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Defect management strategies are vital for enhancing the performance of perovskite-based optoelectronic devices, such as perovskite-based light-emitting diodes (PeLEDs). As additives can fucntion both as acrystallization modifier and/or defect passivator, a thorough study on the roles of additives is essential, especially for blue emissive Pe-LEDs, where the emission is strictly controlled by the n-domain distribution of the Ruddlesden–Popper (RP, L2An–1PbnX3n+1, where L refers to a bulky cation, while A and X are monovalent cation, and halide anion, respectively) perovskite films. Of the various additives that are available, octyl phosphonic acid (OPA) is of immense interest because of its ability to bind with uncoordinated Pb2+ ( notorious for nonradiative recombination) and therefore passivates them. Here, with the help of various spectroscopic techniques, such as X-ray photon-spectroscopy (XPS), Fourier-transform infrared spectroscopy (FTIR), and photoluminescence quantum yield (PLQY) measurements, we demonstrate the capability of OPA to bind and passivate unpaired Pb2+ defect sites. Modification to crystallization promoting higher n-domain formation is also observed from steady-state and transient absorption (TA) measurements. With OPA treatment, both the PLQY and EQE of the corresponding PeLED showed improvements up to 53% and 3.7% at peak emission wavelength of 485 nm, respectively. Ministry of Education (MOE) National Research Foundation (NRF) Submitted/Accepted version This research was funded by National Research Foundation (NRF), Prime Minister’s Office, Singapore under its Competitive Research Programme (CRP Award NRFCRP14-2014-03) and Ministry of Education, Singapore (MOE2019-T2-2-097). The photophysics studies were supported by the grants funded by the Singapore Ministry of Education under its AcRF Tier 2 grant (MOE-T2EP50120- 0004) and the NRF under NRF Investigatorship (NRF-NRFI2018-04).

Growth of the large-sized and high-quality MoS2 single crystals for high-performance low-power electronic applications is an important step to pursue. Despite the significant improvement made in minimizing extrinsic MoS2 contact resistance based on interfacial engineering of the devices, the electron mobility of field-effect transistors (FETs) made of a synthetic monolayer MoS2 is yet far below the expected theoretical values, implying that the MoS2 crystal quality needs to be further improved. Here, we demonstrate the high-performance two-terminal MoS2 FETs with room-temperature electron mobility up to ∼90 cm2 V-1 s-1 based on the sulfurization growth of the bifunctional precursor, sodium molybdate dihydrate. This unique transition-metal precursor, serving as both the crystalline Mo source and seed promotor (sodium), could facilitate the lateral growth of the highly crystalline monolayer MoS2 crystals (edge length up to ∼260 μm). Substrate surface treatment with oxygen plasma prior to the deposition of the Mo precursor is fundamental to increase the wettability between the Mo source and the substrate, promoting the thinning and coalescence of the source clusters during the growth of large-sized MoS2 single crystals. The control of growth temperature is also an essential step to grow a strictly monolayer MoS2 crystal. A proof-of-concept for thermoelectric device integration utilizing monolayer MoS2 sheds light on its potential in low-voltage and self-powered electronics.

International audience; Layered transition metal dichalcogenides (TMDCs) intercalated with alkali metals exhibit mixed metallic and semiconducting phases with variable fractions. Thermoelectric properties of such mixed-phase structure are of great interest because of the potential energy filtering effect, wherein interfacial energy barriers strongly scatter cold carriers rather than hot carriers, leading to enhanced Seebeck coefficient (S). Here, we study the thermoelectric properties of mixed-phase LixMoS2 as a function of its phase composition tuned by in situ thermally driven deintercalation. We find that the sign of Seebeck coefficient changes from positive to negative during initial reduction of the 1T/1T' phase fraction, indicating crossover from p- to n-type carrier conduction. These anomalous changes in Seebeck coefficient, which cannot be simply explained by the effect of deintercalation-induced reduction in carrier density, can be attributed to the hybrid electronic property of the mixed-phase LixMoS2. Our work shows that careful phase engineering is a promising route toward achieving thermoelectric performance in TMDCs.