Acceptor Charge-Transfer States at Two-Dimensional Metal

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Donor/Acceptor Charge Transfer States at 2D Metal Halide Perovskite and Organic Semiconductor Interfaces Lianfeng Zhao, YunHui Lisa Lin, Hoyeon Kim, Noel C. Giebink, and Barry P. Rand ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.8b01722 • Publication Date (Web): 11 Oct 2018 Downloaded from http://pubs.acs.org on October 11, 2018

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ACS Energy Letters

Donor/Acceptor Charge Transfer States at 2D Metal Halide Perovskite and Organic Semiconductor Interfaces Lianfeng Zhao†, YunHui L. Lin†, Hoyeon Kim‡, Noel C. Giebink‡, Barry P. Rand*†§ † Department

of Electrical Engineering, Princeton University, Princeton, New Jersey 08544,

United States ‡

Department of Electrical Engineering, The Pennsylvania State University, University Park,

Pennsylvania 16802, United States § Andlinger

Center for Energy and the Environment, Princeton University, Princeton, New Jersey

08544, United States *

Corresponding Author Email: [email protected]

ABSTRACT Metal halide perovskite semiconductors with small exciton binding energy have been widely used in perovskite solar cells and achieved rapid progress in terms of device performance. However, the strong excitonic nature of 2D perovskites with small n values remains underexploited (n represents the number of inorganic monolayer sheets sandwiched between bulky organic cation layers). In this work, we report experimental evidence of donor/acceptor charge transfer (CT) states formed at 2D metal halide perovskite/organic semiconductor heterojunctions, with a corresponding increase in photocurrent production for these excitonic materials. Furthermore, it is found that the size of the organic cation in the 2D perovskite layer plays a critical role in the CT process. The ability to dissociate excitons in 2D perovskites by interfacing with an organic semiconductor in a donor/acceptor configuration opens up new opportunities for exploiting the excitonic nature of low-

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dimensional perovskites in applications such as solar cells, photodetectors, light emitting devices, and in light-matter interactions.

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Due to the large exciton binding energies (0.3 – 1 eV) in organic semiconductors, organic solar cells utilize donor/acceptor heterojunctions for efficient exciton dissociation.1 Charge transfer (CT) complexes at the donor/acceptor interface play essential roles as intermediates in photocurrent production in organic solar cells.2 In contrast, three-dimensional (3D) hybrid organicinorganic metal halide perovskite semiconductors (e.g. CH3NH3PbI3, or MAPbI3), which have achieved notable progress in solar cell power conversion efficiency (PCE) in the last several years,3 possess low exciton binding energies and therefore do not require donor/acceptor structures to produce photocurrent.4 Recently, there is growing interest in the use of two-dimensional (2D) perovskites, or Ruddlesden-Popper phases, in solar cells,5 due in part to improved moisture stability arising from the inclusion of bulky hydrophobic organic cations.6 Two-dimensional perovskites possess the general formula B2(SMX3)n-1MX4, where B and S represent bulky and small organic cations, respectively, M divalent metal cations, and X halide anions. The number of MX4 monolayer sheets sandwiched between bulky organic cation layers is represented by n.7 The highest performing 2D perovskites used in perovskite solar cells feature n values greater than 3.8 This is primarily because exciton binding energy increases dramatically for small n values due to quantum and dielectric confinement effects9 (e.g. 380 meV and 270 meV for n=1 and 2, respectively,10 compared to ~10 meV in 3D perovskites4), reducing overall photocurrent generation efficiency. Furthermore, the strong excitonic nature of 2D perovskites with small n values remains underexploited, including in light emitting devices where average values of no smaller than 3 are typical.11-14 In this work, we report the observation of donor/acceptor CT states at 2D metal halide perovskite/organic semiconductor heterojunctions. Various 2D perovskite structures with different n values and different bulky organic cations such as ethylammonium (EA), butylammonium (BA)



