Dye-Sensitized Solar Cell with Electrophoretic Deposited Photocatalytic Carbon Nanotube Counter Electrode: Nanotube Density and Cell Performance

Prospective solar cell technologies such as dye-sensitized solar cells (DSSCs), organic photovoltaics (OPVs) and much recently discovered perovskite solar cells compared to the conventional solid-state pn-junction type silicon solar cells are of high demand for solar energy conversion due to their simple fabrication processes, high energy conversion efficiency, low cost, and potential for flexible solar panels^1-4. Among the emerging solar cell technologies, DSSC’s stability and reliability in air is so far most promising compared to OPVs and perovskite cells. While platinum film is used as photocatalyst in the counter electrode of a conventional DSSC, nanostructured alternatives such as carbon nanotubes (CNTs) can further decrease both material and fabrication cost, enhance mechanical flexibility and provide comparable performances^5-6.   

Network of metallic CNTs have demonstrated significant potential for alternate transparent conductors to indium doped tin oxide (ITO). Moreover, its catalytic activity for DSSC counter electrode has also been previously explored to replace platinum⁷.  However, a lower conversion efficiency still persists. Placing the nanotube on counter electrode in a network structure with optimal density is therefore desired for enhances solar cell performance, as the rate of charge transfer reaction on the counter electrode and hence the photo-conversion efficiency is dependent on optimal density of nanotubes. While enormous density of nanotubes in counter electrode can provide large amount of catalytic surface for the I^-/I₃^-reduction reaction, high nanotube density can be an obstacle for fast charge transport. On the other hand, a sparse nanotube network might provide just enough catalytic surfaces without barricading charge transport. The sparse network also has high light transmittance to build transparent solar cells and solar panels.    

In this work, we report an electrophoretic deposition of CNT network on to fluorine-doped tin oxide (FTO) glass substrates. We fabricated DSSCs using these CNT-decorated-electrodes as counter electrodes and tested the cells to evaluate their photocatalytic activities using a solar simulator. We have demonstrated optimal CNTs electrophoretic deposited onto the FTO surface (30-45 seconds deposition at a DC bias of 40V) with conversion efficiency comparable to a DSSC with platinum counter electrode. This work demonstrates the efficient use of catalytic CNTs with optimal density for building DSSCs.

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