Abstract:
Transparent conducting electrodes incorporate an interface layer located on the transparent conducting oxide (TCO) layer of the electrode. The interface layer offers a suitable surface for deposition of further layers in order to fabricate electronic devices such as electrochromic devices or organic light emitting diodes. Problems such as pinholes and short circuiting, associated with the inherent roughness of the TCO layer, are reduced.

Description:
BACKGROUND OF THE INVENTION 
     The invention concerns an interface layer for incorporation in devices produced on a transparent, electrically conducting electrode. 
     A number of electrical devices are known which are concerned with the generation or modification of light and which comprise an active region located between two essentially planar electrodes. In order to facilitate light transmission from (or to) the device, at least one electrode must be transparent. 
     Such devices include organic light emitting diodes (OLED) and electrochromic devices. 
     In OLEDs, an emissive electroluminescent layer comprising one or more layers of organic compound is located between the electrodes and emits light in response to an applied voltage. 
     In electrochromic devices a stack of materials which, in combination exhibit electrochromic properties is located between the electrodes and changes colour and, or opacity in response to an applied voltage. 
     Devices of the type which concern the invention are typically fabricated by providing a transparent conducting electrode comprising a transparent substrate and a conductive coating stack, and building successive layers thereon comprising the active region of the device and a further electrode—which may also be transparent. The transparent conducting electrode is frequently realised by depositing the conductive stack of coatings on the substrate using techniques such as chemical vapour deposition (CVD), which are well known to persons skilled in the art, see for example U.S. Pat. No. 7,968,201. 
     The conductive stack typically comprises a Transparent Conducting Oxide (TCO), i.e. a doped metal oxide, as the uppermost layer (i.e. the furthest layer from the substrate). In addition to offering the requisite electrical properties and mechanical stability, the TCO should offer a suitable surface for deposition of further layers as the rest of the device is fabricated. Examples of transparent conductive oxide materials include fluorine doped tin oxide (SnO 2 :F), Zinc oxide doped with Aluminium, Gallium or Boron (ZnO:Al, ZnO:Ga, ZnO:B), Indium oxide doped with tin (ITO) and cadmium stannate. 
     Unfortunately, these surfaces can be inherently rough which can cause localised short circuits drawing current from an area up to a few millimeters from the point of short circuit. This results in a undesirable aesthetic effect as well as reducing the performance of the device. 
     Furthermore defects such as pinholes in the devices deposited on the TCO can result in similar problems. 
     The current invention addresses both of these problems. 
     According to the invention a transparent electrode comprises the features set out in claim  1  attached hereto. 
     The inventors have shown that incorporation of a thin interface layer between the top of the electrode stack (TCO) and the active region of the device overcomes problems associated with roughness of the TCO. Examples of interface layer materials include TiO 2 , SiO 2 , SnO 2  and ZnO and mixtures comprising any of these. The good device performance obtained when such interface layers are included is surprising because these materials have high electrical resistance. 
     In a preferred embodiment, the interface layer comprises a TiO 2  layer having a thickness greater than 5 nm. 
     In another preferred embodiment, the interface layer comprises ZnO, having a thickness between 25 and 80 nm. 
     In a further preferred embodiment, the TCO layer comprises a fluorine doped tin oxide. 
     In some embodiments, the underlayers comprise a layer of SnO 2  and a layer of SiO 2 . 
     The transparent conducting electrode according to the invention is suitable for incorporation in electronic devices such as electrochromic devices and organic light emitting diodes. 
     According to a second aspect of the invention, a method of manufacturing a transparent conducting electrode comprises the steps set out in claim  8  attached hereto. 
     In a preferred embodiment, the underlayers, TCO and interface layer are deposited by Chemical Vapour Deposition (CVD). The CVD may be done on the float glass ribbon produced during the float glass production process. 
     In another preferred embodiment, the underlayers, TCO and interface layer are deposited by Plasma Enhanced CVD. 
     In another preferred embodiment, the underlayers, TCO and interface layer are deposited by sputtering. 
     Preferably, the interface layer is selected from TiO 2 , SiO 2  and ZnO. More preferably, the interface comprises TiO 2  having a thickness of less than 5 nm. 
     In another preferred embodiment, the interface layer comprises ZnO, having a thickness between 25 and 80 nm. 
     In some embodiments, the TCO layer comprises a fluorine doped tin oxide. 
     In some embodiments, the underlayers comprise a layer of SnO 2  and a layer of SiO 2 . 
     Materials such as TiO 2  and ZnO are is rendered more hydrophilic upon exposure to ultraviolet (UV) radiation. Such treatment of interface layers according to the invention renders the electrode more receptive to deposition of subsequent layers as the device is fabricated, particularly where such layers are deposited by so-called ‘wet’ chemical methods i.e. techniques involving liquid solutions. 
     In another aspect of the invention, use, as a transparent conducting electrode, of an electrically conducting coating stack located on a transparent substrate; 
     the stack comprising at least one underlayer and a transparent conducting oxide (TCO) layer located on the underlayers, 
     is characterised by the electrode having an interface layer located on the transparent conducting oxide layer. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will now be described with reference to the accompanying figures in which: 
         FIG. 1  shows a transparent conducting electrode of the prior art; 
         FIG. 2  shows an electrochromic device incorporating a transparent conducting electrode according to the invention 
         FIG. 3  shows an organic light emitting diode incorporating a transparent conducting electrode according to the invention and 
         FIG. 4  illustrates a so-called ‘dynamic coater’ used for laboratory scale chemical vapour deposition of coatings on a substrate. 
     
