene)-bis(2-cyanoacrylate) ester (PCBB-2CN-2C8),⁹⁰ are used as electron transport materials due to their high electron affinity and strong electron acceptance ability. However, the larger molecular size of C₆₀ (with a van der Waals diameter of ∼7 Å) compared to the [PbI₆]⁴⁻ octahedron (with a size of 6.3 Å) makes it difficult for C₆₀ to fit into the [PbI₆]⁴⁻ octahedron layer to form a 2D structure.⁸¹ To overcome this barrier, Yang et al. synthesized a novel amino-functionalized fullerene derivative called C₆₀-BPAM, in which two amino groups were grafted onto C₆₀ using a pyrrolidine–phenyl linker. They then fabricated a DJ-type (C₆₀-BPAM)PbI₄ 2D perovskite on top of a 3D MAPbI₃ perovskite by drop-casting an anti-solvent to form a 2D/3D heterojunction.⁸¹ As a result, C₆₀-BPAM significantly facilitated electron transport and improved the humidity stability of the 2D perovskite structure due to the highly conductive and hydrophobic C₆₀ tail. This strategy presents a promising new approach for the preparation of 2D/3D heterojunctions (Fig. 9g and h).

3.2.2. Aliphatic spacers. Despite aliphatic spacers are considered to be weaker than aromatic spacers in terms of charge transport, their longer carbon tails endow them with higher structural degrees of freedom and hydrophobic properties, thereby enabling a wide range of applications.⁹¹ In 2018, Huang et al. utilized short-chain aliphatic BA and the corresponding iodide (BAI) as passivating agents to investigate 2D/3D stacking structures.⁹² They found that BA promoted the formation of a pure 2D BA₂PbI₄ perovskite by reacting with MAPbI₃. The pure BA₂PbI₄ exhibited more organic spacers than the mixture of 2D perovskites, indicating superior protective effects. On the other hand, the reaction between BAI and MAPbI₃ resulted in inferior (BA)₂(MA)_n−1Pb_nI_3n+1 with various n values, leading to a rougher 2D perovskite layer compared to the BA-constructed 2D perovskite (Fig. 10a and b). In 2021, Noh et al. developed another method called the solid-state in-plane growth (SIG) process, which involved integrating a pre-formed 2D BA₂PbI₄ perovskite capping layer onto 3D perovskite surfaces through mechanical and thermal pressure.³⁴ The SIG process allowed for controlled deposition of a pure and highly crystalline 2D (BA)₂PbI₄ perovskite layer on top of a 3D perovskite layer without the presence of any quasi-2D phases. As a result, the intact 2D/3D heterojunction enhanced the carrier lifetime and facilitated the design of the local electric field distribution at the junction, leading to a significant improvement in the V_OC of PSCs (Fig. 10c–e). Recently, Mohite et al. explored the use of acetonitrile (MeCN) as a solvent to dissolve 2D perovskites (BA₂MA_n–1Pb_nI_3n+1, n = 1 to 4) without destroying the underlying 3D perovskite.⁹³ MeCN was found to possess an appropriate dielectric constant and Gutmann donor number. With an increase in the n value of the 2D perovskite, a transition from type I heterojunction to type II heterojunction was observed due to changes in energy level alignment. Among them, the type II heterojunction achieved with BA₂MA₂Pb₃I₁₀ (n = 3) and the 3D perovskite exhibited near-perfect alignment of the valence band edges for 3D and 2D perovskites. This alignment was considered ideal for hole extraction and created a significant energy barrier for electron transport. Consequently, the fabricated 2D/3D PSC incorporating a pure n = 3 2D component achieved a champion PCE of 24.5% with an impressive V_OC of 1.20 V.

Fig. 10 Absorption spectra of (a) BA-treated MAPbI₃ films and (b) BAI-treated MAPbI₃ films with different reaction times. Reproduced with permission.⁹² Copyright 2018, American Chemical Society. (c) Time-resolved photoluminescence decay curves of the control and SIG-processed 2D/3D films. (d) Top-view and cross-sectional sketches of the manufacturing of a (BA)₂PbI₄ film on a 3D perovskite substrate via the SIG method. (e) The 2D/3D perovskite film fabricated under SIG60 conditions after 2 min, 5 min, 8 min and 10 min processing times. Reproduced with permission.³⁴ Copyright 2021, Springer Nature.

