A 6-dimensional grand unified theory with the compact space having the topology of a real projective plane, i.e., a 2-sphere with opposite points identified, is considered. The space is locally flat except for two conical singularities where the curvature is concentrated. One supersymmetry is preserved in the effective 4d theory. The unified gauge symmetry, for example SU(5) , is broken only by the non-trivial global topology. In contrast to the Hosotani mechanism, no adjoint Wilson-line modulus associated with this breaking appears. Since, locally, SU(5) remains a good symmetry everywhere, no UV-sensitive threshold corrections arise and SU(5)-violating local operators are forbidden. Doublettriplet splitting can be addressed in the context of a 6d N = 2 super Yang-Mills theory with gauge group SU(6). If this symmetry is first broken to SU(5) at a fixed point and then further reduced to the standard model group in the above non-local way, the two light Higgs doublets of the MSSM are predicted by the group-theoretical and geometrical structure of the model.

Low-temperature processing for high-performance solar cells based on hydrogenated microcrystalline silicon (μc-Si:H) has been developed using a conventional rf plasma-enhanced chemical vapor deposition (PECVD) technique at an excitation frequency of 13.56 MHz under a high deposition pressure condition. Among pin type solar cells, it is found that deposition temperature of i-layer at 140 °C is effective particularly for improving open circuit voltage (Voc), surprisingly without deteriorating short circuit current or fill factor. Carrier density of undoped μc-Si abruptly decreases for deposition temperatures lower than 180 °C, and the improvement of Voc is ascribed to a decrease of shunt leakage current arising from the oxygen-related donors. This implies that oxygen-related donors can be passivated at low deposition temperatures and that hydrogen plays an important role for the passivation. We propose a simple model for the hydrogen passivation of oxygen related donors. We apply this passivation technique to solar cells, and consequently a conversion efficiency of 8.9% (Voc=0.51 V, Jsc=25 mA/cm−1, FF=0.70) has been obtained in spite of an oxygen concentration of 2×1019 cm−3 in combination with device optimization such as a p-layer. Effect of deposition temperature of i-layer upon other solar cell parameter, short circuit current, and fill factor is also discussed.

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The effect of a perforated transparent electrode, which is an electrode punctuated with holes, on light transmittance and light scattering was studied using textured ZnO:Ga formed by wet etching. The perforations were found to play a role not only in improving transmittance but also in maintaining high haze factor. The influence of the surface morphology of textured ZnO:Ga on the growth of hydrogenated microcrystalline silicon and on solar cell performance was also studied. It was found that, while the ZnO:Ga surface is not steep, a height difference is present on the substrate surface at the edge of the perforations. To resolve this problem, an additional thin ZnO:Ga layer was deposited on a substrate which already had a perforated transparent electrode. Improvements in solar cell performance were obtained while high open circuit voltage and high fill factor were maintained.

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