Recombination Processes and Lifetime Measurements in Silicon Photovoltaics

Recombination lifetime is one of the critical parameters in the search for cost-competitive photovoltaic technologies. Each technology has specific materials issues with respect to the role of recombination lifetime in the potential success of that technology. The dominant commercial technology for low-cost deployment of photovoltaics is currently based on various growth methods of bulk silicon. For low-cost terrestrial applications, the objective is to compromise efficiency while maximizing the efficiency-to-cost ratio. A frequent and cost-efficient tactic is to develop low-cost silicon purification and gettering processes, assessing the effectiveness of the latter by lifetime measurements. The recombination mechanism that affects low-cost silicon photovoltaics is the impurity-related Shockley-Read-Hall (SRH) process, and SRH-impurity removal is of primary concern. Here, I will present some results from a photovoltaic device model that links a theoretical efficiency to a given range of recombination lifetimes. Specialized measurement techniques are needed to get meaningful information about recombination lifetimes for these low-cost materials. Described here is a contactless photoconductive decay measurement system that has proven to be successful for most of these materials. Experimental results on a range of low-cost silicon alternatives will be presented.