Over the past decades displays and solar cells technology had substantially evolved. For less than five decades ago the bulk technology was the most widely used worldwide. With the emergence of flat device technology a great revolution has occurred and, nowadays, this is the dominant technology in the field of displays and solar cells. The future technology has begun with the discovery of the organic semiconductor material which makes possible to replace conventional glass substrates for flexible substrates such as polymeric or metallic ones. In this thesis different types of polymeric composite substrates based on commercial high performance thermoplastic polyetherimide (PEI), and a natural and biocompatible polymer commonly used as artificial skin, bacterial cellulose (BC) has been developed. The above mentioned nanocomposites were developed for application as flexible substrates in organic devices. Three types of substrates were studied: PEI/carbon nanotubes (CNTs) nanocomposite; BC/PEI nanocomposite; and BC modified with an aluminum doped titanium dioxide (AlTiO2) layer. The first two substrates were used for the production of organic emitting devices (OLEDs), while the latter one was used for the production of a photodetector device in aqueous medium – retinal prosthesis. The new materials were mainly characterized by its optical and morphological properties and the results were used to determine its possible applications. PEI/CNT nanocomposite presented similar properties to the pure polymer when produced with low CNTs contents. For higher concentrations the results were inferior to those of the pure polymer. BC/PEI nanocomposite has showed comparable or better properties when compared with pure polymers. A highlight was the great improvement in their optical transparency in the visible region of electromagnetic spectrum, and the smooth surface achieved by the nanocomposite – comparable to that found for glass substrates and with better uniformity in relation to PEI substrates. Both substrates were functionalized by depositing a layer of tin doped indium oxide (ITO), which was used as a transparent electrode in the production of OLEDs. The analysis of surface functionalization showed that electrical properties of ITO films onto composites were also comparable to those obtained for glass and PEI substrates. However, BC/PEI substrate presented better ITO film stability in bending tests, showing no changes in its resistivity value even after undergoing 5 mm diameter of bending. The devices produced in the PEI/CNT composite substrate has also similar properties to those obtained by using pure polymer. The higher efficiency achieved by both flexible devices reached 1.45 cd/m2 which is still below the values obtained for the glass substrates – 2.15 cd/m2 in the case of commercial ITO substrate and 2.00 cd/m2 in the case the substrate with deposited ITO. The devices produced onto CB/PEI composite substrates showed excellent efficiency (2.50 cd/m2), a higher value than that obtained for substrates coated with commercial ITO. The CB/AlTiO2 substrate was designed to improve the adhesion of the ITO film onto BC substrate when in contact with water. The result was quite satisfactory, because in addition to maintaining the ITO layer adhered to the substrate it has a 46 percent improvement in surface roughness. This change in surface morphology resulted in a significant improvement of ITO electrical resistivity, a reduction of approximately 63 percent was observed. The modified substrates were used for production of a photodetector device and the results showed a promising substrate for production of biocompatible and flexible retinal prosthesis.