The effect of surface plasmon resonance on the photovoltaic properties of CdS quantum dot sensitized solar cells

Use of surface plasmon resonance effect of the Ag nanoparticles (Ag NPs) towards the photophysical properties of CdS quantum dot sensitized TiO 2 based solar cells (QDSSCs) was investigated under the illumination of 100 mW cm -2 (AM 1.5). Depositions of CdS on TiO 2 electrodes were carried out by successive ionic layer adsorption and reaction (SILAR) method with optimized ten cycles. Current voltage characteristics of QDSSCs fabricated with optimum amount of Ag NPs (0.2 % w/v) in TiO 2 showed ~ 26% increment in the power conversion efficiency from 1.09 to 1.37%. Surface morphological studies of photoanodes were carried out with scanning electron microscopy, high-resolution transition electron microscopy and energy dispersive X-ray measurements (EDX). The existence of spherical shape Ag nanoparticles in TiO 2 with an average particle size of 35 nm was confirmed by EDX analysis. The estimated average size of the TiO 2 and CdS particles appears to be around 40 nm and 4.2 nm respectively.


INTRODUCTION
Quantum dot (QD) sensitized solar cells (QDSSCs) are considered to be one of the promising alternative for low-cost next-generation photovoltaic devices due to the tunable band gap, high absorption coefficient and multiple exciton generation (MEG) of QDs. In QDSSCs, mesoporous TiO2 nanostructures which are usually fabricated on fluorine doped tin oxide (FTO) conducting glass substrates are sensitized with QDs. Among the QDs used as the sensitizers, CdS has attracted a great attention due to its excellent sensitizing ability in the visible regions of the solar spectrum [1][2][3]. It has been well established that metal nanoparticles (NPs) , such as gold and silver can enhance the photovoltaic properties of QDSSCs due to their plasmon resonance effect (SPR) associated with the enhancement of electromagnetic field due to collective electron oscillation [4][5]. Further, these NPs can act as the light trapping agent and electron traps for facilitating charge separation. However, in most of the studies, scientists have investigated the effect of incorporation of nanoparticles into the wide band gap semiconductors like TiO2. In this scenario, SPR effect of Ag NPs on the TiO2 dye sensitized solar cell (DSSC) was extensively studied. In separate studies Saravanan et al [6] and Jeong et al [7] have reported the performance enhancement of DSSCs fabricated with silver nanoparticle incorporated TiO2 photoanodes. According to them, the photo current generation is enhanced due to the increase in the light trapping as a result of the SPR effect of the incorporated AgNPs in the photoanode and there by the enhancement of the overall performances of DSSCs. However, there are very few studies available on the usage of SPR effect on the quantum dot sensitized solar cells. Zhao et al has reported a highly efficient CdS/CdSe QDs sensitized solar cells by incorporating Ag NPs in to the TiO2/ZnO photoanode and observed an increase in light harvesting and a decrease in surface charge recombination while prolonging the electron life time [8]. However, according to our knowledge, there is no any evidence for QD sensitized solar cells with Ag NPs incorporated TiO2 electrodes sensitized with CdS photoanodes. In view of this, here, the possibility of use of SPR in the CdS quantum dot sensitized TiO2 based solar cells and its effect on the photo physical properties of the device have been explored.

Preparation of Ag nanoparticles
Silver nanoparticles were synthesized by using a facile one-pot chemical reduction method as described in the literature [9]. That is briefly by adding 0.25 g of AgNO3 (Sigma-Aldrich) to a 20 ml of Di-methyl formaldehyde (>99%, BDH),(DMF), containing 0.85g of Polyvinyl pyrrolidine (PVP-40,000 M.W, BDH), PVP, at room temperature and heating the reaction mixture at 90 °C for 6 h. Ag colloidal dispersions with red, blue and green colors were collected at different time intervals. Using acetone as the precipitating agent, colloidal Ag nanoparticles were centrifuged at 3000 rpm and dispersed in ethanol to obtain 0.40% (W/V) Ag nanoparticle dispersion.

