Date of Award

Fall 2003

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Materials Science and Engineering - (Ph.D.)

Department

Committee for the Interdisciplinary Program in Materials Science and Engineering

First Advisor

Roland A. Levy

Second Advisor

N. M. Ravindra

Third Advisor

Marek Sosnowski

Fourth Advisor

Trevor Tyson

Fifth Advisor

Yuan-Min Li

Abstract

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.

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