CdTe Barrier type photovoltaic cells with enhanced open-circuit voltage, and process of manufacture

Abstract

A cadmium telluride photovoltaic cell is produced with increased conversion efficiency arising from enhanced open-circuit voltage. Such voltage is achieved by altering the surface of the crystalline cadmium telluride that contacts the barrier metal by heating the cadmium telluride in the presence of oxygen prior to depositing the barrier metal.

What is claimed is: 
     
       1. A photovoltaic cell comprising a layer of crystalline cadmium telluride, an exterior surface portion of said layer having .[.TeO 2  in the crystalline lattice.]. .Iadd.an oxide form of tellurium .Iaddend.to a depth effective to produce an open-circuit voltage that is enhanced over the open-circuit voltage that is produced .[.without said TeO 2  .]. .Iadd.by a photovoltaic cell identical in structure except for the layer of cadmium telluride which has in the exterior surface portion thereof only the oxide forms of tellurium resulting from exposing said surface portion to air at room temperature, .Iaddend.   a layer of metal in contact with at least part of said exterior surface portion, said layer being capable of forming a barrier with said cadmium telluride .[.altered.]. surface .Iadd.portion; .Iaddend.and   an electrode in ohmic contact with said cadmium telluride.   
     
     
       2. The cell as defined in claim 1, and further including an antireflection layer in contact with at least part of said metal layer, said antireflection layer having a thickness that enhances the transmission of light through the cell. 
     
     
       3. The cell as defined in claim 1, and further including a current-conducting grid applied over, and in ohmic contact with, said metal layer. 
     
     
       4. A process for the formation of a photovoltaic cell, the process comprising the steps of a. forming a layer of crystalline cadmium telluride,   b. heating at least one surface of the layer in air for more than about 2 minutes and less than or equal to about 20 minutes at a temperature between about 250° C. and about 350° C., or for a time less than or equal to about 2 minutes but at a temperature in centigrade degrees no lower than about 490/a, where a is the time of heating in minutes; and   c. applying a metal layer in contact with at least part of said one surface, said metal being capable of forming a barrier with cadmium telluride, whereby a cell is produced having an open-circuit voltage which is at least about 50 millivolts greater than a similar cell processed without step (b) above.     
     
     
       5. A photovoltaic cell comprising a layer of crystalline cadmium telluride, a surface of said layer being altered to contain .[.tellurium oxide.]. .Iadd.an oxide form of tellurium .Iaddend.by heating in an oxygen containing atmosphere for between about 1 minute and about 20 minutes at a temperature between about 250° and about 500° C. the oxygen being present .Iadd.in the atmosphere .Iaddend.in an amount that is effective to enhance the open-circuit voltage by at least 50 millivolts .Iadd.compared to the open-circuit voltage of a photovoltaic cell identical in structure except for the layer of cadmium telluride which has in the surface thereof only the oxide forms of tellurium resulting from exposing said surface to air at room temperature,   in contact with at least part of said altered surface, a layer of metal capable of forming a barrier with said cadmium telluride altered surface; and   an electrode in ohmic contact with said cadmium telluride.   
     
     
       6. A photovoltaic cell exhibiting an enhanced open circuit voltage, comprising a layer of crystalline cadmium telluride, a surface of said layer being in an altered state achieved by heating in air for more than about 2 minutes and less than or equal to about 20 minutes at a temperature between about 250° C. and about 350° C., or for a time less than or equal to about 2 minutes but at a temperature in centigrade degrees no lower than about 490/a, where a is the time of heating in minutes;   in contact with at least part of said altered surface, a layer of metal capable of forming a barrier with said cadmium telluride altered surface; and   an electrode in ohmic contact with said cadmium telluride.   
     
     
       7. A process of measuring light, comprising the steps of a. exposing to light, a photovoltaic cell comprising a layer of crystalline cadmium telluride, an exterior surface portion of said layer having .[.TeO 2  in the crystalline lattice.]. .Iadd.an oxide form of tellurium .Iaddend.to a depth effective to produce an open-circuit voltage that is enhanced over the open-circuit voltage that is produced .[.without said TeO 2  .]. .Iadd.by a photovoltaic cell identical in structure except for the layer of cadmium telluride which has in the exterior surface portion thereof only the oxide forms of tellurium resulting from exposing said surface portion to air at room temperature, .Iaddend.   a layer of metal in contact with at least part of said exterior surface portion, said layer being capable of forming a barrier with said cadmium telluride altered surface; and   an electrode in ohmic contact with said cadmium telluride, and   b. measuring either the open-circuit voltage or short-circuit current generated by the cell. .Iadd.   
     
     
       8.  A photovoltaic cell comprising a layer of crystalline cadmium telluride, a surface of said layer being altered by heating in an oxygen containing atmosphere for between about 1 minute and about 20 minutes at a temperature between about 250° and about 500° C., the oxygen being present in the atmosphere in an amount that is effective to enhance the open circuit voltage by at least 50 millivolts compared to the open-circuit voltage of a photovoltaic cell identical in structure except for the layer of cadmium telluride which has in the surface thereof only the oxide forms of tellurium resulting from exposing said surface to air at room temperature, and   in contact with at least part of said altered surface, a layer of metal capable of forming a barrier with said cadmium telluride altered surface; and   an electrode in ohmic contact with said cadmium telluride. .Iaddend. .Iadd.9. A process for the formation of a photovoltaic cell, the process comprising the steps of   a. forming a layer of crystalline cadmium telluride,   b. heating at least one surface of the layer in an oxygen containing atmosphere for a time and at a temperature, and in an amount of oxygen, that are effective to enhance the open-circuit voltage by at least 50 millivolts compared to the open-circuit voltage of a photovoltaic cell identical in structure except for the layer of cadmium telluride which has in the surface thereof only the oxide forms of tellurium resulting from exposing said surface to air at room temperature, said temperature being at least about 250° C. and said time being no greater than about 20 minutes,   c. applying a metal layer in contact with at least part of said one surface, said metal being capable of forming a barrier with cadmium telluride, and   d. ohmically contacting the cadmium telluride with an electrode. .Iaddend.   
     
     
        .Iadd.10.  A process for the formation of a photovoltaic cell, the process comprising the steps of a. forming a layer of crystalline cadmium telluride,   b. heating at least one surface of the layer in an oxygen containing atmosphere for between about 1 minute and about 20 minutes at a temperature between about 250° C. and about 500° C., the oxygen being present in the atmosphere in an amount that is effective to enhance the open-circuit voltage by at least 50 millivolts compared to the open-circuit voltage of a photovoltaic cell identical in structure except for the layer of cadmium telluride which has in the surface thereof only the oxide forms of tellurium resulting from exposing said surface to air at room temperature,   c. applying a metal layer in contact with at least part of said one surface, said metal being capable of forming a barrier with cadmium telluride, and   d. ohmically contacting the cadmium telluride with an electrode. .Iaddend.