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

Doctor of Philosophy, The Ohio State University, 2004, Electrical Engineering

The epitaxial integration of III-V semiconductors with Si is of interest for photovoltaics since Si substrates offer a lighter, stronger, and cost effective platform for device production. By using compositionally step-graded SiGe layers to 100% Ge, the 4% lattice-mismatch between Si and GaAs and In 0.49 Ga 0.51 P is accommodated; this method has produced record low threading dislocation densities (TDD) of 1x10 6 cm -2 in fully relaxed the Ge/SiGe/Si (SiGe) substrates. In this dissertation, this method of III-V/Si integration is used for the development of GaAs and In 0.49 Ga 0.51 P single junction (SJ) solar cells and In 0.49 Ga 0.51 P /GaAs dual junction (DJ) solar cells, integrated on a Si platform. As such, we report that the minority carrier electron lifetime in p-type GaAs grown on Si is lower than that of holes in n-type GaAs at a given TDD and is a consequence of the higher mobility of electrons. This lower lifetime produced higher reverse saturation currents and lower open-circuit voltages for n+/p compared to p+/n configuration GaAs cells grown on SiGe with the same TDD. The higher performance of the p+/n GaAs/Si cell, by virtue of its higher open-circuit voltage, has demonstrated a record terrestrial efficiency of 18.1% and has been produced in areas up to 4 cm 2 with no degradation in cell performance. In 0.49 Ga 0.51 P SJ cells were integrated on Si substrates and an increase in depletion region recombination component of the reverse saturation current with TDD was also measured. A p+/n polarity preference for In 0.49 Ga 0.51 P on Si was demonstrated, although, the lower mobility of both carriers in In 0.49 Ga 0.51 P compared to GaAs, suggests a greater TDD tolerance. Based on these SJ results, the first realization of an In 0.49 Ga 0.51 P/GaAs DJ cell on Si with an output voltage greater than 2 V was demonstrated. A comparison with an identical DJ cell on GaAs found that the DJ cell on Si retained 91% of open-circuit voltage and 99% of short-circuit cu (open full item for complete abstract)

Committee: Steven Ringel (Advisor) Subjects: Physics, Condensed Matter