Method for forming solar cell having active region with nanostructures having energy wells

Abstract

A method and apparatus for solar cell having graded energy wells is provided. The active region of the solar cell comprises nanostructures. The nanostructures are formed from a material that comprises a III-V compound semiconductor and an element that alters the band gap of the III-V compound semiconductor. For example, the III-V compound semiconductor could be gallium nitride (GaN). As an example, the “band gap altering element” could be indium (In). The concentration of the indium in the active region is non-uniform such that the active region has a number of energy wells, separated by barriers. The energy wells may be “graded”, by which it is meant that the energy wells have a different band gap from one another, generally increasing or decreasing from one well to another monotonically.

1 . A method for forming a solar cell, the method comprising:
 growing a plurality of nanostructures that include a III-V compound semiconductor that is present from the bottom to the top of substantially all of the plurality of nanostructures, the nanostructures form at least a portion of an active region of the solar cell, wherein growing the nanostructures comprises:
 adjusting concentration of a band gap altering element incorporated into the III-V compound semiconductor in order to configure substantially all of the nanostructures to have a non-uniform concentration of the band gap altering element, wherein substantially all of the nanostructures have a plurality of segments, and wherein each of the segments has a band gap; 
 wherein the band gap of a particular segment is established by adjusting the concentration of the band gap altering element; and 
 wherein the adjusting the concentration of a band gap altering element includes adjusting the concentration of the band gap altering element to be non-uniform in the nanostructures to create energy wells of different energy levels and barriers between the energy wells. 
   
   
   
       2 . The method of  claim 1 , wherein adjusting concentration of the band gap altering element comprises adjusting the concentration such that energy wells closer to a first side of the active region have higher band gaps than energy wells further from the first side. 
   
   
       3 . The method of  claim 1 , wherein adjusting concentration of the band gap altering element comprises adjusting the concentration such that band gap of energy wells decreases monotonically from one energy well to another. 
   
   
       4 . The method of  claim 1 , wherein adjusting concentration of the band gap altering element comprises adjusting the concentration such that barriers between the energy wells have approximately the same band gaps as each other. 
   
   
       5 . The method of  claim 1 , wherein adjusting concentration of the band gap altering element comprises adjusting the concentration such that barriers between the energy wells have graded band gaps. 
   
   
       6 . The method of  claim 1 , wherein adjusting concentration of the band gap altering element comprises adjusting the concentration such that barriers between the energy wells that are closer to a first side of the active region have greater band gaps than barriers that are further from the first side. 
   
   
       7 . The method of  claim 1 , wherein adjusting concentration of the band gap altering element comprises adjusting the concentration such that width of a particular energy well is configured to affect the band gap of the particular energy well by quantum confinement effect. 
   
   
       8 . The method of  claim 1 , wherein adjusting concentration of the band gap altering element comprises adjusting the concentration such that width of a particular barrier between two of the energy wells is configured to control charge carrier tunneling between the two energy wells. 
   
   
       9 . The method of  claim 1 , wherein the band gap altering element comprises indium. 
   
   
       10 . The method of  claim 1 , wherein the band gap altering element comprises aluminum. 
   
   
       11 . The method of  claim 1 , wherein the III-V compound semiconductor comprises gallium nitride. 
   
   
       12 . The method of  claim 1 , further comprising forming a layer that is configured to reflect photons that were not absorbed when passing through the active region back into the active region. 
   
   
       13 . The method of  claim 1 , wherein the barriers have a bandgap that is selected to control output voltage of the solar cell. 
   
   
       14 . A method for forming a solar cell, the method comprising;
 forming a plurality of nanostructures over a substrate having a conductivity of a first type, the plurality of nanostructures are formed substantially throughout from In x Ga x−1 N, forming the plurality of nanostructures includes adjusting concentration of the indium in the In x Ga x−1 N to create a plurality of segments in substantially all of the nanostructures, wherein each of the segments has a band gap that is established at least in part by the concentration of the indium; and   forming a top junction layer over the plurality of nanostructures, the top junction layer has a conductivity of a second type that is opposite the first type of conductivity;   wherein the concentration of the indium is non-uniform in the plurality of nanostructures to create energy wells of different energy levels and barriers between the energy wells.   
   
   
       15 . The method for forming a solar cell of  claim 14 , wherein forming the plurality of nanostructures includes doping the plurality of nanostructures substantially throughout with a material having the first type of conductivity. 
   
   
       16 . The method for forming a solar cell of  claim 14 , wherein forming the plurality of nanostructures includes forming the plurality of nanostructures without intentionally doping the plurality of nanostructures, wherein the solar cell is either a p-i-n structure or an n-i-p structure. 
   
   
       17 . The method for forming a solar cell of  claim 14 , further comprising forming a window that admits electromagnetic radiation, wherein adjusting concentration of the indium in the In x Ga x−1 N includes adjusting concentration of the indium such that the band gap of the energy wells decreases monotonically from one energy well to the next energy well in a direction away from the window. 
   
   
       18 . A method for forming a solar cell, the method comprising:
 providing a substrate having a conductivity of a first type;   forming a plurality of nanostructures from a III-V compound semiconductor, each of the a plurality of nanostructures has a bottom and a top;   incorporating a bandgap altering element into the plurality of nanostructures, incorporating the bandgap altering element includes adjusting the concentration of the bandgap altering element to create a plurality of segments in substantially all of the nanostructures, wherein each of the segments has a band gap that is established at least in part by the concentration of the band gap altering element, the adjusting the concentration of the bandgap altering element includes creating energy wells of different energy levels and barriers between the energy wells; and   forming a top junction layer over the plurality of nanostructures, the top junction layer has a conductivity of a second type that is opposite the first type of conductivity.   
   
   
       19 . The method for forming a solar cell of  claim 18 , wherein forming the plurality of nanostructures includes doping the plurality of nanostructures substantially from the bottoms to the tops of the nanostructures with a material having the first type of conductivity. 
   
   
       20 . The method for forming a solar cell of  claim 18 , wherein forming the plurality of nanostructures includes forming the plurality of nanostructures without intentionally doping the plurality of nanostructures, wherein the solar cell is either a p-i-n structure or an n-i-p structure. 
   
   
       21 . The method for forming a solar cell of  claim 18 , wherein a first and a second of the segments correspond to a first and a second energy well, wherein adjusting the concentration of the bandgap altering element includes forming the first and second segments to have different lengths to fine tune the band gap of the first and second energy wells by quantum confinement effect. 
   
   
       22 . The method for forming a solar cell of  claim 18 , wherein adjusting the concentration of the bandgap altering element includes configuring the width of a particular energy well to influence electron-hole recombination rate by affecting piezoelectric field effect due to polarization charges at well-barrier interfaces. 
   
   
       23 . The method for forming a solar cell of  claim 18 , further comprising doping the energy wells, doping the barriers or doping both the energy wells and the barriers to influence electron-hole recombination rate by affecting piezoelectric field effect due to polarization charges at an interface between energy wells and adjacent barriers.