Role of Sio at TCO/P Interface on the Electrical Properties of the P/I Junction
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ROLE OF SIO AT TCO/P INTERFACE ON THE ELECTRICAL PROPERTIES OF THE P/I JUNCTION C. CARVALHO, J. M. M. DE NIJS*, R. MARTINS AND L. GUIMARAES Faculty of Science and Technology of New University of Lisbon and Centre of Molecular Physics of Lisbon Universities (INIC), Quinta da Torre, 2825 Monte da Caparica, Portugal. *Faculty of Physics, Eindhoven University of Technology, 5600 MB Eindhoven, The Netherlands. ABSTRACT The reduction of the ITO (Indium Tin Oxide) by the deposition of an a-Si:C:H p layer by means of a Plasma Enhanced Chemical Vapour Deposition (PECVD), is considerably diminished if a thin silicon monoxide layer (estimated to be between 5-50A thick) is applied as a diffusion barrier. The amount of reduced indium diminishes (its concentration is two times lower), while the amount of silicon oxide is less although silicon monoxide was added on purpose. The influence of the silicon monoxide layer on the electrical properties of the p/i junction is shown by its J-V characteristics (in the dark and under illumination). We can see that with a suitable silicon monoxide thickness we can improve the short circuit current density (Jsc) and the rectifying ratio. INTRODUCTION Transparent Conductive Oxides (TCO) like tin oxide, Sn02 and ITO are the more commonly used as contact in optoelectronic devices, such as solar cells, electroluminescence diodes, etc, based on the amorphous silicon technology. In most of these applications, the a-Si:H film is grown by plasma processes. Previous studies (1,2] have shown a degradation of the TCO's due to the reduction of the oxide films, leading to a decrease of the optical transparency of the contact and the electrical properties of the device are deteriorated due to the percolation of the metallic atoms in the bulk of the a-Si:H film. In this paper we study the role of the reduction ITO in the electrical properties of a p/i junction to be used in solar cells, by using a thin SiO layer between the ITO and the subsequently silicon p layer. The SiO layer will act as a remedy for the reduction [3] as the deterioration of the solar cell is mainly attributed to the reduced In and Sn [4,5] and not to the silicon oxides formed at the ITO/a-Si:H interface due to plasma deposition process [2,5]. Besides, the SiO in comparison with In203 and Sn02, is expected to withstand better the reduction caused by the deposition of the p layer. We also emphasize the importance of the SiO layer on the behaviour of the J-V characteristics of the p/i junctions, comparing junctions with and without the SiO layer. EXPERIMENTAL
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The ITO was deposited by reactive thermal evaporation and details concerning the deposition process are described elsewhere [6]. The silicon monoxide layer (SiO) was deposited in the same chamber by thermal evaporation of the SiO powder. The substrates were placed on a metallic mask to prevent half of the substrate Mat. Res. Soc. Symp. Proc. Vol. 219. @1991 Materials Research Society
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area to receive the SiO deposit. This allowed us to simultaneously study the influenc
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