The long-chain in aliphatic organic spacers can provide the possibility of forming more reaction sites, which can strengthen the interaction between the spacers and inorganic [PbI₆]⁴⁻ framework, thereby improving the film's photophysical properties and device performance.^91,94 In 2021, Lan et al. utilized a viscous polymer named polypropylene glycol bis (2-aminopropyl ether) (A-PPG) with a number-average molecular weight (Mn) of 400 to create a quasi-2D layer on the surface of 3D perovskite in situ. A-PPG had the ability to partially dissolve excess PbI₂ and perovskite, and it was applied in a one step spin-coating process with the addition of chlorobenzene as the anti-solvent in advance.⁹⁵ Moreover, unlike other spacer cations, the lone electron pairs of the ether-oxygen in A-PPG can also form coordination bonds with undercoordinated Pb²⁺, significantly contributing to defect passivation and reducing non-radiative recombination. Consequently, the A-PPG-modified 2D/3D PSC exhibited an optimized PCE of 22.24% with a distinguished V_OC of 1.21 V. The device also maintained 90% of its original PCE in an ambient atmosphere after 50 days, demonstrating extraordinary humidity stability. More reported organic spacers applied in surface-treatment type PSCs are summarized in Table 2.

Table 2 An exhaustive list of recently reported organic spacer cations applied in surface-treatment type PSCs

