Dye-sensitized solar cells (DSSCs) are promising alternative to the conventional p-n junction solar cells , due to simple structure and process, low cost fabrication, transparency, color control, and applicability in the flexible DSSCs [2-4]. The DSSCs are composed of a dye-adsorbed nano-porous TiO2 layer on fluorine-doped tin oxide (FTO) electrode, redox electrolytes and the counter electrode .
To generate meaningful electrical power from DSSCs, the electrons need to pass four important interfaces of DSSCs: dye/TiO2, TiO2/FTO, electrolyte/counter electrode, and dye/electrolyte . As the key component of DSSCs, the nano-porous TiO2 electrode shows high surface area, which enables both efficient electron injection and light harvesting .
However, it also provides abundant TiO2 surface sites (direct route) and bare FTO conducting sites (indirect route), where the photo injected electrons may recombine with I3- species in the
The charge recombination mainly occurs at the electrode/electrolyte interface due to the absence of energy barrier layer [11,12]. The recombination will cause the loss of the photocurrent. So the photovoltaic performance of DSSCs is seriously decreased . Recently, the charge recombination can be significantly suppressed by employing a thin blocking layer [14,15] as shown in Fig. 1. Semiconductors, such as TiO2, Nb2O5, ZnO and some insulating materials, such as CaCO3 and BaCO3, have been used as a blocking layer for the fabrication of DSSCs [16-20].
In this work, we systematically investigated and compared the optical, structural, and photo- electrochemical properties based on different types of blocking layers such as TiO2, Nb2O5, and ZnO. We expected that the electrodeposited various blocking layers (TiO2, Nb2O5 and ZnO) at the exposed TiO2/FTO interface contributes to the suppression of the charge recombination.
The FTO glasses (7 Ω/sq.) that we used were cleaned thoroughly in acetone and ethanol for 30 min in each step to remove the organic pollutants and other contamination . After drying by Ar compressed gas, TiO2 nano-particle paste was coated on the FTO electrode by using the doctor-blade technique. And then TiO2 nano-porous film was annealed at 500℃ for 30 min in air. The thickness of TiO2 film was approximately 14 μm. Ru complex dye (N719; 535 bis-TBA : cis-bis (isothiocyanato) and bis (2, 2'-bipyridyl-4, 4'-dicarboxylato) -ruthenium(Ⅱ) bistetrabutylammonium 0.05 g in 100 ml absolute ethanol (99.99%)) was synthesized in advance. To adsorb a photo sensitized dye on the TiO2 surface the TiO2 working electrode was immersed in Ru complex dye (N719) for 24 h at room temperature. Immediately the dye-adsorbed TiO2 working electrode was rinsed with absolute ethanol and dried by Ar compressed gas. The counter electrode was prepared by sputtering (sputter coater; sc 7640) a pt layer on the other FTO coated glass electrode. The Pt-treated electrode was placed over the dye-coated electrode and edges of the cell were sealed with 0.5 mm wide strips of 60 μm-thick surlyn (solarnix, sx 1170, Hot Met). After sealing, an iodide based electrolyte of tri-iodide in acetonitrile (solarnix, AN-50) was injected into the cell through of the two small holes drilled in the counter electrode. The holes were then covered with small cover glass and sealed.
As the blocking layer materials, TiO2, Nb2O5 and ZnO were selected in this study. The blocking layer was deposited between the FTO glass and TiO2 nano-porous film by using RF magnetron sputter. The TiO2, Nb2O5 and ZnO ceramic targets were fabricated (diameter; 2 inch) with high purity TiO2 (99.99%), Nb2O5 (99.99%) and ZnO (99.99%) powders respectively. More details about the sputtering conditions are given in Table 1.
The deposition conditions were maintained carefully stable during the growth of blocking layer. After that, those were fabricated in the same way as the standard DSSCs.
The film thickness were measured using a surface profilier meter (α-step, TENCOR P-2). The crystal structure of the film was analyzed by x-ray diffractometer (XRD; Rigku). The film morphology was observed by field emission scanning electron microscope (FE-SEM; JEOL). The electrical resistivity of the film was obtained using a four-point probe method. The Model name
The fabrication of different blocking layers using RF magnetron sputtering method.
of Nanoscope Ⅲa was used to perform atomic force microscopic (AFM) studies. The photocurrent-voltage (I-V) characteristics of the DSSCs were measured under AM 1.5, 100 mW/m2 (1 sun) using a solar simulator.
Figure 2 showed the XRD patterns of TiO2 nano-porous film and FTO electrode annealed at 500℃ electrode temperature.
In case of the Fig. 2(a), due to the background’s peaks it seems like amorphous phase. But it clearly represented tendency of the crystallinity in TiO2 nano-porous film with (101) preferred orientation at 2 θ = 32.196°. The TiO2 nano-porous film exhibited TiO2 anatase crystalline phase. Fig. 2(b) showed XRD patterns of FTO electrode annealed at 500℃ electrode temperature.
The FTO electrode showed the tendency of the crystallinity with (110) prefered orientation at 2 θ = 12.38°. It believed that the FTO electrode has excellent thermal stability because high temperature does not change the crystallity of FTO electrode.
From the XRD results, it is clear that the structure of the fim such as TiO2 nano-porous film and FTO electrode are completely crystalline phase by annealing at 500℃.
From the AFM images, it is clear that the sputtered blocking layers have smooth surface with RMS roughness value which decreased by ~7 nm.
