Facile Charge Propagation in CdS Quantum Dot Cells

A photoactive electrode of cadmium sulfide (CdS) quantum dots and fullerene (C-60) nanowhiskers (NW), free of a wide-gap semiconducting oxide support, has been fabricated for the first time, by growing CdS quantum dots using the successive ionic layer adsorption and reaction (SILAR) method over a layer of C-60 nanowhiskers. Enhanced excited electron injection efficiency from the CdS quantum dots to C-60 nanowhiskers was ascertained on the basis of maximum fluorescence quenching and shortest emission decay lifetimes in the CdS/C-60 (NW) electrode compared to conventional CdS/C-60 and CdS electrodes. Conducting atomic force microscope (C-AFM) revealed the larger nanoscale electronic conductivity for the CdS/C-60 (NW) electrode relative to CdS/C-60 or neat CdS electrodes. Kelvin probe force microscopy (KPFM) furnished an insight into how the downshift of the quasi Fermi level toward more positive potentials in the C-60 nanowhiskers as compared to neat C-60, is capable of providing an additional driving force for rapid electron transport within the photoanode. Photoelectrochemical cells based on the CdS/C-60 (NW) and CdS/C-60 electrodes were formed by employing a thin film of carboxylate functionalized multiwalled carbon nanotubes (MWCNTs)/poly(dimethyldiallylammonium chloride) (PDDA) as the counter electrode. Rapid electron transport and high effective surface area of the C-60 nanowhiskers manifested in higher photocurrents, photovoltage, and incident photon to current conversion efficiency (IPCE) for the cell based on the CdS/C-60 (NW) electrode. The advantage of using MWCNT/PDDA electrode as the counter electrode was realized in terms of an overall enhancement in short circuit current (J(SC)), open-circuit voltage (V-OC), and IPCE attained for the CdS/C-60 (NW)-MWCNT/PDDA cell as opposed to cells based on platinum (Pt) as the counter electrode. Our method of combining CdS quantum dots with C-60 nanowhiskers to yield an electrode that is superior to C-60-based traditional electrodes on all counts is easily applicable to other visible light absorbing quantum dots and thus opens up exciting possibilities for a plethora of yet unexplored donor-acceptor architectures for high performance photoelectrochemical solar cells.