Improving extraction efficiency of OLEDs by a luminescent polymer embedded in a colloidal crystal matrix

Here, it is reported an enhancement of the light-extraction efficiency in organic light emitting diodes (OLEDs) by growing a colloidal crystal (CC) matrix inside the luminescent layer of single bottom-emitting heterostructures. The CC matrix is obtained by self-assembly of Silica (SiO₂) spheres during the spin-coating procedure used for the deposition of the luminescent polymer Poly [2-methoxy-5-(3,7-dimethyloctyloxy)-1,4-phenylenevinylene] (MDMO-PPV). The SiO₂ sphere diameter is estimated assuming a crystalline face-centered-cubic (fcc) structure and performing first principles calculations with the plane wave expansion method. The calculations provided a 250 nm sphere diameter for an adequate decoupling of the waveguide organic mode. From these considerations both, traditional bilayer and nanostructured OLEDs, were elaborated using ITO/PEDOT:PSS/MDMO-PPV/Ag and ITO/PEDOT:PSS/MDMO-PPV+Colloidal crystal/Ag architectures, respectively. The light-extraction efficiencies of both types of OLED architectures are quantified by measuring the wall plug efficiency. The results show a significant increase (near to 30%) on the average efficiency for nanostructured OLEDs. The improvement in the power efficiency is attributed to a reduction in the effective refractive index of the organic layer through the incorporation of a fcc-colloidal crystal into the emission layer (EML) of the OLED, which modifies their reflection properties. This novel method for obtaining CC in a single-step associated to the spin coating process has also the advantage of introducing the scattering medium directly into the EML in order to modify internal coupling modes, instead of focusing on substrate modification. (paper)