Defect induced non-ideal dark II–VV characteristics of solar cells

The non-ideal behavior of the dark current–voltage (II–VV) characteristics of typical silicon solar cells is characterized by (1) an unexpectedly large recombination current, often characterized by an ideality factor larger than 2, (2) an ohmic characteristic at low reverse bias, and (3) pre-breakdown at a reverse bias far below the expected breakdown voltage. Experimental evidence, especially from lock-in thermography results, shows that all these features are due to currents flowing locally in the edge region, or at certain extended crystal defects like grain boundaries. Detailed investigations on local breakdown sites in industrial solar cells are introduced. Though a realistic theory of these processes is still missing, a unified explanation of non-ideal dark II–VV characteristics is presented and several theoretical approaches to explain different aspects of this non-ideal behavior are discussed.