Reducing exciton-polaron annihilation in organic planar heterojunction solar cells

We investigate the relationship between charge concentration, exciton concentration, and photocurrent generation in fullerene-containing heterojunction diodes. Impedance measurements on C60 diodes reveal a charge buildup at the C60/bathocuproine (BCP) interface that can be swept out under reverse bias. In solar cell structures, a similar charge buildup is observed in dark conditions, and increases as a function of incident light intensity. Photoluminescence measurements reveal that the C60 exciton concentration is voltage dependent, explained via the process of exciton-polaron annihilation. This process has a negative impact on the generated photocurrent of the solar cells and thereby decreases the fill factor. A combination of electroabsorption, photoluminescence, and impedance measurements reveal a decrease in charge buildup and the associated exciton-polaron annihilation through the use of a BCP/3,4,9,10-perylenetetracarboxylic bis-benzimidazole/Ag cathode.