To boost the photoconversion efficiency (PCE) of ever promising quantum dot sensitized solar cells (QDSSCs), and to improve the design of photoanodes, the ability of the counter electrode (CE) to effectively reduce the oxidized electrolyte needs special attention. A composite of a 15 wt% graphene oxide nanoribbon (GOR), obtained by unzipping multi-walled carbon nanotubes (MWCNTs), and Cu x S intersecting hexagonal nanoplates, synthesized by a low cost, facile and scalable microwave synthesis

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... te, is reported as a fascinating CE for QDSSCs. The best performing Cu 1.18 S-GOR CE could notably achieve a record PCE of ∼3.55% for CdS sensitized QDSSCs, ∼5.42% for in situ deposited CdS/CdSe co-sensitized QDSSCs and ∼6.81% for CdTe/CdS/CdS dual sensitized QDSSCs, apart from increasing the PCE of previously reported QDSSCs. A systematic investigation of the CE design revealed the high electrocatalytic activity of GOR due to the presence of organic functional groups, graphitic edge sites and a quasi-onedimensional (quasi-1D) structure, which increases the interfacial charge transfer kinetics from the CE to the polysulfide electrolyte. The highly stable Cu 1.18 S-GOR CE has the added advantage of a favourable energy band alignment with the redox potential of the polysulfide electrolyte, which reduces the loss of charge carriers and thus can increase the PCE of QDSSCs. † Electronic supplementary information (ESI) available. See strips of graphene oxide (GO) with large catalytic edges and prepared by unzipping CNTs. 40,41 GOR has larger surface area for interaction with the electrolyte and chalcogenide nanostructures than the parent CNTs, GO or RGO, conveniently prepared by greener routes. 42 Even if GOR shows unique solution processability, there has been no report offering an optimization of the metal chalcogenide and the GOR composite that can offer high electrocatalytic performance as a CE in QDSSCs. The catalytic performance of the CE in QDSSCs depends on the available active sites and how fast the electrons flow back into the electrolyte from the external circuit, creating electron pathways to complete the circuit. So the carrier mobility of a CE should be good enough to reduce the charge transfer resistance (R CT ). R CT will add up to the overall series resistance (R s ), which determines the most important parameter of QDSSC, the fill factor (FF). So the catalytically efficient CE will reduce R s and improve FF, resulting in a higher PCE. GOR offers one of the greatest intrinsic carrier mobilities at room temperature, with a perfect atomic lattice, a promising mechanical strength, and chemical and thermal stability with additional functionalization of -OH and -COOH groups. Previously the Cu x S CE was prepared via exposing a brass foil to the polysulfide electrolyte, 43,44 electrochemical deposition, 45 electrospinning, 46 solvothermal, 47 successive ion layer adsorption and reaction (SILAR) etc. 48 Unlike the detailed synthesis strategies involving surfactants, organic solvents and an inert atmosphere required for controlling the domain sizes and topologies, 49 in this report a low cost microwave irradiation technique was used for the scalable production of Cu x S nanostructures with a tunable chemical composition, whereas GOR was synthesized by oxidative unzipping of MWCNTs. The superiority of GOR as a composite material over GO and CNTs is discussed based on electrochemical experiments. The CE made by doctor blading a paste of Cu x S-GOR composite on FTO glass shows enhanced catalytic activity, which is stable over several cycles and could improve the PCE of reported QDSSCs. Nanoscale Paper This journal is