Phase Engineering in Quasi-2D Ruddlesden−Popper PerovskitesYani Chen, Shuang Yu, Yong Sun, and Ziqi Liang*

Department of Materials Science, Fudan University, Shanghai 200433, China

ABSTRACT: Quasi-2D Ruddlesden−Popper (RP) halide perovskites have drawn intensiveresearch interest because they possess superior ambient stability while retaining excellentdevice performance as compared to their pure 2D or 3D counterparts. By phase engineer-ing strategy, quasi-2D perovskites can fall into three typeslarge-n 2D perovskite, 2D:3Dmixed perovskite, and 3D/2D bilayer perovskite. This Perspective discusses the modu-lation of phase composition, hierarchical distribution, and crystal orientation in quasi-2Dperovskites, aiming to uncover the correlation between morphological structure, bandalignment, and charge recombination. A perspective of phase engineering in 2D RP-typeperovskite materials is then given toward the concurrent stability and device efficiency.

Owing to their superior ambient stability, two-dimensional(2D) Ruddlesden−Popper (RP) organometallic perovskites

have recently attracted rapidly increasing attention. The generalchemical formula for 2D RP-type perovskites is L2An−1BnX3n+1,where A is a conventional monovalent cation (= CH3NH3

+,HC(NH2)2

+, Cs+, etc.) while L is a larger aromatic or aliphaticalkylammonium organic spacer cation (= 2-phenylethylammo-nium (PEA), n-butylammonium (n-BA), etc.) that tailors the3D perovskite into layered 2D structures, B is a divalent metalcation (= Pb2+, Sn2+, Ge2+, Cu2+, etc.), X is a halide anion(= Cl−, Br−, or I−), and the variable n (= 1, 2, 3, 4, ...) repre-sents the number of inorganic layer within each quantum well.As such, 2D perovskites enable one to readily tailor the opticalbandgap and absorption by varying either the spacer cationstructure or the variable n while the organic spacer largelyincreases stability against oxygen and moisture.1−3

Recently, a few years have witnessed tremendous develop-ment in 2D perovskites-based solar cells. Their power conver-sion efficiency (PCE) has rapidly increased from ∼4%4,5

in 2014 to 13.7% in 2017 through either optimized fabrica-tion (“hot-casting” technique) or composition engineering(Cs-inclusion method).6,7 However, such performance isremarkably worse than that of 3D analogues (up to 22.7%),8

which can be attributed to hindered charge transport by theinsulating nature of inserted long organic spacers and ran-dom packing of crystals in 2D perovskites. Of particularrelevance is that the nominal 2D perovskites are a mixture ofphases with different quantum well widths. Yet, there lacks aneffective means to regulate the ratio and hierarchical distributionof mixed phases to achieve favorable band alignment and chargetransport.Notably, a multitude of research groups have successfully

fabricated 3D/2D perovskite hybrids, resulting in a new era ofperovskite research for high-efficiency thin film photovoltaicswith superior air stability.9−27 These dimensionally mix-phasedperovskites are denoted as quasi-2D perovskites. By far, as

shown in Figure 1, there are mainly three routes to create them:(1) increasing the ratio of spacer cations in the precursor

(Figure 1a), (2) mixing 2D and 3D perovskites in solution,which involves both one-step and two-step processing methods(Figure 1b), and (3) depositing a thin layer of 2D perovskite ontop of a 3D perovskite thin film to generate the 3D/2D bilayerstructures (Figure 1c).This Perspective will first present the synthesis, energetic

structures, optical properties, and carrier dynamics of quasi-2Dperovskites. Next, the three above-mentioned types of quasi-2Dperovskites will be discussed and compared. Finally, the out-look of these hybrid perovskites in optoelectronic devices willbe given and a critical perspective offered to guide the design ofnovel perovskite structures.

Received: March 18, 2018Accepted: April 30, 2018Published: April 30, 2018

Figure 1. Schematic of three types of quasi-2D perovskites: (a) large-n2D perovskite, (b) 2D:3D mixed perovskite, and (c) 3D/2D bilayerperovskite.

