Multicolor organic light emitting devices

• US 5,707,745 A
• Filed: 12/13/1994
• Issued: 01/13/1998
• Est. Priority Date: 12/13/1994
• Status: Expired due to Term

• Current Assignee: The Trustees of Princeton University (Princeton University)
• Original Assignee: The Trustees of Princeton University (Princeton University)
• Inventors: Burrows, Paul Edward, Sapochak, Linda Susan, McCarty, Dennis Matthew, Forrest, Stephen Ross, Thompson, Mark Edward
• Primary Examiner(s): NAKARANI, DHIRAJLAL S
• Application Number:
  US08/354,674
• Time in Patent Office: 1,127 Days
• Field of Search: 257/89, 257/90, 257/94, 257/96, 257/97, 257/98, 257/440, 257/443, 428/336, 428/432, 428/433, 428/434, 428/457, 427/77, 427/78
• US Class Current: 428/432
• CPC Class Codes: C09K 11/06 containing organic luminescent materials
• CPC Class Codes: H01L 24/82 by forming build-up interconnects at chip-level, e.g. for high density interconnects [HDI] interconnection structure between a plurality of semiconductor chips H01L23/5389
• CPC Class Codes: H01L 27/156 two-dimensional arrays
• CPC Class Codes: H01L 2924/00 Indexing scheme for arrangements or methods for connecting or disconnecting semiconductor or solid-state bodies as covered by H01L24/00
• CPC Class Codes: H01L 2924/12033 Gunn diode
• CPC Class Codes: H01L 2924/12041 LED
• CPC Class Codes: H01L 2924/12042 LASER
• CPC Class Codes: H01L 2924/12044 OLED
• CPC Class Codes: H10K 2102/3031 Two-side emission, e.g. transparent OLEDs [TOLED]
• CPC Class Codes: H10K 50/11 characterised by the electroluminescent [EL] layers
• CPC Class Codes: H10K 50/125 specially adapted for multicolour light emission, e.g. for emitting white light
• CPC Class Codes: H10K 50/14 Carrier transporting layers
• CPC Class Codes: H10K 50/828 Transparent cathodes, e.g. comprising thin metal layers
• CPC Class Codes: H10K 59/32 Stacked devices having two or more layers, each emitting at different wavelengths
• CPC Class Codes: H10K 59/80524 Transparent cathodes, e.g. comprising thin metal layers
• CPC Class Codes: H10K 85/10 Organic polymers or oligomers
• CPC Class Codes: H10K 85/111 comprising aromatic, heteroaromatic, or aryl chains, e.g. polyaniline, polyphenylene or polyphenylene vinylene
• CPC Class Codes: H10K 85/113 Heteroaromatic compounds comprising sulfur or selene, e.g. polythiophene
• CPC Class Codes: H10K 85/114 Poly-phenylenevinylene; Derivatives thereof
• CPC Class Codes: H10K 85/146 poly N-vinylcarbazol; Derivatives thereof
• CPC Class Codes: H10K 85/30 : Coordination compounds
• CPC Class Codes: H10K 85/322 : comprising boron
• CPC Class Codes: H10K 85/324 : comprising aluminium, e.g. Alq3
• CPC Class Codes: H10K 85/341 : Transition metal complexes, e.g. Ru(II)polypyridine complexes H10K85/331 takes precedence
• CPC Class Codes: H10K 85/60 : Organic compounds having low molecular weight H10K85/10 - H10K85/50 take precedence
• CPC Class Codes: H10K 85/611 : Charge transfer complexes
• CPC Class Codes: H10K 85/631 : Amine compounds having at least two aryl rest on at least one amine-nitrogen atom, e.g. triphenylamine
• CPC Class Codes: Y10T 428/26 : Web or sheet containing structurally defined element or component, the element or component having a specified physical dimension
• CPC Class Codes: Y10T 428/265 : 1 mil or less
• CPC Class Codes: Y10T 428/31678 : Of metal

Abstract

A multicolor organic light emitting device employs vertically stacked layers of double heterostructure devices which are fabricated from organic compounds. The vertical stacked structure is formed on a glass base having a transparent coating of ITO or similar metal to provide a substrate. Deposited on the substrate is the vertical stacked arrangement of three double heterostructure devices, each fabricated from a suitable organic material. Stacking is implemented such that the double heterostructure with the longest wavelength is on the top of the stack. This constitutes the device emitting red light on the top with the device having the shortest wavelength, namely, the device emitting blue light, on the bottom of the stack. Located between the red and blue device structures is the green device structure. The devices are configured as stacked to provide a staircase profile whereby each device is separated from the other by a thin transparent conductive contact layer to enable light emanating from each of the devices to pass through the semitransparent contacts and through the lower device structures while further enabling each of the devices to receive a selective bias.

