Field-Effect Transistors Based on van-der-Waals-Grown and Dry

Sep 19, 2017 - (1-3) A compatible all-perovskite optoelectronic system (APOS) is possible to be achieved, in which all components, such as solar cells...
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Field-Effect Transistors Based on van-der-Waals-Grown and DryTransferred All-Inorganic Perovskite Ultrathin Platelets Chengxue Huo,# Xuhai Liu,# Xiufeng Song, Ziming Wang, and Haibo Zeng* MIIT Key Laboratory of Advanced Display Materials and Devices Institute of Optoelectronics & Nanomaterials, College of Material Science and Engineering, Nanjing University of Science and Technology, Nanjing 210094, China S Supporting Information *

ABSTRACT: Nowadays, the research on perovskite transistors is still in its infancy, despite the fact that perovskite-based solar cells and light-emitting diodes have been widely investigated. Two major hurdles exist before obtaining reliable perovskite-based transistors: the processing difficulty for their sensitivity to polar solvents and unsatisfactory perovskite quality on the transistor platform. Here, for the first time, we report on high-performance all-inorganic perovskite FETs profiting from both van der Waals epitaxial boundary-free ultrathin single crystals and completely dry-processed transfer technique without chemical contaminant. These two crucial factors ensure the unprecedented high-quality perovskite channels. The achieved FET hole mobility and on−off ratio reach 0.32 cm2 V−1 s−1 and 6.7 × 103, respectively. Moreover, at the low temperature, the mobility and on−off ratio can be enhanced to be 1.04 cm2 V−1 s−1 and 1.3 × 104. This work could open the door for the FET applications based on perovskite single crystals.

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an optimized device configuration and careful substrate surface treatment.11 However, the optical properties and stability of Sn2+-based halide perovskites are inferior to Pb2+-based counterparts. Regarding Pb2+-based OIHPe transistor, Heo et al. presented μ h of only 10 −5 cm 2 V −1 s −1 from a methylammonium lead iodide (MAPbI3) thin film.12 Chin et al. have also reported very low room temperature μh/μe of 237 K), while the interaction of holes and phonon scatterings dominates at low temperature. We believe that future work should be focused on achieving better perovskite quality with no ion vacancies in the transistor applications. Because the crystal quality is essential for the excellent properties of optoelectronic devices, we used van der Waals

epitaxy to grow all-inorganic perovskite ultrathin platelets on fluorphlogopite mica to obtain superb material quality. The growing process is shown in Figure 1a and Figure S1. We heated CsX and PbX2 (X = Cl, Br, I) mixture powder to obtain the CsPbX3 (X = Cl, Br, I) ultrathin platelets (Figure S1).20−23 The detailed synthesis can be found in the Supporting Information. Because the substrate possessed no dangling bonds on their surface, the epitaxial layers were connected with the substrates through weak van der Waals force instead of chemical bonding, as demonstrated by the van der Waals gap in Figure 1a. Therefore, the van der Waals epitaxy can avoid the restriction of lattice matching between epitaxial layers and substrate, enabling the growth of a large area of 2D single crystal with excellent quality compared with other synthetic method.22,24,25 The nucleary is a random process due to the lack of dangling bonds. Once the nucleus begins to grow, the surface energy can play an important role. The adatoms are inclined to incorporate into ultrathin platelets by corners for higher surface energy. Therefore, the perovskites tend to grow along the diagonal direction, as shown in Figure 1a. Figure S2 shows typical optical images of CsPbX3 ultrathin platelets grown on mica. It is noteworthy that several incomplete platelets exist, especially for the larger platelets. The incomplete 4786

DOI: 10.1021/acs.jpclett.7b02028 J. Phys. Chem. Lett. 2017, 8, 4785−4792

Letter

The Journal of Physical Chemistry Letters

Figure 2. Characteristics of FETs based on CsPbBr3 ultrathin platelets. (a) Schematic illustration of completely dry transfer method. (b,c) Output and transfer characteristic of CsPbBr3 FET at room temperature, respectively. Inset of panel b is optical image of FET based on CsPbBr3. (d,e) Output and transfer characteristic of the FET at 237 K, respectively.

Figure 1b confirm the crystal structure of these ultrathin platelets. After subtracting the XRD signal of the pure mica (black line in Figure 1b), we obtained three typical peaks of CsPbBr3 (green line) corresponding to (100), (110), and (200) planes in PDF standard card of cubic phase (red line in Figure 1b), which confirm that the perovskite ultrathin platelets are in cubic phase. Because the logarithmic coordinates are used to describe the intensity of XRD, the intensity of (100) and (200) is much stronger than that of (110), which indicates a preferential orientation along (100) and (200) directions. This suggests that the ultrathin platelets are single crystals.20 The absorption and photoluminescence (PL) spectra of CsPbBr3 are illustrated in Figure 1c. An excitonic absorption peak in the absorption spectrum exists because of the large exciton binding energy of CsPbBr3.20 The PL spectrum shows that CsPbBr3 can emit light at 525 nm with the full width at half-maximum (fwhm) of 16.8 nm. The fwhm is even narrower than that of CsPbBr3 quantum dots, suggesting the high quality and

