Improved cathode buffer layer to decrease exciton recombination in organic planar heterojunction solar cells

We show that an advanced cathode buffer design, consisting of bathocuproine/3,4,9,10-perylenetetracarboxylic bis-benzimidazole/Ag, increases the short-circuit current of organic planar heterojunction cells and reduces the J-V slope at reverse voltages. We study the physical origin of these effects by measuring reflectivity, voltage dependent external quantum efficiency, and voltage dependent photoluminescence. Our findings suggest that the observed effects are mainly associated with a voltage dependent polaron-induced exciton quenching in the C-60 layer. Finally, this improved cathode buffer design is applied to a diindeno[1,2,3-cd:1',2',3'-lm] perylene/C-70 based cell, leading to a considerable planar heterojunction efficiency of 5.7%. (C) 2013 American Institute of Physics. [http://dx.doi.org/10.1063/1.4789852]