Date of Award
Doctor of Philosophy in Materials Science and Engineering - (Ph.D.)
Committee for the Interdisciplinary Program in Materials Science and Engineering
Roland A. Levy
N. M. Ravindra
In this study, single junction p-i-n μc-Si:H solar cells were prepared using plasma of silane diluted by hydrogen in a low-cost, single chamber, non-load-locked RF-PECVD system. Direct structural characterization of μc-Si:H solar cells, rather than stand-alone films, was conducted using Raman Spectroscopy, XRD, and AFM. Strong correlations among device deposition, i-layer structural properties, and device performance have been established. With such correlations, critical issues in fabricating low-cost, large-scale, high performance μc-Si:H solar cells were identified.
The critical importance of seeding processes in determining the microstructure of μc-Si:H i-layers and performance of μc-Si:H solar cells has been demonstrated. Using p-layer seeding methods, stable conversion efficiencies of 5% have been achieved using very simple device configuration. Micro-crystallinity obtained from Raman scattering, presented as Ic/Ia, proved to be sensitive to the microstructure of μc-Si:H i-layers. Strong spatial non-uniformity of i-layer microstructure as well as variations in device performance were observed. A wide variety of i-layer microstructures, from mixed-phase Si:H to highly crystalline μc-Si:H, were revealed by Raman scattering. Generally, solar cells with mixed-phase Si:H i-layers exhibit high open circuit voltages, low fill factors, low efficiencies, and severe light-induced degradation. On the other hand, solar cells with truly μc-Si:H i-layers show low open circuit voltages, high fill factors, high efficiencies, and excellent stability against light-induced degradation. It was shown by XRD experiments that high performance, optimum μc-Si:H solar cells exhibit smaller grain sizes compared to solar cells with i-layers showing higher micro-crystallinity. Correlations among non-uniformity pattern, i-layer micro-crystallinity, and AFM surface morphologies were also observed.
Solar cells with truly μc-Si:H i-layers exhibit excellent stability under both conventional and accelerated light soaking. Mixed-phase Si:H solar cells show much worse stability against light exposure. However, it has been demonstrated that stable, high performance μc-Si:H solar cells can only be obtained with i-layers being μc-Si:H, yet close to the μc-Si:H to mixed-phase Si:H transition edge where an optimum microcrystallinity range (Ic/Ia at around 1.8) was identified. These optimum μc-Si:H solar cells exhibit moderate open circuit voltages at 0.5 V, high fill factors, high efficiencies, and excellent stability against light-induced degradation. Such optimum μc-Si:H i-layers demand a very narrow optimum processing window.
Li, Liwei, "Fabrication and characterization of microcrystalline silicon solar cells" (2004). Dissertations. 607.