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Naphthalene Diimide Ammonium Directed Single-Crystalline Perovskites with ‘Atypical’ Ambipolar Charge Transport Signatures in Two-Dimensional Limit Xiaomin Li, Jin Yang, Ziyi Song, Ruiping Chen, Lulu Ma, Haining Li, Jiong Jia, Jiao Meng, Xuan Li, Mingdong Yi, and Xuan Sun ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b00988 • Publication Date (Web): 11 Sep 2018 Downloaded from http://pubs.acs.org on September 11, 2018
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ACS Applied Energy Materials
Naphthalene Diimide Ammonium Directed Single-Crystalline Perovskites with ‘Atypical’ Ambipolar Charge Transport Signatures in Two-Dimensional Limit Xiaomin Li,1 Jin Yang,2 Ziyi Song,3 Ruiping Chen,4 Lulu Ma,5 Haining Li,1 Jiong Jia,1 Jiao Meng,1 Xuan Li,1 Mingdong Yi,3 Xuan Sun1,* 1
Key Laboratory for Colloid & Interface Chemistry, Shandong University, Ministry of Education, Jinan, 250100, P. R. China School of Microelectronics, Shandong University, Jinan, 250100, P. R. China 3 Key Laboratory for Organic Electronics & Information Displays (KLOEID) and Institute of Advanced Materials (IAM), Nanjing University of Posts Telecommunications, Nanjing 210023, P. R. China 4 State Key Lab of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, 350002, P. R. China 5 School of Material Science and Engineering, Shandong Provincial Key Laboratory of Preparation and Measurement of Building Materials, University of Jinan, 250022, P. R. China 2
ABSTRACT: Single crystal of a lead-iodine based 2D perovskite having naphthalene diimide ammonium (NDIA) molecules as organic layers was developed and the charge transport property was studied using field-effect transistors (FETs) measurements. Structure determination reveals the layered alternative stacking of lead iodide sheets and NDIA bilayers. The presence of NDIA promoted the lead iodide octahedron to form the unique three-point coplanar [Pb3I10]4- unit, which then connected into 2D network in a corner-sharing manner. The NDIA cations closely stacked into 1D chains within the bilayers that being sandwiched between the inorganic layers. FET characteristics of the single crystal obtained at room temperature demonstrate VDS-depended electron and hole transport behavior with mobilities reach up to more than 5×10-3 cm2 V−1 s−1. The 1D stack of NDIAs contributes greatly to the performance improvement for both the charge transport and the stability. Key Words: 2D semiconductor; Organic-inorganic halide perovskites; Naphthalene diimide; charge transport; single crystal; FieldEffect Transistor
Two-dimensional (2D) semiconductors featuring sizable bandgaps and strong in-plane charge transport have opened up new opportunities for nanoelectronics and optoelectronics.1,2 Organic-inorganic halide perovskites (OIHPs) are particularly attractive, which combine the excellent carrier transport inherent to the inorganic semiconductors together with the processability and flexibility of the organic semiconductors, therefore are ideal for wafer-scale fabrication of flexible electronic and optoelectronic devices by bottom-up approaches.3 Though the 3D structured OIHPs, specifically the lead-based hybrids containing organic methylammonium (MA+) or formanidium (MA+) have demonstrated benchmarks in light emission applications and photovoltaic technologies, their instability strictly hampers the large-scale application, while low-dimensional hybrid perovskites are much more stable. Through incorporation of bulkier organic cations, 2D layered hybrid perovskites can be constructed by slicing the connectivity of the 3D-networked perovskites along specific crystallographic planes.4 The dimensionality, the connection, orientation and distortion of the resultant lead halide inorganic frameworks are extremely sensitive to the nature of the organic cations, which in turn strongly affect the quantum confinement of the nanocrystals and have a significant impact on the intrinsic photophysical properties of the compounds, such as the bandgap and carrier mobility, the central elements determining the suitability for specific optoelectronic applications.5,6 The ability to control perovskite sheet orientation
through the choice of organic cation thus provides an important handle for tailoring and understanding the charge transport in 2D limit. Earlier studies on charge transport of the tin(II)-based 2D perovskite analogs have disclosed that either the -oriented perovskite sheets7 or the -terminated ones8 demonstrated metallic-to-semiconducting transition in response to the dimensionality reduction, since charge transport along the lattice direction perpendicular to the large organic cation layer was suppressed. Nevertheless, high carrier mobility was expected for the layered perovskites on account of the extended inorganic framework derived from their 3D counterparts.9,10 In contrast, the intrinsic charge transport parameters of organolead halide perovskites remain rarely disclosed, especially for the low-dimensional-networked perovskites. The known studies on lead-based field effect transistors (FETs, including the phototransistors11,12) were limited to 3D MAPbI3 (MA = CH3NH3), which exhibited an ambipolar but predominantly hole mobility (µh) of only 10−5 cm2 V−1 s−1 at room temperature,13 but a dominant n-type transport with limited electron mobility of less than 10−3 cm2 V−1 s−1 for the single crystal.14 Moreover, large hysteresis due to ion migration or charged point defects11,14-17 remains a serious concern for such 3D perovskite containing small organic cations. Indeed, the above experimental results in revealing the intrinsic transfer characteristics of lead-based perovskite have demonstrated controversy in dominant p- or n-type nature of the carriers.13,14,16,17 According to a DFT analysis,18 the ambipolarity of
