Thin photovoltaic device structure

Figure 9.11 Schematic presentation of the thin-film photovoltaic device structure glass/ITO/...

It is indeed intriguing and very attractive to think of photovoltaic elements based on thin plastic films with low cost but large areas, cut from rolls and deployed on permanent structures and surfaces. In order to fulfil these requirements, cheap production technologies for large scale coating must be applied to a low cost material class. Polymer photovoltaic cells hold the potential of such low cost cells. Flexible chemical tailoring of desired properties, combined with the cheap technology already well developed for all kinds of plastic thin film applications, precisely fulfill the above-formulated demands for cheap photovoltaic device production. The mechanical flexibility of plastic materials is welcome for all photovoltaic applications onto curved surfaces in indoor as well as outdoor applications. 

The elemental Pt(0) Is dispersed throughout the surface polymer as determined by depth profile analysis,(7) and a representation of the interface is given in Scheme V. According to this view there is a certain amount of Pt(0) in contact with the thin SiOx overlayer on the bulk p-type Si. This is a relevant structural feature, since direct deposition of Pt(0) onto photocathode surfaces is known to improve the efficiency for the reduction of H2O to H2> Thus, we expect that, for an interface like that depicted in Scheme V, there will be a certain amount of the H2 evolution occurring by direct catalysis of the reaction of the photoexcited electrons with H2O at the Si0x/Pt(0) interfaces. In the extreme of a uniform, pinhole-free coverage of Pt(0) on p-type Si/SiOx one expects that the photocathode would operate as a buried photosensitive interface and in fact would be equivalent to an external solid state photovoltaic device driving a photoelectrolysis cell with a Pt(0) cathode. 

II-VI semiconductor layers and bulk semiconductors like Si, GaAs, InP, etc. In particular, quantum wells are formed by thin epitaxial multilayered structures like (Zn, Cd)Se/ZnS. Nevertheless, the choice between bulk semiconductors and the layers deposited or between the multilayers is governed by the lattice mismatch between the two components as the lattice mismatch causes the formation of misfit dislocations. In the optical devices these defects are potential non-radiative centres and at worst they can cause the failure of injection lasers. Figure 29 is a map of energy gap versus lattice constants for a variety of semiconductors it can be used to select different heterostructures, not only for optoelectronics applications but also for photovoltaic cells. In the latter application the deposited films are generally polycrystalline and the growth of high-quality epitaxial layers has received little applications. 

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]. 

Thin film organic polymer semiconductors are an attractive option for the development of cost-effective photovoltaic devices. They are uniquely suited to low-cost mass production and provide potential flexibility via property engineering. However, materials with better physical and photovoltaic properties than those currently available are needed. This will require a far better understanding of the structure and transport properties of these materials. 

We have previously shown that when PPV is self-assembled with specific electronically active polyanions such as poly(thiophene acetic acid) (PTAA) or sulfonated fiillerenes (S-C60 )(7), the photoluminescence of the PPV is essentially completely quenched by the polyanion. The mechanism of this quenching is believed to be due to a photoinduc electron transfer process taking place between the excited PPV and the adjacent electroactive polyanion molecules. The quenching process, in this case, is not associated with a Forster type energy transfer since in both cases, the required spectral overlap of a donor emission band with an acceptor absorption band is not fulfilled. In addition, photo-induced electron transfer processes have previously been confirmed in PPV/C60 systems and can be exploited to fabricate thin film photovoltaic devices (77). In order to mediate this electron transfer process, we have constructed multilayer heterostructures in which the PPV donor and the polyanion electron acceptor are separated from each other with electronically inert spacer layers of known thickness. In addition to allowing studies of the electron transfer process, such structures provide important insights into the thermal stability of the multilayer structure. The "spacers" used in this study were bilayers of SPS/PAH with an experimentally determined bilayer thickness of 30 +/-5 A. 

Figure 4.17 (a) Schematic of the photovoltaic device with P3HT GOPITC thin film as the active layer and the structure ITO/PEDOT PSS (30 nm)/P3HT GO-PITC (110 nm)/ A1 (80 nm). (b) Experimental J-V curves of the photovoltaic devices based on P3HT (red curve) and P3HT GO-PITC composites (blue curve, 10 wt.% black curve, 20 wt.%) after postfabrication thermal annealing at 160°C for 20 min (reprinted from [86] with permission from American Chemical Society). 

As the molecular-scale heterogeneity of the active layer greatly influences the power conversion efficiency, it is fundamentally important to identify and control the structural and optical properties and their relation to the function of a photovoltaic device. Ellipsometry is especially useful in determining the complex index of refraction and layer thickness, as well as structural details in thin-film geometry [69-72]. This information is needed to calculate the internal optical electric field distribution and the resulting photocurrent action spectra with respect to the efficiency of thin-film devices [66]. 

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