Organic Photovoltaic Cells Based on Continuously Graded Donor-Acceptor Heterojunctions

We demonstrate enhanced organic photovoltaic cell (OPV) efficiency through the use of continuously graded donor-acceptor (D-A) heterojunctions. Device performance is a strong function of both D-A grading and overall composition ratio. The use of a tunable gradient permits an increase in the D-A interface area for high-exciton diffusion efficiency relative to a planar heterojunction, while also improving the charge collection efficiency relative to a uniform mixture. Using the archetypical D-A pair of copper phthalocyanine and C-60, a power conversion efficiency of eta(P) = (2.1 +/- 0.1)% is realized under 100 mW/cm(2) simulated AM1.5G solar illumination for a graded heterojunction. This represents an improvement in eta(P) of similar to 60% relative to a planar heterojunction OPV and similar to 20% compared to a uniformly mixed heterojunction OPV.