Crystalline thin film photovoltaic cell
A material stack including an ohmic contact layer and a single crystalline semiconductor base substrate of a first conductivity type and having a surface Fermi level pinned close to a band edge (either the conduction band or valence band) is first provided. A stressor layer is then formed above the ohmic contact layer and a material portion of the single crystalline semiconductor base substrate is removed by a process referred to as spalling. A transparent conductive oxide layer is then formed on an exposed surface of the material portion of the single crystalline semiconductor base substrate that was removed by spalling.
1. A method of forming a thin film photovoltaic cell, said method comprising:
providing a material stack of, from bottom to top, a single crystalline semiconductor base substrate of a first conductivity type and having a surface Fermi level pinned close to a band edge, and an ohmic contact layer;
forming a stressor layer above said ohmic contact layer;
removing a portion of said single crystalline semiconductor base substrate by performing a spalling process to provide a thin film structure that comprises, from bottom to top, a spalled single crystalline semiconductor material portion of said single crystalline semiconductor base substrate, said ohmic contact layer and said stressor layer; and
forming at least a transparent conductive oxide layer on an exposed surface of said spalled single crystalline semiconductor material portion.
2. The method of claim 1 , further comprising forming an edge exclusion material on a surface of said ohmic contact layer and at each vertical edge of said material stack prior to forming said stressor layer.
3. The method of claim 2 , wherein said edge exclusion material comprises a photoresist material, a polymer, a hydrocarbon material, an ink, a metal, or a paste.
4. The method of claim 1 , further comprising forming a handle substrate on an exposed surface of said stressor layer prior to performing said spalling process.
5. The method of claim 4 wherein said spalling process further comprises pulling or peeling said handle substrate.
6. The method of claim 1 , wherein said spalling process is performed at room temperature.
7. The method of claim 1 , wherein said stressor layer comprises a metal, a polymer, a spalling inducing tape or any combination thereof.
8. The method of claim 1 , wherein said stressor layer comprises Ni.
9. The method of claim 1 , wherein said ohmic contact layer comprises a semiconductor metal alloy having a dopant of said first conductivity type.
10. The method of claim 1 , wherein said transparent conductive oxide layer is formed directly on said exposed surface of said spalled single crystalline semiconductor material portion.
11. The method of claim 10 , further comprising forming an antireflective coating on an exposed surface of said transparent conductive oxide layer.
12. The method of claim 1 , further comprising forming an amorphous semiconductor layer of a second conductivity type on said exposed surface of said spalled single crystalline semiconductor material portion, wherein said second conductivity type is opposite said first conductivity type.
13. The method of claim 12 , further comprising forming an intrinsic amorphous semiconductor material layer between said amorphous semiconductor layer of said second conductivity type and said exposed surface of said spalled single crystalline semiconductor material portion.
14. The method of claim 1 , further comprising forming a semiconductor layer having a lattice contact that differs from a lattice constant of said spalled single crystalline semiconductor material portion between said exposed surface of said spalled single crystalline semiconductor material portion and said transparent conductive oxide layer.
15. The method of claim 1 , further comprising forming an insulating layer between said exposed surface of said spalled single crystalline semiconductor material portion and said transparent conductive oxide layer.
16. The method of claim 1 , further comprising further forming a light transparent window layer between said exposed surface of said spalled single crystalline semiconductor material portion and said transparent conductive oxide layer.
17. The method of claim 16 , wherein said light transparent window layer is selected from an oxide layer and a semiconductor material having a wider band gap than a band gap of said spalled single crystalline semiconductor material portion.
18. The method of claim 1 , wherein said stressor layer is formed in direct contact with a topmost surface of said ohmic contact layer.
19. The method of claim 1 , wherein said single crystalline semiconductor base substrate is not formed utilizing an epitaxial growth process.
20. The method of claim 1 , wherein said single crystalline semiconductor base substrate is selected from InP, InGaAs and germanium.