    
    
     All of the figures are for illustrative purposes only and the relative thicknesses of the layers are not to scale. 
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to  FIG. 1 , a transparent conducting electrode of the type concerned by the invention typically comprises a glass substrate  1  bearing a conductive stack of coatings  2 ,  3  and  4 . In the example shown, based on NSG&#39;s TEC™ series of coated glass, the stack comprises underlayers of SnO 2    2  and SiO 2    3  and a TCO layer  4  of SnO 2 :F. 
     The TCO layer  4  typically has a rough surface (although the scale of the roughness is exaggerated for illustrative purposes here) which can give rise to problems during device operation as previously alluded to. 
     Referring to  FIG. 2 , an electrochromic device according to the invention comprises a transparent electrode  1 - 4 , an ion storage layer  5 , an ion conductor (electrolyte) layer  6 , a layer of electrochromic material  7  and a further transparent conducting layer  8 . A further layer  9  of transparent material such as glass or plastic would also typically be included. 
     During operation, an electrical potential is applied between transparent conducting layers  4  and  8  which causes redox reactions to occur in the electrochromic and ion storage layers  7  and  5 , with charge compensation by ion migration across the ion conductor layer  6 . Typically, Li +  ions are used in the ion conductor layer  6 . 
     These reactions are accompanied by changes in colour/transmission of the electrochromic layer  7 . 
     The various layers may be deposited by techniques that are well known to a person skilled in the art, such as sputtering, PECVD (Plasma Enhanced Chemical Vapour Deposition) or solution deposition. Moreover the skilled person will be aware that some electrochromic devices may incorporate additional layers to those shown in this example. 
     According to the invention, the device incorporates a further interface layer  10  between the TCO layer  4  and the ion storage layer  5 . In this example interface layer comprises TiO 2  and its inclusion mitigates the problems associated with roughness of the TCO layer  4 . 
     Referring to  FIG. 3 , an OLED device according to the invention comprises a transparent electrode  1 - 4 , a hole injection layer (HIL)  11 , a hole transport layer (HTL)  12 , one or more emissive layers  13 , a hole blocking layer (HBL)  14 , an electron transport layer (ETL)  15  and a further electrode  16 . 
     During operation, a voltage is applied between the electrodes and electron-hole recombination in the region of the emissive layers give rise to excitons&#39; (a bound state of the electron-hole combination) which on relaxation give rise to an emission in the visible region. Light thus generated exits the device through the transparent substrate  1 . 
     According to the invention, the device incorporates a further interface layer  10  between the TCO layer  4  and the HIL  5 . In this example interface layer comprises TiO 2  and its inclusion mitigates the problems associated with roughness of the TCO layer  4 . 
     Examples 1-9 
     A series of samples were prepared on 3.2 mm glass substrate. Each sample incorporated a stack of SiO 2  (example 9 only)/SnO 2 /SiO 2 /SnO2:F layers of varying thickness topped by an interface layer of TiO 2 , SnO 2  or SiO 2 . The samples are detailed in table 1 (layer thicknesses in Angstrom) along with an indication of observed sheet resistance and transmittance. 
     The stack interface layers were deposited by atmospheric pressure CVD. 
     
