Multidimensional perovskite solar cells

Abstract

Organic-inorganic hybrid perovskite solar cells (PSCs) have attracted extensive attention, and their certified power conversion efficiency (PCE) has reached 25.5%. However, the instability of the high-efficiency 3-dimensional (3D) perovskite against ambient conditions (moisture, light and thermal) and the existing defects severely limit its practical applications and commercialization. Unlike 3D perovskites, the large hydrophobic spacer cations in low-dimensional (2D, 1D, and 0D) perovskites are able to effectively improve the stability, but they also weaken the light absorption range and hinder charge transport. The construction of a low-dimensional/3D perovskite multidimensional structure, which can combine the advantages of the high stability of low-dimensional perovskites and the superior efficiency of 3D perovskites, is proposed to achieve high efficiency and ultrastability. Moreover, the proper incorporation of low-dimensional perovskite into 3D perovskite can passivate defects and inhibit ion migration. Herein, this article summarizes the recent research progress of low-dimensional/3D perovskite multidimensional structures for PSCs and provides some perspectives toward developing stable and efficient PSCs.

Graphical abstract

Constructing a low-dimensional/3D perovskite multidimensional structure can combine the advantages of the high stability of low-dimensional perovskites and the superior efficiency of 3D perovskites. Moreover, the proper incorporation of low-dimensional perovskite into 3D perovskite can passivate defects and inhibit ion migration. Herein, a systematic review summarizes recent progress of the multidimensional structure-based perovskite solar cells. Multidimensional perovskite solar cells

1. Introduction

Organic-inorganic hybrid perovskites are considered to be one class of the most promising light harvesting materials for constructing high-efficiency solar cells due to their excellent optoelectronic properties [1], [2], [3], [4]. Among perovskites, 3-dimensional (3D) perovskites (ABX₃, A= MA⁺, FA⁺, Cs⁺; B= Pb²⁺, Sn²⁺; X= Cl⁻, Br⁻, I⁻) are the most promising and studied for use in high-efficiency perovskite solar cells (PSCs) due to their most suitable optical bandgap and excellent carrier mobility. In the past decade, PSCs based on 3D perovskite light-absorbing layers have developed rapidly, and the certified power conversion efficiency (PCE) has reached 25.5% [5], which is comparable to that of other commercial solar cells (such as Si-based solar cell). However, the instability of high-efficiency 3D perovskites under ambient conditions (moisture, light and thermal) and the large number of defects in 3D perovskite films severely limit their practical applications and commercialization. To improve the device stability and realize the theoretical PCE limit of 30%, strong environmental resistance and defect passivation are essential.

Perovskites can be regarded as interactions between A-site cations and corner sharing octahedra [BX₆]⁴⁻. The type and dimensionality of the perovskite will be affected by the coordination chemistry between the A-site cation and the octahedra [BX₆]⁴⁻ [6,7]. Structurally speaking, by completely or partially replacing small A-site cations with different bulky organic cations, 3D perovskites can be converted into low-dimensional (including two-dimensional (2D), one-dimensional (1D), and zero-dimensional (0D)) perovskites. It should be noted that the low dimensionality mentioned here means that the individual components of perovskite have at least one dimension down to the molecular level and does not refer to the final perovskite morphology (such as nanosheets, nanowires, nanocrystals). Unlike 3D perovskites, the large hydrophobic spacer cations in low-dimensional perovskites can effectively improve the thermal stability and suppress moisture invasion. However, the insulating spacer cations also lead to a large bandgap and act as multiple quantum wells to trap carriers and hinder charge transport [8,9]. Thus, despite the significant improvement in stability, single low-dimensional perovskites are not a good choice for the light absorbing layer of photovoltaic applications. Currently, many studies have shown that the incorporation of low-dimensional perovskites with 3D perovskites can prevent moisture intrusion, improve thermal stability, and suppress ion migration. These studies seem to indicate that the low-dimensional/3D perovskite multidimensional structure is an effective strategy to combine the advantage of the high stability of low-dimensional perovskites and the superior efficiency of 3D perovskites prepared by different methods, thereby resulting in high-performance perovskite devices with ultrastability [10], [11], [12], [13]. To date, there have been some reviews focusing on perovskite stability in the literature. However, they mainly start from the perspective of a single type of perovskite (such as Sn²⁺-based perovskite) or perovskite structure (such as 2D/3D) [14], [15], [16]. There are still no articles that systematically review this important field of low-dimensional/3D perovskite-based PSCs.

In this article, we review the recent progress on low-dimensional/3D perovskite multidimensional structure-based PSCs. We systematically classify the applications of 2D/3D, 1D/3D, and 0D/3D multidimensional structured perovskites. In particular, the advantages and disadvantages of different dimensions and different preparation processes are analyzed, and several representative preparation strategies for constructing the low-dimensional/3D perovskite multidimensional structure and improving the performance and stability of PSCs are emphasized. Finally, we introduce the challenges that perovskites still face and provide some perspectives toward stable and efficient low-dimensional/3D perovskite-based PSCs.

2. 2D/3D multidimensional film

2D perovskites are low-dimensional perovskites commonly used in PSCs. In 2D perovskites, the links between octahedra [BX₆]⁴⁻ are separated by large organic cations along crystallographic planes, and the chemical formula is generally described as (A’)_m(A)_n-1B_nX_3n+1 (where A’ represents aliphatic- or aromatic-based cations, m = 1 or 2, and n denotes the layer thickness of metal halide sheets). According to the different value of n, 2D perovskites can be divided into pure 2D perovskites (n = 1) and quasi-2D perovskites (1 < n < 6). It is worth noting that a mixture of 2D perovskites with low-n value (n < 4) and 3D perovskites can form quasi-3D perovskites (n = 30–60). However, since the smaller difference of thermodynamic stability in the high-n structures, the formed quasi-3D perovskites are difficult to be pure phases. Thus, the n value of quasi-3D perovskites is denoted by the precursor composition.

Due to the strong hydrogen bonding between the large organic cations and [BX₆]⁴⁻ and the high humidity resistance of the organic layers, the 2D perovskite is more stable than its 3D phases. However, compared to 3D perovskite-based devices, 2D perovskite-based PSCs do not exhibit ideal performance due to their larger bandgap, low carrier mobility and weaker carrier transport resulting from insulating spacer cations [17,18]. Since the first report in 2014, the highest PCE of reported 2D perovskite-based PSCs is only ∼18%. Therefore, the design of a 2D/3D multidimensional perovskite structure that combines the great stability advantage of 2D perovskites and the superior efficiency of 3D perovskites is more feasible than the direct use of a 2D perovskite.

2.1. 2D/3D perovskite bilayer structure

Depositing a 2D perovskite layer on the surface of a 3D perovskite to form a 2D (top)/3D (bottom) perovskite bilayer is one of the most commonly used methods for preparing 2D/3D perovskite multidimensional bilayer structures. The 2D perovskite top layer can serve as a protective layer to prevent water from invading the 3D perovskite film due to its stronger hydrophobicity, and it can act as a barrier layer to inhibit ion migration and charge carrier recombination between the 3D perovskite and top carrier transport layer (CTL) layer. Besides, preparing a 3D perovskite layer on an organic cationic substrate to induce the formation of an inverted 2D (bottom)/3D (top) bilayer structure is another common strategy. The bottom cation-induced 2D perovskite can modulate the film formation of 3D perovskite and modify the interface between the 3D perovskite layer and the bottom CTL. Currently, posttreatments (postspin-coating, postdipping, and postgas phase treatment), bottom surface modification, and some novel and niche methods are the main methods used to construct 2D/3D perovskite multidimensional structures. In this section, we review 2D/3D bilayer structured perovskites in which different preparation methods are used for the 2D perovskite layers.

2.1.1. Postspin-coating

Organic cation exchange reaction is the most commonly used strategy to prepare 2D/3D perovskite bilayer films, where the bulky organic molecules are diffused into the 3D perovskites to convert the surface 3D perovskites into 2D perovskites. Among the preparation methods based on organic cation exchange reaction, postspin-coating bulky organic molecules, such as 5-ammoniumvaleric acid (5-AVA) [19], phenethylammonium iodide (PEAI) [20], and butylammonium (BA⁺) [21], on 3D perovskites is the most commonly used strategy. The cation exchange process accomplished through the use of a slight solvent can optimize the reaction thickness by controlling the concentration and reaction time without destroying the underlying 3D perovskite. Most importantly, this strategy is applicable to perovskites prepared by different preparation methods, for example, triple cationic perovskite prepared by the two-step spin coating method [22], Sn-Pb mixed perovskite prepared by the blade coating method [23], and MAPbI₃ prepared by the one-step spin coating method [24].

