Characterization of interfacial charge transport and recombination by impedance spectroscopy on SiO2 coated TiO2 based dye sensitized solar cells

Performance of dye sensitized solar cells (DSSCs) was improved 80%, by treating the mesoporous titanium dioxide (TiO₂) surface using atomic layer deposited (ALD) ultra thin silicon dioxide (SiO₂), compared to a DSSC with untreated mesoporous TiO₂ photoelectrode. Effect of SiO₂ treatment on charge transport, recombination and lifetime of electrons in the TiO₂ were investigated by electrochemical impedance spectroscopy (EIS). X-ray photoelectron spectroscopy confirmed the ultra thin growth of SiO₂ on the surface of mesoporous TiO₂. Dark current-voltage (I-V) characteristics of the DSSCs with SiO₂ surface treatment showed that the TiO₂/electrolyte interface quality was better than that of DSSC with untreated mesoporous TiO₂ in terms of defect density. Photovoltaic performance of the DSSC which used SiO₂ surface treatment showed that the effect of reverse electron recombination was significantly suppressed compared to the DSSC which did not use SiO₂ treatment. Dark and illuminated I-V measurements of the DSSCs which used SiO₂ surface treatment suggested that the lower thickness of SiO₂ (5 cycles) did not block electron transport from the dye to TiO₂ but thicker SiO₂ (20 cycles) blocked but still the performance was better than the DSSC with no SiO₂ treatment due to the control on density and activity of TiO₂ surface states on electron transport.