Claims

1. A multicolor light emitting device (LED) structure, comprising:

a plurality of at least first and second light emitting organic devices (LEDs) stacked one upon the other, to form a layered structure, with each LED separated one from the other by a transparent electrically conductive layer to enable each device to receive a separate bias potential to operate to emit light through the stack.

2. The multicolor light emitting device structure of claim 1, wherein each of said LED'"'"'s emits a different wavelength of light and therefore a different color when biased.

3. The multicolor light emitting device of claim 2, wherein the longest wavelength LED is on top of the stack in the vertical direction followed by successively shorter wavelength LED'"'"'s, with shortest wavelength LED on the bottom of the stack.

4. The multicolor light emitting device structure of claim 1, including at least first through third light emitting devices stacked upon one another, respectively.

5. The multicolor light emitting device structure of claim 4, wherein said first device emits the color blue (B), said second device emits the color green (G) and third device emits the color red (R).

6. The multicolor light emitting device structure of claim 5, wherein said devices are stacked in the following sequence along the vertical axis starting from a bottom point and directed upward, wherein the first device emits a blue color, and has the second device for emitting a green color located on top of the upper surface of said blue emitting device, with the third device for emitting a red color located on top of the upper surface of said green emitting device, whereby said blue emitting device of the shortest wavelength is at the bottom with the red emitting device of the longest wavelength on top when the structure is aligned vertically.

7. The multicolor light emitting device structure of claim 4, wherein said at least first, second, and third organic LED'"'"'s are stacked in successive order over a common substrate.

8. The multicolor light emitting device structure of claim 7, wherein said substrate is at the bottom of said LED structure, and a topmost layer of said third organic LED consists of indium tin oxide (ITO) material serving as a contact for an underlying metal material layer.

9. The multicolor light emitting device structure of claim 1, wherein each LED device is a transparent double heterostructure (DH) device fabricated from organic materials.

10. The multicolor light emitting device structure of claim 9, wherein said transparent electrically conductive layer is an indium tin oxide (ITO).

11. The multicolor light emitting device structure of claim 9, wherein said transparent electrically conductive layer comprises a metallic layer having a work function less than four electron volts, and an ITO layer on said metallic layer.

12. The multicolor light emitting device structure of claim 9, wherein said organic material is selected from the group consisting of trivalent metal quinolate complexes, trivalent metal bridged quinolate complexes, Schiff base divalent metal complexes, tin (iv) metal complexes, metal acetylacetonate complexes, metal bidentate ligand complexes, bisphosphonates, divalent metal maleonitriledithiolate complexes, molecular charge transfer complexes, aromatic and heterocyclic polymers, and rare earth mixed chelates.

13. The multicolor light emitting device structure of claim 12 wherein the trivalent metal quinolate complexes have the following formula ##STR1## wherein R is selected from the group consisting of hydrogen, alkyl, aryl and a heterocyclic group, L represents a ligand selected from the group consisting of picolylmethylketone;

salicylaldehyde;

a group of the formula R(O)CO-- wherein R is as defined above;

halogen;

a group of the formula RO-- wherein R is as defined above; and

quinolates.

14. The multicolor light emitting device structure of claim 12 wherein the metal bidentate ligand complexes have the following formula:

space="preserve" listing-type="equation">MDL.sup.4.sub.2

wherein M is selected from trivalent metals of Groups 3-13 of the Periodic Table and the Lanthanides, D is a bidentate ligand and L⁴ is selected from the group consisting of acetylacetonate;

compounds of the formula OR³ R wherein R³ is Si or C and R is selected from the group consisting of hydrogen, alkyl, aryl, a heterocyclic group;

3,5-di(t-butyl) phenol;

2,6-di(t-butyl) phenol;

2,6-di(t-butyl) cresol and a compound of the formula ##STR2##

15. The multicolor light emitting device structure of claim 14 wherein D is selected from the group consisting of 2-picolylketones, 2-quinaldylketones and 2-(o-phenoxy) pyridineketones.