and phenethylammonium (PEA) are used to form heterojunctions with organic acceptors with different highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energies. Charge transfer features are found in the external quantum efficiency (EQE) spectra only for the strongly excitonic n=1 case and, notably, with relatively small organic cations (here, EA and BA) when forming a heterojunction with the strong electron accepting molecule 1,4,5,8,9,11-hexaazatriphenylenehexacarbonitrile (HATCN). No CT features are observed for the PEA case, because PEA cations are apparently too bulky, protecting the 2D metal halide sheet and inhibiting CT processes at the donor/acceptor interface. Additionally, we confirm that CT features are not found at interfaces with insufficient orbital energy offset, such as when BA2PbI4 is paired with tris-(8-hydroxyquinoline)aluminum (Alq3). Furthermore, we show that, as n increases, the 2D perovskites are not sufficiently excitonic (i.e. the exciton binding energy reduces to less than or equal to kT) to show CT states, even when paired with the strong electron acceptor HATCN. Observation of CT states at 2D perovskite/organic heterojunctions points to methods to exploit the excitonic nature of 2D perovskites to broader research areas such as donor/acceptor type solar cells, photodetectors, light emitting devices, and light-matter interactions. External quantum efficiency (EQE) spectra of donor/acceptor heterojunction solar cells can reveal the presence of CT states at energies below individual donor or acceptor photocurrent contributions. The EQE in this low energy region results from photocurrent generated by intermolecular CT state absorption. The HOMO and LUMO energy levels of 2D butylammonium lead iodide (BA2PbI4, n=1) have been measured to be 5.8 eV and 3.1 eV with respect to vacuum, respectively.15 Here, we utilize BA2PbI4 as an electron donor, pairing this layer with a strong electron accepting organic molecule with a deep LUMO. Among various acceptor choices, HATCN is a suitable candidate because of its deep LUMO of 5.2 eV (Figure 1) and large optical


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gap, meaning that photocurrent generated by exciting interfacial CT states can be unambiguously assigned rather than overlapping with intramolecular absorption (Figure S1) and also avoiding the possibility of exciton energy transfer competing with charge transfer from the perovskite to the organic film. Smooth, pin-hole free polycrystalline thin films of BA2PbI4 are prepared using a one-step spin coating method with a solvent exchange step.16-17 To confirm this, surface morphology of the film is studied with a scanning electron microscope (SEM), as shown in Figure S2. A smooth and pin-hole free perovskite surface is evident, and is important for solar cell operation to avoid any short-circuit paths or chemical reactions between the perovskite film and metal contacts.18 Donor/acceptor type BA2PbI4/HATCN heterojunctions are studied as well as control samples of BA2PbI4 single layers, and BA2PbI4 perovskite/Alq3 heterojunctions. Current density – voltage (JV) curves are shown in Figure S3. Major device parameters such as short-circuit current density (Jsc), open-circuit voltage (Voc), fill factor (FF) and power conversion efficiency (PCE) are summarized in Table S1. The HOMO and LUMO of Alq3 (5.8 eV and 3.2 eV with respect to vacuum) are very similar to that of BA2PbI4, such that neither energy transfer nor charge transfer (with associated low energy absorbing CT states) is expected when forming a heterojunction. In Figure 1d, we compare the EQE spectra of the three types of solar cells. As can be seen, the EQE spectrum of the BA2PbI4/HATCN heterojunction contains a Gaussian feature in the spectral region below 2.25 eV that lies below the absorption cutoffs of HATCN and BA2PbI4. We assign this photocurrent contribution to direct absorption from an interfacial CT state based on its qualitative similarity to the intermolecular CT state absorption band commonly observed in organic photovoltaic cells. Within the framework of Marcus theory, photocurrent generation due to direct donor-acceptor CT absorption exhibits a Gaussian lineshape given by:19-21



