Light and Elevated Temperature Induced Degradation of Silicon Solar Cells

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The importance and urgency of reducing carbon dioxide emissions has received much publicity. Electricity generation is responsible for 38% of carbon emissions world wide. Of all sources of global warming electricity generation is probably, technologically, the most easily replaced by carbon free sources. Electricity from sunlight using the photo-voltaic effect, which we will refer to as solar PV, was very much a niche application as little as 15 years ago. However in the last decade silicon solar PV technology has developed with astonishing speed so that today it is the cheapest form of electricity generation in most countries within 45 degrees of the equator. Equally importantly the cost of manufacture is decreasing by 24% for each doubling of production volume, much faster than most products. At the moment Solar PV provides only 2.6% of the world's electricity (in kWh) although a higher percentage in some countries (eg 7.9% in Germany, 5.4% in India). There are a number of factors which delay the take up of this technology. The biggest difficulty is intermittency in countries like the UK where peak load does not match peak solar output necessitating pumped storage hydro or other rapid start up generation which adds to the cost. In tropical and sub-tropical countries solar generation matches the load much better and it is these countries in which electricity demand is increasing most rapidly. However in general there is a reluctance to invest in Solar which in part is due to Solar being regarded as an unproven technology and questions regarding long term reliability of a capital intensive system with a costing based on a projected life of >25 years. It is well known that silicon solar cells degrade. There are two commercially important mechanisms. One is due to a reaction involving boron and oxygen which happens very quickly reducing the efficiency by ~2% in the first 24 hours of operation. This is well enough understood for specialists to be on the way to developing ways of minimising the effect and demonstrating stability. The other mechanism is called "light and elevated temperature degradation" (LeTID). It takes months or sometimes years to produce a degradation of between 2 and 5%. The higher the light intensity and the higher the temperature the faster the degradation although there are large variations between different materials and solar cell designs which are not at all understood despite much behavioural data.The aims of this project are to develop a fundamental understanding of the degradation mechanism, to test proposed methodologies for reducing or eliminating LeTID and to use our understanding of the degradation mechanisms involved to develop meaningful accelerated life tests. Experimental work will be done in Manchester using test devices fabricated by us in Manchester and by the University of New South Wales (Australia). The prime techniques used will be optical, chemical and electrical measurements in Manchester and the Australian National University (Canberra) supported by modelling work at the University of Aveiro (Portugal). These will include lifetime spectroscopy, Deep Level Transient Spectroscopy and variants, admittance spectroscopy, low temperature photo-luminescence, time resolved photo-luminescence, Raman spectroscopy, hydrogen measurements and Secondary Ion Mass Spectroscopy. Materials and devices samples will be supplied by two manufactures active in the silicon solar field.