Polymeric photovoltaic cells

Despite the extensive application of ruthenium complexes in DSSC, transition metal containing polymers have received relatively little attention in the fabrication of polymeric photovoltaic cells. Most of the early works on ruthenium containing polymers were focused on the light-emitting properties.58-60 Several examples of ruthenium terpyridine/bipyridine containing conjugated polymers and their photoconducting/electroluminescent properties were reported.61,62... 

In this section, some ruthenium complex containing polymers incorporated with charge transport functionalities are presented. Being incorporated with both photosensitizing and charge transport units in the same polymer molecule, they are considered promising candidates for polymeric photovoltaic cells. However, the photovoltaic properties have not been reported so far. 

Alternating copolymer 20 derived from 2,7-dibenzosilole and 4,7-dithienyl-2,1,3-benzothiadiazole is an outstanding polymeric electron donor in photovoltaic cells.37 With an active layer made up of copolymer to PCBM in a 1 2 ratio, the solar cell displays a high short-circuit current of 9.5 mA/cm2, an open-circuit voltage of 0.9 V, and a fill factor of 50.7%, under illumination of an AM 1.5 solar simulator at 80 mW/cm2. The calculated energy conversion efficiency is 5.4%, which is one of the highest efficiencies so far reported for polymeric photovoltaic cells. 

The development of effective polymeric photovoltaic cells led to the creation of architectures consisting of phase-separated polymer blends [282-286]. Such systems consist of interpenetrating bicontinuous networks of donor and acceptor phases with domain sizes of 5—50 nm, and provide donor/acceptor heterojunctions distributed throughout the layer thickness (Figure 3.21). 

Yu, G., Gao, J., Hummelen, J.C., Wudl, F., and Heeger, A.J. (1995) Polymeric photovoltaic cells enhanced efficiencies via a network of internal donor-acceptor heterojunctions. Science, 270,1789. 

The role of PEDOT buffer layers have been studied in detail in various device structures. This includes polymeric photovoltaic cells " also in combination with htanium dioxide - dye sensitized photovoltaic cells ... 

Spanggaard H, Krebs FC (2004) A brief history of the development of organic and polymeric photovoltaics. Sol Energy Mater Sol Cells 83 125... 

G. J. Vendura, Improved Thermal Control for a-Si H Photovoltaic Cells Fabricated on Polymeric Substrates, presented at 26th IEEE Photovoltaic Specialists Conference, Los Angeles, CA, USA, 1997. 

The main reasons for the low efficiencies are the incomplete absorption of the solar spectrum by any single material which serves as a colorant. Self-absorption of the fluorescence by the emitting colorant. Critical cone losses of the reemitted centers, absorption and scattering of the host materials, lack of good contact between the LSC and the photovoltaic cell. Reflection of light from the metallic surfaces of which the electrical contacts are made. The decrease of performance efficiency of the collectors made of organic dyes with time are a result of photodecomposition of the colorant and polymeric host material. 

In evaporation-intercalation devices solar energy conversion would, at least in the more efficient case of a thermal system, not be converted by exciting electrons and rapidly separating them from holes, but by transferring atoms or molecules across a phase boundary by evaporation which is usually a very efficient process. It is, consequently, neither necessary to use materials which are well crystallized like those developed for photovoltaic cells nor is it necessary to prepare sophisticated junctions. A compacted polycrystalline sheet of a two-dimensional material which is on one side placed in contact with an electrolyte, sandwiched between the layer-type electrode and a porous counter electrode, as it is used in fuel cells, would constitute the central energy conversion unit. Some care would have to be taken to choose an electrolyte which is suitable for intercalation reactions and which is not easily evaporated through leaks in the electrodes. Thin layers of polymeric or solid electrolytes would seem to be promising. 

The majority of photovoltaic modules use silicon as the photovoltaic cell element, but other materials are, in principle, possible. The last four chapters consider the use of organic polymers (sometimes doped) as the cell element or in some related conducting property acrylonitrile, some polymeric phthalocyanines and polymers of 2-vinylnaphthalene that is doped with pyrene and 1,2,4,5-tetracyanobenzene. The study on the last group of polymers was initiated by the idea that they could be used to transfer solar energy to a reaction center and produce some type of chemical reaction. The final chapter carries this approach further in the consideration of polymeric electrodes that could be used to split water into oxygen and hydrogen. The latter could then be utilized as a source of storable, readily transportable chemical energy. 

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