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

- New push pull dyes based on a benzothiadiazole-π-trisarylamine skeleton are designed for NiO pDSC.
- Highly performant pDSC were obtained owing to properly vectorialized photo-induced hole transfer and intense charge transfer absorption.
- The best performing dye is the PP2-NDI dyad; increased performances are due to decreased geminate charge recombination.
- The association of a PP2-NDI/NiO and a D35/TiO2 photoelectrodes afforded a tandem DSC with a 2.80% efficiency.

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 (Jsc = 5.90 mA/cm2, Voc = 143 mV, 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 (I3−/I− or T2/T− based electrolytes). The latter gave PCE among the best ever reported for tDSC (Jsc = 5.90 mA/cm2; Voc = 896 mV; FF = 63%, PCE = 2.80% in the case of the I3−/I− electrolyte, Jsc = 4.52 mA/cm2, Voc = 840 mV, FF = 60% and PCE = 2.27% for T2/T− based devices) unraveling the significance of the new dyes presented in this study.

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