Jan Kroon and Andreas Hinsch
Ever since the invention of the silicon solar cell in the 1940s, people have acknowledged the enormous potential of photovoltaic systems for large scale electricity production. However, semiconductor grade silicon wafers are expensive so great effort has been put into developing cheaper thin-film solar cells and modules. Such films may be purely inorganic (amorphous silicon, cadmium telluride, copper-indium-diselenide) or contain organic materials as an essential part of the device. Examples of the latter are:
1. dye-sensitized photoelectrochemical solar cells (nc-DSC),
2. molecular organic solar cells (MOSC) made from relatively small organic molecules,
3. polymer organic solar cells mainly based on electrically conductive polymers.
These three types have an enormous potential for future photovoltaic applications. There are several reasons for this:
• Reduction of production costs: amounts and required purity of organic materials are low and large scale production is considered to be relatively easy compared with most inorganic materials, involving low temperature processing at atmospheric pressure, cheap materials and flexibility.
• They can in principle be tailored to all needs due to the infinite variability of organic compounds, and this makes them widely applicable.
The current status of research into the three above types can be summarised as follows. The interest in nc-DSC has increased enormously since the report by O’Regan and Gratzel in 1991 . This photoelectrochemical cell is based on a charge transfer from light-excited dye molecules to an inorganic semiconductor with a large bandgap. By using nanostructured ТІО2 that has pores on the nanometer scale, enough light can be absorbed to achieve useful efficiencies. Indeed, these devices have shown efficiencies up to 11% over small areas (0.25 cm2) and 5-8% over somewhat larger areas (1-5 cm2) . At this stage, fundamental and technologically oriented research are running in parallel. While many academic research groups are investigating the various unknown aspects of this solar cell, several companies and institutes around the world have concentrated their efforts on the technological development of efficient large-area multicell modules that are simple to make and stable in the long term.
Molecular organic solar cells, made in most cases from flat aromatic molecules like phthalocyanines and perylenediimides, have been under investigation since the early 1970s. In 1979 Kodak patented an organic two-layer p-n heterojunction with an efficiency of 0.95% and this remained valid for nearly 20 years. By then, the effect of doping with fullerene (Сбо) molecules had been investigated in a three-layer device and this resulted in an efficiency of 1.1% . Very recently, a significant efficiency enhancement was demonstrated for a laboratory Schottky-type MOSC based on single crystals of pentacene (T] = 4.5%) after molecular doping with bromine .
The youngest and fastest growing field in organic solar cell research is based on the use of electrically conducting polymers as photovoltaic materials. The inherent processing advantages of this technology, already developed for a number of thin film technologies (e. g., light-emitting displays LED, field-effect transistors FET), combined with the flexible possibilities for chemically tailoring desired properties, make polymer-based solar cells very attractive. Presently, external power conversion efficiencies of up to 3% have been achieved for laboratory cells consisting of a bulk heterojunction of light-absorbing polymers such as phenylene-vinylene and fullerene (Ceo) molecules . Although several aspects remain to be investigated (e. g., conducting polymers with better light absorbing properties, film morphology, device stability), these encouraging numbers promise a cheap production process for efficient ‘plastic’ solar cells in the future.
The photoelectrochemical (i. e., liquid-containing) nc-DSC is closer to market introduction than the fully organic/polymeric solar cells of types (2) and (3). For the short term (< 5 years), commercialisation of the nc-DSC technology is expected for low-power indoor applications such as calculators, watches, clocks, and electronic price tags. This should work as a stepping stone for the introduction of mid – and high power applications, which are mainly intended for outdoor use. In principle, the colour and design of the products for large area power applications can be varied to a larger extent than with several other solar cells, but there is always an optimum in the freedom of design and the performance of the cell.
For a successful market introduction of nc-DSC technology, several factors stand out: technical performance and manufacturability, cost, design, market demand and, last but not least, long-term stability.
In order to transfer the results achieved for small laboratory cells to a full production line for dye-sensitised solar modules to be used for indoor and outdoor applications, all process steps and technological parameters relevant for industrial production have to be investigated. Topics that are essential for reliable and cheap production technology are listed below: 
• long term stability,
• evaluation of process steps in terms of costs.
In this chapter, we first describe the basic working principles and components of the nc-DSC, then address a number of technologically related issues like manufacturing and long-term stability.