16. The multicolor light emitting device structure of claim 12 wherein the Schiff base divalent metal complexes are selected from those having the formula ##STR3## wherein M¹ is a divalent metal chosen from Groups 2-12 of the Periodic Table, R¹ is selected from the group consisting of ##STR4## ##STR5## wherein X is selected from the group consisting of hydrogen, alkyl, alkoxy each having 1 to 8 carbon atoms, aryl, a heterocyclic group, phosphino, halogen and amine.

17. The multicolor light emitting device structure of claim 12 wherein the aromatic and heterocyclic polymers are selected from the group consisting of poly (para-phenylene vinylene), poly (dialkoxyphenylene vinylene), poly (thiophene), poly (phenylene), poly (phenylacetylene) and poly (N-vinylcarbazole).

18. The multicolor light emitting device structure of claim 12 wherein the rare earth mixed chelates comprise a Lanthanide bonded to a bidentate aromatic or heterocyclic group.

19. The multicolor light emitting device structure of claim 18 wherein the bidentate aromatic or heterocyclic group is selected from the group consisting of salicylaldehydes, salicyclic acid, quinolates, Schiff base ligands, acetylacetonates, phenanthroline, bipyridine, quinoline and pyridine.

20. The multicolor light emitting device structure of claim 18 wherein the divalent metal maleonitriledithiolate complexes have the formula ##STR6## wherein M³ is a metal having a +2 charge, Y¹ is selected from the group consisting of cyano, and phenyl, and L⁵ is a group having no charge.

21. The multicolor light emitting device structure of claim 20 wherein L⁵ is a group of the formula P(OR)₃ or P(R)₃ wherein R is selected from the group consisting of hydrogen, alkyl, aryl and a heterocyclic group.

22. The multicolor light emitting device structure of claim 12 wherein the bisphosphonates have the formula M²_x (O₃ P-organic-PO₃)_y wherein M² is a metal ion and organic represents an aromatic or heterocyclic fluorescent compound bifunctionalized with phosphonate groups.

23. The multicolor light emitting device structure of claim 12 wherein the trivalent metal bridged quinolate complexes have the formula ##STR7## wherein M is a trivalent metal ion and Z is selected from SiR or P=O wherein R is selected from the group consisting of hydrogen, alkyl, aryl, or a heterocyclic group.

24. The multicolor light emitting device structure of claim 12 wherein the tin (iv) metal complexes have the formula SnL¹₂ L²₂ wherein L¹ is selected from the group consisting of salicylaldehydes, salicyclic acid, and quinolates and L² is selected from the group consisting of alkyl, aryl and a heterocyclic group.

25. The multicolor light emitting device structure of claim 12 wherein the molecular charge transfer complexes comprise an electron acceptor complexed with an electron donor.

26. The multicolor light emitting device structure of claim 1, wherein each LED device is a transparent single heterostructure device fabricated from organic materials.

27. The multicolor light emitting device structure of claim 2, wherein said transparent electrically conductive layer is an indium tin oxide (ITO).

28. The multicolor light emitting device structure of claim 1, wherein said devices are stacked in an order dependent upon and in accordance with their respective emission wavelength and absorption characteristics.

29. The light emitting device structure of claim 1, wherein each of said LEDs emit substantially the same wavelength of light.

30. The multicolor LED structure of claim 1, wherein said plurality of at least first and second LEDs are stacked over a common substrate.

31. The multicolor LED structure of claim 1, further including a reflective metal layer over one end of the stack of said plurality of a LEDs.

32. The multicolor LED structure of claim 31, further including a layer of anti-reflecting material disposed over the other end of said plurality of at least first and second LEDs.

33. The multicolor LED structure of claim 1, further including a layer of anti-reflecting material disposed over one end of the stack of said plurality of at least first and second LEDs.

34. A multicolor light emitting device structure comprising

a transparent substrate layer having deposited on a surface a first transparent electrically conductive coating.

a first light emitting device deposited on said first transparent electrically conductive coating;

a second transparent electrically conductive coating deposited on the surface of said first device not in contact with said first coating;

a second light emitting device deposited on the surface of said second coating;

a third transparent electrically conductive coating deposited on the surface of said second device not in contact with said second coating;

a third light emitting device deposited on the surface of said third coating;

said first, second, and light emitting devices each including an organic emission layer for emitting light of a desired color, respectively; and

a fourth electrically conductive coating deposited on the surface of said third device not in contact with said third coating.

