Category Towards High-Efficiency Organic Solar Cells: Polymers and Devices Development

To characterize lattice defects in a semiconductor, several techniques were used during the second half of the last century. Between these methods, we cite the thermally stimulated
current (TSC) (Leonard & Grossweiner, 1958; Bube, I960), the admittance spectroscopy (Losee, 1974), the increase or decay curves of photoconductivity (Rose, 1951; Devore, 1959), the optically stimulated conductivity (Lambe, 1955; Bube, 1956), and the analysis of space- charge-limited currents as function of applied voltage (Smith & Rose, 1955; Rose, 1955; Lampert, 1956). The basic concept of using the change of capacitance under bias conditions by the filling and emptying of deep levels was already anticipated fifty years ago (Williams, 1966)...

Boussairi Bouzazi1, Hidetoshi Suzuki2, Nobuaki Kijima1, Yoshio Ohshita1 and Masafumi Yamaguchi1

1Toyota Technological Institute 2Myazaki University Japan

1. Introduction

With only 3 % of N and 9 % of In, InGaAsN with a band gap of 1.04 eV was obtained and could be lattice matched to GaAs and Ge. This dilute nitride semiconductor has been selected as a promising candidate for high efficiency multijunction tandem solar cells (Geisz and Friedman, 2002). However, the diffusion length of minority carriers and the mobility are still lower than of that in GaAs or InGaAs and showed a considerable degradation with increasing the N concentration...

Figure 2 shows the PA spectra of the nanostructured TiO2 electrodes adsorbed with combined CdS/ CdSe QDs for different adsorption times, together with that adsorbed with CdS QDs only. The pre-adsorption times for CdS QDs were fixed at 40 min (average diameter: ~ 4.2 nm).

The spectra were normalized to the photon energy of 4.0 eV. With increasing adsorption time, the red-shift of optical absorption at the shoulder point (indicated by arrows) can be clearly observed, implying the growth of CdSe QDs. Also, the comparison between the adsorption of CdSe QDs on the nanostructured TiO2 electrodes with and without a pre-adsorbed CdS QD layer was carried out to evaluate the difference in PA spectra...

Photocurrent measurements were performed in a sandwich structure cell (i. e., in the two- electrode configuration) with Cu2S film on brass as the counter electrode (termed the Cu2S counterelectrode). The applied electrolyte was polysulfide solution (1 M Na2S + 1 M S). It is well known that the electrocatalytic activity of Pt with a polysulfide electrolyte is not satisfactory for photovoltaic cell applications and alternative counter electrode materials with higher activity such as Cu2S and CoS have been reported (Hodes et al., 1980). The higher electro-catalytic activities of these materials are due to a reduction in the charge transfer resistance between the redox couple and the counterelectrode (Gimenez et al. 2009)...

The optical absorption properties of nanostructured TiO2 electrodes adsorbed with combined CdS/CdSe QDs were investigated using PA spectroscopy. The scattering effects in the optical absorption of nanostructured TiO2 electrodes adsorbed with combined CdS/ CdSe QDs can be minimized by employing the PA method. Figure 1 shows the schematic diagram of a photoacoustic spectrometer. Typical gas-microphone method was applied in the PA spectroscopic investigation (Rosencwaig & Gersho, 1977). The PA cell was composed of an aluminum cylinder with a small channel at the periphery in which a microphone (electret condenser type) was inserted (Shen &Toyoda, 2004). The inside volume of the cell was approximately 0.5 cm3. The cell was suspended by four rubber ban

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to prevent interference from external vi...

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 pQ/sq]. The TiO2 electrodes were then sintered in air at 450 °C 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...

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 conversion efficiency) and photovoltaic measurements.

Taro Toyoda and Qing Shen

The University of Electro-Communications

Japan

1. Introduction

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 & Gratzel, 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...