Solar cells Subject

Dye-sensitized solar cells (DSSCs) are photoelectrochemical solar devices, currently subject of intense research in the framework of renewable energies as a low-cost photovoltaic device. DSSCs are based upon the sensitization of mesoporous nanocrystalline metal oxide films to visible light by the adsorption of molecular dyes.5"7 Photoinduced electron injection from the sensitizer dye (D) into the metal oxide conduction band initiates charge separation. Subsequently, the injected electrons are transported through the metal oxide film to a transparent electrode, while a redox-active electrolyte, such as I /I , is employed to reduce the dye cation and transport the resulting positive charge to a counter electrode (Fig. 17.4). 

The recombination of photogenerated electrons and holes is the bane of all solar cells and a major reason for their less than ideal efficiencies. Excitonic solar cells, in which the electrons and holes exist in separate chemical phases, are subject primarily to interfacial recombination. There is, as yet, no theoretical model to accurately describe interfacial recombination processes, and this is an important area for future research. Wang and Suna [91] have laid a possible foundation for such a model by combining Marcus theory with Onsager theory. 

As mentioned earlier, the DSSC is a very attractive and promising device for solar cell applications that has been intensively investigated worldwide, and its photovoltaic mechanism has also intensively investigated [11-20]. Moreover, commercial applications of the DSSC have been under investigation. In this chapter, we describe the DSSC, including its component materials, structure, working mechanism, efficient preparation procedure, current researches, and long-term stabilities. We also introduce the subjects for improvement of its performance and commercial applications. 

Reliable stability data of the p-i-n solar cell itself are not easily obtained, especially for non-encapsulated cells or modules. One of these tests e.g. for EN/IEC 61646 certification of modules is the so-called damp-heat test (85°C, 85% humidity, up to lOOOh). Recent studies were performed by Stiebig et al. [50, 51] exposing different types of cells to harsh conditions. One of the most important results was the excellent stability of silicon thin film solar cells. Remarkably, this is also valid for small area modules even without encapsulation [52]. This is of high interest because costs and efforts for module encapsulation strongly depend on the inherent stability of the solar cells. As a more detailed treatment of this subject is beyond the scope of this chapter, the reader is referred to the original papers [50,51]. 

In the most general situation, the current density in semiconductors and in solar cells is composed of electron and hole contributions jq = jh+ Je The relevant carrier concentrations ne(x) and rih(x) are subject to generation and recombination and have to obey continuity equations... 

Photosynthesis is the inspiration for all efforts to harvest solar energy ranging from solar cells to light-induced water splitting. Historically, synthetic molecular systems that undergo light-induced electron transfer reactions have featured prominently in efforts to functionally mimic photosynthesis.Like the natural system, many of the intermediates and transient products of these light-induced electron transfer reactions are radicals thus, they are subject to spin-chemical effects. 

While progress towards more efficient polymer solar cells has been steady and directions for future work are mapped out, there are few reports of cell performance under continuous illumination. Kroon et al. (2002) studied the response of encapsulated MDMO-PPV cells, similar to that illustrated in Fig. 10.23(a), subjected to thermal stress and continuous illumination. A 30-50% decrease in efficiency was observed for cells stored in the dark at... 

The transfer of a single electron between two chemical entities is the simplest of oxidation-reduction processes, but it is of central importance in vast areas of chemistry. Electron transfer processes constitute the fundamental steps in biological utilization of oxygen, in electrical conductivity, in oxidation reduction reactions of organic and inorganic substrates, in many catalytic processes, in the transduction of the sun s energy by plants and by synthetic solar cells, and so on. The breadth and complexity of the subject is evident from the five volume handbook Electron Transfer in Chemistry (V. Balzani, Ed.), published in 2001. The most fimdamental principles that govern the efficiencies, the yields or the rates of electron-transfer processes are independent of the nature of the substrates. The properties of the substrates do dictate the conditions for apphcability of those fimdamental... 

The development of amorphous silicon solar cells has progressed so well in recent years that they threaten to dominate the field of solar energy conversion. It is difficult to see how wet systems can compete with these cells, the only limitations of which arise from design problems. In the near future, it seems probable that their only failure wOl be the material used to support the cells. The progress and status of this subject does not come within the scope of this review and the interested reader should consult journals directed more towards solid-state physics than photochemistry. In this Section, we consider only aspects of photovoltaics that are directly relevant to photochemistry. 

Investigation of the electrical properties of sub-GBs is an important subject for improving the performance of Si multicrystal solar cells. In particular, the carrier recombination activity of sub-GBs has been well investigated... 

Fig. 3 Number of scientific publications contributing to the subject of polymer solar cell(s) . Search done through ISl, Web of Science, 2007...

The past decades have seen a rapid expansion in the use of polymers in many new fields of applications where they play an essential role. Unfortunately, most polymers are not inherently stable to light, so the studies of their photochemical behavior remain a subject of constant interest. Among the various fields that were the object of interest in the last few years, one has observed some of them emerging as the domains of organic solar cells, of coatings, and of nanocomposites. 

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