Tracing the source, the 2D/3D perovskite bilayer structure is first constructed in 3D MAPbI₃. In 2016, Docampo et al. substituted a small MA⁺ in the top surface of 3D MAPbI₃ with long-chained PEA⁺ and obtained a (PEA)₂(MA)₄Pb₅I₁₆/MAPbI₃ 2D/3D bilayer structured perovskite film (Fig. 1a) [25]. Benefitting from the advantages of 2D perovskites, the 2D/3D heterojunction exhibits an increased hole transfer capacity and a reduced charge recombination rate, which contributes to higher open-circuit voltage (V_oc) and fill factor (FF). Then, the corresponding device based on 2D/3D bilayer structure exhibited an enhanced efficiency of 14.94% with an improved V_oc of 1.08 V, while these values for the pure 3D perovskite device are 13.61% and 0.99 V, respectively. More importantly, the 2D/3D bilayer structure-based device realized strong moisture hindrance under 75% humidity. The novel design proves the feasibility of the 2D/3D bilayer structure, but as this was an initial attempt, the research is not thorough enough and the efficiency is not high enough. In the following years, PSCs based on 2D/3D MAPbI₃ bilayer structures have achieved increasing progress, and the efficiency has been improved by 20% [26,27]. Meanwhile, to broaden the spectral range of light utilization, improve the thermal stability, and heighten the efficiency of the device, a narrower bandgap FAPbI₃ is proposed, and the corresponding 2D/3D bilayer structure is also studied. For instance, Gratzel's group spin-coated isobutylammonium iodide (IBAI) on α-FAPbI₃ to form a 2D IBA₂FAPb₂I₇ protective layer and used grazing incidence wide-angle X-ray scattering (GIWAXS) and X-ray reflectivity (XRR) measurements to verify its usefulness (Fig. 1b-d) [28]. They calculated that the difference in velocity of surface recombination between the 2D/3D bilayer and pristine 3D FAPbI₃ layer is 32 cm s⁻¹, proving that the 2D perovskite can efficiently passivate electronic trap states at or near the surface of the perovskite film. With molecularly tailored IBA₂FAPb₂I₇ applied as a 2D protective layer, the optimal device achieved an enhanced PCE close to 23% with superior stability (Fig. 1e, f), which was the highest value achieved by FAPbI₃-based PSCs at that time and is far superior to devices based on pure MAPbI₃. Of course, the treatment method is also effective for other MA and FA mixed perovskites that combine the advantages of MA and FA-based perovskites. For instance, Sargent selected 4-vinylbenzylammonium (VBA), which has a highly similar structure to PEAI, to build a (VBA)₂PbI₄/(MAPbBr₃)_0.15(FAPbI₃)_0.85 2D/3D bilayer structure (Fig. 1g) [29]. The 2D/3D perovskite film exhibited faster transport (the decay decreases from 842 ns to 51 ns) and lower trap state density compared to pristine 3D perovskite, and the corresponding device exhibited an improved PCE of 20.4% and retained 90% of its initial efficiency in air (∼30% relative humidity (RH)) for 2300 h (Fig. 1h). In addition, noted that the 25.2% recording efficiency work also adopts postspin-coating to generate 2D perovskite on triple cationic perovskite [30]. This work only proved the existence of 2D perovskites by X-ray diffraction (XRD) and did not provide a systematic comparison and characterization to illustrate the role of 2D perovskites. However, we speculate that it may also form a 2D protective layer on triple cationic perovskite to passivate defects and improve stability.

Fig. 1.

2D/3D perovskite bilayer structure prepared by postspin-coating methods. (a) Schematic illustration of the crystal structures of the 2D (PEA)₂(MA)₄Pb₅I₁₆/MAPbI₃ bilayer heterojunction [25]. GIWAXS data for (b) FAPbI₃ film and (c) 2D IBA₂FAPb₂I₇/FAPbI₃. (d) XRR of FAPbI₃ and 2D IBA₂FAPb₂I₇/FAPbI₃ films. (e) J-V curves and (f) stability test for FAPbI₃- and 2D IBA₂FAPb₂I₇/FAPbI₃-based devices [28]. (g) Schematic illustration of the formation process and (h) stability test for 3D (MAPbBr₃)_0.15(FAPbI₃)_0.85- and 2D (VBA)₂PbI₄/(MAPbBr₃)_0.15(FAPbI₃)_0.85-based devices [29].

Although the stability of PSCs has been greatly improved, hysteresis after stability tests are performed is rarely noticed. Bach et al. found that crystal defects and structural defects may occur during the postspin-coating process due to the prompt 2D perovskite formation process [31]. After humidity stability testing, these defects decompose 2D perovskites and generate a large hysteresis even though the reverse-scan PCE is still high. If these findings are correct, the credibility of the stability results of some previous strategies is questionable. These works require more performance characterizations to prove their stability results or better strategies are needed to solve the problem. Here, they introduced alkali cations (K⁺ and Rb⁺) into the butylammonium iodide (BAI)/isopropanol (IPA) posttreatment solution to exquisitely modulate the formation of 2D perovskite crystals, thereby reducing the undesired orientation or crystallization of the 2D perovskite. After adding K⁺ and Rb⁺, the morphology of the perovskite film improved, the mobility of the 3D/2D perovskite further increased from 2.0 × 10⁻³ to 2.3 × 10⁻³ cm² V⁻¹ s⁻¹, and the average lifetime increased from 72 to 98 ns. Finally, the optimal device retained 93% of its initial efficiency of 21.1% and had negligible hysteresis after 1200 h in 30%−60% humidified air. In addition, a p-type 2D perovskite layer is also considered to replace the traditional organic hole transport layer (HTL, spiro-OMeTAD), which is instable and has a high synthesis cost for hole transport. In 2018, Xu et al. converted the top 3D MAPbI₃ surface layer into a p-type 2D BA₂MA_n-1Pb_nI_3n+1 by spin-coating a BAI tert‑butanol solution on a MAPbI₃ perovskite film (Fig. 2a) [32]. The conduction band minimum (CBM), Fermi level and valance band maximum (VBM) of the 2D perovskite surface layer are −3.2, −4.5 and −5.4 eV, respectively, while those of the 3D perovskite are −3.9, −4.1 and −5.5 eV (Fig. 2b). Note that due to the poor conductivity of the 2D perovskite, a suitable amount of formamidinium iodide (FAI) is added to the BAI solution to construct a conducting channel, and a higher hole mobility of approximately 0.30 cm² V⁻¹ s⁻¹ is exhibited compared to the use of pure 3D perovskite (0.35 cm² V⁻¹ s⁻¹) and 3D/2D perovskite without FAI (0.07 cm² V⁻¹ s⁻¹). Finally, benefitting from the reasonable energy band position, enhanced conductivity, and stability of the 2D perovskite, the optimized unencapsulated device achieved an efficiency of 16.13% and maintained an efficiency over 70% over one month under an RH of 65% or a temperature of 85 °C (Fig. 2c).

Fig. 2.

2D/3D perovskite bilayer structure prepared by postspin-coating methods. Schematic diagram of (a) the growth process and (b) a band diagram of the (BA)₂(MA)_n-1Pb_nI3_n+1/MAPbI₃ planar heterostructure. (c) J-V curves, humidity stability, and thermal stability for the control MAPbI_3- and 2D/3D-based devices [32]. SEM images of (d) pure CsPbI₂Br and (e,f) BA₂CsPb₂I₆Br/CsPbI₂Br [34]. (g) Schematic diagram of the templated growth process of CsPbI₃/PEA₂PbI₄ films. (h) Electron and hole trap densities of the CsPbI₃, CsPbI₃/PbI₂, and CsPbI₃/PEA₂PbI₄ films [35].

The abovementioned cation exchange processes for the formation of 2D/3D bilayer perovskites all occur on organic-inorganic hybrid perovskites. This is because the organic cation cannot intercalate into the crystal lattice of all inorganic perovskites and therefore cannot form a new 2D perovskite phase even when the inorganic perovskite is immersed in the exchange solution for a long time [33]. However, this does not mean that postspin-coating is completely unsuitable for inorganic perovskites. There are still some interesting designs that have succeeded. For instance, Lin et al. added a trace amount of dimethyl sulfoxide (DMSO) into BAI chlorobenzene solution [34], where DMSO dissolves a thin layer of the 3D CsPbI₂Br film to enable the reaction with BAI to form a 2D perovskite protective layer (Fig. 2d-f). Finally, the optimized device delivered a PCE of 14.5% with significantly improved moisture stability. However, even though a dynamic spin-coating process is adopted to reduce the damage to 3D perovskites caused by DMSO, the inevitable damage to 3D perovskites may still cause device performance degradation. After that, Jen's group formed a CsPbI₃ film with excess PbI₂ at the grain boundaries by adding excess PbI₂ power to the CsPbI₃ precursor solution and then allowing the excess PbI₂ to react with PEAI to form a 2D perovskite (Fig. 2g) [35]. This process will not destroy the formed 3D perovskite and can passivate the defects at the grain boundaries. Compared to the CsPbI₃ film, the defect states of the CsPbI₃/2D film decreased from 1.15 × 10¹⁶ (electron) and 1.35 × 10¹⁶ cm⁻³ (hole) to 6.42 × 10¹⁵ and 7.08 × 10¹⁵ cm⁻³, respectively, due to the passivation effects of the 2D perovskite (Fig. 2h). The corresponding device efficiency increased from 15.69% to 18.82% with enhanced stability.

2.1.2. Postdipping

As another solution treatment strategy, the principle of the postdipping method is the same as that of the postspin-coating method. Obviously, the reaction rates of different treatment solutions and 3D perovskites are different. Compared to the postspin-coating process, the contact between the 3D perovskite and the posttreatment solution is more uniform in the immersion process, which has a relatively small effect on the treatment efficiency of the general treatment solution. As mentioned above, the prompt 2D perovskite formation process may induce many crystal defects and structural defects. Thus, a postdipping strategy that can reduce the concentration of an extremely reactive solution to slow down the reaction rate and control the formation process of 2D perovskite should be adopted in some cases. However, it should be noted that the n value may change over a long dipping time in some impregnation solutions, which then leads to the formation of a mixture of low-n perovskites [36]. In the preparation process, this phenomenon should be consciously avoided or used instead of ignored.

