Photoconversion in planar-heterojunction organic photovoltaic cells (OPVs) is limited by a short exciton diffusion length (L(D)) 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 L(D) 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 L(D). 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 L(D) and the degree of molecular interaction has wide implications for the design of both OPV active materials and device architectures.

This work demonstrates an approach for measuring the Förster radius of energy transfer between electron donating and accepting materials commonly used in organic photovoltaic cells (OPVs). While energy transfer processes are surprisingly common in OPVs, they are often incorrectly ignored in measurements of the exciton diffusion length and in models of device performance. Here, the effi ciency of energy transfer between an emissive donor and an absorptive acceptor is investigated through complementary experimental and theoretical techniques. This is accomplished by spatially separating the donor and acceptor materials using a wide-energy-gap spacer layer to suppress direct charge transfer and tracking donor photoluminescence as a function of spacer layer thickness. Fitting experimental data obtained for a variety of donor materials allows for the extraction of Förster radii that are in good agreement with predicted values. The impact of donor-acceptor excitonic energy transfer on device performance and on measurements of the exciton diffusion length is also investigated using the archetypical small molecule donor material boron subphthalocyanine chloride (SubPc). An average exciton diffusion length of 7.7 nm is extracted from photoluminescence quenching experiments using SubPc. This value is independent of the quenching material when the role of energy transfer is properly modeled.

We demonstrate enhanced power conversion efficiency in organic photovoltaic (OPV) cells incorporated into a plasmonic nanocavity array. The nanocavity array is formed between a patterned Ag anode and an unpatterned Al cathode. This structure leads to the confinement of optical energy and enhanced absorption in the OPV. Devices characterized under simulated solar illumination show a 3.2-fold increase in power conversion efficiency compared to OPVs with unpatterned Ag anodes. The observed enhancement is also reflected in the external quantum efficiency, and the spectral response is consistent with optical finite-difference time-domain simulations of the structure.

We demonstrate enhanced exciton diffusion in an organic photovoltaic cell through the incorporation of a phosphorescent sensitizer. The increase in exciton diffusion length (LD) is realized using a composite electron donor layer consisting of a N,N′-bis(naphthalen-1-yl)-N,N′-bis(phenyl)-benzidine (NPD) host doped with the phosphorescent guest fac-tris(2-phenylpyridine) iridium [Ir(ppy)3]. The presence of the phosphor at low concentration allows for the population of the long-lived NPD triplet state and an increase in LD. An increase in the NPD LD from 6.5±0.3 to 11.8±0.6 nm is extracted from measurements of the external quantum efficiency for donor layers containing 5 wt % Ir(ppy)3. This enhancement leads to a ∼80% improvement in the power conversion efficiency relative to devices containing an undoped donor layer.

Connecting molecular structure and exciton diffusion length in rubrene derivatives demonstrates how the diffusion length of rubrene can be enhanced through targeted functionalization aiming to enhance self-Förster energy transfer. Functionalization adds steric bulk, forcing the molecules farther apart on average, and leading to increased photoluminescence efficiency. A diffusion length enhancement greater than 50% is realized over unsubstituted rubrene.