The Reduced Recombination and the Enhanced Lifetime of Excited Electron in QDSSCs Based on Different ZnS and SiO 2 Passivation

In this study, we focus on the enhanced absorption and reduced recombination of quantum dot solar cells based on photoanodes which were coated by different ZnS or SiO2 passivations using the successive ionic layer absorption and reaction methods. The quantum dot solar cells based on photoanode multilayers, which were coated with a ZnS or SiO2 passivation, increased dramatic absorption in the visible light region as compared with other photoanodes and reduced rapid recombination proccesses in photovoltaic. As a result, the performance efficiency of TiO2/CdS/CdSe photoanode with SiO2 passivation increased by 150% and 375% compared with TiO2/CdS/CdSe with ZnS passivation and TiO2/CdSe photoanode, respectively. For this reason, we note that the tandem multilayers can absorb more wavelengths in the visible light region to increase a large amount of excited electrons, which are transferred into the TiO2 conduction band, and decrease number of electrons returned to the polysulfide electrolyte from QDs when a ZnS or SiO2 passivation is consumed. Moreover, it is obvious that there was a far shift towards long waves in UV-Vis spectra and a sharp drop of intensity in photoluminescence spectra. In addition, the dynamic process in solar cells was carried out by electrochemical impedance spectra.


Introduction
In recent years, quantum dot-sensitized solar cells (QDSSCs) based on quantum dots (QDs) have been considerable interest as promising candidates to replace the dye-sensitized solar cells (DSSCs) because QDs have potential functions such as a high extinction coefficient, tunable bandgaps, a large intrinsic dipole moment, low processing cost compared to organic dyes [1,2], and generation of multiple excitons [3].The QDs were used in QDSSCs as sensitizers which include PbS [4,5], PbSeS [6], CdS [7], CdSe [8], CdTe [9], and ZnS [10].The highest efficiency, which was achieved by Jara et al., was approximately 2.51% for CuInS 2 [11] because the electrons in the CB of the QDs moved to the electrolyte and recombined or got blocked.Moreover, tandem multilayers can go up the power conversion efficiency in the QDSSCs.
Like for instance, Woo et al. [12] and Ravindran et al. [13] studied the QDSSCs based on the TiO 2 /CdS/CuInS 2 tandem multilayers and the power conversion efficiency was about 1.47%.This photovoltaic performance was higher than those of the CdS and CuInS 2 QDs alone.However, the efficiency was still minor because of increased combination processes at the QDs/TiO 2 surface and plenty of electrons moving to the electrolyte.
Overall, this work illustrates the QDSSCs based on different tandem multilayered photoanodes to enhance absorption in the QDSSCs.In addition, ZnS and SiO 2 coating layer was applied to the TiO 2 /CdS/CdSe electrode to reduce recombination and electron movement to the electrolyte.Therefore, the efficiency of the QDSSCs can be an upward trend, particularly, compared with TiO 2 /CdS and TiO 2 /CdSe photoanodes.Firstly, the TiO 2 film was dipped in 0.5 M Cd 2+ solution for 1 min and 0.5 M S 2− solution for 1 min after being dried in the air to make one SILAR layer.Thickness of the electrode layer showed up in repeat of the three time layers.Secondly, the TiO 2 /CdS film was then dipped in Cd 2+ solution for 5 min and Se 2− for 5 min after rinsing with Milli-Q ultrapure water to make one SILAR layer.Thickness of the TiO 2 /CdS/ CdSe was increased in repeat of the three time layers.Finally, a TiO 2 /CdS/CdSe film was dipped in 0.1 M Zn 2+ solution for 5 min and 0.1 M S 2− solution for 5 min after rinsing with Milli-Q ultrapure water to make one SILAR layer.Thickness of the TiO 2 /CdS/CdSe with ZnS layer had a growth in repeat the two time layers.Similarly, a TiO 2 /CdS/CdSe was immersed in a beaker with an aqueous solution-mixed tetraethyl orthosilicate (0.01 M) and NaOH (0.1 M) during 15 min.After that, the films were rinsed with water and dried in air.The QDSSCs were prepared with an active area of 0.192 cm 2 , a photoanode, and a cathode.The space between photoanode and counter electrode was filled with the polysulfide electrolyte, which consisted of 0.5 M Na 2 S, 0.2 M S, and 0.2 M KCl in Milli-Q ultrapure water/methanol (7 : 3 by volume).

