Photovoltaic Properties of CdSe Quantum Dot Sensitized Inverse Opal TiO2 Solar Cells: The Effect of TiCl4 Post Treatment

Recently, semiconductor quantum dot (QD) sensitized solar cells (QDSSCs) are expected to achieve higher conversion efficiency because of the large light absorption coefficient and multiple exciton generation in QDs. The morphology of TiO2 electrode is one of the most important factors in QDSSCs. Inverse opal (IO) TiO2 electrode, which has periodic mesoporous structure, is useful for QDSSCs because of better penetration of electrolyte than conventional nanoparticulate TiO2 electrode. In addition, the ordered three dimensional structure of IO-TiO2 would be better for electron transport. We have found that open circuit voltage Voc of QDSSCs with IO-TiO2 electrodes was much higher (0.2 V) than that with nanoparticulate TiO2 electrodes. But short circuit current density Jsc was lower in the case of IO-TiO2 electrodes because of the smaller surface area of IO-TiO2. In this study, for increasing surface area of IO-TiO2, we applied TiCl4 post treatment on IO-TiO2 and investigated the effect of the post treatment on photovoltaic properties of CdSe QD sensitized IO-TiO2 solar cells. It was found that Jsc could be enhanced due to TiCl4 post treatment, but decreased again for more than one cycle treatment, which indicates excess post treatment may lead to worse penetration of electrolyte. Our results indicate that the appropriate post treatment can improve the energy conversion efficiency of the QDSSCs.


Introduction
One of the potential candidates for next generation solar cells is dye sensitized solar cells (DSSCs), due to their high energy conversion efficiency exceeding 10% [1]. However, the photovoltaic performance of DSSCs is needed to be further improved in order to replace conventional Si-based solar cell in practical applications. From the viewpoint of sensitizers, semiconductor quantum dot (QD) sensitized solar cells (QDSSCs) have been the focus of much attention as candidates for replacing the sensitizer dyes in DSSCs, due to their specific advantages in solar cell applications [2] [3] [4]. For example, the QDs show tunable band gap by controlling their sizes, so that the absorption spectra of the QDs can be tuned to match the spectral distribution of sunlight. Moreover, the QDs have large extinction coefficients and a potential to generate multiple electron-hole pairs with one single photon absorption [5].
On the other hand, the morphology of the TiO 2 electrode is one of the most important factors in QDSSCs. However, the normal TiO 2 electrode has a disordered assembly of nanoparticle structure, causing the poor penetration of electrolyte. In our previous study, we have demonstrated that inverse opal (IO) structure TiO 2 electrode, which has ordered periodic mesoporous structure, is useful for QDSSCs because of better penetration of electrolyte than conventional nanoparticulate structure [6]. Moreover, this structure has the possibility of enhancing the light harvesting efficiency, due to the slow light effect by photonic band gap which depends on the filling fraction of TiO 2 in the IO structure [7].
We have proposed the use of IO-TiO 2 solar cell sensitized with CdSe QDs by chemical bath deposition (CBD) method [6]. In addition, the CdSe QDs were coated with ZnS for surface passivation. We have found that open circuit voltage Voc of QDSSCs with IO-TiO 2 electrodes were much higher (about 0.7 V) than that with nanoparticulate TiO 2 electrodes (about 0.5 V). But short circuit current density J sc was lower in the case of IO-TiO 2 electrodes because of the smaller surface area of the IO-TiO 2 [6]. In this study, to increase surface area of IO-TiO 2 , we have applied TiCl 4 post treatment on IO-TiO 2 and investigated the effect of the post treatment on photovoltaic properties of CdSe QD sensitized IO-TiO 2 solar cells. We found that the photovoltaic performance changed systematically by increasing the TiCl 4 post treatment cycles on IO-TiO 2 .

Experiment
IO-TiO 2 electrodes were prepared on fluorine-doped tin oxide (FTO) conducting glass (10 Ω/sq) by the sol -gel method [8]. Substrates were cleaned ultrasonically with acetone and methanol. Monodisperse polystyrene latex (PSL) suspensions (304 nm in diameter) were sonicated for 30 min to split the aggregated particles. The synthetic opal templates were assembled by immersing FTO substrate vertically in 0.125 wt% PSL suspension and evaporating the solvent in an oven at 40˚C until the solvent completely disappeared, leaving behind a colloidal PSL film on the substrate. Then TiO 2 was brought into the void of the template by the following method. The substrate was immersed into TiO 2 precursor solution with mixtures of absolute ethanol, hydrochloric acid, tetrabutyl titanate, and deionized water for 10 min [8]. The substrate was subsequently heated at 450˚C for 3 h in air with a heating rate of 1˚C/min to calcine the template and anneal the TiO 2 . For the post treatment on the surface of IO-TiO 2 , the substrate was immersed into 50 mM TiCl 4 solution at 70˚C for 1 h and subsequently heated at 450˚C. The processes were repeated several times (1, 2, 3 cycles). After the TiCl 4 post treatment, CdSe QDs were adsorbed on IO-TiO 2 electrode at 10˚C for 9 h using a chemical bath deposition (CBD) method [6]. The PA method is a useful tool for opaque and scattered solid samples because the signal is directly proportional to the acoustic energy by heat generated through the optical absorption resulting from nonradiative processes [9]. Figure   1 shows the schematic diagram of PA spectroscopy. The light source was a 300 W xenon short arc lamp. Monochromatic light through a monochromator was modulated with a mechanical chopper at frequency of 33 Hz. The PA spectra were normalized by using PA signals from a carbon black. The photocurrent density (J) versus photovoltage (V) characteristic measurements were performed in sandwich structure solar cells with using a CdSe QD sensitized electrode as the working electrode and Cu 2 S as the counter electrode. The effective cell area was 0.25 cm 2 , while the polysulfide electrolyte (1M Na 2 S and 1M S solution) was used as redox couple [10]. The photovoltaic characteristics of the solar cells were measured using a solar simulator (Peccell technologies, Inc.) with 100 mW/cm 2 irradiation AM 1.5.  (c) and with three cycles TiCl 4 post treatment (d) and cross section of IO-TiO 2 (e). As shown in Figure 2, after the calcination of latex template, a honeycomb structure appears with an ordered hexagonal pattern of spherical pores that connect each sphere to its nearest neighbors. The diameter or center to center distance of the pores, referred to as the periodic lattice constant of IO-TiO 2 , was determined to be ~230 nm (Figure 2       The average sizes of the CdSe QDs were about 6 nm calculated using the theory of effective mass approximation [9] [12]. Studies of the Urbach rule [13], which shows the low energy exponential tail depends on the photon energy, give information about disorders and impurities states. The dependence of the PA intensity on photon energy at the lower energy tail can be given by the following

Results and Discussion
where P is the PA intensity, hν is photon energy, k B is the Boltzmann constant, T is absolute temperature, and P 0 , σ, E 0 are fitting parameters. σ is called steepness factor, which show a characteristic of exponential tail. By fitting Equation (1) Table 1. Open circuit voltage (V oc ) is almost the same for all of the IO-TiO 2 electrodes (0.7 V), which is much higher than that of the CdSe QD sensitized nanoparticulate TiO 2 solar cells (0.5 V) [6]. In addition, short circuit current density (J sc ) was enhanced due to the amount of the adsorbed CdSe QDs increased after the TiCl 4 post treatment and has a maximum value of 6.33 mA/cm 2 at one cycle treatment, but decreased again for more than one cycle treatment, which results from excess TiCl 4 post treatment. This is because that the increase of surface area after the post treatment may lead to worse penetration of electrolyte. Therefore, there is an optimum cycle of the post treatment for