Enhanced Photovoltaic Properties of the Solar Cells Based on Cosensitization of CdS and Hydrogenation

The hydrogenated TiO 2 porous nanocrystalline film is modified with CdS quantum dots by successive ionic layer adsorption and reaction (SILAR) method to prepare the cosensitized TiO 2 solar cells by CdS quantum dots and hydrogenation. The structure and topography of the composite photoanode film were confirmed by X-ray diffraction and scanning electron microscopy. With deposited CdS nanoparticles, UV absorption spectra of H:TiO 2 photoanode film indicated a considerably enhanced absorption in the visible region. The cosensitized TiO 2 solar cell by CdS quantum dots and hydrogenation presents much better photovoltaic properties than either CdS sensitized TiO 2 solar cells or hydrogenated TiO 2 solar cells, which displays enhanced photovoltaic performance with power conversion efficiency (η) of 1.99% (Jsc = 6.26mAcm , Voc = 0.65V, and FF = 0.49) under full one-sun illumination. The reason for the enhanced photovoltaic performance of the novel cosensitized solar cell is primarily explained by studying the Nyquist spectrums, IPCE spectra, dark current, and photovoltaic performances.


Introduction
Photoelectrochemical solar cells based on TiO 2 nanocrystalline films sensitized with organic dyes have been studied intensely for the past 20 years as a potential promising lowcost alternative to traditional solid-state solar cells.Semiconductor nanocrystals with narrow band gap such as CdS, CdSe, PbS, and InP were also demonstrated as efficient sensitizers in the spectral range from the visible to mid infrared with advantages such as low-cost fabrication, good stability, multiple exciton generation, and the tunability of optical properties and electronic structure by changing the size of nanocrystals [1,2].The photoelectrochemical cells with semiconductor nanocrystals as sensitizers also were called quantum dot sensitized solar cells (QDSSCs) [1].Among the common semiconductors, CdS is one of the most commonly used inorganic sensitizers [3] due to its optical properties and narrowed band gap adjusted by the particle size [4] and better UV stability [5] and it has reduced dark current [6].However, the efficiencies of CdS sensitized TiO 2 solar cells have still stayed low up to now.It has been extensively studied how to enhance their photovoltaic efficiency.One common effective method is cosensitization of more than one kind of quantum dots with different band gap.CdS and CdSe quantum dots cosensitized nanocrystalline TiO 2 is one of the most common cosensitization structures with a power conversion efficiency of more than 3% [7,8].
The other alternative method is to modify TiO 2 .In order to improve the photoelectric properties of TiO 2 under sunlight, some metal or nonmetal impurities [9,10] were added to generate donor or acceptor states in the band gap and to modulate energy band structure [11].Recently, the hydrogenated TiO 2 has attracted extensive attention.Chen et al. [12] demonstrated that the hydrogenated TiO 2 nanocrystal enhanced solar absorption by introducing disorder in the surface layers of nanophase TiO 2 , and Wang et al. [13] reported hydrogen treatment was a simple and effective strategy to fundamentally improve the photoelectrochemical performance of TiO 2 nanowires for water splitting.Our previous work reported the self-sensitized effect of hydrogenated TiO 2 film which led to enhanced photovoltaic properties in the solar cell with hydrogenated TiO 2 as photoanode without adding any dye [14].
If a photoanode was prepared by depositing CdS quantum dots on the surface of hydrogenated TiO 2 film, the self-sensitized effect of hydrogenated TiO 2 film would work together with quantum dots sensitization, which should also be a kind of cosensitization to be a promising method to enhance the photovoltaic properties of quantum dot sensitized solar cells.In this work, hydrogenated TiO 2 films were fabricated on fluorine-doped tin oxide (FTO) glasses by screen printing and annealing under the specific temperature and time; then CdS quantum dots were attached to the surface of hydrogenated TiO 2 by successive ionic layer adsorption and reaction (SILAR) method.Structural characterization, photoelectrochemical properties, and photovoltaic performances were investigated and discussed.The cosensitized TiO 2 solar cells by CdS quantum dots and hydrogenation present much better photovoltaic properties than either CdS sensitized TiO 2 solar cells or hydrogenated TiO 2 solar cells.The reason for the enhanced photovoltaic performance of the novel cosensitized inorganic solar cell was also explained in detail.

