**6. References**


In this chapter, main effort has been directed to disclose the structure-property relationship for solar cell polymers. The requirements and criteria for an efficient polymer donor in BHJ solar cell have been discussed with representative examples. Key factors are: absorption efficiency, solubility, stability (thermal-, photo-), low band gap, HOMO/LUMO energy level, charge carrier mobility and morphology. In order to achieve high power conversion efficiency, a good balance among these factors should be met. On the other hand, choice of acceptor counterpart and device engineering for the BHJ device also play important roles for power conversion efficiency improvement. Nowadays choice of donor/acceptor combination and device fabrication is still 'a state of art' but more and more rules of thumb have been pointed out. Provided that if BHJ concept still prevails for the next 10 years or longer, newer device design is also urgently required. Tandem solar cell device reported is one example to address the efficiency issue from this point of view. But no matter what kind of new changes will be brought out, the photon flux capture material, which is conjugated

This work was financially supported by National University of Singapore under MOE AcRF

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**4. Conclusion** 

**5. Acknowledgment** 

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**22** 

*Japan* 

**Optical Absorption and Photocurrent** 

**with Photovoltaic Properties** 

Taro Toyoda and Qing Shen

*The University of Electro-Communications* 

**Spectra of CdSe Quantum Dots Adsorbed** 

**on Nanocrystalline TiO2 Electrode Together** 

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

**1. Introduction** 

