Enhancement of Electron Transfer Efficiency in Solar Cells Based on PbS QD / N 719 Dye Cosensitizers

Cosensitized solar cells (CSSCs) have recently become an active subject in the field of sensitized solar cells (SSCs) due to their increasing electronic utilization. However, because of the dye molecules, layer must be single, dye-SSCs cannot be co-sensitized with two different dyes to form two different molecules layer. But it is possible to be cosensitized with quantum dots (QDs) and dyes. Here we designed novel photoanode architecture, namely, PbS QDs and N719 dyes are used as co-sensitizers of the TiO2 mesoporous film. The experimental result shows that PbS QDs/N719 dyes co-sensitized structure can make PbS QDs and N719 dyes mutual improvement. Taking the advantage of PbS not only achieved higher transfer efficiency of photo-excited electron, but also achieved obviously wider range and higher intensity of absorption. The PbS QDs which have been deposited on the TiO2 film was coated by N719 dyes, which can effectively prevent PbS QDs from corroding by I/I3 electrolyte and light. As we expected, the solar energy-conversion efficiency which is showed by CSSCs fabricated following these photoanodes is relatively higher than the PbS QDs or N719 dyes, single-sensitized solar cells under the illumination of one sun.


Introduction
Quantum dots-sensitized solar cells (QDSSCs) and dyesensitized solar cells (DSSCs) have attracted more and more interest due to their characteristics of low cost, surrounding friendly and high energy-conversion efficiency than the conventional silicon-based solar cell.The electrode of DSSCs usually includes a mesoporous oxide layer (such as TiO 2 , ZnO, and SnO 2 ) and a single molecular dye layer [1].The dyes on the surface of the mesoporous oxide film play a role in increasing the optical activity of electrode in visible light region [1,2].In addition to the organic sensitizers, semiconductor quantum dots (QDs), such as CdS [3][4][5][6][7], CdSe [7][8][9][10], PbS [11][12][13], and ZnSe [14] which absorb light in the visible region, can also serve as sensitizers of DSSCs, that is, QDSSCs [15].The specific advantages of QDs over organic materials in light harvest come from the quantum confinement effect [16].Although, the maximum efficiency theoretically predicted for a DQSSCs is higher than that for DSSCs using organic sensitizers.But now their energy conversion is lower than DSSCs.So as to extend absorption range and increase conversion efficiency, cosensitization has been used in QDSSCs.QDSSc constructed with Mn-doped CdS/CdSe co-sensitized delivered highest power conversion efficiency of 5.4% by Santra et al. prepared [17].Lee and Lo have also prepared QDSSCs cosensitization of CdS/CdSe which showed the higher conversion efficiency (4.22%) [7]. Lee et al. have prepared QDSSCs based on CdSe and Mg-doped CdSe nanocrystals which also showed higher performance than that of single-sensitized solar cell [18].However, because of the single dye molecules, layer must be single, DSSCs cannot be co-sensitized with two different dyes forming two different molecules layers.But it is impossible to be co-sensitized with quantum dots and dyes.
The unique property of PbS can effectively inhibit the charge recombination, which is different from other semiconductors [19,20].Taking this advantage of PbS, the novel electrode structure based on PbS QDs/N719 dyes co-sensitization is prepared.It is the main purpose to enhance the transfer efficiency of photoexcited electron, meanwhile, extend absorption range and increase absorption intensity.Firstly, PbS nanoparticles are deposited on the TiO 2 mesoporous film using screen printing by SILAR method.Then, the PbS QDs/TiO 2 mesoporous film is immersed in N719 dyes.In this work, taking the advantage of PbS not only achieves higher transfer efficiency of photoexcited electron, but also achieves wider range and higher intensity of absorption obviously.The PbS QDs which have been deposited on the TiO 2 film were coated by N719 dyes, which can effectively prevent PbS QDs from corroding by I − /I − 3 electrolyte and light.As we expected, the solar energyconversion efficiency which is showed by CSSC fabricated following these photoanode is relatively higher than the PbS QDs or N719 dyes single-sensitized solar cell, under the illumination of one sun (AM 1.5, under 100 mW/cm 2 ).

Experimental and Characterization
TiO 2 mesoporous films were prepared by screen printing of TiO 2 paste on clear Fn-doped SnO 2 oxide (FTO) glass substrates, which have been pretreated by being immers into TiCl 4 aqueous solution and then sintered at 70 • C for 30 min.Followed by sintering at 450 • C for 30 min, successive ionic layer absorption and reaction (SILAR) method was used to assemble PbS QDs onto the TiO 2 mesoporous film.The TiO 2 film was dipped into ethanol solution containing Pb(NO 3 ) 2 (0.1 M/L) which was precursor solution of Pb 2+ for 60 s, rinsed with ethanol (then dried with Nitrogen), and dipped another 60 s in the Na 2 S (0.1 M/L) methanol solution which was precursor solution of S 2− and rinsed again with methanol (then dried with nitrogen), which was one SILAR cycles.Then, the TiO 2 film which have been coated by PbS QDs was immersed into N719 dyes paste for 44 h.The preparation of photoanode was completed.
A field-emission gun scanning electron microscope (SEM, QUANTA 200) with an energy dispersive X-ray spectroscopy (EDS) system was used to characterize the morphology and composition of the samples.Energy dispersive X-ray spectroscopy (EDS) image was recorded by energy dispersive X-ray spectroscopy (EDS) system.The ultraviolet visible (UV-Vis) spectroscopy was recorded in the wavelength range of 350-1000 nm by using the Cary 5000 spectrometer (US Varian).The photocurrent-voltage (J-V) curves were measured under an illumination of a solar simulator at one sun (AM1.5, 100 mWcm −2 ).An Eco-Chemie Autolab potentiostat/galvanostat (Princeton Research, V3-400) was used to record the current-voltage (J-V) curves.