Spacers	Device architecture	VOC (V)	FF (%)	JSC (mA cm^−2)	PCE (%)	Ref.
BnA	FTO/c-TiO2/m-TiO2/SnO2/3D perovskite/BnA-based 2D perovskite/spiro-OMeTAD/Au	1.078	78.80	24.48	20.79	66
PEA	FTO/TiO2/Cs0.05(MA0.12FA0.88)0.95Pb(I0.88Br0.12)3/PEA2PbI4/PEAI-spiro-OMeTAD/Au	1.170	79.00	22.93	21.09	63
PBA	ITO/NiOX/(Cs0.07FA0.95PbI2.95)0.85(MAPbBr3)0.15/PBA-based 2D perovskite/PCBM:C60/Zr(acac)4/Au	1.170	83.92	22.16	21.76	67
FPEA	FTO/c-TiO2/m-TiO2/Cs0.1(FA0.83MA0.17)0.9Pb(I0.83Br0.17)3/FPEA2PbI4/spiro-OMeTAD/Au	1.126	80.00	22.80	20.54	69
FEA	FTO/c-TiO2/m-TiO2/FA0.92MA0.04Cs0.04PbI3/FEA-based 2D perovskite/spiro-OMeTAD/Au	1.096	78.40	25.79	22.16	32
3Cl-BA	FTO/c-TiO2/m-TiO2/MAPbI3/(3Cl-BA)2PbI4/carbon	0.980	63.00	21.31	13.22	73
CF3PMA	ITO/PTAA/MAPbI3/CF3PMA-based 2D perovskite/PCBM/BCP/Ag	1.070	72.22	19.38	15.01	71
CF3PEA	FTO/TiO2/FA0.85MA0.15PbI3/(CF3PEA)2FAPb2I7/spiro-OMeTAD/Au	1.130	80.78	25.29	23.10	72
PTA	FTO/c-TiO2/SnO2/MAPbI3/PTA-based 2D perovskite/spiro-MeOTAD/Ag	1.104	81.90	23.39	21.26	96
OHPEA	ITO/SnO2/Rb0.05Cs0.05[(FA0.83MA0.17)]0.9Pb(I0.83Br0.17)3/OHPEA-based 2D perovskite/spiro-MeOTAD/Ag	1.220	75.50	23.25	21.38	97
ABA	FTO/NiOX/MAPbI3/PCBM/Rhodamine 101/Ag	1.105	81.90	23.42	21.18	70
VBA	ITO/TiO2/(MAPbBr3)0.15(FAPbI3)0.85/VBA-based 2D perovskite/spiro-OMeTAD/Au	1.150	78.00	22.50	20.20	33
TFPhFA	FTO/SnO2/FA0.93MA0.07PbI3/spiro-MeOTAD/Au	1.160	81.26	25.42	24.00	98
PDA	FTO/TiO2/(Cs0.05(FA0.83MA0.17)0.95Pb(I0.83Br0.17)3/PDA-based 2D perovskite/spiro-MeOTAD/Ag	1.145	77.89	23.78	21.15	74
XDA	FTO/c-TiO2/SnO2/MAPbI3/XDA-based 2D perovskite/spiro-MeOTAD/Ag	1.134	76.92	23.78	20.74	99
IZ	ITO/SnO2/FA0.9Cs0.1PbI3/IZ-based 2D perovskite/spiro-MeOTAD/Ag	1.172	80.76	24.57	23.25	84
BIZ	ITO/NiOX/CsPbI2Br/(BIZ)2PbI3Br/PCBM/Ag	1.151	79.79	15.59	14.32	86
FBIm	ITO/SnO2/FAPbI3/(FBIm)2PbBr4/spiro-MeOTAD/MoO3/Ag	1.170	81.80	24.03	23.00	80
ThMA	FTO/c-TiO2/m-TiO2/SnO2/[(FAPbI3)0.87(MAPbBr3)0.13]0.92(CsPbI3)0.08/ThMA-based 2D perovskite/spiro-OMeTAD/Au	1.150	78.00	23.20	20.80	78
3-TMA	FTO/c-TiO2/m-TiO2/SnO2/MAPbI3/quasi-2D 3-TMA perovskite/spiro-MeOTAD/Au	1.132	77.10	23.60	20.59	79
2-TEA	FTO/c-TiO2/m-TiO2/SnO2/MAPbI3/(2-TEA)2PbI4/spiro-MeOTAD/Au	1.117	73.70	23.60	19.42	79
OA	FTO/SnO2/(FAPbI3)0.95(MAPbBr3)0.05/OA-based 2D perovskite/PTAA/Au	1.180	80.80	24.70	23.60	100
1,8-ND	ITO/SnO2/FA0.92MA0.08PbI3/spiro-OMeTAD/Au	1.150	83.00	24.90	23.80	101
C60-BPAM	FTO/NiOX/MAPbI3/(C60-BPAM)PbI4/PCBM/BCP/Ag	1.090	78.53	23.61	20.21	81
CA	ITO/PEDOT:PSS/MAPbI3/PCBM/Rhodamine 101/LiF/Ag	0.920	77.26	19.29	13.86	60
BA	FTO/SnO2/(FAPbI3)0.95(MAPbBr3)0.05/BA2PbI4/spiro-OMeTAD/Au	1.185	83.90	24.70	24.59	34
HA	FTO/SnO2/(FAPbI3)0.80(CsPbI3)0.07(MAPbBr3)0.13/HA-based 2D perovskite/spiro-OMeTAD/Ag	1.140	76.00	23.76	20.62	102
HTA	ITO/SnO2/(FAPbI3)0.95(MAPbBr3)0.05/HTA-based 2D perovskite/spiro-OMeTAD/Ag	1.170	79.55	22.69	21.12	103
4-ABA	FTO/SnO2/Cs0.05(FA0.83MA0.17)0.95Pb(I0.83Br0.17)3/4-ABA-based 2D perovskite/spiro-OMeTAD/Au	1.200	78.55	24.57	23.16	104
AVA	FTO/c-TiO2/m-TiO2/MAPbI3/AVA-based 2D perovskite/spiro-MeOTAD/Au	1.060	76.00	22.30	18.00	105
A43	FTO/c-TiO2/m-TiO2/MA0.9FA0.1PbI3/(A43)2PbI4/spiro-MeOTAD/Au	1.110	79.30	22.87	20.12	106
A-PPG	FTO/TiO2/(FAPbI3)0.84(CsPbI3)0.06(MAPbBr3)0.10/A-PPG-based 2D perovskite/spiro-MeOTAD/Au	1.210	78.97	23.27	22.24	95
DMEDA	ITO/SnO2/MAPbI3/DMEDA-based 2D perovskite/spiro-OMeTAD/Ag	1.140	78.00	22.57	20.18	107
DETA	ITO/NiOX/Cs0.05(FA0.85MA0.15)0.95Pb(I0.85Br0.15)3/DETA-based 2D perovskite/PCBM/BCP/Ag	1.100	83.33	21.73	19.94	94
TETA	ITO/NiOX/Cs0.05(FA0.85MA0.15)0.95Pb(I0.85Br0.15)3/TETA-based 2D perovskite/PCBM/BCP/Ag	1.110	83.90	21.61	20.13	94
TEPA	ITO/NiOX/Cs0.05(FA0.85MA0.15)0.95Pb(I0.85Br0.15)3/TEPA-based 2D perovskite/PCBM/BCP/Ag	1.120	84.81	22.39	21.28	94
PEHA	ITO/NiOX/Cs0.05(FA0.85MA0.15)0.95Pb(I0.85Br0.15)3/PEHA-based 2D perovskite/PCBM/BCP/Ag	1.130	84.90	22.63	21.79	94
EDA	ITO/NiOX/PTAA/Cs0.05(FA0.95MA0.05)0.95Pb(I0.95Br0.05)3/EDA-based 2D perovskite/PCBM/BCP/Ag	1.200	81.90	25.20	24.70	108
ODA	FTO/c-TiO2/m-TiO2/(FA0.85MA0.15)Pb(I0.85Br0.15)3/(ODA)PbI4/spiro-MeOTAD/Au	1.100	81.00	24.20	21.60	109
EDBE	ITO/SnO2/Cs0.2FA0.8Pb(I0.7Br0.3)3/EDBE-based 2D perovskite/spiro-OMeTAD/Ag	1.280	78.50	18.85	18.85	110