In 2019, Grätzel's group immersed 3D triple cation perovskite into a pentafluorophenylethylammonium iodide (FEAI) IPA solution to form a 2D (FEA)₂PbI₄ protective layer on a 3D perovskite (Fig. 3a) [37]. The formation of the protective layer is determined by examining the changing surface texture using scanning electron microscopy (SEM) images and depth elemental analysis from X-ray photoelectron spectroscopy (XPS) (Fig. 3b, c). In addition to replacing A-site cations or occupying the vacancies of A-site cations, the perfluorinated benzene in FEAI is normally hydrophobic and is hardly wetted by water due to the high electronegativity of fluorine, which is helpful to further improve the moisture resistance of perovskite films. Thus, the surface free energy of the 2D/3D perovskite layer was reduced from 54.7 to 24.3 mJ m⁻² compared with that of the bare 3D perovskite layer. Moreover, the nonperovskite phase at the surface of 2D/3D perovskite was also reduced due to the reaction: FAPbI₃(δ) + 2FEAI = (FEA)₂PbI₄ + FAI. Finally, the optimal FEAI-treated device exhibited a higher PCE of 22.1% and an enhanced stability at 90% RH compared to the bare device and PEAI-treated device (Fig. 3d). After that, Han et al. further used the FEAI precursor as a treatment solution to form a 2D (FEA)₂PbI₄ perovskite on 3D FAPbI₃ perovskite to act as an electron blocking layer between the electrode and 3D perovskite for HTL-free PSCs [38]. The CBM and VBM of the 2D perovskite surface layer are −3.36 and −5.78 eV, respectively, which are suitable for preventing the electron transfer/recombination to the carbon electrode. Then, the optimal device exhibited an enhanced PCE of 17.47% compared to the untreated device (16.30%). Moreover, without the traditional intolerant spiro-OMeTAD HTL, the optimized unencapsulated device retained 95% efficiency after 1000 h in air with an RH of 50–70%.

Fig. 3.

2D/3D perovskite bilayer structure prepared by post-dipping methods. (a) Schematic illustration of the formation process of the (FEA)₂PbI₄/3D perovskite bilayer heterostructure. SEM images of (b) pure 3D film, and (c) 2D/3D film. Scale bars, 1000 nm. (d) Performance of the control MAPbI₃ and 2D/3D-based devices. Scale bars, 1000 nm [37]. (e) Schematic illustration of the PTAI intercalation-induced formation of the PTAMAPbI₄ perovskite capping layer on PTAI-MAPbI₃. (f) The ion chromatography spectra of MA cations in an IPA solution, 1 mg mL⁻¹ PEA/IPA, and a 1 mg mL⁻¹ PTA/IPA solution after soaking MAPbI₃ perovskite films for 60 min [39].

The above research is mainly focused on the ion exchange process, and the formed 2D perovskite can be used as a protective layer and a passivation layer. To explore more possibilities, Zhao et al. innovatively proposed a 2D perovskite of PTAMAPbI₄ based on a steric phenyltrimethylammonium (PTA) cation, which can inhibit MAI extraction at the same time (Fig. 3e) [39]. Different from the regular cation exchange process, the PTA cations intercalate into the MAPbI₃ film to form PTAMAPbI₄ rather than substitute the MA cations. Due to the intercalation mechanism, the formed PTAMAPbI₄ can lock MA⁺ and hinder the PTAI from further intercalating with MAPbI₃. Thus, the morphology and thickness of PTAMAPbI₄ is not sensitive to soaking time and should be only ∼10 layers, while MAPbI₃ is highly sensitive to soaking time with other treatment solutions (such as PEAI). The thinner thickness of 2D perovskite should not affect carrier transmission. Surprisingly, the MA⁺ residues in the PTAI/IPA, PEAI/IPA, and pure IPA solutions after soaking MAPbI₃ films for 60 min were 0, 1.57 × 10⁻³, and 1.52 × 10⁻³ mmol (close to the total amount of MA⁺ in MAPbI₃) (Fig. 3f), respectively. This proved that the PTAMAPbI₄ protective passivation layer can further lock volatilized MA⁺ to significantly stabilize MAPbI₃. Finally, the unencapsulated PSCs realized a PCE of 21.16% and retained 93% efficiency after 500 h light illumination.

2.1.3. Vapor-treatment

Although the preparation methods based on the organic cation exchange reaction within posttreatment solutions have achieved breakthrough progress, most high-efficiency (>20%) 2D/3D multidimensional bilayer structured devices are prepared by these strategies. However, the commonly applied solvents (e.g., IPA) in wet-chemical approaches inevitably affect the quality of the underlying 3D perovskite film and may lead to an unavoidable intermixture of 2D and 3D perovskites, and the commonly applied methods (e.g., spin-coating) are unable to produce a large-area uniform layer. Vapor processing is intrinsically additive, so issues such as the solubility limitation of precursors or the need for orthogonal solvents do not exist. In the vapor-treatment process, the molecules in organoamine vapor can induce cation substitution through redox reactions in a large area. Based on this uniqueness, different preparation methods (vapor supply methods) have been developed. For instance, Chen et al. used butylamine vapor (BAV) to treat 3D MAPbI₃ to form a 2D perovskite protective layer, where the BAV was provided by an open bottle filled with butylamine liquid in a N₂ atmosphere [40]. The formation process occurs layer by layer from the surface to the bottom of the 3D film, and the thickness of the 2D layer is tuned by the BAV treatment time. Xie et al. vaporized BAI on 3D MAPbI₃ by a thermal evaporation process under an atmospheric pressure of 10 kPa (Fig. 4a) [41]. As the treatment time increases, the perovskite grains turn to platelet crystals with larger root square roughness (Fig. 4b-d). Bolink and coworkers first deposited PEA₂PbI₄/MAPbI₃ perovskite heterojunctions by dual-source vacuum deposition [42]. Based on different heterojunction structures, a reduction in charge transport is found without direct evidence of surface passivation at the 2D/3D interface. Moreover, there are multiple 2D phases in some works, and the efficiency of all vapor-treated devices is below 20%. The low efficiency may be caused by the interface contact problem or reaction conditions that are not controlled properly. Overall, the gas phase treatment for Pb-based perovskites requires more in-depth research.

Fig. 4.

2D/3D perovskite bilayer structure prepared by vapor-treatment methods. (a) Schematic diagram of the growth of the (BA)₂(MA)_n-1Pb_nI_3n+1/MAPbI₃ bilayer heterojunction structure. SEM images of the perovskite thin films without (b) and with 5 min (c) and 60 min (d) BAI vapor treatment [41]. (e) Schematic diagram of the growth of the PEA₂SnI₄/MASnI₃ bilayer structure. (f) TOF-SIMS depth profile of the 2D/3D perovskite thin film on a PEDOT:PSS/FTO substrate [43].

The solubility of Sn-based perovskites in IPA is much larger than that of Pb-based perovskites, and the effect of the short-term solution treatment process on Pb-based perovskites can be ignored, while damage to the bottom Sn-based perovskite layer may not be neglected. Thus, vapor treatment may be more suitable for Sn-based perovskites than Pb-based perovskites. Moon et al. fabricated PSCs based on a 3D MASnI₃/2D PEA₂SnI₄ bilayer perovskite (Fig. 4e) [43]. Different from the traditional process of forming 2D perovskite on 3D perovskite, they first treated vacuum-deposited SnI₂ with PEAI vapor to form a 2D PEA₂SnI₄ perovskite on the surface of 3D perovskite and then used MAI vapor to diffuse into the film to form 3D MASnI₃ perovskite. This strategy can minimize the exposure time and prevent the oxidation of SnI₂ and MASnI₃, and the bilayer structure has been proven to be feasible by time-of-flight secondary ion mass spectrometry (TOF-SIMS) (Fig. 4f). The amount of Sn⁴⁺ at the film interior, which acts as an electron trap and contributes to carrier recombination, decreased from 49.4% (3D perovskite) to 7.2% (2D/3D bilayer perovskite). Finally, the optimal bilayer perovskite-based device achieved an efficiency of 10.2% with improved stability. However, the actual situation is still not satisfactory, and the efficiency is still reduced by more than 10% after 24 h, even with a 2D perovskite layer. More effective strategies should be studied and used for Sn-based perovskite devices.

2.1.4. Bottom layer modification

All the works mentioned above attempt to construct a 2D perovskite protective layer on surface of a 3D perovskite. However, there are some studies that have constructed an inverted 2D (bottom)/3D (top) bilayer structure. Different from the top surface protective layer, the bottom cation-induced 2D perovskite is used to modulate the film formation of 3D perovskite and modify the interface between the 3D perovskite layer and the bottom CTL, thereby providing a better band alignment for carrier transport and enhanced stability. The earliest research was reported by Li's group, who deposited MAPbI₃ on a PEDOT:PSS/ITO substrate with/without PEI•HI treatment, in which PEI•HI was synthesized by branched polyethylenimine (PEI) and hydroiodic acid (HI) [44]. With the introduction of PEI•HI, the top 3D perovskite layer achieved favorable pore-free surface coverage and better crystallization. Finally, the optimal device achieved a 31% increment of PCE with an average 21% increment of J_sc and a 7% increment of V_oc. After that, Park et al. selected conductive ethylenediamine (EDA), which is useful for carrier transport, for the bottom 2D layer (Fig. 5a) [45]. With the introduction of the optimal amount of EDA, the grain diameters of the 3D perovskite film increased from several hundred nanometers to 2 μm, and the water contact angle increased from 68.9° to 93.1° (Fig. 5b-d). Moreover, the shunt resistance of the 2D/3D device (33 kΩ cm⁻²) and that of the 3D device (16 kΩ cm⁻²) were compared, and the 2D/3D device was proven to have a lower leakage current and fewer trap-assisted recombinations. Benefitting from the fewer defects, better crystallization and hydrophobicity of the 2D/3D perovskite film, the best-performing 2D/3D device obtained a higher PCE of 15.02% compared to pure 3D perovskite (13.1%). The main problem of this strategy is that the bottom surface modification layer needs to withstand the erosion of strong polar solvents (DMSO or DMF) when preparing the upper 3D perovskite. The above reports all believe that a 3D/2D perovskite bilayer structure is formed after the bottom surface modification strategy, but the final existing form (bilayer structure or mixed structure) and content of the 2D perovskite in multidimensional film is actually unclear. If the formed 2D perovskite can be dissolved in these polar solvents, more in-depth exploration and quantitative analyses are needed.

Fig. 5.

