Direct Observation of Perovskite Photodetector Performance

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Surfaces, Interfaces, and Applications

Direct Observation of Perovskite Photodetector Performance Enhancement by Atomically-Thin Interface Engineering Zibo Li, Jieni Li, Dong Ding, Huizhen Yao, Lai Liu, Xue Gong, Bingbing Tian, Henan Li, Chenliang Su, and Yumeng Shi ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b10971 • Publication Date (Web): 28 Sep 2018 Downloaded from http://pubs.acs.org on September 28, 2018

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Direct

Observation

of

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Perovskite

Photodetector Performance Enhancement by Atomically-Thin Interface Engineering Zibo Li,† Jieni Li,† Dong Ding,† Huizhen Yao,† Lai Liu,† Xue Gong,† Bingbing Tian,† Henan Li,‡ Chenliang Su,†,§ Yumeng Shi*†,§ †

SZU-NUS Collaborative Innovation Center for Optoelectronic Science&Technology,

International Collaborative Laboratory of 2D Materials for Optoelectronics Science and Technology of Ministry of Education, College of Optoelectronic Engineering, Shenzhen University, Shenzhen 518060, China. ‡

College of Electronic Science and Technology, Shenzhen University, Shenzhen

518060, China. §

Engineering Technology Research Center for 2D Material Information Function

Devices and Systems of Guangdong Province, College of Optoelectronic Engineering, Shenzhen University, Shenzhen 518060, China. KEYWORDS. perovskite, transition-metal dichalcogenides, photoconductive atomic force microscopy, heterojunction, photovoltaic phenomenon.


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ABSTRACT Lead trihalide perovskites have been integrated with atomically thin WS2 and served as absorption layers to improve photoresponsivity in photodetectors. The combination of perovskites and two-dimensional (2D) transition-metal dichalcogenides (TMDCs) materials provides the platform to study light-matter interactions and charge transfer mechanisms in the optoelectronic devices. Herein, conductive and photoconductive atomic force microscopy (C-AFM and PC-AFM) were used to image the dark current and photocurrent generated in WS2/CH3NH3PbI3 (MAPbI3) heterostructures. Dark current measurement in the applied voltage range displays characteristic diode behavior, which can be well-described by thermionic emission theory. Under laser illumination at 532 nm, the spatially resolved photocurrent images exhibit location dependent photoresponse, where the photocurrent increases remarkably for the WS2/MAPbI3 heterostructures compared with the bare MAPbI3 regions. Furthermore, comparative surface roughness and 2D Fourier analysis of the topographic and current maps reveal the interfacial conditions of the WS2/MAPbI3 heterojunctions play an important role in the charge separation process. In addition, WS2/MAPbI3-based photodetectors have been fabricated. Our study provides direct evidence that atomically thin TMDCs monolayers can effectively assist the charge separation process and improve the light-to-electric energy conversion, that aiding in the design principles and understanding of 2D heterostructured optoelectronic devices.



INTRODUCTION Heterostructure formed by alien semiconducting materials provides opportunities for exploiting new types of optoelectronic and electronic devices with excellent performance. Single or few-layered transition metal dichalcogenides (TMDCs) possess unique electrical and optical properties, which are of technological and fundamental interests.1-8 However, in spite of their superior photoelectrical properties, atomically thin TMDCs monolayers are difficult to absorb sufficient light,9 which hinders the performance of TMDCs-based optoelectronic devices. Efforts have been made to increase the light harvest of TMDCs-based optoelectronics. Integrating TMDCs materials with other nanomaterials such as single-walled carbon nanotubes,10 PbS quantum dots, 11-13 amorphous silicon14 and rhodamine 6G fluorescent dye15 can lead to prominent increase in light absorption and result in an improved responsivity and quantum efficiency. Methylammonium lead halide perovskite with the formula of MAPbX3 (where MA=CH3NH3 and X=halogen) owns interesting electrical and optical properties such as high absorption efficiency and long free carrier diffusion lengths, which enable their outstanding photovoltaic cell performance.16-23 The integration of 2D TMDCs materials and lead halide perovskites provides lots of opportunities for studying light-matter interactions and charge transfer mechanisms in optoelectronic devices.24,25 The photodetectors based on the composites of perovskite and TMDCs usually showed much better performances than those of pure perovskite photodetectors.26 This is mainly due to the improved interfacial charge transfer in the