35. The multicolor light emitting device structure of claim 34, wherein said first, second, third and fourth electrically conductive coatings are adapted to receive individual sources of biasing potential, respectively.

36. The multicolor light emitting device structure of claim 34, wherein said devices and electrically conductive layers are deposited to form a staircase profile, with said transparent substrate being of a greater length than said first device, with said first device being of a greater length than said second device, with said second device of a greater length than said third device, wherein each step is covered by said respective electrically conductive coating adapted for applying operating potentials to said device structures, and wherein said first through third transparent electrically conductive coatings allow light emitted by any of said devices, respectively, to pass through said transparent substrate layer.

37. The multicolor light emitting device structure of claim 34, wherein said forth electrically conductive coating, includes a a metal layer that reflects upward directed light back to said substrate.

38. The multicolor light emitting device structure of claim 37, wherein said fourth electrically conductive coating further includes an indium tin oxide (ITO) layer between said metal layer and said surface of said third device not in contact with said third coating, said ITO layer serving as a contact for an underlying metal material layer of said third light emitting diode device.

39. The multicolor light emitting device structure of claim 34, wherein said transparent substrate is glass, said first conductive coating is indium tin oxide (ITO), and each of said second, third and fourth conductive coatings are each comprised of an ITO layer disposed on a low work function metal layer, respectively.

40. The multicolor light emitting device structure of claim 34, wherein each of said devices are double heterostructures (DH), with said first device operative when biased to emit blue light (B), said second device operative when biased to emit green light (G), said third device operative when biased to emit red light (R).

41. The multicolor light emitting device structure of claim 40, wherein each DH structure is comprised of organic compounds.

42. The multicolor light emitting device structure of claim 34, wherein each of said devices are single heterostructures (SH), with said first device operative when biased to emit blue light (B), said second device operative when biased to emit green light (G), and said third device operative when biased to emit red light (R).

43. The multicolor LED structure of claim 42, wherein

each of said single heterostructures (SH) is comprised of organic material.

44. The multicolor light emitting device structure of claim 34, wherein each of said devices are polymer structures, with said first device operative when biased to emit blue light (B), said second device operative when biased to emit green light (G), and said third device operative when biased to emit red light (R).

45. The light emitting device structure of claim 34, wherein each of said devices are operative when biased to emit light of substantially the same wavelength.

46. The multicolor light emitting device structure of claim 34, further including a layer of anti-reflecting material deposited between said transparent substrate layer and said first transparent electrically conductive coating.

47. The multicolor light emitting device structure of claim 46, further including a reflective metal layer between said fourth electrically conductive coating and the surface of said third device not in contact with said third coating for reflecting upward directed light back to said transparent substrate layer.

48. A method of fabricating a multicolor, light emitting device (LED) structure comprising the steps of:

forming a first transparent electrically conductive layer upon a transparent substrate;

depositing a first hole transporting layer upon said first transparent electrically conductive layer,

depositing a first organic emission layer upon said first hole transporting layer to provide a first emission color;

depositing first electron transporting layer upon said first emission layer;

depositing a second transparent electrically conductive layer upon said first electron transporting layer, said second transparent electrically conductive layer adapted to receive a first bias potential;

depositing a second hole transporting layer upon said second transparent electrically conductive layer;

depositing a second organic emission layer upon said second hole transporting layer to provide a second emission color;

depositing a second electron transporting layer upon said second emission layer; and

depositing a third transparent electrically conductive layer upon said second electron transporting layer, said third transparent electrically conductive layer adapted to receive a second bias potential.

49. The method of claim 48, further including the step of shadow masking a region of said first transparent electrically conductive prior to depositing said first hole transporting layer expose said region of said first transparent, electrically conductive layer thereby enabling said first bias potential to be applied between said second transparent electrically conductive layer and said region of said first transparent electrically conductive layer.

50. The method of claim 48, further including the step of etching away a region of said first hole transporting layer to expose a portion of said first transparent electrically conductive layer thereby enabling said first bias potential to be applied between said second transparent electrically conductive layer and said exposed portion of said first transparent electrically conductive layer.

51. The method of claim 48, further including the step of forming a layer of anti-reflecting material between said first transparent electrically conductive layer and said transparent substrate.

