Solution-processible carbon nanotubes-based thin films as counter electrodes in dye sensitized solar cells.
Date of Issue2013
School of Materials Science and Engineering
Platinum (Pt) and fluorine-doped tin oxide (FTO) are commonly used as counter electrodes (CEs) in dye sensitized solar cells (DSSC), but the high processing temperature of Pt and brittle behavior of FTO makes them unsuitable for application in flexible, bifacial DSSC. Carbon nanotubes (CNT) is a possible alternative as Pt- and/or FTO-free CEs, as the conducting and catalytic properties can be tuned by altering its physical aspects (e.g. tube length, surface defects, functionalization). Thin pristine single-walled CNT films (with 3-5% surface carboxylate side groups) of similar optical transmittance (80-85% @ 550 nm) to Pt/FTO gave comparable DSSC performance only when illuminated under low light intensity (1% Sun), as high CE-to-electrolyte charge transfer resistance (RCT) caused charge recombination to dominate with brighter illumination intensities. To improve RCT and sheet resistances (RS), chemical modification by nitric acid (HNO3) was carried out. RS was reduced, resulting in an increase in the short circuit current density (JSC) of the DSSCs when both FTO and Pt were replaced. However, fill factors (FF) remained low and the VOC increment phenomenon persisted. This indicated that the decrease in RS from HNO3 treatment was not sufficient for thin CNT films as CE. Hence, surface modification of CNT was explored with the formation of Au or Pt nanoparticle clusters on CNT surfaces by spontaneous electroless chemical bath deposition. Au exhibited charge transfer mechanism with CNT and increased the work function from 4.85 eV to 5.12 eV. However, poor Au precursor wetting led to the formation of sparse dendritic Au clusters of up to 150 nm in diameter. This decreased Au-CNT system’s potential to create charge transfer complexes with CNT to improve DSSC performance. Both RS and RCT were found to decrease leading to a small decrease in series resistance of the DSSC and an increase in JSC, improving efficiency from 0.22% to 0.25% under 100% Sun illumination. In the case of Pt decoration, Pt properties were preserved as indicated by the negligible changes in RCT values and the close proximity of oxidation peak onsets of Pt-CNT and pristine Pt from cyclic voltammetry measurements. JSC was most significantly improved (13.7 mA/cm2 to 14.2 mA/cm2 with 100% Sun, and 147 μA/cm2 to 158 μA/cm2 with 1% Sun), giving promising DSSC efficiencies of 3.8% and 5.6% respectively. CoS or NiS were also investigated as Pt alternatives. Unlike NiS, CoS is semiconducting, but better wetting led to a more homogeneous deposition on CNT and a higher DSSC efficiency. The excellent catalytic properties of both sulfides further increased JSC, but FF were low due to limited contact area between the nanoparticles and CNT surfaces. By altering the synthesis method to preform CoS-CNT nanocomposite, at low concentration, both FF and optical transmittance were improved and DSSC efficiency reached 5.78% when illuminated at 100% Sun from the photo anode side, and 2.22% from the CE side. Incident light on both sides of the DSSC may thus be utilised in transparent photovoltaic applications such as power-producing windows.
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