Characterization.
The morphologies of samples were investigated using field emission scanning electron microscopy (FESEM).The crystal structure was analyzed using an X-ray diffractometer (Philips, PANalytical X'Pert, CuKα radiation).The absorption properties of the samples were investigated using a diffuse reflectance UV-Vis spectrometer (JASCO V-670).The current-voltage (J-V) characteristics of the QDSSC were measured under AM 1.5 (100 mW/cm 2 ) illumination, which was provided by a solar simulator (Oriel, USA), and recorded by a Keithley model 2400 digital source meter.Electrochemical impedance spectroscopy (EIS) measurements were obtained by an FRA-equipped PGSTAT-30 from Autolab.The measurements were performed in dark conditions at negative bias, and the frequency range was from 500 kHz to 0.1 Hz with a modulation amplitude of 20 mV.

Results and Discussions
3.1.Structure and Morphology of Material.To obtain information about the QDs' size and crystalline photoanodes, the electrodes were annealed in vacuum at 200 °C.They were measured by using an X-ray diffractometer (XRD).Figure 1(a) illustrates the XRD of the TiO 2 /CdS/CdSe photoanode.As can be seen from Figure 1(a), it is obvious that the XRD of photoanode clearly appears the strongest peak at 25.4 °posi- tion corresponding to the (101) plane which indicates an upstanding TiO 2 anatase [14].In addition, the XRD illustrates three peaks at 26.4 °, 44 °, and 51.6 °corresponding to the (111), (220), and (311) planes of CdS and CdSe cubic phase [10].Similarly, the crystallization of electrode was also determined by Raman spectra.It is noticeable that four modes are observed at 144 cm −1 , 397 cm −1 , 517 cm −1 , and 638.5 cm −1 corresponding to phonon vibrations of TiO 2 anatase [15].Moreover, three peaks also appeared at 206.5 cm −1 and 395 cm −1 , corresponding to phonons of longitudinal optical vibrations in CdSe and at 298 cm −1 corresponding to the longitudinal optical mode in CdS.
To observe shape of the photoanode, FESEM of the TiO 2 / CdS/CdSe electrode was carried out.It is obvious that the white TiO 2 electrode became yellow and red because of the , this is a cross-sectional FESEM of TiO 2 /CdS/CdSe, which is homogeneous and strongly adherent to the substrate and shows cutting shape of film with thickness of 12 μm.This also proves that CdS, CdSe, and QDs have successfully coated on the surfaces of TiO 2 .
3.2.The Optical Photoanodes.The optical CdS/CdSe-cosensitized TiO 2 films can be monitored by studying the absorbance and energy bandgap of the materials.Figure 2(a) shows the UV-Vis spectra with distrinct photoanodes.The absorption of the TiO 2 /CdS/CdSe with ZnS or SiO 2 passivation electrode is expanded through the visible region compared with other photoanodes.The absorbance of the TiO 2 /CdS/CdSe with SiO 2 passivation electrode progresses because the loaded CdS and CdSe concentrations on the TiO 2 film can absorb more photons in the visible region from 400 nm to 700 nm.
We can determine optical bandgap of materials based on the relation between the absorption coefficient (α) and the incident photon energy (hν) using Tauc [16].
Besides, to know materials inside, we can also calculate the position of conduction band and valence band for materials, which depend on energy bandgap of materials as follows: where for CdS or for CdSe, and with M = Cd, S, and Se.Where E o CB is the conduction band potential, E e is a given constant equal 4.5 eV [17].E EA for Cd, S, and Se are 0 eV, 2.077 eV, and 2.02 eV, respectively.Similarly, E ion for Cd, S, and Se are 8.99 eV, 10.36 eV, and 9.75 eV, respectively.The position of conduction and valence bands of CdS and CdSe are recorded in Table 1. Figure 2(b) can plot (αhν) 2 versus the photon energy (hν) using data from the optical absorption spectra.The plotting results are recorded in Table 1.It is immediately remarkable that there is a steadily dropping tendency of energy bandgap from 2.8 eV of pure TiO 2 film to 1.83 eV of TiO 2 /CdS/CdSe/SiO 2 film.The obtained results are in good agreement UV-Vis spectra.However, there is a sharp increase in bandgap of CdS and CdSe in bulk and quantum dots.From data in Table 1, it is obvious that the position of conduction and valence bands is shifted toward more negative potential and more positive potential by decreasing the size of particles.Like for instance, the position of conduction and valence bands for CdS shifted toward from −4.25 eV to −4.11 eV and from −6.5 eV to −6.64 eV, respectively.Likewise, the position of conduction and valence bands for CdSe shifted toward between −4.5 eV and− 4.3 eV and between −6.2 eV and− 6.4 eV, respectively (Figures 3(a) and 3(b)).According to the band-edge alignment of CdS and CdSe in Figure 3, the CdS has a high position of conduction band as that of CdSe.Thus, the transfer of excited electrons from CdS to CdSe and TiO 2 facilitates.

