Nanoscale Characterization of WSe 2 for Opto-electronics Applications
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Nanoscale Characterization of WSe2 for Opto-electronics Applications Nirmal Adhikari; Avra Bandyopadhyay; Anupama Kaul* University of Texas at El Paso, El Paso, TX, United States *E-mail: [email protected] ABSTRACT Two dimensional (2D) thin transition metal dichalcogenides are being widely investigated for optoelectronics applications. Here, we report on the interfacial study of WSe2 with photo-absorber materials for efficient charge transport using Kelvin Probe Force Microscopy (KPFM) for solar cell applications. The WSe2 in these experiments was synthesized using Chemical Vapor Deposition (CVD) with a WO3 powder and Se pellets as the precursors, where the selenium was placed upstream in an Ar carrier gas within the furnace at a temperature zone of 260-270oC. For the interfacial analysis, nanoscale KPFM measurements show an average surface potential of 125 meV for the CVD synthesized WSe2 flakes. KPFM measurements signify that a thin layer of WSe2 can be used to suppress back recombination of carriers between the electron transport layer (ETL) and the absorber layer. A proper band alignment between ETL and absorber layer helps to increase the overall device performance, which we will elaborate upon in this work. Capacitance-voltage and capacitance-frequency measurements were measured to study the role of defects. INTRODUCTION Transition metal dichalcogenide (TMDCs), two-dimensional (2D) materials are good candidates for next-generation nanoelectronic and optoelectronic devices [1- 4]. These materials have tunable bandgap, high photoconductivity, high transparency, thickness-dependent electronic band structure and flexibility for heterostructure devices [5, 6]. Among the various TMDCs materials, WSe2 has been widely researched due to its outstanding electrical properties, including a low subthreshold swing value, a high on/off ratio, good carrier mobility and transport properties. Photodetectors based on few-layered WSe2 have exhibited excellent broadband photodetection properties demonstrating the emerging applications of 2D materials for highefficiency photovoltaic applications [7] and can also be used to increase the stability of the current high efficient perovskite solar cells [8-10]. The structure of WSe2 is composed of covalently bonded Se− W−Se atoms in layers that are closely packed in the hexagonal arrangements. These layers are held together by relatively weak van der Waals forces, which provide an advantage for exfoliating WSe2 film into monolayers. Intrinsic WSe2 offers p-type semiconducting properties, and its band gap can be tuned by controlling the number of layers. Monolayer WSe2 has a direct band gap of Eg = 1.63 eV with hole mobility of ∼500 cm2 V−1 s −1 [11]. WSe2 has a distinct optical response that exhibits a very high light absorption over a broad spectrum of wavelengths. It has been shown that WSe2 has superior optoelectronic property than MoS2 [12]. It is important to explore the electronic properties of WSe2 as prospective candidate for optoelectronic applications such as photovoltaic technolo
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