I. Optimization of interdigitated back contact silicon hetero-junction (IBC-SHJ) solar cell fabrication process; II. Passive tuning of optical couplers
| Author(s) | Nsofor, Ugochukwu J. | |
| Date Accessioned | 2019-06-06T12:03:39Z | |
| Date Available | 2019-06-06T12:03:39Z | |
| Publication Date | 2019 | |
| SWORD Update | 2019-03-19T16:04:36Z | |
| Abstract | The commercial dominance of Si solar cells has been driven by progress in their conversion efficiencies coupled with their long-term durability and widespread knowledge-base for making standardized devices. Recent record efficiency Si solar cells have utilized a relatively new device architecture incorporating Si heterojunction (SHJ) – due to their remarkable high open circuit voltage (Voc) and low thermal budget. The focus of my research involves the optimization of SHJ fabrication processing to improve conversion efficiency and application to another high-efficiency concept – the interdigitated back contact (IBC) structure. ☐ SHJ cells, unlike conventional Si cells made from diffused junctions, have their junctions formed by growing thin-doped and intrinsic amorphous silicon (i.a-Si:H) layers on the silicon surface. Thus, the i.a-Si:H/c-Si interface is an integral part of the junction, which places great emphasis on the purity of the wafer surface. Excellent surface passivation is contingent on a well-prepared surface free from organic contaminants, particles and metallic ions, thus enabling low surface recombination and high open circuit voltage (VOC). This work began with a systematic study on the optimal conditions for silicon surface preparation to ensure excellent passivation at the a-Si:H/c-Si interface of SHJ solar cells. X-ray photoelectron spectroscopy (XPS) was utilized to analyze the elemental composition of impurities on the wafer surface. Passivation quality, characterized by effective minority carrier life time (τeff) and implied Voc (iVOC), was determined using either quinhydrone-methanol solution or ~10 nm of i-a-Si:H deposited using DC plasma process. This study outlines the critical steps in the cleaning process, supporting a wafer cleaning approach that is concise and repeatable while creating minimal chemical waste. ☐ Excellent passivation at the i.a-Si:H/c-Si interface was achieved using bilayer deposition of i-a-Si:H films with post deposition hydrogen plasma treatment (HPT). Correlation between the hydrogen plasma composition and passivation quality was studied using optical emission spectroscopy (OES) during the PECVD process. Fourier transform infrared (FTIR) and Variable angle spectroscopic ellipsometry (VASE), both non-invasive techniques, were used to characterize the ~10 nm films to evaluate the Si-H bonding and optical properties. Development of bilayer deposition of i-a-Si:H with HPT has led to state-of-the-art iVoc exceeding 750 mV. Temperature-dependent conductivity measurements of several p- and n-doped layers were studied. Deposition conditions at low plasma voltage were found to have a better combination of microstructure parameters, conductivities and ΔVoc. Improvements from these layers were transferred to Front HJ and IBC-SHJ structures with conversion efficiencies up to 20% and 15.1 % respectively. Challenges and technical difficulties with the IBC-SHJ structures were also highlighted and possible remedies were proffered. ☐ As an additional research focus, my prior work on the design and fabrication of a dual output modulator for balance detection systems is presented and results of developing a novel method to passively tune the splitting ratio of a Ti-diffused lithium niobate (LiNbO3) 3-dB directional coupler by the addition of a silicon-rich nitride cladding material is highlighted. The sensitivity to slight variations in the fabrication process conditions usually presents a challenge to achieving precise and repeatable 3-dB splitting in a directional coupler made of Ti-diffused LiNbO3 waveguides. In the Mach-Zehnder Interferometer (MZI) modulation configuration used in this work, this deviation in splitting ratio results in poor extinction ratio and low modulation depth. To mitigate this problem, different techniques that involve changing the effective index of the two waveguides were explored and a new approach involving post-fab trimming was developed. | en_US |
| Advisor | Hegedus, Steven S. | |
| Degree | Ph.D. | |
| Department | University of Delaware, Department of Electrical and Computer Engineering | |
| DOI | https://doi.org/10.58088/k824-aq51 | |
| Unique Identifier | 1103709148 | |
| URL | http://udspace.udel.edu/handle/19716/24231 | |
| Language | en | |
| Publisher | University of Delaware | en_US |
| URI | https://search.proquest.com/docview/2202858202?accountid=10457 | |
| Title | I. Optimization of interdigitated back contact silicon hetero-junction (IBC-SHJ) solar cell fabrication process; II. Passive tuning of optical couplers | en_US |
| Type | Thesis | en_US |