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MAPbI3 was revealed to be derived from the presence of the MA, which has contributions to the orbital composition of the band edges. This result provides a new sight for scanning the role of organic cations in structural and transport properties of the lead halide perovskites. Thus, it is meaningful to accommodate organic molecules with distinctive structures into hybrid perovskites, which is significantly underestimated. Particularly interesting is, quite recently, it has been disclosed that quantum confinement reduced the in-plane effective mass,19 which may indicate the improved carriers mobilities in the 2D layered OIHPs. The nature of the organic barrier layer was also suggested to affect the transport gap of the 2D compounds on the depth of the quantum wells and the interlayer electronic communication.19 From the point of organic semiconductor, dye molecules may provide opportunities to enhance the carrier mobilities as long as long-range ordered molecular arrangements are inducted with the template of inorganic plates. For instance, incorporating the monovalent TTF into the lead iodide hybrid induced a high conductivity at room temperature, which is indicative of the improved charge-carrier transport on account of the synergistic relationship between the radical cation system and Pb−I network.20 Naphthalene diimides (NDIs) are known n-type semiconductors which have been intensively studied in organic thin film transistors (OTFTs) and demonstrated high carrier mobilities depending on the crystalline packing.21 Great efforts have been devoted to further improve the charge transport properties of NDIs together with the aim of understanding structure-function relationship by means of molecular modification,22 crystalline stacking adjustment,23 and organicinorganic hybridization.24 Considering the electronic complementarity between electron-deficient NDIs and electron-rich Pb-I polyanions,25 integrating the NDI derivatives with perovskites is highly interesting for structure engineering and photoelectric properties manipulation. Herein we report the single crystal structure of a 2D perovskite (NDIA)4Pb3I10 and disclose the balanced ambipolar transport feature with FET measurements. (NDIA)4Pb3I10 crystals with the shape of oblique hexagonal plates (SEM shown in Figure S1) were obtained solvothermally with starting materials of NDIA-I, PbI2, and HI solution (experimental details are provided in Supporting Information). The EDS mapping showed the presence of C, N, I and Pb (Figure S2, ESI†). Solid (NDIA)4Pb3I10 crystallizes in the acentric monoclinic space group P21/n (Table S1, ESI†). The asymmetric unit cell consists of four NDIA molecules and a Pb3I10 trimer. The whole structure is formed by alternating layers of Pb−I sheets and NDIA cations (Figure 1), which stacks along the b-axis. The noteworthy structural feature is the unusual connection mode of the Pb−I octahedral, which is distinct to the pure corner-sharing bilayer structures that considered as being sliced from the 3D perovskite. Pb3I104- trimer is formed from three face-sharing octahedra linked by iodized vertexes, and then connects into corrugated layers through corner-sharing pattern. Selected bond distances and angles are shown in Table S2. So far, few isostructural organolead halide perovskites have been reported, including a bithiophene monoammonium cation (AESBT) incorporated iodometalate26 and a recently developed broadband emissive (tms)4Pb3Br10 (tms = trimethylsulfonium; (CH3)3S+).27 More interestingly, this 2D layered inorganic sublattice is absolutely different from a previously reported H2DPNDI (N,N’-di(4-pyridyl)-1,4,5,8napthalene diimide) containing lead halide,25 where 1D
[Pb2I6]2− polyanions were formed. The organic NDIA cations adopt a head to tail arrangement by weak inter-molecular interactions to form bilayers and sandwiched between the anionic slabs. In each layer, as the result of the template effect associated with the H-bonds, the NDIA stacks in chains that grow mainly along the a-axis. Eclipsed trimers of NDIA can be figured out in each chain, with an average intra-trimer distance around 3.52 Å. The neighboring trimers are incompletely overlapped with the inter-trimer distance of 3.33 Å. Such extended π-stacking may indicative of the potential charge transport path along the 1D chain of NDIAs. The huge difference between the 2D structure of (NDIA)4Pb3I10 and the 1D (H2DPNDI)Pb2I6 originated absolutely from the substitution effect at the ends of the NDI framework, which highlights the significance of the organic molecules in engineering the hybrids. The n-butyl chain on one end of the NDIA cannot be well solved because of disorder.