𝑓

𝐸𝑄𝐸(𝐸) ∝ 𝐸√4𝜋𝜆𝑘𝑇 𝑒𝑥𝑝 (

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−(𝐸𝐶𝑇 +𝜆−𝐸)2 4𝜆𝑘𝑇

),

where E is energy, k is the Boltzmann constant, T is temperature, 𝜆 is the nuclear reorganization energy, and f is related to the electronic coupling between the initial ground state and the excited CT state. In contrast, no Gaussian CT features are observed for the BA2PbI4/Alq3 heterojunction devices, due to insufficient orbital energy offset at their interface, where the orbital energy offset is insufficient to drive charge transfer across the interface, as discussed above. Finally, we note that the photocurrent output from BA2PbI4 is enhanced in the presence of the CT feature in the BA2PbI4/HATCN heterojunction solar cell compared to the BA2PbI4 single layer and BA2PbI4/Alq3 devices. This observation is consistent with our expectation that, in the same manner as organic solar cells, for highly excitonic 2D perovskites, the presence of a donor/acceptor junction facilitates exciton dissociation and charge generation efficiency. Further, we varied the excitonic character of the perovskite layer by modifying the ratio of BA to MA organic cations and find that as n increases in BA2(MAPbI3) n-1PbI4, the resultant 2D perovskites are too weakly excitonic (i.e. the exciton binding energy reduces to less than or equal to kT) to exhibit CT states in their EQE spectra, even when paired with the strong electron acceptor HATCN (Figure S4 in the Supporting Information). In fact, the EQE of the n = 2 and n = 3 perovskite/HATCN heterojunction solar cells are even lower than that of the perovskite single layer devices, because in the heterojunction solar cells, the presence of the HATCN layer inhibits charge extraction without improving charge generation. In contrast and to reiterate, in the highly excitonic n = 1 perovskite/HATCN solar cell, the HATCN layer is beneficial for charge generation, such that the heterojunction solar cell exhibits higher EQE compared to the single layer perovskite device (Figure 1d). 6 ACS Paragon Plus Environment

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Notably, photocurrent generated by intermolecular CT state absorption from the BA2PbI4/HATCN planar heterojunctions is ~4 orders of magnitude lower than that from intramolecular absorption (Figure 1d). To further enhance the CT process, BA2PbI4/HATCN bulk heterojunctions (BHJs) with various HATCN molar ratios were prepared (Figure S5, Supporting Information). Absorption and XRD measurements were performed to confirm that no side products were formed in the BA2PbI4/HATCN blend films with up to 40 mol% HATCN (Figure S6, Supporting Information). Figure 2a compares the EQE spectra of the BA2PbI4/HATCN planar heterojunction (PHJ) and BHJ devices with 40 mol% HATCN. For the BHJ device, photocurrent generated by intermolecular CT state absorption is only 1.5 orders of magnitude lower than that from intramolecular absorption, which is ~80 times enhanced with respect to that of the PHJ device. Fitting the CT state spectra of these devices according to Marcus theory (Figure S7) yields a higher CT energy (Ect) value for the BHJ devices (1.7 eV) in comparison to that of the PHJ devices (1.5 eV). This is possibly induced by increased disorder and reduced dielectric constant in the BHJ system.22 The f value for the BHJ devices (3.2×10-4 eV2) is also larger than that of the PHJ devices (3.0×10-6 eV2), likely due to increased donor-acceptor interactions in the BHJ. Transient photoluminescence (PL) was also measured for BA2PbI4 single layer, BA2PbI4/HATCN planar heterojunction and BA2PbI4/HATCN bulk (with 20% and 40 mol% HATCN) heterojunction films (Figure 2b, also see Figure S8 and Table S2 in the Supporting Information for more information). Due to the ultrafast CT process, PL decay lifetime is significantly reduced by increasing the molar ratio of HATCN in the bulk heterojunction films. In contrast, the PL decay rate of the BA2PbI4/HATCN planar heterojunction film is very similar to the BA2PbI4 single layer. These observations indicate that the exciton diffusion length in 2D perovskites is very short at least in the out-of-plane direction, such that only those excitons generated very near to the donor/acceptor



