Development of highly conductive n-type μc-Si:H films at low power for device applications

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Abstract
Highly conductive phosphorus‐doped n‐type hydrogenated microcrystalline silicon (μc‐Si:H) films have been prepared by the usual (13.56 MHz) radio‐frequency glow discharge of silane (SiH4), phosphine (PH3), and hydrogen (H2) in an ultrahigh‐vacuum deposition system. The highest conductivity of the films obtained in this study is 100 S cm−1 after optimizing the hydrogen dilution ratio, chamber pressure, substrate temperature, and doping concentration of phosphorus. The formation of microcrystallinity in the material has been studied by transmission electron microscopy, x‐ray‐diffraction studies, and Raman spectroscopy. The volume fraction of microcrystallinity in these amorphous‐microcrystalline mixed‐phase materials has been estimated from Raman spectra. Sizes of the crystallites and volume fraction of microcrystallinity vary with hydrogen dilution, chamber pressure, and substrate temperature. The variations in the properties with deposition parameters have been explained in terms of the growth kinetics. The n‐type μc‐Si:H thin film, thus developed, has been applied in the first cell of a double‐junction amorphous silicon solar cell. The prepared p‐i‐n–p‐i‐n stacked cell employing the n‐type μc‐Si:H film has exhibited appreciable improvement in open‐circuit voltage, fill factor, and efficiency compared to the one with amorphous n layer in the inner n–p contact. Degradation of the cells prepared with and without μc‐n layer has been studied.