The n-modulating strategyrequires a delicate control ofn values in 2D perovskites tocompromise between the

efficiency and stability in devices.

Perspective

pubs.acs.org/JPCLCite This: J. Phys. Chem. Lett. 2018, 9, 2627−2631

© 2018 American Chemical Society 2627 DOI: 10.1021/acs.jpclett.8b00840J. Phys. Chem. Lett. 2018, 9, 2627−2631

Varying the Thickness of Inorganic Lattices (n). With theincrease of the n value from 1 to ∞, the perovskite structureundergoes dimensional evolution, which transits gradually frompure 2D phases to 3D:2D mixtures and finally 3D perovskites,as schematically shown in Figure 2a. It has been reported that

the structures and properties of perovskites varied with then value. For example, for the (PEA)2MAn−1PbnI3n+1 perovskite,the formation energy increased with decreasing n value, sug-gesting enhanced air stability.9 Moreover, the energy bandgaps(Egs) increased linearly with decreasing quantum well thickness(i.e., n value), giving Eg ∝ 1/n at n > 2.10 Meanwhile, thedielectric constants increased largely with n, thus lowering theexciton binding energy (Eb). Furthermore, structural transfor-mation from the 2D to 3D phase brought about a slightlydiminished effective mass of electrons (mc) and holes (mv),indicating that the layer thickness (n) has negligible effect onthe mobility of photogenerated carriers. Later, Barea et al.investigated the influence of n values on exciton splitting.11

It was found that there is no quantum confinement effect in theRP halide perovskites with n ≥ 2, which can be confirmed bythe absence of emission or absorption shifts from UV−vis andelectroluminescence spectra. This phenomenon is similar tothat of the 3D bulk counterpart so that RP perovskites with n >3 can be effectively applied to solar cells. As a demonstration,(CHMA)2(MA)n−1PbnI3n+1 based solar cells (n = 1−5) werecompared for device performance.12 When n < 3, the PCEbarely exceeded 0.1% while it increased up to ∼4% when n ≥ 4.Remarkably, the Etgar group synthesized layered perovskiteswith very large n values (n = 40, 50, 60), which exhibited higheropen-circuit voltage (VOC) and PCE (∼8.5%) than their 3Dcounterparts.13 On the other hand, the Sargent group showedthat utilization of an allylammonium ligand enabled a narrowerdistribution of QW widths (n = 10), thus creating a flattenedenergy landscape that led to ×1.4 and ×1.9 longer diffusionlengths for electrons and holes, respectively.14 In addition, then value impacts greatly the crystal orientation of 2D perovskites.For instance, the (BA)2(MA)n−1PbnI3n+1 2D perovskite crystalswere preferentially aligned parallel to the substrate when n = 1and turned nearly perpendicular when n = 4.15 In short, whenn is small, the ambient stability can be guaranteed in RP

perovskites owing to the dominant contribution of hydrophobicspacing cations while the charge carrier transportation is hindered,and vice versa. There is always a trade-off between device perfor-mance and stability, as indicated in Figure 2b. It is thereforenecessary to determine the optimal n value in RP perovskites tocompromise between the efficiency and stability in devices.2D:3D Mixed Perovskites. For the n-modulating strategy, it is

however inevitable to generate supersmall-n 2D phases (n < 3)that impede charge/energy transfer processes, even when thestarting n value in the precursor solution is very large. This canbe compensated by the presence of 3D phases with favorablecharge transport, which may be formed via self-assembly tosome degree yet in an uncontrollable way. In order to attain adelicate control of 3D phases, several research groups haverecently generated devices based on a mixture of 2D and3D perovskites.16−20 There are two typical routes toward2D:3D mixed perovskites. One is a “one-step” method thatblends the precursors of 2D and 3D perovskites in solution forspin-coating, while the other is “two-step” deposition that spin-casts the mixed solution of an organic cation (A) and spacercation (L) on the top of the PbI2 layer. In the former method,the Snaith group added n-BA+ into benchmark 3D perovskitesof mixed-cation/halide FA0.83Cs0.17Pb(IyBr1−y)3.

