Electronic structure of fullerene-squaraine complexes for photovoltaic devices

Squaraine dyes are promising materials to be used as electron donors in photodevices. We applied quantum chemical methods to investigate the electronic spectrum of three different squaraine dyes and of their complexes with fullerene C₆₀ in the gas phase. Based on the excited-state densities, the electronic states were characterized as localized, delocalized and charge transfer, allowing an understanding of the basic photophysics of these compounds. Comparison with experimental data showed that the measured short-circuit voltage is correlated not only to the energy of the charge-transfer state, but also to the energy of the lowest excited state, localized at the squaraine dyes. We discuss whether this may imply a non-Marcus regime for the charge-pair dissociation at the interface.