Bottom-Up Synthesis of MeS_x Nanodots for Optoelectronic Device Applications

WS_x and MoS_x nanodots are synthesized from (NH₄)₂WS₄ and (NH₄)₂MoS₄ precursors using a solvothermal method, and applied to organic photovoltaic cells (OPVs) and organic light emitting diodes (OLEDs) as hole injection layers (HILs). The optical band gaps of WS_x and MoS_x nanodots are 3.55 and 3.1 eV, respectively, and these nanodots show their strongest photoluminescence (PL) emission at 438 and 436 nm. The work functions of the nanodots increased from 4.3–4.4 to 5.0–5.1 eV following ultraviolet/ozone (UVO) treatment. By sandwiching thin layers of UVO-treated WS_x and MoS_x as HILs, the power conversion efficiency of OPVs dramatically increases from 1.51% to 3.0% and 2.95%, comparable to that of poly(3,4 ethylenedioxythiophene):poly(styrene-sulfonate) (PEDOT:PSS) based devices (3.23%). This increased OPV efficiency is believed to come from the increased work function, large band gap, and PL properties of nanodots. The UVO-MoS_x based OLED shows a higher maximum luminance efficiency (14.7 cd A⁻¹) compared to PEDOT:PSS based devices (13.1 cd A⁻¹). In addition, this study confirms that the stabilities of the OPV and OLEDs in air can be prolonged by using UVO-treated WS_x or MoS_x nanodots as HILs. These results demonstrate the great potential of synthesized WS_x or MoS_x nanodots for use as HILs in optoelectronic devices.