Optoelectronic devices including heterojunction

Abstract

Embodiments of the invention generally relate to optoelectronic semiconductor devices such as photovoltaic devices including solar cells. In one aspect, an optoelectronic semiconductor device includes an absorber layer made of gallium arsenide (GaAs) and having only one type of doping. An emitter layer is located closer than the absorber layer to a back side of the device, the emitter layer made of a different material than the absorber layer and having a higher bandgap than the absorber layer. A heterojunction formed between the emitter layer and the absorber layer, and a p-n junction is formed between the emitter layer and the absorber layer and at least partially within the different material at a location offset from the heterojunction. The p-n junction causes a voltage to be generated in the device in response to the device being exposed to light at a front side of the device.

1 . An optoelectronic semiconductor device comprising:
 an absorber layer made of gallium arsenide (GaAs) and having only one type of doping;   an emitter layer located closer than the absorber layer to a back side of the device, the emitter layer made of a different material than the absorber layer and having a higher bandgap than the absorber layer;   a heterojunction formed between the emitter layer and the absorber layer; and   a p-n junction formed between the emitter layer and the absorber layer at a location offset from the heterojunction, the p-n junction causing a voltage to be generated in the device in response to the device being exposed to light at a front side of the device.   
     
     
         2 . The optoelectronic semiconductor device of  claim 1  wherein the p-n junction is located within two depletion lengths of the heterojunction. 
     
     
         3 . The optoelectronic semiconductor device of  claim 1  wherein the offset of the p-n junction from the heterojunction is provided by an intermediate layer located between the absorber layer and the emitter layer, the intermediate layer having the same type of doping as the absorber layer and including the different material. 
     
     
         4 . The optoelectronic semiconductor device of  claim 3  wherein the intermediate layer includes a graded layer having a material gradation from GaAs at a side closer to the absorber layer, to the different material of the emitter layer, and a back window layer not having the gradation and having an approximately uniform composition of the different material. 
     
     
         5 . The optoelectronic semiconductor device of  claim 1  wherein the material of the emitter layer is Aluminum Gallium Arsenide (AlGaAs). 
     
     
         6 . The optoelectronic semiconductor device of  claim 1  wherein the optoelectronic semiconductor device is a single carrier transport solar cell. 
     
     
         7 . The optoelectronic semiconductor device of  claim 3  further comprising:
 an n-metal contact disposed on a front side of the device; 
 a p-metal contact disposed on the back side of the device, wherein the front side is disposed over the back side; 
 an anti-reflection coating disposed over the n-metal contact; and 
 a cell disposed between the front and back sides, the cell including an n-type film stack disposed over a p-type film stack, wherein:
 the n-type film stack comprises an n-type contact layer comprising gallium arsenide disposed over an n-type front window disposed over the absorber layer comprising n-type gallium arsenide and disposed over the intermediate layer comprising n-type aluminum gallium arsenide; and 
 the p-type film stack comprises the emitter layer comprising p-type aluminum gallium arsenide disposed over a p-type contact layer comprising p-type gallium arsenide. 
 
 
     
     
         8 . The optoelectronic semiconductor device of  claim 3  further comprising:
 an n-metal contact and a p-metal contact disposed on the back side of the device; 
 an anti-reflection coating disposed on a front side of the device, wherein the front side is disposed over the back side; and 
 a cell disposed between the front and back sides and comprising an n-type film stack disposed over a p-type film stack, wherein:
 the n-type film stack comprises an n-type contact layer comprising gallium arsenide disposed over an n-type front window disposed over the absorber layer comprising n-type gallium arsenide, disposed over the intermediate layer comprising n-type aluminum gallium arsenide, 
 the p-type film stack comprises an emitter layer disposed over a p-type contact layer, 
 
 wherein the n-metal contact is coupled to the absorber layer, and the p-metal contact is coupled to the p-type contact layer. 
 