2D/3D perovskite bilayer structure prepared by bottom layer modification methods. (a) Schematic illustration of the induction of 2D growth at the base of a 3D MAPbI₃ film. SEM images of the (b) 3D and (c) 2D/3D perovskite films and (d) their corresponding contact angles [45]. (e) Schematic illustration and (f) cross-section SEM image of n-BAI double-side passivated PSCs. (g) J-V curves of the best-performing control and double-side passivated PSCs [47].

Regardless of the problem mentioned above, combined with the achievements from the 2D top perovskite protective layer, the double-sided surface layers are believed to be a promising strategy to achieve higher performance and stable PSCs. In 2018, Almora et al. first used impedance analysis to systematically study the effect of 2D perovskite interlayers (BI₂PbI₄, AVA₂PbI₄, and PEA₂PbI₄) with CH₃NH₃PbI₃ and Cs_0.1FA_0.74MA_0.13PbI_2.48Br_0.39 [46]. Recently, White's group successively proved that the advantages of upper and bottom double-sided passivation can be combined to further improve the performance by utilizing suitable passivators (Fig. 5e, f). Through using n-butylammonium iodide (n-BAI) as a double-sided passivator [47], they achieved a high efficiency of 22.77% for the Cs_0.07Rb_0.03FA_0.765MA_0.135PbI_2.55Br_0.45 perovskite with a bandgap of 1.6 eV (Fig. 5g). After that, they further achieved a high efficiency of 23.27% for Cs_0.07Rb_0.03FA_0.765MA_0.135PbI_2.55Br_0.45-based PSCs using dual-isomer alkylammonium cations (DIAC) [48]. The efficiency achieved is better than that of almost all single-sided passivated perovskite devices, which seems to indicate that double-sided passivation is a better strategy. However, currently, only their research group has reported an efficiency exceeding 22% or even over 23% for 2D/3D/2D perovskite multilayer structures. Therefore, this strategy requires more verification and research by different researchers around the world.

2.1.5. Other methods

Apart from the widely adopted methods mentioned above, some strategies that have not been extensively studied have also been proven effective. For instance, Noh et al. devised a solid-state in-plane growth (SIG) method to fabricate a 2D (BA)₂PbI₄/3D (FAPbI₃)_0.95(MAPbBr₃)_0.05 bilayer heterojunction structure [49], where a solid-state 2D (BA)₂PbI₄ film is stacked on a 3D (FAPbI₃)_0.95(MAPbBr₃)_0.05 film by pressure and heated to induce an in-plane direction growing 2D perovskite on a 3D perovskite. After optimizing the pressure, heating temperature and contact time, the trap-assisted nonradiative recombination of the SIG perovskite film is reduced, and the carrier lifetime is improved from 0.2 μs for the control device to 1.1 μs. Moreover, the 2D/3D bilayer perovskite is confirmed as a p-p isotype heterojunction structure, and the 2D perovskite has a wider bandgap of 2.38 eV. When the 2D perovskite is thin enough to be completely depleted, the thicker 2D perovskite can induce a higher built-in potential (V_bi). As a result, the optimal device exhibited an enhanced PCE of 24.59% with an improved V_oc of 1.185 V, while these values for the control device are 22.39% and 1.098 V, respectively. The corresponding encapsulated device maintained 94% of the PCE after 1056 h in an environment with a temperature of 85 °C and an RH of 85%. Sargent and coworkers directly introduced the PEAI cation into the ethyl acetate (EA) antisolvent for Pb-Sn mixed perovskite [50], where the approach enables PEA cations to penetrate deeper into the whole film compared to the posttreatment approach. They found that PEA can also mainly exist on the front surface of the perovskite film and does not penetrate deeply into the inside of the perovskite film. The layered perovskite domain is lower than the detection limit of many instruments for the surface-passivated films, which indicates that the passivation layer is still thin enough to achieve surface defect passivation, as proven through the XPS spectra (reduced Sn⁴⁺ signal) results, and maintains efficient charge extraction, as proven through the time-resolved photoluminescence (TRPL) decay results (32 ns for pristine perovskite and 210 ns for in-film surface-passivated perovskite). Moreover, the average surface potential difference between the grain boundary and the grain interior of in-film surface-passivated perovskite (39 mV) is smaller than that of pristine perovskite (72 mV) and traditional posttreated perovskite (75 mV), indicating that this approach can also passivate grain boundaries to reduce charge carrier trapping. Eventually, the optimal device achieved an improved PCE of 18.95% without an efficiency drop under a 200 h continuous working stability test.

In addition, because the cation exchange process is not suitable for inorganic perovskites, Liu et al. directly spin-coated synthesized inorganic Cs₂PbI₂Cl₂ nanosheets on the electron transport layer (ETL) layer and top surface of 3D CsPbI₂Br to obtain double-sided modified all-inorganic perovskite heterojunctions [51]. The main reason for choosing Cs₂PbI₂Cl₂ is its high hole mobility of 1.39 × 10⁴ cm² V⁻¹ s⁻¹, large electron mobility of 9.39 × 10³ cm² V⁻¹ s⁻¹, and better environmental stability (high temperature/humidity conditions) than organic-inorganic hybrid 2D perovskite. Finally, the Cs₂PbI₂Cl₂/CsPbI₂Br/Cs₂PbI₂Cl₂ inorganic heterostructure-based device delivered an enhanced PCE of 16.65% and retained 90% of its initial efficiency under an ambient condition with a temperature of 25 °C and a RH of 35% for 648 h. Zhu and coworkers developed a spontaneous interfacial manipulation (SIM) strategy to fabricate 2D GA₂PbI₄/3D CsPbI_xBr_3-x bilayer perovskite [52], in which guanium (GA⁺) was added into the perovskite precursor. During the thermal annealing process, GA cations are pushed up to the film surface and combine with unsaturated Pb to spontaneously form an ultrathin 2D GA₂PbI₄ structure. Compared to the pristine CsPbI_xBr_3-x film, the grain size enlarged from 100 to 300 to 300–700 nm with fewer cracks, and the recombination lifetimes were prolonged from 60.2 to 213.5 ns, suggesting a higher film quality with efficient charge extraction of the 2D/3D bilayer perovskite. As a result, the PCE was enhanced from 13.64% (control PSCs) to over 18% (SIM-based PSCs). Although the spin-coating method was chosen in this work, we infer that this strategy is also applicable to other large-area preparation methods. At the same time, this strategy can effectively passivate the 3D perovskite surface and simplify the preparation process of bilayered perovskite without using weakly polarized posttreated solvents. Therefore, Guo et al. subsequently added S-benzyl-lcysteine into the MAPbI₃ precursor to fabricate a 2D/3D bilayer perovskite by using a similar strategy [53]. In this process, they chose a vacuum-assisted one-step blade-coating method that can deposit a large-area perovskite film. Finally, the defect density decreased from 2.46 × 10¹⁶ (3D device) to 1.53 × 10¹⁶ cm⁻³ (2D/3D device), and the corresponding PCE increased from 18.46% to 20.14%.

2.2. 2D/3D perovskite mixed structure

In addition to the bilayer structure, the mixed structure is the other type of 2D/3D perovskite heterojunction, where the 2D perovskite can be located at the grain boundaries and incorporated into the 3D perovskite lattice to passivate defects in the corresponding place and reduce ion migration. To date, there are many different methods that can be used to successfully prepare 2D/3D mixed perovskites. Different from the commonly used postprocessing method for preparing bilayer structures, directly introducing 2D perovskite into the perovskite precursor can affect the crystal growth process and tune the morphology and quality of the 3D perovskite film due to the strong hydrogen bonding between the 2D additive and [PbX₆]⁴⁻, thereby improving carrier transmission, decreasing the trap density and suppressing carrier recombination. More importantly, reasonable regulation can improve the stability of 3D perovskite films without causing unacceptable performance degradation. In this section, we start with the preparation steps and review the progress of 2D/3D mixed perovskites.

2.2.1. One-step method

The greatest advantage of the one-step method is that it can simplify the preparation process, of which the one-step spin coating method is the most commonly used. In this process, 2D perovskite and 3D perovskite precursors should be properly mixed in proportion. In 2017, Snaith's group blended the calculated 2D BAPb(I_0.6Br_0.4)₃ and 3D FA_0.83Cs_0.17Pb(I_0.6Br_0.4)₃ perovskite precursor solutions to obtain BA_x(FA_0.83Cs0_.17)_1-xPb(I_0.6Br_0.4)₃ perovskite precursor solutions with different BA contents [54]. With the introduction of an appropriate ratio of 2D perovskite precursor, the perovskite grain size was enlarged, and ‘plate-like’ 2D phase crystallites formed between the 3D perovskite grains. Moreover, they evidenced that through this strategy, the crystal growth is more orderly, the crystallinity is higher, the defects of nonradiative recombination can be reduced and the stability of the device can be improved. Certainly, the strategy is also applicable to all-inorganic perovskites and has been confirmed. For instance, Zhao and coworkers mixed CsPbI₃ precursors with different ratios of EDAPbI₄ precursors to form a CsPbI₃•xEDAPbI₄ precursor solution [55], where 2D EDAPbI₄ was used to promote the formation of high quality and phase stable α-CsPbI₃ perovskite films. Finally, the 2D/3D mixed film realized a reduced surface defects and an increased photoluminescence (PL) lifetime, suggesting a suppression of radiation-less recombination. The corresponding optimal device achieved a 54.8% increment of PCE with a 28.2% increment of J_sc and a 10.6% increment of V_oc.

