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In situ synthesis of graphene molecules on TiO2: application in sensitized solar cells.

We present a method for preparation of graphene molecules (GMs), whereby a polyphenylene precursor functionalized with surface anchoring groups, preadsorbed on surface of TiO2, is oxidatively dehydrogenated in situ via a Scholl reaction. The reaction, performed at ambient conditions, yields surface adsorbed GMs structurally and electronically equivalent to those synthesized in solution. The new synthetic approach reduces the challenges associated with the tendency of GMs to aggregate and provides a convenient path for integration of GMs into optoelectronic applications. The surface synthesized GMs can be effectively reduced or oxidized via an interfacial charge transfer and can also function as sensitizers for metal oxides in light harvesting applications. Sensitized solar cells (SSCs) prepared from mesoscopic TiO2/GM films and an iodide-based liquid electrolyte show photocurrents of ∼2.5 mA/cm2, an open circuit voltage of ∼0.55 V and fill factor of ∼0.65 under AM 1.5 illumination. The observed power conversion efficiency of η=0.87% is the highest reported efficiency for the GM sensitized solar cell. The performance of the devices was reproducible and stable for a period of at least 3 weeks. We also report first external and internal quantum efficiency measurements for GM SSCs, which point to possible paths for further performance improvements.