Outlook

Organic materials are generally considered to offer a range of new possibilities in terms of material use and device concepts. Although it is difficult to make firm statements about the different technologies in this category, there is every reason to believe that they may be produced at (very) low cost.

Compared to organic/polymeric solar cells, higher efficiency and stability are achieved for the dye-sensitized solar cell at the present stage of develop­ment. A number of license holders of EPFL patents and a large number of other groups including several Japanese companies, research institutes and universities are now working towards commercializing nc-DSCs for indoor and outdoor applications and improving our basic understanding.

The first aim is to commercialise the nc-DSC for indoor applications and consumer electronics. It has been demonstrated [15] that several products for this type of application are technically capable of fulfilling all the require­ments, although more technological research is still required for large scale production of these cells. For a successful introduction of nc-DSC technology onto the market, the main challenge is (company) economics. Large through­puts in solar cell production are needed to be price competitive on the already existing (consumer electronics) market. Another route is to aim for a higher value product. This means that dye PV products need to be developed which have a better performance or which permit broader operating conditions than other thin film solar cells. Flexibility in the module design for dye PVs might be an advantage over other inorganic thin film solar cell technologies in terms of applicability and product diversity.

A pilot line for producing large numbers of small modules also provides important knowledge for the future production of much larger solar panels for outdoor use, where the long term cost target should be less than 1 euro/Wp. The most important issue to be solved is intrinsic stability and particularly thermal stability. A major concern remains the use of a liquid electrolyte. Practical experience will have to demonstrate feasibility.

Long term research into sensitised oxide solar cells focuses on solid-state devices, where the liquid electrolyte is replaced by a solid charge transport material. As in ‘solid’ batteries, the electrolyte in the dye cell can be gela­tinised, which should obviate electrolyte leakage over long periods of use. It has been found that organic liquid-phase electrolytes could be gelled with amino acid derivatives showing comparable efficiencies to liquid electrolytes [20]. However, at higher temperatures the quasi-solid gel reversibly changed back into a liquid.

Toshiba developed a novel solid state chemically cross-linked gel elec­trolyte, which makes an irreversible three-dimensional network in the pores of ТЮ2. Interestingly, no significant loss in photovoltaic performance was found compared with cells containing liquid electrolytes [21].

A second approach in the development of solid state dye sensitised solar cells is the use of solid p-type hole conductors interpenetrating the nanocrys­talline ТЮ2 structure. Inorganic p-type semiconductors like Cul [22] and CuSCN [23], and amorphous organic/polymeric hole transport materials [24] have been tested in this regard but so far they have been less efficient (< 3- 4%) than photoelectrochemical solar cells containing liquid phase electrolytes. Likewise, most research efforts on fully organic/polymeric solar cells focus on efficiency and stability and much more fundamental research is needed to develop promising devices. The first applications for this type of solar cell are not expected within a time-frame of 10 years, although this field is cur­rently being explored by a large and rapidly expanding research community. This could lead to faster realisation of the ultimate goal: a very cheap, highly efficient and stable organic thin film solar cell.

Acknowledgements. Some of the work described in this chapter was fi­nanced by the European Commission under contract numbers JOR3-CT97-

0147 (‘Indoor Dye PV’) and JOR3-CT98-0261 (‘LOTS-DSC’). The collab­oration with the co-authors from the partners in the ‘Indoor Dye PV’ and

‘LOTS-DSC’ projects listed in the references is gratefully acknowledged.

[1] G. A. Chamberlain: Solar Cells 8, 47 (1983).

[2] J. Simon, J.-J. Andre: Molecular Semiconductors (Springer, Berlin, 1985).

[3] J. Kanicki: in Handbook of Conducting Polymers, Vol. 1, ed. by T. A. Skotheim (Marcel Dekker, New York, 1985) p. 543.

[4] Federation Internationale de Football Association.

[5] The other isomer is called cis-polyacetylene. This has a slightly different structure with non-degenerate ground state energy.

[6] It is instructive to point out that while OPV 1-Сбо is the smallest member of the homologous series of ОРУїг-Сбо dyads, it lacks a vinylene bond and is therefore formally not an oligo(p-phenylene vinylenej-Ceo derivative.

[7] Fluorescence spectra were corrected for the Raman scattering of ODCB or toluene by subtracting the spectrum of the pure solvents from the spectra of the OPVn – Сбо solutions, after correcting for the absorbed light intensity by measuring the second-order diffraction of the excitation light from the grating of the monochro­mator.

[8] For n = 1 the question of an energy transfer is less relevant because the OPVI moiety cannot be excited without a simultaneous strong absorption of the fullerene moiety.

[9] For n = 1 the question of an energy transfer is less relevant because the

OPVI moiety cannot be excited without a simultaneous strong absorption of

the fullerene moiety.

[12] Formally, such an electron transfer could be considered as a hole transfer in which a positive charge is transferred from the fullerene to the oligo(p-phenylene vinylene).

[13] From similar calculations for the mixture of OPV1 and МР-Сво, free energies

for the charge-separated state of 2.87 and 1.76 eV are estimated in toluene and

ODCB, respectively. Since these energies are much higher than the МР-Сво

triplet energy, no photoinduced electron transfer is to be expected.

[17] Formally, such an electron transfer could be considered as a hole transfer in which a positive charge is transferred from the fullerene to the oligo(p-phenylene vinylene).

[18] across the thickness di, which is determined by the absorption coefficient d > 1 /a with the disadvantage L < d (e. g., amorphous hydrogenated

[19] There is no recombination of excess carriers at the surfaces nor in the area of a possible space-charge region.

[20] The conjugated polymer diluted by a proper host matrix shows less in­terchain interaction than pure films.

• Macroscopic ordering of the conjugated polymer can be performed by mechanical stretching of the host polymer.

• The stability of conjugated polymer-fullerene devices embedded in con­ventional polymers (guest-host approach) are higher due to encapsulation against environmental influences.

• By the choice of a proper host matrix, charge transfer between conjugated polymer and fullerene may be advantageously tuned, by changing either

[21] TlfcTljQj

nfaj + Щяк ‘

[22] large-area deposition of uniform ТіОг layers,

• development of methods for dye-staining and electrolyte-filling,

• internal electrical interconnection of individual cells,

• sealing of modules,

[23] structuring (electrical insulation) of the TCO-glass plates,

[24] hole-drilling on the counter electrode side,

[25] screen-printing of conductive silver lines for adequate current collection,

[26] screen-printing of colloidal ТІО2 and platinum-containing pastes on the front and counter electrodes,

[27] sintering of the ТЮ2 and platinum layers between 400 and 500°C,

[28] coloration of the ТІО2 electrode by chemical bath deposition,

[29] sealing/lamination of the front and counter electrodes,

[30] injection of electrolyte through filling holes and device closure,

[31] electrical contacting and wiring.

Updated: August 25, 2015 — 3:15 am