During the spin coating process, the dripping of antisolvent can quickly remove the residual perovskite solvent, which can further enhance crystallization and reduce the pinholes caused by solvent volatilization. Of course, the one-step anti-solvent method will also make the preparation process more complicated compared to the traditional one-step spin-coating method without an antisolvent. For instance, Song et al. added chlorobenzene as an antisolvent in the last 10 s of the spin-coating process of the perovskite precursor [56]. All the films (pure 3D perovskite and 2D/3D multidimensional perovskite) possess a pinhole-free morphology, and the mixed film has “flake-like” 2D EDBEPbI₄ crystallites vertically standing between the 3D grains (Fig. 6a,b). This is because the exposed amine groups at both faces of the 2D EDBEPbI₄ perovskite effectively interact with the 3D perovskite. Benefitting from the vertical passivation effect, the hole/electron trap densities are reduced from 6.2 ± 0.5 × 10¹⁵/4.4 ± 0.5 × 10¹⁵ for the pure 3D perovskite to 2.8 ± 0.3 × 10¹⁴/2.9 ± 0.3 × 10¹⁴ cm⁻³ for the 2D/3D mixed perovskite. More importantly, vertical passivation does not affect the carrier transport. Finally, the optimal 2D/3D mixed perovskite-based device achieved a high PCE of 21.06% and an enhanced stability (maintaining 90% PCE over 3000 h in air) (Fig. 6c). Kanatzidis dropped toluene as an antisolvent on the rotating perovskite film [57], and all the MA_0.5FA_0.5Pb_0.5Sn_0.5I₃ films with or without a 3-(aminomethyl)piperidinium organic spacer were dense and pinhole-free. The mixed film shows a longer average carrier lifetime (671.6 ns) than that of pure 3D perovskite (204.6 ns) and decreases the oxidation degree of Sn²⁺ to Sn⁴⁺ (serving as a recombination center). Therefore, the optimal 2D/3D device obtained an improved efficiency of 20.09% and an enhanced stability.

Fig. 6.

2D/3D perovskite mixed structure. SEM images of (a) pure MAPbI₃ film and (b) optimal EDBEPbI₄/MAPbI₃ mixed film. The “flake-like” structure in the yellow area is EDBEPbI₄. (c) Stability test of pure MAPbI₃- and EDBEPbI₄/MAPbI₃-based devices [56]. (d) Schematic illustration of the growth of 2D/3D perovskite films based on ArFACl [61]. (e) Schematic illustration of the film fabrication strategy without (route in red arrows) and with (route in blue arrows) PEAI evaporation [64].

Notably, the one-step spin-coating method can produce high-efficiency small devices; thus, it is suitable for laboratory research. However, a commercially available method must be able to produce large-area films. The small-area preparation methods are suitable for exploration and mechanism analysis, and then provide theoretical guidance for the design of PSCs. The large-area preparation methods are used to meet future practical applications. Therefore, the preparation methods of large-area perovskite film are also necessary to be studied. In 2017, Nazeeruddin prepared a 2D/3D (HOOC(CH₂)₄NH₃)₂PbI₄/CH₃NH₃PbI₃ perovskite by using a printable industrial-scale process [58]. In particular, they further substituted the HTL with hydrophobic carbon electrodes. Finally, the device delivered an efficiency of 11.2% for 10 × 10 cm² modules and was stable for one year with zero loss. After that, Guo et al. incorporated 2-(4-fluorophenyl)-ethylammonium iodide (FPEAI) into (MAPbI₃)_0.75(FASnI₃)_0.25 to inhibit the oxidation of Sn²⁺ to Sn⁴⁺ and prepared a high quality 2D/3D multidimensional film by using a vacuum-assisted (1000 Pa) blade coating method [59]. As a result, the optimal 2D/3D multidimensional device achieved an efficiency of 17.51% for small areas and 13.8% for large areas (1 cm²), while an efficiency of only 12.51% was achieved for pure 3D perovskites with small areas.

2.2.2. Two-step method

In the research of PSCs, the two-step method is another type of extensively researched preparation method. Take the two-step spin coating method as an example, in brief, the first layer of precursor reactant (such as PbI₂) is spin-coated on the substrate, then the second layer of precursor (such as MAI) is spin-coated on the first layer and reacts to form a dense perovskite film. For 2D/3D mixed perovskites, the A-site precursor of 2D perovskites needs to be added to the A-site precursor or B-site precursor of 3D perovskites. For instance, Chen et al. added different contents of 2-thiophenemethylammonium (ThMAI) to the A-site cation precursor of 3D perovskites in a solution of FAI:MAI:MACl and then spin-coated the prepared PbI₂ film to form a 2D/3D mixed structured perovskite [60], where the ThMA⁺ cations can form extended organic sheets and hydrogen-bonding interactions with the neighboring octahedra [PbI₆]⁴⁻. They found that the addition of ThMA⁺ by this method can enlarge the grain size (from ∼1.5 to ∼3 μm), increase the film hydrophobicity (water droplet contact angle increase from 57.7° to 99.1°), and induce crystalline growth and orientation (higher intensity and smaller full width at half maximum (FWHM) for the main (110) peak). Then, the mixed film leads to a more balanced carrier mobility (hole and electron mobilities of 2.09 × 10⁻³ and 2.14 × 10⁻³ cm² V⁻¹ s⁻¹) and a longer carrier lifetime (from 1.21 to 1.85 μs). Fimally, the optimized device obtianed a PCE of 21.49% with a notable FF of 81%. Recently, Liu and coworkers introduced aromatic formamidiniums (ArFAs) into organic salt precursor solutions (Fig. 6d) [61], and achieved the highest efficiency (23.39%) among the devices based on 2D/3D perovskite mixed structures. This mainly benefits from the strong hydrogen bonding interactions between the ArFA spacer and the corner-sharing [PbI₆]⁴⁻, which induce better crystalline orientation, fewer defect states (from 1.85 × 10¹⁵ to 9.70 × 10¹⁴ cm⁻³), a longer carrier lifetime (from 421 to 1396 ns), a larger photovoltage decay time (from 49.1 to 198.5 μs), and improved stability (maintaining 56% efficiency for 3D devices and 95% efficiency for 2D/3D devices after 1360 h in air with 30–40% humidity). To add BAI to the B-site cation precursor of 3D perovskites, Li et al. introduced BAI into a PbI₂ precursor solution, where BAI can cut and squeeze the lattice network of PbI₂ and induce a better orientation of 2D PbI₂ [62]. In the following process, the BAI will redissolve in the MAI IPA solution to cause a mesoporous structure of PbI₂ to facilitate the reaction of MAI and PbI₂ crystals and leave a small amount of the BAI to form a 2D perovskite protective layer. Finally, they obtained a high-quality perovskite film with large grains and good crystallinity, and the corresponding carrier lifetime increased from 49.484 (control 3D perovskite) to 99.272 ns (optimized 2D/3D multidimensional perovskite).

In addition to the solution process, the vapor process is another widely studied method due to its advantages mentioned above, such as its large-scale and well-controlled reaction rates. Thus, some researchers have also attempted to adopt the vapor method to fabricate 2D/3D perovskite mixed structures by using a two-step process. In 2018, Guo et al. combined the solution and low-pressure vapor method to prepare a 2D/3D perovskite multidimensional film by using a solid-vapor reaction process [63], where the MAI vapor was pushed to react with the spin-coated PEAI/PbI₂ hybrid film at a pressure of ∼1 torr. Through optimizing the doping amount of PEAI, tiny (PEA)₂(MA)_n-1Pb_nI_3n+1 islands were formed around the grain boundaries of the dense MAPbI₃ perovskite film to lower the trap state density and enlarge the MAPbI₃ grain size, and the efficiency of corresponding device was improved from 17.31% to 19.10%. Wu et al. fabricated a (PEA)₂SnI₄/FASnI₃ bulk heterojunction by continuously evaporating PEAI and SnI₂ on a FAI mixture particles film (Fig. 6e), in which the first evaporated PEAI was attached both on and between FAI particles [64]. The incorporation of PEAI improved the FASnI₃ crystal growth and inhibits Sn²⁺ oxidation to Sn⁴⁺, thereby improving the film coverage, decreasing the trap density from 2.97 × 10¹⁵ to 1.95 × 10¹⁵ cm⁻³ and enhancing the device stability. In general, to date, there are very few studies (only 3 items) based on this strategy, and the efficiencies of the corresponding devices are lower than those of other devices based on the same type of perovskite prepared by other methods. The self-regulating ability of the solid-gas reaction is not as good as that of the liquid-liquid reaction and solid-liquid reaction. The low efficiency may be due to the unreasonable internal 2D/3D distribution and the limited passivation effect caused by possible interface contact problems, which can also be observed from the comparison of the calculated defect state density (only reduced by approximately 30%).

3. 1D/3D multidimensional film

One-dimensional perovskites are another important component of low-dimensional perovskites. In the 1D perovskite, the links between octahedra [BX₆]⁴⁻ are further separated by large organic cations along the [010] plane in the 2D perovskite, and the chemical formula is A₃BX₅. The linearly arranged [BX₆]⁴⁻ can improve the skeleton strength of perovskite lattice and the inactive organic cations can wrap and protect the metal halides. Thus, the 1D perovskite phase is theoretically more stable than its 2D and 3D phases against water, oxygen, and light, so it is also useful to modify the 3D perovskite by constructing 1D/3D heterojunctions and passivating the crystal boundaries for environmentally stable PSCs. However, apart from the impressive stability, the 1D perovskite also brings some unfavorable characteristics, such as reduced carrier mobility and a weakened carrier separation capacity, into 1D/3D multidimensional perovskite-based devices due to the presence of bulky organic cations. Therefore, although great efforts have been dedicated and excellent success has been achieved for 2D/3D heterojunction-based PSCs, 1D/3D heterojunction-based PSCs are still largely underexplored. The main problem is determining how to design a reasonable 1D/3D multidimensional perovskite structure to improve the device stability without sacrificing the efficiency of the device. In this section, we review 1D/3D multidimensional perovskite-based PSCs with different perovskite film layer structures that are similar to 2D/3D multidimensional perovskites (Table 1).

Table 1.

Properties of multidimensional perovskite film and photoelectric performance of corresponding perovskite solar cells.

