The thickness-dependent optical constants and annealed phase transitions of atomic-layer-deposited ZnO ultrathin films with a thickness of less than 50 nm have been demonstrated by spectroscopic ellipsometry. The thickness dependence of refractive index and extinction coefficient was discussed, and the mechanisms were given in the molecule level based on previous reports. Furthermore, the optical properties of ZnO ultrathin films varied with annealing temperatures, and the phase transition was found at high annealing temperature. The thickness of the ultrathin films decreased obviously, and the refractive index of the ultrathin films changed a lot after annealing at high temperature while Zn2SiO4 formed at a temperature above 800 °C. The low phase transition temperature of Zn2SiO4 may be due to the ultrathin scale effect. What’s more, photoluminescence spectra showed the annealing effect on ultrathin films and the enhanced defects luminescence were observed. We believe that these investigations will help improved understanding of essential physical chemistry and optoelectronic devices based on ultrathin oxide films for optical and photoelectric applications.

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Fill factors (FFs) of ~0.87 have been obtained for crystalline Si (c-Si) solar cells based on Ag front contacts after rapid thermal annealing. The usual single PN junction model fails to explain the high FF result. A metal/oxide/semiconductor (MOS) junction at the emitter is found to be inversely connected to the PN one, and when its barrier height/e is close to the open-circuit voltage of the solar cell, very high FF is obtainable. In this work, although the open-circuit voltage (<580 mV) is not high here, the efficiency of c-Si solar cell still reaches the state-of-the-art value (>20 %) due to the high FF achieved.

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