Structural Properties of Microcrystalline Si Solar Cells
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STRUCTURAL PROPERTIES OF MICROCRYSTALLINE SI SOLAR CELLS M. Luysberg, C. Scholten, L. Houben, R. Carius*, F. Finger*, O. Vetterl*, Institut für Festkörperforschung, Forschungszentrum Jülich, 52425 Jülich, Germany Institut für Photovoltaik, Forschungszentrum Jülich, 52425 Jülich, Germany
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ABSTRACT The structural properties of nip-µc-Si:H solar cells are investigated by transmission electron microscopy, X-ray diffraction and Raman spectroscopy. Different structural compositions are obtained by variation of the gas mixture during preparation by plasma enhanced chemical vapour deposition. Nucleation and growth of the n-layer onto textured TCO substrate was found to be similar to the growth on glass substrates. The growth of the i-layer follows a local epitaxy. This implies that the structure of the n-layer is of special importance regarding the control of the microstructure in microcrystalline Si nip solar cells. INTRODUCTION Meanwhile microcrystalline silicon (µc-Si:H) is successfully applied as contact and absorber layer in high efficiency thin film solar cells [1-4]. The electrical and optical properties significantly change as the crystalline volume fraction is varied from highly crystalline to amorphous. Structuralwise this transition is characterized by a decrease in size of crystalline grains, forming columnar structures [5]. In general, each column can be considered as a single crystal with a high density of structural defects, predominantly twins. Thus, all of the grain boundaries within a single column are coherent, whereas incoherent grain boundaries separate the individual columns. Differences in the microstructure are reflected in the solar cell performance. As an example, the silane concentration SC (gas flow ratio [SiH4]/[SiH4+H2]), one of the most important parameter to control the structure of the material in plasma enhanced chemical vapor deposition (PECVD) [5,6], has an optimum near to the transition to the amorphous growth regime [4]. In this paper the microstructure of nip solar cells is investigated by transmission electron microscopy, X-ray diffraction, and Raman spectroscopy. In contrast to our previous studies on µc-Si:H layers, solar cells contain several interfaces, such as the n-i interface or the ZnO-n interface. Special emphasis is put on the nucleation and growth of each individual layer within the solar cell. EXPERIMENT µc-Si:H nip solar cells were deposited onto texture etched ZnO back reflectors [4]. The individual microcrystalline Si layers were prepared from SiH4/H2 mixtures at different concentrations SC ([SiH4]/[SiH4+H2]) with PECVD at 95 MHz, a temperature of 200 °C, a pressure of 0.3 Torr and a discharge power of 5-30 W, yielding deposition rates of 2-5 Å/s. Doping was achieved by gas admixture of PH3 for n-type and B(CH3)3 for ptype layers. The doped layers were always deposited at a SC of about 2%, in order to
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Figure 1: Raman Spectra of three nip solar cells deposited at (a) 5%SC 5 W (b) 6.5% SC 30 W (c) 7% SC 20W recorded at the differen
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