Perovskites	PCE (%)	Trap state density (cm−3)	Carrier lifetime (ns)	Stability (retain 90% efficiency)	Year	Ref.
PEA2SnI4/FASnI3	9.41	-	-	600 h (N2)	2018	10
β-GUA + FA0.95Cs0.05PbI3	22.2	3.69 $\times$ 1016	1100	∼160 h (35% RH)	2020	12
PEA2PbI4/FAMACs triple cationic perovskite	21.31	1.39 $\times$ 1015	-	∼200 h (∼30% RH)	2020	20
OAI + (FAPbI3)0.95(MAPbBr3)0.05	22.9	-	1150	∼30 h (∼65% RH and 65°C)	2019	21
PNAI + FAMACs triple cationic perovskite	22.62	1.98 $\times$ 1015	-	∼1000 h (∼30% RH)	2020	22
PEABr + (FASnI3)x(MA PbI3)y	15.15	-	-	∼50 h (∼50% RH)	2019	23
(PEA)2(MA)4Pb5I16/MAPbI3	14.94	-	-	∼19 days (75% RH)	2016	25
DMEDAI2 + MAPbI3	20.2	-	142	∼15 days (60-80% RH)	2019	27
IBA2FAPb2I7/FAPbI3	22.7	-	-	>700 h (N2, 1 sun)	2020	28
(VBA)2PbI4/(MAPbBr3)0.15(FAPbI3)0.85	20.4	-	51	∼2300 h (∼30% RH)	2019	29
C15H34NBr + FAMACs triple cationic perovskite	25.2	-	3600	3600 h (encapsulated, 30% RH)	2021	30
RbI/KI/BAI + FAMACs triple cationic perovskite	21.1	-	98	>1200 h (30-60% RH)	2020	31
BA2MAn-1PbnI3n+1/MAPbI3	16.13	-	-	∼10 days (65% RH)	2018	32
BA2CsPb2I6Br/CsPbI2Br	14.5	-	4.19	>25 days (25% RH)	2019	34
PEA2PbI4/CsPbI3	18.82	6.42 $\times$ 1015 (e) 7.08 $\times$ 1015 (h)	55.47	∼30 h (40% RH)	2021	35
(FEA)2PbI4/FAMACs triple cationic perovskite	22.1	-	2550	∼1000 h (40% RH)	2019	37
(FEA)2PbI4/FAPbI3	17.47	-	-	>1000 h (50-70% RH)	2021	38
PTAMAPbI4/MAPbI3	21.16	-	-	>500 h (N2, 1 sun)	2020	39
BAV + MAPbI3	∼20	-	165.5	∼600 h (30-40% RH)	2020	40
(BA)2(MA)n‐1PbnI3n+1/MAPbI3	16.5	-	-	∼20 days (55% RH)	2019	41
PEA2PbI4/MAPbI3	18.3	-	-	-	2019	42
PEA2SnI4/MASnI3	9.2	-	-	∼24 h	2020	43
MAPbI3/(PEI)2PbI4	13.8	-	-	∼10 days (∼50% RH)	2015	44
(EDA2)(CH3NH3)PnI3n+1/MAPbI3	15.02	-	-	∼120 h (∼55% RH)	2020	45
n-BAI + Cs0.07Rb0.03FA0.765MA0.135PbI2.55Br0.45	22.7	-	-	20 h (N2, 1 sun)	2020	47
DIAC + Cs0.07Rb0.03FA0.765MA0.135PbI2.55Br0.45	23.27	-	-	>100 h (N2, 1 sun)	2020	48
(BA)2PbI4/(FAPbI3)0.95(MAPbBr3)0.05	24.59	-	1100	1000 h (encapsulated, 85% RH and 85°C)	2021	49
PEAI + Sn-Pb perovskite	18.95	-	210	>200 h	2020	50
Cs2PbI2Cl2/CsPbI2Br	16.65	6.46 × 1015	70.41	648 h (35% RH and 25°C)	2020	51
GA2PbI4/CsPbIxBr3-x	>18	0.7 $\times$ 1014 (e) 0.64 $\times$ 1014 (h)	213.5	∼100 h (40% RH and 25°C)	2020	52
SBLC + MAPbI3	20.14	1.53 $\times$ 1016	247	∼10 days (50% RH)	2020	53
BAPb(I0.6Br0.4)3/FA0.83Cs0.17Pb(I0.6Br0.4)3	20.6	-	-	∼200 h (45% RH, 1 sun)	2017	54
CsPbI3/EDAPbI4	11.8	-	-	∼30 days	2017	55
EDBEPbI4/MAPbI3	21.06	2.9 $\times$ 1014 (e) 2.8 $\times$ 1014 (h)	185	3000 h	2018	56
3AMP + MA0.5FA0.5Pb0.5Sn0.5I3	20.09	-	671.6	∼25 h (20-50% RH, 1 sun)	2020	57
(HOOC(CH2)4NH3)2PbI4/CH3NH3PbI3	11.2	-	-	1 year	2017	58
FPEAI + (MAPbI3)0.75(FASnI3)0.25	17.51	3.58 $\times$ 1015	2190	∼50 h (50% RH)	2020	59
ThMAI + FAMA perovskite	21.49	-	1850	1800 h (encapsulated, 30-50% RH)	2019	60
ArFAs + FAMA perovskite	23.39	9.7 $\times$ 1014	1396	>1360 h (30-40% RH)	2021	61
BAI + MAPbI3	18.01	-	99.3	∼150 h	2020	62
(PEA)2(MA)n-1PbnI3n+1/MAPbI3	19.1	-	-	∼ 20 h (1 sun)	2018	63
EAI + FAMACs triple cationic perovskite	22.3	-	560	>500 h (N2, 1 sun)	2019	65
PyPbI3/MAPbI3	16.65	-	-	>110 days (30-65% RH)	2020	66
(CH3)3SPbI3/(FAMACs) PbI3-xBrx	16.67	-	24	∼100 h	2020	67
TAPbI3/(FAMA)PbI3	18.97	-	114	∼100 h (70% RH)	2019	68
PyPbI3/FA0.9Cs0.1Pb(I0.92 Br0.08)3	19.62	-	259.87	∼7 days (65% RH)	2020	69
PAI + FAMACs triple cationic perovskite	21.19	-	-	>3000 h (N2, 1 sun)	2020	70
PZPY + FAMACs triple cationic perovskite	18.1	7.63 $\times$ 1015 (e) 4.05 $\times$ 1015 (h)	147	30 h (40% RH and 25-85°C cycling)	2018	71
HAPbI2Br + FAMACs triple cationic perovskite	21.2	3.10 $\times$ 1015	1216.57	2520 h (30% RH and 25°C)	2021	72
[(CH3)3NCH2I]PbI3/(FAPbI3)0.85(MAPbBr3)0.15	22.7	-	-	>120 h (50% RH)	2021	73
δ-FAPbI3/CsFAMA1-xGAx	20.29	-	22.7	∼15 days (40% RH and 80°C)	2019	74
PbI2-BPy(II)/FAMACs triple cationic perovskite	21.18	1.43 $\times$ 1016 (e) 6 $\times$ 1015 (h)	-	∼60 h (50% RH, 1 sun)	2020	75
Cs4PbI6/γ-CsPbI3	16.39	-	7.81	>500 h (N2, 1 sun)	2019	76
Cs4Pb(IBr)6/CsPbI3-xBrx	14.77	-	8.70	>60 days (N2)	2020	77

3.1. 1D/3D perovskite bilayer structure

Similar to 2D/3D multidimensional perovskites, posttreatment, especially postspin-coating organic molecules, is the common strategy for constructing 1D/3D bilayer structured perovskites. In 2019, Grätzel's group systematically investigated the interface between the perovskite and the HTL by using this posttreatment method based on an FA-based perovskite device and found that EA/IA/Gua and FA were all microscopically mixed in the same phase with through-space atomic-level contact [65]. The decay lifetime increased from 250 (control sample) to 560, 625, and 333 ns after surface treatment by optimal ethylammonium iodide (EAI), imidazolium iodide (IAI), and guanidinium iodide (GuaI) concentrations, respectively. The larger decay time and higher PL intensity suggest a reduction of the non-radiative recombination losses. Finally, with the surface treatment of an optimal amount of EAI, IAI, and GuaI, the hysteresis of the devices was greatly reduced and the corresponding PCE was enhanced from 20.5% to 22.3%, 22.1%, and 21.0%, respectively. Moreover, this strategy is applicable to different types of perovskites prepared by different preparation methods. For instance, Pham et al. spin-coated pyrrolidine iodide (PyI) on MAPbI₃ prepared by using a one-step antisolvent method to obtain a 1D PyPbI₃/3D MAPbI₃ bilayer perovskite heterojunction structure [66]. Elsenety et al. spin-coated trimethyl sulfonium cations ((CH₃)₃SCl) on 3D (FA/MA/Cs)PbI_3-xBr_x prepared by using a one-step antisolvent method to form a 1D (CH₃)₃SPbI₃ top layer [67]. Gao and coworkers deposited thiazole ammonium (TA) on (MA/FA)PbI₃ obtained by using a gas-pump drying method to form a 1D TAPbI₃/3D (MA,FA)PbI₃ stacked structure [68], and the corresponding device realized a PCE of 18.97% and retained 92% of the PCE in air (∼20% humidity and 20 °C temperature) for 2 months. All of these 1D perovskite top layers serve as moisture resistance layers to prevent water intrusion into the 3D perovskite film and as barrier layers to suppress ion migration and charge carrier recombination. In turn, the stability of the devices is improved.