       
         
               
             
               
               
             
               
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Physical Properties of Transparent Electrodes with Interface Layers 
               
             
          
           
               
                   
                 Example 
               
             
          
           
               
                   
                 1 
                 2 
                 3 
                 4 
                 5 
                 6 
                 7 
                 8 
                 9 
               
               
                   
                   
               
             
          
           
               
                 Glass 
                 3.2 mm 
                 3.2 mm 
                 3.2 mm 
                 3.2 mm 
                 3.2 mm 
                 3.2 mm 
                 3.2 mm 
                 3.2 mm 
                 3.2 mm 
               
               
                 SiO2 
                   
                   
                   
                   
                   
                   
                   
                   
                 150 
               
               
                 SnO2 
                 250 
                 300 
                 250 
                 250 
                 250 
                 250 
                 250 
                 250 
                 250 
               
               
                 SiO2 
                 250 
                 220 
                 250 
                 250 
                 250 
                 250 
                 250 
                 250 
                 250 
               
               
                 SnO2:F 
                 3400 
                 2300 
                 3400 
                 7600 
                 3400 
                 3400 
                 3400 
                 3400 
                 3400 
               
               
                 TiO2 
                   
                 400 
                 200 
                 200 
                 100 
                 50 
                   
                   
                 200 
               
               
                 SnO2 
                   
                   
                   
                   
                   
                   
                 1000 
               
               
                 SiO2 
                   
                   
                   
                   
                   
                   
                   
                 200 
               
               
                 Rs (ohm/sq) 
                 13.6 
                 20.0 
                 13.6 
                 6.0 
                 13.6 
                 13.6 
                 13.6 
                 13.6 
                 13.6 
               
               
                 % T 
                 84.5 
                 65.1 
                 77.3 
                 74.2 
                 82.6 
                 84.1 
                 81.1 
                 85.3 
                 77.3 
               
               
                   
               
             
          
         
       
     
     Note that the resistivities of TiO 2  and SiO 2  are both in excess of 1 MΩ·cm. The transmission values are likely to change when these stacks are incorporated in an actual device as reflectance is affected by the adjacent layers. 
     Example 10 
     A transparent conductive oxide was deposited on a glass substrate with the structure glass/SnO 2 (60 nm)/SiO 2 (15 nm)SnO 2 :F(720 nm). 25 nm of undoped tin oxide was deposited on top. Table 2 shows the properties with and without the additional undoped tin oxide layer. Surprisingly the sheet resistance of the coating is not substantially affected by the addition of a highly resistive layer 
     
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                   
                 Coated side 
                 Sheet 
                   
               
               
                   
                 Tvis 
                 reflection 
                 Resistance 
                 Haze 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                 Reference 
                 82.1% 
                 10.0% 
                 8.7Ω/□ 
                 11.0% 
               
               
                 Tin oxide 
                 81.2% 
                 10.4% 
                 8.7Ω/□ 
                 11.5% 
               
               
                 buffer coated 
               
               
                   
               
             
          
         
       
     
     The TCO was used in a photovoltaic module resulting in an improved Voc and FF as shown in Table 3 
     
       
         
               
             
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 3 
               
             
             
               
                   
               
               
                 Normalised performance data for Photovoltaic 
               
               
                 Device incorporating Interface Layer 
               
             
          
           
               
                   
                 Jsc 
                 Voc 
                 FF 
               
               
                   
                   
               
             
          
           
               
                   
                 Reference 
                 1.00 
                 1.00 
                 1.00 
               
               
                   
                 Tin oxide 
                 1.00 
                 1.01 
                 1.01 
               
               
                   
                 buffer coated 
               
               
                   
                   
               
             
          
         
       
