Introduction: Dye-sensitized solar cells (DSSCs) have been intensively studied with a growing demand as potential alternatives for the next generation solar cells due to their low cost and eco-friendly production, easy processing, and relatively high energy conversion efficiency when compared with conventional solar cells. In contrast to the conventional solar cells, which relies on high purity substrates grown at very high temperatures using high cost processes in a specially designed environments such as clean room, DSSCs do not require such costly and complex processes and can be prepared in a simple laboratory environment without much concern on materials purity and ambient atmosphere. Purpose: The purpose of this study is growth of ZnO nanostructures with different surface morphology such as nanowire, nanorod and nanoflower in order to obtain higher efficiency dye sensitized solar cells (DSSCs). Scope: A typical DSSC consists of a wide band-gap semiconductors as photoanode deposited on fluorine doped tin oxide (FTO) coated glass substrate and sensitized by dye molecules and Pt coated FTO counter electrode (CE) with a I-/I3- redox electrolyte filled in between photoanode and CE. Under solar irradiation, excited electrons in the Ruthenium-dyes are injected into the conduction band of semiconductors and diffuse into the FTO/semiconductors interface and eventually are extracted to an external load. The extracted electrons flow through the load and reach the CE. I-/I3- redox electrolyte accept the electrons from Pt CE and those are transferred to the dye molecules to refill the holes in the HOMO. Limitation: It is widely believed that the ruthenium dyes N719 are regenerated by iodide with near unity quantum yield following photo-oxidation in dye-sensitized solar cells (DSSCs). However, the incident photon-to-current efficiency (IPCE) of DSSCs using these dyes decreases with increasing forward bias, limiting power conversion efficiency (η) compared to the hypothetical constant-IPCE case. This phenomenon could arise due to incomplete regeneration, but despite the important implications for cell efficiency, it has received little attention. The results strongly suggest that this is the case, even for abnormally high iodide concentrations, where η is reduced by as much as 30% by the effect. Method: Growth was carried out on FTO (SnO2:F) substrate by hydrothermal method. The average diameters of the structures were determined to be 50-250 nm and lengths in the range of 1-10 μm. Detailed structural and optical characterizations were performed by using scanning electron microscopy, x-ray diffraction, photoluminescence, absorbance and Raman spectroscopies. Results: DSSCs were constructed using ZnO nanorods, nanowires and nanoflowers structures. The current density-voltage (J-V) characteristics of solar cells fabricated using the different ZnO nanostructures, under 100 mW/cm2 of AM-1.5 illumination were compared each others. Active electrode area was modified as 0.25 cm2. We show that there is a considerable increase in the short-circuit current density (Jsc) of the ZnO nanoflower DSSCs in comparison with the other 2 nanostructure DSSCs. This is attributed to the superior light absorbing characteristics, provided by the larger surface areas of the nanoflower structure. Although, the open-circuit voltage (Voc) of the different DSSCs are almost identical, the higher Jsc of the nanoflower structure results in higher solar conversion efficiency () of 3.1 %, while it is obtained 1.59 % and 2.04 % for nanorod and nanowire samples, respectively. ZnO nanoflower structures achieve values that are almost 50 % higher than that of the ZnO nanowires and about 85% higher than ZnO nanorods structures. This is a significant result showing the dependence of solar cell performance on surface morphology of the ZnO nanostructure. Achieved larger surface area also results in improved dye adsorption. In comparison with nanoflowers, both nanowire and nanorod structures have gaps that are inherent in their morphology, which results in lower internal surface area leading to incomplete absorption of photons. However, ZnO nanoflowers structures have nanoscale branches that stretch to fill these gaps and also, provide both a larger surface area and a direct pathway for electron transport along the channels resulting in higher Jsc. Higher solar conversion efficiency in the present study was obtained compared to the given values for the ZnO nanostructures DSSCs in the literature. The ZnO nanoflower DSSCs show much higher IPCE (34% at about 510 nm) compared to the ZnO nanowires (25% at about 520 nm) and ZnO nanorods (18% at about 510 nm), mainly due to the aforementioned effect of larger surface area (and dye-loading). Conclusion: ZnO nanostructures were used as the wide band-gap semiconducting photo-electrode in DSSCs. Solar conversion efficiencies and incident photon to current conversion efficiencies (IPCE) were measured for different ZnO nanostructures. The highest solar conversion efficiency of 3.1% and IPCE of 34% was obtained in ZnO nanoflowers/N719 dye/I-/I-3 electrolyte, while it is 2.04% and IPCE of 25% for ZnO nanowires and 1.59% and IPCE of 18% for the nanorods, respectively.
Anahtar Kelimeler: Dye-sensitized Solar Cell, Nanowires, Nanorods, Nanoflowers, ZnO
|
