Tailoring porphyrin-based electron accepting materials for organic photovoltaics.

Abstract

The syntheses, potentiometric responses, optical spectra, electronic structural properties, and integration into photovoltaic devices are described for ethyne-bridged isoindigo-(porphinato)zinc(II)-isoindigo chromophores built upon either electron-rich 10,20-diaryl porphyrin (Ar-Iso) or electron-deficient 10,20-bis(perfluoroalkyl)porphyrin (Rf-Iso) frameworks. These supermolecules evince electrochemical responses that trace their geneses to their respective porphyrinic and isoindigoid subunits. The ethyne linkage motif effectively mixes the comparatively weak isoindigo-derived visible excitations with porphyrinic π-π* states, endowing Ar-Iso and Rf-Iso with high extinction coefficient (ε ∼ 10(5) M(-1)·cm(-1)) long-axis polarized absorptions. Ar-Iso and Rf-Iso exhibit total absorptivities per unit mass that greatly exceed that for poly(3-hexyl)thiophene (P3HT) over the 375-900 nm wavelength range where solar flux is maximal. Time-dependent density functional theory calculations highlight the delocalized nature of the low energy singlet excited states of these chromophores, demonstrating how coupled oscillator photophysics can yield organic photovoltaic device (OPV) materials having absorptive properties that supersede those of conventional semiconducting polymers. Prototype OPVs crafted from the poly(3-hexyl)thiophene (P3HT) donor polymer and these new materials (i) confirm that solar power conversion depends critically upon the driving force for photoinduced hole transfer (HT) from these low-band-gap acceptors, and (ii) underscore the importance of the excited-state reduction potential (E(-/*)) parameter as a general design criterion for low-band-gap OPV acceptors. OPVs constructed from Rf-Iso and P3HT define rare examples whereby the acceptor material extends the device operating spectral range into the NIR, and demonstrate for the first time that high oscillator strength porphyrinic chromophores, conventionally utilized as electron donors in OPVs, can also be exploited as electron acceptors.