**Optical Absorption and Photocurrent Spectra of CdSe Quantum Dots Adsorbed on Nanocrystalline TiO2 Electrode Together with Photovoltaic Properties**

Taro Toyoda and Qing Shen *The University of Electro-Communications Japan* 

### **1. Introduction**

474 Solar Cells – New Aspects and Solutions

Zhou, E., Nakamura, M., Nishizawa, T., Zhang, Y., Wei, Q., Tajima, K., Yang, C. &

Zhou, Q., Hou, Q., Zheng, L., Deng, X., Yu, G. & Cao, Y. (2004). Fluorene-based low band-

Vol.41, No.22, (November 2008), pp. 8302-8305, ISSN 1520-5835

Vol.84, No.10, (March 2004), pp. 1653, ISSN 1077-3118

Hashimoto, K. (2008). Synthesis and photovoltaic properties of a novel low band gap polymer based on N-substituted dithieno[3,2-b:2',3'-d]pyrrole. *Macromolecules*,

gap copolymers for high performance photovoltaic devices. *Applied Physics Letters*,

There is a great deal of interest in the technological applications of titanium dioxide (TiO2) to dye-sensitized solar cells (DSCs) made from nanostructured TiO2 electrodes because of their high photovoltaic conversion efficiency, which exceeds 10% (Chiba et al., 2006). Since the initial pioneering work on DSCs (O'Regan & Grätzel, 1991), they have often been proposed as a sustainable energy source. In DSCs, the applications of organic dye molecules as a photosensitizer, nanostructured TiO2 as an electron transport layer, and an iodine redox couple for hole transport dramatically improve the light harvesting efficiency. With Rubased organic dyes adsorbed on nanostructured TiO2 electrodes, the large surface area enables more efficient absorption of the solar light energy. The main undertaking for those developing next-generation solar cells is to improve the photovoltaic conversion efficiency, together with the long time stability. Nowadays, there exists an intense effort aimed at developing third-generation solar cells. One of a promising approach is to replace the organic dyes by inorganic substances with strong optical absorption characteristics and longer stability over time. Recently, as an alternative to organic dyes, semiconductor quantum dots (QDs) have been studied for their light harvesting capability (Niitsoo et al., 2006; Diguna et al., 2007; Mora-Seró, 2009). The enormous potential of science and technology on nanoscale to impact on industrial output has been recognized all over the world. One emerging area of nanoscience being at the interface of chemistry, physics, biology and materials science is the field of semiconductor QDs, whose unique properties have attracted great attention by researchers during the last two decades. Different strategies for the synthesis of semiconductor QDs have been developed, so that their composition, size, shape, and surface protection can be controlled nowadays with an exceptionally high degree. The surface chemistry of semiconductor QDs is another key parameter, in many respects determining their properties related to their assembly. Semiconductor QDs exhibit attractive characteristics as sensitizers due to their tunable bandgap (or HOMO-LUMO gap) by size control (Yu et al., 2003), which can be used to match the absorption spectrum to the spectral distribution of solar light. Moreover, semiconductor QDs possess higher extinction coefficients than conventional metal-organic

Optical Absorption and Photocurrent Spectra of CdSe Quantum

conversion efficiency) and photovoltaic measurements.

to be ~ 5 μm by examining the cross sectional SEM images.

sample preparation conditions as mentioned above.

**2. Experimental procedure** 

**2.1 Sample preparation** 

40 min.

Dots Adsorbed on Nanocrystalline TiO2 Electrode Together with Photovoltaic Properties 477

sample surface by modulated light by PT effect cause periodic pressure changes in the enclosed gas, which creates acoustic waves. The intensities of the acoustic waves are converted to an electrical signal by the microphone. The PA method can be applied to evaluate the optical absorption properties of combined CdS/CdSe QDs adsorbed on nanostructured TiO2 electrodes. In order to investigate the subsequent photosensitization of combined CdS/CdSe QDs adsorbed on nanostructured TiO2 electrodes after optical absorption, photoelectrochemical current measurements were carried out and the incident photon-to-current conversion efficiency (IPCE, namely the quantum efficiency of photocurrent) was evaluated. Also, photovoltaic properties were characterized by solar

Experimental sections are divided into three sections, 1) sample preparations on TiO2 electrodes and CdS/CdSe quantum dots, 2) optical absorption measurements by photoacoustic technique, and 3) photoelectrochemical current (incident photon to current

The method for the preparation of nanostructured TiO2 electrodes has been reported in a previous paper (Shen & Toyoda, 2003). A TiO2 paste was prepared by mixing 15 nm TiO2 nanocrystalline particles (Super Titanai, Showa Denko; anatase type structure) and polyethylene glycol (molecular weight: 500,000) in pure water. The resultant paste was then deposited onto transparent conducting substrates [F-doped SnO2 (FTO), sheet resistance: 10 μΩ/sq]. The TiO2 electrodes were then sintered in air at 450 ºC for 30 min to obtain good necking and to sublimate polyethylene glycol. The highly porous nanostructure of the films (the pore sizes were on the order of a few tens of nanometers) was confirmed from scanning electron microscopy (SEM) images. The thicknesses of the films were measured and found

At first, CdS QDs were adsorbed onto nanostructured TiO2 electrodes (pre-adsorbed layer) from the common NH3 bath with a solution composition of 20 mM CdCl2, 66 mM NH4Cl, 140 mM thiourea, and 0.23 M ammonia to obtain a final pH ~9.5 (Niitsoo et al., 2006; Jayakrishnan et al., 1996). The TiO2 electrodes were immersed in a container filled with the final solution. The adsorption was carried out at room temperature in the dark for

