Bottom‐Up Synthesis of MeS_x Nanodots for Optoelectronic Device Applications

WS x and MoS x nanodots are synthesized from (NH 4 ) 2 WS 4 and (NH 4 ) 2 MoS 4 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.