Solution-processable copper-doped molybdenum oxide films as hole injection interfacial layer in polymer light-emitting diodes

Solution-processable copper-doped non-stoichiometric molybdenum oxide (Cu:MoO_x) films were prepared by a sol–gel method and applied as hole injection layer (HIL) in polymer light-emitting diodes (PLEDs). Optical transmittance spectra and scanning electron microscopy images showed that both MoO_x (pristine oxide, without Cu) and Cu:MoO_x films are transparent and homogeneous, and an increase in surface roughness and electrical conductivity was observed after the Cu incorporation. According to Kelvin probe force microscopy images, the average surface potential value is more negative for the Cu:MoO_{x(1 and 5%)} films than for the MoO_x film (control), suggesting that there is an increase in the number of hole carriers. In fact, it was demonstrated that the electric conductivity, surface roughness, work function, and morphology of the HIL could be tuned by varying the concentration of Cu in the MoO_x films. To demonstrate possible applications of these materials, PLEDs were assembled using MoO_x and Cu:MoO_x films as HIL and two different emissive layers: poly[9,9-dioctylfluorenyl-2,7-diyl]-end capped with dimethylphenyl (PFO, blue light emitting polymer) and poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV, red light emitting polymer). MEH-PPV-based PLED composed of Cu:MoO_x(1%) HIL delivered the best performance, with a current efficiency (η_curr) of 313.9 mcd A⁻¹ (at 10 V), an intense red color (CIE 0.493; 0.506), and a turn-on voltage (V_ON) of 4.8 V. PFO-based PLED containing the Cu:MoO_x(5%) film showed the best results: η_curr of 124 mcd A⁻¹ (at 10 V), an intense blue color (CIE 0.168; 0.218) and V_ON of 4.8 V. We observed that the Cu:MoO_x films promoted an enhancement in brightness and overall performance, and caused a significant change in the morphology and electroluminescence profile of the diodes. Those results opened a new range of possibilities to investigate in detail the electrical, optical, and morphological properties of MoO_x and Cu:MoO_x films as HIL in PLEDs.