On Calculating the Current-Voltage Characteristic of Multi-Diode Models   for Organic Solar Cells
  We provide an alternative formulation of the exact calculation of the current-voltage characteristic of solar cells which have been modeled with a lumped parameters equivalent circuit with one or two diodes. Such models, for instance, are suitable for describing organic solar cells whose current-voltage characteristic curve has an inflection point, also known as an S-shaped anomaly. Our formulation avoids the risk of numerical overflow in the calculation. It is suitable for implementation in Fortran, C or on micro-controllers. 
Ken Roberts, S. R. Valluri
physics.comp-ph math.NA