Enhanced Performance of DSSCs Based on the Insertion Energy Level of CdS Quantum Dots

Cadmium sulfide (CdS) quantum dots (QDs) are assembled onto the TiO 2 films by chemical bath deposition method (CBD). And theQDs size is controlled by the times of CBD cycles.They are characterized byUV-visible absorption. To avoid the photo corrosion and electrolyte corrosion, CdS and N719 are sequentially assembled onto the nanocrystalline TiO 2 films to prepare a CdS/N719 cosensitized photo electrode for the dye-sensitized solar cells. In the structure of TiO 2 /CdS/N719 electrode, the reorganization of energy levels between CdS and N719 forms a stepwise structure of band-edge levels which is advantageous to the electron injection and hole recovery of CdS and N719 QDs. The open circuit voltage (Voc), short circuit current density (Jsc), and efficiency are increased.


Introduction
Due to its renewable and clean-energy characteristics, solar energy is destined to be an important energy source for the next generation.Recently, dye-sensitized solar cells (DSSCs) have attracted lots of attention because of their lower cost compared to the silicon based solar cells.DSSCs are based on photosensitization of mesoporous TiO 2 films by absorbed sensitizers.The molecular engineering of porphyrin sensitizers is recently used in DSSCs and energy conversion efficiencies up to 13% have been reported [1].In addition to organic dyes, semiconductor quantum dots (QDs) which absorb light in the visible spectrum are also good candidates to be a sensitizer of a DSSC cell.And there are some specific advantages of using semiconductor QDs as sensitizers.Due to the quantum confinement effect, the optical property and the band gap of a QD can be adjusted by changing the size of the QDs [2][3][4].In addition, the QDs open up a way to utilize hot electrons and to generate multiple electron-hole pairs with a single photon through impact ionization [5,6].But there are some disadvantages about the QDs, such that the QDs often undergo photo corrosion and electrolyte corrosion [7,8].And we know that certain organic molecules are able to reduce the corrosions [7,9].In this work, we make the TiO 2 sensitized by both CdS QDs and the N719 in DSSCs to improve the performance of DSSC and also to explore the principles involved.

Experimental
In our experiment, F-doped tin oxide was used as the substrate.Firstly, FTO glass substrates were ultrasonically soaked in deionized water, acetone, and absolute ethanol subsequently.Then the glass was soaked into the TiCl 4 solution with the concentration of 0.04 mol/L for 30 min at 70 ∘ C. The TiO 2 was coated on the FTO substrates and then sintered at 450 ∘ C for 30 min.The thickness of the TiO 2 film was 8 m which was controlled by the times of the screen printing.Here we used chemical bath deposition (CBD) to assemble QDs onto the TiO 2 photoelectrode.And the particle size of QDs was controlled by adjusting the dipping time.The TiO 2 film was firstly dipped into the Cd(NO 3 ) 2 ethanol solution (0.3 M) for 5 min, rinsed with the ethanol, and then dipped for another 5 min in the Na 2 S ethanol solution (0.1 M) and rinsed again with the ethanol.This is called one cycle,  and here we assembled QDs for 2∼10 cycles to discuss the effect of the particle size on the light absorbance.For the total dissolution of the solute, the ethanol and the deionized water are mixed as the solvent in which the proportion of the volume is 7 : 3.
The TiO 2 film with CdS QDs was soaked in the N719 for 44 hours.Then the TiO 2 electrodes were incorporated into thin-layer sandwich-type cells with Pt-coated FTO as counter electrode and electrolyte solution.The working area of the sample was 0.25 cm 2 .The surface morphology of the samples was conducted with the SEM.Optical absorption spectra of the TiO an illumination of xenon lamp.The irradiance is 100 mW/cm 2 which is measured by a radiometer (FZ-A, Photoelectric Instrument Factory of Beijing Normal University).