Figure 1. Crystal structure of (NDIA)4Pb3I10. (A) View down the c axis showing the corrugated Pb3I104− layers and NDIA bilayer consisting of 1D chains stacking along a axis. (B) View down the a axis showing infinite stacks of NDIA molecules, with large overlap in a trimmer (NDIA in yellow, orange, and green color) and between the neighbor trimers.
Figure 2. Comparison of the UV–Vis electronic absorption spectra of (NDIA)4Pb3I10 and NDIA-I. The Kubelka–Munk transformation of the diffuse reflectivity of (NDIA)4Pb3I10 crystals and the starting material, NDIA-I, is shown in Figure 2. (NDIA)4Pb3I10 crystals shows a broad absorption down to the near-infrared range, whereas the NDIA-I powder presents a parallel absorption covering the area from UV-Visible to the near-infrared. The overlapping bands in the region of 300-400 nm derived from the perylene ring can be identified from the solution spectrum of NDIA-I (Figure S3), but are not well resolved for (NDIA)4Pb3I10, which was ascribed to be obscured by inorganic−organic charge transfer. The strong absorption from 450 to 700 nm in the absorption feature of NDIA-I powder is highly suggestive of charge transfer between iodine and NDIA.28 Tauc plots corresponding to the direct and indirect bandgap fits of (NDIA)4Pb3I10 are provided in Figure S4, respectively. The spectra plotted as [hνF(R)]2 (panel a) does not exhibit a well-
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ACS Applied Energy Materials defined direct optical band edge within the examined energy range. While that plotted as [hνF(R)]1/2 (panel b) shows a linear region that indicates an indirect optical band gap of 1.45 eV when extrapolated to zero absorption. Savory et.al29 has investigated the charge transfer between the organic and inorganic networks in the 1D (H2DPNDI)Pb2I6 hybrid by ab initio DFT calculations, which demonstrated that the valence band is distinctly comprised of the lead s and iodine p orbitals, while the conduction band sits firmly on the organic DPNDI. There will be the same situation for the present NDIA involved hybrid, suggesting a direct electron transition from the lead iodide layers to the organic network as indicated in the absorption spectra. In this case, lead iodide may behave as electrondonating groups to make the hybrid ambipolar semiconductor. It is worthy to note that p/n dual channel ambipolar transport was indeed achieved by crystal engineering of a donor dipyrrolopyridine with NDI acceptors by H-bonding, where the chargetransfer interactions between the donor and acceptor could 30 be finely modulated.
The preferential-alignment of the (NDIA)4Pb3I10 crystals on substrate was firstly determined by X-ray diffraction (XRD) and atomic force microscopy (AFM) measurements. The peak at 5.65° in the XRD patterns (Figure S5) is attributed to the (040) reflection and corresponds to a lattice plane distance of ca. 15.55 Å, which matches well with a quarter of the crystallographic b-axis lattice parameter (Table S1, ESI†), indicating that the 2D planes parallel to the substrate with the b-axis perpendicular to the substrate. Accordingly, the NDIA molecules stood up in line on the substrate surface with a dihedral angle of roughly 60° (Figure 3a). AFM images of the surface of (NDIA)4Pb3I10 crystal (Figure S6) present clear layered structure, corresponding to the layered stacking of inorganic and organic sheets. The height of each layer is roughly 3 nm, coincides with the half length of the unit cell in [010] direction (Table S1, ESI†). The overall height of the plates is variable and cannot be accurately measured within the range of AFM. The indexed schematic diagram of (NDIA)4Pb3I10 single crystal simulated by Bravais-Friedel-Donnay-Harker (BFDH) method (Figure S7) indicates that the [010] direction is perpendicular to the substrate, while the (010) plane of the crystal, containing the net Pb–I matrix, is parallel to the substrate, which is completely consistent with the indications of the XRD and AFM. [100] direction is the fastest crystal-growth direction, leading to the crystalline plates elongated along this direction as shown in SEM observation (Figure S1). In this direction (a-axis), NDIA stacks with largest overlap to provide potential charge transfer path.