16 The resultingmixture-based solar cells exhibited a stabilized PCE of ∼17.5%.It is interesting to note that the 2D phases tended to interspersebetween highly oriented 3D perovskite grains, resulting in sup-pressed nonradiative charge recombination. This phenomenonwas also found in the NH2C4H9COOH (termed Ava) spacerdication-incorporated MAPbI3-based quasi-2D perovskite.17

Zhao et al. recently introduced this Ava dication into a 3DMAPbBr3 perovskite to form NH3C4H9COO(CH3NH3)nPbnBr3n,where the protonated NH3

+ groups displaced the MA+ whilethe COO− groups took the place of unoccupied Pb2+, therebypromoting the formation of smooth planar films with a notablyhigh PLQY up to ∼80%.18 More recently, they reportedanother organic spacer, ethylenediamine (EDA)-based quasi-2D perovskites, bearing double active sites.19 It was found thatthe addition of EDAPbI4 aided in avoiding the formation of theundesirable nonperovskite δ phase, which can be attributed tothe cross-linking capability of EDA. As a result, the devicesbased on CsPbI3·0.025EDAPbI4 perovskite yielded a PCE of11.8% with excellent stability at room temperature for months.On the other hand, Jen and his co-workers for the first timeemployed two-step deposition to introduce phenylethylammo-nium iodide (PEAI) into FAPbI3 perovskite and generate quasi-2D FAxPEA1−xPbI3 structures.20 The PEA+ ions around thecrystal grain boundaries presented dual functions: (i) tighteningthe FAPbI3 domains via migration into the FAPbI3 grain bound-aries to form organic shells with induced π−π interaction and(ii) raising the phase transition energy and passivating thesurface to improve both phase and moisture stability. Theresulting quasi-2D perovskite solar cells gave a high PCE of17.7% with superior phase stability in air.

In the 2D:3D mixed perovskites, however, it is highly dif-ficult to determine their n values, as illustrated in Figure 3,

Figure 2. (a) Schematic of 2D (PEA)2MAn−1PbnI3n+1 perovskite struc-tures with varying n value from 1 to ∞. (b) Device performance andstability variation of PCE with n value. Reprinted with permission fromref 9. Copyright 2016 American Chemical Society.

Morphological randomness of2D:3D mixed perovskites

increases the difficulties of deviceoptimization and bench-to-bench

reproduction.

The Journal of Physical Chemistry Letters Perspective

DOI: 10.1021/acs.jpclett.8b00840J. Phys. Chem. Lett. 2018, 9, 2627−2631

2628

which displays three possible band structure alignments. It ismore likely for those mixed perovskites to exhibit a randomdistribution of 2D and 3D phases (Figure 3a), which leadsto mismatched band alignment and thus unfavorable chargerecombination, rather than forms favorably flat or ordered bandalignments (Figure 3b,c).3D/2D Bilayer Perovskites. In order to resolve the random

band alignment in the 2D:3D mixed perovskites, the 3D/2Dbilayer heterojunction structure has received exponentiallyincreasing attention. Such structure can be formed by a “cationexchange” method in solution-cast or thermal evaporation.21−27

On the other hand, such heterostructures can be varied by modi-fying the composition of precursors. For example, the Huanggroup investigated the chemical reaction mechanism of forming3D/2D stacking structures.21 They compared two kinds of pre-cursor solution for “cation exchange”: by means of either BAdissolved in chlorobenzene (CB) or BAI dissolved in isopropylalcohol (IPA), as shown in eqs 1 and 2, respectively. Theresulting 2D perovskites possessed distinctive compositions andphases. Intriguingly, reversible conversions can be obtained bysuch cation exchange reactions with BAI (3D to 2D) or MAI(2D to 3D) solutions.

