in the embodiment shown in fig1 , no space charge zone occurs at the hole - injecting contact . this embodiment calls for a low energy barrier for hole injection . sometimes , this may not be achieved with available materials . hence the injection of holes from the anode is less effective than for the conventional structure , where the work - function of the anode can be modified . thus , the oled exhibits an elevated operating voltage . according to the invention , the disadvantage of prior structures is avoided by inverted oleds with doped injecting and transporting layers , optionally in combination with block layers 4 , 6 . fig2 shows an exemplary apparatus in which the hole - injecting and - conducting layer 7 is doped , so that the boundary layer to the contact ( anode 8 ) constitutes a space charge zone . the doping is high enough so that this space charge zone can be easily tunneled through . that such dopings are possible has already been shown in the literature for non - inverted light - emitting diodes ( e . g ., x . q . zhou et al ., appl . phys . lett . 78 , 410 ( 2001 ); j . blochwitz et al ., organic electronics ( 2001 ), in press ). the apparatus according to the present invention has the following advantages : ( 1 ) excellent injection of charge carriers from the anode into the p - doped charge carrier transporting layers ; ( 2 ) independence from detailed preparation of hole - injecting anode contact material 8 ; and ( 3 ) possibility of choosing a material for layer 8 with a comparatively high barrier to hole injection in a preferred embodiment , by way of example , a solution will here be specified in which the combination of p - doped injecting and transporting layer and block layer 4 , 6 is employed on the hole side only . the oled exhibits the following layer structure : substrate 1 , e . g ., glass cathode 2 : 1 nm lif in combination with aluminum ( lif improves injection at the contact ) electroluminescent and ( in this case ) conventional electron - conducting layer 5 : 65 nm alq , possibly to be replaced by 30 nm alq and 15 nm alq doped with about 1 % of an emitter dye such as quinacridone hole - side block layer 6 : 5 nm triphenyldiamine ( tpd ) p - doped layer ( hole transporting layer 7 ): 100 nm starburst tdata 50 : 1 doped with f 4 - tcnq transparent electrode ( anode 8 ): indium tin oxide ( ito ) the mixed layer 7 is produced in a process of vapor deposition under vacuum in mixed evaporation . in principle , such layers may also be produced by other methods , such as for example a vapor deposition of the substances one upon another , possibly with ensuing temperature - controlled diffusion of the substances one into another , or by other application ( e . g . spin - coating ) of the already mixed substances within or outside of the vacuum . the block layer 6 was also vapor deposited under vacuum but may alternatively be produced otherwise , e . g ., by spin - coating within or outside of the vacuum . in another preferred embodiment by way of example , there may be an additional n - doped layer 3 between the cathode 2 and the electroluminescent layer 5 . the doping may , as described above , consist in a mixed vapor deposition of two organic materials ( species of molecules ) or by incorporation of metal atoms in the organic layer ( as described in , e . g ., patent application ser . no . de 100 58 578 . 7 filed nov . 25 , 2000 ). as an example pursuant to the invention , mention may here be made of the doping of an aluminum complex of tris ( 8 - quinolinolato ) (“ alq ”) with lithium ( li ) ( u . s . pat . no . 6 , 013 , 384 ( j . kido et al .) filed jan . 22 , 1998 ; j . kido et al ., appl . phys . lett . 73 , 2866 ( 1998 )). the use of such a layer , according to the present invention , in an inverted oled structure , results in the positive effect of a lithium fluoride layer between the electron - transporting layer 3 and the metal layer of the cathode 2 , which is achieved in an uninverted structure of the oled ( the vapor deposition of ‘ hot ’ metal atoms on lif achieves the desired effect on the electron injection ) see e . g . m . g . mason , j . appl . phys . 89 , 2756 ( 2001 )), is achieved for the inverted structure as well . the lif layer lowers the work - function of the cathode 2 material ( e . g ., aluminum ) and the li - doping of the electron - transporting layer 3 ( typically , and in conformity with the invention , in a concentration between 5 : 1 and 1 : 10 li atoms to alq 3 molecules ) makes possible a band bending at the boundary surface of the cathode , permitting an efficient injection of electrons , much as in the case of hole injection into the p - doped hole - transporting layer . according to the present invention , the use of doped layers makes it possible to achieve the same low operating voltages and high efficiencies in an inverted structure with light emission through the cover contact as in a conventional structure with emission through the substrate 1 . this is due , as described , to the efficient hole injection , which , due to the doping , is relatively independent of the exact work - function of the transparent anode material . thus , highly efficient displays can be built on conventional semiconductor substrates . although only some preferred applications have been described , it will be obvious to those skilled in the art that many modifications and variations of the invention presented are possible in the spirit of the invention . for example , other transparent contacts than ito may be used as anode materials ( e . g ., as in h . kim et al ., appl . phys . lett . 76 , 259 ( 2000 ); h . kim et al , appl . phys . lett . 78 , 1050 ( 2001 )). furthermore , it would be in conformity with the invention to compose the transparent anode of a sufficiently thin layer of a non - transparent material ( e . g ., silver or gold ) and a thick layer of a transparent conductive material .