**4. Conclusion**

486 Solar Cells – New Aspects and Solutions

Fig. 9. Dependence of the photovoltaic parameters( (c) FF and (d) η ) on the adsorption time,

CdSe QD-sensitized solar cells without a pre-adsorbed CdS QD layer increase with the increase of adsorption times up to 24 h, indicating the difference of the crystal growth and the

Figure 9 shows the preliminary ultrafast photoexcited carrier dynamics characterization of combined CdS/CdSe and CdSe without a pre-adsorbed CdS layer (average diameters of both CdSe QDs are ~ 6 nm) using a improved transient grating (TG) technique (Katayama et al., 2003; Yamaguchi et al., 2003; Shen et al., 2010). TG signal is proportional to the change in the refractive index of the sample due to photoexcited carriers (electrons and holes). TG method is a powerful time-resolved optical technique for the measurements of various kinds of dynamics, such as carrier population dynamics, excited carrier diffusion, thermal

both of combined CdS/CdSe (●) and CdSe without a preadsorbed CdS QD layer (○).

formation of recombination centers in combined CdS/CdSe and CdSe QDs.

We have described the performance of quantum dot-sensitized solar cells (QDSCs) based on CdSe QD sensitizer on a pre-adsorbed CdS layer (combined CdS/CdSe QDs) together with the basic studies of optical absorption and photoelectrochemical current characteristics. It can be observed from optical absorption measurements using photoacoustic (PA) spectroscopy that the CdSe QDs on the nanostructured TiO2 electrodes with a pre-adsorbed CdS layer grow more rapidly during the initial adsorption process than those without a pre-adsorbed CdS layer. Photoelectrochemical current in the visible light region due to the adsorbed CdSe QDs can be observed, indicating the photosensitization by combined CdS/CdSe QDs. The maximum IPCE value (~ 75%) of the CdSe QDs on the nanostructured TiO2 electrodes with a pre-adsorbed CdS QD layer is 30% greater than that without a pre-adsorbed CdS layer. It indicates the possibilities of a decrease in recombination centers, interface states, and inverse transfer rate that is suggested by the preliminary ultrafast photoexcited carrier carrier dynamics characterization owing to the possibilities of active CdSe QDs by the excess Cd remaining after CdS adsorption and passivation effect of CdS QDs on the nanostructured TiO2 surface. The short-circuit current (Jsc) in combined CdS/CdSe QD-sensitized solar cells shows

**23** 

**Investigation of Lattice Defects in GaAsN** 

**Grown by Chemical Beam Epitaxy Using** 

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. These electrical properties are insufficient to insure the current matching in the multijunction solar cell structure AlInGaP/GaAs/InGaAsN/Ge (Friedman et al., 1998). An obvious reason of such degradation is the high density of Nrelated lattice defects that can be formed during growth to compensate for the tensile strain caused by the small atomic size of N compared with that of arsenic (As) and to the large miscibility of the gap between GaAs and GaN. These defect centers are expected to act as active recombination and/or scattering centers in the forbidden gap of the alloy (Zhang & Wei, 2001). However, no experimental evidence has yet been reported. On the other hand, the conductivity of undoped *p*-type InGaAsN or GaAsN and their high background doping (Friedman et al., 1998; Kurtz et al., 1999; Moto et al., 2000; Krispin et al., 2000) prevent the design of wide depletion region single junction solar cell and the fabrication of intrinsic layer to overcome the short minority carrier lifetime. This serious problem was expected in the first stage to the density of unintentional carbon in the film (Friedman et al., 1998; Kurtz et al., 1999; Moto et al., 2000). However, the carrier density in some InGaAsN semiconductors was found to be higher than that of carbon (Kurtz et al., 2002). Furthermore, the high density of hydrogen (up to 1020 cm−3) and the strong interaction between N and H in InGaAsN to form N-H related complex were confirmed to be the main cause of high background doping in InGaAsN films (Li et al., 1999; Janotti et al., 2002, 2003; Kurtz et al., 2001, 2003; Nishimura et al., 2007). In addition, N-H complex was found theoretically to bind strongly to gallium vacancies (VGa) to form N-H-VGa with a formation energy of 2 eV less than that of isolated VGa (Janotti et al., 2003). These predictions were supported experimentally using positron annihilation spectroscopy results (Toivonen et al., 2003).

**1. Introduction** 

**Deep Level Transient Spectroscopy** 

Boussairi Bouzazi1, Hidetoshi Suzuki2, Nobuaki Kijima1,

Yoshio Ohshita1 and Masafumi Yamaguchi1

*1Toyota Technological Institute* 

*2Myazaki University* 

*Japan* 

maxima with the increase of CdSe QDs adsorption times between 2 h and 24 h, also indicating the decrease of recombination centers, interface states, and the increase in quasi Fermi level. The open-circuit voltage (Voc) and fill factor (FF) are independent of adsorption times. The photovoltaic conversion efficiency (η) of the combined CdS/CdSe QD-sensitized solar cell shows a maximum value of 3.5%.