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3.3. Bulk incorporation and surface treatment coexisting model

In view of the advantages of 2D/3D perovskite heterojunctions in modifying grain boundaries and passivating surface defects, researchers have started exploring the combination of these two methods to create a coexisting type 2D/3D heterojunctions. Specifically, the process involves adding spacers to the perovskite precursor, resulting in a 2D/3D heterojunction within the bulk of the deposited 3D perovskite film. Subsequently, a solution containing spacers (or 2D perovskites) is dropped onto the film surface after annealing, creating a capping 2D perovskite layer after post-treatment. Furthermore, there exist many other approaches for constructing the same structure, which we will discuss in the following discussion and mainly focus on aromatic and aliphatic organic spacers.

3.3.1. Aromatic spacers. Lan et al. designed a pyridinyl organic ammonium salt named 2-(2-pyridyl)ethylamine iodide (2-PyEAI) and applied the above method to prepare a 2D/3D hybrid PSC with a high efficiency of 23.2%.¹¹¹ Meanwhile, Liu et al. made groundbreaking advancements by developing 2-thiopheneformamidinium bromide (ThFABr) to passivate the inevitable defects present in both the bulk and surface of perovskite films, achieving an ultralong carrier lifetime exceeding 20 μs and carrier diffusion lengths longer than 6.5 μm (Fig. 11a and b).¹¹² Typically, the carrier lifetime and diffusion length are closely related to the density of defects. As shown in Fig. 11c, the PL intensity enhanced more pronouncedly in the coexisting type 2D/3D PSC as the excitation fluence increased, indicating that the trap states were filled more rapidly during the excitation process, confirming the effective passivation of defects. Importantly, the passivation of defects was facilitated by the incorporation of ThFABr, resulting in the formation of 2D perovskites in the bulk and on the surface of 3D perovskites (Fig. 11d–f). Moreover, the thickness of perovskite films was generally less than 1 μm due to the limited carrier diffusion length. The ultra-long carrier lifetime and diffusion lengths made it possible to fabricate film thickness-insensitive devices for their future commercialization. Consequently, the coexisting type 2D/3D PSCs achieved a champion PCE of 24.69% (film thickness < 1 μm) and an excellent PCE of 24.15% (film thickness > 1.1 μm (Fig. 11g and h).

Fig. 11 (a) TRPL decay spectra, (b) the diffusion length of holes and electrons, (c) pump fluence dependent PL intensity and (d) GIWAX measurements for the control, bulk incorporation type (target-1) and coexisting type (target-2) 2D/3D films using ThFABr. J–V curves of PSCs based on (e) control, target-1 and target-2 perovskites and (f) a thick perovskite film. Reproduced with permission.¹¹² Copyright 2023, John Wiley & Sons, Inc. (g) Schematic diagram of the DGPF film. (h) ToF-SIMS depth profile of the DGPF perovskite film deposited on an ITO substrate. J–V curves based on the 1.63 eV(i) and 1.53 eV(j) perovskite. Reproduced with permission.¹¹³ Copyright 2021, Springer Nature. (k) Schematic demonstration of why BDA treatment can induce respective 2D hybrid perovskite formation, while ODA stays only at the surface or grain boundaries without affecting bulk properties. Reproduced with permission.¹¹⁴ Copyright 2016, American Chemical Society.