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hybrid structure and the high charge mobility of TMDCs. Moreover, TMDCs own tunable bandgaps, which can be well aligned with perovskite, so they can be used to suppress the dark current of hybrid photodetectors. Thus, the photodetectors based on TMDCs/perovskite hybrid structure exhibited higher performance and better environmental stability than those of pure perovskite photodetectors. Among well-exploited TMDCs materials, the band structure of WS2 (valence band edge is about −5.82 eV and conductive band is about −3.84 eV) is well aligned27 with the MAPbI3 perovskite (valence band edge is about −5.35 eV and conductive band edge is about −3.75 eV28). Therefore, WS2 has been integrated with MAPbI3 perovskite

to

fabricate

planar

photodetectors

with

significantly

improved

photo-detecting capability. WS2/MAPbI3 based planar photoconductors have shown high responsivity (≈17 AW−1) and high on/off ratios (≈105), which pave their way for future applications in digital electronic components and circuits.28 As the size of electronic and optoelectronic devices are developing towards nanoscale for highly integrated circuits, the electrical contacts of heterojunction must be reduced accordingly. The WS2/MAPbI3 heterostructure at nanometer size may show dramatically different properties compared with their macroscopic counterparts. In previous reports,29,30 conductive and photoconductive atomic force microscopy (C-AFM and PC-AFM) have been used to study the layer-dependent optoelectronic and electrical properties of TMDCs. However, few studies have been published so far to investigate the electrical contacts and charge transport process in vertical stacked WS2/MAPbI3. Meanwhile, in view of the outstanding flexibility of single-layer



TMDCs, it must showed inhomogeneous contact status after single-layer TMDCs transferred onto the rough perovskite surface. So the fluctuant interface properties of atomically thin TMDCs and perovskite materials may differ in micro-regions and may have a significant influence on the performance of optoelectronic devices.31-33 Therefore, an in-depth study of the electrical properties of the interface between atomically thin WS2 and MAPbI3 is demanding. In this contribution, we utilize C-AFM and PC-AFM to study the charge generation and transfer mechanisms at WS2/MAPbI3 interface. The spatially resolved photoresponse study of WS2/MAPbI3 heterostructure provides a unique way for the direct comparison and characterization of the various material regions under identical experimental conditions without complicated device fabrication process. The scanning current images provide sufficient measurement points reflecting the local optoelectronic and electrical properties directly with nanoscale spatial resolution. The photocurrent images combined with topographic measurement further indicate the contact properties between WS2 and MAPbI3 play a dominant role in the physical charge separation and transport.

EXPERIMENTAL SECTION Material Synthesis. Through chemical vapor deposition (CVD) method,2 monolayer WS2 with triangular-shape was fabricated on C-plane sapphire substrates.7 A quartz boat with WO3 powder (300 mg, Sigma–Aldrich) was located at the center of a tube furnace. The sapphire substrates were put at the downstream ~10 cm near the WO3 powders. A quartz boat with S powers was placed at the upper stream where


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the annealing temperature can be set independently. During the CVD process to grow WS2 crystals, the pressure was maintained at 5 Torr and Ar/H2 gas flow were set as Ar=50 sccm, H2=10 sccm. In order to promote the growth of WS2 crystals, the temperature of the heating zones for S and WO3 were raised to 240 °C and 900 °C in 30 min. Then the growth process maintained at the temperature for 15 min and followed by a natural cooling to room temperature. The MAPbI3 films were fabricated by spin-coating.34,35 The MAPbI3 precursor solution was made by dissolving 461 mg PbI2 and 159 mg MAI in 630 µL N,N-dimethylformamide (DMF) and 70 µL anhydrous dimethylsulfoxide (DMSO). Then 100 µL precursor solution was spun onto indium tin oxide (ITO)-glass substrate at 2,000 rpm for 2 s and 4,000 rpm for 20 s, and the sample was quickly washed with 120 µL toluene at 13 s during spin-coating. Subsequently, the sample was heated at 80 ℃ for 30 min. All the perovskite samples were prepared in Ar gas filled glove box with ultra-low H2O and O2 level (175 µm in Solution-Grown CH3NH3PbI3 Single Crystals. Science. 2015, 347, 967−970.



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Direct Observation of Perovskite Photodetector Performance

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