Define the model for charge injection and surface recombination at the contacts as described in the papers [1,2]. Phib is the injection barrier potential and N0 is the site density. The optional parameters Phib_h and P0 are the barrier potential and site density for holes, if values are not provided the same values as for electrons are assumed.
 J.Barker, C.Ramsdale, and N.Greenham. Modeling the current-voltage characteristics of bilayer polymer photovoltaic devices. Physical Review B, 67:075205 (9pp), 2003.  J.Campbell Scott and G.Malliaras. Charge injection and recombination at the methal-organic interface. Chemical Physics Letters, 299:115-119, 1999.