Fabrication of CdS quantum dot sensitized solar cells using nitrogen functionalized CNTs/TiO2 nanocomposites

Abstract

Effect of using nitrogen functionalized carbon nanotubes in photoanode materials of quantum dot sensitized solar cells (QDSSCs) was studied. Several SWCNT/TiO2, MWCNT/TiO2 and N-doped MWCNT/TiO2 nanocomposites were prepared including different weight percents (0.25–0.40wt%) of CNTs to TiO2. The nanocomposites were applied as the photoanodes of QDSSCs. All photoanodes were fabricated by CdS and ZnS doping using successive ionic layer adsorption and reaction (SILAR) method. The measured optical band gaps of all photoanodes were approximately equal to 2.5eV. The photoluminescence (PL) spectra of all photoanode nanocomposites illustrated four maxima at around 310, 365, 440 and 535nm plus two shoulders near 285 and 720nm which could be assigned to the emission bands of TiO2, CdS and ZnS nanoparticles (NPs) as well as CNTs. The highest power conversion efficiency (η) was measured under one illumination of sun (AM 1.5, 100mW/cm2) for the 0.35% N-MWCNT loaded cell (1.78%) indicating 24.47% increase in the η value relative to that of the cell made up of bare TiO2 (1.43%). The IPCE of 0.35% N-MWCNT was the greatest (~54% near 450nm) among other cells containing 0.35% of SWCNT and MWCNT indicating it can convert the most incident light into electrical energy due to the most loading of CdS quantum dots on the N-MWCNT. The electrochemical impedance spectroscopy (EIS) evidenced that the Rs of the photoanode including 0.35%N-MWCNT (23.78Ω) was smaller than that of the blank photoanode (32.28Ω) and this was due to the increased charge transfer in the nanocomposite photoanode. Besides, a greater recombination resistance (R2) was measured for the 0.35% N-MWCNT photoanode (494.2Ω) compared with that of the bare TiO2 photoelectrode (206.8Ω).