Crystalline Molybdenum Oxide Thin-Films for Application as Interfacial Layers in Optoelectronic Devices

Abstract

The ability to control the interfacial properties in metal-oxide thin films through surface defect engineering is vital to fine tune their optoelectronic properties and thus their integration in novel optoelectronic devices. This is exemplified in photovoltaic devices based on organic, inorganic or hybrid technologies, where precise control of the charge transport properties through the interfacial layer is highly important for improving device performance. In this work, we study the effects of in situ annealing in nearly stoichiometric MoO x (x~3.0) thin-films deposited by reactive sputtering. We report on a work function increase of almost 2 eV after inducing in situ crystallization of the films at 500°C, resulting in the formation of a single crystalline α-MoO 3 overlaid by substoichiometric and highly disordered nanoaggregates. The surface nanoaggregates possess various electronic properties, such as a work function ranging from 5.5 eV up to 6.2 eV, as determined from low-energy electron microscopy studies. The crystalline underlayer possesses a work function greater than 6.3 eV, up to 6.9 eV, characteristic of a very clean and nearly defect-free MoO 3 . By combining electronic spectroscopies together with structural characterizations, this work addresses a novel method for tuning, and correlating, the optoelectronic properties and microstructure of device-relevant MoO x layers.

Crystalline Molybdenum Oxide Layers as Efficient and Stable Hole Contacts in Organic Photovoltaic Devices

“…Clearly, devices using as-deposited MoO Sp show very poor 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 6 performance, which could be explained by a low electrical conductivity 28 and/or a low work function for these non-annealed sputtered MoO x films, as previously reported. 29 In this device configuration, efficient hole extraction at the anode contact requires that the work function of the MoO x layer should be equal to or larger than the HOMO energy level of DBP (5.5 eV 33 ).…”

“…Crystallization of nearly stoichiometric MoO Sp films on silicon surfaces has been shown to be followed by large work function increases, from around 4.8 eV for asdeposited films to around 6.3 eV (on average) for films annealed at 500°C. 29 In order to probe the work function of the super-oxidized and annealed MoO Sp films formed on ITO surfaces in this study, LEEM measurements of the films annealed at 350°C were conducted, as shown in 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 clearly show a superior air stability compared to those developed from the MoO Th films, and an impressive 80% of the initial PCE for the MoO Sp devices is maintained after 240 hours of light soaking under 1000W/m 2 irradiation at ~60°C. This is in strong contrast to the MoO Th devices, which at best (for 10 nm MoO x film thickness) maintain just below 50% device performance…”

The Feasibility of Using Magnetron Sputtered MoO x as Effective Hole Injection Layer in Organic Light-Emitting Diode

“…Oka et al reported possibility of improving hole injection performance of MoO x films deposited by reactive sputtering in an atmosphere of argon (Ar) and oxygen (O 2 ). [23] Cauduro et al demonstrated crystalline MoO x (x $ 3) with tunable WF through reactive sputtering and post-annealing at 500 C. [24] Some applications of magnetron sputtered MoO x to solar cells, [25,26] gasochromic switching [27] and micro-supercapacitors [28] are also demonstrated. In this study, rather than using vacuum thermal evaporation or solution-processed technology, we propose using magnetron sputtering for depositing MoO x films and cast successful application to OLEDs.…”

Bottom-Up and Top-Down Approaches for MgO

“…Nanoparticles and thin films are very common form of materials for utilization in different applications [1][2][3][4]. Synthesis approaches play vital role to determine characteristics of nanoparticles [5] and thin films [6].…”