Pt-free transparent counter electrodes for dye-sensitized solar cells prepared from carbon nanotube micro-balls

Since their initial invention, dye-sensitized solar cells (DSSCs) have offered cost-effective photovoltaic systems. For their counter electrodes, DSSCs generally employ Pt nanoparticles. However, Pt is expensive, rare, and already widely in demand as catalyst in various chemical and electrochemical fields. Substitutes for Pt have been sought among carbon materials, such as activated carbon, carbon black, and carbon nanotubes. Carbon nanotubes (CNTs) are the most appealing candidates, because of their favorable electrochemical catalytic activities. Unfortunately, as with other carbon materials, CNTs cannot provide high charge exchange currents. To obtain performances comparable to Pt counter electrodes, large surface areas are required, resulting in thick electrodes. We have found that transparent Pt-free counter electrodes suitable for DSSCs can be prepared using MWCNT micro-balls deposited on transparent substrates. The deposition density (i.e., the number of CNT micro-balls per unit area) can be controlled, allowing transparency and DSSC performance to be tuned. For a counter electrode transparency of 70%, the efficiency of a DSSC using CNT micro-balls is more than 80% of one using Pt nanoparticles. The prepared CNT micro-balls can be usefully applied in other electrochemical devices, such as battery and supercapacitors.