52. The method of claim 51, further including the step of forming a metal layer on said third transparent electrically conductive layer, for reflecting upward directed right back to said substrate.

53. The method claim 48, further including the step of forming a metal layer on said third transparent electrically conductive layer, for reflecting upward directed light back to said substrate.

54. A multicolor, energizable, light emitting structure, comprising:

at least three layers of electrically conductive material;

a transparent, energizable, light emitting device including an emission layer of organic material, (OLED), being disposed between adjacent ones of said layers of electrically conducive material, respectively, so that said OLEDs are stacked on each other with one of said layers of electrically conductive material disposed between each two of said OLEDs and the other layers of electrically conductive material are disposed on the outside of said OLEDs;

said layers of electrically conductive material disposed between adjacent ones of said OLEDs and one of said outside layers being substantially transparent; and

means on each of said layers of electrically conductive material for being connected to a bias for selectively energizing each of said OLEDs.

55. The structure of claim 54, wherein each of said OLEDs emits a different color.

56. The structure of claim 55, wherein said OLEDs are stacked in a vertical array.

57. The structure of claim 56, further including:

a third OLED in said stack;

the middle OLED being operative to emit light of a predetermined wavelength;

the top OLED being operative to emit light of a longer wavelength than the predetermined wavelength; and

the bottom OLED being operative to emit light of a shorter wavelength than the predetermined wavelength.

58. The structure of claim 56, further including:

a transparent substrate;

said stack of OLEDs and layers of electronically conductive material being supported by transparent substrate in an order that corresponds to the length of the light wave that said OLEDs emit; and

said OLED emitting the shortest wavelength is closest to said transparent substrate so that the light emitted from each of said OLEDs when it is energized is transmitted through the other OLEDs and through said through said transparent substrate.

59. The structure of claim 58, further including:

a layer of anti-reflecting material disposed between said OLED emitting the shortest wavelength and said transparent substrate so that the light emitted from each of said OLEDs when it is energized is not reflected from said transparent substrate.

60. The structure of claim 58, further including:

a layer of reflective material adjacent said OLED emitting the longest wavelength for reflecting light emitted from said OLED back through said substrate.

61. The structure of claim 54, wherein said layer of electrically conductive material includes indium-tin-oxide (ITO) and a metal.

62. The structure of claim 61, wherein said metal has a work function of less than four electron volts.

63. The structure of claim 54, further including:

a transparent substrate;

said stack of OLEDs and layers of electrically conductive material being supported by said transparent substrate in an order that corresponds to the length of the light wave that said OLEDs emit; and

said emitting the shortest wavelength is closet to said transparent substrate for maximizing light emitted from said structure from each of said OLEDs respectively, when energized.

64. The structure of claim 63, further including:

a layer of anti-reflecting material disposed between said OLED emitting the shortest wavelength and said transparent substrate so that the light emitted from each of said OLEDs when it is energized is not reflected from said transparent substrate.

65. The structure of claim 64, further including:

a layer of reflective material adjacent said OLED emitting the longest wavelength.

66. The structure of claim 64, wherein said layer of electrically conductive material includes an indium-tin oxide (ITO) layer and a metal layer.

67. The structure of claim 66, wherein said metal has a work function of less than four electron volts.

68. The structure of claim 63, further including:

a layer of reflective material adjacent said OLED emitting the longest wavelength for reflecting light emitted from said LED back through said substrate.

69. The structure of claim 54, wherein each of said OLEDs is a double heterostructure.

70. The structure of claim 69 wherein each of said LEDs is comprised of organic material.

71. The structure of claim 54, wherein each of said OLEDs is a single heterostructure.

72. The structure of claim 71, wherein each of said LEDs is comprised of organic material.

73. The structure of claim 54, wherein each of said OLEDs emit light of substantially the same color.

74. An energizable, light emitting structure, comprising:

a transparent substrate;

a first layer of transparent, electrically conductive material supported on said substrate;

a transparent, energizable, light emitting device (LED) supported on said first layer of transparent, electrically conductive material, said LED including an emission layer of organic material;

a second layer of transparent electrically conductive material supported by said LED; and

said LED being operative to produce light and transmit it through said transparent substrate when energized.

