Impurities and Metastable Centers in Amorphous Silicon Solar Cells
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IMPURITIES AND METASTABLE CENTERS IN AMORPHOUS SILICON SOLAR CELLS D.E. Carlson Solarex Thin Film Division,
826 Newtown-Yardley Road,
Newtown,
PA 18940 USA
ABSTRACT Amorphous silicon solar cells are adversely affected by impurities through the creation of traps, recombination centers and metastable centers. The microstructure of discharged-produced, hydrogenated amorphous silicon (a-Si:H) appears to be strongly affected by the presence of impurities in the discharge atmosphere. A model is developed in which impurities create microvoids in a-Si:H, and traps, recombination centers and metastable centers are associated with the internal surfaces of the microvoids. In this model, hydrogen plays an important role in determining the electronic activity and diffusivity of impurities, and metastable centers are created by the trapping of holes near microvoids and the induced motion of hydrogen on the internal surfaces of the microvoids. INTRODUCTION There is considerable experimental evidence that impurities such as oxygen create traps [1,2], recombination centers [1-3] and metastable centers [4,5] in a-Si:H. These defects degrade the performance of a-Si:H solar cells by decreasing the diffusion length and distorting the internal field distribution [2]. Other impurities such as carbon, nitrogen and chlorine have also been shown to decrease the performance of a-Si:H solar cells [5,6,7]. In addition, there is evidence that light-induced degradation of a-Si:H solar cells is enhanced by the presence of oxygen, carbon and nitrogen in concentrations greater than about 10+ 2 0cm- 3 1[6,7,8]. In this paper, we develop a model for impurity incorporation that can explain many of the properties of a-Si:H films contaminated with impurities during film growth. The discussion is centered on the growth of a-Si:H from a glow discharge in silane but many of the arguments can be extended to other growth processes. INCORPORATION OF IMPURITIES DURING GROWTH A silane discharge atmosphere is likely to contain impurity molecules such as H2 0, 02, N2 , COs C02 , CH4 and other hydrocarbons. The discharge will create new species such as disiloxane ((SiH 3 ) 2 0), NH3, SiCH6 , etc. as well as radicals such as CH3 , NH2 , SiH 3 , SiH 2 , etc. One common factor is that most of the species impinging on the growing surface will be complexed with hydrogen. Another aspect of film growth from a silane discharge is that the surface kinetics are dominated by dehydrogenation reactions that are thermally activated. This surface dehydrogenation creates dangling bonds that react with radicals impinging on the surface to form chemical bonds. Whenever hydrogenated impurities are incorporated into a-Si:H films, microvoids will be formed if these impurity-hydrogen complexes do not readily dehydrogenate at the growing surface. If the impurity is not complexed with hydrogen or if the impurity-hydrogen bond is weak, then the impurity may be incorporated into a substitutional or valence-accommodating site. As a specific example consider the case of carbon incorporat
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