Efficient Electrocatalysts for Dye-Sensitized Solar Cells

Abstract

Cathode materials play a crucial role in dye-sensitized solar cells (DSSCs). Herein we present the applications of graphene based composites, conductive polymers and metal selenides in DSSCs as the cathode. Monolayer of PDDA/graphene/PDDA/H2PtCl6 is fabricated on conductive glass using electrostatic layer-by-layer self-assembly technique, which is then converted to graphene/Pt monolayer for use as counter electrode in DSSC. As compared to the sputtered Pt counter electrode, the self-assembled monolayer reduces the Pt amount by about 1000-fold but exhibits comparable photovoltaic performance. Close-packed graphene suffers from a low surface area, which limits its electrocatalytic activity as the counter electrode in DSSCs. To solve this problem, we have synthesized a porous graphene/SiO2 nanocomposite converted from graphene oxide mixed with SiO2 nanoparticles through facile hydrazine hydrate reduction. Graphene and graphene/SiO2 films prepared by drop-casting the suspensions onto the conductive glass substrates at room temperature without heat treatment have been applied directly as cathodes in DSSCs. As compared to the nonporous graphene film, the porous graphene/SiO2 composite film demonstrates much better electrocatalytic performance for the reduction of triiodide in DSSCs due to the remarkably enhanced surface area. As a consequence, the incorporation of SiO2 nanoparticles in the graphene film improves fill factor and power conversion efficiency by 65% and 69%, respectively. Oriented polyaniline nanowires array has been successfully grown in situ on conductive glass substrates without templates and applied as the cathode of DSSCs mediated with a Co(bpy) 3+/2+ 3 (bpy = 2,2′-bipyridine) redox couple. As compared to the polyaniline film with a random network, the oriented polyaniline nanowires array exhibits much better electrocatalytic performance, which is even outperforming the typical Pt electrode in both electrocatalytic performance and electrochemical stability when exposed to the acetonitrile solution of Co(bpy) 3+/2+ 3 redox couple. We have also proposed a facile one-step low temperature hydrothermal approach for in-situ growth of metal selenides (Co Se and Ni 1 0.85 0.85Se) on conductive glass substrates. The as-prepared metal selenides on conductive substrates can be used directly as transparent cathodes for DSSCs without any post-treatments. It is found that graphene-like Co0.85Se exhibits higher electrocatalytic activity than Pt for the reduction of triiodide. As a consequence, the DSSC with Co0.85Se generates higher short-circuit photocurrent and power conversion efficiency (9.40%) than that with Pt. These findings provide new routes to fabrication of cheap and efficient counter electrodes for DSSCs.