Solar cell with epitaxially grown quantum dot material

Abstract

A monolithic semiconductor photovoltaic solar cell comprising a plurality of subcells disposed in series on an electrically conductive substrate. At least one subcell of the plurality of subcells includes an epitaxially grown self-assembled quantum dot material. The subcells are electrically connected via tunnel junctions. Each of the subcells has an effective bandgap energy. The subcells are disposed in order of increasing effective bangap energy, with the subcell having the lowest effective bandgap energy being closest to the substrate. In certain cases, each subcell is designed to absorb a substantially same amount of solar photons.

1 . A monolithic, multijunction, semiconductor photovoltaic solar cell comprising:
 a plurality of subcells disposed in series, each subcell having formed therein a p-n junction or a p-i-n junction, the plurality of subcells having a quantum dot subcell, the quantum dot subcell being a subcell with strained epitaxially-grown semiconductor layers that include self-assembled quantum dots, the quantum dot subcell being pseudomorphically grown on another subcell.   
     
     
         2 . The solar cell of  claim 1 , further comprising:
 tunnel junctions formed between the subcells, the tunnel junctions for electrically connecting the subcells.   
     
     
         3 . The solar cell of  claim 1 , further comprising:
 an electrically conductive substrate upon which the plurality of subcells is formed.   
     
     
         4 . The solar cell of  claim 3  wherein each of the subcells has an effective bandgap energy, the subcells being disposed in order of increasing effective bandgap energy, a subcell with a lowest effective bandgap energy being closest to the electrically conductive substrate. 
     
     
         5 . The solar cell of  claim 1 , wherein each of the subcells is for absorbing a substantially same fraction of solar photons. 
     
     
         6 . The solar cell of  claim 4 , wherein the plurality of subcells consists of three subcells, a first subcell having the lowest effective bandgap energy, a third subcell having a highest effective bandgap energy, and a second subcell disposed between the first subcell and the third subcell, the second subcell being the quantum dot subcell. 
     
     
         7 . The solar cell of  claim 6  wherein:
 the electrically conductive substrate is a Ge or a GaAs substrate; 
 the first subcell includes Ge; 
 the strained epitaxially-grown semiconductor layers include strained InGaAs quantum dot layers intercalated with GaAs, AlGaAs, or GaPAs layers; and 
 the third subcell includes GaInP, AlGaAs or AlGaInP. 
 
     
     
         8 . The solar cell of  claim 7  wherein the first subcell is epitaxially grown on the electrically conductive substrate. 
     
     
         9 . The solar cell of  claim 7  wherein the electrically conductive substrate is a Ge substrate and the first subcell is an interdiffused portion of the Ge substrate. 
     
     
         10 . The solar cell of  claim 7  wherein:
 the electrically conductive substrate is n-doped; and 
 an n-side of each of the p-n junction or p-i-n junction formed in each subcell is closer to the substrate than a respective p-side of the p-n junction or the p-i-n junction. 
 
     
     
         11 . The solar cell of  claim 7  wherein:
 the electrically substrate is p-doped; and 
 a p-side of the p-n junction of p-i-n junction formed in each subcell is closer to the substrate than a respective n-side of the p-n junction or p-i-n junction. 
 
     
     
         12 . The solar cell of  claim 1  wherein:
 one of the subcells includes a Bragg reflector or a distributed Bragg reflector to modify an absorption characteristic of the solar cell. 
 
     
     
         13 . A monolithic, multijunction, semiconductor photovoltaic solar cell comprising:
 a plurality of subcells disposed in series, each subcell having formed therein a p-n junction or a p-i-n junction, the plurality of subcells having a quantum dot subcell, the quantum dot subcell being a subcell with strained epitaxially-grown semiconductor layers that include self-assembled quantum dots.