Micro-photovoltaic materials and systems for solar energy harvesting and light management

Potter, Maggie M.

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https://hdl.handle.net/2142/113167 Copy

Description

Title

Micro-photovoltaic materials and systems for solar energy harvesting and light management

Author(s)

Potter, Maggie M.

Issue Date

2021-07-13

Director of Research (if dissertation) or Advisor (if thesis)

Nuzzo, Ralph

Doctoral Committee Chair(s)

Kenis, Paul

Committee Member(s)

• Diao, Ying
• Braun, Paul

Department of Study

Chemical & Biomolecular Engr

Discipline

Chemical Engineering

Degree Granting Institution

University of Illinois at Urbana-Champaign

Degree Name

Ph.D.

Degree Level

Dissertation

Keyword(s)

• Photovoltaics
• microcells
• electrochromic
• photoelectrochromic
• luminescent solar concentrators
• edge recombination
• silicon heterojunction

Abstract

Photovoltaics (PV) enable energy harvesting of Earth’s most abundant resource: the sun. Electricity generation using silicon-based PV results in significantly less CO2 emissions than that from fossil fuel-based sources, and moderate commercial efficiencies (15 – 20%) as well as reductions in cost at the system-level have culminated in a installed global PV capacity in excess of 500 GW. Restrictions on land use, electrical transmission and distribution losses, and grid discontinuities, however, suggest solar energy harvesting benefits from a balanced approach to expansion of both remote (i.e., solar farm) PV and on-site form factors. Transparent PV (TPV) for power generation on-site expands the reach of PV beyond the traditional building rooftop and façade applications to include electrochromic (EC) windows, low power displays, window-integrated PV, automotive-integrated PV, and multimedia mobile displays for portable consumer electronics. Specifically, silicon solar microcells (µ-cells) offer a variable degree of transparency when assembled in sparse or dense arrays on transparent substrates. This dissertation focuses on i) the design of silicon solar µ-cells for improved device efficiencies, and ii) the integration of silicon µ-cells with systems to enable transparent, flexible, and on-site energy harvesting. In this work, we first integrate silicon solar µ-cells with an EC window application to demonstrate a photoelectrochromic (PEC) device with tunable transmission throughout visible wavelengths. Here, we design and assemble an EC device stack based on WO2 as the EC layer and V2O5 as the ion storage (IS) layer, both of which are fabricated using sol-gel methods on transparent, indium tin oxide (ITO)-coated substrates. A semi-solid polymer based on polymethyl methacrylate serves as the electrolyte layer. The transparency of the EC device is dynamically controlled by the application of an external power source, and exhibits switching times on the order of state-of-the-art (8 seconds), with a modulation in transmission as high as 33%. Integration with two silicon solar µ-cells in series enables autonomous operation limited by the PV short-circuit current density (JSC) with switching times less than 3 minutes and a modulation in transmission of up to 32%. Next, we explore the synthesis and tunability of photophysical properties of polyhydrofuran, an EC polymer, based on post-polymerization modifications of polyketones. The absorption of polyhydrofuran is controlled by reaction time subsequent to addition of hydrogen bromide (HBr) gas and is correlated with the presence of chromophoric endgroups, as dictated by the molecular weight. The polyhydrofuran exhibits blue emission with a quantum yield of ca. 14%, and the fluorescence is largely quenched with precise addition of an acidic dopant. Notably, dopant addition reversibly extends absorption to wavelengths throughout the entire visible regime, and absorption intensity increases according to amount of dopant added. Future work in this field involves integrating this polyhydrofuran with an EC device stack and a silicon microcell array to enable a polymer-based PEC system. As a result of the high surface area to volume ratio inherent to the small size of silicon solar µ-cells, we next develop device structures to improve microcell device performance by introducing a new architecture of the silicon heterojunction (SHJ) variety. SHJ PV materials exploit the high-quality passivation properties of the intrinsic amorphous silicon (a-Si) layers deposited on the top and bottom surfaces of the crystalline silicon base, allowing for minimized surface recombination and high open-circuit voltage (VOC). We employ semiconductor