Analysis of surface plasmon polariton mediated energy transfer in organic photovoltaic devices

Organic photovoltaics (PV) are constrained by a tradeoff between exciton diffusion and optical absorption. The short exciton diffusion length within organic semiconductors demands the use of extremely absorptive materials. Unfortunately, the excitonic character of most organic materials yields highly structured absorption spectra, with regions of strong and weak absorption. Here, we describe a device architecture that decouples light absorption and exciton diffusion in organic PV through the addition of a light absorbing 'antenna' layer external to the conventional charge generating layers. Radiation absorbed by the antenna is transferred into the thin charge generating layers via surface plasmon polaritons (SPP) in an interfacial thin silver contact and radiation into waveguide modes. SPPs are a particularly effective energy transfer mechanism as they propagate in the plane of the PV rather than parallel to the incident radiation, thereby providing a more efficient means of pumping thin charge generating structures. We exploit efficient SPP-mediated energy transfer by attaching a resonant cavity antenna to a conventional small-molecular weight organic PV. We find that the resonant cavity antenna boosts the performance of a phthalocyanine-based PV in the absorption gap between the phthalocyanine Q and Soret bands. Off resonance the antenna serves as a mirror, but near the resonant wavelength, the antenna absorption is significantly enhanced, and energy is fed back into the PV cell via SPP-mediated energy transfer. Thus, the resonant antenna may be employed to supplement the performance of the PV cell at resonance, with no degradation off-resonance.