International Journal of Photoenergy
To study the relative injected electrons in the TiO 2 film from CdS and CdSe QDs, photoluminescence (PL) was carried out (Figures 4(a) and 4(b)).It reveals that there is a significant drop in the PL of TiO 2 /CdS, TiO 2 /CdSe, TiO 2 /CdS/ CdSe with ZnS, and TiO 2 /CdS/CdSe with SiO 2 photoanodes as compared with that of QD films under identical conditions.The bandgap of TiO 2 (3.2 eV) limits its absorption range below 400 nm.The CdS QDs have a lower CB than CB of CdSe QDs (3 nm size).Therefore, the electron injection from the CB of CdSe QDs to the CB of CdS QDs is effective because CdS and CdSe QDs have a higher quasi Fermi levels than that of TiO 2 .Yella et al. reported that when CdS and CdSe QDs were made a cascade structure, the electrons were injected from CdSe to CdS QDs and then to TiO 2 nanoparticles [18].The redistribution of electrons results in a stepwise band structure.The insertion of a CdS layer between TiO 2 and CdSe elevates the CdSe QDs' CB and generates higher driving force for electron transportation.In addition, the quantum confinement effect makes the energy levels of the CB more negative with decreasing the size of particle [19].The PL intensity of all photoanodes decreased sharply because the CB of CdSe QDs is as high as that of CdS QDs and TiO 2 nanoparticles.Therefore, the electron injection from the CB of CdSe QDs to the CB of CdS QDs and TiO 2 is effective.Besides, the results reveal that the PL of the anode film loaded with SiO 2 is higher than that of the anode film loaded with ZnS.The possible reason is reduced recombination process in nanoparticles, leading to 5 International Journal of Photoenergy increase the PL of nanoparticles.The increase of PL can also boost the emission quantum yield (QY) of QDs, which is helpful for producing more excitons.