interface undergoes the CT process. This is consistent with the enhanced CT feature for the bulk heterojunction devices in the EQE spectra. We also conducted electroluminescence (EL) and PL measurements on these samples. However, neither EL nor PL from CT states was observed, possibly due to the limitation of our detection sensitivity. To study the role of the bulky organic cation on the CT process, we study additional 2D perovskite systems including PEA2PbI4, EA2PbBr4, BA2PbBr4, PEA2PbBr4, and the 3D system MAPbBr3. Figure 3a presents the X-ray diffraction (XRD) patterns of these films. The XRD patterns of the 2D perovskite films exhibit peaks over multiple harmonics, indicating reasonable long-range order and texturing of the films oriented with the c-axis out-of-plane, with diffraction dictated by the bulky organoammonium cation. The characteristic diffraction peaks corresponding to the (002) plane at ~6.4° for BA2PbI4 and BA2PbBr4 and ~5.4° for PEA2PbI4 and PEA2PbBr4 correspond to interlayer distances of ~1.38 nm and ~1.63 nm, which are consistent with the bulkier PEA cations compared to the BA cations. As the EA cations are even smaller than BA cations, the diffraction peak corresponding to the (002) plane for EA2PbBr4 is measured to be 7.95°, corresponding to an interlayer distance of 1.11 nm. Figure 3b shows absorption spectra of these perovskite films. The strong absorption peak at approximately 515 nm for PEA2PbI4 and BA2PbI4, 402 nm for PEA2PbBr4 and BA2PbBr4 and 389 nm for EA2PbBr4 are associated with absorption to excitonic states, whose corresponding energies are slightly less than the band-gap energy of the respective material. In contrast, the MAPbBr3 film exhibits no strong excitonic absorption feature due to the small exciton binding energy in 3D perovskites. Figure 4a compares the EQE spectra of the PEA2PbI4/HATCN heterojunction and PEA2PbI4 single layer devices. Although PEA2PbI4 is strongly excitonic and has a similar absorption spectrum compared to that of BA2PbI4, unlike the BA2PbI4/HATCN heterojunction (cf.


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Figure 1d), no CT features are observed at the PEA2PbI4/HATCN heterojunction. Furthermore, the peak EQE of the PEA2PbI4/HATCN heterojunction is slightly reduced compared to that of the PEA2PbI4 single layer device. All of these effects are consistent with the fact that PEA cations are bulkier compared to BA cations. The bulkiness of the PEA cations prevents efficient molecular orbital overlap between excitons formed on the 2D metal halide octahedral sheet and neighboring HATCN molecules. As a result, the EQE spectrum of the PEA2PbI4/HATCN solar cell does not exhibit significant CT features despite PEA2PbI4 being a strongly excitonic 2D perovskite. Figure 4b shows the EQE spectra of the various bromide-based perovskite/HATCN heterojunction solar cells. Consistent with the observations for the iodide-based perovskites, CT features are only observed for 2D bromide perovskites that contain the relatively small BA and EA cations (BA2PbBr4 and EA2PbBr4), whereas no CT features are observed for the PEA2PbBr4/HATCN heterojunction. Fitting the CT state spectra of these devices according to Marcus theory (Figure S9) yields a higher f value for the EA2PbBr4/HATCN heterojunctions (5.6×10-4 eV2) in comparison to that of the BA2PbBr4/HATCN devices (2.1×10-5 eV2). This is consistent with the fact that EA cations are smaller than the BA cations, resulting in a smaller donor-acceptor distance and stronger intermolecular overlap. These observations call for attention on the influence of bulky organic cations on the properties of 2D perovskites. Finally, no CT features are observed for the 3D MAPbBr3/HATCN heterojunction because MAPbBr3 is not sufficiently excitonic, similar to what was observed for the Ruddlesden-Popper phase 2D perovskites with higher n values. In summary, we have demonstrated the formation of CT states between excitonic 2D perovskites and organic semiconductors. In addition, we have identified several factors that affect the strength of the CT transition. In particular, we have found that the size of the organic cation in the 2D perovskite plays a critical role in its ability to couple to an organic acceptor. Charge



transfer features were observed in the EQE spectra of donor/acceptor solar cells comprised of 2D perovskite with relatively small organic cations (here, EA and BA), paired with the strong electron accepting molecule HATCN, whereas no CT features are present when the 2D perovskite layer contains bulkier aromatic PEA cations. Bulkier cations prevent contact between the 2D perovskite metal halide sheet and the organic layer, inhibiting CT processes at the donor/acceptor interface. Furthermore, the ability to form CT states decreases with increasing n value of the 2D perovskite, which is linked to reduced excitonic character. The observation of CT states at 2D perovskite/organic heterojunctions opens up numerous opportunities for exploiting the excitonic nature of 2D perovskites in various applications such as donor/acceptor type solar cells, photodetectors, light emitting devices, and light-matter interactions. Experimental Methods Experimental details can be found in the Supporting Information. ASSOCIATED CONTENT Supporting Information Experimental details, Supporting Figures S1-S9 and Supporting Tables S1-S2 (PDF) AUTHOR INFORMATION Corresponding Author *

