Materials Integrations for High-Performance Photovoltaics by Wafer Bonding
<p>The fundamental efficiency limit for state of the art triple-junction photovoltaic devices is being approached. By allowing integration of non-lattice-matched materials in monolithic structures, wafer bonding enables novel photovoltaic devices that have a greater number of subcells to impro...
| Summary: | <p>The fundamental efficiency limit for state of the art triple-junction photovoltaic devices is being approached. By allowing integration of non-lattice-matched materials in monolithic structures, wafer bonding enables novel photovoltaic devices that have a greater number of subcells to improve the discretization of the solar spectrum, thus extending the efficiency limit of the devices. Additionally, wafer bonding enables the integration of non-lattice-matched materials with foreign substrates to confer desirable properties associated with the handle substrate on the solar cell structure, such as reduced mass, increased thermal conductivity, and improved mechanical toughness. This thesis outlines process development and characterization of wafer bonding integration technologies essential for transferring conventional triple-junction solar cell designs to potentially lower cost Ge/Si epitaxial templates. These epitaxial templates consist of a thin film of single-crystal Ge on a Si handle substrate. Additionally, a novel four-junction solar cell design consisting of non-lattice matched subcells of GaInP, GaAs, InGaAsP, and InGaAs based on InP/Si wafer-bonded epitaxial templates is proposed and InP/Si template fabrication and characterization is pursued.</p>
<p>In this thesis the detailed-balance theory of the thermodynamic limiting performance of solar cell efficiency is applied to several device designs enabled by wafer bonding and layer exfoliation. The application of the detailed-balance theory to the novel four-junction cell described above shows that operating under 100 suns at 300 K a maximum efficiency of 54.9% is achievable with subcell bandgaps of 1.90, 1.42, 1.02, and 0.60 eV, a material combination achievable by integrating two wide-bandgap subcells lattice matched to GaAs and two narrow-bandgap subcells lattice matched to InP.</p>
<p>Wafer bonding and layer transfer processes with sufficient quality to enable subsequent material characterization are demonstrated for both Ge/Si and InP/Si structures. The H-induced exfoliation process in each of these materials is studied using TEM, AFM, and FTIR to elucidate the chemical states of hydrogen leading to exfoliation. Additionally, the electrical properties of wafer-bonded interfaces between bulk-Ge/Si and bulk-InP/Si structures are show Ohmic, low-resistance electrical contact. Further studies of p-p isotype heterojunctions in Ge/Si indicate that significant conduction paths exist through defects at the bonded interface. The first known instance of epitaxy of III-V compound semiconductors on wafer-bonded Ge/Si epitaxial templates is demonstrated. Additionally InGaAs is grown on InP/Si templates that have been improved by removal of damage induced by the ion implantation and exfoliation processes.</p> |
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