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GaAs0.75P0.25/Si Tandem Solar Cells: Design Strategies and Materials Innovations Enabling Rapid Efficiency Improvements

Lepkowski, Daniel Leon
http://orcid.org/0000-0001-8899-4844
http://rave.ohiolink.edu/etdc/view?acc_num=osu1616613409578001

Abstract Details

2021, Doctor of Philosophy, Ohio State University, 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 thermal expansion coefficient between III-V compounds and Si [3]–[6]. The most prevalent defects as a result of this heterogeneous integration are threading dislocations which propagate from the III-V/Si interface and extend all the way to the surface of the III-V material. These threading dislocations are detrimental to top cell performance [7]–[10] and thus overall device efficiency. This research in this dissertation explores various aspects of ideal bandgap pairing (1.1 eV/1.7 eV) GaAs0.75P0.25/Si tandem solar cells with the intent of gaining scientific understanding about the impact of crystalline defects on GaAs0.75P0.25 top cell performance, extracting maximum performance for a given TDD through defect resilient designs, and of course improving overall tandem efficiency. To understand the impact of crystalline defects on GaAs0.75P0.25 cell performance, the GaAs0.75P0.25 top cell was first coarsely optimized through model informed design changes resulting in high performance GaAs0.75P0.25 cells on a low TDD GaAs-based virtual substrate. These low TDD virtual substrates were used as an analog to the technologically relevant Si-based virtual substrate [11]. Porting this identical, semi-optimized design to the higher TDD Si-based virtual substrate allowed for the extraction of performance as a function of TDD [12]. Analytical modeling was used to properly quantify the impact of TDD on transport properties for better informed model-based design throughout the remainder of the work. To create defect resilient designs, both transport- and optics-based strategies were employed to improve performance at a given TDD. Critical aspects of recombination associated with threading dislocations were exploited by using a rear-emitter design to improve voltage [13], [14]. During this study, the critical role of the window layer properties on passivation was discovered [15]. Optically speaking, a model-based exploration, informed by experimental data, of a Ga0.64In0.36P/Al0.66In0.34P DBR was performed in order to enable a reduction of the base thickness and provide resilience against TDD-induced diffusion length shortening. This strategy shows real promise for further experimental exploration in the near future. Lastly the improvement of tandem solar cells was driven by a combination of the device design and material quality improvements discussed throughout the dissertation. These advancements raised the AM1.5G efficiency of the GaAs0.75P0.25/Si tandem solar cell from 13.3% to 23.4%, the current verified world record for this technology. Notably this work resulted in the first certified demonstration of efficiency >20% for a monolithic III V/Si cell, even outperforming the best 3-junction III-V/Si device at the time. Based on the recent reduction in TDD down to 3×106 cm-2 and defect resilient device designs on these new virtual substrates discussed herein, the near-term expectation is for efficiency of the GaAs0.75P0.25/Si tandem cell to exceed 27% in the next round of tandem devices. These devices are currently being fabricated. Finally, the realistic pathway to >30% efficient tandem cells was explored, as a function of TDD, through data informed analytical modeling. These results indicate that if TDD can be reduced to ~1×106 cm-2, then efficiencies of greater than 30% should be achievable. This is a critical mark for technology viability in an ever changing technoeconomic landscape.
Steven Ringel (Advisor)
Tyler Grassman (Advisor)
Sanjay Krishna (Committee Member)
323 p.
Electrical Engineering; Energy; Engineering
Solar Cells; II-V Si; Tandem Solar Cells; GaAsP; Semiconductor Devices

Recommended Citations

Citations

  • Lepkowski, D. L. (2021). GaAs0.75P0.25/Si Tandem Solar Cells: Design Strategies and Materials Innovations Enabling Rapid Efficiency Improvements [Doctoral dissertation, Ohio State University]. OhioLINK Electronic Theses and Dissertations Center. http://rave.ohiolink.edu/etdc/view?acc_num=osu1616613409578001

    APA Style (7th edition)

  • Lepkowski, Daniel. GaAs0.75P0.25/Si Tandem Solar Cells: Design Strategies and Materials Innovations Enabling Rapid Efficiency Improvements. 2021. Ohio State University, Doctoral dissertation. OhioLINK Electronic Theses and Dissertations Center, http://rave.ohiolink.edu/etdc/view?acc_num=osu1616613409578001.

    MLA Style (8th edition)

  • Lepkowski, Daniel. "GaAs0.75P0.25/Si Tandem Solar Cells: Design Strategies and Materials Innovations Enabling Rapid Efficiency Improvements." Doctoral dissertation, Ohio State University, 2021. http://rave.ohiolink.edu/etdc/view?acc_num=osu1616613409578001

    Chicago Manual of Style (17th edition)

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This open access ETD is published by The Ohio State University and OhioLINK.
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