Abstract
An important reason for the relatively low efficiency of dye-sensitized solar cells (DSSCs) is the low open-circuit voltage (VOC) of about 0.7 V for a standard solar cell with a dye that has an absorption onset at 1.6 eV. We report an enhancement of the VOC of about 0.10 V with respect to a TiO2-based DSSC modified with ZnO nanoflowers that we prepared by a new and facile method. An additional increase of the VOC of about 0.08 V was achieved by modifying the ZnO nanoflowers with Au nanoparticles, resulting in a DSSC with an efficiency of 2.79%, highlighted by a high VOC of 0.89 V. Detailed analysis with electrochemical impedance spectroscopy and intensity-modulated photovoltage and photocurrent spectroscopies (IMVS and IMPS) reveal that the main reason for the increase of VOC is related to the shift of the band edges upon coupling TiO2 with ZnO nanoflowers, even though the electron lifetime at the same charge density actually decreases. These results show the intricate interplay between band edge shift, recombination kinetics, and DSSC performance and illustrate that a higher voltage DSSC can be fabricated by modification of the photoanode materials.
Highlights
Dye-sensitized solar cells (DSSCs) have been the center of attention for almost 3 decades after the report of Graẗ zel and O’Regan in 1991,1 where they demonstrated the assembly of a DSSC based on TiO2 nanoparticles that reached an efficiency of 7.1%
The ZnO nanoflower material was blended with TiO2 anatase nanoparticles in order to prepare the pastes denoted as 5% ZnO (5Z), 10Z, and 15Z, where the number corresponds to the weight percentage of ZnO, and DSSCs were assembled and characterized
The influence of the incorporation of ZnO nanoflowers into the photoanode nanomaterial on the performance of TiO2based DSSC has been evaluated in detail using both standard and specialized small-signal modulation methods, electrochemical impedance spectroscopy (EIS), and IMVS/IMPS
Summary
Dye-sensitized solar cells (DSSCs) have been the center of attention for almost 3 decades after the report of Graẗ zel and O’Regan in 1991,1 where they demonstrated the assembly of a DSSC based on TiO2 nanoparticles that reached an efficiency of 7.1%. Even though the performance improvement in a TiO2/ZnO nanoflower DSSC has already been shown, most of the reported devices require multistep synthesis methods, additives, or long reaction times to obtain ZnO nanoflowers or extra steps to fabricate the solar cell. These factors increase the fabrication cost and affect the largescale production of the DSSC. We use advanced electrochemical characterization techniques including electrochemical impedance spectroscopy (EIS) and intensity-modulated photovoltage and photocurrent spectroscopies (IMVS/IMPS) to determine the mechanisms behind the improved performance
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