Processing and Performance of Polyaniline/Multi-walled Carbon Nanotubes / Graphene Composites as Counter Electrodes for Dye-sensitized Solar Cells

This paper is mainly concerned with the replacement of platinum with polyaniline (PANi), graphene and multi-walled carbon nanotube (MWCNT) composite films used as a catalysis layer in counter electrode of dye-sensitized solar cells (DSSC). First, we used aniline sulfate solution as an efficient dispersing agent to debundle MWCNTs and to avoid graphenes aggregated. The composite films were grown on fluorine-doped tin oxide (FTO) and glass substrates by using electro-chemical deposition and chemical/electro-chemical deposition respectively. From the results of four-point probe and thermo-gravimetric analysis, the surface resistance and the weight loss percentage of the films were decreased with the addition of graphenes and MWCNTs. Accordingly, the PANi/Graphene/MWCNTs composite films were successfully fabricated by electro-chemical deposition and chemical/electro-chemical deposition.
In the first part of this research, the PANi film were grown on the FTO-coated glass as counter electrodes of DSSCs by using electro-chemical deposition. The power conversion efficiency of as-fabricated DSSC was 5.55±0.05 % .When adding the proper amount of graphenes and MWCNTs (aniline/graphene=1/0.0045；aniline/MWCNTs=1/0.0045 ) , the power conversion efficiency were raised to 7.29±0.08% and 7.21±0.08% . The highest power conversion efficiency was 7.67±0.05%, when the weight ratio of aniline/graphene/MWCNT was 1/0.0030/0.0045. In addition , the addition of graphene and MWCNTs could enhance the Jsc of DSSC from 15.48 to 18.21 mA/cm2.
In the second part of this research, the PANi film was grown on the glass slide as a counter electrode of DSSC by using chemical/electro-chemical deposition. The highest power conversion efficiency of as-fabricated DSSC was 0.58±0.02%. When adding the proper amount of graphenes and MWCNTs (aniline/graphene=1/0.0060；aniline/graphene=1/0.0060) , the power conversion efficiency were raised to 2.04±0.08% and 2.03±0.07%. The highest power conversion efficiency was 3.58±0.06%, when the weight ratio of aniline/graphene/MWCNTs was 1/0.0045/0.0060. In addition , the addition of graphene and MWCNTs could enhance the Jsc of DSSC from 3.64±0.06 to 9.38±0.07 mA/cm2.
From the result of cyclic voltammetry (CV) analysis, the PANi/Graphene, PANi/MWCNTs, and PANi/Graphene/MWCNTs composite films compared to neat PANi have higher catalysis of converting tri-iodide (I3－) to iodide (I－) due to their higher conductivity and higher redox current density. The increase of redox current density resulted in higher Jsc, higher power conversion efficiency, and better charge collection efficiency of DSSCs. In addition, the open-circuit voltage of DSSCs was higher with the PANi/Graphene/MWCNTs counter electrode because of the higher reduction potential for I－/ I3－ redox couples.