On the other hand, in 2021, He et al. reported a heterogeneous 2D/3D tin-halide PSC (FASnI₃) through the substitution of FAI with FPEABr. They demonstrated that 2D components can be transferred from the bulk phase to the surface of the 3D perovskite during the grain growth process.¹¹⁵ It was observed that, on one hand, there was an extensive oxidation process from Sn²⁺ to Sn⁴⁺ during the aging of FASnI₃. The authors proved that the introduction of FPEABr, which formed a 2D phase, prevents the oxidation of Sn²⁺ to Sn⁴⁺. On the other hand, the 2D tin-perovskite layer based on FPEA⁺ blocked the water penetration due to the presence of bonded F atoms, resulting in significantly improved humidity stability. Consequently, the highly oriented and stable 2D/3D PSC achieved an outstanding PCE of 14.81%, with a certified PCE of 14.03%, setting a new record for FASnI₃-based PSC at that time.

3.3.2. Aliphatic spacers. For aliphatic spacer cations, in 2021, Li et al. designed a dimensionally graded perovskite formation (DGPF) approach to achieve a homogeneously distributed self-passivated 2D/3D perovskite structure in the bulk. This structure was further capped by a stepped mixed-dimensional 2D perovskite with a wider-band gap through thermal annealing of the 3D perovskite layer covered with a spin-coated n-butylammonium bromide (BABr) solution.¹¹³ The introduction of the BA moiety immensely passivated antisite defects like Pb_Br and Pb_I, which were known as deep-level defects that can cause carrier recombination. Using the DGPF technique, the 2D/3D hybrid PSC demonstrated a maximum hysteresis-free PCE of 21.54% with a band gap-voltage offset (W_OC) of 0.39 V in a 1.63 eV perovskite system (RbCsMAFA-based) and a champion PCE of 23.78% with a low W_OC of 0.326 V in an approximately 1.53 eV perovskite system (Cs_0.03(FA_0.97MA_0.03)_0.97Pb(I_0.97Br_0.03)₃) (Fig. 11g–j). However, not all organic spacers can be applied to generate 2D perovskites within the 3D perovskite bulk through post-treatment. Jen et al. found that the introduction of BDA with a short carbon chain (the same number of carbon atoms as BA) could promote the transformation from 3D perovskite to 2D perovskite during post-treatment, whereas the octane-1,8-diaminium (ODA) (with twice the number of carbon atoms compared to BA and BDA) could only insert into the grain boundaries within the 3D perovskite bulk without any dimension transformation.¹¹⁴ The fast dimension transformation caused by BDA could be attributed to the high thermodynamic stability of 2D perovskites and the low activation energy of the transformation process. In contrast, the failure of ODA to facilitate dimensional transformation could be ascribed to two factors: (1) thermodynamically, there was a huge activation energy involved in the transformation process from the MAPbI₃ 3D perovskite to ODAPbI₄ 2D perovskite, although the anti-gauche isomerization of ODA may increase the Gibbs free energy of the ODAPbI₄ 2D perovskite (still lower than that of MAPbI₃); (2) kinetically, the transition from 3D to 2D perovskite occurred too slowly to be observed within a 10-second post-treatment time. This study emphasized the importance of reasonable molecular design for the surface modification and bulk passivation of perovskites (Fig. 11k).

3.3.3. Alicyclic spacers. In contrast to aromatic compounds, which also have a ring structure, alicyclic spacers, such as cyclohexylammonium, exhibit stronger N–H bonding due to the localized electron density in the cycle structure. The stronger bonding is more favorable for defect passivation compared to aromatic spacers such as phenylammonium.^116,117 As a result, in addition to long-chain aliphatic compounds, alicyclic spacer cations have been found to be suitable for producing 2D/3D heterojunctions.^118,119 For example, in 2022, Wu's group applied an alicyclic organic salt called (R)-(−)-1-cyclohexylethylammonium iodide (R-CEAI) to fabricate multiple 2D/3D perovskite homojunctions using the DGPF method.¹²⁰ Grazing incidence X-ray diffraction (GIXRD) characterization revealed residual stress in the R-CEAI-based perovskite films. The alleviated lattice distortion resulting from this stress was beneficial for carrier dynamics and device performance. Consequently, an R-CEAI-based PSC showed an improved PCE of 22.52% in comparison with the pristine PSC with a lower PCE of 19.43%.

