Substrate-Free Thermoelectric 25 μm-Thick Ag₂Se Films with High Flexibility and In-Plane <i>zT</i> of 0.5 at Room Temperature

Thermoelectric inorganic films are flexible when sufficiently thin. By removing the substrate, that is, making them free-standing, the flexibility of thermoelectric films can be enhanced to the utmost extent. However, studies on the flexibility of free-standing thermoelectric inorganic films have not yet been reported. Herein, the high thermoelectric performance and flexibility of free-standing thermoelectric Ag2Se films are reported. Free-standing Ag2Se films with a thickness of 25.0 ± 3.9 μm exhibited an in-plane zT of 0.514 ± 0.060 at room temperature. These films exhibited superior flexibility compared to Ag2Se films constrained on a substrate. The flexibility of the Ag2Se films was systematically investigated in terms of bending strain, bending radius, thickness, and elastic modulus. Using free-standing Ag2Se films, a substrate-free, flexible thermoelectric generator was fabricated. The energy-harvesting capacity of the thermoelectric generator was also demonstrated

Conductivity Enhancement of Nickel Oxide by Copper Cation Codoping for Hybrid Organic-Inorganic Light-Emitting Diodes

We demonstrate a Cu­(I) and Cu­(II) codoped nickel­(II) oxide (NiO_<i>x</i>) hole injection layer (HIL) for solution-processed hybrid organic-inorganic light-emitting diodes (HyLEDs). Codoped NiO_<i>x</i> films show no degradation on optical properties in the visible range (400–700 nm) but have enhanced electrical properties compared to those of conventional Cu­(II)-only doped NiO_<i>x</i> film. Codoped NiO_<i>x</i> film shows an over four times increased vertical current in comparison with that of NiO_<i>x</i> in conductive atomic force microscopy (c-AFM) configuration. Moreover, the hole injection ability of codoped NiO_<i>x</i> is also improved, which has ionization energy of 5.45 eV, 0.14 eV higher than the value of NiO_<i>x</i> film. These improvements are a consequence of surface chemical composition change in NiO_<i>x</i> due to Cu cation codoping. More off-stoichiometric NiO_<i>x</i> formed by codoping includes a large amount of Ni vacancies, which lead to better electrical properties. Density functional theory calculations also show that Cu doped NiO model structure with Ni vacancy contains diverse oxidation states of Ni based on both density of states and partial atomic charge analysis. Finally, HyLEDs of Cu codoped NiO_<i>x</i> HIL have higher performance comparing with those of pristine NiO_<i>x</i>. The current efficiency of devices with NiO_<i>x</i> and codoped NiO_<i>x</i> HIL are 11.2 and 15.4 cd/A, respectively. Therefore, codoped NiO_<i>x</i> is applicable to various optoelectronic devices due to simple sol–gel process and enhanced doping efficiency