platelets usually possess at least one right-angle corner, which suggests that they are part of unfinished squares. This phenomenon proves that the perovskites tend to grow along the diagonal direction. Because CsPbBr3 ultrathin platelets exhibited more superior morphology compared with CsPbCl3 and CsPbI3, we chose CsPbBr3 ultrathin platelets as the active materials for FET investigation. The CsPbBr3 ultrathin platelets can also grow on other substrates, such as SiO2/Si, GaN, FTO glass, and graphene (Figure S3), which can confirm the growth process is van der Waals epitaxy. The lateral dimension of the perovskites ranges from 10 to 100 μm, which is significantly larger than the previous reports.20−22 In particular, the largest ultrathin platelet we obtained was larger than 300 × 300 μm2 (Figure S4). The square shape of the ultrathin platelets coincided with the cubic crystal structure of CsPbBr3. The high preparation temperature of ultrathin platelets could lead to this cubic phase of perovskites.20,21 The X-ray diffraction (XRD) patterns in 4787

DOI: 10.1021/acs.jpclett.7b02028 J. Phys. Chem. Lett. 2017, 8, 4785−4792

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Figure 2b,c provides room-temperature p-type output and transfer characteristics, respectively, from a typical perovskite ultrathin platelet transistor. All of the characteristics were obtained at a scan rate of 3.5 V s−1.11 Output characteristics are functions of drain current (Id) versus drain-source voltage (Vds) at different gate voltages (Vg), whereas transfer characteristics are described as Id versus Vg at different Vds. Relatively small hysteresis can be obtained from the output characteristics, as shown in Figure 2c. In contrast, the transfer characteristics exhibit larger hysteresis, as illustrated in Figure S9. The strong hysteresis in transfer characteristics could be attributed to screening effects arising from the ionic migration.11,13 Specifically, the continuously swept Vg induces more dynamic ion accumulation at the perovskite/dielectric interface in the forward scan, and a severe Id decrease can be expected in the reverse scan due to the previous ion accumulation. Therefore, we calculated linear μh using the transfer characteristics (Vg = [−40 V, −60 V], Vds = −4 V) measured in the forward scan by eq 1

excellent optical properties of these van-der-Waals-grown ultrathin platelets.26,27 No surfactant is involved in the van der Waals epitaxy process, whereas the surfactant inevitably introduces ligands and residues during the solution preparation process of perovskites.28 This could be of critical importance to determine the better optical properties of van-der-Waals-grown perovskites compared with the solution-processed counterparts. We carried out more measurements to further study the quality of the ultrathin platelets. Figure 1d shows the optical bright-field image of a typical CsPbBr3 ultrathin platelet, which possesses a smooth surface with sharp rectangular corners. No grain boundary can be observed from the optical image of the platelet or even from the high magnification scanning electron microscope (SEM) image (see Figure S5), which confirms the good quality of the perovskite ultrathin platelets. Figure 1e illustrates the optical image of the PL corresponding to Figure 1d. It indicates that the nanoplatelets prepared by van der Waals epitaxy are of high quality with few defects and traps that can prejudice radiative recombination of photon-generated electron−hole pairs. Furthermore, the edges of these ultrathin platelets appear to be brighter than the central part, suggesting a wave-guiding effect in these square perovskites.29 Figure 1f shows the atomic force microscope (AFM) image, in which the thickness of this platelet is ∼148 nm. Figure S6 presents the partial enlarged AFM image of CsPbBr3, and the average roughness was 104, respectively. The results indicate that the mobility in perovskite FETs is determined by the interaction of the charge carriers, ions, and phonons at the semiconductor-dielectric interface. The ion migration plays an important role in the electrical-transport process at relatively high temperature (above ∼237 K), while the interaction of charge carriers and phonon scatterings dominates at low temperature (below ∼237 K) in CsPbBr3 ultrathin platelet transistors. This work has shed new light on the importance of developing high-quality perovskite nanostructures in the optoelectronic devices. The devices also showed stability during 1 month in air, which can be attributed to the excellent quality of the ultrathin platelets and the protection of PVA film during fabrication of the devices.



#

C.H. and X.L. contributed equally to this work.

Notes

The authors declare no competing financial interest.



ACKNOWLEDGMENTS This work was financially supported by the National Basic Research Program of China (2014CB931702), NSFC (61704082), NSFC-RGC (5151101197), the National Natural Science Foundation of China (51502139), the Fundamental Research Funds for the Central Universities (30917015106, 30917014107), the Natural Science Foundation of Jiangsu Province of China (BK20170851), China Postdoctoral Science Foundation funded project (2014M560425), and PAPD of Jiangsu Higher Education Institutions.



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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.7b02028. Preparation of CsPbBr3 ultrathin platelets and fabrication of FETs (PDF)



REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Haibo Zeng: 0000-0002-0281-3617 4791

DOI: 10.1021/acs.jpclett.7b02028 J. Phys. Chem. Lett. 2017, 8, 4785−4792

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