Layer-by-Layer CdS-Modified TiO 2 Film Electrodes for Enhancing the Absorption and Energy Conversion Efficiency of Solar Cells

A layer-by-layer assemble method was used to fabricate CdS quantum dots (QDs) sensitized electrodes. Scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), and transmission electron microscopy (TEM) have been utilized to characterize the samples. The absorption spectra and photovoltaic measurement confirmed that much more effective deposition of QDs in TiO2 matrix and much better power conversion performance were achieved for these multilayer electrodes compared with the ones fabricated by traditional single-layer assembly method.


Introduction
In recent years, dye-sensitized solar cells (DSSCs) have attracted a lot of scientific and technological interests owing to their high energy conversion efficiency, low production cost, and a facile fabrication process [1][2][3].Besides conventional ruthenium dyes, inorganic short-band-gap semiconductor quantum dots, such as CdS, CdSe, CdTe, PbS and others, have been researched as the good candidate sensitizers in the so-called quantum-dot-sensitized solar cell (QD-SSC) [4][5][6][7].The advantage of inorganic QDs over conventional ruthenium dyes is that QDs can generate multiple electron-hole pairs per proton and improve the efficiency of the solar cells [8].In addition, QDs' optic and electronic properties can be adjusted to match the solar spectrum much better by changing their shape and size [9][10][11].However, the difficulty of penetrating QDs into the mesoporous TiO 2 matrix hindered QD-SSCs from achieving higher energy conversion efficiency.The QDs assembly difficulty can mainly be attributed to the two following reasons: (1) QDs are generally not bound effectively to the TiO 2 crystalline surface to form a firmly anchoring QDs monolayer on TiO 2 nanoparticles [12].(2) It is relatively difficult for nanoscale QDs to penetrate deep into the TiO 2 mesoporous films and deposit in the underlayer of TiO 2 film.To solve the first problem, the various bifunctional surface modifiers are adopted as linker molecules to anchor the QDs on TiO 2 surface, which indeed enhance assembly of QDs [13][14][15].But little attention focused on the second problem.It is still unachievable to obtain an entire coverage of QDs on the surface of TiO 2 crystal throughout the mesoporous films.
In this work, we used multilayered quantum dots assembly method to fabricate CdS QDs sensitized solar cells.The increased coverage ratio, incorporation amount of QDs, and effective deposition of QDs in underlayer of TiO 2 mesoporous films were expected for these multilayer CdS quantum-dots-sensitized solar cells.

Experimental Details
8 g TiO 2 powder (P25, Degussa) was thoroughly mixed with 100 mL distilled water by vigorous stirring and ultrasonic.The slurry was then sprayed on transparent conductive oxide (TCO) glass (14 Ω/squ, Nippon Sheet Glass) and the films were dried at 120  The as-prepared samples were cleaned sequentially with distilled water.The spraying-drying-depositing procedure was termed as one SDD cycle.One to five cycles were applied to fabricate the TiO 2 (n)-CdS(n) electrodes (n = 1-5).The process used for the preparation of the TiO 2 (n)-CdS(n) electrodes was displayed in Figure 1.For comparison, onestep (abbr.OS) preparation of TiO 2 film (TiO 2 (OS)) with almost same thickness as TiO 2 (5) was also fabricated.CdS deposition process was similar to the procedure ascribed above except the longer CdS bath time of 10 h.TiO 2 (OS)-CdS and multilayer TiO 2 (n)-CdS(n) (n = 1-5) electrodes were all annealed at 400 • C for 30 min in nitrogen.
A surlyn spacer (30 μm thick, DuPont) was sandwiched between the CdS quantum-dot-sensitized electrode and the Pt-coated counterelectrode.The space between the electrodes was filled with the polysulphide electrolyte which consisted of Na 2 S (0.5 M), S (0.125 M), and KCl (0.2 M), using water/methanol (7 : 3 by volume) solution as co-solvent.A 0.25 cm 2 active area was defined by a hole punched through the surlyn frame and was additionally masked from illumination by black electrical tape to the same size.
The morphology and chemical composition of photoanode were examined by field-emission scanning electron microscopy (FESEM, JSM-6330F) equipped with an energydispersive X-ray spectrometer (EDS) analyzer.The detailed microscopic characterization of CdS QDs was analyzed using transmission electron microscope (TEM, JEM-2010HR).Absorption spectra of samples were recorded using a spectrophotometer (Hitachi U-4100).The photocurrentvoltage (I-V ) curve of each cell was measured by Keithley 2400 source meter under an illumination of a solar simulator (Newport Oriel 91192) at 100 mW cm −2 .Electron impedance spectroscopy (EIS) measurements were carried out by applying a 10 mV AC signal over the frequency range of 10 −2 -10 5 Hz under 100 mW cm −2 illumination at open circuit voltage by using an electrochemical workstation (CHI760C).