E-mail: [email protected]

Notes The authors declare no competing financial interest. ACKNOWLEDGEMENTS


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This work was supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering under Award Nos. DE-SC0012458 and DE-SC0012365 as well as the ONR Young Investigator Program (Award #N00014-17-1-2005). REFERENCES (1) (2) (3) (4)

(5) (6)

(7) (8)

(9)

(10)

(11)

(12)

(13)

Gao, F.; Inganas, O. Charge Generation in Polymer-fullerene Bulk-heterojunction Solar Cells. Phys. Chem. Chem. Phys. 2014, 16, 20291-20304. Deibel, C.; Strobel, T.; Dyakonov, V. Role of the Charge Transfer State in Organic Donor–Acceptor Solar Cells. Adv. Mater. 2010, 22, 4097-4111. Green, M. A.; Ho-Baillie, A.; Snaith, H. J. The Emergence of Perovskite Solar Cells. Nat Photon 2014, 8, 506-514. Miyata, A.; Mitioglu, A.; Plochocka, P.; Portugall, O.; Wang, J. T.-W.; Stranks, S. D.; Snaith, H. J.; Nicholas, R. J. Direct Measurement of the Exciton Binding Energy and Effective Masses for Charge Carriers in Organic–inorganic Tri-halide Perovskites. Nat. Phys. 2015, 11, 582. Chen, Y.; Sun, Y.; Peng, J.; Tang, J.; Zheng, K.; Liang, Z. 2D Ruddlesden–Popper Perovskites for Optoelectronics. Adv. Mater. 2018, 30, 1703487. Smith, I. C.; Hoke, E. T.; Solis-Ibarra, D.; McGehee, M. D.; Karunadasa, H. I. A Layered Hybrid Perovskite Solar-Cell Absorber with Enhanced Moisture Stability. Angewandte Chemie 2014, 126, 11414-11417. Saparov, B.; Mitzi, D. B. Organic–Inorganic Perovskites: Structural Versatility for Functional Materials Design. Chem. Rev. 2016, 116, 4558-4596. Zhang, X.; Munir, R.; Xu, Z.; Liu, Y.; Tsai, H.; Nie, W.; Li, J.; Niu, T.; Smilgies, D.M.; Kanatzidis, M. G.; et al. Phase Transition Control for High Performance Ruddlesden–Popper Perovskite Solar Cells. Adv. Mater. 2018, 30, 1707166. Straus, D. B.; Kagan, C. R. Electrons, Excitons, and Phonons in Two-Dimensional Hybrid Perovskites: Connecting Structural, Optical, and Electronic Properties. J. Phys. Chem. Lett. 2018, 1434-1447. Blancon, J.-C.; Tsai, H.; Nie, W.; Stoumpos, C. C.; Pedesseau, L.; Katan, C.; Kepenekian, M.; Soe, C. M. M.; Appavoo, K.; Sfeir, M. Y.; et al. Extremely Efficient Internal Exciton Dissociation Through Edge States in Layered 2D Perovskites. Science 2017, DOI: 10.1126/science.aal4211 Yang, X.; Zhang, X.; Deng, J.; Chu, Z.; Jiang, Q.; Meng, J.; Wang, P.; Zhang, L.; Yin, Z.; You, J. Efficient Green Light-emitting Diodes Based on Quasi-twodimensional Composition and Phase Engineered Perovskite with Surface Passivation. Nat. Commun. 2018, 9, 570. Yuan, M.; Quan, L. N.; Comin, R.; Walters, G.; Sabatini, R.; Voznyy, O.; Hoogland, S.; Zhao, Y.; Beauregard, E. M.; Kanjanaboos, P.; et al. Perovskite Energy Funnels for Efficient Light-emitting Diodes. Nat Nano 2016, 11, 872-877. Wang, N.; Cheng, L.; Ge, R.; Zhang, S.; Miao, Y.; Zou, W.; Yi, C.; Sun, Y.; Cao, Y.; Yang, R.; et al. Perovskite Light-emitting Diodes based on Solution-processed Selforganized Multiple Quantum Wells. Nat. Photonics 2016, 10, 699-704. 11 ACS Paragon Plus Environment