device physics simulations to examine the current-voltage (IV) characteristic curves of square-shaped SHJ µ-cells in the dark across a range of sizes (200 µm x 200 µm x 80 µm – 1000 µm x 1000 µm x 80 µm) and edge recombination velocities (ERV). Total recombination mapping reveals that recombination currents at the device perimeter (intersection of depletion region with exposed surface) are dominant at low voltages, while recombination currents at the device edge (intersection of the quasi-neutral region with the exposed surface) are dominant at high voltages. With an understanding of the origins of recombination currents in SHJ µ-cells, we simulate performance under standard AM1.5G illumination conditions with varying device size and ERV. We further develop fabrication routes to SHJ µ-cells based on two distinct microelectronics processing methods: deep reactive ion etching (DRIE) and laser cutting. We compare measured results to simulated results before and after damage removal along the microcell edges subsequent to the dicing procedure, and report the highest microcell VOC to date for an exceptional device (588 mV, for a 400 µm × 400 µm × 80 µm size) that employs native oxide as the passivation material along the sidewalls. Strategies to further improve VOC are demonstrated through passivation of the microcell sidewalls using both atomic layer deposition (ALD, TiO2) and plasma-enhanced chemical vapor deposition [PECVD, a-Si/silicon nitride (SiNx)], achieving an increase in VOC of up to 55 mV relative to native oxide passivation. Proposed optimizations to processing conditions paired with performance predictions provide pathways to SHJ µ-cells with efficiencies surpassing 15% at the 400 µm × 400 µm × 80 µm device size. Next, the performance of a luminescent solar concentrator (LSC) embedded with a SHJ microcell array for window-based applications is modeled through semiconductor device physics simulations and Monte Carlo ray tracing. Given the large contribution of edge surface area to the total surface area of PV µ-cells, we simulate SHJ microcell performance with varying device size under the unusual illumination conditions found within the luminescent waveguide form factor, wherein the microcell edge surface is subject to significant irradiance from the luminophore. Remarkably, we find that at certain device sizes, performance parameters for illumination along microcell edges are equal to or greater than that at traditional illumination along the microcell top surface. Further improvements in performance for edge illumination conditions are achieved via the addition of an anti-reflection coating along the microcell edges and improvements in sidewall passivation quality (via decreasing ERV). We simulate the LSC system performance using near-infrared emitting quantum dots as the luminophore, varying photoluminescence quantum yield, visual transparency, and geometric gain (GG, ratio of illuminated waveguide area to illuminated solar cell area). We find such an LSC design to reach approximately 7% power conversion efficiency at unity quantum yield, 30% visual transparency, and a GG of 20. Finally, we determine the detailed balance limit of performance for the aforementioned single-junction LSC system, as well as a tandem system consisting of the single-junction system with a silicon sub-cell underneath for harvesting photons that would otherwise be transmitted. Varying each the luminophore and PV microcell bandgap, we find an optimal luminophore to cell bandgap offset of 180 nm and system efficiency limits of 27% and 36% for the single-junction and tandem systems, respectively. Notably, sandwiching with short-pass filters that exhibit perfect photoluminescence (PL) trapping via reflection is crucial to realizing system limits for the single-junction system. PL trapping requirements are eased in the case of the tandem system as a result of light harvesting by the silicon sub-cell. For the single-junction system, we also examine the influence of luminophore optical characteristic influence power conversion efficiency. We fabricate prototypes for both the single-junction and tandem LSC system; we note system performances of the single-junction and tandem devices of 8.2% (GG = 1.6) and 11.4% (GG = 1.7), respectively. Further optimization of materials could lead to increases in efficiency and performances that approach the detailed balance limits as predicted by our model.

Graduation Semester

2021-08

Type of Resource

Thesis

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Copyright 2021 Maggie Potter

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