Arylamine-Based Squaraine Donors for Use in Organic Solar Cells

Squaraine (SQ) dyes are notable for their exceptionally high absorption coefficients extending from the green to the near-infrared. In this work, we utilize the functionalized SQ donor: 2,4-bis [4-(N-phenyl-1-naphthylamino)-2,6-dihydroxyphenyl] squaraine (1-NPSQ) by substitution of isobutylamines in the common "parent SQ" with arylamines to improve stacking and hence exciton and charge transport. The strong electron-withdrawing arylamine group results in a highest occupied molecular orbital energy of 5.3 eV, compared to 5.1 eV for the parent SQ, making 1-NPSQ a suitable donor when used with a C-60 acceptor in an organic photovoltaic cell. Optimized and thermally annealed, nanocrystalline heterojunction 1-NPSQ/C-60/bathocuproine solar cells with an open circuit voltage of 0.90 +/- 0.01 V, fill factor of 0.64 +/- 0.01, and short circuit current of 10.0 +/- 1.1 mA/cm(2) at 1 sun, AM1.5G illumination (solar spectrally corrected) result in a power conversion efficiency of 5.7 +/- 0.6%. Crystallograpnic data suggest that the intermolecular stacking of 1-NPSQ molecules is closer than that of the parent SQ, thereby reducing the device series resistance and increasing its fill factor.