+ + → + ↑2BA MAI MAPbI (BA) PbI 2MA3 2 4 (1)

+ ↔ +− +n2BAI MAPbI (BA) (MA) Pb I MAIn n n3 2 1 3 1 (2)

The solar cell applications of such 3D/2D bilayeredstructures were initiated by the Pablo group.22 They fabricated3D/2D bilayer perovskite-based solar cells by a “cationinfiltration” method that spin-coated a layer of mixed PEAI:MAIor BAI:MAI in IPA solution on top of the MAPbI3 layer.As shown in Figure 4a, the partial reorientation of crystallitesoccurred upon formation of the PEAMAPI layer, therebyyielding a PCE of 16.84% in solar cells. The same result of suchordered band alignment was also found in MAPbI3/(BA)2(MA)n−1PbnI3n+1 3D/2D bilayers by the Jin group usingsteady-state and transient spectroscopic measurements.23 Mul-tiple perovskite phases were arranged in the sequence order oftheir n values along the direction perpendicular to the substrate.Carrier transfer within the unique superstructure occurred mostlikely from the small-n to the large-n phase.24 Moreover, theoptimal alignment of graded energy bands provided a contin-uous upshift of the LUMO level in the 3D/2D perovskitestructure, which facilitated charge transfer from the perovskiteto the electron-transport layer (ETL) and also preventedbackflow of electron carriers, thus suppressing both chargerecombination and ion migration (Figure 4b,c).25,26

Alternately, this bilayer structure has manifested as aneffective strategy to fabricate high-quality Pb-free perovskitefilms for achieving promising photovoltaic performance.

For instance, when a PEAI layer was thermally evaporatedonto the FASnI3 thin film, the oxidation of the Sn2+ could beeffectively avoided even in the absence of reductant additivesuch as SnF2 and hydrazine.27 As a result, the resulting solar cellsexhibited a promising PCE of 6.98% with good reproducibility.

Table 1 summarizes the up-to-date device performance of3D/2D bilayer perovskites-based solar cells. It is worth notingthat the most adopted 3D perovskite in Table 1 is MAPbI3, forwhich further enhancement of PCEs can be realized by using amixed cation or halide, similar to the methods used in the pure3D counterpart.26 For instance, an ultrahigh PCE up to 18.51%was acquired for Cs0.05(FA0.83MA0.17)0.95Pb(I0.83Br0.17)3/PEA2Pb2I4planar solar cells.26

The original motive of developing RP 2D structures forperovskite solar cells is to improve the ambient stability withthe aid of additionally inserted hydrophobic cations. Theresultant 2D perovskites present the anisotropic structural fea-tures, in particular, microscopic structures including phaseseparation and crystal orientation, which play a determiningrole in solar cells by influencing light harvest, charge transport,and device stability as compared to the 3D counterparts. Thishas been widely proved by both experimental studies and funda-mental mechanistic investigations in recent years.9−15 Therefore,we believe that phase engineering would be the core issue inthe upcoming research of 2D RP perovskites-based solar cells.In this context, 3D/2D bilayer perovskites have demonstratedgreat potential where the superior photovoltaic performance of3D perovskites and the enhanced stability of 2D perovskitesare complementary. Despite the promising published results,there remains great challenges in this direction: How to derivethe relationship between the charge carrier dynamics and the

Figure 3. Schematic band alignment in 3D:2D mixed perovskites in(a) random, (b) flat, and (c) ordered fashions.

Figure 4. (a) Schematic interpretation of the surface reorganization ofMAPbI3 films upon the PEA2MA4Pb5I16 formation. Adapted fromref 22. Copyright 2016 by the American Chemical Society. (b) Energylevel alignment at the interface of MAPbI3/PCBM. Reprinted fromref 25. Copyright 2017 by the Wiley-VCH. (c) Proposed thermaldegradation routes of different devices. Reprinted with permissionfrom ref 25. Copyright 2017 by Wiley-VCH.

The 3D/2D bilayer perovskitestructure appears to be the mostviable avenue to significantlyimprove air stability withoutnotably sacrificing PCEs.

