Evaluation of counter electrodes composed by carbon nanofibers and nanoparticles in dye-sensitized solar cells

A series of counter electrodes (CEs) for dye-sensitized solar cells (DSSCs) was fabricated using different weight ratios of electrospun carbon nanofibers (ECNs) and carbon nanoparticles (CNPs). The conductivity of neat ECN was 838 S/m, which is more than twice than that of neat CNP, and the bulk resistance of CEs decreased as the ECN ratios increased in the composite, leading to lower transport resistance in the CEs. However, as the concentration of CNPs increased, the surface area of CEs also improved because CNPs have a much smaller dimension than ECNs, leading to higher electrocatalytic property. The CEs with higher ratio of CNPs possessed several superiorities compared with those with higher ratio ECNs, such as larger surface area for triiodide reduction, faster reaction rate, and less charge transfer resistance at the interface of CE and electrolyte. Evidenced from cyclic voltammograms and electrochemical impedance spectroscopy, the devices with higher ratio CNPs exhibited lower Nernst diffusion impedance and higher efficiency electrocatalytic performance than those with higher ratio ECNs. When the materials of CE switched from neat ECN to those with a higher concentration of CNPs, the DSSC fill factor, current density, and efficiency were improved.