In particular, the formation of coexisting 2D/3D heterojunctions opens up the possibility of combining different types of organic spacers. For instance, in 2022, Mai et al. selected oleylammonium (OAm) and PEA as perovskite bulk and surface passivation agents, respectively. These agents not only promoted the crystal growth in the out-of-plane orientation but also passivated defects at grain boundaries and the perovskite surface, leading to a reduction in trap density and nonradiative recombination.¹²¹ Consequently, the modified 2D/3D hybrid device with the architecture of FTO/NiO_x/PTAA/Al₂O₃/perovskite/PCBM/BCP/Ag achieved a PCE of 20.52% without any hysteresis. More reported organic spacer cations applied in bulk incorporation and surface treatment coexisting type PSCs can be found in Table 3.

Table 3 An exhaustive list of recently reported organic spacer cations applied in bulk incorporation and surface treatment coexisting type PSCs

Spcers	Device architecture	V OC (V)	FF (%)	J SC (mA cm^−2)	PCE (%)	Ref.
FPEA	ITO/PEDOT:PSS/FASnI3/ICBA/BCP/Al	0.840	70.76	24.91	14.81	115
2-PyEA	ITO/SnO2/FA0.92MA0.08PbI3/spiro-OMeTAD/Au	1.140	82.10	24.80	23.20	111
DMA	FTO/TiO2/Cs0.05FA0.85MA0.10Pb(I0.97Br0.03)3/spiro-OMeTAD/MoO3/Ag	1.140	78.00	25.43	22.61	122
ThFA	ITO/SnO2/FA0.84MA0.16PbI3/spiro-OMeTAD/MoO3/Ag	1.210	81.62	25.07	24.69	112
BA	ITO/SnO2/Cs0.03(FA0.97MA0.03)0.97Pb(I0.97Br0.03)3/spiro-OMeTAD/Au	1.210	80.00	24.57	23.78	113
OA	FTO/c-TiO2/PMMA:PCBM/Cs0.07Rb0.03FA0.765MA0.135PbI2.55Br0.45/spiro-OMeTAD/Au	1.240	78.46	24.24	23.62	123
TBA	FTO/PEDOT:PSS/FAPbI3/spiro-OMeTAD/Au	0.990	71.20	24.23	17.02	124
CME	FTO/c-TiO2/m-TiO2/Cs0.05FA0.80MA0.15Pb(I0.85Br0.15)3/spiro-OMeTAD/Au	1.110	77.00	23.45	20.33	125
Gu	ITO/SnO2/(FA0.93MA0.07Pb(I0.93Br0.07)3/spiro-OMeTAD/Au	1.120	81.15	24.79	22.53	126
TBH	(FAPbI3)0.84(CsPbI3)0.06(MAPbBr3)0.10	1.186	84.10	22.60	22.54	127
R-CEA	FTO/TiO2/(FAPbI3)0.84(CsPbI3)0.05(MAPbBr3)0.11/spiro-OMeTAD/Au	1.195	80.50	23.42	22.52	120
BDA	ITO/Cu:NiOX/MAPbI3/PCBM/C60/Ag	1.110	76.00	14.48	11.97	114
EDBE	ITO/Cu:NiOX/MAPbI3/PCBM/C60/Ag	1.160	71.00	4.33	3.44	114