The surface roughness is one of the important factors that determined optical properties. The thin film with flat and low surface roughness value shows a low reflectivity because the constructive interference and destructive interference are clearer according to the thickness.
From FE-SEM images, bare FTO showed the texture surface morphology which has diameter above 1 μm. In case of blocking layers deposited FTO electrodes in Fig. 3(b), Fig. 3(c) and Fig. 3(d) showed that TiO2, Nb2O5 and ZnO nano-particles were covered on the FTO electrode respectively. It is clear that the blocking
layers which have diameter up to 40 nm were deposited on FTO glass.
Figure 5 showed the cross sectional SEM images of bare FTO and various blocking layer deposited FTO electrodes. As the thickness of the TiO2 nano-porous film is above 14 μm, the images was not shown the material interface clearly. However, before sputtering the blocking layers, we calculated the deposition rate under the same condition. So, we are confident that the thickness of blocking layers are deposited approximately 50 nm even if the SEM images does not shown the thickness clearly. And also, in case of FE-SEM images in Fig. 3, it is clear that the difference from bare FTO was seen. It means that the blocking layers were deposited on FTO glass. It could be confirmed that TiO2, Nb2O5 and ZnO blocking layers are approximately 50 nm respectively.
Figure 6 showed the sheet resistance of bare FTO electrode and blocking layers of TiO2, Nb2O5 and ZnO sputtered on FTO electrode.
The sheet resistance of bare FTO electrode was about 7 Ω/sq. However, after deposited the blocking layers on FTO electrodes, the sheet resistances were obtained such as FTO/TiO2 = 18 Ω/sq., FTO/Nb2O5 = 10 Ω/sq., FTO/ZnO = 8 Ω/sq. respectively.
Especially, the most increased value of sheet resistance was obtained from TiO2 deposited FTO electrode. These results, we expected that the TiO2 deposited FTO electrode will negatively affect the current density (Jsc) and the conversion efficiency (η).
Figure 7 showed the photocurrent density-voltage (J-V) curves obtained under 100 mW/m2 illumination with AM 1.5 conditions of DSSCs consisting of TiO2 electrode without and with blocking layer of TiO2, Nb2O5 and ZnO.
The characteristic parameters for DSSCs can be obtained from the photocurrent density-voltage curve, such as the short circuit current (Jsc), the open circuit voltage (Voc), fill factor (FF) and conversion efficiency (η).
The efficiency η of the DSSCs can be calculated from the following equation;
where, Jsc is the integral photocurrent density(current obtained at short circuit conditions divided by the area of the cell), Voc is the open circuit voltage, FF is the fill factor, FF = (I × V)max/IscVoc (related to the serious resistance for a potential solar cells), and Pin is the intensity of the incident light.
The characteristic parameters for the DSSCs corresponding to Fig. 7 are summarized in Table 2. The DSSC fabricated on the bare FTO/glass electrode showed Voc of 0.6846 V, Jsc of 4.0175 mA/cm2, FF of 75.9849, and calculated power conversion efficiency of ηAM1.5 = 2.08% deposited with Nb2O5 blocking layer, showed maximum efficiency. As compared with standard DSSC, the conversion efficiency (η), short circuit current (Jsc) of the DSSC with the Nb2O5 blocking layer were improved in 26.9% and 35.5% respectively. On the other hand, the open circuit voltage (Voc) was slightly diminished by 1.6%. Another feature seen in this figure, the DSSCs with the TiO2 and ZnO blocking layers showed the also enhanced efficiency, η (2.24%), η (2.64%) and short circuit current, Jsc (4.2048 mA/cm2), Jsc (4.6245 mA/cm2) respectively.
The reason for the enhancement of Jsc with blocking layer is associated with the result of increase of protection effect against ionic penetration from electrolyte through the blocking layer. Consequently, the cell performance under illumination seems to indicate a decrease of the recombination rate in the cell when TiO2, Nb2O5 and ZnO were used as the blocking layer.
However, the DSSC with the TiO2 blocking layer showed lower efficiency than DSSCs with the Nb2O5 and ZnO blocking layer. The lower efficiency of DSSC with the TiO2 blocking layer possibly attributed to the fact of increment of the sheet resistance. As mentioned earlier, the sheet resistance value of FTO/TiO2 electrode is more than twice larger than the sheet resistance values of Nb2O5 and ZnO. For this reason, we considered that short circuit current, Jsc was electronically lower than those of Nb2O5 and ZnO. Also we might infer that TiO2, Nb2O5, and ZnO deposited on FTO electrodes were effectively reduce the electron recombination by minimizing the direct contact between the redox electrolyte and the conductive FTO surface.
In summary, the characteristics of the RF sputter grown various blocking layers (TiO2, Nb2O5 and ZnO) on the FTO electrode were investigated for the possible application of DSSCs. The DSSC deposited with Nb2O5 blocking layer, showed maximum efficiency. Compared with standard DSSC, the conversion efficiency (η), short circuit current (Jsc) of the DSSC with the Nb2O5 blocking layer were improved in 26.9% and 35.5% respectively. On the other hand, the open circuit voltage (Voc) was slightly
The fabrication of different blocking layers using RF magnetron sputtering method.
diminished by 1.6%. Among the DSSCs with the blocking layers, the DSSCs with the TiO2 blocking layer showed lower efficiency than DSSCs with the Nb2O5 and ZnO blocking layers.