     
     
         9 . A solar cell comprising:
 an absorber layer made of gallium arsenide (GaAs) and having only one type of doping;   an emitter layer made of a different material than the absorber layer and having a higher bandgap than the absorber layer;   an intermediate layer provided between the absorber layer and the emitter layer, the intermediate layer having the same type of doping as the absorber layer, wherein the intermediate layer includes a material gradation from GaAs at a side closer to the absorber layer, to the different material of the emitter layer at a side closer to the emitter layer;   a heterojunction formed between the emitter layer and the absorber layer; and   a p-n junction formed between the emitter layer and the absorber layer and at least partially within the different material at a location offset from the heterojunction, the p-n junction causing a voltage to be generated in the cell in response to the cell being exposed to light at a front side of the cell.   
     
     
         10 . The solar cell of  claim 9  wherein the intermediate layer includes a graded layer having the gradation, and a back window layer not having the gradation and having an approximately uniform composition of the different material. 
     
     
         11 . The solar cell of  claim 10  wherein the graded layer is located adjacent to the absorber layer, and wherein the back window layer is located between the graded layer and the emitter layer. 
     
     
         12 . The solar cell of  claim 9  wherein the material of the emitter layer is Aluminum Gallium Arsenide (AlGaAs). 
     
     
         13 . The solar cell of  claim 11  wherein the intermediate layer includes a material gradation of about 30% Aluminum 70% GaAs to about 100% GaAs, in a direction of the emitter layer to the absorber layer. 
     
     
         14 . The solar cell of  claim 9  wherein the emitter layer is located closer than the absorber layer to a back side of the device, such that single carrier transport is provided in the device. 
     
     
         15 . A photovoltaic cell comprising:
 an absorber layer made of gallium arsenide (GaAs) and having only one type of doping;   an emitter layer made of a different material than the absorber layer and having a higher bandgap than the absorber layer;   a heterojunction formed between the emitter layer and the absorber layer; and   a p-n junction formed between the emitter layer and the absorber layer and at least partially within the different material at a location offset from the heterojunction, wherein a majority of the absorber layer is outside of a depletion region formed by the p-n junction, the p-n junction causing a voltage to be generated in the cell in response to the cell being exposed to light at a front side of the cell.   
     
     
         16 . The photovoltaic cell of  claim 15  wherein the p-n junction is located within two depletion lengths of the heterojunction. 
     
     
         17 . The photovoltaic cell of  claim 15  wherein the material of the emitter layer is Aluminum Gallium Arsenide (AlGaAs). 
     
     
         18 . The photovoltaic cell of  claim 15  wherein the offset of the p-n junction from the heterojunction is provided by an intermediate layer located between the absorber layer and the emitter layer, the intermediate layer having the same type of doping as the absorber layer, wherein the intermediate layer includes a material gradation from GaAs at a side closer to the absorber layer, to the different material of the emitter layer. 
     
     
         19 . The photovoltaic cell of  claim 15  wherein the emitter layer is located closer than the absorber layer to a back side of the cell, such that single carrier transport is provided in the cell. 
     
     
         20 . A method for forming an optoelectronic semiconductor device, the method comprising:
 forming an intermediate layer on an absorber layer, wherein the absorber layer is made of gallium arsenide (GaAs) and has only one type of doping, the intermediate layer having the same type of doping as the absorber layer; and   forming an emitter layer on the intermediate layer, wherein the emitter layer is located closer than the absorber layer to a back side of the device, the emitter layer being made of a different material than the absorber layer and having a higher bandgap than the absorber layer, wherein the intermediate layer includes the different material of the emitter layer,   wherein a heterojunction is formed between the emitter layer and the absorber layer, and   wherein a p-n junction is formed between the emitter layer and the absorber layer at a location offset from the heterojunction, the p-n junction causing a voltage to be generated in the device in response to the device being exposed to light.   
     
     
         21 . The method of  claim 20  wherein the intermediate layer includes a material gradation from GaAs at a side closer to the absorber layer, to the different material of the emitter layer at a side closer to the emitter layer. 
     
     
         22 . The method of  claim 20  wherein the optoelectronic semiconductor device is a single carrier transport solar cell.