Results and Discussion
The thickness of bare TiO 2 film prepared using TiO 2 powders of P25 was about 5 m. Figure 1 shows the SEM of TiO 2 with different amount of CdS, and (a) was TiO 2 with CdS after 4 CBD cycles and (b) was 10 CBD cycles.From these images we can see that, with the increase cycles of the CdS, the interspaces of the TiO 2 particles are becoming smaller and smaller; this result indicates that the CdS was assembled onto the surface of TiO 2 particles, and the amount of the CdS QDs is increased with increasing CBD cycles.Figures 1(a) and 1(b) are SEM of TiO 2 with different amount of CdS; Figures 1(c) to 1(e) are typical XRD patterns and TEM images of the photo anode.
Figure 2 shows the absorbance spectra of CdS/TiO 2 films in the visible region.From this figure we can see that, with the increase of the cycles of CdS, the absorbance intensities are becoming stronger and stronger, and this suggests the increased number of CdS QDs.The extrapolations at the absorption edges show a red shift in the spectra.The shift suggests a growth in QD size with increasing CBD cycles.And this can be explained by the decrease in band gap energy arising from the increasing particle size in the quantum size regime.
The parameters of QDSSCs were tabulated in Table 1, and the corresponding trend of the parameters with increasing CBD cycles was shown in Figure 3.
From Table 1 and Figure 3 we can see that with the increasing cycles of CBD, the short circuit current ( sc ), open circuit voltage ( oc ), and the efficiency are improved before the CBD cycle is 6 and fill factor (FF) before 4, but these parameters are decreased with the continuing increase of CBD cycles.This suggests, with the increasing CBD cycles (before the cycles of 6), the size of the QDs is becoming larger and the energy level has been adjusted.This makes the  oc increase with the increase of the CBD cycles.This is according to the absorbance spectra of CdS/TiO 2 films.And also the more CBD cycles are, the bigger  sc is becoming, and this suggests the additional amount of CdS QDs.So is the efficiency.But when the CBD cycles are 8 or more, the  sc ,  oc , FF, and efficiency are decreased.This may be because the spaces between the TiO 2 particles are becoming blocked, and the electrolyte getting into the spaces is becoming harder, and this makes the hole and electron recombination easier, so the  sc decreases.This also causes the FF to decrease and so does the efficiency.So we can get the best efficiency with the CdS sensitized TiO 2 , which is 0.745% with the CBD cycle of 6.But we can see that the efficiency is still low, and the QDs also undergo photo corrosion and electrolyte corrosion.So here we make the cosensitized DSSC.Plots of photocurrent density against voltage of the cosensitized DSSC are shown in Figure 4.And the corresponding readings are tabulated in Table 2.When the TiO 2 was sensitized both by CdS and N719, the efficiency increases  as shown in Table 2.We can see that, with the assembly of CdS QDs, the efficiency first increases from 4.39% to 5.37%, but with the CBD cycle of 4 it decreases to 2.74% and when the cycle is 10, the efficiency decreases to 0.24%.The trend of the parameters of the DSSC is shown in Figure 5. From this figure we can see that when the CBD cycle is 2, the  sc ,  oc , FF, and efficiency are the best, and when the cycles increase these parameters become smaller.
The phenomena observed above can be explained as follows [9][10][11][12][13]: with the assembly of the CdS QDs, an energy level is inserted between the conduction band of TiO 2 and N719 as shown in Figure 6.When the CBD cycle is 2, the conduction band of the CdS is becoming closer to the conduction band of TiO 2 .Then the electron transition is becoming easier, so the  sc increases.But with the increase of the CBD cycles (such as 4), the spaces between the TiO 2 particles are becoming blocked, and the electrolyte getting into the spaces is becoming harder.What is more is that the electrons can transfer between the TiO 2 particles, so the recombination between the electron and hole is becoming easier, so the  sc decreases.This also causes the FF to decrease and so does the efficiency.So we can see that when CBD cycle is 2, the best efficiency can be obtained.The performance to other related cosensitized DSSCs is compared in Table 3.

Conclusions
The CdS QDs alone can sensitize TiO 2 films.In order to avoid photo corrosion and electrolyte corrosion, the TiO 2 films are cosensitized by CdS and N719.The band gap of CdS QDs was tuned by CBD cycles.With the inserting energy level of CdS, the  oc and  sc were increased and so was the efficiency.The efficiency was improved from 4.39% to 5.37% when the CBD cycle was 2.

Figure 1 :
Figure 1: SEM of TiO 2 with different amount of CdS, (a) 4 CBD cycles of CdS, (b) 10 CBD cycles of CdS, (c) typical XRD patterns, and (d) and (e) typical TEM images of the photo anode.

Figure 2 :
Figure 2: The absorbance spectra of CdS/TiO 2 films in the visible region.

Figure 6 :
Figure 6: Representation of energy level diagram in a cosensitized DSSC by N719 and CdS.

Table 1 :
The parameters of QDSSCs with CBD cycles of 2∼10.

Table 2 :
Parameters of DSSCs constructed with different sensitization conditions.

Table 3 :
The comparisons of the performance to other related cosensitized DSSCs.