Synthesis and properties of new benzothiadiazole-based push-pull dyes for p-type dye sensitized solar cells

We report the synthesis of three new push-pull dyes specially designed for the sensitization of mesoporous p type semi-conductor NiO for the construction of p type dye sensitized solar cells (pDSC). Their structure stems from the conjugation of the well-known tris(carboxy-arlyamine) anchoring moiety to a benzothiadiazole acceptor through a fluorene (PP1) or thiophene-fluorene linker (PP2). Appending a NDI secondary acceptor onto PP2 yielded a dyad (PP2-NDI). The push-pull structure is here particularly adapted to the sensitization of p-type semi-conducting materials. All three dyes were investigated by spectroscopic and electrochemical techniques, and their electronic properties were modelled by TD-DFT. In all cases, the new sensitizers exhibit adequate absorption features (spanning from 400 nm to 600 nm, extinction coefficients between 20000 and 27000 M-1 cm(-1)) and their thermodynamic parameters are all in favor of efficient interfacial charge transfers involving NiO. pDSC were constructed with all three dyes; high photoconversion efficiencies (PCE) were reached, especially in the case of dyad PP2-NDI Usc = 5.90 mA/cm(2), V-oc = 143 my, FF = 33.8%, PCE = 0.29%), due to a combination of outstanding electronic properties and increased charge separation state lifetime. Tandem DSC (tDSC) composed of a TiO2 photoanode sensitized by the dye D35 and a NiO photocathode sensitized by PP2-NDI were assembled with two different electrolytes (I-3(-)/I- or T-2/T- based electrolytes). The latter gave PCE among the best ever reported for tDSC Usc = 5.90 mA/cm(2); Voc = 896 my; FF = 63%, PCE = 2.80% in the case of the I-3(-)/I- electrolyte, J(sc) = 4.52 mA/cm(2), V-oc = 840 mV, FF = 60% and PCE = 2.27% for T-2/T- based devices) unraveling the significance of the new dyes presented in this study. (C) 2017 Elsevier Ltd. All rights reserved.