Addition of regiorandom poly(3-hexylthiophene) to solution processed poly(3-hexylthiophene):[6,6]-phenyl-C61-butyric acid methyl ester graded bilayers to tune the vertical concentration gradient

Donor-acceptor vertical concentration gradient in the active layer is of crucial importance in graded bilayer poly(3-hexylthiophene):[6,6]-phenyl-C61-butyric acid methyl ester (P3HT:PCBM) solar cells. We demonstrate that upon addition of regiorandom P3HT to graded regioregular P3HT:PCBM bilayers, we are able to tune the vertical concentration gradient. With the help of energy-dispersive x-ray spectroscopy elemental mapping of the device cross-sections, we find a strong relationship between the concentration gradient profile and the device performances. Upon addition of regiorandom P3HT, the devices exhibit power conversion efficiencies up to 3.83% (compare to 3.09% for regioregular P3HT devices)