However, although some excellent studies have been reported, the effect of 1D perovskites on the phase transition behavior of 3D perovskites in bilayer perovskite structures is still unclear. The main problem is that the formed 1D perovskites only serve as a protective layer to prevent water molecule invasion or affect the 1D/3D bilayer structure through thermodynamic and kinetic reliability. Thus, Xu and coworkers further investigated the effect based on an in situ 1D/3D FA-based perovskite bilayer structure [69]. In this process, pyrrolidinium hydroiodide (PyI), which can provide ammonium groups coordinated with the lead vacancy in 3D perovskite, is spin-coated on surface of the 3D FA_0.9Cs_0.1Pb(I_0.92Br_0.08)₃ perovskite layer prepared by the two-step spin-coating method to form a thin PyPbI₃ capping layer. The prolonged lifetime of the 1D/3D perovskite is 30–40 times that of the 3D perovskite due to the enhanced activation barrier in the presence of 1D PyPbI₃ (Fig. 7a). According to DFT calculations, the formation thermodynamic energy differences (ΔE) between the α- and δ-phases are calculated to be −0.04 eV (1D/3D perovskite) and −0.16 eV (3D perovskite) (Fig. 7(b), c). They first proved that the 1D perovskite in the 1D/3D bilayer structure is kinetic and thermodynamically reliable and helps stabilize the black perovskite phase of the 3D perovskite. Finally, the optimal 1D/3D structured PSCs obtained a PCE of 19.62% and maintained 70% efficiency after 27 days in an ambient environment with an RH of 65 ± 5%.

Fig. 7.

1D/3D perovskite bilayer structure. (a) Transformation fraction plots for 3D and 1D/3D films. (b) DFT-computed energy difference between the α- and δ-phase FA-based perovskites with and without a 1D PyPbI₃/perovskite interface. (c) Thermodynamic and kinetic analysis of the 3D perovskite and 1D/3D perovskite phase transition processes [69]. (d) Schematic diagram of the in situ cross-linking progress of PA⁺ in 1D ‘‘perovskitoids’’. (e) TRPL spectra of reference, 1D/3D and cross-linked 1D/3D perovskite/HTM films. GIXRD of the (f) reference films and (g) 1D/3D before and (h) after cross-linking films [70].

Note that the efficiencies of most of the 1D/3D bilayer perovskite structured PSCs mentioned above are all below 20%, which is more unbearable than that based on 2D/3D bilayer perovskites. The low efficiency is due to the shorter carrier diffusion length, lower carrier mobility, and weaker charge transport caused by the insulating spacer cations within the 1D perovskite. To simultaneously improve the carrier transport and the PCE based on 1D perovskites, Yang et al. first combined 1D perovskites with cross-linking to modify 3D perovskites [70]. The propargylammonium (PA⁺) with a cross-linkable alkynyl terminal is cross-linked surrounding the 1D perovskitoid, allowing the carrier to effectively provide transport between the 1D chains of face-sharing octahedra [BX₆]⁴⁻ (Fig. 7d). The decay time is 48.34 ns (Fig. 7e), and the hole mobility value is 1.89 × 10⁻³ cm⁻² V⁻¹ s⁻¹ for the cross-linked 1D/3D perovskite, which is far better than that of the 1D/3D perovskite (84.71 ns, 8.42 × 10⁻⁴ cm⁻² V⁻¹ s⁻¹) and the reference (142.64 ns, 6.70 × 10⁻⁴ cm⁻² V⁻¹ s⁻¹). Moreover, this design can transform the residual tensile strain (a source of instability) to compression strain in the 3D perovskite film, as confirmed by grazing incident X-ray diffraction (GIXRD) (Fig. 7f-h). With the cooperation of 1D perovskite and the cross-linking strategy, the optimal device with complete cross-linked PA⁺ delivered an enhanced PCE of 21.19% and an improved stability (maintaining 93% efficiency after continuous illumination for over 3000 h).

3.2. 1D/3D perovskite mixed structure

In addition to the perovskite bilayer structure, the perovskite mixed structure is the other type of 1D/3D perovskite heterojunction, where the 1D perovskite can be incorporated into the 3D perovskite lattice to reduce ion migration and avoid the collapse of the 3D perovskite structure frame. In this type of 1D/3D perovskite heterojunction, the common fabrication process introduces 1D perovskite as an additive to 3D perovskite precursor solutions. Similar to the 2D/3D mixed structure, it can affect the growth process and tune the morphology and quality of the 3D perovskite film due to the strong hydrogen bonding between the 1D additive and [PbX₆]⁴⁻, thereby improving carrier transmission, decreasing the trap density and suppressing carrier recombination. More importantly, reasonable regulation can improve the stability of 3D perovskite films without causing unacceptable performance degradation. For instance, Mai and coworkers introduced 2-(1H-pyrazol-1-yl)pyridine (PZPY) into the Cs_0.04MA_0.16FA_0.8PbI_0.85Br_0.15 precursor and then spin-coated it on substrates to obtain 1D/3D (including 1D PbBr₂-PZPY and 1D PbI₂-FAI-PZPY heterojunction films) (Fig. 8a) [71]. The grain size decreases as the content of 1D perovskite increases because the molecule may distort and/or distribute the crystal lattice in the interjunction domain in the self-assembly process. The decay time obtained by TRPL is enhanced to 147 ns from 34.1 ns (pure 3D perovskite film), and the PL lifetime obtained by fluorescence lifetime imaging microscopy (FLIM) is enhanced to 10 ns from 3 ns for the pure 3D perovskite domain (Fig. 8b, c). The electron trap density is suppressed from 1.03 × 10¹⁶ to 7.63 × 10¹⁵ cm⁻³, and the hole-trap density is suppressed from 1.10 × 10¹⁶ to 4.05 × 10¹⁵ cm⁻³. Then, the optimal 1D/3D perovskite mixed structure-based device realized an improved PCE of 18.1%, and the PCE is reversible under temperature cycling (25–85 °C) at a RH of 55% for 30 h, while the control device can only maintain five cycles. Similarly, Li's group incorporated hydrazinium cations (HA⁺) into FA-based perovskites to form a 1D/3D multidimensional structure (Fig. 8d) [72]. The optimal 1D/3D multidimensional film exhibited a larger grain size (from ∼250 to 724 nm) with better crystallinity and a smoother surface due to the enhanced continuity between grains and slower crystallization process (Fig. 8e-g). Compared to the pure FAPbI₃ perovskite, the trap state density decreased from 1.06 × 10¹⁶ to 3.10 × 10¹⁵ cm⁻³, and the carrier lifetime was enhanced from 434.6 to 1216.57 ns. More importantly, the corresponding device realized a PCE of 21.20% and maintained 90% of its initial efficiency for 2520 h under ambient conditions (∼25 °C temperature and ∼30% humidity), which is far better than the pure FA-based perovskite-based device with a PCE of 19.36% and a stability of ∼500 h (Fig. 8h, i). On these bases, to further improve the performance of the PSCs, Zhan et al. mixed appropriate [(CH₃)₃NCH₂I]I (TMIMI) in the PbI₂ precursor of the two-step spin coating method to incorporate room temperature 1D ferroelectric [(CH₃)₃NCH₂I]PbI₃ with 3D (FAPbI₃)_0.85(MAPbBr₃)_0.15, in which the ferroelectric polarization of 1D ferroelectric perovskite plays a dominant role in the mixed PSCs for charge separation and transport [73]. The mixed structure is confirmed by high resolution transmission electron microscope (HRTEM) and the ferroelectricity is proved by piezoresponse force microscopy (PFM)-based hysteresis loop measurement. Benefitting from the recognized advantages of 1D perovskites, the 1D/3D mixed perovskite film achieves a smooth and pinhole-free surface, and the optimized 1D/3D PSCs improve the PCE values from 21.3% to 21.9%. After being positively polarized with +1 V, the surface recombination is further reduced by a ferroelectricity induced built-in field, and the device efficiency is further improved to 22.7% with a V_oc close to the S-Q limit.

Fig. 8.

1D/3D perovskite mixed structure. (a) HRTEM image of 1D/Cs_0.04MA_0.16FA_0.8PbI_0.85Br_0.15 heterostructure perovskite. Bulk fluorescence lifetime imaging of (b) pure 3D and (c) optimal 1D/3D heterostructure perovskite thin films [71]. (d) Schematic of with (FAMACs)_0.85HA_0.15 hybrid film. SEM images of (e) (FAMACs)- and (f) (FAMACs)_0.85HA_0.15-based perovskite films. (g) XRD patterns of (FAMACs)_1-xHA_x perovskite films. (h) J-V and (i) stability test of pure (FAMACs)- and (FAMACs)_0.85HA_0.15-based devices [72]

In addition to the reaction of organic molecules with PbI₂ to form a one-dimensional perovskite, which then constitutes a 1D/3D multidimensional heterojunction structure, the perovskite phase control method is also a novel method. In 2019, Daoud et al. added guanidinium (GA) into an FA-based perovskite precursor and prepared PSCs by a one-step spin-coating process [74]. The large tolerance factor (t) of pure GA-based perovskite reveals the GA is commonly used to form low-dimensional perovskites as protective or passivation layers. However, a moderate concentration of GA can also be incorporated into the lattice of MAPbI₃ forming a 3D GA_xMA_1-xPbI₃ perovskite phase to enhance the environmental stability and efficiency. In this study, they proposed that an appropriate concentration of GA can be incorporated into the crystal unit to form a 3D perovskite (abbreviated as CsFAMA_1-xGA_x). Slightly excessive GA will induce the formation of 1D yellow phase FAPbI₃ (δ-FAPbI₃), where the 1D new phase can passivate the defects. By tuning the GA concentration, the optimal perovskite film exhibited a longer carrier lifetime of 22.7 ns and a smaller trap-filled limit voltage of 0.129 V compared to the perovskite film without GA (11.1 ns and 0.177 V). Finally, benefitting from the passivation effect and the incorporation of hydrogen bonds provided by GA, the optimal device realized a high PCE of 20.29% and enhanced stability (ambient, thermal, light). Notably, most studies on stability are focused on the final film without in situ technology. To elucidate the dynamic process and mechanism of 1D/3D multidimensional perovskite decomposition, Fan et al. inventively self-built an in situ characterization system to explore the influence of external conditions (water, oxygen, electric field, and pressure) on 1D/3D perovskite films (morphology, composition, and structure) [75]. Based on the system, they inferred that the bipyridine (BPy) in 1D perovskite can improve the migration activation energy to alleviate ion migration and quench oxygen radicals due to the existence of hydrogen bonds and its role as an electron donor. Therefore, the optimal device exhibited an enhanced PCE of 21.18% with high stability against water, oxygen, and light.