75. The structure of claim 74, wherein said first and second layers comprise indium-tin-oxide.

76. The structure of claim 75, wherein said second layer further comprises a layer of metal that has a work function that is less than four electron volts.

77. The structure of claim 76, wherein said metal is from the group consisting of magnesium, arsenic and magnesium-gold alloy.

78. The structure of claim 74, further including:

said second layer of electrically conductive material being substantially transparent;

a second transparent, energizable, light emitting device (LED) supported on said second LED including an emission layer of organic material;

a third layer of electrically conductive material supported by said second LED; and

said second LED being operative to produce light and transmit it through said first LED and through said transparent substrate when energized.

79. The energizable light emitting structure of claim 74, wherein said emission layer includes at least one material is selected from the group consisting of trivalent metal quinolate complexes, trivalent metal bridged quinolate complexes, Schiff base divalent metal complexes, tin (iv) metal complexes, metal acetylacetonate complexes, metal bidentate ligand complexes, bisphosphonates, divalent metal maleonitriledithiolate complexes, molecular charge transfer complexes, aromatic and heterocyclic polymers and rare earth mixed chelates.

80. The structure of claim 74, wherein said LED is double heterostructure (DH).

81. The structure of claim 80 wherein said LED is comprised of organic material.

82. The structure of claim 74 wherein said LED is single heterostructure (SH).

83. The structure of claim 82, wherein said LED is comprised of organic material.

84. The structure of claim 74, further including a layer of anti-reflecting material between said transparent substrate and said first layer of transparent electrically conductive material.

85. The structure of claim 84, further including a metal layer on said second layer of transparent electrically conductive material, for reflecting light emitted from said LED back though said transparent substrate.

86. The structure of claim 74, further including a reflective metal layer on said second layer of transparent electrically conductive material, for reflecting light emitted from said LED back through said transparent substrate.

87. A method of fabricating a multicolor, energizable light emitting structure, comprising the steps of:

providing a transparent substrate;

providing a first substantially transparent electrically conductive layer on said transparent substrate;

providing a first transparent, light emitting diode (LED) on said substrate, said first LED including an emission layer of organic material and, being operable when energized to emit light of a first predetermined wavelength;

providing a second substantially transparent, electrically conductive layer on said first LED;

providing a second transparent, light emitting diode (LED) on said second substantially transparent, electrically conductive layer, said second LED including an emission layer of organic material and being operable when energized to emit light of a second predetermined wavelength, that is longer than said first predetermined wavelength; and

an electrically conductive layer on said second (LED).

88. A method as in claim 87, wherein

said steps of providing said first and second LEDs comprises the steps of forming each of said LEDs by:

depositing a hole transporting layer on said first and second substantially transparent, electrically conductive layers;

depositing an emission layer on each of said hole transporting layers; and

depositing an electron transporting layer on each of said emission layers.

89. The method of claim 88, wherein each of said emission layers includes a material selected from the group consisting of trivalent metal quinolate complexes, trivalent metal bridged quinolate complexes, Schiff base divalent metal complexes, tin (iv) metal complexes, metal acetylacetonate complexes, metal bidentate ligand complexes, bisphosphonates, divalent metal maleonitriledithiolate complexes, molecular charge transfer complexes, aromatic and heterocyclic polymers and rare earth mixed chelates.

90. A method as in claim 89, wherein each of said substantially transparent electrically conductive layers and said layer of electrically conductive layer are comprised of indium-tin-oxide.

91. A method as in claim 88, further including:

the step of providing a layer of substantially transparent metal between said LEDs; and

a layer of said substantially transparent electrically conductive material on each of said layers of substantially transparent metal.

92. A method as in claim 91, wherein said metal has a work function of less than about four electron volts.

93. A method as in claim 91, wherein said metal is from the group consisting of magnesium, arsenic and magnesium/gold alloy.

94. A method as in claim 87, wherein said layer of electrically conductive material has a reflective surface for reflecting light emitted from said LEDs through said transparent substrate.

95. A method of claim 94, further including the step of providing a layer of anti-reflective material between said substrate and said first transparent electrically conductive layer.

96. A method as in claim 87, further including the step of:

providing an electrical contact on each of said layers of substantially transparent, electrically conductive material, and on said layer of electrically conductive material so that each of said layers can be connected to a source of bias potential.

97. A method as in claim 87, further including the step of providing a layer of anti-reflective material between said substrate and said first transparent electrically conductive layer.