Results and Discussion
Figures 2(a) and 2(b) are the SEM images of TiO 2 film before and after CdS quantum dots deposition, respectively.The bare nanocrystalline TiO 2 film was exhibited in Figure 2(a).After CdS modification, there was a coating assembled on the surface of TiO 2 nanoparticles (Figure 2(b)).The EDS analysis (Figure 2(c)) confirmed that this coating consist of CdS. Figure 2(d) shows the TEM image of the CdS nanocrystal.The distance of 0.337 and 0.245 nm between the adjacent lattice fringes can be assigned to the interplanar distance of hexagon CdS (002) and (102) face, respectively.The diameter of QDs was about in the range of 5∼10 nm.
Figure 3 shows the comparison of the absorption spectrum of multilayer electrodes.The absorbance intensities of electrodes increased with SDD cycles.The absorption peaks of CdS-modified electrodes with different cycles were almost at same position of 450 nm, which was consistent to the value reported in the literature [16].For TiO 2 (5)-CdS(5), the absorption edge was approximately 510 nm, which was obtained from the intersection of the sharply decreasing region of a spectrum with its baseline.Corresponding to this absorption edge, the band gap was calculated to be 2.43 eV.That was higher than 2.25 eV, the value reported for CdS in bulk [16].It indicated that the size of the CdS particles deposited on the TiO 2 films were still within the scale of quantum dots.The size of CdS particles was possible to be estimated from the excitonic peaks of the absorption spectra based on the empirical equations proposed by Yu et al. [17].Then the mean diameter of CdS particles deposited on the TiO 2 films was calculated to be ca.8.23 nm.The size of CdS calculated based on empirical equations was consistent with that observed in TEM image.For comparison, the UV-Vis absorption spectra of TiO 2 (OS)-CdS was also showed in Figure 3.The absorbance of TiO 2 (OS)-CdS was lower than that of TiO 2 (5)-CdS(5) although their film thickness were almost the same, which displayed the superiority of multilayered electrodes in depositing quantum dots.The absorption edge of TiO 2 (OS)-CdS was also at ca. 510 nm, indicating that the size of CdS particles on TiO 2 (OS) was the same as that on multilayered electrodes.
The effect of SDD cycles on the device performance has also been studied.The photocurrent-voltage (I-V ) curves of the QD-SSCs using TiO 2 (n)-CdS(n) (n = 1-5) and TiO 2 (OS)-CdS as photoelectrodes were showed in Figure 4.It was considered that the absorption spectrum of photoelectrodes played an important role in determining the energy conversion efficiency of DSSCs.Inferred from Figures 3 and 4, the I-V properties of TiO 2 (n)-CdS(n) QD-SSCs (n = 1-5) were consistent to the absorbance of TiO 2 (n)-CdS(n) electrodes (n = 1-5).Table 1 summarizes the open circuit potential (V OC ), short circuit current density (J SC ), fill factor (FF), and overall energy conversion efficiency (η) of these cells.The V OC , J SC and η increased with cycles and the QD-SSC using TiO 2 (5)-CdS(5) photoelectrode exhibited the best performance due to the most incorporated amount of CdS on electrode.The V OC , J SC , and η of TiO 2 (OS)-CdS QD-SSC were only 0.31 V, 0.91 mA cm −2 , and 0.11%, respectively, which were much lower than those of TiO 2 (5)-CdS(5) QD-SSC (0.44 V, 7.12 mA cm −2 , and 1.06%), although its absorbance was little lower than that of TiO 2 (5)-CdS (5).It may be due to the difficulty for CdS QDs diffusing and depositing in the underlayer of TiO 2 mesoporous film, which resulted in overloading of CdS QDs in the surface of TiO 2 film.Overloading of QDs was reported to be disadvantageous to the cell performance because mesopores were blocked by extra loading of CdS QDs and the electrolyte diffusion was also hindered [18].So the advantage of multilayer TiO 2 films in cell performance over the traditional TiO 2 films was evident.The sealed multilayer CdS QDSSCs displayed good stability still retaining over 90% of their initial efficiency after 720 h of storage in ambient conditions.