4. 0D/3D multidimensional film

In addition to 2D and 1D perovskites, 0D perovskites are also an important type of low-dimensional structured perovskite, and the chemical formula is A₄BX₆ or A₃B₂X₉. In the 0D perovskite, the [BX₆]⁴⁻ octahedra are completely isolated from each other and usually result in self-trapping excitons, which leads to a high photoluminescence quantum yield (PLQY) and a low carrier mobility. Thus, it is generally considered to be more suitable for light emission than photovoltaic devices. On this basis, many research groups have reported CsPbX₃/Cs₄PbX₆ composite structures as luminescent materials and have achieved good performance. The 0D structured Cs₄PbBr₆ not only passivates the 3D structured CsPbBr₃ due to their good lattice match but also helps to improve the stability of the hybrid material system. More importantly, the 2D/3D and 1D/3D multidimensional structures mentioned above introduce organic molecular groups, which substantially limit the high-temperature thermal stability of the devices. Therefore, reasonable coordination of the structural advantages and energy band arrangement of 0D Cs₄PbX₆ and 3D CsPbX₃ is expected to produce all-inorganic PSCs with enhanced thermal stability and moisture stability.

Recently, some excellent studies have attempted to prepare high-performance PSCs based on 0D/3D multidimensional films. Notably, the dimensions of perovskite can be controlled by adjusting the stoichiometry between Cs, Pb, and X, and even an alloy composed of 0D Cs₄PbX₆ and 3D CsPbX₃ can be obtained without additional additives. Thus, these studies were all used to adjust the ratio of CsI in the precursor to prepare 0D/3D multidimensional perovskite films. For instance, Jen et al. fabricated a Cs₄PbI₆/γ-CsPbI₃ heterostructure by a facile one-step spin-coating process, where the excessive CsI suppresses the growth of perovskite to result in decreased crystallite sizes and the optimal precursor was prepared by dissolving a 1.2:1 ratio of CsI/PbI₂ in a DMSO and DMF mixed solvent [76]. They found that in situ-formed 0D Cs₄PbI₆ crystals are distributed around the γ-CsPbI₃ grain boundaries and serve as a molecular lock to improve the phase stability of 3D CsPbI₃. In addition, the introduction of Cs₄PbI₆ around γ-CsPbI₃ can reduce shallow defects and passivate the defects in the grain boundaries; however, excessive introduction may also cause a thin Cs₄PbI₆ layer to form at the bottom, hindering charge collection. The results are confirmed by the blueshift of the PL peaks, which first increased and then decreased the carrier lifetimes and longitudinal element distribution. Benefitting from the molecular-locking effect and defect passivation effect, the all-inorganic Cs₄PbI₆/γ-CsPbI₃ heterostructure-based device obtained a high PCE of 16.39% and was stable in air over 7 days, exhibiting a 30% PCE increment and greatly improved stability compared to the pure CsPbI₃-based device. After that, Li and coworkers fabricated Cs₄Pb(IBr)₆/CsPbI_3-xBr_x mixed dimensional-based PSCs (Fig. 9a) [77]. Different concentrations of CsI were added to the precursor solution consisting of 1 M PbI₂, 0.5 M CsBr and DMSO solvent (labeled as x-CsI, where x represents the concentration of CsI). As a result, the 0D Cs₄Pb(IBr)₆ crystals passivated the defects and facilitated the (100) preferential crystal orientation of the 3D perovskite. The optimal device realized a PCE of 14.77% and retained 93.9% of its initial efficiency in a N₂-filled atmosphere for 60 days. More importantly, they directly proved that the 0D Cs₄Pb(IBr)₆ grains were spontaneously dispersed in the grain boundaries of the 3D CsPbI_3-xBr_x and observed a smooth transition area at the (040)_0D/(002)_3D interface through electron back-scatter diffraction (EBSD) and HRTEM measurements (Fig. 9b-g). The direct evidence of the 0D/3D mixed dimensional coexistence state and its influence on phase stability and optoelectronics performance is conducive to further study on the influence of the 0D phases relative to the crystal growth, stability, and carrier transport of 3D CsPbX₃.

Fig. 9.

0D/3D perovskite multidimensional structure. (a) Schematic coating and formation processes for the Cs₄Pb(IBr)₆-CsPbI_3-xBr_x films. (b) HRTEM image of the 0D/3D film and (c) a zoomed-in image of the red rectangle in (b). SEM images of (d) the control 0.5-CsI film and (e) the 1.0-CsI film. EBSD measurements of (f) the 0.5-CsI film and (g) the 1.0-CsI film [77].

5. Summary and outlook

Driven by the excellent optoelectronic properties of 3D perovskites and the excellent stability of low-dimensional perovskites, tremendous efforts have been made to construct low-dimensional/3D perovskite multidimensional structures for PSCs in the past few years. In this review, we systematically summarize the recent research progress of low-dimensional/3D perovskite multidimensional structured PSCs and discuss the respective advantages of different low-dimensional/3D perovskite multidimensional structures. In particular, the advantages and disadvantages of the different dimensions and different preparation processes are analyzed. The proper incorporation of low-dimensional perovskite into 3D perovskite can improve the stability, passivate defects and suppress ion migration, which directly affects the device efficiency and stability. As shown in the review, with a reasonable design, the highest PCE of low-dimensional/3D perovskite multidimensional structured PSCs reached 25.2%, and many devices can maintain 90% efficiency for thousands of hours in air. These results show that a reasonable combination can improve stability without sacrificing efficiency. To unlock the potential of low-dimensional/3D multidimensional perovskites, a better understanding of their underlying mechanisms, including the crystal growth occurring during the synthesis process, the crystal structure and the optoelectronic properties after synthesis, is needed.

In addition, it should be noted that there are still many issues that need to be solved, thereby ensuring the practical application of low-dimensional/3D multidimensional perovskite devices in the future. The first major problem is that the current research on low-dimensional/3D multidimensional perovskite structures is mainly focused on small-area (below 0.1 cm²) perovskite devices, and large-area (over 10 × 10 cm²) perovskite devices only achieve an efficiency of ∼12%. Although some commercial companies have shown that they have achieved an efficiency of >15% in a large area (over 1000 cm²), this is still far below the requirements for practical applications. It is relatively easy to achieve high efficiency on a small-area substrate due to the very mature spin coating method and controllable film quality. The small-area preparation methods are suitable for exploration and mechanism analysis, and then provide theoretical guidance for the design of PSCs. The large-area preparation methods are used to meet future practical applications. Thus, the existing large-area preparation methods (printing method, vapor method, and blade coating method) should be optimized or a cheaper and simpler way to obtain high-quality large-area perovskite films should be developed in the following research process. The second major problem is that the most stable unencapsulated 2D/3D perovskite-based device has been able to retain 90% of its initial efficiency in ambient conditions for thousands of hours, which is far better than the achievable pristine 3D perovskite-based devices. However, this is still far from reaching commercial standards (∼20 years). Thus, strategies that can further improve stability must be provided. For example, combined with a suitable encapsulant, it is worth noting that the encapsulant used for commercial silicon solar cells can be used as a reference but cannot be replicated due to the different material properties between silicon and perovskite. The quality of the low-dimensional perovskite protective layer (e.g., denser and more hydrophobic) was optimized to insulate water and oxygen from contact with the 3D perovskite. The third major problem is the toxicity of Pb, and some effective strategies must be adopted to ensure that Pb cannot escape easily. We speculate that forming stronger chemical bonds to trap Pb ions is a promising potential method, and this area has not been extensively studied. Another promising potential method is to use other metal elements (such as Sn) instead of Pb. Of course, it is necessary to solve the problems the new types of perovskites face, such as the easy oxidation of Sn²⁺ in Sn-based perovskites. The fourth major problem is the production cost. The current mainstream HTL layer is a high-cost organic material Spiro-OMeTAD, and the mainstream top electrode is the noble metal Ag and Au. Furthermore, the commonly used doped Spiro-OMeTAD will lose its conductivity in air and moisture condition and cannot withstand high temperatures, which is also one of the reasons for the instability of PSCs. The fourth problem is also the significance of studying HTL-free PSCs and carbon electrodes, and we speculate that these are promising research directions to reduce costs. In summary, there are still many problems that PSCs need to face, especially the four problems pointed out above. Nevertheless, we still believe PSCs are one class of the most promising solar cells. In particular, we believe that constructing a low-dimensional/3D perovskite multidimensional structure is a promising method to achieve efficient and stable PSCs, and the above-mentioned problems should be solved on the basis of this strategy.

Declaration of Competing Interest

The authors declare that they have no conflicts of interest in this work.

Biographies

Fengren Cao received his B.S. degree and Ph.D. degree from Soochow University in 2014 and 2019, respectively. Since 2019, he is an assistant researcher in the School of Physical Science and Technology, Soochow University, China. His research interests lie in the field of optoelectronic conversion materials, especially with a focus on nanostructure-based photodetector.

Liang Li received his Ph.D. degree from the Institute of Solid State Physics, Chinese Academy of Sciences, in 2006. From 2007 to 2012, he worked in the National University of Singapore (NUS), Singapore; National Institute of Advanced Industrial Science and Technology (AIST), Japan; National Institute of Materials Science (NIMS), Japan; and the University of Western Ontario (UWO), Canada. Since August 2012, he is a Full Professor in Soochow University, China. His research group (http://ecs.suda.edu.cn) focuses mainly on energy conversion materials for solar cells, photodetectors, and electrochemical batteries.