Browsing by Subject "semiconductor device"

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    Copper zinc tin sulfide (Cu2ZnSnS4) photovoltaic material development and thin film solar cells
    (2016-03) Zhang, Liyuan
    Copper zinc tin sulfide (Cu2ZnSnS4, or CZTS) is emerging as an alternative light absorbing material to the present thin film solar cell technologies such as Cu(In,Ga)Se2 and CdTe. All the elements in CZTS are abundant, environmentally benign, and inexpensive. In addition, CZTS has a band gap of ~1.5 eV, the ideal value for converting the maximum amount of energy from the solar spectrum into electricity. CZTS has a high absorption coefficient (>104 cm-1 in the visible region of the electromagnetic spectrum) and only a few micron thick layer of CZTS can absorb all the photons with energies above its band gap. A two-stage process of CZTS thin film synthesis is presented, which consists of sequential thermal evaporation of copper, tin and zinc layers followed by a heat treatment in the presence of sulfur vapor (sulfurization) in a sealed quartz ampoule. The metal precursor stacking order, deposition rate and thickness of each metal layer can be adjusted to give uniform metal precursor stacks of controlled morphology and composition. The effects of sulfurization temperature, time, substrate material, metal precursor stacking order, and back contact layer on the morphological and structural properties of the CZTS films are investigated. Observations of grain size changes and compositional modification are made and explained in terms of the likely secondary phases present. CZTS thin film solar cells were fabricated and the effects of chemical composition were studied both on the absorber layer properties and on the final solar cell performance. It is confirmed that CZTS thin film chemical composition affects the carrier concentration profile, which then influences the solar cell properties. Only a small deviation from the optimal chemical composition can drop device performance to a lower level, which confirms that the CZTS solar cells with high conversion efficiency existed in a relatively narrow composition region. Besides CZTS absorber chemical composition study, post deposition rapid thermal annealing (RTA) was conducted and its influence on solar cell performance was studied. It is observed that post deposition RTA would lead to an increase of device performance. Through C-V measurement results, we have shown that post RTA of CZTS solar cell affects the CZTS/CdS interfacial defect concentration and zero bias depletion depth, which means the defect-related charge at CZTS/CdS interface reduces and it improves Voc and the fill factor.