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 C₆₀, a power conversion efficiency of η_P = (2.1 ± 0.1)% is realized under 100 mW/cm² simulated AM1.5G solar illumination for a graded heterojunction. This represents an improvement in η_P of ~60% relative to a planar heterojunction OPV and ~20% compared to a uniformly mixed heterojunction OPV.