Preparation of Hydrogenated TiO 2 Films and CdS
Quantum Dots.Hydrogenated TiO 2 (H:TiO 2 ) films were prepared on fluorine-doped tin oxide (FTO) glasses with an area of 0.16 cm 2 via the same processing as reported previously [14].Then CdS quantum dots were attached to the surface of hydrogenated TiO 2 by successive ionic layer adsorption and reaction (SILAR) method [15,16].In brief, 0.1 M Cd(NO 3 ) 2 in methanol was used as the cation source and 0.1 M Na 2 S in 1 : 1 methanol and water as the anion source.The H:TiO 2 films with FTO substrates were successively dipped into the cation source and anion source for 5 min each.Following each dip, the films were rinsed for 1 min or more using pure ethanol to remove excess precursor, and the electrode was dried for 10 min before the next dipping.This dip cycle was repeated several times to obtain desirable CdS quantum dots on the surface of the H:TiO 2 films as photoanodes.According to the preliminary experimented research on the effect of SILAR cycles for CdS quantum dots, an optimized SILAR cycle was determined as 9 cycles.For comparison, the H:TiO 2 films without any quantum dots and TiO 2 films with CdS quantum dots were also fabricated as photoanodes.

Assembly of Solar Cells.
To assemble solar cells, the mixed solution of water and methanol with volume ratio of 3 : 7 consisting of 0.2 M KCl, 0.5 M Na 2 S, and 2.0 M S was used as polysulfide electrolyte.The photoanode, a platinized counter electrode, and the polysulfide electrolyte were sealed together with a hot-melt polymer film (Surlyn 1702-25, DuPont) to constitute a sandwich-like solar cell to measure photoelectrochemical properties.The active area of the cell is 0.16 cm 2 .

Testing Device and Characterization
Method.The crystal structure of TiO 2 films was characterized by an X-ray

Characterization of TiO 2 Films Cosensitized by CdS and
Hydrogenation.X-ray diffraction patterns of the H:TiO 2 film, CdS/TiO 2 film, and CdS/H:TiO 2 film are shown in Figure 1.TiO 2 in all the films is in the anatase phase, the same as reported in our previous work [14].With the successive ionic layer adsorption in 9 cycles, the peaks at 27.9 ∘ and 43.8 ∘ corresponding to (100) and (110) planes of CdS quantum dots, respectively, can be observed in the patterns of CdS sensitized TiO 2 and H:TiO 2 films as shown in Figures 1(c) and 1(d), respectively.No obvious difference is observed between the XRD patterns of the CdS sensitized TiO 2 films with or without hydrogenation.Figure 2 shows SEM surface morphology of pure H:TiO 2 film and CdS/H:TiO 2 film.The

Absorption of TiO 2 Films Cosensitized by CdS and Hydrogenation.
The UV-visible absorption spectra of H:TiO 2 film, CdS/TiO 2 film, and CdS/H:TiO 2 film are showed in Figure 3.It has been demonstrated that hydrogenation treatment can improve light absorption of TiO 2 due to the generated dangling bonds and disordered surface layers on the surface of TiO 2 nanophase [12].Compared with the H:TiO 2 film with a band gap of about 3.10 eV, the CdS/TiO 2 film has a much stronger absorption in the range of 300∼800 nm in that CdS has a much smaller band gap (2.25 eV in bulk) than H:TiO 2 [17].When CdS quantum dots are deposited on the surface of the H:TiO 2 films, the obtained CdS/H:TiO 2 film combines the advantages of both hydrogenation treatment and CdS quantum dots and thus reveals the strongest absorption as shown in Figure 3.

