Category Organic Photovoltaics

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 ...

In order to predict outdoor module lifetimes, appropriate accelerated ageing tests are needed to make useful extrapolations to realistic outdoor conditions and to identify possible degradation mechanisms.

A systematic investigation of intrinsic chemical stability was carried out on devices specially designed for high power applications [7]. For this purpose, accelerated ageing test procedures were developed for nc-DSCs and it turned out that, to first order, a separation can be made between the effects of visible light soaking, UV illumination and thermal treatment on long term stability.

• Visible light soaking alone is not a dominant stress factor, which means that the dye (a ruthenium bipyridyl complex) used in these tests is sur­prisingly stable [7,18,19].

• UV light exposure often ...

Environmental and accelerated ageing tests were performed on the indoor dye modules described in Sect. 7.6 in order to detect failure mechanisms. Contrary to expectations, many modules survived humidity/freeze cycling tests (10 cycles, 85%, 20 h at 55°C per cycle) without major degradation, demonstrating the capability of the sealing concept (see Fig. 7.8). This was also true for temperature cycling (between —5 and 55°C). However, it has

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Ise (ЦА) Voc

Maximum Power (jaW) Fill Factor

been shown from accelerated testing (strong continuous illumination under a fluorescent lamp, including UV-B light) that UV light in particular is a serious degradation factor...

Besides the establishment of a reliable processing technology for nc-DSC, the long term stability of the cells/modules has to be guaranteed. The overall stability of the cell is controlled by the intrinsic and extrinsic stability. The intrinsic stability is related to irreversible (photo-)electrochemical degrada­tion of the dye and/or components of the electrolyte solution. Evaporation of the electrolyte solvent and intrusion of water and oxygen determine the extrinsic stability of the device. This requires hermetic sealing materials and reliable sealing methods, as discussed in Sect. 7.4. Standard testing according to IEC standards (environmental and accelerated) can be applied to nc-DSCs...

As mentioned in the introduction, nc-DSCs are expected in the short term to be available for low-power applications, competing with other thin film technologies, among which amorphous silicon is already an established tech­nology.

One aspect of indoor applications which makes sealing somewhat less critical is the low temperature range under operation and storage, as well as the possibility of using electrolytes with high viscosity and high boiling point. For commercialisation of indoor dye devices, it is of the utmost importance to simplify and optimize the currently known process steps.

In the above-mentioned European project [15], a number of partners have investigated every aspect of the production and testing of nc-DSC for indoor applications...

A number of groups are working on upscaling nc-DSC technology for indoor and outdoor applications. Details of technological processing are not normally provided at this stage, but some general information reported in various papers is summarised in this section.

The INAP/Germany consortium has been working on the upscaling and improvement of nc-DSC technology since 1995 [10-12]. The goal of the work at ШАР is to reach the pilot production stage for modules > 100 cm2 for high power applications. Their focus is on automating most process steps and reducing fluctuations in module properties introduced by manual handling and variable material quality...

The presence of a liquid electrolyte requires hermetic sealing of the module in order to prevent evaporation of the solvent as well as intrusion of water and oxygen. The sealing materials have to meet several requirements:

• chemical stability in contact with the liquid electrolyte,

• excellent barrier properties in order to minimise solvent losses (typically nitriles) during the service time of the module, and intrusion of water and oxygen,

• good and stable adhesion of substrates to the TCO and glass,

• processing compatible with other components of the nc-DSC.

Inorganic glass frits can be used to seal glass/glass modules...

An interesting and innovative approach to the industrial fabrication of nc – DSCs is to use a single-faced monolithic structure of consecutive porous lay­ers. In terms of production costs, the advantage of this concept over the ‘standard’ glass/glass design is that only one TCO-glass plate is required for a series connection of integrated cells, as is also known from amorphous silicon technology. This construction principle was first suggested in the liter­ature by Kay and Gratzel [8]. An AM 1.5 efficiency of 5.3% has been reported for a small module (20 cm2) containing 6 series-connected monolithic cells of

4.7

x 0.7 cm2.

Figure 7.5 illustrates the nc-DSC construction principle in the monolithic setup...