Figure 3. (a) Schematic illustration of the bottom-gate and top-contact devices with the (NDIA)4Pb3I10 crystalline plate as
the charge transport layer. (b) Optical image of the channel direction relative to the (NDIA)4Pb3I10 crystalline slab of the FET devices. Transfer characteristics of the single-crystal plate device along the a-axis (L = 85 µm, W = 565 µm) at (c) VDS = 15 V and (d) VDS = -30 V. The intrinsic charge-carrier transport properties of (NDIA)4Pb3I10 crystal was evaluated by FET measurements. The individual (NDIA)4Pb3I10 crystalline microplates served as the semiconducting channel of FET devices. Devices were fabricated in bottom-gate and “top contact” geometry as shown in Figure 3a, using silicon substrate as the gate electrode. The gold (Au) source and drain electrodes (100 nm thick) were thermally evaporated through a shadow mask on the crystal plate. Figure 3b displays an optical image of a typical FET device, where the thickness of the perovskite microplate is more than 10 µm. According to the above analysis on the molecular orientation in crystal, the device channel was made to nearly perpendicular to a-axis (Figure 3b). All measurements were performed in air. Figures 3c and 3d displayed representative transfer characteristics for a FET device having channel length, L = 85 µm and channel width, W = 565 µm. The transfer characteristics showed the source–drain voltage (VDS) depended carriers nature. At VDS = 15 V, the transfer curve demonstrates dominant p-type transport (Figure 3c); when VDS = -30 V, n-type transport is dominant (Figure 3d). The ambipolar nature of charge transport is well demonstrated in the corresponding output curves provided in Figure S8. When VG increased from 0 to 20 V, strong n-conduction was presented in Figure S8a, where IDS enhanced along with VG when VDS less than 15 V. Figure S8b showed the IDS changes corresponded to VG, which indicate typical p-conduction when VDS was higher than 10 V and n-type transport when VDS was less than -15 V. This VDS-depended transport behavior has ever been reported for the ambipolar MAPbI3−xClx hybrid halide perovskite without discussing the origin.15 For (NDIA)4Pb3I10 crystal, it is speculated to be the indications of the synergistic effect between the organic NDIA and the inorganic framework. According to theoretical simulation,18,29 CBM main contribution is given by C and N orbitals of the organic cations, which is associated with the electron transfer. Namely, NDIA contributes mostly to the n-type transport, which is consistent with the previous reports.21-23,30 Overall, this atypical ambipolar behavior may be ascribed to the combination of the p-type transport of the Pb-I layer and the n-type conduction of the NDIA layer integrated by H-bonding as indicated in the crystal structure. The maximum electron mobility (µe) is 5.44×10-3 cm2 V−1 s−1, and 5.83×10-3 cm2 V−1 s−1 for hole transportation (µh), obtained based on 10 devices (Figure S9). The on/off ratio is 103, 10 times higher than that of CH3NH3PbI3−xClx hybrid.15 The overall mobility was relying on the thickness of the crystalline plates, and reduced for the thicker ones. This is a common feature for the organic transistors since charge transport typically takes place in the first few monolayers adjacent to the gate dielectric. Hence, the mobilities can potentially be promoted by controlling the thickness of the crystals in the future work. To the best of our knowledge, this is the first report on the carrier mobilities of the 2D perovskite crystals by FET measurement. It is interesting to find that the mobility of this 2D structure containing large organic cations is higher than that of the spincoated layers of the 3D perovskite MAPbI3, whose field-effect hole mobility (µh) was only 10−5 cm2 V−1 s−1 at room temperature.13 Even in single-crystalline MAPbI3 devices, moderate mobili-
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