Photovoltaic
Performance of the QDSSCs.The characterization by J-V curves provides limited resources to analyze the mechanisms that govern the performance.A thorough analysis requires discriminating several factors such as series resistance, recombination parameters, and carrier energetics.These factors are not commonly available simply by analyzing J-V curve data, notably because J-V curves are elastic with respect the models and can be equally described by many different frameworks.Figure 4(c) illustrates the J-V curves of the QDSSCs with distinct photoanodes with sensitized TiO 2 nanoparticles (active area: 0.192 cm 2 ) at AM 1.5 (100 mW/cm 2 ).All QDSSCs has appeared the J-V characteristic.The performance parameters of QDSSCs are listed in Table 2.The thickness of the QDs was optimized with CdS (3 layers), CdSe (3 layers), and SiO 2 (2 layers) in this work.The TiO 2 /CdS/CdSe/SiO 2 exhibits the highest performance: η = 3 01% (we used Pt counter electrode), V OC = 0.54 V, J SC = 12 mA/cm 2 , and FF = 0.46.It is immediately noticeable that J SC of the QDSSCs improved significant after SiO 2 coating and is attributed to the excited electrons characteristics, such as separation, collection, injection, and recombination.The TiO 2 /CdS/CdSe was coated with SiO 2 or ZnS passivation which increased the significant light harvesting, separation, collection, and injection but reduced recombination.
3.4.EIS of the QDSSCs.The EIS characteristics were found by Mora-Sero et al. [20] to investigate the dynamic processes in the QDSSCs.We can know the information about charge processes through the QDs/TiO 2 /FTO contacts and diffusion processes of the electrons in electrolyte and TiO 2 film.Moreover, the lifetime of electrons can be determined as follows [21].Moreover, we also focused on the dynamic resistance in the QDSSCs, which is determined from the J-V curves.The external resistance is the internal resistance is and the shunt resistance is where V and I are the voltage and current at the point on the I-V curve; V OC , I ph , and I o are the open voltage, short current, and current density saturation, respectively [22].All resistances are recorded in Table 3.
As can be seen from Figure 5, it is noticeable that the radius of the semicircles increases in the following order: TiO 2 /CdS, TiO 2 /CdSe, TiO 2 /CdS/CdSe with ZnS, and TiO 2 / CdS/CdSe with SiO 2 passivation.The semicircles for the QDSSCs with TiO 2 /CdS/CdSe with ZnS and TiO 2 /CdS/CdSe with SiO 2 retained their shapes, but the radius of the semicircles expanded more because semicircles 1 and 3 were mixed onto semicircle 2. Therefore, we focus on the change of semicircle 2 at the intermediate frequency.

Conclusions
In

Figure 3 :
Figure 3: Energy levels alignment of the photoanode: (a) bulk and (b) quantum dots.

Figure 5
Figure 5 illustrates the EIS of the QDSSCs based on the different photoanodes.The Nyquist plots have three semicircles with different frequencies when the concentration of the photoanodes changes.The semicircle at high frequency (95 Hz-1000 kHz) is small because of the resistance against the electron diffusion in FTO/TiO 2 and Pt/electrolyte surfaces (denoted by R ct1 ).The semicircle at the intermediate

Table 1 :
The parameters obtained from UV-Vis.
Table 3 illustrates that R ct2 increased gradually and then peaked at 132 Ω corresponding to TiO 2 /CdS/CdSe with SiO 2 passivation because

Table 2 :
[23][24][25][26][27][28]different QDSSCs obtained from J-V curves.On the contrary, the result from Table2gives that the efficiency of QDSSCs-based TiO 2 /CdS/CdSe with SiO 2 photoanode is the highest (3.01%).This means that SiO 2 layer is a passivation to protect CdS and CdSe QDs from electrolyte[23][24][25][26][27][28]and reduced recombination processes at surface contacts and diffusion in the TiO 2 film.In addition to this, I suppose that there was an increase of the electron lifetime of TiO 2 /CdS/CdSe with SiO 2 photoanode-sensitized solar cells and which is near twice that of other photoanode-sensitized solar cells.With this lifetime, the excited electrons are enough time to transfer from CdSe and CdS to TiO 2 .Thus, the current density of QDSSCs increased sharply.This result is also consistent with that of UV-Vis spectra, PL spectra, and J-V curves.In the same way, the values of resistance dynamic R s , R D , and R d enhanced minor while the thickness of photoanodes went up.The results show that the resistance values depended on the thickness of the QD films.External and internal resistances at the same bias voltage under similar illumination conditions are distinct.This is due to the voltage-dependent nature of R d .The dissimilar nature of R d at different illuminating conditions is also noted.This can be expected as the values of diode factor at different illuminations are not the same.R d and R D of each illumination condition can differ approximately 40%.Therefore, a clear distinction should be made when one studies and measures the two types of dynamic resistance.
summary, we have successfully enhanced performance QDSSCs based on the distinct electrodes with ZnS or SiO 2 passivation.The highest efficiency of the QDSSCs based on the TiO 2 /CdS/CdSe with SiO 2 passivation electrode is approximately 3.01%, and the short current density is 12 mA/cm 2 compared with that of other electrodes because ZnS or SiO 2 passivation protected CdS and CdSe QDs from electrolyte and reduced recombination processes at surface contacts and diffusion in the TiO 2 film.

Table 3 :
Parameters obtained from the EIS measurements and J-V curves.