Impedance Spectra of the Solar Cells Based on Cosensitization of CdS and Hydrogenation.
In order to analyze the internal electron transport process of solar cells, the electrical impedance spectra (EIS) for the sensitized solar cells based on CdS/H:TiO 2 , CdS/TiO 2 , and H:TiO 2 are shown in Figure 4(a), and Figure 4(b) shows the relevant equivalentcircuit model.Similar to a typical DSSC system [18,19],  ct1 and CPE1 represent the electron transfer resistance and interfacial capacitance at the interface between counter electrode and electrolyte, respectively. ct2 and CPE2 are electron transport resistance and interfacial capacitance at the interface between electrolyte and photoanode, respectively.  is ohmic series connection resistance of the whole cell.  is the electrolyte Nernst diffusion impedance.Theoretically, the solar cell impedance spectroscopy has three semicircles representing high-, middle-, and low-frequency features, associated with  ct1 and CPE1,  ct2 and CPE2, and   , respectively [20].In Figure 4(a), the largest semicircle at middle-frequency almost covers the other two, which is attributed to carrier transportation and recombination at photoanode/electrolyte interfaces.The fitted values of  ct2 are 18 Ω, 32 Ω, and 56 Ω for the solar cells with photoanode of CdS/H:TiO 2 , CdS/TiO 2 , and H:TiO 2 , respectively.According to our previous work, hydrogenation treatment of TiO 2 can reduce  ct2 because the band gap is narrowed down and meanwhile the oxygen vacancy density increases.The loading of CdS nanoparticles on pure TiO 2 also decreases  ct2 of the CdS/TiO 2 /electrolyte interfaces significantly.For the CdS/H:TiO 2 photoanode, both of hydrogenation selfsensitization and CdS sensitization work together and result in the lowest  ct2 .The lower  ct2 reflects quicker electrons transport at the interface between electrolyte and photoanode, which implies that smaller probability of interface recombination occurs at the photoanode/electrolyte interface.

