Recent Progress in Non-Platinum Counter Electrode Materials for Dye-Sensitized Solar Cells

Dye-sensitized solar cells (DSSCs) have gained increasing attention with regard to photovoltaic devices, because of their low cost and simple fabrication methods; they are mostly investigated in indoor light-harvesting and portable applications. The focus has been on three main parameters of photovoltaic devices, that is, lifetime, and cost effectiveness. A DSSC consists of four prominent components including a photoanode, a photosensitizer, a redox electrolyte, and a counter electrode. The counter electrode is a crucial component, in which triiodide is reduced to iodide by electrons flowing through the external circuit. An effective approach to improve the performance of a counter electrode is to enhance the power conversion efficiency and to reduce the cost of the device. Platinum-coated conducting glass electrodes give the best performance, but their high cost and the scarcity of platinum restricts large-scale application in DSSCs. This has prompted researchers to develop low-costing platinum-free electrodes for DSSCs. In this review, we focus mainly on counter electrode materials for the electrocatalytic redox reaction for the I-/I-3(-) electrolyte, and apart from this, other counter electrode materials for iodine-free redox electrolytes are discussed. Different counter electrode materials are highlighted in different categories such as carbon materials, conducting polymers, oxide and sulfide materials, transition-metal nitrides and carbides, and composite materials. The stability of counter electrodes in DSSCs is also presented.