Fabrication of TiO2 photoanodes
In the preparation of photoanodes, at first, a compact layer of TiO2 (P90) was spin coated on pre-cleaned fluorine-doped, tin oxide (FTO) glass substrates (8 Ω cm-2, Solarnoix) by using a mixture prepared with TiO2 P90 (Evonik) powder and HNO3. Spin coating of this layer was done for 1 minute at 3000 rpm. The spin coated substrate was sintered at 450 °C for 45 minutes. Subsequently a coating of TiO2 P25 (Degussa), active layer was "doctor bladed" on the P90 layer of the above substrate by using a mixture of 0.25 g of TiO2 P25 powder, 1 ml of 0.1 M HNO3, 0.05 g of polyethylene glycol (99.8%, Sigma-Aldrich) and 1 drop of triton X-100 (Sigma-Aldrich). Finally, the substrates with all these two layers were resintered at 450 °C for 45 minutes and allowed to cool down to room temperature. Different amounts of previously prepared silver NPs were drop casted on some of the above TiO2 photoanodes and dried at 100 °C for 45 minutes.

Characterization of materials
Fabricated FTO/TiO2 photanodes were characterized by the following techniques. Highresolution transmission electron microscopy (HRTEM) imaging was carried out with a JEOL JEM-2100 TEM at an accelerating voltage of 200 kV. Energy dispersive X-ray (EDX) spectra were obtained from Ametek EDX module with Octane T Optima-60 EDX detector in TEM mode.

Figs 1(a) and 1(b) show the HRTEM micrograph and EDX spectra of Ag nanoparticles used
for the doping of TiO2 electrodes respectively. As can be seen form Fig 1 (a), silver nanoparticles are spherical in shape having smooth surface and are well dispersed. The average diameter of silver nanoparticles is found to be approximately 35 nm. This image also shows that the produced nanoparticles have more or less narrow size distribution.
Elemental composition analysis by EDX presented clearly by Fig 1 (b) confirmed the presence of Ag showing a strongest signal from silver (Ag) region. Apart from the Ag peaks additional peaks assigned to carbon can also be seeing in the figure. This could be due to the residual organic compounds used in the preparation of Ag nanoparticles. Therefore, the unusual large particle appears in the HRTEM micrograph (Fig 1a) could be correlated with these additional C peaks or due to an aggregate formed by the Ag NPs. The surface topology SEM image of TiO2 nanoparticle layer (P25) deposited on FTO with P90 layer is shown in Fig. 1 (c). The estimated size of the TiO2 particles appears to be around 40 nm. Fig. 1 (d) shows the EDX analysis of the TiO2 P25 layer and it confirms the presence of Ti as well as O with no any other element percent. While Fig. 1. (e) shows the SEM image of the surface of the Ag doped (0.3 w/v%) TiO2 (P25), Fig. 1 (f) Table 1. The enhancement of photovoltaic conversion efficiency,  predominantly depends on the short-circuit-current density JSC, as shown in Table 1. with Ag NPs shows nearly ~ 39 % enhancement in the efficiency. This enhancement could be due to the increased amount of photo generated carriers between the QDs and TiO2 particles due to the SPR effect of Ag nanoparticles as observed by others in the cases of gold.

Current -voltage characteristics
It should be noted that the energy conversion efficiency of the CdS-sensitized solar devices fabricated with the present strategy is comparable and even higher than some of the efficiency values reported in the literature by using different strategies such as doping with different materials [7][8][9][10][11]. Therefore, it can be concluded that efficiency of CdS sensitized solar cells can be significantly enhance by incorporation of Ag NPs in the TiO2 photoande with a simple techniques.

CONCLUSIONS
We developed CdS quantum dot (QD)-sensitized TiO2 photoanodes with effectively enhanced photovoltaic performance by using plasmatic silver nanoparticles and demonstrated that significant enhancement in the efficiency can be obtained by using this simple technique. As observed in dye sensitized solar cells, the light scattering and trapping of plasmonic silver nanoparticles and the near-field coupling of plasmonic silver with the TiO2 and CdS semiconductors could increase the effective path length of incident light in absorber layer and hence the efficiency of the device.