oled displays are made up of a plurality of diodes : for convenience , the rest of the description will refer to the production of a single diode . in the drawings , the proportions among the various parts have not been respected in order to point out the details of greater interest . the inventors have found that in an oled display manufactured according to the invention , the diffusion phenomena of the electron - donor metal are reduced with respect to what occurs in known displays . although the phenomenon has not yet been studied in depth , it is believed that the presence of an already doped etl reduces the diffusion of the metal layer in contact with the cathodes , and consequently maintains its functionality for a longer time . similarly , it is believed that the presence of the metal layer reduces the diffusion of the metal from the etl toward the interface with the cathodes . the consequence is a lower shift over time of the electric characteristics of the oleds of the invention . fig1 shows an oled diode 10 used to form the display of the invention . the diode is made up of a sequence of superimposed layers deposited onto a transparent support 11 , generally made of glass . on this support anodes 12 are deposited ( the drawing shows only one anode ), being transparent in turn , generally made of ito and manufactured by screen - printing or by cathode deposition with a suitable masking . on the anodes , an organic htl layer 13 is present , generally manufactured with nitrogenated aromatic compounds ( aryl amines , derivatives of pyridines or pyrazines , etc .). the eml layer 14 of organic material is then provided , wherein the luminescence is generated upon recombination of electrons and holes transported by etl and htl layers , respectively . this layer may be manufactured , for example , from tris ( 8 - hydroxyquinoline ) aluminum ( often indicated in the field with the abbreviation alq ). on layer 14 the electron transport layer etl 15 is provided and the electron - donor metal layer 16 is present over etl layer 15 . finally , on the external surface of layer 16 , the cathode 17 is provided , generally made of aluminum , to which the electrical contact ( not shown ) for the supply of diode 10 is connected . typical thicknesses for the different layers are : about 150 nanometers ( nm ) for anodes 12 ; about 120 nm for htl layer 13 ; between 5 and 10 nm for eml layer 14 ; between 30 and 80 nm for etl layer 15 ; between 0 . 2 and 5 nm for electron - donor metal layer 16 , and between 200 and 300 nm for cathodes 17 . the characteristic elements of the diode of the invention are layers 15 and 16 . layer 15 may be manufactured with the same alq material of the eml layer and is formed of a portion 15 ′ directly contacting the eml layer and of a portion 15 ″. the portion 15 ′ is not intentionally doped with the electron - donor metal , although this may partially diffuse into portion 15 ′ during the life of the display . for the operation of the display , it is necessary to avoid having the electron - donor metal come into contact with and penetrate layer 14 . thus , the height of portion 15 ′ must be sufficient to ensure that the electron - donor metal is unable to pass through this entire height during the life of the device . this minimum height may be extrapolated from known data or from accelerated diffusion tests of the specific metal into the specific organic material . for instance , in the case in which layer 15 is made of alq and the metal is lithium , it has been observed that a thickness of about 40 nm for portion 15 ′ ensures the required properties . on the contrary , portion 15 ″ is intentionally doped with the electron - donor metal during the manufacture of diode 10 . the molar ratio between the doping metal and the organic molecules in portion 15 ″ is preferably between 1 : 100 and 2 : 1 and more preferably between 1 : 6 and 1 : 1 . the layer 16 of electron - donor metal is preferably made of lithium or cesium . the metal used for doping portion 15 ″ and the one used to form layer 16 are not necessarily the same . for example , it is possible to use cesium for doping layer 15 ″ and lithium for forming layer 16 . in a second aspect thereof , the invention relates to a process for manufacturing diodes of type 10 and an oled display comprised of a plurality of such diodes . as it is known , anodes 12 are generally formed on the transparent support 11 via screen - printing techniques starting from hydroalcoholic suspensions of particles of a mixed oxide of indium and tin having submicronic size . all the other layers are generally produced via evaporation , commonly by positioning the support ( on which the anodes are already present ) in an upside - down position in the upper portion of an evacuated thermostated chamber , wherein the sources of the various components of the oled are provided . the evaporation of the various components from these sources may be controlled by mechanical elements ( known in the field as “ shutters ”) opening or closing the specific source , by the control of the temperature , or by both these means at the same time . by means of calibration tests it is possible to determine the deposition rates of the various layers and thereby control their thickness by the evaporation time . alternatively , it is possible to resort to gauges of the thickness of the material deposited , typically quartz microbalances ( known as “ quartz crystal monitor ”, qcm ) arranged in the chamber in proximity to support 11 . fig2 shows the essential steps of the process of the invention , i . e . the formation of layers 15 and 16 . for ease of representation , the evacuation chamber is not shown in the drawing , whereas the evaporation sources used for producing the characterizing components of the invention are shown . in this case , the details of the drawing are not to scale . fig2 a shows a support 11 on which anodes 12 , the htl layer 13 , and the eml layer 14 have already been formed in a known way . fig2 b schematizes the manufacturing operation of portion 15 ′, which is obtained by evaporation of the organic material of the etl layer ( alq , for instance ) from source 20 , for example a heated crucible . during this step , every other evaporation source provided inside the chamber is inactive . in fig2 c , the manufacturing step of portion 15 ″ is shown . in this step , both source 20 of the organic material of the etl and source 21 of the electron - donor metal are active . the simultaneous evaporation of the two materials occurs , thus depositing a homogeneous mixture of both of them . the source of the electron - donor metal may in turn be a simple crucible , possibly closed by a cover with an orifice , or an evaporator of a more complex shape , such as those shown in u . s . pat . no . 6 , 753 , 648 and in international patent application no . wo 2006 / 057021 , both in the applicant &# 39 ; s name . the achievement of the desired ratio between the organic component and the metal is accomplished through the control of the ratio of the evaporation rates of the two components , which may be controlled through the ( different ) temperatures at which sources 20 and 21 are kept and possibly through the size of apertures provided in covers arranged on the sources . finally , fig2 d shows the manufacture of layer 16 . in this step , the source 20 of the organic material is made inactive ( by interrupting its heating or by means of a shutter ), while the evaporation of the metal of source 21 is continued for the time needed to obtain the desired thickness of layer 16 . in fig2 b - 2 d , the dashed zones between sources 20 and 21 and the layers under formation represent the “ cones ” of the vapors of the various materials . it will be appreciated by those skilled in the art that changes could be made to the embodiments described above without departing from the broad inventive concept thereof . it is understood , therefore , that this invention is not limited to the particular embodiments disclosed , but it is intended to cover modifications within the spirit and scope of the present invention as defined by the appended claims .