(603b) Engineering Nanoscale Film Morphology In Organic Photovoltaic Cells Using Graded Donor-Acceptor Heterojunctions

Organic photovoltaic cells (OPVs) have received considerable attention due to their potential low-cost, and compatibility with roll-to-roll fabrication and flexible substrates. The excitonic nature of OPVs necessitates the use of a donor-acceptor (D-A) heterojunction in order to dissociate the exciton into its constituent charge carriers. While the optical absorption length in most organic semiconductors is ~100 nm, excitons may typically diffuse only over a distance of ~10 nm. This trade-off between absorption and exciton diffusion is often overcome by increasing the area of the dissociating D-A interface using engineered film morphologies. This work presents an approach to realize optimized film morphology in OPVs based on small molecule organic semiconductors using a highly tunable, continuously graded D-A heterojunction (GHJ). The GHJ leads to an increase in the D-A interface area allowing for an enhanced exciton diffusion efficiency, while also preserving the charge collection efficiency. This simultaneous optimization leads to a significant improvement in device performance relative to that of conventional planar and uniformly mixed OPVs. Here, the performance of GHJs containing boron subphthalocyanine chloride (SubPc) and C₆₀ as the electron donating and accepting materials, respectively, is optimized based on supporting electrical and morphological characterization of uniform mixtures. Electrical characterization suggests optimum charge transport in mixed films containing 80 wt.% C₆₀. The origin of this C₆₀-rich optimum composition is explained in terms of morphological changes in the active layer upon diluting SubPc with C₆₀. While neat SubPc is found to be amorphous, mixed films containing 80 wt.% C₆₀ show clear nanocrystalline domains of SubPc. Using this optimum composition in GHJ OPVs, we demonstrate a power conversion efficiency of η_P=(4.5±0.1)% under 100 mW/cm² AM1.5G simulated solar illumination.