Method of fabricating back surface point contact solar cells

• US 4,927,770 A
• Filed: 11/14/1988
• Issued: 05/22/1990
• Est. Priority Date: 11/14/1988
• Status: Expired due to Term

• Current Assignee: Electric Power Research Institute, Board of Trustees of the Leland Stanford Junior University (Stanford University)
• Original Assignee: Electric Power Research Institute Corporation of District of Columbia, Board of Trustees of the Leland Stanford Junior University (Stanford University)
• Inventors: Swanson, Richard M.
• Primary Examiner(s): Weisstuch, Aaron
• Application Number:
  US07/272,130
• Time in Patent Office: 554 Days
• Field of Search: 437/2-5, 136/256, 136/261, 357/30 J, 357/30 Q
• US Class Current: 438/83
• CPC Class Codes: H01L 31/022441 Electrode arrangements specially adapted for back-contact solar cells
• CPC Class Codes: H01L 31/061 the potential barriers being of the point-contact type H01L31/07 takes precedence
• CPC Class Codes: H01L 31/0682 back-junction, i.e. rearside emitter, solar cells, e.g. interdigitated base-emitter regions back-junction cells
• CPC Class Codes: H01L 31/1804 comprising only elements of Group IV of the Periodic Table
• CPC Class Codes: Y02E 10/547 Monocrystalline silicon PV cells
• CPC Class Codes: Y02P 70/50 Manufacturing or production processes characterised by the final manufactured product

Abstract

A back surface point contact silicon solar cell having improved characteristics is fabricated by hydrogenating a silicon-silicon oxide interface where hydrogen atoms are diffused through silicon nitride and silicon oxide passivating layers on the surface of a silicon substrate. In carrying out the hydrogenation, the substrate and passivation layers are placed in a hydrogen atomsphere at an elevated temperature of at least 900° C. whereby hydrogen atoms diffuse through the two passivation layers. Self-alignment techniques are employed in forming small-geometry doped regions in the surface of the silicon substrate for the p-n junctions of the solar cell. Openings are formed through the passivation layers to expose first surface areas on the substrate, and a doped silicon oxide layer is then formed over the passivation layers and on the exposed surface areas. Portions of the first doped layer on the two passivation layers are removed and then second portions of the two passivation layers are removed, thereby exposing second surface areas. A second doped silicon oxide layer is then formed over the passivation layers and on the second exposed surface areas. Dopants from the two doped silicon oxide layers are then diffused into the first and second surface layers to form p and n diffused regions in the surface of the substrate. Thereafter, the first and second doped silicon oxide layers are removed by a preferential etchant which does not remove the silicon nitride layer, thereby exposing the first and second surface areas. A two-level metal interconnect structure is then formed for separately contacting the first surface areas and the second surface areas.

Claims

1. A method of fabricating a back surface point contact solar cell, comprising the steps of

(a) providing a substrate of silicon having two major opposing surfaces,

(b) forming a silicon oxide layer on at least a first of said major surfaces,

(c) forming a silicon nitride layer on said silicon oxide layer,

(d) removing first portions of said layers thereby exposing first surface areas on said first major surface,

(e) forming a first doped layer with a first conductivity type dopant therein on said surface areas,

(f) removing second portions of said layers, thereby exposing second surface areas on said first major surface,

(g) forming a second doped layer with a second conductivity type dopant therein over on said second surface areas,

(h) heating said substrate and thereby diffusing dopant from said first layer into said first surface areas and diffusing dopant from said second doped layer into said second surface areas,

(i) subjecting said substrate and said layers to an elevated temperature of at least 900°

C. in a hydrogen atmosphere, thereby hydrogenating the interface between said major surface and said silicon oxide layer,

(j) removing said first and second doped layers by a preferential etchant which does not remove said silicon nitride, and

(k) forming a two-level metal interconnect structure for separately contacting said first surface areas and said second surface areas.

2. The method as defined by claim 1 wherein step (k) includes forming a first metal layer contacting said first and second surface areas selectively etching said first metal layer to electrically isolate all of said first surface areas forming an insulative layer over said first metal layer, and forming a second metal layer over said insulative layer and in electrical contact with all of said first surface areas.

3. The method as defined by claim 1 wherein step (k) includes forming a first aluminum layer contacting said first surface areas, oxidizing said first aluminum layer, and forming a second aluminum layer contacting said second surface areas.

4. The method as defined by claim 1 wherein said first and second doped layer comprise silicon oxide.

5. The method as defined by claim 1 wherein the steps of forming said first and second doped layers and the diffusing of dopants therefrom are performed sequentially.

6. The method as defined by claim 1 wherein steps (h) and (i) are carried out concurrently.

7. The method as defined by claim 1 wherein step (b) includes forming a silicon oxide layer on the second major surface.

8. The method as defined by claim 7 wherein step (b) further includes doping the silicon oxide layer on the second major surface.

9. The method as defined by claim 7 wherein step (b) further includes establishing an electrical charge in the silicon oxide layer on the second major surface.

10. In the fabrication of a back surface point contact solar cell including a silicon substrate and employing silicon oxide and silicon nitride passivation layers, a method of reducing the recombination velocity of electron-holes pairs at the interface of the silicon substrate and the silicon oxide layer, comprising the steps of

placing the silicon substrate with silicon oxide and silicon nitride layers thereon in a hydrogen atmosphere, and

heating said silicon substrate in said atmosphere at a temperature of at least 900°

C. to diffuse hydrogen atoms through said silicon nitride layer and through said silicon oxide layer to the interface of said silicon oxide layer and said substrate.

11. A method of forming first and second diffused regions of opposite conductivity type in a surface of a silicon substrate for fabricating a back surface point contact solar cell, comprising the steps of

(a) providing a substrate of silicon having two major opposing surfaces,

(b) forming a silicon oxide layer on a first of said major surfaces,

(c) forming a silicon nitride layer on said silicon oxide layer,

(d) removing first portions of said layers, thereby exposing first surface areas on said first major surface,

(e) forming a first doped silicon oxide layer with a first conductivity type dopant therein over said layers and on said first surface areas,

(f) removing second portions of said layers, thereby exposing second surface areas on said first major surface,

(g) forming a second doped silicon oxide layer with a second conductivity type dopant therein over said layers and on said second surface areas,

(h) heating said substrate and thereby diffusing dopant from said first doped layer into said first surface areas and diffusing dopant from said second doped layer into said second surface areas, and

(i) removing said first and second doped layers by a preferential etchant which does not remove said silicon nitride.

12. The method as defined by claim 11 and further including:

(j) forming a two-level metal interconnect structure for separately contacting said first surface areas and said second surface areas.

13. The method as defined by claim 12 wherein step (j) includes forming a first metal layer contacting said first and second surface areas selectively etching said first metal layer to electrically isolate all of said first surface areas forming an insulative layer over said first metal layer, and forming a second metal layer over said insulative layer and in electrical contact with all of said first surface areas.

14. The method as defined by claim 12 wherein step (j) includes forming a first aluminum layer contacting said first surface areas, oxidizing said first aluminum layer, and forming a second aluminum layer contacting said second surface areas.