fig3 shows in diagrammatic form a cross - sectional structure of a top emitting oled according to the invention . an anode material 32 such as ito may be situated on a metal mirror 35 which is positioned over an active matrix back plane 31 . a layer hole transporting material 36 is pedt / pss and is situated between the anode 32 ( ito ) and an emissive layer 33 . optionally , a further intermediate layer 37 may be applied between the electron - injecting layer and the light emitting layer . a low work function metal ( ba ) layer 34 is deposited over the light emitting layer 33 by electron beam evaporation or thermal evaporation . over this layer is deposited a silver layer 38 , also by electron beam evaporation or thermal evaporation . optionally , sio 2 or zns is deposited over the silver layer to form a transparent encapsulation layer so as to protect the device from ingress of oxygen and moisture . the encapsulation layer is generally a dielectric or polymer - dielectric composition . according to a first measurement technique , the bilayer is deposited onto a cleaned 0 . 7 mm blank glass substrate by evaporation of each layer . following evaporation , the substrate is transferred to a glove box associated with the evaporation apparatus to avoid any exposure of the oxygen and moisture - sensitive ba metal to the atmosphere . transparency of the bilayer on glass is measured in the glove box using a he - ne 635 nm laser diode and silicon photodiode detector . transparency of the blank glass is also measured and transparency of the bilayer alone is calculated as a ratio by dividing the transparency of the bilayer on glass by the transparency of the blank glass . according to a second measurement technique , the first measurement technique is followed except that a layer of silicon dioxide is evaporated over the bilayer in order to further minimize any exposure of the bilayer to oxygen or moisture , and transparency of a layer of silicon dioxide on glass is measured instead of transparency of blank glass for the purpose of the ratio calculation . poly ( ethylene dioxythiophene ) / poly ( styrene sulfonate ) ( pedt / pss ), available from h c starck of leverkusen , germany as baytron p ® is deposited over an indium tin oxide anode supported on a glass substrate ( available from applied films , colorado , usa ) by spin coating . a hole transporting layer of f8 - tfb ( shown below ) is deposited over the pedt / pss layer by spin coating from xylene solution to a thickness of about 10 nm and heated at 180 ° c . for one hour . a blue electroluminescent polymer as disclosed in wo 03 / 095586 is deposited over the layer of f8 - tfb by spin - coating from xylene solution to form an electroluminescent layer having a thickness of around 65 nm . a 5 nm thick layer of ba is formed over the electroluminescent layer by evaporation of ba until the desired thickness is reached . a 5 nm thick layer of ag is then similarly formed over the ba layer . finally , the device is sealed from the atmosphere by placing a glass plate over the device such that the device is located within a cavity formed within the center of the glass plate and gluing the glass plate to the substrate . in order to maximise light output through the cathode , a reflective layer may also be provided on the substrate . devices may be prepared in accordance with the process of example 1 , except that the electroluminescent layer is formed from a red electroluminescent polymer comprising 50 mol % 9 , 9 - di - n - octylfluorene - 2 , 7 - diyl , 17 mol % “ tfb ” repeat units ( illustrated below ), 30 mol % 1 , 3 , 2 - benzothiadiazole - 4 , 7 - diyl , and 3 mol % 4 , 7 - bis ( 2 - thiophen - 5 - yl ) - 1 , 3 , 2 - benzothiadiazole . materials of this type are disclosed in wo 00 / 46321 and wo 00 / 55927 . devices may be prepared in accordance with the process of example 1 , except that the electroluminescent layer is formed from a green electroluminescent polymer as disclosed in , for example , wo 00 / 55927 and wo 00 / 46321 . a full color device may be prepared according to the method of example 1 except that the pedt / pss and f8 - tfb layers are deposited by inkjet printing into inkjet wells formed by photolithography defining red , green and blue subpixel areas followed by inkjet printing the aforementioned red , green and blue electroluminescent polymers fig4 a shows a plot of transmission vs thickness of ba and al . this illustrates how sensitive transmission of light through the cathode is to al thickness . if the minimum acceptable ba thickness is taken to be 5 nm , this plot indicates that al must be kept below 1 nm to keep the transmission above 75 %. each additional nm of aluminum thickness drops the transmission by an additional 5 % approximately . fig4 b shows an analogous plot replacing the al with ag . according to this plot , silver thicknesses of up to 7 nm give an acceptable transmission of 75 %. the transmission is much more tolerant to small changes in the thickness of ag as compared to that of al . the results in fig4 a and 4 b were obtained for light at the red end of the visible spectrum ( 633 nm ). fig5 shows the difference between transmission of a device with a ba : al cathode compared to one with a ba : ag cathode . in each case , the cathode has a bilayer , each component of which having a layer thickness of 5 nm . the figure demonstrates that significantly greater light transmission across the entire visible range may be achieved using a ba : ag cathode . this demonstrates the suitability of such cathodes for full color displays and their potential for use as common cathodes . fig6 shows a plot of transparency vs resistance for ba / ag cathodes as compared with ba : al cathodes . it will be apparent from this figure that a ba : al cathode can give a suitable combination of transparency and resistance whereas the ba : al system suffers from low transmission and moderate resistance . devices according to the invention therefore use silver to provide full cathode transparency control over achievable and sensible thickness ranges . this allows tuning of the cavity strength to suit different device requirements such as color gamut , angular color shift and outcoupling efficiency . thicker metal capping layers may be used to achieve adequate transparency and this may provide more protection for the underlying light emissive layers if further layer deposition is required . by allowing a greater metal thickness for a given transparency , there is a greater tolerance during manufacture to errors in cathode thickness for achieving a desired transparency .