Functionalized squaraine donors for nanocrystalline organic photovoltaics.

We study a family of functionalized squaraine (fSQ) donors for absorbing in the near-infrared (NIR) and green spectral regions. The NIR-absorbing materials are the symmetric molecules 2,4-bis[4-(N-phenyl-1-naphthylamino)-2,6-dihydroxyphenyl]squaraine (1-NPSQ), 2,4-bis[4-(N,N-diphenylamino)-2,6 dihydroxyphenyl]squaraine, and 2,4-bis[4-(N,N-dipropylamino)-2,6-dihydroxyphenyl]squaraine. The green light absorbing donors are asymmetric squaraines, namely, 2,4-bis[4-(N,N-diphenylamino)-2,6-dihydroxyphenyl]squaraine and 2-[4-(N,N-diisobutylamino)-2,6-dihydroxyphenyl]-4-diphenylamino]squaraine. Substitution of the arylamine groups enhances intermolecular packing, thereby increasing hole transport and the possibility of forming extended nanocrystalline junctions when annealed. Nanocrystalline solar cells based on fSQ and a C(60) acceptor have V(oc) = 1.0 V and fill factors 0.73 ± 0.01. Solar cells incorporating annealed 1-NPSQ films result in a power conversion efficiency of 5.7 ± 0.6% at 1 sun, AM1.5G illumination.