Synthesis of CoSe2/RGO Composites and Their Application in a Counter Electrode of Dye-sensitized Solar Cells

Abstract

Nowadays, the energy issues regarding to fossil fuels have become serious problems. Though fossil fuels such as coal, natural gas and petroleum are cheap and convenient to use, they have triggered the air pollution caused by the emission of harmful gas after burning them. To resolve these problems, the renewable energy can be a good option, which use eco-friendly resources such as sunlight, wind, water, and etc. Solar cells have emerged as one of the promising renewable energy devices due to the usage of solar energy which can satisfy the future global need for renewable energy sources. After the development of single crystalline silicon-base solar cells that were the first solar cells, many types of solar cells have been extensively studied and researched to further improve efficiency and make up for the disadvantages. As such efforts, thin film solar cells such as CIGS and CdTe, and next-generation solar cells such as polymer solar cells have been developed for low-cost and high efficiency solar cells. Dye-sensitized solar cells (DSSCs), one of the next-generation solar cells, have attracted great attention since Grätzel’s group introduced them in 1991 first due to their easy fabrication and cost-effectiveness compared with silicon based solar cells. A typical DSSC comprises nanocrystalline titanium dioxide (TiO2) films, dye molecules as light-senstizers, iodide (I-) and triiodide (I3-) redox couple electrolyte, and platinum (Pt) electrocatalysts covered onto transparent conductive oxide (TCO). Among the parts of DSSCs, electrocatalyst materials play roles as the catalyst for reduction of I3- to I- in the liquid-based electrolyte. Due to its superior catalytic effects for I3- reduction and chemical stability, Pt has been widely used as a counter electrode (CE) material in DSSCs. However, the scarcity of rare Pt metal severely makes fabrication cost of DSSCs high and hinders the large-scale production of DSSCs. Because of these reasons, finding more abundant and cheaper materials than Pt is necessary. In this study, cobalt diselenide (CoSe2) and the composites of CoSe2 and reduced graphene oxide (CoSe2@RGO) were synthesized by a facile hydrothermal reaction of cobalt ions and selenide source with or without graphene oxide. Analyses with X-ray, Raman and morphology revealed the formation of CoSe2 compounds and reduction of graphene oxide. Electrochemical analyses demonstrated that CoSe2@RGO composites have excellent catalytic activity for reduction of I3-, exhibiting synergetic effect between CoSe2 and RGO. As a consequence, the DSSC using the CoSe2@RGO CE showed the comparable power conversion efficiency (7.01 %) to the Pt-based CE device (6.77 %). Therefore, the composites of CoSe2 with RGO can be good candidates as catalyst materials instead of Pt.