CdS Q-Dot-Impregnated TiO<sub>2</sub>-B Nanowire-Based Photoanodes for Efficient Photovoltaic Conversion in ‘Q-Dot Co-sensitized DSSC’
Debajyoti Das, Pronay Makal
Abstract
Cadmium sulfide (CdS) as a narrow-band-gap semiconductor has been coupled in 0–15 wt % to hydrothermally grown titanium dioxide bronze-phase (TiO2-B) nanowires (TNWs), reducing the optical band gap from 2.96 to 2.71 eV. In CdS Q-dot-impregnated TNW/CdS composite nanowires, absorption of light extends toward the visible range of the solar spectrum and effective light-harvesting improves via increased specific surface area; furthermore, the properly aligned band edges between CdS and TiO2-B ascertain efficient separation of the photogenerated charge carriers. Overall, an optimum photoanode material characteristic has been demonstrated in TNW/CdS-10 nanocomposite that achieves a typical photovoltaic (PV) conversion efficiency, η ∼ 1.63%, in Q-dot-sensitized solar cells (QDSSCs), compared to the η ∼ 0.86% of the simple electrochemical cell with a pristine TNW photoanode. N3 dye sensitization of the pristine TNW photoanode leads to a conversion efficiency of η ∼ 3.88% in dye-sensitized solar cells (DSSCs). When the CdS Q-dot-sensitized TNW/CdS-10 photoanode is further co-sensitized by the N3 dye, an enhanced PV conversion efficiency of η ∼ 8.04% is attained in the “Q-dot co-sensitized DSSC” device by virtue of the optimum light absorption facilitated by the maximum dye-loading capacity retained by the widest available surface area on the composite photoanode, contributing to a superior incident-photon-to-current conversion efficiency (IPCE) of ∼ 69.60% at 520 nm. Interfacial electron injection and the recombination dynamics of the cells studied using impedance spectroscopy demonstrate that the DSSC performance increases due to minimization of the electron–hole recombination rate and the increase of the conduction of charge carriers via reduced charge-transfer resistance of the photoanode(Q-dots/dye)/electrolyte interface at an optimum 10 wt % CdS loading. Sequential sensitization of TiO2-B nanowire photoanodes via CdS Q-dot impregnation and further by N3 dye molecule adsorption improves light-harvesting, facilitates efficient photocarrier generation, and prevents interfacial charge recombination, and their cumulative electron injection results in a better photovoltaic performance via novel “CdS Q-dots and N3 dye co-sensitization of TiO2-B NWs” in Q-dot co-sensitized DSSCs.