This work demonstrates that carbon nanotube (CNT) fiber electrodes consisting of a mesoporous and crystalline network of few-layer CNTs can be used as free-standing, unipolar, and dual counter-electrode (CE)/current-collectors in highly efficient and stable planar dye-sensitized solar cells (DSSCs). After device optimization, particularly in terms of photoanode thickness, the solar energy conversion efficiency reached 8.8%, which is comparable to devices with platinum CE (8.7%). Through study of symmetric cells using electrochemical impedance spectroscopy (EIS) measurements, this work discloses the first clear identification of the different processes involved in the use of CNTf electrodes as CE, namely bulk ion diffusion, ion diffusion inside the mesoporous electrode, and charge transfer at the CNT surface. These results provide clear directors for a further understanding of the fundamental catalytic properties of CNTf fibers towards improving their photoelectrochemical features. Finally, we assemble new device architectures, namely i) a free-standing CNT-CEs (unipolar electrode) device with two photoanodes facing each other to circumvent the lack of transparency of CNTs without losing performance, and ii) large-area solar cells (>10 cm2) with 10 devices interconnected in parallel along 1 m continuous CNTf-CE, featuring efficiencies comparable to the-state-of-the-art carbon-based DSSC modules.
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