Self‐Assembled Graphene/MWCNT Bilayers as Platinum‐Free Counter Electrode in Dye‐Sensitized Solar Cells

We describe the preparation and properties of bilayers of graphene‐ and multi‐walled carbon nanotubes (MWCNTs) as an alternative to conventionally used platinum‐based counter electrode for dye‐sensitized solar cells (DSSC). The counter electrodes were prepared by a simple and easy‐to‐implement double self‐assembly process. The preparation allows for controlling the surface roughness of electrode in a layer‐by‐layer deposition. Annealing under N2 atmosphere improves the electrode's conductivity and the catalytic activity of graphene and MWCNTs to reduce the I3− species within the electrolyte of the DSSC. The performance of different counter‐electrodes is compared for ZnO photoanode‐based DSSCs. Bilayer electrodes show higher power conversion efficiencies than monolayer graphene electrodes or monolayer MWCNTs electrodes. The bilayer graphene (bottom)/MWCNTs (top) counter electrode‐based DSSC exhibits a maximum power conversion efficiency of 4.1 % exceeding the efficiency of a reference DSSC with a thin film platinum counter electrode (efficiency of 3.4 %). In addition, the double self‐assembled counter electrodes are mechanically stable, which enables their recycling for DSSCs fabrication without significant loss of the solar cell performance. The straightforward, low‐temperature, and cheap production of metal‐free counter electrodes for dye‐sensitized solar cells (DSSCs) is presented. The self‐assembly of carbon materials such as graphene flakes and multi‐walled carbon nanotubes (MWCNTs) leads to high‐performing counter electrodes. Recycling of counter electrodes in DSSCs is demonstrated and compared for different electrolyte compositions.