Photoconversion in planar-heterojunction organic photovoltaic cells (OPVs) is limited by a short exciton diffusion length (LD) that restricts migration to the dissociating electron donor/acceptor interface. Consequently, bulk heterojunctions are often used to realize high efficiency as these structures reduce the distance an exciton must travel to be dissociated. Here, we present an alternative approach that seeks to directly engineer LD by optimizing the intermolecular separation and consequently, the photophysical parameters responsible for excitonic energy transfer. By diluting the electron donor boron subphthalocyanine chloride into a wide-energy-gap host material, we optimize the degree of interaction between donor molecules and observe a ∼50% increase in LD. Using this approach, we construct planar-heterojunction OPVs with a power conversion efficiency of (4.4 ± 0.3)%, > 30% larger than the case of optimized devices containing an undiluted donor layer. The underlying correlation between LD and the degree of molecular interaction has wide implications for the design of both OPV active materials and device architectures.
Bibliographical noteFunding Information:
This work was supported primarily by the National Science Foundation (NSF) Program in Solid State and Materials Chemistry (DMR-1006566). Partial support was also received from the University of Minnesota NSF Materials Research Science and Engineering Center (DMR-0819885) and the University of Minnesota Initiative for Renewable Energy and the Environment. The authors wish to acknowledge helpful discussions with D. A. Blank.