Highly Efficient Iodide/Triiodide Dye-Sensitized Solar Cells with Gel-Coated Reduce Graphene Oxide/Single-Walled Carbon Nanotube Composites as the Counter Electrode Exhibiting an Open-Circuit Voltage of 0.90 V

To increase the open-circuit voltage (V-oc) of dye-sensitized solar cells (DSCs), it is crucial to enhance the photovoltaic efficiency of DSCs. Here, we report an effective method to significantly improve the V-oc and photovoltaic efficiency of DSCs by using gel-coated composites of reduced graphene oxide (rGO) and single-walled carbon nanotubes (SWCNTs) as the counter electrode. Gel-coated rGO-SWCNT composites outperform Pt, rGO and SWCNTs in catalyzing the reduction of I-3(-) and functioning as the counter electrode of DSCs. The V-oc and power conversion efficiency (PCE) are 0.86 V and 8.37% for fresh DSCs with the composite of 80 wt % rGO and 20 wt % SWCNTs, significantly higher than those (V-oc = 0.77 V, PCE = 7.79%) of control DSCs with Pt fabricated by pyrolysis as the counter electrode. The Voc value of DSCs with rGO-SWCNT composites as the counter electrode further increases to 0.90 V after one week. The high V-oc and PCE are ascribed to the synergetic effects of rGO and SWCNTs in reducing the overpotential of the I-3(-) reduction. RGO with high specific surface area can have high electrocatalytic activity, whereas SWCNTs give rise to high conductivity for the composites and facilitate the penetration of the redox species into rGO sheets by preventing the agglomeration of the rGO sheets. To the best of our knowledge, this is the first time to report iodide/triiodide DSCs with both high V-oc and PCE.