Influence of base pressure and atmospheric contaminants on a-Si:H solar cell properties

Abstract

Articles you may be interested inHydrogenated amorphous silicon oxide containing a microcrystalline silicon phase and usage as an intermediate reflector in thin-film silicon solar cells Deposition of microcrystalline silicon prepared by hot-wire chemical-vapor deposition: The influence of the deposition parameters on the material properties and solar cell performance J. Appl. Phys. 98, 024905 (2005); 10.1063/1.1957128 Role of growth temperature and the presence of dopants in layer-by-layer plasma deposition of thin microcrystalline silicon (μc-Si:H) doped layersThe influence of atmospheric contaminants oxygen and nitrogen on the performance of thin-film hydrogenated amorphous silicon ͑a-Si: H͒ solar cells grown by plasma-enhanced chemical vapor deposition at 13.56 MHz was systematically investigated. The question is addressed as to what degree of high base pressures ͑up to 10 −4 Torr͒ are compatible with the preparation of good quality amorphous silicon based solar cells. The data show that for the intrinsic a-Si: H absorber layer exists critical oxygen and nitrogen contamination levels ͑about 2 ϫ 10 19 atoms/ cm 3 and 4 ϫ 10 18 atoms/ cm 3 , respectively͒. These levels define the minimum impurity concentration that causes a deterioration in solar cell performance. This critical concentration is found to depend little on the applied deposition regime. By enhancing, for example, the flow of process gases, a higher base pressure ͑and leak rate͒ can be tolerated before reaching the critical contamination level. The electrical properties of the corresponding films show that increasing oxygen and nitrogen contamination results in an increase in dark conductivity and photoconductivity, while activation energy and photosensitivity are decreased. These effects are attributed to nitrogen and oxygen induced donor states, which cause a shift of the Fermi level toward the conduction band and presumably deteriorate the built-in electric field in the solar cells. Higher doping efficiencies are observed for nitrogen compared to oxygen. Alloying effects ͑formation of SiO x ͒ are observed for oxygen contaminations above 10 20 atoms/ cm 3 , leading to an increase in the band gap.

Impurities in thin-film silicon: Influence on material properties and solar cell performance

“…3). These findings were in agreement with previously published results from other groups [2,4,5,14]. Differences between a-Si:H and μc-Si:H solar cells were also reported.…”

“…This is in agreement with the Hall measurements on μc-Si:H [2,15]. It was found that μc-Si:H films contaminated with oxygen and nitrogen have stronger n-type behavior compared to a-Si:H. Furthermore, for a-Si:H at very high oxygen concentration a decrease of the dark conductivity attributed to Si-O alloying was found [4]. A corresponding effect was not observed for nitrogen within the studied range (see Fig.…”

“…For 350 nm thick a-Si:H solar cells the oxygen and the nitrogen do not influence J SC (~13 mA cm − 2 ) and V OC (~900 mV) [4]. In contrast, for thicker μc-Si:H solar cells (~1.2-1.3 μm) the J SC is strongly affected by the impurities.…”

“…The effect of oxygen and nitrogen impurities on the optoelectronic properties of thin film silicon was the subject of several previous studies. An increase of dark conductivity with increasing oxygen and nitrogen was reported for films of a-Si:H [4][5][6][7][8][9] and μc-Si:H [2, [10][11][12]. Kinoshita et al [5] and Isomura et al [6] proposed that these impurities lead in a-Si:H (to a small degree) to donor-like states which shift the Fermi level towards the conduction band.…”

“…For the fabrication of a-Si:H and μc-Si:H p-i-n solar cells with high efficiency a low concentration of impurities in the intrinsic layer (i-layer) are required. Atmospheric contaminants in the deposition process can originate from air leaks, outgassing of the deposition chamber walls or from impure deposition gasses (silane and hydrogen) [1][2][3][4]. The effect of oxygen and nitrogen impurities on the optoelectronic properties of thin film silicon was the subject of several previous studies.…”