The Journal of Physical Chemistry Letters Perspective

DOI: 10.1021/acs.jpclett.8b00840J. Phys. Chem. Lett. 2018, 9, 2627−2631

2629

morphological structures in quasi-2D perovskites? How to improveelectronic coupling between 2D and 3D at their interfaces?How to simplify the processing procedures while maintainingthe reproducibility of device performance? These are the mosturgent issues waiting to be resolved in the near future.

■ AUTHOR INFORMATIONCorresponding Author*E-mail: zqliang@fudan.edu.cn.ORCIDZiqi Liang: 0000-0003-4581-6487NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThis work was supported by the Inter-Governmental Interna-tional Cooperation Project of Science and TechnologyCommission of Shanghai Municipality (STCSM) underGrant No. 17520710100 (Z.L.).

■ REFERENCES(1) Chen, Y.; Sun, Y.; Peng, J.; Tang, J.; Zheng, K.; Liang, Z. 2DRuddlesden−Popper Perovskites for Optoelectronics. Adv. Mater.2018, 30, 1703487.(2) Pedesseau, L.; Sapori, D.; Traore, B.; Robles, R.; Fang, H. H.; Loi,M. A.; Tsai, H.; Nie, W.; Blancon, J. C.; Neukirch, A.; Tretiak, S.;Mohite, A. D.; Katan, C.; Even, J.; Kepenekian, M. Advances andPromises of Layered Halide Hybrid Perovskite Semiconductors. ACSNano 2016, 10, 9776−9786.(3) Etgar, L. The Merit of Perovskite’s Dimensionality: Can ThisReplace The 3D Halide Perovskite? Energy Environ. Sci. 2018, 11,234−242.(4) Smith, I. C.; Hoke, E. T.; Solis-Ibarra, D.; McGehee, M. D.;Karunadasa, H. I. A Layered Hybrid Perovskite Solar-Cell Absorberwith Enhanced Moisture Stability. Angew. Chem., Int. Ed. 2014, 53,11232−11235.(5) Cao, D. H.; Stoumpos, C. C.; Farha, O. K.; Hupp, J. T.;Kanatzidis, M. G. 2D Homologous Perovskites as Light-AbsorbingMaterials for Solar Cell Applications. J. Am. Chem. Soc. 2015, 137,7843−7850.(6) Tsai, H.; Nie, W.; Blancon, J. C.; Stoumpos, C. C.; Asadpour, R.;Harutyunyan, B.; Neukirch, A. J.; Verduzco, R.; Crochet, J. J.; Tretiak,S.; Pedesseau, L.; Even, J.; Alam, M. A.; Gupta, G.; Lou, J.; Ajayan, P.M.; Bedzyk, M. J.; Kanatzidis, M. G. High-Efficiency Two-DimensionalRuddlesden-Popper Perovskite Solar Cells. Nature 2016, 536, 312−316.(7) Zhang, X.; Ren, X.; Liu, B.; Munir, R.; Zhu, X.; Yang, D.; Li, J.;Liu, Y.; Smilgies, D.-M.; Li, R.; Yang, Z.; Niu, T.; Wang, X.; Amassian,