Results and Discussion
In order to understand the action of PbS QDs in the PbS QDs/N719 dyes, co-sensitizer, the SILAR cycles which were used to deposited PbS QDs range from 0 to 3 cycles and deposition time was 30 seconds.Samples were named as N719, 30PbS1-N719, 30PbS2-N719, and 30PbS3-N719.
For the sake of ensuring that PbS have assembled on the TiO 2 mesoporous film before coated N719 dyes, the surface morphologies of bare TiO 2 mesoporous film using screen printing and the PbS-sensitized TiO 2 mesoporous film using SILAR method were characterized by SEM.The SEM images are shown in Figure 1; not very apparent difference in the surface morphology was observed from the SEM images after PdS QDs were assembled on the TiO 2 film.This result indicates that particle size of PbS is so surprisingly small that the presence of these little particles cannot be distinguished under resolution of the SEM.But if carefully observed, we find that the mesoporous have become smaller after deposited PbS QDs.The EDS image of PbS QDs/TiO 2 mesoporous film which is in the upper right corner of Figure 1   By using SILAR process, PbS could be deposited onto the TiO 2 mesoporous film and the incorporated amount of QDs is raised with increasing SILAR cycles.Then films were dipped into N719 dyes pate and can get the PbS QDs/N719 dyes co-sensitized electrode.The variation of UVvis spectrum is shown in Figure 2(a).We can observe that absorption of PbS QDs/N719 dyes, co-sensitized electrode shifts to long, wavelength range, known as red shift in the meantime, have obviously higher absorption intensity than PbS QDs or N719 dyes single sensitized.That means the PbS QDs/N719 dyes co-sensitized photoanode has an increased absorption coefficient which is advantageous to properties of Co-sensitized solar cells (CSSCs).
Those photoanodes, namely, N719, 30PbS1-N719, 30PbS2-N719, and 30PbS3-N719, were used to prepare SSCs.Measured J-V curves are shown in Figure 2(b), responding to parameters listed in Table 1.As shown as Figure 3, PbS and N719 are brought in contact as a cascade structure and existing energy levels alignment that make conduction band levels of N719/PbS/TiO 2 forming stepwise structure, which is conducive to electron transport.More importantly, PbS QDs not only can efficiently prevent electrons and holes recombination [19,20], but also increase the specific surface area where will be deposited more N719 dye molecules.Based on above superiors, a higher transfer efficiency of photoexcited electron is achieved.In addition, PbS QDs which have been coated by N719 dyes can be effectively prevented from corroding by I − /I − 3 electrolyte and light.But we find that the energy conversion efficiency of 30PbS1-N719 is lower than N719.That is because the PbS QDs have very little effect on the specific surface area when, deposited one layer QDs, but instead make N719 dyes absorption decreased, which reduce the short-circuit current density (J SC ).The SILAR cycles increase to 3 layers and more will increase the number and the particle size of PbS QDs, then dipped N719 dyes, which will clog the pores of TiO 2 film.As a result, the difficulties of electrolyte diffuse into pores, higher recombination of photoexcited electrons, and reduction of J SC , so energy that conversion efficiency (Eff.) of 30PbS3-N719 is lower than of 30PbS2-N719.
To achieve a better result, we tailored the deposition time of SILAR cycles to control particle size of PbS QDs.The PbS deposition SILAR cycles were all 2 layers, the deposition time is 0 S, 30 S, 60 S, and 90 S, respectively.Samples are named as N719, 30PbS2-N719, 60PbS2-N719, and 90PbS-N719.
From Figure 4 and Table 2, we find that the increase in deposition time will raise J SC , open-circle voltage (V oc ), and energy, conversion efficiency.The best deposition time of PbS using SILAR method is 60 s an energy conversion efficiency of 5.14% is achieved.Continue to increase the time, the J SC , V oc , and Eff. will reduce.Because particle size of PbS QDs will become too larg to clog the small mesoporous after dipped N719 dyes, this is like 30PbS3-N719.

Conclusion
The PbS QDs/N719 dyes co-sensitized photoanode as the work electrode of SSCs is better than, PbS QDs or N719 dye, single-sensitized photoanodes.PbS QDs/N719 dyes cosensitized structure can make PbS QDs and N719 dyes mutualy improved.Taking the advantage of PbS not only achieved higher transfer efficiency of photoexcited electron, but also achieved obviously wider range and higher intensity of absorption.The PbS QDs which have been deposited on the TiO 2 film were coated by N719 dyes, which can effectively prevent PbS QDs from corroding by I − /I − 3 electrolyte and light.The solar energy-conversion efficiency which is showed by CSSC fabricated following PbS QDs/N719 dyes cosensitized photoanode is relatively higher than the PbS QDs or N719 dyes single-sensitized solar cell, under the illumination of one sun.

Figure 1 :
Figure 1: (a) SEM image of bare TiO 2 mesoporous film and (b) SEM image of PbS QDs-coated TiO 2 mesoporous film, where SILAR cycles are 2 cycles.EDX image of PbS QDs/TiO 2 mesoporous film which is contained (b).
(b) clearly indicates that PbS QDs have successfully deposited onto the TiO 2 film.

Table 2 :
Performance parameter of PbS QDs different deposition time.