Heteroatom-doped graphene and its application as a counter electrode in dye-sensitized solar cells

The most frequently used counter electrode (CE) in dye-sensitized solar cells (DSSCs) is platinum on fluorine-doped tin oxide glass. This electrode has excellent electrical conductivity, chemical stability, and high electrocatalytic affinity for the reduction of triiodide. However, the high cost of metallic platinum and the poor electrochemical stability pose a major drawback in the commercial production. This has necessitated a search for a non-precious metal and metal-free electrocatalyst that demonstrates better catalytic activity and longer electrochemical stability for practical use in DSSCs. Graphene has been at the centre of attention due to its excellent optoelectronic properties. However, a defect-free graphene sheet is not suitable as a CE in DSSCs, because of its neutral polarity which often restricts efficient charge transfer at the graphene/liquid interface, irrespective of the high in-plane charge mobility. Hence, heteroatom-doped graphene-based CEs are being developed with the aim to balance electrical conductivity for efficient charge transfer and charge polarization for enhanced reduction activity of redox couples simultaneously. The elements commonly used in chemical doping of graphene are nitrogen, oxygen, boron, sulfur, and phosphorus. Halogens have also recently shown great promise. It has been demonstrated that edge-selective heteroatom-doping of graphene imparts both efficient in-plane charge transfers and polarity, thereby enhancing electrocatalytic activity. Thus, heteroatom-doped graphene serves as a good material to replace conventional electrodes and enhance power conversion efficiency in DSSCs. The focus is to reduce the cost of DSSCs. This review explores the performance of DSSCs, factors that influence the power conversion efficiency, and various physicochemical properties of graphene. It further outlines current progress on the synthetic approaches for chemical doping (substitutional and surface transfer doping) of graphene and graphene oxide with different heteroatoms in order to fine-tune the electronic properties. The use of heteroatom-doped graphene as a CE in DSSCs and how it improves the photovoltaic performance of cells is discussed.