Here we used only CdS QDs as multilayer sensitizer in this work.It is very promising to deposit different kinds of QDs, such as CdSe, CdTe, and PbSe and the others in multilayers of electrodes.The extending of absorption range, the increase of absorbance, and improvements in the cell efficiency could be possible for solar cell fabricated by multilayer TiO 2 film sensitized by multiple QDs with different absorption spectra range.
Electron impedance spectroscopy of multilayer QD-SSCs, shown in Figure 5, were measured to investigate the kinetics of charge transfer and recombination.An equivalent Table 1: A summary of photovoltaic and photoelectron transport properties of CdS QD-SSCs under AM1.5 condition.circuit used to fit the experimental data was shown as inset in Figure 5.By modeling and fitting the Nyquist plots, the electron transport parameters such as charge transfer resistance related to recombination of electrons at the TiO 2 /electrolyte interface (R k ), the first-order reaction rate constant for loss of electrons (k eff ), electron life time (τ eff ) and effective diffusion coefficient (D eff ) were extracted and also presented in Table 1.It was clear that R k decreased with the SDD cycles, for which there are two possible reasons: (1) an increase in film thickness and (2) an increase in the electron density owing to the increasing current density.k eff decreased with cycles varying from 1 to 3. Thus, the recombination rate decreased with cycles when cycle was below 3.This showed the reason the value of J SC and η increases greatly when cycles were within 3.But k eff increased when cycle was more than 3.The increasing recombination rate means that electrons were lost rapidly by recombination.Inferred from Table 1, although FF decreased with cycle changing from 1 to 3 and then increased when cycle was above 3, the growth rate of η for cells with cycle above 3 decreased comparing with that for ones with cycle below 3 under considering the rate of increase for film thickness.It was deduced that the increase of electron recombination rate may contribute to the decreasing  growth rate of η above 3 SDD cycles.D eff increased with the increasing film thickness, which was consistent with the result reported by Adachi et al. [19].As for TiO 2 (OS)-CdS, the R k value was similar with that of TiO 2 (1)-CdS (1), which should due to similar current density of these two cells.

Conclusion
We have demonstrated a layer-by-layer assembling method to improve the coverage of QDs on the surface of TiO 2 crystal throughout the mesoporous films effectively and its application in QD sensitized solar cells.The multilayer electrodes showed much more effective deposition of QDs in TiO 2 matrix and better power conversion performance, compared with the ones fabricated by traditional method.
The open circuit voltage, short circuit current, and overall power conversion efficiencies of multilayer CdS QD-SSCs increased with the layers and reached maximum value, 0.44 V, 7.12 mA cm −2 , and 1.06%, respectively, for 5 layers.

Figure 2 :Figure 3 :
Figure 2: SEM images of TiO 2 nanocrystalline film (a) before and (b) after CdS quantum dots deposition.(c) EDS spectra of CdS-modified TiO 2 .(d) TEM image of CdS quantum dots.

Figure 5 :
Figure 5: Experimental (dots) and fitted (solid curve) Nyquist plots of CdS QD-SSCs under AM1.5 condition.The equivalent circuit used to fit the experimental data was shown as inset.