Organic solar cells conversion efficiencies

Within the vast field of solar cell devices, organic solar cells [258-261] are experiencing important progresses in their conversion efficiencies that will possibly make them, in the near future, a competitive alternative to silicon solar cells. Moreover, this class of solar cells presents several advantages with respect to their inorganic counterpart such as their low-cost production, easy fabrication, and lightweight, all features that could allow for the fabrication of large-area, foldable, and flexible photovoltaic devices. 

Organic solar cells have reached efficiencies exceeding 4%. In fact power conversion efficiencies of organic solar cells have reached an impressive 5%. This has been possible because of the discovery of bulk heterojunction solar cells. Solar cells are discussed in Chapter 5. 

The absorption, emission, and redox properties of squaraines make them highly suited for applications as photosensitizers. In view of this, the early studies on squaraines were focused on thin photovoltaic and semiconductor photosensitization properties [1,4,5,91-97], Champ and Shattuck [98] first demonstrated that squaraines could photogenerate electron-hole (e-h) pairs in bilayer xerographic devices. Subsequently, extensive work has been carried out on the xerographic properties of squaraines [2,24,34,47,48,99,100], and these properties have been reviewed recently [11]. In an extensive smdy on the correlation s between cell performance and molecular structure in organic photovoltaic cells, squaraines were found to have much better solar energy conversion efficiencies than a variety of other merocyanine dyes [4,5]. 

Furthermore, the production is expected to be easily scalable. This technology is currently developed by many researchers around the world, but has not yet reached the marketplace. In order for polymer solar cells to become economic their efficiency must be improved. The power conversion efficiency of a solar cell is dictated by three factors (i) the fraction of sunlight that can be absorbed, (ii) the fraction of absorbed photons that lead to extracted charges ( internal quantum efficiency ), and (iii) the energy that is retained by the extracted charges (ideally close to the open-circuit voltage ). In this review we will refer often to factors (ii) and (ui). Their interplay is not well understood and at present these have not both been optimized simultaneously even in state-of-the-art organic solar cells. 

Incorporation of organic solar cells into textiles has been realized reaching encouraging performances. Stability issues need to be solved before future commercialization can be envisaged. The mechanical stability of the devices was not limiting the function of the devices prepared. It would seem that low power conversion efficiency much more pertinent than the mechanical stability on the timescale of commercial. 

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