A.; Zhao, K.; Liu, S. Stable High Efficiency Two-DimensionalPerovskite Solar Cells via Cesium Doping. Energy Environ. Sci. 2017,10, 2095−2102.(8) Best Research Cell Efficiencies. https://www.nrel.gov/pv/assets/images/efficiency-chart.png (Accessed Oct 2017).(9) Quan, L. N.; Yuan, M.; Comin, R.; Voznyy, O.; Beauregard, E.M.; Hoogland, S.; Buin, A.; Kirmani, A. R.; Zhao, K.; Amassian, A.;Kim, D. H.; Sargent, E. H. Ligand-Stabilized Reduced-DimensionalityPerovskites. J. Am. Chem. Soc. 2016, 138, 2649−2655.(10) Zhang, L.; Liang, W. How the Structures and Properties of Two-Dimensional Layered Perovskites MAPbI3 and CsPbI3 Vary with TheNumber of Layers. J. Phys. Chem. Lett. 2017, 8, 1517−1523.(11) Rodríguez-Romero, J. s.; Hames, B. C.; Mora-Sero, I. n.; Barea,E. M. Conjugated Organic Cations to Improve the OptoelectronicProperties of 2D/3D Perovskites. ACS Energy Lett. 2017, 2, 1969−1970.(12) Koh, T. M.; Huang, J.; Neogi, I.; Boix, P. P.; Mhaisalkar, S. G.;Mathews, N. High Stability Bilayered Perovskites through Crystal-lization Driven Self-Assembly. ACS Appl. Mater. Interfaces 2017, 9,28743−28749.(13) Cohen, B.-E.; Wierzbowska, M.; Etgar, L. High Efficiency andHigh Open Circuit Voltage in Quasi 2D Perovskite Based Solar Cells.Adv. Funct. Mater. 2017, 27, 1604733.(14) Proppe, A. H.; Quintero-Bermudez, R.; Tan, H.; Voznyy, O.;Kelley, S. O.; Sargent, E. H. Synthetic Control over Quantum WellWidth Distribution and Carrier Migration in Low-DimensionalPerovskite Photovoltaics. J. Am. Chem. Soc. 2018, 140, 2890−2896.(15) Venkatesan, N. R.; Labram, J. G.; Chabinyc, M. L. Charge-Carrier Dynamics and Crystalline Texture of Layered Ruddlesden−Popper Hybrid Lead Iodide Perovskite Thin Films. ACS Energy Lett.2018, 3, 380−386.(16) Wang, Z.; Lin, Q.; Chmiel, F. P.; Sakai, N.; Herz, L. M.; Snaith,H. J. Efficient Ambient-Air-Stable Solar Cells with 2D−3DHeterostructured Butylammonium-Caesium-Formamidinium LeadHalide Perovskites. Nat. Energy 2017, 2, 17135.(17) Grancini, G.; Roldan-Carmona, C.; Zimmermann, I.; Mosconi,E.; Lee, X.; Martineau, D.; Narbey, S.; Oswald, F.; De Angelis, F.;Graetzel, M.; Nazeeruddin, M. K. One-Year Stable Perovskite SolarCells by 2D/3D Interface Engineering. Nat. Commun. 2017, 8, 15684.(18) Zhang, T.; Xie, L.; Chen, L.; Guo, N.; Li, G.; Tian, Z.; Mao, B.;Zhao, Y. In Situ Fabrication of Highly Luminescent BifunctionalAmino Acid Crosslinked 2D/3D NH3C4H9COO(CH3NH3PbBr3)nPerovskite Films. Adv. Funct. Mater. 2017, 27, 1603568.(19) Zhang, T.; Dar, M. I.; Li, G.; Xu, F.; Guo, N.; Gratzel, M.; Zhao,Y. Bication Lead Iodide 2D Perovskite Component to StabilizeInorganic a-CsPbI3 Perovskite Phase for High-Efficiency Solar Cells.Sci. Adv. 2017, 3, e1700841.(20) Li, N.; Zhu, Z.; Chueh, C.-C.; Liu, H.; Peng, B.; Petrone, A.; Li,X.; Wang, L.; Jen, A. K.-Y. Mixed Cation FAxPEA1−xPbI3 withEnhanced Phase and Ambient Stability toward High-PerformancePerovskite Solar Cells. Adv. Energy Mater. 2017, 7, 1601307.