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3.4. Other models

In addition to the three mainstream 2D/3D heterojunction structures mentioned above, 2D perovskites can also be constructed at the bottom of 3D perovskites through self-assembly and/or in situ formation to passivate the defects of the buried interface in 3D PSCs.¹²⁸ Yu et al. achieved this by fabricating 2D perovskite in situ at the buried interface through the interaction between the PEA spacers and 3D perovskite. They first deposited PEABr on an ITO substrate and then spin-coated the perovskite precursor. Moreover, Chen et al. demonstrated that the introduction of chiral cations, namely α-methylbenzylamine (S-/R-/rac-MBA), can induce the formation of 2D perovskites at the interface between PEDOT:PSS and FASnI₃, resulting in better matched energy levels and improved interface contacts and leading to improved hole extraction from FASnI₃ by PEDOT:PSS.¹²⁹ To achieve this, MBA was added in advance to prepare a quasi-2D perovskite precursor with n = 5. The precursor primarily formed a 3D structure from the upper surface rather than quasi-2D perovskite when the anti-solvent was dropped, due to the lower formation energy of 3D perovskite. Subsequently, the MBA molecules acted as templates to guide the downward growth of 3D perovskite. Finally, at the end of solvent evaporation, the excess MBA spacers interacted with [PbI₆]⁴⁻ octahedron layers at the bottom of the 3D perovskite, thus forming a 2D perovskite at the buried bottom interface. Additionally, considering the significance of buried interface passivation, White et al. proposed a design of 2D/3D/2D structure for double-side surface modification using the BA spacer, achieving in a remarkable PCE of 22.77%.¹³⁰ These studies have some implications for constructing 2D perovskites at the buried interface to reduce buried interface defects and enhance charge extraction.

4. Summary and outlook

In the past several years, significant advancements have been made in constructing 2D/3D heterojunctions by incorporating bulky and hydrophobic organic spacers into or onto 3D perovskite films. These advancements aim to enhance the photovoltaic performance and long-term stability of PSCs. However, due to the lack of a deep understanding in this field, we have presented the structural characteristics of both 3D and 2D perovskite materials, discussed the transformation process from 3D to 2D perovskite, summarized the key organic spacers used in fabricating 2D/3D PSCs, highlighted their roles in the formation mechanisms of different types of 2D/3D heterojunctions, and drawn the following conclusions:

2D/3D heterojunctions can be fabricated through bulk incorporation and surface treatment methods. In the bulk incorporation process, the formed 2D/3D heterojunctions play several roles: (1) passivating defects in bulk of 3D perovskite; (2) forming a distinctive structure, such as a porous structure for the PbI₂ film; (3) providing seeds and serving as a template to assist 3D perovskite growth; and (4) regulating the crystallization rate and controlling the crystal orientation; and (5) forming type I heterojunctions and reducing carrier recombination at grain boundaries. On the other hand, in the surface treatment process, 2D components containing different organic spacers are deposited on top of 3D perovskite through post-treatment. The resulting 2D/3D heterojunctions formed by this method exhibit the following effects: (1) passivating defects on the perovskite film surface; (2) forming a hydrophobic capping layer to improve humidity stability; (3) inhibiting ion migration; and (4) generating type II heterojunctions to promote carrier transport. Furthermore, some researchers have combined these two methods to prepare coexisting 2D/3D heterojunctions, which offer advantages of both types of 2D/3D heterojunctions.

Looking ahead, the construction of 2D/3D heterostructures remains a crucial method to improve the comprehensive performance of PSCs, including high efficiency and long-term stability. While the stability of 2D/3D hybrid PSCs is higher than that of 3D PSCs, the maximum PCE of 2D/3D hybrid PSCs (PCE = 24.9%)³⁰ is still behind that of the 3D PSCs (PCE = 26.1%).² To bridge this efficiency gap, researchers need to dedicate more attention and efforts. Based on the contents of this review, we propose several possible strategies for enhancing 2D/3D hybrid PSCs. (1) Prioritizing aromatic spacers over aliphatic spacers: aromatic spacers possess π-conjugated structures with a higher dielectric constant, which benefits charge transfer. (2) Focusing on modifying 2D seed crystals rather than simple salt treatments: recent studies have shown that 2D seed crystals provide more advantages compared to salt treatments alone. (3) Developing a coexisting 2D/3D heterojunction model instead of a single 2D/3D heterojunction model: as described in this review, the coexisting 2D/3D heterojunction model can take advantages of the two other single models. (4) Constructing 2D/3D heterojunctions in perovskite solar modules or tandem solar cells: in order to realize commercialization, the preparation of the perovskite solar modules and tandem solar cells is an inevitable trend for the future. Therefore, further studies are necessary to develop preparation technologies for constructing 2D/3D heterojunctions in perovskite solar modules and tandem solar cells.

In conclusion, we believe that with the joint efforts of researchers, significant progress can be made in improving the efficiency and stability of 2D/3D PSCs, ultimately leading to their commercialization.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (Grant No. 52273182) and Natural Science Foundation of Hebei Province of China (Grant No. B2022408005).

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