     
     Examples 11-16 
     A series of experiments were conducted to deposit ZnO layers on various substrate to further investigate the suitability of this material as an interface layer according to the invention. 
     For examples 11-15, the deposition was done using a ‘dynamic’ laboratory scale coater as illustrated in  FIG. 4 . In this apparatus, the premixed precursors move towards the coater through a heated line  17  before they reach baffle section  18  which equalises the precursor flow before it enters the sealed coating section. The glass substrate  19  sits on a heated carbon block  20  which is heated to the desired temperature using either heating elements (not shown) inserted inside the carbon block or by an induction coil (not shown) around the sealed coating section. Any unreacted precursor or by products are then directed towards fish tail exhaust  21  and continue towards the incinerator  22 . The arrows show the direction in which the gaseous mixture moves. 
     The reactants (DEZ and t-butyl acetate) were delivered by passing N 2  carrier gas through bubblers (not shown). The DEZ and t-butyl acetate bubbler temperatures were 100° C. and 85° C. respectively. Reactant quantities are expressed in terms of the total gas flow that reaches the substrate surface. 
     Example 16 was done ‘on-line’ by atmospheric pressure CVD, on a float glass ribbon produced during the float glass manufacturing process. In this case, a thin film evaporator as described in U.S. Pat. No. 5,090,985 was used to deliver the reactants, whose quantities are again expressed as a percentage of total gas flow. 
     Table 4 summarises the reaction conditions used to generate examples 11-16. SLPM=Standard Liters per Minute; TEC10FS™ and TEC10™ are Nippon Sheet Glass Group products comprising coated glass substrates providing a transparent conducting electrode based on a fluorine doped tin oxide. 
     
       
         
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 4 
               
               
                   
               
               
                   
                   
                   
                 DEZ bubbler 
                 t-butyl acetate 
                 Total gas 
                   
                   
               
               
                   
                   
                 % t-butyl 
                 flow 
                 bubbler flow 
                 flow 
                 Linespeed 
               
               
                 Sample 
                 % DEZ 
                 acetate 
                 (SLPM)* 
                 (SLPM) 
                 (SLPM) 
                 (inch/min) 
                 Substrate 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 11 
                 1 
                 5 
                 0.28 
                 0.91 
                 36 
                 200 
                 SiO2 coated 
               
               
                   
                   
                   
                   
                   
                   
                   
                 glass 
               
               
                 12 
                 1 
                 10 
                 0.28 
                 1.82 
                 36 
                 200 
                 SiO2 coated 
               
               
                   
                   
                   
                   
                   
                   
                   
                 glass 
               
               
                 13 
                 1.3 
                 10 
                 0.37 
                 1.82 
                 36 
                 200 
                 TEC10FS ™ 
               
               
                 14 
                 3 
                 4 
                 0.85 
                 0.62 
                 36 
                 150 
                 TEC10 ™ 
               
               
                 15 
                 3 
                 4 
                 0.85 
                 0.62 
                 36 
                 150 
                 SiO2 coated 
               
               
                   
                   
                   
                   
                   
                   
                   
                 glass 
               
               
                 16 
                 1.5 
                 4 
                   
                   
                 630 
                 318 
                 TEC10 
               
               
                   
               
             
          
         
       
     
     Table 5 summarises, for examples 11-16, measured values for ZnO thickness (in Angstrom), percentage haze, percentage visible transmission (Tvis), and sheet resistance of the sample Rs. 
     
       
         
               
               
               
               
               
             
           
               
                   
               
               
                   
                 ZnO 
                   
                   
                   
               
               
                 Sample 
                 thickness 
                 % Haze 
                 % Tvis 
                 Rs 
               
               
                   
               
             
             
               
                 11 
                 246 
                 0.33 
                 92.1 
                 — 
               
               
                 12 
                 383 
                 0.43 
                 91.4 
                 — 
               
               
                 13 
                 416 
                 3.25 
                 86.0 
                 10.65 
               
               
                 14 
                 660 
                 1.13 
                 85.1 
                 12.70 
               
               
                 15 
                 800 
                 0.26 
                 85.3 
                 — 
               
               
                 16 
                 251 
                 2.12 
                 86.4 
                 18.62 
               
               
                   
               
             
          
         
       
     
     The transmission levels for all of examples 11-16 indicate that the addition of a Zinc oxide layer has not caused any significant absorption. Haze levels on the silica coated substrates indicate an inherently smooth coating, and the roughness levels when coated on TEC 10 are similar to the substrate values. 
     Sheet resistance has been increased slightly by the over coat, but still in the range suitable for PV devices.