New Materials Promise Cheaper Photovoltaics

by L Stolt, University of Uppsala, Sweden

Introduction

Stand-alone photovoltaic energy systems are established technology in many applications, such as satellites, lighthouses or remote holiday homes. Systems connected to the public supply have been demonstrated in a large number of development projects, but are not yet financially viable. Researchers at the University of Uppsala and elsewhere are working to develop new solar cell materials that will lead to cheaper, more efficient solar systems suitable for grid-connection.

Background

The prices of solar cell modules must be reduced substantially if solar systems are to become financially viable. Today, factory gate prices of solar cells are about $4-5/Wp (where $ is the US dollar). This price would have to be reduced by two-thirds before such cells could be considered for even the most favourable mains-connected systems. A cost reduction to one-tenth of the present level, which is expected in the long term, would make solar cells economic for electricity generation for the majority of applications where storage of electricity is not required.
 It is also important that the cells' efficiency is sufficiently high, to restrict the area required for a given power output. The target efficiency generally quoted is 15%. The commonest types of present-day cell modules have an efficiency of 10-13%, but their production costs are too high.
 Maintenance and operating costs for mains-connected solar cell systems are negligible. The cost of the electricity is due mainly to the capital cost of the installation. It is therefore important that the system, and particularly the solar cell modules, have a sufficiently long life. Cost calculations are usually based on a 20-30 year lifetime.
 A decisive factor in determining how quickly large-scale utilisation of solar cells for mains-connected power generation becomes financially viable is how quickly the prices of the modules fall. In principle, the other aspects of the technology (such as the performance of the solar cell modules and costs of other system components) are already sufficiently good.

Thin-film Solar Cells

At present, the dominant solar cell technology is based on the use of crystalline silicon. This technology is mature and there is only limited potential for further cost reductions. In the future, thin-film cells could replace crystalline silicon cells, bringing the price down to a level low enough for large-scale mains-connected solar electricity production. One type of thin-film solar cell, based on amorphous silicon, has been available on the market for 6-7 years, but its efficiency is low and its life too short. Research into two other materials, CIS (CuInSe2) and CdTe, is well advanced, and both materials have achieved over 15% efficiency in laboratory tests.
 In principle, all thin-film technologies offer the same potential for low cell production costs. At present, three types are available: amorphous silicon, CdTe and CIS.

  • Amorphous silicon is the only thin-film technology that is established on the market. Its main problem is that, despite substantial R&D efforts, its efficiency is still low, at about 4-6% (stabilised) for commercial modules and 11% for the best laboratory cells.

 CdTe cells have achieved almost 16% efficiency, and the technology is becoming commercial. However, there are two problems with these cells. One is the use of cadmium which, as a heavy metal, presents an environmental hazard; the other is uncertainty as to the cells' long-term stability.

 CIS cells have achieved efficiencies of 17% in the laboratory. Module prototypes with efficiencies of 11% have been produced, and long-term stability has been found to be good. However, this is the most complicated material to use, and production methods for CIS are the least well developed.

 Research

 Research into CIS solar cells being carried out at the University of Uppsala is at the leading edge in the thin-film field. Efficiencies of over 17% have been achieved; the world record for all types of thin-film solar cells is 17.7%, achieved by an American group. Research is providing the knowledge that is important in the coming industrialisation of the technology.

The Market

 Solar cell technology is already established for certain niche applications. As costs fall, solar cells will become competitive in areas where more significant amounts of energy are needed. In the next few years, one of the most important markets will be in developing countries, where most of the demand will be for stand-alone systems.
 By the end of the century, it is expected that mains-connected solar electricity production will have begun to become more significant, mainly for supplying peak power in areas where air conditioning is widely used, although other applications are of interest, such as for the provision of system support to avoid the necessity of building additional power lines.

Conclusions

 In the next few years, new solar cell technologies will be transferred from the research laboratory to industrial production. The major breakthrough will happen when thin-film technology is successfully put into large-scale production and 10-15% efficient photovoltaic modules are produced at low costs. Perhaps the most promising candidate technology is CIS solar cells, but the development of appropriate manufacturing equipment and methods is expected to take 3-5 years.
 For more information contact the CADDET Swedish National Team in Stockholm.

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