Doctor of Philosophy, The Ohio State University, 2021, Electrical and Computer Engineering

The solar energy industry has been growing rapidly due to decreasing costs of manufacturing and increasing panel efficiencies. As the industry dominant crystalline Si solar cell technology approaches its fundamental efficiency limits, new strategies for achieving higher efficiency while maintaining low cost is a necessity to continue the industries growth. The only demonstrated way to surpass the fundamental single junction efficiency barrier is through the use of multijunction photovoltaic cells. The multijunction cell architecture splits the solar spectrum to be absorbed by two or more different semiconductor materials with the highest energy photons being absorbed by the wider bandgap top cells, and lower energy light being absorbed by the narrower band gap bottom cells. This technology has been very successfully demonstrated in the III-V materials system and scaled by the space solar industry; however, terrestrial power generation has much stricter requirements on production cost than the space solar industry. Thus, arises the potential for an interesting marriage of technologies. Imagine combining the highly scaled Si manufacturing infrastructure, the low-cost Si wafer materials, and high efficiency III-V multijunction cells. This is the precise combination present in III-V/Si tandem solar cells. To date, two fabrication methods, mechanical stacking and surface activated wafer bonding, have demonstrated impressive 3-junction solar cells with efficiencies of 35.9%[1] and 34.1%[2] respectively. The issue is that these techniques are not largely considered scalable for terrestrial scale power generation; however, these prototype devices demonstrate great promise for this III-V/Si device architecture. The most scalable method of III-V/Si integration is through the use of epitaxy to monolithically integrate the III-V and Si solar cells; however, this approach can lead to defect formation due to differences in the lattice parameter, crystal structure, and th (open full item for complete abstract)

Committee: Steven Ringel (Advisor); Tyler Grassman (Advisor); Sanjay Krishna (Committee Member) Subjects: Electrical Engineering; Energy; Engineering

Doctor of Philosophy, The Ohio State University, 2020, Materials Science and Engineering

III-V/Si multijunction photovoltaics possess the potential for high power conversion efficiencies (surpassing the single-junction limit) at low cost by leveraging the inexpensive and scalable Si platform. Both space and terrestrial markets can benefit from this technology; however, multiple materials-related obstacles must first be overcome in order to truly demonstrate this potential and enable adoption of this new technology. For terrestrial usage, low-cost III-V deposition techniques are necessary to remain cost competitive with current photovoltaics. Given a Si bottom cell, the optimal series-connected dual-junction photovoltaic efficiency for both space and terrestrial solar spectra is achieved with a 1.7-1.8 eV top cell, which is most conveniently provided by GaAsyP1-y (y ~0.7-0.8) at around 3% lattice mismatch to the Si substrate. While epitaxial integration of GaAsyP1-y alloys on Si is favorable over high-cost and non-scalable wafer bonding or stacking techniques, controlling (minimizing) dislocation content within the strain-relaxed metamorphic materials, and/or minimizing its impact on top cell performance via careful device design, is key to achieving optimal performance. Materials challenges related to the heterovalent, lattice-mismatched GaP/Si interface and the thermal expansion coefficient mismatch of GaAsyP1-y to Si all complicate the production of low-defect density GaAsyP1-y materials. To this end, we have undertaken metalorganic chemical vapor deposition (MOCVD) growth studies of GaP/Si nucleation layers and GaAsyP1-y step-graded buffers, demonstrating substantial progress by reducing defect densities by over an order of magnitude in the course of this recent work. These efforts have largely been enabled via rapid feedback regarding crystalline defect populations obtained from electron channeling contrast imaging (ECCI). From these experimental studies, a deeper understanding of dislocation dynamics in these metamorphic materials is reac (open full item for complete abstract)

Committee: Tyler Grassman (Advisor); Suliman Dregia (Committee Member); Michael Mills (Committee Member); Roberto Myers (Committee Member) Subjects: Materials Science

Doctor of Philosophy, The Ohio State University, 2018, Electrical and Computer Engineering

III-V multijunction solar cells (MJSC) are capable of the highest conversion efficiencies among all solar cell classifications. These devices are thus of major interest for both terrestrial and space applications. However, the economics of the terrestrial and space markets leads to significantly different design requirements for III-V MJSCs to become more economically viable in each market. In the terrestrial market, despite their high efficiency, the high manufacturing cost of III-V MJSCs currently limits their applicability in a market that is currently dominated by crystalline silicon. Thus, lower cost III-V MJSC approaches must be developed for them to become more competitive. This intuitively leads to the concept of merging III-V MJSCs with Si solar cells to demonstrate III-V/Si MJSCs. Such an approach simultaneously takes advantage of the high conversion efficiency of III-V MJSCs and the low-cost manufacturing of Si. In the space market, III-V MJSCs are already the dominant technology due to their high efficiency, radiation hardness, and reliability in extreme conditions. However, new III-V MJSC approaches must be developed if they are to push the boundary of conversion efficiency even further. An approach to improve the efficiency and thus economic viability is through the use of additional high-performance sub-cells at optimal bandgaps to more ideally partition the solar spectrum. Although the design requirements for improving the economic viability of III-V MJSCs in the terrestrial and space markets differ drastically, the design of III-V MJSCs can be altered to meet the design requirements for both markets by using the versatile technique of III-V metamorphic epitaxy. This is the growth of relaxed (i.e. unstrained) III-V compounds at a lattice constant that differs from that of the substrate. The major advantage of III-V metamorphic epitaxy is that it provides an additional degree of freedom for III-V MJSC device design. Traditional lattice-matche (open full item for complete abstract)

Committee: Steven Ringel (Advisor); Tyler Grassman (Committee Member); Sanjay Krishna (Committee Member); Lei Cao (Committee Member) Subjects: Electrical Engineering