The CdSe QDs were prepared by using a chemical bath deposition (CBD) technique (Shen & Toyoda, 2004; Shen et al., 2004; Gorer & Hodes, 1994). First, for the Se source, an 80 mM sodium selenosulphate (Na2SeSO3) solution was prepared by dissolving elemental Se powder in a 200 mM Na2SO3 solution. Second, an 80 mM CdSO4 and 120 mM of a trisodium salt of nitrilotriacetic acid [N(CH2COONa)3] were mixed with the 80 mM Na2SeSO3 solution in a volume ratio of 1: 1: 1. TiO2 electrodes adsorbed with CdS QDs were placed in a glass container filled with the final solution at 10 ºC in the dark for various times (from 2 to 24 h) to promote CdSe QDs adsorption. To investigate the role of pre-adsorbed layer of CdS QDs, the CdSe QDs only were adsorbed directly on nanostructured TiO2 electrodes with the same

After the adsorption of CdSe QDs, the samples were coated with ZnS for surface passivation of the QDs by successive ionic layer adsorption and reaction (SILAR) for three times in 0.1

simulator under the illumination of one sun intensity (AM 1.5: 100 mW/cm2).

dyes, and larger intrinsic dipole moments leading to rapid charge separation (Underwood et al., 2001). The demonstration of multiple exciton generation (MEG) by impact ionization has fostered an interest in colloidal semiconductor QDs (Schaller et al, 2006; Trinh et al., 2008). One of the most attractive configurations to exploit these fascinating properties of semiconductor QDs is the quantum dot-sensitized solar cell (QDSC) (Nozik, 2002; Klimov, 2006). The efficient formation of more than one photoinduced electron-hole pair (exciton) upon the absorption of a single photon is a process not only of a great current scientific interest but is potentially important for optoelectronic devices that directly convert solar radiant energy into electricity. The demonstration of MEG by impact ionization in colloidal semiconductor QDs could push the thermodynamic photovoltaic conversion efficiency limit of solar cells up to 44% (Klimov, 2006) from the current 31% of the Shockley-Queisser detailed balance limit (Shockley & Queisser, 1961). The optimization of QDSCs can benefit from the intensive effort carried out with DSC. Although the photovoltaic conversion efficiencies of QDSCs lag behind those of DSCs and the use of semiconductor QDs as light absorbers requires the development of new strategies in order to push the performance of QDSCs, QDSCs have attracted significant attention among researchers as promising thirdgeneration photovoltaic devices.

In this chapter, we describe the performance of QDSCs based on CdSe QD sensitizer on nanostructured TiO2 electrode with a pre-adsorbed layer of CdS QDs (termed combined CdS/CdSe QDs) proposed by Niitsoo et al (Niitsoo et al, 2006) and developed by other groups (Lee & Lo, 2009; Sudhagar et al., 2009). They showed that a pre-adsorbed layer of CdS prior to CdSe adsorption improved the QDSC's performance. Hence it is interesting and useful to investigate the detailed function of combined CdS/CdSe QDs sensitizer on performance of QDSCs, together with the basic studies of optical absorption and photocurrent characteristics. Information regarding the optical absorption properties is initially necessary in order to investigate the electronic states of combined CdS/CdSe QDs for future photovoltaic cell applications. However, few accurate studies of the optical absorption properties of combined CdS/CdSe QDs adsorbed on nanostructured TiO2 electrodes have been carried out. The main reason for this is the difficulty in using the conventional transmission method because of strong light scattering by the highly porous structure of the nanostructured TiO2 electrodes. However, scattering effects can be minimized by employing the photothermal (PT) technique. In general, an optically excited solid relaxes to thermal equilibrium by the emission of photons (radiative processes) or phonons (nonradiative processes). In the PT technique, the signal detected is directly proportional to the thermal energy (heat production) induced by the absorbed photons through nonradiative processes (emission of phonons). Heat production by nonradiative processes has been detected by several methods (Tam, 1986). The PT signal is less sensitive to light scattering effects than conventional spectroscopy signals, and the ability of the PT technique to produce optical absorption spectra from strongly scattering media has been demonstrated, in particular using photoacoustic (PA) method which is a PT technique (Inoue et al., 2006; Toyoda et al., 2009). Thus, the PT technique is a useful technique for studying the optical absorption spectra of the strongly scattering and/or opaque samples with which we are dealing in our investigations. PA method detects the acoustic energy produced by heat generation through nonradiative processes in materials (Rosencwaig & Gersho, 1977). The PA cell, which is a small gas-tight enclosure with a sensitive acoustic microphone built in one wall, monitors the temperature changes in the sample produced by absorbed photons through nonradiative processes. Periodic temperature changes in the sample surface by modulated light by PT effect cause periodic pressure changes in the enclosed gas, which creates acoustic waves. The intensities of the acoustic waves are converted to an electrical signal by the microphone. The PA method can be applied to evaluate the optical absorption properties of combined CdS/CdSe QDs adsorbed on nanostructured TiO2 electrodes. In order to investigate the subsequent photosensitization of combined CdS/CdSe QDs adsorbed on nanostructured TiO2 electrodes after optical absorption, photoelectrochemical current measurements were carried out and the incident photon-to-current conversion efficiency (IPCE, namely the quantum efficiency of photocurrent) was evaluated. Also, photovoltaic properties were characterized by solar simulator under the illumination of one sun intensity (AM 1.5: 100 mW/cm2).