(14)

(15)

(16) (17)

(18)

(19) (20) (21)

(22)

Xiao, Z.; Kerner, R. A.; Zhao, L.; Tran, N. L.; Lee, K. M.; Koh, T.-W.; Scholes, G. D.; Rand, B. P. Efficient Perovskite Light-emitting Diodes Featuring Nanometresized Crystallites. Nat. Photonics 2017, 11, 108-115. Silver, S.; Yin, J.; Li, H.; Brédas, J.-L.; Kahn, A. Characterization of the Valence and Conduction Band Levels of n = 1 2D Perovskites: A Combined Experimental and Theoretical Investigation. Adv. Energy Mater. 2018, 8, 1703468. Kerner, R. A.; Zhao, L.; Xiao, Z.; Rand, B. P. Ultrasmooth Metal Halide Perovskite Thin Films via Sol-gel Processing. J. Mater. Chem. A 2016, 4, 8308-8315. Zhao, L.; Yeh, Y.-W.; Tran, N. L.; Wu, F.; Xiao, Z.; Kerner, R. A.; Lin, Y. L.; Scholes, G. D.; Yao, N.; Rand, B. P. In Situ Preparation of Metal Halide Perovskite Nanocrystal Thin Films for Improved Light-Emitting Devices. ACS Nano 2017, 11, 3957-3964. Zhao, L.; Kerner, R. A.; Xiao, Z.; Lin, Y. L.; Lee, K. M.; Schwartz, J.; Rand, B. P. Redox Chemistry Dominates the Degradation and Decomposition of Metal Halide Perovskite Optoelectronic Devices. ACS Energy Lett. 2016, 1, 595-602. Marcus, R. A. On the Theory of Oxidation‐Reduction Reactions Involving Electron Transfer. I. The Journal of Chemical Physics 1956, 24, 966-978. Vandewal, K. Interfacial Charge Transfer States in Condensed Phase Systems. Annual Review of Physical Chemistry 2016, 67, 113-133. Graham, K. R.; Erwin, P.; Nordlund, D.; Vandewal, K.; Li, R.; Ngongang Ndjawa, G. O.; Hoke, E. T.; Salleo, A.; Thompson, M. E.; McGehee, M. D.; et al. Re-evaluating the Role of Sterics and Electronic Coupling in Determining the Open-Circuit Voltage of Organic Solar Cells. Adv. Mater. 2013, 25, 6076-6082. Bernardo, B.; Cheyns, D.; Verreet, B.; Schaller, R. D.; Rand, B. P.; Giebink, N. C. Delocalization and Dielectric Screening of Charge Transfer States in Organic Photovoltaic Cells. Nat. Commun. 2014, 5, 3245.


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FIGURES

Figure 1. Molecular structures of (a) HATCN and (b) Alq3. (c) Energy diagrams of BA2PbI4, Alq3, and HATCN. (d) External quantum efficiency (EQE) spectra of BA2PbI4 single layer, BA2PbI4/HATCN and BA2PbI4/Alq3 heterojunction devices.



Figure 2. (a) Normalized external quantum efficiency (EQE) spectra of BA2PbI4/HATCN planar heterojunction and BA2PbI4/HATCN bulk (with 40 mol% HATCN) heterojunction devices. (b) Transient photoluminescence for BA2PbI4 single layer, BA2PbI4/HATCN planar heterojunction and BA2PbI4/HATCN bulk (with 20% and 40 mol% HATCN) heterojunction devices. EQE and PL are normalized to their respective maximum value.

Figure 3. (a) XRD patterns of various 2D perovskite and MAPbBr3 films. (b) Absorption spectra of various 2D perovskite and MAPbBr3 films.


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Figure 4. (a) External quantum efficiency (EQE) spectra of PEA2PbI4/HATCN heterojunction and PEA2PbI4 single layer devices. (b) EQE spectra of EA2PbBr4/, BA2PbBr4/, PEA2PbBr4/ and MAPbBr3/HATCN heterojunction devices.


Acceptor Charge-Transfer States at Two-Dimensional Metal

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