Abstract
Creating a unidirectional electron transport is one of the key factors of reaching high power energy conversion efficiency in dye-sensitized solar cell. Unfortunately, undesired electron pathway called recombination exists in real devices. The recombination causes photocurrent loss and mainly occurs at TiO2/electrolyte as well as FTO/electrolyte interface. Herein we focus on retarding the recombination at latter interface by using Nb2O5 blocking layer. Owing to slightly higher conduction band position of Nb2O5 comparing to TiO2, which can suppress charge recombination and enhance power conversion efficiency. In this study, the Nb2O5 blocking layer was prepared by the spin coating 5mM niobium ethoxide ethanol solution on FTO substrate. The film was then characterized by X-ray diffraction (XRD), X-ray photoelectron spectrometer (XPS) and transmission electron microscopy (TEM), which indicated a small part of TT-phase crystalline was formed. The uniformity of such blocking layer was scrutinized by optical microscope (OM) and scanning electron microscope (SEM). UV/Vis spectroscope was used to determine the transmittance of so-prepared Nb2O5 thin film on FTO. Band structure was positioned by XPS and UV/Vis spectroscope. In order to evaluate the recombination extent, the electrochemical impedance spectroscopy with transmission line model was employed and the recombination resistance (Rct) can be obtained. At low forward bias, considering the same condition as low-sun irradiation, recombination takes place primarily at FTO/electrolyte interface. In the case of volatile electrolyte based devices, the Rct with the Nb2O5 blocking layer increases 3 times higher than the blank one. As non-volatile electrolyte was applied, more significant effect could be observed for 14 times improvement in Rct value in the device fabricated with this Nb2O5 blocking layer. As the result, the Nb2O5 blocking layer could suppress the recombination at FTO/electrolyte interface, especially under low-sun condition. An increase in power conversion efficiency from 11.1% to 11.8% and 7.87% to 8.53% were obtained in volatile and non-volatile electrolyte based devices, respectively. This research reveals the importance of blocking effect under low illumination, which is the crucial issue of indoor application for DSSC and this niobium oxide thin film acts as a remarkable blocking layer.