Table 1. Device Performance of 3D/2D Bilayer Perovskites-Based Solar Cellsa

3D/2D bilayer structure device configuration Jsc (mA/cm2) Voc (V) FF PCE (%) stabilityb

MAPbI3/PEA2Pb2I4 FTO/NiO/perovskite/PCBM/PN4N/Ag

21.80 1.17 0.78 19.8925 60%/30 d, RH 20−30%

MAPbI3/PEA2MA4Pb5I16 or BA2MA3Pb4I13 FTO/c-TiO2/ perovskite/spiro-OMeTAD/Au

18.63 1.08 0.73 14.9422 76%/19 d, RH 75%

16.56 1.08 0.62 11.4922

MAPbI3/(BA)2PbI4 or (BA)2(MA)n−1PbnI3n+1 ITO/PTAA/perovskite/PCBM/C60/BCP/Cu

22.49 1.11 0.78 19.5621 96.5%/100 h

22.59 1.09 0.77 18.8521 88.2%/heat for 100 hCs0.05(FA0.83MA0.17)0.95Pb(I0.83Br0.17)3/PEA2PbI4 FTO/c-TiO2/m-TiO2 /perovskite/

spiro-OMeTAD/Au22.89 1.11 0.73 18.5126 90%/1000 h, RH 60 ± 10%

FASnI3/(PEA,FA)SnI3 ITO/PEDOT:PSS/perovskite/C60/BCP/Ag

20.07 0.47 0.74 6.9827 n/a

aNote that JSC is the short-circuit current density, VOC is the open-circuit voltage, and FF is the fill factor. bStability: the remaining percentage ofinitial efficiency/storing time, relative humidity (RH) or heat.

The Journal of Physical Chemistry Letters Perspective

DOI: 10.1021/acs.jpclett.8b00840J. Phys. Chem. Lett. 2018, 9, 2627−2631

2630

(21) Lin, Y.; Bai, Y.; Fang, Y.; Chen, Z.; Yang, S.; Zheng, X.; Tang, S.;Liu, Y.; Zhao, J.; Huang, J. Enhanced Thermal Stability in PerovskiteSolar Cells by Assembling 2D/3D Stacking Structures. J. Phys. Chem.Lett. 2018, 9, 654−658.(22) Hu, Y.; Schlipf, J.; Wussler, M.; Petrus, M. L.; Jaegermann, W.;Bein, T.; Muller-Buschbaum, P.; Docampo, P. Hybrid Perovskite/Perovskite Heterojunction Solar Cells. ACS Nano 2016, 10, 5999−6007.(23) Wang, J.; Leng, J.; Liu, J.; He, S.; Wang, Y.; Wu, K.; Jin, S.Engineered Directional Charge Flow in Mixed Two-DimensionalPerovskites Enabled by Facile Cation-Exchange. J. Phys. Chem. C 2017,121, 21281−21289.(24) Li, L.; Zhou, N.; Chen, Q.; Shang, Q.; Zhang, Q.; Wang, X.;Zhou, H. Unraveling the Growth of Hierarchical Quasi-2D/3DPerovskite and Carrier Dynamics. J. Phys. Chem. Lett. 2018, 9, 1124−1132.(25) Bai, Y.; Xiao, S.; Hu, C.; Zhang, T.; Meng, X.; Lin, H.; Yang, Y.;Yang, S. Dimensional Engineering of a Graded 3D−2D HalidePerovskite Interface Enables Ultrahigh VOC Enhanced Stability in Thep-i-n Photovoltaics. Adv. Energy Mater. 2017, 7, 1701038.(26) Chen, P.; Bai, Y.; Wang, S.; Lyu, M.; Yun, J.; Wang, L. In SituGrowth of 2D Perovskite Capping Layer for Stable and EfficientPerovskite Solar Cells. Adv. Funct. Mater. 2018, 28, 1706923.(27) Ran, C.; Xi, J.; Gao, W.; Yuan, F.; Lei, T.; Jiao, B.; Hou, X.; Wu,Z. Bilateral Interface Engineering toward Efficient 2D−3D BulkHeterojunction Tin Halide Lead-Free Perovskite Solar Cells. ACSEnergy Lett. 2018, 3, 713−721.

The Journal of Physical Chemistry Letters Perspective

DOI: 10.1021/acs.jpclett.8b00840J. Phys. Chem. Lett. 2018, 9, 2627−2631

2631