Fabrication of CuInS₂-sensitized solar cells via an improved SILAR process and its interface electron recombination

Tetragonal CuInS₂ (CIS) has been successfully deposited onto mesoporous TiO₂ films by in-sequence growth of In_xS and Cu_yS via a successive ionic layer absorption and reaction (SILAR) process and postdeposition annealing in sulfur ambiance. X-ray diffraction and Raman measurements showed that the obtained tetragonal CIS consisted of a chalcopyrite phase and Cu-Au ordering, which related with the antisite defect states. For a fixed Cu-S deposition cycle, an interface layer of β-In ₂S₃ formed at the TiO₂/CIS interface with suitable excess deposition of In-S. In the meantime, the content of the Cu-Au ordering phase decreased to a reasonable level. These facts resulted in the retardance of electron recombination in the cells, which is proposed to be dominated by electron transfer from the conduction band of TiO₂ to the unoccupied defect states in CIS via exponentially distributed surface states. As a result, a relatively high efficiency of ∼0.92% (V_oc = 0.35 V, J_sc = 8.49 mA cm^-2, and FF = 0.31) has been obtained. Last, but not least, with an overloading of the sensitizers, a decrease in the interface area between the sensitized TiO₂ and electrolytes resulted in deceleration of hole extraction from CIS to the electrolytes, leading to a decrease in the fill factor of the solar cells. It is indicated that the unoccupied states in CIS with energy levels below E _F0 of the TiO₂ films play an important role in the interface electron recombination at low potentials and has a great influence on the fill factor of the solar cells. © 2013 American Chemical Society.