Effects of Cr₂O₃ modification on the performance of SnO₂ electrodes in DSSCs

In this paper, we demonstrate that Cr₂O₃, a visible absorbing insulator, can be used as an efficient blocking layer material for the anode of dye-sensitized solar cells (DSSCs). We prepared SnO₂ electrodes surface-modified with Cr₂O₃ with various Cr/Sn ratios and studied the effect of the modification on the performance of DSSCs. DSSCs with Cr/Sn ratios 0.02, 0.05, and 0.10 showed increased overall photon-to-electricity conversion efficiency from that of pure SnO₂. Especially, the DSSC with the Cr/Sn ratio 0.02 showed a remarkably large increase by 55%. The electrode materials were analyzed by powder X-ray diffraction, transmission electron microscopy, N₂ adsorption studies, and UV-Vis diffuse reflectance spectroscopy. The Cr-containing species appears to be Cr₂O₃ nanoparticles, spread evenly on the SnO₂ nanoparticles and filling the gap space between SnO₂ particles. The electrochemical properties of the electrodes were characterized by Mott-Schottky plots and electrochemical impedance spectroscopy. As the Cr-content increases, the flat-band potential is negatively shifted. The impedance spectroscopy data show that Cr/Sn = 0.02 and 0.05 samples have lower charge transport resistance at the electrode, which can be explained by the rise of the conduction level due to the charge transfer from the more basic Cr₂O₃ nanoparticles to SnO₂ nanoparticles. These observations corroborate with the trends of the short-circuit current and the open-circuit voltage of the DSSCs.