Reverse heterojunctions for solar cells
In a solar cell having one or more subcells, at least one subcell is provided with a reverse heterojunction, the reverse heterojunction being formed with an emitter and an adjacent base, wherein the emitter has a band gap that is at least 10 meV lower than that of the adjacent base in order to reduce sheet resistance of the emitter and/or increase the subcell current with minimal effect on the open-circuit voltage. Because of the increase in current, the decrease in emitter sheet resistance, and relatively unchanged open-circuit voltage of the subcell, the efficiency of a solar cell employing one or more subcells with reverse heterojunctions is enhanced.
1. A photovoltaic cell comprising at least one reverse heterojunction subcell, wherein the reverse heterojunction subcell comprises:
an emitter characterized by an emitter bandgap and by a single emitter alloy composition; and
a base characterized by a base bandgap and by a single base alloy composition; wherein,
the emitter is adjacent the base;
the emitter bandgap is 10 meV to 150 meV less than the base bandgap; and
the emitter alloy composition and the base alloy composition comprise different alloy compositions of the same alloy system.
2. The photovoltaic cell of claim 1 , wherein the at least one reverse heterojunction subcell is a top subcell of the photovoltaic cell.
3. The photovoltaic cell of claim 2 , wherein,
the emitter alloy composition comprises InGaP and the base alloy composition comprises InGaP; or
the emitter alloy composition comprises AlInGaP and the base alloy composition comprises AlInGaP.
4. The photovoltaic cell of claim 3 , wherein,
the emitter has a band gap between 1.8 eV and 1.9 eV; and
the base has a band gap between 1.9 eV and 2.1 eV.
5. The photovoltaic cell of claim 4 , wherein,
the emitter has a band gap of 1.9 eV; and
the base has a band gap of 2.0 eV.
6. The photovoltaic cell of claim 1 , wherein,
the emitter alloy composition is selected from InGaAs, GaAs, AlInGaAs and AlGaAs; and
the base alloy composition is selected from InGaAs, GaAs, AlInGaAs and AlGaAs.
7. The photovoltaic cell of claim 1 , wherein the reverse heterojunction subcell is lattice matched to another subcell.
8. A method for increasing the power conversion efficiency of a photovoltaic cell comprising the steps of:
forming a base, wherein the base is characterized by a base bandgap and by a base alloy composition; and
forming, adjacent the base, an emitter, wherein,
the emitter is characterized by an emitter bandgap and an emitter alloy composition;
the emitter band gap is 10 meV to 150 meV less than the base bandgap; and
the emitter alloy composition and the base alloy composition comprise different alloy compositions of the same alloy system.