Core-shell PbS/Sn:In₂O₃ and branched PbIn₂S₄/Sn:In₂O₃ nanowires in quantum dot sensitized solar cells

Core-shell PbS/Sn:In₂O₃ and branched PbIn₂S₄/Sn:In₂O₃ nanowires have been obtained via the deposition of Pb over Sn:In₂O₃ nanowires and post growth processing under H₂S between 100 °C-200 °C and 300 °C-500 °C respectively. The PbS/Sn:In₂O₃ nanowires have diameters of 50-250 nm and consist of cubic PbS and In₂O₃ while the PbIn₂S₄/Sn:In₂O₃ nanowires consist of PbIn₂S₄ branches with diameters of 10-30 nm and an orthorhombic crystal structure. We discuss the growth mechanisms and also show that the density of electrons in the n-type Sn:In₂O₃ core is strongly dependent on the thickness of the p-type PbS shell, which must be smaller than 30 nm to prevent core depletion, via the self-consistent solution of the Poisson-Schrödinger equations in the effective mass approximation. The PbS/Sn:In₂O₃ and PbIn₂S₄/Sn:In₂O₃ nanowire networks had resistances of 100-200 Ω due to the large carrier densities and exhibited defect related photoluminescence at 2.2 eV and 1.5 eV respectively. We show that PbS in contact with polysulfide electrolyte has ohmic like behavior but the PbS/Sn:In₂O₃ nanowires gave, rectifying current voltage characteristics as a counter electrode in a quantum dot sensitized solar cell using a conventional ITO/TiO₂/CdS/CdSe photo anode, an open circuit voltage of ≈0.5 V, and short circuit current density of ≈1 mA cm^-2. In contrast the branched PbIn₂S₄/Sn:In₂O₃ nanowires exhibited a higher current carrying capability of ≈7 mA cm^-2 and higher power conversion efficiency of ≈2%.