Photovoltaic Performances of the Solar Cells Based on Cosensitization of CdS and Hydrogenation.
The incident photon to current conversion efficiencies (IPCE) spectra of the solar cells with different photoanodes are shown in Figure 5.
The IPCE can be expressed as [21] IPCE where  LHE is the light harvesting efficiency,  inj is the electron injection yield, and  cc is the charge collection efficiency.It can be seen that the profile of IPCE plot corresponds well with the UV-vis absorption spectra in Figure 3.The IPCE of TiO 2 nanocrystal film without any sensitizer is low in the UV region and negligible in the visible region.The H:TiO 2 increases oxygen vacancy density and has much more trap states near the conduction band, leading to the enhancement of the absorption as reported in our previous work [14].
The absorption enhancement implies the increase of the light harvesting efficiency, which increases the IPCE value according to the above formula.Significantly enhanced IPCE can be observed in the CdS sensitized solar cells, which can be attributed to the fact that CdS quantum dots have much higher absorption coefficients of 10 5 to 10 6 M −1 cm −1 above the band gap energy [22].The CdS/H:TiO 2 solar cell exhibits the strongest IPCE due to the cosensitization of CdS and selfsensitization of hydrogenated TiO 2 .The maximum IPCE of the CdS/H:TiO 2 solar cell can reach 41.7% using a polysulfide electrolyte, suggesting that the cosensitization of CdS and hydrogenation has the potential to enhance the photoelectric properties.
Figure 6 shows the illuminated and dark - characteristics of the solar cells with different photoanodes, and the photovoltaic performances of them are listed in Table 1.The self-sensitization effect of H:TiO 2 photoanode with polysulfide electrolyte is very weak, exactly as our previous results of H:TiO 2 photoanode with the I − /I 3 − electrolyte.The CdS/TiO 2 QDSSC also displays the ordinary photovoltaic performance as others' work with an energy conversion efficiency of 1.1%.Compared with the other two solar cells, the CdS/H:TiO 2 solar cell, which is cosensitized by CdS and hydrogenation, exhibits enhanced photovoltaic characteristics with short circuit current density ( sc ) of 6.26 mA/cm 2 , open-circuit voltage ( oc ) of 0.65 V, fill factor (FF) of 0.49, and power conversion efficiency () of 1.99%.The enhancement of  sc can be explained by the equation:  sc =  inj −  rec , where  inj and  rec are the electron injection current density and the recombination current density [23].
The stronger absorption at the UV-vis (as shown in Figure 3) and the higher IPCE (as shown in Figure 5) of CdS/H:TiO 2 hold out the potential of increasing  inj and consequently enhancing the photoelectric effect.On the other hand, the recombination current is derived from the interface recombination mostly caused by the reduction reaction with the electron in the conduction band of TiO 2 and S  2− in the electrolyte, which would form the dark currents and reduce photovoltaic performance of solar cells.As shown in Figure 6(b), the dark current density of the solar cell with CdS/H:TiO 2 photoanode is obviously lower than that of the CdS/TiO 2 or H:TiO 2 solar cell, which should be attributed to lower  ct2 of the CdS/H:TiO 2 solar cell than that of the other two solar cells.The enhanced  inj and the suppressed dark current density of the CdS/H:TiO 2 solar cell lead to the enhancement of the  sc .
Under constant illumination, the solar cell reaches a photostationary situation, and  oc corresponds to the increase of the quasi-Fermi level of the semiconductor (  ) with respect to the dark value ( 0 ), which equals the electrolyte redox energy ( 0 =  redox ). oc can be determined by the following equation [24]: where  is the positive elementary charge;    is the thermal energy;  is the electron concentration in conduction band of the semiconductor photoanode under illumination;  0 is the electron concentration in the dark condition.Here,  and  0 can be characterized by IPCE and dark current density, respectively.The stronger IPCE as shown in Figure 5 implies the higher  of the CdS/H:TiO 2 solar cell than the H:TiO 2 solar cell and the CdS/TiO 2 solar cell, while the lower dark current density as shown in Figure 6(b) manifests the lower  0 of the CdS/H:TiO 2 solar cell than the other two solar cells.As a result, the higher  and the lower  0 lead to the improvement of  oc for the CdS/H:TiO 2 solar cell as shown in Figure 6(a).

Conclusions
In summary, a cosensitized TiO 2 photoanode by CdS quantum dots sensitization and self-sensitization of hydrogenated TiO 2 film was achieved by depositing CdS quantum dots on the surface of hydrogenated TiO 2 film.By comparing solar cells with different photoanodes of H:TiO 2 , CdS/TiO 2 , and CdS/H:TiO 2 , the cosensitization effect by CdS and hydrogenation in the CdS/H:TiO 2 solar cell enhanced photovoltaic performance with power conversion efficiency () of 1.99%, which was increased by more than 80% compared with CdS/TiO 2 solar cells.The cosensitization effect combined the quantum dots sensitization and self-sensitization of hydrogenated TiO 2 films and caused larger extension absorption in the visible light range, quicker electrons transport, smaller probability of interface recombination, and consequently better photovoltaic performance.This study will give some useful enlightenment to the development of novel inorganic low-cost solar cells.

Figure 4 : 2 Figure 5 :
Figure 4: (a) Electrochemical impedance spectra of the solar cells with different photoanodes; (b) the corresponding equivalent circuit.

Figure 6 :
Figure 6: The - characteristics of the solar cells with different photoanodes (a) under simulated AM 1.5 solar spectrum irradiation at 100 mW cm −2 and (b) in the dark.

Table 1 :
The photovoltaic performances of the solar cells with different photoanodes.